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Wet Chemical Fire Extinguishers

How do fight burning cooking oil and fat fires? This is important for all restaurants and fish and chip shops, although most households also have small deep fat fryers. Burning fat or oil in pans or any deeper container are difficult to tackle as they only start burning once the entire content has reached the combustion temperature where no source of ignition is required. That means that the oil/fat will re-ignite, even if you temporarily extinguish the flames, as you would with a CO2 fire extinguisher. The burning fat can also not be extinguished with water or foams of the conventional type as the water droplets sink under the surface and ‘explode’ due to the extremely rapid expansion of the water into steam. This ‘explosion’ carries burning oil out of the container and can injure as well as start secondary fires.

Due to these restrictions of traditional extinguishers, a different fire extinguisher was developed. They are called wet chemical fire extinguishers. They produce a light foam that rests on the surface and spreads itself over it until the entire surface is covered by foam, cutting off the oxygen supply. The chemicals also react with the liquid to inhibit the chemical reaction between the liquid and oxygen. This process is called saponification. The wet chemical is usually deployed with a long lance to avoid a spray jet hitting the burning surface, carrying off burning oil. Wet chemical extinguishers also have a cooling effect. However, care must be taken, as even though the flames will have gone the oil/fat might still be hot enough to self-ignite should oxygen reach the surface. It is therefore important to remove or switch off the source of heat as soon as possible.

Wet Chemical fire extinguishers are marked by a yellow label on the red extinguisher. They also have their own classification called ‘F’-rating. The higher the ‘F’- rating, the more burning oil or fat can be extinguished. For example a wet chemical fire extinguisher with a 40F rating can fight a fat/oil fire with 0.06 square meter surface while a 75F wet chemical fire extinguisher can fight a fire with 0.11 square meter surface. Larger surfaces will require at least 2x 75F fire extinguishers.


Reviewed: 01/02/2020 (doc:500 V1.0). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.


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