Fire Classification & Extinguishing Methods PDF

OBJECTIVE 1 Explain the classifications of fires, and describe the extinguishing media that are appropriate for each classification. CLASSIFICATION OF FIRES Fires are classified according to the material that is burning. Each type of fire is unique and how the fire can be safely extinguished. See f...

OBJECTIVE 1 Explain the classifications of fires, and describe the extinguishing media that are appropriate for each classification. CLASSIFICATION OF FIRES Fires are classified according to the material that is burning. Each type of fire is unique and how the fire can be safely extinguished. See figure one for fire classification and the related symbols of each classification. The following are the five classification of fires from the National Fire Protection Association (NFPA) 10, Standard for Portable Fire Extinguishers: Class A Class A fires occur in ordinary combustible materials such as wood, cloth, and paper. Class B Class B Fires occur in the vapor-air mixture over the surface of flammable liquids such as greases, clean, and lubricating oils. Class C Class C Fires occur in energized electrical equipment. Class D Class D Fires occur in combustible metals such as magnesium, titanium, zircronium, and sodium. Class K Class K fires occur in kitchens was cooking oils and grease. Page 3 of 44 FIRE EXTINGUISHING AGENTS 1. Dry Chemicals The following are the most common types of fire extinguishing agents in use today: • • • • • Dry chemicals Gaseous Dry powders Water Foams The following sections explore the types of fires that these agents are used to extinguish. 1. Dry Chemicals Dry chemical fire extinguishing agents stop the chemical chain reaction sequence that is associated with fire. On a weight basis, dry chemical extinguishing agents are probably more effective than even the halons (a type of gaseous extinguishing agent) in extinguishing fires. As such, dry chemical extinguishing ages are mainly used in portable and wheeled extinguishers and also in some stationary equipment. Dry chemical fire extinguishing agents are generally used where significant fire extinguishment capability is required from a relatively small quantity of material. This is the reason that dry chemical fire extinguishing agents are mostly used in portable and wheeled extinguishers with capacities up to 160 kg. There are also special applications that involve stationary extinguishing equipment without two1360 kg of capacity. Sodium Bicarbonate The first dry chemical fire extinguishing agent to be created used a base of sodium bicarbonate. In this extinguishing agent, sodium bicarbonate is compounded with other materials to make them water repellent so that it is capable of flowing from a pressurized container. Sodium bicarbonate based extinguishing agents are effective on class B and C fires, but not on class A and D. The effectiveness of sodium bicarbonate based extinguishing agents is approximately 50% greater than that of water applied to the same fire. Potassium Bicarbonate Research conducted at the United States Naval Research Laboratory led to the development of a second generation dry chemical fire extinguishing agent based on potassium bicarbonate, rather than sodium bicarbonate. This material is commonly referred to as Purple-K. Extinguishing agents Page 4 of 44 with a potassium bicarbonate base are found to be about twice as effective as those with a sodium bicarbonate based. Potassium bicarbonate extinguishing agents are effective on class B and C type fires only. Multi-Purpose A third type of dry chemical evolved that was quite unique in its effectiveness on class a fires in addition to the normal class B and C Extinguishing capabilities of dry chemical agents. Refer to as multi-purpose dry chemical, this extinguishing agent is based on mixtures of ammonium phosphates, or ammonium phosphates and sulfates. 2. Gaseous Gaseous extinguishing agents alter the vapour phase concentration of the fuel’s oxidizing agent so that it is either below the lower flammability limit or above the upper flammability limit. There are two categories of gaseous extinguishing agents, which are used on class C fires to prevent the possibility of electric shock: • Inert type agents, such as nitrogen or carbon dioxide • Halons or halogenated hydrocarbon type fire extinguishing agents. However, these types of extinguishing agents have now been banned by the Montreal Protocol due to ozone layer depletion. High levels of halon can also cause an oxygen-deficient environment and lead to suffocation. Halons have been replaced by carbon dioxide, dry chemical, foam, hydrochlorofluorocarbons (HCFCs), powdered aerosols, and water mist. 3. Dry Powder Dry powders are extinguishing agents that are developed specifically for use on class D combustibles. Class D combustibles are reactive and combustible metals such as sodium, potassium, magnesium, and aluminum. 4. Water Water is used on class A fires. The primary mechanism of extinguishment by water is its ability to cool the fuel and oxidizing agent mixture below the ignition temperature of the fuel. The volume of water present, as a liquid, is expanded by a factor of approximately 1600 times in converting it to steam. Steam displaces oxygen in the immediate atmosphere and removes the heat of combustion. Page 5 of 44 5. Foams Foam is the result of adding certain materials to water to improve its ability to wet certain fuel surfaces. NPFA 11 Standard covers firefighting foams. Foam extinguishing agents can be divided into two categories: • Chemical foams • Mechanical foams Chemical Foams Chemical foams are produced by a chemical reaction between substances such as sodium bicarbonate and aluminum sulfate. In this chemical reaction, carbon dioxide is released and acts as the blowing agent to form a mass of foam bubbles. Chemical foams are mostly obsolete in North America. Mechanical Foams Mechanical foams are produced by mechanically mixing air with a proportioned foam solution. The solution is a mixture of water and foam concentrate at an appropriate dilution. The two most common dilutions are 6% and 3%, (that is, 6 parts foam concentrate to 94 parts water, and 3 parts foam concentrate to 97 parts water). Foam agents (all kinds) are most often employed in extinguishing class B flammable and combustible liquids. Mechanical foam agents place a barrier, or effective separation, between the fuel and the oxidizing agent (usually air). A secondary mechanism of extinguishment is associated with the boiling of water to produce a cooling effect. All foam extinguishing agents can be used on class A combustibles. The most commonly-used foams for class A combustibles are based on synthetic type concentrates that use hydrocarbon surfactants (detergents). The following are types of mechanical foam concentrates: • • • • • Protein Fluoroprotein foam concentrates Aqueous film-forming foam (AFFF) Synthetic Alcohol resistant concentrates Page 6 of 44 Protein Foam Protein foam is derived from a naturally-occurring chemical found in the hoofs and horns of animals. Chemicals are added to the protein to protect it from freezing and decomposition. The chemicals also make the protein less corrosive. Protein foams result in a thick mass of foam bubbles that have excellent burn back resistance, but they spread slowly over a fuel’s surface. A downside to protein foams is that they also tend to pick up the fuel to which they are being applied, which can help spread the fire. Fluoroprotein Foam Fluoroprotein foam was successfully developed to overcome two of the drawbacks of regular protein foams. Unlike protein foams, the fluoroprotein foam blanket spreads easily across a fuel surface, and it picks up less fuel. Fluoroprotein foam differs from protein foam in that a fluorocarbon surfactant is added at relatively low concentrations to provide better extinguishment speed and burn back resistance. Fluoroprotein foams are commonly used in both topside and subsurface applications for the protection of flammable and combustible liquid storage tanks. Aqueous Film Forming Foam (AFFF) Aqueous film forming foam (AFFF) was developed at the U.S. Naval Research Laboratory primarily to provide very rapid-fire extinguishment (or knockdown) capabilities. This foam consists of fluorocarbon and hydrocarbon surfactants that can be used in both aspirating and non-aspirating mechanical foam hardware. Aspirating nozzles are specifically designed to entrain air in certain proportions into the diluted foam water solution. Non-aspirating type foam hardware is designed primarily for the application of water in either spray or straight-stream patterns. Synthetic Foam Synthetic foam concentrates are based on synthetic surfactants. The two main types of synthetic foam are aqueous film forming foams and alcohol-resistant aqueous film forming foams. Synthetic foams are divided into the following three categories, based on their expansion ratio: • Low expansion: An expansion ratio of 20:1 or less • Medium expansion: An expansion ratio greater than 20:1, but less than 200:1 • High expansion foam: An expansion ratio of 200:1 or greater Page 7 of 44 Alcohol-Resistant Concentrates (ARC) Alcohol-resistant concentrates have been specially formulated for extinguishment of fires that involve water-soluble fuels. All of the foam agents discussed up to this point are effective on non-water-soluble fuels such as gasoline, diesel fuel, crude oil, kerosene, and toluene. If any of these foam agents is used on a water-soluble fuel, such as methyl alcohol or acetone, the foam will simply dissolve because of the high solubility of the fuel in water. Most of the alcohol-resistant concentrates (ARC) currently in use are based on formulating AFFF in such a way as to allow it to be used on a water-soluble fuel. This type of AFFF is achieved by adding a chemical that forms an insoluble membrane (similar to an egg white) between the fuel and the foam blanket. In this way, alcohol-resistant concentrates based on AFFF are now widely used. Page 8 of 44