Many cruisers have expressed interest in foam fire extinguishers, so here is some information.
All foam extinguishers contain water and foam concentrate, so any type of foam extinguisher can extinguish a Class A, B or AB fire. The amounts vary by the % type of concentrate, which for Class B fires is either 1,3 or 6%. What this means is that if you had a 100-gallon foam fire extinguisher and were using 3% concentrate, the extinguisher would contain 3-gallons of foam concentrate and 97-gallons of water.
The concentrate/water mixture is expelled from the extinguisher by compressed air (usually 100-psi to 150-psi) and is discharge thru a nozzle that either creates a spray or one that mixes the solution with air. This mixing with air is what creates the “foam bubbles”, or what we think of as “foam”.
Foam can float on top of Class B fuels and it is this ability that allows the burning fuel
vapors to be sealed off and the fire extinguished. Foam can also be used to cover (seal) un-ignited fuel
surfaces that are producing flammable vapors and prevent them from igniting.
The ability to totally seal off the burning surface and prevent re-ignition of the vapors in windy or rolling conditions as found at sea is generally the single
big advantage that foam has over dry chemical, Halons and CO2.
Foams can be “thin” or “thick”. Thin types that can rapidly spread over a Class B surfaces are in the AFFF “lightwater” family, while the thicker types are in the FPPP family.
The thick foams can adhere slightly better to overhead or vertical surfaces, which can help extinguish this location of fire better and also form a barrier to help prevent ignition of flammable materials – which are normally Class A materials. This could be useful in an engine
compartment or cockpit
The thin foams can rapidly spread over the surface of Class B liquids and can flow around obstacles in the fire. This is very useful in any type of bilge
or contained space fire as any foam applied at a single location will end up spreading over the entire surface.
AFFF foam was developed by the US Navy
to fight aircraft crash and machinery space fires on ships and airfields. It is still in use today and anytime you see a airplane crash on TV you will see AFFF foam being applied. If you go back to page 4 of this section, you will see a 2-1/2 gallon AFFF extinguisher “at work” extinguishing yacht size Class AB fires.
AFFF is the most common type of foam found in “yacht size” fire extinguishers. They range in size from 1-liter to 9-liters (2-1/2-gallons). The most common size you will find in the US is the 2-1/2 gallon size. It is made out of stainless steel
and is about the same size and weight as a 20-lb dry chemical extinguisher, so it is a fairly big and heavy (about 35-lbs total weight).
On fuel docks you may find 36-liter or 33-gallon “trolley” or “wheeled” units that come equipped with a long hose.
AR:AFFF foam is sort of a “dual agent” for the two major types of Class B fuels. Fires involving alcohol (the AR part of the foam), ethanol, keytones, etc, tend to rapidly destroy the AFFF foam bubbles, so an additional chemical is mixed with the AFFF concentrate to “toughen” up the bubbles to prevent their destruction.
If you have unknown Class B fuels burning, or have spilled a lot of alcohol, acetone or keytones into the engine
spaces, then AR:AFFF will handle the fire. In all other respects, AR:AFFF works and spreads just like AFFF.
FPPP is the “thick” foam and is rarely found on yachts. It’s benefits are primarily for fighting aircraft fires that are off the run way or away from the airport
where grass or dirt are present and will rapidly absorb AFFF - so a foam blanket cannot easily form.
The European made foam extinguishers normally made from steel
and have a plastic bladder inside of them to prevent the foam solution from rusting the steel. These bladders can fail, and when they do you have to replace the entire extinguisher.
US made foam extinguishers are made from stainless steel and normally never have a corrosion
When using a foam extinguisher you should always try to apply the foam as gently as possible onto the burning fuel surface. You can do this by “bouncing” the foam onto the fire, letting it fall on the fire like “raindrops”, or applying it to a vertical surface and letting it “run down and flow” onto the surface. If these surfaces are on fire, then the "falling" or "running" foam will extinguish these surfaces.
Since most of the engine compartments on yachts of the 25 to 55-foot range are fairly small, applying the foam to an overhead, vertical surface or running down a surface (the engine block would work well for doing this).
Since the foam is a liquid, it will try to flow to the lowest point (usually the bilge or an engine catch pan), so when applied to burning vertical surfaces it will extinguish that fire and continue on to extinguish any fire in a lower location.
If you have a fractured or leaking fuel line or injector that is spraying or letting fuel drip (a pressurized fire), foam will NOT work on that portion of the fire – you will need a dry chemical unit to extinguish it. However, the foam will work on the un-pressurized portion of the fire. And, you can use both foam and dry chemical at the SAME time on a fire for rapid knockdown and total securment, Again, look at my previous posting
for pictures of this occurring.
With the small extinguishers, foam does not obscure your view or create a toxic atmosphere, so it is generally very user friendly.
Post fire, foam is easy to clean up as it is basically a “soapy” water. You can use manual pumps to pump it overboard
, or rags and sponges to mop it up.
You can go to Yahoo or U-Tube and search for foam fire extinguishers or foam fire fighting and find several short videos on foam fire extinguishers.