Foam Systems

Fire retardant is a chemical mixture dropped from aircraft — fixed-wing air tankers and helicopters equipped with external tanks — to slow or stop the spread of wildland fires. The red slurry commonly seen on television during major wildfire coverage is long-term retardant, so named because it remains effective even after the water in the mixture evaporates. The primary active ingredient is ammonium polyphosphate or diammonium phosphate, which acts as a fertilizer-like coating on vegetation that prevents it from igniting. The red iron oxide dye (or fugitive dye in newer formulations that fades after exposure to sunlight) marks the drop area so pilots and ground crews can see where retardant has been applied. The U.S. Forest Service maintains a Qualified Products List (QPL) for wildland fire retardants and evaluates products through the Wildland Fire Chemical Systems program at the Missoula Technology and Development Center. Phos-Chek (manufactured by Perimeter Solutions) and FireTrol (manufactured by Perimeter Solutions, formerly ICL Performance Products) are the dominant long-term retardant brands. Retardant is mixed with water at air tanker bases operated by the Forest Service and state agencies and loaded into aircraft at designated air tanker bases and reload facilities. Environmental concerns about retardant include its toxicity to aquatic organisms — the Forest Service prohibits retardant drops within 300 feet of waterways (with exceptions for immediate threat to life). Short-term retardant (essentially foam or gelled water) provides only temporary fire suppression effect that diminishes as the water evaporates, making it less effective for creating firebreaks than long-term retardant.

Firefighting foam is a water additive that dramatically improves water's ability to extinguish certain types of fires, particularly flammable liquid (Class B) fires involving fuels like gasoline, jet fuel, and crude oil. Foam works by floating on the surface of a burning liquid, forming a blanket that smothers the fire by separating the fuel from oxygen and suppressing flammable vapor release. There are several foam types. Aqueous Film-Forming Foam (AFFF) has been the standard for military and airport firefighting since the 1960s — it spreads a thin aqueous film across the fuel surface for rapid knockdown. Alcohol-Resistant AFFF (AR-AFFF) adds a polymer membrane that prevents polar solvents like ethanol and methanol from breaking down the foam blanket. Class A foam is used on ordinary combustible (Class A) fires — it reduces water's surface tension, allowing it to penetrate into the fuel load more effectively, and is widely used in structural and wildland firefighting. However, AFFF contains per- and polyfluoroalkyl substances (PFAS, often called forever chemicals) that do not break down in the environment and have been linked to serious health concerns including cancer. This has driven a major industry shift toward fluorine-free foam (F3), which uses alternative surfactants to achieve film formation without PFAS chemistry. The U.S. Department of Defense is transitioning away from PFAS-containing AFFF under the National Defense Authorization Act (NDAA), with a deadline originally set for October 2024. NFPA 11 (Standard for Low-, Medium-, and High-Expansion Foam) governs foam system design, and foam must meet performance standards under MIL-SPEC (MIL-PRF-24385 for AFFF) or UL 162 for listing.

Fixed foam systems are permanently installed fire protection systems that proportion and deliver firefighting foam to protect specific high-hazard areas where flammable liquids are stored, processed, or handled. These systems are found in petroleum refineries, fuel tank farms, aircraft hangars, chemical plants, loading racks, and marine terminals. When a fire or spill is detected, the system automatically mixes foam concentrate with water at the correct proportioning rate and discharges it through fixed piping to foam chambers mounted on storage tanks, monitors, or overhead foam-water sprinkler heads. For large atmospheric storage tanks containing flammable liquids, NFPA 11 specifies the application rate and duration — for example, a cone-roof hydrocarbon tank requires a minimum foam solution application rate of 0.10 GPM per square foot of liquid surface area for a minimum of 55 minutes for a Type II discharge device. The foam proportioning method varies by system size and design: bladder tanks (a rubber bladder inside a pressure vessel forces concentrate into the water stream), balanced-pressure proportioners with foam concentrate pumps, in-line balanced-pressure proportioners, and around-the-pump proportioners are all used. Large tank farm systems may use tens of thousands of gallons of foam concentrate stored in dedicated tanks. The shift away from PFAS-containing AFFF has major implications for fixed foam systems, as converting to fluorine-free foam may require system modifications and retesting. FM Global, the major industrial property insurer, publishes Data Sheet 7-14 for the protection of flammable liquid storage and has specific requirements for foam system design and testing.

Fixed suppression systems using foam in industrial settings are permanently installed fire protection systems designed to protect enclosed spaces and specific equipment where flammable liquid fires are the primary hazard. Unlike tank farm foam systems that protect open-top storage, fixed foam suppression in industrial settings protects areas like turbine enclosures in power plants, machinery spaces on ships, aircraft hangars (where both floor-level spill fires and three-dimensional running fuel fires must be addressed), transformer containment areas, and underground parking garages where fuel spills may occur. Aircraft hangar foam suppression is governed by NFPA 409 (Standard on Aircraft Hangars) and uses either low-level foam discharge through nozzles at floor level or overhead foam-water deluge sprinkler systems — or both. The system design must deliver foam solution to the hangar floor fast enough to cover a spill fire before it can spread to the aircraft. For power plant turbine enclosures, foam is used alongside or as an alternative to CO2 or clean agent suppression. Marine applications follow International Maritime Organization (IMO) standards and use either high-expansion foam generators (expansion ratios of 200:1 to 1,000:1) that can fill an entire engine room with foam to smother a fire, or medium-expansion systems for deck protection. High-expansion foam is particularly effective in enclosed spaces because the enormous volume of foam blanket displaces the air/oxygen supply and insulates unburned surfaces from radiant heat. NFPA 11A (now consolidated into NFPA 11) covers medium- and high-expansion foam systems.

Foam delivery equipment includes all the hardware used to get foam solution from the proportioner to the fire — eductors, nozzles, monitors, and applicators. A foam eductor (also called an inline inductor) is the simplest and most commonly carried foam proportioning and delivery tool on fire apparatus. It works on the Venturi principle — water flowing through a constricted throat creates a low-pressure zone that draws foam concentrate from a bucket or pail through a pickup tube and mixes it into the water stream at a set ratio (typically 3% or 6%). Standard eductors are rated at specific flow rates (60 GPM, 95 GPM, or 125 GPM) and require a specific inlet pressure (typically 200 PSI) to function correctly — operating outside these parameters will result in incorrect proportioning or no foam pickup. Foam nozzles are different from standard fire nozzles because they aspirate air into the foam solution stream, producing finished foam with an expansion ratio suitable for the application. Low-expansion foam nozzles (expansion 2:1 to 20:1) produce a wet, flowing foam for Class B flammable liquid fires. Medium-expansion nozzles (expansion 20:1 to 200:1) produce a drier foam for enclosed space flooding. Master stream foam monitors — large-capacity remote-controlled or manually operated nozzles mounted on apparatus, trailers, or fixed positions — can deliver 500 to 2,000 GPM or more of foam solution for large-scale fuel spill or tank fires. Foam hand applicators and subsurface injection systems are used for gentle application onto burning fuel surfaces in storage tanks. Major manufacturers include Elkhart Brass, Task Force Tips (TFT), Akron Brass, and Williams Fire & Hazard Control.

Foam equipment is the general category encompassing all hardware used in the generation, proportioning, and application of firefighting foam. This includes portable equipment carried on fire apparatus, such as 5-gallon pails and 55-gallon drums of foam concentrate, eductors, foam nozzles, and compressed air foam system (CAFS) backpacks, as well as larger vehicle-mounted systems. Fire apparatus may carry foam in several ways: some engines have a built-in foam tank (typically 20 to 100 gallons) with an onboard proportioner connected to the pump discharge, while others carry foam concentrate in pails and use portable eductors. Class A foam equipment for structural firefighting is typically simpler — a small foam tank with a proportioning valve set at 0.1% to 1.0% feeding into the pump discharge. Class B foam equipment for flammable liquid fires requires more precise proportioning (typically 1%, 3%, or 6%) and foam-specific nozzles that aspirate air to create a proper foam blanket. Compressed Air Foam Systems (CAFS) inject compressed air into the foam solution stream, creating a lightweight, high-energy foam that clings to vertical surfaces and penetrates into the fuel load. CAFS is used for both structural and wildland firefighting. Testing equipment like foam refractometers (which measure concentrate percentage in the foam solution) and foam quality test kits (which measure expansion ratio and 25% drain time) are used to verify that the equipment is producing foam within specifications. NFPA 11 and NFPA 1901 specify requirements for foam equipment installed on fire apparatus.

Foam proportioners are the devices that mix foam concentrate with water at the exact ratio required for effective fire suppression. Getting the proportioning ratio right is critical — too little concentrate and the foam will not form properly or extinguish the fire; too much is wasteful and can actually reduce effectiveness. Common proportioning ratios are 1% (1 gallon of concentrate per 99 gallons of water), 3% (3 per 97), and 6% (6 per 94), depending on the foam concentrate type and the fuel being fought. There are several proportioning technologies. Venturi eductors are the simplest — water flowing through a restriction creates suction that draws in concentrate. Around-the-pump proportioners use the pressure differential between the pump discharge and suction sides to inject concentrate. In-line balanced-pressure proportioners use a separate concentrate pump (usually driven by water motor or electric motor) that maintains concentrate pressure equal to the water line pressure, metering concentrate through an orifice. Bladder tank systems use water pressure on one side of a rubber bladder inside a steel tank to squeeze concentrate out on the other side and into the water stream. Electronic proportioning systems use variable-speed pumps controlled by flow meters and microprocessors to adjust concentrate injection in real time — these are the most accurate and versatile systems but also the most expensive. Compressed Air Foam Systems (CAFS) represent a distinct approach — they inject both foam concentrate and compressed air into the water stream simultaneously, producing finished foam in the hose. All proportioning systems should be tested regularly with a refractometer or conductivity meter to verify the correct mix ratio.

Foam proportioning technology encompasses the engineering principles and system designs used to achieve accurate mixing of foam concentrate with water across varying flow rates and pressures. This is a deeper technical topic than the devices themselves — it addresses why certain proportioning methods are chosen for specific applications and how they maintain accuracy. The fundamental challenge is that most fire suppression scenarios involve constantly changing water flow rates and pressures (as hoselines are opened and closed, as pump pressures change), and the proportioner must maintain the correct concentrate ratio throughout these fluctuations. Balanced-pressure proportioning is the most common method for fixed systems — a foam concentrate pump (driven by a water motor, electric motor, or diesel engine) supplies concentrate at a pressure matched to the water pressure, and a ratio controller or metering orifice maintains the correct percentage. This method works across a wide flow range and is used in most refinery, tank farm, and airport foam systems. Pressure proportioning (bladder tank) systems are simpler and require no external power — water pressure acts on one side of a bladder, pushing concentrate out at the same pressure as the water supply. These are self-contained but limited in capacity by the bladder tank size. Premixed foam (foam concentrate pre-mixed with water in a tank) is the simplest method but is only practical for small, dedicated systems because the premixed solution has a limited shelf life. CAFS (Compressed Air Foam Systems) add a third element — compressed air from an onboard compressor — that produces a higher-quality finished foam with better adhesion, longer drain time, and less water usage. The CAFS concept was developed for wildland firefighting by the USDA Forest Service but has been adopted for structural applications as well. NFPA 11 provides design criteria for proportioning systems used in fixed installations.

Class A foam and fire-blocking gel are used in wildland firefighting to protect structures and slow fire spread through vegetation. Class A foam works by reducing water's surface tension (from about 72 dynes/cm to approximately 30 dynes/cm), allowing it to soak into fuels like wood, brush, and duff rather than running off. When applied through a CAFS or aspirating nozzle, it creates a visible white foam blanket on vegetation and structures that insulates the material from radiant heat and prevents ignition. Class A foam concentrates are typically proportioned at 0.1% to 1.0% — a very low ratio compared to Class B foam — meaning a small amount of concentrate treats a large volume of water. Common Class A foam concentrates include Phos-Chek WD881 (now Perimeter Solutions), Novacool, Silv-Ex, and various formulations from National Foam and Angus Fire. Fire-blocking gels (like Barricade and Thermo-Gel) are thicker polymer-based products that are applied to the exterior of structures (siding, roofing, decks, fencing) ahead of a wildfire, creating a heat-resistant coating that can protect the structure for hours as the fire front passes. Gel is typically applied using a proportioner and garden-hose-style nozzle from a small trailer-mounted unit. During major wildfire events, structure protection groups (a division of the incident management team) use engines equipped with Class A foam and gel to prepare threatened homes in the wildland-urban interface (WUI). NFPA 1145 (Guide for the Use of Class A Foams in Manual Structural Fire Fighting) provides guidance on Class A foam operations.

Wildland fire retardant encompasses all chemical agents specifically formulated for wildland fire suppression, including both long-term retardants and short-term suppressants. Long-term retardant (the red aerial drops described in the fire-retardant category) uses ammonium phosphate salts that undergo a chemical reaction when exposed to heat, releasing water vapor and leaving a fire-resistant char layer on vegetation. This chemical action persists even after the water carrier evaporates, which is why it is called long-term — it remains effective for hours or days until washed away by rain. Short-term retardants are water enhancers (Class A foam, gels, and thickening agents) that improve water's effectiveness but lose their suppression capability once the water evaporates. Ground-applied retardant can be delivered by engines, tenders, and specialized retardant delivery vehicles that mix and spray product along containment lines and around structures. The U.S. Forest Service manages the testing and qualification of all wildland fire chemicals through its Wildland Fire Chemical Systems (WFCS) program, maintaining the Qualified Products List (QPL) that agencies reference when procuring retardant. Environmental stewardship is a major consideration — retardant application must comply with the Forest Service's Record of Decision and Aerial Application of Fire Retardant Environmental Impact Statement, which establishes avoidance areas near waterways, threatened and endangered species habitat, and cultural sites. The cost of aerial retardant application is substantial — a single air tanker load (typically 2,000 to 4,000 gallons depending on aircraft type) costs several thousand dollars in chemical alone, not including aircraft operating costs. Ground retardant operations are more economical and are the primary method for structure protection in the wildland-urban interface.