Class D fire extinguishers are a specific, specialist type of fire-suppressant derived from the dry powder extinguishers used on Class A, B and C fires, used solely for the purpose of extinguishing Class D fires. These fires are caused by burning metals that combust on reaction with the air. Due to the very specific nature of this type of fire, the portable extinguisher used to tackle them needs to be very specialist too.
Class D fires are caused by processes involving various metals in manufacturing or scientific laboratories, such as cutting, drilling or mining, and often only a very small amount of metal dust (or swarf) can cause a combustion. Industries where this type of fire is a significant risk have to take special precautions so that the risk of fire is limited. The types of metals that can be involved in Class D fires are sodium, lithium, magnesium, potassium, steel and others. Magnesium and titanium fires in particular are more common than would be thought as they are often used in the batteries of computers. When a combustible metal ignites, it can easily spread to other combustible materials around it.Aside from those metals that combust with air or water, combustible metals are not a large fire risk in larger pieces as they are generally unable to maintain the heat of a combustion, so a lot of heat is required to maintain the fire. Usually in the case of a fire involving a laptop battery, for example, the combustible metal starts the fire which is spread and maintained by the other combustible materials around the starting point.
What is a Class D fire extinguisher made of?
A Class D extinguisher has a specialist chemical makeup and application as well as that of its fire fighting capabilities. The extinguishing agent absorbs the heat and smothers the flames, which restricts the oxygen, thus putting out the fire. Like ordinary dry powder extinguishers, a Class D fire extinguisher is red in colour with a band of blue located in the top half of the canister which covers no more than 5% of the body. This kind of specialist extinguisher will have some kind of indication on the label that it is designed for Class D fires so it can be identifed.
These extinguishers will also be equipped a long hose and usually a rounded lance style applicator to allow the operator to keep well back from the fire and allows for easy application of the chemical powder. The applicator is designed so that the chemical will be applied at a low velocity, so it will land efficiently and effectively onto the fire, preventing any swarf from spreading. The canisters of a Class D extinguisher usually come in a 9kg size, which makes them heavier than some but effective for the type of fire to be dealt with.
Different compositions of the chemical within the canister will have limited effect on the composition of certain fires, for example a copper powder variety will have limited effect on a lithium fire. For this reason, the exact specification of the dry powder must be considered when a Class D fire extinguisher is placed. The extinguishing agent, when released, can affect the health of a user or individual in the vicinity, so a Class D fire extinguisher should never be used in a confined space. Due to the nature of this extinguisher, those located near one should be suitably trained in using this specialist piece of portable fire-fighting equipment.