Case Studies - Flash Fires

Accidental releases of flammable liquids or gases can result in the formation of a cloud of flammable vapour. This cloud is often denser than air and can drift away from industrial sites, keeping close to the ground. If the cloud encounters an ignition source a fire may occur with combustion propagating through the cloud. The characteristics of the fire will vary depending on the release scenario and may result in a flash, jet or pool fire, a fireball or even a vapour cloud explosion.

Current guidance on vapour cloud fires advises that anyone caught in the 'footprint' of the ignited cloud will suffer fatal injuries, the 'footprint' being defined as the region in which the concentration of vapour is greater than half its lower flammability limit. However, evidence from a number of incidents has indicated that the guidance is unreliable, with some victims surviving relatively unharmed and the burn area being less clearly defined.

An international project with industry was set up to investigate further the behaviour of vapour cloud fires. The aim was to produce experimental data for the development of improved computer models for predicting the behaviour of the fires. HSL undertook trials to study the ignition of vapour clouds formed from the release of liquefied petroleum gas (LPG).

The trials demonstrated how the concentration of gas within the cloud varied with the atmospheric environment and the release conditions. The presence of obstructions in the path of the vapour cloud (for example, a fence) dramatically altered the concentration of LPG vapour. The concentration of vapour in the cloud was generally low and the fires produced were relatively lean. The results obtained from the experiments are being used to validate a flash fire computer model developed for HSE. The findings are also being incorporated into an improved vapour cloud fire model for industry use when assessing both on and off site hazards.

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