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Liquid Fuel

The flammable liquids most commonly found are lubricating oil, diesel oil, kerosene, oil-base paints and their solvents.

Vaporization. Flammable liquids release vapour in much the same way as solid fuels. The rate of vapour release is greater for liquids than solids, since liquids have less closely packed molecules. In addition, liquids can release vapour over a wide temperature range. Gasoline starts to give off vapour at -43 °C (-45 °F). This makes gasoline a continuous fire hazard; it produces flammable vapour at normal temperatures. Heating increases the rate of vapour release.

Heavier flammable liquids such as lubricating oil must be heated to release sufficient vapour for combustion. Lubricating oils can ignite at 204 °C (400 °F). A fire reaches this temperature rapidly, so that oils directly exposed to a fire will soon become involved. Once a light or heavy flammable liquid is burning, radiation feedback and the chain reaction quickly increase flame production.

The vapour produced by a flammable liquid is heavier than air. This makes the vapour very dangerous, because it will seek low places, dissipate slowly, and travel to a distant source of ignition. For example, on board vapour escaping from a container can travel along a deck and down deck openings until it contacts a source of ignition (such as a spark from an electric motor). If the vapour is properly mixed with air, it will ignite and carry fire back to the leaky container. The result can be a severe explosion and fire.

Burning. Pound for pound, flammable liquids produce about 2,5 times more heat than wood. This heat is liberated 3 to 10 times faster from liquids than from wood. These ratios illustrate quite clearly why flammable liquid vapour burns with such intensity. When flammable liquids spill, they expose a very large surface area, release a great amount of vapour and thus produce great amount of heat when ignited. This is one reason why large open tank fires and liquid-spill fires burn so violently.

Flash Point. The flash point of a liquid is the temperature at which it gives off sufficient vapour to form an ignitable mixture near its surface. An ignitable mixture is a mixture of vapour and air that is capable of being ignited by an ignition source, but usually is not sufficient to sustain combustion.

Sustained combustion takes place at a slightly higher temperature, referred to as the fire point of the liquid. The flash points and fire points (temperatures) of liquids are determined in controlled tests.

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