The Hydrocracker Explosion and Fire at BP Oil, Grangemouth Refinery. 22nd March 1987

Accident summary

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.

Failures in technical measures

Operators did not trust the reading given by the float gauge in the HP separator and the nucleonic level indication was misread due to an offset on the chart recorder.
Maintenance Procedures: fault reporting systems
Control Systems: sensors
Operating Procedures: provision of comprehensive written operating procedures
The low low level trip on the HP separator had been disconnected several years earlier when a plant modification had been made.
Plant Modification / Change Procedures: HAZOP
Alarms / Trips / Interlocks: level measurement
The LP Separator pressure relief had been sized for a fire case, not a gas breakthrough from the HP separator.
Relief Systems / Vent Systems: sizing of vents, relief valves
There were no means of isolating the HP separator following an incident in the LP separator.
Isolation: emergency isolation

References

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.