Mechanical Foam: ITS CARE AND USE!
INCREASED HAZARDS of flammable liquid storage and movement of such liquids by road transportation, plus the amount of fuel used in air travel today have forced many fire departments to be more dependent on foam for extinguishment of fires in these liquids than ever before. Flammable liquids with flash points below 100°F (gasoline-50°F; JP-4 -26°F to -36°F) cannot be extinguished with water alone. Mechanical foam, a hydrolized protein material, made up of closely formed bubbles having waterretention properties and stability, coupled with its ability to float readily on the lightest flammable liquids, makes it an excellent extinguishing agent for flammable liquid fires.
Foam is lighter than the lightest oils or hydrocarbon fuels. This enables it to float on the burning liquid, smothering as it covers the burning fuel and at the same instance cooling with the water suspended in the foam, which slowly drains out onto the surface of the fire. Preventing flash back of burning flammable liquid is important. Flash back can occur if the vapors of the fuel are released. Foam keeps these vapors under its blanket thus preventing reignition.
There are two types of foam liquids available today: Low-expansion protein foam (expansion ratios up to 12) and high-expansion non-protein foam (expansion ratios up to 30 or more). This article will discuss low-expansion protein foam.
In the low-expansion family of mechanical foam liquids we have three basic types:
- A3 per cent foam liquid used on hydrocarbon flammable liquids such as gasoline, oils, JP-4, benzene, rubber cement, etc.
- A 6 per cent foam liquid also used on hydrocarbon flammable liquids as described above. When employing 6 per cent foam liquid twice as much as 3 per cent foam liquid is required.
- A 6 per cent all-purpose foam liquid. Used on polar solvent or water-soluble flammable liquids, such as alcohol, acetone, methyl acetate, ketones, etc.
When fighting a solvent or alcohol fire with foam, only the alcohol type should be used. Applying regular foam to a water-miscible flammable liquid will usually destroy the hydrocarbon-type foams on contact. This is caused by extraction of the water suspended in the foam.
It is necessary to form an insoluble barrier within the bubble walls of the foam in order to physically separate the solvent from the water. This is achieved by a chemical reaction which takes place when the all-purpose alcohol foam liquid is mixed with water and air at the time of foam formation, depositing a solid within the foam film. This chemical reaction takes place rapidly, making it undesirable to prepare premixed solutions prior to aeration. Best results are achieved by introducing the foam at the nozzle by means of a pickup tube to assure zero premix time.
To produce 1,000 gallons of expanded foam, a combination of 97 gallons of water with 3 gallons of foam concentrate and 900 gallons of air is necessary. This combination is for 3 per cent foam liquid. When using a 6 per cent foam liquid, the water and foam rate differ inasmuch as twice as much foam liquid is used (94 gallons of water with 6 gallons of foam concentrate). The amount of air used is the same, and the net result of expanded foam is the same with either.
To estimate foam production, use this rule of thumb with 3 per cent foam liquid (keep in mind that 6 per cent foam will require twice as much foam concentrate): Produce 1,000 gallons of foam using 3 gallons of foam concentrate (3 per cent) using water and air. One gallon of foam equals 0.134 cubic feet. Multiplying a height of 12 inches by 0.134 gives a blanket thickness of 1.6 inches over each square foot. The result is a little more than 1 ½ inches over an area of 1,000 square feet. Three gallons of foam liquid will cover an area of approximately 20 feet by 50 feet, 1 ½ inches thick. If a 3-inch blanket is formed the area covered will be about one-half.
Properties of a good foam liquid
Foam should not be affected by temperature in storage. Recommended foam storage temperature range is from 40 F to 110 F. No sludge should separate from the liquid in storage, nor should separation take place even though the stored foam has been frozen or kept at high temperatures or alternately both.
Foam should have the ability of forming with soft, hard, or sea water. The temperature of the air or water should not affect the foam or its properties. The foam must be sufficiently fluid to form a continuous blanket and seal off the entire ignited area despite any obstacles which protrude above the fuel surface.
Foam liquid is usually available in 5-gallon shipping containers or 55gallon drums and the liquid itself is not corrosive. Deterioration of the container, if any, will result from exposure to the elements or poor storage.
When storing foam liquid containers in a fire station, select a dry heated area. Though some 3 per cent foam liquids have a freezing point of —20°F, others do not (6 per cent foams freeze at 10°F). Care should be taken to shelter the containers from the elements. Foam liquid frozen solid can be thawed back to its liquid state and still retain its original fire fighting properties.
When stacking foam containers never place the cans directly on a concrete floor. If this is done, moisture in the trapped air between the floor and upper recess on the bottom of the container condense due to temperature changes and cause corrosion. This situation can also occur when foam containers are stacked on top of each other. A recommended procedure in stacking is to separate the containers from the floor and each other with 1 x 2 wooden strips.
Foam liquid containers normally are sealed on arrival and depending on the treatment received they are airtight. Quite often these containers are treated roughly and occasionally develop pin hole leaks between the cap and the container. Continuous condensation at this point, introduced with the air changes above the foam liquid surface, encourages scaling.
Foam liquid can now be obtained in containers with an epoxy coating which is baked on the inside when the container is made. This epoxy coating prevents scaling from starting when the foam container breathes moist air. With epoxy-lined foam containers there is no need to upend them; in fact, it is doubtful that any good comes from upending containers even without this epoxy coating.
There is always a layer of air between the surface of the foam liquid and top of the container and it is thoroughly saturated with water vapor. That part of the container is subject to corrosion. If scaling has occurred, leaving the containers upright will retard further corrosion. Upending a container repeatedly will remove this scaling and expose a fresh surface for further corrosion. After a time this scale can build up deep enough at the bottom of the container to clog a pickup tube or an eductor.
Training with foam liquid
Drills with foam are as important as training with dry chemical, fog or other fire extinguishing medium. Applying foam to a flammable liquid fire requires some training. It is necessary to apply the foam gently to the fire by means of a backboard or gentle application so that the foam floats gently on the burning liquid surface. Once a blanket is formed the operator can accumulate foam in front of the fire and float the foam readily in a continuous blanket across the burning fuel surface.
Often flammable liquid fires involve tanks, trucks or other obstructions including exposures. For this problem training witli fog-foam is extremely helpful. Hot drills are a necessity and the more of them, the surer a fireman is in his movements around a hot fire.
In selecting foam liquid for training purposes it is suggested that the oldest liquid in storage be used. The best method of keeping track of the age of your foam is to number the containers as they are received. Then use the lowest number for training. The nozzle selected must also be considered for training. One with straight stream and fog-foam discharge is helpful as it gives fire fighters allaround experience.
Amount of foam to store
A word of caution here. In estimating how long your liquid concentrate will last, remember that expanded foams vary upon pressures, mixing, and the type nozzle used. Even the type of fire involved has a direct requirement on the amount of foam used. It is impossible to predict the amount of liquid to be used on a given fire as flammable liquid fires of similar size may present different problems to the firemen due to location, weather, etc. Each situation must be handled by experience and proper judgment. In order to assure sufficient foam liquid for adequate fire protection, one can refer to NFPA Standard No. 11 describing recommended foam solution rates. (Note: Foam solution is the amount of foam and water before expansion; 1 to 1.6 gpm per 10 square feet of flammable liquid surface area is recommended—with a foam supply of 20 to 75 minutes.)
An example of this is a flammable liquid spill fire on a street or field, deep in depth, covering an area of approximately 20 x 50 feet or 1,000 square feet. For extinguishment of a fire of this size using 1-gpm foam solution rate per 10 square feet of exposed liquid surface, the required foam supply will be between 60 to 225 gallons of 3 per cent foam concentrate (6 per cent foam concentrate needed would be doubled). A more difficult fire with deeper depth of fuel might take 96 to 360 gallons of 3 per cent foam concentrate (again the amounts are to be doubled for 6 per cent foam concentrate). Of course the same area with no depth of flammable liquid burning could be extinguished with foam concentrate requirements below 60 gallons.
Rate of application
Technically, the rate of application is the amount of foam solution applied to a fire in gallons per minute over the square foot area of the burning flammable liquid: 1 gallon foam solution per 10 square feet in 1 minute. This, of course, is the rate before expansion of the foam solution. To extinguish a flammable liquid fire of 1,000 square feet in size, a minimum foam solution rate of .1 gpm per square foot is required, and occasionally .2 or .3 gpm per square foot may be required. In terms of nozzle requirements, gallonage-wise, the discharge of foam solution should not be less than 160 gpm (three 1 ½-inoh foam nozzles or one 2 1/2-inch foam nozzle) and up to 480 gpm (two 2 1/2-inch foam nozzles and one l 1/2-inch foam nozzle). The rate of application is important in extinguishing a fire. If the rate is too low, the fire will be difficult to extinguish, no matter how much foam you have available. The faster the foam is applied to a fire in volume the faster extinguishment is accomplished.
A 1 ½-inch preconnected all-purpose nozzle on a pumper is usually used as a foam nozzle also. But having 2 ½-inch foam nozzles with sufficient capacity (200 gpm) as a backup is considered good practice.
Keep in mind the higher the foam volume discharged from a nozzle, the greater the horizontal range or distance of throw. At times this has a distinct advantage which enables fire fighters to play foam effectively on a blaze from a safe distance.
When using foam in conjunction with compatible dry chemical, the application rate recommended by the Underwriters’ Laboratories is .5 gpm per square foot. This is five times greater than that discussed above.
Mutual aid for foam needs
The amount of foam liquid needed for emergencies depends on the individual fire department’s everyday exposures. These include tank farms within community or district limits, gasoline and flammable liquid transports through the city or town, air traffic around airports, available manpower and nozzles, etc. However, one procedure which should be planned is the obtaining of foam through mutual aid from stocks of other departments and existing foam supplies elsewhere. Air Force bases, major oil companies and fire equipment dealers are usually a good source.
It is interesting to note that flammable liquid fires have involved as little as 80 gallons of foam to as much as 5,000 gallons. A potential like this makes mutual aid a must. Plan for your needs and seek out reliable sources. A procedure in writing covering available sources and made available to the various officers of your department is a good start.
- Don’t use corrosion inhibitors in your booster tank water if you intend to employ it for foam production. The inhibitor affects the fire fighting properties of the foam.
- Don’t expose foam liquid concentrate to oil.
- Don’t forget to flush out your pumps, lines, or pickup tubes after use. If you have an eductor, dip the pickup tube in a bucket of clean water and operate the nozzle. The water will flush it clean.
- Don’t permit dissimilar metals in your built-in foam system. This will cause galvanic action and corrosion.
- Don’t upend or turn cans upside down in storage.
- Don’t leave the cap off a foam container in storage. This will start rapid evaporation.
- Don’t mix foam liquid with wetwater or high-expansion non-protein foams. They are not compatible.
- Don’t practice premix storage. If premixing of water and foam is necessary, do not store any longer than two to four weeks.
- Don’t believe your firemen can handle an emergency requiring foam without training.
- Don’t forget that a foam liquid not bearing the UL label should be examined carefully before using.