|
| |
|
|
|
Home  Fire Special Hazards
Water Spray Systems
|
Water Spray
Water most widely used and available fire extinguishing material. The form in which it is applied to
a fire, that is, solid stream, coarse has a strong influence on its spray, or fine spray, effectiveness
and the efficiency with which it controls or extinguishes fires.
Types of Water Spray
Applied as a solid stream to burning oil, water is not effective because it plunges below the oil
surface and the oil floats on top of the water continuing to vaporize and burn. When broken up into
coarse and fine sprays, water becomes our most economical and effective means of extinguishing fire in
the medium and heavy oils. When it is broken up into fine, misty spray and when the entire surface of
vaporizing liquid can be covered simultaneously, water spray is effective in extinguishing the flames
above light oils such as kerosene and, under especially favorable conditions, even some of the lighter
petroleum fractions. This is because water in spray form exerts its extinguishing effect at the oil
surface before it sinks into the body of the oil.
Water Spray Applications
Advances in fire extinguishment research have broadened and increased applications for which water
spray is both a highly effective and the most economical means of extinguishing fire. A combination
of its cooling effect when it vaporizes and the fact that its vapors are non-flammable has made it
effective in preventing the spread of fire from volatile flammable liquids too dangerous in their vapor
forms to be extinguished safely. This method is called controlled burning.
The design of SimplexGrinnell water spray systems is based upon the National Fire Protection
Association, Standard #15, which has been accepted by Factory Mutual and most other independent
insurance organizations.
SimplexGrinnell has been supportive and extremely involved in the development of the standard based
upon a half century of testing by our Research and Development Division and field tests of nearly all
special hazard systems.
This theoretical and actual test experience is passed on to our designers and creates a firm basis
for water spray system design.
Water has been and is the most widely used and available fire extinguishing material. Its application
as a solid steam coarse spray or fine spray and density determines its effectiveness. The application
method of water spray and density must be selected by the trained Fire Protection Designer to accomplish
the intended goal.
Basic theoretical objectives include cooling, controlled burning, vapor distribution and emulsion
extinguishment but it is usually not possible, nor is it advisable, to try to accomplish all of these
objectives simultaneously. In the case of liquefied flammable gases, extinguishment may create a more
destructive hazard.
To achieve any of the objectives, the designer must determine the method of application considering the
inherent mechanical equipment constraints and the possible physical interferences to the spray patterns,
water rundown adherence, thermal updrafts, wind conditions, and general mechanical obstructions which
shield the fire or sources of ignition.
The design complexity of a water spray system can be additionally better understood when hydraulic
design is considered where theoretically each nozzle could be utilized to provide a definitive spray
pattern and volume of water.
Solid stream is rarely utilized with fixed automatic systems since its advantage is greatly dependent
upon judgment. It is best utilized with manual hoses and adjustable hose nozzles which require a human
decision as to the direction and type of discharge.
Special hazards protection generally assumes the fire cannot, or should not, be extinguished and the
control theory is based upon limiting heat input to equipment.
Water properly applied to equipment surfaces will limit heat input by cooling to 6,000 BTU per hour per
square foot of exposed surface. This assumes the equipment has a venting rating to permit a rate of heat
input of 6,000 BTU per square foot per hour.
Controlled burn water application is basically similar to exposure protection. Where there is a leak of
product but no fire, the water spray and associated turbulence promotes dilution. Should the leak ignite,
the double effect of the water spray allows the controlled burn by cooling the container but allowing the
product to burn. In the case of the liquefied gas leak, the controlled burn allows control until the leak
can be stopped thus preventing the possible pocketing of the explosive gas.
Special hazards, such as transformers and various pieces of oil handling equipment within electrical
generation plants, can be protected by cooling and if the equipment is located in a contained (dike)
area with adequate drainage and any liquids involved have a flash point of 200 deg. F or higher, it may
be possible to design the fire protection system to also extinguish the fire.
Years of first hand experience in the extinguishment of hundreds of test fires with water spray have
shown the need for matching the type of water spray discharge to the hazard protected. The importance
of droplet sizes, velocities, area coverage, and optimum application rates have been extensively studied
and resulted in our development of a complete line of normally-open, small droplet spray heads;
normally-closed, automatic opening small droplet spray heads; and coarse droplet projectors. Proper
protection for any specific hazard requires selection of the most suitable spray producers in respect
to such variables as water pressure available, indoor or outdoor location, flammable material involved
in the hazard, distances of projection from nozzle to hazards, rate of application, nearness of other
exposures, etc.
Vapor Dilution - The factor which distinguishes emulsion extinguishment of oil fires from other methods
using water spray mist is that, while all work by vapor dilution, emulsion extinguishment adds a second
source of water vapor. The ordinary source is from droplets partial vaporized while going through the flame
zone. Emulsion extinguishment adds to this water vapor from the film suspended in the emulsion at the
surface of the oil. It is obvious that to get the maximum effect of vapor dilution, the droplets used must
be large enough to travel through the flame zone before being completely vaporized. By vaporizing at the
surface of the oil, where the flammable oil vapors are too rich to burn, the flames are starved by being
fed an unburnable mixture of water vapor, oil vapor and air... so the fire goes out.
Effective Percentage of Water Vapor - Laboratory studies of the flammable vapors from common oils have
determined the minimum percentage of water vapor required to make the resulting mixture non-flammable
in air. More than this minimum percentage of dilution is provided by the emulsion extinguishment method.
With oils lighter than kerosene emulsion extinguishment is not practical since no residual emulsions
are formed. Extinguishment of fire in the lighter oils depends largely upon water vapor from droplets
vaporized on their way through the flames to the oil surface. Enclosures around the hazardous areas
assist this action.
Oil Hazard Extinguishment - With the medium and heavy oils on which emulsion extinguishment is commonly
used, it is an advantage to create a surplus of emulsion. This will remain on the surface for a
considerable period of time and give an additional factor of safety against flash-back or re-ignition
of the fire.
Cooling Applications - Cooling applications of water spray owe their success to the ready absorption of
heat by water and its dissipation at relatively low temperatures by vaporization and radiation. A gallon
of water applied at 50 deg. F which is all vaporized into steam at 212 deg. F will remove over 9,000 BTUs
of heat. When water spray is used to maintain a protective film over the surface of a tank containing a
liquid or compressed flammable gas, heat will flow from the warmer tank to the water film and the tank
temperature will tend to reduce to that of the water. This cooling is so effective that tanks whose
temperatures have been raised by exposure to fire before the water spray is turned on may be cooled
considerably, even while exposed to flames, when water spray is applied. Rate of application and
distribution of the water spray over the protected surface are important factors in determining the
cooling effectiveness and are, in turn, determined by the design of the system.
Gas and Liquid Protection - For the protection of highly-flammable volatile liquids and liquefied gases,
which are kept in the liquid state by pressure and temperature conditions within their containers,
controlled burning is made possible by using both water's capacity for cooling and the diluting
effectiveness of its vapor. Mixtures of flammable gas or vapor with air can exist in three conditions:
(1) too rich to burn, (2) within the flammable or explosive limits; (3) too lean to burn.
Effects in Interface Area - When such a fuel as liquefied petroleum gas is released upon the ground
in the open, it promptly vaporizes and starts mixing with the air to form mixtures which are too rich
to burn where the vapor is concentrated. There is always an interface between to too-rich-to-burn
concentration and the surrounding air where mixing with the air produces mixtures within the flammable
or explosive limits. Ignition at this interface produces immediate flame spread. Released vapor or gas
is only safe against ignition after it has passed beyond its first two conditions, too rich to burn and
within the flammable limits and reached its third condition in which it is part of a mixture too lean
to burn.
Promoting Air Dilution - Air by itself can dilute flammable gases or vapors to a safe condition, too
lean to burn, provided enough air is available and it is moving fast enough and with sufficient
turbulence, because air is approximately four-fifths inert gases (79%). Water vapor is a better
dilutant because it is all non-flammable and it is abundant. It can be produced at the base of the
fire for maximum effectiveness. One gallon of water expands to form approximately 1,675 gallons of
water vapor. By applying water in the form of spray to the outer surface of a container of liquefied
gas, the cooling effects of evaporation and conduction prevent the buildup of dangerous pressures
within the tank. The water vapor produced by heating the water dilutes nearby mixtures to reduce the
intensity of the fire and control the burning rate.
Controlled Burning - Where there is a leak but no fire, the water spray produces air entrained turbulence,
promoting dilution by air plus the water vapor resulting from spraying.
This double effect protects Liquefied Petroleum Gas storage tanks in cases of leakage or fire. If fire
follows the leakage, it can be permitted to burn itself out without developing bursting pressures in
the tank. Such fires should only be extinguished by stopping the flow of L.P.G. If permitted to drift
away downwind or downhill, the vapors would collect in pockets and form explosive mixtures capable of
far greater destruction than would result from controlled burning near the tank where the leak occurs.
Selection of Nozzles - Years of first hand experience in the extinguishment of hundreds of test fires
with water spray have shown the need for matching the type of water spray discharge to the hazard protected.
The importance of droplet sizes, velocities, area coverage, and optimum application rates have been
extensively studied and resulted in our development of a complete line of normally-open, small droplet
spray heads; normally-closed, automatic opening small droplet spray heads; and coarse droplet projectors.
Giving proper protection to any specific hazard requires selection of the most suitable spray producers
in respect to such variables as water pressure available, indoor or outdoor location, flammable material
involved in the hazard, distances of projection from nozzle to hazards, rate of application, nearness
of other exposures, etc.
Fire Records - Many potentially serious fires are extinguished while they are still so small that no
published report is made of them. For this reason only partial records are available on the performances
of the special hazard water spray fire protection systems which SimplexGrinnell has installed.
In contrast to this meager record, news services across the nation broadcast the stories of fire losses
which start in unprotected special hazard areas, quickly gain power and momentum, to end up as major
fire catastrophes.
An excellent record for control or extinguishment of electric power transformer fires has been built up
since the first installation of water spray for electric transformer fire protection was made in America
by Grinnell in 1935.
Other reported electric transformer fires successfully extinguished have included fires which started
when an explosion blew the handhole cover off a high voltage disconnect chamber; when rain and wet coal
dust on an insulator caused a flash-over; when a primary winging failed to ground; when a fault on a 26
KV ring bus arced from a defective pot head to a conservator tank, producing 2 small holes; when repeated
operations of automatic circuit breakers during an electric storm ignited oil spray and spillage; and
when an arc flashed below the oil surface from low voltage leads to the iron core.
 Dry Chemical Systems
Foam Protection
Gaseous Systems
Pilotex® Ultra High Speed Deluge System
Water Spray Systems
|
For more information on fire products, visit
www.tycosafetyproducts-us.com
or
www.tyco-fire.com
|
|
|
|
|
|
Fire
