A Hazop meeting is generally scheduled for 3 hours. 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.
|NO OR NOT||Complete negation of the design intent|
|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||Partly||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|
|Vibrations||too low||too high||none||wrong frequency|
|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 but separate roles are now generally recommended. A minimum team size of 5 is recommended. 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 but is recommended that no more than 8 are involved at any one time. Software is now available from several suppliers to aid the Study Leader and the Recorder.
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. In 1977 the Chemical Industries Association published a guide. 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. By this time, hazard and operability studies had become an expected part of chemical engineering degree courses in the UK.