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This page discusses the catastrophic incident that occurred at the Flixborough chemical plant in England.
The catastrophe led to the development of many of the elements of
Process Safety Management. |
Details of the Event
The Caprolactam Process
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The Flixborough plant had been in operation since the year 1967. It was operated by Nypro (a joint venture between Dutch State Mines and the British National Coal Board). It produced caprolactam, which is in turn used to manufacture nylon. The process used six large pressurized reactors containing cyclohexane. (Cyclohexane is comparable to gasoline; it is a liquid at ambient conditions, but vaporizes easily and release about the same amount of energy as gasoline when ignited.)
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Prior to the Event
In March 1974 a vertical crack had appeared in Reactor #5. It was decided to remove this reactor and to install a bypass between Reactors 4 and 6. This bypass failed due to lateral stresses in the pipe, probably during a pressure surge. The bypass had been designed by engineers who were not experienced in high-pressure pipework, no plans or calculations had been produced, the pipe was not pressure-tested, and was mounted on temporary scaffolding poles that allowed the pipe to twist under pressure. Moreover, the by-pass pipe was a smaller diameter (20") than the reactor flanges (24") so, in order to align the flanges, short sections of steel bellows were added at each end of the by-pass. Due to the dog-leg shape of the bypass these bellows probably squirmed when under pressure.
The Event Itself
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During the late afternoon on 1 June 1974 the 20 inch bypass system ruptured. This may have been caused by a fire at a nearby 8 inch pipe. The rupture resulted in the release of a large (about 40 tons) cyclohexane to the atmosphere. The cyclohexane/air mixture and subsequently found a source of ignition. It caused a massive explosion. |
All eighteen persons in the control room died as a result of the windows shattering and the collapse of the roof.
Long-Term Impact
The Flixborough event was seminal; nothing in the process industries in the United Kingdom was the same afterwards. The event greatly increased public concern over industrial plant safety, leading to increased regulations in the UK regarding hazardous industrial processes (COMAH regulations).
Within industry, Process Safety Management (PSM) techniques to do with hazards analysis and were developed. For example, the first paper on the topic of HAZOPs was presented by Lawley in the year 1974.
Operational Excellence Impact
The elements of operational excellence that were particularly ineffective are highlighted in the Elements of PSM Table below.
- Process Safety Culture
- Compliance
- Competence
- Workforce Involvement
- Stakeholder Outreach
- Knowledge Management
- Hazard Identification and Risk Management
- Operating Procedures
- Safe Work Practices
- Asset Integrity / Reliability
- Contractor Management
- Training / Performance
- Management of Change
- Operational Readiness
- Conduct of Operations
- Emergency Management
- Incident Investigation
- Measurement and Metrics
- Auditing
- Management Review
Hazard Identification and Risk Management
Had a
Process Hazards Analysis been carried out on the modified system, it is likely that the analysis team would have called for more rigorous engineering to be carried out on the bypass.
Asset Integrity / Reliability
Although Management of Change received the most attention in terms of the elements of PSM, the issue of Asset Integrity was also critically important. After all, if the leak in the pressure vessel had not occurred, there would have been no need to install a bypass.
In addition, the control room was not blast-resistant. Everyone in it died. Had this building been hardened, their chance of survival would have been much greater.
Management of Change
The Flixborough event is well known as being the driver for implementing
Management of Change programs in the process industries.
Emergency Management
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If a man's not there, he can't be killed.
Ian Sutton |
One of the saddest aspects of this incident was the fact that there was probably a considerable gap between the time of the large leak and of the explosion itself. During that time, the operators were presumably trying to control the leak and to take other emergency actions. Yet they should have realized that they had "lost it". The best thing that they could have done would have been to quickly remove themselves from the scene. After all, although the explosion damaged nearby houses, no members of the public was killed. Therefore, had the operations personnel escaped the scene, they would likely have survived. The plant would have been destroyed - but that happened anyway.
Incident Investigation
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For every complex problem there is an answer that is clear, simple — and wrong.
H.L. Mencken (1880-1956)
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One of the recurring issues to do with the investigation of large accidents is that the causes are often more complex and subtle than originally thought. For example, in the case of the Flixborough event, one of the original investigators, the scientist Ralph King believes that the cause was more than a simple mechanical failure. He states that the event was triggered by a process upset caused by the addition of water to the reactors and a failure of the agitation system.
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