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Fire Protection Handbook

Page 1: Fundamentals of Fire

A Definition of Fire

One generally accepted definition of combustion or fire is a process involving rapid oxidation at elevated temperatures accompanied by the evolution of heated gaseous products of combustion, and the emission of visible and invisible radiation. Oxidation occurs all around us in the form of rust on metal surfaces, and in our bodies by metabolising the food we eat. However, the key word that sets combustion apart from other forms of oxidation is the word “rapid”.

The combustion process is usually associated with the oxidation of a fuel in the presence of oxygen with the emission of heat and light. Oxidation, in the strict chemical sense, means the loss of electrons. For an oxidation reaction to occur, a reducing agent, the fuel, and an oxidizing agent, usually oxygen, must be present. As heat is added, the ignition source, the fuel molecules and oxygen molecules gain energy and become active. This molecular energy is transferred to other fuel and oxygen molecules which creates a chain reaction. A reaction takes place where the fuel loses electrons and the oxygen gains electrons. This exothermic electron transfer emits heat and/or light. If the fire is in a fire grate or furnace we refer to this process as a controlled fire, and in a building it is an uncontrolled fire.

The Fire Tetrahedron

A tetrahedron can be described as a solid pyramid which is a solid having four plane faces. These four face represent the four elements that must be present for fire to occur: fuel, heat, oxygen, and a chemical chain reaction. Removal of any one of these essential elements will result in the fire being extinguished.

The four elements are: oxygen to sustain combustion, sufficient heat to raise the material to its ignition temperature, fuel or combustible material and subsequently an exothermic chemical chain reaction in the material.

Theoretically, fire extinguishers put out fire by taking away one or more elements of the fire tetrahedron.

The symbol is a good analogy of how to theoretically extinguish a fire, by creating a barrier using foam for instance and prevent oxygen getting to the fire. By applying water you can lower the temperature below the ignition point, or in a flammable liquid fire by removing or diverting the fuel. Finally, interfering with the chemical chain reaction by mopping up the free radicals in the chemical reaction by using halon extinguishers creates an inert gas barrier.

Stages of a Fire

There are three generally recognized stages to a fire: the incipient stage, smoldering stage, and flaming stage.

The incipient stage is a region where preheating, distillation and slow pyrolysis are in progress. Gas and sub-micron particles are generated and transported away from the source by diffusion, air movement, and weak convection movement, produced by the buoyancy of the products of pyrolysis.

The smoldering stage is a region of fully developed pyrolysis that begins with ignition and includes the initial stage of combustion. Invisible aerosol and visible smoke particles are generated and transported away from the source by moderate convection patterns and background air movement.

The flaming stage is a region of rapid reaction that covers the period of initial occurrence of flame to a fully developed fire. Heat transfer from the fire occurs predominantly from radiation and convection from the flame.

Classes of Fire

	Class A Fires The most common type of fire, these occur when a solid, organic material such as wood, cloth, rubber, or plastic becomes heated to its flash point and ignites. At this point the material undergoes combustion and will continue burning as long as the four components of the fire tetrahedron (heat, fuel, oxygen, and the sustaining chemical reaction) are available. This class of fire is commonly used in controlled circumstances, such as a campfire, match or wood-burning stove.This class of fire is fairly simple to fight and contain - by removing the heat, oxygen, or fuel, or by suppressing the underlying chemical reaction, the fire tetrahedron collapses and the fire dies out.
	Class B Fires Fires involving flammable liquids or gaseous fuels. These fires follow the same basic fire tetrahedron as Class A fires, except that the fuel in question is a flammable liquid or gas. Water should never be used to extinguish this type because it can cause the fuel to scatter, spreading the flames. Extinguish by inhibiting the chemical chain reaction of the fire, which is done by dry chemical and FM-200 extinguishing agents, Smothering with CO2 or foam is also effective.
	Class C Fires Fires involving potentially energized electrical equipment. This sort of fire may be caused by short-circuiting machinery or overloaded electrical cables. Electricity may be conducted from the fire through water, foam, and other conductive agents to the firefighter’s body, therefore must never be used. While the fire is, or could possibly be electrically energized, it can be fought with any agent rated for electrical fire, such as CO2, FM-200 and dry chemical powder extinguishers such as PKP and baking soda are especially suited to extinguishing this sort of fire. Once electricity is shut off to the equipment involved, it will generally become an ordinary combustible fire.
	Class D Fires Fires involving metals. Certain metals are flammable or combustible, including sodium, titanium, magnesium, steel, lithium, and calcium. When one of these combustible metals ignites, it can easily and rapidly spread to surrounding ordinary combustible materials. With the exception of the metals that burn in contact with air or water (for example, sodium), it requires a lot of heat to ignite a mass of combustible metal. Generally, metal fire risks exist when sawdust, machine shavings and other metal ‘fines’ are present. Generally, these fires can be ignited by the same types of ignition sources that would start other common fires. Water and other common firefighting materials can excite metal fires and make them worse. The NFPA recommends that metal fires be fought with ‘dry powder’ extinguishing agents. Dry Powder agents work by smothering and heat absorption. The most common of these agents are sodium chloride granules, graphite powder, and powdered copper. Some extinguishers are labeled as containing dry chemical extinguishing agents. This may be confused with dry powder. The two are not the same. Using one of these extinguishers in error, in place of dry powder, can be ineffective or actually increase the intensity of a metal fire. Metal fires represent a unique hazard because people are often not aware of the characteristics of these fires and are not properly prepared to fight them. Therefore, even a small metal fire can spread and become a larger fire in the surrounding ordinary combustible materials.
	Class K Fires Fires involving cooking oils and fats. Though such fires are technically a subclass of the flammable liquid/gas class, the special characteristics of these types of fires are considered important enough to recognize separately. Saponification can be used to extinguish such fires. Appropriate fire extinguishers may also have hoods over them that help extinguish the fire.

Summary

A fire begins by an external ignition source in the form of a flame, spark, or hot ember. This external ignition source heats the fuel in the presence of oxygen. As the fuel and oxygen are heated, molecular activity increases. If sufficiently heated, a self-sustaining chemical chain reaction or molecular activity occurs between the fuel and oxygen. This will continue the heating process and the resulting chain reaction will escalate without the need for an external ignition source. Once ignition has occurred, it will continue until one of the following occurs:

1. All the available fuel or oxidant has been consumed
2. The fuel and/or oxygen is removed
3. The temperature is drastically lowered
4. The number of excited molucules is reduced, breaking the chain reaction

FIRE SAFETY, AT ITS MOST BASIC, IS BASED UPON THE PRINCIPLE OF KEEPING FUEL SOURCES AND IGNITION SOURCES SEPARATE.