Elements of Fire

The phenomena of fire are complex, but they may be described by major characteristics: rapid oxidation with the evolution of heat and light. A knowledge of the conditions that determine whether this rapid oxidation will take place with any material is essential for understanding the principles underlying fire prevention, control and extinguishment.

Every combustible substance has temperatures at which combustion can take place. They fall into three categories, namely: flash point, fire point and spontaneous ignition temperature.

The flash point of a substance is the lowest temperature at which it will give off enough vapours to produce a momentary flash of a small fire. There are several types of apparatus for determining the flash point of a material.

The fire point is the lowest temperature at which a substance will continue to burn when ignited. The fire point is generally slightly above the flash point.

Spontaneous ignition temperature of a substance is the lowest temperature at which its vapour, usually a gas generated through the application of heat, will ignite independently of any external source of ignition. A substance or its vapour, however, does not have to be heated in order to be ignited.

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Solid Fuel

Before solid fuel will burn, it must be changed to the vapour state. In a fire situation, this change usually results from the initial application of heat. The process is known as pyrolysis, which is generally defined as "chemical decomposition by the action of heat". In this case, the decomposition causes a change from the solid state to the vapour state. If the vapour mixes sufficiently with air and is heated to a high enough temperature (by a flame, spark, hot motor, etc.), combustion results.

The burning rate of a solid fuel depends on its configuration. Solid fuels in the form of dust or shavings will burn faster than bulky materials (that is, small wood chips will burn faster than a solid wooden beam). Finely divided fuels have much larger surface area exposed to the heat. Therefore, heat is absorbed much faster, and vaporization is more rapid. More vapour is available for ignition, so it burns with great intensity and the fuel is quickly consumed. On the other hand, a bulky fuel will burn longer than a finely divided fuel.

Dust clouds are made up of very small particles. When a cloud of flammable dust (such as grain dust) is mixed well with air and ignited, it burns extremely quickly, often with explosive force. Such explosions have occurred for instance on ships during the loading and discharging of grains and other finely divided materials.

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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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Gaseous Fuel

There are both natural and manufactured flammable gases.

Burning. Gaseous fuels are already in the required vapour state. Only the proper intermix with oxygen and sufficient heat are needed for ignition. Gases, like flammable liquids, always produce a visible flame; they do not smoulder. Radiation feedback is not necessary to vaporize the gas; however, some radiation feedback is still essential to the burning process, to provide continuous resignation of the gas.

Explosive Range (Flammable Range). A flammable gas or the flammable vapour of liquid must mix with air in the proper proportion to make an ignitable mixture. The smallest percentage of a gas (or vapour) that will make an ignitable air-vapour mixture is called the lower explosive limit (LEL) of the gas (or vapour). If there is less gas in the mixture, it is too lean to burn. The greatest percentage of a gas (or vapour) in an ignitable air-vapour mixture is called its upper explosive limit (UEL). If a mixture contains more gas than UEL, it is too rich to burn. The range of percentages between the lower and upper explosive limits is called the explosive range of the gas or vapour.

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Fire Spread

If a fire is attacked early and efficiently, it can easily be confined to the area in which it started. If it is allowed to burn unchecked, it can generate great amounts of heat that will travel away from the fire area, igniting additional fires wherever fuel and oxygen are available. As the original fuel source is consumed, the heat, and thus the fire, will extend to new fuel sources.

Heat from a fire is transferred by one or more of three met­hods: conduction, radiation and convection.

Conduction is the transfer of heat through a solid body. For example, on a hot stove, heat is conducted through the pot to its contents. Wood is ordinarily a poor conductor of heat, but metals are good conductors.

In many cases the skilful application of water, particularly in the form of a spray, will retard or halt the transmission of heat by conduction. A water spray pattern absorbs heat more efficiently than a solid stream, because the smaller water droplets present more surface to the heat source. At the same time, less water is used.

Heat radiation is the transfer of heat from a source across an intervening space: no material substance is involved. The heat travels outward from fire in the same manner as light, that is, in straight lines. When it contacts a body, it is absorbed, reflected or transmitted. Absorbed heat increases the temperature of the absorbing body. For example, radiant heat that is absorbed by a sealing will increase the temperature of that sealing, perhaps enough to ignite its paint.

Heat radiates in all directions unless it is blocked. Radiant heat extends fire by heating combustible substances in its path, causing them to produce vapour, and then igniting the vapour.

Convection is the transfer of heat through the motion of heated matter, i.e. through the motion of smoke, hot air, heated gases produced by the fire.

When it is confined, convicted heat moves in predictable patterns. The fire produces lighter-than-air gases. Heated air, which is lighter than cool air, also rises, as does the smoke produced by combustion. As these heated combustion products rise, cool air takes their place; the cool air is heated in turn and then also rises to the highest point it can reach. As the hot air and gases rise from the fire, they begin to cool; as they do, they drop down to be reheated and rise again. This is the convection cycle.

To prevent fire spread, the heat, smoke and gases should be released into the atmosphere. It is imperative, that the fire be confined to the smallest possible area. If a fire is discovered, attempts should be made to close off all openings to the fire area until firefighting personnel and equipment can be brought into position to fight the fire.

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National Fire Protection Association
N F P A

«To promote the science and improve the methods of fire protection and prevention; to obtain and circulate information on these subjects and to secure the cooperation of its members and the public in establishing proper safeguards against loss of life and property by fire».

The National Fire Protection Association has two functions: one, to provide standards under the guidance of which fire waste may be checked; the other, to educate the public, so that loss of life and injury from fire will be reduced.

The Association's technical committee members are expert fire prevention engineers who serve without pay. Since 1924 the Association has maintained a staff of engineers to organize, stimulate and encourage fire prevention committees. These men have frequently recommended definite programs for the improvement of municipal fire hazard conditions. One of the Association's engineers is employed solely to promote wide recognition of the National Electrical Code, which governs the safe installations of electric wiring and equipment.

All the members take part in the great work of public education. Realizing that loss of life and injury by fire is a major cause of human suffering the Association's members are devoted to showing people how this destructive force can be controlled and fire waste eliminated. To make the public understand the tragic human suffering and economic burden of fire loss is the prime task of the Association and its members. The public must, therefore, be reached by the press, by the schools, from the lecture platforms, by means of radio, television, motion pictures and by general educational programs sponsored by fire departments, with message of fire protection and prevention.

Fire waste is also a serious economic waste. Property destroyed by fire is a total economic loss. The burden of fire loss is borne by every citizen. Much of the Association's literature is prepared with the object of educating the people in fire prevention.

The Association has sections composed of state, provincial and city fire experts. The Electrical Section of the Association, open to all members interested in electrical matters, is responsible for the development of new editions of the National Electrical Code, and provides a forum for general discussions of electrical hazards.

The Railroad Section of the Association, including representatives of large railroad systems, provides a medium for the consideration of activities in railroad fire protection.

The «Quaterly» magazine is unique in its special articles. The important fires occurring in the United States and Canada, with special reference to their origin, their extent and the manner of their extinguishment are described in the «Quaterly». The articles show how the majority of fires start and how, in the cases given, they were extinguished.

UNIT 3

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