cf lecture 10 fault - event analysis

7/27/2019 CF Lecture 10 Fault - Event Analysis

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Fault/Event Analysis


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FAULT TREE ANALYSIS

Graphically displays a sequence of

faults and causes leading to ahazardous event

Uses Boolean logic with basic AND and

OR gates


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Reactor quench system example


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QUENCH

TANK

REACTOR

Inlet L2

L1

V-2

T1

T2

L3

Outlet

V-1

M

V-2 open button

V-1 close button

Temperature

Alarm


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Reactor quench system Highly unstable reaction carried out in Reactor is sensitive to small increases in

temp. Hence quench system to control reaction.

Reactor temp monitored by 2 independent sensors T1 and T2

Quench tank outlet valve V-2 activates automatically when T1 detects a certain

increase in temp.

Independently T2 activates alarm to alert operator for loss of control of reaction

Operator should then press a button to close valve V-1 to stop reactor feed.

On hearing alarm the operator is also instructed to press a button that opens V-2

in case T1 sensor failed to operate

If either V2 opens or V1 closes the reactor enters a stable shut-down condition

without damage to the system.

Top event = Reactor damage due to high temp

Analysis limited to equipment in diagram i.e. upstream and downstream of

reactor not considered.

Power failure, wiring or push button failures not taken into account.

The normal state of V-1 is open and V-2 is closed.


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Fault tree analysis advantages

Graphical - Easy to follow

Simple and logical overview of causes andinitiating events - Particularly useful when youare considering a small number of major

outcomes

Extremely powerful tool that is capable ofhandling most combinations of events -concentrates on multiple causes

Good basis for quantification


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Fault tree analysis limitations

Trees can grow rapidly

Needs experience

It is not a model of the causes of all possiblesystem failures

Can be time consuming

It is not suitable for analysing time sequences of

events such as batch processing


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Developing a Fault Tree 1

Determine the Top Event, the occurrence

which triggered the incident or accident, or

might do so.

Identify all the possible faults which mightcontribute to creating the Top Event

Write the faults on cards or sticky notes

Sort them into connected groups Arrange in logical sequences


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Building fire Central London


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Example. Top Event is - Fire Sprinkler System

did not operate

Faults might be:

Water supply to system failed

Heat detection system did not work

Water pump did not start

Sprinkler nozzles were blocked

Smoke detectors did not function


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Typical sprinkler nozzle


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Typical smoke detector


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Group items together.

Water supply group contains:

Water supply to system failed

Water pump did not start Sprinkler nozzles were blocked

Detection failure group contains:

Heat detection system did not work

Smoke detectors did not function


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Fire Sprinkler system failed to operate

Fire detection system failure Water supply to system failed

Smoke

detectors failed

Heat detectors

failed Water pump

did not start

Sprinkler nozzles

blocked


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Smoke

detectors failed

Heat detectors

failed Water pump

did not start

Sprinkler nozzles

blocked

Pump filter

missingDirt in water


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Smoke

detectors failed

Heat detectors

failed Water pump

did not start

Sprinkler nozzles

blocked

Pump filter

missing

Dirt in waterPOSSIBLE MAINTENANCE FAILURES?


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Minimal Cut Sets

Cut Sets:

A list of those failure events which will cause

the top event to occur

Minimal Cut Sets:

A list of minimal, necessary and sufficient

conditions for the occurrence of the top

event.


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Set Cuts example

Set cuts are 1,2,3 and 1,2,4, and 1,2,3&4.Minimal set cuts are 1,2,3 and 1,2,4.


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EVENT TREE ANALYSIS

Graphically displays a sequence of

possible consequences resulting froman undesirable event

Uses Boolean logic but in opposite

direction to Fault Trees, which usually

work backwards to define the causes.


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Event Tree example

Release ofRelease of

flammableflammable

gasgas

Immediate

Immediate

ignition

ignition

Gasdetector

Gasdetector

fails

fails

Firedetection

Firedetection

fails

fails

ESDfails

ESDfails

Delugefails

Delugefails

Explosion?

Explosion?

Delayed

Delayed

ignition

ignition

YesYes

NoNo

NoNo

NoNo

NoNo

NoNo

NoNo

NoNo

NoNo

NoNo

YesYes

YesYes

YesYes

YesYes

YesYes

YesYes

NoNo

NoNo

NoNo

NoNo

NoNo

YesYes

NoNo

YesYes

NoNo

NoNo

YesYes

YesYes


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Event Tree construction

Start with the initiating event

Consider the impact of every credibleoperation and the positive or negative

outcome Each outcome must be taken to its ultimatecompletion

It may be possible to reduce the number of

branches by combining those with the sameoutcome.


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Event tree analysis advantages

Graphical - Easy to followSimple and logical overview of events and

consequences

Extremely powerful tool that is capable of

handling most combinations of events

Good basis for quantification

Potential single point failures and system

weaknesses can be identified


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Event tree analysis limitations

Trees can grow rapidly

Partial outcomes are not modelled. Yes or No

only

Initiating events are considered singly

Multiple trees are needed for multiple eventsIt is not suitable for analysing time sequences of

events such as batch processing


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Event Tree example

Comparative GasReleaseEventTree (courtesyCBISS)

GasRelease Immediate Vapour Cloud forms Liquid rainout Explosion Toxic Result

Ignition? and ignites? and ignition? occurs? Chemical?

Yes Jet Fire

Yes Vapour Cloud Explosion

Yes

No FlashFire

No

Yes Pool Fire

No

Yes Toxic Exposure

No

No Possible environmentalimpact


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Terminal fire Australia

cf lecture 10 fault - event analysis

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