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Purple-K, also known as PKP, is a dry-chemical fire suppression agent used in some dry powder fire extinguishers. It is the most effective dry chemical in fighting class B (flammable liquid) fires, and can be used against some energized electrical equipment fires (USA class C fires). It has about 4–5 times more effectiveness against class B fires than carbon dioxide, and more than twice that of sodium bicarbonate. Some fire extinguishers are capable of operation in temperatures down to −54 °C or up to +49 °C. Dry Chemical Powder works by directly inhibiting the chemical chain reaction which forms one of the four sides of the fire tetrahedron (Heat + Oxygen + Fuel + Chemical Chain Reaction = Fire). To a much smaller degree it also has a smothering effect —by excluding oxygen from the fire.
Purple-K powder has an acrid taste and odor, is free-flowing, floating on most liquids, non-abrasive, does not wet with water and is compatible with most foam concentrates. It has violet color, to distinguish it from other dry agents. Its principal component is potassium bicarbonate (78–82% by weight), with addition of sodium bicarbonate (12–15%), mica (1–3%), Fuller's earth (1–3%), amorphous silica (0.2–%), and is made hydrophobic by methyl hydrogen polysiloxane (0.2–1%).
Purple-K is commonly used in oil refineries, airport ramps, service stations, military facilities, naval warships, power plants, and other places where flammable liquids are handled. It is often paired with foam in twin agent systems, usually found fitted to airport fire appliances.
Purple-K is used in many forms, from small handheld fire extinguishers to large mobile and stationary units, including fixed-nozzle piping systems.
Cleanup of spent agent can be difficult, as it creates a mess when discharged. If the spent agent is dry it can be removed by suction, but when combined with water, hydrocarbons and other liquids, it forms a thick crusty scum that can be challenging to remove.