Selection and Use of Foams to Meet Challenges of Flammable Liquid Fires
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Fighting fires involving flammable liquids demands special considerations not required when combating fires in class A fuels.
The basic technique used to extinguish a class A fire is to cool the fuel below its ignition temperature, but this alone will not be totally adequate for fires involving significant quantities of flammable liquids. Flammable liquids give off vapors—even at normal temperatures—that may ignite. The successful control of flammable liquid spills and fires requires a suppression tactic that will prevent the ignition or reignition of these vapors.
The difference between flammable liquids and combustible liquids is the flash point ranges of the fuels. The flash point of a fuel is roughly the lowest temperature at which the liquid gives off vapors that can be ignited. “Flammable” fuels are those that have a flash point below 100°F. Flammable liquids, such as gasoline, alcohol, ethers, etc., are constantly producing vapors that will burn.
“Combustible” liquid fuels, like diesel fuel, kerosene and fuel oil, have a flash point at 100°F or above. They must be preheated or vaporized in order to burn. Using water fog to extinguish a combustible liquid fire is effective in most cases because fog cools the fuel below its flash point and the vapor production is not sufficient to allow reignition.
Reignition possible
While water fog may extinguish a flammable liquid fire, the fuel will float on the surface of the water and flammable vapor production will continue. If an ignition source is present, then an explosive reignition may result that could injure or kill personnel present. The successful control of a flammable liquid fire or spill requires the sealing of the fuel with a foam blanket to prevent vapor production.
In the fire service, we generally encounter flammable liquids hazards of two basic types. Hydrocarbon fuels, such as gasoline, are the most common flammable liquids. These fuels do not mix with water and generally the fuel floats on top of the water.
Polar solvents, such as alcohols and ethers, are the other major type of flammable liquid. Polar solvents mix with water readily. Because of this mixing, polar solvents attack and destroy conventional types of foam blankets (protein, fluoroprotein and AFFF). Special foams must be used on polar solvent flammable liquids.
Today, the fire service has a compounded problem in dealing with hydrocarbon fuels mixed with various polar solvents. These liquid fuels, unleaded gasoline and gasohol, make the flammable liquid hazard more complex than past experience has exhibited.
Transportation major risk
Flammable liquid fire hazards are in all areas of communities and range from production to use risks. The major risk for most fire fighters is in transportation incidents. Ruptured fuel tanks of autos involved in accidents are a minor problem compared to that of semi-trailer tank trucks that may carry as much as 8000 gallons of flammable liquids and be involved in spills and fires. Railroad tank cars carry 33,000 gallons of these fuels. Regardless of whether these flammable liquids are hydrocarbons, polar solvents or a mixture of both, bulk transportation vehicles will display DOT flammable placards.
Another major risk for fire personnel is spills and fires at service stations and other loading or unloading facilities. Failure of automatic shutoff nozzles or fill hoses in vehicle fueling operations or the rupture of off-loading hoses during the filling of underground tanks by bulk transport vehicles can cause spills and fires involving hundreds or perhaps thousands of gallons of flammable liquids.
Because of the increasing emphasis on the conservation of hydrocarbon type fuels, the petroleum industry had introduced gasohol, a blend of 90 percent unleaded gasoline and 10 percent ethyl alcohol (grain alcohol), a polar solvent. Efforts to limit air pollution caused the development of the unleaded gasolines that contain less than 10 percent of certain aromatic polar solvents (tertiary butyl alcohol, tertiary amyl methyl alcohol, methyl tertiary butyl ether and others) that are required to increase octane ratings. These unleaded fuels come in various blends, depending on the manufacturer.
As efforts continue to conserve gasoline and reduce pollutants, the fire service can expect to see increasing use of blends of hydrocarbon fuels and polar solvents. In some countries, gasolines contain 20 percent alcohol and may soon be blended to 25 percent alcohol.
Blended in tank truck
Unleaded gasohols are currently blended at the time of distribution by mixing 800 gallons of ethyl alcohol with 7200 gallons of unleaded gasoline in the tank truck. This alcohol must be transported to tank farms in large quantities and the fire service should expect an increase in the bulk transportation of polar solvents by rail and highway. Of the nine flammable liquids listed in the 1978 edition of the United States Department of Transportation’s “Emergency Action Guide for Selected Hazardous Materials” (a collection of information concerning frequently transported hazardous materials), five are listed as polar solvents that cannot be controlled by conventional foams in the event of a fire or spill.
Most fire companies carry low expansion foams designed for use on hydrocarbon fuel fires and spills. Indeed, all engine companies should have foam capability. Foam capability includes not only the necessary foam and application equipment, but also a thorough understanding and practice in its use.
Fire fighting foam is a mass of airfilled bubbles made from water solutions and special foaming agents. Foams create an air-excluding, cooling, vaporsealing blanket that halts combustion and prevents the release and reignition of fuel vapors. To be effective, foam blankets must be highly stable, resist heat and resist burnback—the ability of flame to shrink the foam blanket. However all foam blankets will break down to some degree under the effect of fire and heat.
There are three basic types of low expansion foams commonly in use by fire companies. Protein foam concentrate is a combination of protein polymers created from the chemical digestion and hydrolysis of natural protein solids, with organic solvents added to improve the foam blanket. This foam blanket is stable on hydrocarbon fuels and resists heat and burnback well.
Fluoroprotein foam is similar to protein foam, but it also contains fluorinated surface active agents that make this foam blanket shed fuel if it becomes coated during application. This foam is effective on deep flammable liquid fuel fires and it possesses superior vapor-sealing and burnback resistance. It also is compatible with dry chemical agents. This foam has a low viscosity so that it will penetrate into deep-seated fires in class A fuels.
The third type of foam concentrate commonly in use is aqueous film-forming foam (AFFF), a synthetic concentrate that is similar to the other foams in this class. AFFF also produces a water solution film over the surface of hydrocarbon flammable liquids that seals the surface and prevents the release of flammable vapors even if the foam blanket is disturbed. This foam also has deep penetration characteristics for class A fuels. AFFF is more readily affected by heat and may have some trouble sealing the fuel surface in the vicinity of hot metal.
Foam for polar solvents
Alcohol-type foam concentrates have been developed for use in fires and spills involving polar solvents, such as alcohols and ethers. These foams are not generally found in the fire service due to technical factors involving the generation and application of the foam blanket. Makers of fire fighting foam concentrate have developed foams known as polar solvent liquid (PSL) foams. PSL is a protein-based foam with special additives which produce a barrier in the foam bubbles to prevent the breakdown of the foam blanket in the presence of alcohols, ethers and other polar solvents. This type of foam blanket is stable and long lasting and is also suitable for hydrocarbon fuel fires.
Many fire departments have portable high expansion foam generating equipment and some departments have special hazards protected by either a total flooding or local application high expansion foam system. High expansion foam is a blend of surface active and synthetic detergent foaming agents mixed with water in a 1 to 2 percent solution and expanding to a ratio of 100 to 1 (100X) up through 1000 to 1 (1000X).
Effective on spill fires
High expansion foam is recommended for fires in class A fuels in confined areas, such as basements, windowless buildings and underground shafts. It has good penetration and insulation qualities, but the foam blanket created is light and may be disrupted by the effects of weather. Most flammable liquid fires and spills are outdoors and a high expansion foam blanket is easily disrupted by wind or heavy rain.
High expansion foam blankets are effective on spill fires of hydrocarbon liquids and some tests have shown them to be effective on gasohol fires as well. Polar solvents readily attack and destroy high expansion foam blankets.
A unique feature of high expansion foam is its ability to rapidly move—in an intact mass and large volume—into an inaccessible or confined space. This may be of great value in a flammable liquid spill or fire that has entered a stream bed, culvert, basement of a structure or similar situation. A high expansion foam blanket may be used to cover the fuel and extinguish the fire out of sight and reach of fire fighters.
High expansion foam is developed by a generator that is usually portable but may be mounted on a trailer or vehicle. The foam solution is created by an eductor that can be a component of the foam generator or it may be remote from the generator location. The water and foam solution is sprayed onto a net or screen in the generator and a fan blows air through this net or screen to create the soapsuds-like bubbles of finished high expansion foam. The fan may be either water powered or electrically driven.
Contamination avoided
Except for hand-held units, finished foam is delivered to the fire area through a 25 to 50-foot tube approximately the same diameter as the discharge opening of the generator. This chute allows the generator to be located where it can draw fresh air to create the finished foam. The use of air contaminated by products of combustion should be avoided.
The output of high expansion foam generators is measured in cubic feet per minute (cfm), not in gallons per minute as with other foams. Hand-held generators produce about 1250 cfm of foam at a ratio of up to 250:1. Larger portable units produce from 3000 up to 7000 cfm of high expansion foam. There are wide differences in foam production capability of the various models of high expansion foam generators and a thorough knowledge of the specifics of any high expansion foam generator is vital to its proper use.
When high expansion foam is used in a confined space, ventilation must be provided opposite the point of foam introduction. Otherwise, pressure may build in front of the foam blanket and prevent the extension of foam into the fire area.
Foam affected by heat
High expansion foam blankets are affected by high heat levels, as are other foams, and a rate of application must be developed to overcome this. Because of the high ratio of water to foam concentrate, high expansion foams break down and drain more rapidly than other foams and, when used on flammable liquid fires and spills, may allow flammable vapors to be released.
In general, high expansion foam has advantages that make it a desirable extinguishing agent for fires in confined areas, but most fire departments use another type of foam for flammable liquid fires and spills in open areas.
Protein foam concentrate is the least expensive type of foam available and AFFF is the most expensive of the three common fire fighting foams. The fluoroprotein foams are generally moderately priced and can use existing foam eductors for their production. PSL foam concentrates exceed AFFF price scales and some are priced at rates less than AFFF. The PSL foams of some manufacturers require special delivery equipment (nozzles and eductors) but other PSL foams can use existing equipment. PSL foam concentrates are being rapidly developed and improved to meet expanding fire service needs and their use should be thoroughly investigated.
High expansion foams come in both 3 and 6 percent concentrates. Both are equally effective, but the 6 percent concentrate is generally less expensive per gallon. However, the use of 3 percent concentrate will sustain the foaming operation for twice as long (or make twice as much finished foam) as the same amount of 6 percent concentrate.
Generation of foam
To produce a foam blanket, the liquid concentrate must be mixed in the proper proportion with water and air and it must be delivered through the proper nozzle.
The proportioner is a device that passes water under pressure through a venturi to pick up the foam concentrate. The proportioner may be a portable unit that uses a flexible hose to pick up the concentrate from 5-gallon cans or it may be built into the fire apparatus pump discharge piping and fed from a fixed tank of foam concentrate. When portable cans are used, care should be taken to monitor the supply so that the concentrate containers can be changed promptly. If large quantities of foam are being used, the concentrate can be poured into a mixing tub at the proportioner site so that there will be no need to move the pickup tube from can to can. A clean salvage tub or a large bucket can be used for this purpose.
The movement of this foam concentrate and water mixture should be kept to short runs of hose. As a rule of thumb, a hose line from a proportioner to a nozzle should not be longer than 150 feet.
Proper nozzle needed
The nozzle that is used to apply the foam solution is critical to the proper application of the foam blanket. Fire fighting foam is a combination of foam concentrate, water and air. The nozzle is the device that introduces the air into the foam solution. For best results, an air-aspirating foam nozzle should be used. Protein, fluoroprotein and polar solvent liquid foams must have an airaspirating nozzle to be generated effectively. Aqueous film-forming foam can be used with a fog nozzle, but the foam blanket is of poor quality and the chief fire control element then becomes the aqueous film that seals the fuel surface. If this film is attacked by the alcohol or ethers in unleaded gasoline or gasohol, fire control will be difficult.
Careful matching of the nozzle and proportioner is a must for foam operations. The proportioner and the nozzle must have the same flow rate. If a 95-gpm proportioner is teamed with a 125-gpm nozzle, then the foam solution will be weak and watery. If the 95-gpm proportioner is teamed with a 60-gpm nozzle, then the water flow will not be adequate to draw the foam concentrate into the water stream.
Pump pressures for foam blanket production are also important. The pressure at the proportioner must not exceed 200 psi. There is approximately a 35 percent pressure loss at the proportioner. With a 200-psi intake pressure, the proportioner will be discharging foam solution at about 130 psi. Nozzle pressures should be approximately 100 psi. At this pressure, a 95-gpm foam nozzle has an effective reach of 50 to 70 feet. Larger hand line nozzles have reaches of up to 100 feet.
Foam breakdown problem
Foam must be applied at a rate that will compensate for the foam blanket breakdown due to heat, mechanical damage, water runoff, etc. The foam blanket must maintain an unbroken layer over the extinguished portion of the flammable liquid. An adequate rate-of-application is vital to the control of flammable liquid fires. Without a sufficient application rate, the fire and heat will destroy the foam blanket faster than it is being applied and the fire will not be controlled.
An adequate rate-of-application is considered to be 1 gpm of foam solution for every 10 square feet of burning fuel surface. The application of foam on non-burning spills of flammable liquids can be at a lesser rate due to the absence of heat with less breakdown of the foam blanket. Using this guideline, a 95-gpm foam line of AFFF, fluoroprotein foam or a polar solvent liquid foam, equipped with an air-aspirating foam nozzle operating at 100 psi nozzle pressure, will control a fire of 950 square feet. A 250-gpm foam line will control a spill fire of 2500 square feet. Protein foam blankets require a higher rate-of-application.
To control a flammable liquid fire with foam, a supply of concentrate must be available to meet this foam rate demand. A 95-gpm foam line will use about 6 gallons of 6 percent foam concentrate per minute. This is more than one can per minute! A 250-gpm foam line will use about 15 gallons (three cans) of 6 percent concentrate each minute of operation. To sustain foam application for 10 minutes, a 95-gpm line must have 60 gallons, or 12 cans, of foam concentrate available. A 250-gpm foam line will require 150 gallons, or 30 cans, of foam concentrate.
Maintenance of seal
The foam blanket will begin breaking down as soon as it is applied, so careful attention must be paid to proper maintenance of the foam seal. After fire control, some foam should be periodically applied to replace that which has broken down until all the fire hazard is eliminated. Special care should be taken to maintain the foam blanket if fire fighters must move through it, as in the case of rescue operations after vehicle accidents.
The method of application is also important to successful foam use. The foam must be applied gently to the fuel surface. It should never be plunged into the fuel, and water streams should not be operated so as to dilute and wash away the foam. The foam blanket should be rolled onto the fuel surface by directing the stream onto the ground in front of the fire and allowing the foam blanket to build and move away from the nozzle. The foam can also be banked off a wall or other obstruction with the same effect.
Use on gasohol
The unleaded gasolines and gasohols are combinations of hydrocarbon fuels and polar solvents. Current research by the foam industry indicates that conventional fluoroprotein foams and AFFF can control small spills of these mixed fuels. Large or in-depth spill fires may not be readily controlled by these foam agents because of the effect of the alcohols and ethers on the foam blankets.
The breakdown of these foam blankets will be accelerated because of the presence of the polar solvents. Reignition potential exists in as little as 10 minutes after extinguishment so care should be taken to maintain the foam blanket seal with frequent reapplication of foam.
Another measure to improve the foam blanket stability is to apply the foam solution with an air-aspirating nozzle. The polar solvents will destroy the aqueous film in AFFF, so it is imperative to create a good AFFF blanket with an air-aspirating nozzle.
Fire departments may wish to explore the potential of using a new type of foam when restocking existing supplies. A polar solvent liquid foam may be desirable for the projected increase in incidents involving unleaded gasolines and gasohols as well as the bulk transport of ethyl alcohol and other polar solvents. Careful investigation of this option is necessary due to compatibility with existing equipment and cost factors.
Unleaded gasolines and gasohols do complicate the fire fighting problem, but existing foam concentrates now carried on engine companies will be effective on the usual spills and fire that the fire service experiences. The chief danger is the more rapid breakdown of the foam blanket after the fire is controlled. The use of an air-aspirating foam nozzle to create the best possible foam blanket and the continual concern for maintaining the foam seal with periodic application of new foam during post-fire operations will enable officers to counter this danger.
Summary
The following factors must be fully understood in order to plan for and handle flammable liquid fires:
- Flammable liquids are a more complex fire control problem because of their flammable vapor production capabilities.
- Flammable liquids are of two basic types—hydrocarbons and polar solvents. Polar solvents will destroy conventional fire fighting foams.
The flammable liquid fire control problem is more difficult due to the introdution of unleaded gasolines and
- gasohol—a mixture of hydrocarbon and polar solvent fuels.
- The most common fire fighting foams—protein, fluoroprotein and AFFF—have some differences that must be understood.
- New foams—polar solvent liquid foams—have been developed to handle fires and spills in both hydrocarbon and polar solvent fuel fires.
- The generation of a good foam blanket requires a careful mix of water, air and foam solution applied under special conditions.
- An air-aspirating foam nozzle should be used to apply all foams including AFFF.
- Rate-of-application is critical in flammable liquids fire fighting. A rate of I gpm of foam solution per 10 square feet of fuel surface must be maintained until the fire is controlled.
- Special care must be taken to maintain the foam blanket seal in all cases and especially in cases involving unleaded gasolines and gasohols.
A thorough comprehension of the effective use of foam is necessary for tactical decisions of flammable liquid incidents. Complete training and practice in foam use is necessary for the effective application of this agent.
Some information for this paper was supplied through the cooperation of National Foam Systems, Inc., and Angus Fire Armour Corporation.
