The six major types of fire extinguishers and the Classes of fires they are suitable for
Elements of a fire
There are four necessary elements of a fire: oxidizer, heat, fuel, and an uninhibited chemical chain reaction, all represented in the “fire tetrahedron”. Heat promotes the ignition temperature of the fuel, the fuel undergoes combustion with an oxidizer (commonly atmospheric oxygen), and an uninhibited chemical chain reaction provides continuous energy (heat) to ignite more fuel.
As heat is applied, the molecules of a combustible material (fuel) break down into smaller volatile free radical fragments in a process called pyrolysis. These free radical fragments readily bond with encountered oxygen in an exothermic redox (combustion) reaction, which supplies the heat necessary for pyrolysis to continue.
Four methods of extinguishing a fire
Fires can be extinguished by removing any one of the four sides of the fire tetrahedron.
(i) Heat removal: To start a fire, the ignition temperature must be reached. One method of controlling fires is by cooling it, so the temperature drops below the ignition point. This can be achieved by applying water, dirt, or other appropriate cooling agents.
(ii) Oxygen removal: At least 16% of the oxygen in air is required to start and grow a fire.1 A foam extinguisher, which “coats” the fuel is commonly used to suffocate a fire, preventing its exposure to oxygen. A water extinguisher can also be used with a “fog nozzle”, which increases its surface area, but a limitation is that it can evaporate and run out more readily than other methods.
Always refer to ‘Section 5: Firefighting measures” of the SDS to determine the correct extinguisher to use. Drenching amounts of water for instance may be the agent of choice for oxidizer-fed fires, which don’t require atmospheric oxygen.2
(iii) Minimize the fuel source: Fuel elimination is a way to contain a fire until the remainder of the fuel is consumed. Fires can be starved by using chemical retardants at the base of the flame, which coat the fuel, making it inaccessible to the fire.
(iv) Chemical chain reaction interruption: Free radicals produced through burning can be removed via a halon extinguisher, which creates an inert gas barrier. However, many authorities have discontinued the use of these extinguishers as halon is listed as a Montreal Protocol ozone-depleting material.
Five classes of fires
Fires are generally grouped into five classes (not including oxidizer-fed fires), based on the fuel that is involved. The United States, Europe, and Australia use slightly different classification systems: the system below is the American one:
Class A: Solid non-metal combustible material including wood, paper, cloth, and many plastics
Class B: Flammable or combustible liquids (or gases) including gasoline, oil and acetone
Class C: Energized electrical equipment such as appliances or faulty wiring
Class D: Combustible metals including potassium, magnesium, and sodium
Class K: Commercial kitchen fires with cooking oils and grease
Six major types of extinguishers3
Portable extinguishers are useful for putting out small fires, but they are not effective against large, spreading fires. For larger fires, close all doors and get professional assistance.
Water extinguishers are used to cool and suffocate Class A fires. There are various types including water jet, water spray, water extinguishers with additives, and water mist or fog extinguishers.
Foam extinguishers use a mechanical or chemical reaction to create foam that cools and suffocates Class A and B fires.
Dry chemical extinguishers extinguish the fire primarily by interrupting the chemical chain reaction at the fuel’s surface. Multipurpose dry chemical extinguishers are suitable for Class A, B, C fires, while ordinary ones are for Class B and C fires only.
Dry powder extinguishers suitable for Class D fires separate fuel from oxygen and dissipate the heat from the burning metal.
Carbon dioxide extinguishers can be used to cool and suffocate (through oxygen displacement) Class B and C fires.
Wet chemical extinguishers cool and create a barrier between the oxygen and fuel of Class K commercial cooking fires. They may also be used on Class A fires in commercial kitchens.
The extinguisher must be compatible with the Class of fire
Only use an extinguisher that is suitable for that Class of fire to prevent fire aggravation; familiarize yourself with the ones available in your environment prior to working with reagents.
|
Class |
Water |
Foam |
Dry chemical |
Dry Powder |
Carbon dioxide |
Wet Chemical |
|
Class A Ordinary flammable materials |
Yes |
Yes |
(Multi-purpose extinguishers only)
|
No |
No |
Only in commercial kitchens
|
|
Class B Flammable liquids/gases
|
No |
Yes |
Yes |
No |
Yes |
No |
|
Class C Live electrical equipment
|
Only water mist extinguishers |
No |
Yes |
No |
Yes |
No |
|
Class D Combustible metals
|
No |
No |
No |
Yes |
No |
No |
|
Class K Commercial kitchen fires with cooking oils & grease
|
No
|
No |
No |
No |
No |
Yes |
F) How to use a fire extinguisher
Remember the acronym PASS1:
Pull - the pin on the extinguisher
Aim - the nozzle at the base of the fire
Squeeze - the lever to release the agent
Sweep – Slowly sweep the nozzle from side to side
G) When not to use a fire extinguisher
Always send for help before considering the use of a fire extinguisher. Do not use an extinguisher if a substantial amount of smoke is being generated, the fire is near other flammable or combustible materials, the fire is too large for one extinguisher, you cannot keep your exit at your back, or if you don’t know how to use one.1
H) How to minimize the risk of laboratory fires
1. Improper storage:
- Flammables should be stored in a flammable cabinet (with a “flammable” label) that is kept away from incompatible materials such as compressed gases, oxidizers, or sources of heat.
2. Improper handling:
- Flammables should be handled in a clutter-free fume hood to decrease the chance of bottles being knocked over.
3. Improper disposal:
- Flammables should be disposed of in a flammable waste container and should not be mixed with incompatible chemicals.
4. Improper use of electrical equipment:
- Avoid plugging too many electronics into a single outlet and ensure wires are not dangling or near water and other chemicals.
5. Improper use of PPE:
- Always secure loose clothing and long hair to prevent them from catching fire.
- When working with flammables, wear a flame resistant lab coat.
6. Improper use of fire-fighting equipment:
- Become familiar with the available fire-fighting equipment before beginning an experiment. This includes how to secure, activate, and aim it.
- Review SOP and reagent SDS information (Section 5: Firefighting measures) to identify the appropriate class of extinguisher that should be used.
- Ensure that your fire extinguishers are well maintained and replaced when expired.
7. Improper housekeeping:
- Remove clutter from fume hoods to prevent spills and label labware accordingly.
- Clear clutter from hallways and doorways as this may prevent people from accessing the exit during an emergency.
- Self-closing fire doors may be required depending on your institution. Do not prop them open as smoke, toxic gas, and fire will be allowed to spread.
