Dry Powder – Class D Extinguishers

For metallic flammable solids (e.g., phosphorus, sodium, lithium, magnesium) do not use water, foam or carbon dioxide as a fire suppressant. Dousing metallic fires with water may generate hydrogen gas, an extremely dangerous explosion hazard, particularly if fire is in a confined environment.  Additionally, fires involving flammable metals (lithium, sodium, potassium etc.) or flammable metal compounds (butyllithium, diethylzinc etc.) can be fueled by using water or carbon dioxide fire extinguishers. You must have a class D – Dry Powder extinguisher on hand if you are using these materials.

Dry powder extinguishers are available in stored pressure  and cartridge operated units  containing one of several available dry powder agents.  This type of extinguisher is available in 30-pound (13.6 kilo) cartridge operated units.  The agent is also obtainable for manual application with a scoop. Discharge times range from 30 to 60 seconds, and the discharge range is from 6 to 8 feet (1.8 to 2.4 meters).  Dry powder extinguishers are also available in 150 and 350 pound (68 and 159 kilo) wheeled units.  Dry powder extinguishers are suitable only for class D fires and are the only effective  fire control agents for this type of fire. Dry powder agents must be evaluated  in relation to the specific combustible metal that is being protected.   These extinguishers  can be recharged in-house.

dry-powder-catridge-type

dry-powder-catridge-type

Dry powder agents are designed to control fires in combustible metals (class D).   The two most common agents in this category are G-1 and Met-L-X. Dry powders function by creating a crust over the surface of the burning metal. To extinguish the fire, this crust must completely cover the burning surface. These agents are usually applied by hand scoop or portable fire extinguisher. Graphite and sodium chloride are two common examples of these agents. There are several Class D fire extinguisher agents available, some will handle multiple types of metals, others will not.

* Sodium Chloride (Super-D, Met-L-X or METAL.FIRE.XTNGSHR) – contains sodium chloride salt and thermoplastic additive. Plastic melts to form a oxygen-excluding crust over the metal, and the salt dissipates heat. Useful on most metals, magnesium, titanium, aluminium, sodium, potassium, and zirconium.

* Copper based (Copper Powder Navy125S)-developed by the U.S. Navy in the 70s for hard to control lithium and lithium alloy fires. Powder smothers and acts as a heat sink to dissipate heat, but also forms a copper-lithium alloy on the surface which is non-combustible and cuts off the oxygen supply. Will cling to a vertical surface-lithium only.

* Graphite based (G-Plus, G-1, Lith-X, Pyromet or METAL.FIRE.XTNGSHR)-contains dry graphite that smothers burning metals. First type developed, designed for magnesium, works on other metals as well. Unlike sodium chloride powder extinguishers, the graphite powder fire extinguishers can be used on very hot burning metal fires such as lithium, but unlike copper powder extinguishers will not stick to and extinguish flowing or vertical lithium fires. Like copper extinguishers, the graphite powder acts as a heat sink as well as smothering the metal fire.

* Sodium carbonate based (Na-X)-used where stainless steel piping and equipment could be damaged by sodium chloride based agents to control sodium, potassium, and sodium-potassium alloy fires. NA-X agent is recommended wherever stress corrosion of stainless steel must be kept to an absolute minimum. Limited use on other metals. Smothers and forms a crust.

Most Class D extinguishers will have a special low velocity nozzle or discharge wand to gently apply the agent in large volumes to avoid disrupting any finely divided burning materials. Agents are also available in bulk and can be applied with a scoop or shovel.

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