What Is Fire Fighting Foam and How Does It Work?
Fire fighting foam is a mass of small air-filled bubbles made from water, a foam concentrate and air, which is lighter than any flammable liquid and so floats on top of it. Sprayed onto a burning pool of gasoline, diesel or crude, the foam spreads into a blanket that cuts the fuel off from the oxygen above it, stops flammable vapor rising, cools the liquid surface as water drains out of the bubbles, and keeps doing all three while the blanket holds together. It is the standard agent for large flammable liquid fires under NFPA 11 and the one agent that leaves a pool secure after the flame is out.
What is in the foam
Foam starts as a concentrate, a liquid bought in drums or totes, mixed into water at a fixed percentage, usually 1, 3 or 6 percent by volume, to make foam solution. The solution is aerated, either by drawing air in at the nozzle or by mechanical generators, and the finished foam has an expansion ratio that describes how much its volume has grown. Low expansion foam, under 20:1, is wet and heavy and is what goes on pool fires. Medium and high expansion foams above that are lighter and drier and are used for filling volumes or suppressing vapor.
The concentrate chemistry decides what the foam can do. Hydrocarbon foams form a blanket on gasoline, diesel and crude. Alcohol resistant foams add a polymer that forms a membrane over polar solvents such as methanol, ethanol or acetone, which would otherwise dissolve the blanket. AFFF, the film-forming foam that dominated for decades, is being withdrawn in the US because of PFAS content, and fluorine-free foams have taken its place on new orders.
The four things a foam blanket does
Smothering is the obvious one: the blanket is a barrier that stops air reaching the fuel, and without oxygen at the surface the flame cannot continue. Separation is the same barrier from the other direction, keeping fuel vapor from rising into the flame zone. Cooling comes from the water, which drains out of the bubbles over the blanket's drain time and absorbs heat from the fuel surface. Vapor suppression is what keeps the pool safe after the fire is out, since a sealed surface releases no flammable vapor to find an ignition source.
The numbers that matter are the application rate and the duration. NFPA 11 sets a minimum of 0.10 gpm of solution per square foot for fixed foam outlets on a hydrocarbon pool fire and 0.16 where a monitor is doing the work over a diked area. On an open spill the rate depends on the concentrate: 0.10 for AFFF and other film-forming foams, 0.16 for protein, fluoroprotein and the fluorine-free products replacing them. Durations run from 15 minutes on a spill to 55 minutes on a tank of gasoline or crude. A 60 gpm handline at 0.16 gpm per square foot covers about 375 square feet. A blanket applied too thinly breaks up faster than it forms.
Where foam stops working
Foam needs a surface to sit on. A jet of gas or liquid burning at a flange, fuel spraying from a pump seal, or a fire running down a pipe rack gives it nothing to cover, and foam applied to those fires is wasted. Foam solution conducts electricity, so it stays off energized switchgear, motors and transformers. It is slower than dry chemical to put out an established flame, and wind, heat and splashing fuel all break the blanket. Water-based foam is not used on an LNG pool, where it would increase vaporization; high expansion foam on LNG is for vapor and radiant heat control, with potassium bicarbonate dry chemical as the extinguishing agent.
Those limits are why a loading rack, a pump row or a jetty is usually protected by foam and dry chemical together, either as separate units or on one twin-agent skid where Purple K knocks the flame down and foam follows to secure the pool. Compatibility between the powder and the concentrate has to be confirmed before the two are put on one frame, since some agents break a blanket faster than they help.
Frequently asked questions
Is fire fighting foam just soapy water?
No. Detergent bubbles collapse in seconds and do not resist fuel. Fire fighting concentrates are formulated for drain time, fuel tolerance and heat resistance, and are listed against UL or FM test fires.
How long does a foam blanket last?
Minutes, not hours. Drain time, the period for a quarter of the liquid to drain out, runs from a few minutes for a fast-flowing foam to well over ten for a stable one. On a secured pool the blanket is reapplied until the fuel is removed or cooled.
Can foam be stored ready-mixed?
Yes. Premixed solution is held in pressure vessels on skids and trailers and expelled by nitrogen, with no proportioner or water supply at the point of use. The trade-off is a shelf life on the solution and a tank that freezes in cold weather unless heated.
See how the dry chemical half of that pairing is built, filled and supplied on a dry chemical powder skid. Or talk to a Salus engineer about your hazard.
