A fire fueled by chemical substances presents a unique hazard, distinguishing it from fires involving ordinary solid materials like wood or paper. The chemical nature of the fuel dictates the specific approach required to safely extinguish the flames. Understanding the science behind this combustion is paramount because using the wrong suppression method can intensify the fire or create a dangerous explosion. The response must be precisely matched to the material that is burning.
Defining Chemical Fires: The Role of Oxidation
A fire is a chemical process known as combustion, a rapid form of oxidation that releases energy as heat and light. For any fire to occur, four elements must be present simultaneously: fuel, heat, an oxidizing agent (typically oxygen), and a self-sustaining chemical chain reaction. This relationship is often visualized as the fire tetrahedron; removing any single element results in extinguishment.
In an ordinary fire, the fuel is a carbon-based solid, and the oxidation rate is relatively contained. Chemical fires differ because the fuel source is volatile, highly reactive, or self-oxidizing, leading to a much faster and more energetic reaction. The chemical chain reaction is often more vigorous, demanding specialized suppression techniques to break the cycle of combustion. These substances may also contain their own oxidizers, meaning they do not require atmospheric oxygen to burn.
Categorizing Chemical Fire Hazards
Chemical fires are grouped into classifications based on the type of fuel, which determines the hazard and the correct response. While Class A fires involve common combustibles, chemical fire hazards focus primarily on three other categories.
Class B fires involve flammable liquids and gases, such as gasoline, propane, alcohol, or oil-based paints. Because these fuels easily vaporize and mix with air, they ignite readily and spread quickly.
Class C fires involve energized electrical equipment, where the electrical current is the initial hazard. Although the fuel may be insulation or plastic housing, the electrical energy complicates the response by creating a shock risk. Once power is disconnected, a Class C fire typically becomes a Class A fire, but the initial response must address the electrical component.
The third major category is Class D, consisting of combustible metals like magnesium, lithium, potassium, and sodium. These metals burn at extremely high temperatures, often reaching between 2,000 and 4,000 degrees Fahrenheit. The intense heat and highly reactive nature of these elements necessitate a unique suppression strategy.
Why Standard Extinguishing Methods Fail
Using water to cool a fire is ineffective and often hazardous when dealing with chemical fires. For Class B fires involving flammable liquids like oil or gasoline, water is denser than the fuel and sinks below the burning surface. This causes the lighter, burning liquid to float on top of the water, spreading the fire across a wider area.
Applying water to a Class C fire involving energized electrical equipment creates a severe electrocution hazard. Because ordinary water conducts electricity, the person applying the water can complete the circuit, suffering a potentially fatal electrical shock. Even foam, which contains water, is unsafe until the electrical current has been de-energized.
The most violent reaction occurs when water is applied to a Class D fire involving combustible metals. The intense heat causes the water (\(\text{H}_2\text{O}\)) to dissociate, or split, into its constituent elements: hydrogen and oxygen. The released oxygen fuels the fire, while the highly flammable hydrogen gas often results in an explosive reaction, making the fire significantly worse.
Selecting the Correct Suppression Agent
The correct approach involves selecting an extinguishing agent designed to interrupt the fire tetrahedron based on the fuel class. For Class B fires, suppression agents like fire-fighting foam create a physical barrier or blanket over the flammable liquid. This blanket separates the fuel from atmospheric oxygen, effectively smothering the fire and preventing the release of flammable vapors.
Carbon dioxide (\(\text{CO}_2\)) extinguishers are effective for both Class B and Class C fires because the agent is non-conductive and displaces oxygen, removing the oxidizer from the tetrahedron. Dry chemical agents, which contain substances like sodium or potassium bicarbonate, are also used on Class B and C hazards. These chemicals work by directly interfering with the fire’s chemical chain reaction, causing combustion to cease.
For Class D combustible metal fires, specialized dry powder agents must be used, as they are the only safe and effective solution. These agents, which may include materials like graphite or sodium chloride, are applied to smother the burning metal and absorb the heat. The powder forms a crust over the molten metal, cutting off the oxygen supply and cooling the fuel below its ignition temperature.