FIGHTING FLAMMABLE LIQUID FIRES: A PRIMER, PART 3
To understand how Class B foams are produced, you must be familiar with certain “foam terminology.” The process starts when foam concentrate is properly mixed or proportioned with water to make “foam solution.” Foam concentrate is the foam liquid that comes from the foam manufacturer in five-gallon buckets or 55-gallon drums. Foam solution is the proper mix of foam concentrate and water. You must understand what foam solution is because minimum foam application rates are all based on applying a certain amount of foam solution on each square foot of burning liquid surface for a certain amount of time. One of the most common mistakes firefighters make at large flammable liquid fires is using an insufficient application rate.
PROPORTIONING
Mixing the right amount of concentrate with the right amount of water is called “proportioning.” Proportioning devices range from simple inlineeductors or premixed systems to more complicated balanced pressure or around-the-pump systems. (Different types of foam-proportioning systems and equipment will be discussed in greater detail in another article.) The two most commonly used proportioning concentrations/percentages are three percent and six percent. A three percent foam solution is created by mixing three parts or gallons of a three percent concentrate with 97 parts or gallons of water to make 100 parts or gallons of foam solution. A six percent foam solution is created by mixing six parts or gallons of a six percent concentrate with 94 parts or gallons of water to make 100 parts or gallons of foam solution. (Note: Fresh, brackish, or salt water can be used to proportion foam solution. Some foams even perform better when proportioned with salt water.)
What is the difference between a three percent and a six percent AFl-‘F, film-forming fluoroprotein (FFFP), or fluoroprotein concentrate? Is one better, more efficient, or more cost effective? Even foam concentrates are mostly water. By comparison, a three percent AFFF concentrate looks darker than a six percent AFFF concentrate. It is more concentrated. The first AFFF developed was six percent. The foam manufacturers later made a more concentrated three percent AFFF by removing half the water from the six percent AFFF concentrate. This water is added back during the proportioning process, when 97 percent or parts of water are added instead of 94 percent or parts, as with a six percent concentrate.
The advantage of three percent foam concentrate is that only half as much of it as six percent concentrate is needed to make the same amount of finished foam. One gallon of three percent foam concentrate will make twice as much finished foam as one gallon of six percent concentrate. Less foam concentrate will have to be carried on fire department apparatus, and what is carried will go much farther.
Most foam-proportioning systems do not work perfectly. A six percent foam concentrate leaves more room for error. A six percent foam that is l ½ percent off or proportioned at 4½ percent may be slightly better than a three percent foam that is 1 ½ percent off and proportioned at 1 ⅛ percent. Properly proportioned, finished three percent and six percent AFFF foams are identical in capability. They look, smell, and perform exactly the same. The only difference is that it took twice the amount of six percent concentrate to make the same amount of three percent foam solution There are greater cost savings and storage advantages with three percent foams, compared with more room for proportioning error with six percent. Most tire departments are going to three percent foams. Because of the cost factor, many tire departments are waiting until they use up their six percent foam concentrate inventory before switching to three percent. Some foam manufacturers sell a one percent AFFF, which leaves no room for proportioning error and probably should be used only for premixed systems.
FOAM EXPANSION OR AERATION
After the foam concentrate and water are properly proportioned, air is turbulently introduced into the foam solution. This happens at the nozzle or discharge device and is called “expansion” or “aeration.” This process produces a watery suspension of air in liquid solution—foam bubbles. It is now “finished foam” or just “foam.” There is a “foam triangle”: The three necessary ingredients—foam concentrate, water, and air—have to be brought together in the proper quantities to make finished foam.
Two types of nozzles or discharge devices are used to add air to the foam solution and make finished foam for flammable liquid firefighting: the airaspirating foam nozzle and the nonairaspirating, standard fog nozzle. Each has its advantages, limitations, and recommended uses. A nonair-aspirating, standard fog nozzle will expand or aerate the foam solution approximately four times (a 4:1 expansion ratio). The typical air-aspirating foam nozzle usually will expand the foam solution 10 times (a 10:1 expansion ratio). Most Class B foams can be expanded up to a 25:1 ratio using medium-expansion air-aspirating foam nozzles. Such a relatively high expansion ratio will allow for verygentle application of a thick foam blanket and may be used in situations, such as nonfire vapor control, that permit firefighters to get close to the spill and in which thermal updraft is not an issue.
Air aspirating foam nozzles. An air-aspirating foam nozzle can be used only to make foam. It is a long, narrow, tube-like nozzle. It aspirates or induces air into the foam solution as it passes through the nozzle. Air is draw n into the barrel of the nozzle by a venturi action through holes near its base or inlet. The air is added to the foam solution in the nozzle. Usually screens or other obstructions in the nozzle tube further turbulently mix and aerate the foam solution beyond the level accomplished by the venturi effect. Because of the obstructions and turbulence inside the nozzle, some velocity and stream range arclost. A foam nozzle will not project a stream of foam as far as a standard fog nozzle. Stream range and distance arcbeing sacrificed for better aeration, more bubbles, and a thicker and longer lasting foam blanket.
Remember that the bubbles hold the foam solution. The thicker the foam layer, the more bubbles will be present to replace those that are popping and releasing their water content at the point where the blanket touches the Hamm able liquid spill. The more bubbles, the longer the foam blanket will last. A thicker, more aerated, and expanded foam blanket makes for a longer-lasting foam.
(Photos by author.)
Because a foam nozzle expands the foam solution approximately 10 times, more air is in the foam, and it is much lighter than foam applied through standard fog nozzles. Although effective on fuel fires, it has a greater tendency to be carried aloft by a fire’s thermal updraft, which sometimes can equate to a 50 to 60 mile-per-hour wind.
Some foam nozzles have variable discharge patterns; others do not. Foam nozzles also will not protect personnel as well as would a wide angle stream from a fog nozzle.
Standard nonair aspirating fog nozzles. A standard fog nozzle works differently than a foam nozzle. Air is not induced into or mixed with the foam solution inside the nozzle; rather, air is added to the foam solution after it leaves the nozzle. The firefighter operating the nozzle has some control over how well the foam is aerated by adjusting the nozzle setting. A fog nozzle expands best—and makes the best foam —on a straightstream setting. The stream is so narrow that it actually crosses itself while sailing through the air. The particles of foam solution turbulently bounce off each other and mix with air as they travel to the fire in a straight stream.
Unless absolutely necessary, it is recommended that firefighters do not use wider than a 30-degree fog when aerating foam with a standard fog nozzle. Keep the stream in a level or slight arc so that the foam gently falls onto the flammable liquid surface. This is called the “rainfall” or “lob method.” Gently is the key word in foam application. Never aim or plunge the foam stream directly into the fuel spill. This will stir up more fuel vapors and greatly reduce the effectiveness of the foam.
Foam also picks up more air as it impacts objects in the fire area, such as an aircraft fuselage or storage tank shell. This is called the “deflection method.” This impact technique expands the foam better and provides a very gentle application as the foam flows off the object onto the fuel spill. The foam stream also can be bounced off a hard surface, such as pavement, in front of the fire. The firefighter can build up a wall of foam that can be pushed or flows gently across the burning liquid surface. This is called the “bounce method.”
Fog nozzles probably are most often used for foam application. They provide more flexibility during firefighting, with greater reach and stream distance. They also allow for a protective wide fog setting for high radiant heat situations. Foam from a fog nozzle is soupier, wetter, and heavier because it is not expanded or aerated as much as that from a foam nozzle. Because of this, the foam will penetrate and better resist the effects of the fire’s thermal updraft, as well as allow aqueous film of AFFF to drain out faster. This usually means a morerapid fire knockdown and extinguishment.
Both the air-aspirating and standard fog nozzles can be used for fire attack. Both should be available for use byresponding firefighters. The fog nozzle often is better for fire extinguishment because of greater reach, heavier foam, and wide fog protection. A foam nozzle usually allows for gentler application; a thicker, longer lasting foam blanket; and better spill ignition prevention. Some nozzle manufacturers offer attachments that will convert their fog nozzles into air-aspirating foam nozzles. Synthetic foams such as AFFF can be used with either fog or foam nozzles because they require very little energy to expand their solution. Older type protein and fluoroprotein foams can be used only through foam nozzles because they require the greater expansion energy supplied by these nozzles.
MINIMUM FOAM APPLICATION RATE
Besides misuse of water, the most common mistake firefighters make at large flammable liquid incidents is failing to put enough foam on the fire for a long enough time. Successfully using foam depends on supplying the minimum application rate or more. Do not go “bear hunting with a BB gun.” In most cases, make sure you have enough foam concentrate, water supply, and foam-delivery devices before attempting fire extinguishment.
“Minimum foam application rate” is the minimum amount of foam solution and application time necessary to extinguish a flammable liquid fire. It can be quickly determined using simple mathematical formulas. If less than this minimum rate is used, the fire most likely will not be extinguished. The fire will keep burning up the foam as it is applied and will be controlled only after all the flammable liquid is consumed by the flames. If more than this minimum is applied, the fire usually will be extinguished. If the minimum application rate is increased, the fire will be extinguished faster (up to a point). This is called “mass application” and is a principle used in aircraft rescue and firefighting.
Foam application rates are directly related to the surface area of burning flammable liquid. The more area on fire, the greater the application rate and the more foam needed. There are different application rates for different flammable liquid fire situations, such as spill, storage tank, and aircraft firefighting situations. A certain gpm flow of foam solution is needed on each square foot of burning liquid surface for a certain period of time. Application rates will tell the firefighter how much foam concentrate and water supply will be needed, as well as the type and number of nozzles to use.
Application rates are intended for and are more accurate on large fires. The smaller the fire, the less heat energy and less thermal updraft and therefore the less foam needed. Application rates were determined by the National Fire Protection Association and addressed in NFPA 11, Standard for Low Expansion Foam and Combined Agent Systems. They are based on studies of actual fire incidents as well as simulated test burns.
Since application rate is dependent on the surface area of the liquid on fire, minimizing the surface area involved by closing valves, diking, and other containment operations is advantageous. Quantity or depth of liquid is not important when determining foam application rates. Even if the quantity is great and the spill surface small, only a small amount of foam is needed.
Minimum application rates apply only to the quantity of foam solution-expansion, aeration, and bubbles have nothing to do with it. All fire departments should preplan target hazards in their jurisdictions and determine how much foam would be required to mitigate a flammable liquid/vapor release/fire from these installations. (Account for the foam needed for extinguishment, reflash protection, and protective/backup foam hoselines.) It is also essential to preestablish where you can get more foam if necessary, such as from airports, military bases, and oil refineries.
Spill fire application rates. This is when firefighters actually can walk up to (circumstances permitting and all safety considerations accounted for) and apply foam directly on a flammable liquid spill fire. Even if a gasoline tank truck or several vehicles also are burning in the involved area, it is still a spill fire and requires the same minimum foam application rate. NFPA 11 states that for spill fires, firefighters must be prepared to apply a minimum of 0.1 gpm of synthetic foam solution per square foot of burning liquid surface for an uninterrupted period of 15 minutes and 0.16 gpm of protein foam solution per square foot of burning liquid surface for an uninterrupted period of 15 minutes. 1 find it easier to understand and compute by changing 0.1 gpm/sq.ft. to 1.0 gpm/ 10 sq.ft., or 0.16 gpm/sq.ft. to 1.6 gpm/ 10 sq.ft.
For example, a 50by 100-foot pool of gasoline is on fire, and you want to compute the minimum application rate of AFFF from solution. First determine the total surface area of liquid on fire (50 feet X 100 feet = 5,000 square feet on fire). Next determine how many 10-square-foot units are in the total area on fire (5,000 divided by 10 = 500 10-square-foot units on fire). We must apply a minimum of one gpm of AFFF foam solution on every 10-square-foot unit (500 X 1 gpm = 500 gpm). The minimum fire flow needed to extinguish this fire is 500 gpm of AFFF foam solution. Remember that foam solution is only foam concentrate properly proportioned with water. No expansion/aeration— no air—is involved yet.
This 500-gpm fire flow tells us we will need five 1 1/2-inch foam hoselines (5 X 100 gpm = 500 gpm) or three 1 3/4-inch foam lines (3 x 200 gpm = 600 gpm) or two 2 1/2-inch lines just to put the fire out. If we use less than this, the fire may not be extinguished. We may get lucky and extinguish it with less foam and fewer lines. But if the minimum application rate or more is applied, firefighters usually will be sure to extinguish the fire.
As per NFPA 11, the foam application must continue uninterrupted for a minimum of 15 minutes: 500 gpm x 15 minutes = 7,500 gallons of foam solution that will be needed. How much of this is foam concentrate and how much is water? If three percent concentrate is being used, multiply “,500 gallons of foam solution by 0.03; 225 gallons of three percent foam concentrate will be needed). (7,500 – 225 = 7,275 gallons of water will he needed).
Remember that these are only minimum amounts. As the foam application rate is increased, the fire will he extinguished faster, up to a point where it levels out. Also, do not forget that additional foam and foam lines will be needed for backup and protective safety hoselines and to maintain the foam blanket to prevent a reignition. Make sure your proportioning device is set for the proper percentage of the foam concentrate being used. If a three percent concentrate is used at six percent, it will be too rich, and expensive foam agent will be wasted. If six percent is used at three percent, it will be too lean and may not put the fire out or keep it out.
If the above fire scenario were tackled with protein foam solution, 800 gpm (500 x 1.6) of foam solution would be required from appropriate application devices. Twelve thousand gallons of protein foam solution would be needed for a 15-minute application (800 gpm x 15 minutes 12,000 gallons).
If this scenario involved a polar solvent (alcohol, acetone, MEK, etc.), multipurpose (alcohol -resistant) foam would be used at up to double the minimum foam application rate. Use up to 0.2 gallons of foam solution per square foot of burning liquid surface area, or two gallons per 10 square feet. Some multipurpose synthetic foam concentrates will have to be proportioned at six percent on polar solvents. Refer to the manufacturer’s specifications and directions for the multipurpose foam your department uses for recommended application rates.
Storage tank fires. Again, liquid surface area on fire is what is important Hie depth or amount of liquid in the tank means nothing when figuring the minimum foam application rate. First determine the number of square feet on fire by multiplying the diameter of the tank squared by 0.8. For example, an 80-foot-diameter tank would have 5,120 square feet of liquid surface area (80 feet x 80 feet X 0.8 = 5,120 square feet).
Certain storage tanks are equipped with fixed foam protection systems, such as subsurface injection systems or foam chamber systems that require fire departments to make a connection and pump foam into the system. NFPA 11 recommends a 0.1 gpm/sq. ft. application rate for such cases. In the above scenario, 512 gpm of foam fire flow will be needed (0.1 X 5,120 = 512 gpm). If a flammable liquid, such as gasoline, is on fire, foam application will have to be continued uninterrupted for 55 minutes for a total fire flow of 28,160 gallons of foam solution (512 gpm x 55 minutes = 28,160 gallons). A total of 845 gallons of three percent foam concentrate (28,160 X 0.03 = 845) or 1,690 gallons of a six percent foam concentrate (28,160 x 0.06 = 1,690) will be needed at minimum. If the tank contains a combustible liquid, such as diesel or kerosene, the application time would be only 30 minutes.
When there is no fixed foam fire protection system on the tank, the system is unusable, or one cannot be installed during the fire (foam towers, portable foam chambers, or hot tapping a subsurface injection system into a product line), foam usually will be applied with portable devices such as monitors and deluge sets. This foam application method requires, as per NFPA 11, a minimum of 0.16 gpm of foam solution per square foot of burning liquid surface. I often round this off to two gpm/10 sq. ft. The 80-footdiameter tank, with 5,120 square feet of liquid surface area, would require an 819-gpm foam fire flow in this situation (5,120 X 0.16 = 819.2 gallons per minute). This means that one 1,000-gpm or two 500-gpm monitors should extinguish this tank fire. If a flammable liquid, such as gasoline, is burning, the uninterrupted application time is 65 minutes. A minimum of 53,248 gallons of foam solution will be needed to extinguish this fire (819.2 gpm X 65 minutes = 53,248 gallons). This means that 1597 gallons of three percent foam concentrate (53,248 gallons X 0.03 = 1,597) or 3,195 gallons of six percent foam concentrate (53,248 gallons x 0.06 = 3,195) will be needed. If the tank contains a combustible liquid, the application time is only 30 minutes.
OTHER CLASS B FOAM FACTS
Do not mix different types of foam concentrates, such as an AFFF with a fluoroprotein concentrate, in the same storage tank or container. The mixed concentrates sometimes can solidify, crust up, or otherwise clog proportioning equipment. Foam systems on fire apparatus have failed to work at incidents because of this. Do not even mix different percentages of the same type of foam, such as mixing three percent and six percent AFFF in the same foam tank. Finished foam could be too lean, thin, or rich. Foam concentrate may be wasted, and the foam system may not perform as designed. Foam concentrate tanks and systems should be thoroughly flushed out, usually under the direction of the apparatus or foam manufacturer, before a different type of foam liquid is reloaded. An exception to this is during a major fire where a different type of foam concentrate is loaded into a foam tank immediately after the previous type is used up. This is allowable, but all the different foam concentrates should be used at the fire or drained out and the system flushed after the fire.
Although it can be done with certain AFFFs, I do not recommend mixing different brands of the same type of foam concentrate. Once two different manufacturers’ foam concentrates have been mixed, which manufacturer do you turn to if there is a problem with the concentrate? Most manufacturers will analyze a sample of a department’s concentrate to determine its condition. This is accomplished by sending a sample in a clean, pint-size paint can to the maker of the foam. It should be done periodically or when foam concentrate supplies are suspected of being defective. Once you have mixed different brands of foam concentrate, to which manufacturer would you send the sample for evaluation?
Different foam concentrates can he proportioned from different proportioned and applied in succession or at the same time on the same fire. This is usually the case at large mutual-aid fires, where many different types of foam concentrates will show up. All can be applied to the same fire at the same time with few adverse effects.
Never direct or plunge a straight stream of foam into the center of a flammable liquid spill. This can cause fuel saturation of the foam and will stir up the flammable liquid, releasing even more vapor. This action can reduce the effectiveness of the foam by more than 60 percent and can splash or spread the burning liquid farther. It also may disrupt an existing, functioning foam blanket.
If possible, approach and begin applying foam from upwind and uphill. Foam operations conducted with the wind at your back provide improved visibility and breathing, as well as decreased stress from radiant heat and smoke. Better application of foam and reduced extinguishing time usually will result, because the wind or inrushing air will help carry the foam into the fire area. It is much easier to throw foam bubbles with the wind than against it
Start applying foam at the upwind edge of the fire and move in as the fire recedes. This is where the fire is breathing. If multiple nozzles or application devices are being used, position all of them so their discharge is applied initially in approximately the same area of the fire. 1’his provides a greater fire knockdown punch at the proper application rate. Except in aircraft situations, where the entire fuselage must be protected, do not spread application devices around the involved fire area.
Exercise caution when using water streams in conjunction with foam streams or blankets. Use water streams in such a way as to not physically disrupt the foam blanket Improper or careless w’ater application may drive or wash away the foam blanket. Use minimum amounts of cooling water to protect exposures. Firefighters tend to use too much water at flammable liquid incidents and often make the situation worse. If the exposure has direct flame impingement, its paint is blistering or turning black, or water applied to it turns to steam, it needs constant cooling. If the exposure stays or glistens wet after water application, it needs only an occasional cooling spray.
All personnel involved in the foam operation or anywhere near the hazard area should be in full protective gear and SCBA. litis includes rubber boots, turnout pants, flash hoods, gloves, helmets, and coats. Bunkers should have proper liners in place and be completely buttoned up. Turnouts or bunkers are a protective system; use and wear them properly. Reflective proximity or approach gear only makes the firefighter more comfortable w hen closer to the fire and is not a necessity.
Foamed areas can be slippery. Firelighters should walk in shuffling steps to maintain balance and stability. Never turn your back on a flammable liquid spill or fire. Back out in the same manner as you approached the fire until in a safe area. Safety’ backup hoselines should be charged with foam, should be properly positioned, and should be staffed with attentive and prepared personnel.
Maintain the foam blanket with regular reapplications. I prefer a foam blanket that covers the toes of my boots when I step in it (three to four inches thick). Foamed areas should be examined constantly and additional foam applied where the foam blanket has been interrupted or broken down. Consider a constant foam application if personnel are working in the foamed area. I se the same techniques as during initial foam application. The fresh foam will reestablish the vapor sealing blanket.
Obviously, never enter an unfoamed fuel spill. Do not even enter a foamed spill unless absolutely necessary. Have a good reason for being there. Clear the immediate operational area of all but the necessary personnel involved in the tasks at hand. This means especially news media and nonfire personnel.
The fire must be totally extinguished before foam application is stopped. Any remaining fire can regain its Original intensity and burn up foam already applied. Once application is started, continue until the fire is extinguished. Again, this often requires determining the minimum foam application rate and making sure that sufficient foam concentrate and water supplies are on scene and ready prior to starting the fire attack. Exceptions to this are rescue and critical exposure situations, where firefighters may have to “go for it” with what foam is immediately available at the scene.
What are the environmental and cleanup considerations associated with foams? Anything not normally found in the environment should not be discharged or left in it. Ideally, residual foam should be cleaned up with the spilled flammable liquid and contaminated earth. Refer to the manufacturer of your foam concentrate for more information.
Firefighters must consider and understand many items when using Class B foams at a flammable or combustible liquid fire. The type of material on fire, its hazardous characteristics, and the number of square feet of liquid surface area involved are very important. The minimum application rate must be determined, and sufficient quantities of foam concentrate, water, and discharge devices must be on scene and in place prior to fire attack. Firefighters should be properly protected. The proper and safe application technique, as well as equipment, must be used to gently apply the foam blanket. The resulting foam blanket also should be monitored and reapplied as necessary
The next article in this series will carer the various types of Class B foam proportioning and application equipment available.
