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Fire Extinguishment Theory & Classes of Fire (A, B, C, D, K)
The four ways to put a fire out, why water works, and how to match the agent to the class of fire.
- Extinguishment means removing a side of the tetrahedron: heat, oxygen, fuel or the chain reaction.
- Water works mainly by absorbing heat; you need enough flow to beat the fire's heat output.
- Do not extinguish a gas fire until you can shut off the fuel.
- Fire classes A, B, C, D and K match the fuel to the right agent. Other countries use different letters.
- Sprinklers extinguish or control fire in the first minutes, before anyone arrives.
Four ways to put out a fire
Extinguishment means breaking the tetrahedron. A flaming fire can be put out by reducing temperature, removing fuel, removing or diluting oxygen, or interrupting the chemical chain reaction. A smoldering fire has no flame chain reaction, so only three options apply: cool it, remove the fuel, or cut off the oxygen.
| Method | Side of the tetrahedron attacked | Typical agents | Works best on |
|---|---|---|---|
| Cooling | Heat | Water | Solid fuels and liquids with high flash points |
| Smothering (oxygen exclusion or dilution) | Oxygen | Foam, carbon dioxide, inert gases, covering the fuel | Flammable liquids, enclosed spaces |
| Fuel removal | Fuel | Closing a valve, removing material, letting it burn out | Gas and liquid fuel fires, fire in the path of spread |
| Chain reaction interruption | Chain reaction | Dry chemical, halogenated and clean agents | Flaming gas and liquid fires |
Cooling: why water works
Water is the fire service’s main agent because it is cheap, plentiful and absorbs a great deal of heat, especially when it turns to steam. Water expands roughly 1,700 times as it becomes steam, so it also takes up room that air and smoke would occupy. To put a fire out by cooling, you must absorb heat faster than the fire produces it. That is a negative heat balance, and it takes an adequate flow of water in the right form. Too little water just makes steam.
Cooling puts out solid fuels and liquids with high flash points by cooling them until they stop producing enough vapor to burn. It does not work on low flash point liquids or flammable gases, because they keep giving off vapor no matter what the temperature of the water.
Smothering: cutting off or diluting oxygen
This reduces the oxygen reaching the fire by introducing an inert gas or by separating fuel from air, as foam does. It will not work on materials that supply their own oxygen, or on certain metals. Magnesium, for example, can keep burning in carbon dioxide or nitrogen.
Fuel removal
You can stop a fire by shutting off the fuel, moving combustibles out of its path, or letting it burn out. The classic rule for a burning gas leak: do not extinguish the flame until you can shut off the flow of gas. A flame you put out without stopping the gas leaves an unburned fuel cloud looking for an ignition source.
Interrupting the chain reaction
Dry chemical and halogenated (and halon replacement) agents interrupt the chain reaction in the flame, which can bring about very quick knockdown. This works on gas and liquid fuels because they burn only in flame. It does not extinguish glowing, smoldering material, which also needs cooling.
Classes of fire
In the United States, NFPA 10 sorts fires into classes by fuel so the right agent is picked. Other countries use different letters, so read the label on the equipment.
| Class | Fuel | Typical agents | Watch out for |
|---|---|---|---|
| A | Ordinary combustibles: wood, cloth, paper, rubber, many plastics | Water, foam, multipurpose dry chemical | Deep-seated fires may rekindle; overhaul thoroughly |
| B | Flammable and combustible liquids, greases and gases | Foam, dry chemical, carbon dioxide; stopping the flow for gas fires | A straight water stream can spread a burning liquid; gas fires call for fuel shutoff |
| C | Energized electrical equipment | Non-conducting agents such as carbon dioxide or dry chemical | De-energize if possible. Once the power is off, treat it by the fuel that is burning |
| D | Combustible metals: magnesium, titanium, zirconium, sodium, potassium, lithium metal | Special dry powder agents made for each metal | Water can make some metal fires worse. No single agent works on all of them |
| K | Cooking oils and fats in commercial cooking appliances | Wet chemical agents, often in fixed hood systems | Added to NFPA 10 in 1998; very hot oils can reignite after flames are knocked down |
Fixed systems: the extinguishment you do not have to carry
Automatic sprinklers work mainly by cooling, and they are usually on the fire in seconds to minutes, long before an engine arrives. Fires that grow large and kill people are very often fires where sprinklers were missing or did not work. That is why the two worst fires on the previous pages, in Las Vegas and in Charleston, keep coming back in code discussions.
Charleston revisited: what sprinklers could have changed
The NIST study of the 2007 Sofa Super Store fire recommended that model building and fire codes require sprinklers in all new retail furniture stores, regardless of size, and in existing ones with any single display area larger than about 2,000 square feet (190 square meters). In the team’s computer simulations, sprinklers on the loading dock where the fire began likely would have maintained conditions people could survive.
The NIST authors said that if today’s model codes had been in place and rigorously followed in 2007, the conditions that led to the rapid fire spread probably would have been prevented. The lesson: the best extinguishment is the one that starts in the first minutes, automatically.
Fireground talk
- Knockdown
- Reducing a fire's intensity so it is under control and no longer growing.
- Negative heat balance
- Absorbing heat faster than the fire produces it.
- Rekindle
- A fire that flares up again after it appeared to be out.
- Straight stream / fog
- Two ways to deliver water: a solid reach stream or a fine spray with more heat absorption.
- Stop the leak first
- The rule for gas fires: shut off the fuel before putting out the flame.
Practice it in the simulators
Study questions
What are the four methods of fire extinguishment?
Cooling (reducing heat), smothering (removing or diluting oxygen), fuel removal, and interrupting the chemical chain reaction.
Why can't water extinguish low flash point liquids by cooling?
Because they continue to give off flammable vapor even when cooled, so vapor production cannot be reduced enough.
What are the NFPA fire classes?
Class A: ordinary combustibles. Class B: flammable liquids and gases. Class C: energized electrical equipment. Class D: combustible metals. Class K: cooking oils and fats.
Why should you not put out a burning gas leak before shutting off the gas?
Because unburned gas keeps flowing and can collect, and then reignite or explode when it finds an ignition source.
Sources and further reading
- Charleston, SC Furniture Store: Technical Study of the Sofa Super Store Fire — NIST
- NIST Study on Charleston Furniture Store Fire Calls for National Safety Improvements — EHS Today
- Portable Fire Extinguishers information sheet (fire classes) — Sanford (NC) Fire Department
- NFPA 10, Standard for Portable Fire Extinguishers — NFPA (print standard)
- Essentials of Fire Fighting, IFSTA — Fire Protection Publications (print textbook)
About this guide. The Fire Behavior & Chemistry series was originally informed by LBFD Training Division recruit training material. This edition has been independently rewritten and expanded using the sources listed on each page, and its diagrams were drawn for this site. Free Fire Simulator is not affiliated with or endorsed by LBFD. Spotted an error or have a better source? Use the Contact Us button.