Civil Engineering Reference
In-Depth Information
body of the extinguisher. The size of the extinguisher
provided for any particular location depends upon the
fi re risk and the persons who may need to use it. There
are seven basic types extinguishing equipment:
and therefore creates a quicker knockdown with fewer
media. AFFF extinguishes the fi re by cooling.
Foam
- foam extinguishers again are similar in
construction to water type extinguishers, the foam
being expelled from the body of the cylinder by stored
pressure. The foam is created by the premixed foam
being aerated through a special attachment at the end of
the discharge hose. Because the foam is less dense than
water it fl oats on the burning surfaces of oils and other
fl ammable liquids. It extinguishes the fi re by smothering,
i.e. by preventing air getting to the fuel.
Dry powder
- dry powder extinguishers are
constructed as shown in Figure 9.69. The dry powder is
expelled from the extinguisher by the stored pressure in a
CO
2
cylinder. When the CO
2
is operated, the dry powder
is transformed into a liquid state and discharges through
the hose to the nozzle. Dry powder extinguishes fi re by
smothering, i.e. by forming a barrier between the burning
fuel and the air. It is the only extinguishing media that can
successfully deal with fl owing fuel fi res.
Carbon dioxide (CO
2
)
- CO
2
extinguishers are
constructed as shown in Figure 9.70. They contain only
CO
2
, which has been pressurised to such an extent that
it is a liquid. When the operating lever is depressed,
the liquid CO
2
moves out through the discharge tube. At
the end of the tube is a horn which is provided to slow
the rush of pressurised CO
2
and direct it at the fi re. These
extinguishers work by smothering, i.e. they replace the
immediate atmosphere of air around a fi re with an inert
gas that is one that does not support combustion. CO
2
is the only medium that can be used to safety fi ght fi res
➤
Water
➤
Aqueous fi lm forming foam (AFFF)
➤
Foam
➤
Dry powder
➤
Carbon dioxide
➤
Wet chemical
➤
Fire blanket.
Water
- water extinguishers are most commonly found in
offi ces and other places of work where the combustible
material is carbonaceous, i.e. wood, paper, plastic, etc.
Water extinguishes the fi re by cooling. Modern extinguish-
ers are of the stored pressure type where the pressure is
supplied by a small CO
2
cylinder within the extinguisher,
in much the same way as a soda siphon; alternatively the
body of the extinguisher can be pressurised. The water is
applied to the fi re through a discharge hose which allows
the water to be played on the fi re without having to bod-
ily move the extinguisher backwards and forwards. The
pressure within the extinguisher is indicated on the neck
of the cylinder.
AFFF
- AFFF extinguishers are almost identical to
the water type extinguisher apart from the fact that they
have a foam additive in the water (aqueous fi lm-forming
foam) that increases the effi ciency of the extinguisher
Operating lever
Carrying handle
CO
2
Gas cartridge
piercer
Gas inlet tube
CO
2
Gas cartridge
Powder
Discharge tube
Nozzle
Figure 9.68
Pressure gauge on the neck of a water
extinguisher
Figure 9.69
Dry powder extinguisher
