4.3 Foam and Ignitable-Liquid Fire Control
Key Takeaways
- Class A foam reduces water's surface tension so it soaks into ordinary combustibles; Class B foam forms a vapor-suppressing blanket on ignitable (flammable/combustible) liquids.
- AFFF (aqueous film-forming foam) floats a film of water across hydrocarbon fuels; alcohol-resistant (AR) foam is required for polar/water-miscible fuels like ethanol that would break a standard blanket.
- Foam proportioning percentages (typically 0.1-1% Class A, 3% or 6% Class B) must match the fuel; the wrong concentrate or wrong percentage produces a blanket that fails.
- Apply foam gently so the blanket is not plunged through the fuel - use roll-on, bank-down, or rain-down methods, never a hard direct stream into the liquid.
Why foam, and why it is a Firefighter II skill
Water alone fails on most flammable-liquid fires: it is heavier than hydrocarbons, sinks beneath them, can spread burning fuel, and flashes to steam without securing the surface. Foam solves this by floating a blanket on the fuel that separates the fuel from oxygen, suppresses flammable vapors, and cools. Extinguishing an ignitable-liquid fire with correctly selected and applied foam is a Firefighter II job performance requirement, so the exam pushes past "foam exists" into selection, proportioning, and application technique.
Class A versus Class B foam
| Property | Class A foam | Class B foam |
|---|---|---|
| Target fuel | Ordinary combustibles (wood, paper, brush) | Ignitable liquids (gasoline, diesel, alcohols) |
| How it works | Lowers water's surface tension so it penetrates and clings | Forms a vapor-suppressing film/blanket over the liquid |
| Typical proportioning | About 0.1% to 1% | Commonly 3% or 6% (read the fuel and the concentrate) |
| Common use | Structure fires, wildland, overhaul | Vehicle fuel fires, fuel spills, storage tanks |
Class A foam makes water "wetter" - useful in overhaul and on deep-seated Class A fuels. Class B foam is the ignitable-liquid agent and is the focus of the Firefighter II skill.
Matching Class B concentrate to the fuel
The single most-tested foam concept is matching concentrate to fuel type:
- Hydrocarbon fuels (gasoline, diesel, oil, jet fuel) do not mix with water. AFFF (aqueous film-forming foam) works well - it releases a thin water film that spreads ahead of the foam blanket and seals the surface.
- Polar solvent / water-miscible fuels (ethanol, methanol, acetone, many alcohols) will break down a standard foam blanket by pulling the water out of it. These require alcohol-resistant (AR) foam, which forms a tough polymeric membrane on the fuel.
Using a standard hydrocarbon foam on an alcohol fire is a classic exam trap - the blanket dissolves and the fire reignites. Ethanol-blended motor fuels (E10, E85) increasingly demand AR foam.
Proportioning: getting the percentage right
Proportioning is mixing the right amount of foam concentrate into the water stream to make a foam solution, which is then aerated into finished foam. The percentage is the volume of concentrate per volume of solution: a 3% concentrate means 3 parts concentrate to 97 parts water. Common errors the exam probes:
- Using a 3% concentrate at a 6% setting (or vice versa) - the blanket is too thin or wastes concentrate.
- Forgetting that the concentrate must match BOTH the fuel and the rated percentage marked on the container.
- Assuming finished foam quality does not depend on aeration; a proper eductor or proportioner plus an air-aspirating nozzle (or compressed-air foam) builds the bubble structure that makes the blanket stable.
Application technique
Even the right foam fails if it is applied wrong. The goal is an unbroken blanket built gently so the stream does not plunge through the fuel surface and submerge the foam (which destroys the blanket and can splash burning fuel). Three accepted methods:
| Method | How | When |
|---|---|---|
| Roll-on (roll-in) | Direct the stream onto the ground just in front of the fuel so foam rolls across the surface | Pool fire on open ground |
| Bank-down (bank-shot) | Bounce the stream off a wall, tank shell, or object so it flows down onto the fuel | Fire against a vertical surface or in a container |
| Rain-down | Loft the stream into the air so foam falls gently onto the surface like rain | Large pools where you cannot reach the far side directly |
After knockdown, maintain the blanket - flammable vapors keep rising and the blanket slowly breaks down, so reapply as needed and do not assume a covered spill is permanently safe. The exam answer for an ignitable-liquid fire is: select the correct concentrate for the fuel, proportion to the rated percentage, apply gently to build an unbroken blanket, and maintain it - not a hard direct stream that plunges into the fuel.
How foam is made: the equipment chain
Finished foam is built in stages, and the exam expects you to know the chain. Foam concentrate is the liquid in the pail or tank. A proportioner (commonly an in-line eductor that uses the Venturi effect, or an apparatus-mounted balanced-pressure system) meters the concentrate into the water to make foam solution. The solution is then aerated at the nozzle - air mixed in to create the bubble structure of finished foam. Skipping or under-aerating a step yields a watery, short-lived blanket.
An in-line eductor has two practical limits the exam may probe: the nozzle flow must match the eductor's rating, and the distance/elevation between eductor and nozzle is limited or the concentrate will not pick up. Compressed-air foam systems (CAFS) inject air mechanically for a drier, longer-reaching, more durable foam.
Expansion ratio and foam types
Foam is also classified by expansion ratio - the volume of finished foam produced per volume of solution.
| Type | Expansion ratio | Typical use |
|---|---|---|
| Low-expansion | Up to 20:1 | Most fireground Class B work; flows across fuel and travels well |
| Medium-expansion | 20:1 to 200:1 | Vapor suppression on spills, some hazmat |
| High-expansion | 200:1 to 1000:1 | Filling confined spaces, basements, warehouses |
Low-expansion foam is the workhorse for ignitable-liquid fires because it holds together and flows across the fuel surface. A note on agents: legacy fluorinated AFFF (PFAS-containing) is being phased toward fluorine-free foams (F3) for environmental reasons, but the application principles - match the fuel, proportion correctly, apply gently, maintain the blanket - are unchanged.
Vapor suppression without fire
Foam is not only an extinguishing agent. On an unignited flammable-liquid spill, a foam blanket suppresses vapors and reduces the ignition hazard while the spill is controlled - a defensive, protect-in-place use. The exam may describe a fuel spill with no fire and ask the action; covering it with an appropriate foam blanket and eliminating ignition sources is a valid Firefighter II response, again applied gently and maintained until the hazard is removed.
A tanker spill of E85 ethanol-blend fuel is burning in a shallow pool. Which foam choice and reasoning is correct?
A firefighter is applying Class B foam to a burning fuel pool on open ground. Which application reflects correct technique?