Foam Fire Suppression Systems: A Guide for Flammable Liquid Hazards

A foam fire suppression system puts a fire out by floating a blanket of aerated foam solution over the surface of a burning liquid. The blanket separates the fuel from the air above it, holds the vapor down, cools the surface as water drains out and stays in place long enough for the fuel to be isolated or cooled below its flash point. No other agent does all of that on a large pool, which is why foam is the agent of record for tank farms, dike areas and spill hazards under NFPA 11. Foam also has a hard limit that a specification has to respect: it cannot sit on a pressurized jet, a running fuel fire or energized equipment, and on those hazards it needs dry chemical beside it.

What a foam system is made of

Every foam system, from a handline on a skid to a fixed installation on a 150 ft diameter tank, is the same chain of components: a water supply at adequate pressure and flow, which for a fixed system is the fire main or a dedicated pump; a concentrate tank sized for the discharge duration plus reserve; a proportioner that mixes concentrate into the water at a fixed percentage, normally 3 or 6 percent, and for alcohol resistant products often 3 percent on hydrocarbons and 6 percent on polar solvents; piping to the hazard; and a discharge device that aspirates the solution into finished foam and puts it where it will do some good.

The proportioner is where most of the engineering sits. An in-line eductor uses the venturi effect to draw concentrate into the water stream and is cheap and simple, but it is sensitive to back pressure and inlet pressure and is limited to handlines and small monitors. A balanced pressure proportioner uses a concentrate pump or a bladder tank to match concentrate pressure to water pressure, so the mix stays accurate across a range of flows, and is the usual choice for fixed systems. The discharge device sets the expansion ratio: a foam chamber on a tank shell or a low expansion nozzle gives 5:1 to 10:1 and a dense, wet blanket for pool fires; a medium expansion generator gives 20:1 to 200:1; a high expansion generator gives several hundred to one and fills enclosed volumes.

How a foam blanket stops a pool fire

Four mechanisms work together. Separation is the main one: the blanket is a physical barrier between fuel and oxygen. Vapor suppression follows from it, since a sealed surface stops the flammable vapor that would otherwise feed the flame or find an ignition source downwind. Cooling comes from the water content, which drains out of the bubbles over the blanket's drain time. With film-forming foams, a thin aqueous film spreads ahead of the blanket and seals the fuel surface before the bulk of the foam arrives, which is what made AFFF fast on low flash point fuels.

Drain time, the period before a quarter of the solution has drained out of the foam, is the usual measure of blanket stability. A blanket is damaged by hot steel, by fuel splashing through it, by a water stream applied on top, and by some dry chemical agents, which is why powder and concentrate compatibility has to be confirmed on any twin-agent unit. Polar solvents, including methanol, ethanol, acetone and MEK, dissolve a conventional blanket and require an alcohol resistant concentrate that lays down a polymer membrane between the fuel and the foam.

The numbers that size a system

NFPA 11 sizes a foam system on three quantities: the application rate, the discharge duration and the area to be covered. For a fixed roof tank with Type II discharge outlets delivering foam onto the fuel surface, the minimum rate is 0.10 gpm per square foot, and the duration is set by flash point: 30 minutes for a hydrocarbon flashing above 100 degrees Fahrenheit, 55 minutes for anything below 100 degrees and for crude. Spills worked with monitors or portable nozzles run 15 minutes, at 0.10 gpm per square foot with a film-forming foam and 0.16 with protein, fluoroprotein or a fluorine-free concentrate. Multiply rate by area by duration and add the supplementary handline allowance and you have the solution quantity; multiply by the proportioning percentage and you have the concentrate.

A 100 ft diameter cone roof tank of gasoline has a surface area of about 7,850 square feet. At 0.10 gpm per square foot the solution flow is 785 gpm. Gasoline flashes well below 100 degrees, so the duration is 55 minutes, which comes to roughly 43,000 gallons of solution and about 1,300 gallons of concentrate at 3 percent for the tank alone, before handlines, before the dike, and before the reserve that most owners and insurers require. Those figures are why tank farm foam systems are fixed installations and not something carried on a skid.

Hazard	Typical minimum rate	Typical duration	Usual delivery
Cone roof tank, hydrocarbon	0.10 gpm per sq ft	30 min above 100 F flash, 55 min below and for crude	Foam chambers on the shell
Floating roof seal area	0.30 gpm per sq ft	20 minutes	Foam dams and pourers
Diked area	0.10 gpm per sq ft from fixed low level outlets, 0.16 from monitors	30 min below 100 F flash, 20 min from 100 to 140 F	Fixed outlets or monitors
Nondiked spill, loading rack	0.10 gpm per sq ft with film-forming foam, 0.16 with protein, fluoroprotein or fluorine-free	15 minutes	Monitors, handlines, twin-agent skids

Those are the standard's floors for hydrocarbons. Fluorine-free products generally sit above them, and by more than most retrofit budgets assume, with the gap widest on gasoline and ethanol blends. Size a real system from the concentrate data sheet and its listing, not from a generic table.

Concentrate choice and the PFAS transition

AFFF, the aqueous film-forming foam that dominated flammable liquid protection for fifty years, is being withdrawn in the US because of its PFAS content. Fluorine-free foams, labeled FFF or SFFF depending on the supplier, have replaced it on new systems and are being retrofitted into existing ones. They extinguish by the same blanket mechanism but without the film, which means they depend more on aspiration, flow less easily across the fuel, and generally need a higher rate or a longer duration to match the old performance. A retrofit is not a drop-in, and the concentrate tank on an existing system is often the part that turns out to be too small.

Class A foams, used on wildland and structural fires, are wetting agents at low concentration and have no place on a flammable liquid hazard. Whatever the concentrate, it has a shelf life, is tested annually under NFPA 11, and must be stored above freezing and below about 120 degrees Fahrenheit.

Where foam needs dry chemical beside it

Foam protects a surface. A flammable liquid that is spraying from a failed seal, a gas that is burning at a flange, or a fuel running down a pipe rack has no surface for the blanket to sit on, and foam applied to those fires is wasted. Foam is also conductive and stays off energized switchgear, transformers and motors. Dry chemical covers exactly those gaps. Potassium bicarbonate interrupts the flame chain reaction in seconds, works on three-dimensional and pressurized fires and is safe on live equipment, and its own weaknesses, no cooling and no vapor seal, are the two things foam does best.

That is why loading racks, jetties, pump rows and transfer areas are usually protected by the pair rather than by foam alone, either as a foam monitor system with a dry chemical skid placed for the jet, or as one twin-agent skid carrying both. The Salus Global skid range carries Purple K in 350, 1,000 and 2,000 lb capacities for the dry chemical side, and the same nitrogen-expelled design can be paired with a foam premix tank on one frame.

Limits an honest specification should state

Foam needs water, usually a great deal of it, and the water supply is the first thing that fails at a remote site. Foam is slow compared with dry chemical on an established flame, and it does nothing against a jet. Wind and heat break a blanket up, and a tank fire that has been burning for hours can boil the foam off as fast as it arrives. LNG is not a foam hazard in the usual sense: water-based foam on an LNG pool increases the vaporization rate, high expansion foam is used for vapor control and radiant heat reduction, and dry chemical is the extinguishing agent. Concentrate expires, freezes and, in the case of legacy AFFF, creates a disposal liability for any discharge, including a test discharge. None of this argues against foam on a pool hazard; it argues for a specification that says what the foam will do and what covers the rest.

Frequently asked questions

How long does a foam system need to discharge?

NFPA 11 sets minimum durations by hazard: 30 minutes on a cone roof tank holding a liquid that flashes above 100 degrees Fahrenheit, 55 minutes below that and for crude, 20 minutes for a floating roof seal fire, 20 to 30 minutes for a diked area and 15 for a spill. A manual skid is not held to those figures but should be sized to hold the blanket until isolation is complete.

Can fluorine-free foam replace AFFF in an existing system?

Usually, but not as a simple refill. The proportioner has to be checked or recalibrated for the new concentrate's viscosity, the discharge devices may need to aspirate more to form a stable blanket, and the application rate and duration in the design basis have to be reworked against the new listing.

Does foam work on an electrical fire?

No. Foam solution is conductive and must not be applied to energized equipment. Dry chemical is the agent for Class C hazards, and a site with pumps, motors or switchgear inside a foam-protected area should have a dry chemical unit placed to cover them.

Salus Global dry chemical skids are UL listed and assembled, filled and inspected at our facility in Texas. Call +1 360 901 9828 or email info@salusglobal.co.