Foam's effectiveness is directly related to its expansion ratio and surface tension. The expansion ratio is the volumetric ratio of the foam to the water used to create it. A higher expansion ratio means the foam can cover a larger area with a smaller amount of water. Effervescence in this context refers to the rapid expansion of the foam, which is essential for it to quickly and effectively blanket the fire. The foam's surface tension is a critical property because it allows the foam to spread evenly and rapidly across the burning oil surface. A lower surface tension enables the foam to flow and cover a large area efficiently, preventing oxygen from reaching the fuel. The foam works in three primary ways:

1. Smothering: It forms a blanket that separates the fuel (oil) from the oxygen in the air.
2. Cooling: The water content of the foam absorbs heat from the fire, converting to steam and providing a cooling effect.
3. Radiation Shielding: The foam blanket provides a barrier that prevents radiant heat from the flames from reaching and further heating the fuel source.

The choice between low and high expansion foam depends on the type and location of the fire.

Low Expansion Foam

Low expansion foam typically has an expansion ratio of up to 12:1. It is best suited for localized fires where the burning oil is contained within a horizontal surface, such as a save-all or a confined area. Its dense, heavy nature allows it to effectively smother fires on flat surfaces. It is not effective against fires originating higher up in a space, such as from a burst fuel line, as it cannot reach these elevated sources of ignition.

High Expansion Foam

High expansion foam has a much higher expansion ratio, often up to 1000:1. This foam is light and voluminous, making it ideal for filling an entire compartment, such as an engine room or pump room. This capability makes it highly effective against major conflagrations where the fire may not be confined to a horizontal plane. The foam is typically discharged from overhead ducts, filling the space from the top down, which allows it to reach and extinguish fires at all levels, including those originating from elevated fuel lines and hot surfaces.

In the absence of foam appliances, water jets can be used on oil fires, but with caution and a specific technique. The key is to use a fine water spray rather than a solid jet.

• Cooling and Smothering: The fine water droplets cool the burning vapors by absorbing heat and converting to steam. The steam produced also has a smothering effect. This technique is especially important for oils with low flash points, such as crude oil or gasoline.
• Preventing Spluttering: A solid jet of water would be counterproductive, as the large water droplets would sink into the hot oil. The rapid conversion of water to steam would cause the oil to splutter and possibly spread the fire. The fine spray, however, cools the vapor before it can ignite.
• Cooling Hot Surfaces: Water spray can also be used to cool surrounding hot metal surfaces, preventing the re-ignition of flammable vapors.