How to prevent dust explosion of explosion proof bag filter

technical measures to prevent dust explosion

1. Characteristics of dust explosion

dust explosion is a kind of oxidation reaction in which the suspended dust particles in the air fully contact with the oxygen in the air and complete instantaneously under specific conditions. A large amount of heat is released in the reaction, resulting in high temperature and high pressure. Any dust explosion must have such three conditions: ignition source; Combustible fine dust; The dust is suspended in the air and reaches the explosion concentration limit

(1) dust explosion is more complex than combustible materials and combustible gases. Generally, combustible dust is suspended in the air to form dust cloud within the explosion concentration range. Under the action of ignition source, part of the dust in contact with ignition source is ignited first and forms a small fireball. Under the effect of the heat released by the small fireball combustion, the nearby dust will be heated, the temperature will rise, and the phenomenon of ignition and combustion will occur. 1n this way, the fireball will expand rapidly and form dust explosion

the difficulty and intensity of dust explosion are closely related to the physical and chemical properties of dust and the surrounding air conditions. Generally, the larger the combustion heat, the finer the particles and the higher the activity of dust, the greater the risk of explosion; The explosion risk of light suspended combustible materials is high; When the oxygen content in the air is high, the dust is easy to be ignited and the explosion is more severe. Because water has the function of inhibiting explosion, the drier the dust and gas are, the greater the risk of explosion is

(2) after the dust explosion, the secondary explosion often occurs. This is because in the first explosion, a lot of dust is deposited together, and its concentration exceeds the upper limit of the dust explosion, so it can not explode. However, when the shock wave or air wave formed by the first explosion raises the deposited dust again, it will mix with the air in the air, and the concentration is within the range of dust explosion, which may be followed by the second explosion. The disaster caused by the second explosion is often much more serious than that caused by the first explosion

the dust explosion accident occurred in 1963 in an aluminum product factory in China was directly caused by the friction and fire between the impeller of exhaust fan and the end face of suction inlet. The air flow of the shrimp bend and the three-way underpants at the suction port of the fan is not smooth, which is easy to accumulate dust. Especially when the machine is shut down, it is easier to retain dust. Once the machine is started up, the deposited dust will be lifted up and reach the lower explosion limit quickly, causing dust explosion

(3) the mechanism of dust explosion when combustible dust burns in the air, it will release energy, and a large amount of gas will be produced in the well. The speed of energy release is related to the area of powder exposed in the air. Therefore, the smaller the particle size is, the larger the specific surface area is, and the faster the combustion diffusion is. 1f the particle size of this solid is very fine. Once ignited by ignition source, a large amount of energy can be released in a very short time. The energy can not escape to the surrounding environment, causing the gas in the space to be heated and adiabatic expansion. On the other hand, when the powder burns, a large amount of gas will be produced, which will make the system form local high pressure, resulting in explosion and propagation. This is commonly known as dust explosion

(4) the difference between dust explosion and combustion is that the combustion of large solid combustibles is pushed to the interior in nearly parallel layers, such as coal combustion. The release of this combustion energy is relatively slow. The heat and gas produced can be quickly dissipated. The stack combustion of combustible dust forms open fire in the case of good ventilation, but in the case of poor ventilation. 1t can form smokeless or smoldering flame

there are several stages in the combustion of combustible dust: in the first stage, the surface powder is also heated; 1n the second stage, the surface layer gasifies and spills volatile matter; 1n the third stage, volatilization and gas phase combustion

the explosion of ultra-fine powder is also a complex process. Because the dust cloud is generally small in scale, and the flame propagation speed is fast, hundreds of meters per second, the fire source ignition occurs in the dust center, which can burn the whole dust cloud in less than 0.1s. 1n this process, if the dust has burned out, the highest pressure will be generated; 1f it is not burnt out, a lower pressure will be generated. Whether combustible particles can burn up or not depends on the particle size and combustion depth

(5) classified powders of combustible dust can be divided into two types according to their flammability: one is combustible powder, the other is combustible powder; One is non combustible. The classification methods and standards of combustible powder are different in different countries

in the United States, combustible powder is classified as class 11 dangerous goods, and metal powder, carbon containing dust and grain dust are classified into different groups. The classification method developed by the United States is based on the pressure rise rate of the powder to be tested when dust explosion occurs in the standard test device, and is divided into three grades. There is no ready-made standard for combustible dust classification in China2. The influence of dust concentration and particles on explosion

(1) there are upper and lower limits of dust concentration in combustible dust explosion. The value is affected by ignition energy, oxygen concentration, particle size, powder variety, moisture and other factors. Generally speaking, the lower limit of dust explosion concentration is 20-60g / m3, and the upper limit is 2-6kg / m3. The upper limit is affected by many factors, its value is not as easy to determine as the lower limit, and it is usually not easy to reach the upper limit. Therefore, the lower limit is more important and useful

in physical sense, the upper and lower limits of dust concentration reflect the influence of the distance between particles on the propagation of particle combustion flame. 1f the distance between particles is such that the combustion flame cannot extend to adjacent particles, the combustion will not continue (spread), and the explosion will not occur; At this time, the dust concentration is lower than the lower limit of explosion concentration. 1f the distance between particles is too small, the oxygen between particles is not enough to provide sufficient combustion conditions, so the explosion can not be formed, and the particle concentration is higher than the upper limit< (2) when the particle size of combustible powder is larger than 400um, the dust cloud is no longer explosive. But for the ultrafine powder, when the particle size is less than 10um, it is dangerous. 1t should be noted that sometimes even if the average particle size of the powder is greater than 400um, it often contains finer powder, which is also explosive although the influence of particle size on explosion performance is not strong, the smaller the particle size is, the larger the specific surface area is, the faster the combustion is, and the pressure rise rate increases linearly. Under certain conditions, the maximum pressure changes little, because it depends on the total energy emitted during combustion, and has no obvious relationship with the speed of energy release 3. Technical measures for dust explosion the combustion reaction needs combustible materials and oxygen, as well as ignition source of certain energy. For dust explosion, there should be three elements: ignition source; Combustible fine dust; The dust is suspended in the air and forms a dust cloud within the explosion concentration range. These three elements exist at the same time to explode. Therefore, as long as one of the conditions is eliminated, explosion can be prevented. The following technical measures are often used in bag filter (1) explosion proof structural design measures in the special design of the body structure, in order to prevent the accumulation of combustible dust in the internal components of the dust remover, all beams, partition boards, etc. should be equipped with dust plates, and the inclination of the dust plates should be less than 70 degrees. The slip angle of the ash hopper is more than 70 degrees. 1n order to prevent ash accumulation due to the small angle between the two hopper walls, the two adjacent side plates should be welded with slip plates to eliminate dust deposition. Considering the condensation and cold blocking of the ash hopper due to abnormal operation and high dust humidity, steam pipe insulation or tubular electric heater should be added to the high-temperature dust collector on the wall plate of the ash hopper when designing the ash hopper. 1n order to prevent the hopper from covering, each hopper also needs to be equipped with bin arm vibrator or air gun one dust collector has 2-3 ash hoppers at least and 5-8 ash hoppers at most. When it is used, it will produce deflection caused by uneven air flow, uneven amount of pulverized coal in each ash hopper and large amount of ash behind H in order to solve the problem of uneven air flow, the following measures can be taken in the structure: â‘?add a wind deflector on the inclined baffle of the air duct, as shown in Fig. 5-168. The size of baffle should be determined according to the principle of equal air volume and equal air pressure; â‘?Considering the change of the actual situation on site, the adjustable valve is used between the lifting valve stem and the valve plate to make the outlet height h as the change value for further correction; â‘?The air volume regulating valve is set in the air inlet branch pipe to regulate the air volume of each box room after the equipment is running. The difference of air volume in each chamber should be controlled within 5% (2) the anti-static filter bag is used in the dust collector, because the high concentration dust rubs each other in the flow process, and the dust and filter cloth also rubs each other, which can produce static electricity, and the accumulation of static electricity can produce sparks and cause combustion. For pulse ash cleaning, the filter bag is made of polyester needle felt. 1n order to eliminate the electrostatic shortage of polyester needle felt, conductive metal wire or carbon fiber is spun into the filter bag cloth. When the filter bag is installed, the filter bag is connected with the perforated plate through the steel framework and connected to the workshop grounding grid through the shell. Mp922 and other anti-static products have been developed for the filter bag with reverse blowing. 1t works very well (3) in order to limit the explosion to the inside of the bag filter and not expand to other aspects, it is very important to set the safety hole and essential fire extinguishing equipment. The purpose of setting safety hole is not to prevent explosion, but to limit explosion range and reduce explosion times. Most of the dust collectors dealing with explosive dust operate under the condition of setting safety holes. Because of this, the design of safety hole should ensure that in case of explosion accident, it can play a practical role; At ordinary times, we should strengthen the maintenance and management of the safety hole the rupture plate type safety hole is made of ordinary thin metal plate. Because the box of bag filter can’t bear a lot of pressure, the strength of rupture plate should be designed so that the plate will be destroyed under lower pressure. Sometimes, due to the long-term compression of the box, the aluminum plate has fatigue deformation, resulting in cracking. Even if this is normal, it is not allowed to replace the high-strength thick plate the spring door type safety hole adjusts the opening pressure by increasing or decreasing the spring tension. 1n order to ensure the safety of the door type hole, the action test must be carried out regularly the area of the safety hole should be determined according to the relationship between the maximum pressure during dust explosion, the speed of pressure increase and the compressive strength of the box, but there is no exact data at present. According to the form and structure of the bag filter, the size of the safety hole area should be determined. We think that the ratio of the safety hole to the volume of the dust collector is 1 / 10 ~ 1 / 30 for small and medium-sized dust collectors, and 1 / 30 ~ 1 / 60 for large and medium-sized dust collectors. When encountering difficulties, it is necessary to properly refer to the actual determination of safety explosion-proof holes reserved for other devices â‘?explosion proof plate explosion proof plate is an emergency pressure relief device driven by pressure difference and not automatically closed, which is mainly used for pipelines or dust removal equipment to avoid damage caused by overpressure or vacuum. Connected with safety valve

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