4AE3 U4 C4 Obj 2.pdf

Unit A-4 • Introduction to Plant and Fire Safety ======================================== OBJECTIVE 2 Explain the five classes of fires, and describe the types of fire extinguishing media and how they act on these fires. ( CLASSES OF FIRES There are five classes of fire. 1. Class A 2. Class B...

Unit A-4 • Introduction to Plant and Fire Safety ======================================== OBJECTIVE 2 Explain the five classes of fires, and describe the types of fire extinguishing media and how they act on these fires. ( CLASSES OF FIRES There are five classes of fire. 1. Class A 2. Class B 3. Class C 4. Class D 5. Class K Class A Class A fires consist of ordinary combustible materials such as: • Wood • Cloth • Paper • Rubber • Many plastics Usually a Class A fire is anything that leaves behind ash, though some plastics may burn completely. These fires are extinguished by: a) Cooling the fire with water and water solutions. This is the primary method associated with extinguishing a Class A fire. Each litre of water placed on a fire removes about 2257 kJ of heat as it evaporates. The evaporating water removes heat from the fire so rapidly that the fire can no longer remain hot enough to continue burning. b) Coating the fuel with dry chemicals which retard combustion by excluding oxygen. The dry chemical from this type of extinguisher coats the fuel, separating it from the oxygen in the air. Unlike water, the dry chemical does not penetrate and soak the fuel. Also, dry chemical does not lower the temperature of the fuel like water does. c) Interruption of the combustion chain with dry chemicals. The dry chemicals from this type of extinguisher interfere with the self-Sl!Staining chain reaction of the fire, causing it to go out. Dry chemicals do not reduce the temperature of the fuel like water does. d) Interruption of the combustion chain by halogenated agents. Halotron is a gas that interferes with the self-sustaining chain reaction of the fire. In this way, the fire is extinguished. Like dry chemicals, halotron does not soak or reduce the temperature of the fuel. NOTE: Halotron is an environmentally acceptable replacement for halon gases, which have been banned in Canada due to their harmful effect on the ozone layer. e) Removing oxygen from the area. Fire needs oxygen to burn. Class A fires can be smothered with fire blankets. Carbon dioxide (C02 ) gas can also smother fires. However, it is not recommended for use on a Class A fire because C02 does not cool the fuel. Once discharged, the C02 dissipates quicldy and allows oxygen to access the hot fuel, which can then re-ignite the fire. 4-10 <(-------------------------------4-t_h_C_Ias_s_E_d_if_fo_n 3•_P _a _r_t_A ( Plant Fire Safety • Chapter 4 Class B ( Class B fires involve flammable or combustible liquids such as: • Oil • Gasoline • Greases • Or similar material such as solvents Class B fires do not include cooking oils or grease. These fires are extinguished by: a) Smothering the fire by covering the liquid fuel with an inert gas, to remove and separate the oxygen from the fuel. b) Interruption of the combustion chain reaction with dry chemicals. c) Coating the fuel with dry chemicals which retard combustion by excluding oxygen d) Inhibiting the release of combustion vapours. Note: Water must not be used to put out a Class B fire. Burning oil floats on water. Applying water will cause the oil fire to spread. Water rapidly boils when applied to fires. The boiling action will cause burning oil to be sprayed on the surrounding surfaces. However, in some cases water can be used as a fine mist (fogging) from a special nozzle used by trained and experienced individuals. Class C A Class C fire is an electrical fire, which means that there is electricity present in the burning area. An electrical fire is usually caused by electrical connections that overheat or by an electrical arc which ignites combustible material. To put out this type of fire, non-conducting extinguishing elements must be used. Otherwise, the fire fighter runs the risk of electrocution. For example, water conducts electricity. This allows the electrical current to flow through the water and shock the person fighting the fire. If the power is shut off, what remains is a Class A or Class B fire which can be extinguished by the methods described above. C02 is a commonly used medium for extinguishing Class C fires, because it is non-conductive and leaves no residue behind. However, if the power is not shut off, this type of fire may re-ignite after the C02 dissipates. Class D Class D fires involve combustible metals including: • Magnesium • Potassium • Sodium • Zirconium • • • • Titanium Calcium Lithium Uranium Although Class D fires are not very common, understanding the potential risks is important. Most people do not expect metals to catch on fire. These fires can spread rapidly from a small fire into a dangerous situation if appropriate actions are not taken quickly. -------------------------s 4th Class Edition 3 • Part A Z 4-11 Unit A-4 • Introduction to Plant and Fire Safety ============== ==================== As well, extinguishing media used for Class A, B, and C fires are ineffective for Class D fires, and can actually further spread or intensify the fire. Metals that are burning require a heat absorbing extinguishing media to put out the fire, such as graphite in powder form and sodium chloride in ( granular from. Class K Class "K" can be taken to mean "Kitchen:' A Class K fire occurs with oils and greases common to ldtchens such as: a) Animal or vegetable fats b) Oil or grease on a grill or in a deep fryer To extinguish a Class K fire, these methods can be used: a) Class K fires are successfully extinguished in the same way as Class B fires. A Class B extinguishing product that smothers and removes the oxygen from the area is recommended. b) A practical method used in ldtchen fires is an extinguishing medium called a wet chemical, which can be used on a deep fryer fire, for example. The wet chemical extinguishing agent has a cooling effect, and creates a barrier between the hot grease/oil and the air (oxygen). EXTINGUISHING MEDIA 7 s E 4-12 T h e m e a n s o f e x t i n g u i s h i n g a f i r e r categories: 1. Cooling 2. Reduction in oxygen content 3. Removal of fuel 4. Chemical flame inhibition Fuel removal refers not only to the physical removal of the fuel, but also to the cutting off of fuel vapours from combustion (while in flaming mode). It may also consist of the covering of the glowing surface to isolate the fuel from the oxidizing agent. The various broad classes of extinguishing media available use one of the following materials to extinguish a fire: • • • • Water Carbon Dioxide Dry Chemicals Wet Chemicals • Aqueous Film-forming Foam (AFFF) • Dry Power • Halogenated Agents Water Water is an efficient and readily available extinguishing agent used for Class A fires. It does so by cooling the combustible material, and by lowering its temperature to a point where combustion is no longer sustained. The water may also produce steam which will reduce the ambient oxygen supply. It is a common misconception that water "smothers" a fire. Certainly if a deluge of water completely covering the fire occurs, then smothering does separate the air from the fire. However, water primarily reduces the fuel temperature by quicldy drawing away the heat as it evaporates. This lowers the temperature to below that at which the fuel will burn, thus putting out the fire. c a -----n --------- b e s u m m a r i z e d i n t o f o u ( - - --- - 4th Class Edition 3 • Part A ( - ---- Plant Fire Safety • Chapter 4 Carbon Dioxide (C02) ( Carbon dioxide is noncombustible and therefore does not react with most substances. It primarily extinguishes by reducing the oxygen content of the atmosphere to a point where combustion can no longer be supported. It is used for Class B (flammable liquids) fires and for Class C (electrical) fires as it does not conduct electricity. The C02 gas displaces the oxygen required for combustion. C02 has a minor cooling effect as well. Under some conditions of application, the available cooling effect can be helpful, especially where the C0 2 is applied directly to the burning material. The principal advantage of C02 is that it does not leave a residue. This consideration is important in laboratories, areas where food is prepared and where there is electronic equipment. Dry Chemicals Dry chemical extinguishing agents are either regular or multipurpose. The regular dry chemicals are used for Class B or Class C fires. The multipurpose dry chemicals can also be used on Class A fires. Note that the dry chemical extinguishing agents do not penetrate some Class A materials that are on fire as well as water does (such as burning mattresses). Wet Chemicals Wet chemical extinguishing agents are a newer form of fire-fighting media. They are designed primarily for fighting Class K (kitchen) types of grease fires, such as the oil in deep fryers. Wet chemicals can also be used for Class A fires. Aqueous Film-Forming Foam (AFFF) This extinguishing agent is an "aqueous" (liquid water) film-forming "surfactant:' Essentially, this is a soapy water foam that not only provides water to put out a Class A fire, but also spreads out as a film and covers the burning material to separate the fuel from the air. Therefore, AFFF both cools and smothers the fire. Due to its filming and foaming qualities it will adhere to the burning material rather than run off as water would. This results in the foam not only cooling the fire, it smothers it also and provides a seal so that the air cannot return and reignite the fire. When used properly, AFFF can be effective on Class B fires. It can be directed to spread foam over the oil or grease fire. Water on the other hand, would splash and cause a bigger fire. AFFF is conductive, so it cannot be used on electrical fires. Dry Powder Dry power extinguishing agents are used on Class D (combustible metals) fires. Metals will burn, particularly if they are in a finely divided form. Some metals burn when exposed to external heat. Other metals burn from contact with moisture or reaction with other materials. Dry powders.are often a combination of chemicals that are determined by the type of combustible metal that is on fire. For example, Graphite in dry powder form is an effective extinguishing media for a Class D fire. The dry powder covers the metal and smothers it so that oxygen is separated from the burning surface. Use the appropriate extinguisher according to the manufacturer instructions. It is expected that additional training for these types of fires and their appropriate extinguishing agents will be provided at the worksite. This training should also include the warning that dry powder extinguishers are not be confused with dry chemical. -------------------------s 4th Class Edition 3 • Part A Z 4-13 Unit A-4 • Introduction to Plant and Fire Safety ================================== Halogenated Agents and Halotron Halon extinguishers were commonly used from the 1970's onward in control rooms and computer rooms to rapidly extinguish fires without leaving damaging residue. Halons were much more effective than C02 . However, halon is quite damaging to the environment and harmful to people when breathing the halon fumes. Therefore, the use of halon has been abandoned. ( It is unlikely that Power Engineers will use or even encounter halon fire extinguishers. Most, if not all, jurisdictions have completely phased out halon use. Halotron is a replacement medium that operates in the same way as halon but with fewer negative environmental effects. Like halon, it interferes with the uninhibited chain reaction that occurs in the flaming mode of the combustion process. It is effective on Class A, B and C fires, does not conduct electricity, and is not corrosive. The extinguishing medium is a pressurized halocarbon. It discharges as a rapidly evaporating liquid that boils at a higher temperature than many other fire-fighting liquids (27°C in the atmosphere). This allows the medium to reach farther into a fire before evaporating, malting it an effective extinguishing agent. ( ( 4-14 / S 4th Class Edition 3 • Part A