RR937 - Flammable vapour cloud risks from tank overfilling incidents

This report documents research undertaken in 2007 and 2008 to examine the important factors affecting the production of flammable vapour in incidents where large storage tanks of volatile liquids are overfilled. This is an important output from the Buncefield response programme that was not published on completion due to constraints on HSL staff involved in the investigation arising from the legal case. The work was summarised for the Process Safety Leadership report but has not been published in full.

It includes much 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. It also contains the results of some early Computational Fluid Dynamics (CFD) modelling studies into the vaporisation of volatile components of multi component hydrocarbon mixtures. Simulations are presented that examine the effect of a number of factors on the vapour cloud behaviour, including the bund shape and location, tank proximity, presence of obstacles and spray pattern from the overfilling tank.

The work 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.

This work 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 completed, most notably splashing and pool evaporation. These issues have been explored in a later research report RR908.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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Updated 2022-05-10