The series of events that led to the accident started on the morning of Sunday 24 July 1994. A severe electrical storm caused plant disturbances that affected the vacuum distillation, alkylation and butamer units as well as the fluidised catalytic cracking unit (FCCU). The crude distillation unit that provided feed to the Pembroke Cracking Company (PCC) units was shut down as a result of a fire, which had been started by a lightning strike. During the course of the morning all PCC units except the FCCU were shut down.
However the direct cause of the explosion that occurred some five hours later was a combination of failures in management, equipment and control systems during the plant upset. These led to a release of about 20 tonnes of flammable hydrocarbons from the outlet pipe of the flare knock out drum of the FCCU.
The explosion was caused by flammable hydrocarbon liquid being continuously pumped into a process vessel that, due to a valve malfunction, had its outlet closed. The only means of escape for this hydrocarbon once the vessel was full was through the pressure relief system and then to the flare line. The flare system was not designed to cope with this excursion from normal operation and due to liquid breakthrough at the FCCU flare knock out drum, a failure occurred in the outlet pipe.
A total of 20 tonnes of a mixture of hydrocarbon liquid and vapour was released, which found a source of ignition about 110 m from the flare drum and subsequently exploded. This caused a major hydrocarbon fire at the flare drum outlet itself and a number of secondary fires. The fires were effectively contained and escalation prevented by cooling nearby vessels that contained flammable liquids. As the explosion had incapacitated the flare relief system, the safest course of action was to allow the fires to continue to burn which they did, finally being extinguished on the evening of Tuesday 26 July 1994.
Health and Safety Executive, ‘The explosion and fires at the Texaco Refinery, Milford Haven, 24 July 1994: A report of the investigation by the Health and Safety Executive into the explosion and fires on the Pembroke Cracking Company Plant at the Texaco Refinery, Milford Haven on 24 July 1994’, ISBN 0 7176 1413 1, 1997.