The LWF Blog

Fire Safety for Facilities Management Personnel – How fire works – Part 58

August 10, 2020 1:49 pm

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 57, LWF began to discuss the combustion of flammable liquids. In part 58, we continue looking at flammable liquids.

When considering how to measure the level of hazard posed by a flammable liquid, it is usual for it to be defined by its flashpoint. However, there are other ways of measuring the hazard which are useful, although none as relevant. The flammability limits of the vapor and the auto-ignition temperature can also be taken into consideration. The flammability limit is the proportion as percentage in which the flammable vapour must be present in air for an explosion to occur on ignition of the vapour.

For example, with methane gas, a mixture between 5 per cent and 15 per cent in air is flammable, but if the mixture is either lesser or greater than that range, it will not burn if an ignition source is introduced. Another interesting example is that the headspace of a tank of petrol would contain vapour of too great a percentage for it to burn.

A flammable liquid with a wide flammability limit would clearly be a greater hazard than one with a very narrow limit.

An understanding of flammability limits is useful when considering such fire safety measures as ventilation and in ascertaining the maximum acceptable concentration of vapour in an environment.

The auto-ignition temperature is commonly of less significance, but should still be basically understood when flammable liquids are in use. It is the temperature at which a flammable liquid will spontaneously ignite without an outside source of ignition. This might be seen if a pan of cooking oil was left on a source of heat.

Usually, the auto-ignition temperatures of flammable and highly flammable liquids are high – in the region of 200 ºC–600 ºC – but this refers to the onset of flaming.

Some flammable liquids, fish oils and linseed oil among them, can undergo spontaneous heating at room temperature when absorbed onto rags etc., despite the fact that their flashpoints and auto-ignition temperatures may be high.

In part 59, LWF will begin to look at the combustion of flammable gases. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.