Process Safety · Case study

Flixborough, 1974: the temporary bypass

A makeshift 20-inch pipe installed to keep the plant running failed. Hot cyclohexane escaped and exploded, destroying the site.

Date1 June 1974

WhereNypro, Flixborough, UK

ProcessCyclohexane oxidation (nylon intermediate)

Deaths28 on site

Injuries36 on site, 53 reported off site

DamageAbout 2,000 properties

What happened

The plant oxidised cyclohexane in a train of six reactors at about 155 °C and 8.8 bar. In March 1974 a crack was found in reactor 5, so it was removed and a 20-inch bypass pipe was installed between reactors 4 and 6 to keep production going. Because the reactor nozzles were at different heights, the bypass had a “dog-leg” shape and was supported on scaffolding, with bellows at each end.

No design calculation was made, no drawing beyond a chalk sketch on the workshop floor, no pressure test of the assembly, and the bellows manufacturer’s guidance was not followed. On 1 June the bypass failed. About 30–40 tonnes of hot cyclohexane flashed into a vapour cloud, which ignited and exploded.

Sequence of events

Mar 1974Crack found in reactor 5. Decision to bypass it and restart quickly.

Days laterBypass designed and built by site staff; no qualified mechanical engineer in post.

Apr–MayPlant runs with the bypass.

1 Jun, 16:53Bypass ruptures during a pressure rise. Vapour cloud forms and explodes within about a minute. The control room is destroyed.

Why it happened

No management of change: a major modification treated as a quick repair; no hazard review.
No engineering design: forces on the dog-leg and bellows not calculated; no reference to the design code; no pressure test.
Missing expertise: the works engineer post was vacant and nobody with the right competence was involved.
Production pressure: restarting fast took priority over understanding the original crack.
Large inventory under pressure and an occupied control room close to the plant.

Lessons for your plant

Every modification, temporary or permanent, must go through MOC and be designed, built and tested to the same standard as the original plant.
When a key post is vacant, decisions that need that expertise must get it elsewhere.
Find the cause of a failure (the crack) before restarting.
Locate control rooms and occupied buildings away from large flammable inventories, or make them blast-resistant.

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Sources

Summary written for training. Figures are as published by the investigating bodies and widely cited sources; see the sources for full details.