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SPRINKLER
SYSTEM
That
fires can be extinguished by the application of water must
have been one of man's earliest discoveries and even before
our early ancestors learned to make fire they knew that water
the antinode having undoubtedly seen accidental fires extinguished
by rain.
 The
subject is not new, it is only in the comparative recent history
that this phenomenon has been scientifically investigated
and the results supplied to the design of Fire Extinguishing
Equipment. Only in the past 100 years or so has a system been
developed for automatic control of fire.
Principles
of Water Application for Fire Fighting
There
are three main applications for water in fire control, each
with a somewhat different objective.
a)
Extinguishment
The extinguishment of fire by water application involves four
major mechanisms.
- Cooling
If the
surface of the burning materials is sprayed with water, heat
is absorbed from it in the conversion of water into vapour
and by conduction. When the temperature has fallen below that
at which the material will give off sufficient vapour to support
combustion, the fire will go out. The efficiency of the cooling
effect depends on complete coverage of the surface by water
spray of suitable drop size and density of application.
- Dilution
The cooling
process may be assisted by the dilution of flammable gases
with water vapour, thus weakening the flammable mixture to
a point where burning can not continue.
Water spray may extinguish fires involving some water soluble
liquids by diluting them below the point at which sufficient
vapour is given off to support combustion. The volume of water
and the time necessary for extinguishment will vary greatly.
- Smothering
Each droplet
of water when vapourised occupies approximately 1,700 times
its former volume. If sufficient steam is generated by the
heat of the fire, oxygen is displaced or excluded and the
fire is smothered by lack of oxygen support combustion.
- Emulsification
Water sprayed
in a certain manner on medium and heavy oils agitates the
surface sufficient to produce an emulsion. The effect is to
make the surface of the oil (the seat of fire) temporarily
incombustible and stop and inhibit the evolution of flammable
vapoures. This principle is used in high velocity water spray
systems such as the Mulsifyre System.
b)
Control of Burning
Fires involving liquids with flash points below 660 Centigrade
can not always be extinguished by water. Gases liberated from
highly volatile liquids are usually heavier than air and form
explosive mixtures which can ignite at considerable distance
from the source of leakage.
Properly designed medium velocity water spray can be successfully
employed for flame control and controlled burning of liberated
gases to create a safe situation until the necessary action
has been taken to seal leakage, drain burning liquids etc.
It is important in these circumstances to use the correct
density of application of spray and suitable drop size and
velocity to minimise agitation of the burning liquid and prevent
undue air entrainment.
c)
Exposure protection
In many plants and processes the exposure of certain pressure
vessels, structures and tanks surrounding fire presents an
extremely serious hazard to life and property particularly
if by nature of the risk and process involved, fire fighting
operation are likely to be protected and difficult. Examples
are solvent recovery plants, LPG storage etc.
Properly designed water spray is effective in protecting vessels,
structures and plant exposed to heat from surrounding fire,
preventing dangerous temperature rise and distortion and collapse
of structures. Water spray of the correct drop sizes, drop
velocity and rate of application of the risk should be designed
to civer the whole of the surface concerned.
In all but the most exceptional situations the principles
of automatic operation should be adopted whether the objective
be extinguishment, control of burning of exposure protection.
Even though plants are continuously manned by trained personnel
it is vital for maximum effectiveness to achieve water application
quickly and independently of the human element.
Scope
and Limitations
It
is difficult to imagine a fire protection problem involving
industrial buildings or plant which can not effectively be
solved by the use of a properly designed water spray system
along the principles described earlier. There are a few special
circumstances, however, where the application of water
spray
might result in an effect, dangerous to plant and
personnel and these may be briefly described as :
- Materials
which react chemically with water, sometimes violently to
produce substances dangerous to life and which can not be
cooled to a temperature below that at which chemical decomposition
is self-sustaining.
Examples
a) Nitrate
and other fuse salts
b) Certain carbides
c) Flammable peroxides
d) Alkali metals such as sodium and potassium
e) Certain hydrides
f) Certain anhydrides
- Flammable
liquids in open containers and at temperature higher than
the boiling point of water. Penetration of water below the
surface would cause rapid steam generation and possible
violent boil-over.
- Molten
solids in open containers and at temperatures higher than
the boiling point of water. Certain of these materials such
as asphalt, or tar, can be extinguished by medium velocity
small drop size water spray system of limited duration due
to the formation of crust on the surface. Careful consideration
of the material and type of water spray is essential.
Low flash
point flammable liquids in open containers without adequate
overflow and drainage facilities. Suitable bounding is necessary
to prevent spread of fire beyond the protected area.
- Equipment
operating at very high surface temperatures without lagging
or similar protection from direct water application.
There
is a wide range available of fixed protection equipment using
water as the medium.
Continuing
development of such systems is directed to improving the form
of water application for the increasing diversity of risks
and achieving extinguishment or control with the minimum amount
of water. The following is a list of the main types of systems
available with a brief description of the main applications.
Automatic
Sprinklers
Although
automatic sprinkler installations have been commercially
available since 1882 and have since been installed in a steadily increasing
range of industrial and commercial premises, is true to say
that many facets of sprinkler protection are little understood.
It may be worthwhile, therefore, to describe briefly the
basis principles involved. Sprinklers fixed on pipe work charged
with water spaced at regular intervals installed throughout
the protected building and are connected to a reliable water
supply. When a fire occurs, only the sprinklers in the immediate
vicinity of the fire automatically operate the discharged
water to control the fire. Sprinklers are not so sensitive
that they operate at a stage when prompt manual first aid
operations still have a chance of success. These factors
limit the damage to very low proportions. The operation of any one
sprinkler causes an alarm to sound to bring help to the scene
so that the water may be shut off as soon as the fire has
been extinguished. The value of the automatic sprinkler system
in the eyes of Fire Insurance Companies whose estimate is
based on actual calculations, is shown by the high annual
rebates on fire insurance premium which are allowed to the
owners of sprinklered premises. The rebates vary according
to the risk and it is not possible to lay down any hard and
fast rules. For approved installations conforming with the
standards laid down by the Tariff Advisory Committee of India,
the saving is seldom less than about 40% of the premium normally
charged for the insurance of a corresponding risk without
sprinklers.
Maintenance,
Fire Drill & Training
Just by
installation of sprinkler system does not ensure safety.
It
has been observed that if maintenance is not done, at the
time of emergency, the system installed, fail to deliver.
Regular
fire drills should be conducted to create the awareness/preparedness
among the occupants.
Testing
of systems and training should also be imparted to the occupants
during such drills.
Sprinkler
System Components
- Sprinkler
- Installation
Control / Deluge / Alarm Valve
- Water
Motor Gongs
- Flow
Switches
- Solenoid
Valves
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