In an ignitable liquid fire, water alone is often not enough: the vapour layer above the fuel keeps feeding the flames and water does not easily suppress it. Low-expansion foam forms a water-based blanket over the liquid surface that separates the fuel from air, stops vapour release and cools the surface. FM Global DS 4-7N Low-Expansion Foam Systems covers the selection, design logic, component approval and maintenance of these systems.
Scope and Expansion Classes
Foam systems are grouped by expansion ratio — the volume of finished foam divided by the volume of foam solution that produced it. Under NFPA 11, up to 20:1 is low expansion, 20:1 to 200:1 is medium and above 200:1 is high expansion. DS 4-7N focuses on low-expansion foam, which is heavy, can be projected over a distance, resists wind reasonably well and flows across a liquid surface to form a blanket.
| Application | Typical system | Point to watch |
|---|---|---|
| Fixed-roof storage tank | Foam outlets on the shell or subsurface injection | Subsurface injection is not suitable for polar solvents |
| Floating-roof tank | Rim seal protection | Foam dam and seal type |
| Bunds and pool fires | Fixed foam pourers or monitors | Bund area and drainage |
| Loading racks, pump areas | Foam-water sprinklers or nozzles | Detection speed and coverage |
| Aircraft hangars | Foam-water deluge, monitors, floor nozzles | Coverage under wings |
Key Loss-Prevention Principles
The concentrate must suit the fuel
Water-miscible (polar) liquids such as alcohols, ketones and esters break down ordinary foam quickly and need alcohol-resistant (AR) concentrates. For hydrocarbons, protein, fluoroprotein, AFFF, FFFP and fluorine-free foams can be used, but each is sensitive to the application method in its own way. Selection should never happen before the fuel list is drawn up.
The system is approved as a whole
Foam performance depends on the concentrate, the proportioner and the discharge device (foam chamber, sprinkler, monitor, nozzle) working together. FM expects these to be used as combinations tested and approved together. Using a concentrate with a discharge device outside its approval can change foam quality and, with it, extinguishing performance.
Proportioning accuracy
Mixing concentrate into water at the correct percentage (for example 1, 3 or 6 per cent) is the heart of the system. Balanced pressure (pump or bladder tank), in-line inductors, around-the-pump and direct injection each work over a limited flow range. Where one proportioner feeds several systems with very different flows, proportioning must be verified at both the lowest and the highest flow.
Adequate application rate and duration
Foam must be applied at a sufficient rate and for a sufficient time to extinguish the fire and prevent re-ignition. If the rate is too low, foam breaks down on the burning surface faster than it forms and the fire does not go out. For example, NFPA 11 requires at least 4.1 L/min·m² (0.10 gpm/ft²) for fixed foam outlets on fixed-roof tanks storing hydrocarbons, with the duration depending on flash point and outlet type. Values in the current revision of the FM data sheet should be confirmed for each project.
Water supply and cooling
Foam systems consume water, and in the same fire tank cooling, exposure protection of neighbouring tanks and hose demand all run too. Total water demand is the sum of these. Concentrate stock must also cover the design duration plus a reserve.
Acceptance Testing and Operation
A flow test on water alone is not enough to accept a foam system. The proportioning percentage is measured on samples of foam solution (by refractometer or conductivity), and where possible foam is collected from the discharge device to check expansion and drainage time. Where test foam and waste water will go must be planned beforehand, because foam solution should not be released straight into the storm drain. In operation, concentrate tank level, valve positions and annual laboratory analysis of concentrate samples are tracked.
Moving Away from Fluorinated Foams
AFFF and similar fluorinated foams are being restricted in many countries because of their PFAS content, and the European Union has adopted a phased restriction on PFAS in firefighting foams. Switching to fluorine-free foam is more than changing the concentrate: fluorine-free foams usually need gentler application, aspirating discharge devices and sometimes higher application rates. The existing system must be cleaned, the old concentrate disposed of safely and the approval of the new combination checked. The transition plan should be agreed with the insurer.
How It Differs from NFPA 11 and EN 13565
NFPA 11 and, in Europe, EN 13565-2 give application rates, durations and design rules, while concentrate performance is tested to product standards such as the EN 1568 series. FM differs on two points: first, strict adherence to approved concentrate–device combinations; second, questioning where foam sits within the whole hazard. In an ignitable liquid warehouse, for example, foam-water sprinklers are assessed together with containment and drainage; in an area without drainage, foam alone may not be enough to confine the fire.
Field observation: the most common defects in foam systems are proportioners that have never been tested, ageing concentrate or stocks mixed with a different type, and concentrate line valves left closed. The system appears to work on water but produces no foam.
Practical Checklist
- All protected fuels are listed and AR concentrate is chosen for polar solvents.
- Concentrate, proportioner and discharge device form an approved combination.
- Proportioning is verified at the lowest and highest flows.
- Application rate and duration are calculated to the current data sheet and standard.
- The water supply meets the combined foam, cooling and hose demand.
- Concentrate stock covers the design duration and reserve; types are not mixed.
- Annual concentrate sampling and proportioning tests are carried out.
- Drainage and containment are designed together with the foam system.
- The fluorine-free transition and waste disposal have been discussed with the insurer.
Frequently Asked Questions
What does low-expansion foam mean?
Foam with an expansion ratio of up to 20:1. It is heavy, can be projected and forms a blanket on ignitable liquid surfaces. Medium and high expansion foams are used mainly to fill volumes.
Can I simply replace AFFF with fluorine-free foam?
Usually not. Fluorine-free foams need different discharge devices and sometimes different application rates. Check the approval of the combination and clean the pipework.
Can concentrates from different manufacturers be mixed?
The general rule is no. Different types or brands can cause sludge and loss of performance; mixing is considered only with a manufacturer's compatibility statement.
Why test the proportioner?
Proportioning errors are invisible from outside. Too little concentrate means weak foam; too much means the stock runs out early. An annual test reveals these deviations.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet 4-7N, Low-Expansion Foam Systems; NFPA 11, Standard for Low-, Medium-, and High-Expansion Foam; EN 13565-2; EN 1568 series. This page is a summary in our own words, not the data sheet text. Download and design to the current revision from fmglobal.com.