FLAMMABILITY:
When petroleum is ignited, it is the gas that is progressively given off by the liquid which burns as a visible flame. The quantity of gas available to be given off by a petroleum liquid depends on its volatility.
Petroleum gases can be ignited and will burn only when mixed with air in certain proportions. If there is too little or too much petroleum gas, the mixture cannot burn. The limiting proportions, expressed as a percentage by volume of petroleum gas in air, are known as the Lower and Upper Flammable Limits.
For gas mixtures from petroleum liquids likely to be encountered in normal tanker trades, the overall range is from a minimum Lower Flammable Limit of about 1% gas by volume in air to a maximum Upper Flammable Limit of about 10% gas by volume in air.