There is nothing magical about fire fighting foam. We are simply creating a mechanical barrier that shuts out the oxygen and the fire goes out. Unfortunately there are situations in which this system will not work; usually these are instances wherein there is a reaction between the foam and the fuel. One of the most common on these is the interaction of foaming agents with non polar fuels such as alcohols. To enable firefighters to effectively combat these fires manufacturers developed “ATC” (Alcohol Type Concentrates) foaming agents. Originally these were foam concentrates fortified with metallic ions and organo-metalic substances such as calcium stearate. Generally these work well on fires involving non-polar liquids but they tend to leave something to be desired in the case of fires involving “gasohol” (motor fuel containing ethanol) which is a mixture of compounds, some of which are miscible and some of which are immiscible with water. As a result several manufacturers have introduced new formulations specifically designed for use on these materials. They are expensive but they do work.
As the firefighting world gained experience with foam technology it became apparent that there were attributes of foam that were not being exploited. For example a foam blanket is made up of billions of small bubbles or “cells” which enclose minute amounts of the gas used to “generate the foam. This gas is usually air but if we should substitute another gas in place of the atmospheric mixture we would have an extinguishing agent suitable for use on pyrophoric and/or hypergolic materials such as ethylene oxide. Simple as it sounds, the production of a foam containing a gaseous component other than air does present some technical difficulties. These have largely been overcome with the advent of the compressed air foam system (CAF). In these systems foam solution passes through a mixing chamber where pressurized gas supplied by a compressor is entrained in it. Since this is a closed system it is a relatively simple matter to connect the intake of the compressor to a supply of the desired inert gas, usually nitrogen, (N2)or carbon dioxide (CO2). The result is a blanket of foam made up of cells containing an inert, non-oxygen gas which can be utilized in combating fires involving materials that will react with oxygen.
Another nontraditional use for firefighting foam which has been made possible by the introduction of CAF units is the rendering of enclosed spaces such as tanks inert. The CAF units allow the three parameters of foam, gas, water and foam concentrate to be adjusted independently by the operator. As a result the physical characteristics of the finished foam can be made to fit almost any set of conditions. The foam can be wet and heavy or dry and light; it can be fluid so as to flow across a pool of burning liquid or it can be made stiff enough to bridge a fracture in an overturned tank truck. Since the foam is visible, complete filling of the vacant space can be ascertained with certainty and as mitigation proceeds additional foam can be pumped into the void space for replenishment as needed to replace liquid or other lading that has been removed.
Fire fighting foam is just that, foam, and the insulating properties of such materials are well known. The great capacity of foams to block heat is due to its physical structure, not its chemical properties. Each one of the millions of tiny bubbles that make up the body of the foam absorbs energy as they are broken down by radiant heat or flame impingement. All, or at least most of us have seen the demonstration in which the presenter covers his hand with a thick layer of shaving foam and then holds it in front of a propane blow torch without any discernable discomfort. This is “for real”; it is not a parlor trick. Shaving foam is made up of very tiny bubbles and as each one is ruptured by the flame from the blow torch it absorbs a small amount of energy. True, this amount is extremely small, but consider the number of bubbles involved. After all, even the oceans are made up of drops of water. Of course the foam on the demonstrator’s hand will eventually be consumed and evaporated and will have to be replenished. The same is true of the foam that is being used as a thermal barrier during a fire. Because fire fighting foam is such a good insulator or thermal barrier, If it can be made to adhere to a surface such as a tank wall or a supporting girder, the temperature may be kept low enough to prevent an explosion or BLEVE or perhaps forestall a building collapse. CAF units have made this a reality. If one is using protein foam, (the old standby, cheap, dirty and odoriferous, known far and wide as “GI” foam ) it can be made to stick to hot metal surfaces in the same way that meat will stick to an ungreased frying pan (after all, meat is protein). Of course repeated applications of foam will probably be required but if the temperature, and therefore the pressure, of an involved container such as an LPG tank can be maintained below the point of criticality and thus an explosion or BLEVE be avoided it is certainly worth the trouble. To accomplish this, the CAF operator should set his controls to deliver a stiff foam which can be sprayed onto the hot sides of the tank and allowed to adhere. If the foam is thick (dense and dry) enough it will stick to the hot walls and remain in place offering thermal protection to the underlying object.
Fire fighting foams are amazingly strong mechanically. One method of containing a potentially explosive device is to place it inside a box surrounded by very thick and heavy fire fighting foam. In one demonstration at Texas A&M an explosive charge was placed in a plywood box, enclosed in a wire mesh safety net. The box was then filled with foam and detonated. There was no significant damage to the box. Next the same box was emptied and the foam blown out with an air jet. An identical charge was placed in the box and detonated. This time the box was obliterated. There was not a piece of it larger than a soup can lid to be found. The foam had absorbed the explosive energy.
Another problem with the use of firefighting foam has been that of “reach”. Foam, by its very nature, is light and because there is very little concentration of mass it is difficult to propel over any significant distance. As the foam gets lighter the problem gets worse; it is particularly true when using high expansion foams. The old “chemical foams” were not as bad as modern ones in this respect. With these foams the agent left the nozzle as a liquid and the chemical reaction that generated the foam took place while the stream was in flight so that a finished foam was delivered at the point of impact. Anyone who has observed a beverage squirting from a shaken can has seen this phenomenon. The same is true of CAF units to an extent. The foam is under pressure in the unit and when it is expelled from the nozzle this pressure is released. The gas within the foam then expands and the foam is generated as it travels to the target. Because there is a greater concentration of mass, more energy can be applied and the fluid can be propelled farther. Anyone wishing to demonstrate this is referred to the toy known as a “nerf” football. It is almost impossible to throw one of these hard enough to do any significant damage to persons or property. This is why these spongy toys were developed in the first place.
When CAF units were first developed by the Texas Forest Service, primarily for small rural departments, they were known as WEPS (Water Expanding Pump System) units. The key word here is expanding; these units would expand a gallon of foam solution (water plus foam concentrate) something like a thousand times and in some cases, more. This was a great advantage in the rural areas for which these units were developed. The fires encountered in these areas usually involved wild-land areas and ordinary structures with an occasional small hydrocarbon incident thrown in. These, with the exception of those involving hydrocarbon fuels, can easily be extinguished with ordinary water streams; but, the problem is, where do we get our water? Tanker operations can be effective if the haulage distance is not excessive and if adequate water supplies are available. Unfortunately this is not always the case. CAF (or WEPS) units expand water and effectively increase the available supply. For example, if we have a tanker deployed with a thousand gallons of water on board (8600 lbs.) and, with a CAF unit we expand it by a factor of 500 (to be conservative) we wind up with a half million gallons (500 X 1000) of firefighting agent. This is enough to do serious social work at anybody’s fire. The key here is not the density of the foam or the fact that it will float on the surface of a burning liquid but the fact that the quantity available has been dramatically increased.
Electrical hazards emanating from involved structures are no strangers to fire fighters. Some claim that water fog will not conduct electricity due to the separation of the water droplets. Others vehemently challenge this statement. The same thing holds true for foams though experience has shown that when any significant electrical charge flows through a foam the cells (bubbles) tend to collapse and eliminate the electrical pathway. This would indicate that foams may be safer to use in locations where there is a risk of electric shock than are fogs or water streams but extreme caution should be observed in any case.
Fire fighting foams were developed to meet a need engendered by the advent of the automobile. Since that time they have been re-designed and re-engineered to do a variety of things and to do them well; firefighting would be much less successful without them. From the simple and often home made “soap suds” solutions that were stirred up in a barrel behind the firehouse and would enable the firefighters to control a gasoline fire to the modern multi-stream behemoths producing thousands of gallons of foam per minute the underling principles have not changed. Foam still floats on water and cuts off the oxygen to put out the fire and it will continue to do so to serve the needs of our technology based culture for the foreseeable future.