Fire is a chemical reaction requiring three components: fuel, heat, and an oxidizing agent (typically oxygen), which together form the fire triangle. Successfully suppressing a fire means removing one or more of these elements to stop combustion. Selecting the correct extinguishing agent is paramount for effective suppression and safety, as the choice depends entirely on the material that is burning.
The Standard Classification of Fires
The National Fire Protection Association (NFPA) categorizes fires based on the nature of the fuel source to ensure the correct suppression method is used. This system identifies five distinct classes of fire, each presenting a unique challenge. Class A fires involve ordinary combustible materials, such as wood, paper, cloth, and many common plastics.
Class B fires are fueled by flammable liquids, including gasoline, oil, and paint. Class C fires involve energized electrical equipment, where the hazard lies in the presence of an electrical current. Combustible metals, like magnesium, titanium, and sodium, are the fuel source for Class D fires, which require specialized extinguishing agents. Class K fires are specific to cooking oils and fats, commonly found in commercial kitchens.
Common Fires Water Can Extinguish
Water is an extremely effective and common extinguishing agent, but only for fires involving ordinary solid combustibles, known as Class A fires. These fires include burning items like wood furniture, paper products, textiles, and household rubbish. The power of water lies in its ability to rapidly cool the burning material, which is a process known as quenching.
When water is applied to a Class A fire, it absorbs a tremendous amount of thermal energy through both sensible heating and latent heat of vaporization. This absorption quickly reduces the temperature of the fuel source below its ignition point, effectively removing the heat component of the fire triangle. The transformation of liquid water into steam is highly efficient, requiring approximately 2,260 kilojoules of energy per kilogram of water, which cools the fire and prevents reignition. For deep-seated fires in porous materials like upholstered furniture, water also penetrates the material, cooling internal hot spots that might otherwise cause the fire to smolder and flare up again.
Types of Fires Water Should Never Be Used On
Using water on fires other than Class A can be ineffective and often dangerous, making the situation worse. Class B fires, involving flammable liquids like gasoline or oil, should never be treated with water because water is denser than these fuels. Water sinks beneath the burning liquid, causing the fuel to float and spread across the surface, rapidly expanding the fire.
Water also presents a significant electrical hazard when used on Class C fires involving energized electrical equipment. Since water conducts electricity, applying it to a live circuit creates a path for the current to travel back to the person holding the extinguisher, posing a severe risk of electrocution. The correct procedure is to disconnect the power source first, allowing the fire to be safely extinguished with a non-conductive agent.
Class K fires, involving superheated cooking oils and fats, react violently to water. When water hits grease that is burning at a high temperature, it instantly vaporizes into steam. This rapid expansion violently pushes the burning oil out of the container in a steam explosion, often referred to as a “flashback,” causing the fire to splatter and spread uncontrollably.
Class D fires, involving combustible metals such as magnesium or sodium, are also highly reactive to water. The intense heat of these metal fires can chemically react with the water, stripping the oxygen from the water molecules and producing highly flammable hydrogen gas. This reaction can result in an explosion and intensify the fire, demanding the use of specialized dry powder agents to smother the flames and cut off the oxygen supply.