Flixborough (Nypro UK) Explosion 1st June 1974

Accident 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 recognised 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 were hampering the rescue work.

Failings in technical measures

A plant modification occurred without a full assessment of the potential consequences. Only limited calculations were undertaken on the integrity of the bypass line. No calculations were undertaken for the dog-legged shaped line or for the bellows. No drawing of the proposed modification was produced.
Plant Modification / Change Procedures: HAZOP
Design Codes - Pipework: use of flexible pipes
No pressure testing was carried out on the installed pipework modification.
Maintenance Procedures: recommissioning
Those concerned with the design, construction and layout of the plant did not consider the potential for a major disaster happening instantaneously.
Plant Layout: positioning of occupied buildings
Control Room Design: structural design to withstand major hazards events
The incident happened during start up when critical decisions were made under operational stress. In particular the shortage of nitrogen for inerting would tend to inhibit the venting of off-gas as a method of pressure control/reduction.
Operating Procedures: number of critical decisions to be made
Inerting: reliability/back-up/proof testing

References

Health and Safety Executive, 'The Flixborough Disaster : Report of the Court of Inquiry', HMSO, ISBN 0113610750, 1975.