Extinguishing Agents

Water
Water is by far the most commonly used and readily available extinguishing agent.  It is used in portable fire extinguishers, manual hose lines, installed systems, and as a base for foam-water systems. Water works well because it has a large capacity for absorbing heat. As we mentioned earlier, absorbing the heat cools the burning material to below its ignition temperature, thus causing the fire to go out. Water absorbs the most heat during its conversion to steam. For example, a pound of water at a room temperature of 70ºF (21ºC) will absorb approximately 150 Btu (158 kilojoules) to raise the temperature to 212ºF (100ºC), the boiling point of water. When water is
vaporized into steam, it absorbs approximately another 970 Btu (1023 kilojoules).  It is during the conversion to steam that the maximum benefits from the application of water are gained. Vaporization during effective use of water also reduces water damage.  Under actual fire fighting conditions, it is not possible to get 100% vaporization or even come very close to it, but the goal is to apply water in a way that will provide the highest possible vaporization.

Dry Chemical
Dry chemical agents are fine powders (about the consistency of talcum powder) that are based on several chemical compounds. They are available in two categories:  regular dry chemical agents which may be used on class B and C fires,  and multipurpose dry chemical agents for use on class A, B, and C fires.  They both function primarily by interrupting the chemical chain reaction.  The multipurpose dry chemicals are compounds that allow the agent to adhere to surfaces, which is why they are effective on class A fires. Unless the class A rating is needed, regular dry chemicals should be used in class B and C hazard areas.  The adhesion mentioned previously is a disadvantage in equipment and machinery fires because it makes cleanup much more difficult. Even with the regular dry chemical agents, the clean-up and salvage problem is a major disadvantage. This agent class generally provides the most rapid knockdown of flammable liquid fires available. Dry chemicals are used in portable fire extinguishers, wheeled fire extinguishers, vehicle and stationary hose systems, and local application installed systems both in structures and mounted on vehicles.  Used by a trained individual, dry chemicals offer a potent fire-fighting agent for  flammable liquid fires.
The most common regular dry chemical agent is sodium bicarbonate, baking
soda.  The most common multipurpose dry chemical agent is mono-ammonium phosphate. A common dry chemical used primarily for protecting significant flammable liquid exposures is potassium bicarbonate, typically referred to as Purple K.  Purple K is approximately twice as effective, pound for pound, on flammable liquid fires as a regular dry chemical.

Halons
Halogenated hydrocarbon agents, usually referred to as halons, are a group of  gaseous agents that are effective in fire control. The two most common halons used for fire control are 1211 and 1301.  Halon 1211 is typically used in portable fire extinguishers, and 1301 is normally used in installed systems.  The halon agents extinguish fires primarily by interrupting the chemical chain reaction.  Their major advantage is that they leave no residue,  which makes them especially suited to computer and delicate equipment protection. The smaller units, those under seventeen pounds, are only rated for class B and C fires.   The larger units, seventeen pounds and
over, are rated for class A, B, and C fires.   Halon is stored under pressure as a liquid.  When discharged, it rapidly vaporizes to a gas. This behavior is referred to as a vaporizing liquid.   Another advantage these agents offer is their holding ability.   If a room is filled to the proper concentration with halon, usually about 7%, a fire cannot burn as long as that concentration is maintained. Halon ranks between dry chemical and carbon dioxide with respect to speed of fire control.   The main disadvantage  of halon is its environmental impact and cost.   It is the most expensive of the extinguishing agents.   Halon is also one of the chemicals connected with the depletion  of the ozone layer.

This agent is used in portable fire extinguishers, wheeled fire extinguishers, vehicle and stationary hose systems, local application installed systems both in structures and mounted on vehicles, and total flooding installed systems.

Carbon Dioxide
Carbon dioxide (CO2) is a gaseous fire control agent that is stored under pressure as a liquid. It is rated for class B and C fires. The major advantages of carbon dioxide are that it leaves no residue and is nonconducting. It does not afford as much fire control capability as halon or some halon substitutes, but it is much less costly.  It functions by excluding oxygen from the fire.
The main disadvantage of carbon dioxide is that it can create an oxygen deficient area where it has been used that poses a significant risk to personnel.  This agent is used in portable fire extinguishers, wheeled fire extinguishers, vehicle and stationary hose systems,  local application installed systems both in structures and mounted on vehicles, and total flooding installed systems.

Foam
Foam is a general category of extinguishing agents that includes a wide variety of  specific foams for special purpose application.   Foam is used in portable fire extinguishers, wheeled extinguishers, manual hose lines, fixed hose systems, and a variety of installed systems.   Two major types of foam are chemical and mechanical.
Chemical foams are created by a chemical reaction and are rarely used. Mechanical foams are created by mixing foam concentrate with a specific proportion of water to form a foam solution. Several types of proportioning devices are used.   One of the most common is the in-line eductor. The pickup tube is placed in the foam concentrate, and water flows through the eductor.   The passing water creates a venturi,  which draws the foam concentrate into the stream.   The metering valve controls the percentage
of concentrate to ensure a proper mixture.   The foam solution flows through
the hose to a nozzle.   Air is introduced to the foam solution at the nozzle in a process  called aeration to form the finished foam.   The finished foam is a bubbly substance that is similar to soap suds in appearance.
Foam is suitable for use on class A and B fires,  but is specifically designed for
class B hazards.   Foam involves several of the extinguishing methods we discussed.  It is mostly water, so it offers a cooling capability. It is designed to float on the surface  of a flammable liquid, forming a barrier between the fuel surface and the air,  so it excludes oxygen.   This barrier acts to remove the fuel from the fire situation by sealing it.

Foam must be able to flow freely over the surface of a liquid.
This flow ability enables the foam to spread over the liquid’s surface and aids in achieving rapid, complete coverage of the liquid. The foam should provide a stable blanket once applied. Foam will begin to lose water as soon as it is applied,  and the speed at which the water is lost is the foam’s drainage or drain down rate.   The slower this drain down rate, the longer the foam blanket will last.   Since the foam is used as a fire control agent, heat resistance is an important characteristic.   The foam should be able to withstand the heat from a fire and hot surfaces in the area.   This ability is
sometimes referred to as burnback resistance.   More specifically, it is the capability of the foam to maintain an effective blanket in the proximity of a fire.   The foam should also resist contamination from the product to which it is applied.   This is referred to as fuel pickup.

Examples of foam types include: Protein foam, Aqueous film forming foam (AFFF),  Alcohol-type foam (for Polar Solvent Fires)

Dry Powder
Dry powder agents are designed to control fires in combustible metals (class D).  The two most common agents in this category are G-1 and Met-L-X. Dry powders function by creating a crust over the surface of the burning metal. To extinguish the fire, this crust must completely cover the burning surface. These agents are usually applied by hand scoop or portable fire extinguisher. Graphite and sodium chloride are two common examples of these agents.

Wet Chemicals
Wet chemical agents, typically potassium acetate, are designed specifically for cooking oil and grease fires.  They are available in portable fire extinguishers and installed systems.

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