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HSE publishes Buncefield-related research on flammable vapour

17 July 2012

The Health and Safety Executive-funded research report ‘Flammable vapour cloud risks from tank overfilling incidents’ has recently been published in full. The report documents research undertaken in 2007 and 2008 but which was not published on completion due to constraints on Health and Safety Laboratory staff involved in the investigation arising from the legal case. 

The huge blast at the Buncefield fuel depot in Hertfordshire on 11 December 2005 was caused by the overfilling of a large petrol-storage tank and subsequent formation of a flammable vapour cloud, which spread off-site and ignited.

A series of investigations and reports were carried out on the incident; this latest report (RR937) includes useful information arising from a joint study of tank designs carried out with Shell Global Solutions. The analysis describes how different tank designs are likely to behave in an overflow situation, and the impact on the production rate of flammable vapour. 

Work in the area of Computational Fluid Dynamics (CFD) provides a first step towards developing a mathematical model to predict the size of flammable vapour clouds from overfilling releases, based on simple measures such as the tank dimensions, tank design type, pumping rates and liquid composition.

It also highlights the fact that there are important processes occurring at the bottom of the cascade near to the tank base that were not well understood at the time the report was originally completed, most notably splashing and pool evaporation. These issues were explored in research report RR908.

A related report also recently published – RR936 Buncefield investigation: Liquid flow and vapour production – looks at the connection between the loss of containment and the formation of a flammable vapour cloud. Practical and numerical investigations have demonstrated that the bulk of fuel vaporisation and entrainment of air occurred during the cascading of fuel from the top of the tank into the bund.

The work involved the construction of a fullscale replica of a section of top of the tank involved at Buncefield and also a full-height section of the tank wall, to enable liquid flow experiments to be carried out.

For its research, HSE constructed a full scale replica of a section of top of the tank involved at Buncefield and also a full height section of the tank wall. This enabled it to simulate the development of the vapour cloud.

Roughly half of the liquid released flowed out over the edge of the tank and formed a freely falling spray, the study found. The other liquid was initially deflected by an inclined plate and ran down the tank wall.

Around 9 metres above the bund, liquid running down the wall hit a circumferential stiffening girder (wind girder) and was projected out, away from the tank, passing through the cascade of liquid from above.

The findings also showed that overall liquid flow results in a relatively fine spray, with droplets a few millimetres in diameter.

Analysis of heat, mass and momentum transfer found that the fuel cascade drives a significant downward flow of air, which was contaminated by high concentrations of light hydrocarbons.

“The hydrocarbon mixture entering the vapour cloud that surrounds the tank is fuel rich. The volume flow is sufficient to establish a cloud of depth 1.6 - 2 m deep over the area affected by the vapour cloud explosion,” said HSE.

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