Flixborough (Nypro UK) Explosion 1st June 1974
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:
- Design Codes - Pipework: use of
- No pressure testing was carried out on the installed pipework
- Maintenance Procedures:
- Those concerned with the design, construction and layout of the plant
did not consider the potential for a major disaster happening
- Plant Layout: positioning of
- 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
- Operating Procedures: number of
critical decisions to be made
- Inerting: reliability/back-up/proof
Health and Safety Executive, ‘The Flixborough Disaster : Report of the
Court of Inquiry’, HMSO, ISBN 0113610750, 1975.