HAZOP - Class Notes

8/14/2019 HAZOP - Class Notes

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plant. If considered necessary, action is then taken to

remedy the situation.

Example: Consider a scenario

You were driving a car at 100 km/h in the middle of

the night. It was raining heavily. Whilst replying to a

text message the car hits a deep hole and one of your

tire blows.

You hit the brake, but due to slippery road and thintire thread, the car skidded and was thrown off the

road.

We ask the following questions:

What was the cause of the accident?What was the consequence of the event?

What could we have done to prevent all those things

happening?

What other possible accidents might happen on the

trip?

Can we be prepared before the accident occurs?

A systematic approach

PARAMETER GUIDE

WORDCAUSE CONSEQUENCE SAFEGUARDS ACTION


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Parameter

Relevant for the condition(s) of the process (e.g.

pressure, temperature, composition).

Guideword

A short word to create an imagined deviation of the

design/process intent. Most common: no, more, less,

as well as, part of, other than, and reverse.

Deviation

from design intent by combining guide words (No,

more, less, etc.) with process parameters resulting in a

possible deviation from design intent. e.g., when the

guide word "less" is combined with the parameter

"flow" the deviationless flow" results.Guide-word + Parameter = Deviation


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Cause

The reason(s) why the deviation could occur. Several

causes may be identified for one deviation.

Consequences

The results of the deviation, in case it occurs. May be

both process hazards and operability problems,

like plant shut-down or reduced quality of the

product.

Several consequences may follow from one causeand, in turn, one consequence can have several causes

Safeguard

Facilities that help to reduce the occurrence frequency

of the deviation or to mitigate its consequences. Thereare, in principle, five types of safeguards:

1. Identify the deviation (e.g., detectors and alarms)

2. Compensate for the deviation (e.g., control system

that reduces feed to a vessel in case of overfilling it -

usually an integrated part of the process control)

3. Prevent the deviation from occurring (e.g., an inertgas blanket in storages of flammable substances)

4. Prevent further escalation of the deviation (e.g., by

(total) trip of the activity.)

5. Relieve the process from the hazardous deviation

(e.g., pressure safety valves (PSV) and vent systems)


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FLASH DRUM EXAMPLE

Consider the Flash drum system. The purpose is to

separate multiple mixture of mostly A and B plus

some other heavy components to produce main

product leaving the top at 90 mol % of A. Steam at 5

bar is used to bring the temperature at the desired

saturation temperature of 130oC.

Nominal operating conditions are as follows:

T1= 70oC

T2= 120oC

T3= 100o

CT4= 110

oC

P9= 3 bar


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Consider the Flash Drum as the HAZOP Study Node,

Conduct HAZOP

A nodeis the specific location in the process in which

(the deviations of) the process intention are evaluated.

Examples might be: separators, heat exchangers,

scrubbers, pumps, compressors, and interconnecting

pipes with equipment

Separator as a node

Other nodes: process to process heat exchanger,

utility heat exchanger

Alternatively, each process line can be taken as anode.


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Design Intent

Description of how the process is expected to behave

at the Study Node.

To separate light components from the heavies (or tocollect light component at the top product stream)

Deviation

The guide word "less" is combined with the

parameter "level" the deviation " less level" results.

Causes

The reason(s) why the DEVIATION could occur

Flash Drum Example

Causes for LESS LEVEL in V-40

FCV8 stuck open

LESS inlet FlowMORE Flow in line 8, etc


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Consequences

The results of the DEVIATION, in case it occurs

Consequence for LESS LEVEL in V40

V-40 empty, leading to pump P8 running dry

No separation

Safeguard

Sometimes termed as Protection, or Existing

These are facilities that help to reduce the occurrencefrequency of the DEVIATION or to mitigate its

CONSEQUENCES.

Safeguard against LESS LEVEL in V40

LIC8 controlling the liquid level

Action/Recommendation

Where a credible cause results in a negative

consequence, it must be decided whether some action

should be taken.

At this stage consequences and associated safeguardsare considered. If protective measures are adequate,

then no action need be taken.

Actions fall into two groups:

Actions that remove the cause.

Actionsthat mitigate or eliminate the consequences.


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Existing Safeguard for LESS LEVEL is considered

not adequate.

Actions proposed that mitigate or eliminate the

consequences.

Add Low level Alarm (LAL and LALL)

Add trip to stop pump P8 when LALL triggered.

Flash Drum: Less Level

Deviation Causes Consequences Safeguards ActionLessLevel

(Low

Level)

ValveFCV8 stuck

open

Less feedLIC8

malfunction

Level dropleading to dry

separator,

hence noseparation

Risk ofpump running

dry (damage)

LIC8 Install LAL

Install

LALL with

SIS to stoppump P8

Now, consider MORE PRESSURE as deviation

Deviation Causes Consequences Safeguards ActionMore

Pressure

(High

Pressure)

Valve FCV9

stuck Close

PIC

Malfunction

increase in

pressure

leading to risk

of explosion

PIC8 Install

PAH

Install

Pressure

relievevalve

More

temperature

in Feed

TIC7


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Process HAZOP worksheet

Hazards and Operability Review

Project Name: Date: Page of

Process :

Section: Ref. Drawing:

Item Study

node

Process

Parameter

Deviations

(guide

words)

Possible

causes

Possible

consequences

Action Required

`

Deviation- realistic if there are sufficient causes to

believe the deviation can occur

Three basic types of causes that could result in arelease of hazardous or flammable material:

1. Human error - acts of omission or commission

creating a hazard.

2. Equipment failure - a mechanical, structural or

operating failure.3. External Events - items outside the unit affects the

operation of the unit.

External events include upsets on adjacent units

affecting the safe operation of the unit (or node) being

studied, loss of utilities, and exposure from weather

and seismic activity.

Modes of operation to consider

The following modes of plant operation should be

considered for each node:

Normal operationReduced throughput operation


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Routine start-up

Routine shut-down

Emergency shutdown

Commissioning

Special operating modes

Documents

HAZOP - Class Notes