
The water spray fire extinguishing system is developed based on the automatic sprinkler system. It is mainly used in places where the fire spreads quickly and is suitable for water but the automatic sprinkler system is difficult to protect. The system is a fixed fire extinguishing system that uses a water mist nozzle to decompose the water flow into fine water mist droplets under a certain water pressure for fire extinguishing or protective cooling. The water spray fire extinguishing system can not only extinguish solid, liquid, and electrical fires, but also provide protective cooling for liquefied hydrocarbon storage tanks and other facilities or equipment with high fire hazards and difficult fire fighting. It is widely used in petrochemical, electric power, metallurgy, and other industries. In recent years, the water spray fire extinguishing system has been popularized and applied in the brewing industry. In addition, the protection object of the water spray fire extinguishing system covers industrial facilities and equipment such as the electric power and petrochemical industry, which is different from the automatic sprinkler fire extinguishing system.
Terms Related to Water Spray Fire Extinguishing Systems
- Water spray fire protection system
It is composed of a water source, water supply equipment, pipeline, deluge alarm valve (or electric control valve, pneumatic control valve), filter and water mist nozzle, etc., and sprays water mist to the protected object for fire extinguishing or protective cooling. - Transfer pipe
A pipeline that detects fires using closed nozzles and transmits signals using changes in air pressure or water pressure. - Supply intensity application density
- Response time response time
From the start of the water supply facilities of the system to the time from the most unfavorable point in the system to the spraying of water mist from the water mist nozzle. - Spray nozzle
Under certain pressure, the water flow can be decomposed into water droplets with a diameter of less than 1mm in the set area, and the nozzles can be sprayed out according to the designe - Effective range effective range
When the nozzle sprays horizontally, the horizontal distance between the highest point of the water mist and the plane where the nozzle is located is perpendicular to the axis of the nozzle. - Water spray cone
The cone is formed by the water mist within the effective range of the water mist nozzle. - Deluge alarm valves unit
It is a device composed of a deluge alarm valve, solenoid valve, pressure switch, hydraulic alarm bell, pressure gauge, and supporting general valves.
Basic Design Parameters
The basic design parameters include design supply intensity, continuous spray time, protection area, working pressure of water mist nozzle, and system response time. The basic design parameters need to be selected according to the protection purpose of the water spray fire extinguishing system and the category of protection objects.
The supply intensity, response time, and continuous spraying time of the water spray fire extinguishing system are the basic design parameters to ensure the fire extinguishing or protective cooling effect. The respective supply intensity, continuous spray time, and response time are stipulated for different protection objects.
(1) The protection purpose of the protected object
1) Water spray protection for oil-immersed transformers
Transformer oil is a hydrocarbon liquid mixture mainly composed of cyclic hydrocarbons extracted from crude oil. The initial boiling point is greater than 300℃, and the flash point is generally above 140℃. Cracking will produce a small amount of hydrogen and light hydrocarbons, and these gaseous combustible substances are prone to explosion. When water spray causes a transformer fire, the suffocation effect of the water vapor formed by the evaporation of water mist is obvious, and the fire can be controlled quickly. When the transformer has fewer openings, good ventilation conditions are not formed inside and outside the transformer, and the fire scale is small. Water spray can The fire was successfully extinguished; when the transformer has many openings, good ventilation conditions are easily formed inside and outside, and the fire scale is large, it is difficult to extinguish the fire with a large spray intensity.
2) Water spray protection for liquefied hydrocarbon storage tanks or similar liquid storage tanks
Hydrocarbon gases (C1~C4) that are gaseous at room temperature are called liquefied hydrocarbons after they are pressurized or (and) cooled and become liquid. Other similar liquids refer to liquids with similar physical and chemical properties to liquefied hydrocarbons, such as ethylene oxide, Dimethyl ether, liquid ammonia, etc. For liquefied hydrocarbon storage tanks or similar liquid storage tanks, the purpose of setting up a water spray fire extinguishing system is mainly to cool down the storage tanks and prevent boiling liquid steam explosions. If the LPG storage tank leaks, the superheated liquid will quickly vaporize to form an LPG vapor cloud. After the steam cloud meets the fire source and explodes, it will backfire and ignite the leak source to form a jet fire, exposing the storage tank to the flame. If at this time If the storage tank cannot be effectively cooled, the liquid in the tank will rapidly expand and boil, the temperature of the tank wall (drywall) above the liquid level will rise rapidly and the strength will decrease. At the same time, the vapor pressure will increase abnormally. After a certain period, the drywall will produce thermoplastic cracks, the pressure in the tank will drop sharply, the liquid will be in a deep superheated state, and the rapid expansion and gasification will generate a large amount of steam, which will cause the boiling liquid steam explosion.
(2) Supply intensity and continuous spray time
US NFPA15 and API 2030 treat the continuous spraying time of the water spray fire extinguishing system as an engineering judgment problem, and the protective cooling system requires continuous spraying for several hours without interruption.
The Japanese Insurance Association stipulates that the continuous spraying time of the water spray fire extinguishing system should not be less than 90 minutes. The Japanese Fire Protection Law and Japan’s “Liquefied Petroleum Gas Security Rules” stipulate the continuous spraying time of specifically protected objects as follows: communication room and places where combustibles are stored, garages and parking lots require water sources to ensure that the water volume of continuous spraying is not less than 20 minutes.
prEN14816 stipulates the spray time for various protection objects of water spray, the shortest is 30 minutes, and the longest is 120 minutes.
(3) Main basis for response time
The water spray fire extinguishing system is generally used for protecting objects with high fire risk, fast fire spread, and difficult fire extinguishing. When a fire breaks out, if the fire is not extinguished or protective cooling is not carried out in time, great losses will be caused. Therefore, the water spray fire extinguishing system must not only ensure sufficient supply intensity and continuous spray time but also ensure that the system can start quickly. Response time is a performance index to evaluate the start-up speed of the water spray fire extinguishing system, and it is also one of the basic parameters that must be considered in system design. When the water spray fire extinguishing system is used for fire extinguishing, it is required that the system can start quickly to extinguish the fire at the initial stage. Therefore, the system response time is stipulated to be no more than the 60s. When the system is used for protective cooling, different response times are stipulated according to the degree of danger of the protective place and the operability of the system. For example, a liquefied hydrocarbon storage tank with high risk, in case of fire, needs to be cooled as soon as possible to avoid an explosion of boiling liquid vapor. Therefore, the response time is stipulated to be no more than 120s. For liquid storage tanks with a relatively low degree of danger, after a fire occurs, the fire will not cause great harm to the storage tank in a short period. Therefore, the specified response time is not greater than 300s.
Arrangement of nozzles and pipes
- The plane layout of the water mist nozzle can be rectangular or rhombus. When arranged in a rectangular shape, the distance between the water mist nozzles should not be greater than 1.4 times the radius of the water mist cone bottom of the water mist nozzles; when arranged in a diamond shape, the distance between the water mist nozzles should not be greater than 1.7 times the water mist nozzles The radius of the bottom circle of the water mist cone.
- When the protection object is an oil-immersed power transformer, the layout of the water mist nozzles should meet the following requirements:
1) The orifice of the transformer insulator rising seat, the oil conservator, the radiator, and the oil sump shall be protected by water mist nozzles;
2) The horizontal distance and vertical distance between water mist nozzles should meet the requirements of the water mist cone intersection. - When the protected objects are liquid and combustible gas storage tanks, the distance between the water mist nozzle and the outer wall of the protected storage tank should not be greater than 0.7m.
- When the protection object is a cable, the arrangement of the water mist nozzle should make the water mist surround the cable.
- When the protection object is the conveyor belt, the arrangement of the water mist nozzle should make the water mist completely envelop the head, tail, and upper surface of the upward belt of the conveyor on fire.
- When the protected objects are indoor oil-fired boilers, electro-hydraulic devices, hydrogen-sealed oil devices, generators, oil circuit breakers, steam turbine oil tanks, and coal mill lubricating oil tanks, the water mist nozzles should be arranged around the top of the protected objects, and should Make the water mist spray directly to and completely cover the protection object.
- Systems for protecting Class A, B, and C liquid storage tanks
1) The cooling water ring pipe of the fixed roof storage tank and the internal floating roof storage tank treated as the fixed roof storage tank should be arranged along a single ring at the top of the tank wall. The water supply intensity should be calculated twice.
2) When the wind resistance ring or reinforcement ring of the storage tank has no flow diversion facilities, a cooling water ring pipe should be installed under it.
3) When the cooling water ring pipe on the storage tank is divided into two or more arc-shaped pipe sections, the arc-shaped pipe sections should not be connected, and the water pipes should be connected from the outside of the fire embankment, and the water pipes should be connected outside the fire dike respectively. A control valve that can identify the opening and closing state is installed on the water pipeline.
4) The cooling water standpipe should be fixed on the tank wall with pipe clips, and the distance between them should not be greater than 3m. The lower end of the standpipe should be equipped with a rust cleaning port, the distance between the rust cleaning port and the top surface of the tank foundation should be greater than 300mm, and the length of the pipe section collecting rust should not be less than 300mm. - The connection between the standpipe and the horizontal pipeline in the tank group used to protect the liquefied hydrocarbon or similar liquid storage tank should be able to eliminate the stress caused by the settlement of the storage tank.
System Components
(1) The selection of the mist nozzle should meet the following requirements:
1) Centrifugal atomization water mist nozzles should be used to fight electrical fires;
2) The water mist nozzles installed in indoor dusty places should be equipped with dustproof caps, and the water mist nozzles installed outdoors should be equipped with dustproof caps;
3) The centrifugal atomization water mist nozzle should be equipped with a columnar filter.
(2) For systems with a response time not greater than 120s, a deluge alarm valve group should be installed, and the function and configuration of the deluge alarm valve group should meet the following requirements:
1) The deluge alarm valve group receiving the electric control signal should be able to be opened electrically, and the deluge alarm valve group receiving the transmission pipe signal should be able to be opened hydraulically or pneumatically;
2) It should have remote manual control and on-site emergency mechanical start-up functions;
3) The opening and closing status of the deluge alarm valve should be displayed on the control panel;
4) It is advisable to drive the hydraulic alarm bell to alarm;
5) Pressure gauges should be installed at the inlet and outlet of the deluge alarm valve;
6) A flushable filter should be installed before the solenoid valve.
(3) When the system water supply control valve adopts electric control valve or pneumatic control valve, it shall meet the following requirements:
1) It should be able to display the open and closed state of the valve;
2) It should have the function of receiving control signals to open and close the valve;
3) The opening time of the valve should not be greater than 45s;
4) It should be able to alarm when the valve fails, and display the cause of the failure;
5) It should have the on-site emergency mechanical start function;
6) When the valve is installed in the valve well, the stem of the valve should be lengthened, and the electric actuator should be higher than the top of the well;
7) The pneumatic valve should be equipped with a reserve gas tank. The volume of the gas tank can be calculated according to the gas volume required for opening and closing all the pneumatic valves connected to the gas tank 3 times.
(4) The pipeline before the deluge alarm valve should be equipped with a flushable filter, the filter screen should be made of corrosion-resistant metal material, and the basic size of the mesh should be 0.600mm ~ 0.710mm.
(5) The water supply pipeline shall meet the following requirements:
1) The pipes between the filter and the deluge alarm valve and behind the deluge alarm valve shall be hot-dip galvanized steel pipes, stainless steel pipes, or copper pipes inside and outside; seamless steel pipes shall be used for pipes that need to be bent;
2) The working pressure of the pipeline should not exceed 1.6MPa;
3) When the system pipeline adopts galvanized steel pipe, the nominal diameter shall not be less than 25mm; when stainless steel pipe or copper pipe is used, the nominal diameter shall not be less than 20mm;
4) The system pipes should be connected by grooved pipe joints (clamps), flanges, or threaded connections, and ordinary steel pipes can be welded;
5) For grooved pipe joints (clamps), the material of the shell should be ductile iron with a grade not lower than QT 450-12;
6) The grooved pipe joint (clamp) sealing ring and non-metallic flange gasket in the protection zone shall pass the dry burning test specified in Appendix A of this specification;
7) A water discharge valve or a sewage outlet should be set at the lower part of the pipeline.
