Water mist extinguishing system introduction and comparison of advantages and disadvantages

First, the origin and development of water mist

The so-called “water mist” has mainly water mist , water fog , and fine water spray in English, and water mist is the most used in the literature. The application of water mist in fire protection began in the 1940s when it was mainly used in special places such as transportation vehicles. Due to environmental issues, halogenated fire extinguishing agents have been phased out, and the potential advantages of water mist as an extinguishing agent for the environment have led to continuous expansion of its application. Water mist fire suppression systems are used in residential buildings, flammable liquid storage facilities and electrical appliances. Research on equipment has yielded encouraging results.

In 1993 , representatives from the engineering and research sectors, manufacturers of water mist systems, insurance companies, administrative departments, and industrial users formed the NFPA Technical Committee on Water Mist. Fire Suppression Systems ), the committee began to develop the NFPA standard for the specification of fine water mist technology as a basis for design and installation.

In 1996, in Boston, Massachusetts, United States each year at the annual meeting May 20-23, the water mist fire suppression technology committee submitted a water mist system specification (NFPA 750, Standard on Water mist Fire Protection Systems), and received The United States Fire Protection Association approved. The specification on July 18 the same year promulgated by the National Institute of Standards with effect from August 9, the same year on July 26, 96 NFPA 750 version was approved by the US national norms.

Second, the definition of fine mist

"Mist" (water mist) with respect to the "water spray" (water spray) concept, called a fine mist, using a special nozzle, high pressure water through the water particles produced. In NFPA 750 , the definition of fine water mist is: at the minimum design working pressure, the diameter of the water particle Dv0.99 measured in the thickest part of the water mist at a distance of 1 meter from the nozzle is not greater than 1000 μ ( the meaning of Dvf) . : Dvf is the ratio of the cumulative diameter of the water particle diameter, its particle diameter from 0 to the diameter of a certain particle and the corresponding total cumulative volume, eg Dv0.99 )

According to the size of the water particles in the water mist, the fine water mist is divided into three levels. The first stage is a water mist with Dv0.1 = 100 μ Dv0.9 = left connection portion 200 μ, which represent the most fine mist.

Mist Level 2, Level 1 is the limit of the water mist Dv0.1 = 200 μ connection portion between 400 μ with Dv0.9 =. This fine water mist can be generated by high pressure nozzles, dual-flow nozzles, or many impact nozzles. Due to the presence of larger particles of water, Class 2 fine mists are more likely to generate larger flows than Class 1 fine mists .

The third -stage water mist is greater than 400 μm in Dv0.9 , or the right-to- Dv0.99=1000 μm in the second stage fine water mist boundary . This water mist is mainly produced by medium pressure, small hole showerheads, various impact nozzles, and the like.

Studies have shown that it is necessary to extinguish water mist particles in Class B fires that are smaller than 400 μm, while larger particles are effective for Class A fires because fuel is wetted. Because of this, the definition of water mist includes a Dv0.99 of 1000 μ. Mist defined in NFPA 750, contains both a portion of the water spray system (Water Spray) NFPA 15 defined, but also contains a water mist at a high pressure common sprinkler system (Sprinklers) produced. In general, water mist refers to water fog with Dv0.9 less than 400 μ.

Third, the mechanism of water mist fire extinguishing and its application

The key to the success of a water mist fire extinguishing system is to increase the surface area per unit volume of water particles. After the water is micronized, even the same volume of water can increase the total surface area. The increase in surface area makes it easier to absorb heat and cool the combustion reaction. Heat-absorbing water particles are easily vaporized and increase in volume by approximately 1700 times. The generation of water vapor not only dilute the oxygen concentration near the flame, suffocates the combustion reaction, but also effectively controls the heat radiation. It can be considered that water mist fire extinguishing is mainly through the dual effects of efficient cooling and anoxia.

The relationship between the distribution of the diameter of fine water mist particles and the ability to extinguish fire is a complex issue. Generally, the first-stage and second-stage water mist for fire extinguishing effect of the liquid pool of fuel is preferably, but not stirring pool level. Under normal circumstances , it is more difficult to extinguish Class A combustibles with Class 1 water mist. This may be because the water mist cannot penetrate the carbonized layer and soak the burning material. However, because of the high spray velocity of fine water mist, it is still possible to extinguish Class A combustibles when the combustion is on the surface or in an enclosed space, which is conducive to the reduction of oxygen. This shows that for a certain combustion, the particle diameter of the water mist is not the only factor that determines the fire extinguishing ability. The fire extinguishing effect of the system is also closely related to the spray direction, speed, and spray strength of the fine water mist relative to the flame.

NRL (Naval Research Laboratory) had the following three kinds of fire-extinguishing effect of water mist system to do a series of tests: a single low-pressure flow system (Single-Fluid Low-Pressure Systems ), single-stream high-pressure system (Single-Fluid High-Pressure Systems and Twin-Fluid Systems .

Tests have shown that a single-flow low-pressure system produces large particles of water and shows good results in extinguishing deep Class A fires. This is due to its relatively large flow rate ( 3 to 4 times that of a single-flow high pressure system ) , which produces a surface wetting effect, but reduces the ability of the system to extinguish an occluded fire. The fire suppression effect of the single-flow high-pressure system is completely opposite. Because of the smaller diameter of water particles, the ability to extinguish blocked fires is enhanced, and the effect of extinguishing deep Class A fires is reduced . Tests have shown that when a fire in a large space is blocked and the concentration of oxygen in the fire drops below 18% , the flame can be extinguished.

The water mist fire extinguishing system has a much longer time for extinguishing a fire than a halogenated or other gas fire extinguishing system, the former being about 100-200 seconds and the latter being about 10-20 seconds. In addition, the size of the closed space has little effect on the extinguishing effect of the fine water mist. In the open space, the fine water mist system has a distinct advantage over the gas fire extinguishing system.

The size of a fire also affects the fire extinguishing effect of a water mist fire extinguishing system. A burning fire is more likely to be extinguished than a slow burning fire. This is due to the fact that the former consumes oxygen quickly and is accompanied by a large amount of steam and turbulence. While the water mist system sprays water, the temperature of the fire field drops sharply, which helps to artificially extinguish the fire and reduce the property loss caused by heat.

With regard to the electrical insulation of water mist, a foreign company has used the Securipex Fire-Scope 2000 water mist fire extinguishing system to inject the system into an enclosed room with an electric motor, a generator and a switchboard. The voltage is 220~440V . The results showed that during the release process, the resistance reading decreased significantly, but as usual, the equipment was operating normally. In general, as the device becomes dry, the resistance value will return to normal. Whether the water mist system can be used for electrical equipment, the key lies in the size of water mist particles. When a certain kind of fine water mist system is used for electrical equipment, it should be certified by the relevant department.

The water mist system can be used to protect valuable factory production equipment, oil-filled power transformer rooms in high-rise buildings, self-contained diesel generator rooms and oil storage rooms, and fuel oil and gas-fired boiler rooms. High-pressure and dual-flow medium water mist systems can be used in high- and low-voltage power distribution rooms in important high-rise buildings, important electronic communication equipment rooms, power plant control rooms, and gas turbines.

IV. Classification of Water Mist Fire Extinguishing System

A water mist nozzle is a device that contains one or more orifices that atomize water droplets. It is the most critical part of the system. The principle of the water mist nozzle generating water particles is one of the following five ways. The liquid is released at a very high speed relative to the surrounding air, and it is shredded into water particles due to the speed difference between the liquid and the air. On a fixed surface, water particles are generated due to collisions; two streams of similar constituents collide with each other, and each stream of water forms water particles; the liquid vibrates or electronically pulverizes into water particles (ultrasonic and electrostatic atomizers); the liquid is in a pressure vessel It is heated above the boiling point and suddenly released to atmospheric pressure (burst liquid sprayer).

According to different standards, the water mist system can be divided into the following systems:

Low-pressure systems, working pressures less than 12.1 bar (175 psi) ; medium-pressure systems, operating pressures from 12.1 bar ( 175 psi) to 34.5 bar ( 500 psi ) ; high-pressure systems, operating pressures greater than 34.5 bar ( 500 psi ) .

Topical application (Local Application Systems), bearing protective steam turbine unit, the heat pump refinery; total flooding system (Total Flooding Application Systems), to protect the entire protected area, such as the gas turbine plant room. ( 1 psi=0.0689 bar , 1 bar=105 Pa )

Prefabricated system (Preengineered Systems), as the compressed gas cylinder of the double drive system, there are several cylinders and tank specifications, are processed in the plant, selected according to the size of the design protection area; Engineering System (Engineered Systems) , Its composition and working methods are similar to conventional Deluge systems .

Single stream media system (Single Fluid Media Systems) and dual medium spray system (Twin Fluid Media Systems). The single-flow medium system uses only water as an extinguishing agent and is supplied to the nozzles by one-way piping. The system consists of a nozzle, a pump deluge, a detector and a controller. Dual-flow medium spray system, water and atomizing medium are supplied by different pipelines and mixed and collided at the nozzle to generate water particles. The appearance of the water mist nozzle is shown in Figure 3 . The atomizing medium is divided into two ways, one for pushing the water in the tank and the other for the nozzle. The water in the water storage tank is at atmospheric pressure. When the fire is extinguished, it is pushed by the high-pressure gas and enters the protection zone. Atomizing media are high pressure air or other gases that can be mixed with water and produce a fine mist of water.

The water mist fire extinguishing system in practical projects is often a combination of the above categories. The Securipex Fire-Scope 2000 water mist fire extinguishing system, which is widely used, is used as an example. The system is a low pressure, dual flow, and prefabricated system. The working pressure of the gas and water medium in the nozzle is 5.5bar , the water particle size is 200 μ, the nozzle characteristic coefficient is K=4.5L/min.bar1/2 , the nozzle spacing is 3.0m , and the corresponding water spray intensity is 0.95. L/ min. m2 .

Fifth, the advantages and disadvantages of water mist fire extinguishing system

Advantages: Fine water mist technology can use its nitrogen-containing water vapor to effectively separate air from combustibles, thereby rapidly preventing the spread and deterioration of the fire. Its high-speed spray of water can also efficiently clean smoke in the air, which is conducive to the evacuation of people surrounded by fire. Moreover, because of its high speed, its penetrating power is extremely strong. It can reach the root of the fire source and completely control and extinguish the fire, effectively preventing the renewed fire. Moreover, only the nozzles of the whole water mist system are relatively expensive, so the water mist fire extinguishing equipment is relatively cost-effective relative to other fire fighting equipment. Moreover, during the entire process of extinguishing the fire, the water mist utilization technology is also very high in terms of utilization of water, resulting in less water stains and being completely green and environmentally friendly. In addition, the water mist system also has a wide range of characteristics, such as electronic fires, forests, and dangerous solid-liquid gas fires. In these more complex and difficult-to-control areas, the water mist system can play a very important role. Great fire extinguishing effect.

Disadvantages There are mainly two factors that affect the working efficiency of the water mist fire extinguishing system. One is to see if it can produce small and fast water particles, and the other is to see if the water mist can spread to a sufficiently wide space. To meet two requirements, on the one hand, the pressure resistance of the nozzle is very high, so the water mist system is more expensive than the traditional water sprinkler system in terms of cost. On the other hand, the water quality requirements are very high, because if the water quality is poor, it will cause nozzle clogging, which will not only affect the performance of the nozzle, but also cause damage to the nozzle. The water mist system has a longer time for extinguishing a fire than some gas fire extinguishing systems. It is not suitable for objects that can strongly react with water, such as solid potassium, sodium hydroxide, and the like.

Water, as the most original fire extinguishing agent, has been limited in its use due to its own characteristics. People began to search for halon, carbon dioxide, and other fire extinguishing agents. However, water is being re-introduced when environmental protection is increasingly valued. Pay attention. As long as they avoid weaknesses and conduct in-depth studies, water will certainly play a greater role in the field of fire protection.

Some of the information in this article comes from the Internet and is reorganized and edited by China Rescue Equipment Network.