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.
Toxicity is the degree to which a substance or mixture of substances can harm humans or animals.
Toxic substances can affect humans in four main ways: by being swallowed (ingestion); through skin contact; through the lungs (inhalation) and through the eyes.
Static electricity presents fire and explosion hazards during the handling of petroleum and during other tanker operations such as tank cleaning, dipping, ullaging and sampling. Certain operations can give rise to accumulations of electric charge which may be released suddenly in electrostatic discharges with sufficient energy to ignite flammable hydrocarbon gas/air mixtures. There are three basic stages leading up to a potential static hazard:
- Charge separation;
- Charge accumulation; and
- Electrostatic discharge.