How Firefighting Foam is Generated
How do firefighting foams work in fire suppresion?
Foam concentrate is based on hydrocarbon surfactants. They have different compositions:
Synthetic firefighting foams:
- Fluorine-Free Foams (F3, also known as FFF): creating a foam blanket full of bubbles
- Fluorinated foams: Aqueous Film Forming Foam (AFFF) and Alcohol-Resistant Aqueous Film Forming Foam (AR-AFFF also called AFFF-ARC)
- Protein foam
- Film Forming Fluoroprotein (FFFP)
- Alcohol-Resistant Film Forming Fluoroprotein (AR-FFFP)
The foam solution, also known as premix solution, is the appropriate mix of water and foam concentrate. A 3% ratio means that 3 parts of foam concentrate are added to 97 parts of water to obtain 100 parts of foam solution.
A foam concentrate is formulated to be efficient at the nominal concentration. Appropriate foam proportioning is crucial for foam performance and efficient fire extinguishment. Then, this foam solution is expanded with air in the foam-generating equipment to produce finished foam.
This ratio depends on the foam concentrate type or the fuel type. When a foam concentrate is described as 1 x 3, it means that the ratio of concentrate to water have to be 1% on a hydrocarbon fire and 3% on a polar solvent fire.
The finished foam is an amount of bubbles which forms a stable foam blanket which separate fuel from the oxygen supply. It provides fire extinguishment and prevents flammable vapours release.
The expansion ratio is the volume of the finished foam produced with premix solution. It could be low, medium or high expansion. It is closely related to foam-generating equipment.
Foam blanket with AFFF foams is composed of a thin aqueous film which spreads rapidly above the fuel surface separating the fuel from oxygen supply. Whereas, Alcohol resistant foams (AR-AFFF and AR-F3) form a polymer layer to efficiently extinguish class B polar solvent fires. It prevents foam destruction by the fuels.
A long drainage time will guarantee an efficient and long-lasting foam blanket. The drainage rate is based on how long the foam blanket remains stable and keeps more than 75% of the surface safe from burn back.
HOW TO EXTINGUISH A FIRE WITH FIREFIGHTING FOAM?
Firefighting foam is perfectly suited to suppress fire. It is composed of premix solution (a ratio of water and foam concentrate) mixed with air. It forms a stable foam blanket which spreads above the fuels and adheres to surfaces. Some types of foam forms a film above the fuel surface and seals vapours. The use of fire fighting foam concentrate prevents fuel contact with oxygen, resulting in fire suppression. It also has a high cooling effect and will avoid reignition.
To extinguish a fire, firefighting foam acts in two phases:
- Separation effect: the foam blanket suppresses oxygen supply to the flammable material and prevents combustible evaporation
- Cooling effect: a lot of energy is required to transform water included in the foam blanket into vapour. This contributes to reduce or remove the energy required by the combustion. This water vapour cools down fire high temperature and prevents from thermal radiation which could enhance the fire.
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