chapter 04-extinguishing agents

© 2009 Delmar, Cengage Learning

Extinguishing Agents

Chapter 4


Objectives

• Review and examine the basic components of the fire extinguishment process

• Review the five basic classifications of fire and explain the various types of agents used to extinguish or control fires in these five classifications


Objectives (cont’d.)

• Examine in detail the variety of agents used for fire extinguishment and explain the application methods for each of these agents

• Identify and explain the benefits of using the latest technological advances in fire extinguishing agents such as compressed air foam and ultrafine water mist systems


Introduction

• Fire combustion process modeled by using a fire tetrahedron

• Fire professionals use this model daily to extinguish fires

• Fire professionals’ task is to select the method that will stop the progress of the fire quickly and safely


Fire Extinguishment Theory

• Heat and temperature are not the same• Heat: form of energy

– Defined as the molecular motion of the material

• Temperature: measure of average molecular velocity or degree of intensity of the heat


Interruption of the Combustion Process

• Combustion is interrupted by removing one or more components of the fire tetrahedron

• Four extinguishment processes:– Temperature reduction– Fuel removal– Oxygen removal– Breaking release of additional fuel from material


Temperature Reduction/Heat Removal

• Water cools temperature of fuel to below ignition temperature

• Insufficient cooling can lead to rekindling • Flammable vapors may continue to be released• If fuel temperature is above flash point, a source

of ignition with sufficient energy will cause reignition


Fuel Removal

• Stop flow of liquid or gaseous fuel • Wildland fires

– Remove vegetation in front of the fire– Backfire or burn fuel in front of main fire


Oxygen Depletion

• Reduce the amount of oxygen available to the combustion– Cover pan containing burning food– Flood area with inert gas– Blanketing fuel with foam


Chemical Flame Inhibition

• Extinguishing agents interrupt the combustion reaction

• Exact mechanisms have not been completely determined


Extinguishment and Classification of Fires

• Classification of fire based on the type of fuel being burned

• Five fire classifications: A, B, C, D, and K– Each has own specific requirements for fire

extinguishment


The Process and Agents of Extinguishment

• Water has ability to absorb heat energy more than all other elements except mercury– Non-conforming characteristic in processes of

expansion and contraction

• Latent heat of vaporization: quantity of heat absorbed by a substance when it changes from a liquid to vapor


Agents to Improve Plain Water

• Water not always best extinguisher for fires– Gasoline fires or other hydrocarbon liquids with flash

points below 100°F should not use water– Water not effective on most metallic dusts and

shavings, or in pyrophoric metals


Additives to Improve Water Applications

• Several chemicals developed to increase water’s effectiveness when combined– Temperature can be lowered– Can be made thicker– Surface tension can be reduced to decrease the

amount of friction loss


Wet Water

• Water with a wetting agent added• Wetting agent reduces surface tension of water

– Allows it to flow and spread better

• Used when need to penetrate deeply to reach seat of a fire in tightly baled materials – Cotton– Tightly packed cardboard boxes


Slippery Water

• Uses polymers, a plastic like additive• Increases amount of water moved through hose

line• Reduces friction loss

– Pressure lost by fluids while moving through pipes, hoses, lines, or other limited spaces

• Flowing larger quantities of water will increase the reaction backpressure


Thick Water

• Designed to improve water’s ability to provide insulating barrier surface of a solid fuel– Expensive– Reports on use have not been very positive


Viscous Water

• Thickening agents added to water:– Clings and adheres to surfaces of fuel by providing a

continuous coating thicker than untreated water over the fuel surface

– Projects further when discharged from nozzle – Better resists wind and air currents


Water Application Methods

• Straight stream• Fog or spray• Water mist systems• Foams


Straight Stream Water Applications

• Should not impact the thermal layer zones • Cools temperature of ceiling area over fire• Highly effective on a high flash point liquid fire


Fog or Spray Water Applications

• Divides water stream into very small drops resulting in more water surface area

• Absorbs great deal of heat• Can produce thermal imbalance between hot

and cool smoke layers• Purges an area of smoky and noxious gases


Water Mist Systems

• New research and technological improvements in nozzles and pressure systems demonstrate possibility of efficient fire suppression

• Dependant on:– Mist droplet size– Mist stability– Transportation behavior of mist– Unobstructed nozzles– Efficiency and rate of droplet vaporization


Foams

• Long history of success• Two-powder foam to single-powder foam to

mechanical foam• Now surfactant or detergent foam is also used

– Better penetrates a porous solid fuel source as it reduces surface tension of water

– Known as wet water or wetting agent


Foam Classifications

• Class A– Extinguish Class A fires, or fires in combustible solids

• Class B– Extinguish Class B fires

• Special foams– Compressed air foam systems

• Low, medium, or high expansion foam


AFFF Concentrate

• Combination of water and perfluorocarboxylic acid

• Loses water content rather rapidly • May provide less burn-back resistance

compared to other protein-based foams


Application of Class A Foams

• Mixture of water, foam concentrate, and air• Can be made wetter or dryer• Size of bubbles determined by air pressure• Wet, dry, and fluid foams


Figure 4-4 Foam expansion ratio and drain time


Wet Foam

• Smaller bubbles• Less expansion • Fast drain times

– Drain time: amount of time for the water to drain away from the foam solution

• Good for initial fire suppression, overhaul, and penetration into deep-seated fires


Dry Foam

• High expansion ratio • Fluffy and consists mainly of air• Slow drain times • Hold shape for a long period of time• Good for exposure protection


Fluid Foam

• Medium to smaller bubbles• Moderate drain times• Works well for:

– Direct attack– Exposure protection– Mop-up operations


Fire Extinguishing Chemicals and Other Agents

• Water is not always the best extinguishing agent• Agents that have been developed to fill in for

water: – Dry chemicals– Carbon dioxide– Halogenated agents


Dry Chemicals

• Principle chemicals used for dry chemical agents:– Sodium bicarbonate, potassium bicarbonate,

potassium chloride, and urea-potassium bicarbonate

• Used on Class B, D and K fires• Discharged under high pressure by gas

expellant • Forms blanket over top of the fire • Extinguishes fire by excluding oxygen


Dry Chemicals (cont’d.)

Figure 4-6 Stored pressure dry chemical extinguisher


Application of Dry Chemicals

• Two common arrangements:– Stored pressure– Separate pressure container

• Activation can be manual, automatic, or both• Location of discharge nozzles need to be

located in the center above the cooking surface


Carbon Dioxide

• Consist of high-pressure cylinders or low-pressure tanks containing carbon dioxide under pressure

• Colorless, odorless, electrically non-conducive gas that is non-corrosive to most metals


Application of Carbon Dioxide

• Known as “dry ice”• Extinguishes fires by smothering• Creates a refrigerating effect• Creates shrill noise• Creates static electricity


Halogenated Agents

• Halon 1301 emerged as effective extinguishing agent not serious toxic threat to humans– Seriously depleted the ozone layer surrounding the

earth

• Production of certain halons used for extinguishing purposes banned in certain countries (Halon 1211,a liquid, and Halon 1301, a gas)

• New “environmentally clean” agents developed


Alternative (Clean) Halogenated Fire Extinguishing Systems

• Not effective against fires in certain chemicals or mixtures of chemicals

• Designed primarily for areas where persons are prohibited

• Monthly inspection of the enclosure is needed


Clean Agent Application Methods

• Triggering detectors or manual release starts an alarm sequence

• Control head provides gas escape route into atmosphere through discharge control valve

• Escape of gas creates imbalance of pressure• Nozzles designed to distribute extinguishing

agent at controlled rate, smoothly and evenly in fan-shaped patterns


Special Extinguishment Situations

• Many metals and some chemicals are incompatible with water, dry powder, and carbon dioxide extinguishing agents when burning– May react violently when incompatible agents used– Specialized extinguishing agents are required


Combustible Metal Fires

Table 4-2 Extinguishing materials for some metal fires


Chemical Fires

• Encountered in high school or college labs, leather tanning plants, and wool dying processes

• Runoff of contaminated water may impact groundwater

• Imperative that firefighters make provisions to have persons who can provide technical expertise and advice respond to the incident


Pressurized Gas Fires

• Difficult to extinguish because the gas is under pressure

• Damaged tank, fitting, or valve will provide a continuous supply of fuel

• Where there is no fire, immediate evacuation required

• Need to ensure gas supply cut off• Tank must be cooled


Pressurized Gas Fires (Cont’d.)

Figure 4-10 Diagrams of BLEVE events


Summary

• Fires extinguished by:– Cooling, depleting the oxygen, removing fuel, or

breaking chain of combustion

• Water is not the only extinguishing agent– Can be made more effective by making it slippery,

reducing its surface tension, and making it thicker

• Carbon dioxide, dry chemicals, and foam also used as extinguishers

chapter 04-extinguishing agents

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