Foam is typically the fire suppression agent of choice in situations where combustible or flammable liquid is stored in tanks or bulk storage facilities. It is especially effective when the flammable liquid has a surface where the foam can be applied. Unlike water, which is heavier than most flammable liquids and will sink ineffectively to the bottom, fire suppressing foam is lighter and will rise to the top. There, it creates a blanket on the surface, reducing vapor formation, preventing oxygen from reaching the combustible liquid and effectively smothering the potential fire.
High expansion foam can also be used to extinguish a fire in a contained space. When used in a railway tunnel, aircraft hangar or other defined space, foam is effective at quickly filling an area and smothering the flames. Finally, foam also has a cooling effect like water as the foam solution consists mostly of water (~ 97%).
Another way to categorize fire protection foam is by rate of expansion. Foam is produced by mixing foam concentrate with water to create a foam solution. This solution then passes through a discharge device (such as a nozzle) which introduces air into the mixture and greatly increases the volume of the finished foam.Low-expansion foam describes foam that expands in volume 2-20 times from water density to finished foam. This is typically used where the object is to create a fire-suppressing film on the surface of a flammable liquid.
Medium (20-200x expansion) and high expansion foams (200-1000x expansion) are typically used to quickly fill a large volume of confined space, such as a basement, mine tunnel, or aircraft hangar.
Each type of foam and expansion rate calls for different equipment and/or different settings on the equipment. Designers and maintenance personnel should check with foam and equipment manufacturers to ensure proper use and compatibility.