EXTERNAL PUBLICATIONS

SUMMARY

Accidental releases of flammable liquids or gases can result in the formation of a cloud of flammable vapour. This cloud is often dense relative to the ambient conditions. If this cloud encounters an ignition source, a fire may occur with combustion propagating through the cloud. The characteristics of the resulting fire depend on the release conditions and the environment into which the vapours are released. Various scenarios may result, including flash fires, fireballs, pool or jet fires or even vapour cloud explosions if the release is into congested areas.

This report describes trials undertaken by the Health and Safety Laboratory (HSL) as part of a Joint Industry Project with partners in Canada, France, Japan, Spain and the United Kingdom. The aim of the project is to produce data for validation of computer models, which are designed to predict the behaviour of vapour cloud fires under different environmental conditions. To date, relatively few experimental investigations have been undertaken.

Two experimental programmes were undertaken as part of the project. Investigations of ignition of vapour clouds formed from Liquefied Natural Gas (LNG) were undertaken by Gaz de France at their site near Nantes, France. The aim was to study the characteristics of fireballs produced from ignition of a highly concentrated vapour cloud at various positions. The trials undertaken at HSL involved the investigation of ignition of vapour clouds formed from horizontal, momentum-jet releases of Liquefied Petroleum Gas (LPG).

In the HSL trials, LPG was released at rates of up to 4.9 kg s-1. The resulting vapour cloud was characterised to determine the cloud temperature and concentration of LPG vapour at different locations. The vapour cloud was ignited at various positions and the behaviour of the resulting fire was observed. Heat flux sensors were used to assess the severity of the vapour cloud fires produced. Ten different cases were examined, based on the requirements to validate a flash fire model developed by WS Atkins for the Health and Safety Executive (HSE). The 10 cases comprised 6 unobstructed releases and 4 releases in the presence of a fence positioned normal to the flow. The main characteristic varied between each case was the point of ignition.

The results of the trials demonstrated how the concentration of gas within the vapour cloud varied with atmospheric conditions and the release environment. The presence of obstructions in the path of the vapour cloud was found to dramatically alter the concentration of LPG vapour in the cloud with, in this case, significant decreases in the vapour concentration downwind of the fence.

The concentration of gas in the vapour clouds formed was generally low and the vapour cloud fires produced were relatively lean. The flames were therefore often invisible. Ignition of the cloud was observed at concentrations below the Lower Flammability Limit (LFL) of 2.2 vol.%. In some cases the cloud was ignited, but the flame did not propagate throughout the cloud, resulting in the formation of isolated pockets of ignition. In no cases were fireballs observed.

The results obtained in these trials are to be used to validate the flash fire model developed for HSE. This work is to be undertaken by WS Atkins and will be reported separately.

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 author(s) alone and do not necessarily reflect HSE policy