Class A and Class B foams both produce foam. From the standpoint of formulating for performance on these two types of fuels, that is where the similarity ends. In fact, they are diametrically opposed. Clarence Grady is absolutely correct when he points these differences out.

Class B foams are formulated to prevent fuel pickup or fuel emulsification. We want the foam to float on the surface of the liquid fuel and not interact with the fuel layer below it. In contrast, when we formulate a Class A foam, we want it to be very surface-active and interact with the fuel layer below it.

The fact that both foams are capable of producing a foam blanket means that the solutions from which they're produced have low surface tension compared to just plain water. The key to the performance difference comes not in the reduced surface tension but in what we do to control the interfacial tension.

Interfacial tension is the “attractive force” between the bubbles and the substrate that they rest on. For a Class B foam we look to minimize this attractive force, so we formulate to have high interfacial tension. For a Class A foam we look to maximize this attractive force, so we formulate to have low interfacial tension.

We control this interfacial tension through the selection of the type of foaming agents used in the formulations. The types of foaming agents we choose fall into two different categories.

For Class B formulations we use foaming agents that are said to be oleophobic or oil-fearing. The molecules of these foaming agents line up on the bubble wall so that they shed the Class B fuel (oil or gasoline). In contrast, the foaming agents we for Class A formulations are said to be oleophilic or oil-loving. The molecules of these foaming agents line up on the bubble wall so as to attract Class B fuel. Because these bubbles attract fuel rather than shed it, they act like a bunch of miniature wicks. The bubble walls themselves, because they are wicking fuel, can become flammable.

Because of the way foams are formulated, the ability to extinguish and prevent re-ignition of a Class B fuel when using a Class A foam depends on a number of factors. First and foremost is probably how deep the fuel layer is. For spill fires, where the fuel layer depth is an ⅛ inch or less, a Class A foam will probably emulsify the entire layer of fuel. Under this condition, the fire will be extinguished and the fuel will not re-ignite as long as the fuel remains emulsified, assuming that the Class A foam was applied at a high-enough application rate and with enough force to cause the emulsification to occur.

A second very important consideration is the type of Class B fuel. If it's a high vapor-pressure fuel such as gasoline, it will be much more difficult to extinguish with a Class A foam than if it's a lower vapor-pressure fuel such as kerosene or fuel oil. If it is a water-miscible fuel such as alcohol, Class A foam may not work at all.

Another way to minimize the fuel wicking effect of using a Class A foam on a Class B fuel is to control the application method by using medium-expansion foam-generating nozzles or CAFS equipment. Because these methods minimize the amount of foam bubbles in contact with the fuel surface, they minimize the amount of the total foam blanket that is wicking fuel.

Ultimately, we do not recommend the use of Class A foams on Class B fuels. We do, however, recognize that they may be successfully employed if proper consideration is given to type of fuel, fuel geometry, foam solution application rates, proper foam application techniques and proper application hardware.