HAZARD ANDOPERABILITY STUDY

• Brainstorming, Multidisciplinary Team Approach• Structured Using Guide Words• Problem Identifying• Cost Effective

When to Use?

Optimal from a cost viewpoint

1.when applied to new plants at the point where the design is nearly firm and documented or

2. to existing plants where a major redesign is planned.

It can also be used for existing facilities.

Results

Types: The results are the team findings. Which include: (1) identification of hazards and operating problems, (2) recommended changes in design, procedure, etc., to improve safety; and (3) recommendations for follow-on studies where no conclusion was possible due to lack of information.

Nature: Qualitative.

Requirements

Data: The HazOp requires detailed plant descriptions, such as drawings, procedures, and flow charts. A HazOp also requires considerable knowledge of the process, instrumentation, and operation, and this information is usually provided by team members who are experts in these areas.

Staff: The HazOp team is ideally made up of 5 to 7 professionals, with support for recording and reporting. For a small plant, a team as small as two or three could be effective.

Time and Cost

The time and cost of a HazOp are directly related to the size and complexity of the plant being analyzed. In general, the team must spend about three hours for each major hardware item. Where the system analyzed is similar to one investigated previously, the time is usually small. Additional time must be allowed for planning, team coordination, and documentation. This additional time can be as much as two three times the team effort as estimated above

HAZOP STUDY - TEAM COMPOSITION

A Team Leader, an expert in the HAZOP Technique

Technical Members, for example

New Design Existing Plant

Design or Project Engineer Plant Superintendent

Process Engineer Process Supervisor (Foreman)

Commissioning Manager Maintenance Engineer

Instrument Design Engineer Instrument Engineer

Chemist Technical Engineer

Principles of HAZOP

Concept

•Systems work well when operating under design conditions.

•Problems arise when deviations from design conditions occur.

Basis

•a word model, a process flow sheet (PFD) or a piping and instrumentation diagram (P&ID)

Method

•use guide words to question every part of process to discover what deviations from the intention of design can occur and what are their causes and consequences may be.

PRINCIPLES OF HAZOPS

GUIDE WORDS*NONE

MORE OF

LESS OF

PART OF

MORE THAN

OTHER

CAUSE DEVIATION CONSEQUENCES (from standard (trivial, important, condition catastrophic) or intention) -hazard -operating difficulties*COVERING EVERY PARAMETER RELEVANT TO THE SYSTEM UNDER REVIEW: i.e. Flow Rate. Flow Quantity, Pressure, Temperature, Viscosity, Components

STUDY NODES

The locations (on P&ID or procedures) at which the process parameters are investigated for deviations. These nodes are points where the process parameters (P, T, F etc.) have an identified design intent.

INTENTION

The intention defines how the plant is expected to operate in the absence of deviations at the study nodes.

DEVIATIONS

These are departures from the intension which can be discovered by systematically applying the guide words.

•Process conditions•activities•substances•time•place

GUIDE WORDS

Guide Words

No, None

More Of

Less Of

As Well As (More Than)

Part Of

Reverse

Other Than

Meaning

Negation of Intention

Quantitative Increase

Quantitative Decrease

Qualitative Increase

Qualitative Decrease

Logical Opposite of Intention

Complete Substitution

Deviations Generated by Each Guide Word

Guide word Deviations

NONE No forward flow when there should be, i.e. no flow.

MORE OF

More of any relevant physical property than there shouldbe, e.g. higher flow (rate or total quantity), highertemperature, higher pressure, higher viscosity, etc.

LESS OF Less of any relevant physical property than there should be,e.g. lower flow (rate or total quantity), lower temperature,lower pressure, etc.

PART OF Composition of system different from what it should be,e.g. change in ratio of components, component missing, ect.

MORE THAN More components present in the system than there shouldbe, e.g. extra phase present (vapour, solid), impurities (air.Water, acids, corrosion products), etc.

OTHER THAN What else can happen apart from normal operation, e.g.start-up, shutdown, uprating, low rate running, alternativeoperation mode, failure of plant services, maintenance,catalyst change, etc.

REVERSE: reverse flow

EXAMPLE

The flowsheet shows that raw material streams A and B are transferred by pump to a reactor, where they react to form product C. Assume that the flow rate of B should not exceed that of A. Otherwise, an explosion may occur. Let’s consider the flow of A in line 1:

NONE No flow of AMORE Flow of A greater than design flowLESS Flow of A less than design flowAS WELL AS Transfer of some component additional to APART OF Failure to transfer a component of AREVERSE Flow of A in a direction opposite to design directionOTHER THAN Transfer of some material other than A

A B

C

B

AB FF

1

2

4

3

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

Select a vessel

Explain the general intention of the vessel and its lines

Select a line

Explain the intention of the line

Apply the first guide words

Develop a meaningful deviation

Examine possible causes

Examine consequences

Detect hazards

Make suitable record

Repeat 6-10 for all meaningful deviations derived from first guide words

Repeat 5-11 for all the guide words

Mark line as having been examined

Repeat 3-13 for each line

Select an auxiliary system (e.g. Heating system)

Explain the intention of the auxiliary system

Repeat 5-12 for auxiliary system

Mark auxiliary as having been examined

Repeat 15-18 for all auxiliaries

Explain intention of the vessel

Repeat 5-12

Mark vessel as completed

Repeat 1-22 for all vessels on flow sheet

Mark flow sheet as completed

Repeat 1-24 for all flow sheets

Beginning

End

Figure 8.9 Hazard and operability studies : detailed sequence of examination

(Chemical Industry Safety and Health Council, 1977 Item 6)

HAZOP DISPLAY

Guide Word Deviation Possible Causes Consequences Action Required

No

More

No Flow

MoreFlow

Pump Fail

Line Blockage

OperatorStops Pump

ExcessivePump Speed(Control System)

System Over-Heated

Over-CooledProduct(IncompleteReaction)

ShutdownSystem

ProductUnacceptable;Dump

EXAMPLE

An alkene/alkane fraction containing small amounts of suspended water is continuously pumped from a bulk intermediate storage tank via a half-mile pipeline into a buffer/settling tank where the residual water is settled out prior to passing via a feed/product heat exchanger and preheater to the reaction, is run off manually from the settling tank at intervals. Residence time in the reaction section must be held within closely defined limits to ensure adequate conversion of the

alkene and to avoid excessive formation of polymer.

Results of hazard and operability study of proposed olefinedimerization unit: results for line section from intermediate storage to buffer/settling tank

Guide word Deviation Possible causes Consequences Action required NONE No flow (1)No hydrocarbon available

at intermediate storage.

(2)J1 pump fails (motor fault, loss of drive, impeller corroded away etc.)

(3)Line blockage, isolation valve closed in error, or LCV fails shut.

(4)Line fracture

Loss of feed to reaction section and reduced output. Polymer formed in heat exchanger under no flow conditions.

As for (1)

As for (1) J1 pump overheats.

As for (1)

Hydrocarbon discharged into area adjacent to public highway.

(a) Ensure good communications with intermediate storage operator

(b)Install low level alarm on settling tank LIC.

Covered by (b)

Covered by (b) (c)Install kickback on J1 pump.

(d)Check design of J1 pump strainers.

Covered by (b)

(e)Institute regular patrolling & inspection of transfer line.

(1)

Results of hazard and operability study of proposed olefinedimerization unit: results for line section from intermediate storage to buffer/settling tank

Guide word Deviation Possible causes Consequences Action required MORE OF More flow

More pressure

More temperature

(5)LCV fails open or LCV bypass open in error.

(6)Isolation valve closed in error or LCV closes, with J1 pump running.

(7)Thermal expansion in an isolated valved section due to fire or strong sunlight.

(8)High intermediate storage temperature.

Settling tank overfills.

Incomplete separation of water phase in tank, leading to problems on reaction section.

Transfer line subjected to full pump delivery or surge pressure.

Line fracture or flange leak.

Higher pressure in transfer line and settling tank.

(f)Install high level alarm on LIC and check sizing of relief opposite liquid overfilling.

(g)Institute locking off procedure for LCV bypass when not in use.

(h)Extend J2 pump suction line to 12’’ above tank base.

(j)Covered by (c) except when kickback blocked or isolated. Check line. FQ and flange ratings and reduce stroking speed of LCV if necessary. Install a PG upstream of LCV and an independent PG on settling tank.

(k)Install thermal expansion relief on valved section (relief discharge route to be decided later in study).

(l)Check whether there is adequate warning of high temperature at intermediate storage. If not, install.

(2)

Results of hazard and operability atudy of proposed olefinedimerization unit: results for line section from intermediate storage to buffer/settling tank

Guide word Deviation Possible causes Consequences Action required LESS OF

PART OF

MORE THAN

OTHER

Less flow

Less temperature

High water concentration in stream.

High concen- tration of lower alkanes or alkenes in stream.

Organic acids present

Maintenance

(9)Leaking flange of valved stub not blanked and leaking.

(10)Winter conditions.

(11)High water level in intermediate storage tank.

(12)Disturbance on distillation columns upstream of intermediate storage.

(13)As for (12)

(14)Equipment failure, flange leak, etc.

Material loss adjacent to public highway.

Water sump and drain line freeze up.

Water sump fills up more quickly. Increased chance of water phase passing to reaction section.

Higher system pressure.

Increased rate of corrosion of tank base, sump and drain line.

Line cannot be completely drained or purged.

Covered by (e) and the checks in (j).

(m)Lag water sump down to drain valve and steam trace drain valve and drain line downstream.

(n)Arrange for more frequent draining off of water from intermediate storage tank. Install high interface level alarm on sump.

(p)Check that design of settling tank and associated pipework, including relief valve sizing, will cope with sudden ingress of more volatile hydrocarbons.

(q)Check suitability of materials of construction.

(r)Install low-point drain and N2 purge point down- Stream of LCV. Also N2 vent on settling tank.

(3)

HAZOP PREPLANNING ISSUES

Preplanning issues addressed in a typical refinery unit HAZOP include the following:

• Verification of as-built conditions shown on the P&IDs• Line segment boundaries set; markup of P&IDs• List of support documents compiled • P&IDs (base study document) • Process flow diagrams (PFDs) • Process description • Operating manuals/procedures • Processing materials information • Equipment and material specifications• Tentative schedules of time to be spent per P&IDs sheet• Recording technique (computer program or data sheet) determination• List of standard abbreviations and acronyms compiled• Criticality rankings devised• HAZOP training given to all team members (one day)• Arrange for system or process briefings for team before work begins.

C

HAZOP STUDY LOGISTICS

Logistical development of this refinery unit HAZOP included the following:

• Preplanning issues were addressed the prior week.• The team include three core team members and four part-time members.• The study included 16 moderately busy P&Ids.• The study took three and one-half weeks.• The team met 4 hours per day in morning review sessions and spent 2 hours per day

on individual efforts for reviews, follow-ups, and field checks.• Dedicated space was required for storing the large number of documents.• The study resulted in 170 data sheets.• The team recorder used a personal computer to record, sort, and retrieve data. The

Stone & Webster proprietary program PCHAZOPa was used.• The plant operator was the key contribution plant member of the team.• Key operating procedures were reviewed relative to the P&Ids and safe engineering

practices.