FIGHTING FLAMMABLE LIQUID FIRES: A PRIMER PART 1: THE FAMILY OF FOAMS
(Photo by Phill Queen.)
The goal of this three-part series on Class B firefighting foams is to provide information for firefighters to safely, effectively, and efficiently use foams in flammableand combustible-liquid situations.
The information in this article was gathered from a wide variety of sources, including product brochures, reference books, training curriculums, articles, interviews, and years of personal experience with these agents in actual and training fire scenarios.
Case study #1. An older largeframe aircraft has crashed, and several people are trapped in the wreckage. The left wing is ripped off during the impact, and a considerable amount of highly flammable aviation fuel is spilling around the aircraft fuselage. The fire department arrives on scene, and its only alternative is to enter the hazardous area and attempt to extricate the victims. Firefighters cover the spill with a foam blanket and set up protective hoselines. As the rescue team cuts into the flight deck area with a rescue saw, firefighters apply water to the forcible-entry area to cool and improve the cutting operation. Cutting the aluminum aircraft body usually will not create a spark. Suddenly, however, the saw blade strikes a steel accessory on the aircraft, sparks are generated, the fuel spill erupts in flames, and six firefighters are severely burned.
Lesson learned: A foam blanket will not last forever. It consists of millions of bubbles that eventually will pop, and the solution will drain away. Firefighters walking and dragging hoselines through the foam blanket, as well as the wind, sun, and other weather factors, hasten the breakdown of the foam blanket. The water being applied to the cutting operation and the continuous fuel leak from the ruptured right wing act to further break down the foam blanket and release more flammable vapors.
After covering a flammable-liquid spill with foam, you must maintain and regularly replenish the foam blanket. It is also imperative that you not misuse water at a flammable-liquid incident. Entry into even a foamed fuel spill should be done only if necessary. If entry is required, all personnel should be in full turnout gear with SCBA.
Case study #2. A large floating roof storage tank is being filled with gasoline. The terminal personnel underestimate the inventory of fuel, and the tank begins to overfill. A waterfall of gasoline pours down the side of the tank, and a huge vapor cloud forms and finds an ignition source. The resulting flashback ignites the tank and surrounding fuel spill, killing two employees. The fire department responds and attempts to attack the fire and protect exposed tanks with a combination of water and foam. In one area two hose teams are working together to extinguish spot fires in the flooded containment area. Attempting to put out a spot fire, one firefighter aims his nozzle at the feet of the other team. Instead of controlling the fire, the foam stream breaks up the foam blanket, and the fire dramatically intensifies. The two firefighters abandon their hoseline and begin to run for their lives, breaking up the foam blanket even more. Enveloped in flames, they trip and fall into the pool of water, fuel, and foam. No longer being disturbed, the foam blanket reseals, the fire again is secured, and the firefighters scramble out to safety.
The key word in foam application is gently. The lesson here is to avoid disturbing the fuel surface and the foam blanket. Again, regularly maintain and replenish the foam blanket and stay out of the foam and fuel spill unless absolutely necessary. If firefighters must enter this area, they must wear full protective gear, be experienced and/or trained in the task at hand, and do it safely and expeditiously. Consider playing foam onto the entry crew as they perform their tasks.
Case study #3. In another city’, a storage tank of Jet A, a kerosene fuel used in commercial jet aircraft, is on fire. In addition to the well-involved tank, there is also a significant spill fire in the containment area. The fire department uses aircraft rescue and firefighting (ARFF) foam vehicles to attack and attempt to control the fire. ARFF apparatus are designed for a quick, mass application: They usually dump their entire load of agent in less than two minutes. According to the National Fire Protection Association, a spill fire may require a 10-minute foam application and a tank fire up to 65 minutes of continual application. In this scenario, the fire almost is extinguished when the ARFF units run out of agent. During resupply operations, the fire burns up the existing foam blanket and regains its original intensity. Also, several spraying fuel fires that the foam cannot extinguish are in the involved area. Eventually, an experienced firefighting contractor is hired. Enough foam concentrate and appropriate delivery devices are set up to extinguish the spill and tank fires and keep them out. Dry chemical then is used to extinguish the spraying fuel fires. (Foam usually will extinguish only a flat, two-dimensional pool of fuel, whether it’s on the ground or in a container such as a tank. Foam usually w ill not extinguish a moving, spraying, three-dimensional fire. These types of fires require an agent such as halon or, most likely, c ry chemical that will stop the combustion chain reaction.
(Photo by author.)
Firefighters at large, flammable-liquid fires, such as in storage-tank facilities, must determine the minimum foam application rate, w’hich indicates the amounts of foam concentrate, water, foam solution, and application devices (nozzles) needed to extinguish the fire. If not enough foam is applied to the fire for a long enough time, the fire w ill not be extinguished. If more than the minimum application rate is applied, the fire will be controlled faster, up to a point.
Case study #4A fire department responds to a vehicle accident with trapped casualties. Gasoline has spilled around the wrecked vehicles, and the incident commander calls for a foam application to the hazard area. An in-line foam eductor is set up and firefighters attempt to foam the area, but no “white stuff” (foam) comes out of the nozzle. The officer in charge notices that the bale on the nozzle is gated down (partially closed). When the firefighter fully opens the nozzle, a nice, thick blanket of foam is applied.
In-line foam eductors are relatively easy to use, but there are many ways firefighters can cause them to fail to make foam. In-line eductors work on strict hydraulic requirements, and anything that changes the hydraulic flow and pressure through the eductor, such as a partially closed nozzle, a mismatched nozzle/eductor combination, kinked hose, too much hose between the eductor and nozzle, or inadequate engine pressure, prevents foam production.
The same scenario, but this time the engine company has a prepiped foam system on the apparatus. The hoseline preconnected to the foam system is dropped and charged. Unfortunately, the crew is unable to get any “white stuff” from the nozzle. There could be several causes of this problem. The department may have purchased several different types of foam concentrate and mixed them together in the concentrate tank on the apparatus: Never mix different types of foam concentrate in the same storage container; doing this can cause storage problems such as solidification, jelling, and crusting, which can plug up the foam system. The foam system may not have been properly flushed after its last use. In most cases, foam equipment rarely is used. When foam concentrate sits in system piping for long periods of time, it can jell or dry up, plugging and restricting the foam system. I also have seen foam systems improperly installed by the apparatus manufacturer so that foam comes out of the nozzle but not in the proper percentage.
MISUNDERSTOOD RESOURCE “
These real-life situations indicate just a few of the problems you can encounter when dealing with Class B fuels and firefighting foams. Flammable and combustible liquid incidents are probably the most common types of hazardous-materials incidents to which fire departments respond. Firefighters may encounter flammable liquids when dealing with incidents involving tank trucks, truck terminals, truck-loading racks, storage tanks, railroads, pipelines, refineries, chemical plants, aircraft, vessels, gas stations, residential garages, and automobiles. There is a tremendous potential for flammable-liquid incidents and a need for Class B foam in every community. Preparation and proper training are essential for firefighters handling incidents involving flammable liquids.
Plain water rarely will extinguish a flammable-liquid fire; dry chemicals, halons, carbon dioxide, and foam will. However, foam is the only agent that will extinguish a fuel fire and keep it out. Analyses of many incident case studies reveal that firefighters often misuse water, fail to use firefighting foams properly, or don’t use them at all. In some cases, the fire is not extinguished, but rather the fuel supply is burned away. Foam is one of the least understood agents available to the fire service.
Some information offered to the fire service may be less than complete or factual. There are a lot of test claims; accept those from certified, recognized testing agencies.
How confident are you in the foam equipment and foam concentrate your department uses? How comfortable are you with your knowledge of and experience with using foam? Could your department successfully handle the incident scenarios described at the beginning of this article”’ Flammable-liquid incidents are extremely dangerous. Flammable and explosive vapors are usually invisible; and after you have been around them for a few minutes, they will numb your sense of smell. When things go wrong with flammable liquids, they go wrong fast—there is rarely a second chance to mitigate the hazard differently. Many firefighters have been killed, seriously burned, and disfigured by flammable liquids.
Many types of firefighting foam are available, and no one extinguishing agent or foam is perfect for every fire situation. Each has positive and negative aspects. You must understand the chemistry and dynamics of the particular fire scenario and the recommended foam agents as well as the appropriate application techniques.
EVOLUTION OF FOAMS
There are two categories of firefighting foams: chemical and mechanleal. Descriptions follow.
Chemical foam. It was developed in the late 1800s to combat coal and oil fires. The foam bubble is the result of a chemical reaction involving sodium bicarbonate powder, aluminum sulfate powder, and water. The reaction forms carbon dioxide gas, which is trapped inside the bubble.
Many problems are associated with chemical foams: They are expensive, are difficult to produce and use, and create a very stiff foam blanket that does not reseal well when disrupted. Also, foam quality is somewhat dependent on the temperature of the solution. Although some chemical foam systems and equipment still may be around, most have been replaced by more efficient mechanical foams.
Mechanical foam. This foam is the choice of modern firefighters and makes up the majority of the family of firefighting foams. Mechanical foam equipment is easier to use, can be handled by fewer personnel, and can be placed in operation more quickly than chemical foam equipment. It is created by mechanically agitating or bringing together three components: water, concentrated foam liquid, and air in a turbulent state. The energy to make the foam comes from a pumping source, such as gas pressure or the apparatus pump.
All mechanical foam systems, whether using an in-line eductor or a huge fixed foam system for an aircraft hangar, consist of four parts: water supply, foam concentrate supply, proportioning device, and discharge (aeration) device.
Firefighting foam is mostly water; only a small amount of concentrate is needed to make a large amount of mechanical foam. Foam solution is made by proportioning or mixing foam concentrate with water in a specific ratio. Expansion, or aeration, is the ratio of air-to-foam solution that produces a “finished foam.” Air is required to create the blanketing characteristic of firefighting foam. (Proportioning and expansion will be discussed in greater detail later in this series.)
LOW-EXPANSION FOAMS
Low-expansion foams are the predominant agents used for flammableliquid firefighting. Typically, low-expansion foam solutions consist of three parts (three percent) concentrate mixed with 97 parts (97 percent) water or six parts concentrate mixed with 94 parts water. These foam solutions then are expanded with air up to a 20:1 ratio; however, most low-expansion foam proportions range from 4:1 to 12:1.
Protein foam. The first mechanical foam concentrate was protein foam, which was developed in the 1930s. It is made from a chemical digestion or acid/alkaline hydrolysis of natural, organic, protein solids such as animal horn, fish by-products, and blood. Vegetable materials also are used.
Protein foam produces a dense, thick, viscous, and very stable foam blanket. Some foam manufacturers claim that protein foam’s heat and burnback resistance (the foam’s ability to resist being consumed by the fire) is approximately twice that of aqueous film-forming foams and that it has better expansion qualities and a slower drain-out time than synthetic foams.
(Photo by author.)
Some of the disadvantages of protein foam are slower fire knockdown and extinguishment times than with synthetic foams; protein foam lacks fuel tolerance and is easily contaminated by the fuel; fuel reportedly saturates, coats, and burns on protein foam bubbles; sedimentation or solids in suspension in the concentrate settle out; it has limited shelf life; it is not compatible with some dry chemical agents; and it must be used through air-aspirating foam nozzles. Protein foams do not provide an aqueous film —if a hole or separation is created in a protein foam blanket, it will expose the flammable liquid and its vapors.
Fluoroprotein foam. This foam was developed in the 1960s by adding synthetic fluorocarbon surfactants to a protein foam concentrate. This addition gives fluoroprotein foam an oleophobic or fuel-shedding quality, which provides a resistance to fuel contamination. This feature allows it to be used for subsurface foam injection, a method of flammable liquid storage tank fire protection in which foam is injected at the bottom of the tank and then floats up through the fuel to the surface, where it extinguishes the fire. (We will talk more about this type of foam application in the final article.) The surfactants also make fluoroprotein foam dry-chemical compatible. Except for these characteristics, it has the same advantages and disadvantages as regular protein foam. Both protein and fluoroprotein foams provide a layer of bubbles that acts to smother a Class B fire and prevent vapor emission. Both foams can be purchased in threeor sixpercent concentrates.
Aqueous film-forming foam (AFFF). AFFF was developed by the U S. Navy and the 3M Company during the 1960s. It is a combination of fluorocarbon surfactants, synthetic foaming agents, and stabilizers. It is commonly available in one-, three, and six-percent foam concentrates.
AFFF revolutionized and added a new dimension to flammable-liquid firefighting—an aqueous film. The foam bubbles spread over the fuel surface in the same manner as the protein foams. In addition and more important, as the foam drains, part of the solution forms a layer on the surface. This film floats on and rapidly spreads across the flammable liquid, producing faster fire knockdown. (Note: As long as AFFF foam bubbles are visible on the fuel surface, an aqueous film will continue to drain out and suppress vapors. This aqueous film is invisible, so do not assume that it is there. Do not enter any flammable liquid hazard area unless there is a continuous, unbroken blanket of foam bubbles covering the entire spill. Consider continuously bouncing or playing the foam around and off any personnel working in the spill area)
(Photo by author.)
(Photo by author.)
The AFFF blanket should be constantly monitored and fresh foam reapplied as necessary. This reapplication may be necessary every few minutes, depending on the thickness of the blanket, the conditions at the scene, and the number or types of disruptive forces on the foam layer. The foam also has excellent rehealing and resealing capabilities.
AFFF extinguishes flammable-liquid fires faster than protein or fluoroprotein foams. (Refer to each foam manufacturer for the exact fire control and drain-out times of its foams.) Putting out the fire is usually the number one priority; keeping it out is less of a problem. Faster knockdown means lower minimum application rates. The minimum application rate for AFFF is 0.1 gallons per minute of foam solution per square foot of burning liquid surface, compared with 0.16 gpm/ft for protein or fluoroprotein foams. Due to AFFF’s greater effectiveness in controlling fuel fires, when airports were still using fluoroprotein foam on aircraft rescue and firefighting (ARFF) vehicles, the Federal Aviation Administration allowed one-third less AFFF foam concentrate than fluoroprotein foam to be carried. (Minimum application rates will be discussed more in another article.)
AFFF commonly is used with standard fog nozzles and usually forms a soupy, heavy, wet foam that penetrates a fire’s thermal updraft more effectively than protein or fluoroprotein foams. The fire thermal updraft is the movement of flame and heat upward into the smoke columm and can equal up to a 50-mph wind. Into this updraft we are throwing foam bubbles. Protein and fluoroprotein foams, which usually are expanded or aerated more and are lighter than AFFF, have a greater tendency to be carried off by this thermal updraft.
The low surface tension of AFFF solution provides a rapid penetration on Class A, ordinary combustibles. It greatly increases the effectiveness of water, much like a wetting agent. AFFF also mixes rapidly with water, can be premixed, and will stay in solution for long periods of time. If necessary, the proper amount of AFFF concentrate can be dumped into the water tank of an engine, pumper, or water tender to make a tank full of foam solution. This is called the “dump-and-go” method and either wastes foam concentrate or gives the fire a chance to regain its original intensity during resupply operations. This is a last-resort action —the first choice is always to use some type of foam proportioner. AFFF is a powerful detergent and a strong penetrant. When dumped into an apparatus water tank, it may cause pump seals to leak and may lift rust and scale that could run through the pump and clog small orifices. Undiluted AFFF concentrate also can strip and remove apparatus paint down to the bare metal. Even diluted foam solution can streak vehicle paint jobs. Because of this penetrating ability, AFFF as well as other foam concentrates usually are stored in specially designed and constructed stainless steel or plastic concentrate tanks or containers. Always thoroughly wash all apparatus and equipment used, and rinse foam systems and equipment for a minimum of five minutes. AFFF and some other foam concentrates are difficult to wash off. Continual contact with AFFF, as with most other foam concentrates, can dry irritate, and crack skin. Wash exposed areas with plain water and apply hand cream.
AFFF is compatible with dry chemical agents and often is used in conjunction with Purple K Dry Chemical in twin-agent systems. A twinor dualagent system, popular at many airports, is two nozzles “married” together. One nozzle discharges the dry chemical to knock down the fire, and the other nozzle follows up with an AFFF foam blanket to seal the fuel surface and prevent reflash.
AFFF solution requires relatively low energy to expand into foam, compared with protein or fluoroprotein foam mixtures, and can be used with air-aspirating foam nozzles as well as standard, nonair-aspirating fog nozzles. Because it is completely synthetic, it has a much longer shelf life than protein foams. A fire department wants its foam concentrate inventory to last as long as possible and does not want to have to worry about its condition. AFFF is the recommended foam agent for airport ARFF apparatus because of its good storage characteristics and effectiveness on aircraft fuel fires.
Film-forming fluoroproteins (FFFP) were developed in the 1980s to combine the qualities of fluoroprotein and aqueous film-forming foams—-to provide a foam with the reported burnback resistance and slower drain-out time of a fluoroprotein and the quicker extinguishing ability and aqueous film of an AFFF. They are popular outside the United States and are available in threeand six-percent concentrates. Like AFFF, this foam may be used with foam nozzles or fog nozzles. Check the manufacturer’s approvals and listings to be sure.
Multipurpose synthetic concentrates. These foams are effective for both hydrocarbon and polar solvent fires. They often are called “alcoholtype” foams, not because they contain alcohol, but because they can be used effectively on alcohol-type polar solvents, such as methyl ethyl ketone and acetone, for example, without being destroyed.
“Regular” low-expansion foams, such as AFFF, fluoroprotein, and the others listed above, while effective on (nonpolar) hydrocarbon fires, are ineffective on polar solvent fires: Polar solvents dissolve and destroy foam blankets of AFFF and other “regular” foams. Polar solvents have an affinity for water, which is also polar. Polar liquids actively mix with each other. Polar solvents “pull” the water right out of a “regular” foam blanket. Since foam blankets are 94 to 97 percent water, no foam is left after this occurs.
Multipurpose synthetic concentrates are “regular” foams, such as AFFF, with a special polymer additive that forms a physical barrier between the foam bubbles and the polar solvent liquid. This additive makes these concentrates look thicker and more jelly-like than other low-expansion foam concentrates. The polymer is water soluble but not soluble in polar solvents. When multipurpose synthetic foam solution is applied to a polar solvent, the polymer and polar solvent react almost instantaneously to form a thin, tough layer that protects the foam and its aqueous film from breakdown by the moisture-seeking polar solvents. If this protective layer becomes disrupted, as long as there is a bubble blanket a new polymeric layer w ill be produced by a regenerative action called “self healing.” Avoid unnecessary activity’ in the spill area, and immediately repair disturbed areas with fresh foam.
While most low-expansion, Class B foams are ineffective on polar solvent fires, multipurpose synthetic concentrates produce a tough, polymeric, selfhealing barrier over these water-seeking liquids.
Multipurpose synthetic foams usually are found in a three-percent/sixpercent (3/6) concentrate. They are proportioned at three percent for use on nonpolar hydrocarbons (gasoline, diesel, etc.) and act like regular foam. In this situation, the polymer does not form a polymeric layer; it just goes along for the ride in the foam. It is proportioned at six percent for polar solvents (alcohol, acetone, etc.). The foam solution needs to be richer when dealing with the tougher polar solvents, to form an adequate polymeric layer. (Some foam manufacturers market a multipurpose syntheticfoam concentrate that can be used at three percent on polar solvents.)
MEDIUMAND HIGH-EXPANSION FOAMS
Mediumand high-expansion foams are blends of highly specialized synthetic surfactants and foam stabilizers. Typical proportioning percentages vary from 1½ to three percent. The resulting foam solutions are aerated or expanded in special expansion airaspirating devices.
Although medium-expansion foams have been used with some success in flammable-liquid firefighting, their use is very limited. These foams have an expansion ratio of between 20:1 and 200:1 and therefore produce a lighter, drier, and less dense foam blanket than the low-expansion variety. All low-expansion foams can be expanded through foam devices such as air-aspirating nozzles to create medium-expansion foam. Remember, however, that medium-expansion foams are adversely affected by wind, and their vapor-sealing, burnback-resistance, and fuel-tolerance capabilities are less than those of low-expansion foams.
High-expansion foams with expansion ratios of between 200:1 and 1,000:1 primarily are intended for use on Class A, ordinary combustibles in confined areas such as basements, vessel cargo holds, coal mines, and other places with limited accessibility for firefighters. They are not recommended for flammable-liquid firefighting. High-ex foams are very dry and as such exhibit extremely limited vapor-sealing characteristics, burnback resistance, fuel tolerance, and wind resistance. High-expansion foam may make it to the seat of the fire but may be unable to control it. There have been reports of firefighters walking through high-expansion foam bubbles, all cool and protected, and then suddenly stepping out of the bubbles into the fire. It is difficult and dangerous to work in these foams.
HAZARDOUS-MATERIAL FOAMS
During the past several years, several foam manufacturers experimented with and marketed hazardous-material foams. Although they probably were able to control a flammableliquid fire, hazardous-material foams were not intended or designed for this use. They were intended primarily for vapor, dust, or odor suppression and control. As with all foams, their solutions contained predominantly water, but since so many chemicals are water reactive, most manufacturers have ceased offering this type of agent.
(Photo by author.)
(Photo by author.)
One Class B foam manufacturer still sells a liquid stabilizer that can be proportioned with multipurpose synthetic concentrate. It requires a special proportioning device with two pick-up tubes. One pick-up tube is for the stabilizer and the other is for the foam concentrate. The resulting solution can be applied with a standard fog nozzle but works best with a medium-expansion foam nozzle. When the foam sets up, which usually takes a few minutes, it forms a rubberlike blanket, several inches thick, that can last for several days. Eventually the moisture will dry out of the foam blanket. Foam-making and application devices must be flushed out immediately after use, or there is the risk that the foam will set up inside the equipment. Any contaminated foam must be hauled off to a hazardous-waste dump site with the dirt, chemical, and other contaminated debris. This foam can be used to put a longer lasting vapor barrier over spilled flammable liquids until clean-up can be completed. Other low-expansion, Class B foams require regular reapplications to maintain their blanket and vaporcontrol capabilities.
Another foam manufacturer made two types of haz-mat foam—one for acids and the other for alkaline materials—but these products no longer are manufactured or marketed.
CLASS A FOAMS
Class A foams, as their name implies, are intended for use on ordinary combustible or Class A materials such as wood, plastic, rubber, and vegetation. They initially were used in vegetation fires and were referred to as “wildland foams.” They also are being used more and more on structure fires, with very positive results.
Compressed-air Class A foam will stick to vertical surfaces, a quality that differentiates it from other foams. These adhering bubbles will hold the moisture on the burning materials longer, allowing the surface to absorb more water. Class A foams insulate, protect, and cool the materials they cover and prevent the spread of fire by radiation. These foams also lower the surface tension of water so it penetrates into the fuel deeper and faster. Firefighters get more extinguishment and exposure protection from their available water, and there usually is less water damage. Several manufacturers produce cost-effective Class A foams.
Class A foams probably could extinguish a flammable liquid fire in some limited applications, but they are not designed, intended, or marketed for that use.
The next article will discuss the nature and characteristics of flammable and combustible liquids, foam proportioning, application techniques, application rates, tactics and strategies, and safety considerations.
