Hazard and operability study

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A hazard and operability study (HAZOP) is a structured and systematic examination of a complex plan or operation in order to identify and evaluate problems that may represent risks to personnel or equipment. The intention of performing a HAZOP is to review the design to pick up design and engineering issues that may otherwise not have been found. The technique is based on breaking the overall complex design of the process into a number of simpler sections called 'nodes' which are then individually reviewed. It is carried out by a suitably experienced multi-disciplinary team (HAZOP) during a series of meetings. The HAZOP technique is qualitative, and aims to stimulate the imagination of participants to identify potential hazards and operability problems. Structure and direction are given to the review process by applying standardised guide-word prompts to the review of each node. The relevant international standard[1] calls for team members to display 'intuition and good judgement' and for the meetings to be held in 'a climate of positive thinking and frank discussion'.

The HAZOP technique was initially developed in the 1960s to analyze major chemical process systems but has since been extended to other areas, including mining operations and other types of process systems and other complex systems such as nuclear power plant operation and software development. It is also used as the basis for reviewing batch processes and operating procedures.


The method is applied to complex 'processes' for which sufficient design information is available, and not likely to change significantly. This range of data should be explicitly identified and taken as the ‘design intent’ basis for the HAZOP study. For example, a prudent designer will have allowed for foreseeable variations within the process creating a larger design envelope than just the basic requirements and the HAZOP will be looking at ways in which this might not be sufficient.

For processes plant, the nodes are chosen so that for each a meaningful design intent can be specified and they are commonly indicated on piping and instrumentation diagram (P&IDs) and process flow diagram (PFD). The extent of each node should be appropriate to the complexity of the system and the magnitude of the hazards it might pose. However, it will also need to balance between "too large and complex" (fewer nodes, but the team members may not be able to consider issues within the whole node at once) and "too small and simple" (many trivial and repetitive nodes, each of which has to be reviewed independently and documented).

For each node in turn the HAZOP team uses a list of standardised guide-words and process parameters to identify potential Deviations from the design intent. For each deviation, the team identifies feasible Causes and likely Consequences then decides (with confirmation by subsequent risk analysis where necessary) whether the existing safeguards are sufficient, or whether an Action to install an additional safeguard is necessary to reduce the risks to an acceptable level.

The degree of preparation for the HAZOP is critical to the overall success of the review - 'frozen' design information provided to the team members with time for them to familiarise themselves with the process, an adequate schedule allowed for the performance of the HAZOP, provision of the best team members for their role. Those scheduling a HAZOP should take into account the review scope, the number of nodes to be reviewed, the provision of completed design drawings and documentation and the need to maintain team performance over an extended time-frame. The team members may also need to perform some of their normal tasks during this period and the HAZOP team members can tend to lose focus unless adequate time is allowed for them to refresh their mental capabilities.

The team meetings should be managed by an independent, trained HAZOP Facilitator who is responsible for the overall quality of the review, partnered with a dedicated Scribe to minute the meetings. "The success of the HAZOP study strongly depends on the alertness and concentration of the team members and it is therefore important that the sessions are of limited duration and that there are appropriate intervals between sessions. How these requirements are achieved is ultimately the responsibility of the study leader."[1]

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.[2] Various software programs are now available to assist in meetings.

Guide words and parameters[edit]

In order to identify deviations, the team applies (systematically, in order [a]) a set of Guide Words to each node in the process. To prompt discussion, or to ensure completeness, it may also be helpful to explicitly consider appropriate parameters which apply to the design intent. These are general words such as Flow, Temperature, Pressure, Composition. The current standard[1] notes that Guide Words should be chosen which are appropriate to the study and neither too specific (limiting ideas and discussion) nor too general (allowing loss of focus). A fairly standard set of Guide Words (given as an example in Table 3 of [1]) is 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 / INSTEAD 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

(The last five guide words are applicable to batch or sequential operations.) Where a guide word is meaningfully applicable to a parameter e.g. NO FLOW, MORE TEMPERATURE, their combination should be recorded as a credible potential deviation (from the design intent) that requires review.

HAZOP-type studies may also be carried out by considering applicable guide words and identifying elements to which they are applicable[1] or by considering the parameters associated with plant elements and systematically applying guide words to them; although this last approach is not mentioned in the relevant standard, its examples of output include a study (B3) recorded in this way.[1] 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 [b] 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 should then be subject to another HAZOP, to ensure that no new problems have been added. [c]

The technique can also be applied where design information is not fully available and doing so may be useful in eliminating alternative designs, before too much time is invested in them. However, where a design is required to have a HAZOP performed to meet legislative or regulatory requirements, such an 'early' meeting cannot be considered to comply with this requirement.

"The term HAZOP has been often associated, in a generic sense, with some other hazard identification technique. The use of the term with such techniques is considered to be inappropriate and is excluded from this document."[1]


A HAZOP study is a team effort. The team should be as small as possible consistent with their having relevant skills and experience [d] A minimum team size of 4[1]-5 [3] is recommended. In a large process there will be many HAZOP meetings and the individuals within the team may change as different specialists are required and deputies are required for the various roles. As many as 20 individuals may be involved[4] but is recommended that the team should not exceed 7[1]-8[3] at any time (a larger team will make slower progress adding considerably to the costs). Each team member should have a definite role as follows [1] Note that duplication of roles (e.g. Client, Contractor & Project Management representatives) should be avoided:

Name Role Comment
Study Leader / Chairman / Facilitator

(Full-time attendee)

To manage the team meetings Someone experienced in leading HAZOPs, who is familiar with this type of process but is independent of the design team. Responsible for progressing through the series of nodes, moderating the team discussions, maintaining the accuracy of the record, ensuring the clarity of the recommended actions and identifying appropriate actionees.
Recorder / Secretary / Scribe

(Full-time attendee)

To minute the team meetings To document the Causes, Consequences, Safeguards and Actions identified for each deviation, to record the conclusions of team discussions (accurately but comprehensibly), [e] to document problems and recommendations
Process Designer / Engineer

(Full-time attendee)

Representing the team which has designed the process To provide design intent details or explain any further information
Operator / User

(Full-time attendee)

Representing those who will operate the process [f] To consider the operation, and the potential causes and consequences of deviations

To question its operability of the process

Discipline / Vendor Specialist

(Part-time attendee)

Providing specialist guidance to the team; e.g. Instrumentation, Human Factors Specialist, 3rd-party equipment To provide specialist technical knowledge not available within the team, e.g. instrumented control systems, human reliability analysis, design & operation of 3rd-party (vendor) equipment

(Part-time attendee)

Providing specialist guidance to the team on maintainability issues To consider the maintenance of the plant equipment and question its maintainability.

In earlier publications it was suggested that the Study Leader could also be the Recorder[4] but separate roles are now generally recommended.

The use of computers and projector screens can enhance the recording of meeting minutes (the team can see what is minuted and ensure that it is accurate), the display of P&IDs for the team to review, the provision of supplemental documented information to the team and the logging of non-HAZOP issues that may arise during the review, e.g. drawing/document corrections and clarifications. Specialist software is now available from several suppliers to support the recording of meeting minutes and tracking the completion of recommended actions.


Although a number of companies were addressing this issue, the technique is generally considered to have originated in the Heavy Organic Chemicals Division of Imperial Chemical Industries (ICI), which was then a major British and international chemical company. The history has been described by Trevor Kletz[4][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.[4] By this time, hazard and operability studies had become an expected part of chemical engineering degree courses in the UK.[4]

See also[edit]


  1. ^ If an individual team member spots a problem before the appropriate guideword is reached it may be possible to maintain rigid adherence to order; if most of the team wants to take the discussion out of order no great harm is done if they do, provided the Study Leader ensures that the secretary is not becoming too confused, and that all guidewords are (eventually) adequately considered
  2. ^ This relates to the Distributed Control System (DCS) hardware only; software (unless specially carefully written) must be assumed to be capable of attempting incorrect or inopportune operation of anything under its control
  3. ^ ie the modifications (and their possible effect on other plant items) should undergo re-HAZOP
  4. ^ and affiliation "Where a system has been designed by a contractor, the HAZOP team should contain personnel from both the contractor and the client."[1]
  5. ^ e.g. he is unclear what conclusion has been reached against a guideword (or he suspects the Study Leader has missed one)
  6. ^ If similar plant exists, its users should also be represented


  1. ^ a b c d e f g h i j k 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."
  2. ^ 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"
  3. ^ a b 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
  4. ^ a b c d e Kletz, T. A., (1983) HAZOP & HAZAN Notes on the Identification and Assessment of Hazards IChemE Rugby
  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[edit]