Caprolactam (an intermediate product in the production of nylon) was being manufactured by oxidation of cyclohexane with air in a series of 6 mild steel, inter-connected reactors. A temporary 20″ NS (DN 500) bypass pipe assembly incorporating expansion joints (bellows units) had been installed around one of the reactors to enable it to be taken off-line to repair a large crack. On the day of the incident (Saturday), while the plant was on hot circulation pending restart, the bypass line ruptured releasing 30 tonnes of hot cyclohexane that formed a flammable cloud and subsequently found an ignition source. A huge unconfined vapour cloud explosion (UVCE) occurred and 28 employees were killed instantly (18 of them in the control room). The entire plant was destroyed and 1821 homes and 167 business premises suffered significant damage. The resulting fire burned for 3 days. The loss of life would have been greater if the explosion had occurred on a weekday.

Basic Cause:
Hot cyclohexane release to atmosphere due to squirm and rupture of a bellows unit in the temporary reactor bypass pipe assembly.