Buncefield blast linked to faulty controls
24 Jun 2006
Investigators have identified failures of the automatic tank gauging (ATG) and safety switch systems on a fuel storage tank as root causes of the massive explosions and fire at the Buncefield oil depot on 11 December last year.
According to a report from the Buncefield Investigation Board, at around 7.00pm the previous evening Tank 912 in bund A of the Hertford Oil Storage Ltd (HSOL) West site at Buncefield started receiving unleaded motor fuel from a 14-inch pipeline, pumping at a rate of 550m3/h.
From around 3.00am on 11 December, the level gauge on the tank recorded an unchanged reading, even though filling continued at the same rate. The tank should have been full by 5.20am, but 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 that flowed off-site. Meanwhile, fuel continued to be pumped into Tank 912, reaching a rate of 890m3/h, when flow from the pipeline to another part of the site stopped.
This led to the creation of a highly flammable fuel mist, which travelled around the site before reaching a source of ignition at 6.01am, when the first explosion occurred.
Experts are still investigating why the safety switch and associated trip instrumentation failed to prevent Tank 912 from overfilling. Information from the relevant SCADA system has indicated that no ultimate high level alarm was received. The actual switch, which has only recently been located at the blast site, is now undergoing detailed forensic investigation.
One industry expert told PE that the alarm system for tanks such as those at Buncefield are checked periodically, but perhaps no more frequently than every 12 months. However, he said high level controls of this type have been a source of problems for many years, forcing some companies to resort to dipping in order to check levels.
Another interim report on the incident is expected by the first week of July.
