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Focus on Hazmat
Foams, froth and concentrates
Volume 24 Number3

Foam, like many other innovations within the realm of firefighting paraphernalia, came about in response to a perceived need. In this case it was the need for a method of extinguishing liquid hydrocarbon fires which in turn were caused by the need to store large quantities of motor fuel in the community, and this was engendered by the advent of the motor car and its internal combustion engine.

To have a fire one must have the "Fire Triangle" -- heat, fuel and oxygen (or an equivalent oxidizer). Remove any one of these and out goes the fire. Foam can eliminate all of these; it can separate the fire from the fuel, it can shut out atmospheric oxygen and it is an excellent heat shield.

Foam is, in the final analysis, a fairly simple creation. It consists of water, a wetting agent and air. The earliest commercially produced foams were exactly that, soap suds. These early foams were supplied as two powders ("A" and "B," naturally) one was sodium bicarbonate (NaHCO3) or baking soda and powdered soap (Grandma's famous lye type). In some formulations, sodium carbonate (Na2CO3 ) or washing soda was employed instead of the baking soda. The other powder was alum or hydrated aluminum potassium sulfate (KAl(SO4)2.12H2O) though the sodium salt can also be used as can aluminum sulfate (AL2 (SO4) 3 which is commonly, though incorrectly, referred to as "alum." Saponin and licorice were often included to stabilize the bubbles.

In practice the two powders were fed into an in-line mixing device similar in principle and function to the present day in-line eductors used for liquid foam concentrates. As the powders were mixed with water, the chemical reaction set in with CO2 being liberated and the soap or other wetting agent being dissolved. As the fluid moved down the hose line the constituents mixed and the liberated gas became trapped in the fluid much as does the gas in a cold drink can. When the fluid arrived at the nozzle, the confining pressure was released and the entrained gas began to expand on the way to the fire and the stream struck the flames as finished foam rather than as a liquid.

This foam, introduced during the final decade of the 19th century was simple to make and cheap to produce; alum is a common water treating reagent, soda, washing or baking, is a common industrial material and soap was a household commodity. The main problem with this type of foam was that it was a hygroscopic powder and such powders tend to clump during storage, especially in damp climates. No one was ever able to come up with a thoroughly reliable automated system for using these foams to protect large installations such as refineries or tank yards; even after the foam was reformulated into a single powder. The problem was not in the efficacy of the foam but limitations of mechanical reliability. As a result, in the 1940s these foams were supplanted by the more dependable pump and pipe systems that used liquid foam concentrates. Soaps and detergents can make very effective fire fighting foams. One of the best high expansion concentrates around for use on hydrocarbon fires is a commercial dishwashing formulation.

In the 1940s Percy Lavon Julian developed a foam concentrate based on protein hydrolysates, originally from soy but later employing animal proteins derived from slaughter house wastes. This foam was stabilized by the addition of formaldehyde and/or a high molecular weight glycol. It was messy, it had a horrific odor, especially in the case of the versions derived from slaughterhouse by products but it did and does work for general use on hydrocarbon fires as well as Class "B" fires.

This was the first in a line of protein based foams; it was followed in the early 1960s by fluoroprotein and shortly thereafter by aqueous film-forming foam (AFFF).

The main problem with the protein based foam agents is that of "protein denaturation" by non-polar solvents such as alcohols. The solution of this problem was the addition of metallic stearates to the foam concentrates. The result was AR (Alcohol Resistant) or ATC (Alcohol Type Concentrate) and later by the AR AFF concentrates though, in most cases, this latter derives its resistance to non-polar substances through means other than the presence of metallic stearates.

The mechanisms by which the two types of foams are formed differ. Chemical foams depended on the reduction of the surface tension of water by the wetting agent and the production of carbon dioxide by the reaction of the sodium bicarbonate with the alum (an acidic salt). The gas evolved then aerates the soap solution to produce the finished foam. The liquid concentrates, on the other hand, produce foam by being mixed with atmospheric air in the same way as egg whites are foamed to make meringue dessert toppings and depend on the formation of long chain polymeric molecules, usually proteinaceous in nature, to produce bubbles. The foam produced is simply the result of entraining air in the mixture of concentrate and water and agitating the mixture as it exits the nozzle. There is no chemical reaction analogous to that found in the formation of chemical foams.

The aerated foams are usually heavier than the chemical foams and the blankets they produce tend to last longer. This is due to the entwining of the polymeric chains to form a mat-like matrix over the surface of the burning liquid. This property has been carried to the extreme in the case of the AFFF concentrates wherein the polymer chains form an actual membrane over the liquid surface, effectively isolating the fuel from oxygen. It also prevents, or at least reduces, incidents of "flashback" or rekindle.

Foams are chemical substances but their action is mechanical. They act, as mentioned previously, by isolating the fuel from the oxidizer (normally the atmosphere) and they absorb heat, thus reducing the rate of vaporization of fuel. How they do this is of less importance than that it is accomplished. "Foams are Foams," and once the foam is generated and applied to the surface of the burning liquid if it forms a blanket which puts out the fire, absorbs heat and isolates the fuel supply it really makes no difference whether the foam is protein, AR, AFFF or even chemical. The mode of application also makes very little difference; high expansion, subsurface injection, in-line aspirated or compressed air foam once they are on the surface of a burning liquid will, if present in sufficient quantities and in the absence of an adverse reaction with the fuel, effect extinguishment.

The problem is of course getting the foam on the fire and forming and maintaining a blanket once it gets there. Foam is a very light material and propelling it over any significant distance is difficult. The problem is similar to that encountered when one tries to throw a "nerf" ball around the house. These balls are so light and the mass is so dispersed that it is almost impossible to throw them hard enough to do any significant damage. This is explained in the equation F = MA, where F stands for force, M denotes mass and A is acceleration. If M is very small then A must be extremely large in order to generate an "F" large enough to carry a foam any significant distance, then too, the surface area of finished foam is so great that wind conditions can sometimes render it ineffective, especially high expansion foams.

The solution to this problem is to have, in conventional foam systems, the foam solution exit the nozzle as a liquid containing entrained air under pressure. As this fluid leaves the nozzle, its mass (M) is concentrated enough so that a moderate pressure (F) can accelerate it to a degree which enables it to span a considerable distance. This is exactly what happens when one squirts a stream from a beverage can. The beverage leaves the container as a liquid, and as it flies through the air toward the objective, the entrained gas expands so that by the time the stream hits the target the liquid has been transformed into foam. Since this expansion takes place within the generator before the finished foam is ejected, high expansion foams have almost no range and must be applied directly onto the fire. This can be demonstrated even more graphically by means of an aerosol container of shaving cream. When the valve is opened the liquid with its entrained and/or dissolved gas is released through the small hole in the nozzle. Immediately upon escaping from the confines of the dispenser the stream expands into a foam which is much larger in dimension than the opening in the package.

Subsurface injection systems offer another illustration of this. When the foam solution (Water, Air and Foam Concentrate) is injected at the bottom of a tank of fluid, it is a liquid and the hydrostatic pressure of the tank contents tends to keep it in that state. However, as it begins to rise toward the surface this pressure decreases and the entrained air begins to expand. By the time the solution reaches the surface, it is a foam and will spread over the burning liquid effecting extinguishment. A transparent demonstration tank reveals an elongated cone of foam starting at the injection nozzle and expanding as it rises until it emerges at the surface as an expanding blanket of foam.

Foams have always done what they were meant to do; namely put out fires on burning liquids. The early chemical foams did this very well so long as there were no mechanical problems with stoppages due to caking. The regular protein foams worked equally well. The problem arose when new developments in chemistry introduced large quantities of non-polar solvents into the market place. This problem has recently been exacerbated by the introduction of ethanol (ethyl alcohol C2H5OH) into motor fuel. Due to the denaturation of the proteins in their formulations, the older foams just do not work with these fuels. The answer to this need was, of course, found in the various "alcohol type" concentrates and the AR AFFF foams currently in use. Given that research for new products continues, it is not unlikely that we will see the introduction of new materials into the market place and into the transportation stream of general commerce that will in turn require the development of new types of foams for use in combating fires involving these materials.

It is not impossible that at some point there will be a return of the chemical foam concept. These foams had two characteristics which are lacking in the air aspirated foams which now predominate the field. In the first place, there was no protein in the chemical foams, hence there would be no denaturation of protein and consequent degradation of the foam. Secondly, the foam would be blown with carbon dioxide, a non-flammable gas which itself finds use as a fire suppressant, rather than air which, of course, contains oxygen. This would make such a foam more effective on fires involving hypergolic or pyrophoric materials such as aluminum alkyls or ethylene oxide.

Foams have another property which, until fairly recently, has generally been overlooked; they expand available water. If one has a tanker containing 1,000 gallons of water, he can sustain a stream of 100 gallons per minute for only ten minutes. But if he expands that water 100 times by making it into foam, he multiplies his fire suppression capability at least 100 times and perhaps more since the foam will stay put rather than running off or being absorbed by the substrate. This has a decided impact on tanker supplied operations especially in the case of wild land fires or those occurring in suburban areas where development has stretched ahead of fire main construction thus requiring tanker supplied operations. Foam wet lines which can quickly be established from moving apparatus have proven very effective in controlling and combating wildland fires.

Foams have also proven effective for inerting confined spaces such as overturned tank trailers or other containment or process vessels. Foam will stay put in these applications in spite of any small cracks or fissures that may be present in the vessel. Also foam is visible, viewers can tell where it is and is not. When carbon dioxide is used as an inerting agent, there is no indication of its location, how full the space in question actually is or the concentration of the inerting agent present. Not so with foam. We can see it and thus determine just how much of the void has been filled. We can be sure that our inerting agent has not leaked away or been diluted due to the action of gaseous diffusion. Also, determine when additional foam should be added, advantages that should not go unnoticed.

Fire fighting foam has evolved in parallel with evolving needs; from the simple soap suds that extinguished the early gasoline fires to the complex chemical matrices that smother giant tank fires involving a variety of chemicals. Whatever new materials may emerge from the laboratories of our ingenious chemists, it is reasonable to expect that other equally talented researchers will devise an equally efficacious foam to control them.

 
 

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