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Suitable Extinguishers for use on Class F Fires - Fires Involving Cooking Oils And Fats
A class F fire is a fire that involves cooking oils or fats. Traditionally this type of fire was covered by the use of a normal foam extinguisher, class B, but due to the high temperatures involved these extinguisher mediums are very in effective.
Flammable liquids will only burn once the flash point has been exceeded. The flash point of any liquid is the lowest temperature at which a liquid hydrocarbon gives off enough vapour to form an explosive mixture with air. Above this temperature the gases will ignite when a ignition source is applied. If the liquid is continued to heat past its flash point it will eventually ignite itself, this is called its auto ignition temperature.
As an example the flash point of petrol is below 21°C, but cooking oils and fats have a flashpoint typically in excess of 340°C and are extremely difficult to extinguish using a convention class B extinguisher.
The use of a non class F extinguisher can be extremely dangerous a poses a real risk of injury. A good example would be using a standard water jet extinguisher on a cooking oil fire will result of an explosion, the water entering the very hot fat turns to steam and results in the burning oil being thrown about and possibly expanding the fire or hurting the extinguisher operator. Normal Foam extinguishers will extinguish the flames but due to the high temperature of the cooking oil destroys the foam blanket very quickly and exposes the oils surface to the atmosphere allowing the flammable vapour to once again mix with the air. Both Co2 And ABC Powder extinguishers will extinguish the flames but because there is no barrier being formed between the oil and the air, the cooking oil will rapidly re-ignite.
Class F extinguishers are typically a "Wet Chemical", "Dry Chemical" or are foam based with special additives. The materials within these extinguishing mediums react with the hot burning oil to create a thick soapy heat resistant crust on top of the cooking oils surface, preventing the flammable vapours reaching the surface. this process is called saponification. The soapy layer is caused by the reaction of the alkalinity extinguishing agent and the burning oil.
Some of the "foam based with special additive" extinguishers work by covering the hot burning oil with a thick heat resistant crust on top of the surface as above whilst at the same time cooling the burning oil by converting the extinguishing water into steam in a controlled manner. The special additives, which are added to the basic AFFF mixture, are based on nitrogenated derivatives and ammonium salts of phosphoric acid.
The make up of the materials used in wet chemical extinguisher media are typically based on alkaline potassium salts of citrate, acetate, lactate or carbonate or mixtures. The potassium helps to quickly knockdown the flame whilst the radical of citrate, acetate or carbonate provides the ingredients to form the soap layer. The wet chemical can be water based or added to AFFF or FFFP to create a saponifying media. The commonly used materials arepotassium citrate or acetate that provide good extinguishing characteristics.
Sodium or potassium bicarbonate powders are the typically used Dry Chemical extinguisher media used. The powder reacts in the same way as wet chemicals to create a soapy layer. The main disadvantage of these this extinguisher medium is that it reduces visibility and causes contamination of the surrounding area.
Fire blankets can be used on class F fires but only ones involving small amounts of oils or fats, typically 3 litres or less. The operator is required position the blanket over the fire and then allowing the oil or fat to cool down.
Larger commercial fat fryers will require an extinguisher or the relevant size or even a fire suppression system to be installed on the fryer itself
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