Frying tonight – the ‘f’ class fire classification

Over 10 years ago a new classification of fire was introduced for fires involving cooking oils, however there is still some ignorance around the need to have specialised equipment to safely tackle fires in this category and many establishments remain unprotected against this dangerous fire risk

Classifications

For fire fighting purposes fires have been classified, first under BD 4547 and now EN2, according to the main type of fuel involved.

Class A – fires involving flammable solids

Class B – fires involving flammable liquids and liquefiable solids

Class C – fires involving flammable gases

Class D – fires involving flammable metals

Class F – fires involving cooking oils and fats

There used to be a Class E for electrical fires, but this was discontinued as electricity doesn’t burn – an electrical fire can involve any of the above classes, with the presence of electricity being an additional hazard rather than a fuel. Fires where electricity are present are still categorised separately on extinguisher labels so that a non-conductive agent that is safe for use where a live electrical supply is involved can be identified.

Cooking oils and fats were traditionally part of Class B, along with substances like petrol and spirits. However a major difference in how they burn means that normal agents for Class B fires are ineffective. Most flammable liquids, such as petrol or heptane (which is used for fire test rating of Class B extinguishers), burn around 50 degrees Centigrade. However, cooking oil fires commonly start when the oil is heated to past it’s auto-ignition temperature, usually between 285 & 385 degrees Centigrade, but sometimes up to 499 degrees Centigrade. The oil burns at this temperature, but it’s auto-ignition temperature is reduced (by this burning) by about 30 degrees Centigrade. Thus, the fire will be self sustaining unless it’s temperature is significantly reduced. This high temperature makes the fires very dangerous and presents problems when trying to use normal extinguishing methods on these fires. A tble at the end of this article summarises the problems with traditional agents.

Case studies

The following case studies illustrate the problem when using traditional agents.

1. South Mimms Service Area – A fast food restaurant’s fryer caught fire and was immediately tackled with CO2 extinguishers. These had no effect and the fire spread to the ductwork. The entire service area burnt down as a result.
2. Heathrow Terminal One – A fast food restaurant’s fryer caught fire and was immediately tackled with CO2 extinguishers. These had no effect and the fire spread to the ductwork. The terminal was severely damaged. Millions of pounds were lost through damage and disruption.
3. Royals Shopping Centre – A fryer caught fire in one of the units and was immediately tckled with two fire blankets. One was too small, the other burnt through. Despite the rapid attendance of the fire service and the use of a fixed BC Powder extinguishing system the fire was only just contained with great difficulty.

Following lobbying by the fire industry, Class F, a new fire classification, was introduced for all fires involving cooking oils.

A new symbol for use on extinguishers, depicting a burning frying pan with the letter F in the top right corner, was also introduced.

The only type of extinguisher to be Class F rated is the Wet Chemical extinguisher. This contains an alkaline liquid solution of up to 20% potassium salts (potassium acetate, potassium citrate, potassium carbonate) and it is the unique effect of these salts on the fats in cooking oils that is the key to their effectiveness.

Wet Chemical saponifies the oil, i.e. by hydrolysis rapidly converts the burning substance to a non-combustible soap. This process is endothermic, meaning it absorbs thermal energy from its surroundings, decreasing the temperature and eliminating the fire. The soapy scum formed also secures vapours and generates steam, assisting the extinction further.

Even a raging fryer with 75 litres of oil can be extinguished in under 2 seconds, however the entire extinguisher must be discharged onto the oil to ensure a complete crusting of the surface and maximise cooling.

Potassium salts had been used in the US for many years in the form of ‘loaded stream’ extinguishers for greater effectiveness on Class A fires and it was the US who developed the first Wet Chemical extinguishers for cooking oil fires in the 1990’s

Chubb Fire first introduced US made Badger Wet Chemical extinguishers to the UK in the late 1990’s, followed by the UK base of the US firm Amerex and currently there are several manufactures of these extinguishers around the world.

In the UK Wet Chemical extinguishers are made to BS 7937: 2000 and are identifiable by a Canary Yellow panel to the front of the extinguisher and the class F symbol. They bear a Fire Rating based on the maximum size, in litres of oil, the extinguisher can extinguished when used by a skilled operator. A typical 6 or 9 litre extinguisher would be rated 13A:75F (the water base makes it suitable for Class A fires as well)

Because they discharge as a fine mist they pass the 35kv conductivity test for extinguishers and can be used in the presence of energised electrical equipment, although care needs to be taken with pools of agent on the floor.

The cost of the agent makes them slightly dearer than traditional extinguishers and to make them available to smaller establishments are produced in a range of sizes with ratings from 25F to 75F. The German manufacturer Total has a compact 2 litre model with an impressive 40F rating.

Comparison of agents

This is a guest post from Anthony Buck of http://sm-ms.co.uk

Anthony’s knowledge of UK fire extinguishers and the Regulations is unsurpassed