Fire fighting foam
From Wikipedia, the free encyclopedia
Fire Retardant Foam, or fire suppression foam, is a foam used for fire suppression. Its role is to cool down the fire and to coat the fuel, preventing its contact with oxygen, resulting in suppression of the combustion. The surfactants used have 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 used as well, eg. lauryl alcohol. Other chemicals are used as well, eg. corrosion inhibitors.
Low-expansion foams are foams with expansion rate lower than 20 times. Foams with expansion ratio between 20-200 are medium expansion. The low-expansion foams, eg. AFFF, are low-viscosity, mobile, able to quickly provide coverage of large areas.
High-expansion foams have expansion rate over 200. They are suitable for cases when an enclosed space, eg. a hangar, has to be quickly filled.
Alcohol-resistant foams contain a polymer that forms a protective layer between the burning surface and the foam, preventing the foam breakdown by alcohols present 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 practical experiences led to its increasing acceptance for fighting other types of class A fires, including structure fires. [1] Class A foams facilitate wetting of the class A fuels, lowering the surface tension of the water and assisting with saturating them with water, which helps suppressing the fire and preventing reignition.
[edit] Class B foams
Class B foams are designed for firefighting on class B fires - burning flammable liquids. Using class A foam for extinguishing of a class B fire may lead to unexpected results, as the class A foams are not designed to contain the explosive vapors produced by the flammable liquids. Class B foams have two major subtypes.
[edit] Synthetic foams
Synthetic foams are foams based on synthetic surfactants. Synthetic foams provide better flow, faster knockdown of the flames, but limited post-fire security.
Aqueous film forming foams (AFFF) are water-based foams, frequently containing alpha-olefin sulfonates, and/or perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) 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 the protective film even in their presence.
[edit] Protein foams
Protein foams contain naturally occurring proteins as the foaming agents. Protein foams 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).
[edit] Applications
Every type of foam has its best application. High-expansion foams are used when an enclosed space, e.g. a basement or a hangar, has 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-FP is suitable for burning alcohols. The most flexibility is achieved by AR-AFFF or AR-FFFP. AR-FFFP has to be used in areas where gasolines are blended with oxygenates, which prevent the formation of the film between the AFFF foam and the gasoline and break down the foam, rendering the AFFF foam virtually useless.
[edit] History of Fire Fighting Foams
Water has long been the universally accepted agent for suppressing fires. However, water is not the most effective agent in all cases. For example, water is typically not effective on an oil fire, and can be positively dangerous. The development of fire fighting foams was a positive step toward solving the problem of 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. Chemical foam is a stable solution of small bubbles containing carbon dioxide with lower density than oil or water that exhibits persistence for covering flat surfaces. Because it’s lighter than the burning liquid, it flows freely over the burning liquid surface and extinguishes the fire by a smothering action. Chemical foam is considered obsolete today because of the many containers of powder required for even the smallest fires.
In the 1940s, Percy Julian developed an improved type of foam called Aerofoam. By using mechanical action, a liquid protein-based concentrate, made from natural animal 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 its ease of handling made it very popular. Protein foam is easily contaminated by some flammable liquids; therefore, care should be used so that the foam is only applied above the burning liquid. Protein foam has slow knockdown characteristics but it is very economical for post fire security.
In the 1960s, National Foam, Inc. developed fluoroprotein foam. It contains an active agent called 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 a fire fighting foam called AFFF which is short for aqueous film-forming foam. This is synthetic foam with low viscosity that spreads rapidly across the surface of most hydrocarbon fuels. A water film forms under the foam bubbles which cools the liquid fuel and will stop the formation of flammable vapors and provide a 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 synthetic foam developed for both hydrocarbon and polar solvent materials. Polar solvents are combustible liquids that are destructive to conventional fire fighting foam. These solvents extract the water contained in the foam and break down the foam blanket. Therefore, these fuels require an alcohol or polar solvent resistant foam. Alcohol resistant foam must be "bounced" off a surface and allowed to flow down and over the liquid to form its membrane. Differing from standard AFFF that can be sprayed right onto the fire. Since AR-AFFF is effective on a variety of fuels; it is far and away the most accepted standard for American industry today.
[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 [www.fireworld.com/site/articles/innovation.html]
