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Extinguisher
Types
Carbon
Dioxide Type
 Carbon
dioxide (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 fire. Carbon dioxide fire extinguishers are used for putting
out fires in oils, petroleum products, gaseous substances
under pressure, and also on sophisticated electrical and electronic
apparatus.
Carbon
dioxide extinguishers are not to be used in
i)
Fires involving chemicals that contain their own oxygen supply
(such as cellulose nitrate).
ii)
Fires invotving reactive metals such as sodium, potassium
and magnesium.
The
common type of portable carbon dioxide extinguisher covered
by IS: 2878- 1976 is discussed here.
Construction
The principal parts of the extinguisher are, as shown in the
figure above. Carbon dioxide is retained in the cylinder as
liquid under pressure. The cylinder is filled with the charge
to about two-thirds by weight of its total water capacity.
The gas at the time of discharge makes considerable noise.
The user should therefore be well conversant with its operation
to prevent the jet from being misdirected during the first
few vital seconds.
How
Does Carbon Dioxide Extinguish Fire
When the extinguisher is actuated carbon dioxide from the
cylinder comes out at a considerable velocity into the atmosphere
and forms a layer of gas which is about one and half times
heavier than air. The vapour blanket puts out fire by displacing
the air around the fire and reducing the oxygen supply needed
to continue combustion.
Dry
powder type
 Dry
powder type fire extinguishers are suitable for tackling petroleum
fires, gas fires, fires in electrical equipment and for controlling
surface fires in textile fibres, These extinguishers are noted
for the speed with which they put out fires.
The
chemical powders employed are usually sodium based and when
applied to a fire, undergo 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.
Special 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 dry powders
used should conform to IS: 4308-1982 specification.
There
are two types of ordinary dry powder extinguishers available
viz.
(1)
Cas Cartridge type and
(2) Stored Pressure type.
The
first type being the most common will be discussed here. These
extinguishers are available in 1, 2, 5 and 10 Kg capacities.
Construction
The construction of this type of fire extinguisher is shown
in the figure. The chemical powder is contained in the main
shell of the extinguisher and CO2 gas is held under
high pressure in a sealed cartridge. When the extinguisher
is operated, the cartridge is broken allowing the CO2
gas to escape to the main shell and push out the powder in
the form of fog.
Method
of 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 actuate the piercing mechanism which in turn breaks
the seal1ng 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 the extinguisher from
the upwind side of the fire to extend the effective range
of the spray.
Foam
Type
 Portable
extinguishers expelling foam are best suited to put out class
B fires involving flamable liquids like oils, solvents, petroleum
products, varnishes, paints, etc. The foam expelled by actuating
the extinguisher forms a blanket over the surface of the liquid
on fire and cuts out the contact of the burning liquid with
air thus extinguishing the fire.
The
most common type of the portable foam extinguisher covered
by the Indian Standard IS: 933-1976 is discussed here.
Construction
The extinguisher shown in the figure consists essentially
of two containers, the inner and the outer. These containers
when filled to the specified level contain 9 litres liquid,
The outer container holds a solution of Sodium bicarbonate
to which a foam stabilizer is added, The inner container (a
long brass/plastic tube) contains solution of aluminium sulphate,
When operated, the solution intermixes producing foam that
is expelled from the extinguisher.
Method
Of Operation
1.
Remove the extinguisher from the socket.
2.
Pull the plunger, rest it on notch and turn the extinguisher
over shaking well, to ensure the mixing of the two liquids.
3.
Where a liquid on fire is in a container, direct the jet
at
the far inside edge of the container, or at an adjoining
vertical surface above the level of the burning liquid.
This breaks
the jet and allows the foam to build up and flow across the
surface of the liquid.
Water
Type
 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
maybe induced by chemical reaction or by mechanical means.
Water
expelling fire extinguishers are used mainly in class 'A'
fires (IS: 2190-1979) involving ordinary combustible materials
like wood, paper, textiles, etc. which are put out by the
cooling action of water. Besides, 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-energising them. The various types of water-expelling extinguishers
are:
1.
Soda acid type IS: 934-1976
2. Gas Pressure actuated type IS: 940-1976
3. Constant Air Pressure type IS: 6234-1971.
The
soda acid type is the most commonly used.
Soda
Acid Type Extinguisher
Construction
The various parts and contents of a soda acid extinguisher
are shown in the figure. The total liquid capacity of the
body (or the solution containers) when filled to the specified
level, should be 9 litres. During manufacture, the body is
required to be tested to an internal hydraulic pressure of
25 kgf/cm2 for 5 minutes.
Method
of operation:
Before actuating the extinguisher the operational instructions
given on the body of the extinguisher should be read carefully.
At this stage, it would be advisable to know whether soda
acid extinguisher are of up-right type or turn-over type depending
on their method of working. One must know the type of the
extinguisher provided at a given place. To operate the extihguisher,
remove the guard cap and strike the plunger against a hard
surface like the floor. Direct the jet emerging from the nozzle
on the base of fire.
Principle
of working:
When the plunger is struck, it breaks the acid phial (bottle).
The sulphuric acid and sodium bicarbonate solution react together
to release carbondioxide (CO2). The CO2
generated creates pressure which forces the water out of the
extinguisher. The CO2 acts only as a propellant
and the water extinguishes the fire by cooling effect.
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