2.2.1 A mixture of hydrocarbon gas and air cannot
ignite unless its composition lies within a range of gas in air concentrations
known as the “flammable range”. The lower limit of this
range, known as the “lower flammable limit” is any hydrocarbon
concentration below which there is insufficient hydrocarbon gas to
support combustion. The upper limit of the range, known as the “upper
flammable limit” is any hydrocarbon concentration above which
there is insufficient air to support combustion.
2.2.2 The flammable limits vary somewhat for different
pure hydrocarbon gases and for the gas mixtures derived from different
petroleum liquids. In practice, however, the lower and upper flammable
limits of oil cargoes carried in tankers can be taken, for general
purposes, to be 1% and 10% hydrocarbon by volume, respectively.