From time immemorial mankind has used water to control fire. From the first time that some nameless aboriginal ancestor discovered that water would extinguish fire until the present day the basic principle of firefighting has been to "put the wet stuff on the red stuff". This approach served well when virtually all combustibles were solids. There was the occasional bit of oil used as lamp fuel, various lubricants and of course the odd container of "spirituous liquors" that would burn but other than that there were very few flammable liquids in the every day environment and those that were around were present in rather small quantities. In the nineteenth century America, and the world, was geared to solid fuel. The industrial revolution had arrived but it was powered by the steam engine and that engine was nearly always fueled by coal or wood.
All this changed in August of 1859 when Col. Edwin Drake brought in the first oil well at Titusville, Pennsylvania. Originally petroleum was used to produce "lamp oil" or kerosene to replace the scarce and more expensive whale oil then in use; gasoline was essentially a waste product since, because of its high volatility, it was prone to explosion and therefore deemed to unsafe for use in the wick type lamps of the day. Then, in 1898, the Duryea brothers hitched an Otto gasoline engine to a buggy to produce the first automobile in America and when Henry Ford introduced the Model T in 1903, liquid fuels were off to the races.
The coming of the automobile and with it the introduction of large quantities of liquid fuels and lubricants into virtually every community opened up a whole new world for firefighters. For the first time they were being called on to deal with fires that could not be put out with ordinary water streams and, at first, there was nothing with which to meet this challenge. Liquid hydrocarbon fuels and most other organic liquids (carbon disulphide CS2 being the most common exception) are generally lighter than water; thus when water is applied to these materials they simply float to the top of the container and continue burning. If more water is applied the container may overflow and spread the fire. In addition, if the temperature of the burning liquid has exceeded the boiling point of the applied water it may flash into steam causing a "slop over" which again spreads the fire.
The popular legend has it that the idea for firefighting foams came to an off-duty firefighter who noticed that the suds produced in the soapy water of his wife's laundry tub floated on the surface of the water sealing it from the atmosphere. He reasoned that if he could apply soap suds to a hydrocarbon fire the water in the suds would remain on top of the liquid and smother the fire; thus fire foam was born.
Regardless of their composition all foams are basically a system composed of three components: Water, a wetting agent or detergent and a gas (usually air). To obtain foam the three components must be mixed prior to application. This mixing can be done in two ways: 1.) the foaming agent or "concentrate' can be mixed with water in a tank either when the tank is filled or when the apparatus arrives at the scene and it is determined that the nature of the fire is such that foam will be needed. This works well in the case of small, self contained first response units but it has the drawback that the tank has to be refilled periodically unless a backup engine can arrive before the onboard supply of premixed foam solution is exhausted. 2.) The concentrate is carried in a tank or container and added to the fire fighting water by means of an eductor or proportioning pump. This system has the advantage of being able to supply foam in a continuous stream, so long as the supplies of water and foam concentrate hold out. In either case the foam solution (foam concentrate and water) is aerated by passing it through some sort of air aspirating nozzle or by the application of compressed air as in a ("CAF" unit). The finished foam is then applied to the fire.
Some of the original fire/foam was generated in 2? gallons fire extinguishers. This was referred to as Chemical Foam. It was generated when two chemicals mixed together. The fire extinguishers had an inner chamber and an outer chamber. The chemicals were referred to as A & B. As long as the extinguisher was upright, things were normal. When the extinguisher was uprighted and turned upside down, the two chemicals would mix. The chemical reaction would do two things. One, it would create a chemical reaction that would create pressure which forced the foaming agents out the hose of the extinguisher. It also caused bubbles which were filled with Co2 gas.
These extinguishers were very similar to the Soda Acid extinguishers of their day. The major difference was the foam extinguishers created bubbles so that the foam bubbles could foam over the surface of the flammable spill. They worked very well and because of the Co2 in the bubble, it helped to extinguish the fire.
One major problem with both the Foam and the Soda Acid extinguisher was that the chemicals were very corrosive. Also the extinguisher containers were not pressurized and there was no standard requiring that they be hydro tested. Too many times when the extinguisher was pressurized by turning it upside down, the container shell would rupture from the pressure and kill or injure people. They also could not be used on electrical fires as the chemicals were good conductors of electricity. These types of extinguishers were banned about the late 1960's.
If any of these extinguishers seem to still be full of liquid, do not invert an old Soda Acid or Foam extinguisher. There is no way to know if the chemicals are still inside the extinguishers and the container could fail with terrible results.
Since their original introduction, numerous "improved" foaming agents have been introduced. Some were developed for the sake of economy, others to deal with specialized situations; polar liquids such as alcohols are a classic example. Protein foams, originally based on hydrolyzed waste animal tissue from slaughter house operations in various glycols, were introduced because of their longevity, their cheapness and the fact that they would not harm plant life. They had one major drawback however; proteins are neutralized or "denatured" by non-polar solvents such as alcohol. The protein foams also had a short shelve life. If they were stored in a hot environment, their shelf life could be measured in months. As the "Age of Chemistry" developed, more and more non-polar chemicals were introduced into transit and the market place for use as solvents and chemical feed stocks.
The first move away from the protein based foams was when the US Navy in cooperation with 3M Company developed AFFF (Aqueous Film Forming Foam) This foam contains some Fluro Florinated chemicals which cause the AFFF to form a very thin film on the surface of the fuel. The film was strong enough to prevent the fuel vapors from rising through the film. It worked very rapidly and that was one of its major reasons to exist. The Navy wanted foam that worked faster than the protein foams in use at the time.
AFFF originally was used in conjunction with Dry Chemical, often referred to as a Twin Agent system. The AFFF world and the US Navy were the original users of this new type of foam and fire fighting system. The next major users were the airport fire departments.
The AFFF foams were not recommended by the NFPA 11 Standard for tank fire protection. Some resistance was political more that technical. In the late 1970s a 160 foot (50 meter) gasoline tank was extinguished. Due to this extinguishment and the results of many fire tests performed by 3M, the NFPA 11 Committee dropped the restriction on AFFF.
In the 1980s the 3M Company purchased the rights from National Foam to manufacture a new type of foam called ATC. (Alcohol Type Concentrate). In just a few years the new type of foam became the foam of choice in the US and successfully extinguished most of the tank fires in the US and around the world. Today that foam is almost exclusively used in refineries and chemical plants around the world.
Today the use of traditional foams is being challenged by a new fuel, Ethanol. Ethanol is in the process of being used in the majority of gasoline sold. The challenge that most fire departments have is that many of them use AFFF or emulsifier types of foams. None of them will work on 10 percent or 85 percent ethanol. To extinguish fires involving this fuel requires the use of AR types of foams or AR/AFFF.
Foam consists of a lattice of tiny bubbles which are bound together by means of static, or magnetic, attraction and the weaker Van derVaal's forces. As long as it is intact, this lattice has tremendous insulating power working in exactly the same way as does the foam insulation in a domestic refrigerator. Foam can be used as an insulting blanket. The normal challenge is that most foams will not adhere very well to vertical surfaces. As long as the foam is there, it insulates the surface very well from fire and heat. Along comes a new agent and system, called CAFS (compressed air foam system). This foam is made by mixing a foam agent with air and water in the correct amounts. This foam is a very thick bubble and will adhere to vertical surfaces. If a fire department sprays the foam onto a tank shell, it will protect it very well from heat and flames for hours. It has been used very successfully in the wild fires out west. So if we apply a thick blanket of adherent foam to an iron beam or tank wall that is being subjected to extreme heat in a fire we may well delay or even prevent its weakening and/or collapse. This technique has been used to protect a tank of flammable liquid, adjacent to a conflagration, from radiated heat or flame impingement. If the blanket can be replenished as needed the temperature of the tank contents can be maintained at a safe level until the fire can be extinguished or the contents of the tank removed from the danger zone. That compressed foams will adhere to hot metal is a definite plus and makes it possible to apply foam blankets to vertical or near vertical surfaces where their function is to protect from heat rather than to smother the fire.
The insulating qualities of foams can also be utilized to retard vaporization of volatile liquids. A thick foam blanket applied to the surface of a volatile liquid will protect against heating and, since the rate of vaporization and the vapor pressure are both proportional to the temperature of the liquid, diminish the concentration of hazardous or explosive vapors above the liquid.
Firefighters are some of the world's greatest innovators. Leave anything lying around a firehouse and, (if it is not nailed down) they will "acquire" the article and find a new and different use for it. Foams have been no exception. It wasn't long after their introduction that firefighters began to find numerous uses for foams that were over and above what the original innovators had in mind.
The very property that made foams necessary in the first place can be utilized to apply them to a tank fire. Water and "oil" (most hydrocarbons) do not mix and foam is lighter than "oil" and thus will float on top of it. So, if foam, containing entrained air, is injected near the bottom of an oil filled tank it will float to the surface. As the foam rises to the surface of the liquid the hydrostatic pressure becomes less and the air in the foam expands thus, when it gets to the surface, the foam has expanded and flows out over the surface of the oil forming a blanket that seals the oil from the atmosphere, cutting off the oxygen required to sustain combustion and effecting extinguishment. If a little pre-piping and preplanning (remember P5) are done, this procedure can be implemented from a remote location thereby keeping the response personnel out of harms way. The main difficulty encountered by those trying to implement this methodology in the past was the need for an apparatus that could generate and pump foam against the hydraulic head, i.e. back- pressure, generated by the tank contents.
Additional experiments have been tried using foams in which the liquid component is something other than water for use on fires involving water reactive materials such as calcium carbide (CaC2) or metallic alkyls such as triethyl aluminum ( Al(C2H5)3. While the experimental foams have worked they are expensive to use and have not yet been proven practical for general use though some who are involved with rocket fuels have done work in this area.