Foam: From Soap Suds to AFFF
Vol 21 No 3
Foam was introduced into the armamentarium of firefighting about the time of the advent of the automobile. Its advent was in response to the need to control and extinguish fires involving liquids which were lighter than water and immiscible with it, i.e. the gasoline, used to fuel the growing number of automobiles. As the number of motorcars increased so did the amount of fuel that was stored in communities and, not unexpectedly, the number of fires involving volatile organic liquids, mostly hydrocarbons.
The original foams were nothing more than soapsuds. Indeed legend has it that the original idea for foam was conceived by a firefighter as he watched the soapsuds float on his wife's wash tub. Thus it comes as no surprise that the first foam extinguishers were filled with a soap solution containing sodium bicarbonate (baking soda) and another solution of alum which was mixed with the bicarbonate solution when the extinguisher was inverted. This mixture produced the carbon dioxide gas which propelled the foam solution out of the extinguisher.
Indeed one of the most effective "high expansion foams" around, for use on hydrocarbon fires, is a commercially available dishwashing detergent. The "AFFF" foams developed after the disastrous fire aboard one of our Navy's aircraft carriers is also very effective as a heavy duty detergent for such tasks as cleaning machinery.
The early foams were what might be termed "low expansion", they were semi-liquid or "soupy" but they could be flowed over the surface of a burning liquid and do what they were intended to do, namely put out the fire.
As the number of different flammable volatile liquids increased as America entered the age of chemistry it was discovered that many of these would react with the soap based foams then in use and deactivate or destroy them leaving the liquid burning. Also, the fire equipment companies realized that there was a buck to be made selling foam and the race was on to see who could make a better, more efficient and more versatile foam and thus corner the market. Since it was really a tough sell to patent ordinary soap suds, many different substances were tried. Among them, metallic stearates were added to the soap. This is still seen today when one looks at the variety of specialty foams displayed at any fire equipment show.
One of the most successful and one still used in large quantities is known as "protein" or "GI" foam. This is an "aqueous hydrolysate" of proteinaceous material otherwise known as packing house waste. Anything that couldn't be put into the sausage was thrown into the vat to make fire foam. After a while the liquid was removed and a little propylene glycol was added along with a small amount of formaldehyde to retard spoilage. Believe it or not, this witches brew was and still is highly effective against hydrocarbon fires. It is relatively cheap and readily available; thus it has become the mainstay of the smaller volunteer departments for use on vehicle fires.
Fire foams are a system composed of air, water and the foaming agent. By altering the ratio of these ingredients in the foam mix the physical properties of the finished foam can be greatly altered. Less air produces a more liquid, though heaver, foam. More air produces a stiff foam reminiscent of canned shaving foam or the egg white topping seen on top of lemon pies.
Protein or "GI" foam will stick to hot metal in the same way that meat sticks to an ungreased skillet. When used in a compressed air foam (CAF) system, this material will produce a long lasting stiff foam that will adhere to hot metal such as the girders supporting a roof or second story. This adherent foam coating absorbs the impinging heat and helps to prevent or delay the collapse of the burning structure.
Firefighting foams can absorb a tremendous amount of heat and they are exceptional insulators. The classic demonstration of someone holding a hand coated with a thick layer of shaving foam in the flame of a propane torch without feeling the effects of the heat is well known. This is no mere "parlor trick" it is very real and more than one structural collapse has been averted or delayed by shielding the supporting members with foam until occupants could be evacuated or the contents removed from a tank.
The "hallmark" of the American firefighter, if there is one, is ingenuity. The vast majority of specialized firefighting apparatus was developed from an idea conceived in the imagination of some obscure firefighter. This ingenuity has not been absent from the area of foam utilization. Foam has been used to fill void spaces so as to drive out or prevent the accumulation of flammable or noxious vapors. It has been used as a covering to absorb heat and prevent vaporization of a liquid. In at least one case to the author's knowledge' it has been used to cover an exceedingly odoriferous material (a tanker load of methyl mercaptan) and alleviate the need for the evacuation of a dense metropolitan residential area.
Fire foam under pressure acts much like a bottle of soda water or other beverage we won't bother to mention. As long as the cap is in place the liquid is clear and there is no gas in evidence. Remove the cap, however and the liquid begins to foam as the gas absorbed in the liquid starts to expand. This principle is utilized to provide "reach" for a foam stream. If one were to attempt to propel foam for a long distance the task is much like throwing a "nerf" football; it is very difficult to achieve any real distance. This is because force (F) is equal to mass (M) times acceleration (A) thus: F=MA; if the mass (M) is extremely small for the volume involved (density) and actually approaches zero, then even a very large force produces very little acceleration. If however we create a solution of water, foaming agent and compressed gas we can generate a fair amount of force capable of propelling a stream of liquid for a respectable distance. As this stream leaves the nozzle the pressure holding the gas in solution is released (the cap is removed from the beverage bottle) and the dissolved gas begins to expand producing the foam which impacts and covers the target. We have actually generated our foam on its journey from nozzle to target. This principle was originally utilized for fire fighting apparatus by the Texas Forest Service.
As refinery tanks got larger and the dangers inherent in placing ladders against them increased, another innovative and ingenious firefighter got the idea of pumping a pressurized solution of foaming agent, water and air into a storage tank near ground level. Since the solution contained a dissolved gas it would expand when released, lose density and float to the surface. As it rose through the column of liquid in the tank, the hydrostatic pressure became less allowing the dissolved gas to expand and begin to form bubbles in the foam liquid. Upon reaching the surface the entrapped gas would expand fully creating a foam that would flow over the surface of the burning liquid and extinguish the fire while the firefighter remained safely out of harm's way. Thus subsurface injection was born.
Foam has long been thought of as a tool for fighting fires involving flammable volatile organic liquids, particularly hydrocarbons. In truth a great deal of foam is still utilized in this manner but our ever inventive firefighters continue to find new and innovative use for this material.
As cities continue to expand at an ever increasing rate the need for fire protection expands with the population. Often this involves protecting areas beyond the reach of municipal water mains. For the firefighter this means a tanker operation. Such operations rarely are able to supply the need for fire streams required by even a moderate conflagration. Even an eighteen wheeled tanker holding 10,000 gallons will not last long when hooked to a couple of 2? inch nozzles. However, if we convert this water into foam and in so doing expand it several hundred times; we are now able to do some serious social work on the fire This is the underlying principle of the CAF systems and these have become serious workhorses in many rural departments. Since the water content of a foam is considerably less than that of a straight stream, the amount of water damage and the subsequent cleanup operation is greatly reduced.
Foams hold water in place very effectively and this property has been exploited very effectively in the case of wild-land fires where foam has been used to form a wet band or barrier to stop the spread of the fire. This is often done by placing foam dispensing nozzles ahead of the front wheels of an engine. When the wheel passed over the first application of foam it forces the foam and its contained water deep into the grass and other vegetation. A second set of nozzles ahead of the rear wheels repeats the process. And a final set aft of the rear wheels leaves a band of foam on top of the wet line to help maintain it. These wet lines will last for several hours but their three most important properties are that they can be laid down quickly, almost as fast as an engine can travel over rough ground, they can be moved or renewed with fresh foam if this is needed, and they do little or no damage to the ecosystem.
Foams have been used to fight fires involving water reactive materials. The foam releases its water slowly and keeps any reaction that happens between the water in the foam and the water reactive material to proceed in a slower and more controlled manner. Even so some materials are hypergolic or air reactive; ethylene oxide comes to mind. Fires involving these materials cannot be readily extinguished by the application of a conventional foam. Since many of these substances are water reactive in addition to being hypergolic, water streams are not effective either.
To combat fires involving hypergolic materials and/or water reactives researchers have experimented with foam that contains or is "blown" with compressed nitrogen in place of air. This is easily accomplished with a CAF unit since all that is required is to hook the compressor intake port to a liquid nitrogen caddy or the outlet of a dry ice chamber. These foams have proven effective when applied to substances such as ethylene oxide or some metallic alkyls.
Foams occupy space and, since no two objects can occupy the same space at the same time, they have proven to be an effective means of eliminating void spaces such as the interior of a tank trailer which has been emptied prior to its being uprighted. The lading must be removed but doing so creates a confined space in which explosive vapors can accumulate.
The classic method of neutralizing these confined spaces is to toss in some dry ice and allow it to sublime, filling the space with carbon dioxide in the process. This method has two major disadvantages. In the first place carbon dioxide is a colorless and odorless gas. Since it cannot be seen nor smelled how can the responder ascertain when the confined space is really full? He can test with a meter, of course, but that entails a close approach to the tanker; something that may not be exactly safe at the moment. The use of foam, since it is visible to even the untrained eye, allows the firefighter to see when the space is filled. Secondly, foam can be made very stiff and this stiff material will bridge over even a good sized gash or hole in the tank and remain in place while carbon dioxide, being heavier than air, will leak out of a container as soon as it is positioned so that the gas can exit from a gash on the bottom of the tank.
Other researchers have experimented, with more or less success, with foams made of a plastic material which will harden after being applied as a liquid. These materials work and are perhaps useful for limited applications but as of the present time their high cost precludes their wide-spread use.
The world of foams is expanding, even as we write, new foaming agents are being introduced as well as new methods of preparing and applying them. The list of uses as well as the methods of application for these materials continue to grow. We cannot foresee the future but as long as there are imaginative and ingenious firefighters there will be new and innovative developments for using this "jack-of-all-trades" of the fire service.