fault tree analysis 1

8/17/2019 Fault Tree Analysis 1

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FAULT TREE ANALYSIS


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Fault Tree Analysis (FTA)

A FTA is a deductive (top-down) approach that

graphically and logically represents events at a lower

level which can lead to a top undesirable event.

A common tool using graphics and statistics toanalyze an event and predict how and how often it

will fail.

Fault tree analysis is a graphical representation of the major faults or critical failures associated with

a product, the causes for the faults, and potential

countermeasures.


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Applying Fault Tree Analysis

EVENT SYMBOLS:

Basic Event

Conditioning Event

Undeveloped Event

External Event

Primary Event Symbols Intermediate Event Symbol


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EVENTS:

The BASIC event indicates a basic initiating event at the limit of

resolution; i.e. we do not wish to develop this fault any further

in this particular analysis.

The UNDEVELOPED event is undeveloped because there we

either lack information, or the event is of no consequence.

The EXTERNAL event is an event that is expected to happen in

the course of normal operation of the system.

The INTERMEDIATE event is neither the top-most undesiredevent, nor a primary event. Further events are attached to the

INTERMEDIATE event, usually via gates.

The final event is the CONDITIONING event. It is used

primarily with the INHIBIT and PRIORITY AND gates


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GATE SYMBOLS

Gate Symbols

AND

OR

Exclusive OR

Priority AND

Inhibit


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GATES

The AND gate indicates that the output fault (drawn

above the gate) only occurs if the two (or more) input

faults (drawn below the gate) occur.

The OR gate indicates that the output fault occurs if at least one of the two (or more) input faults occur.

The EXCLUSIVE OR gate indicates that the output fault

occurs if exactly one of the two (or more) input faults

occur.

The PRIORITY AND gate indicates that the output fault

only occurs if all the input faults occur in a specified

order (left to right on the page).


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GATES

The INHIBIT gate is another special case of the

AND gate: The output event is caused by a single

input event, but a particular condition (given by a

CONDITIONING event) must be satisfied beforethe input can produce the output.

This condition may be probabilistic. The difference

from the AND gate here is one of emphasis: the

condition given is not a fault or external event as

such, but something to do with the environment,

e.g. a particular temperature.


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FTA:

The two transfer symbols, TRANSFER IN and

TRANSFER OUT allow a large tree to be broken upinto multiple pages; it also allows for the analysis

to be broken into more manageable parts, and for

duplication to be reduced.

Transfer Symbols

Transfer IN Transfer OUT


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FTA:EXAMPLE

AND OR

PSV does not

relieve

Process

pressure

rises

Control

fails high

PSV too

small

Set point

too high

PSV stuck

closed

Fouling inlet

or outlet

Pressure

rises

Process

vessel over

pressured

AND


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PRILIMINARY STEPS

Define precisely the top event. Events such as "high

reactor temperature" or "liquid level too high" are

precise and appropriate. Events such as "explosion of

reactor" or "fire in process" are too vague, whereas anevent such as "leak in valve” is too specific.

Define the existing event. What conditions are sure to

be present when the top event occurs?

Define the unallowed events. These are events that areunlikely or are not under consideration at the present.

This could include wiring failures, lightning

tornadoes, and hurricanes.


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PRILIMINARY STEPS

Define the physical bounds of the process. What

components are to be considered in the fault tree?

Define the equipment configuration. What valves are

open or closed? What are the liquid levels? Is this anormal operation state?

Define the level of resolution. Will the analysis

consider just a valve, or will it be necessary to

consider the valve components?


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PROBABILITY

Once a fault tree has been constructed, then we

can consider calculating probabilities of failure.

Essentially, for each event, we attempt to assign a

probability. For primary events, this probabilitymust be derived from external information.

For all other events, their probability is calculated

from their immediate descendant events and theintervening gate.


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ADVANTAGES

Quantitative - defines probabilities to each event

which can be used to calculate the probability of

the top event

Easy to read and understand hazard profile

Easily expanded to bow tie diagram by addition of

event tree.

Effective way to diagram problems in a system Helps to organize possible causes of a problem in

the system.


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LIMITATIONS

Time-consuming in constructing the graphs

Many systems involve many dependencies

Difficult to detect inconsistencies

Difficult to focus only on the most critical parts of the design on high

coupling systems

Not always easy to spot

Fault trees also assume that failures are "hard," that a particular item of

hardware does not fail partially. Leaking valve is a good example of a

partial failure.

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