Foam Proportioners, Monitors and Nozzles: The Hardware Behind a Foam System

Component chain of a foam system from water supply through proportioner, concentrate tank and piping to the monitor and nozzle

A foam system is a water system with two additions: a proportioner that meters concentrate into the water at a fixed percentage, and discharge hardware that turns the resulting solution into foam and puts it on the fuel. Get the proportioner wrong and the foam is either too lean to hold a blanket or so rich the concentrate runs out early. Get the nozzle wrong and the foam arrives as a wet spray that sinks into the fuel instead of floating on it. The hardware between the water supply and the fire is where most foam system failures originate, and it is also where the switch to fluorine-free concentrates bites hardest, because viscous concentrates and non-film-forming foam both demand different equipment from what an AFFF system was built with.

What the proportioner has to achieve

Concentrate is supplied for use at a stated percentage, typically 1, 3 or 6 percent by volume, and the listing of the foam is only valid in a narrow band around it. NFPA 11 requires the proportioning system to deliver between the rated percentage and 30 percent above it, or one percentage point above it, whichever is less, across the full range of flows the system will see: 3 to 3.9 percent for a 3 percent concentrate, 6 to 7 percent for a 6 percent product. Under-proportioning produces thin foam with poor burnback resistance; over-proportioning exhausts the concentrate supply before the design discharge time is met. The difficulty is that water flow varies with the number of devices open and with supply pressure, and concentrate viscosity varies with temperature and with product. Every proportioner design is a different compromise between accuracy across that range, cost, the need for external power and the complexity of maintenance.

Proportioner types and where each fits

The inline eductor is the simplest. A venturi in the water line creates a low pressure zone that draws concentrate from an atmospheric tank through a pick-up tube. It has no moving parts and costs little, but it loses around 35 percent of inlet pressure across the venturi, works accurately at only one flow rate, and the back pressure from hose length and elevation has to be held within the device's limits. Hand line eductors and small skid systems use them. Viscous alcohol-resistant concentrates can starve an eductor, which is one of the first items to check in a fluorine-free conversion.

The bladder tank proportioner is the standard for fixed systems without power. Concentrate sits inside a rubber bladder in a pressure vessel; a ratio controller in the water line diverts a portion of the water pressure onto the outside of the bladder, squeezing concentrate into the controller's metering orifice at the same pressure as the water. Because the pressures are matched, proportioning stays accurate across a wide flow range without pumps or electricity. The costs are a pressure vessel that has to be built to ASME Section VIII, a bladder that cannot be refilled while the system is running, and the need to drain the water side to service it. Bladder tanks from 50 to several thousand gallons protect loading racks, tank farms and hangars.

Balanced pressure proportioning uses a concentrate pump, driven by an electric motor or a water motor, with a diaphragm valve that balances concentrate pressure against water pressure at the ratio controller. It handles the widest flow range, can proportion multiple risers from one tank, and the atmospheric concentrate tank can be topped up during a discharge. The pump is the price: it needs reliable power or a water-driven motor, a spill-back loop, and maintenance on a par with any other fire pump. Refineries and large terminals with deluge foam on many hazards use it. Around-the-pump proportioning, where concentrate is injected into the suction side of the fire pump, is common on vehicles and simple skids but is limited to a single pump and cannot feed a plain water line from the same pump at the same time.

Piping, concentrate storage and the things that go wrong between them

Concentrate piping and tanks have to be compatible with the product. Most concentrates are mildly corrosive to carbon steel and many manufacturers call for stainless, brass, bronze or lined steel in contact with neat concentrate, with no galvanized components at all. Foam solution piping downstream of the proportioner can be carbon steel, since it is wet only during discharge, but it must drain fully so that residual solution does not corrode from the inside. Storage temperature matters: most concentrates are listed between roughly 35 and 120 degrees F, and a tank that has frozen or sat in the sun for a decade will have separated or degraded. NFPA 11 calls for annual concentrate sampling and laboratory testing against the original specification, and the sample results are the only real evidence that the stored product will still make listed foam. A fifteen year old drum of AFFF in a hot pump house is the most common hidden defect on an industrial foam system.

Monitors: putting solution on a distant target

A monitor is a mounted, directable waterway that lets one person deliver 500 to several thousand gpm at a target they cannot safely approach with a hose. Fixed monitors around a tank farm or loading rack are the usual way of meeting NFPA 11 application rates, and the rate depends on the delivery method as much as on the fuel. A diked area protected by fixed low-level foam outlets is designed at 0.10 gpm per square foot; the same dike protected by monitors is designed at 0.16, because a stream thrown across an open area loses foam to wind, thermal updraft and overshoot. Both run for at least 30 minutes on a Class I hydrocarbon. At the monitor rate a 1,000 gpm device covers roughly 6,000 square feet, so a large dike needs several. The other number a specifier has to know is a prohibition rather than a rate: NFPA 11 does not accept monitors as the primary protection for a cone roof tank over 60 feet in diameter, because the stream cannot be relied on to land inside the shell and lie down gently enough to build a blanket. Above that diameter the tank gets foam chambers or subsurface injection, and monitors become supplementary.

Manual monitors are oscillated by hand; electric or hydraulic remote control moves the operator out of the radiant heat, which at a tank fire can be the difference between using the monitor and abandoning it. Portable ground monitors on a trailer or skid serve a similar purpose for sites that do not justify fixed pipework. Throw distance falls quickly with foam aspiration, with wind and with elevation, and the only way to know what a monitor reaches on a given site is the manufacturer's range chart at the actual supply pressure.

Nozzles and the aspiration question

A foam nozzle does the final job of mixing air into solution. Air-aspirating nozzles draw air in through openings behind the orifice and produce foam with an expansion of 6:1 to 10:1 and a 25 percent drainage time of several minutes, the properties that give a stable blanket. Non-aspirating nozzles are ordinary fog or straight-stream water nozzles; with AFFF they produce a low quality 2:1 to 4:1 foam that still works because the film-forming surfactant does most of the job. With fluorine-free foam there is no film, so a non-aspirating nozzle delivers solution that sinks into the fuel, and the listing data for most FFF products specify aspirating discharge devices at higher application rates. Foam chambers on tank rims, foam makers feeding subsurface injection, and high back pressure foam makers are all aspirating devices that belong to fixed tank protection. Hand line nozzles, which protect the operator and secure a small spill, come in both types, and a crew switching concentrate needs to check which they have.

Assembling the chain on a skid or trailer

On a mobile or skid-mounted unit the whole chain above is compressed onto one frame: a water tank or suction connection, a pump with a driver, an around-the-pump or balanced pressure proportioner, a concentrate tank, a hose reel and a nozzle, sometimes a small monitor. The sizing logic is the same as for a fixed system, just applied to a smaller area and a shorter duration. What changes is the failure mode: a fixed system fails on a corroded orifice or degraded concentrate, a skid fails on the pump driver or on freezing. Dry chemical skids sidestep most of this because they carry no water, no pump and no proportioner, just an agent tank and nitrogen cylinders, which is why they pair so well with a foam unit rather than competing with it. On a twin agent unit the two sit on one frame and share one nitrogen bank, so an operator knocks the flame down with powder and then seals the fuel surface with foam without moving to a second piece of equipment.

Frequently asked questions

Can an existing bladder tank be used with fluorine-free concentrate?

Often the tank and bladder are fine, but the ratio controller usually needs a different metering orifice or a complete replacement because the new concentrate is more viscous. The bladder material must also be compatible with the new product, and the old concentrate must be completely removed and the system flushed, as residual AFFF will contaminate the new charge and the foam produced by a mixture is unlisted.

How is proportioning percentage checked in the field?

The usual methods are refractometer or conductivity measurement of the foam solution compared against a calibration curve prepared from the stored concentrate, per NFPA 11. A flow test at the design flow is run, solution is sampled from a test connection, and the reading is converted to a percentage. Results outside the listed band are a system fault, not a tolerance.

What is the difference between a monitor and a monitor nozzle?

The monitor is the swivel base and waterway that aims the stream; the monitor nozzle is the discharge device fitted to it, which sets the flow rate, pattern and whether the stream is aspirated. They are specified separately and the nozzle must be matched to the monitor's flow and pressure rating.