The UK HSE has prosecuted two companies after an explosion killed four workers and seriously injured another at an oil refinery in Pembrokeshire in 2011. Five workers were emptying a tank using a vacuum tanker when the explosion and subsequent fire took place. The explosion resulted in a fireball which severed the 5-tonne tank roof, and this was projected 55 metres to impact against a butane storage sphere. The roof narrowly missed a pipe track where a range of flammable materials were carried.
Investigation found the explosion was most likely to have been initiated by the ignition of a highly flammable atmosphere within the tank, during what should have been a routine emptying operation [IMCA emphasis] in preparation for further cleaning and maintenance. The investigation also found there had been longstanding failures within the refinery safety management systems (SMS) and as a result the risks posed by flammable atmospheres …were not understood or controlled. [IMCA emphasis]
Please see here for the full UK HSE press release.