On Saturday 21 March 1987 the hydrocracker unit was being recommissioned following a routine shut down. During the recommissioning a plant trip occurred. This was thought to be a spurious trip and the plant operators started to bring up the unit to normal operating conditions. From then on until the time of the incident the plant was held in stand by condition with no fresh feed.
At 07:00 hours the following morning there was a violent explosion and subsequent fire. The explosion could be heard and felt up to 30km away. The explosion centred on a low pressure (LP) separator vessel, V306, which was fabricated from 18mm steel plate and weighed 20 tonnes.
The investigation of the accident suggested that an air operated control valve on the high pressure (HP) separator had been opened and closed on manual control at least three times. Liquid level in the LP separator fell and the valve was opened. This action allowed the remaining liquid in the HP separator to drain away and for high pressure gas to break through into the LP separator. As the pressure relief on the LP separator had been designed for a fire relief case, not gas breakthrough the vessel subsequently exploded.
The control valve did not close automatically because the low low level trip on the HP separator had been disconnected several years earlier. The operators did not trust the main level control reading and referred to a chart recorder for a back up level reading. There was an offset on this chart recorder which led them to assume that the level in the HP separator was normal.
Health and Safety Executive, ‘The fires and explosion at BP Oil (Grangemouth) Refinery Ltd. A report of the investigations by the Health and Safety Executive into the fires and explosion at Grangemouth and Dalmeny, Scotland, 13 March, 22 March and 11 June 1987’, ISBN 0 11 885493 3, 1989.