What is a suppressant?

Wildland Class A fire fighting liquid concentrates consisting of a surfactant, corrosion inhibitor and stabilizers. A brief description of all Class A foams is detailed below:

Foam is made when air is forced into water containing a foam concentrate. The air is forced into an Airtanker load during the drop process [after the load is released] and forms a “foam blanket” after settling on the ground.

One of the primary effects of this foam blanket is to smother and cut off the fire from oxygen and air supply. As the blanket absorbs the heat from the fire, it will slowly break down into a liquid state. The rate of breakdown is in direct proportion to the heat to which it is exposed. The water released from the blanket retains wetting and penetrating qualities and continues to exert a cooling effect.

When applied in adequate quantities foam resists disruption due to wind or heat and to flame attack and is capable of resealing. These properties last for varying periods of time depending on the foam solution mixture and the fire environment.

Unlike long term retardant, once the water evaporates from this “short term retardant” the suppressing action ends.

The enhancing fire fighting ingredients are the surfactant and stabilizer components of this product [water is the active ingredient]. Expansion is another measurable property and is described after the stabilizer description. The following briefly describes how these components function when added to water:

1.1. Surfactant

This component causes water to soak into the fuels.

• Surfactant is a surface-active ingredient that has the ability to decrease the surface tension of water.
• Also known as a wetting agent.
• Surface tension of water is decreased when only a small amount of concentrate [i.e. 0.3%] is added.
• Has a greater ability to penetrate because less run off occurs, as more of the water soaks into the surface of woody material.

1.2. Stabilizer

This chemical increases the stability of the foam [i.e. the life of the bubble].

• Stability of the foam is referred to as the percentage of drain time or the time in minutes for water to drain from the foam bubble.
  Mixtures for aerial application range from:
  • 0.1% [wet/fluid consistency] to
  • 0.3% [dripping consistency] and
  • 0.5% [dry consistency].

1.3. Expansion

Expansion is the ratio of the volume of foam to the volume of solution used to make the foam. Most foam used for wildfire suppression is in the range of 10:1 to 20:1. For example, one gallon of solution can be expanded to yield 10 to 20 gallons of foam

Listed below is a description of foam at various consistencies and their general firefighting uses:

• Dry foam holds its shape, adheres well, and slowly releases the water contained in the bubbles, making it a good insulator. This consistency should be used primarily as an insulating blanket.
  • Example: If ground follow up is going to be delayed and 0.3% [dripping] loads were dropped on the fire, drier consistency foam [0.4%] can be applied to increase the life time of the foam blanket.
    Dry foam has a tendency to be intercepted by the tree canopy and releases water slowly.
• Dripping foam exhibits characteristics intermediate in performance between wet and dry foams. This consistency at approximately 0.3% is considered the most ideal fire fighting foam. This mixture penetrates the tree canopy; adheres to both vertical and horizontal fuels; and forms an insulating blanket that is visible from the air.
• Wet foam is fluid, flows easily and releases water quickly and is a good coolant. Suggested uses for wet foam are as a wetting agent for mop-up with close ground support, and initial drops on a fire to penetrate the canopy and wet the fuels. If ground support is going to be delayed a second drop with a drier foam consistency should be made to seal the blanket.

Short term retardants are usually applied directly to the base of the fire and can easily be termed as suppressants.

When relative humidity is low and fire intensity is high the effectiveness of the foam blanket is short lived.