Types of Fire Extinguishers
There are different types or classes of fire extinguishers, each of which extinguishes a specific type of fire. These fire extinguishers are placed and used throughout the workplace. They are a basic part of the firefighting system. Fire extinguishers are the first line of defence in the event of a fire.
Dry chemical extinguishers
Contain a powder that is expelled under nitrogen or carbon dioxide (CO2) pressure. It puts out the fire by smothering or blanketing the fire and cutting off the oxygen supply. Dry chemical extinguishers are used to extinguish most types of fire. However, they are the best for paper, rubbish, electrical, oil, gas and chemical fires.
Dry chemical extinguishers are usually rated for multiple purpose use. They contain an extinguishing agent and use a compressed, non-flammable gas as a means of getting the chemical on to the fire.
Dry powder is generally the best extinguishing medium for use on Class B fires.
Extinguishers containing dry powder are capable of dealing with burning flammable liquids, spread over large areas, more effectively than other extinguishers of comparable size.
They are effective too on fires involving free-flowing liquids, e.g. spillage over vertical surfaces.
Since dry powder is a non-conductor of electricity it is safe to use on Class C fires.
Varies in size 1 kg and 14 kg handheld models. Wheeled units of 68 kg.
Handheld models are further subdivided into two type’s i.e.
Stored Pressure Type
- The container is charged with either compressed air or Nitrogen.
- The container is fitted with a pressure indicator.
Cartridge type
A cartridge charged with Carbon Dioxide or Nitrogen supplies pressure.
The extinguisher works on the following principles:
- The container contains a charge of dry chemical powder.
- A pressure cartridge containing Carbon Dioxide gas is mounted either internally or externally with some of the modern extinguishers.
- When the puncture lever is actuated the gas, pressure is released and enters the chamber through the gas ports in the gas tube
- The gas entering the chamber agitates the powder and the gas pressure forces it out through the nozzle
Limitations of dry chemical extinguishers
Dry powder has no cooling properties and will therefore not prevent re-ignition.
For this reason, it is not as effective as foam on contained flammable liquid fires which have been burning for a long time.
Generally, powders are messy and some form sticky deposits on surfaces and must be scraped and washed away after the fire. These deposits can have a detrimental effect on delicate machinery and equipment.
Re-ignition of Class A fires is prevented by multi-purpose powder.
CO² extinguishers
Contain liquid carbon dioxide (CO2) under pressure. When released, the CO2 turns into a gas, lowering the temperature of the fire and smothering it. This extinguisher can be used effectively on any type of fire. Carbon dioxide (CO2) extinguishers are most useful on Class B and C (liquids and electrical) fires. These extinguishers are only effective from a short distance, about one to two meters from the fire.
The carbon dioxide is stored as a compressed liquid in the extinguisher. As it expands when it comes out, it cools the surrounding air. The cooling will often cause ice to form around the “horn” where the gas is expelled from the extinguisher. Since the fire could re-ignite, continue to spray the fire even after the fire looks as though it could be out.
The gas will not contaminate foodstuffs or cause a mess and because it is emitted as a gas, will not cause unnecessary damage through fierce impact as in the case of solid material e.g. dry powder.
Limitations of CO² extinguishers
The cooling properties of CO² gas are limited and it therefore provides no protection against re-ignition. Since CO² is a gas, drifty or windy conditions will affect its performance.
The effective range of a CO² extinguisher is also limited. Direct impingement of CO² onto delicate electrical or electronic equipment could cause additional damage through cold shock. Under dry conditions, the discharge of CO² extinguishers generates static electricity, which can be uncomfortable to the user.
Water extinguishers
Contain water under pressure. They put out a fire by soaking or wetting the burning material. Water extinguishers may be used on paper or wood fires. It is very important that you do not use a water extinguisher on electrical fires, due to the possibility of electrocution.
“Did you know that you could keep a box of bicarbonate of soda in your house to put out fires? This works very well on fires that start while you are cooking.”
Water has better cooling properties than any other agent and is therefore best on Class A fires. Class A fires can re-ignite if not adequately cooled. Water will also penetrate readily to reach deep-seated fires.
There are two types, which are both gas propelled viz:
1.Gas cartridge type
- Pressure supplied by a small Carbon Dioxide cylinder
- When activating the extinguisher, the cylinder is pierced, thus putting the water container under pressure.
- Various makes are used either in the upright or inverted position
2.Stored pressure type
- Air or inert gas, pressure being done by a motorcar type valve pressurizes the container
- Fitted with a pressure gauge
- Usually only used in an upright position.
3. Light Water Extinguishers (For use on class “B” fires)
- The light water type extinguisher is charged with a low expansion type foaming agent called Aqueous Fluid Forming Foam or A.F.F.F.
- “Light water” has the ability to make water float on oil and the appearance of a thin soapy solution.
- It spreads over oil faster than protein-type air foams.
- It is compatible with dry powder.
Limitations of water extinguishers.
- Water is a conductor of electricity and can be highly dangerous if used on Class C fires, i.e. fires in the vicinity of live electrical current
- It will also cause Class B fires to flare up and spread and if used on Class D fires may cause violent explosions
4. Vaporising Liquid Extinguishers
- Has a very wide application – effective on class “A”, “B” and “C” fires
- Handheld and automatic models of various capacities are available
- The majority of modern vaporising liquid extinguishers utilize the chemical BCF (Bromochlorodiflumethane)
5. Foams
- Foam extinguishers Class B fires by forming a blanket of bubbles on the surface of the liquid and therefore has a smothering effect on the fires.
- Foam extinguishers are suited for dealing with small-contained flammable liquid fires or for fires, which have been burning for some time causing the container to become very hot and increasing the chances of re-ignition.
Limitations of foam extinguishers
- Most foam has to form a blanket to extinguish the fire, and since it is not possible to cover flowing flammable liquids, foams are not effective.
- Foam is water based and therefore a conductor of electricity. It is dangerous to use it on live electrical equipment. Foam will also tend to break down when coming into contact with liquids such as alcohols, and this will prevent the production of an effective blanket.
Halogenated hydrocarbons – Halon 1211 (BCF)
- Extinguishers containing Halon 1211 are suitable for small Class B fires and because the medium is electrically non-conductive it is particularly suitable and safe for use on live electrical equipment.
- Being a liquid, which vaporizes into a dense heavier than air gas the medium, leaves no mess after discharge. Its liquid state on discharge permits a longer range of throw than does CO².
- The medium has limited cooling properties and is therefore not effective on deep-seated smouldering fires. It should not be used in confined spaces where there is a risk of someone inhaling the products of combustion when the gas decomposes in a fire. The resultant acrid smell could force the user to evacuate the fire area before the fire is successfully extinguished.
- The direct impingement of the medium onto delicate electrical or electronic equipment from close range could cause damage through cold shock.