- Foam (firefighting)
Firefighting foam is a stable mass of air-filled bubbles used for fire suppression. Firefighting foam perform two functions;
- cool the fire; and
- prevent enough oxygen from the fire to suppress or stop a fire.
Firefighting foam generally consists of three ingredients; water, foam concentrate and air. When mixed as a foam solution in the correct proportions, these three ingredients form a homogeneous foam blanket.
Water along with foam concentrate is designed for a nominal proportion ratio of foam concentrate and water. For example, a 3% proportion ratio means that 3 parts of foam concentrate are added to 97 parts of water to obtain 100 parts of foam solution.
Both synthetic-based foam concentrate and protein-based have a shelf life when stored within the manufacturer's guidelines, including temperature limits, and in its original container.
Hydrocarbon vs Polar Solvent Fires
The fire-fighting foam used for hydrocarbon fires and polar solvent fires differ in their composition and application. Polar solvent fires (Class A) involve alcohols, ketones, and esters while hydrocarbon fires (Class B Foam) involve fuels such as gasoline, diesel, and other petroleum-based products.
- Polar solvent fires require the use of Class A foams, which are designed to penetrate the fuel and create a barrier between the fuel and the air to prevent re-ignition. Class A foams are made up of fluorinated surfactants, which have a stronger attraction to polar solvents than hydrocarbon surfactants, making them more effective on polar solvent fires.
- Hydrocarbon fires require the use of Class B foams, which are designed to spread rapidly over the surface of the fuel, form a vapor-sealing blanket, and extinguish the fire by separating the fuel from the air. Class B foams are made up of hydrocarbon surfactants, which are designed to work best on hydrocarbon-based fires.
It is essential to use the correct type of foam concentrate for each type of fire to ensure that the foam is effective and does not create any additional hazards. It is also important to note that some foam concentrates can be used on both polar solvent fires and hydrocarbon, while others are designed for specific fire types.
Expansion Ratio
The expansion ratio is the volume of finished foam produced versus the volume of premix solution supplied. The more air is introduced, the higher expansion ratio you get. For example: when a foam generator supplied with 100 litres of water + foam solution produces 1000 litres of foam, the expansion ratio is therefore 1000/100 = 10.
Perfluoroalkyl and Polyfluoroalkyl Substances ("PFAS")
Firefighting foams are an essential tool used to combat some classes of fire, and it's important to note that not all foams contain per- and polyfluoroalkyl substances (PFAS). PFAS are a group of synthetic chemicals known for their resistance to heat, water, and oil, which made them effective for firefighting purposes in the past. However, due to environmental concerns, efforts have been made to develop alternative foams that are PFAS-free. These PFAS-free foams are designed to be just as effective in suppressing fires while reducing the potential environmental impact.
History of Fire-Fighting Foams
The use of foam in firefighting dates back to the 1800s, when firefighters used water mixed with soap to create a foam that could smother fires. However, it wasn't until the 1960s that modern foam concentrates began to be developed. Early foam concentrates were made from animal-based proteins, which had limited fuel tolerance and were susceptible to fuel pick-up.
A breakthrough came with the development of aqueous film-forming foams (AFFF), which used synthetic detergents to improve fuel tolerance and create a vapor-sealing film across the surface of the fuel. Over the years, foam concentrates have continued to evolve, with the introduction of film-forming fluoroprotein (FFFP) and fluorine-free foams (F3), among others.
- Early foam concentrate made from animal-based protein such as ground hoof and horn;
- Synthetic Aqueous Film-Forming Foams (AFFF) were developed in the 1960s to create a vapor-sealing film across the surface of the fuel. These foams provide improved fuel tolerance compared to protein-based foams. Used for fires involving flammable liquids. This group of synthetic foams also include Film-Forming Fluoroprotein (FFFP) that were later developed in the 1980s, and later alcohol-resistant (AR-FFFP) foams were developed to combat water-soluble alcohols.
- Fluorine-Free Foams (F3) foams have been subsequently developed in as a fluorine-free alternative, increasingly important in the fuel-storage arena due to its environmental benefits.