Foam suppresses a flammable liquid fire by covering the fuel surface and cutting off vapour release. The whole design rests on that blanket being able to form and to last.
Step 1 — Define the fuel
The first distinction driving foam selection is whether the fuel is polar (water-miscible):
- Hydrocarbons (petrol, diesel, crude): a standard foam blanket stays on the surface.
- Polar solvents (alcohols, ketones, esters, ethanol-blended fuels): these dissolve ordinary foam. An alcohol-resistant (AR) type is required.
Ethanol-blended fuels have made this distinction critical on site; where the fuel composition has changed, the foam type must be reassessed.
Step 2 — Choose the concentrate
| Type | Use | Note |
|---|---|---|
| AFFF | Hydrocarbons | Forms an aqueous film; fast knockdown |
| AR-AFFF | Hydrocarbons plus polar solvents | Forms a polymer membrane on polar fuels |
| F3 (fluorine-free) | Hydrocarbons; polar depending on type | PFAS-free; application technique and approval must be verified |
| Protein / fluoroprotein | Tanks and fuel depots | Heat resistant, durable blanket |
Every concentrate is approved at a defined induction rate (for example 1%, 3%, or 3×6%). The rate must match the system's proportioning equipment; changing concentrate often means changing hardware.
Moving to F3 is not a "product swap". Switching to fluorine-free foam means reassessing the proportioning equipment, application rate, nozzle type and cleaning of the pipework together. The new concentrate must be verified by certificate as approved for the fuel and application type it will serve.
Step 3 — Choose the proportioning method
- Venturi (in-line inductor): simple and cheap, but with a narrow flow range and high pressure loss.
- Bladder tank: needs no electricity; common in fixed installations.
- Balanced pressure: accurate over a wide flow range; requires a pump.
- Direct injection: precise control at variable flow; the control system is complex.
The decisive factor is the flow range the system will work over. An arrangement that gives the right rate at one point can lose it when flow changes.
Step 4 — Determine the application type
- Surface application. Foam laid gently onto the fuel surface; the most common method.
- Subsurface application. Foam injected at the tank base and rising to the surface; used on fixed-roof tanks.
- Floating roof rim seal. Protecting the rim space on floating roof tanks.
- Foam-water sprinklers. In production and filling areas handling flammable liquids.
- High expansion foam. Volume filling in enclosed spaces.
Step 5 — Flow, duration and stock calculation
Application density and duration are taken from the standard according to fuel type, application method and protected area. Concentrate stock is set as: application flow × induction rate × duration, plus the reserve the standard requires. That reserve allows a second attack after the first application.
Step 6 — Set up the ancillary systems
- Retention and drainage. Foam solution must be controlled environmentally; a collection basin and interceptor are part of the design.
- Cooling. Water cooling of adjacent tanks during a tank fire is a separate system.
- Bunds and falls. Bunding and floor gradients that limit liquid spread.
- Remote activation. The operator must be able to start the system from a safe distance.
Step 7 — Testing and maintenance
- Periodic laboratory analysis of a concentrate sample (degradation, sedimentation, concentration).
- Proportioning test: measuring the actual induction rate in a sample drawn from the system.
- Nozzle and pipework flow testing.
- Bladder tank membrane inspection.
- Valve and activation chain testing.
- Collection and disposal of foam must be planned for any discharge test.
Do not skip the proportioning test. Having concentrate in the system does not mean it is being inducted at the right rate. A blocked injector or a wrongly sized venturi produces a system that appears to work and is ineffective. The rate must be measured from a sample.
Five common mistakes
- Not reviewing the foam type after a change in fuel composition (ethanol blending).
- Proportioning equipment that does not match the system's operating flow range.
- No reserve allowance in the concentrate stock.
- Retention and drainage left unresolved.
- Pipework not cleaned and compatibility not verified when changing concentrate.
Frequently Asked Questions
Why is AR foam needed on polar solvents?
Alcohols, ketones and ethanol-blended fuels dissolve ordinary foam. An alcohol-resistant type forms a polymer membrane on the polar fuel surface and preserves the blanket.
What decides the proportioning method?
The flow range the system will operate over. An arrangement accurate at one point can lose the rate when flow changes.
What to watch when moving to F3?
Proportioning equipment, application rate, nozzle type and cleaning of the pipework are assessed together, and the new concentrate verified by certificate as approved for that fuel and application.
Why is a proportioning test essential?
Having concentrate in the system does not mean it is inducted at the right rate. A blocked injector or wrong venturi produces a system that appears to work and is ineffective.
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Download MEP Calc on the App StoreNFPA 11 · NFPA 16 · NFPA 30 · NFPA 75 · NFPA 2001 · EN 1568 series · EN 13565 · FM Global DS 4-9, DS 7-88, DS 5-32. This guide is a general road map; the binding text is the relevant standard itself.