Hazop guide to best practices

HAZOP: Guide to Best Practice
Guidelines to Best Practice for the
Process and Chemical Industries

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HAZOP: Guide to Best Practice
Guidelines to Best Practice for the
Process and Chemical Industries
Third Edition
Frank Crawley
Atkins, University of Strathclyde
Brian Tyler
S&T Consultants
(based upon the earlier editions by
Frank Crawley, Malcolm Preston, and Brian Tyler)
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
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Elsevier
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Third Edition 2015
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Notices
Knowledge and best practice in this field are constantly changing. As new research and
experience broaden our understanding, changes in research methods or professional practices,
may become necessary.
Practitioners and researchers must always rely on their own experience and knowledge in
evaluating and using any information or methods described herein. In using such information or
methods they should be mindful of their own safety and the safety of others, including parties for
whom they have a professional responsibility.
To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors,
assume any liability for any injury and/or damage to persons or property as a matter of products
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instructions, or ideas contained in the material herein.
ISBN: 978-0-323-39460-4
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CONTENTS
Foreword............................................................................................. ix
Acknowledgments.............................................................................. xiii
Chapter 1 Introduction .........................................................................1
1.1 Aims and Objectives.....................................................................1
1.2 Essential Features of HAZOP Study............................................2
Chapter 2 Process Hazard Studies........................................................4
2.1 HS 1—Concept Stage Hazard Review........................................4
2.2 HS 2—HAZID at Front-End Engineering Design (FEED) or
Project Definition Stage..............................................................5
2.3 HS 3—Detailed Design Hazard Study........................................5
2.4 HS 4—Construction/Design Verification....................................5
2.5 HS 5—Pre-Commissioning Safety Review..................................6
2.6 HS 6—Project Close-Out/Post Start-Up Review.........................6
2.7 HS 0—Consideration of Inherently Safer or Less Polluting
Systems .......................................................................................6
2.8 HS 7—Demolition/Abandonment Reviews ................................6
2.9 Overview of Hazard Studies .......................................................7
2.10 Illustrative Checklist for HS 2 ....................................................8
Chapter 3 The HAZOP Study Method ..............................................10
3.1 Essential Features.......................................................................10
3.2 The Purpose................................................................................11
3.3 Limitations .................................................................................11
Chapter 4 The Detailed HAZOP Study Procedure.............................13
4.1 The Description and Design Intention......................................13
4.2 Generating a Deviation ............................................................15
4.3 Identifying Causes.....................................................................17
4.4 Evaluating Consequences..........................................................19
4.5 Safeguards (Protection).............................................................19
4.6 Risk Assessment .......................................................................20
4.7 Recommendations/Actions .......................................................21

4.8 Recording .................................................................................22
4.9 Continuing and Completing the Analysis .................................22
4.10 An Illustration of the HAZOP Study Process...........................23
Chapter 5 Organizing a HAZOP Study .............................................29
5.1 Defining the Scope and Objectives of the Study and Establishing
the Boundaries............................................................................29
5.2 Appointing a Team Leader and Selecting the Team...................31
5.3 Preparation.................................................................................36
Chapter 6 Carrying Out a Study ........................................................43
6.1 Premeeting with Client ...............................................................43
6.2 Planning the Meetings ................................................................45
6.3 The Study Meetings....................................................................46
6.4 Coordinating and Reviewing Responses.....................................47
6.5 Completing and Signing Off the Report(s) .................................48
6.6 Follow-Up of Actions and Management of Change...................48
Chapter 7 Recording and Auditing......................................................50
7.1 Background Information ............................................................51
7.2 Section Headings ........................................................................51
7.3 The Recording Format for the Detailed Examination................51
7.4 The Level of Recording..............................................................52
7.5 The Content................................................................................54
7.6 Computer Recording ..................................................................54
7.7 Auditing a HAZOP Study..........................................................55
Chapter 8 Training .............................................................................58
8.1 Team Members...........................................................................58
8.2 Scribe..........................................................................................58
8.3 Team Leader ..............................................................................59
Chapter 9 Company Procedures for HAZOP Study...........................60
Chapter 10 Advanced Aspects of HAZOP Study................................62
10.1 HAZOP Study of Computer-Controlled Processes...................62
10.2 Human Factors.........................................................................69
10.3 Linking HAZOP Studies to LOPA...........................................74
vi Contents

Chapter 11 Specific Applications of HAZOP .....................................77
11.1 Modification of Existing Operations.......................................77
11.2 Repeat Designs—HAZOP-by-difference.................................78
11.3 Periodic Hazard Studies and the HAZOP of an Existing
Plant........................................................................................78
11.4 Operating Procedures..............................................................79
11.5 Pilot Plant and Laboratory Operations...................................80
11.6 Drains, Vents, and Other Interconnections Between Plants....82
11.7 Commissioning and Decommissioning ...................................84
11.8 Start-Up and Shutdown..........................................................85
11.9 Construction and Demolition .................................................89
11.10 Contract Operations ...............................................................90
Chapter 12 Factors for a Successful HAZOP Study...........................92
12.1 Throughout the Study...............................................................92
12.2 Before the Study .......................................................................92
12.3 During the Study ......................................................................93
12.4 After the Study .........................................................................94
Appendix 1: The Guideword-First Approach to HAZOP.....................95
Appendix 2: The Use of Checklists Within HAZOP Study .................99
Appendix 3: An Illustration of HAZOP Study for a
Continuous Operation....................................................101
Appendix 4: An Illustration of HAZOP Study for a
Batch Operation ............................................................123
Appendix 5: An Illustration of HAZOP Study for a Procedure.........139
Acronyms and Abbreviations .............................................................151
References and Bibliography .............................................................153
Index.................................................................................................155
vii
Contents

FOREWORD
FOREWORD TO THIRD EDITION
It is with great pleasure that I have been invited to offer a preface to
this, the third edition of HAZOP Guide to Best Practice which is cer-
tainly one of the most popular IChemE texts that has been developed
by EPSC members since the inception of the Centre in 1992.
This particular book has fond personal memories because several
years ago when working in industry I attended an IChemE HAZOP for
Team Leaders course with one of its authors, Brian Tyler, which was
held at the former UMIST campus in Manchester. The opportunity pre-
sented during that course of managing a HAZOP study team gave me
the necessary confidence back at the workplace to train frontline staff in
the use of the technique, then lead a team in a study on a fully function-
ing gin distillery, and finally present the recommendations to the site
executive team. I still have the course folder, and I am glad to see that
much of that content still forms the core of this current text.
Nevertheless much has happened since the first edition and nothing
stands still for long and so for the technique of HAZOP. There is now
much greater appreciation in HAZOP studies of human error in acci-
dent causation and more broadly human factors and the role of
automation. HAZOP studies are now performed routinely on continuous
plants at various stages of operation such as start-up and shutdown,
batch processing plants, and even packaging plants. The technique
of deviation analysis inherent in the method lends itself with care and
imagination to many diverse major hazard environments.
As for this third edition, the authors, Brian Tyler and Frank Crawley,
are to be praised for their collective efforts in revising this book yet again
and keeping the content as fresh and topical as possible. HAZOP pro-
vides both a structure for the team identification of hazards, accident sce-
narios, and operability issues while offering the chance for an element
of creative thinking for a team whose time and effort is well managed.
If anything the pressure in recent years has grown on the typical HAZOP
team to identify and address all conceivable hazards arising from

dangerous operations which makes this new edition particularly welcome
for students and practitioners alike.
I have no doubt that EPSC members who contributed to the first
edition will be immensely proud to see that this book has become a
standard reference among the process safety community.
Lee Allford
EPSC Operations Manager
IChemE
Davis Building
Rugby CV21 3HQ
December 2014
FOREWORD TO EARLIER EDITIONS
Hazard and Operability Studies (later shortened to HAZOP) were
devised by ICI in the late 1960s, following some major problems with
new, large process plants. The study was an evolution of method study
and was used during the design stage of a project to identify and correct
design faults which might lead to Hazard or Operability problems. Over
the last few decades, the need for high standards in safety and the environ-
ment is fully recognized by the Regulator, the industry, and the public.
HAZOP is now the first choice tool for the identification of weaknesses
in the process design and is used worldwide within the process industry.
It has been used in a modified form outside the process industry.
The first definitive guide on Hazard and Operability Studies was
issued by the Chemical Industries Association in 1974 when the tool
had been fully developed. This remained the main guidance for 26 years.
However, in that period, new ideas on HAZOPs had been developed
and equally some poor practices had been adopted. In 1998, it was
decided that a new guide, using best practice, should be written. The
first edition of this guide was published in 2000. For the second edition,
the authors took the opportunity of reviewing that guide and incorpo-
rating better practices and giving more guidance on how these might be
applied. In particular the new guide addresses Computer-Controlled
Processes.
Although the basic approach of HAZOP is unchanged, there is now
considerable experience in how the technique can be used most
x Foreword

effectively. This experience has been drawn upon in preparing this
guide, with a total of 31 companies contributing to its preparation.
Finally, the guide is important as a joint project which has the sup-
port of the Institution of Chemical Engineers (IChemE), the Chemical
Industries Association (CIA), and the European Process Safety Centre
(EPSC).
Our thanks are due both to the authors of the guide and to the
many industrial members who assisted in its development.
Richard Gowland
Technical Director
European Process Safety Centre
xi
Foreword

ACKNOWLEDGMENTS
This first edition of this guide would not have been possible without
the contributions of the following individuals and the support of their
companies:
M. Dennehy Air Products plc
J. Snadden Air Products plc
A Jacobsson AJ Risk Engineering AB
D. Hackney Akcros Chemicals
K. Brookes Akcros Chemicals
R. Vis van Heemst Akzo Nobel Engineering bv
K. Gieselburg BASF AG
K-O. Falke Bayer AG
V. Pilz Bayer AG
H. Crowther BP Chemicals
H. Jenkins BP Chemicals
J-H. Christiansen Borealis
M. Scanlon Chemical Industries Association
F. Altorfer Ciba Specialty Chemicals
T. Gillard Consultant
S. Duffield EC JRC
J-C. Adrian Elf Atochem
P. Lawrie EniChem UK Ltd
M. Powell-Price EPSC
R. Turney EPSC
R. Carter Eutech
A. Ormond Eutech
C. Swann Eutech
S. Turner Foster Wheeler
H. Wakeling Foster Wheeler
T. Maddison Health and Safety Executive (UK)
A. Rushton Health and Safety Executive (UK)
K. Patterson Hickson and Welch
J. Hopper Huntsman Polyurethanes
M. Wilkinson Institution of Chemical Engineers

J. Geerinck Janssen Pharmaceutica NV
G. Gillett Kvaerner Water
A. Poot Lyondell Chemie Nederland Ltd
E. Dyke Merck
C. Downie Merck and Co Ltd
K. Dekker Montell Polyolefins bv
J. Ytreeide Norsk Hydro AS
D. Moppett Proctor and Gamble European Technical Centre
P. Rouyer Rho^
ne-Poulenc
C. Caputo Rohm and Haas Italia
G. Dilley Rohm and Hass (Scotland) Ltd
F. de Luca Snamprogetti S.p.A
C. Bartholome
Solvay SA
P. Depret Solvay SA
J. Ham TNO
K. Ling Zurich International
The initiative of the IChemE Safety and Loss Prevention Subject
Group and the support of the IChemE Safety Health and Environmental
Policy Committee are recognized.
While all of the above contributed to this guide, the authors would
like to give special thanks to the following for their contributions and
support: Rob Carter, Howard Crowther, Martin Dennehy, Julian
Hooper, Hedley Jenkins, Ken Patterson, Chris Swann, Simon Turner,
and Robin Turney.
The authors would also like to give special thanks to Phil Aspinall
of VECTRA Group Limited for his help and contribution in preparing
the second edition of this guide, in particular for the development
of Section 10.1 on HAZOP Study of Computer-Controlled Processes.
The authors of the third edition wish to thank Jerry Lane (Optimus)
for his help during the preparation of this revision.
xiv Acknowledgments

CHAPTER 1
1
Introduction
1.1 AIMS AND OBJECTIVES
This book is intended to provide guidance on a specific technique
developed for use in the process and chemical industries. The technique
described is HAZOP (hazard and operability) study, a detailed method
for systematic examination of a well-defined process or operation,
either planned or existing.
The HAZOP study method was developed by ICI in the 1960s and
its use and development was encouraged by the Chemical Industries
Association (CIA) Guide published in 1977. Since then, it has become
the technique of choice for many of those involved in the design of
new processes and operations. In addition to its power in identifying
safety, health, and environmental (SHE) hazards, a HAZOP study can
also be used to search for potential operating problems. Not sur-
prisingly, the method has been applied in many different ways within
the process industries.1
While it is frequently used on new facilities, it is now often applied
to existing facilities and modifications. It has also been successfully
applied to process documentation, pilot plant, and hazardous labora-
tory operations as well as tasks such as commissioning and decom-
missioning, emergency operations, and incident investigation.
The objective here is to describe and illustrate the HAZOP study
method, showing a variety of uses and some of the approaches that
have been successful within the process industry. An important input
has come from European Process Safety Centre (EPSC) members
where 22 member companies responded in a survey carried out prior
to the first edition of this Guide (2000). This identified many features
generally regarded as essential to a good study. In addition, many
common variations were described. These variations are in part due
to the range of problems encountered within industry but also reflect
individual choices of style. HAZOP study is a versatile technique and
HAZOP: Guide to Best Practice. DOI: http://dx.doi.org/10.1016/B978-0-323-39460-4.00001-3
Copyright © 2015 Elsevier Ltd. All rights reserved.

good results may be achieved by several different approaches provided
the basic principles are followed. It is hoped that this Guide will help
maintain a high standard for HAZOP study within the industry, both
by raising quality and encouraging flexibility without putting any
unnecessary constraints upon its use and development.
The HAZOP study method is well developed and is useful in most
applications. There are other methods, however, that may have to be
considered depending on the complexity and hazards of the installation
being constructed and the state of the design. This publication does
not address these methods in detail but their importance is discussed
in Chapter 2. A fuller account is given in the IChemE Guide, Hazard
Identification Methods.2
Finally, three illustrations of process industry applications are given
in Appendices 3, 4 and 5. These examples cannot fully represent all the
possible applications and process industries and readers new to HAZOP
study are advised to consult the reference list,3-6
the International
Electrotechnical Commission (IEC) standard,7
or guidelines written
specifically for other industries.
It is hoped that this guide will help people within the process
industries, including all those with responsibilities within safety man-
agement systems. Although it is primarily written for HAZOP study
leaders, scribes, and members, it may also be of use to those involved
in training and plant management.
1.2 ESSENTIAL FEATURES OF HAZOP STUDY
A HAZOP study is a structured analysis of a system, process, or
operation for which detailed design information is available, carried
out by a multidisciplinary team. The team proceeds on a line-by-line
or stage-by-stage examination of a firm design for the process or
operation. While being systematic and rigorous, the analysis also aims
to be open and creative. This is done by using a set of guidewords in
combination with the system parameters to seek meaningful deviations
from the design intention. A meaningful deviation is one that is physi-
cally possible—for example, no flow, high pressure, or reverse reaction.
Deviations such as no temperature or reverse viscosity have no sen-
sible, physical meaning and are not considered. The team concentrates
2 HAZOP: Guide to Best Practice

on those deviations that could lead to potential hazards to safety,
health, or the environment. It is important to distinguish between the
terms hazard and risk. They have been defined8
as follows: a “hazard” is
a physical situation with the potential for human injury, damage to pro-
perty, damage to the environment, or a combination of these. A “risk” is
the likelihood of a specified undesirable event occurring within a speci-
fied period or in specified circumstances. It can also be expressed as a
combination of likelihood and severity, as illustrated in Figure 1.1.
In addition to the identification of hazards, it is common practice for
the team to search for potential operating problems. These may concern
security, human factors, quality, financial loss, or design defects.
Where causes of a deviation are found, the team evaluates the
consequences using experience and judgment. If the existing safeguards
are adjudged to be inadequate, then the team recommends an action
for change or calls for further investigation of the problem. The con-
sequences and related actions may be risk-ranked. The analysis is
recorded and presented as a written report which is used in the
implementation of the actions.
To avoid misunderstanding and confusion with other forms of
process hazard study or project hazard review, the term HAZOP study
is reserved for studies which include the essential features outlined
earlier in this section. Descriptions such as “coarse scale HAZOP” or
“checklist HAZOP” should be avoided.
Likelihood
Severity
High
risk
Medium
risk
Low
risk
Figure 1.1 Illustrative risk graph.
3
Introduction

CHAPTER 2
2
Process Hazard Studies
Within the process industries, significant attention has been given to the
development of comprehensive safety management systems (SMSs) or
SHE management systems with the objective of protecting workers, the
public, and the environment. There are also requirements within legislation
such as the Seveso II Directive, European Union (EU) Directive 96/82/
EC,9
and subsequent country-specific legislation, requiring those compa-
nies handling hazardous materials to have in place an adequate SMS and
to fulfill specified obligations. These requirements range from the prepara-
tion of Major Accident Prevention Policies to submission of detailed safety
reports to a competent authority. Other non-EU countries have similar leg-
islation—for example, the Office of Safety and Health Administration
(USA) (OSHA) regulation 29CFR, Part 1910.119 (1992), Process Safety
Management (PSM)10
in the USA. An integral element of such systems is
the use of systematic techniques for the identification of hazards. In addi-
tion to meeting legal requirements, there are considerable business benefits
to be gained from the use of a systematic and thorough approach to
hazard identification. These benefits include improvement of quality, faster
start-up, and a reduction of subsequent operability problems.
For a new project, the greatest benefit is obtained by carrying out
a number of studies throughout the design process. One such sequence
is the Hazard Study (HS) methodology developed by ICI which used
six stages.4,11
Each study verifies that the actions of previous studies
have been carried out and signed off, and that the hazard and environ-
mental issues have been identified and are being addressed in a timely
and detailed manner.
2.1 HS 1—CONCEPT STAGE HAZARD REVIEW
In this first study, the basic hazards of the materials and the operation
are identified and SHE criteria set. It identifies what information is
needed and the program of studies required to ensure that all SHE
issues are adequately addressed. The aspects covered may include

reaction kinetics, toxicity data, environmental impact, and any special
process features that need further evaluation. In addition, any con-
straints due to relevant legislation are identified. A decision may be
taken on which of the remaining hazard studies (two to six) should
also be undertaken. It is also important at this early stage to apply
the principles of inherent SHE12,13
within the design. This aims to
eliminate, avoid, or reduce potential hazards in the process.
2.2 HS 2—HAZID AT FRONT-END ENGINEERING DESIGN
(FEED) OR PROJECT DEFINITION STAGE
This study typically covers hazard identification and risk assessment,
operability and control features that must be built into the detailed
design, and any special environmental features to be covered.
It is important that the safety integrity levels (SILs)14,15
of any safety
instrumented systems (SISs) are addressed during this study as the
design will still be flexible and simple design changes may be applied
which will reduce the SILs and so simplify the design.
At the end of HS 2, the level of development of design and piping
and instrumentation diagrams (PIDs) would be “approved for design”
(AFD). All the main features should have been added but the finer ones
will not. It is helpful to examine the AFD diagrams for the more blatant
errors using a form of checklist. An example is given in the final section
of this chapter.
2.3 HS 3—DETAILED DESIGN HAZARD STUDY
This normally involves a detailed review of a firm design aimed at the
identification of hazard and operability problems. Relief and blow-
down studies, area classification, personal protection, and manual han-
dling may, if appropriate, be included at this stage. HAZOP studies
are normally carried out at this stage.
2.4 HS 4—CONSTRUCTION/DESIGN VERIFICATION
This review is performed at the end of the construction stage. The
hardware is checked to ensure it has been built as intended and that
there are no violations of the designer’s intent. It also confirms that the
actions from the detailed design hazard study are incorporated, and
operating and emergency procedures are checked.
5

2.5 HS 5—PRE-COMMISSIONING SAFETY REVIEW
This examines the preparedness of the operations group for start-up and
typically covers training, the final operating procedures, preparation pro-
cedures, and readiness for start-up including function testing, cleanliness,
and purging. Confirmation of compliance with company and legislative
standards is done at this stage, for example, under the Pre Start-up Safety
Review (PSSR) required under the OSHA PSM legislation in the USA.
2.6 HS 6—PROJECT CLOSE-OUT/POST START-UP REVIEW
This study, carried out a few months into the production phase,
confirms that all outstanding issues from the previous five studies are
complete and seeks any lessons that might give useful feedback to
future design work.
In addition to these six studies, two more may be included. These
are usually referred to as study zero and study seven, to fit with the
numbering scheme used above.
2.7 HS 0—CONSIDERATION OF INHERENTLY SAFER
OR LESS POLLUTING SYSTEMS
Study zero takes place between the Research and Technical Departments
before the concept stage. It attempts to identify and to incorporate the
inherently safer and greener ideas as early as possible so that they will be
part of the final design.
2.8 HS 7—DEMOLITION/ABANDONMENT REVIEWS
This study can take place before the final shut down but the objective
of the study is to identify those issues which should be dealt with
during the demolition process. It should address issues such as cleaning
methods and standards, size reduction, recovery and recycle of work-
ing inventories, recycle of equipment, safe disposal of nonrecyclable
materials/equipment, and location of potentially harmful/toxic materials
in the equipment or soil. In addition it should address the integrity of lift-
ing devices/brackets, access routes, and the sequence of removal bearing
in mind that some equipment may be supporting other equipment.

2.9 OVERVIEW OF HAZARD STUDIES
The relationship of HS 1 6 to the project life cycle is shown in
Figure 2.1. Experience shows that the use of HS 1 and 2 ensures key
conceptual issues are dealt with early in the life of the project and not
left to the HAZOP study. Use of HS 1 and 2 makes the HAZOP study
easier and faster.
While HAZOP is one of the most flexible techniques for hazard
identification, there are other identification and assessment techniques
which can be used at the detailed design stage to supplement HAZOP
(Figure 2.2).
Stage 5
Pre-
commis-
sioning
Project phase
Stage 3
Stage 1 Stage 2
Process
design
Concept Detailed
engineer-
ing
Stage 4
Construct-
ion
Stage 6
Post-
commis-
sioning
Conceptual
Process
develop-
ment
Project
sanction
Design, engineering
construction
Hand
over
Operation
Figure 2.1 Relationship of six-stage process study system to project life cycle. (Source: EPSC, 1994, Safety
Management Systems (IChemE, UK))
Hazard potential
Replicated
design
Yes
Consider
Checklists Checklists
What if study
HAZOP
Checklists
What if study
HAZOP
HAZOP
QRA
Safety reviews
Job safety analysis
FMEA/FMECA
FMEA/FMECA
FMEA/FMECA
Risk ranking
Risk ranking
Fault trees
Hazard analysis
LOPA
LOPA
HAZOP by difference
(see section 11.2)
Design codes
Safety reviews
Job safety analysis
Task analysis
HAZOP by difference
(see section 11.2)
Consider Consider Consider
No Replicated
design
Yes No
Low High
Figure 2.2 Possible alternatives to replace or supplement HAZOP study as a detailed design hazard study.
7

Whenever HAZOP study is the chosen technique, its use should be
justified on the basis of complexity, inherent hazards, or the costs of
the operation. While HAZOP study is ideally suited to novel processes,
hazardous process or complex processes, it can equally be used in
simple and repeat designs although there may be fewer benefits.
Considerable benefits can also be found from its use for modifications
or for change of use of plant.
More information on the techniques used for hazard identification
and the way in which these relate to an overall process for risk assessment
is given by Pitblado16
and by Crawley and Tyler.2
2.10 ILLUSTRATIVE CHECKLIST FOR HS 2
An example of a checklist for use at conclusion of HS 2 to ensure com-
mon problems have been considered and are covered before the
detailed design work begins.
• Are piping materials of construction in compliance with codes?
• Are spec breaks in the correct place? Are high- to low-pressure
interfaces identified and given the correct treatment?
• Is the mass balance measured and achieved by the controls?
• Do any control parameters require independent verification?
• Do the protective systems (and SIS where appropriate) give adequate
protection against the known hazards?
• Do SIS shut downs have pre-alarms?
• Have low flow conditions at start-up and shut down been addressed?
Are any vents, recycle lines, and bypasses required?
• Are the standards of isolation appropriate for the risks?
• Is lagging appropriate to the piping codes?
• Are maintenance vents and drains shown?
• Are there traps in vessels that may need special features for entry?
• Are there any possible settling out points in piping or equipment
which may need treatment?
• Has over pressure protection been applied as appropriate?
• Does the over/under pressure relief indicated look “adequate”?
• Is the design of the relief collection consistent? Can incompatible
materials mix in the system?
• Do the vents go to a safe location?
• Do the drains go to a safe capture system?

• Is the design of the vent collection consistent? Can incompatible
materials mix in the vent system?
• Is the design of the drain collection consistent? Can incompatible
materials mix in the drain system?
• Are the main parameters (pressure, temperature, flow, and level)
adequately controlled and are there adequate diagnostics to assess
problems?
• Are line slopes shown and are they appropriate?
• Are there any pockets in the lines which may require drains?
• Are liquid capture bunds required anywhere for S or E requirements?
9

CHAPTER 3
3
The HAZOP Study Method
3.1 ESSENTIAL FEATURES
A HAZOP study is a structured and systematic examination of a
planned or existing process or operation. At the outset of the study,
the team creates a conceptual model (design representation) of the
system or operation. This uses all available, relevant material such as a
firm, detailed design, an outline of operating procedures, material data
sheets, and the reports of earlier hazard studies. Hazards and potential
operating problems are then sought by considering possible deviations
from the design intention of the section or stage under review. The
design intention is a word picture of what should be happening and
should contain all of the key parameters that will be explored during
the study. It should also include a statement of the intended operating
range (envelope). This is usually more limiting than the physical design
conditions. For those deviations where the team can suggest a cause,
the consequences are estimated using the team’s experience and exist-
ing safeguards are taken into account. Where the team considers the
risk to be nontrivial or where an aspect requires further investigation,
a formal record is generated to allow the problem to be followed up
outside the meeting. The team then moves on with the analysis.
The validity of the analysis obviously depends upon having the
right people in the team, the accuracy of the information used, and
the quality of the design. It is normally assumed that the design work
has been done in a competent manner so that operations within the
design envelope are safe. Even where this is the case, the later stages of
the project must also be carried out correctly—that is, engineering
standards are followed and there are proper standards of construction,
commissioning, operation, maintenance, and management. A good
HAZOP study tries to take account of these aspects and of the changes
that can reasonably be expected during the lifetime of the operation.
A study will sometimes identify problems that are within the design
limits as well as problems which develop as the plant ages or are
caused by human error.

A key feature of timing of a HAZOP study is that the design must
be firm and the PIDs must be frozen—a situation that requires
management commitment and forward planning.
3.2 THE PURPOSE
One purpose of a HAZOP study is to identify and evaluate any
remaining hazards within a planned process or operation that were not
identified or designed out in earlier stages. The hazards may be several
types, including those to people and property, both on- and off-site. It
is also important to consider the potential effects to the environment.
Regardless of the type of hazard, many have directly related financial
consequences.
HAZOP studies are also normally used to identify significant
operability or quality problems and this will be included as a defined
objective of a study. A survey of EPSC members carried out in 1999 as
part of the preparation for the first edition of this Guide found that
over 90% of the respondents included significant operability problems
in the scope of the search. Operability problems arise through the
reliability as well as the manner of the plant operation, with con-
sequences such as downtime, damaged equipment, and the expense of
lost, spoilt, or out-of-specification product leading to expensive re-run
or disposal costs. The need to consider quality issues varies greatly
with the details of the operation but in some industries it is a crucial
area. Of course, many operability problems also lead to hazards,
giving a dual reason for identifying and controlling them. A HAZOP
study may also consider quality issues in the proposed design.
It is advisable to cover aspects of maintenance operations, including
isolation, preparation, and removal for maintenance since these often
create hazards as well as an operability problem. Where there are
manual operations or activities, it may be necessary to analyze the
ergonomics of the whole operation or activity in detail.
3.3 LIMITATIONS
Difficulties may be caused by inadequate terms of reference or poor
definition of the study scope. The intention of a HAZOP study is not
to become a re-design meeting. Nevertheless, some actions may result
11
The HAZOP Study Method

in changes to the design and potential problems may be found within
the intended range of operation.
The analysis of problems within a HAZOP study is normally
qualitative although, increasingly, simple risk assessment is used to
help the team to decide on the need for action and the action itself.
Some of the problems may need a fuller quantitative analysis, includ-
ing quantitative risk assessment (QRA). This would be done outside
the HAZOP meeting.
A HAZOP study is not an infallible method of identifying every
possible hazard or operability problem that could arise during the
actual operations. Expertise and experience within the team is crucial
to the quality and completeness of a study. The accuracy and extent
of the information available to the team, the scope of the study, and
the manner of the study all influence its success. Only a systematic,
creative, and imaginative examination can yield a high-quality report
but even then, not every potential problem will necessarily be found.
Additionally, the study will only be effective if the issues identified
during the study are resolved and put into practice. Some important
factors for success are listed in Chapter 12.

CHAPTER 4
4
The Detailed HAZOP Study Procedure
The actual study must proceed in a carefully planned, systematic manner
to cover all of the selected aspects of the process or operation. It is normal
to cover a continuous operation by dividing it into sections and work-
ing from an upstream starting point. A batch process or a procedure is
divided into sequential steps and these are taken in a chronological order.
The division of a process into sections or steps is described in more detail
in Section 5.3 and illustrated in Appendices 3 5. The pattern of analysis
for an individual section or stepa
is shown in Figure 4.1, and its main
elements are described in the following sections.
4.1 THE DESCRIPTION AND DESIGN INTENTION
It is essential the team begins with a full understanding of the section or
stage to be analyzed, either knowing the existing situation or having
sufficient information to be able to form an adequate conceptual model.
A full description should be developed, including all the key parameters,
and the HAZOP report should include the design description.
Next, a design intention for the step is formulated and recorded. This
should include a statement of the intended operational range (envelope)
so that the team can recognize any situations lying outside this range as
deviations. The design intention may be interlinked with the step
description and hence to the design parameters of the equipment.
It is good practice to develop a comprehensive design intention,
clearly linked to the drawings being used, which can be referred to during
the search for deviations. A design intention may refer to equipment
items in the section, to materials, conditions, sources, and destination, to
changes or transfers, as well as to the means of control and timing of a
step. It not only refers to plant equipment but covers what is intended to
be done within the section being analyzed.
a
Other terms sometimes used in place of section or step include node, stage, and part.

Figure 4.1 Flow diagram for the HAZOP analysis of a section or stage of an operation—the parameter-first
approach.

The recording of the design intention should include sufficient infor-
mation to enable a later user of the records to understand the picture
developed and used by the HAZOP team during their study.
4.2 GENERATING A DEVIATION
The next step is to generate a meaningful deviation by coupling a
guidewordb
and a parameter.c
A deviation can be generated by taking
a parameter and combining it with each guideword in turn to see if a
meaningful deviation results (the parameter-first approach). This is the
method described in Figure 4.1. The alternative approach is to take a
guideword and try each parameter in turn (the guideword-first
approach). More details of the guideword-first approach are given in
Appendix 1, pages 95 97.
The standard set of guidewords for process plant is listed in Table 4.1,
alongside their generic meanings. The first seven are normally used,
with the others included if appropriate. As the purpose of the guide-
words is to assist the team in a creative and thorough search for
meaningful deviations, it is important to select a set that works well
for the problem being studied. Variations of the standard set may be
tried or others added to the list. Some companies have developed
their own set of guidewords for particular technologies.
While clear recommendations can be made as to which guidewords
should be considered, it is not possible to provide such firm advice
regarding parameters. The selection of parameters is a task each team
must address for each system studied. Table 4.2 gives examples of
parameters that might be used in the analysis of a process operation.
This list is not exhaustive but is intended to show the depth and
breadth of the parameter and guideword search that can be used. It
must be emphasized that many of the parameters listed will not apply
to every issue or process as parameters relate to the individual system,
process, or operation being studied.
b
The term “guideword” is used here for an action word or phrase such as “no,” “more of,” and
“as well as.” Other authors have used alternative terms such as keyword.
c
The term parameter is used here as the generic name for a variable, component, or activity
referred to in the stage under study—for example, flow, pressure, transfer, and measure. Many
alternative terms have been used, including keyword, property word, element, and characteristic.
We discourage the use of keyword for either guideword or parameter as it may lead to confusion.
15

The extent of this list emphasizes the need for the team to form a
clear conceptual model of the step and to use it to decide which
parameters should be used in the search for possible deviations. When
seeking deviations it must be remembered that not every guideword
Table 4.2 Examples of possible parameters for process operations
• Flow
• Pressure
• Temperature
• Mixing
• Stirring
• Transfer
• Level
• Viscosity
• Reaction
• Composition
• Addition
• Monitoring
• Separation
• Time
• Aging
• Phase
• Speed
• Particle size
• Measure
• Control
• pH
• Sequence
• Signal
• Start/stop
• Operate
• Maintain
• Diagnostics
• Services
• Communication
Table 4.1 Standard guidewords and their generic meanings
Guideword Meaning
No (not, none) None of the design intent is achieved
More (more of, higher) Quantitative increase in a parameter
Less (less of, lower) Quantitative decrease in a parameter
As well as (more than) An additional activity occurs
Part of Only some of the design intention is achieved
Reverse Logical opposite of the design intention occurs
Other than (other) Complete substitution—another activity takes place OR
an unusual activity occurs or uncommon condition exists
Other useful guidewords include:
Where else Applicable for flows, transfers, sources, and destinations
Before/after The step (or some part of it) is effected out of sequence
Early/late The timing is different from the intention
Faster/slower The step is done/not done with the right timing
Interpretations of the guidewords for computer-controlled systems (programmable
electronic system, PES) are given in the IEC HAZOP Application Guide.7

combines with a parameter to give a meaningful deviation. It is a
waste of time to discuss combinations which do not have a physical
meaning. Some examples of meaningful combinations are given in
Table 4.3. Many parameters will emerge from the step description and
statement of the design intention, provided it is explicit and com-
prehensive. In addition, a good team is likely to identify further para-
meters during the examination, particularly for the later guidewords
“as well as,” “part of,” and “other than.” It is good practice to apply
all of the guidewords to the design intention before leaving a node.
Most of the combinations in Table 4.3 have obvious meanings but,
as an example of the subtlety possible in HAZOP study, reverse pressure
is included. It may apply to the situation in twin tubing where the pres-
sure in the annulus between the outer and inner tube may be sufficient
to crush the inner tube.
HAZOP study is most effective when it is a creative process, and the
use of checklists for guidewords or parameters can stultify creativity.
Nevertheless, checklists can be helpful for an experienced team.
Illustrations are given in Appendix 2, pages 99 100.
4.3 IDENTIFYING CAUSES
Once a meaningful deviation has been identified, the team then seeks a
cause. It is worth noting at once if the consequences are trivial as there
Table 4.3 Examples of meaningful combinations of parameters and guidewords
Parameter Guidewords That Can Give a Meaningful Combination
Flow None; more of; less of; reverse; elsewhere; as well as
Temperature Higher; lower
Pressure Higher; lower; reverse
Level Higher; lower; none
Mixing Less; more; none
Reaction Higher (rate of); lower (rate of); none; reverse; as well as/other
than; part of
Phase Other; reverse; as well as
Composition Part of; as well as; other than
Communication None; part of; more of; less of; other; as well as
17

is then no point in searching for causes. If there are likely to be several
causes, as with the deviation “no flow” in a pipeline, it is very helpful
to have a short brainstorming session to identify as many causes as
possible, remembering that causes may be related to human factors as
well as to hardware items. In seeking causes (and evaluating con-
sequences), it is essential that all members of the team take a positive and
critical, but not defensive, attitude. This is particularly important for any
members responsible for the design. It can be useful to create and use a
databank of frequently occurring causes to ensure no common causes are
overlooked. If this is done, however, it should not be allowed to affect the
creativity of the team or become the principal source of causes.
Although only realistic causes need to be discussed in detail, a
judgment on this cannot be made without taking account of the
nature and seriousness of the consequences. Acceptable risk involves
an assessment of both frequency and severity so it is impractical to
completely separate the discussion of cause and consequences in a
HAZOP analysis. Sometimes this results in an action to assess the risk
by more detailed analysis outside the HAZOP study meeting, for
example, where a major consequence could occur as the result of a
combination of causes. The term “realistic” implies a consideration of
the likely frequency of a cause. If only minor consequences ensue, then
even high-frequency causes may be ignored. In effect, a risk assessment
is made based on a combination of the frequency of the event and the
seriousness of the consequences. Experienced teams have little diffi-
culty in this for most events. However, judgments as to the frequency
at which causes are described as “realistic” are likely to differ from
company to company and will certainly alter between countries due to
different legislative approaches. In some circumstances, it may be best
to analyze and record for even very low-frequency causes, perhaps
with all the causes identified.
An alternative approach is to ignore the safeguards when evaluating
consequences so that the ultimate effects are understood. Then each
cause is considered in turn. Now the adequacy of the safeguards can
be evaluated and the need for action determined.
It is important that causes are clearly described, as broadly similar
causes may have distinctly different consequences. In these circum-
stances, it is necessary to distinguish and treat each cause separately.
For example, pump failure due to a mechanical cause may cause loss of

containment as well as loss of flow while pump failure due to an elec-
trical cause may simply lead to loss of flow. So while it may sometimes
be possible to group causes together, this should only be done where the
team is sure that the consequences are identical for every cause.
Finally, before the discussion of a particular deviation is concluded,
the team should consider all of the possible causes suggested.
4.4 EVALUATING CONSEQUENCES
The consequences of each cause must be carefully analyzed to see
whether they take the system outside the intended range of operation.
It is essential to fully identify all of the consequences, both immediate
and delayed, and both inside and outside the section under analysis. It
often helps to analyze how the consequences develop over a period of
time, noting when alarms and trips operate and when and how the
operators are alerted. This allows a realistic judgment on the likelihood
and influence of operator intervention.
Where an effect occurs outside the section or stage being analyzed,
the team leader must decide whether to include the consequences in
the immediate analysis or to note the potential problem and defer the
analysis to a later, more suitable point, in the overall HAZOP study.
Whichever approach is adopted it is important that consequences outside
the study section are fully covered, however distant they may be.
4.5 SAFEGUARDS (PROTECTION)
There are variations in practice as to when the existing safeguards and
protection are noted and taken into account. One approach is first to
analyze the outcome ignoring the existence of any safeguards such as
an alarm, trip, or vent. Then, when the worst outcome has been identi-
fied, the safeguards are noted and the team moves to considering the
need for action. This approach has the advantage that the team is
alerted to possible serious consequences and misjudgments of the need
for protection are less likely. Against this, it can be argued that it is
unrealistic to ignore the in-built safeguards of a well-designed opera-
tion. Whichever approach is adopted, it is good practice to make note
of the safeguards in the detailed records of the study.
19

4.6 RISK ASSESSMENT
Originally, little or no risk assessment was done in a HAZOP study, its
purpose being the identification of hazard and operability problems.
This is still a valid approach. However, if risk assessment is to be done
during the study, the team needs an agreed approach covering:
• whether all problems will be assessed or only the high-severity ones;
• how it will be done;
• when it will be done.
It can be very time-consuming to do a risk assessment for every
problem. However, if the team has a familiar, well-constructed risk
matrix which is appropriate to that particular industry, they will become
efficient at assigning likelihood and severity categories. A good software
package helps by providing an easily viewed reminder of the matrix and
may also allow different risks to be recorded for different categories of
consequences such as environmental, process, or personnel injury.
The estimations of likelihood and severity are normally qualitative,
typically in order of magnitude bands. They rely on the team’s experi-
ence and judgment of similar events and will be uncertain, perhaps by
as much as a factor of 3 (i.e., about one-half of an order of magnitude).
A good team will quickly estimate frequencies as low as once in 10 years
for common events. For lower frequencies, it may be necessary to make
some analysis of the conditions needed for the event to occur and to do
a rough quantification to get to lower frequencies. Inevitably, the uncer-
tainty in the estimate will be greater for very low frequencies. When
events of very low frequency, of one in 100 years or less, need to be con-
sidered, it is better to refer the problem to outside analysis by QRA or
full Hazard Analysis and not to lose focus on the identification exercise.
The assessment is probably best made either after the team has clar-
ified the consequences or following the discussion of the safeguards.
Some companies choose to assess the risk at three stages:
1. unmitigated;
2. after safeguards;
3. after actions.
The advantage of this approach is that it shows the worst case
consequences, the extent to which these are alleviated by existing

safeguards, and then the effects of the proposed actions. This sequence
makes it very clear how serious the problem is, the reliance on existing safe-
guards, and hence the need to ensure these are maintained during opera-
tions and the benefit, and hence the justification, for the proposed actions.
A further benefit of risk assessment after the consideration of
the consequences is that minor problems are apparent and further
discussion can be terminated.
4.7 RECOMMENDATIONS/ACTIONS
Several different approaches are in common use:
• After a potential problem is identified, it is always referred for inves-
tigation outside of the HAZOP meeting.
• At the other extreme, the team attempts, whenever possible, to deal
with the problem and record a recommended solution to that problem
whether engineering or procedural.
• The norm is for an intermediate approach where the team
recommends a solution to the problem only if there is a breach of
standards or if the team has unanimously agreed a solution which is
within their authority to make. All other problems, particularly if
there is no unanimity, are referred for further investigation outside
the HAZOP meeting. This approach has the benefit that agreed hard-
ware changes can be immediately marked on the working drawing
and taken into account during the remainder of the study.
The approach used should be agreed in the definition of the study.
Whichever approach is adopted, it is important that there is consensus
among the team on any positive action, as well as on the causes and
consequences. Also further causes, consequences, and deviations that
might be associated with a change should be considered and covered
within the HAZOP study. It is essential that all recommendations/
actions are unambiguous and clearly recorded so that they can be
understood at a later stage in the project by non-team members.
Actions may be either specific or generic. The former is more
common but, where a change might apply at several points within the
design, it is simpler to make a generic action, so avoiding repetition
and the possibility of different actions for similar problems in different
parts of the process.
21

It is good practice to have an entry in the action column for every
deviation and cause discussed, even if the entry simply states that no
action is required because the existing safeguards are considered
adequate, to show that the team concluded their discussion.
4.8 RECORDING
The conclusions reached by the team must be fully recorded, and it
should be remembered that the HAZOP report typically represents the
only comprehensive record of the study and of the operating strategy
intended by the designers of the plant. The report should be regarded as
one of the suite of key documentation handed forward to the operators
of the project.
The selection of items to be included in the record are agreed during
the planning of the study. It is important that sufficient detail is
recorded for the potential problem to be understood outside the meeting
by persons who were not present. The details of recording are discussed
in Chapter 7. During the examination process, the team members
should be aware of the details of the current record, either by it being
displayed or by the leader stating what is to be recorded. In addition,
team members should have an early opportunity to check the first draft
of the meeting records.
4.9 CONTINUING AND COMPLETING THE ANALYSIS
In the parameter-first approach, the normal sequence is to consider in
turn all causes of a particular deviation. When that is complete the same
parameter is considered with another guideword to see if a meaningful
deviation can be generated. This continues until all the guidewords have
been tried. In practice a team quickly recognizes which guidewords to
consider with each parameter. When all meaningful deviations have been
examined, the team moves on to another parameter and considers this
with all appropriate guidewords. The HAZOP analysis of the section or
step is complete when the team can suggest no further parameters.
To get the best results from a HAZOP study, it is essential that the
group functions as a team throughout, with every member feeling free to
contribute and actually doing so. It is expected that a consensus will be
reached at every stage of the analysis. If any team member is not satisfied
with a conclusion or recommendation, then the team should aim to resolve

the issue before moving on or turn it into an action for further discussion
outside the meeting.
4.10 AN ILLUSTRATION OF THE HAZOP STUDY PROCESS
This simple example shows how a HAZOP study works. It is applied to
a familiar task. The early stages are set out in full but the analysis is not
completed, only going far enough to show at least one line of analysis
for each guideword. You can easily add some more yourself.
Consider filling the fuel tank of a diesel-engined car as part of the
operation of a new filling station. Assume the design of the filling sta-
tion is complete and that it has been subjected to a full set of Hazard
Studies. The intention here is to look at one function of the design.
Consider a car driver arriving to take on fuel. Having selected this
filling station, we consider what the driver has to do. A minimum set
of steps is:
1. Select a filling bay that is not occupied.
2. Park so the filling hose can reach the inlet to the car’s fuel tank.
3. Remove the cap from the fuel tank.
4. Determine which fuel is required—95-octane lead-free petrol, diesel,
high-octane petrol, etc.
5. Place the fuel nozzle into the car’s fuel tank inlet.
6. Start the flow of fuel.
7. Monitor the flow, stopping it when enough has been added.
8. Replace the fuel nozzle on the pump stand.
9. Replace the cap on the car fuel tank.
10. Pay for the fuel taken.
11. Drive away.
These could be made more precise but initial drafts of operating
instructions rarely cover all situations.
Information must be collected for the study. This should include:
• The layout of the filling station showing entry and exit lanes, the
number, position, and spacing of the pumps, and related buildings
(the shop and pay point, tanker supply area and filling connections,
the car wash, the compressed air and water supply station, etc.).
Drawings and photographs of equipment items are required.
• The details of each typical pump station (if there is more than one
style) with information on the number of fuel types available, the
control system to be used, the display, and the flowrates. Drawings,
23

specifications, and photographs are the minimum; a team visit to
the site would be useful. Normally a PID would be included.
• fuel properties;
• site drainage details and plans;
• fire safety measures and firefighting equipment;
• details on typical usage—fractional occupation of the available
pump spaces, time per visit, range of amounts transferred,
other traffic to and from the site (e.g., visits for shop pur-
chases only);
• number of operators on-site and their general duties;
• frequency of supply tank filling and any restrictions placed on
customer access during resupply;
• typical nonavailability of pumps, for example, due to shortage of
fuel or individual pump failure;
• history of filling station incidents (specific to the operating company
and in general).
We will assume that an experienced HAZOP study leader has been
appointed to lead this study. The leader will review this information for
general suitability and coverage and then think about the division of the
steps of the operation (1 11 above) into stages for the study. The initial
suggestion might be:
Stage 1 Steps 1 2 Arrival and preparing for transfer
Stage 2 Steps 3 9 Filling the tank
Stage 3 Step 10 Paying
Stage 4 Step 11 Leaving
We will look here at stage 2.
The team leader will need to assemble a suitable team. This might be:
• team leader (TL);
• site architect (SA);
• member of the site management (SM);
• representative from the pump manufacturers (PM);
• local operator (LO);
• representative user (RU);
• petrol company health and safety adviser (HS);
• scribe (TS).

After familiarization with the study data the team would discuss
what is involved in stage 2 and draw up a design intention. This
could be:
To transfer diesel fuel from the selected fixed pump into the fuel tank of
the car at the fastest rate compatible with safety. The amount trans-
ferred may be a chosen volume, a chosen value or the full tank capacity.
The transfer will be controlled by an untrained member of the public
and may be terminated manually or by automatic cut-off when the tank
is full.
The team, on advice from the leader, is using the standard set of
seven guidewords, namely:
• No;
• More;
• Less;
• As well as;
• Part of;
• Reverse;
• Other than;
plus the additional ones of
• Where else;
• Before/after.
An initial consideration by the team of possible parameters gave the
following ones (which may be extended by ideas suggested during
the study itself):
• composition;
• flow rate;
• quantity;
• temperature;
• safety;
• control.
The following section gives examples of the team discussion, the
first relating to high fuel flow:
TL “I would like to discuss high flow of fuel into the car’s fuel tank
a little more. What are the implications of the failure of the Dead
Mans’ Handle on the filler or the failure to shut off in the case of high
level in the car fuel tank?”
25

LO “The fuel will spill out of the tank in an uncontrolled manner and
go into the drain system where it will be caught in the interceptor.”
RU “Do we know if there is any level measurement or warning of
overload of the interceptor?”
HS “I think that there is.”
TS “I am making a note of the action on HS to verify this.”
LO “This raises some issues about the emptying of the interceptor both
‘how?’ and ‘how often?’.”
TS “I am making an action on this between the LO and the HS.”
TL “Are there any more consequences associated with these causes
of high flow?”
At a later stage, the team has a short brainstorming session to start
the guide word “other/other than.”
TL “I suggest we start the use of the guideword ‘other/other than’
by brain-storming for possible deviations. Any ideas?”
LO “A non-standard fuel container is filled.”
PM “Perhaps using a different fuel.”
RU “A car jacking is attempted.”
HS “Safety—an engine fire.”
TS “Car has a trailer or caravan attached.”
RU “Car won’t restart or a puncture is noticed.”
LO “Leak of coolant, engine oil or other fluid from the vehicle.”
SA “Driver taken ill or appears so (drink, drugs).”
SM “Extreme weather conditions—wind, frost, lightning, snow.”
Table 4.4 shows extracts from the report.
Table 4.4 This is a selection from the report that could result from the study. Enough has
been included here to illustrate each of the main guidewords at least once. An action placed
on two team members means that they are both expected to be involved in resolving the
problem. However, the responsibility for the response is placed upon the ﬁrst named member
Ref. Deviation Cause Consequence Safeguards Action On
1 No flow Wrong
initiating
sequence used
by the customer
Delay.
Possible
damage from
wrong
sequence. Sale
may be lost.
Required sequence is
usual for the UK
and uses illuminated
buttons on the pump
panel. The site
operator can select
and speak to each
station.
A1: Consider installing
an alert to the operator
whenever delay between
removing hose and start
of pumping exceeds
selected time (say 20 s).
PM
2 No flow Supply tank at
low cutoff level
Delay and
frustration for
customer as
cause not
apparent.
Alarm to site
operator of
impending loss of
supply. Operating
procedure to cone
off pumps with
prepared signage.
A2: Review restocking
arrangements against
the expected demands to
minimize this situation.
SM
A3: Review operator
training and testing.
HS
(Continued)

Table 4.4 (Continued)
8 More
quantity
Customer error Customer
cannot pay;
delay at till
and at pump.
None A5: Cover in training
procedures.
SM
9 More
(high) fuel
flow
Dead man’s
handle on
pump fails or
the flow fails to
shut off on high
level in tank
Fuel spillage
over side of
car, onto
ground, and
into drain
system.
Possible fire.
Maintenance of the
pumps. Interceptor
within the drains.
A6: Check on the
recommended
maintenance
procedures.
PM
and
SM
A7: Check for level
indicator and warning
of interceptor overload.
HS
A8: Review location
and effectiveness of the
first aid firefighting
facilities.
HS
10 More time Driver leaves
car unattended
(e.g., to shop in
main store)
Pump blocked
to other users.
Uncertainty
over
“abandoned”
vehicle.
None A9: Establish
procedure to deal with
“abandoned” vehicles
including emergency
evacuation of the area.
HS
13 Less
quantity
Low level in
main supply
tank
Customers
cannot get
fuel.
Low level warning on
main supply tank.
A11: Check that
resupply arrangements
cover all likely rates of
sale. See also A2.
SM
Operator training to
cone off the affected
pumps.
A12: Review SM
16 Reverse
entry of
car into
the pump
lanes
Driver mistake
or deliberate
short cut taken
Confusion
among other
users and
increased
likelihood of
on-site
collision.
Signage A13: Review the
position and
instructions on signs.
SA
and
SM
A14: Consider if
routing of entry/exit
slip lanes can reduce
occurrence.
SA
18 As well
as—
customer
uses
mobile
phone
Customer
ignores
warnings
Possible
ignition
source—not
likely with
diesel but
could be with
petrol.
Warning notice at
every pump station.
A15: Check on reality
of the rumors of fuel
ignition from mobile
phones.
HS
A16: Consider
requiring till operator
to warn phone users
over built in speakers
on pump station.
SM
(Continued)
27

Table 4.4 (Continued)
19 Only part
of
sequence
completed
Customer does
not properly
replace fuel
nozzle on its
stand
Transfer
pump
continues to
run against
closed valve.
Payment
cannot be
made and
customer must
return to the
pump.
Till operator can
notify customer using
the pump speaker
but is unlikely to
spot the problem
before customer
leaves the pump.
A17: Check with
manufacturer how
likely this is with the
chosen design and
what the alternatives
are.
SM
and
PM
21 Other fuel
container
filled
Customer uses
fuel can
(perhaps as well
as filling car
fuel tank)
Pump is
stopped and
then restarted.
May attempt
to pump a
different fuel.
High-level
cutoff may
not work if
container has
a wide neck.
Not possible to
pump separate fuel
until payment made
for first and pump
zeroed. Restart with
same fuel is possible
provided nozzle not
replaced first.
A18: Decide whether a
timed cutoff should be
included so restart is
not possible after a
selected time.
SM
A19: Check whether
high-level cutoff works
in wide necked
containers.
PM
22 Other
event—
carjacking
attempted
Planned
criminal
activity
Risk of
violence with
injury (or
death).
Warning to
customers to remove
car keys and not to
leave car unlocked.
A20: Check wording
and prominence of
notices.
HS
and
LO
Bad publicity
inevitable.
A21: Put up clear
notice that CCTV is in
use as a deterrent.
SM
A22: Review
emergency procedures
to ensure this
eventuality is covered
and that training is
provided.
SM
and
LO

CHAPTER 5
5
Organizing a HAZOP Study
The detailed HAZOP study procedure described in the previous chap-
ter needs to be fitted into an overall scheme. This chapter covers the
preliminary organization while Chapter 6 covers the study itself.
5.1 DEFINING THE SCOPE AND OBJECTIVES OF THE STUDY
AND ESTABLISHING THE BOUNDARIES
These two aspects are not independent of each other although they are
discussed separately. This part of the planning contributes significantly
to the eventual quality of a HAZOP study. A clear definition of the
scope and objectives is crucial to the whole study and should define
the responsibilities and authority of the team (see also Section 6.1). It
should also cover responsibility for initiation of the study, for follow-
up and implementation of the actions, and how any rejected actions
are dealt with. A well-defined set of objectives helps prevent the team
straying into areas that are not relevant to the study. Quality assurance
(QA) and auditing of the study should also be planned from the outset
so that the study and the recording facilitate any later checks.17,18
It is normal practice to include the identification of SHE risks as a
principal objective, including risks to persons both on- and off-site, risks
to property and plant, and all types of environmental risks. It must be
made clear whether a further objective is to search for potential oper-
ability problems that by themselves have no SHE implications. These
include reliability and maintenance issues, product quality or loss, and
factors affecting plant life and productivity. Meeting regulations, com-
pany standards, and contractual performance requirements may also be
a defined objective. Consideration should also be given to the intended
uses for the study reports, in particular the need for audit and demonstra-
tion that a high standard has been reached in the identification of hazards
and the assessment of risk. This may be required by both outside regula-
tors and by insurance assessors. If it is expected that QA is required, the
issues must be addressed during the definition of the scope.17

The boundaries for the study must be clearly defined as part of the
specification of the study, including how any problems that extend
beyond these boundaries will be handled. The first step is to identify
those sections of the physical plant which are to be included. When
boundaries are drawn, consideration is given to factors such as the
nature of the process and the inherent hazards, the novelty of the opera-
tion, the complexity of the control system, and the relationship to other
operating units. Where an existing plant is being considered, perhaps
with a new process, it may be possible to omit from the study those
sections that are used in a standard manner (although it may still be
necessary to consider the effect that changes in the new section can have
on the “standard” section). Also, if a standard unit or a proprietary unit
is to be studied, it may be more effective to concentrate on the interfaces
between that unit and other plant and operations. The unit may then be
covered by comparison with a previously completed, full HAZOP study
of the unit—an approach described as HAZOP by-difference. However,
this should be done with care as there may be some small differences
which could have a significant impact on operability.
When the study boundaries are drawn, there are likely to be
interfaces with off-section elements of the plant such as drains, recycle
lines, vents, and effluent treatment and perhaps sections of plant
intended for occasional use for special purposes such as start-up lines.
Consideration must be given to the inclusion of these within the
HAZOP study, recognizing that interfaces with systems handling
process materials are of potential importance. It is essential that such
elements of the system are not overlooked simply because they are not
on the drawings being used. The team needs to have an overview of the
whole system so that important off-section causes and consequences are
not overlooked, especially when associated with unusual events such as
a plant trip. An interface review may be needed to clearly identify what
crosses the boundaries.
There can be difficulties in setting boundaries when a modification to
an existing plant is to be studied. As well as covering the modified sec-
tions of plant, it is usually necessary to extend the boundaries on either
side to ensure that related sections, where causes of deviations that affect
the modification and where consequences of deviations appear, are
covered by the HAZOP study. It is a matter of judgment for the
team leader.

Another aspect to be established at the outset of a study is the range
of operational modes covered. For a continuously operating plant the
main condition is the steady-state operation, but it is necessary to cover
other modes such as start-up and shutdown (see Section 11.8), high and
low rate running, hold conditions—particularly those anticipated during
start-up—and changeover from one state to another. These may involve
quite different pressure, temperature, or flow conditions compared to
steady-state operation and may need detailed examination. Since many
of them are discrete operations, they need to be examined using the
batch HAZOP approach. For a batch operation, the additional modes
could be special batches at the start and conclusion of a campaign or
alternative control modes, such as fully automatic and partially manual.
It is useful to think through the life cycle of the operation when deciding
which alternative conditions should be included. It must be decided
whether these alternative operational modes need separate examination
or whether they can be covered under the guideword “other.”
It is not possible to estimate the length of a study or to plan in
detail until these boundaries have been established.
5.2 APPOINTING A TEAM LEADER AND SELECTING THE TEAM
5.2.1 The Team Leader/Facilitator
The selection of a team leader should be made at an early stage of the
planning for a HAZOP study. An essential role of the team leader is to
ensure the HAZOP methodology is used effectively and productively
and so the leader needs to have a deep understanding and consider-
able experience of HAZOP studies. HAZOP study teams work best
when there is clear leadership from an experienced leader. Desirable
attributes for the leader include:
• wide experience of all the stages of process hazard studies, including
QRA;
• extensive experience as a HAZOP study member and, preferably,
some as a HAZOP study scribe;
• training in the leadership of HAZOP studies;
• technical competence and the ability to quickly understand the
system and its operation;
• meticulous attention to relevant detail;
• good analytical thinking;
31

• motivational skills including the encouragement of creativity and
open speaking;
• independence from the project itself with no other direct responsibi-
lity to the project manager other than completing the scope of
the HAZOP study. The leader should be able to concentrate on the
application of the method and the working of the team.
Independence from the project allows the leader to stand back and
be able to take an objective, unbiased view. The leader acts, and is
recognized to act, as an impartial person within the team. Freedom
from other responsibilities is important because of the multiplicity of
tasks already resting upon the leader. It is also advisable to check the
suitability of the leader for the magnitude of the task, with only experi-
enced and proven leaders given studies involving major hazards or
other difficulties.
A good leader will have interpersonal skills that can be deployed to
help the study group to function as a true team. These include the
ability to listen, guide, and encourage individual contributions as and
when required, to sense the unspoken feelings of individuals—perhaps
indicated by their body language—to move the group toward a con-
clusion when a consensus is emerging and to work efficiently with the
scribe. Within the HAZOP process, the leader directs the route taken
through the stages, the parameters and guidewords. It may be necessary
to vary from the preset plan by introducing new parameters and guide-
words, to question doubtful issues and to defer examination—perhaps
to gather more information—and to periodically review and remind the
team of the key issues. The leader has responsibility for ensuring that
the HAZOP study is functioning to a high standard and, if it is not, to
call for changes which will restore these standards. In an extreme case,
this could mean terminating a study until acceptable drawings, project
information, or other team members are available.
A good leader working with a badly-selected team may be able to
produce an adequate HAZOP study. The quality of the leader is criti-
cal and a good team cannot always negate the impact of a poor leader.
If the team loses confidence in the leader, the study is doomed.
5.2.2 Scribe (Scribe/Recorder)
The scribe is another key individual in the team. The person chosen
must be familiar with the HAZOP study method and usually has a

technical background so that special explanations are not required.
The scribe must be a good listener, always paying attention to details.
In a major study, the scribe has no other role, as recording is a
full-time task. In a small study—for example, a modification—or on
a self-sufficient small site, the recording may be done by the team
leader or a team member. Note that experience shows that the scribe
cannot be contributing to a discussion and writing at the same time so
this inevitably slows the study.
The scribe should establish a working relationship with the leader
to become a helper, not just a recorder. Help can be given by noting
suggestions and deferred problems and bringing them up at a later
time, and by assisting in the selection of guidewords and parameters.
More directly, the scribe starts to record as a consensus appears, with-
out waiting for instruction or dictation from the leader. To do this the
scribe must learn when to write if recording manually (or to type if
recording electronically)—starting too soon is a waste of effort while
waiting for a final version to be dictated can slow the progress of the
study and interferes with the creative flow of the discussion. The team
should know exactly what has been recorded. This may be done by the
leader reviewing the entry, the scribe reading it back or, if a computer
recording program is in use, having it projected so all of the team can
see the record.
The choice of computer recording over a simple written record is a
matter of preference as both methods can produce good records. If
a projector is used—as is common practice—it must be understood
that during the meeting the priority is to capture the essential points of
the analysis; spelling is not a high priority as it can be improved later
but there must be no ambiguity or incompleteness in the first record.
It is important the study records are produced consistently and
speedily after each session, particularly in a lengthy study, and that
final and agreed versions are produced.
5.2.3 Team Size and Membership
The size and composition of the team varies according to the complexity
of the study. The minimum size for even the smallest study is probably
four with some of the roles combined. Usually there is a core team
of five or six. Occasionally, the number may be as large as eight or
33

nine; team membership should be reviewed if this is exceeded. A typical
team might involve the following personnel:
• team leader;
• scribe;
• process engineer;
• project/design engineer;
• plant/operations manager;
• operator (or commissioning team representative for a new design);
and one or more, as and when required, from;
• SHE expert (mandatory in some countries);
• research chemist;
• control/instrument engineer;
• maintenance/mechanical engineer;
• other specialists.
The essentials for the team members are that they have sufficient
experience in the area of the operation and, as a group, have a
comprehensive knowledge of the intended operation. In addition, the
basic disciplines represented should ensure that a fundamental analysis
of identified problems can be made. The key is the assembly of indivi-
duals who, together, provide the correct combination of basic dis-
ciplines and expertise, and general experience of the type of operation
to cover all stages of the project. The disciplines needed may be a selec-
tion from chemical, mechanical, and electrical engineering, control
systems engineering, chemistry, biochemistry, and other disciplines
which offer specialist knowledge, when necessary, of reactors, piping,
instrumentation, software, and metallurgy. General experience within
the industry should average several years per member, including direct
relevance to the intended operation with regard to the site, the method
of control, and type of process. These latter aspects may be covered by
the inclusion of a senior supervisor. Direct project involvement ranges
from the initial research and development, earlier hazard studies, pro-
cess design and the intended construction, commissioning, operations,
control and maintenance groups.
It is possible that in complex processes which have a number of
operating interests that the membership may be “dynamic.” If this is
the case the new team members must be given sufficient time to
appreciate the direction and dynamics of the HAZOP study and be
able to fit into the team at the appropriate time. Likewise a departing

member should be satisfied that there is no further use for his/her team
membership before departing from the team.
Compromises often have to be made when selecting the team. The
ideal persons may not be available and there can be difficulties if a big
team results from an attempt to meet all of the criteria suggested
above. In a large team—say nine or more—separate conversations
may start, making it difficult for the leader to capture everything and
disrupting the unity of the team. The leader must make a balance to
achieve the best possible results. The potential for conflict is greater in
a large team; it is a responsibility of the leader to be alert to developing
problems and to prevent them from becoming major issues.
Whenever a new design or a modification to an existing plant is
studied, the team should always include a representative from the
design group, from the operations team, and it is likely to include an
instrumentation engineer. Other team members are then chosen accord-
ing to the nature of the project. Possible further members could be a
mechanical engineer, development chemist, electrical engineer, software
specialist, and safety specialist. It may be possible for the safety special-
ist to also act as the team leader. A good team is likely to have several
members who are closely involved with the project and one or more
who have little or no previous connection. It should not consist solely
of those who have worked full time on the design nor should it be
totally independent of the project. The interaction between these two
groups can improve the quality of the study. Some companies like to
have an independent process engineer present to act as a critical ques-
tioner and challenger of assumptions, another role that can be filled by
a truly independent team leader who has the necessary background.
In a normal HAZOP study where several meetings are required, it is
important to have the core team members present at all the study
meetings. Apart from the development of a team style and spirit, this
maximizes the benefits from the growing understanding of the operation
that comes as the study progresses. If one member of the team is from an
operating team working on shifts, however, it may be impractical to have
the same individual present at all the meetings. Specialists are added to
the team when their expertise is needed for a particular section or stage.
It is important that some of the team have previous experience of
HAZOP study; if some have not participated before, then training
35

should be considered. Provided a few members are experienced, the
others can be introduced by a short example or, if a single individual is
involved, by a briefing and then by participation within the otherwise
practiced team.
5.3 PREPARATION
It is important for the team leader to assemble and review the
necessary data for the study and, if necessary, to reorganize the mate-
rial for use by the team or to call for additional data. In this work, the
leader may be assisted by a member of the design team and by
the scribe. A site visit by the whole team can be very useful when an
existing plant is studied but this is not always practical.
Knowledge of previous incidents on the plant or process being
studied is important. A search should be made of appropriate accident
databases to identify historic incidents relating to the type of process
and to any specialized equipment being used. These may be corporate
or international databases.
The boundaries for the study must be known and clearly stated. In
the case of a continuous process, the boundaries will start at the feed-
stock and end at the final product distribution. It may be appropriate
for a member of the plant producing the feedstock and another from
the final user to partake in the appropriate phase of the HAZOP study.
Minor changes in product specification as the result of a modification
must be treated with care.
It is also necessary to plan the route the study will follow through
the process; this must be done with care. There will be a number of
branches and also heat exchangers which have process fluids on both
sides. As a general guidance it is best to examine the route using the
logical process flow but where there are branches or interactions, such
as heat exchangers, it is appropriate to discuss the major issues that
must be examined at the branch or on the other side of the heat
exchange and then to make note of these for discussion at a later date.
For example, the reduction in flow (low flow) on one side may have an
impact on the other side of the heat exchanger and that process stream.
Likewise a spared pump is a potential branch. It is appropriate to
examine the interaction between the pumps but not to carry out a full
HAZOP on each unit as that would be a repetition of effort. A note to

the effect that the units are identical and only one plus the interactions
was studied would be appropriate, provided the PID is truly identical
in each case. Another issue might be the use of spared heat exchangers
on fouling duty. The changeover and the impact of two heat sources
on the process and pressure relief must be considered in detail. (See
also the notes on vents and drains in Section 11.6.)
As a generalization a continuous process will start up (and shut
down) in a “low flow mode.” This must be part of the study. Likewise
the start-up and shut down will involve dynamic changes in pressure
and temperature (see Section 11.8). The HAZOP study must examine
the whole envelope of these parameters. In some cases, particularly
compressors and pumps, the low flow mode may require special start-
up facilities which will only be used very occasionally. These must be
examined with the same rigor as the rest of the process.
Following shut down the HAZOP study must examine the disposal
or storage of off-specification materials which may contain contaminants
including corrosive and toxic by-products.
This preplanning work is normally undertaken by the team leader,
often with help from a member of the design team.
5.3.1 Continuous Processes
For a good HAZOP study to be possible, it is necessary for the team to
have a complete and accurate process representation. For continuous
operations this is based upon the PID, supplemented by design specifi-
cations and other details used in the design work, including flow
diagrams, and material and energy balances. Further items are the
intended operating conditions—usually expressed as a range—the oper-
ating and control philosophy and methods, and equipment and instru-
ment specifications. Relief settings are essential to allow verification of
pipe and vessel specifications, and alarm and trip settings should be nor-
mally available. In addition, there should be material properties,
hazards and known operating problems together with the basis for
safety, site details, and demands on the site services, and the results of
earlier hazard studies including any material and reaction investigations.
Information on vendor packages needs to be available.
Once the required level of detail has been obtained, the next
planning task is the division of the PIDs into sections and items to
37

which the guidewords can be applied. Alternative terms may be used
to describe the blocks selected for examination. The term “section” is
used in this guide for part of a continuous process to which the guide-
words are applied; another frequently used term is “node.” The term
“item” is sometimes used for vessels, exchangers, and so on, to which
restricted or modified guidewords are applied. In a batch process, the
terms step or stage are commonly used. A HAZOP study is a rigorous
but time-consuming, and hence expensive, exercise. The higher the
number of sections or steps, the more time-consuming is the study.
Ideally, no sections should feature a process line junction and no steps
should cover more than one element of the batch sequence. Experience
suggests that some branching within a section and some step combina-
tion can be tolerated on systems other than those of very high hazard
potential. Defining the intention helps demonstrate that the team have
selected a worthwhile manageable node. Designation of the sections
and steps is best undertaken by the leader at the outset of the study.
Since this task is critical to the integrity of any HAZOP study, it is
essential the leader has a proven track record in HAZOP management
(see Section 5.2.1, page 31).
The selection of the sections is usually based upon the line diagram
together with a description of the operating conditions. Often there are
several possible ways of doing this division based upon the variation
of parameters such as flow, pressure, temperature, and composition, at
control points or junctions of lines with vessels. It is essential each section
can be given a clear design intention and that a conceptual model can be
constructed for the system in its given mode of operation. The size of the
section contributes to effective analysis as there are problems in choosing
either too small or too large a section. In the former case, there will
probably be causes of deviations outside the section and consequences
occurring upstream or downstream. There is a danger of these being
overlooked. If too large a section is taken, then the design intention may
be imprecise or very complicated so that it becomes difficult for the team
to cover all possible meanings of each deviation. No simple, universal
method can be given for the division into sections; experience is essential.
The example in Appendix 3 provides an illustration.
It can be useful to mark each line and item of plant equipment as it
is studied during the operating sequence. A final check can then be
made to ensure all lines and items have been considered.

A further feature of continuous processes is that a high proportion
of incidents and near misses occur when the plant is operated at con-
ditions other than the steady state—for example, during start-up after
maintenance, during commissioning, or shutdown. Therefore, it is
important for the plan to give sufficient attention to these condi-
tions and to the deviations that might occur in these situations—for
example, too rapid heating or cooldown to the operating temperature,
inadequate purging, or protective systems overridden. The team should
also be mindful of the need for diagnostic instruments that will facili-
tate process performance monitoring and allow the assessment of local
and wider heat and mass balances. These may be needed to allow
assessment of heat exchanger performance, efficiency of pumps and
compressors, and fouling of filters and piping, and can be of great
importance during incidents.19
During start-up, it is sometimes necessary to override protective
systems such as low temperature or pressure trips during what is an
unsteady operation with a high risk of upset. The HAZOP study must
pay particular attention to the process dynamics and risks with the
protective system overridden and how the protective system will be
reset (see also Section 4.5, page 19).
5.3.2 Batch Processes and Sequential Operations
The preparation work for a batch or sequential operation is usually
more demanding than for a continuous process. In addition to the
same range of background material, descriptions are also required of
the detailed method of operation, the draft operating procedure, and
the outline of the control sequence for a computer-controlled process.
This time-dependent information is used in the division of the opera-
tion into stages for study, as illustrated in the example in Appendix 4.
If a reaction is studied, the team needs information on the reaction
process including heat and gas flows, exotherm onset temperatures,
and the physical and chemical properties of the mixtures as well as the
individual components. Again, the ability to define a design intention
for each stage is an essential requirement.
One major difference between continuous and batch operations is
that a physical item may need to be considered several times in a batch
HAZOP study as the status, conditions, and design intention change.
Thus, a reaction vessel may be considered during the addition of each
39

component of the reaction mixture, during the reaction stage itself,
and then during cooling and discharge. Another example is the
common pipeline operation of pigging, where a typical sequence
involving the loading of a scraper pig into a pig launcher at the start
of the operation might be:
• Check the initial conditions.
• Open the LP flare valve and vent the vessel.
• Open the drain valve. Allow the launcher vessel to drain for 5 min.
Then close the drain valve, leaving the launcher drained and at LP
flare pressure.
• Close the LP flare valve and purge the pig launcher with inert gas
by repeated pressurization to 5 barg and then venting to the LP flare
until hydrocarbon gas tests show it is gas-free as measured at a
representative sample point.
• Lock open the atmospheric vent valve and lock closed the drain valve
and master isolation valves, leaving the vessel at atmospheric pressure.
• Check double block valve vents for any sign of leakage.
• Open closure door, load the scraper, and then shut the closure door.
The scraper is now loaded but not launched.
The pig launcher is at the center of each of these steps but each one
involves a different configuration and usually different contents and
pressures. The main issues will differ from step to step as will the design
intention. Hence, the parameters will vary between the steps. Each
of these steps should be considered separately, using the full round of
guidewords each time to ensure that the hazards are fully examined.
Further requirements for batch process HAZOP studies are
intended trip settings for both the control system and the safety system,
including those that take place between successive stages of the
process. The HAZOP team must take into account the manner and
timing of the resetting of the control and trip systems, covering both
safety and operability aspects.
Another difference from continuous processes is the need for the
team to picture the whole operation and the changing conditions
throughout the plant at all stages of the batch. Without this under-
standing, interactions and consequences may be overlooked. Indeed, as
the progress of one batch is followed through its various stages, a
previous batch may still be in a connected section of the plant and

preparations for a following batch may be underway. Thus, a
deviation in the batch being followed through may cause an interaction
with the previous or the following batch and so have consequences for
these batches. The HAZOP study team needs to have a clear overall
picture to enable it to identify such cross-connections; time-related
flow charts, for example, a bar chart, are needed to give this picture.
Such charts are unlikely to be available from the design work and, if
needed, must be prepared for the HAZOP study. Good preparation
and organization of the data can greatly enhance the prospects of
success in batch operations.
5.3.3 HAZOP Study of a Procedure
Many operations, both simple and complex, are dynamic and are done
by following a strict, predetermined procedure. Any failure to strictly
adhere to the procedure or to ensure all the steps are done in the
correct order may have the potential for a serious consequence. Start-
up and shut down operations normally follow a sequence set out in a
procedure, and these operations are known to be common occasions
for problems and incidents. Hence, all procedures should be considered
for evaluation by HAZOP study. The general principles of HAZOP
study, as discussed in this monograph, hold good but the approach
has to be altered to get the best results. This is described below and
illustrated in Appendix 5.
The major difference from the examination of a steady-state operation
is that procedures are carried out in a chronological order and so each
step must be considered in turn and be challenged by appropriate
guidewords. Thus, instead of traditional parameters such as flow, and
guidewords such as more, potential problems are more likely to be
uncovered by deviations such as operation mistimed or guidewords
such as too early/too late or out of sequence. However, this does not
preclude the use of the usual parameters and guidewords used in the
HAZOP study of steady-state processes.
As ever, the team must have a clearly defined node and design
intention for the step being examined. The node may be a piece of
equipment such as a pump or a pig launcher. It may be a section
of line, as might be selected for steady-state HAZOP study, but where
the flow is not fixed but is dynamic due to moment-by-moment
changes during the procedure. The node may include a number of
41

simple operations that comprise a clear section of the procedure, for
example, a series of valve changes in starting a pump or changes of
flow, quantity, and temperature for the start-up, shut down, or condi-
tioning of a reactor. Arriving at the best selection of nodes is often more
difficult than when planning for a continuous process HAZOP study.
The choice of guidewords is also more challenging. The guidewords
too early/too late, out of sequence or incomplete, could identify missed
venting in the pump start-up or residues left in the reactor. Further
possible guidewords are tabulated in Section 10.2 on Human Factors.
Of course the standard guidewords given in Table 4.1 should also
be used if they generate meaningful deviations with the parameters
of the node.
Good practice, as with HAZOP study of steady-state operations, is
to identify appropriate guidewords and parameters before the study
is commenced, and it is the responsibility of the team to adapt the
method of study to suit the problem being examined.

CHAPTER 6
6
Carrying Out a Study
6.1 PREMEETING WITH CLIENT
It is important that the HAZOP team knows what is required of it,
and the Client (Project) has had an opportunity of discussing this with
the team. This is particularly important when the Facilitator is an
external consultant.
The starting point of the HAZOP should be a meeting between
the Client and the Facilitator. During this meeting, there should be a
discussion such that the Facilitator knows what the Client requires
from the HAZOP and what can be delivered (“the Deliverable”). It is
quite possible that each Client will have specific requirements. This
meeting should prevent any possible misunderstanding which may
affect the quality of the final deliverable. This meeting should draw up
a Scope and Terms of Reference.
Consideration should be given to the following when defining the
Scope of the study:
• The start and end points, which might be outside the immediate
PIDs (particularly for a modification or a plant in a sequence
of many).
• That which is included and that which is excluded from the study.
For example with a modification some operability issues may already
be recognized by Operations. Should they be noted? Likewise should
any “mothballed equipment” be inside or outside the scope bearing
in mind that it could be brought back into operation without further
analysis?
• The likely Parameter and Guideword Matrix to be used.
• Any particular issues that the Client feels might require special
attention.
• The availability and competence of future team members.
• The spread of disciplines.
• In the case of a modification, the boundaries of the study both up-
and downstream of that change.

• In the case of a modification, the Facilitator should satisfy him/
herself that the PIDs are of “Approved for HAZOP” quality. In
the case of a modification, the Facilitator should satisfy him/herself
that any interconnecting drawings (such as services) are both
available and to the correct standard.
• The requirements or otherwise for risk ranking (outside the study)
and against what criteria.
It is vital to note the status and revision of the information (e.g.,
procedure or drawings) to be used as the basis for the study as close
out of actions normally requires them to be updated and significant
changes may necessitate re-HAZOP.
Equally it is vital that team membership is limited to those who
can contribute constructively and can contribute without external
interference.
Consideration should be given to the following when defining the
Terms of Reference:
• The method to be adopted (corporate or other).
• The recording format, spread sheet or other computer-based systems.
• In the case of a modification, how potential issues identified outside
the immediate bounds of the study should be handled, recognizing
that causes could be within the scope but that the consequences
identified may be outside the scope of the system under review.
• The delivery date and reporting format. This must take account of
how the actions will be passed on and tracked. It is good practice to
ensure that the reason for any recommended action can be understood
when that line of the HAZOP study record is read in isolation.
• The requirements for the follow-up HAZOP Study following resolu-
tion of the actions.
Finally, the Facilitator should have a view of the room to be used
for the study and to satisfy him/herself that:
• the room is a suitable size allowing the tables to be arranged to the
Facilitator’s preference (e.g., horseshoe, round, or conference style);
• the heating/air conditioning, lighting, and ventilation are adequate;
• there is sufficient display space for large-scale drawings and ade-
quate storage for supporting information such as line lists, pipe
specs, and data sheets;

• there are sufficient power points and projection screen or a suitable
wall area;
• there is communication from the meeting room but not necessarily to
it. Wi-Fi links are useful to allow internet access to reference material;
• good refreshment and break facilities are to hand.
The facilitator may also wish to review the general support
arrangements for the study such as the availability of the members and
possible alternates, the travel and accommodation arrangements for
those from off-site, and the possibility of changes to PIDs during
long studies.
6.2 PLANNING THE MEETINGS
The first requirement is a good estimate of the number of meetings
required for the study. This can be made once the boundaries for the
study are defined and the preliminary planning work, including the selec-
tion of the stages, has been done. The length of time needed to analyze a
section or stage depends on its size, complexity, and the associated
hazards. Experience from similar studies provides a good guide to the
length of time to be allowed and hence the number of meetings needed.
An ideal arrangement is to have no more than three or four sessions
a week, each limited to half a day. This is often impractical, however,
and many studies are done as a full-time activity. In this situation,
the leader must monitor the team’s performance to ensure an
acceptable standard is maintained. There should be an arrangement
that allows all the team members to work from central documentation.
Regular short breaks are advisable and interruptions to meetings
should be prevented except for emergencies.
Team members should be provided with background details of the
planned study in advance of the first meeting. This should include details
of the scope and purpose of the study, essential design information, and
an indication of the HAZOP approach to use, including a first list of
guidewords and parameters. Normally, the team membership and details
of planned meetings are included.
Full details of options and methods of recording are covered in
Chapter 7. It is helpful to have a standard template for recording to
ensure that none of the intended entries are overlooked. The style of
45

recording should have been agreed at the premeeting (see Section 6.1),
and the whole team should then be briefed.
6.3 THE STUDY MEETINGS
6.3.1 The Initial Meeting
In a long study, the first meeting differs from the bulk of the working
meetings. After a reminder of the study objectives and scope, a brief
overview of the project should be given, preferably by the individual
most closely associated with the development work. This should cover
the plant and intended operations as well as its relationship to the site,
the services, and neighboring units. For a reaction system, the over-
view includes the process chemistry and basis for safe operations. It
may be useful to give a short review of the HAZOP working method
adopted, including discussion of the guidewords and parameters. This
is particularly important if the Project is in any way out of the ordi-
nary. Any queries about the precirculated material can be dealt with.
This preliminary discussion aims to ensure the team has a common
understanding of the project and it helps to establish the group as a
functional team. It is also an opportunity for team members to learn
of the skills and special competencies of each other.
If the team members have not worked together before, it can be
helpful to discuss and agree a set of “ground rules.” They may range
from behavioral aspects such as listening to others, giving everyone a
chance to speak, and not having simultaneous conversations to technical
aspects such as how they will help the scribe and whether solutions will
be sought to the problems identified. To be effective they need to be
accepted (“owned”) by the members although the leader, probably
having most experience of HAZOP study, may suggest many. Working
within the rules is best done by self-discipline of the members but the
leader may, at times, have to remind the team of them.
6.3.2 The Detailed HAZOP Study Meetings
These follow an agreed plan, working as described in Chapter 4,
concentrating on the identification and recording of potential problems
for all SHE hazards and, if agreed, operating problems as well. It is
recommended that the project’s process engineer suggests a design
intention for each section or step and specifies the safe design envelope
for operation. The team can then discuss the design intention and

refine it if necessary. The extent to which the problems are evaluated,
ranked and solved, varies according to company policy and require-
ments. In the planning and definition of objectives for a study, it must
be made absolutely clear what the responsibility and authority of the
team is in these respects as failure to do so results in confusion and
wasted time within the meetings.
There should be periodic reviews of the work, either at the completion
of a section or stage or at the end of a meeting. As well as confirming the
recorded details of the analysis, this encourages a check upon the
progress of the study against the study plan. Reasons should be sought
for any significant deviation. If ranking of frequencies or consequences is
to be done, then the most efficient way is to do this is at the end of a
session. Similarly, this is probably the best time for an outline discussion
of SIL ratings if this is required. The whole team should receive a copy of
the meeting records for checking as soon as possible after the end of a
meeting in addition to any action notes assigned to them.
6.4 COORDINATING AND REVIEWING RESPONSES
The need to receive, check, and incorporate action responses arises as
a study progresses. In a short study, this may be done at a special meet-
ing after the whole study is complete. In a long study, it is normally
done by periodically using part of a meeting to review the responses,
after which they can be incorporated into the formal final record.
It is essential that when a recommendation for change is accepted,
whether a hardware or a software change, the team agrees that it is an
adequate solution to the original problem and also that it does not
cause further problems by introducing new deviations. However,
responsibility for the accuracy and adequacy of the response lies with
the individual to whom the action was assigned, not the team. The
responses generated from the action sheets should not be accepted
unless they provide sufficient detail of their basis, including any neces-
sary calculations, references, and justification for the proposal. These
will become part of the HAZOP study records and hence will be part
of any audit or review to show complicity with regulations.
Where a response generates significant design or operational change
affecting the design intent then it may be appropriate to HAZOP study
these as well.
47

6.5 COMPLETING AND SIGNING OFF THE REPORT(S)
The simplest and most common definition of completion for a
HAZOP study is when all of the selected plant and operations have
been examined and all of the problems identified during the examina-
tion have been considered. This includes collection of the responses
and actions, and review and acceptance by the team (or by an autho-
rized person or subgroup) as a satisfactory response to the identified
problem. At this point a delegated person signs the detailed HAZOP
study report as complete, that is, that responses have been received for
all of the actions and that these have been reviewed and are considered
to be satisfactory. It does not mean that all the recommendations have
been carried out—that has to be covered under a separate management
procedure. In major studies, the signing off may be staged. If a few
responses remain outstanding, it is still possible to sign off the remain-
der provided company procedures include a secure follow-up mecha-
nism. In this case, it is advisable that each outstanding action is
allocated to a category—for example, to be done before start-up or
may be completed during commissioning and so on.
6.6 FOLLOW-UP OF ACTIONS AND MANAGEMENT
OF CHANGE
Responsibility for the implementation of actions, including any
rejections, passes to the line management for the project, where an autho-
rized person has the responsibility for signing off the actions as they are
implemented. Although individual members of the team may have
responsibilities for the implementation of some actions, the team as a
group should not. If an action is rejected, the records should include the
reasons for this, including a signed authority. There should be a formal
check before the facility is commissioned to ensure that all actions identi-
fied during the study have been implemented or resolved. An established
tracking system for actions is needed on major projects, and it may be
done as part of a computer-based HAZOP recording package.
Once the HAZOP study is complete, it is important to introduce a
system to minimize and control any further changes to the design. The
implications of such change to the safety of the process must be
considered by a structured Management of Change Procedure and, in
some circumstances, it may be necessary to reconvene all or part of the
HAZOP team.

The management of change must be a live system which captures
and tracks all changes both before start-up and through the operational
phase of the project.
It can also be beneficial to conduct a follow-up review, perhaps 12
months after the study, to draw out the “lessons learned.” Beneficial
changes can then be used to improve the Corporate Design Guides as
in HS 2 (see Chapter 2).
49

CHAPTER 7
7
Recording and Auditing
The final records are all that exist to show the work done by the team.
They may be revisited at a future date for a number of reasons, the
most likely being as part of a Safety Case. All future uses of the
records should be identified in order that the recording and reporting
system is designed to meet these needs in an efficient way. The final
reports should cover the why, how, when, and by whom of the study,
as well as recording the necessary details of the analysis and actions of
the team. They must be understandable by nonmembers of the team,
even some years later.
The study team will use the records in a number of ways. Draft
versions of the records are used by the team for checking after each
meeting and generating action notes as well as for reference during
the study. The final reports are needed for the implementation of the
actions and to link to later process hazard studies. There may be
regulatory and contractual obligations to be met and a requirement
for audit, and they will be a key part of the plant safety dossier.
They may represent the only detailed record of the intended opera-
ting strategy as developed by the design team. They may also be
needed as a starting point for the HAZOP study of modifications. If
a significant incident occurs during the lifetime of the operation, it is
likely that regulatory authorities will wish to examine the records.
Thus, the team should be mindful of the uses for which the final
report may be required.
In addition, the records should provide an easy and clear understand-
ing of the process and the equipment, be of especial use in the preparation
of Operating Instructions as well as for troubleshooting and operator
training and have the potential to improve the management of change
(MOC). Hence, it is essential to anticipate the intended uses so that the
HAZOP study file contains the necessary information, detail, and clarity
to meet the requirements of each use. As a minimum, the marked up
PIDs, HAZOP tables, and all action responses are archived.

7.1 BACKGROUND INFORMATION
The amount of material in the final HAZOP study file depends
upon the company practice for archiving of related materials. Items
which are securely archived—for example, all the revisions of the
PIDs, the Material Safety Data Sheet (MSDS), and reaction hazard
investigations—can be referenced without putting duplicates into the
HAZOP study file. Otherwise all drawings, including the version used
at the start of the study, and all other documentation are included
or referenced. This includes any previous hazard studies referred to
during the HAZOP study, site drawings, specially prepared material,
outline operating or control procedures, and so on. Draft versions of
the detailed meeting records need not be included provided the full
final version is incorporated.
Additionally, a statement of the HAZOP study procedure should be
given—for example, by reference to a company protocol. The name
and a statement of company role should be included for each person in
the team, including an attendance record. The remit of the study
should be made clear and the hardware boundaries stated. The records
should show the operational modes selected for examination and
indicate how this was done—that is, by separate examination or by
grouping a range of conditions within one examination.
7.2 SECTION HEADINGS
For each section or stage given a complete round of guideword
examination, there is a header which explains the model used by the
team. This identifies the section limits, its status and contents, and the
means of control. There is also a full statement of the design intention,
as developed by the team in their search for deviations. If the design
intention is complex or extensive, then reference to an appropriate
design brief or specification may be used.
7.3 THE RECORDING FORMAT FOR THE DETAILED
EXAMINATION
The discussions of the team are normally recorded in a tabular format
with a series of main columns, perhaps with some supplemental
sections for each entry. The minimum set of columns is:
Deviation Cause Consequence Action
51

However, it is commonplace to include an initial column for the
“parameter,” and after the “consequence” column it is also good prac-
tice to have one headed “protection” (or “safeguards”), particularly if
these offer significant risk reduction. It is essential to have a numbering
system, either numbering separately each entry or each action, and it is
usual to link the action to the person responsible in an “action on” column.
Thus, the norm is likely to be:
Ref. Parameter Deviation Cause Consequence Safeguards No. Action Action On
Further items to be considered are “response,” “comment,” “hazard
category,” “event frequency,” and “event magnitude.” From these last
two it is possible to develop a risk ranking scheme. The choice of
which items to include depends on the company style and the uses to
be made of the records, as well as on the recording system used.
A risk matrix is needed if the event likelihood and severity
classifications are to be used in risk ranking—indeed if this is to be
done it is likely that the company will have an established risk assess-
ment matrix. This may range from a simple three-by-three matrix to
more complex methods. Not all users of HAZOP rank potential
problems and, if it is to be done, a suitable matrix must be agreed
before the study is started. As described in Section 4.6, three levels of
risk may be shown: for the worst case, then after allowing for the exist-
ing safeguards, and finally after the recommended actions have been
incorporated. The merit of this approach is that it explicitly shows the
importance of maintaining the safeguards and of implementing the
action. It must also be remembered that HAZOP studies are not well
suited to the identification of high-consequence low-frequency events.
These should have been identified in the earlier hazard studies.
7.4 THE LEVEL OF RECORDING
Once the format is determined, the level of detail of the recording is
decided. Three levels are possible:
1. record by exception—only when an action results;
2. intermediate record—where an action results, where a hazard exists,
or where a significant discussion takes place;
3. full record—all meaningful deviations are noted even if no realistic
causes are found.

Recording by exception requires an entry only when the team
makes a recommendation. This method serves the immediate needs of
the study and provides a basis for implementation of the actions but
is of little value in any subsequent uses. It is not recommended for
general use. It may lead to shorter meetings and simpler reports, and
be superficially attractive if there is pressure to complete the study
within the project time constraints, but any economies are likely to be
false ones as the fuller levels of recording have many later benefits.
At the intermediate level, a record is generated whenever there is
any significant discussion by the team, including those occasions
where there is no associated action. These include deviations identi-
fied by the team which, though realistic and unanticipated in the
original design work, happen to be adequately protected by the exist-
ing safeguards. There are some important gains by recording at
this level. One is the increased likelihood that these safeguards are
maintained during the plant lifetime since their purpose is spelt out
in the records. A second benefit is that the ground covered by the
team is clear during an audit and to any later reader of the HAZOP
study file; any deviation not recorded was either not considered a
realistic combination of guideword and parameter or was thought to
have no significant causes or consequences. A third benefit is the
ease with which modifications can be analyzed by HAZOP study at
a later date.
In full recording, an entry is included for every deviation considered
by the team, even when no significant causes or consequences were
found. At this level, each parameter is recorded with each guideword
for which the combination is physically meaningful. It may even
extend to listing the guidewords that were not considered as they
did not give a meaningful deviation with the parameter examined.
Reasons for recording in full are to conform to a company standard,
or the high hazards involved, or to meet a legislative requirement, such
as the OSHA legislation in the USA. Some shortening of the records
may be possible by having standard entries to cover some common
cases. For example, if no causes can be found by coupling a parameter
with a group of guidewords, the term “remainder” can be written with
the parameter and the phrase “no causes identified” put in the cause
column or “no significant risk” in the consequence column. Full
recording is obviously more expensive and leads to a very substantial
53

file but does permit a full audit. It is therefore the preferred option for
those industries that need to demonstrate the highest possible standard
of safety management.
7.5 THE CONTENT
It is essential that all entries, whether causes, consequences, protection,
or actions, are as clear as possible and identify unambiguously the
items to which they refer, using vessel, equipment, and line numbers. If
the records are too brief or otherwise inadequate, they may be open to
misinterpretation so creating difficulties in the implementation of
safety and operability into the final design.
7.6 COMPUTER RECORDING
Dedicated computer recording systems have been available for many
years and are widely used, particularly for large HAZOP studies
although standard word processing or spreadsheet programs are
perfectly adequate. A list of commercial software has not been
included here due to the problems of giving comprehensive coverage
and maintaining an up-to-date listing. Gillett4
gives some information
while further sources are software houses and consultancy companies.
There are few disadvantages to using a computer recording system
provided it is done well, and it is certainly worth considering if a
handwritten record will later be word-processed. Probably the major
disadvantage is that the package may force the recording, and perhaps
even the HAZOP study, to be done in the way envisaged by the pro-
gram designer. It is essential that the package allows the chosen style
to be followed. It is also important to have a scribe who is familiar
with the recording program and is able to type fast enough to avoid
any delays to the meeting. Also, if the records are displayed on screen
for the team to see, the display system must be powerful enough to
avoid the need for a dimly-lit room. As with hand records, the forms
can usually be customized to suit a company style. A great deal of
preparative work can be done beforehand by the leader and the scribe
and, even if changes are needed during the meeting, this is easily done.
There are some advantages with computer recording. During the
study, the headers, which include the design intention, and earlier

sheets are easily consulted and seen by all of the team. Single keystroke
entries are made for parameters and guidewords and for frequently
used phrases. If the team can view what is being recorded, then any
disagreements or possible ambiguities are dealt with immediately.
Databanks of possible causes, effects, and frequencies are held on the
computer and consulted when needed. Draft records for checking are
available for circulation by printing or email shortly after the end of a
meeting and action notes can also be generated without delay.
However, general circulation should only be done after the leader and
scribe have checked them for spelling and meaning. Responses to
action notes are easily incorporated into the records. Spell-checking
facilities are normally available and it may be possible to search the
program for individual words, names, or combinations—for example,
to list all the individual records where responses are overdue. Some
programs are used as a management tool for the study, and the more
sophisticated programs are written for use with other process hazard
studies. It is also worth noting that the recommendations/actions can
be captured from the electronic records and quickly transferred into
the MOC or HAZOP actions tracking system.
A different aspect of computer use in HAZOP studies is the
expert system, designed to “conduct” a HAZOP study. A number of
programs have been developed, but the present view is that a fully
satisfactory system has yet to be written; indeed some think the creati-
vity of a good team will never be duplicated by a computer. However,
such programs do have some potential as a preliminary screening tool,
for example, on PIDs at the “approved for review” stage, since they
can ensure no known cause or deviation is overlooked.
7.7 AUDITING A HAZOP STUDY
An audit or review of a completed HAZOP study may be done
internally, to show conformity with company standards and to learn
from the study, or externally. In this latter case, it is likely to be done
by a regulator to confirm compliance with national codes or in the
aftermath of an incident. All reviews will largely depend on the study
records, and it is important this is recognized at the outset of the study.
The list below covers some of the major points that should be exam-
ined in an internal audit. The key question is “did we do it properly?”
whereas in an external audit it is “was it done properly?”
55

• General features
• clear terms of reference and authority for the team;
• scope of the study—start and end points; interfaces and links to
facilities as a whole;
• timing of the study within the project and facilities for the study
team;
• links to other project hazard studies;
• the team
• qualification of leader and scribe;
• selection, competence, and experience of the team members;
• continuity of attendance and use of specialists.
• time available.
• Preparation and overview
• PIDs revision number and date. Should be either final design or
as-built;
• modes of operation selected for study (e.g., steady state, start-up,
and shutdown);
• other documentation made available (e.g., cause and effect
diagrams, equipment specifications, isometrics, operating and
control sequences, material hazard and data sheets, site plans,
reports from earlier hazard studies);
• any special preparative work done for a batch process or a pro-
cedure (e.g., batch progress charts, plant status chart);
• node selection.
• Detailed report
• style of recording which should include, as a minimum, clear
reference number for each line, deviation, cause, consequence,
safeguards, action, and action on;
• marked off matrix of guidewords and parameters showing a
comprehensive and imaginative use;
• entries that can be precisely related to the PIDs and are
sufficiently detailed for the auditor to understand the meaning
and outcome of the discussions;
• a good design intention for each node, realistic causes, appropriate
consequences, understanding of the design envelope and existing
safeguards, etc.;
• sufficient and appropriate depth in the search for causes.
No excess of trivial items;
• clarity when no causes are found for possible deviations or no
action is required;

• significant perceived risks referred for more detailed risk assessment;
• archiving.
• Post study work
• clear links to follow-up so all recommended actions can be traced
to a final decision and implementation. This may require an audit
of the formal closeout procedure;
• evidence that the team has been able to review the outcomes of
actions where further investigation was recommended.
57

CHAPTER 8
8
Training
8.1 TEAM MEMBERS
It is good practice for team members to have been trained in the
HAZOP study technique. This is usually done through a formal train-
ing course covering the principles and the methodology of the tech-
nique. Experience can be given by the use of simple examples which
allow the key points to be illustrated, as well as showing important
aspects of preparing, leading, and recording the study. It is best if the
course is quickly followed by experience in an actual study so that the
learning points are reinforced and developed. Where an organization
regularly uses HAZOP and has many experienced and practiced team
members, it is possible to introduce a new member to a team after a
brief introduction to the method. Subsequent learning is then done by
working within a group of experienced practitioners. A recent develop-
ment has been the creation of online courses for training team
members. While these lack the interactions of group training methods,
they can be useful for individuals with an urgent need for training or
for those unable to access a group course.
8.2 SCRIBE
Formal training is not required provided the scribe understands the
categories and level of recording needed. However, it is helpful if the
scribe has been trained as a member and had sufficient exposure to
HAZOP studies so that issues requiring to be recorded can be recog-
nized promptly without instruction. If a computer recording program
is chosen, it is essential the scribe is trained in its use so that the
recording process does not delay the progress of the team. In a long
study, it is helpful to have a scribe who is also a trained leader; this
can allow roles to be rotated periodically.

8.3 TEAM LEADER
A team leader needs to develop and practice skills to:
• understand and use the HAZOP method and structure;
• manage the team effectively to optimize the contribution of all
participants;
• use their own technical knowledge and experience lightly so as not
to become the technical expert.
Thus, previous experience as a team member in a number of studies
is important, preferably covering a range of different operations. Not
all experienced HAZOP study members, however, can become success-
ful leaders. It is also of value for the leader to have experience as a
scribe in order to appreciate the particular problems with this task.
Some formal training in leading, including practice undertaken along-
side an established leader, is recommended. Initial experience should
be gained on a low-risk process. As studies are led on higher risk pro-
cesses, the degree of mentoring from a more experienced leader should
be increased until the trainee becomes confident of how to address the
nature of the hazards faced in such studies across a range of processes.
Obviously, the team leader must fully understand the HAZOP
methodology and, during the meetings, must focus on applying this
and making it effective. Equally important is man management—
getting the best from the team members and the scribe. Having this
skill is essential for good leadership; without it a leadership candidate
should not be allowed to progress.
HAZOP study leaders should always critically review their perfor-
mance after each study, if possible with the help of another experienced
person from the team. It is important to maintain the skills of leading by
regular usage, and this is helped if the company keeps a list of trained
leaders with a record of the hazard study work done by each one.
More details20
are available of the range of training methods that
have been used.
59
Training

CHAPTER 9
9
Company Procedures for HAZOP Study
If HAZOP studies are regularly used as part of a company’s process
hazard studies, a company procedure on the execution of a HAZOP
study is recommended. This helps greatly in the uniformity of applica-
tion and maintenance of standards and simplifies the reporting of any
study carried out in accordance with the procedure. All of the relevant
issues described in Chapters 2 8 of this guide can be included, as can
those covered in the IEC guide.7
It is important to give advice on the
selection of suitable techniques for process hazard studies so that a
HAZOP study is considered whenever it might be appropriate and is
used when it is best suited.
A company procedure might cover:
• responsibility for initiating a study;
• where HAZOP might be applied and how it relates to other
Hazard Studies;
• the purpose and the range and depth of study;
• any special requirements, for example, checking cause and effect
diagrams;
• timing of the study, within the overall project and the detailed
meetings;
• terms of reference and scope;
• detailed HAZOP methodology to be used;
• follow-up and implementation of actions;
• how records will be maintained;
• how the study integrates with the company’s MOC procedure.
Further important aspects to be considered are:
• appointing a leader and scribe;
• team selection, composition, and training;
• documentation needed for the study;
• facilities for the team meetings;
• preliminary briefings;

• style and level of recording and the recording format;
• application (or not) of risk assessment with, if necessary, a
suitable (corporate) risk matrix;
• any summary reports in addition to the main, full report;
• signing off of individual actions and the final report.
The HAZOP methodology could be expected to cover:
• approaches to continuous and batch processes;
• node selection;
• design intention;
• guidewords;
• parameters;
• identification of causes;
• consequence types (hazards, operability, financial, quality, reputa-
tion, etc.);
• consideration of safeguards;
• risk and SIL assessments;
• actions and recommendations—company preferences;
• human factor issues.
A company procedure must cover the general, basic HAZOP
methodology, as indicated above, but it should also be tailored to the
particular activities and types of process and materials used, as well as
to the specific company arrangements for administration and organiza-
tion. The standard guidewords should always be considered but there
may be additional ones that have proved useful in previous studies.
Likewise a list of parameters can be produced to act as a minimum set
to be considered during a study. This can be particularly useful with
the guideword “other/other than.” There can be a list of specific issues
that must be addressed, for example, in drilling where the clays can
become agglomerated and then may choke the mud gutters. Another
possible special issue is how shutdowns are addressed—there may be
multiple levels of shutdown from the process shutdown (or “hold”) to
the ultimate shutdown for a major incident.
61
Company Procedures for HAZOP Study

CHAPTER 10
10
Advanced Aspects of HAZOP Study
10.1 HAZOP STUDY OF COMPUTER-CONTROLLED PROCESSES
The use of computers to control chemical processes in part or in
entirety is now widespread, and many control devices contain some
form of programmable logic. The number of reported incidents21,22
in
such systems demonstrates their need for effective hazard study. The
introduction of computers has occurred since the original development
of the HAZOP study method and so the technique has had to be
adapted to cover such processes.
The reliability of PESs in safety-related applications is now covered
by international standards such as IEC 6150823
and 61511.14
In the
UK, the PES Guidelines24
give detailed advice and point out the many
types of problems that must be covered and give advice on how to
deal with them. As well as anticipating random hardware failures, it is
necessary to identify, and eliminate or minimize, systemic failures
including those due to:
• errors or omissions in system specifications;
• errors in design, manufacture, installation, or operation of hardware;
• errors or omissions in software.
The specification and planning of a computer-controlled process is
normally examined during HS 2; the detailed search for undetected
errors and omissions occurs during HS 3.
Where safety-related systems are computer-controlled, they must be
designed, installed, operated, and maintained to the specified standard for
their purpose. The performance of the whole system must be demon-
strable, including all elements—from the sensors, through the logic proces-
sors, to the actuators and final process hardware elements such as valves.
HAZOP study has proved to be an effective method of critical
examination of computer-controlled systems. But first there must have

been an earlier HS 2 to identify the key issues and set out design
requirements. Then the HAZOP methodology must be adapted to
bring out the computer-related problems as well as the conventional
hardware and human factor problems—the recommended approaches
are described here.
10.1.1 Hazard Study 2
The title “Safety and Operability Review” (SOR) has commonly been
used for the study carried out on a computer-controlled system at the
time of HS 2. This should be done at an early stage in the project
definition and design stages, as described in Chapter 2, in order to
highlight the main hazards and to identify critical safety functions. It
is carried out by a small team that includes at least one expert on
computer systems.
The team uses a checklist or a set of questions that will help to
highlight the key issues—it is not, therefore, following the traditional
HAZOP method. Key topics to be considered include:
• safety critical protective functions;
• interactions between control and protection systems;
• the extent of hardwiring of controls, alarms, and trips;
• the degree of redundancy and/or diversity required;
• independence and common cause failures;
• input/output (I/O) arrangements for the control system;
• routing of data highways and their vulnerability;
• communication links and speeds;
• program storage method and the security;
• positioning and security of the computer hardware;
• likely consequences of system failure and of site power failure;
• construction of screen pages and alarm displays to assist
troubleshooting.
Where significant hazards are present there will be a layered protec-
tion system starting with alarms allowing operator intervention, then
actions by the control computer and, ultimately, hardwired or software
initiated actions through SISs or safety-related protection systems and
demands on the passive protection such as relief valves or actions
driven by an independent computer.
63

10.1.2 Functional Safety and IEC 61508
IEC 6150823
has implications for HAZOP study of computer-
controlled processes. Its title has three key terms:
1. Functional safety—that part of the overall safety of the plant,
process, or piece of equipment that depends on a system or equip-
ment operating correctly in response to inputs.
2. The systems covered—any electric, electronic, or PES.
3. Safety-related—that is, systems that are required to perform a
specific function to ensure risks are kept at an acceptable level.
The requirements placed on a safety-related system depend firstly on
its function. This will be determined during Hazard Studies 2 and 3
when possible hazardous conditions are identified. Then the perfor-
mance required for this safety function must be determined by risk
assessment. A key part of the IEC 61508 methodology is the establish-
ment of SILs for each hazardous condition, giving the level of reliability
required of the protective systems. These can then be specified to match
the SIL requirement, and individual components specified accordingly.
The standard uses four SILs. SIL1, the least demanding, specifies a
probability of failure on demand (PFD) for low-demand events
of between 1021
and 1022
; the PFD range decreases by an order of
magnitude for each step up in the SIL rating. To achieve SIL 3 or 4 is
very demanding, usually requiring a high-integrity protective system
with multiple sensors and actuators, voting systems, diversity, and
redundancy. This is difficult to design, prove, and maintain and so
most systems in the chemical process industries tend to be designed to
operate with protective systems up to, but not beyond, SIL2.
In order to ensure effective implementation of a safety-related
system, it is important to consider it throughout the life cycle of the
project and process, including: scope, specification, validation, installa-
tion, commissioning, operation, maintenance, and modifications. IEC
61508 covers all these life cycle activities. This standard forms the basis
for developing other standards for particular sectors of operations,
including BS IEC 6151114
for the process industries.
It is essential that these systems are defined early in the project
so that the detailed design can satisfy the overall design intentions and
to ensure that all computer-controlled safety-related functions are
clearly recognized. If a positive decision is taken to operate with
computer-controlled safety-related functions, then very high standards

of design, proving, and maintenance will be required, and a Computer
HAZOP (CHAZOP) study will almost certainly be necessary, to meet
the reliability required by the Standards.
As well as meeting the required SIL, the systems should be designed
to minimize demands on the protection systems from control failures,
for example, by spurious trips. It is usual to address the requirements
of the standard by having a combination of hardwired or passive
protective devices while having the early warning alarms, initial actions,
and duplicated safety actions passing through the control computer.
10.1.3 Enhanced HAZOP for Computer-Controlled Systems
The enhanced HAZOP extends the usual conventional HAZOP study
to include aspects of the computer-controlled systems. It is suitable
when the control system does not have safety-related functions and
where the hazards are not exceptionally high and where there is a final
level of hardwired protective devices.
The purpose of a conventional HAZOP is to consider possible
deviations, using the full PID and other relevant design and operating
information, and to single out for action those that have significant
consequences not adequately controlled by the existing design. When a
computer-controlled system is studied, the whole control loop from the
field sensor through the logic solver and the final element must be con-
sidered. There will also be additional deviations that are due to random
failures within the computer hardware. These could be the system as a
whole, an individual input and output board, or the operator consoles.
Attention must also be given to the control sequences, and these must
be defined in sufficient detail for the exact operating sequence to be
understood for each section or stage being reviewed. In addition, it is
important to know what status checks are being made by the computer
and what actions will be taken by the control system if a fault is detected.
The team needs to review and question these actions for each stage.
For an enhanced HAZOP, the information needed in addition to that
normally used in a study of a conventionally controlled process includes:
• The specification of the computer hardware, the control language,
and the programming method.
• The QA checks for the software.
• The signals from the plant instrumentation to the computer and the
computer outputs to controls, valves, alarms, etc.
65

• The interface between the operators and the computer and the
means of interaction allowed to the operators.
• Which safety features are hardwired.
• An outline of the control sequence on which the code will be
based—probably as a logic diagram. Sufficient information is
needed so that the intended progression is known and the response
of the control system to a deviation can be evaluated.
The HAZOP follows the conventional method, but common differ-
ences encountered with computer-controlled systems include:
Guidewords: The usual set of guidewords should be used, but the
sequence-related guidewords, such as sooner and later, and before
and after, become important. Other possibilities are more or less
often and interrupt.
Parameters and deviations: While considering only meaningful
deviations, a sensible selection must be combined with lateral
thinking and imaginative suggestions. A computer-controlled
system could introduce new parameters such as data flow, data rate,
and response time.
Causes: As well as the usual HAZOP causes, there may now be
additional ones that originate in the computer system, such as the reli-
ability of power supplies, the possibility of complete or partial hard-
ware failure, plus the links and handover sequence. Human factors
during interactions with the computer should also be considered.
Consequences and safeguards: The evaluation must ask what
information is provided to the computer, how it will be interpreted,
and how the control system will react. Again, the interaction between
the operators and the computer system may enter the analysis.
It is helpful to ask four key questions when assessing a developing
event:
Does the computer know?
What does the computer do?
Does it tell the operator?
What can/does the operator do?
Clearly the HAZOP study team must have sufficiently detailed know-
ledge of the intended control and emergency sequences if an accurate
evaluation of the consequences is to be made. The team should include
both the person responsible for creating the control program and
someone who understands the coding process to ensure that there is
no uncertainty or ambiguity between the different disciplines involved.

Actions: Many actions will concern the draft control sequence, per-
haps to provide better control, more checks, further information for
the operators, or more alarms. These can benefit from the capacity
of a computer to carry out complex checks and actions with speed
and reliability. There may also be some actions to correct problems
of timing identified in the draft control sequence.
Reporting: The normal requirements for good reporting apply. In
particular, it is essential that all actions are written so those carry-
ing out the programming or coding will be clear what is required.
The report must be clear and meaningful to individuals who were
not part of the team, bearing in mind that the implementation will
rely not only on those versed in the usual engineering disciplines
but will now involve software specialists.
10.1.4 Computer HAZOP (CHAZOP) Study
This staged approach is recommended for complex control systems, when
there are major hazards or when safety-related functions come under
computer control. An enhanced HAZOP is usually carried out first but, if
all the necessary information is available, the two studies may overlap.
The timing of the second study can be a problem, in that the detailed
coding may not be available until late in the overall development.
10.1.4.1 Detailed Study of Computer Hardware
Details of the second stage computer HAZOP are given in a Health
and Safety Executive (HSE) Research Report,25
and only a brief sum-
mary is given here. If done in full, it is a very comprehensive review of
the computer hardware and software. It is very time-consuming, espe-
cially for batch processes, and therefore should only be used for high
hazard activities or new or unusual processes.
All the normal plant documentation used for the first HAZOP
study is needed as well as the detailed design specifications for control
schemes and protective systems. For this study, the detailed design
specifications for all the computer hardware will also be needed,
including cabinet details, I/O configurations, alarm and trip schedules,
communication links, watchdogs, backups, power supplies, security
considerations, and provisions for software modifications. Many of
these will have been identified by HS 2.
The team work through the computer system to build up a picture of
how it is intended to work and what will happen if any element fails,
67

identifying all reasonable causes of failure both internal and external
to the computer system. In many ways this parallels failure modes
and effects analysis (FMEA), by considering all of the failure modes of
the key elements in turn. The four key questions used in enhanced
HAZOP (see above) will help the team to identify the adequacy of the
required response.
This stage of the examination need not be lengthy if equivalent
hardware has been considered before for other projects or if there is a
consistent, standard policy over hardware, and there are common, tested
subroutines in the computer code for actions that recur regularly.
10.1.4.2 Detailed HAZOP of Computer Sequences
This follows the conventional HAZOP method of using guidewords,
but in this case it applies them to the steps of the control sequence and
the computer actions. It is therefore most usually called a CHAZOP.
It can obviously only be carried out when the sequence (but probably
not the coding) has been fully developed. It can also be applied to the
functional steps of a continuously acting control loop.
The sections or nodes for the HAZOP will follow the steps of the
control sequence, and as usual a design intention will be built up for
each node. But each design intention is now written from the point of
view of the computer functions, for example, “Open Valve XSV0001”
or “Sense level in Tank T0002.”
The conventional HAZOP guidewords are then used to generate
deviations such as no/more/less signal or no/more/less/reverse driven,
high/low/bad signal, input other than expected, sooner/later valve move-
ment, etc. As always, the team needs to use all the guidewords creatively
to create meaningful deviations.
The guideword other can be used to include global effects on the
computer system such power failures or environmental impacts, if
these have not already been picked up under the earlier guidewords. It
can also include key overrides of functions and MOC in control logic,
and the important security issues associated with both of these.
For each meaningful deviation, the team then looks for realistic
causes and examines the possible consequences, following the con-
ventional HAZOP structure. For significant consequences, the safe-
guards are identified, and again the four key questions will help the
team confirm if these are adequate. The result will be a sequence which

has been reviewed systematically for possible deviations at each stage,
and therefore will be better able to deal with problems when in opera-
tion, neither being unable to recognize what is happening, nor getting
stuck in some loop in its internal logic.
For a batch system, this depth of study is very time-consuming but it
reflects the fact that the system has to cope with many different circum-
stances, each providing opportunities for failure or having different
consequences that may not have been identified elsewhere. But, despite
the time involved, this method has been demonstrated to provide a
thorough way of identifying potential problems in the logic and the
components of the computer system.
10.2 HUMAN FACTORS
The purpose of a HAZOP study is to examine possible deviations from
the design intention, to find any previously unconsidered causes of
deviations, evaluate their potential consequences, and to then review the
relevant safeguards before suggesting appropriate actions. Each of these
steps may involve people. This may occur through an error that
contributes to a hazardous event or reduces the reliability of a control
measure that is intended either to prevent the hazard or limit the
consequences. Thus, it is essential that team members take account
of human behavior and have a realistic understanding of typical human
performance in both normal and abnormal conditions. There are several
useful documents26 28
relating humans and risk. They give many exam-
ples of both large and small incidents where human factors played a
significant role, describe the main types of human error and how human
behavior relates to these, and include guidance on ways to minimize
such errors. Indeed, one approach to the management of human failures
is described as a “human-HAZOP.” There is no doubt that the regu-
lator attaches importance to the qualitative assessment of human fail-
ure, backed up where necessary by quantitative assessment. Also there
is emphasis on the need to evaluate low-frequency high-consequence
events adequately since experience has shown that major process safety
incidents are often triggered by human error and organizational failures.
This section is intended to provide basic guidance on the human aspects
that should be considered in a HAZOP study.
Human behavior falls into three broad patterns. For much of the
time, humans operate in a skill-based mode, carrying out familiar tasks
and actions without having to think consciously about them. This will
69

apply in many industrial operations once they become familiar. For non-
standard or less familiar tasks, humans move to a rule-based mode—using
the available information and trying out a response that seems to fit or
has worked in the past. Finally, if nothing easier has worked, they move
to a knowledge-based mode, seeking further information and trying to
find an explanation that will allow a suitable response. Each of these
modes is associated with particular types of error. A HAZOP study team
needs to be alert to the possibilities of these errors causing or contributing
to unwanted outcomes. The team must try to anticipate possible slips,
lapses, mistakes, and deliberate violations.
It should be recognized that errors do not occur because people are
stupid or incompetent but as a result of fundamental limitations of human
performance that are influenced by equipment design features and opera-
tional conditions. An HSE guide26
identifies three contributing aspects—
the individual, the job, and the organization. The individual’s competence
involves skills, personality, attitudes, and risk perception. The job covers
the task, workload, environment, display and controls, and procedures.
Finally, the organization can affect outcomes through culture, leadership,
resources, and communications. However, even accounting for these, it
should be recognized that the possibility of human error cannot be
absolutely eliminated by training and procedures—these are not adequate
control measures for human fallibility.
When carrying out a routine procedure, an operator will mostly
work in the skill-based mode. The likely errors here are slips or lapses.
It may be that there is an array of buttons, labeled A E, to operate
similar valves on different vessels. Pressing C when it should be B
would be a slip. How easily this might occur will depend on many fac-
tors such as the layout and design of the control panel (e.g., where
equipment elements from different suppliers have different operating
controls or philosophies) as well as external influences such as time
pressure and fatigue. The consequence could vary from a trivial loss of
material to causing a catastrophic runaway reaction. Clearly this is a
cause that the HAZOP team should consider. A lapse might happen in
a multistep start-up procedure where, say, after completing step 13 the
operator is distracted by a phone call or has to briefly attend to
another task. Returning to the sequence it is resumed at step 15 and
thereby step 14 is omitted. This may have a trivial consequence; it
may be recoverable; but it should be considered by the HAZOP team
whenever the consequences matter.

The next level of operation in the hierarchy is rule based. When an
uncommon event occurs, humans take the available information to see
if it fits some previously experienced or learned rule. The sequence
followed is “if the symptoms are X then the problem is Y; if the pro-
blem is Y then do Z.” More than one rule may be tried if the first does
not work. If no rule works, then the knowledge-based mode must be
tried. New data must be sought and an attempt made to model the pro-
cess and use this to select the best actions, improvising in an unfamiliar
and possibly critical situation. Not surprisingly mistakes are more likely
in these modes. If the knowledge-based mode is called for in a complex
system, especially in a critical situation where individuals are highly
stressed, the likelihood of successful control and recovery is very low.
The HAZOP team must recognize that people under pressure are sus-
ceptible to predictable errors due to natural biases within the human
cognitive system. People are very bad at recognizing new situations and
will tend to jump to hypotheses based on more familiar situations. This
can mean that operators will be slow to react to a potentially hazardous
mode of operation and assume that the system will, as it usually does,
operate safely. People will even try to rationalize weak signals of failure
to explain away potential problems; they may even focus inappro-
priately on evidence that appears to support their assumptions rather
than acknowledge that they may be witnessing a new problem.
An example of these modes of behavior—skill based, rule based,
and knowledge based—would be a control room operator realizing
that during a routine transfer between vessels that the connecting line
instrumentation shows a rising pressure—a skill-based level of opera-
tion. Applying experience the first rule might be that a valve is closed
in the transfer line and this would be immediately checked. If the valve
is found to be open, then there is no other obvious cause. With the
pressure still rising, further information and a new model are needed.
This material has a high melting point—if the operator knows this,
then a line blockage may be suspected and then appropriate actions
can be tried. Again, knowledge and experience are crucial to raising
the chance of a successful intervention.
Finally, the HAZOP team should be alert to possible violations
(i.e., deliberate breaches of rules and procedures). There are many
possible reasons why violations may occur. It could be to save time, to
make the work physically easier, because it simplifies a procedure or
seems more efficient. If done during normal, everyday operations,
71

deliberate violations may play a part in eroding safety margins. Where
shortcuts in maintenance and calibration tasks, for example, are con-
doned as accepted practice, the reliability of designed safety measures
can be reduced and may one day lead to a major incident. These vio-
lations are most likely to take place if employees have the perception
that management want corners cut to save time or to achieve the
production schedule. Good design of plant and procedures, involve-
ment and education of the operators as well as good management and
supervision reduce the likelihood of routine violations, although in an
emergency it is possible that irregular steps will be tried.
Within HAZOP study, it is often necessary to assess the likelihood of
event frequency. This is usually done by experienced judgment, occasion-
ally by semiquantitative assessment and, rarely, by referral for QRA.
These approaches can also be applied to human error. For relatively
frequent events, an experience-based approach will work. Estimates may
also be derived by task analysis methods using a quantitative Human
Reliability methodology but this takes considerable effort and requires
considerable expertise. At the intermediate level of estimation, there are
some helpful observations from within the nuclear industry,29
and it is
useful if the team leader or at least one member of the team has know-
ledge of these documents. They suggest that there is no task, however
simple, for which the failure rate is zero. For the simplest task listed, the
selection of a key switch operation rather than a non-key one, the quoted
error rate is 1 in 104
—this implies that no task is error free. So a study
team should never assume that a problem can be eliminated completely
by an action that relies entirely upon an operator. At the other extreme,
for example, the high-stress situation of large loss of coolant in a nuclear
reactor, the probability for “operator fails to act correctly in the first 60 s”
is 1. That is it should be assumed that there is no chance at all of correct
remedial actions in that time. The situation does not improve greatly over
the next 5 min and is not negligible several hours later. Another source of
human error can occur at a shift handover where communication and
records of previous actions may be poor and an error rate of 1 in 10
is quoted for “personnel on different work shift failing to check the
condition of the hardware unless required by checklist.”
While these are useful guidelines, it is important to recognize the
many other factors that influence human error rates. A comprehensive
set of performance-influencing factors (PIFs) has been established.30
These include training, control panel design, competence and

motivation, environment, level of demand and suddenness of onset of
events, management attitude to safety, procedures, and communica-
tions. There are many more. Understanding these may influence a
HAZOP team’s suggestions for action. In a modern computer-
controlled plant, it can be easy to add an alarm but if this is to be
done it must be within the overall design of the alarm and trip system
so that the operator is not subjected to alarm and/or mental overload
when a major event occurs. When an individual is overloaded with
information, they are less likely to separate the critical, top-level
information from the unimportant and the trivial, resulting in either
inaction or the wrong action. Another state is mind set where the
individual uses the information to create an initial, but erroneous,
scenario and rejects critical information which shows it to be incorrect.
The HSE document, Identifying Human Failures,31
gives a list (in the
following table) of failure types in the form of HAZOP style guidewords
which may be used in the search for human error leading to a deviation.
Action errors Checking errors
A1: Operation too long/short C1: Check omitted
A2: Operation mistimed C2: Check incomplete
A3: Operation in wrong direction C3: Right check on wrong object
A4: Operation too little/too much C4: Wrong check on right object
A5: Operation too fast/too slow C5: Check too early/too late
A6: Misalign
A7: Right operation on wrong object Information retrieval errors
A8: Wrong operation on right object R1: Information not obtained
A9: Operation omitted R2: Wrong information obtained
A10: Operation incomplete R3: Information retrieval incomplete
A11: Operation too early/too late R4: Information incorrectly interpreted
Selection errors Information communication errors
S1: Selection omitted I1: Information not communicated
S2: Wrong selection made I2: Wrong information communicated
I3: Information communication
incomplete
Violations I4: Information communication
unclear
V1: Deliberate actions
73

A HAZOP study team would seldom find it necessary to systemati-
cally examine all of these possible deviations. In many operations and
procedures, the use of appropriate guidewords, which may be problem
specific, will help the team decide which of the possible deviations
could lead to potential problems. It is unlikely that all of these devia-
tions would be found using just the conventional combinations of
guidewords and parameters.
The example of a HAZOP study of a procedure (Appendix 5) shows
some of the ways that human factors may be identified by the team.
In summary, all HAZOP study teams need to be aware of the
potential for human error to generate causes and to influence con-
sequences. They need to use the present understanding of human
behavior, influencing factors, and the typical probabilities for different
types of error. It is also good practice to examine the design of control
screens from the perspective of the operator. This will reveal design
inadequacies such as when separate elements that should be monitored
as part of a routine task are actually presented on separate screens.
Such arrangements add workload and complexity and introduce
opportunities for confusion and error. In formulating actions, they
should consider the required level of human behavior—the skill-based,
the rule-based, or the knowledge-based mode—and the actions should
reflect the needs for further diagnostics, training, second line of super-
vision, or simply an addition in a standard operating procedure (SOP)
as illustrated in Appendix 5.
10.3 LINKING HAZOP STUDIES TO LOPA
Layer of protection analysis—LOPA—is a widely used technique to
determine the level of protection needed to provide adequate
safeguards against major hazards that could arise on a plant or pro-
cess. The method, developed in the early 1990s, is well documented32
and is accepted by regulators in many countries as an appropriate
method of analyzing identified hazards and assessing if sufficient
protective systems are in place to achieve a tolerable risk.33
The first
step in LOPA is the classification of the severity of hazard con-
sequences if this has not already been carried out by a Hazard
Identification process such as HAZOP. For the major consequences
such as injury, fatality, or major accident to the environment, the

magnitude of the ultimate, unmitigated consequences (a scenario) is
estimated, then a maximum tolerable frequency is assigned from either
the company standards or from publically available suggested levels.33
The frequency of the initiating cause(s) is estimated using either a
generic value for this type of event such as control system failure or
human error or, preferably, by fault tree analysis of the expected
sequence for the individual event. Other factors such as “time at risk”
for hazards which exist for part of the time and “conditional modifiers”
such as probability of ignition are considered. It is then possible, taking
into account all independent protection layers, to estimate the frequency
of occurrence of the scenario, that is, the frequency of the top event.
Comparison with the target tolerable frequency shows whether the
protection is adequate and, if not, the magnitude of the necessary
improvements. Where instrumented safety systems are used, their
reliability (PFD) is often expressed as an SIL. The further measures to
achieve a tolerable frequency may include addition of an SIS—a system
designed and evaluated from sensor through control loop to actua-
tors—to have a demonstrable PFD at levels such as SIL1 (a PFD
between 1% and 10%) or SIL2 (a PFD between 0.1% and 1%). LOPA
may be used as a part of HS 2 to ensure that the PFD of each SIS can
be covered in the detailed design. Alternatively, or additionally, it can
follow HS 3, especially where this involves a full HAZOP study.
As the prime aim of HAZOP study is the identification of hazardous
events and evaluation of the consequences, it can clearly link to a
LOPA study, since it produces deviations, causes, consequences, and
safeguards which feed directly into LOPA. To realize the potential of
this link, it is important that the team fully evaluate and record in
detail the consequences for each cause of each deviation as well as the
safeguards already present in the design. Thus, any possibility of severe
injuries or fatalities, of major fire, explosion and toxic releases, or of
substantial plant damage and disruption, need to be recorded in the
consequence column of the HAZOP report. This highlights scenarios
that should be included in the LOPA study. An additional useful item
is the teams’ view of the likely frequency of the event.
Two outcomes are common during the HAZOP study. If the
scenario has already been considered at the HS 2 stage, then the team
can immediately review the design and recommended safeguards to
confirm their adequacy. If the scenario is new then, as a potential
75

major event, it will need to be examined using the LOPA method-
ology. This requires a LOPA study to follow the HAZOP study either
as a resumption of the earlier one or a new study. To be effective the
HAZOP team must be on the lookout for potential major con-
sequences, even when these are anticipated to be very-low-frequency
events. The ultimate consequence should be clearly recorded so that
when the records are reviewed they stand out as needing review in
the LOPA study. All possible causes should be recorded as these are
essential inputs to the LOPA study. Similarly, the existing safeguards
should be clearly recorded as they may serve as independent layers of
protection against a developing event.
Thus, LOPA and HAZOP study are natural partners in identifying
hazards, determining whether existing safeguards are adequate and, if
not, specifying the additional levels of protection that are needed for
each possible cause of the event. However, if the full benefits are to
be realized, it is essential that the HAZOP study has been planned
to provide optimal information for the LOPA study.

CHAPTER 11
11
Specific Applications of HAZOP
The main uses of HAZOP study in the process industries are for new
designs, processes, and operations, both continuous and batch, and for
modification and reuse of existing plant and processes. This chapter
comments on the special aspects of these uses and also considers some
other, less frequent, applications.
11.1 MODIFICATION OF EXISTING OPERATIONS
It is important to have in place a procedure for the management of change
(MOC) to ensure that all modifications are reviewed before any variation
in plant, process, or operation is made. The review should recommend an
appropriate method of hazard identification. Where there are significant
hazards, this may be a HAZOP study. The company MOC procedure
should include criteria for deciding if a HAZOP study should be done.
A very wide view should be taken as to what is a “modification.”
Anything which changes a plant or a process in any way must be treated
as a modification. Such changes could be to materials, catalysts, solvents,
conditions, sequences, quantities, procedures, software, and so on.
When a HAZOP study is used for a modification, the basic
principles of HAZOP are retained and applied. For major modifi-
cations, the study follows the steps taken for a new design. For small
modifications, it is possible to proceed more quickly, using a smaller
team and combining some of the roles within the team—for example,
a member may act as the scribe and it may even be acceptable for
the leader to have another role. It is particularly important to have the
operating team represented within the HAZOP study team.
If the system was previously studied by HAZOP, then the original
report may provide a useful starting point. There can be problems
with defining the boundaries for the study since it is unlikely that the
whole operation will be reviewed. The boundaries may have to be
some distance from the point of the modification to ensure that all

relevant causes and consequences are considered. The boundaries
should be agreed between the project team and the HAZOP leader
with the leader given the authority to extend the boundaries, if felt
necessary, during the study.
11.2 REPEAT DESIGNS—HAZOP-BY-DIFFERENCE
In some branches of the process industry, it is commonplace to install
designs which are essentially the same as an earlier installation or
which are made up of standard units, varying only in size from other
installations. In these cases, it may be possible to do an effective
hazard identification study by detailed comparison to an earlier, full
HAZOP study, concentrating on any differences from the previous
case. When this method is used, the team must be particularly aware
of any variations in size, site, services, and interfaces with other plant.
11.3 PERIODIC HAZARD STUDIES AND THE HAZOP
OF AN EXISTING PLANT
Periodic hazard studies are process hazard analyses to ensure a process
plant continues to operate and be monitored to appropriate SHE
standards throughout its life. While MOC reviews provide a record of
incremental changes over a period of time, it may become necessary to
review a system as a whole, particularly when the multiple changes
may interact adversely with each other.
Such a review is particularly important if any changes to operating
procedures, feeds, or products and/or modifications have been made.
The requirement for such periodic studies can be legal—for example,
OSHA—or company policy as best practice. There are several techni-
ques available for such studies, including the retrospective use of
Hazard Studies 1 and 2. HAZOP study should be considered as a pre-
ferred approach if the following have occurred:
• major incidents;
• many modifications;
• the original studies were inadequate;
• significant design deficiencies have been revealed;
• the plant has not run smoothly.
HAZOP is necessarily more time-consuming than most of the alter-
native techniques, but has the advantage of a comprehensive outcome.

Other techniques2
can be valuable in identifying key issues and the need
(or otherwise) for more detailed studies such as HAZOP. The choice of
method will depend among other factors on the available experience base,
the sophistication of the process, and regulatory requirements.
Whichever technique is used, it is important that target dates are set for
completion of actions and for review of progress and subsequent periodic
studies. Proper action progressing and specific periods for subsequent study
may be a legal requirement where such studies are mandated.
11.4 OPERATING PROCEDURES
The HAZOP methodology for an operating procedure is essentially the
same as for a batch process. Such a detailed study is normally only
applied to critical procedures. It requires a well-defined procedure to be
available, including all significant steps and actions, an up-to-date PID
and, ideally, structural drawings to locate valve positions. For existing
plant, a tour of the process area is recommended. In preplanning, it has
to be decided whether the HAZOP of the procedure comes before or after
the HAZOP of the process—the normal case would be for the process
HAZOP study to be done first. Also, the study must not degenerate into
a procedure-writing meeting. The team composition must be correctly
balanced to get the best results and must include some members who are
familiar with the process, including an operator. Before the start of the
study, the procedure is reviewed for clarity and the aims of the study
defined. Normally, these are to identify potential hazards, operability
problems, and environmental problems which may result from deviations
from the procedure, especially those due to human factors.
The actual analysis follows the batch HAZOP methodology,
working through the procedure stage by stage. Each stage, which may
consist of a number of individual actions, is examined using the guide-
words to prompt the team members to suggest meaningful deviations
which are then analyzed in the usual way. In addition to the standard
guidewords, “out of sequence” and “missing” can be productive.
“Missing” is interpreted to mean that a step is missing from the proce-
dure at or just before the stage examined—although such deviations
could equally well be found using the guideword “no.” In the list of
parameters, the phrase “complete the step” can be used to good effect,
as it combines meaningfully with the guidewords “no,” “more,” “less,”
79

“reverse,” “part of,” “as well as,” “out of sequence,” and “missing.” An
unusual but occasionally useful question is “verification of success”—
how is it known that a valve is closed or that a vessel is depressurized.
This last point is very important in pigging and filter operations.
A major difference from process studies is that many of the causes
of deviations are related to human factors. These may be of omission
or commission. The importance of Human Factors in HAZOP studies
has been considered in Section 10.2. Other possible causes include
poorly written procedures, difficulties caused by poor layout, bad
lighting, parameter indicators with limited or poor ranges, or too
many alarms. The latter, a cause of information overload, is a topic of
concern and advice is available.9
In assessing safeguards, a reasonable
allowance can be made for the presence of the operator if close
involvement with the system allows for the possibility of immediate
detection and correction of the deviation. Experience of the actual
conditions and the style of operation is important when making such
a judgment about human factors.
Actions may suggest a change in the procedure but need not be
limited to this option; instrument or equipment modifications should
be recommended if they offer the best solution to a problem.
11.5 PILOT PLANT AND LABORATORY OPERATIONS
Pilot plants and laboratories typically differ from full-scale plants and
processes by their smaller scale, diversity and greater degree of human
interaction. Nevertheless, there are many exceptions. Some refinery or
petrochemical pilot plants can dwarf full-scale fine chemical plants. Pilot
plants built to test scale-up may be single continuous stream plants and
highly automated. However, the hazard study approach for all pilot
plants, semi-technical plants, and experimental plants follows the same
pattern. A preliminary hazard analysis—for example, Hazard Studies 1
and 2 (see Chapter 2)—should be carried out, and if the potential for
significant process hazards is identified then a HAZOP study can be
recommended. The same HAZOP methodology for continuous processes
or batch processes, as described earlier, is used but with greater emphasis
placed on the process and design uncertainty and human factors.
The design intent and limitations of the pilot plant or laboratory
should be clearly defined. Constraints need to be established on the

experiments, materials, and process conditions allowed in the pilot
plant or laboratory. A system needs to be established which registers
and evaluates new experiments. If an experiment falls outside the
established constraints, then the experiment should be subjected to
a preliminary hazard analysis which decides whether a HAZOP
study is required. The preliminary hazard analysis should also
establish whether extra studies are required—for example, reaction
stability,34
reactor relief requirements, emergency measures, and
occupational health. Changes to the building, laboratory, plant,
process, or equipment are covered by a modification procedure, as
discussed in Section 11.1, page 77.
It is recommended that the HAZOP study of large continuous pilot
plants initially studies the process as a continuous one (see Section 5.3.1,
page 37). During the HAZOP study, extra emphasis is required on the
possible inadequacies of the design. For example, a heater control may
not just fail but the heater may be grossly over- or under-sized as a
result of uncertainties in the process stream properties. Since starting,
stopping, and aborting experimental runs is a regular feature, the
process steps involved in these operations should also be subjected to a
batch HAZOP study (see Section 5.3.2, page 39).
For multifunctional experimental plants (typically batch or
semi-batch processes), it is recommended that a typical process is selected
and subjected to a HAZOP study. The other processes are then hazard-
studied in as far as they deviate from the established process in conditions
(e.g., temperature, pressure, and concentration), materials (e.g., additives
and solvents), and process steps. For all types of unit, the operating bands
must be clearly established at the start.
For typical research laboratories, support systems should be
subjected to a HAZOP study if they are critical for the security, safety,
health, or environment of the laboratory. For example, the ventilation
and extraction systems for a laboratory designed to handle highly toxic
materials or biologically active agents should be effective and reliable—
a HAZOP study of these systems should identify the potential hazards.
A HAZOP study of gas supplies identifies hazards caused by failure of
the equipment or operation not covered by standard designs.
For pilot plant and laboratory HAZOP studies, an appropriate
team is assembled. A good team might involve the laboratory
81

manager, operating technician or laboratory analyst, equipment
specialist, maintenance manager or technician, and HAZOP leader.
If the purpose is scale-up, then the respective idea developer—for
example, chemist, physicist, and engineer—and a technical representa-
tive of the subsequent development stage are useful additions.
Due to the greater degree of human interaction, a greater emphasis
is placed on the skill, knowledge, training, and experience of the oper-
ating technician or laboratory analyst. Involving these in the HAZOP
study ensures relevance, simplifies training, and aids motivation, as
well as improving the documentation of procedures.
A HAZOP study of proprietary equipment—for example, analytical
test equipment—is generally not required provided the equipment has
been subjected to a risk analysis by the manufacture and is installed
and used only as intended by the manufacturer.
While no laboratory experiment is too small to be hazard-studied, the
benefits of HAZOP study are more likely to be achieved if there is a poten-
tial for fire, explosion, significant release of a hazardous material, or other
major loss. A HAZOP carried out at this stage can help to ensure that
hazards are addressed at an early stage in the development of the process.
11.6 DRAINS, VENTS, AND OTHER INTERCONNECTIONS
BETWEEN PLANTS
The vent, relief, and drains systems often link many pieces of equipment,
sometimes different plants, through a common piping network. The
individual pieces of equipment may operate at significantly different
pressures, some equipment may be starting up while others may be shut
down, and some of the fluids may be mutually incompatible. The design
of these systems is often complex to reduce release to the environment
and may be spread over a number of process PIDs or split between
process PIDs and a separate set of vent and drain PIDs.
The HAZOP therefore requires special skills:
• interface management between PIDs (of which there may be many);
• analysis of fluid incompatibilities;
• analysis of the potential for simultaneous releases (particularly vent
and relief systems);
• assessment of the potential for dynamic, static, or other induced chokes.

The interface management requires at a minimum that all inter-
faces (lines entering and leaving a PID) are positively identified on
all drawings and that there are no mismatches or exclusions. If there
is a set of vent and drain PIDs, each interface on the vent or drain
PID should be labeled with a fluid description. The following data
may be recorded:
Condition in
Equipment
Condition in
Vent/Drain
Flow ü ü
Phase ü ü
Pressure ü
Temperature ü ü
Potential incompatibilities
(solids, water, acid/alkali)
ü ü
If the vent and drain are included on the process PIDs and there
are many drawings, consideration should be given to preparing a
special interface drawing to link all process PIDs onto one sheet. If
this is not possible, the interfaces can be labeled as above and the
vent or drain treated as a system. The main parameters and guide-
words are:
Flow More/reverse/no
Pressure More
Temperature Higher/lower
Phase Change
However, there are likely to be secondary issues such as:
Pressure drop High
Line drainage No/less
Dynamic choke More
Static chokes due to debris More
Imposed back pressure on the relief valve More
Reaction forces High
Isolation standards Less
Material compatibility Less
83

A study such as the relief and blowdown review examines the
dynamics within a subsection and the total system, but this does not
eliminate the need to examine the total system with a HAZOP study.
11.7 COMMISSIONING AND DECOMMISSIONING
Commissioning and decommissioning occurs only once on any process
and the issues are often unique. The problems of commissioning are
usually dealt with during the main HAZOP study, either by inclusion
as a parameter under the guideword OTHER in simple cases or by
full examination in more complex cases. They usually have to be trea-
ted as a sequential operation rather than as part of a continuous
process. Decommissioning is seldom considered during early project
stages; it can be a complex process which merits its own HAZOP study
before it is undertaken.
The main features of commissioning are:
• removal of construction debris;
• purging;
• test runs for equipment.
Where the process is very critical or involves complex or high-cost
machinery such as major compressors, the HAZOP procedure can be
used to follow the cleaning process, for example, to:
• verify that no debris is moved from dirty to clean systems;
• identify where debris may lodge and/or block restrictions (valves,
flow meters, instruments);
• ensure that cleaning proceeds from small to larger piping and not
the reverse.
The purging routes can be managed in a similar manner but,
obviously, the rule is to ensure purging proceeds in one direction only.
Test running equipment has the potential to operate outside the
normal envelope. Fluid velocities may be higher (or lower) than nor-
mal, the test fluid may have a different density, viscosity, or tempera-
ture and, in the case of gases, a different ratio of specific heats. If
water is used instead of a lower density fluid, the static and dynamic
heads and power draws may be excessive and the static loads on piping
may be higher. If air is used for test-running compressors, there may
be seal problems, horsepower limits, and high discharge temperatures.

When HAZOP is used in connection with commissioning, it is
necessary to select suitable parameters in order to develop meaningful
deviations. Some examples are given below:
Density Higher/lower
Molecular weight Higher/lower
Pressure ratio Higher/lower
Power demand Higher/lower
Gamma Higher
Noise Higher
Debris Some/more of
Contamination Oxygen/inerts—source of/disposal of
Process contaminants As well as
Water Consequences of/formation of
Cleanliness More/less
Pressure Over/under
Load/stress Higher
Other Projectiles: more/less/velocity
Other Ice/mass balance/static load
In the case of decommissioning—leading to demolition—the
sequence with which the equipment is decontaminated is followed by
the HAZOP procedure. Furthermore, the procedure can follow poten-
tial issues associated with:
• catalysts plus contamination reactivity/deactivation;
• pockets or potential traps;
• special procedures for the demolition contractor.
Ultimately, as much of the equipment as possible should be recycled
and the manner in which it is decommissioned may affect the demoli-
tion. There may be some form of process waste (even residual working
inventories) which has to be processed further elsewhere. This requires
an abnormal operation which should be studied in detail.
11.8 START-UP AND SHUTDOWN
In general a process plant is designed for steady-state operation. The
piping configurations and instruments are designed for that objective.
But there are also two dynamic modes which also must be
85

considered—start-up and shutdown—especially important since many
recorded incidents have occurred during these phases.
The main purpose of a HAZOP Study is the analysis of possible
deviations outwith the design envelope during steady-state operation of
a plant or process and has not handled Start-Up and Shutdown
adequately. The structure of a Start-Up and Shutdown will be in the
form of a procedure or set of steps which start at the introduction
of inventory and end with the specification product from that step.
The Start-Up and Shutdown HAZOPs are therefore best studied as a
Procedural HAZOP (Appendix 5).
During these operations, pressures and temperature, and hence
compositions of the process fluids, may be well outside the steady-state
range or even outside the specification of the materials of construction.
Also, as the illustration in Appendix 5 shows, if a step is missed or is
done out of sequence there is the potential for a deviation.
This section must be treated as an overview as each unit operation must
be treated separately. It should be noted that control is designed for
steady-state operation and not necessarily for the dynamics of Start-Up
and Shutdown. Attention must be paid to the control of a potentially
unsteady state as well as specific flows only experienced during these
operations.
11.8.1 Start-Up
Start-up is a stepwise process. The division or specification of the nodes is
case specific and best defined by the Facilitator and team; they may well
be different from that of a steady-state study. Appendix 5 gives some ideas.
Start-up begins with a pressurizing cycle for that step followed by a
conditioning phase ending in specification product leaving that step.
During a pressurizing cycle, the deviations:
• pressure low;
• temperature low;
• velocity high
can be used to cover potential problems related to vapor liquid equilibrium
data. There can often be problems due to effects in vent and relief systems.
Low temperature may create fluid handling issues due to viscosity, particu-
larly in pockets or traps, or metallurgical issues due to the properties of the

materials of construction. Also during start-up, inerting materials such as
nitrogen (as well as) may create difficulties with condensation.
Consideration should be given to the various steps in the conditioning
operations using guidewords such as:
• out of sequence;
• incomplete;
• too early or too late;
• too fast or too slow;
• step missed out;
• nonsteady-state control.
These may be treated as generic and applicable to all unit operations.
As the example in Appendix 5 shows, these are appropriate to any
procedural HAZOP study. As the Start-Up is followed until steady-
state operation is achieved, attention must be paid to the product
composition at any part of the process using guidewords:
• as well as;
• other than.
Some unit operations may require a second set of guidewords.
For a crystallization operation, the following might be appropriate:
• too small or too large (crystal size);
• mass balance.
Attention must be given to how the off-specification materials pro-
duced during the Start-Up can be stored, reduced, reused, or recycled,
bearing in mind the potential upset that might occur if off-specification
materials enter the downstream section of the process. Guidewords
such as the following can be used:
• How much?
• Where?
• As well as
• Metallurgy.
11.8.1.1 Special Start-Up Conditions
It is important that the team follows the dynamics of the process
during Start-Up. There are many potential hazards that must be con-
sidered and for which no listing would be adequate. As an example the
87

start-up of a refrigeration compressor which also acts as a heat pump
will require a special recycle loop which could be operated manually
for only an hour. The loop is not required for steady state but if not
fitted the compressor cannot be started up if “flash gas” is normally
used for the heat pump cycle. Likewise during start-up, it may be nec-
essary to circumnavigate the flammability diagram bearing in mind
that it is both pressure and temperature sensitive.
It should be noted that any Start-Up may require a total or partial
Shutdown. There will be a number of upsets which will require the
identification of a “Safe Holding Position” which may not necessarily
be the total shutdown and could be at a stable part of the Start-Up
cycle. The team should consciously address this Safe Holding Position
in the procedure from where the restart could be attempted.
The team should select an appropriate set of Guidewords.
11.8.2 Shutdown
Shutdown is less of a stepwise process and is not necessarily the reverse
of start-up. The division or specification of the nodes is case specific and
best defined by the Facilitator and team; they may well be different
from that of a steady-state study. Appendix 5 gives some ideas.
Inventories must be run down to an operating minimum. Then the
process is depressured and residual and often contaminated inventory
recovered. Once again guidewords such as follows may be appropriate:
• low temperature;
• low pressure;
• increased viscosity;
• phase change (phase change could be ice);
• vacuum.
Consideration must be given to how and where the residual
contaminated inventory might be stored, recovered for reuse, and
recycled during a turn-around or destroyed when the process is shut
down permanently and decommissioned (see Section 11.9).
Guidewords such as the following can be used:
• Where?
• How much?

• Too much (the volumes of contaminated inventory at the final shut-
down may be critical)
• Other than
• As well as
• Metallurgy
11.8.2.1 Special Shutdown Conditions
As with start-up it might be necessary to operate a flash gas recycle
line or to circumnavigate a flammability envelope.
11.8.2.2 Emergency Shutdown
Emergency shutdown by a SIS can be tracked through the Cause and
Effects Diagrams to ensure that the sequence is correct. Even if correct
hazards could still occur due to faulty operation of the SIS. Guidewords
such as the following should be applied to each operation:
• incomplete operation;
• partial operation;
• nonoperation;
• reverse operation.
11.9 CONSTRUCTION AND DEMOLITION
Construction is not a steady-state condition, and equipment may be
delivered out of sequence; furthermore, there may be the need to impose
unusual loads—for example, in the hydraulic testing of a steam main or
gas main. The issues are likely to be the following:
• sequence of equipment (out of sequence);
• access for lifting equipment into place;
• loads on piping/foundation.
Where construction is undertaken close to an operating plant, the
potential for interaction may need to be considered.
Demolition is not the reverse of construction and contains its own
SHE risks. Safety issues involve access and overhead/underground
operations. Health hazards include toxic/flammable contamination,
possible fires, and asphyxiation; some of these hazards are produced
in the demolition process—for example, in the hot cutting of materials.
Environmental issues could involve the disposal of lagging, spent
catalysts, residues found in the equipment, and possible spills on site.
89

The restoration of the site for other uses is influenced by the mode
of demolition if it does not capture all residues. The classification of
materials for recycling and the verification of cleanliness are equally
important.
Specialist study methods have been developed for construction and
demolition activities using checklist and/or HAZOP study approaches.
11.10 CONTRACT OPERATIONS
It is important to define within the contract all aspects of any planned
HAZOP study, including the extent to which operability problems
are sought, the responsibility for cost of change, and the control of
team membership. The latter may be a problem if clients request
several places in the HAZOP team in addition to the contractors and
representatives of licensors. The team leader should still aim to keep
the team number below 10 and should consider the responsibilities of
each prospective team member to ensure their presence is needed for
the sections being discussed.
11.10.1 Advice to Users of Contractors
Where a company wishes to use a contractor to carry out a HAZOP
study for systems on process plant, reference may be made to this
guide in any invitation to bid. It should be made clear who is respon-
sible for the HAZOP study, including closing out all of the actions.
Contractors may have their own corporate HAZOP method document
(procedure or standard) ready for a client to review and comment on
before contracts are signed. If this is either more or less than required,
it must be made clear prior to signing. Comments should make it clear
whether and where the scope is to be changed. Any changes suggested
to the way in which a contractor does the work, after contract award,
may be grounds for a contract change. An alternative approach is to
give the company HAZOP study procedure to the contractor so that
the work can be priced to meet the specified requirements.
In some countries, the contractor may have a statutory obligation
with respect to the health and safety requirements of their deliver-
ables and so it may not be practical to reduce the scope of work for
the HAZOP study.

When the study is done it may happen that the team is confronted
by a package unit which is defined only as a box. The team must make
every effort to find the correct PIDs and to study them in the proper
manner. Some of the PIDs may not follow the same style or give the
full details found on process PIDs.
11.10.2 Advice to Contractors
It is good practice to have a corporate procedure or specification
which covers the preparing, running, reporting, and following up a
HAZOP study and to state how it relates to this guide to best practice.
Such a procedure should cover the training requirements and qualifica-
tions stipulated for the HAZOP team members, particularly the leader.
When contractors bid for a project, it is important that their corporate
procedure is submitted, or is referred to, in the contractor’s bid. Once
it has been given to the relevant client for review (confidentially if
necessary), it serves as a benchmark for what will actually be done on
the job. Any changes to this procedure required by the client should be
documented in a job-specific procedure, approved by contractor and
client, for that project. The scope of the systems to be studied should
be defined, and any sections not included must be clearly identified.
Where an external consultant is used to lead the team, it is important
to make arrangements for access after the HAZOP is completed to
resolve any uncertainties which may arise.
91

CHAPTER 12
12
Factors for a Successful HAZOP Study
There are a number of pitfalls in the HAZOP process which must be
addressed and eliminated throughout the study process; before, during,
and after. Those listed in this chapter are some of the more common
ones that may affect the quality and value of the study. The listing is
not comprehensive but serves to indicate the detail required to achieve
a good study. Further advice is given by Kletz.1
12.1 THROUGHOUT THE STUDY
• The HAZOP study should be an integral part of an overall SMS
that includes the other appropriate studies described in Chapter 2.
• The process must have the full backing and support of senior
management.
12.2 BEFORE THE STUDY
• The study must be initiated by a person who has authority and who
will also receive and implement the actions. If the person does not
have authority and the actions are not implemented, the study is a
waste of time.
• The design must be well developed and “firm”—that is, the sections
examined are not being simultaneously developed. In the case of a
modification, the PIDs must be verified as “as built” with the
changes highlighted within a “cloud.” A study cannot be carried out
on a partly-developed design as the subsequent changes will under-
mine the HAZOP study. Freezing of the PIDs is critical to a
study. Also, the drawings must be well prepared. The drawings are
the record of what was studied, and if they are inaccurate or incom-
plete the HAZOP study is worthless. Equally, the study must not be
delayed too long as the options for change will become very limited.
Premature study where the PIDs are still not finalized is wasteful
of time and effort. Equally, the freeze of the PIDs stops further

development of the drawings and may hinder further design work.
The balance is a careful judgment.
• A skilled and suitably experienced team leader should be chosen.
• The leader must be given a clear scope, objectives, and terms of ref-
erence from the initiator for this study (including delivery date and
recipient); if this is not done, the study may be incomplete in some
aspects and not fulfill the requirements of the initiator.
• The leader should choose a route plan (see Section 5.3) through the
PIDs to ensure that all necessary sections are covered effectively
with special care taken at branches and interfaces with services. The
route should be clearly defined with well thought out starts and ends.
• The study should not be required to make project decisions; nor
should the design team adopt the approach of “leave it to the
HAZOP study to decide what should be done!” If a problem is
known, then it needs to be addressed during the design.
• The study team must be balanced and well chosen to combine knowl-
edge and experience. A study group that is drawn entirely from the
project team will not be capable of critical creative design review.
Equally, a team which has no operations input may lack objectivity.
• The team must be given adequate notice of the study so that they
can carry out their own preparation in readiness for the study itself.
This may require some preliminary reading of any relevant hazard
databases and analysis of the PIDs.
• The extent to which problems are evaluated, ranked, and solved
should be defined.
12.3 DURING THE STUDY
There are a number of important factors for success during the actual
study process:
• The team must be motivated, committed, and have adequate time
to complete the examination.
• Team continuity is important—only essential variations and substi-
tutions should be accepted (see Section 5.2.3).
• The boundary of the study must be clearly analyzed and studied. A
change on item “one” may have an effect on item “two.” The item
may be two different processes or an operation upstream or down-
stream on the same process. If the potential impact is not perceived
correctly, the boundary may be placed wrongly.
93
Factors for a Successful HAZOP Study

• The boundary of a study on a modification is equally complex—a
change in the temperature of a reactor may affect the by-product
spectrum and have a more far-reaching impact than the immediate
modification.
• A clear description, design intention, and design envelope must be
given to every section or stage examined.
• The study uses a creative thought process. If it becomes a mechanistic
process and simply works through a checklist, or if fatigue sets in, the
study must be halted and restarted when the team is refreshed.
• Proposing, developing, and finalizing actions is the responsibility of
the team, not the leader.
• Each action must be relevant, clearly defined, and worded with no
ambiguity. The person who follows up the action may not have
been at the meeting and could waste time and effort if there is a
misunderstanding.
• The study must accept a flexible approach to actions. Not all
actions are centered on hardware changes—procedural changes may
be more effective.
• The study team members must be aware that some problems ranked
and identified during the study may be caused by human factors.
• There are potential pitfalls, which must be treated individually,
when planning the route around branched systems. These branches
may be recycle lines, junctions in the process, or vents and drains.
12.4 AFTER THE STUDY
• Every action raised must be analyzed and answered accurately.
• Many of the actions raised will require no further change but all
must be signed off as “accepted” for action or no action, as
described in Sections 6.4 and 6.5.
• Actions that require a positive change should be subject to an MOC
process (which may require a new HAZOP of the change) and put
into a tracking register.
• Action reply sheets should be clearly linked to the original study
reports, including the reference number, the node, the intent, and
the deviation. This greatly helps follow-up and audit. They should
also reference any calculations carried out.

APPENDIX 1
1
The Guideword-First Approach to HAZOP
In Chapter 4, a detailed description is given of the parameter-first
approach to HAZOP study, perhaps the most widely used approach
particularly for continuous processes. An alternative is the guideword-
first approach, which is also widely used, particularly for batch pro-
cesses. This was the approach used in many early studies. The two
approaches do not differ in any basic essential, only in the order in
which deviations are developed and analyzed. The guideword-first
sequence is shown in Figure A1.1.
In the parameter-first approach, a parameter is chosen and then
considered in combination with all of the guidewords that give a mean-
ingful deviation. Thus, the parameter flow might be combined with the
guidewords “none,” “more,” “less,” “reverse,” and “elsewhere.” Then,
another parameter such as “pressure” or “temperature” might be
taken, each one being combined with “more” and “less.”
In the guideword-first approach, the guideword “none” is taken first
and combined with “flow” (plus any other parameters which sensibly
combine with it). Next, the guideword “more” is combined with the
parameter “flow,” then “pressure” and “temperature.” These three
parameters are taken in turn with the guideword “lower.” Table A1.1
contrasts the examination sequence under the two approaches.
With a good leader, the only difference between the two approaches is
the order in which deviations are considered. In these circumstances, the
choice of the approach is no more than a matter of preference or
convention.
One advantage of the parameter-first approach is that all aspects of a
parameter are taken together instead of being interspersed between devia-
tions which involve other parameters. Therefore, all possible deviations
associated with flow are sought before another parameter is considered.
This is well suited to the main parameters of continuous processes.

Figure A1.1 Flow diagram for the HAZOP analysis of a section or stage of an operation—the guideword-first
approach.
96 Appendix 1: The Guideword-First Approach to HAZOP

However, it can be argued that creativity is better encouraged by
the guideword-first approach, particularly for the later guidewords
such as “part of,” “as well as,” and “other than.” The reason for this
is that while appropriate guidewords are easily selected, it is a greater
challenge to get a fully comprehensive and imaginative coverage of the
parameters. If the parameter-first approach is used, there is a tendency
to list the parameters at the start of the analysis and, when the original
list is exhausted, to move on to the next section without a final search
for any more parameters. This is not good practice.
Consequently, the parameter-first approach may provide conve-
nience, but it demands a greater understanding and application by the
team leader and the team members if the best results are to be
obtained.
Table A1.1 A comparison of the order of considering deviations
with the guideword-first and parameter-first methods
Guideword-First Method Parameter-First Method
No flow No flow
More flow More flow
More pressure Lower flow
More temperature Reverse flow
Lower flow Flow elsewhere
Lower pressure More pressure
Lower temperature Lower pressure
Reverse flow More temperature
................ Lower temperature
Flow elsewhere ................
................ ................
97
Appendix 1: The Guideword-First Approach to HAZOP

APPENDIX 2
2
The Use of Checklists Within HAZOP Study
The use of checklists is contrary to the principles of the HAZOP study
method but for an experienced leader, a carefully constructed checklist
for a commonly encountered unit can be a helpful way of ensuring
that less common deviations are fully considered. With less experience,
however, there is a danger that these become a substitute for the
creative thinking and analysis from first principles that are essential
characteristics of a good HAZOP study.
The examples given here illustrate two ways in which checklists
may be used. Table A2.1 lists some aspects which might be considered
under the guideword “other (other than).” Table A2.2 gives checklists
for vessels and for vents/drains. Neither is comprehensive, nor will all
the items in the list need consideration in every analysis. Thus, it is
recommended that if lists are used at all they are the responsibility of
the team leader who brings out items from the list which might apply
in the circumstances of the study. Furthermore, it is important that
such lists are customized and expanded to ensure relevance to each
industry and application.

Table A2.2 Illustrative HAZOP study checklists
General List for Vessels Vents/Drains
Pressure deviations Isolation
Level deviations/bunding Cross flow/reverse flow
Temperature deviations Pressure rise/restriction/choke
Vortex High back pressure in vent line
More/less mixing Flashing/cooling
More/less layering Luting
Increased velocity Elevation change
Sediment Sequence in the vent/drain header
Start-up/shutdown/purging Air ingress
Fouling Two-phase flow
Capacity 1Piping checklist
Dip legs
Table A2.1 An illustrative parameter list for the guideword “other (other than)”
Maintenance
Testing and calibration
Instrumentation, alarms, and trips
Hold conditions
Services
Relief
Sampling
Static
Start-up and shutdown
Low and high rate running
Nonstandard batches
Manual operation of automatic plant
100 Appendix 2: The Use of Checklists Within HAZOP Study

APPENDIX 3
3
An Illustration of HAZOP Study for a
Continuous Operation
The model used for this illustration of the HAZOP study is of an off-
shore wellhead gas platform linked to a central process facility (CPF)
by a 15 km subsea pipeline. There is no chemistry but it is essential the
team understands both the physics and physical chemistry of the
process.
Gas is trapped in a loose sandstone formation 2000 m below the
sea bed. It is in hydrostatic equilibrium, trapped by an impervious
rock over and around the sandstone rock but in water contact at the
bottom. The gas exists as a dense phase, mostly methane, saturated
with water vapor at 80
C and 200 bar (20 MPa). The gas flows to
the surface in a production tubing of 15 cm diameter, made up from
a number of threaded sections, and the pressure falls due to both fric-
tional losses and the reduced gas static head; the flowing pressure at
the top of the well is about 125 bar. The production tubing is sur-
rounded by a number of threading casings of increasing diameter
which are used in the drilling program, the number and size of cas-
ings is a function of the local geology. The effective pressure contain-
ing capacity of each casing is a function of the strength of the rocks
and the cement bond between the rock and tubing. If there is a leak
of gas into the annulus between each casing, there is the potential for
collapse of the inner casing due to pressure reversal, so it is essential
to ensure a pressure gradient “in to out” and, if leakage occurs
through the threaded sections of casing, it must be depressurized.
Likewise, in the upper sections of the casing, multiple path leakage
could lead to a fracture of the cement. There is one major barrier
(flap-type valve) set in the production tubing 250 m below the sea
bed. This is called the sub-surface safety valve (SSSV—sometimes
called a down hole safety valve (DHSV)) and is held open by a
hydraulic signal. Loss of the signal causes valve closure, and the
valve is difficult to open under high pressure differential.

The casings are terminated on a “wellhead” (Figure A3.1, pages
108 109) which is bolted to the “Christmas tree.” Within the tree is
a master valve (MV) and, at an angle to the flow, a wing valve
(WV); both are held open by a hydraulic signal. The emergency roles
of each valve vary—the SSSV is protection against a main process
event or failure of the tree, the WV is the main process valve, and
the MV is used during downwell operations. Depending
upon the level of emergency, the WV closes first, then the MV, and
finally the SSSV. There are five wells in total in the field feeding the
CPF. The flow of gas is controlled by a metal-to-metal seated manu-
ally operated valve called a choke. This is usually left in a fixed
position and only adjusted occasionally. As the pressure drops across
the choke, the temperature falls and two phases (condensate and
gas) are produced. If the pressure drop is from 200 bar to less than
about 50 bar, the temperature can fall below 0
C and ice and/or
hydrocarbon hydrate solid can be formed which is controlled by
injection of methanol. There is every potential for temperatures as
low as 250
C during the initial start-up of the process when the gas
column in the production tubing loses its heat to the rocks surround-
ing the well and the initial temperature of the flowing gas could be
as low as 15/20
C.
The two phases flow into a collection manifold through a safety
shut-off valve (ESDV2) into the subsea pipeline, about 30 m below sea
level, linking the wellhead platform to the CPF, 25 m above sea level.
The pipeline is rated for the maximum closed wellhead pressure (about
180 bar). There is some phase separation at low flow rates but for the
most part transport is in mist or annular flow. The gas flows onto the
CPF, 55 m above the sea bed, through a second safety shut-off valve
(ESDV3) at the edge of the platform and then a process shutdown
valve before entering a two-phase separator (Figure A3.2, pages
110 111) with a design pressure of 120 bar. Gas and liquid phases are
metered separately, and the two phases then pass through a third
safety shut-off valve (ESDV6) into a main subsea pipeline connecting
the CPF to a shore terminal where it is processed. The data from the
two flows, gas and liquid is used for reservoir performance monitoring
and also apportioning products at the onshore terminal to each
supplier. The main subsea pipeline has a pressure rating equal to the
separator. A pig launcher can be fitted for pipeline monitoring
(Figure A3.3, pages 112 113).
102 Appendix 3: An Illustration of HAZOP Study for a Continuous Operation

As the reservoir ages, the reservoir pressure falls, the flow rates
decrease, and the water tends to increase. Ultimately, the reservoir
tends to produce sand due to high-level pressure differentials and this
is very abrasive.
Water associated with the gas is very saline and can be corrosive, so
corrosion inhibitors are injected with the methanol used for hydrate
suppression. The infield pipeline (and main pipeline) is protected from
corrosion from seawater by sacrificial anodes, likewise the wellhead
and CPF structures. There is therefore a potential electrochemical link-
age between the three central elements. The pipe work is protected
from corrosion internally by the corrosion inhibitor in the methanol
injected for hydrate suppression.
The gas produces 1 m3
of liquids per 20,000 standard m3
gas during
the separation process. The separator is designed for some “slugging”
capacity and has some offline washing facility to remove sand. The
liquid phase is level controlled and the gas phase is pressure controlled
into the pipeline. The separator is fitted with high level alarms and
shutdowns which close a shutdown valve inlet to the separation. The
separator is protected against overpressure by a full-flow relief valve,
discharging to a vent, sized for the maximum steady-state well flows.
There are also two levels of pressure protection which first close the
process shutdown valve and finally the WV.
There are other technical issues which are not discussed in this illus-
tration as they do not serve to illustrate the study technique. These will
need to be addressed in a real study.
A3.1 METHANOL INJECTION
Methanol is used as a hydrate suppressant and is pumped by a positive
displacement pump to injection pressure at the shore terminal.
Corrosion inhibitor is mixed with methanol at the terminal and there
is an emergency shut-off valve in the feed line at the wellhead platform
(ESDV1). The main pump is fitted with a recycle pressure relief valve
set at 240 bar. Each well is dosed continuously for 1 week with corro-
sion inhibitor. The changeover is carried out manually during the
weekly inspection giving a 5-week cycle. The flow is controlled onshore
to prevent hydrate formation in the pipeline.
103
Appendix 3: An Illustration of HAZOP Study for a Continuous Operation

A3.2 GENERAL PROCESS DATA
In this model, the gas flow is taken as 105
sm3
/h and the pipeline is
12 in. (30 cm) diameter. The closed-in system pressure is 180 bar for
which all piping on the wellhead platform and the subsea piping is
designed. The CPF is designed for a pressure of 120 bar downstream
of the process shutdown valve (ESDV3).
All process piping is designed for 230
C at the appropriate pres-
sure, and the separator is designed for 240
C as it will contain liquid
hydrocarbon. During process blowdown, the gas temperatures can fall
to 240
C (or lower). Slugs and liquid equivalent to three riser lengths
can be produced on increasing flow.
There is a fire and gas detection system which isolates the valves
into and out of the CPF and depressure the process—level two. Such
an event on this wellhead platform involves closure of the export valve
and the WV plus MV but no depressuring. Loss of pressure in the
manifold on the wellhead platform results in closure of the SSSV.
A3.3 THE ISSUES
The inflow of gas is generally limited by the productivity index (PI) of
the well. It self-limits at high demands and probably produces sand.
Once the well is flowing, it must be managed to avoid sand (and water)
production by fixing the choke position. Sand production can be
detected by sand probes, and excessive sand production leads to erosion
of the choke and piping and eventually settles out in the main pipeline.
Corrosion is detected by a probe. More corrosion inhibitor is injected.
The SSSV is self-closing and cannot be opened with a pressure dif-
ferential of more than a few hundred psi. SSSVs are available with
compensating features and there are thousands of wells in UKCS. The
MV and WV can be opened with a pressure differential and the choke
operates with a pressure differential. The choke is not a shut-off valve
and tends to wear and leak with time due to sand erosion.
Hydrates—a loose formation of hydrocarbon and liquid water—form
above about 600 psi (4 MPa) and temperatures of about 15
C; this is con-
trolled by methanol. Expansion of gas can produce both a water and
hydrocarbon liquid phase. Temperatures as low as 250
C are possible
with throttling but are composition and pressure/temperature dependent.

When the infield pipeline is isolated, it slowly pressures due to
choke valve leakage and could reach the shut-in well pressure. It is
possible to open the shutdown valves with this pressure differential but
valve seat damage may result.
The steady-state issues are generally sand and erosion; the dynamic,
start-up, and shutdown issues are hydrants and low-temperature for-
mation. The transient states involve the potential to move from wavy
flow to mist flow with slug potential, which may be exacerbated by sea
bed contours. The operating pressures are high and close to the piping
design pressure limits.
There are effectively six blocks for this HAZOP study:
1) methanol pumps—onshore not included
2) wellhead platform Figure A3.1
3) subsea infield line Figures A3.1 and A3.2
4) production platform and vent system Figures A3.2 and A3.3
5) main pipeline to the shore Figure A3.3 (interface)
6) onshore terminal not included
Block six—the onshore terminal has a slug catcher and gas processing
equipment as well as the methanol pumps. These are out of the direct
scope of this study, but it is very likely that this study will contain some
actions for the hand over of information to the onshore HAZOP team.
A3.4 METHODOLOGY
There are four distinct operations:
1. start-up—low-pressure downstream;
2. start-up—system pressurized;
3. shutdown and blowdown;
4. process transient.
There is little point in analyzing transients when the process cannot
be started, so the logical approach is to analyze the start-up first
(Tables A3.1 and A3.2, pages 114 120). Experience shows that many
of the problems associated with the continuous processes occur during
the dynamic phases of upset, start-up, and shutdown. The first part of
105

the study illustrates this point and then in the second part, number 5
onward, moves on to the steady-state part of the study. It will be noted
that the issues are quite different. It can be assumed that methanol is
charged up to ESDV1, the process is air freed, and liquids are dis-
placed so far as is possible prior to start-up.
Team members
Facilitator Abe Baker
Project Manager Charlene Doig
Platform Superintendent Ed Fox
Process Engineer Geoff Hughes
Instruments Iain Joules
Scribe Keith Learner
Petroleum Engineer Mike November
The design intent is to flow five gas wells at the combined rate of
105
sm3
/h (85 mmscfd) of gas from the wellhead platform, with as low
sand content as practicable, into a production separator on the CPF.
The team has the following available:
• a general description of the wellhead installation and the CPF;
• a selection of PIDs;
• the “cause and effects” drawings for the shutdown system
(Tables A3.3 and A3.4, page 121);
• the operating intent from which the detailed operations are written.
The outline operating intent is as follows:
• open SSSV using methanol to form a pressure balance;
• open MV and WV the choke valve and thereby pressure up the
infield pipeline monitoring for evidence of chokes/hydrates;
• slowly pressure the separator and then also the main pipeline to the
shore;
• each well is brought online in sequence.
The shutdown is on three levels.
Level one—process upset
Close an appropriate shutdown valve to arrest the cause of this event.

Level two—major event
In general, this means a detected fire or detected gas leakage and
closes all valves around the process and blowdown all vessels—pipe-
lines remain pressurized.
Level three—potential for major escalation
The SSSV on the wellhead platform can be closed by a manual sig-
nal from the control center on the central platform.
The riser ESD valves on the wellhead platform are closed by a
manual signal from the control center on the central platform.
The riser ESD valves on the central platform are closed by fire or
high-level gas detection local to the valve or by a manual signal from
the control center.
Failure mode
All valves are controlled by hydraulic power (not air) and all fail
closed except for the blowdown valve ESDV7 which fails open.
Piping code (for Figures A3.1 A3.3)
Fluid
G—gas
V—vent
M—methanol
D—drain
C—condensate
Pipe sizes are in inches
Pressure rating
1—ANSI class 150
9—ANSI class 900
15—ANSI class 1500
AP1 5000—special well piping design pressure 5000 psig
Materials
CS—carbon steel
SS—stainless steel
See also the cause and effects tables (Tables A3.3 and A3.4).
107

Figure A3.1 P1D 1 (to be used in conjunction with Table A3.1).

Figure A3.1 (Continued).
109

111

113

Table A3.1 Demonstration of the HAZOP study conducted on node 1 (To be used in conjunction with figure A3.1)
DATE: 13/02/2015 INTENT 1: Pressure wellhead side of SSSV with methanol to allow SSSV to be opened
NODE 1: Reservoir to Choke Valve Start-Up
Operation
INTENT 2: To flow 2 3 104
sm3
/h of gas into the collection manifold G1-12v 15 CS at a pressure of 100 bar and 15 20
C
PID 1 STATUS: SSSV closed, MV closed, WV closed, choke closed, ESDV1 open, ESDV2 open, methanol pump running sequence
valve open
ATTENDEES: AB, CD, EF, GH, IJ, KL, MN
No. Parameter Guideword Deviation Cause Effect Protective Systems Action Action on
1 Flow No SSSV closed
LINE OUT THE METHANOL FROM THE INJECTION PUMPS THROUGH 2v 15 CS TO EQUALIZE THE PRESSURE DIFFERENCE ACROSS THE SSSV
2 Pressure Low SSSV
cannot be
opened
• Pressure
drop in M1
2v 15CS is
too high due
to the need
to inject
methanol
into online
wells
• The pump
capacity is
inadequate
SSSV cannot
be opened
RV on methanol
pump is set to avoid
overpressure of the
methanol. Pump,
pipeline and M1 2v
15 CS (this pump is
on shore)
2.1 Verify that the pressure drop
in the subsea pipeline when
dosing other wells is less
than the valve set pressure
minus 180 bar
CD
2.2 Verify that the methanol
pump has adequate capacity
to dose other wells and
pressurize the wellhead and
down hole section of piping
CD
3 Pressure Low SSSV
closed—no
flow of
methanol
Closed in
system with a
PD pump
Potential system
overpressure
RV on pump will
lift
3.1 Verify that the relief valve
setting on the pump is
correctly set to ensure all
piping—on shore, offshore
and subsea—is not over
pressured
CD
3.2 Ensure that the HAZOP of
the methanol pump reflects
that the pumps may run with
a no flow case—consider the
need for a pressure control
spill valve round this
methanol pump
CD

SSSV OPENED, MV OPENED, AND WV OPENED
4 Pressure Higher Wellhead
pressure at
closed-in
condition
Normal start-
up
• Potential for
reverse flow of gas
to the shore if the
methanol pump
stops
• Pump does not
inject methanol
• Potential reverse
flow to
production
platform if NRV
fails to open
• NRV fitted in
methanol feed
lines M1-2”,
15 CS, M2-2” 15
CS
• PD pump is a
form of NRV
4.1 Petroleum engineering to
review maximum SITP and
discuss with the project team
CD/MN
Shut in
wellhead
pressure is
higher than
anticipated
Poor reservoir
predictions
4.2 Verify the methanol pump
relief valve is set at the
correction pressure for both
processes and piping
CD/MN
4.3 Ensure the HAZOP of
methanol pumps reflects the
potential reverse flow
through relief valve if fitted
CD
Pressure
Temperature
Flow
Higher or
lower
Higher
Pressure
discussed
above
No
meaningful
deviations
during the
opening of
the SSSV and
pressuring to
the choke
4.4 Ensure the HAZOP of the
methanol pumps reflects the
hydraulic link from the well
to the methanol pump with
the potential for system over
pressure if the suction
isolation valve is closed
CD/EF
NORMAL OPERATION
5 Flow Lower Restriction in
reservoir or
downstream
of choke
• Poor PI
• Hydrate/ice
Loss of production 1. None 5.1 Noted
2. Methanol 5.2 Ensure methanol injection
rates are monitored and
recorded daily at the shore
EF
6 Flow Higher Choke opened
too far
Possible sand
production leading
to erosion in piping
and the choke
Sand probe (AE) 6.1 Ensure the peak flow
characteristics are recorded
in operating instructions
MN/EF
6.2 Monitor sand probe on
routine and more frequently
early in the field life
IJ/EF

7 Flow As well as Sand
production or
well debris
from drilling/
perforation
• Erosion on
piping or choke
• Possible choking
of condensate
control valve in
V1
Sand probe (AE) 7.1 Consider if a well cleanup
program can be set in place
MN
7.2 Operating instructions
should note the need to
monitor for debris build-up
in V1
EF
8 Pressure Lower None
9 Pressure Higher Production
higher than
off-take
Production
platform upset
or shutdown
and ESDV3
and 4 closed
• Pipeline
pressurized to
180 bar
• On restart there
is a high
pressure drop
over ESDV3 or
4 which may
cause valve seat
damage
• Initial gas flow
through ESDV4
could overload
the relief valves
on V1 and over
pressure V1 with
a high transient
flow
• Relief valves on
V1
• Pipelines full
pressure rated
• PSHH on V1
9.1 Verify ESD3 has hard seats IJ
9.2 Consider the need for a
pressuring line around
ESDV4
EF/CD
9.3 Analyze the flow
characteristics into V1 as
ESDV4 is opened and the
pressure/time profile in V1
CD
9.4 Dependent upon 9.4
determine a means of
establishing a steady
dynamically limited flow
which will not overpressure
V1
EF/GH
10 Pressure Lower Choke leaks
and ESDV4
leaks plus
platform blow
down plus WV
closed
• Lower
temperatures
• scenario unlikely
None 10.1 Verify there is no thermal
implication in the choke
MN
10.2 Review this scenario with
respect to the pipeline later
in the study
AB

See 10.1 and 10.2
11 Temperature Lower See 10.1
and 10.2
12 Maintenance None Can the items
up to and
including the
choke be
maintained
Poor isolation
standards
Loss of production Isolation valve 12.1 Can the sand probe and
corrosion probe be removed
safely; are they fitted in self-
isolation pockets?
IJ/CD
12.2 Review the need for double
isolation on each well at the
manifold
MN/CD
12.3 Determine if wear on the
choke is likely to be
significant at any phase of
the field life
MN/CD
It will be noted that, sometimes, there are two persons in the “actions on” part of the table. This is because these two were the leaders of the discussion and are the most likely to
understand the issues. The first person (initials underlined) is the one who is accountable for the action.
Please note: v 5 inches.
No other meaningful parameters and deviations were found and the study of section/mode was completed.
Others to be analyzed:
• corrosion;
• erosion;
• access for maintenance;
• maintenance features—standards of isolation;
• purging features—vents and drains, location and termination points;
• diagnostic features.

Table A3.2 Demonstration of the HAZOP study conducted on node 2 (To be used in conjunction with figures A3.2 and A3.3)
DATE: 13/02/2015 INTENT 1: To flow 105
sm3
of gas to the production platform at a pressure of 100 bar and 15 20
C
NODE 1: Subsea Pipeline from Choke to ESDV3 Start-Up/
Operation
STATUS: All SSSV open, all MV, WV open, ESDV3 and 4 open and chokes closed.
Initial state 0 bar, nitrogen filled
PID 2 and 3 ATTENDEES: AB, CD, EF, GH, IJ, KL, MN
No. Parameter Guideword Deviation Cause Effect Protective
Systems
Action Action on
Slowly open choke
13 Pressure Low/lower Pipeline
pressure low
Pressuring Low-temperature ice
or hydrate
formation
Methanol
injection
13.1 Review the temperature/time
profile as the pipeline is pressured
taking into account the thermal
mass of the pipework—the lowest
temperature will be at the choke
CD
13.2 Pursue means of pressuring the
system from the onshore terminal
GH, CD,
EF
14 Pressure High Pipeline
fully
pressure
rated
Noted
15 Temperature Low See 13 See 13 See 13 See 13 See 13. Consider again under
higher temperature
AB
16 Temperature High Adiabatic
compression
of nitrogen
Nitrogen
piston
compressed
by incoming
gas
None None Noted GH, CD,
EF
16 Review means of displacing
nitrogen—a potential
contamination in gas as part of
13.2

17 Flow Low/no/
high
No logical
meaning
during start-
up
18 Phase Change Production
of ice,
hydrate,
condensate
Expansion of
gas into the
pipeline
Potential choke Methanol
injection
Noted
18.1 Ensure the operating instructions
record the need for continuous
methanol upstream of the choke
while pressuring the line
EF
NORMAL OPERATION
20 Flow Low Low
production
Low off take
at terminal
Potential slugging
regime
None 20.1 Review line slug size and
separation/hold up in capacity in
V1
GH
21 Flow Higher Rate
increase
Higher off
take at the
terminal
Potential slug
formation and
reactions forces on
the riser
21.1 Include in 20.1 GH
21.2 Review the riser support against
slugging
CD
22 Flow High High
demands
Possible sand
formation and
erosion
Sand
probes in
each well
22.1 See 6.1 and 6.2
23 Flow Lower Restricted
flows of the
pipeline
Hydrate
formation
Line choked ice/
hydrate slug may
move causing
reactions forces on
the riser and sudden
high flow into V1
23.1 Devise means of avoiding hydrate
plugs moving during recovery
from a hydrant plug
EF
23.2 Monitor methanol injection daily
on shore
EF

24 Flow High High flow Hydrate slug
moves when
under high
pressure
differential
Higher pressure in
V1
Methanol 24.1 See 21.2 EF/GH
24.2 See 9.4 and 9.5
25 Temperature Lower Pipeline
depressured
Possible hydrate
formation
25.1 Review the temperature in the
pipeline during depressuring—
verify if it does not go out of the
spec limits. Allowance should be
made of heat flow into the line.
See 23.1 and 24.1
GH
26 Temperature Higher Pipe warmer
than when
laid
Hotter fluids
flowing in
pipeline after
start-up
Thermal expansion
of the pipeline
26.1 Consider the potential for
upheaval buckling and the need
for trenching or rock dump
EF/GH
27 Electro
potential
High
differential
Possible loss
to cathodic
protection
Localize
corrosion
outside the
pipeline
• Sacrificial anodes
• Isolation flanges
27 Routinely monitor the
performance of the insulating
flanges at the wellhead and
production platform
EF/IJ
28 To be continued
It will be noted that, sometimes, there are two persons in the “actions on” part of the table. This is because these two were the leaders of the discussion and are the most likely to
understand the issues. The first person (initials underlined) is the one who is accountable for the action.

Other issues are:
• corrosion internally and externally on the process piping and subsea
pipelines. The process and pipelines are electrically insulated by a
special flange arrangement;
• erosion;
• nitrogen disposal at start-up.
Table A3.3 Cause and effects for wellhead platform
Detected Gas (Low
Level)
Detected Gas High Level 60%
LEL
Detection
Fire
Vibration
(Impact)
WVs C C C C
UM valves C C C
SSS valves C C
ESDV1 C C
ESD2 C
C—Closed
O—Open
Table A3.4 Cause and effects for CPF
Local Fire or Gas Detected
at ESDV3 and 6
V1 High
Pressure
V1 High
Level
General Gas Detection
High Level 60%
General
Fire
ESDV3 C
ESDV6 C
SD
wells
C
WV C C C C
ESDV4 C C C C
ESDV5 C C C C
ESDV7 C C O O
C—
Closed
O—
Open
121

APPENDIX 4
4
An Illustration of HAZOP Study for
a Batch Operation
A4.1 INTRODUCTION
This example illustrates the following aspects of a HAZOP study:
• an overall description of the plant and process;
• the selection of the stages of the process for study;
• a list of the documentation available to the team;
• the relevant PID;
• the team membership;
• for one stage:
the detailed description and the design intention;
a list of the parameters and guidewords used;
a part of the HAZOP study report, illustrating some of the devia-
tions with consequences, safeguards, and actions.
The example has been fabricated, although it is intended to resem-
ble a real operation. For simplicity, the main reactants are simply
labeled A and B.
A4.2 THE COMPANY, SITE, PLANT, AND PROCESS
The company is a long-established general chemical manufacturing
company, employing over 2000 people at several sites in the UK. It has
a good safety record and routinely uses HAZOP study for new plant,
new processes, and major modifications. A central safety group oversees
this activity and provides trained leaders for all process hazard studies
on new plant or processes. The six-stage process hazard study system is
used, with HAZOP study as the usual method used at stage three.
The site concerned with this process is on the outskirts of an indus-
trial city and employs 350 people. It lies between a river and a major
road and, on one side, is close to an old housing estate. There are a
number of continuous processes and several general-purpose batch

units on-site. The operation to be examined is for a new process in one
of the batch units which will be adapted to the needs of this process.
Laboratory work has been done to determine the batch size and
conditions. Reaction hazard investigations have been carried out to
identify the reaction hazards and to define a basis for safety.
The essential elements of the process are as follows. The sodium
salt of an organic reactant, A, is formed by adding caustic soda solu-
tion to A in a large reaction vessel. The process is mildly exothermic; a
slight excess of caustic is used to maintain the pH around 11. The salt
is then reacted with a second organic material, B, added at a controlled
rate from a measure vessel. This reaction is very exothermic, and cool-
ing is required. To obtain a high-quality product, the reaction needs to
be carried out at between 55 60
C.
The section of the batch plant to be used for the reaction stage con-
sists of two measure vessels at level two that will be used for the caus-
tic soda solution and for reactant B. The required amounts are taken
from drums on weigh scales, using vacuum. The feed to the reactor at
level one is by gravity. Component A is pumped from drums directly
into the reactor followed by a line flush with water. After the reaction
is complete, as confirmed by analysis of a sample, the products are
pumped to another vessel for further processing.
Component A is a solid, mp 30
C, flash point 45
C, with a long-
term exposure limit (LTEL) of 10 ppm. Component B is a liquid, mp
5
C, bp 122
C, flash point 10
C, with a LTEL of 2 ppm and it causes
chemical burns on skin contact. The solution of A in caustic soda and
the product solution after reaction of B are all single-phase systems.
A4.3 THE PROCESS STAGES SELECTED FOR HAZOP STUDY
From consideration of the draft operating procedure, based on the labora-
tory investigations, the stages marked ü were selected for HAZOP study:
1 Check plant set-up
ü 2 Measure 250 kg of 30% caustic solution to vessel F1
3 Melt two drums of A in a drum heater
ü 4 Pump 425 kg of A to reaction vessel F3
5 Flush line with 100 L of water
6 Heat F3 and contents to 55
C
ü 7 Run caustic from F1 to F3
124 Appendix 4: An Illustration of HAZOP Study for a Batch Operation

ü 8 Measure 375 kg of component B to vessel F2
ü 9 Reaction stage: controlled feed of B from F2 to F3
ü 10 Workout for 20 min (combined with step 9 for HAZOP study) (see Table A4.1, pages
132 137)
11 Sample and check product
ü 12 Pump contents of F3 to F4
13 Wash F3 with 200 L of water
14 Pump wash water to F4
The basis for not selecting some stages is that they are simple,
familiar steps of low hazard potential, with little chance of incorrect
execution or omission by the operators.
During the HAZOP study, the team is expected to consider all
potential SHE hazards as well as operability problems.
A4.4 HAZOP STUDY OF STEPS 9 AND 10, THE REACTION
STAGE AND WORKOUT
The HAZOP study team
Name Discipline Job Title Role/Represents Years
Mike Stopner Chemist Safety Advisor Leader 25
Jennie Howard Chem Eng Project Engineer Design team 7
Tom Bailey Shift supervisor Operations 17
Bob Teryl Chemist RD Chemist Process development 12
Andy Wires Electrical Eng Control Engineer Control/instruments 3
Frank Laycrew Mech Eng Site Engineer Services/maintenance 14
Karl Jones Chem Eng Trainee Project Engineer Scribe 1
All are full-time employees of the company and, apart from KJ,
have previous experience of HAZOP study, including training on a
2-day in-house course. MS has also attended an external 4-day training
course on HAZOP study leadership and has been leading studies
throughout his time in the central safety department (5 years).
A4.4.1 Documentation for the Study
Documents include a set of PIDs showing the plant as it will be set
up for the operation including spaded lines and any new connections.
A separate PID is used for each stage of the HAZOP, marked up
showing the actual items of the plant involved in that stage and
125
Appendix 4: An Illustration of HAZOP Study for a Batch Operation

Figure A4.1 PID for steps 9 and 10; drawing on AB01/Rev 2 (to be used in conjunction with Table A4.1, pages
132 137).

127

showing which valves are open (Figure A4.1 for steps 9 and 10). Full
details of plant and equipment specifications are available if required.
The other items made available to the team are:
• site plan;
• process description and outline operating procedure;
• reaction hazard review covering laboratory studies of the chemistry,
reaction kinetics and thermodynamics. Results from differential
scanning calorimetry analysis of components A, B, and the reaction
product; adiabatic calorimetry data, including reaction simulation.
The basis of safety was derived from this work;
• material safety data sheets for all reactants; the available hazard
data for the product;
• alarm, interlock and trip schedule;
• reports from Hazard Studies 1 and 2.
A4.4.2 Plant Conditions and Step Description
As a consequence of the previous steps, the state of the plant at
the start of this step is that the reaction vessel, F3, contains 425 kg
of component A, 100 L of water, and 250 kg of 30% caustic.
Component A has been converted to the sodium salt and sufficient
excess caustic is present to raise the pH to 11. The stirrer is running
and the mixture is controlled at 55 60
C, using hot water to the vessel
jacket. The vessel is open to the scrubber through a condenser, set to
return any condensed liquids to F3. Measure vessel F2 contains 375 kg
of component B that is to be run down to F3 over a period of about
3 hour at 2 kg min21
. The flow rate is controlled by the orifice plate,
OP1. Both F2 and F3 are operated at atmospheric pressure under
nitrogen to prevent the formation of a flammable atmosphere. The
continuous feed of nitrogen into each vessel is vented through the
scrubber. The scrubber is operated to reduce the levels of vapor A and
B to below their LTEL values to permit safe discharge at a high level.
The basis for safety was developed from work initiated in HS 2.
It requires:
• The rate of heat evolution from the amount of B present in the reac-
tor does not exceed the cooling capacity of the reactor. For this to
be achieved, all the following conditions must be maintained:
the rate of addition of B is controlled at 2 kg min21
;
there is continuous stirring to avoid accumulation of B;
the temperature is kept above 40
C to minimize accumulation of B;

• If runaway reaction does occur, the reaction vessel is protected by
the rupture of the bursting disc which relieves to a dump tank from
which the resulting vent is acceptable at the expected frequency.
From the earlier hazard reviews, the worst case event is identified as
a runaway reaction which is not fully relieved by the bursting disc. In
this circumstance, it is estimated that the reactor design pressure could
be exceeded by a factor of 1.5. Reactor rupture is possible but is of low
probability. The consequences of vessel rupture would be the possibility
of operator fatality and severe local contamination which, if an aerosol
cloud forms, could be blown off-site. The worst effect of this is if the
wind direction was toward the local housing estate. The operation of
the bursting disc is crucial to avoidance of this event, and it has been
sized on the basis of small-scale experiments providing data for use with
the DIERS design rules.
As the previous steps have been subjected to HAZOP study, it is
assumed that there is negligible chance that they are not as described
unless new causes are found.
Step nine is started by opening valve AV203, all other conditions
having been set previously. The operator initiates the step from a
control panel through the computer control system. Apart from occa-
sional checks during the 3 hour addition, the operator relies on the
alarms to indicate any deviation from the set conditions. Alarms are
set to indicate stirrer motor failure, and low and high temperatures of
50
C and 65
C, respectively. If the temperature reaches 70
C, the
valve AV203 is automatically tripped to close. Step 10 (working out
for 20 min) follows on directly from step nine with no change in the
system or the settings. At 31
/2 hour from the start of the reaction stage
the computer closes AV203, puts the system to hold and awaits an
operator input.
A4.4.3 Parameter and Guidewords
The following guidewords (and abbreviations) are used.
N No (not, none)
M More (more of/higher)
L Less (less of/lower)
R Reverse
PO Part of
129

AWA As well as (more than)
WE Where else
EL Early/late
O Other (other than)
The team leader has prepared preselected combinations of para-
meters and guidewords that give meaningful deviations, as shown in
Table A4.2 (page 138). Team members are given opportunities to
extend this list. Only those combinations generating a significant
discussion are recorded in the HAZOP tables as no realistic mean-
ings or likely causes were found for some. It was found that some of
the later combinations had been adequately examined under earlier
pairings—for example, “part of composition” was mostly dealt with
by “more/less quantity.” Some additional deviations came up during
the analysis.
The following “parameters” are also considered under the guide-
word “other (other than)”:
• services (including failure modes of valves);
• maintenance;
• safety;
• process interruption/hold/recovery;
• drainage;
• trips;
• corrosion;
• ignition sources (e.g., static electricity).
Action in emergencies—for example, fire, explosion, and toxic
leak—is considered for the whole process at the completion of the
HAZOP study of all the individual stages.
A4.5 A SECTION OF THE HAZOP STUDY REPORT FOR THE
BATCH REACTION (SEE TABLE A4.1)
Step—reaction and workout, steps 9 and 10 (see Table A4.1, pages
132 137);
Team—MS (leader), JH, TB, BT, AW, FL, KJ (scribe);
Drawing—PID (see Figure A4.1, pages 126 127);
Meeting date: 28/02/15; Revision: 0.

A4.5.1 Step Description
A measured quantity of component B is added by gravity feed from
F2 to the prepared mixture in F3 at 55 60
C. The step is initiated by
the operator using the computer control system and the addition is
started by opening valve AV203, all other valves being preset. The
addition takes about 3 hours followed by a short workout period. The
total time for these steps is 31
/2 hour after which AV203 is closed and
the system held awaiting an operator command.
A4.5.2 Design Intention
To transfer by gravity from the measure vessel F2, and to completely
react, 375 kg of component B with the stirred aqueous solution of the
sodium salt of 425 kg of component A in F3. The transfer rate is to be
controlled at 2 kg min21
by orifice plate OP1. The reaction tempera-
ture in F3 is to be controlled in the range 55 60
C. On completion of
the addition, the reaction mixture is stirred for 20 30 min before sam-
pling. A nitrogen atmosphere is maintained in F2 and F3 at the flow
rates established in previous steps.
Note 1
This report is from the first analysis by the team. Since the earlier steps in
the process have already been studied, some deviations—for example,
wrong amount of A is present—have already been considered. Entries
will only occur for these deviations for new causes or new consequences
suggested by the team. The team took one session developing this report.
Note 2
The numbering system adopted is to have an item number for every
row of the analysis and to relate the actions to that number. Where
two or more actions result they are numbered as, for example, 14.1
and 14.2. Some action numbers are not used such as 15 and 16.
Note 3
A response/comment column is available but not shown. It is used to
enter the responses to the actions and to record any further comments
by the HAZOP study team.
131

Table A4.1 HAZOP study on steps 9 and 10 (reaction and workout) (to be used in conjunction with figure A4.1, pages 126 127)
Ref. Parameter Deviation Possible Cause Consequence Safeguard/Protection No Action On
1 Quantity/
step
No B is
added—
step
omitted
Operator error,
for example, at
shift handover.
MV307 closed
after maintenance
Spoilt batch Detected at sampling and
can easily be corrected.
Batch sheet requires
analysis to be signed off
by supervisor
1.1 Start-up check to confirm that
MV307 is open
TB
1.2 Operating procedure to include
a sight glass check that flow is
established
TB
2 Quantity Excess of
B is added
F2 not fully
emptied from last
batch
Excess of B in product: batch
will be out of specification
Detected at sampling but
a special procedure will
then be required
a check on vessel F2 before B is
measured out
TB
3 Quantity Too little
of B is
added
Blockage in line
or at OP1
Batch out of specification and
process delay
Detected at sampling 3.1 Check procedure for clearing
line and OP1 when transfer line
holds component B
FL
3.2 Batch sheet to require a check
that F2 is empty at end of the
addition stage
TB
4 Quantity Too much
of A is
present
Error at earlier
stage resulting in
small excess
(double charging
covered in
HAZOP of
addition step)
process delay
Detected at sampling 4.1 Check that procedure will be
written to cover this case and
include in training program
JH
5 Quantity Too little
A is
present
Error at earlier
stage resulting in
small deficiency
process delay. Not easily
corrected
Detected at sampling 5.1 Evaluate likelihood of this
deviation and, if necessary,
draw up procedure
MS
6 Flow (rate) Too fast Corrosion/erosion
of OP1
Reaction rate and heat release
increased. May eventually
exceed vessel cooling capacity
leading to over-temperature
Independent alarms
TICA 32/33 located in a
manned control room
6.1 Check that OP1 material is
compatible with component B
BT

7 Flow (rate) Too fast Wrong OP fitted
at OP1 after
maintenance
Could quickly exceed the vessel
cooling capacity, causing a
reaction runaway and demand
on BD2
TICA32/33 located in a
manned control and BD2
relieving to dump tank.
Good control of
maintenance
7.1 Specify OP1 size in operating
procedure and ensure problem is
covered in operator training
TB
7.2 Confirm flowrate at OP1 at the
water trials stage
FL
7.3 Control sequence to include trip
closure of AV203 and fully open
CV303 in the event of over-
temperature
AW
8 Flow (rate) Too fast MV306 is open
and so orifice
plate OP1 is
bypassed
Will very quickly exceed the
vessel cooling capacity and lead
to a reaction runaway and
demand on BD2
TICA32/33 to manned
control room and BD2
relief to dump tank. BD2
is sized for addition at
maximum possible flow
rate in a 25 mm line
8.1 MV306 to be locked closed as it
is not used in this process
FL
8.2 Include sensing of BD action to
give alarm and to close AV203
AW
8.3 Consider removal of OP2 from
the cooling water inlet line so
full cooling capacity will be
available. Take into account the
original purpose of OP2 in
controlling heating rates/cooling
profiles/blowdown of
condensate
FL
9 Flow (rate) Too slow Partial blockage
in line or at orifice
plate OP1
Batch time extended Operator will note
problem when seeking to
move to next stage
9.1 Covered by actions 3.1 and 3.2 FL
TB
10 Flow Elsewhere Crack or leak at
BD2 (action 8.2
only detects full
burst)
Loss of contaminated nitrogen
to dump tank and eventually to
atmosphere
None 10.1 Put BD2 on a regular checking
schedule
FL
(Continued)

Table A4.1 (Continued)
11 Temperature High Control problem
or faulty
temperature signal
(reads low)
Overheating will occur, with
contributions from the heating
system. Most serious condition
would be common effect since
both temperature probes are in
the same pocket in F2
None unless the fault also
leads to a low
temperature alarm when
operator intervention
could be expected
11.1 Check whether it is possible to
physically separate the two
temperature probes (control and
protection) to reduce common
cause effects
FL
12 Temperature High Loss of cooling
water (a low
probability event)
Overheating. Runaway if
cooling water is not restored or
the addition halted
TICA32/33 are located in
the manned control room
and BD2 relieves to
dump tank
12.1 Covered by action 7.3 AW
13 Temperature High Jacket not
switched from
steam to cooling
water after earlier
step
Overheating with possible
reaction runaway
TICA32/33 to manned
control room and BD2
relieves to dump tank
13.1 Control program to include
checks that valve CV301 on the
steam line is closed
AW
14 Temperature Low Control problem
or faulty
temperature signal
(reads high)
Poor quality batch. Extreme
outcome is cessation of reaction
and accumulation of unreacted
B
TAL from TICA 32 14.1 Take TAL from both the
control and the protection
temperature sensors
AW
14.2 Determine suitable interval for
calibration checks on TICs
FL
15 Pressure High/low No causes
identified in
addition to the
runaway
situations
discussed above
16 Reaction
rate
High/low No additional
causes found

17 Mix No mixing Mechanical
coupling fails or
agitator blade
becomes detached
Risk of accumulation of
unmixed B leading to
uncontrolled reaction
Possibly detected by low
motor current alarm
17.1 Add a rotation sensor to the
shaft of the stirrer; interlock to
reactant feed valve AV203
AW
18 Mix No mixing Motor failure Risk of accumulation of
unmixed B leading to
uncontrolled reaction
Alarm on motor current
(low)
18.1 Existing safeguard adequate
provided action 17.1 is
implemented
AW
18.2 Develop a safe operating
procedure for restarting a batch
after accumulation has occurred
BT
19 Mix Less
mixing
Viscous mixture
formed
Stirring becomes inefficient and
unmixed B may accumulate
May be alarmed by
sensor added in action
17.1
19.1 Check viscosity under extreme
conditions to decide if action is
needed. If so, include an alarm
on high motor current
BT
20 Mix Reverse Incorrect
connection after
maintenance
Stirring becomes inefficient and
unmixed B may accumulate
None 20.1 Include a check on stirrer
operation in the commissioning
trials and in the maintenance
procedures
TB
21 Composition Part of Wrong ratio of
reactants covered
under high/low
quantity
22 Composition As well as Wrong drum used
when charging
component B
Unpredictable but minimum will
be a spoilt batch
Covered in HAZOP of
the charging step
22.1 Review actions from earlier
HAZOP and ensure that the
purchasing department specifies
a distinct drum color
MS
23 Control None Complete loss of
control computer
System moves to fail safe
condition
Design assumes a period
of operation of the
computer on its UPS.
Ultimate protection is
provided by BD2
23.1 Check that fail safe settings
include isolation of feed of B,
continued stirring and full
cooling to vessel jacket
AW
(Continued)

24 Control Part of Selective failure.
Most serious
would be loss of
temperature
sensors/control
Possible undetected overheating Ultimate protection is
provided by BD2
24.1 Check that the temperature
sensors connect to different
input boards
AW
24.2 Include temperature comparison
(TICA32/22) in the checks and
add a difference alarm
AW
25 Operator
action
Sooner Step started early Starting temperature is low.
Reactant may accumulate and
then cause runaway reaction
once mixing starts
Ultimate protection is
provided by BD2
25.1 Specify the lowest safe starting
temperature
BT
25.2 Provide software interlock to
prevent low temperature start
AW
26 Operator
action
Part of Workout period is
shortened if the
addition is slow
(for any reason)
Uncertain—basis for inclusion
of the workout period is not
clear
26.1 Carry out further laboratory
work to determine the
importance of the workout and
to define the minimum
allowable time
BT
27 Services Loss of
instrument
air
All valves move to assigned
failure positions
27.1 Review the failure modes of all
valves to ensure specification is
correct
JH
28 Services Power loss Unpredicted
failure, cut cable,
and so on
Stirrer stops. Computer moves
plant to a safe hold position
Computer has its own
UPS
28.1 Include this condition in the
check under 27.1
AW
28.2 Consider need for planned
restart procedure after such an
interruption
JH
29 Maintenance Work on
AV203
Valve problem on
AV203 during the
transfer
AV203 cannot be isolated from
F2 for safe maintenance
None 29.1 Put additional manual valves in
the F2/F3 line
FL
29.2 As a general action, review the
PID to ensure all key items
can be isolated
FL

30 Vessel entry
(F3)
Other
activity
Inspection or
other requirement
for entry to vessel
Risk to operator from inert
atmosphere, especially nitrogen
Spades installed on all
lines
30.1 Review the isolation of F3,
including possible insertion of
flexible section into the nitrogen
line so that it can be
disconnected and blanked off.
Need to cover F2 as well since it
has its own nitrogen supply and
is linked to F3
MS
31 Drainage Leak of B Leaking flange on
transfer line from
F2 to F3
Some loss of component B into
process area
All spillages in this area
run to a common sump
31.1 Check the materials in use on
adjacent units for potential
incompatibility
FL
a routine inspection of the
transfer line at the stage of the
process
TB
32 pH High/low Imbalance in
quantities of A or
caustic added
previously
Batch quality affected unless
initial pH is range 10 11.5
None 32.1 Operating procedure to include
a check on pH before this step
is initiated
TB
32.2 Consider need for a procedure
for correction of pH
BT
33 Trip action Out of
range
condition
Any Control system moves the plant
to a predetermined state based
on the trip signals
33.1 Prepare matrix to show which
valves act in each trip scenario.
Review the matrix at next
HAZOP meeting
JH
34 Operator
PPE
Exposure Leakage or
spillage
Contamination Standard procedures 34.1 Confirm that procedures exist
for all materials handled in the
process
TB

Table A4.2 Preliminary list of applicable combinations of parameters
and guidewords
Guidewords
Parameter N M L R PO AWA WE EL O
Quantity ü ü ü ü
Flow ü ü ü ü ü
Temperature ü ü
Pressure ü ü
Reaction ü ü ü ü ü ü
Mix ü ü ü
Step ü ü
Control ü ü ü
Composition ü ü
Operator action ü ü ü ü

APPENDIX 5
5
An Illustration of HAZOP Study for a Procedure
This study is loosely modeled on an article in the ICI Safety
Newsletter No 32 August 1971. As the study is “hypothetical,” the
working parameters of the up- and downstream processes are not
available but this should not detract from demonstrating the study pro-
cess. Also, as the study is clearly short and operations oriented, it does
not justify a full team and the Facilitator may also act as Scribe. With
the limited number of team members, some of the actions must be
dealt with by someone outside of the study group who has the skills so
to do. It is the responsibility of the person named in the study records
to ensure that a competent person answers them and that they are
implemented properly.
A5.1 BACKGROUND
This example is based upon the HAZOP study of a planned modifica-
tion of an existing process operation (Figure A5.1).
An intermediate storage tank (IST) receives a C6 hydrocarbon
stream (averaging 25 m3
/hour) from the reflux drum of an atmospheric
pressure distillation column, run down on exit level control via the
reflux pumps into the 250 m3
, nitrogen-blanketed tank. This conical-
roofed tank serves as a buffer and temporary storage for the material
before the C6 material is pumped by the J1 centrifugal pump, on level
control, to the plant petrol blending unit. The IST operates at ambient
temperature and at 500 Pa on split range pressure control and is
inerted by nitrogen from the 1.3 bar site nitrogen supply. The tank is
protected by a pressure (vacuum) valve (PV) set at 2250/ 1 750 Pa. It
is in a bunded enclosure with an overflow, sealed with glycol, which
empties into the bund. There is adequate instrumentation, including
level indication with high- and low-level alarms and high-level trip plus
temperature and pressure indication, all to the site control room.

LA2
Hi
TI
250 m
3
intermediate
storage tank (IST)
Normal working:
ambient temp.
500 Pa
LT
PT
PIC
LI
LA1
Lo
C6 hydrocarbons
from reflux drum
on exit level
control (25 m3/hour)
Nitrogen
10 psig
split range
N2 blanket
system set
at 500 Pa
PV valve
set at
–250 Pa
+750 Pa
flame arrester
IST intermediate storage tank
LA level alarm (Hi, Lo)
LCV level control valve
LI level indicator
LIC level indicator/controller
LT level transmitter
PA pressure alarm (Hi, Lo)
PG pressure gauge
PIC pressure indicator /controller
PT pressure transmitter
ROP restriction orifice plate
PV pressure/vacuum valve
TI temperature indicator
Intermediate storage tank and link to the petrol blending system
LA1 50 m3
m3
LA2 200
ROP
seal pot
LT
PG
to petrol
blending system
J1 centrifugal transfer pump
LCV
LIC
NRV
V3
V2
V1
V4
Figure A5.1 PID for the existing process.

Figure A5.1 gives sufficient detail for the Procedural HAZOP and any
deficiencies are outwith the scope of the study.
The modification is planned to remove the 100 m length of 100 mm
diameter piping between the tank master isolation valve, V1, and the first
pump isolation valve, V2, and to refit the pump closer to the tank, but
first the flammable fluid (about 0.8 m3
) must be removed. Consideration
was given to draining it into a drum but the risks were considered to be
unacceptable. In line with the corporate management of change policy, a
Hazards Study approach (see Chapter 2) was adopted.
1. The inherently safer option (HS 0) of displacing the fluids with
nitrogen has been adopted. There is a nitrogen ring main on the site
which can be connected below valve V4. This link is fitted with a
non-return valve at the hose connection.
2. During the HS 2 analysis (FEED), it was recognized that this was
a non-standard operation with potential human factors (see
Section 10.2). It was recommended that all valves should be clearly
labeled (V1, etc.) and that a dummy run practice should be carried
out to debug the procedure and to familiarize the crew with the
operation.
3. During the HS 3, it was decided that a HAZOP study should be
carried out.
The final arrangement below (Figure A5.2) shows the 100 mm
diameter suction line, the 25 mm diameter nitrogen header and a flexi-
ble hose, and 18 mm diameter depressuring line with an isolation valve
V6. All piping other than the hose will be hard piped.
An operational procedure has been drawn up which, in accordance
with the MOC policy, is to be the subject to a Procedural HAZOP.
PG
to petrol
blending system
J1 centrifugal transfer pump
LCV
V5
LIC
flex
blow-off
N2 supply
NRV
NRV
V6
V3
V2
V1
V4
Figure A5.2 Nitrogen supply connection for the line flushing procedure.
141
Appendix 5: An Illustration of HAZOP Study for a Procedure

A5.2 DETAILED PROPOSED SEQUENCE
The operation will be carried out by an operator stationed near J1
who will be the lead operator and a second operator, in radio commu-
nication, at the tank to operate valve V1. The lead operator will con-
trol the procedure.
The initial set-up is for all valves V1 V6 closed and with the line
between V1 and V2 containing C6 liquid.
1. Open V6 then open V5 to prove line clear of debris and to displace
any air in the hose.
2. Close V6 then open V4.
3. The operator at the J1 pump should open V2 slowly until fully
open.
4. The operator at the tank is then instructed to open V1 slowly by
one or two turns.
5. The operator at V1 should wait until nitrogen is heard passing
through the valve into the IST then the tank operator will close V1.
6. The tank operator should then cautiously reopen V1 by one or
two turns to ensure as much liquid as possible has been blown
back to the IST.
7. Close V4.
8. Close V1 after allowing any residual N2 in the line to depressure
into IST.
9. Pump-based operator to close V2.
10. Close V5.
11. Verify V2, V4, and V5 are all closed.
12. Open V6 to depressurize the line.
13. Disconnect the hose at V4.
A5.3 THE HAZOP STUDY
HAZOP study team
• Mike Manchester (MM) Facilitator and Scribe
• Brenda Bolton (BB) Production Manager
• Sandy Southport (SS) Senior Operator
• Wally Wigan (WW) Safety Officer
142 Appendix 5: An Illustration of HAZOP Study for a Procedure

Division into nodes
Node 1
• Steps 1 2: Connect and prove the nitrogen supply. V5 V6.
• Design intention: To prove that the N2 supply is fitted and to
displace any air in the hose.
Node 2
• Steps 3 9: Clear the line by blowback to IST. V5 V1.
• Design intention: To completely clear petroleum feedstock from the
100 m line between the J1 pump and the IST by blowback to the
IST using N2 from the 1.3 barg nitrogen ring main via a temporary
connection fitted to an existing drain by the J1 pump. Manual con-
trol by operators positioned at each end of the line. After the main
clearance, a brief second flush will be applied.
Node 3
• Steps 10 13: Depressurize and disconnect. V1 V6.
• Design intention: Line previously containing C6 but now containing
N2 to be depressured.
Node 4
• V3-J1-LCV: Line to petrol blending (not completed).
Guide words:
• Out of Sequence—too early, too late
• Rate—too fast, too slow
• Magnitude—more, less
• Pressure—more
• Communication
• Reverse
• Incomplete
• Other
143

Table A5.1 HAZOP study report for node 1 (to be read in conjunction with figures A5.1 and A5.2)
Steps 1 2: Connect and prove the nitrogen supply. V5 V6.
Design Intention: To prove that the N2 supply is fitted and to displace any air in the hose.
Initial status: All valves V1 V6 closed.
Attendees: MM, BB, SS, and WW (Note: BB
means BB is actioned to refer to a competent person). Date: 2/1/15.
Ref.
No.
Guideword Effect Cause Consequence Safeguards Actions On
1.1 Out of
sequence
No flow N2 V6 closed, V5 open Some (small amount)
of air left in the hose
Valve labeling
and practice
1.1.1 Reinforce practice
1.1.2 Confirm that traces of air in
IST are not a safety issue
BB
WW
1.2 Too fast High flow N2 V5 too far open Waste of N2 and
local noise
Valve labeling
and practice
1.2.1 Consider the jet reaction at the
vent and secure
BB
1.2.2 Can stones be sprayed about?
Is there an “impact hazard” for
humans?
BB
Jet reaction on stones
at vent, V6
1.2.3 Procedure to emphasize that
the V5 should be opened slowly
WW
1.3 Out of
sequence
Possible
contamination of
downstream process
or rotation of the
pump
V3 open by mistake, V6
closed, and V4 opened
Possible
contamination of
downstream process
or rotation of the
pump
Valve labeling
and practice
1.3 Consider locking V3 closed as
part of the preparation process
BB
1.4 Out of
sequence/
reverse
Reverse flow from
(upstream) process to
V6
V3 left open, V4 left open,
V6 open during the
depressuring
Possible release of
“materials” from
downstream plant
Pump NRV
Valve labeling
and practice
Many valves in
route
Very low risk. See 1.3 above.
Many valves have to be incorrectly
set and NRV passing
Noted
No
actions
arising
1.5 Out of
sequence/
reverse
Reverse flow from
IST
V6 left open after blowdown
and V1 and V2 opened ready
for displacement of C6. V5
still closed
IST drains through
V6
Valve labeling
and practice
1.5.1 Consider the need for NRV on
the pump side of V2
BB
Environmental
impact
Potential fire
1.5.2 This part of the procedure
should have “one on one”
supervision
BB
1.6 Other guidewords No effects identified

Table A5.2 HAZOP study report for node 2 (to be read in conjunction with figures A5.1 and A5.2)
Steps 3 9: Clear the line by blowback to IST. V5 V1.
Design intention: To completely clear petroleum feedstock from the 100 m line between the J1 pump and the IST by blowback to the IST using N2 from the 1.3 barg nitrogen ring main
via a temporary connection fitted to an existing drain by the J1 pump. Manual control by operators positioned at each end of the line. After the main clearance a brief second flush will
be applied.
Status: As at end of Node 1.
Attendees: MM, BB, SS, and WW.
Date 2/1/15.
Ref.
No.
2.1 Out of
sequence
Too early
(valve
operation)
N2 flow out of V6 V6 left open N2 losses Valve
labeling and
practice
2.1.1. Obvious, take corrective action
on V6
BB
Possible release of
C6 at V6
Human factors. V6 opened
and V1 and V2 opened for
displacement. V5 not yet
opened
Environmental impact and
possible fire
Valve
labeling and
practice
2.1.2. See 1.5.1 and 15.2 BB
2.2 Too late (valve
operation)
N2 in next
(downstream)
operation
V2 not open, V3 left open None 2.2 Action to be corrected BB
See 1.3
2.3 Out of sequence C6 released from V6 V1 and V2 open, V6 left
open, and V5 closed
possible fire
Valve
labeling and
practice
2.3.1 Consider the need for an NRV at
the N2 side of V4
BB
2.3.2 Review how this operation should
be supervised. This part of the
procedure should have “one on
one” supervision. See 1.5.2
WW
2.4 Too fast (valve
opening)
V5 too far open Human factors Possible overpressure of IST due
to high N2 flow
PRV on IST 2.4.1 Assess the capacity of IST PRV
against blow by
BB
More flow
pressure—more
in IST
Poor understanding of
operation
As above 2.4.2 Consider the need for a flow
restrictor in N2 supply
BB
Magnitude
(more than two
turns on V1)
More N2 flow into
IST
Poor understanding of
operation
As Above 2.4.3 If a flow restrictor is inserted how
will it be controlled as it is now a
“Safety Critical Item”?
BB
(Continued)

Ref.
No.
2.5 Too slow (valve
V5 opening)
Low flow N2
and C6
C6 not displaced No true indication of N2
flow rate
Slower displacement of C6 due
to N2 “slippage”—wavy flow
may result in limited C6 removal
None 2.5.1 The flow of C6 will not necessarily
be plug flow. In what two-phase
flow regime is the displacement
expected to operate?
BB
2.5.2 How can the regime be
controlled?
BB
2.6 More flow N2
High flow
See 2.4 and 2.5 See 2.4 See 2.4 See 2.4 2.6.1 See 2.5.1
2.6.2 See 2.5.2
2.6.3 See 2.4.2/2.4.3
BB
BB
BB
2.7 Incomplete C6 left in line Line not true, hogs and
hollows plus elevation
changes
Some C6 trapped in the line at
the end of the final blow
through. Environmental impact
and possible fire
None
obvious
2.7.1 Check the line slope and sags
2.7.2 Is there too much line distortion
to make the blow out viable? A
site visual check should be carried
out
BB
BB
2.8 Reverse flow of
C6
C6 released from V6 V6 left open and V5 closed
at the end of cycle. Some
C6 still in the line
possible fire
Valve
labeling and
practice
As 2.3.1 BB
2.9 Communication As above As above As above Valve
labeling and
practice
2.9.1 Review how this operation should
be supervised
How long might it take?
BB
Misinterpretation Human factors
What is the significance of
a change in the noise?
What will it sound like?
Wavy flow may produce a
sound like gas passing into the
IST.
2.9.2 Ensure the operators are trained
in the use of radios
BB
2.9.3 Review this parameter. Is it really
safe for operation and a credible
control parameter?
BB
(Continued)

Ref.
No.
2.10 Communication As above Misunderstanding of the
point in the sequence
without a clear lead
operator
Possible upset not easy to define Lead
operator is
specified in
the
procedure
2.10.1 Ensure that one operator is
clearly the lead operator
controlling the actions and the
other takes instructions from the
leader
BB
Misunderstanding of the
point in the operation due
to poor radio protocol
As above 2.10.2 Ensure that the operators are
competent in the use of radios
and language protocol
WW
Possible source of
ignition
Radios not compatible
with Hazardous Area
Classification
Possible fire (remote possibility) 2.10.3 Verify that the radios are
compatible with the area
classification
WW
2.11 Incomplete Possible reverse flow
from IST during the
step 8 depressuring
cycle
Hydrostatic head in IST Live V1 V2 is recontaminated
with C6
None 2.11.1 Consider closing V1
IMMEDIATELY the gas flow is
detected and then depressure via
V6
BB
Possible environmental impact
and fire during final blow down
through V6
2.11.2 Review the operation step 8 in
the procedure. Is it viable?
BB
Other
guidewords
No effects identified

Table A5.3 HAZOP study report for node 2, final blow through, steps 6 9 (to be read in conjunction with figures A5.1 and A5.2)
(Immediate continuation of node 2 after first nitrogen flush of the line, i.e., completion of step 5.)
Status: As at the end of main blow through. V1, V3, and V6 closed; V2, V4, and V5 open.
Team/date as Table A5.2.
Ref.
No.
2.12 Less flow
Incomplete
Line V1 V2
incompletely cleared.
C6 still in line
Flow regime
uncertain and line
slopes uncertain
Significant final C6 left in line
which has to be drained.
Environmental impact and risk
of fire
None 2.7 (see 2.5.1 and 2.5.2; 2.6.1 and
2.6.2)
BB
2.13 More flow (N2)
(V5 too far open)
(See 2.3)
V5 too far open Human factors Possible overpressure of IST PRV on
IST
2.12.1 Assess the capacity of IST
PRV against blow-by. See
2.4.1
BB
Poor
understanding of
operation
2.12.2 Consider the need for a flow
restrictor in N2 line. See 2.4.2
BB
2.12.3 If a flow restrictor is inserted
how will it be controlled as it
is now a “Safety Critical
Item”? See 2.4.3
BB
2.14 Communication C6 still in line. Could
result in a major spill
later in the process
“Sound” is the
only variable.
What will it
“sound like”?
possible fire if incompletely
drained
None See 2.5 and 2.9.3
Review this parameter. Is it really
safe for operation?
BB
No other
differences
between first and
final clearing

A5.4 FINAL HAZOP STUDY REPORT
Obviously, the final report cannot be written until the full HAZOP
study has been completed. However, it is clear that there are a number
of steps with potential for errors (human factors), it being a one-off
operation and unfamiliar to the operations staff. There are also a num-
ber of “unknowns.” Displacement (or blowing through) is a standard
operation but it has significant implications when carried out within
the constraints of this procedure. Is it a viable solution?
To date the key findings from nodes 1 and 2 are:
1. The procedure has missed the natural hogs and hollows in the line
between V1 and V2 and any elevation changes between the nitro-
gen injection point and the IST which may make the procedure
nonviable.
2. There is potential for release of C6 at V6.
3. A possible overpressure of the IST following a N2 blow-by.
4. Step 4 is vague—one or two turns is not a measurable parameter
so there is potential for human error/factors.
5. The flow regime in the line from V2 to V1 is uncertain, and it is
not clear that the contents can be displaced in a controlled man-
ner. There is a potential conflict between transport of C6 and IST
integrity. Is there a better alternative?
6. The compatibility of the radios with the Hazardous Area
Classification.
7. It is a new one-off operation which needs some training (human
factors).
8. There is a need for labeling of all the valves (human factors).
9. The interpretation of the “end of clearing” using a subjective
“noise” (human factors) is not inherently safe.
10. There is a possibility of recontamination of the line following the
final blow out due to hydrostatic head in IST if the closure of V1
is delayed. The line V1 V2 must not be blown down into IST due
to the risk of recontamination. V1 must be closed first.
A5.5 AUTHORS NOTES ON THIS PROCEDURE
Valve V1 is a gate valve as the procedure says “open two turns.”
Assuming that it is mounted vertically, the flow regime in the line
from V1 to V2 will be very uncertain as the gap for liquid flow will be
149

at the bottom of the line, and this will not necessarily be the low
point—there could be “hogs and hollows” in the line especially if it
slopes to the pump J1. A foam pig would pass the fully open gate
valve V2 and be stopped by the partially open gate valve V1 (open one
or two turns). This method would give a more complete line clearance
and so a pig run may be preferable.
The objective of this exercise was to demonstrate the use of
HAZOP in a procedure, but it has produced more issues than
expected! This shows the strength of HAZOP.
150 Appendix 5: An Illustration of HAZOP Study for a Procedure

ACRONYMS AND ABBREVIATIONS
AFD approved for design
AIChE American Institution of Chemical Engineers
BS British Standard
CFR Code of Federal Regulations
CHAZOP computer HAZOP (study)
CIA Chemical Industries Association
CPF central process facility
DHSV down hole safety valve
EC European Community
EPSC European Process Safety Centre
ESDV emergency shutdown valve
EU European Union
FEED front-end engineering design
FMEA failure modes and effects analysis
FMECA failure modes and effects criticality analysis
HAZID hazard identification (method)
HAZOP hazard and operability (study)
HS hazard study
HSE Health and Safety Executive (UK)
IChemE Institution of Chemical Engineers
IEC International Electrotechnical Commission
I/O input/output
LOPA layer of protection analysis
LTEL long-term exposure limit
MOC management of change (system)
MSDS material safety data sheet
MV master valve
OP orifice plate
OSHA Office of Safety and Health Administration (USA)
PID piping and instrumentation diagram
PES programmable electronic system
PFD probability of failure on demand
PI productivity index
PIF performance-influencing factor

PSM process safety management
PSSR pre start-up safety review
QA quality assurance
QRA quantitative risk assessment/analysis
SHE safety, health, and environmental
SIL safety integrity level
SIS safety instrumented system
SMS safety management system
SOP standard operating procedure
SOR safety and operability review
SSSV sub-surface safety valve
WV wing valve
152 Acronyms and Abbreviations

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(IEC 61882, Guide for Hazard and Operability (HAZOP) Studies).
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154 References

INDEX
Note: Page numbers followed by “f” and “t” refers to figures and tables
respectively.
A
Accidents, 36
Actions, 3, 21 22, 67
and recommendations, 21 22
Alarms, 19, 63, 65, 67, 80
Approved for design (AFD), 5
Area classification, 5
Audit, 29, 47, 50, 53, 55 57, 94
Auditing
external, 55 57
internal, 55 57
B
Batch, 13
Batch processes, 13, 37 41, 61, 67, 80
Batch/sequential operations, 123
Blowdown, 104
Boundaries, 30, 36
C
Cause and Effects Diagrams, 89
Causes, 17 19, 66, 74 75
Central process facility (CPF), 101 102
Checklists, 7f, 17, 90, 99 100
Chemical Industries Association (CIA), 1
Clients, 90
Commissioning, 1, 10, 34, 39, 48, 64, 84 85
Company procedures, 60
Complexity, 2, 8, 45
Compressors, 37, 39, 84
Computer control, 39
Computer controlled processes, 62 69
Computer HAZOP (CHAZOP) study,
64 65, 67 69
enhanced HAZOP, 65 67
legislation, 64 65
use of HS2, 62 63
Computer recording, 33, 54 55
Conceptual model, 10, 13, 16 17, 38
Consequences, 3, 10 11, 17 19, 21, 66
evaluation of, 17 19
Construction, 5, 89 90
Continuous operation, 101
Continuous processes, 37 39
Contract operations, 90 91
Contractors, 90 91
Control, 5, 16t, 34
D
Decommissioning, 1, 84 85
Demolition, 89 90
Design, 1
Design intention, 2 3, 13 15
Deviations, 2 3, 10, 13
generating, 15 17
Down hole safety valve (DHSV), 101
Drains, 82 84
E
Enhanced HAZOP, 65 67
Environment, 2 4
Environmental risk, 29
Ergonomics, 11
European Process Safety Centre (EPSC), 1 2
F
Failure modes and effects analysis (FMEA),
67 68
Functional safety, 64 65
G
Generic actions, 21
Guideword-first approach, 15, 95 98
Guidewords, 2 3, 15, 16t, 17t, 22, 37 38, 42,
53 54, 61, 66, 68, 79 80, 130, 138t
H
Hardware, 21, 47, 62, 67 68
Hazard identification, 2 5, 7 8, 74 75,
77 78
Hazard studies, 4
checklist, 8 9
HS 0-7, 4 6
overview, 30
Hazards, 2 4, 63, 74 75, 82, 89 90

HAZOP, 1 3, 13, 29, 55 57, 60, 62, 77, 92
analysis, 14f, 18, 22
applications, 77
existing plant, 78 79
pitfalls, 92
purpose, 11
HAZOP example batch operation, 123
documentation, 125 128
guidewords, 129 130
PID, 125 128
parameters, 130
plant and process, 123
process description, 128
reaction stage description, 131
team members, 123, 130
HAZOP example continuous process, 101
issues, 104 105
methodology, 105 121
node 1 records, 114t
node 2 records, 118t
PIDs, 109f, 111f, 113f
process data, 104
process description, 101
team members, 105 106
HAZOP example procedure, 139
description, 139 141
operating sequence, 142
PID, 140f
review, 149 150
study records, 149
team members, 139
HAZOP sequence flow chart, 14f, 96f
HAZOP study
alternatives, 7f
background, 1
batch processes, 39 41
boundaries, 29 31, 43, 77 78
commissioning, 84 85
completion, 22 23
computer-controlled process, 62 69
conceptual model, 10
construction, 89 90
continuous processes, 37 39
contract operations, 90 91
decommissioning, 84 85
demolition, 89 90
description, 13 15
design envelope, 10
design intent, 13 15
deviations, 15 17
drains and vents, 82 84
enhanced, 65 67
essential features, 2 3, 10 11
examples, 2, 16t
existing plant, 10 11
ground rules, 46
illustration, 23 26
interconnections, 82 84
laboratory operations, 80 82
legislation, 4
limitations, 11 12
links to LOPA, 74 76
meetings, 35, 46 47
methodology, 10, 31 32, 59, 61, 79,
105 121
modifications, 50, 53, 77 78
objectives, 29 31
operating procedures, 79 80
operational modes, 31
pilot plants, 80
pitfalls, 92
planning, 31 32, 45 46
procedures, 13, 41 42
purpose, 11
recording, 22, 50, 52 55
records, 15, 47, 53 57
repeat designs, 78
reports, 29, 50, 53, 94, 128
scope, 11 12, 29 31
scribe, 32 33, 58
shutdown, 88 89
stages/steps, 20, 24, 124 125
start-up, 85 89
successful, 92
team, 31 32, 40 41, 66, 69 70, 74, 125
team leader, 31 36, 58 59, 90, 130
team members, 32, 34 35, 45, 58, 91
team membership, 33 36, 90
team size, 33 36
HAZOP study meetings
follow up, 48 49
initial meeting, 46
planning, 45 46
responses, 47
signing off, 48
study sessions, 46 47
Human behavior, 71, 74
knowledge-based mode,
69 70
rule-based mode, 69 70
skill-based mode, 69 70
Human error, 10, 69 70, 72 74
Human factors, 69 74, 139 150
Human Reliability methodology, 72
156 Index

I
ICI, 1
ICI six stage hazard studies, 4, 7f
IEC 61508, 64 65
Inherent safety, 6
Interfaces, 30, 83
Interfaces high/low pressure, 83
International Electrotechnical Commission
(IEC), 2, 64 65
L
Laboratory operations, 80 82
Layer of protection analysis (LOPA), 74 75
Licensors, 90
M
Maintenance, 10 11, 26t, 60, 64 65, 71 72
Major Accident Prevention Policies, 4
Management of change (MOC), 50, 54 55,
68, 77 78, 94
Management of Change Procedure, 48
Material properties, 37
Material Safety Data Sheet (MSDS), 51
Meetings
duration, 45 46
initial, 46
planning, 45 46
Mentoring, 59
Modifications, 50, 53, 64, 77 78
N
Near misses, 39
Node, 37 38, 41 42, 68, 86
O
Office of Safety and Health Administration
(OSHA), 4, 53 54, 78
Operability, 4 5, 11 12, 29 30, 40, 79, 90, 125
Operability problems, 4
Operator, 19, 66
Operator training, 50
P
Parameter-first approach, 14f, 15, 22
Parameters, 10, 13, 15, 16t, 17t, 22, 32, 37 38,
41, 66, 83, 138t
Performance-influencing factors (PIFs), 72 73
Periodic hazard studies, 78 79
Pilot plant, 80 82
Pipe, 37, 44, 103
Piping, 82, 84 86, 89, 104
Pitfalls, 92, 94
Planning, 37 38
Plant safety dossier, 50
Post-HAZOP work, 94
Pre Start-up Safety Review (PSSR), 6
Pre-meeting with client, 43 45
inclusions, 43
terms of reference, 43
Preparation, 36 42
Probability of failure on demand (PFD), 64,
74 75
Procedure HAZOP, 139
Procedures, 13, 60, 79 80
Process and instrumentation diagram (PID),
5, 37, 43 44, 65, 79, 82 83, 92 93, 109f,
111f, 113f, 125 128, 140f
Process hazard studies, 4
Process safety management (PSM), 4
Programmable electronic systems (PES), 62
Project life cycle, 7
Protection, 19
Pump, 18 19
Q
Quality, 12
Quality assurance (QA), 29
Quantitative risk assessment (QRA), 12
R
Reaction stability, 80 81
Realistic causes, 18
Recommendations, 21 22
Record, content, 54
Recording, 22, 50 55
background information, 51
computer recording, 54 55
content, 54
format, 51 52
level, 52 54
section headings, 51
uses, 50
Relief, 5, 82
Repeat designs, 78
Report completing, 48
Reporting, 67
Responses
coordinating and reviewing, 47
Risk, 2 3, 64, 70
assessment, 5, 20 21
environmental, 29
Risk graph, 3f
Risk ranking, 52
157
Index

S
Safe Holding Position, 88
Safeguards, 3, 10, 18 21, 52 53, 66
safety, health, and environmental (SHE)
hazards, 1, 4 5
Safety and Operability Review (SOR), 63
Safety instrumented systems (SISs), 5, 8, 63,
74 75
Safety integrity levels (SILs), 5, 64, 74 75
Safety management systems (SMSs), 4
Safety-related systems, 62, 64
Scribe, 24, 32 33, 58
Sequential operations, 39 41
Seveso II Directive, 4
Shutdown, 31, 61, 85 89, 106
Software, 20, 54, 67
Special skills, 82 83
Start-up, 6, 31, 39, 85 89
Sub-surface safety valve (SSSV), 101 102, 104
System parameters, 2 3
Systems, 7f, 30, 39, 54 55, 62 67, 81 82
T
Team, 10, 12, 15, 17 18, 22, 29, 32 36,
40 41, 43, 50, 53, 56
leader, 24 25, 31 36, 58 59, 90
members, 22, 33 36, 45, 58
membership, 33 36
scribe, 32 33
selection, 31 36
size, 33 36
Training, 58
leader, 58 59
members, 58
scribe, 58
team leader, 58 59
Trips, 19, 30, 39, 65
V
Valves, 40 42, 71
Vents, 19, 30, 36 37, 42, 82 83
Vessels, 37 40
158 Index

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