Buncefield blast linked to faulty filling controls
9 May 2006
According to a report from the Buncefield Investigation Board, at around 7pm the previous evening, Tank 912 in bund A of the Hertford Oil Storage Ltd West site at Buncefield started receiving unleaded motor fuel. The supply, from a 14-inch pipeline, was pumped in at a rate of 550 cubic metres/hour.
From around 3.00am on 11 Dec, the level gauge on the tank recorded unchanged readings even though filling continued at the same initial rate. While the tank was estimated to have been filled by 5.20am, failure of the ATG allowed pumping to continue, causing fuel to overflow from the tank.
The flow rates of fuel cascading down the tank and through the air promoted rapid formation of a fuel-rich vapour. Meanwhile, fuel continued to be pumped into Tank 912, rising to a rate of 890 cubic metres/hr when flow from the pipeline to another part of the site stopped.
The resulting overflow continued despite various visual and audible alarms being in place. This led to a highly flammable fuel mist, which traveled around the site before reaching a source of ignition at 6.01am -- when the first explosion occurred. Investigators believe the ignition source was most likely located in a nearby car park or on the site itself.
Experts are still investigating why the safety switch and associated trip instrumentation failed to prevent Tank 912 from overfilling as well as other underlying causes of the accident. The actual switch, which has only recently been located at the blast site, is to undergo detailed forensic investigation, the report noted.
