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HALON
SYSTEM
Halogenated
hydrocarbons represent a class of extinguishing agents that
are superior to many other extinguishing agents in terms of
their effectiveness.
Halon
1301 is one such halogenated hydrocarbon which is used widely
(in the west) as an extinguishing agent of a class because
of its low toxicity and great effectiveness as a flame inhibitor
and explosion suppressant. The Halon 1301 fire extinguishing
agent has existed for more than 40 years and has been commercially
used worldwide as a fire extinguisher for nearly 20 years.
The fire extinguishing properties of Halon 1301 were discovered
in a series of evaluation tests conducted by the US Army following
World War II. It is considered as a 'clean' agent, which does
not wet or leave residual material.
 Halon
1301, a gas under normal conditions, is able to penetrate
hard-to-reach places that may not be possible in case of other
gaseous fire extinguishing agents. Halon 1301 is a chemical
compound composed of carbon, fluorine, and bromine. Its formula
is CBrF3; it is a pure compound and not a mixture of chemicals.
Table 1 compares the effectiveness of Halon 1301 in controlling
fire with carbon dioxide - another extinguishing agent.
Under
normal conditions, it exists as a gas, about 5 times heavier
than air. When it is compressed, it can be stored as a liquid
having density about 50 per cent heavier than that of water.
Important
Properties
- Appearance:
Colourless, odourless gas
- Molecular
Weight : 148.9
- Specific
heat at constant pressure 1 atm 25°C : 870 J/kg°C Boiling
Point at 1 atm(Liq.) : -57.75°C
- Freezing
Point: -168°C
- Vapour
Pressure at 21°C : 14.062 kg/cm2
- Latent
Heat of Vaporisation at Boiling Point: 118.81 Kj/kg Vapour
Density at 21.1 °C at I atm : 6.36 kg/m3
- Density
of the Liquid at 21.1 °C : 1566.45 kg/m3
Uses
And Applications
Halon
1301 tops fire very rapidly when introduced into the fire
area in proper concentration. It is effective on 2 main classes
of fire
- Common
combustibles such as wood, paper, etc.
- Flammable
liquids.
It
is also used very effectively in controlling fires involving
electrical/electronic circuits. Since low concentrations of
Halon 1301 are required to extinguish most fires, and as the
agent has a low degree of inhalation toxicity in its natural
state, it can be successfully used to attack fires quickly
in normally occupied areas.
The
exact process by which Halon 1301 extinguishes fires is not
clearly understood, but it is generally thought to be a chemical
reaction rather than a physical effect. It perhaps interacts
with the chemical chain required to support combustion and
stops the fire quickly without leaving any residue. However,
it will not extinguish fires involving burning metals, metal
hydrides or materials that contain their own oxidising agents
(such as nitrate, gun powder, solid rocket propellant,etc.).
Although
the agent will extinguish all fires involving ordinary combustibles,
special design considerations must be observed to make the
system effective.
The
application of Halon 1301 fire protection system can be categorised
into the following :
- Total
Flooding Systems
in
which a supply of the agent is discharged into all enclosure
to fill it uniformly with a concentration of the agent
sufficient to extinguish the fire. The basic components
of a Total
Flooding System are shown the figure. A supply of the agent
contained under pressure is connected by piping to one
or
more discharge nozzles located within the enclosure. Upon
detection of a fire, a control valve on the agent supply
container releases the agent through the piping to one
or more discharge nozzles within the enclosure to fill
it uniformly
with a concentration sufficient to extinguish the flame.
Total
Flooding Systems can be of two types :
Explosion
Suppression Systems
In
which the agent is rapidly injected into a developing
flame front to suppress it before it has reached damaging
proportions in an enclosure. The detection and suppression
is normally achieved within 0.06 seconds.
Inerting
Systems
In
which a concentration of the suppressing agent is
developed
within an enclosure to prevent ignition by an ignition
source. That is, upon detection of presence of flammable
gas in an enclosure, the enclosure is filled up quickly
with Halon 1301 as per required concentration and
turning
the atmosphere of the enclosure non flammable or inert.
- Local
Application Systems
apply
Halon vapour directly on the burning surface manually
or
automatically from fixed systems. The ability of a local
application system to extinguish the fire depends on
the
rate of the application of the agent on the burning surface.
As the rate of application of Halon 1301 increases,
the
time required for extinguishment is reduced by an amount
greater than the increase in the discharge rate. As
a result,
the total quantity of agent required for extinguishing
is also reduced. However, at some point the proportionality
reverses. Further, increase in the discharge rate results
in a small decrease in extinguishing time, so that the
total
quantity required to extinguish begins to increase again.
Hence, the optimum discharge rate should be determined
to
find out the total minimum agent requirement. However,
since the use of this system is very limited, not much
work has
been done in the area of framing comprehensive standards
for such systems. Most of the extinguishing systems
using
Halon 1301 as agent are of Total Flooding type. The typical
applications include computer rooms, magnetic tape storage
vaults, electronic control rooms, telephone exchanges,
storage areas for works of art, etc. The application
of Halon
1301
as explosion suppressant and as inerting medium include
storage areas for solvents and other flamable liquids,
refineries,
flamable gas handling systems and grain elevators.
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