Table of Contents

  1. Purpose
  2. General
  3. Responsibilities
  4. Procedure
  5. Flowchart
  6. References
  7. Attachments


This guide describes the recommended method of preparing, executing, implementing and reporting of a Hazard and Operability (HAZOP) Study.


2.1 Objective

The objective of a HAZOP Study is to identify potential hazards and operability problems of (part of) a new or revamped plant or unit (further referred to as “plant”) by means of a formal systematic review of the Piping and Instrument Diagrams (P&ID’s) or referred to as Engineering Flow Diagrams (EFD’S). As part of a full HAZOP study design changes are proposed and implemented to upgrade the safety and operability of the plant.

2.2 Technique

The HAZOP Study technique makes use of a series of Parameters and Guide Words. By means of combinations of these Parameters and Guide Words, conceivable deviations of the proposed process design conditions are systematically searched for and discussed in a team for the possible causes and consequences. Actions are recommended when judged necessary. Implementation of the recommended actions is normally part of the scope of the HAZOP Study as well.

A HAZOP Study is carried out in a number of HAZOP Study sessions by a team of engineers from both the Contractor, the Client and, when appropriate, (a) representative(s) from the Process Licensor.

The execution of a full HAZOP Study consists of the following basic steps:

  • preparation of the HAZOP Study;
  • systematic review of the P&ID’s during the of HAZOP Study sessions;
  • recording of the HAZOP team findings during the sessions;
  • implementation of the recommendations made during or after the HAZOP Study sessions;
  • issuing of the final HAZOP Study Report.

2.3 Timing

A HAZOP Study can in principle be executed at any point in time of the project provided that sufficiently detailed P&ID’s are available. Examples of suitable timings for a HAZOP Study are:

  • at the end of the basic engineering phase of a project;
  • at the start of the detailed engineering phase of a project;
  • in the detailed engineering phase when information from equipment and instrumentation vendors, piping layout, etc., are available and have been incorporated in the P&ID’s;
  • at any time when significant design changes have been made to the plant.

2.4 Quantitative Risk Analysis

In most cases the recommendations of a HAZOP Study are agreed upon on the basis of qualitative considerations only. However, in some cases it may be judged necessary to base the recommended implementation on a quantitative basis. This requires a quantitative risk analysis to be made. The guidelines for the execution of such an analysis is not part of the scope of this Guide.


A HAZOP team includes representatives from the Client, the Contractor and possibly the Process Licensor. A typical HAZOP team may have the following members with the tasks and responsibilities as described below:

HAZOP Team Member

Tasks and Responsibilities

HAZOP Team Leader

The task of the team leader is to prepare and conduct the HAZOP Study sessions. The Team Leader’s responsibility is to define the HAZOP Study objectives and ensure a proper execution of the study sessions. The team leader shall be independent of the project.

Process Design Engineer(s)

The process design engineer(s) is (are) responsible for the process design of the plant. The lead process engineer is responsible for the implementation of the HAZOP Study recommendations and the preparation of the final HAZOP Study Report.


The scribe performs the administrative tasks during the HAZOP Study sessions and records the findings and possible recommendations of the HAZOP team.

Process Control Engineer or Control Systems Engineer

The process control engineer or control systems engineer is responsible for the design of the process control and safeguarding by instrumentation of the plant.

Engineering Specialist(s)

The engineering specialist(s) are normally not participating full time in the HAZOP Study sessions. However they may be on call and asked for advice if required.

Operations Representative or Commissioning Engineer

The operations representative or commissioning engineer will in particular advise on practical and operational aspects that need to be incorporated in the design of the plant.

Licensor Representative

In case the process design is based on licensed technology, consideration shall be given to having (a) representative(s) of the licensor participate in the HAZOP Study sessions.

Other Representatives

Other disciplines may participate part time or full time in the HAZOP Study sessions, like (a) Safety Specialist(s) Maintenance Representative(s), Research Chemist, etc.

Authority Engineer

Depending on the HAZOP Study objectives, the Authority Engineer may assist the HAZOP Study Team Leader to determine any authority requirements of the study. The Authority Engineer may attend part of the meetings when required.

It is the responsibility of the Project Manager to ensure the timely assignment of the HAZOP Team Leader and those other HAZOP Team members that are part of the project team.


4.1 General

Basis Requirements

The procedure for the execution of a HAZOP Study presented below is an outline of the most important aspects of such a study. The execution of an actual HAZOP Study may differ in certain aspects from the format described. However, in all cases it remains essential that:

  • the actual HAZOP procedure followed has been formally agreed upon by the Client;
  • if applicable, the actual procedure fulfils the authority requirements.

Batch Operating Plants

The procedure below has been written for continuously operating chemical and petrochemical plants.

In the case of a batch operating plant, the same procedure can in principle be followed. However the search for conceivable deviations of the proposed process design conditions (by means of combinations of Parameters and Guide Words) shall take place at each possible step in the batch process. In addition, the deviations may not only originate from human errors, or incorrectly working instrumentation, etc., however also from erroneous recipes or failures of the computer. Additional deviations as “Missed Step”, “Step out of Order” may be used to identify potential hazardous situations.

A detailed HAZOP Study of a batch operating plant may therefore require a substantially larger effort relative to a HAZOP Study of a continuously operating plant.

4.2 Preparation

Prior to commencing a HAZOP Study, the HAZOP Team leader shall reach formal agreement with the client upon a number of basic aspects regarding the execution of the study, such as:

4.2.1 Authority Requirements

Depending on the objectives of the HAZOP Study, the HAZOP team leader shall ensure that the HAZOP Study execution complies with the requirements of the authorities. He may be assisted in this by the Authority Engineer.

4.2.2 Client Requirements

The HAZOP team leader shall ensure that the HAZOP Study execution complies with requirements of the Client.

4.2.3 Basis and Scope of the HAZOP Study

The basis and scope of the HAZOP Study must be agreed upon by all participating parties prior to commencing the HAZOP Study sessions. The basis may be defined by means of a list of agreed assumptions, such as:

  • the process will work in accordance with its design;
  • the equipment is suitable for the intended use;
  • written operating procedures will be followed;
  • preventive maintenance and inspections will be performed;
  • operators are competent and well trained.

The scope of the HAZOP Study may be defined by:

  • a list of the P&ID’s (EFD’s) that will be reviewed;
  • a list of proposed systems to be “hazopped”;
  • a summary of possible exclusions from the study scope, such as existing parts of a (revamped) plant, utility systems, any quantitative risk analysis, etc.;
  • a list of documents to be available during the HAZOP Study sessions such as safety properties of hazardous materials, process control and safeguarding by instrumentation design documents, basis of the material selection, operating instructions, equipment data sheets, instrument data sheets, plot plans, reports of previous safety reviews, etc.

4.2.4 HAZOP Study Documents

It is the task of the project team to ensure that the documents that are to be formally reviewed in the HAZOP Study are formally issued, for example as “for HAZOP Study”. Documents that only need to be available during the HAZOP session, may or may not be formally issued, depending on the importance of those documents.

4.2.5 HAZOP Study Timetable

For planning purposes all members of the HAZOP Study team shall be timely informed on the expected duration of the HAZOP Study sessions. The duration of these sessions depends amongst others on the number and complexity of the P&ID’s to be reviewed and the experience of the participants in the HAZOP Study sessions.

For planning purposes and manhour estimates, the following typical figures may be used:

  1. Preparation of the HAZOP Study.

  2. Review of the P&ID’s, whereby:
  • the review time is 4-12 hours per P&ID;
  • the duration of the HAZOP sessions is 4-6 hours per day;
  • the number of HAZOP sessions is 3-5 days per week.
  1. Implementation of the HAZOP Study recommendations.

  2. Writing of the HAZOP Study Report.

4.2.6 HAZOP Team Composition

The format and degree of detail of recording and the tools to be used (manual, computer program, etc.) shall be agreed in advance as well. An example of a recording sheet is given in Attachment 2. Some possible recording tools are mentioned in Section 4.4.

4.3 HAZOP Study Sessions

4.3.1 Definition of a System

The formal examination of a process requires that the plant is split into “Systems”. These systems may be a single line, a series of interconnecting lines, a combination of a series of lines, equipment and instrumentation, etc. As part of the preparation, an agreed list of systems has already been obtained at the start of the HAZOP Study sessions.

During the sessions the HAZOP team makes an exact definition of the system before it is being “hazopped”. A system may be defined by:

  • a list of the P&ID’s on which the system is shown;
  • a list of all lines, that are part of the system;
  • a list of all equipment that are part of the system.

The definition of a system requires the following judgment.

  • a system shall perform a logical process function, like a column overhead system, a column reboiler system, a feed system of a plant, etc.;
  • too small a system may cause tedious repetitive analyses, too large a system may cause key safety points to be overlooked.

An example of a system definition is shown in Attachment 3.

4.3.2 Review of a System

After the definition of a system has been agreed by all HAZOP Team members, the (lead) process design engineer explains the objective of that system, the design process conditions, any foreseeable abnormal conditions and the design philosophy used. The process control or control system engineer may explain the control and safeguarding aspects of the system.

The HAZOP Team Leader then selects a part of the system (a line, a piece of equipment, or a combination of elements small enough for an effective analysis) and faces the team with the first foreseeable deviation from the design conditions based on the combination of a Parameter and a Guide Word, normally “No flow”.

The HAZOP team will now determine:

  • the possible “Causes” of the deviation;

  • the possible “Consequences” of the deviation;

  • the presence of “Existing Safeguards” to protect against the deviation;

  • any “Recommended Actions” to improve the safety and/or operability of the system;

  • identification of the party (in the column “By”) that is responsible for the follow-up and implementation of the recommendation.

All of the above is recorded by the Scribe during the HAZOP Sessions.

Once the discussion and the recording of the first deviation has been completed for the entire System, the HAZOP Team Leader proceeds with the following deviation. The review of the System is complete when all deviations that were to be investigated have been dealt with. The HAZOP Team Leader will then conduct the review of the next system in the same manner.

4.4 Recording

The proceedings of the HAZOP Study sessions are recorded by the scribe.

4.4.1 Clearness of the Recordings

It is essential that the recordings are understood not only by the team members during the HAZOP Study sessions, but also by a person outside the HAZOP Team long after the HAZOP Study has been concluded.

For this purpose it is recommended that;

  1. all recordings are concise and clear;

  2. if useful, use is made of equipment numbers, instrument tag numbers, etc.

4.4.2 Detail of the Recordings

The level of detail of the recordings of the HAZOP Study shall be as agreed upon prior to commencing the HAZOP Study sessions:

  • If there are no authority requirements to be fulfilled, it may be acceptable to only fully record those “Deviations” for which additional safeguarding is agreed to be necessary.
  • In case the HAZOP Study has to comply with authority requirements, the recordings must be complete: for each “Deviation” all “Possible Causes”, all “Potential Consequences”, all “Existing Safeguards”, etc. must be mentioned, also in case no additional safeguarding is added.
  • If time permits, it is advisable to produce a complete recording. This will allow individuals that did not participate in the HAZOP, to be informed about the actual details of the HAZOP itself. Furthermore it will provide evidence that a particular scenario has been screened even if no consequences resulted.

During the HAZOP Study sessions the scribe fills out the columns “Deviation”, “Possible Causes”, “Potential Consequences”, “Recommended Actions”, “Item No” and “By”. These recordings shall be agreed upon by all members of the HAZOP Team, preferably during the session or, if this is not possible, at a later point in time.

The column “Actual Implementation” is filled out by the Lead Process Engineer during the follow-up phase of the HAZOP Study in which the HAZOP Team recommendations are being implemented.

4.4.3 Recording Tools

For the recording of the HAZOP Study findings and results, either of the following three types of reporting tools may be used:

  • dedicated HAZOP Study Reporting Programs such as “HAZOP-PC” (Primatech). Leader (JBF Associated Inc.), and Hazsec (Technica);
  • recording sheets that make use of non-dedicated computer programs such as a spreadsheet or word-processing program;
  • hand-made recording sheets, manually filled out during the HAZOP sessions.

The use of a dedicated HAZOP Study Reporting Program is recommended above the other reporting tools as they are designed specifically for this purpose.

4.5 Implementation

After completion of the HAZOP Study sessions, the project team will be faced with a list of recommendations, made by the HAZOP Team. It is noted that the HAZOP Team Leader is not involved anymore in this part of the HAZOP Study and that the responsibility for the implementation of the HAZOP Study recommendations lies with the lead process design engineer.

The recommendations of the HAZOP Team may be divided in two types:

  • a specific recommendation how to eliminate the effect or how to reduce the effect and/or likelihood of an identified potential hazard;

  • a general recommendation to further investigate an identified potential hazard.

In case the HAZOP Team came up only with a general recommendation, the actual implementation shall be formally agreed upon with the Client and, if applicable, the Process Licensor. The formal agreement may be laid down in a written communication (letter, fax) or a final HAZOP Study Report signed by all parties involved.

It is noted that the implementation of the HAZOP Study recommendations may affect not only the P&ID’s but also other documents. Any document that has been revised as a result of the HAZOP Study, shall be formally reissued, for example as “HAZOP Study Incorporated”.

Finally it may be considered to show the HAZOP Action Item Number at the revised part(s) of the document.

4.6 Major Design Changes

Major design changes and field changes made after the HAZOP Study will be subject to an additional HAZOP Study by the home office team.

The HAZOP Study after design changes will be planned prior to a pre-determined issue of the EFD’s. This is also reflected in the Project HO SHWE Plan.

The final HAZOP Study report is to be issued once all design and construction have been completed.




6.1 Hazard and Operability Study – Why? When? How?
Directoraat Generaal van de Arbeid, The Netherlands,
Report R-3E, The Hague (1979)

6.2 Dow Chemical Company
Dow’s Fire and Explosion Index Hazard Classification Guide
AIChE New York

6.3 Germany
Storfall Verordnung

6.4 Belgium
Decreet Milieuverordening (Chapter 4, Articles 7 through 16)

6.5 The Netherlands

6.6 The Netherlands
Besluit Risico Zware Ongevallen

6.7 The Netherlands
Besluit Verplichtstelling Extern Veiligheidsrapport

6.8 The Netherlands
Besluit Verplichtstelling Arbeidsveiligheidsrapport

6.9 Operability Studies and Hazard Analysis
H.G. Lawley
Chemical Engineering Progress, Vol.70, No.4, April 1974

6.10 Size Up Plant Hazards This Way
H.G. Lawley
Hydrocarbon Processing, Vol.55, No.4, April 1976

6.11 Eliminating Potential Process Hazards
T.A. Kletz
Chemical Engineering April 1, 1985

6.12 CM-PE-317 Procedure for Conducting Project Hazard Reviews Level 2


  1. Parameters and Guide Words

  2. Recording Sheet

  3. System Definition

Parameters and Guide Words

This attachment shows some examples of:

  • frequently used Parameters and Guide Words.

  • examples of resulting deviations of the anticipated process design conditions.

Parameters and Guide Words


Guide Words



















Other Parameters could be:

  • Reaction

  • Composition

  • Leak

  • Rupture

  • Utility

  • Relief

  • Safety

  • Maintenance

  • Corrosion, Erosion

  • Start-up, Shutdown

  • Special Operations, etc.

The selected Parameters and Guide Words depend on the phasing of the HAZOP Study, the scope and the degree of desired detail of the study.

Deviations from the Anticipated Process Design Conditions

Foreseeable deviations from the anticipated process design conditions are searched for by combining Guide Words and Parameters as follows:

Flow + No


No Flow

Flow + Reverse


Reverse Flow

Pressure + Lower


Low Pressure

Level + High


High Level

The Parameter "Other" may be used as follows:

Reaction : Occurrence of chemical reactions (not) foreseen in the design e.g. runaway reactions.
Composition : Change of the composition and/or phase of the fluid and the resulting change of the physical properties.
Leak : Leak into our out of the process via heat exchanger tubes, pump seals, pipe flanges, sampling, etc.
Utility Failure : Confirm consequences of a failure of a utility stream.
Material Selection : confirm the selected materials of construction against the design as well as abnormal conditions (e.g. cold embrittlement, corrosion, erosion).
Relief : Listing of all applicable relief cases per relief valve. Check if a sufficient number of relief valves are installed.
Safety : Confirm any relevant safety related item, e.g. control valve fail safe positions, static electricity, insulation, tracing, winterizing, jacketing, flushing, hot spare pump arrangement, sloping requirements, autorestart requirements, equipment distances/ elevations/accessibility, distance to other facilities, vibration, pulsation, liquid hammer, toxic releases, earthing, fire, explosion, response times (e.g. instrumentation) gas detection, fire protection, fire proofing, sprinkler systems, escape routes.
Maintenance : Confirm equipment sparing and isolation requirements, accessibility to and into (manholes) equipment, crane access.
Start-up : Review of the start-up procedure as a separate exercise.
Shutdown : Review of the (emergency) shut-down procedure as a separate exercise.
Special Operations : Review special operations, such as loading, regeneration and unloading of catalyst, drainage, sampling, etc.

Click image for pdf:

System Definition:

Client : Speciality Products BV Issue : 1
Project : Ardon ® Revamp Issue date : 7/11/2004
Unit : 100 Job No. :  
System : 07 Client Project No. :  


System Name : Product Rundown Section
Equipment : Product Vessel MS-107
  : Product Stroage Tank MF-103
  : Product Pump PP-113 A/B
  : Product Cooler TT-105
Lines : PR-1046
  : PR-1047
  : PR-1048
  : PR-1049
  : PR-1052
  : CWR-1010
  ; CWR-1011
P&ID's : 00107B, issue 4
  : 00107C, issue 3