Modern fire fighting technology has afforded us with a wide array of specialised fire extinguishing agents, and a diverse array of fire extinguishers are available to tackle every type of fire. One of the most specialised types of extinguishing agent is the group of dry powder extinguishers collectively known as class D extinguishers—these are intended to fight fires caused by various flammable metals, which are difficult or even dangerous to fight with less sophisticated agents such as water or foam.
As you can imagine, class D extinguishers are an integral part of a fire safety plan for laboratories and industrial applications, but even the amateur mechanic or metalworker can benefit from having one of these extinguishers around, especially if they work with flammable metals (such as magnesium and lithium) which are commonly used in mechanical components. However, choosing the right class D extinguisher for your needs is essential, as a number of powder types are available, each with differing applications and specialising in tackling different types of fire.
Sodium chloride extinguishers
These extinguishers contain sodium chloride salt crystals not unlike ordinary table salt—however, when applied to a metal fire the salt cakes together, forming an airtight, fire extinguishing crust over the burning materials.
The great advantage of sodium chloride extinguishers is their versatility, and the salt crystals can effectively tackle burning magnesium, potassium and aluminium, as well as more exotic flammable metals used in heavy industry such as titanium and even uranium. Sodium chloride is particularly effective at extinguishing fires caused by swarf, the fine powder of aluminium filings produced when drilling or milling aluminium, making it particularly useful for car body shops.
However, these extinguishers can do more harm than good if used without care and attention. Steel and other metals vulnerable to rust can be heavily damaged by the rust-promoting properties of the salt, and after a fire has been extinguished a thorough clean of the area will be required to minimise salt damage. Sodium chloride extinguishers are also unsuitable for using on other types of fire, as they contains special thermoplastic bonding materials which can themselves catch fire when not used on metal.
Sodium carbonate extinguishers
Changing one word can make a whole world of different, and sodium carbonate does not provoke rust like sodium carbonate does. As such these extinguishers are particularly useful for applications where metal damage should be avoided at all costs, and are widely used in steel-heavy industries. They function in largely the same was as sodium chloride extinguishers, forming a crust over the fire to deprive it of oxygen. Sodium carbonate can also be used to extinguish flammable liquids, although they are not as effective as foam or carbon dioxide and can be difficult to apply safely.
However, losing the metal damaging properties of sodium chloride also means losing its versatility with metal fires, and carbonate can only be used to fight sodium and potassium fires.
These extinguishers contain a fine graphite powder that smothers fires in the same way as other class D materials. However, graphite powder is also a terrific heat conductor, and when applied to a fire it acts as an effective heat sink, reducing heat damage to surrounding objects and structures. This also allows graphite to be used on most metal fires, as it has high enough temperature tolerances to deal with even hot-burning metals such as zirconium.
Unfortunately, graphite powder can be tremendously difficult to clear up once the danger of a fire has passed—more worryingly, the fine powder can easily find its way into the moving parts of equipment and machinery, potentially causing damage and accelerated wear to expensive equipment.
For more information, contact companies like The Fire Protection Specialist Company Pty Ltd.