5.a) What is a fire extinguisher? Classify fire extinguishers and explain in detail.
Answer:
FIRE EXTINGUISHERS
A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user (i.e., no escape route, smoke, explosion hazard, etc.), or otherwise requires the expertise of a fire department. Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire.
TYPES OF FIRE EXTINGUISHERS
To deal with the multitude of different fire classes, a range of fire extinguishers have been
developed. Types of fire extinguishers are as follows:
- Water type fire extinguisher.
- Foam type fire extinguisher.
- Dry chemicals type fire extinguisher.
- Carbon-di-oxide type fire extinguisher.
- Halon type fire extinguisher.
1. WATER TYPE FIRE EXTINGUISHER
Water expelling fire extinguishers have water as an extinguishing agent which is released in the form of a jet by means of gas pressure in the upper part of the container. The gas pressure may be induced by chemical reaction or by mechanical means. Water expelling fire extinguishers is as shown in figure, are used mainly in Class ‘A’ fire, water when applied to burning material is converted to steam which reduces the percentage of available oxygen. Water expelling type extinguishers should not be used on fires involving electrical equipment without de- energing them.
The various types of water expelling extinguishers are:
i. Soda acid type.
ii. Gas pressure activated type.
iii. Constant air pressure type.
SODA ACID EXTINGUISHER
The operational instructions given on the body of the extinguisher should be read carefully. It should be ascertained whether it is of upright of turnover type. To operate the extinguisher, remove the cap and strike the plunger, direct the jet emerging from the nozzle on the fire.
WORKING PRINCIPLE
When the plunger is struck, it breaks the acid phial (bottle). The sulphuric acid and sodium bicarbonate solution react together to release CO2. The CO2 generated creates pressure which forces the water out of extinguisher. The CO2 acts only as a propellant and the water extinguishes the fire by cooling effect.
GAS PRESSURE TYPE
The gas pressure type essentially has an outer container similar to that of the soda acid type. There is gas cartridge filled with CO2 under pressure which forms the inner compartment. When the cartridge is priced open, CO2 under pressure is released into the body of extinguisher driving water out through the discharge tube.
CONSTANT AIR PRESSURE TYPE
The constant air pressure type extinguisher is filled with water and dry air is introduced through air lines till the desire pressure is built up. When air lines are removed, the container is hermetically sealed. While activation safety pin is withdrawn and valve lever is depressed resulting in a jet of water through the hose, duration of discharge is 90 seconds to 120 seconds, effective jet length is 6m.
2. FOAM TYPE FIRE EXTINGUISHER
Foam type fire extinguisher is as shown in figure; this type is suited for class B fires. The foam expelled by actuating the extinguisher forms a blanket over the surface of the liquid on fire and gets over the contact of the burning liquid with air thus extinguishing the fire.
OPERATION
- Remove the extinguisher from the socket.
- Pull the plunger, rest it on the notch and turn the extinguisher over shaking well to
ensure the mixing of the two liquids. - Direct the foam jet above the level of turning liquid. This allows the foam to build up
over the surface of the liquid. The jet from the foam extinguisher should have a length
of at least 6 m. - Do not direct the jet directly into the liquid because this will drive the foam beneath the surface and render it ineffective. In addition, it may splash burning liquid on to the surrounding and spread the fire.
Foam solution is electrically conductive and therefore is not recommended for use on electrical fires.
3. DRY CHEMICALS TYPE FIRE EXTINGUISHER
Dry chemicals type fire extinguisher is as shown in figure, this type is suitable for tracking gas fire, fires is electrical equipment. The chemical powder employed is usually sodium based and when applied to a fire undergoes chemical reaction. The free radicals which are responsible for sustaining any fire are put out of action by the dry chemical powders and because of this, the fire dies out very fast. Chemical dry powders containing mixtures of sodium, potassium and barium compounds have been found useful in extinguishing fires in metals such as sodium and magnesium.
The chemical powder is contained in the main shell and CO2 gas is held under high pressure in a sealed cartridge. When the extinguisher is operated, the cartridge is broken allowing CO2 gas to escape to the main shell and push out the powder in the form of fog.
OPERATION
Carry the extinguisher to the place of fire and keep it upright. Remove the safety clip and strike the knob located in the cap to activate the piercing mechanism which in turn breaks the sealing disc of the cartridge. Direct the stream of escaping powder at the base of the flame. For effective result stand about 5 to 8 feet away and direct the stream near the seat of the fire, progress forward, moving the nozzle rapidly with a side to side sweeping motion. When using on outdoor fires always operate extinguisher from the upwind side for the fire.
4. CARBON-DI-OXIDE TYPE FIRE EXTINGUISHER.
Carbon-di-oxide type fire extinguisher is as shown in figure; CO2 is effective as an extinguishing agent primarily because it reduces the oxygen content of air to a point where combustion cannot continue. CO2 is non-combustible and does not react with most substances. Being a gas, it can penetrate and spread to all areas affected by fires.
Carbon-di-oxide fire extinguishers are used for putting out fires in petroleum products, gaseous substances under pressure and also in sophisticated electrical and electronic apparatus. Carbon-di-oxide extinguishers are not to be used in
- Fires involving chemicals that contain their own oxygen supply.
- Fires involving reactive metals such as sodium, potassium and magnesium.
OPERATION
Carry the extinguisher to the place of fire. Remove the safety pin and operate the discharge device or unscrew the valve depending on the design. CO2 is delivered by means of discharging horn through a high-pressure flexible hose. Direct the jet at the base of fire, starting at one edge and sweeping across the surface of the burning material. When used in open air, the operator should stand on the up-wind side of the fire and should apply the gas in a downwind direction as close as possible to the fire. On fires in electrical equipment first switch off the current then direct the jet or horn straight at the fire.
HALON TYPE FIRE EXTINGUISHER
Halon is the most effective extinguishing agent than any other extinguishing agent. It is ideal for intense and rapid flammable fires. Halon is non-conductive and leaves no traces, when applied. So, it is also suitable for electrical fires, computer rooms etc. Halon extinguishing the fire by interrupting the chain reactions which takes place at the flame zone for continuance of fire. Halon is twice as effective as carbon-di-oxide on a weight basis and five times as effective as carbon-di-oxide on volume basis.
OPERATION
Carry the extinguisher to the place of fire, remove the safety pin and operate the discharge device. Halon is discharged by means of the discharge nozzle or hose. When operated it is discharged as a semiliquid jet at a high velocity and evaporate rapidly to cover a fire in a blanket of mist. The halogenated extinguishing agents are known as Halons. Halogenated extinguishing agents are hydrogen in which one or more hydrogen atoms have been replaced by atoms from the halogen series i.e., fluorine, chlorine, bromine and iodine. There are many halogenated but only two are most effective and less toxic. They are Halons 1301 and Halon 1211.
21CV653 – Occupational Health and Safety Solved Model Question Paper