Fire Extinguishment

It is known that three factors are essential to combustion, namely:

1). The presence of a fuel, or combustible substance.

2). The presence of oxygen (usually as air) or other supporter of combustion.

3). The attainment and maintenance of certain minimum temperature.

Fire extinction, in principle, consists in the limitation of one or more of these factors, and methods of the extinction may therefore be conveniently classified under the following headings:

1) Starvation, or the limitation of fuel.

2) Smothering, or the limitation of oxygen.

3) Cooling, or the limitation of temperature.

The primary aim of the Fire Service is to save life, then to extinguish fire.

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Extinguishing Media

Nitrogen has a boiling point of 195⁰ С and a critical temperature 147°C and therefore cannot be kept as a liquid under pressure alone. It is used as a compressed gas for an expelling with many other extinguishing media. In some fixed installations it is used as a compressed gas in the cylinders. In hand extinguishers as a compressed gas it has never been popular because carbon dioxide is so readily available in liquid form.

Nitrogen can be supplied in bulk as a cryogenic liquid in insulated tankers and stored in vacuum insulated evaporators for use in industrial processes. In such circumstances the nitrogen could well be used as a fire - fighting medium in fixed installations. But although tests have shown that liquid nitrogen is superior to carbon dioxide as an extinguishant, for all practical purposes carbon dioxide is more convenient. For all situations where damage to equipment or fabric cannot be permitted carbon dioxide remains the ideal extinguishing media.

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Alcohol Resistant Foams

For fires involving polar solvents it is necessary to use special foams. Normal protein and synthetic foams of medium and high expansion are quite ineffective against these water soluble flammable liquids since their foam bubbles are broken down immediately on contact with the solvent surface.

Among the most important polar-solvents in wide - spread industrial use are alcohol (e. g. ethanol, methanol, isopropanol) and Ketones.

Alcohol resistant foam compounds contain special additives capable of forming a protective barrier in the bubble walls to retard destruction of the foam. They should be applied gently onto the fuel surface, particularly where deep layers of fuel are involved such as storage tanks.

Alcohol resistant foams may also be used against hydrocarbon liquid fires but generally are not as effective as protein foams and thus are not to be recommended where a major hydrocarbon risk exists.

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Carbon Dioxide

At normal temperatures carbon dioxide is a gas 1,5 times as dense as air. It is easily liquefied and bottled when it is contained under a pressure. When the cylinder is opened the liquid boils off rapidly as gas, extracting the latent heat from the surrounding atmosphere. Intense refrigeration occurs with some of the gas appearing as snow.

It should be noted that materials which supply their own oxygen will continue to burn, as anу material that tends to decompose the carbon dioxide, such as burning magnesium. Also, when used in the open air a strong breeze will tend to disperse the gas before the smothering effect can control the fire. Apart from these considerations, carbon dioxide is quick and clean, non-conductive, non-toxic and does not harm to most fabric.

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