Selection of Fire Extinguishers Based on Extinguishing Agents

By Mark Conroy

One of the most important factors in selecting the right extinguisher for a specific hazard is in choosing the best extinguishing agent for that application. Every fire extinguisher is effective on at least one type of fire, but certain extinguishers are better on some fires than others. An understanding of each extinguishing agent and why certain agents are a match for specific fire hazards will help you select the most appropriate extinguisher for each hazard area and result in the highest level of safety.

Dry Chemical Extinguishers

• Dry chemical is best suited where quick extinguishment can be accomplished and re-ignition sources are not present. Dry chemical is a powder with additives to improve flow, minimize the possibility of caking, and prevent packing. There are three dry chemical agents used in extinguishers.
• A multi-purpose dry chemical extinguisher is charged with ammonium phosphate base agent and is the only dry chemical that is suitable for Class A, B, and C fires (ABC rated). Almost all buildings have these extinguishers installed throughout for protection of the building and contents.
• An ordinary dry chemical extinguisher is charged with sodium bicarbonate or potassium bicarbonate (more effective) extinguishing agents. These extinguishers are in¬tended for Class B fires and are compatible with energized electrical equipment (BC rated).
• The most effective high flow dry chemical extinguisher for flammable liquid fires is potassium bicarbonate (Purple K) with a discharge rate of at least 1 lb/sec. High flow extinguishers were specifically developed for flowing and pressurized flammable liquid fires. The large amount of agent and high flow rate provides fire extinguishment effectiveness on these difficult fires.

Clean Agent Extinguishers

• Halogenated agents and carbon dioxide are clean agents, meaning they are electrically non-conducting and evaporate quickly, leaving no residue. Halogenated agents fall into two categories: Halon 1211 and halocarbons, such as Halotron I.
• Halon 1211 and Halotron® I extinguishers are typically selected for the protection of delicate electronic equipment, such as copiers, computers, servers, robotics, and telecommunications equipment (smaller extinguishers are BC rated and larger extinguishers ABC rated).
• Carbon Dioxide is a colorless and odorless inert gas that is suitable for Class B and is compatible with energized electrical equipment (BC rated). Carbon dioxide extinguishers are normally installed for the protection of flammable liquids since they don’t contaminate the product.

Foam Extinguishers

• Foam extinguishing agents include aqueous film-forming foam (AFFF) and film-forming fluoroprotein foam (FFFP). Foam is mostly water and can extinguish common combustibles. Although AB rated, foam is intended for flammable liquid spill fires or pool fires.
• Foam extinguishing agent floats on the surface of flammable liquids. Foam spreads across the surface of the flammable liquid and creates a foam blanket, which prevents flammable vapors from reaching oxygen in the air, thereby extinguishing flames and preventing re-ignition.

Wet Chemical

• Wet chemical is specifically intended for cooking oil fires (K rated). A wet chemical extinguisher is charged with an aqueous potassium acetate solution or similar chemical that is alkaline and will chemically react with fats in cooking oil to form foam that smothers the fire.
• Starting in 1998, Class K extinguishers were required for new commercial cooking applications. Then in 2002, existing dry chemical extinguishers for kitchens were to be replaced with Class K extinguishers at the next 6-year maintenance or hydrostatic test. By 2008, dry chemical had been completely phased out for Class K hazards. Today, all cooking appliances using cooking oil are now protected with Class K wet chemical extinguishers.

Water Extinguishers

• Water extinguishers are charged with plain water, but some water extinguishers are also listed for use with loaded stream agent. Water extinguishers are intended for Class A fires only (A rated).
• Starting in 1998, Class K extinguishers were required for new commercial cooking applications. Then in 2002, existing dry chemical extinguishers for kitchens were to be replaced with Class K extinguishers at the next 6-year maintenance or hydrostatic test. By 2008, dry chemical had been completely phased out for Class K hazards. Today, all cooking appliances using cooking oil are now protected with Class K wet chemical extinguishers.

Dry Powder Extinguishers

• A dry powder extinguisher is charged with extinguishing agent made up of powder or granular solids intended only for Class D combustible metal fires (D Rated).
• Dry powder extinguishers that use sodium chloride are effective on fires involving magnesium, sodium, potassium, and sodium-potassium alloys. Heat from a fire causes the extinguishing agent to form a crust, which prevents oxygen in the air from reaching the burning metal.
• Dry powder extinguishers that use a copper extinguishing agent are effective on lithium and lithium alloy fires. Application of the extinguishing agent smothers the fire.

Extinguisher Ratings and Applications

Careful selection of the most appropriate extinguisher and extinguishing agent is critical for many applications. Your knowledge of the extinguishing agents will enhance the safety of your customer’s facilities and help improve your bottom line.