Introduction to HAZOP Study

What is HAZOP?

The full form of HAZOP is a Hazard and Operability Study. This is a comprehensive multi-disciplinary team exercise to critical review (Study) the piping design (Layout/ Routing/Placement of branches/inline instrumentation items/equipment, etc.) with respect to Hazardous and Operational considerations and requirements.

A Hazard and Operability (HAZOP) study is a structured and systematic examination of a planned or existing process or operation in order to identify and evaluate problems that may represent risks to personnel or equipment or prevent efficient operation.

The HAZOP technique was initially developed to analyze chemical process systems but has later been extended to other types of systems and also to complex operations and to software systems.

A HAZOP is a qualitative technique based on guide-words and is carried out by a multi-disciplinary team (HAZOP team) during a set of meetings. Few of the common HAZOP guidewords are listed below:

  • No or not
  • Other than
  • More
  • Early
  • Less
  • Late
  • As well as
  • Before
  • Part of
  • After
  • Reverse (of intent)
  • Others can be crafted as needed

HAZOP Mechanism

HAZOP is now a mandatory activity. It is a qualitative, experience intensive exercise as of now. It is in the form of deviation analysis. After the process design, the steady-state specifications of each stream in the flowsheet are known.

The HAZOP team exhaustively asks itself questions as to what will happen if this specification deviated on the positive or negative side of the expected steady-state value. It debates the possible causes and consequences of each such eventuality. Anything which appears to them as likely to lead to hazardous situations is debated further and possible means of avoiding the same or raising alarm if it happens so that remedial action can be taken etc. are recommended. This may lead to the recommendation of additional instrumentation on lines and equipment, Hi-Lo alarms and trips, etc. may be required to be provided.


The idea of HAZOP is to foresee a hazardous situation and take measures and abundant precaution to avoid them and increase process safety.

Hazop is a structured analysis, conducted after the design review, to ensure the design is suitable for all the intended operating conditions and complies with the HSE requirements. This process is also to ensure that the fundamentals of the design are thoroughly explained, understood, and examined.

Basically, HAZOP is performed in four phases as listed below:

  1. Definition Phase
  2. Preparation Phase
  3. Examination and
  4. Documentation and follow-up phase.

Important characteristics of all four HAZOP phases are provided in the image below.

HAZOP Phases
HAZOP Phases

The examination phase is the most important HAZOP phase among the four phases. A typical flowchart of HAZOP Examination phase is provided below:

HAZOP Examination phase flow-chart
HAZOP Examination phase flow-chart

Advantages of HAZOP

The benefit of the HAZOP is that early identification and assessment of the critical hazards provide essential input to project development decisions.  This leads to a safer and more cost-effective design with a minimum cost of change penalty. Other benefits of HAZOP are

  • It works as a qualitative risk assessment tool
  • During the brainstorming session based on past experiences, all lessons learned safety points are taken care of to prevent re-occurrence.
  • HAZOP is a very simple and effective method.
  • As HAZOP is a cross-functional team effort; experiences from all disciplines combine into the design for highly safe operation.
  • HAZOP is a powerful communication tool for obtaining early buy-in on the approach.

Key Documents required for HAZOP

Important documents required for during a HAZOP Study are:

  • PFD / PFS’s (Process Flow Diagram/ Process Flow Schemes)
  • P&ID / PEFS’s (Piping & Instrument Diagrams/ Process Engineering Flow Schemes)
  • Basis of Design
  • Operating, Control and safeguarding philosophy
  • Plot plans & Hazardous area classification drawings
  • Cause & Effect diagrams

HAZOP Team Composition

Normally a HAZOP review team consists of

  • Chairman (independent)
  • Lead engineers from Process, Instrumentation & Control (both from Design team and Client/ End-user)
  • Operations engineer (from Client/ End-user)
  • Lead engineers from Mechanical/Piping, Pipeline, HSE, and Electrical as required. (Both from Design team and Client/ End-user)

The following image shows a typical team composition for HAZOP Analysis.

Typical HAZOP Team
Typical HAZOP Team

HAZOP Timings

The HAZOP study should preferably be carried out as early in the design phase as possible to have an influence on the design. On the other hand; to carry out a HAZOP we need a rather complete design. As a compromise, the HAZOP is usually carried out as a final check when the detailed design has been completed.

A HAZOP study may also be conducted on an existing facility to identify modifications that should be implemented to reduce risk and operability problems.

Typically HAZOP in EPC design companies is performed 3 to 4 weeks after the Design review meeting, once the design review points are incorporated in the key documents.

Application of HAZOP Analysis

In recent times, HAZOP has got world-wide acceptance across various industries due to its inherent benefits. The main industries are:

  • Chemical,
  • Petro-chemical
  • Pharmaceutical
  • Refinery
  • Food
  • Oil and gas
  • Nuclear
  • Fertilizer
  • Power plants

Few more useful resources for you…

What is Engineering Process Safety?Opens in a new tab.
Safety Rules during A Field Visit By A Design EngineerOpens in a new tab.
An article on Crane safety during ConstructionOpens in a new tab.
HAZOP (Hazard and Operability) Study: A brief introductionOpens in a new tab.
An article on Excavation Hazards at Construction SitesOpens in a new tab.
Hazardous Area- Theory, Classification and Equipment selection: A short presentationOpens in a new tab.
An article on THE HAZARDS OF PRESSURE TESTINGOpens in a new tab.
Few job opportunities for youOpens in a new tab.

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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