Introduction to HAZOP

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Introduction to HAZOPIntroduction to HAZOPHAZARD and OPERATABILITYHAZARD and OPERATABILITY

HAZOP StudiesHAZOP Studies

BackgroundBackground

A HazOp study identifies hazards and A HazOp study identifies hazards and operability problems.operability problems.The concept involves investigating The concept involves investigating how the plant might how the plant might DeviateDeviate from the from the design intent.design intent.If, in the process of identifying If, in the process of identifying problems during a HazOp study, a problems during a HazOp study, a solution becomes apparent, it is solution becomes apparent, it is recorded as part of the Hazop result.recorded as part of the Hazop result.

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HazOP study was developed to HazOP study was developed to supplement experience-based supplement experience-based practices when a new design or practices when a new design or technology is involved.technology is involved.

Its use has expanded to almost all Its use has expanded to almost all phases of a plant’s life.phases of a plant’s life.

ConceptConcept

The HazOp concept is to review the plant The HazOp concept is to review the plant in a series of meetings, during which in a series of meetings, during which multidisciplinary team methodically multidisciplinary team methodically “brainstorms” the plant design following “brainstorms” the plant design following the structure provided by the guide words the structure provided by the guide words and the team leader’s experience.and the team leader’s experience.The best time to conduct a HazOp is when The best time to conduct a HazOp is when the design is fairly firm, at this point the the design is fairly firm, at this point the design is well enough defined to allow design is well enough defined to allow meaningful answers to the questions meaningful answers to the questions raised in the HazOp process.raised in the HazOp process.

ContinueContinueAt this point it is still possible to At this point it is still possible to change the design without a major change the design without a major cost.cost.HazOp can be done at any stage after HazOp can be done at any stage after the design is nearly firm. For the design is nearly firm. For example: many older plants are example: many older plants are upgrading their control and upgrading their control and instrumentation systems.instrumentation systems.It is very effective to examine a plant It is very effective to examine a plant as soon as the control system as soon as the control system redesign is firm.redesign is firm.

The success or failure of HazOp The success or failure of HazOp depends on several factors:depends on several factors:

1.1. The completeness and accuracy The completeness and accuracy of drawings and other data.of drawings and other data.

2.2. The technical skills of the teamThe technical skills of the team3.3. The ability of the team to use the The ability of the team to use the

approach as an aid to their approach as an aid to their imagination in visualizing imagination in visualizing deviations, causes and deviations, causes and consequences.consequences.

4.4. The ability of the team to The ability of the team to concentrate on the more serious concentrate on the more serious hazards which are identified.hazards which are identified.

DefinitionsDefinitions

STUDY NODESSTUDY NODES

The locations (on piping and The locations (on piping and instrumentation drawings and instrumentation drawings and procedures) at which the process procedures) at which the process parameters are investigated for parameters are investigated for deviations.deviations.

DeviationsDeviations

These are departures from the These are departures from the intention which are discovered by intention which are discovered by systematically applying the guide systematically applying the guide words (e.g. “more pressure”) words (e.g. “more pressure”)

CausesCauses

These are the reasons why deviations These are the reasons why deviations might occur. Once a deviation has might occur. Once a deviation has been shown to have a credible cause, been shown to have a credible cause, it can be treated as a meaningful it can be treated as a meaningful deviation. These causes can be deviation. These causes can be hardware failure, human errors, an hardware failure, human errors, an unanticipated process state (e.g. unanticipated process state (e.g. change of composition), external change of composition), external disruptions (e.g. loss of power), etc.disruptions (e.g. loss of power), etc.

ConsequencesConsequences

These are the results of the These are the results of the deviations should they occur deviations should they occur (e.g., release of toxic materials).(e.g., release of toxic materials).

Guide WordsGuide Words

These are simple words which are These are simple words which are used to qualify or quantify the used to qualify or quantify the intention in order to guide and intention in order to guide and stimulate the brainstorming process stimulate the brainstorming process and so discover deviations.and so discover deviations.Each guide word is applied to the Each guide word is applied to the process variables at the point in the process variables at the point in the plant (study node) which is being plant (study node) which is being examined.examined.

For exampleFor example

Guide WordGuide Word ParameterParameter DeviationDeviation

NONO FLOWFLOW NO FLOWNO FLOW

MOREMORE PRESSUREPRESSURE HIGH HIGH PRESSUREPRESSURE

AS WELL ASAS WELL AS ONE PHASEONE PHASE TWO PHASETWO PHASE

OTHER THANOTHER THAN OPERATIONOPERATION MAINTENANCEMAINTENANCE

HAZOP GUIDE WORDS HAZOP GUIDE WORDS AND MEANINGSAND MEANINGS

Guide WordsGuide Words MeaningMeaningNoNo Negation of the design Negation of the design

intendintend

LessLess Quantitative DecreaseQuantitative Decrease

MoreMore Quantitative IncreaseQuantitative Increase

Part ofPart of Qualitative DecreaseQualitative Decrease

As Well AsAs Well As Qualitative IncreaseQualitative Increase

ReverseReverse Logical opposite of the Logical opposite of the intendintend

Other ThanOther Than Complete SubstitutionComplete Substitution

•It is not unusual to have more than one deviation from the application of one guide word. For example:

“more reaction” could mean either than a reaction takes place at a faster rate, or that a greater quantity of product results.

•When dealing with a design intention involving a complex set of interrelated plant parameters (e.g., temperature, reaction rates, composition, or pressure), it may be better to apply the whole sequence of guide words to each parameter individually than to apply each guide word across all of the parameters as a group

Guidelines for Using ProcedureGuidelines for Using Procedure

1.1. Define the purpose, objectives, and Define the purpose, objectives, and scope of the studyscope of the study

2.2. Select the teamSelect the team3.3. Prepare for the studyPrepare for the study4.4. Carry out the team reviewCarry out the team review5.5. Record the resultsRecord the results* Some of these steps can take place at * Some of these steps can take place at

the same time.the same time.

1- Purpose, Objectives and Scope 1- Purpose, Objectives and Scope of Studyof Study

Check the safety of a designCheck the safety of a design

Decide whether and where to buildDecide whether and where to build

Develop a list of questions to ask to Develop a list of questions to ask to suppliersupplier

Check operating/safety proceduresCheck operating/safety procedures

Improve the safety of an existing Improve the safety of an existing facilityfacility

Verify that safety instrumentation is Verify that safety instrumentation is reacting to best parameters.reacting to best parameters.

Specific consequences to be Specific consequences to be consideredconsidered

Employee safetyEmployee safetyLoss of plant or equipmentLoss of plant or equipmentLoss of productionLoss of productionLiabilityLiabilityInsurabilityInsurabilityPublic safetyPublic safetyEnvironmental impactsEnvironmental impactsFor example, a HAZOP might be For example, a HAZOP might be considered to determine where to build a considered to determine where to build a plant to have the minimal impact on public plant to have the minimal impact on public safety. In this case, the HAZOP should safety. In this case, the HAZOP should focus on deviations which result in off-site focus on deviations which result in off-site hazards.hazards.

2- Select the Team2- Select the Team5 to 7 members5 to 7 membersDesign EngineerDesign EngineerProcess EngineerProcess EngineerOperations SupervisorOperations SupervisorInstrument Design EngineerInstrument Design EngineerChemistChemistMaintenance SupervisorMaintenance SupervisorSafety Engineer (team leader)Safety Engineer (team leader)

3- Prepare for the Study3- Prepare for the Study

Obtain the necessary data (drawings, Obtain the necessary data (drawings, line diagrams, flow sheets, plant line diagrams, flow sheets, plant layouts, isometrics, etc. layouts, isometrics, etc.

Convert the data into a suitable form Convert the data into a suitable form and plan the study sequenceand plan the study sequence

Arrange the necessary meetingsArrange the necessary meetings

4- Carry Out the Team Review4- Carry Out the Team Review

5- Record The Results5- Record The Results

The recording process is an The recording process is an important part of the HAZOP.important part of the HAZOP.

It is useful to have the team It is useful to have the team members review the final report and members review the final report and then come together for a report then come together for a report review meeting.review meeting.

The first HAZOP form should be filled The first HAZOP form should be filled out during the meeting.out during the meeting.

ExampleExample

Consider, as a simple example, the Consider, as a simple example, the continuous process shown in the figure.continuous process shown in the figure.

In this process, the phosphoric acid and In this process, the phosphoric acid and ammonia are mixed, and a non-ammonia are mixed, and a non-hazardous product, diammonium hazardous product, diammonium phosphate (DAP) results if the reaction of phosphate (DAP) results if the reaction of ammonia is complete.ammonia is complete.

If too little phosphoric acid is added, the If too little phosphoric acid is added, the reaction is incomplete and ammonia is reaction is incomplete and ammonia is producedproduced

For Study Node 1For Study Node 1

1.1. NO:NO: a: no flow at study node number 1 b: consequences: excess ammonia in

reactor and release to work area c: causes: * valve A falls closed * phosphoric acid supply exhuasted * plug in pipe, pipe rupture d: suggested action: automatic closure of

valve B on less of flow from phosphoric acid supply

II. LESSII. LESSA. less & flow – reduced flow at A. less & flow – reduced flow at

study node 1study node 1B. B. Consequences:Consequences: excess ammonia excess ammonia

in reactor and release to work in reactor and release to work area.area.

C. C. Causes: Causes: - valve A partially closed- valve A partially closed - Partial plug or leak in pipe- Partial plug or leak in pipe

D. D. Suggested Action: Suggested Action: Automatic closure of valve B based on reduced flow in pipe from phosphoric acid supply.

III. MOREIII. MORE

A. More & FlowA. More & Flow Increased flow at Increased flow at study node 1study node 1

B. Consequences: B. Consequences: excess phosphoric excess phosphoric acid degrades product but presents acid degrades product but presents no hazard to workplace.no hazard to workplace.

IV. PART OFIV. PART OF

A. Part of & Flow: A. Part of & Flow: Decreased Decreased concentration of phosphoric acid at study concentration of phosphoric acid at study node 1node 1

B. consequences:B. consequences: excess ammonia excess ammonia

C. causes:C. causes:

- vendor delivers wrong material or - vendor delivers wrong material or concentrationconcentration

- error in charging phosphoric acid supply - error in charging phosphoric acid supply tanktank

D.suggested action: D.suggested action: add check of add check of phosphoric acid supply tank concentration phosphoric acid supply tank concentration after charging procedure.after charging procedure.

V. Other Than V. Other Than A. Other than & flow:A. Other than & flow: material other than material other than phosphoric acid in line Aphosphoric acid in line AB. consequences:B. consequences: depend on substitution, depend on substitution, team member assigned to test potential team member assigned to test potential substitutions based on availability of substitutions based on availability of other materials at site and similarity in other materials at site and similarity in appearance.appearance.C. Causes:C. Causes:

- Wrong delivery from vendor- Wrong delivery from vendor - wrong material chosen from plant - wrong material chosen from plant

warehousewarehouseD. Recommended Action:D. Recommended Action: plant procedures plant procedures to provide check on material chosen to provide check on material chosen before charging phosphoric acid supply before charging phosphoric acid supply tank.tank.

ExampleExampleConsider a chemical reaction between two Consider a chemical reaction between two substances X and Y to form a product Z, substances X and Y to form a product Z, where the concentration of raw material Y where the concentration of raw material Y must never exceed that of X, otherwise an must never exceed that of X, otherwise an explosion will occur.explosion will occur.Starting with the pipeline from the inlet of Starting with the pipeline from the inlet of the feed pump that delivers the raw the feed pump that delivers the raw material X to where it enters the reaction material X to where it enters the reaction vessel.vessel.The design is examined and the operation The design is examined and the operation analyzed for possible malfunction.analyzed for possible malfunction.Keys or guide words (Does not, more) are Keys or guide words (Does not, more) are used to conduct the examination of the used to conduct the examination of the systemsystem..

The collapse of the interior of a steam-jacketed chemical The collapse of the interior of a steam-jacketed chemical vessel.vessel.

The purpose of the vessel was to mix various The purpose of the vessel was to mix various chemicals in the presence of catalyst.chemicals in the presence of catalyst.At the time of failure the steam valve on the inlet At the time of failure the steam valve on the inlet to the jacket was closed since steam heating was to the jacket was closed since steam heating was not required for the particular part of the process not required for the particular part of the process being carried out that time.being carried out that time.At a certain point in the process the mixing of At a certain point in the process the mixing of various chemicals resulted in a high temperature various chemicals resulted in a high temperature being generated.being generated.On the day of the accident ass the steam heating On the day of the accident ass the steam heating was not required, the valves on the steam jacket was not required, the valves on the steam jacket inlet and the condensate outlet were in the inlet and the condensate outlet were in the closed position and condensate water lying in the closed position and condensate water lying in the jacket became heated by the chemical process jacket became heated by the chemical process within the vessel.within the vessel.The water in the jacket was flashed off into The water in the jacket was flashed off into steam and it was estimated that a pressure in steam and it was estimated that a pressure in excess of 28.5 kg per square cm could have been excess of 28.5 kg per square cm could have been produced resulting in the collapse of the inner produced resulting in the collapse of the inner bottom of the vessel. bottom of the vessel.

A hand-operated vent valve was fitted A hand-operated vent valve was fitted directly to the jacket and the normal directly to the jacket and the normal procedure when shutting off the steam procedure when shutting off the steam heating was to leave the valve open.heating was to leave the valve open.On this occasion it had inadvertently been On this occasion it had inadvertently been left closed.left closed.The steam line to the jacket was protected The steam line to the jacket was protected by a suitable safety valve but as the by a suitable safety valve but as the steam inlet valve on the jacket was closed steam inlet valve on the jacket was closed the safety valve was isolated.the safety valve was isolated.The above accident could have been The above accident could have been prevented by the provision of a separate prevented by the provision of a separate safety valve fitted to the jacket.safety valve fitted to the jacket.

Guide Guide WordWord

DeviationDeviation Possible CausesPossible Causes ConsequencesConsequences Action Action RequiredRequired

More ofMore of Steam Steam pressure in pressure in jacket jacket exceeds exceeds safe working safe working pressurepressure

a)a) Increase in Increase in pressure in pressure in steam mainsteam main

b) Product reaction b) Product reaction heats heats condensate in condensate in jacket to 230 jacket to 230 degree C with degree C with valves A, B, C valves A, B, C closed closed

Safety valve Safety valve opensopens

Collapse of Collapse of vesselvessel

Regularly Regularly inspect inspect safety safety valvesvalves

Install Install safety safety valve in valve in jacketjacket

ReverseReverse Flow of Flow of heat heat reverses reverses from from reactor to reactor to jacketjacket

Heats any Heats any condensate present condensate present in jacket to reaction in jacket to reaction temperature of 230 temperature of 230 degree C degree C (equivalent to 400 (equivalent to 400 psi)psi)

Collapse of Collapse of vessel if vessel if valves A, B, C valves A, B, C are closedare closed

Install Install safety safety valve in valve in jacketjacket