A Hazard and Operability (HAZOP) study is a qualitative process risk assessment technique. 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.


Objective of carrying out a HAZOP study:
  • To check a design
  • To decide whether and where to build
  • To decide whether to buy a piece of equipment
  • To obtain a list of questions to put to a supplier
  • To check running instructions
  • To improve the safety of existing facilities

When to perform a HAZOP?

The HAZOP study should preferably be carried out as early in the design phase as possible – to have 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.  Ideally re-hazop should be prformed on all existing process once in every three years or before every change in process/ design.

HAZOP studies may also be used more extensively, including:

  • At the initial concept stage when design drawings are available
  • When the final piping and instrumentation diagrams (P&ID) are available
  • During construction and installation to ensure that recommendations are implemented
  • During commissioning
  • During operation to ensure that plant emergency and operating procedures are regularly reviewed and updated as required

HAZOP background

  • The basis for HAZOP was laid by ICI in 1963 and was based on so-called “critical examination” techniques First guide:
  • “A Guide to Hazard and Operability Studies”, ICI and Chemical Industries Associations Ltd. 1977.
  • First main textbook: Kletz, T. A.: “Hazop and Hazan – Identifying and Assessing Process Industry Hazards”, Institution of Chemical Engineers.
  • See also: Kletz, T. A.: “Hazop – past and future”. Reliability Engineering and System Safety, 55:263-266, 1997

Standards and guidelines

  • IEC 61882. “Hazard and operability studies (HAZOP studies) – Application guide”. International Electrotechnical Commission, Geneva.
  • Crawley, F., M. Preston, and B. Tyler: “HAZOP: Guide to best practice. Guidelines to best practice for the process and chemical industries”. European Process Safety Centre and Institution of Chemical Engineers, 2000
  • Kyriakdis, I.: “HAZOP – Comprehensive Guide to HAZOP in CSIRO”, CSIRO Minerals, National Safety Council of Australia, 2003

HAZOP team members

The basic team for a process plant will be:

  • Project engineer
  • Commissioning manager
  • Process engineer
  • Instrument/electrical engineer
  • Safety engineer
  • Operating team leader
  • Maintenance engineer
  • Suppliers representative
  • Other specialists as appropriate


Prerequisites of Process Hazop

As a basis for the HAZOP study the following information should be available:

  • Process flow diagrams
  • Piping and instrumentation diagrams (P&IDs)
  • Layout diagrams
  • Material safety data sheets
  • Provisional operating instructions
  • Heat and material balances
  • Equipment data sheets Start-up and emergency shut-down procedures






Guide Word Meaning
No Negation of the Design Intent
Less Quantitative Decrease
More Quantitative Increase
Part Of Qualitative Decrease
As Well As Qualitative Increase
Reverse Logical Opposite of the Intent
Other Than Complete Substitution


Advantages of Hazop

  • Systematic examination
  • Multidisciplinary study
  • Utilizes operational experience
  • Covers safety as well as operational aspects
  • Solutions to the problems identified may be indicated
  • Considers operational procedures
  • Covers human errors
  • Study led by independent person
  • Results are recorded

The success or failure of the HAZOP depends on several factors:

  • The completeness and accuracy of drawings and other data used as a basis for the study
  • The technical skills and insights of the team
  • The ability of the team to use the approach as an aid to their imagination in visualizing deviations, causes, and consequences
  • The ability of the team to concentrate on the more serious hazards which are identified.