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3.11.5    Flammability of gases and vapours

The main parameters which describe the ability of mixtures of substances in air to propagate a flame and/or explode are flammable limits, flash points and auto-ignition temperatures. None of these parameters are basic physico-chemical properties of a substance or mixture but are defined by standard tests and, for this reason, values are given to the nearest integer. If the test conditions are varied, then the values of the parameters will change. In particular, values will change considerably if the concentration of oxygen in the mixture is changed.

The flammable limits; lower flammable limit and upper flammable limit, are the values, by volume, of the concentrations in air, at normal temperature and pressure, between which a mixture can propagate a flame.

The flash point is the lowest temperature, at one atmosphere pressure, at which a liquid gives off flammable vapour in sufficient quantity to ignite when mixed with air at or near the surface of the liquid on application of a flame or spark. Substances with a flash point <21 °C are termed ‘extremely flammable’. Substances with flash points between 21 °C and 55 °C are termed ‘flammable’ whilst the term ‘combustible’ applies to all other combustible solids and liquids, including those with flash points above 100 °C.

The auto-ignition temperature is the lowest temperature of a glass surface at which droplets of a combustible liquid falling onto the surface will undergo spontaneous combustion. A glass surface is assumed to be chemically inert and in practice, because of catalytic effects, the values for other types of surfaces can be lower.

Aerosols of combustible solids will also propagate a flame in the appropriate concentration range. However, the flammable limits of aerosols are affected by the size of the particles although, in general, the smaller the particle size, the closer the behaviour of the aerosol resembles that of a vapour.

References

For further information see:

Walsh (1989) Chemical Safety Data Sheets, Roy. Soc. Chem., Cambridge.
Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids (1984). NFPA 325M, National Fire Protection Ass., Quincy, M. A.

J.G. Firth

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