The Hydrocracker Explosion and Fire at BP Oil, Grangemouth Refinery.
22nd March 1987
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
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
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
- 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:
- Alarms / Trips / Interlocks: level
- 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
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.