Fault Tree Analysis in Maintenance Principles

Fault Tree Analysis
Part 1: Introduction

General Description
•Fault Tree Analysis (FTA) is a deductive reasoning technique that
focuses on one particular accident event.
•The fault tree itself is a graphic model that displays the various
combinations of equipment faults and failures that can result in the
accident event.
•The solution of the fault tree is a list of the sets of equipment
failures and human/operator errors that are sufficient to result in
the accident event of interest.
•The strength of FTA as a qualitative tool is its ability to break
down an accident into basic equipment failures and human errors.
This allows the safety analyst to focus preventive measures on
these basic causes to reduce the probability of an accident.

Purpose: Identify combinations of equipment failures
and human errors that can result in an accident event.
When to Use:
a. Design: FTA can be used in the design phase of
the plant to uncover hidden failure modes that
result from combinations of equipment failures.
b. Operation: FTA including operator and procedure
characteristics can be used to study an operating
plant to identify potential combinations of failures
for specific accidents.

Type of Results: A listing of sets of equipment and/or
operator failures that can result in a specific accident.
These sets can be qualitatively ranked by importance.
Nature of Results: Qualitative, with quantitative
potential. The fault tree can be evaluated quantitatively
when probabilistic data are available.

Data Requirements:
a. A complete understanding of how the plant/system
functions.
b. Knowledge of the plant/system equipment failure
modes and their effects on the plant/system.

Staffing Requirements
• One analyst should be responsible for a single fault tree,
with frequent consultation with the engineers, operators,
and other personal who have experience with the
systems/equipment that are included in the analysis.
• A team approach is desirable if multiple fault trees are
needed, with each team member concentrating on one
individual fault tree. Interactions between team members
and other experienced personnel are necessary for
completeness in the analysis process.

Time and Cost Requirements: Time and cost
requirements for FTA are highly dependent on the
complexity of the systems involved. Modeling a
small process unit could require a day or less with an
experienced team. Large problems, with many potential
accident events and complex systems, could require
several weeks even with an experienced analysis team.

FRC
TIS
)
MATERIAL
A
FLOW
CONTROL
VALVE
MATERIAL
B
FLOW
CONTROLLER
EMERGENCY
SHUT-OFF
VALVE
HIGH TEMP
INTERLOCK
BURSTING
DISC

REACTOR EXPLOSION
RUNAWAY
REACTION
BURSTING
DISC FAILS
FLOW CONTROL
LOOP FAILS
TEMPERATURE
INTERLOCK FAILS
FLOW
CONTROLLER
FAILS
THERMO -
COUPLE &
RELAY FAIL
VALVE
STICKS
OPEN
VALVE FAILS
TO CLOSE
3.6  10-4 F/YR
1.8  10-2 F/YR
0.3 F/YR
0.2 F/YR 0.1 F/YR
0.02
Probability
of failure
on demand
0.05
Probability
of failure
on demand
0.01
Probability
of failure
on demand
0.06

Gate Symbol Gate Name Causal Relation
1
2
3
AND gate
OR gate
Inhibit gate
Output event occurs if all input events occur
simultaneously.
Output event occurs if any one of the input events
occurs.
Input produces output when conditional event
occurs.
Table 2.1 Gate Symbols

Gate Symbol Gate Name Causal Relation
4
5
6
Priority
AND gate
Exclusive
OR gate
m
Out of
n gate
(voting or
sample gate)
Output event occurs if all input events occur in the
order from left to right.
Output event occurs if one,but not both, of the
input events occurs.
Output event occurs if m out of n input events
occur.
Table 2.1 Gate Symbols
m
n inputs

Event Symbol Meaning of Symbols
1
2
3
Basic event with sufficient data
Undeveloped event
Event represented by a gate
Table 2.2 Event Symbols
Circle
Diamond
Rectangle

Event Symbol Meaning of Symbols
4
5
6
Conditional event used with inhibit gate
House event. Either occurring or not occurring
Transfer symbol
Table 2.2 Event Symbols
Oval
House
Triangles

Classification of Failures
• Sudden versus gradual failures
• Hidden versus evident failures
• According to effects (critical, degraded or
incipient)
• According to severity (catastrophic, critical,
marginal or negligible)
• Primary failure, secondary failure and
command fault

Component Failure Characteristics
• Primary failure: component within design
envelope (natural aging)
• Secondary failure: excessive stresses
(neighboring components, environment,
plant personnel)
• Command fault: inadvertent control signals
or noises (neighboring components,
environment, plant personnel)

COMPONENT FAILURE CHARACTERISTICS
Primary Faults and Failures
Primary faults and failures are equipment malfunctions that occur in the
environment for which the equipment was intended. These faults or failures are
the responsibility of the equipment that failed and cannot be attributed to external
force or condition
Secondary Faults and Failures
Secondary faults and Failures are equipment malfunctions that occur in an
environment for which the equipment was not intended. These faults or failures
can be attributed to some external force or condition.

COMPONENT FAILURE CHARACTERISTICS
Command Faults and Failures
Command faults and failures are equipment malfunctions in which the component
operates properly but at the wrong time or in the wrong place. These faults or
failures can be attributed to the source of the incorrect command.
when the exact failure mode for a primary or secondary failure is identified, and
failure data are obtained, primary and secondary failure events are the same as
basic failures and are shown as circles in a fault tree.

[ EXAMPLE ]
1) Primary
2) Secondary
3)Command
• Tank rupture due to metal fatigue
• Fuse is opened by excessive current
• Earth quake cracks storage tanks
• Pressure vessel rupture because some faults external to the vessel
causes the internal pressure to exceed the design limits.
• Power is applied inadvertently to relay coil.
• Noisy input to safety monitor randomly generate spurious shutdown
signals.

Boolean Algebra
• AND: all the inputs
are required to cause
the output.
A
AND
B C
A
AND
C B
=

Boolean Algebra
• Inclusive OR: any
input or combination
of inputs will cause
the output.
A
OR
B C
A
OR
C B
=

Boolean Algebra
A
EOR
B C
Exclusive OR: B or C
but not both cause the
the output A.

Boolean Algebra
EOR OR
= =
A
B
A
B
A
B

Boolean Algebra
A
AND
B AND
A
AND
B D
=
C D
C

Boolean Algebra
A
OR
B OR
A
OR
B D
=
C D
C

Boolean Algebra
A
EOR
B EOR
A
“EOR”
B D
=
C D
C
ODD COMBINATIONS

Boolean Algebra
A
AND
B OR
A
OR
AND AND
=
C D
B C B D

Boolean Algebra
A
OR
B L
A
OR
=
(very low
probability)
B

Boolean Algebra
A
AND
B L
A
AND
C L
=
(very low
probability)
(very low
probability)
(very low
probability)

Boolean Algebra
A
OR
B AND
A
OR
=
C L
B
(very low
probability)

Boolean Algebra
A
AND
B H
A
=
(very high
probability)
B

Boolean Algebra
A
OR
B H
A
OR
C H
=
(very high
probability)
(very high
probability)
(very high
probability)

Boolean Algebra
A
AND
B OR
A
=
C H
B
(very high
probability)

Fault Tree Analysis in Maintenance Principles

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