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Foam is available with two different and distinct purposes. One is for use on
flammable liquid spills and fires, while the second is for use on Class "A" fires.
Foam is a blanketing and cooling agent. When the proper foam solution is used on
flammable or combustible liquid fires, it does the following:
- It smothers the fire and prevents air from mixing with the flammable vapours.
- It actually suppresses the vapours and prevents their release.
- It separates the flames or ignition sources from the fuel surface.
- It cools the fuel and adjacent metal surfaces.
In the case of a Class "A" fire, the foam solution:
- Cools the burning surface.
- Breaks water surface tension to permit deeper penetration.
- Leaves a blanket that continues to release water.
- Insulates and prevents re-ignition.
Mechanical foam needs three components to work effectively--water, foam concentrate,
and air. The most common way to combine the foam concentrate with the water is to
pump water under pressure through an eductor/proportioner which meters the proper
amount (percentage) of foam concentrate into the water stream. The resulting
solution, under pressure but yet to have air added, passes through a
predetermined length of hose to the nozzle. The velocity of the discharge entrains
air into the solution giving you finished foam. The amount of air added is
referred to as the expansion ratio. An ordinary fog nozzle will produce an acceptable
foam. However, with the addition of a foam tip, the ratio and the quality of foam
is greatly enhanced. This foam is referred to as low or medium expansion.
Other methods of mixing the foam concentrate with the water include around
the pump proportioners, powered in-line inductors and water pressure bladder
tanks. Check the appropriate sections for information.
Several general rules apply for the application of foam to flammable liquid fires.
- If using an eductor, the gallonage of the eductor and nozzle must match.
- A pressure loss of approximately 65 psi must be taken into consideration
for the eductor.
- Foam application should be applied gently onto the burning surface and not plunged
into the liquid.
- Ensure that you have enough foam concentrate to handle the problem.
This is known as the minimum application rate.
MINIMUM APPLICATION RATE FORMULA
| Hydrocarbon Hazards |
Polar Solvent Hazards |
.10 gal/min./sq.ft.
(4 L/min./sq.m.) |
±.20 gal/min.sq.ft.
(8 L/min./sq.m. |
How to Calculate Minimum Application Rate
- Determine hazard: Hydrocarbon or polar solvent.
- Calculate surface area (LXW).
- Multiply area by application rate formula to determine flow rate required.
- Multiply flow rate by foam percentage to find the amount of concentrate
required for one minute supply.
- Multiply this result by 15 (minutes) to meet NFPA recommendation for total
foam required.
Example using 5000 sq. ft. gasoline spill
5000 x .10=500 gallons of foam
500 x .03=15 gallons foam/min.
Fireground Figures for Extinguishment of Polar Solvent Liquids
Fuel surface area approximately 1000 sq. ft. (100 sq. metres) using 3%-6% AFFF
at 6% with fog nozzle.
Methanol: 380 L/min. (100 USGPM)
Ethanol: 285 L/min. (75 USGPM)
Acetone: 475 L/Min. (125 USGPM)
Figures based on 100 sq. metre spill, low wind, ambient temperature 20ºC.
Foam proportioning at 6% with 5 minute pre-burn time.
Fireground figures for vapour suppression of 100 sq. metres polar solvent spill.
Minimum Foam Requirements for 30 Minute Securement
- Methanol: 90 litres (4 pails)
- Ethanol: 45 litres (2 pails)
- Acetone: 135 litres (6 pails)
Figures based on 6% proportioning through handlines with expansion ratio of 6:1. Quantities
shown are required for 30 minute duration. Based on vapour suppression of 100 sq. metre
spill using AFFF multi-purpose agent. Other synthetic concentrates may require larger
amounts than shown, depending on expansion ratio and type of nozzle used.
Higher expansion ratios will give longer securement time, provided foam used is
suitable for application on polar solvent liquid spills.
These figures are based on actual performance tests establishing minimum
application rates under controlled conditions. These figures have been converted
into working fireground figures based on a 100 sq. metre spill.
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