A hazard and operability study (HAZOP) 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.



The method applies to processes (existing or planned) for which design information is available. This commonly includes a process flow diagram, which is examined in small sections, such as individual items of equipment or pipes between them. For each of these a design Intention is specified. For example, in a chemical plant, a pipe may have the intention to transport 2.3 kg/s of 96% sulfuric acid at 20°C and a pressure of 2 bar from a pump to a heat exchanger. The intention of the heat exchanger may be to heat 2.3 kg/s of 96% sulfuric acid from 20°C to 80 °C. The HAZOP team then determines what are the possible significant Deviations from each intention, feasible Causes and likely Consequences. It can then be decided whether existing, designed safeguards are sufficient, or whether additional actions are necessary to reduce risk to an acceptable level.

When HAZOP meetings were recorded by hand they were generally scheduled for three to four hours per day. For a medium-sized chemical plant where the total number of items to be considered is 1200 (items of equipment and pipes or other transfers between them) about 40 such meetings would be needed.[1] Various software programs are now available to assist in meetings.

Parameters and guide words

The key feature is to select appropriate parameters which apply to the design intention. These are general words such as Flow, Temperature, Pressure, Composition. In the above example, it can be seen that variations in these parameters could constitute Deviations from the design Intention. In order to identify Deviations, the Study Leader applies (systematically, in order) a set of Guide Words to each parameter for each section of the process. The current standard[2] Guide Words are as follows:

Guide Word Meaning
NO OR NOT Complete negation of the design intent
MORE Quantitative increase
LESS Quantitative decrease
AS WELL AS Qualitative modification/increase
PART OF Qualitative modification/decrease
REVERSE Logical opposite of the design intent
OTHER THAN Complete substitution
EARLY Relative to the clock time
LATE Relative to the clock time
BEFORE Relating to order or sequence
AFTER Relating to order or sequence

(Note that the last four guide words are applied to batch or sequential operations.) These are therefore combined e.g. NO FLOW, MORE TEMPERATURE, and if the combination is meaningful, it is a potential deviation. In this case LESS COMPOSITION would suggest less than 96% sulfuric acid, whereas OTHER THAN COMPOSITION would suggest something else such as oil.

The following table gives an overview of commonly used guide word - parameter pairs and common interpretations of them.

Parameter / Guide Word More Less None Reverse As well as Part of Other than
Flow high flow low flow no flow reverse flow deviating concentration contamination deviating material
Pressure high pressure low pressure vacuum delta-p explosion
Temperature high temperature low temperature
Level high level low level no level different level
Time too long / too late too short / too soon sequence step skipped backwards missing actions extra actions wrong time
Agitation fast mixing slow mixing no mixing
Reaction fast reaction / runaway slow reaction no reaction unwanted reaction
Start-up / Shut-down too fast too slow actions missed wrong recipe
Draining / Venting too long too short none deviating pressure wrong timing
Inertising high pressure low pressure none contamination wrong material
Utility failure (instrument air, power) failure
DCS failure failure
Maintenance none
Vibrations too low too high none wrong frequency

Once the causes and effects of any potential hazards have been established, the system being studied can then be modified to improve its safety. The modified design must then be subject to another HAZOP, to ensure that no new problems have been added.


HAZOP is normally carried out by a team of people, with roles as follows[2] (with alternative names from other sources):

Name Alternative Role
Study leader Chairman someone experienced in HAZOP but not directly involved in the design, to ensure that the method is followed carefully
Recorder Secretary or scribe to ensure that problems are documented and recommendations passed on
Designer (or representative of the team which has designed the process) To explain any design details or provide further information
User (or representative of those who will use it) To consider it in use and question its operability, and the effect of deviations
Specialist (or specialists) someone with relevant technical knowledge
Maintainer (if appropriate) someone concerned with maintenance of the process.

In earlier publications it was suggested that the Study Leader could also be the Recorder[3] but separate roles are now generally recommended. A minimum team size of 5 is recommended.[4] In a large process there will be many HAZOP meetings and the team may change as specialists are brought in for different areas, and possibly different members of the design team, but the Study Leader and Recorder will usually be fixed. As many as 20 individuals may be involved[3] but is recommended that no more than 8 are involved at any one time.[4] Software is now available from several suppliers to aid the Study Leader and the Recorder.


The technique originated in the Heavy Organic Chemicals Division of ICI, which was then a major British and international chemical company. The history has been described by Trevor Kletz[3][5] who was the company's safety advisor from 1968 to 1982, from which the following is abstracted.

In 1963 a team of 3 people met for 3 days a week for 4 months to study the design of a new phenol plant. They started with a technique called critical examination which asked for alternatives, but changed this to look for deviations. The method was further refined within the company, under the name operability studies, and became the third stage of its hazard analysis procedure (the first two being done at the conceptual and specification stages) when the first detailed design was produced.

In 1974 a one-week safety course including this procedure was offered by the Institution of Chemical Engineers (IChemE) at Teesside Polytechnic. Coming shortly after the Flixborough disaster, the course was fully booked, as were ones in the next few years. In the same year the first paper in the open literature was also published.[6] In 1977 the Chemical Industries Association published a guide.[7] Up to this time the term HAZOP had not been used in formal publications. The first to do this was Kletz in 1983, with what were essentially the course notes (revised and updated) from the IChemE courses.[3] By this time, hazard and operability studies had become an expected part of chemical engineering degree courses in the UK.[3]


  1. Swann, C. D., & Preston, M. L., (1995) Journal of Loss Prevention in the Process Industries, vol 8, no 6, pp349-353 "Twenty-five years of HAZOPs"
  2. 2.0 2.1 British Standard BS: IEC61882:2002 Hazard and operability studies (HAZOP studies)- Application Guide British Standards Institution. “This British Standard reproduces verbatim IEC 61882:2001 and implements it as the UK national standard.”
  3. 3.0 3.1 3.2 3.3 3.4 Kletz, T. A., (1983) HAZOP & HAZAN Notes on the Identification and Assessment of Hazards IChemE Rugby
  4. 4.0 4.1 Nolan, D.P. (1994) Application of HAZOP and What-If Safety Reviews to the Petroleum, Petrochemical and Chemical Industries. William Andrew Publishing/Noyes. ISBN 978-0-8155-1353-7
  5. Kletz, T., (2000) By Accident - a life preventing them in industry PVF Publications ISBN 0-9538440-0-5
  6. Lawley, H. G.,(1974) Chemical Engineering Progress, vol 70, no 4 page 45 "Operability studies and hazard analysis" AIChE
  7. Chemical Industries Association (1977) A Guide to Hazard and Operability Studies

Further reading

See also