What they are

The fire retardants are chemical compounds that significantly increase the ignition point of the fuels treated with them to reach temperature thresholds above normal. These limits vary depending on the type of additive used.

Composition and how they act

There are several types of retardant additives, which can be classified into:

- Short-term retardants (foams, viscosifiers, gels, etc.). The short-term retardants works by slowing the evaporation of water. Under average summer´s evaporation conditions, 2/3 of the fixed water subsist for 30 minutes. After one hour, the remaining water would only be 1/3 of the total amount. Therefore, the average duration of the short-term retardant will be only 30 minutes.

- Long-term retardants (ammonium salts, such as ammonium sulphate, ammonium phosphate, pyrophosphates and polyphosphates, included within the set of agricultural fertilizers). They function as the short-term retardants, in the first stage of application, until the water evaporates. When this disappears and they are subjected to the action of heat, they decompose by releasing ammonia (NH3) which has a marked reductive character, hindering combustion which is the opposite process (oxidation), at the same time as they leave a coating, making difficult the entrance of oxygen.

Application mixtures

Long-term fire retardants are diluted in water in a ratio of 1/4 to 1/5 (parts of concentrate / parts of water by volume).

The mixtures are prepared in mixing and loading stations, located in the airstrips from which the ground filled fixed-wing aircrafts operate. These charging stations basically consist of a low-pressure centrifugal pump with high capacity (approximately 1,500 l/min.) whose function is to suck up water and retardant while impelling the mixture to the tank of the aircraft.

To adjust the retardant dose on the water flow, the station has a calibrated retardant tank that allows to calculate the correct amount of product applied.

The final blend ready for application has a density of 1.08 to 1.10 g/cm3.

Aerial Application

- In direct attack: The delivering of retardant should be done directly ahead and parallel to the fire edge, ensuring that a third of the width of the drop falls on the flames, with the objective to lower the intensity of the flames and leave impregnated with retardant vegetation immediately ahead.

- In indirect attack: Creation of a firewall at a certain distance from the fire front whose situation on the ground will depend on the established control line. Aircraft drops will be carried out immediately one after the other, so a pilot can see the result of the discharge and support or correct any defects. Drops will overlap with an overlapping index of 25%.

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