Understanding and Preparation of Cause & Effect Diagram

Understanding and Preparation of Cause & Effect Diagram

Sreejesh.S

©2012 QuEST Global Services Pte Ltd. The information in this document is the property of QuEST Global Services Pte Ltd. and may not be copied or communicated to a third party or used for any purpose other than that for which it is supplied without the written consent of QuEST Global Services Pte Ltd.

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©2012 QuEST Global Services Pte Ltd. Confidential 2

Abstract

Carrying out safety analysis is important to prevent accidents and helpCarrying out safety analysis is important to prevent accidents and helpin the design of control and protective systems for process plants.Control and sensor devices and their related control actions can bepresented in a Cause & Effect chart to help with the analysis process.p p y p

The "Cause & Effect" chart is the preliminary document used for thedevelopment of Interlock. It is applicable to Emergency shutdowns /Process shutdowns. It lists the manual and automatic actions (Cause)and the equipment such as control valves, pumps etc. (Effect) to beput into a safe state.


Safety actions Classification

"ESD" (Emergency Shut Down) This causes aESD (Emergency Shut Down). This causes acomplete or partial shutdown of a process unit. AnESD can also isolate a process equipment item asESD can also isolate a process equipment item aswell as a "package" equipment.

"PSD" (Process Shut Down). This is anautomatic action initiated by one or severali t t i t ll d i th i d tinstruments installed in the process in order toprotect the equipment.


Logic identification

All causes and effects associated with a specificAll causes and effects associated with a specificunit of a process must be shown on the C & EMatrix.Matrix.

Several Process units may also be grouped on thesame C & E Matrix.

I t l k b h ll b i di t d i C & EInterlock numbers shall be indicated in C & Ematrix as per PID as a base for logic identification


Cause

The causes are defined by the "TAG Number"and by the "SERVICE DESCRIPTION“and by the SERVICE DESCRIPTION

"SERVICE DESCRIPTION": "Low pressure, Highlevel etc". need not be shown in the C & E matrix.

"NOTES" shall be used to explain special"NOTES" shall be used to explain specialrequirements and to be indicated on the matrix.The section reserved for notes may be modifiedThe section reserved for notes may be modifiedaccording to the volume of information to bedetailed.


Other Logic representation

When the “Cause” of a shutdown comes fromanother logic system it will be indicated in thisanother logic system it will be indicated in thiscolumn: C&E

When several causes generate an action indifferent process units only the reference of thisdifferent process units, only the reference of thislogic will be shown in the logic systemsconcernedconcerned.


Override

Only "Operational" bypasses will be shown on the C&E.C&E.

The by-pass has a direct action on the cause:


Effects

The effects are defined by the "TAG Number” byThe effects are defined by the TAG Number”, bythe "SERVICE DESCRIPTION" and by the type ofaction detailed on the "MATRIX"action detailed on the MATRIX

It may be necessary to double the effects whendue to the causes a command can have an openor close or stop or start action.


Service description

It will not normally be necessary to indicate in theIt will not normally be necessary to indicate in thedescription information shown on the matrix suchas "Open", "Start".as Open , Start .

When the cause of a shutdown is used ondifferent pages of a C&E for the same logicfunction, the continuation is indicated on the lineSERVICE DESCRIPTIONSERVICE DESCRIPTION.


Actions

Description Remarks

A Activate Indicates an action on another logic system (another C&E)

C Close Closes the valveO Open Opens the valveSt Start Starting a pump or Unit or sequenceSp Stop Stopping a pump or UnitSp Stop Stopping a pump or Unit T Trip Trip a motor or UnitI Inhibit Forbids the action when the cause is presentP Permissive Authorises the action when the cause is present. For example authorises the

opening of a valve if other valves are closed. The action will be initiated by theoperator or the system.

R Reset Authorises a re-start, a re-opening after an ESD and return to normal conditions.p gThis action is generally carried out by the operator.

V OVerride This is a special case of inhibition.


Procedure

1 Identify the various interlock section unit and1. Identify the various interlock section, unit andplant based on the Process description and PID’s.

2. Identify the “Causes” based on Section / Unit /Plant. Provide Tag numbers, if required.

3. Identify the “Effects” and relate the same toi b d th d i tivarious causes based on the process description

& PID’s.


Procedure …….

4 “Effects” due to the presence of all causes4. Effects due to the presence of all causes(Single Cause should not lead to the effect) shallbe adequately defined through notes.be adequately defined through notes.

5. “Causes” which are to be effectuated onrunning equipment, the same shall be adequatelydefined through notes. For eg. In a Cooling water

t Wh fi t i i dsystem, When first pump is running andmaintaining the pressure, a sudden fall in coolingwater pressure should start the second pump in


water pressure should start the second pump inorder to maintain the header pressure.

Special Case – Operational bypass

This function is indicated in the column OVERRIDE TAGnumber in the section CAUSE of the diagram.g

Its function is to temporarily inhibit a cause:At unit start up (start up override) i e an FSLL or PSHHAt unit start-up (start-up override), i.e. an FSLL or PSHHwhich closes a valve on the same circuit on which it isinstalled. It may be:

• Automatic, i.e. the bypass is activated by a "Start" or"Reset" command.Reset command.• Manually activated by the operator i.e. HS.

FSLL - Flow Switch Low Low


FSLL - Flow Switch Low Low

PSHH - Pressure Switch High High

Special Case – Operational bypass….

Automatic cancellation of the bypass is mandatoryafter a time delay and if the operating conditionsafter a time delay and if the operating conditionshave returned to normal. This will be defined in a"typical" drawing and will not be shown on theC&E. This is applicable during startup.

The bypass of a cause is either applicable to allThe bypass of a cause is either applicable to alleffects or to a limited number of effects.

An operational bypass may be necessary to allowcertain unit operational cases. When this is the


case it will be detailed in a note.

Voting

Only the result of a vote is indicated as a cause.

A note will indicate the type of vote:

1 out of 2 2 out of 2 2 out of 3

The tag numbers of the instruments are listed in the instrument index and the data base.


Information external to Cause & effect chart

Certain information found in the project jobspecifications need not be shown on the C&E suchspecifications need not be shown on the C&E suchas:

Maintenance bypasses (Maintenance Override)


Sample Cause & effect chart


Reference

Instrument Engineers' Handbook Volume 2 by Bella G LiptakInstrument Engineers' Handbook – Volume 2 by Bella G Liptak

http://www.aidic.it

http://instrumentationportal.com


Thank YouSreejesh.S

©2012 QuEST Global Services Pte Ltd. The information in this document is the property of QuEST Global Services Pte Ltd. and may not be copied or communicated to a third party or used for any purpose other than that for which it is supplied without the written consent of QuEST Global Services Pte Ltd.

[email protected]

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Understanding and Preparation of Cause & Effect Diagram