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Active In SP

Posts: 43
Joined: Jun 2009
14-06-2009, 08:58 AM

Email “ rk_fire@rediffmail.com
Depending on system design pressure, water
mist system classified into three categories
a) Low pressure water mist system: where
the pressure encountered by the system is
175 psi or less, the same pressure range
used for most of standard water spray
b) Intermediate
system: where the pressure encountered
by the system is 175 psi or 500 psi.
c) High pressure water mist system: where
the pressure encountered by the system is
500 psi & above.
Depending on droplet size water mist system
classified into three categories.
Class 1: the 90% of drops in water spray are less
than 200 microns.
Class 2: the 90% of drops in water spray are less
than 400 microns.
Class 3: a water mist meeting the foregoing
definition that is neither class 1 nor class2
2. Flux density: it is the number of droplets or
mass of water mist per volume of space within a
geometric voids, that interact with fire to achieved
heat absorption, oxygen displacement and radiant
heat attenuation. Flux density is usually perceived
as a volume of fog or water mist in a given space
and is expressed as liter per cubic meter per
extinguishment, correct flux density is required.
Flux density is greatly determined by orifice size,
nozzle type and the operating pressure of water
mist system.
Definition: Water mist is a fine water spray with
droplets size less than 1000 microns at a distance
of one meter from the discharge nozzle. This water
has been found to be effective for
1. Fire Extinguishment: where the fire intended
to suppressed with no further burning of
combustible materials.
2. Fire suppression: where the rate of heat rate
release expected to be sharply reduced to the
point that re-growth would not occur.
3. Fire Control: Where the fire growth has been
limited by pre-wetting combustible materials in the
vicinity of fire.
4. Temperature control: Where the temperatures
of confined place are reduced during combustible
material to allow safe egress and reduced
5. Exposure protection: where combustible
materials adjacent to the fire are wetted to delay
their ignition.
There are four characteristics that determine the
effectiveness of water Mist as fire extinguishing
1. Drop size distribution:
2. Flux Density
3. Spray momentum
4. Ingredient Additives
1. Drop size distribution: The drop size
distribution refers to the range of drop contained in
a representative sample or geometric void,
existing in the form of water mist or fog.
The droplets size distribution of water spray
system depends on several factors. The droplets
size varies on the area of spray, the distance from
nozzle, type of nozzle, system design pressure,
colloidal action of droplets and ambient
temperature. Page 36

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 27
3. Spray momentum: spray momentum is a
function of the mass of air and water per unit
volume of spray having regard of the velocity of
that volume. For this reason, a water mist system
that yields a mixture of finer and coarser droplets
will facilitate spray momentum. To achieve
extinguishment, direct impingement is required as
the downward momentum of a water mist spray
must be sufficient to
the upwards
convection current of the fire plume (heat vector
turbulence), and other gases or phenomena
originating from combustion. The water mist must
be capable of penetrating the combustion zone
and prevent flame propagation
4. Ingredient Additives: water mist system is
capable of operating with fresh or sea water or
with additives. The type of extinguishing media
depends on fire risk area and availability of
resources. Generally fresh water is more desirable
for water mist system because of it is less
corrosive on machinery component and other
equipment. However, for protection of offshore
platform and installation, processed water or even
sea water can be used because as per
international standard (SOLAS-74), water mist
system capable of running continuously for 30
Additive such as foam compound can enhance
extinguishing performance of water mist system.
Additives as foam compound with 3% to 6%
performance with class A and class B fire fighting
foam, which have great heat absorption, wetting
and film forming capabilities. Performance result
shows that addition of foam compound gives
excellent result on hydrocarbon fires, plastic,
fibrous & rubber fire and extinction achieved in
much shorter time spans then with plain water.
Possibility and Feasibility of Water Mist
System for Protection of Oil & Gas Installation:
1. Local Application:
a) Trolley
mounted portable
water mist
Extinguisher (50liter Capacity):
It is similar to
trolley mounted DCP
extinguisher. This will be replacement of
50 kg DCP & 50 litre Mechanical foam
Ultra-high pressure water mist jeep
(500 ltr Water & 50 ltr foam capacity):
It is already introduced in ONGC. It is very
helpful to fight medium oil, gas and grass fire. It
is also very useful for attending stand by duties
during Hot Job & HOC/ CTU Job. It consists of
high pressure pump, water & foam storage tank,
high pressure hose & water mist gun, mounted
on a Jeep.
2. Semi-fixed high pressure water mist
It is similar to Jeep mounted high pressure water
mist system, consists of high pressure pump,
water & foam storage tank, and high pressure
hose & water mist gun. It is fixed on wheel or any
particular structure at location. It will extremely
helpful for protection of drilling rig, work over
Riggs &GCS and offshore installation in place of
foam water hose reel.
3. Fixed water mist suppression system:
Fixed water mist suppression system with
automatic detection and actuation facility will be
installation/platform including
1. Offshore platform structure & vessels from gas
jet, flammable liquids fires.
2. Gas turbines and generator rooms.
3. Accommodation and public spaces.
4. Electronic equipments
e.g. telecommunication equipment, electrical
5. Electrical hazards, such as transformers,
switches, circuit breakers and rotating
6. Kitchen, kitchen vent hood.Page 37

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 28
Conclusion: In addition to its strong potential as
Halon replacement, the potential uses for water mist
technology are very diverse, which create a solid
basis for its induction in onshore & offshore
installation, as halon is being phase out from 2010.
Since future of halon alternate is also under suspicion
after Kyoto Protocol, water mist is becoming
alternative for fire suppression in future.
1. Design of special hazard and fire alarm
System by Robert Gagnon

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