is incident occurred on 11th December 2005 on the Hertfordshire Oil Storage Terminal near Hemel Hempstead in England. The incident was the result of a storage tank overfill that formed a dense vapour cloud which subsequently ignited and exploded. The MIIB has officially stated that the reason the overfill occurred was because the level measurement gauge on the tank did not alter in a three-hour period, despite the fact it was being continuously fed unleaded petrol via a pipeline from the Lindsey Oil Refinery in Lincolnshire. The third progress report as presented in The Buncefield Incident 11 December 2005, The Final Report of the Major Incident Investigation Board, Volume 2, stated that findings of the investigation into the instrumentation and controls confirmed this1. It emphasised that in the three-hour period prior to the incident, the level gauge of this tank remained static, despite there being a continuous transfer to it.
This loss of containment (LOC) incident was in part due to shortcomings in the control and instrumentation, and in particular the failings of the tank gauging system in place on the tank that monitored the level of fuel stored in that tank. A major outcome from this investigation was the issuing of a safety notice on the type of switch used on this tank gauging system. The safety alert issued by HSE on 4th July 2006 can still be viewed online at http://www.hse.gov.uk/comah/alerts/sa0106.htm. The safety alert note names the actual type of switch and its manufacturer, and strongly advises against their use on COMAH oil/fuel storage sites. The various switches commonly used in tank gauging applications will be discussed later in this report.