There are several types of “dry powder” fire extinguisher, also known as dry chemical extinguishers, that are filled with powder and pressurised with nitrogen.
The effect of applying the powder is to "knock down" the flames which cause the powder to expand and congeal to create a barrier layer between the fire and the oxygen in the air - effectively smothering the flames.
Powder extinguishers are normally described by the Class of fire they are designed to control.
BC - The standard type of dry powder fire extinguisher, which is filled with sodium bicarbonate or potassium bicarbonate was initially developed for liquid fires (Class B) involving burning petrol, solvents, oils etc. and Class C fires involving flammable Gases. They have generally been superseded by the more versatile ABC powder extinguishers. Like all powder extinguishers, after use they leave a residue which may cause damage to sensitive electrical / electronic equipment.
ABC - This is the most used and most popular multipurpose powder fire extinguisher and as the name implies can be used on class A (burning solids), B (liquid fires) and C (Gases) fires, and is filled with mono-ammonium phosphate powder. The powder also leaves a residue that could cause damage to electrical equipment. (Use CO2 extinguishers for electrical fires).
It is advisable that powder extinguishers are not used in small, confined spaces, due to the reduction in visability that can be caused and respiratory irritation.
Although extremely versatile ABC powder fire extinguishers are not a "catch all" solution. Importantly they are not effective on cooking oil and fat fires. A special Wet Chemical extinguisher has been introduced specifically for this fire risk (Class F) for use in commercial kitchens and food processing areas. ABC dry powder does not extinguish metal fires where the specialist L2 and M28 extinguishers should be used.
Monnex Powder is a superior Dry Powder based on a potassium bicarbonate-urea complex suppressant specifically developed for Class B & C liquid and gas fires. Used for its superior knock down performance it is intended for fuel stores, garages, motor racing events, airports and any similar location where flammable liquids and gases are frequently used or stored. It is more expensive than standard ABC powder equivalents but its superior performance can prove invaluable in these particularly hazardous environments.
M28 & L2 Powder
These special powder extinguishers are designed specifically to fight combustible metal fires (Class D), such as sodium, magnesium and aluminium typically in the form of machined swarf or powder. The M28 powder is not effective on burning Lithium. L2 Powder is also designed for Class D metal fires but unlike M28, is also effective on Lithium fires. These types of extinguishers are equipped with a special low velocity applicator to ensure the powder is applied gently and efficiently without risk of the discharge pressure spreading or splashing the burning metal.
In summary Dry Powder Fire Extinguishers offer significant protection over a range of fire risks but care should be taken to ensure you choose the correct type for your application and to be aware of their limitations.