BRITAIN: Investigators inquiring into last year's huge explosion at the Buncefield oil depot north of London said yesterday the blast was probably caused by the ignition of a vapour cloud after a storage tank overflowed. An automatic safety system should have prevented the overflow but had failed to cut off the fuel supply, they added.
A series of explosions at the depot, in Hemel Hempstead, Hertfordshire, early on December 11th caused a three-day inferno which became one of Britain's biggest ever peacetime fires.
The Buncefield Major Investigation Board said in a report that the tank - number 912 - had been overflowing for more than 40 minutes before the blast.
But extensive damage meant it had not been possible to identify what sparked the cloud of fuel vapour that would have formed. Possible candidates were a nearby emergency generator, a fire pump-house or even a spark from a passing car.
Unleaded fuel was being piped into the tank for 11 hours before the blast and it would have been full by 5.20am, some 41 minutes before the explosion, the report said. The tank's rising level and subsequent overflow failed to trigger alarms in the control room.
The report said gauges monitoring the level in the tank had stuck, showing it to be only two-thirds full, even as more fuel was being pumped in. An automatic override designed to stop any overflow failed to cut the supply.
"At the time of the incident, automatic shutdown did not take place," the report added. "Tank 912 would have been completely full at approximately 5.20am, overflowing thereafter."
Seismological evidence showed the main explosion occurred at precisely 32 seconds after 6.01am. Eyewitness accounts described the main blast being followed by a number of smaller explosions, probably some minutes later. The report said the subsequent blasts were probably internal tank explosions or further release of fuel from damaged tanks and pipework.