Date post: | 06-May-2015 |
Category: |
Education |
Upload: | snoshoesam |
View: | 2,954 times |
Download: | 1 times |
© 2009 Delmar, Cengage Learning
Extinguishing Agents
Chapter 4
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Chemical Flame Inhibition
• Extinguishing agents interrupt the combustion reaction
• Exact mechanisms have not been completely determined
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Water Application Methods
• Straight stream• Fog or spray• Water mist systems• Foams
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Figure 4-4 Foam expansion ratio and drain time
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Fluid Foam
• Medium to smaller bubbles• Moderate drain times• Works well for:
– Direct attack– Exposure protection– Mop-up operations
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Dry Chemicals (cont’d.)
Figure 4-6 Stored pressure dry chemical extinguisher
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Application of Carbon Dioxide
• Known as “dry ice”• Extinguishes fires by smothering• Creates a refrigerating effect• Creates shrill noise• Creates static electricity
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Combustible Metal Fires
Table 4-2 Extinguishing materials for some metal fires
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Pressurized Gas Fires (Cont’d.)
Figure 4-10 Diagrams of BLEVE events
© 2009 Delmar, Cengage Learning
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