What is Class F?

Class "F" fires are fires involving cooking oils or fats, they differ from conventional fires due to the very high temperatures involved. In order for any flammable liquid to burn the temperature must exceed the flash point Above this temperature the liquid will ignite when an ignition source is applied.

For a flammable liquid to spontaneously ignite the auto ignition temperature need to be reached. Typical flammable liquids e.g petrol have a low flash and auto ignition temperatures and are relatively easy to extinguish.

Cooking oil or fat fires have auto ignition temperatures in excess of 340°C and are very difficult to extinguish using conventional extinguishers having a class B capability The industry recognised the difficulties and inadequacies of conventional class B extinguishers and therefore created a new standard BS7937:2000 to cover the special risks involved.

To extinguish a fire created by auto ignition the flames must be extinguished and the temperature of the burning liquid reduced below the auto ignition temperature. The amount of heat involved with the liquid above 340°C is high and the use of the incorrect extinguisher can be extremely dangerous, for example a water jet extinguisher directed at the surface of burning cooking oil will create an explosion as the water is rapidly converted into steam resulting in the expulsion of burning oil possibly spreading the fire and harming the operator.

Conventional foam extinguishers have been proven to extinguish the flame, but the heat involved rapidly destroys the foam blanket, exposing the surface of the oil, allowing re-ignition. Carbon dioxide and ABC powder extinguishers are effective at extinguishing the flame but without sealing the surface of the liquid from the oxygen allowing the oil to rapidly re-ignite. The best method of extinction would be a Class F fire extinguisher.