psv specifications

RELIEF DEVICES - SELECTION, SIZING AND
SPECIFICATION
DEP 80.36.00.30-Gen.
January 2010
DESIGN AND ENGINEERING PRACTICE
This document is restricted. Neither the whole nor any part of this document may be disclosed to any third party without the prior written consent of Shell Global
Solutions International B.V., The Netherlands. The copyright of this document is vested in this company. All rights reserved. Neither the whole nor any part of this
document may be reproduced, stored in any retrieval system or transmitted in any form or by any means (electronic, mechanical, reprographic, recording or otherwise)
without the prior written consent of the copyright owner.

DEP 80.36.00.30-Gen.
January 2010
Page 2
PREFACE
DEP (Design and Engineering Practice) publications reflect the views, at the time of publication, of:
Shell Global Solutions International B.V. (Shell GSI)
and/or
Shell International Exploration and Production B.V. (SIEP)
and/or
other Shell Service Companies.
They are based on the experience acquired during their involvement with the design, construction, operation and
maintenance of processing units and facilities, and they are supplemented with the experience of Shell Operating Units.
Where appropriate they are based on, or reference is made to, international, regional, national and industry standards.
The objective is to set the recommended standard for good design and engineering practice applied by Shell companies
operating an oil refinery, gas handling installation, chemical plant, oil and gas production facility, or any other such
facility, and thereby to achieve maximum technical and economic benefit from standardization.
The information set forth in these publications is provided to Shell companies for their consideration and decision to
implement. This is of particular importance where DEPs may not cover every requirement or diversity of condition at
each locality. The system of DEPs is expected to be sufficiently flexible to allow individual Operating Units to adapt the
information set forth in DEPs to their own environment and requirements.
When Contractors or Manufacturers/Suppliers use DEPs they shall be solely responsible for the quality of work and the
attainment of the required design and engineering standards. In particular, for those requirements not specifically
covered, the Principal will expect them to follow those design and engineering practices which will achieve the same
level of integrity as reflected in the DEPs. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting
from his own responsibility, consult the Principal or its technical advisor.
The right to use DEPs is granted by Shell GSI, in most cases under Service Agreements primarily with Shell companies
and other companies receiving technical advice and services from Shell GSI or another Shell Service Company.
Consequently, three categories of users of DEPs can be distinguished:
1) Operating Units having a Service Agreement with Shell GSI or other Shell Service Company. The use of DEPs
by these Operating Units is subject in all respects to the terms and conditions of the relevant Service
Agreement.
2) Other parties who are authorized to use DEPs subject to appropriate contractual arrangements (whether as part
of a Service Agreement or otherwise).
3) Contractors/subcontractors and Manufacturers/Suppliers under a contract with users referred to under 1) or 2)
which requires that tenders for projects, materials supplied or - generally - work performed on behalf of the said
users comply with the relevant standards.
Subject to any particular terms and conditions as may be set forth in specific agreements with users, Shell GSI
disclaims any liability of whatsoever nature for any damage (including injury or death) suffered by any company or
person whomsoever as a result of or in connection with the use, application or implementation of any DEP, combination
of DEPs or any part thereof, even if it is wholly or partly caused by negligence on the part of Shell GSI or other Shell
Service Company. The benefit of this disclaimer shall inure in all respects to Shell GSI and/or any Shell Service
Company, or companies affiliated to these companies, that may issue DEPs or require the use of DEPs.
Without prejudice to any specific terms in respect of confidentiality under relevant contractual arrangements, DEPs shall
not, without the prior written consent of Shell GSI, be disclosed by users to any company or person whomsoever and
the DEPs shall be used exclusively for the purpose for which they have been provided to the user. They shall be
returned after use, including any copies which shall only be made by users with the express prior written consent of
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GSI may at any time require information satisfactory to them in order to ascertain how users implement this
requirement.
All administrative queries should be directed to the DEP Administrator in Shell GSI.

DEP 80.36.00.30-Gen.
January 2010
Page 3
TABLE OF CONTENTS
1. INTRODUCTION ........................................................................................................4
1.1 SCOPE........................................................................................................................4
1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS .........4
1.3 DEFINITIONS .............................................................................................................4
1.4 CROSS-REFERENCES .............................................................................................5
1.5 CHANGES SINCE PREVIOUS EDITION...................................................................5
1.6 COMMENTS ON THIS DEP.......................................................................................5
2. RELIEF DEVICE SELECTION ...................................................................................6
2.1 GENERAL CRITERIA .................................................................................................6
2.2 MECHANICAL DESIGN..............................................................................................7
2.3 SPECIFIC SELECTION CRITERIA ............................................................................7
3. RELIEF DEVICE CALCULATIONS .........................................................................14
3.1 PRESSURE RELIEF VALVE SIZING.......................................................................14
3.2 NON-RECLOSING RELIEF DEVICE SIZING ..........................................................19
4. REQUISITIONING ....................................................................................................21
4.1 GENERAL.................................................................................................................21
4.2 PRESSURE RELIEF VALVES..................................................................................21
4.3 RUPTURE DISKS.....................................................................................................21
5. RELIEF DEVICE SPECIFICATIONS .......................................................................22
5.1 GENERAL.................................................................................................................22
5.2 PRESSURE RELIEF VALVE SPECIFICATION .......................................................22
5.3 SET PRESSURE CONSIDERATIONS.....................................................................23
5.4 ADDITIONAL REQUIREMENTS FOR SPECIFIC SERVICES.................................25
6. PAINTING.................................................................................................................26
7. IDENTIFICATION .....................................................................................................27
7.2 RUPTURE DISKS.....................................................................................................27
8. PROTECTION AND PACKAGING...........................................................................28
8.1 GENERAL.................................................................................................................28
8.3 RUPTURE DISKS.....................................................................................................28
8.4 BUCKLING PINS ......................................................................................................28
9. DOCUMENTATION ..................................................................................................29
9.1 MANUFACTURER....................................................................................................29
9.2 ELECTRONIC FILES................................................................................................29
9.3 EQUIPMENT FILES..................................................................................................29
10. REFERENCES .........................................................................................................31
11. BIBLIOGRAPHY ......................................................................................................33

DEP 80.36.00.30-Gen.
January 2010
Page 4
1. INTRODUCTION
1.1 SCOPE
This DEP specifies requirements and gives recommendations for the selection, sizing, and
specification of relief devices.
The selection, sizing, and specification of relief devices shall be in accordance with the
following standards, as clarified, amended or supplemented by this DEP:
• ASME VIII, Division 1 or 2;
• API Std 520, Part I and Part II;
• ISO 23251;
• API Std 526;
• ISO 28300.
NOTES: 1. API Std 521, 5th
edition is identical to ISO 23251:2006.
2. API 2000, 6th
edition is identical to ISO 28300:2008.
Refer to DEP 80.45.10.10-Gen. to determine the hydraulics and other requirements of the
relief and flare system.
This DEP is a revision of the DEP of the same number dated December 2008; see (1.5)
regarding the changes.
1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS
Unless otherwise authorised by Shell GSI, the distribution of this DEP is confined to Shell
companies and, where necessary, to Contractors and Manufacturers/Suppliers nominated
by them.
This DEP is intended for use in oil refineries, chemical plants, gas plants, exploration and
production facilities and supply/distribution installations.
When DEPs are applied, a Management of Change (MOC) process should be
implemented; this is of particular importance when existing facilities are to be modified.
If national and/or local regulations exist in which some of the requirements may be more
stringent than in this DEP, the Contractor shall determine by careful scrutiny which of the
requirements are the more stringent and which combination of requirements will be
acceptable with regard to the safety, environmental, economic and legal aspects. In all
cases the Contractor shall inform the Principal of any deviation from the requirements of
this DEP which is considered to be necessary in order to comply with national and/or local
regulations. The Principal may then negotiate with the Authorities concerned, the objective
being to obtain agreement to follow this DEP as closely as possible.
1.3 DEFINITIONS
1.3.1 General definitions
The Contractor is the party which carries out all or part of the design, engineering,
procurement, construction, commissioning or management of a project or operation of a
facility. The Principal may undertake all or part of the duties of the Contractor.
The Manufacturer/Supplier is the party which manufactures or supplies equipment and
services to perform the duties specified by the Contractor.
The Principal is the party which initiates the project work and ultimately pays for its design
and construction. The Principal will generally specify the technical requirements. The
Principal may also include an agent or consultant, authorised to act for the Principal.
The lower-case word shall indicates a requirement.
The capitalised term SHALL [PS] indicates a process safety requirement.

DEP 80.36.00.30-Gen.
January 2010
Page 5
The word should indicates a recommendation.
1.3.2 Specific definitions
For pressure relief valve (PRV) terminology and related terms, refer to API Std 520 and
DEP 80.45.10.10-Gen.
1.4 CROSS-REFERENCES
Where cross-references to other parts of this DEP are made, the referenced section
number is shown in brackets. Other documents referenced in this DEP are listed in (10).
1.5 CHANGES SINCE PREVIOUS EDITION
This DEP is a revision of the DEP of the same number dated December 2008.
In this revision, process safety requirements have been indicated by the use of the
capitalised term "SHALL [PS]".
Other than that, this DEP has been a completely revised and it is impractical to summarise
the changes here.
1.6 COMMENTS ON THIS DEP
Comments on this DEP may be sent to the DEP Administrator at standards@shell.com.
Shell staff may also post comments on this DEP on the Surface Global Network (SGN).

DEP 80.36.00.30-Gen.
January 2010
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2. RELIEF DEVICE SELECTION
2.1 GENERAL CRITERIA
Pressure relief valves (PRVs) shall be selected from the standard sizes listed in API 526.
For standardisation purposes, and to reduce the quantity of spares, the Principal may
require the number of standard sizes at the site to be restricted; the Contractor shall submit
the proposed valve sizes for the approval of the Principal.
Inlet and outlet flanges shall form an integral part of the body unless otherwise approved by
the Principal.
Flange rating (ASME Class) requirements are summarized in Table 1.
Table 1 Inlet Flange Ratings
System rating Inlet Flange Rating
(1)
150 150 or 300
( 2,3)
300 300
(3)
600 600
900 900
1500 1500
2500 2500
NOTES: 1. For orifices types “P” and larger with inlet flanges ASME Class 300 and higher,
the maximum available set pressure limit in API 526 is not suitable for the full
ASME rating. Therefore, for these sizes the next higher rated inlet flange
SHALL [PS] be selected if so dictated by the design pressure of the system.
2. Generally, an ASME Class 300 flange should be selected as it allows use in
higher pressure applications and is consistent with the efforts of some sites to
standardize inlet flange classes.
3. For valves with ASME Class 300 inlet flanges, the valve with the highest
pressure limit available in API 526 shall be selected.
Outlet flange ratings for PRVs that relieve to a flare system or atmosphere shall be in
accordance with API 526 unless the Manufacturer’s standard is a higher rating. For PRVs
that relieve to a pressurised system, the outlet flange SHALL [PS] be selected according to
the maximum back pressure it is subjected to.
Pipe class and flange face finish of the relief valve discharge SHALL [PS] be in accordance
with the line specification of the connected piping.
PRVs for steam service discharging to atmosphere shall be of the exposed-spring type if
operating above 250 °C (480 °F).
Manual lifting devices should be avoided. Typical design codes only require lifting devices
on PRVs in air, hot water (over 60 °C or 140 °F), or steam service.
NOTE: ASME VIII, Division 1, paragraph UG-136(a)(3) requires a lifting device on a PRV in air, hot water, or
steam service. Code Case 2203 allows omission of the lifting device under certain provisions. These
provisions are: (1) there is a documented inspection and maintenance program (2) the omission is
documented on the data sheet, and (3) the omission is approved by the authority having jurisdiction (if
applicable).
Where the application of Code Case 2203 is permitted by local regulations, a lifting device
should not be provided. If local regulations do not permit application of Code Case 2203,
then a lifting device shall be provided only for the fluids specified in the relevant pressure
vessel design Code.

DEP 80.36.00.30-Gen.
January 2010
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Test rods (gags) are seldom justified and shall only be provided with the approval of the
Principal.
An example of where the installation of test rods could be approved is for PRVs installed on
a steam drum, to prevent them from lifting during in-situ testing of the PRVs installed on the
superheater.
NOTE: Test rods prevent the stem moving and are sometimes used to block the PRV during hydrotesting the
equipment or are used when transporting the PRV.
2.2 MECHANICAL DESIGN
1. For new installations, the inlet and outlet connections SHALL [PS] be consistent with the
pipe specifications for the systems to which they connect. PRVs with threaded inlet
nozzles shall not have threaded inlet bushing connections.
NOTE: On PRVs with inlet bushings the valve is liable to come apart during removal if the pipe wrench is
applied to the PRV body (instead of the PRV inlet bushing). PRVs with inlet bushings also require
additional inspection/maintenance of the threaded connection).
2. Seal welding of threaded connections SHALL [PS] not be allowed.
3. Slip-on flanges SHALL [PS] not be used.
4. If PRV fabrication requires welding, the welding procedures SHALL [PS] be submitted for
approval prior to start of fabrication.
5. Pressure relief device design temperature:
The design temperature SHALL [PS] be determined by the inlet and outlet temperatures
that result from extreme operating and relieving conditions. The temperature reduction
resulting from possible valve leakage (e.g., flashing of liquid) SHALL [PS] be included in
the determination of the design temperature. Elevated temperature arising from an
external fire, however, is not a basis for the design temperature and is not a material
selection criterion.
To determine the minimum temperature, an isentropic flash to the choking pressure
followed by an isenthalpic flash to the calculated back pressure shall be carried out – this
is described in more detail in (3.1.7). If this results in a minimum temperature that is only
slightly below the threshold at which low-temperature (fine grain) material is required, more
detailed calculations should be carried out.
2.3 SPECIFIC SELECTION CRITERIA
2.3.1 General
PRVs shall normally be of the spring-loaded type. For special applications, pilot-operated
PRVs, rupture pin valves, and where permitted by local regulations, air-assisted PRVs may
be employed.
For a description of the various types of PRVs, reference is made to API Std 520 Part I.
Capacities established and guaranteed by the PRV Manufacturer for the applicable service
conditions SHALL [PS] be used for selecting the PRVs.
2.3.2 Non-balanced spring-loaded pressure relief valves
If the built-up back pressure plus the variable superimposed back pressure [see (3.1.7.1)
for definitions] does not exceed the allowable overpressure, PRVs shall be conventional
safety relief valves unless other considerations justify the use of a balanced PRV (e.g.,

DEP 80.36.00.30-Gen.
January 2010
Page 8
corrosion or fouling of PRV valve stem).
The allowable overpressure, as defined in API Std 520 Part I, is the pressure at the PRV
inlet (“relieving pressure”) minus the set pressure.
NOTE: Local regulations might not accept non-balanced PRVs set at equipment design pressure if the actual
opening pressure could at times be higher as a result of the back pressure generated by other PRVs
blowing at the same time. In that case, the use of balanced valves, or a slightly lower setting of the
non-balanced valves, should be considered for those valves which may blow as the result of a
common relief contingency.
Spring-loaded PRVs connected to a closed discharge system SHALL [PS] always have a
closed bonnet.
2.3.3 Balanced type pressure relief valves
This type of valve (or pilot operated valve) SHALL [PS] be used when the built-up back
pressure plus the variable superimposed back pressure is higher than that allowed for a
non-balanced valve. Nevertheless, the back pressure SHALL [PS] not exceed the
maximum specified by the Vendor, with account being taken of the flow capacity reduction
and the bellows pressure rating. The maximum back pressure provided by the
Manufacturer for the bellows SHALL [PS] not be exceeded.
PRVs of the balanced type SHALL [PS] have a continuously vented bonnet. See
DEP 80.45.10.10-Gen. regarding the need to route to a safe area.
Balanced-bellows valves, with a bonnet vent open to atmosphere, shall not be used with
fluids which may cause the valve to freeze if there is a leak. If this happens, the bonnet and
balancing bellows may fill up with ice due to condensation and subsequent freezing of the
moisture in the air, making the valve inoperative. Instead of balanced-bellows PRVs, the
use of an auxiliary balancing piston should be considered. In clean service, pilot-operated
PRVs, of which the pilot stays relatively warm, may be selected.
Balanced-bellows valves should not be used on reciprocating compressor discharges,
because the pulsation can cause fatigue cracking of the bellows.
Balanced-bellows valves should not be used in coking or fouling services because the
bellows can become fouled and may not operate properly.
A suitably corrosion-resistant material shall be specified for the bellows since, in the event
of a bellows failure, fluid could enter the bonnet space and escape through the bonnet vent.
Bellows shall be made of Alloy 625 LCF unless otherwise specified in the materials
selection report. If no suitable corrosion-resistant material can be selected or if the chance
of leakage is present, the use of an auxiliary balancing piston should be considered.
2.3.4 Pilot-operated pressure relief valves
Use of pilot-operated PRVs requires approval by the Principal. Where required, permission
for use of pilot-operated PRVs shall be obtained from local authorities.
1. For proper operation, pilot-operated PRVs shall only be selected for clean gas or liquid
service so that the pilot valve or sensing line cannot be blocked with hydrates, ice, wax,
solids or dirt. The pilot valve shall be of the non-flowing type. With this type, the flow
through the pilot lines and pilot valve ceases immediately when the main valve opens,
thus minimising the likelihood of foreign matter being drawn into the small sections of
the pilot valve. Heat tracing or purging SHALL [PS] be installed where cooling of
process fluids to temperatures as low as the Lowest One-Day Mean Ambient
Temperature (LODMAT) can cause plugging in the pilot, sensing line, pilot-operated
valve (POV) inlet, or dome. Where single tracing and/or purging are installed, alarms
SHALL [PS] also be installed to alert operators of any problem with these systems. The
sensing line shall also be as short as practical while still limiting the effect of high inlet
pressure drop on the pilot-operated PRV.
2. Situations in which pilot-operated PRVs may be suitable are:

DEP 80.36.00.30-Gen.
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• where the pressure loss between the protected equipment and the inlet flange of the
PRV exceeds 3 % of the set pressure of the PRV; in such situations the pilot sensor
tapping shouldl be located close to the protected equipment. The pilot line SHALL
[PS] be installed with no low points in the sensing line or its take-off so that
liquid/debris cannot collect between the pilot valve and the main PRV;
• where seat leakage may be a problem with spring loaded relief devices, since pilot
operated relief devices have superior sealing qualities;
• where high back pressures limit even the applicability of balanced valves;
• in large capacity and high set-pressure service due to their smaller size and weight
and where, especially for the larger sizes, a higher operating pressure is allowed;
• if the margin between the maximum operating pressure (MOP) of the protected
system and the PRV set pressure is less than 10 % of the PRV set pressure, since
the process pressure provides a higher seating force than if a spring were used.
If pilot-operated PRVs are used, the options in Table 2 should be considered.
Table 2 Options for pilot-operated pressure relief valves
Option Provided Comments
Modulating pilot Normally Less disruptive to process
Field test connections No
(1)
If required, special installation precautions are
required to assure that the test connection is
not plugged.
Pilot filters and/or purge Yes Unless the service is absolutely clean.
(2)
Pressure spike snubbers Yes Cyclic services (e.g., positive displacement
compressors)
Backflow preventors Yes Whenever discharging into a closed system,
where the closed system pressure might
exceed the design pressure of the protected
equipment.
(3)
NOTES: 1. Where in-situ pop-testing is required, field test connections shall be provided.
2. Pilot filters require periodic maintenance in order to control the risk of blockage. Where a
spare filter is used, the switching valve SHALL [PS] be designed and installed so that it can
be flow tested to ensure proper line-ups. Operating procedures for switching filters SHALL
[PS] be developed to avoid inadvertent closure of the pilot sensing line.
3. Malfunction of backflow preventors could cause the PRV not to reseat.
3. If pilot-operated PRVs are used, the following issues should be considered:
a. Minimum process pressure required to seat the main valve to accommodate the
elasticity of the seat.
NOTE: Can cause commissioning problems while air-freeing equipment
b. Pilot-operated valves are more complex than spring-loaded valves. This can result in
lower reliability and more opportunities for error during repair.
c. The soft goods for main valve and pilot SHALL [PS] be compatible with the process
composition, temperature (relieving and heat tracing temperatures, if applicable), and
pressure. Soft goods can be affected by substances in the process at low
concentration. Some materials are susceptible to explosive decompression in certain
process environments.
NOTE: Where a high pressure gas is present, the gas can permeate into an o-ring. If rapid
depressurization occurs, and if the gas cannot escape quickly enough, the o-ring can be

DEP 80.36.00.30-Gen.
January 2010
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physically damaged (known as “explosive decompression”). This should be addressed by
appropriate o-ring material selection and/or o-ring gland design, in consultation with the
Manufacturer.
d. Low temperature service can cause brittle failure or loss of elasticity of the soft
goods.
e. Use of diaphragm-style pilot-operated PRVs should normally be limited to
pressurized storage rather than process equipment. They should only be used on
process equipment with approval by the Principal.
2.3.5 Air-assisted pressure relief valves
Air-assisted PRVs are spring-loaded PRVs equipped with a pneumatic actuator.
Air-assisted PRVs are not described in most pressure vessel codes and their use may
require the approval of local authorities.
The function of the actuator is to keep the PRV closed when operating close to set
pressure. In this way the operating margin between set pressure and maximum operating
pressure (MOP) may be smaller, and a margin of 5 % (instead of 10 %) may be used for
design purposes. To achieve this goal the following options are possible:
1) The actuator assists to open the PRV exactly at set pressure. In this case the spring
setting of the PRV is 105 % of design pressure. This means that the PRV will open at
105 % of design pressure even if the actuator fails to perform.
2) The actuator assists to keep the PRV closed up to the latter's set point (supplemental
load). The spring setting of the PRV is 100 % of design pressure. Opening of the PRV
also relies on the operation of the instrumentation. However in the event of
instrumentation malfunction the PRV shall in any case open at 110 % of set pressure.
3) The action of the actuator is a combination of 1) and 2). But since a supplemental load
is applied, the spring setting of the PRV may be 100 % of design pressure.
The correct operation of the PRV depends on the reliability of the instrumentation and,
therefore, the air-assist instrumentation should be classified as an Instrumented Protective
Function.
The PRV Manufacturer is responsible for the overall PRV assembly (valve complete with
actuator and/or accessories).
2.3.6 Thermal expansion relief valves (TERVs)
The standard TERV size for piping systems shall be (25 mm x 25 mm) or (20 mm x 25 mm)
[(1”x1”) or (¾”x1”)], flanged, with a minimum orifice area of 0.71 cm
2
(0.110 in
2
).
The body-to-bonnet connection shall not be screwed; the body and bonnet are preferably to
be of one piece.
2.3.7 Rupture disks
2.3.7.1 General
1. As a general rule, rupture disks should not normally be installed since once they have ruptured,
flow continues until all the pressure is relieved. Moreover, once burst, they have to be replaced.
2. Rupture disks (also known as “bursting disks”) should be considered:
! to accomplish a fast response time, which cannot be achieved with a PRV. This could be
required to cope with a sudden gas breakthrough due to a heat exchanger tube burst or
malfunctioning of a level control valve into a liquid-filled system;
! to prevent PRVs in vacuum service from drawing gas or air back into the process;
! to protect PRVs from being in continuous contact with a corrosive, solidifying, or
polymerizing process fluid;

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! to protect PRVs from accumulation of solids and dusts;
! to prevent leakage of very toxic substances through the PRV that discharges to the
atmosphere;
NOTE: Very toxic substances are defined in DEP 01.00.01.30-Gen. Dispersion modeling can be done if
concerns are raised.
! to prevent leakage of very corrosive substances through the PRV that discharges to a
closed system.
3. Rupture disk installations with burst pressures less than 100 kPa (ga) [15 psig] shall be analyzed
for their susceptibility to fatigue failure due to vortex shedding.
4. Rupture disks SHALL [PS] be sized and specified in accordance with the disk Manufacturer’s
recommendations and the relevant design code. Additional detail on rupture disk sizing is given
in (3.2.1).
NOTE: ASME VIII changed substantially as from the 1997 Addendum, requiring a “UD” code stamp, and in
2005 a requirement was added for a code stamp on the rupture disk holder.
5. Capacities to be used for selecting the rupture disks SHALL [PS] be those established and
guaranteed by the disk Manufacturer for the applicable service conditions.
6. Marked burst pressure of rupture disks shall not exceed the Maximum Allowable Working
Pressure (MAWP) of the equipment to be protected except as allowed by the design code.
7. The specified rupture disk burst temperature shall be the ambient temperature, unless the
system is heat traced.
NOTE: Field measurements show that many rupture disk installations are very close to ambient temperature.
Specifying normal operating or relieving temperature for the burst temperature can result in an
increased burst pressure if temperatures are significantly lower, and premature bursting of the disk(s)
if the temperatures are significantly higher.
8. Superimposed back pressure shall be part of the disk design specification.
NOTE: Neglecting superimposed back pressure can cause the rupture disk to relieve at a higher pressure
than desired. However, if the superimposed back pressure is assumed to be lost, the rupture disk can
relieve at a lower pressure than desired.
9. Rupture disks shall be designed so that process pressure pulsations or spikes do not cause
premature failure of rupture disks.
NOTE: Liquid-full applications can be susceptible to pressure spikes (for example, from starting or stopping a
pump) that can lead to premature bursting of a rupture disk.
10. If a rupture disk is used in lieu of a PRV, the risk of disk failure (e.g., pin hole or premature burst)
shall be reduced by
! Selecting rupture disks that are resistant to fatigue, creep, and corrosion in the specific
application.
! Consulting the Manufacturer to determine an initial time-based replacement interval.
! Providing an alarm to alert operations personnel that a disk has burst (whether
premature or legitimate). Where a large liquid flow could be passed, this alarm shall
inform the operators in the unit containing the protected equipment and the operators
managing the flare KO drum.
! Where it is important to limit further the possibility of premature burst, specifying a
properly engineered system consisting of two disks in series.
To indicate when a rupture disk has burst or is leaking, a pressure gauge (in vacuum
service) or a pressure alarm (in pressure service) should be provided between the two
disks in dual disk applications. If bursting or leaking of the first rupture disk could lead to
downstream fouling, then installation of an alarm between the rupture disks should be
considered.
NOTE: If two disks are installed in series, small pin-holes in the upstream disk could create an equal
pressure upstream and downstream of the disk, which will prevent the disk from bursting at its
marked bursting pressure.

DEP 80.36.00.30-Gen.
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2.3.7.2 Rupture disk selection
1. Rupture disks SHALL [PS] be adequate for any constant or variable vacuum and for the
maximum back pressure conditions. This may require the use of vacuum support.
2. Non-fragmenting rupture disk designs SHALL [PS] be used where fragmentation could affect the
proper operation of equipment downstream of the disk such as PRVs, control valves, and
exchangers.
3. To avoid premature failure, a sufficient operating margin shall be provided between the maximum
operating pressure of the vessel and the design bursting pressure of the disk. This operating
margin could be at least 30 % of the nominal bursting pressure for some tension-loaded disks
and could be at least 10 % of the minimum bursting pressure for reverse-buckling disks. The disk
bursting pressure is substantially influenced by temperature, and care should be taken when
specifying a rupture disk to ensure that it protects the equipment under all possible working
conditions.
The disk Manufacturer's recommended operating ratio (the ratio of maximum operating pressure
to marked burst pressure) shall be indicated on the disk data sheet. If the marked burst pressure
is less than 275 kPa (ga) [40 psig], subtract 14 kPa (2 psi) from the lower limit of the marked
burst pressure to determine the allowable operating ratio.
NOTE: This is in line with API Std 520 Part I.
The specified burst pressure SHALL [PS] be determined, taking into account the operating ratio,
the manufacturing range, and superimposed back pressure. The minimum burst pressure shall
be recorded in the rupture disk data sheet.
NOTE: Some forward acting rupture disks have a large manufacturing range and a large operating ratio, resulting in an
operating range far below the MAWP.
4. Reverse buckling rupture disks are generally preferred on account of their smaller operating
margins and manufacturing tolerances.
Reverse buckling rupture disks may be used in vapour relief systems, but SHALL [PS] not be
used in liquid full systems unless the disk is designed for such service.
Reverse buckling rupture disks that use knife blades in the disk holder SHALL [PS] not be used,
because blunt knife blades could prevent the 100 % rupture of the disk.
2.3.7.3 Disk holders
1. Rupture disks shall be installed in the corresponding Manufacturer’s rupture disk holder.
2. The Manufacturer’s holder installation instructions SHALL [PS] be followed.
NOTE: Some Manufacturers use proprietary designs to insure proper orientation of the rupture disk holder and
rupture disk.
3. The rupture disk holder shall shield the disk from contact with the piping flanges, when inserted.
2.3.8 Rupture disk and pressure relief valve combinations
In pressure service, the compartment between the disk and the PRV should have an open
outlet through a restriction orifice or excess flow check valve to the atmosphere at a safe
location. Additionally, where a rupture disk is installed upstream of a PRV to safeguard the
integrity of the system, a pressure gauge (in vacuum service) or a pressure alarm (in
pressure service) should be provided between the disk and the PRV. This will give an
indication when the rupture disk has burst or is leaking. If bursting of leaking of the rupture
disk could lead to downstream fouling, then installation of an alarm between the rupture
disk and PRV should be considered.
NOTE: If a rupture disk is installed upstream of a PRV, small pin-holes in the disk could create an equal
pressure upstream and downstream of the disk, which will prevent the disk from bursting at its marked
bursting pressure (PRV set pressure).
When a rupture disk is installed upstream of a PRV, the published combination capacity
factor (derating factor) for the specific rupture disk/PRV combination, in accordance with
ASME VIII, Division 1, UG-127 and API Std 520 Part I, SHALL [PS] be used in sizing the

DEP 80.36.00.30-Gen.
January 2010
Page 13
PRV. If a published value for the rupture disk/PRV combination is not available, a
combination capacity factor of 0.9 shall be used.
2.3.9 Pressure/vacuum vents
1. Capacities established and guaranteed by the pressure/vacuum vents (PVVs) Manufacturer
SHALL [PS] be used.
Since a weight-loaded pressure vacuum vent requires a pressure that is 80 % to 100 % higher
than set pressure to achieve full rated capacity, the set points of such devices SHALL [PS] be
low enough that the tank design pressure and vacuum rating comply with the allowable
accumulation and underpressures.
2. PVV data sheets shall clearly state the maximum relieving pressure limit.
3. Flame arrestors shall not be used in conjunction with PVVs, unless required by local regulation or
justified by risk analysis.
4. If PVV screens are required, they shall be metallic.
5. Moderate pressure storage tanks that can have pressures exceeding 18 kPa (ga) [2.5 psig] and
all refrigerated storage tanks SHALL [PS] have pipe-away PVVs to limit the thermal radiation at
the tank roof or other affected structure to 15 kW/m
2
(4758 Btu/h.ft
2
) or less. This is consistent
with requirements such as OSHA 1910.106 and ISO 28300. Typical PVVs are not the pipe-away
type and consequently this can cause flame impingement if released vapours ignite.
2.3.10 Manhole covers
1. Manhole covers (emergency tank vents) shall be provided with ASME flange bolt patterns or API
Std 650 flange bolt patterns, or as specified by the tank design standard.
2. Manhole covers shall be suitably restrained so that the cover cannot be blown off from the tank.

DEP 80.36.00.30-Gen.
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3. RELIEF DEVICE CALCULATIONS
3.1 PRESSURE RELIEF VALVE SIZING
3.1.1 General
For each pressure relief valve (PRV), the relevant relief conditions SHALL [PS] be
established in accordance with the design code. The size of the PRV SHALL [PS] be
determined from the largest relief area required for overpressure protection for all scenarios
considered. Sizing of the inlet and outlet piping of the relief valve is covered in DEP
80.45.10.10-Gen.
Combinations of a PRV with a rupture disk or a PRV with a buckling pin have capacity
reduction factors as described in (2.3.7.1) and (3.2.2), respectively.
Two phase flow can exist under relieving conditions when both liquid and vapour or gas
flow from the protected equipment to the relief device or develop in the relief device. Two
phase flow can also form due to flashing of a liquid relief or retrograde condensation of a
vapour relief. This should be checked by thermodynamic calculations.
For reporting the calculations of the PRV size for single-phase flow, the standard
calculation sheet DEP 31.36.90.94-Gen. shall be used. Use of alternative calculation
sheets requires approval by the Principal.
For reporting the calculations of the PRV size for two-phase flow, calculation sheet
DEP 31.36.90.95-Gen. shall be used. Use of alternative calculation sheets requires
approval by the Principal.
3.1.2 Vapour and gas
For calculating the PRV size for single phase vapour or gas flow, the formulae in the
current API Std 520 Part I SHALL [PS] be applied. The applicability of the sizing equations
for pressure relief devices in vapour or gas service is limited to the compressibility range of
0.8 to 1.1. If the vapour or gas is outside this compressibility range, the integral presented
in (3.1.4.1) should be evaluated.
NOTE: API interpretation 520-I-05/03 includes guidance that, for vapour/gas service in the 0.8 to 1.1
compressibility range, the use of the ratio of specific heats at relieving temperature, as an estimate of
the isentropic expansion coefficient (k), along with a compressibility (Z) of 1 will be sufficiently accurate.
3.1.3 Liquid
Where liquid certified valves are specified, the sizing method for certified relief valves (refer
to API Std 520 Part I) should be used. This ensures a more accurate size determination,
avoids oversizing and thereby minimises the probability of chatter. If chatter is a problem,
the application of a friction damper may be considered where allowed by the relevant codes
and regulations.
If a vapor trim valve is specified and there is a liquid relief scenario, then the non-certified
valve equation from API Std 520 Part I SHALL [PS] be used to determine the required liquid
relief capacity for the liquid relief scenario – see also (5.2.2).
3.1.4 Two-phase fluids
3.1.4.1 General
The mass flux of all fluids in equilibrium through a PRV is governed by the following
equation:
( )[ ]
v
dPv
v
hh
A
W
G
P
Po o
5.0
5.0
2
2 







⋅−
=
−
==
∫
[3.1]

DEP 80.36.00.30-Gen.
January 2010
Page 15
where:
G = total mass flux (kg/m
2
s) or (lb/ft
2
-s)
W = discharge rate (kg/s) or (lb/s)
A = relief area (m
2
) or (ft
2
)
h = fluid specific enthalpy (J/kg) or (Btu/lb) at P
ν = fluid specific volume (m
3
/kg) or (ft
3
/lb) at P
P = downstream pressure (N/m
2
absolute) or (psia)
subscript o indicates the upstream condition. Appropriate conversion factors are needed
when working in US Customary units.
To determine the required relief area, the calculated area SHALL [PS] be divided by the
applicable correction factors and coefficients (e.g., discharge coefficient, Kd).
For vapour, gas, and two phase venting, the relieving fluid can “choke” before reaching the
downstream pressure. When this occurs, a minimum fluid pressure is reached (“choking
pressure”) and the mass flux attains a maximum (“critical mass flux”). For a gas or vapour,
the ratio of the choke pressure to the relieving pressure, ηc, is typically about 0.55. For two-
phase venting, it is roughly 0.8 to 0.9.
For flashing and two-phase flow through a PRV, the integral equation SHALL [PS] be
evaluated. Two common methods acceptable for evaluating the integral equation are
described in (3.1.4.2) and (3.1.4.3).
NOTE: The sum-of-the-areas method for two-phase relief sizing that appeared in API Std 520 Part I through
the 6th
Edition has been shown to be non-conservative and SHALL [PS] not be used.
3.1.4.2 Integral method (numerical)
If a suitable equation of state or if sufficient thermodynamic/physical property data are
available, the integral in Equation 3.1 can be evaluated by computerised numerical
methods based on the classical homogeneous equilibrium model (HEM), which assumes
no slip between the liquid phase and the vapour phase. This integral is evaluated by
determining the fluid specific volume, ν, as the downstream pressure, P, decreases. This
can be achieved by successive isentropic flashes of the fluid starting at the PRV upstream
pressure, Po, and continuing to successively lower pressures. The integral is evaluated
over a range of downstream pressures, disregarding the back pressure, to find the
maximum mass flux. The pressure at which the maximum mass flux occurs is the critical
(choke) pressure, PC. If the choke pressure is higher than the back pressure, the integral is
evaluated to the choke pressure; otherwise, the integral is evaluated to the back pressure
(subcritical flow).
A Shell-developed module RVTP, which runs under PRO/II, may be used to carry out the
above calculation routine; see (11).
NOTE: This software is available only to Shell companies and their authorised Contractors, and to separate
licensees.
3.1.4.3 Omega method
An alternative method specified in API Std 520 Part I Appendix D, the Leung Omega
method, may be used as an approximate solution of the above integral. This method has
been developed in co-operation with DIERS (Design Institute for Emergency Relief
Systems) based on the HEM. A parameter, ω, describes the flashing tendency of the fluid
and relates the fluid specific volume to the pressure for an isentropic expansion. Omega, ω,
should be calculated by finding the specific volume at 90 % of the PRV inlet pressure (v9),
based on an isentropic flash, and inserting into the following equation:

DEP 80.36.00.30-Gen.
January 2010
Page 16
9
9 1
o
v
v
ω
 
= − 
 
[3.2]
where,
v9 = two-phase specific volume evaluated at 90 % of the relieving pressure Po
vo = two-phase specific volume at the relieving pressure
Once ω has been evaluated, the critical mass flux, Gc, can be calculated via the following
algebraic equation
ω
η
0
0
v
P
G cc ⋅= [3.3]
where
ηc = critical pressure ratio, Pc/ P0 , and can be expressed as
[3.4]
P0 = the relieving pressure
v0 = two-phase specific volume of fluid at the relieving pressure
For subcritical flow, an analogous equation is presented in API Std 520 Part I Appendix D.
For cases where the exit pressure ratio is other than 90 %, ω can be evaluated as follows,






−
−
= 1
1 0v
v
η
η
ω [3.5]
where,
η = ratio of the exit pressure to the PRV inlet pressure (Po) at which specific volume is
evaluated
v = specific volume at exit conditions
Table 3 summarises the range of Omega values and their physical significance.
Table 3 Ranges of Omega values
Omega value Description Example
ω = 0
Non-flashing liquid(s)
with no gas/vapour
silicone oil
0 < ω < 1
where
ω ≈ α0 /k
Gas/vapour (non-condensing)
with non-flashing liquid
air + silicone oil
ω = 1/k
Gas/vapour (non-condensing)
with no liquid
air
ω ≈ 1 Vapour (condensing) saturated steam
ω > 1 Contains flashing liquid water (near boiling point)
where
α0 = initial (inlet) void fraction;
k = ratio of ideal gas specific heats (approximation of isentropic expansion coefficient).

DEP 80.36.00.30-Gen.
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3.1.4.4 Special considerations
Solubilised gas: Since depressurization of a fluid with solubilised gas produces an effect
similar to flashing, API Std 520 Part 1, Appendix D, Equation D.15 shall be used to
calculate ω when using the Omega method with process simulator data.
Reactive hazards: Two-phase relief SHALL [PS] be examined. The Shell reactive hazards
specialists SHALL [PS] be consulted to determine the relief size which could include a
dynamic relief evaluation.
Two-phase sizing tools: Two-phase PRV sizing tools are often available in process
simulation packages and hydraulics packages. Only packages meeting the DIERS two
phase benchmarks and approved by the Principal shall be used (many applications have
been found to be incorrect).
Two-phase relief exclusions: For non-reactive, liquid-filled vessels exposed to a fire, the
basis for relief sizing should be vapour-only venting. Although the relief is liquid-phase
initially and two phase subsequently, at the maximum allowable relieving pressure, vapour
relief ultimately determines the relief device size.
3.1.5 Supercritical fluid
3.1.5.1 General
A fluid above its critical temperature is considered to be “supercritical”. Typically, fluids
referred to as supercritical are often above their critical pressure. If the fluid is a mixture of
components, the mixture critical temperature and pressure are referenced.
For fluids that do not undergo a phase change upon heating while at relieving conditions,
the relief load and required relief area during external fire exposure are based on
accommodating thermal expansion of the fluid. This approach applies to vapours, gases,
and dense phase (“supercritical”) fluids.
3.1.5.2 Pressure relief valve sizing
The integral equation [3.1] used for finding the mass flux in the flashing two-phase flow
section also applies to supercritical fluids. Three approaches may be used to evaluate the
mass flux:
1. Use a process simulator to find a numerical solution –
This method consists of finding the rate of volumetric expansion at the allowable relieving
pressure. The relief device SHALL [PS] be able to accommodate the maximum volumetric
expansion rate.
2. Use an isentropic expansion equation to solve the integral analytically –
This approach leads to a sizing equation that looks like the API Std 520 Part I equation
for Gas/Vapour venting. However, an applicable isentropic expansion coefficient is
used in place of the specific heat ratio.
3. Use the Leung Omega Method –
Omega should be evaluated with a process simulator by flashing the fluid at its choke
pressure (if that is not known, 70 % of the relieving pressure should be taken).
3.1.5.3 Near-critical liquid
For a liquid approaching its critical temperature, dramatic changes in properties will occur.
For example, the latent heat of vaporization approaches zero, the liquid density becomes
more vapour-like, and the heat capacity rises significantly. Common practice is to assume a
latent heat of vaporization of 116 kJ/kg (50 Btu/lb) as a conservative basis for relief load.
For revamping or expansion of existing facilities or for new facilities, a more-rigorous
evaluation should be performed, accounting for changes in fluid heat of vaporization, heat
capacity, and density in response to possible fluid heating and surpassing of the critical
temperature.

DEP 80.36.00.30-Gen.
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3.1.6 Discharge coefficients
The applicable discharge coefficient, Kd, depends on the type of fluid (at relieving
conditions) passing through the orifice as well as the relief device geometry. For preliminary
sizing, the discharge coefficients given in API Std 520 Part I (or the Manufacturer’s certified
discharge coefficients with corresponding orifice area, if available), shall be used, as
summarized in Table 4.
Table 4 Discharge coefficients
Discharge Coefficient, Kd Vapour certified Liquid certified
For gas/vapour only
relief
0.975 N/A
For all subcooled
liquid relief (including
non-flashing liquids)
0.62 0.65
For two-phase relief
(including saturated
liquids) in non-reactive
service
0.85 N/A
More detail regarding PRV certification can be found in (5.2). For any of the above cases,
the Manufacturer is responsible for verifing the sizing and substitutes its device-specific
discharge coefficients, as applicable.
Other regional or local codes might require different discharge coefficients.
3.1.7 Back pressure
3.1.7.1 General
Back pressure is the pressure that is present at the outlet of a pressure relief device as a
result of the pressure in the discharge system. It is the sum of the superimposed and built-
up back pressures. Superimposed back pressure is the static pressure that is present at the
outlet of a pressure relief device at the time the device is required to operate. It is the result
of pressure in the discharge system coming from other sources and normally includes both
constant and variable components. Built-up back pressure is the increase in pressure at the
outlet of a pressure relief device that develops as a result of flow from that relief device
after the pressure relief device opens.
3.1.7.2 Considerations
For calculation of the back pressure, refer to DEP 80.45.10.10-Gen.
For the impact of back pressure on PRV selection (conventional vs. balanced), refer to
(2.3).
3.1.7.3 Pressure relief valve back pressure correction factor
1. Balanced PRVs: For preliminary sizing, methods in API Std 520 Part I should be used.
Where available, the Manufacturer’s published capacity correction factor due to back
pressure (Kb or Kw) can be used. The Manufacturer is responsible for the final sizing of
balanced PRVs.
2. Conventional PRVs: For critical flow, the allowable built-up back pressure for a
conventional PRV is limited to the allowable overpressure and the back pressure
correction factor (Kb) is 1. For example, if the valve's allowable overpressure is 21 % of
set, then the allowable built-up backpressure is also 21 % of set. For subcritical flow,
the allowable built-up back pressure is still limited to the allowable overpressure; in this

DEP 80.36.00.30-Gen.
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Page 19
case, the coefficient of subcritical flow (F2) described in API 520 Part I is used. For
conventional PRVs in liquid service, the built-up back pressure is limited to the
allowable overpressure and the back pressure correction factor (Kw) is 1.
3. Pilot Operated Valves (POVs): The backpressure only affects POV capacity if the back
pressure causes subsonic flow. For critical flow, the back pressure correction factor (Kb)
for a pilot-operated pressure relief valve is 1. For subcritical flow, the coefficient of
subcritical flow (F2) described in API 520 Part I or the PRV Manufacturer’s curves
SHALL [PS] be used. For POVs in liquid service, the back pressure correction factor
(Kw) is 1.
3.2 NON-RECLOSING RELIEF DEVICE SIZING
3.2.1 Rupture disks
3.2.1.1 General
This section describes relief device sizing for a rupture disk providing overpressure
protection and not for a combination of a PRV and rupture disk in series. See (2.3.7.1) for
additional requirements for sizing a PRV and rupture disk in series.
For each rupture disk, the relevant relief conditions SHALL [PS] be established in
accordance with the design code.
Two sizing methods are available for rupture disks. These methods are the Coefficient of
Discharge Sizing Method and the Flow Resistance Sizing Method.
3.2.1.2 Coefficient of discharge sizing method
Sizing by the coefficient of discharge method is only allowed if all of the following conditions
are met:
! the rupture disk discharges directly to atmosphere,
! is installed within 8 pipe diameters from the vessel nozzle entry,
! has a length of discharge not greater than 5 pipe diameters, and
! has nominal diameters of the inlet and outlet discharge piping equal to or greater
than the nominal pipe size of the device.
Every one of these conditions SHALL [PS] be met; otherwise, the device SHALL [PS] be
sized according to the Flow Resistance Sizing Method.
A coefficient of discharge of 0.62 SHALL [PS] be used with this method. The required
discharge area, A, can be calculated by the appropriate PRV sizing equation for the flowing
fluid from API Std 520 Part 1. Equations include those for critical and subcritical gas/vapour
relief, steam relief, and liquid relief; Appendix D applies for two-phase relief.
3.2.1.3 Flow resistance sizing method
Sizing by the flow resistance method requires analyzing the total system resistance to flow.
This analysis SHALL [PS] take into consideration the flow resistance of the rupture disk
device, piping and other piping components, entrance and exit losses, and valves. The
effects of choking in the total system SHALL [PS] be taken into account.
The calculated relieving capacity for gas/vapour/steam/liquid venting shall be multiplied by
a factor of 0.90 to allow for uncertainties inherent with this method. For two-phase flow, a
smaller multiplicative factor might be appropriate and the Principal should be consulted to
determine this value. Flow resistance for rupture disk devices SHALL [PS] be obtained from
the Manufacturer.
3.2.2 Buckling pins
Either a “UV” and “UD” stamped buckling pin device can be used as a relief device for

DEP 80.36.00.30-Gen.
January 2010
Page 20
overpressure protection in lieu of a PRV or a rupture disk. A “UD” stamped buckling pin
device can be installed between a PRV and the protected equipment. For each buckling pin
device, the relevant relief conditions SHALL [PS] be established in accordance with the
design code. Capacities established and guaranteed by the device Manufacturer SHALL
[PS] be used in buckling pin device selection for the applicable relief conditions.
Buckling pin devices SHALL [PS] be sized and specified in accordance with the device
Manufacturer’s recommendations, ASME VIII and ASME Code Case 2091-3. The same
considerations SHALL [PS] also apply to capacity reduction factors for combinations of a
PRV and a buckling pin device.
NOTE: The sizing requirements for “UD” and “UV” stamped buckling pin devices differ; see ASME Code
Case 2091-3.

DEP 80.36.00.30-Gen.
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4. REQUISITIONING
4.1 GENERAL
The information and specifications for the pressure relief valves (PRVs) in a project shall be
entered into data sheet DEP 31.36.90.93-Gen. The information and specifications for the
rupture disks shall be entered into data sheet DEP 80.46.20.93-Gen. These data sheets
shall be used to requisition the relief devices. Use of alternative data sheets requires
approval by the Principal.
The choice of relief device Manufacturer shall be subject to the approval of the Principal.
The Manufacturer is responsible for:
- verifying the capacity of the requested relief device and confirm that the relief device will
perform as premised in requisitioning;
- completing those portions of the relief device data sheets that are the responsibility of
the Manufacturer;
- providing quotations for relief devices that meet the requirements of the relevant
requisition;
- informing the purchaser of any irregularities found in the relevant requisition.
The Manufacturer is responsible for the design and construction of the supplied relief
devices for the services and conditions specified in the requisition. Available capacities of
the selected relief devices SHALL [PS] be those established and guaranteed by the relief
device Manufacturer for the applicable service conditions. These SHALL [PS] be greater
than the required capacities.
4.2 PRESSURE RELIEF VALVES
If a PRV is provided with an actuator and/or accessories from another source, but supplied
as part of the PRV requisition, the PRV Manufacturer alone is responsible for the overall
valve assembly (valve complete with actuator and/or accessories).
4.3 RUPTURE DISKS
If a rupture disk is provided with a holder and/or accessories from another source, but
supplied as part of the rupture disk requisition, the rupture disk Manufacturer alone is
responsible for the overall disk assembly (disk complete with holder and/or accessories).

DEP 80.36.00.30-Gen.
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5. RELIEF DEVICE SPECIFICATIONS
5.1 GENERAL
The Manufacturer is responsible for supplying relief devices with material grades as
specified on the requisition.
Unless otherwise specified, the Pressure Relief Device material SHALL [PS] conform to the
project material selection report (instrument materials selection table). If no materials
selection report is available, a corrosion and materials engineer SHALL [PS] be consulted.
Any soft goods SHALL [PS] be compatible with the process at normal operating conditions
as well as during relief events and unit upsets.
5.2 PRESSURE RELIEF VALVE SPECIFICATION
5.2.1 General
Pressure relief valves (PRVs) shall be in accordance with API Std 526 and with ASME I (for
power boilers) or ASME IV (for heating boilers) or ASME VIII (for pressure vessels).
The springs of PRVs shall be given a suitable coating to protect them against general
corrosion and/or sulphide stress corrosion cracking. Coatings of cadmium or zinc shall not
be used due to the risk of liquid metal embrittlement during service and/or hydrogen
embrittlement during the galvanising process. Suitable aluminium coatings may be used.
The PRV shall be ASME VIII, UV stamped. The PRV shall be ASME vapour flow certified or
ASME liquid flow certified, consistent with applicable scenarios. These are summarised in
Table 5.
Table 5 Pressure relief valve certification
Scenario Acceptable valve
Only vapour to be relieved Vapour-certified
Only liquid to be relieved Liquid-certified
(1)
The controlling relief is vapour, but
there are non-controlling liquids to be
relieved
Vapour-certified
(2)
Controlling relief is liquid, but there
are non-controlling vapours to be
relieved
Vapour-certified
(3)
Controlling relief is two-phase Vapour-certified
NOTES: 1. Because liquid-certified valves are not certified for vapour, such valves
SHALL [PS] be limited to applications where there are no vapour relief
scenarios (including fire) unless it can be demonstrated that the valve also
provides adequate capacity for the vapour relief scenarios..
2. If blowdown is adjustable, it shall be adjusted for vapour service.
3. Specify vapour-certified valve sized for liquid reliefs, using equations for
valves not requiring capacity certification. PRVs that are only liquid flow
certified might have large blowdowns when relieving vapour. In order to
minimize the amount of product relieved, vapour-certified valves should be
selected.
Liquid only certified valves do not have certified vapour capacities. Therefore, it may be
difficult for the designer and/or the Manufacturer to determine valve capacity while relieving
vapour.

DEP 80.36.00.30-Gen.
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Page 23
Spring-loaded PRVs shall have metal-to-metal seats unless otherwise approved by the
Principal.
NOTE: Although soft-seated PRVs are often available at no extra cost, they are historically less reliable and
more costly to maintain.
5.2.2 Liquid relief
Since specific issues can arise during liquid relief, additional requirements for liquid relief
scenarios warrant special attention. If there are multiple credible liquid relief cases, the
dominant one shall be identified in the PRV data/requisition sheet (DEP 31.36.90.93-Gen.),
even if the controlling case for sizing is vapour relief.
NOTES: 1. Where the liquid certified valve equations are used in relief sizing, this should be indicated on the
PRV data/requisition sheet (DEP 31.36.90.93-Gen.) to ensure that the desired valve is purchased.
2. The discharge coefficient of a liquid certified PRV while relieving vapour can be much smaller than
the discharge coefficient of vapour-certified PRVs. The Manufacturer may not have obtained the
vapour discharge coefficient for a liquid certified valve.
5.3 SET PRESSURE CONSIDERATIONS
5.3.1 General
There shall be an adequate margin between the set pressure and the Maximum Operating
Pressure (MOP). DEP 01.00.01.30-Gen. more specifically defines the margins that should
be maintained.
Set pressures (SP) and maximum relief pressures, expressed in relation to the MAWP (or
design pressure (DP), if the MAWP is not available) of the protected equipment, all
expressed in gauge pressures, shall not exceed the MAWP, except where allowed by the
design code, see Table 6.
NOTES: See DEP 01.00.01.30-Gen. for definitions of design pressure and maximum allowable working
pressure.
Applicable codes, standards, and recommended practices provide requirements and guidance based
on equipment MAWP. Where available, the MAWP is used in the design of overpressure protection
facilities. However, the MAWP of equipment typically cannot be established during the design phase of
new facilities and the design pressure is used in lieu of the MAWP.

DEP 80.36.00.30-Gen.
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Table 6 Set and maximum accumulated pressures for ASME VIII equipment
Set pressure (SP) Maximum allowable accumulated
pressure
(3)
Non-fire conditions Fire conditions Non-fire
conditions
Fire conditions
Single
valve
100 % of DP 100 % of DP 110 % of DP 121 % of DP
Multiple
valves
At least one valve
100 % of DP
maximum settings
at 105 % of DP
(2)
110 % of DP
(1)
116 % of DP 121 % of DP
(1) PRVs for fire protection only may be set as high as 110 % of DP if they are installed in addition to adequate
relief protection of the process equipment against non-fire situations.
(2) Where a number of PRVs are provided in parallel, the set pressures may be staggered in accordance with the
Table to avoid valve chatter. However, for set pressures below 1 MPa (ga) [145 psig], staggering of set
pressures becomes impracticable because the difference between the set pressure tolerance of 3 %
(according to ASME VIII, Division 1, UG 134) and the value of 5 % of the DP becomes too small.
(3) Maximum allowable accumulated pressure is the sum of the design pressure and the maximum allowable
accumulation (as defined in DEP 01.00.01.30-Gen).
NOTE: MAWP, where available, should be substituted for DP.
Table 6 is strictly related to ASME VIII, Division 1 and Division 2. If equipment is built in
accordance with another code, that code should be applied. For example, PD 5500 does
not allow an accumulation of more than 10 % of the design pressure in any situation,
including fire; also, ASME I allows a maximum accumulation of 6 %.
NOTE: Selecting higher design pressures for equipment than prescribed by operating conditions according to
DEP 01.00.01.30-Gen. will result in lower volumetric relief rates and consequently smaller PRVs and
discharge piping. Therefore, the use of higher design pressures should be considered if it results in an
overall cost saving (e.g. if the wall thickness of the pressure vessel is not determined by internal
pressure, but by external loads, wind, transport, handling, or design for full vacuum).
The blowdown pressure (reseating pressure) shall be above the maximum operating
pressure. However, if the blowdown pressure is set too close to the set pressure, the PRV
may open and close rapidly, causing damage to the valve. For most services, the
blowdown pressure will usually be 5 % to 7 % below the valve set pressure.
5.3.2 Liquid relief
For vessels in liquid full service, relief device set pressures shall be adjusted to
compensate for static head between the relief device and the protected equipment.
For vessels that normally have a vapour space and are subjected to a liquid relief scenario,
the relief device set pressure shall not be lowered to compensate for the static head, but
the effects shall be understood when determining the relief device area required.
5.3.3 Cold differential test pressure
Conventional PRVs where normal back pressure is greater than 3 % of set pressure shall
have their cold differential test pressure (CDTP) adjusted to account for superimposed back
pressure.
It is normally not necessary to take into account the temperature correction factors in
determining CDTP unless provisions are made to maintain the valve at an elevated
temperature (e.g., heat tracing the PRV).

DEP 80.36.00.30-Gen.
January 2010
Page 25
5.4 ADDITIONAL REQUIREMENTS FOR SPECIFIC SERVICES
5.4.1 Oxygen service and high pressure air service
DEP 31.10.11.31-Gen. SHALL [PS] apply to PRVs in oxygen service and PRVs in air
service above 5 MPa (ga) [725 psig].
PRVs for oxygen service SHALL [PS] be clearly marked and SHALL [PS] be packed
separately from other PRVs.
5.4.2 Wet hydrogen sulphide service
ISO 15156 or NACE MR0103 SHALL [PS] apply.
NOTES: 1. ISO 15156 SHALL [PS] apply to oil and gas production facilities and natural gas sweetening
plants. NACE MR0175 is equivalent to ISO 15156.
2. NACE MR0103 SHALL [PS] apply to other applications (e.g., oil refineries, LNG plants and
chemical plants).

DEP 80.36.00.30-Gen.
January 2010
Page 26
6. PAINTING
Relief devices shall be prepared and painted according to the Manufacturer's standard that
is suitable for severe service in an industrial and/or marine environment at the operating
temperature and is subject to engineering review and approval, unless otherwise specified
by the Principal.

DEP 80.36.00.30-Gen.
January 2010
Page 27
7. IDENTIFICATION
Each flanged pressure relief valve (PRV) SHALL [PS] have a unique tag number (e.g., xxx-
PRV/PSV/RV-xxxx as stated in the requisition). It shall be stamped legibly on the edge of
the inlet flange by means of die stamps with characters at least 12 mm (1/2 in) high. Each
valve with a threaded inlet SHALL [PS] have a unique tag number. It shall be stamped
legibly on the valve body.
The PRV shall be fitted with a stainless steel nameplate, attached to the bonnet with
stainless steel wire. At least the following information shall be clearly stamped on the plate:
- markings specified in API Std 526 (but in SI units unless otherwise specified by the
Principal);
- tag number.
PRVs for oxygen service SHALL [PS] be tagged: "SUITABLE FOR OXYGEN SERVICE"
7.2 RUPTURE DISKS
Each rupture disk SHALL [PS] have the tag number (e.g., xxx-PSD/RD-xxxx as stated in
the requisition) stamped legibly on the Manufacturer’s disk tag attached to the downstream
side of the disk by means of die stamps with characters at least 3 mm high. If the tag
cannot be attached to the disk it should be provided separately with a hole to be attached to
the companion flange.
The markings shall be legible from the downstream side of the disk. Blockage of the tag
view should be avoided after the full assembly of the disk.
The rupture disk holder shall have a stainless steel nameplate, attached to the outside of
the holder by a spot weld or stainless steel wire. At least the following information shall be
clearly stamped on the plate:
– markings specified in ASME VIII, including size, class rating, material.
– tag number.
The flow direction SHALL [PS] be clearly marked on the disk holder.

DEP 80.36.00.30-Gen.
January 2010
Page 28
8. PROTECTION AND PACKAGING
8.1 GENERAL
All necessary precautions shall be taken for adequate protection of the relief devices during
shipment and storage. All relief devices shall be handled carefully in order to avoid
damaging them or upsetting their adjustment.
After the pressure relief valve (PRV) has been tested, metallic pipe plugs compatible with
the valve body material shall be installed in any vents or drains in the pressure-containing
sections of the valve, whether on the inlet or the outlet side. On bellows valves, the required
bonnet opening shall be provided with a permanent, screened vent fitting.
Each PRV openings shall be closed to prevent ingress of dirt and moisture. Flanges shall
be protected by wooden covers or gasketed metal covers, bolted in place. Threaded
connections shall be closed with metal pipe plugs or plastic protectors, except that the
bonnet insect-screen vent opening on balanced-bellows PRVs shall have the vent fitting
installed with weatherproof adhesive-backed tape over the screen.
Unless otherwise specified, all internal parts shall be treated with a suitable rust
preventative. See DEP 31.10.11.31-Gen for requirements that apply to PRVs in oxygen
service.
PRVs which have been tested shall be secured in the vertical upright position (bolted or
wired to the pallet) for transportation and handled with care. Protective storage shall be
provided before and after the testing and until delivery at the site.
8.3 RUPTURE DISKS
The Manufacturer is responsible for providing the rupture disk and holder as a single,
assembled unit.
The Manufacturer is responsible for riveting or welding together any vacuum supports, seal
or disk liners, and protective rings as a single unit and including the required ASME Code
data on a permanently attached tab.
Unless otherwise specified, disk shipping covers SHALL [PS] be attached to the packaging
instead of to the disk. This is to prevent inadvertent installation of shipping covers with the
disk.
8.4 BUCKLING PINS
Precautions shall be taken to prevent damage of the buckling mechanism during
transportation and installation. There SHALL [PS] be an obvious indication that the securing
device is installed and it shall be removed before commissioning of the buckling pin relief
valve.

DEP 80.36.00.30-Gen.
January 2010
Page 29
9. DOCUMENTATION
9.1 MANUFACTURER
The Manufacturer is responsible for submitting the following documents with the quotation:
- Calculations of relief device capacity;
- Dimensional outline drawing of the relief device;
- Details of where the full relief device specifications will be held.
The Manufacturer is responsible for making available the results of the inspections and
tests to the purchaser as part of a package of final certified documents and drawings.
The Manufacturer is responsible for completing a spare parts list and interchangeability
record (E-SPIR) for all equipment supplied. See DEP 70.10.90.11-Gen.
9.2 ELECTRONIC FILES
1. Documentation produced with PC-based software shall be provided in electronic form.
2. Microsoft Word and Excel shall be used to produce documentation, as applicable.
3. Unless otherwise specified, individual electronic documents that require more than one
program to produce shall be provided in a single file (e.g., Excel spreadsheet integrated
into a Word file).
4. For software used to size PRVs, the documentation that is included in the PRV folder
shall show the underlying equations.
5. Documents provided in Adobe Acrobat files shall also be provided in the native format
(e.g. Word, Excel, AutoCAD) so that future modifications can be made.
9.3 EQUIPMENT FILES
9.3.1 General
1. Equipment files referred to in this DEP include those for all pressure relief devices (e.g.,
PRVs, pressure vacuum vents, rupture disks, and buckling pins).
2. Unless otherwise specified, one set of pressure relief device equipment files shall be
issued to the Principal.
9.3.2 Calculations and narratives
Equipment files shall include calculations and narratives as follows:
1. A narrative description of any set pressure considerations that result in a cold
differential test pressure that is different from the design pressure of the limiting
component in the system.
NOTE: Examples include adjustments for static head, constant back pressure, use of MAWP in excess of
design pressure, system hydraulic losses, reactive hazards, compensation for inlet losses, etc.
2. All required area calculations or supporting data as specified in (3.1) and (3.2). This
includes pressure vacuum vents.
3. The above calculations and narratives shall be combined with documentation of relief
scenarios, relief load calculations, and hydraulic calculations derived from
DEP 80.45.10.10-Gen.
9.3.3 Miscellaneous
Equipment files shall include miscellaneous documentation as follows:
1. New relief device data sheets

DEP 80.36.00.30-Gen.
January 2010
Page 30
a. Data sheets shall be consistent with those used by the Principal.
b. Data sheets shall be provided “as built.”
2. Certified drawings of the relief device that identify all components and materials of
construction.
3. Certified Material Test Reports and Positive Materials Identification as specified in
DEP 31.10.00.10-Gen.
Unless otherwise specified by the Principal, only the minimum necessary quantity of
documentation shall be provided. For example, if a number of relief devices of the same
type are ordered, a separate operating and instruction manual is not required for each
device.

DEP 80.36.00.30-Gen.
January 2010
Page 31
10. REFERENCES
In this DEP, reference is made to the following publications:
NOTES: 1. Unless specifically designated by date, the latest edition of each publication shall be used,
together with any amendments/supplements/revisions thereto.
2. The DEPs and most referenced external standards are available to Shell staff on the SWW (Shell
Wide Web) at http://sww.shell.com/standards/.
SHELL STANDARDS
Definition of temperature, pressure, and toxicity levels DEP 01.00.01.30-Gen.
Material traceability and positive material identification (PMI) DEP 31.10.00.10-Gen.
Gaseous oxygen systems DEP 31.10.11.31-Gen.
Data requisition sheet for safety/relief valves DEP 31.36.90.93-Gen.
Relief valve calculation sheet DEP 31.36.90.94-Gen.
Relief valve calculation sheet for two-phase flow DEP 31.36.90.95-Gen.
Spare parts DEP 70.10.90.11-Gen.
Design of pressure relief, flare and vent systems DEP 80.45.10.10-Gen.
Overpressure and underpressure – Prevention and protection DEP 80.45.10.11-Gen.
Data requisition sheets for rupture disks DEP 80.46.20.93-Gen.
AMERICAN STANDARDS
Sizing, selection and installation of pressure-relieving
devices in refineries
API Std 520
Parts I and II
Petroleum, petrochemical and natural gas industries –
Pressure-relieving and depressuring systems
API Std 521
Flanged steel pressure relief valves API Std 526
Welded steel tanks for oil storage API Std 650
Issued by:
American Petroleum Institute
Publications and Distribution Section
1220 L Street Northwest
Washington DC 20005
USA
Pipe flanges and flanged fittings, NPS 1/2 through
NPS 24
ASME B16.5
ASME boiler and pressure vessel code:
Rules for construction of power boilers ASME I
Rules for construction of heating boilers ASME IV
Rules for construction of pressure vessels ASME VIII
Issued by:
American Society of Mechanical Engineers
345 East 47th Street
New York NY 10017
USA
Materials resistant to sulfide stress cracking in corrosive
petroleum refining environments
NACE MR0103
Petroleum and natural gas industries – Materials for
use in H2S-containing environments in oil and gas
production
NACE MR0175

DEP 80.36.00.30-Gen.
January 2010
Page 32
Issued by:
NACE International
1440 South Creek Dr.
Houston, TX 77084-4906,
USA
Occupational Safety and Health Standards – Hazardous
Materials – Flammable and combustible liquids
OSHA 1910.106
Issued by:
U.S. Department of Labor,
Occupational Safety & Health Administration,
200 Constitution Avenue, NW
Washington, DC 20210,
USA
BRITISH STANDARDS
Unfired fusion welded pressure vessels PD 5500
Issued by:
British Standards Institution
389 Chiswick High Road
London W4 4AL
United Kingdom
INTERNATIONAL STANDARDS
Petroleum and natural gas industries – Materials for use in
H2S-containing environments in oil and gas production
ISO 15156
Petroleum, petrochemical and natural gas industries –
Pressure-relieving and depressuring systems
ISO 23251
Petroleum, petrochemical and natural gas industries —
Venting of atmospheric and low-pressure storage tanks
ISO 28300
Issued by:
ISO Central Secretariat
1, ch. de la Voie-Creuse
Case postale 56
CH-1211 Genève 20
Switzerland
Copies can also be obtained from national standards organizations.

DEP 80.36.00.30-Gen.
January 2010
Page 33
Last page of this DEP
11. BIBLIOGRAPHY
NOTE: The following are for information only and do not form an integral part of this DEP.
Process simulation program PRO/II
Issued by:
Simulation Sciences Inc.,
601 S. Valencia Ave.,
Brea, CA 92621
USA
Relief valve calculation module for two-phase flow RVTP
Issued by:
Shell Global Solutions International B.V.
GSEMH
PO Box 541
2501 CM The Hague
The Netherlands

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