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Foams are a widely used chemical to fight and control wildland fires. But how do they work?
How much should you use? And, when does it become the most effective weapon in your arsenal? How do
foams work? In essence, foams contain surfactants. That is surface active chemical agents that reduce
the surface tension in water. This allows the solution to soak into fuels better than
water alone. Foams have different mix ratios. Lower mix ratios mean thinner foams and faster
drainage times, that is, the wetting ability. These solutions are better for soaking or wetting fuels,
for example, in mop-up operations. One gallon of foam concentrate per 999 gallons of water
is sufficient to lower the water's surface tension. Higher mix ratios mean thicker foams
and slower drain times. The higher mix ratios are good for creating insulating barriers.
For example, in protecting structures or log decks. Remember, as the concentration increases,
so does the expansion causing more foaming and slower drain time that is slower soaking. But
contrary to popular belief, a thick foam is not required for wetting or soaking fuels.
Foams can be used to fight a wild land fire by a direct, or indirect, attack. Let's look at both:
in a direct attack, a foam solution, or wet foam, works best. The foam solution should be applied
directly to the base of the linear flame front. And when you attack the edge, direct a portion
of the foam stream onto any adjacent unburned fuels. On wide hot spots, secure the edges and
move toward the center. Be sure you apply enough foam for extinguishment. As steam becomes visible,
continue to apply the foam solution to wet and cool the fuel. Also leave a foam blanket
over the hot fuels to smother them and continue the wetting process. And don't forget to return
to the treated area after the foam dissipates to check for any missed spots. In an indirect attack
the foam line should be at least two and a half times as wide as the expected flame length. Wet
the entire fuel layer so that the fire will not burn under the foam line. All foam types may be
effectively used for indirect attack, but as the water evaporates the foam loses its effectiveness.
Thus the effectiveness of a foam line in indirect attack is limited by the time it
takes the water to evaporate, and the foam line becomes useless once the water has evaporated.
To learn more about proper foam usage techniques please refer to these websites
In addition to direct, and indirect, attacks foams can be used for prescribed fires,
structure and resource protection, and mop-up operations. Let's briefly look at each. When
other barriers are not available, foams can be used to create a temporary control line during
a prescribed fire, and backfire, until a permanent black line can be built with an ignition device.
Foams used for this purpose should be wet and quick draining. That is, a lower ratio
of foam concentrate to water. Depending on the brand of foam concentrate you use,
start at point two percent to point three percent and adjust accordingly. Then when you apply it,
the width of the foam line should be 1.5 to 2.5 times the expected flame lengths.
Thicker foams can be used to protect specific trees, such as seed trees, snags, and wildlife
trees. They're also used to protect other resources and properties, such as log decks,
wooden structures, and telephone poles. Treat surrounding fuels a distance of 25 to 50 feet
out from the base of the object. In general, the more adverse the burning conditions,
the shorter the time between foam application and ignition should be. The effectiveness and
life of the foam blanket is also dependent on the foam's dryness, and the depth of its application.
At least a half an inch of foam should remain on all surfaces, even if an excess amount runs off.
The penetration of the foam solution can be used soon after flames subside to extinguish the fire
before it burns underground. This also eliminates residual smoke, reduces reburn potential,
and minimizes soil erosion. When you conduct your mop-up operation, avoid applying dry
thick foam. Wet foams penetrate quickly to cool and smother, while dry foams can form a
lid over the fire and trap flame and heat. Also, take extra care when applying foam
to prevent the potential spread of embers from air movement created by the impact of the fire
chemical. Having the right equipment is essential to delivering the foam where and when you want it.
In addition, the equipment you use can influence consistency and structure of the foam.
There are a number of systems to choose from. For example, proportioning equipment adds the
right amount of foam concentrate to your water, batch mixers use manual or automatic regulators,
aspirating nozzles use energy from a water pump to create low, medium, or high expansion foam, while
compressed air foam systems inject compressed air into a foam water mixture. You can contact the
foam companies on the Qualified Products List for specific recommendations of application equipment.
Other fire equipment manufacturers or vendors could identify additional foam dispenser options.
Regardless of what equipment you use, cleanup is essential. Foams are strong detergents
which may dislodge existing rust and scale and possibly clog equipment. So, be sure to flush
internal parts with clean water to help prevent potential clogging of equipment.
Also, your equipment may require lubrication after cleaning. The USDA and US Forest
Service have produced a series of videos that discuss wildland fire chemicals in
more depth. We hope you'll view each one to learn more about these important topics.
You can also find more details on the Wildland Fire Chemical Systems website. Here you'll find
information on safety, environmental issues, product quality, mixing ratios, aerial delivery
systems, and other topics related to knowing when and how to use Wildland Fire Chemicals.