Some, such as gasoline, will burn readily as they have a rather low flash point. Others, such as diesel fuel, will burn with difficulty as they have a higher flash point. Flammable liquids account for 85 percent of all hazardous materials shipments. You may not have a large chemical footprint in your jurisdiction, but you will be dealing with flammable liquids in your career as an emergency responder. You cannot have a conversation about flammable liquids without talking about foam.
What exactly is foam and how does it work? Finished foam is a combination of foam concentrate and water, also known as foam solution, that is mixed with air through agitation — usually accomplished with a foam nozzle. Finished foam is what we see in the form of a white blanket of what looks like soap bubbles that rests on the top of a flammable liquid spill or is used to extinguish a flammable liquid fire. The finished foam blanket works by excluding air from flammable vapors, eliminating vapor release from the fuel surface, separating the flames from the fuel surface and cooling the fuel surface and the surrounding surfaces.
Foam is generally classified by its expansion ratio as either low, medium or high expansion foam. Low expansion foam has an expansion ratio of 20:1 and is designed for flammable liquids. Low expansion foam is what most of the fire service has experience with. Medium expansion foam has an expansion ratio that ranges from 20:1 to 200:1 and is specifically designed for hazardous chemicals. Medium expansion foams are generally used within the chemical industry. High expansion foams have expansion ratios greater than 200:1 and are designed for areas such as confined spaces or large open expanses such as aircraft hangers. High expansion foam requires specialized equipment such as a high expansion foam generator. These generators are either designed into a structure as part of its fire protection system or carried on specialized firefighting apparatus.
Foam is further classified by the type of material the concentrate is made from. This is where the phrase “use the right tool for the right job” is very important. Some foam concentrates work better on certain materials than others. Some foam concentrates will not work at all on certain materials — such as trying to use a Class A concentrate on a Class B fire.
Protein foam concentrate is one of the oldest foam concentrates and was the first mechanical foam. It is designed to be used on hydrocarbon fuels only. Protein foam provides advantages such as a stable foam blanket with excellent heat resistance, burn back and drainage — more on these terms later. Disadvantages include a slow fire knockdown and an inability to be used with fog nozzles. Protein foam concentrate has a distinctive odor as it is made of hoof and horn meal, chicken feathers, etc.
Fluoroprotein (FP) foam concentrate is a refinement of protein foam concentrate and it has the same uses and characteristics as protein foam concentrate, including only being able to be used on hydrocarbon fuels and the inability to be used with fog nozzles. Fluoroprotein foam concentrates contain fluorochemical surfactants that provide for a faster fire knockdown, less fuel wicking and a dry chemical compatibility, making it an excellent choice for a dual agent suppression strategy.
Film Forming Fluoroprotein (FFFP) foam concentrate is also limited to hydrocarbon fuels only. It combines fluorochemical surfactants with protein foam in order to release an aqueous film on the fuel surface. FFFP concentrate offers an increased knockdown over regular fluoroprotein foam.
Aqueous Film Forming Foam (AFFF) concentrate is the foam most firefighters are familiar with as it offers the greatest versatility for flammable liquid firefighting. While it can only be used on hydrocarbons, it offers a fast knockdown, quickly flows around debris — making it the ideal choice for accidents — and is dry powder compatible. AFFF can be used with regular fog nozzles eliminating the need for a special foam nozzle. The foam blanket may not be of as high a quality, but the end result will be the same. AFFF concentrate contains fluorochemical surfactants and synthetic foaming agents.
Alcohol Resistant Aqueous Film Forming Foam (AR-AFFF) is the current “state of the art” in foam concentrates. For years, alcohols required their own foam concentrates separate from hydrocarbons. Research and development has produced a “one foam fits all” concentrate that can be used on all flammable liquids, regardless of their chemical composition. The only difference is in the flow rates. Hydrocarbon fuels require a three percent concentration — three gallons of foam and 97 gallons of water. Polar solvents (alcohols) require a six percent concentration — six gallons of foam and 94 gallons of water. Hydrocarbon flow rates can be accomplished with regular fog nozzles. Polar solvent flow rates require specialized foam nozzles. AR-AFFF concentrates are dry chemical compatible and contain synthetic detergents, fluorochemicals and a polysaccharide polymer.
When selecting the proper foam concentrate, a number of characteristics must be examined.
— Knockdown Speed and Flow is the time required for a foam blanket to spread across a fuel surface and around obstacles.
— Heat Resistance is the foam’s ability to resist the destructive effects of radiant heat from the burning fuel or metal objects.
— Fuel Resistance is the foam’s ability to minimize fuel pick-up by the foam blanket so that the blanket doesn’t become saturated and ignite.
— Vapor Suppression is the foam’s ability to suppress flammable vapors to prevent re-ignition.
— Alcohol Resistance is the ability to resist destruction by alcohol to the blanket’s composition of more than 90 percent water and alcohol’s affinity for water. Alcohol resistance is increasingly important as more and more ethanol is blended with gasoline.
Some basic guidelines must be followed when it comes to the use and storage of foam. For foam concentrate storage it is imperative to follow the manufacturer’s recommended guidelines. Doing so will result in a shelf life measured in years. When mixing foam concentrate with water, fresh water at a temperature of 35°F – 80° F is preferred. Salt water or brackish water — such as pond water used in a drafting operation — or warmer water can be used, but the quality of the finished foam may not be as high. Contaminants such as detergents, oily residues, corrosion inhibitors, etc. may affect foam blanket quality.
Clean air yields best foam blanket, but the effect of contaminated air on low expansion foam blankets is negligible. Nozzle pressure should be 50-200 PSI with 100 PSI at the nozzle being optimal if using a regular fog nozzle. Follow the manufacturer’s recommendation when utilizing a specialized foam nozzle. Foam quality deteriorates at higher pressures. Range decreases at lower pressures.
On un-ignited flammable liquid spills, ignition can be prevented by prompt application of foam. Additional applications may be required to maintain foam blanket quality depending on the foam concentrate being used and the flammable liquid in question. Foam use on electrical fires is NOT RECOMMENDED as the foam blanket is mostly water. Electrical systems should be de-energized prior to foam application.
Foam is also NOT RECOMMENDED for materials stored as a liquid but are vapors at ambient conditions. Foam is also not recommended for materials that react with water. Foam is not effective on three-dimensional fires. It is recommended to first control the spill fire and then extinguish the flowing fire using a dry chemical agent. Three dimensional fires are routinely found in loading racks and aircraft fire situations.
Foam is not effective on pressurized gases such as acetylene, LPG and propane as their vapor pressure is too high for foam to be effective. Foam is also not effective on combustible metals as combustible metals usually react with water and the finished foam blanket consists of mostly water. Fires involving combustible metals require specialized techniques and extinguishing agents.
Foam equipment is a specialized area of fire suppression that needs to be addressed when equipping a piece of apparatus for flammable liquids firefighting. Proportioners introduce the proper percentage of foam concentrate into the hose stream. Eductors are extremely reliable and simple. They have a GPM flow rating and must be matched with a similar nozzle for optimal performance. An adequate inlet pressure (180-200 PSI) is required and back pressure must be kept to a minimum. It is imperative that eductors be kept clean.
We have touched on the use of specialized foam nozzles versus using normal fog nozzles. Both can be used and both have their advantages and disadvantages. Foam nozzles produce aspirated foam while fog nozzles produce non-aspirated foam. Foam nozzles produce finished foam that is of the highest quality, is the most stable and has the longest drainage time. Less frequent application required and less foam concentrate is needed. Non-aspirated foam can be applied through any fog nozzle for greater penetrating power and reach and faster knockdown. Nozzle flow must be practically a straight stream (<10° fog pattern) for maximum effect.
Dedicated foam nozzles have a shorter reach than their structural firefighting cousins and are more subject to wind conditions. Foam nozzles have a fixed nozzle pattern and cannot provide firefighter protection. They are bulky and can only be used for foam. Fog nozzles provide a foam blanket not as thick with less vapor suppression capability. This results in more frequent application and reduced cooling capability.
There are three techniques commonly used for foam application.
The Bounce-Off Technique applies foam as gently as possible by banking the foam stream off of a wall or other obstruction whenever possible.
The Raindown Technique utilizes a foam nozzle that is directed upward allowing finished foam to “rain down” on the surface in question. This technique allows for a fast and effective knockdown, but it may not be effective in high winds or with extensive pre-burns.
The Roll-In Technique has the firefighter hit ground in front of spill with foam, allow foam to pile up in front of spill and then use the foam stream velocity to roll the foam onto the spill surface.
Regardless of whichever foam application technique that you decide to use, you must remember one thing — NEVER PLUNGE THE FOAM! Use most gentle application possible. Plunging the foam stream can splash fuel causing fire to spread and can penetrate the existing foam blanket allowing vapor to escape. Never use water streams in conjunction with foam streams. Water streams can dilute your finished foam blanket rendering the entire operation pointless.
Foam operations, particularly larger ones, are rarely seen in the fire service. Think about the last time you actually flowed foam on an incident. Has it been a while? Have you ever flowed foam at all in your career? Foam proficiency is something that must be maintained, just like all of the other tools in our tool box. If this article has sparked your interest in an under-utilized skill, great! Get out there and learn more about the foam concentrate carried by your department and particularly the foam system used on your apparatus.
Until next time, stay safe!
Mark Schmitt is a Captain/HazMat Specialist for the Greensboro Fire Department assigned to the Foam/ARFF Task Force and a veteran of 25 years in the fire service. The majority of his career has been spent in Special Operations. He holds a Master of Public Administration in Emergency Management and is a graduate of the National Fire Academy’s Executive Fire Officer Program. He has taught numerous hazardous materials courses for the Greensboro Fire Department, local community colleges and the North Carolina Office of the State Fire Marshal in addition to serving as a contract instructor with the National Fire Academy.