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Saturday
Sep142013

Foam 104-How Foam Works

      In Foam 102 we learned about the “Foam” tetrahedron and how the elimination of any one (1) side or component will not extinguish or suppress any hazardous vapors (see Foam 102 article from past posts). Properly “Finished Foam” is designed to mitigate both these issues. Finished Foam smothers the fire separating air from the flammable vapors thus removing oxygen from the fire tetrahedron, extinguishing the blaze.

Finished Foam accomplishes three (3) additional functions which eliminate fire and or greatly reduce the evolution of hazardous vapors from the materials surface.

1] Finished Foam suppresses vapors and more importantly “prevents” their subsequent release.

2] Finished Foam will separate flames from the fuels surface. This also prevents production of further vapors to ignite and continue the burning process.

3] Finished Foam cools the fuels surface and adjacent pre-heated metal surfaces that could re-ignite the fire increasing additional damage and risk to nearby responders. This cooling effect comes from the water in the Foam bubble and removes the “heat” side of the fire tetrahedron.

The affect of the various constituents added to varying Foam types determines the “degree” as to which these properties influence, and in what manner, the fire triangle. The three (3) dominating differences that need discussion are the effects of this chemistry on:

1] AFFF

2] FFFP

3] AR-AFFF

These three types commonly see the effect of these additives in terms of their individual performance. Since they were originally developed for the market to “fit” a particular hazard area, the following trade-off of characteristics was deemed acceptable to accomplish the original design goal.

AFFF or Aqueous Film Forming Foam concentrate was primarily designed for the US Navy to achieve the property of a fast “knockdown”. Shipboard firefighting is akin to fighting a fire inside a paint can, extremely hot, fast heating of contact surfaces, and very fast traveling from one area of the ship to another. This Foam was developed for two reasons; to extinguish “quickly” skin fires on air-craft carriers, and these same type of “skin” fires involving land based operations. With this in mind, the fire attack could;

1] Facilitate the rescue of personnel and

2] Reduce the exposure of live ordinances to fire.

As stated in past lessons, the addition of this fluorochemical surfactant afforded fast flow while sacrificing fuel tolerance. This was acceptable as the final goal was “speed” and not the fuels tolerance.

FFFP or Film Forming Fluoro-Protein Foam concentrate, was an attempt to produce a concentrate that was superior in “all” areas of Foam concentrate performance. The result of the additional protein component had a somewhat reverse affect. This is a practical example of how you cannot combine a fast flowing Foam concentrate that also has rigidity. We will discuss this concept more clearly when we take a look at Class “A” Foams. The end result was a Foam concentrate that has “some” protein durability and “some” fluid film formation. It does provide dependable fire protection that simply “averages” these two technologies.

AR-AFFF or Alcohol Resistant-Aqueous Film Forming Foam introduced a Foam concentrate that chemically reacted with the higher content “alcohol” fuels and solvents coming into the market for enhanced performance and environmental sustainability. In this type of Foam concentrate, a polymeric membrane ONLY forms when the concentrate is used on “polar” or alcohol based fuels or solvents. If it is used on “non-polar” fuels this membrane does NOT form, and the Finished Foam FILMS just like regular AFFF. The higher the % of alcohol present, the better the formation of this polymeric membrane. This type of additive will make the initial concentrate more “viscous” at rest but becomes less viscous when mixed, educted, and flowed into Foam solution on its way to becoming finished Foam. These are referred to as “non-Newtonian” or (unlike water) liquids. The increase through mechanical agitation lessens this concentrated “higher viscosity” state.

                          Haz Mat Mike   

 

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