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HAZOP, Principles of examination of HAZOP

What is HAZOP?

HAZOP is the acronym for Hazard & Operability Studies. It is one of the methods of hazard identification. This technique was first used in the early seventies and is described in the papers” Hazard & Operability Studies” by Lel (1974): HAZOP is carried out by a multi-disciplinary’ team. Which reviews the process to identify potential hazards and operability problems? It essentially involves the application of the technique of critical examination and has its origins in methods study.


What are the Principles of examination of HAZOP?

The basic concept of hazard and operability study is to take a full description of the process and critically examine every part of it through proper questions so as to discover the possible deviations from the intended design, that can occur. Once a deviation is identified. Its causes and consequences are analyzed. This is done systematically by applying suitable guide words.

Hazard & Operability Studies.

Important features of the study are:

  • Consequences
  • Hazards
  • Intention
  • Deviation
  • Causes
  • Operating difficulties

The main guide words and their meanings are:

NO or NOT or DOT’T         :        Negation of intention

MORE         :      Quantitative increase

LESS            :      Quantitative Decrease

AS WELL AS      :          Quantitative increase

PART OF      :              Quantitative Decrease

REVERSE         :            Logical opposite of intention

OTHER THAN      :       Complete substitution

These guide words may be applied to:

  • Process condition
  • Activities
  • Substances
  • Time
  • Place

Detailed Sequence of Steps in A HAZOP Study:

A sample showing the application of the guide’ words is illustrated by the following example.

Example of HAZOP study:

The flow sheet of a reactor system shows that raw material streams A and B are transferred by a pump to a reactor, where they react to produce the product C. Also given that the flow of B should not exceed the flow of A otherwise an explosion may occur. The flow sheet thus shows an intention to Transfer A Transfer B To the reactor at the design flow, with an intention to Produce C.

Let us apply the guide words to the first stream:

IntentionPossible Deviations
(Based on guide Intention words)
Transfer A At design RateNO flowTank empty flow Pump has failed Line disconnected Suction valve dose Discharge valve ClosedExplosion due to excess of B
 MORE flowFaculty control valve Faulty meter Loss of material A Poor quality of Product
 LESS flowValves are passing Low voltageExplosion due to excess of B Poor quality of Product
 Transfer of A As WELL As other components Explosion due to excess of B Poor quality of Product
 PART of A is transferredSame of LESS flow 
 Transfer of A is in REVERSE direction  
 Transfer of some material OTHER  THAN A   

What is the procedure for a study in HAZOP?

The principles described above put into practice in a procedure which consists of the following steps:

  • Carry out the examination
  • Follow-up
  • Record the results
  • Define objectives and scope
  • Select the team
  • Prepare for the study

What is the definition of objectives in HAZOP?

The objectives and scope of a study should be made explicit as early as possible Some examples of reasons for a study are:

  • To Check a design
  • To decide whether and where to build
  • Decide whether to buy a piece of equipment
  • To obtain a list of questions to put to a supplier
  • Check running instructions
  • To improve the safety of existing facilities.

It is also necessary to decide the types of hazard to be considered for example:

  • To the general public
  • To the environment
  • For people working in a plant
  • To plant and equipment
  • To or from product quality

The physical limits of the plant to be studied must be decided and whether interactions with neighboring units or buildings should be included. Any time or financial constraints must be specified. It will also be necessary to state whether any aspects such as the civil engineering 0f the chemistry can be taken for granted and deliberately excluded.

The general objectives for a study are normally set by the person responsible for the project or for the plat; for example the project manager, project engineer or the plant manager. Usually assisted in this definition by a study leader.

Wat is the team composition in HAZOP?

Hazard and Operability Studies are normally carried out by multi-disciplinary teams. There are two types of team members, those, who win make a technical contribution and those who playa supporting and structuring role.

Technical team members

The examination require the team to have a detailed knowledge of the way the plant intended to work. This means a blend of those concerned with the design of the plant and those concerned with its operation. The technique of using guide words generates a very large number of questions for most to answer the majority of those questions without recourse to further expertise.

As an example, a typical small chemical plant would examined by a team consisting of each of the following:

  • Mechanical engineer
  • Chemical ‘engineer
  • Project manager responsible for the project as a whole.
  • R&D chemist
  • Production manager

This group should contain sufficient expertise to provide the necessary technical input. Additionally if some members of the team are drawn from those who also have some responsibility for the design of a plant, they will be’ particularly motivated to produce a successful design and a safe operating procedure. Normally these members of the +cam will have to necessary authority to make changes. The blend of disciplines will vary with the type of project.

Some projects will require the inclusion of different disciplines, for example.

  • Civil engineers
  • Chemists, Pharmacists, etc.
  • Instrument
  • Electrical engineers

The team should not be too large, ideally between three and five technical members. If a study seems to require a large number of people it is worthwhile trying to break it down into several disparate parts with some variation of team composition for each part.

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