There are many different types of firefighting media and many different ways in which to apply them, depending on the nature of the incident encountered.
The media chosen for a given type of fire will depend on the nature of the materials involved and the size and intensity of the fire.
When applied to a fire, an extinguishing medium undergoes changes as it absorbs heat from the fire:
- Its temperature will rise
- It may evaporate
- It may chemically decompose
- It may react chemically with the burning material
To achieve maximum effect, the quantity of heat energy absorbed when these changes occur should be as large as possible.
Water is the cheapest, most efficient and readily available way of extinguishing fires of a general nature. With a high latent heat of vaporisation, it takes about six times as much heat to convert a given mass of water at its boiling point into steam as that required to raise the temperature of the same amount of water to boiling point. The most efficient use of water is where as much as possible is converted into steam. The smothering effect of the steam produced at the seat of the fire is thought to play a part in assisting the extinguishing process.
Submerging the burning materials in water can be effective, particularly when considering cooling the remnants of fire. This can be achieved using a variety of container types, sizes and methods, such as buckets, large refuse skips and improvised methods. Consideration will need to be given to containing the resultant contaminated water.
There are occasions when water in any form is not effective and occasions when it is dangerous to use, particularly where there are materials that react unfavourably with water, potentially with explosive effects. Examples include magnesium, aluminium, lithium, potassium, sodium and other combinations of these substances; they are commonly used in manufacturing processes. It is important that specific sites that may store or use these materials are identified and emergency responders are made aware of the associated hazards, control measures and planning arrangements.
Firefighting foams have been developed primarily to deal with the hazards posed by liquid fuel fires. Although water is used for most incidents, it is generally ineffective against fires involving liquid fuels. This is because the density of water is greater than that of most flammable liquids so when applied it quickly sinks below their surfaces.
Finished foams consist of bubbles produced from a combination of foam concentrate and water that has been mixed with air. These air-filled bubbles form a blanket that floats on the surface of flammable liquids, knocking down and extinguishing fires by:
- Excluding air (oxygen) from the fuel surface
- Separating the flames from the fuel surface
- Restricting the release of flammable vapour from the surface of the fuel
- Forming a radiant heat barrier which can help to reduce heat feedback from flames to the fuel and hence reduce the production of flammable vapour
- Cooling the fuel surface and any metal surfaces as the foam solution drains out of the foam blanket; this process also produces steam which dilutes the oxygen around the fire
A variety of foam concentrates can be categorised into two main groups: protein or synthetic-based, depending on the chemicals used in their production. The characteristics of each concentrate and the finished foam they produce vary, making them suitable for some applications and unsuitable for others.
The main properties of firefighting foams include:
- Expansion: the amount of finished foam produced from a foam solution when it is passed through foam-making equipment
- Stability: the ability of the finished foam to retain its liquid content and to maintain the number, size and shape of its bubbles; in other words, its ability to remain intact
- Fluidity: the ability of the finished foam to be projected onto, and to flow across, the liquid to be extinguished or protected
- Contamination resistance: the ability of the finished foam to resist contamination by the liquid to which it is applied
- Sealing and resealing: the ability of the foam blanket to reseal should breaks occur, and its ability to seal against hot and irregular shaped objects
- Knockdown and extinction: the ability of the finished foam to control and extinguish fires
- Burn-back resistance: the ability of the finished foam, once formed on the fuel, to stay intact when subjected to heat and/or flame
The most common foam in use is in a compressed air foam system, which can be carried in combination with traditional water appliances. The foam attacks all three sides of the fire triangle simultaneously; the foam blankets the fuel, thereby reducing the fuel's capacity to seek out a source of oxygen and adheres to ceilings and walls, more readily aiding rapid reduction in heat. Also, the opaque surface of the foam, as it adheres to walls and ceilings, shields the fuel source from radiant energy.
Compressed air foam systems can deliver a range of useful foam consistencies, labelled from type 1 (very dry) to type 5 (wet), which are controlled by the air-to-solution ratios and, to a lesser extent, by the concentrate-to-water percentage. Types 1 and 2 foams have long drain times, meaning the bubbles do not burst and give up their water quickly. Wet foams, such as types 4 and 5, drain more quickly in the presence of heat.
Compressed air foam systems can produce a wide range of foam qualities or foam types, providing the most appropriate foam response to individual fire situations. This gives the incident commander the advantage of tailoring the best foam type to the tactical use and fire problem at hand. Generally, the environmental effects of foams are considered in terms of their toxicity and their biodegradability. It is the total volume of the foam concentrate that is released into the environment that is of concern; it does not matter by how much it has been diluted. See National Operational Guidance: Environmental protection for further information.
Fire and rescue services also use foam for other purposes in addition to firefighting. See National Operational Guidance: Hazardous materials
Dry chemical powders
The basis of most dry powder extinguishers is sodium bicarbonate. With the addition of a metallic stearate as a waterproofing agent, it is widely used as an extinguishing agent in portable extinguishers and for larger application. Dry powder is very effective at extinguishing flame (rapid knockdown), and is particularly valuable in tackling a fire involving an incident in which clothes have been soaked in flammable liquid and ignited.
Dry chemical powders are expelled from containers by gas pressure and directed at the fire in a concentrated cloud through specially designed nozzles. Dry chemical powders are also tested for their compatibility with foam because early powders tended to break down foam. The two can complement each other at fires where foam is the standard extinguishing agent.
Ternary eutectic chloride powders have been developed for some metal fires. This type of foam melts, and then flows to form a crust over the burning metal, effectively sealing it from the surrounding atmosphere and isolating the fire.
Some burning materials, such as metals that cannot be extinguished by water, may be dealt with by using dry earth, dry sand, soda ash or limestone, all of which act as smothering agents.
Carbon dioxide, vaporising liquids and inert gases
Halons (halogenated hydrocarbons) vaporise rapidly when released from their pressurised container. The vapours are heavier than air, but when drawn into the flames, they inhibit the chain reactions and suppress flaming. Halons have now been largely replaced with inert gases or fine water mists because of environmental concerns.
At normal temperatures, Carbon dioxide (CO2) is a gas 1.5 times as dense as air. It is easily liquefied and bottled in a portable cylinder where it is contained under approximately 51 bars pressure. When discharged, cold CO2 vapour and some solid CO2 are expelled from the horn, which rapidly cools in the process. The solid quickly turns to gas, and some of the liquid CO2 evaporates to maintain the pressure in the cylinder. The gas, however, extinguishes by smothering, effectively reducing the oxygen content of the air. About 20 to 30% is necessary to cause complete extinction, depending on the nature of the burning material.
Carbon dioxide is quick and clean, electrically non-conductive, non-toxic and non-corrosive. It is however an asphyxiate at the concentrations necessary to extinguish a fire. The operation of total flooding CO2 systems requires prior evacuation of all personnel.
Another fire extinguishing method is blanketing, which deprives the fire of oxygen. This is especially useful if someone's clothes are burning. For dealing with fires such as cooking fat fryers, the best method is to smother the fire with a fire resisting blanket.
Small fires in textile materials may often be extinguished by beating them out, or by rolling and screwing up the material tightly to exclude the air. Beating is also the method normally employed to extinguish heath, crop and similar fires in rural areas when water is not readily available.
See also National Operational Guidance: Environmental Protection - Fire water run-off