Dry powders use a different extinguishing method than do standard dry chemicals. Instead of breaking the chain reaction to suppress a fire, dry powders smother a fire by forming a barrier between the fuel and the air.
Application of agent to a fire should result in a thick blanket, usually at least 1 inch (25 mm) thick over the entire surface of the fuel. The application of agent must be approximately 10 lb/ft² (4.5Kg/0.9m2 ) of area for most dry powders. The blanket of agent traps heat and may develop a crack that will allow air to pass through allowing the fuel to continue to burn. Anytime the agent blanket is disturbed, more agent must be applied to maintain at least a 1-inch (25 mm) blanket.
It is very important that overhaul of the hazard be delayed until the fuel has had sufficient time to cool below its ignition temperature. This may require 30 minutes or more to help assure that the metal has cooled enough to be removed without re-ignition.
The most common “Class D” dry powders are composed of a sodium chloride base or a special graphite base with additives that enable it to flow freely.