On 1 June 1974 the Nypro (UK) site at Flixborough was severely damaged by a large explosion that killed 28 people. 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. Within a minute, about 40 tonnes of the plant's 400 tonne store of cyclohexane leaked from the pipe and formed a vapour cloud of 100 - 200 m diameter. The cloud, on finding an ignition source (probably a furnace at a nearby hydrogen production plant) exploded, started numerous fires on the site completely destroying the plant. The fuel-air explosion was estimated to be equivalent to 15 tonnes of TNT and it killed all employees in the nearby control room.

The Impact

Twenty-eight workers were killed and a further 36 were severely injured. Around the site, 1800 buildings within a mile radius of the site were damaged. The fires burned for several days and even after ten days the fire hampered the rescue work immensely. Since, the incident took place on a Saturday, the number of casualties were less, 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. The loss was approx. $ 66 mill which is equivalent with approx. $ 200 mill at present value.

The Lessons Learnt from the Disaster

There was no professional engineer in the plant at the time of the accident. The temporary modification was constructed by people who did not know to design large pipes equipped with bellows. The Flixborough plant contained approx. 400 MT of inventory of which 40 or 50MT escaped. The inventory was large because the conversion was low and most of the material had to be recovered and recycled. The most important lesson that Flixborough taught is the need to minimize inventories of hazardous materials.

The disaster led to a widespread public outcry over industrial plant safety, and significant tightening of the UK government's regulations covering hazardous industrial processes. All refineries and related petrochemical industries were shocked from the accident, although they were aware of the risk of plant modifications and immediately improved their procedures and checklists in order to approve plant modifications. Temporary modifications were even excluded in some cases or approved after thorough examination. Although many modification accidents occurred, Flixborough is still the stock example, the most disastrous of them all..