Fire fighting foam
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Fire Retardant Foam, or fire suppression foam, is a foam used for fire suppression. Its role is to cool the fire and to coat the fuel, preventing its contact with oxygen, resulting in suppression of the combustion. The surfactants used need to produce foam in concentration of less than 1%.
Other components of fire retardant foams are organic solvents (eg. trimethyltrimethylene glycol and hexylene glycol. Foam stabilizers are also used, for example lauryl alcohol. Other chemicals are used as well, such as corrosion inhibitors.
Low-expansion foams have an expansion rate less than 20 times. Foams with expansion ratio between 20-200 are medium expansion. Low-expansion foams such as AFFF are low-viscosity, mobile, and able to quickly cover large areas.
High-expansion foams have an expansion rate over 200. They are suitable for enclosed spaces such as hangars, where quick filling is needed.
Alcohol-resistant foams contain a polymer that forms a protective layer between the burning surface and the foam, preventing foam breakdown by alcohols in the burning fuel. Alcohol resistant foams should be used in fighting fires of fuels containing oxygenates, eg. MTBE, or fires of liquids based on or containing polar solvents.
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[edit] Class A foams
Class A foams were developed in mid 1980s for firefighting wildfires. Favorable experiences led to its acceptance for fighting other types of class A fires, including structure fires. [1] Class A foams facilitate wetting of class A fuels, lowering the surface tension of the water and assisting saturation of them with water, aiding fire suppression and preventing reignition.
[edit] Class B foams
Class B foams are designed for class B fires - flammable liquids. The use of class A foam on a class B fire may yield unexpected results, as class A foams are not designed to contain the explosive vapors produced by flammable liquids. Class B foams have two major subtypes.
[edit] Synthetic foams
Synthetic foams are based on synthetic surfactants. Synthetic foams provide better flow, faster knockdown of flames, but limited post-fire security.
Aqueous film forming foams (AFFF) are water-based, frequently containing sodium alkyl sulfate, and/or perfluoro telomer as surfactants. They have the ability to spread over the surface of hydrocarbon-based liquids. Alcohol-resistant aqueous film forming foams (AR-AFFF) are foams resistant to the action of alcohols, able to form a protective film when they are present.
[edit] Protein foams
Protein foams contain natural proteins as the foaming agents. Unlike other synthetic foams, protein foams are bio-degradable. They flow and spread slower, but provide a foam blanket that is more heat resistant and more durable.
Protein foams include regular protein foam (P), fluoroprotein foam (FP), alcohol resistant fluoroprotein foam (AR-FP), film forming fluoroprotein (FFFP), and alcohol-resistant film fluoroprotein (AR-FFFP).
Protein Foam from non-animal sources is preferred because of the possible threats of biological contaminants like prions.
[edit] Applications
Every type of foam has its best application. High-expansion foams are used when an enclosed space, such as a basement or hangar, needs to be quickly filled. Low-expansion foams are used on burning spills. AFFF is best for spills of jet fuels, FFFP is better for cases where the burning fuel can form deeper pools, AR-AFFFF is suitable for burning alcohols. The most flexibility is achieved by AR-AFFF or AR-FFFP. AR-AFFF must be used in areas where gasolines are blended with oxygenates, since the alcohols prevent the formation of the film between the FFFP foam and the gasoline, breaking down the foam, rendering the FFFP foam virtually useless.
[edit] History of Fire Fighting Foams
Water has long been the universal agent for suppressing fires, but is not best in all cases. For example, water is typically ineffective on an oil fire, and can be dangerous. Fire fighting foams were a positive development in extinguishing oil fires.
In the late 1800s, a method of extinguishing flammable liquid fires by blanketing them with foam was introduced. The original foam was a mixture of two powders and water produced in a foam generator. It was called chemical foam because of the chemical action to create it. Generally, the powders used were sodium bicarbonate and aluminium sulfate, with small amounts of saponin or liquorice added to stabilise the bubbles. Hand-held foam extinguishers used the same two chemicals in solution: to actuate the extinguisher, a seal was broken and the unit inverted, allowing the liquids to mix and react. Chemical foam is a stable solution of small bubbles containing carbon dioxide with lower density than oil or water, and exhibits persistence for covering flat surfaces. Because it’s lighter than the burning liquid, it flows freely over the liquid surface and extinguishes the fire by a smothering action. Chemical foam is considered obsolete today because of the many containers of powder required, even for small fires.
In the 1940s Percy Lavon Julian developed an improved type of foam called Aerofoam. Using mechanical action, a liquid protein-based concentrate, made from soy protein, was mixed with water in either a proportioner or an aerating nozzle to form air bubbles with the free flowing action. Its expansion ratio and ease of handling made it popular. Protein foam is easily contaminated by some flammable liquids, so care should be used so that the foam is only applied above the burning liquid. Protein foam has slow knockdown characteristics, but it is economical for post fire security.
In the 1960s National Foam, Inc. developed fluoroprotein foam. Its active agent is a fluorinated surfactant which provides an oil-rejecting property to prevent contamination. It is generally better than protein foam because its longer blanket life provides better safety when entry is required for rescue. Fluoroprotein foam has fast knockdown characteristics and it can also be used together with dry chemicals which destroy protein foam.
In the mid 1960s the US Navy developed aqueous film-forming foam (AFFF) . This synthetic foam has a low viscosity and spreads rapidly across the surface of most hydrocarbon fuels. A water film forms beneath the foam which cools the liquid fuel, which stops the formation of flammable vapors. This provides dramatic fire knockdown, an important factor in crash rescue fire fighting.
In the early 1970s National Foam, Inc. invented Alcohol Resistant AFFF technology. AR-AFFF is a synthetic foam developed for both hydrocarbon and polar solvent materials. Polar solvents are combustible liquids that destroy conventional fire fighting foam. These solvents extract the water contained in the foam, breaking down the foam blanket. Therefore, these fuels require an alcohol or polar solvent resistant foam. Alcohol resistant foam must be bounced off of a surface and allowed to flow down and over the liquid to form its membrane, compared to standard AFFF that can be sprayed directly onto the fire. Since AR-AFFF is effective on a variety of fuels, it the most accepted for American industry.
[edit] References
- Associated Fire Protection 16 Sept. 2006 [www.afpfire.com/media/docs/foambasics.pdf]
- Clark, William E. Firefighting Principles and Practices. New Jersey: Saddle Brook, 1991.
- Hawthorne, Ed. Petroleum Liquids: Fire and Emergency Control. New Jersey: Englewood Cliffs, 1987
- Riecher, Anton. ? Innovation: Ideas Advance Fire Fighting. ? Vol. 20 No. 6, Industrial Fire World Magazine. 05 Oct. 2006 [2]
- Reney, Varghese Bio-degradable Fire Fighting Foams. Dubai:Journal,2007.
