Until the 1960s, the safety review of process plant designs was largely an exercise based on checklists and experience of what had happened before in similar situations. Much reliance was placed on design codes of practice for each unit operation (e.g. heat transfer, distillation, boilers, etc) but took no account of system interactions or non-standard requirements. Most problems were likely to be missed because of the system complexity rather than because of lack of knowledge by the design team. If a safety review was carried out, it was unlikely to consider the operational aspects of the design in any great detail.

Some approaches such as the Dow Index examined the intrinsic hazards of the materials and the nature of process, but this was largely judgemental. It was applied to fire and explosion hazards, but could not be used to identify process hazards at a detailed level. The Mond Index expanded the technique to include toxic effects, plant layout and storage facilities.

ICI had adopted a ‘critical examination' technique to ask questions such as ‘what is achieved ?' or ‘what else is achieved ?'. In practice, it was cumbersome and time-consuming but it was eventually refined in the 1960s into the present HAZOP technique. The use of HAZOP then spread throughout the chemical industry. Lawley published the first paper on HAZOP in 1974 and a formal guide to Hazard and Operability Studies was produced by the Chemical Industries Association in 1977. The technique has since remained largely unaltered, although the European Process Safety Centre published an updated guide (ISBN 0 85295 4271) in 2000 in conjunction with the UK 's Institution of Chemical Engineers. In the USA , HAZOP is incorporated into the Process Safety Management regulations [OSHA 29 CFR Part 1910]. Today, it is universally recognised throughout many processing industries as the single most effective technique for identifying hazards and poor operability.



HAZOP involves systematically questioning every part of a process to establish how deviations from the design intent can arise.  Once identified, an assessment is made as to whether such deviations and their consequences can have a negative effect upon the safe and efficient operation of the plant.  If the protective measures are insufficient to prevent or control the deviations, action is then taken to remedy the situation.

The HAZOP team has to use their collective experience and imagination to discover credible causes of deviations.  In practice, many of the causes will be fairly obvious but the technique encourages the team to consider other less obvious ways in which a deviation may occur, however unlikely they may seem at first.  In this way, the study has advantages over a mechanistic check-list type of review.  The result is that there is a good chance that potential failures and problems will be identified which had not previously been experienced in the type of plant being studied.