Three Hazop Questions:"What could go wrong?""How would we know it?""What could we do about it?"OverviewProcess Safety Management, within which the Hazard and Operability (HAZOP) discipline is a key component, has been unquestionably successful in reducing the incidence and mitigating the consequences of major accidents in all industries dealing with toxic, reactive, flammable and explosive substances. There has not been quite another Flixborough much less a Bhopal type incident since the widespread advent of these procedures.  This means protecting the communities adjacent to such facilities as well as the workers within them.For the current practitioner, there is an emphasis on this site to the founding purposes and the excellence observed in the initial work. This seems significant now when there have been fewer catastrophes on the front pages of the newspapers.  And especially now with a new generation seeking to ensure safe operations of what are by their component nature, although providing essential goods to the societies we live in, potentially dangerous facilities.For the many others in far-flung spheres of operation who are seeking to mitigate risks, especially against what Robert K. Merton {1}, {2} calls "unintended" or "unanticipated consequences", please consider the general principals applied so successfully in the Process Industries.  Here, (unfortunately) real life and death have been on the line.FlixboroughAccident summary"At about 16:53 hours on Saturday 1 June 1974 the Nypro (UK) site at Flixborough was severely damaged by a large explosion. Twenty-eight workers were killed and a further 36 suffered injuries. It is recognized that the number of casualties would have been more if the incident had occurred on a weekday, as the main office block was not occupied. Offsite consequences resulted in fifty-three reported injuries. Property in the surrounding area was damaged to a varying degree."Prior to the explosion, on 27 March 1974, it was discovered that a vertical crack in reactor No.5 was leaking cyclohexane. The plant was subsequently shutdown for an investigation. The investigation that followed identified a serious problem with the reactor and the decision was taken to remove it and install a bypass assembly to connect reactors No.4 and No.6 so that the plant could continue production."During the late afternoon on 1 June 1974 a 20 inch bypass system ruptured, which may have been caused by a fire on a nearby 8 inch pipe. This resulted in the escape of a large quantity of cyclohexane. The cyclohexane formed a flammable mixture and subsequently found a source of ignition. At about 16:53 hours there was a massive vapour cloud explosion which caused extensive damage and started numerous fires on the site."Eighteen fatalities occurred in the control room as a result of the windows shattering and the collapse of the roof. No one escaped from the control room. The fires burned for several days and after ten days those that still raged