To properly understand foam basics some definitions are in
FOAM: A fire fighting foam is simply a stable mass of small air-filled bubbles, which have a lower density than oil, gasoline or water. Foam is made up of three ingredients - water, foam concentrate and air. When mixed in the correct proportions, these three ingredients form a homogeneous foam blanket.
FOAM SOLUTION: This is a solution of water and foam concentrate after they have been mixed together in the correct proportions.
FOAM CONCENTRATE: This liquid concentrate is supplied from the manufacturer which when mixed with water in the correct proportion forms a foam solution.
FINISHED FOAM: Foam solution as it exits a discharge device, having been aerated.
DRAINAGE RATE: This is the rate at which the foam solution will drain from the expanded foam mass or how long it will take for 25% of the solution to drain from the foam. This is often called the quarter life or 25% drain time. Foam that has a fast drain time is normally very fluid and mobile, spreading across the fuel surface very quickly. While foams with longer drain times are normally less mobile, they move across the fuel surface slowly.
EXPANSION RATE: Volume of finished foam divided by the volume of foam solution used to create the finished forma; i.e., a ration of 5 to 1 would mean that one gallon of foam solution after aeration would fill an empty 5-gallon container with the expanded foam mass.
LOW EXPANSION FOAM: Foam aerated to an expansion ratio of between 2 to 1 and 20 to 1.
MEDIUM EXPANSION FOAM: Expansion ratio between 20 to 1 and 200 to 1.
HIGH EXPANSION FOAM: Expansion ratio above 200 to 1.
DILUTION RATE, MIXING RATE, OR PROPORTIONING RATE (Correct amount of foam concentrate to be mixed with water): The amount is normally shown on the pail or drum of concentrate. If the container of foam concentrate has 3% shown, it means that for every 100 gallons of foam solution required, 3 gallons of the foam concentrate must be used in the solution with the balance being 97 gallons of water.
SEAWATER COMPATIBLE: Some foam concentrates can be used successfully with either sea, fresh or brackish water.
The types of foam used by our customers can be basically broken into two categories, Class A and Class B. These are the same classes as used in identifying fire extinguishers.
Class "A" Foam
Class "A" foam concentrates are generally mixed with water in ratios from 0.1% to 1.0%. They can be added to the fire stream by premixing in the tank, mechanical eductors, or electronic injection systems. As a rule, the electronic injection systems will be the most accurate means of mixing the concentrate with water but mechanical methods are most commonly used.
It is often said that adding a little dish soap is the same thing as using Class "A" foam. That simply is not true. Yes, it will make the water foam somewhat but that is all. Foam concentrates are much more complex than simple dish soap. The results will look like fire fighting foam but will not perform like foam.
Class "B" Foam
Fire departments generally equip themselves with a type of foam that will handle multiple flammable liquid types that are commonly encountered. Generally that would be AR-AFFF (Alcohol Resistant, Aqueous Film Forming Foam.) AR-AFFF is a multi-use, multi-purpose foam concentrate. This type of foam can be used in low concentrations (3%) to fight a common gasoline or diesel fire at an accident scene. At higher percentages (6%) the same foam can be used to fight fires that contain polar solvents such as alcohol. (Polar solvents are compounds that readily mix with water which will cause a foam blanket to dissolve rapidly.)
Aqueous Film Forming Concentrate (AFFF)
AFFF generated foams extinguish hydrocarbon flammable liquid fires the same way as the protein or fluoroprotein foams; however, there is an additional feature. An aqueous film is formed on the surface of the flammable liquid by the foam solution as it drains from the foam blanket. This film is very fluid and floats on the surface of most hydrocarbon fuels. This gives the AFFF unequaled speed in fire control and knockdown when used on a typical hydrocarbon spill fire. In certain circumstances, it is possible to notice the fire being extinguished by the "invisible" film before there is complete foam blanket coverage over the surface of the fuel.
AFFF foam solutions can be applied to a flammable liquid fire using either aspirating or non-aspirating discharge devices. The difference between the two is that the air-aspirating device entrains air and causes it to mix with the foam solution within the device. The non air-aspirating device is incapable of this process.
When flow rates and pressures are similar, AFFF solutions used with a non air-aspirating discharge device will generally discharge/throw the foam a greater distance than the foam that is discharged from the air-aspirating discharge device. A non-aspirating AFFF will generally extinguish a low vapor pressure fuel spill fire slightly faster than the foam discharged from an air-aspirating device. This is because the non-aspirated nozzle generated foam has a lower expansion and will be more fluid; therefore, it will move faster across the fuel surface.
When using AFFF foams, application technique is not as critical as with Proteins or Fluoroproteins. AFFF foam can also be used successfully with the sub-surface injection method. NOTE: The sub-surface method of discharging foam into a storage tank can only be used with tanks that contain standard hydrocarbon fuels, NOT polar solvent/alcohol type fuels.
The recommended application rate for AFFF 3%-6% generated foam solution on a hydrocarbon spill fire with low water solubility is .10 GPM/Sq. Ft. Remember the protein and the fluoroprotein foam solutions require an application rate of .16 GPM/Sq. Ft.
AFFF is suitable for use in a premix state and is suitable for use with dry chemical extinguishing agents. (In major tank farm fires it is common to use dry chemical and foam simultaneously for extinguishment.)
AR-AFFF concentrates are very viscous. Initial impression of this type of foam concentrate my lead one to believe that the concentrate has "gelled" and somehow gone bad. However, a thick, gel-like appearance is normal. This appearance is caused by the presence of polymers, which are the main components required for polar solvent applications. Modern AR-AFFF concentrates are designed to work through proportioning equipment such as in-line eductors, bladder tanks, and balanced pressure pump systems.
AR-AFFF 3%/6% type of concentrate is designed to be used at the 3% application rate when used on a standard hydrocarbon fuel fire and 6% when used on a polar solvent/alcohol fuel. Current 3% AR-AFFF type concentrate is designed for 3% application on either type group, i.e. 3% on hydrocarbons and 3% on polar solvent fuels.
When AR-AFFF is used at the correct proportioning rate on hydrocarbon fuel, fire fighting performance and application rate are the same as for standard AFFF agents. An "invisible" film is formed, the speed of covering a fuel spill with the foam blanket is similar and the application technique using either air-aspirating or non air-aspirating nozzles can be used. When used on an alcohol fire, an air-aspirating nozzle will give a better performance over the non air-aspirating nozzle. The increased expanded foam mass generated by the air-aspirating nozzle will give a more gentle application onto the surface of the alcohol liquid fire than will the non-aspirating nozzle. The intensity of the fire, distance the foam must be thrown, and the application rate also play an important part in determining the type of nozzle and method of extinguishment. The application technique and performance factors are the same for both the 3% and the 3%-6% types of AR-AFFF concentrates.
Synthetic/Detergent (High Expansion) Foam Concentrate
In areas such as a basement, mine shaft or a ship's hold where volume fire control is required, a high expansion foam generator can be used to fill an entire room with large amounts of very light expanded foam bubbles. Depending on the generator being used, high expansion ratios of 400 to 1 up to 1,000 to 1 can be achieved.
Fire control and extinction is achieved by rapid smothering and cooling. Fires involving solid material as well as flammable liquids can be controlled and extinguished using high expansion foam. It also has a special value for dealing with spillages of liquefied natural gas (LNG). A deep layer of 500 to 1 expanded foam will provide a thermal insulation barrier around the LNG spill, which reduces the heat intake, and therefore the rate of evaporation is decreased. Because of the high expansion ratios being achieved, there is very little water used; even with large discharges of the high expansion foam. High expansion foam has little water content within the bubble wall making it very light and not suitable for outdoor use. Medium expansion foam normally has an expansion of around 50-60 to 1. This foam is denser and can be used outdoors but is still affected by weather conditions.
Class "A" Foam Concentrate
Fluoroprotein Foam Concentrate
It makes the fluoroprotein foam more resistant to fuel contamination/pickup and makes the foam blanket more mobile when discharged onto the flammable liquid. Because the fluoroprotein foam is more resistant to fuel contamination, it allows the discharging foam to be applied directly to the fuel surface and the foam blanket will not become as saturated by fuel vapor. This type of foam can be used with a high back pressure foam maker by utilizing the sub-surface method of forcing expanded foam into the base of a cone roof storage tank containing a hydrocarbon fuel. The expanded foam enters the base of the storage tank then floats up through the flammable liquid to the surface where it covers the surface with a foam blanket. Fluoroprotein foam is sometimes used in the hydrocarbon processing industry for storage tank fire fighting. It is necessary to use with air-aspirating discharge devices. The recommended foam solution application rate on hydrocarbon spills is .16 gpm/sq.ft.
Film Forming Fluoro-Protein (FFFP)
Protein Foam Concentrate
The application rate for Protein foam solution on a hydrocarbon spill fire having low water solubility is .16 gpm/sq.ft. Protein foam, because of its stability, is relatively slow moving when used to cover the surface of a flammable liquid.
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