fire science chapter 7 portable fire extinguishers
dry chemical extinguishers
*dry chemical agents are for use on class A-B-C fires and/or class B-C fires; dry powder agents are used on class D fires only. dry chemical extinguishers are among the most common portable fire extinguishers in use today. there are two basic types: 1. regular B-C rated 2. multipurpose and A:B:C rated commonly used dry chemicals include sodium bicarbonate, potassium bicarbonate, urea-potassium bicarbonate, potassium chloride and mono ammonium phosphate. *never mix or contaminate dry chemicals with any other type of agent the dry chemical agents themselves are nontoxic; however, the cloud of chemicals may reduce visibility and create respiratory problems just like any other airborne particulate. many dry chemical agents can be mildly corrosive to all surfaces.
wet chemical stored-pressure extinguishers
similar in appearance to standard stored-pressure water extinguishers, wet chemical fire extinguishers are intended for use on class K fires involving cooking fats, greases and vegetable and animal oils in commercial kitchens. these fire extinguishers contain a special potassium-based, low pH agent formulated to operate on the principle of saponification in which the agent combines with the oils to create a soapy foam surface over the cooking appliance
class C
energized electrical equipment. because water and water-based agents will conduct electrical current, they cannot be used on class C fires until the electrical energy has been eliminated. class C extinguishing agents will not conduct electricity making them suitable for electrical fires.
wet chemical system
extinguishing system that uses a wet-chemical solution as the primary extinguishing agent usually installed in range hoods and associated ducting where grease may accumulate.
CO2 extinguishers
found both as handheld and portable units. they are most effective at extinguishing class B and class C fires. because their discharge is in the form of gas, they have a limited reach and the gas can be dispersed by the wind. they do not require freeze protection. CO2 is stored under its own pressure as a liquified gas ready for release. when released, the CO2 displaces the oxygen and smothers the fire. it produces no vapor-supressing film on the surface of the fuel and so reignition is always a danger. *when CO2 is discharged, a static electrical charge builds up on the discharge horn. touching the horn before the charge has dissipated can result in a shock.
types of portable fire extinguishers
-smothering: excluding oxygen from the burning process -cooling: reducing the burning material below its ignition temperature -chain breaking: interrupting the chemical chain reaction in the burning process -saponification: forming an oxygen-excluding soapy foam surface extinguishing agents that work by smothering are ineffective on materials that contain their own oxidizing agent. for example, a fire in magnesium or other combustible metals will flare and intensify if water is applied. all portable fire extinguishers expel their contents using one of the following mechanisms: -manual pump -stored pressure -pressure cartridge
NFPA 1001 and 10
according to NFPA 1001, those qualified as FF1 level must know the following about portable fire extinguishers: -classifications of the types of fires -risks associated with each class of fire -operating methods of portable fire extinguishers -limitations of portable fire extinguishers NFPA 10, Standard for portable fire extinguishers
stored-pressure water extinguishers
also called air-pressurized water (APW) extinguishers or pressurized water extinguishers, are useful for all types of small class A fires. water is stored in the tank along with either compressed air or nitrogen. a gauge located on the side of the valve assembly shows when the extinguisher is properly pressurized. class A foam concentrate is sometimes added to pump-type or stored-pressure water extinguishers to increase their effectiveness. the addition of class A foam concentrate serves as a wetting agent that aides in extinguishing deep-seated fires by reducing the surface tension of water, allowing water to quickly penetrate the surface.
water-mist stored-pressure extinguisher
although very similar in appearance to standard stored-pressure water extinguishers, water-mist extinguishers use deionized water as the agent and nozzles that produce a fine spray instead of a solid stream. because it is the impurities in water that conduct electricity, the deionized water also makes these class A extinguishers safe to use on energized electrical equipment (class C).
selecting the proper fire extinguisher
because of their corrosive particulate residue, do not select dry chemical extinguishers for use in areas where highly sensitive computer equipment is located. clean agent or CO2 extinguishers are better choices.
portable fire extinguisher rating system - class A ratings
class A portable fire extinguishers are rated from I-A through 40-A. the class A rating of water extinguishers is primarily based on the amount of extinguishing agent and the duration and range of the discharge used in extinguishing test fires. for a I-A rating, 1.25 gallons of water are required.
portable fire extinguisher rating system - class K ratings
class K rated extinguishers must be capable of saponifying (converting the fatty acids or fats to a soap or foam) vegetable oil, peanut oil, canola oil and other oils with little or no fatty acids. wet chemical agents containing an alkaline mixture, such as potassium acetate, potassium carbonate, or potassium citrate work by suppressing the vapors and smothering the fire. any of these agents capable of extinguishing a fire from a deep fryer using these light oils with a surface area of 2.25 square feet meet the minimum criteria for a class K rating.
portable fire extinguisher rating system - class B ratings
classified with numerical ratings ranging from 1-B through 640-B. the rating is based on the approximate square foot area of flammable liquid fire that a non expert operator can extinguish using one full extinguisher.
clean agent extinguishers
clean agents have been developed to replace halogenated extinguishing agents, which are chemical compounds that contain carbon plus ore or more of the halogens (fluorine, chlorine, bromine, or iodine). the two most common Halon extinguishing agents are Halon 1211 and Halon 130. halogenated extinguishing agents are harmful to the ozone layer; however, pressurized with Argon gas, Halotron extinguishers are approved by the EPA. designed specifically for Halon 1211, clean agents are discharged as a rapidly evaporating liquid that leaves no residue. these agents effectively cool and smother fires in class A and class B fuels, and the agents are nonconductive so they can be used on energized electrical equipment (class C) fires.
class K
combustible cooking oils such as vegetable or animal fats and oils that burn at extremely high temperatures. wet chemical systems and portable fire extinguishers are used to control and extinguish class K fires.
dry powder extinguishers
developed to control and extinguish fires involving class D combustible metals. no single extinguishing agent will control or extinguish fires in all combustible metals. some powdered agents can be applied with portable extinguishers, but others must be applied with either a shovel or a scoop. class D portable extinguishers come in both handheld and wheeled models. it must be applied in sufficient depth to completely cover the burning area in order to create a smothering blanket. * water applied to a combustible metal fire results in a violent reaction that intensifies the combustion and causes bits of molten material to splatter in every direction. if a small amount of burning metal is on a combustible surface, the fire should first be covered with powder. then, a layer of powder 1 to 2in deep should be spread nearby and the burning metal shoveled into this layer with more powder added as needed.
aqueous film forming foam (AFFF) extinguishers
intended for class B fires. they are particularly useful in combating fires in or suppressing vapors from small liquid fuel spills. the AFFF extinguisher tank contains a specified amount of AFFF concentrate mixed with the water to produce a foam solution. it has an air-aspirating foam nozzle that aerates the foam solution, producing a better-quality foam than a standard extinguishing nozzle provides . the resulting finished foam floats on the surface of fuels that are lighter than water. the vapor seal created by the film of water extinguishes the flame and prevents reignition. to prevent disturbance of the foam blanket when applying the foam, it should not be applied directly onto the fuel. AFFF extinguishers are not suitable for fires in class C, D or K fuels. they are most effective on static pools of flammable liquids
pump-type water extinguishers
intended primarily for use on ground cover fires although they may be used for small class A fires. the nozzle produces a straight stream, fog, or water-mist pattern
class D
involve combustible metals and alloys such as lithium, magnesium, potassium and sodium. these types of fires can be identified by the bright white emissions during their combustion process. *the use of water or water-based agents on class D fires will cause the fire to react violently, and emit bits of molten metal. class D dry powder extinguishers work best on these types of fires; however, do not confuse dry powder extinguishers with dry chemical units used on class A, B, and C fires
class A fire
involve ordinary combustibles such as textiles, paper, plastics, rubber, and wood. these fuels can easily be extinguished with water, water-based agents such as class A foam, or dry chemicals
class B
involves flammable and combustible liquids and gases , such as alcohol, gasoline, lubricating oils, and liquified petroleum gas (LPG). agents used to extinguish special hazard class B fires are CO2, dry chemical , and class B foam
multiple markings
portable fire extinguishers are identified in two ways. one system used geometric shapes of specific colors with the class letter shown within the shape. the second system, currently recommended in NFPA 10, uses pictographs to make the selection of the most appropriate fire extinguisher easier.
inspection, care and maintenance
specified by NFPA 10. requires fire extinguishers be inspected at least once a year. when inspecting, there are three factors that determine the value of a fire extinguisher: 1. serviceability 2. accessibility 3. simplicity of operation -carry extinguishers diagonally across the body with one hand on the handle and the other on the bottom edge -shake dry chemical extinguishers monthly to loosen the agent and prevent it from settling. -clean the extinguisher after each use or periodically. use warm water and soap to remove dirt, grease and other foreign material. remove any corrosion with steel wool or sandpaper -portable fire extinguishers should be removed rom service for annual maintenance. -they must be hydrostatically tested periodically and the test results must be affixed to the exterior shell. -every six years the dry chemical extinguishing agent should be emptied and the extinguisher refilled.
portable fire extinguisher rating system - class C ratings
there are not any fire extinguishing capability tests specifically conducted for class C ratings. because electricity does not burn, extinguishers for use on class C fires are essentially class A or B fires involving energized electrical equipment. the extinguishing agent is tested for electrical non conductivity. the class C rating confirms that the extinguishing agent will not conduct electricity. it is assigned in addition to the rating for class A and/or B fires.
dry chemical extinguishers - wheeled units
they are rated for class A, B and C fires based on the dry chemical in the unit. when the extinguisher is in position at the fire, the hose should first be stretched out completely. the pressurizing gas should be introduced into the agent tank and allowed a few seconds to fully pressurize the tank before the nozzle is opened. the operator should be prepared for significant nozzle reaction. *the top of the extinguisher should be pointed away from the operator and other nearby personnel when pressurizing the unit.
dry chemical extinguishers - handheld units
two basic types: cartridge-operated and stored-pressure. the stored-pressure type is similar in design to the air-pressurized water extinguisher. a constant pressure of about 200psi is maintained in the agent storage tank. both types of extinguishers use either nitrogen or CO2 as the pressurizing gas. *when pressurizing a cartridge-type extinguisher, do not place your head or any other part of your body above the top of the extinguisher. if the fill cap was not properly screwed back on, the cap and/or a cloud of agent can be forcibly discharged.
portable fire extinguisher rating system - class D ratings
vary with the type of combustible metal being tested. class D agents cannot be given a rating for use on other classes of fire.
using portable fire extinguishers
wear full PPE when using an portable fire extinguisher. after selecting the appropriate size and type of extinguisher for the situation, make a quick visual inspection of the external condition, the hose/nozzle, note the weight of the bottle, and check the pressure. use the PASS application method: P - pull the pin A - aim the nozzle at the base of the fire S - squeeze the handles together to release the agent S - sweep the nozzle back and forth, covering the material apply the agent from a point where it reaches but does not disturb the fuel surface. lay empty fire extinguishers on their sides after use to signal to others that it is empty.
