NFPA 11 Foam Concentrate Supply and Proportioning

The part of a foam system most often found short is the part nobody looks at: how much concentrate is really in the tank, where the reserve that puts the system back into service is kept, and whether the concentrate is being mixed into the water at the right percentage. NFPA 11-2024 deals with all of this in Chapter 4: concentrate storage and reserve (4.3), compatibility (4.4), proportioning methods (4.5) and concentrate pumps (4.6). This page walks through those clauses in plain language.

Clause numbers are from the 2024 edition, read from the standard's PDF pages. Proportioners are described by their generic type; no products or brands are named.

How much concentrate? (4.3.2.2, 4.3.2.5.1)

The concentrate on hand must meet the discharge requirements of the largest single hazard protected, or of the group of hazards that must be protected at the same time (4.3.2.2). Consumption is based on the percentage used in the design, for example 3 % or 6 %, or another figure if listed or approved by the AHJ (4.3.2.5.1).

The annex flags a trap: the hazard with the largest solution flow does not necessarily set the total amount of concentrate (A.4.3.2.2). Duration matters too. Our own example:

Tank B, with the smaller flow, governs the stock because it runs longer, while pumps and pipework are still sized on Tank A's flow. The demand of supplementary hose streams is added on top (5.2.2, 5.9). The annex also recommends monitoring the concentrate level in the storage tank so an adequate supply is always available.

Storage tank rules (4.3.2.1, 4.3.2.3)

• Location: concentrate and equipment are kept where they are not exposed to the hazard they protect (4.3.2.1.1); if housed, in a noncombustible structure (4.3.2.1.2).
• Material: bulk tanks are made from, or lined with, materials compatible with the concentrate (4.3.2.3.1), solidly and permanently mounted, and designed to minimise evaporation.
• Suction inlet: in atmospheric tanks the suction is at least 25 mm (1 in.) above the tank bottom, and concentrate below the suction level does not count as usable (4.3.2.3.1.4, 4.3.2.3.1.5).
• Sediment pocket and expansion: the tank holds the required quantity plus space for thermal expansion (4.3.2.3.2.1); the sediment pocket volume is added to the quantity needed (4.3.2.3.2.3); outlets are placed so sediment is not drawn into the system.
• Access for inspection: conservation-type vents, handholes or manholes allow inspection of the interior, pump suction and test connections, level devices and fill and drain connections (4.3.2.3.2.4). The tank outlet gives a positive head on the pump suction (4.3.2.3.2.5).
• Pressure (bladder) tanks: need means for filling, gauging, draining, cleaning and inspecting the interior and the bladder or diaphragm (4.3.2.3.3.1), and must be stamped to the pressure vessel code (4.3.2.3.3.2). The annex warns that sight glasses can show false levels with more viscous concentrates.
• Storage conditions: concentrate is kept within its listed temperature limits (4.3.2.4.2), and vessels are marked with the concentrate type and intended percentage (4.3.2.4.3). The annex mentions freezing and deterioration from long storage at high temperature.

Reserve supply: the 24-hour rule (4.3.2.5.2)

A reserve supply of concentrate is required, enough for the design demand of the worst-case fire scenario and to put the system back into service after operation (4.3.2.5.2.1). It may be held in separate tanks or compartments, in drums or cans on site, or be obtainable from an approved outside source within 24 hours (4.3.2.5.2.2). Other storage methods, places or time frames need written AHJ approval (4.3.2.5.2.3). The annex warns that relying on a single outside source may not satisfy the 24-hour resupply requirement (A.4.3.2.5.2.2).

Other items needed to recommission the system, such as nitrogen or carbon dioxide bottles for premixed systems, must also be obtainable (4.3.2.6). For outdoor non-automatic systems the AHJ may allow concentrate to be stored off site where it is always available; loading and transport facilities are then provided and the required quantity is gathered on site before equipment is started at a fire (4.3.2.1.3–4.3.2.1.6).

Compatibility (4.3.1, 4.4)

• The concentrate is listed for use with the system's proportioning equipment and discharge devices, and replacement stock is listed with the system components (4.3.1.1, 4.3.1.1.1).
• It is listed for the specific ignitable liquid protected; water-miscible and polar liquids need alcohol-resistant concentrate (4.3.1.2, 4.3.1.2.1).
• Different types are never mixed in storage. Different brands of the same type are mixed only when manufacturer data prove compatibility and the AHJ accepts it (4.4.1.1, 4.4.1.2).
• Separately generated low-expansion foams (protein, fluoroprotein, FFFP, AFFF, SFFF and alcohol-resistant) may be applied to a fire in sequence or together (4.4.1.3). That is not permission to mix them in a tank.
• Where dry chemical is used alongside, both manufacturers confirm the products are mutually compatible (4.4.2.1).

The listing must publish acceptable ranges for density or specific gravity, pH, refractive index and viscosity, which are used to judge the concentrate test results under NFPA 25 (4.3.1.4).

Proportioning methods (4.5)

NFPA 11 lists seven methods (4.5): self-educting nozzle, in-line eductor, pressure proportioner (with or without bladder), around-the-pump proportioner, direct-injection variable-output foam pump, coupled water-motor-driven pump proportioner and balanced-pressure pump-type proportioner. The proportioning system must meet its listed minimum flow over the system's minimum and maximum discharge flows (4.5.1).

For fixed systems balanced-pressure and positive-pressure injection are the preferred methods (4.5.2.1). Balanced-pressure injection takes one of four forms (4.5.2.2):

1. A concentrate pump discharges through a metering orifice into a proportioning controller, with a pressure-balancing valve keeping concentrate and water pressures equal.
2. A pressure proportioning tank with a bladder or diaphragm separating water and concentrate discharges through a metering orifice into a proportioning controller.
3. An in-line balanced-pressure system using a concentrate pump or bladder tank, with a pressure-regulating device in the pump return line holding the concentrate line at a constant pressure above the maximum water pressure at every design flow.
4. A concentrate pump discharges through a metering orifice straight into the riser, with the concentrate pressure upstream of the orifice exceeding the riser water pressure by a set design margin.

Other listed methods are also permitted: balanced positive-pressure injection with a pump whose output tracks water flow, around-the-pump proportioners and in-line eductors (4.5.2.3). The annex says in-line eductors should be used only on open-head or deluge systems. The exposed orifice plate tab or nameplate must be permanently marked with the orifice diameter and, where relevant, the flow direction (4.5.2.4).

Concentrate pumps (4.6)

• Pump design and materials are approved for the concentrate type (4.6.1), and seals or packing are compatible with it (4.6.7).
• Rated capacity is at or above the maximum system demand (4.6.2.1), and discharge pressure at design flow exceeds the highest water pressure possible at the injection point (4.6.2.2).
• A pump able to overpressurise the system has pressure relief from discharge back to suction, and overpressure may not exceed the working pressure of the concentrate piping (4.6.3).
• There is a means of flushing and a drain cock or valve (4.6.4); the pump starts automatically when the system operates (4.6.5) and is listed for the service (4.6.6).

The annex notes that concentrate pumps are usually positive displacement, as centrifugal pumps may not suit high-viscosity concentrates (A.4.6). Concentrate piping may be neither carbon steel nor galvanised (4.7.1.2, 4.7.1.3), and friction loss for alcohol-resistant concentrates uses the manufacturer's data (4.7.1.7).

Acceptance tolerance on percentage

A proportioner's induction rate, expressed as a percentage of total solution flow, must lie within −0 % to +30 % of the listed percentage, or within +1 percentage point, whichever is less (12.6.5). For a 3 % concentrate that means 3.0 % to 3.9 %. Test details are in NFPA 11 Acceptance Tests and ITM.

For quick concentrate arithmetic use the Foam Concentrate Calculator; for the tank-side demand see NFPA 11 Supplementary Hose Streams and Monitors; for the change in concentrate families see AFFF to F3 Foam Transition.

Frequently Asked Questions

Does NFPA 11 require a reserve supply of foam concentrate?

Yes. A reserve enough for the worst-case design demand and to return the system to service is required (4.3.2.5.2.1). It can be in separate tanks, in drums or cans on site, or obtainable from an approved outside source within 24 hours (4.3.2.5.2.2).

Which hazard sets the quantity of concentrate?

The largest single hazard, or the group of hazards protected at the same time (4.3.2.2). The hazard with the largest flow does not always need the most concentrate, because discharge time also counts (A.4.3.2.2).

Can different brands of foam concentrate be mixed in one tank?

Different types never. Different brands of the same type only when manufacturer data prove compatibility and the AHJ accepts it (4.4.1.1, 4.4.1.2).

Which proportioning method does NFPA 11 prefer?

For fixed systems, balanced-pressure or positive-pressure injection (4.5.2.1). Balanced pressure is achieved with a pump and balancing valve, a bladder tank, an in-line balanced-pressure arrangement, or direct injection into the riser (4.5.2.2).

Where should the suction be in an atmospheric concentrate tank?

At least 25 mm (1 in.) above the tank bottom; concentrate below it is not usable (4.3.2.3.1.4, 4.3.2.3.1.5). The sediment pocket volume is added to the required quantity (4.3.2.3.2.3).

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