Four leaf clover teaches you about dust explosion prevention knowledge

With the strengthening of environmental awareness, people have become more and more strict on the pollutant gases emitted by enterprises into the atmosphere. Bag dust collectors are the nemesis of smoke and dust pollution and are efficient dust removal equipment for controlling air pollution. The biggest advantage of the bag dust collector is its high dust removal efficiency, which can reach 99.9% in practical applications. The dust emission concentration can reach less than 10 mg per m3, or even 1 mg per m3. It is precisely because of these advantages that bag dust collectors have been used in many companies. However, with the popularity of bag dust collectors, bag dust collector dust explosion accidents are also on the rise. Therefore, the explosion-proof design of bag dust collectors has been mentioned on the agenda.

1 Conditions and mechanisms for dust explosions
Three conditions are required for a dust explosion to occur:
① When the concentration of dust in the air reaches a certain level, it is within the upper and lower explosion limits. Generally, the lower explosion limit attracts most attention;
②An ignition source with sufficient energy;
③Enough air or oxygen mixing contact.
     Dust explosion is a very complex process, affected by many factors, so the explosion mechanism is not very clear yet. It is generally believed that dust explosion goes through the following development process: Dust explosion firstly involves the surface heating of dust particles after obtaining energy from the ignition source (thermal conduction, thermal radiation). In addition, after the dust particles obtain energy, the inside and outside are heated successively to produce melting and vaporization, and burst out hot tiny proton particles or sparks, which will form the ignition source of dust. Due to the large surface area of ​​the dust, it is fully in contact with the air, and due to the existence of the dust layer, the surface temperature of the dust particles rises sharply, which accelerates the decomposition or gasification of the dust particles. When mixed and contacted with air, gas phase ignition can be formed, so that there are both gas phase and solid phase in the dust, and the combustion will be more violent when the two phases exist at the same time. Moreover, the accumulation of static electricity and friction can also form a source of ignition. When the concentration of dust mixed with air reaches the lower explosion limit, it can burn when it encounters an ignition source. During the initial combustion, due to the sufficient mixing of dust and air, the dust can explode and release energy in the form of pressure waves. Therefore, the dust in the mechanical device is often blown out, and the dust layer on the ground is blown up, forming a flying dust cloud. These dust clouds are instantly re-ignited by the hot residue of the initial explosion, and then a second explosion occurs, and air turbulence can be generated at the same time. Because this explosion blew up a large amount of deposited dust again, its explosion was much more powerful than the initial explosion. The dust explosion was affected by many factors such as the formation of the ignition source, the minimum ignition energy, and the dust concentration. It can be seen from the explosion process that the diameter of the dust particles determines the difficulty of detonation, the ignition source is the key factor in detonation, and the secondary explosion caused by igniting the dust cloud is the greatest hazard.

2 Explosion-proof technology
    Explosion-proof technology can be divided into preventive technical measures and design measures. Preventive measures refer to reducing the concentration of dust below the lower explosion limit and the accumulation of large amounts of dust, and eliminating effective ignition sources; design measures refer to the measures taken to reduce the degree of explosion hazard to a safe level when measures to prevent explosions are difficult to achieve, so that the explosion will not cause casualties and the equipment can be restored to use in a short time after the explosion. It includes: anti-riot, explosion suppression, explosion-proof, explosion venting, etc.
   The bag dust collector is used as a dust collection place to keep the dust concentration below the lower explosion limit at all times, which is not cost-effective and difficult to achieve; but it can be achieved without allowing a large amount of dust to accumulate. Effective ignition sources should be distinguished from the following two types of ignition sources: ordinary ignition sources (such as ignition sources caused by welding, grinding, and fumigation) and ignition sources caused by mechanical failures (mechanical sparks, high-temperature surfaces, flameless combustion heat accumulation, and electrostatic sparks). Among them, various types of motion friction sources are the most common, and most accidents of dust explosions are caused by them. In addition, after investigation and testing, corona discharge mainly occurs in dust clouds, and lightning discharge has not yet occurred in industrial production. In addition, it is difficult for this kind of lightning discharge to occur in industrial production equipment, unless the volume of the dust cloud formed is greater than 60 m3 or the diameter is greater than 3m.
Explosion resistance means that the structure itself can withstand the explosion pressure without bursting. There are two types of explosion-resistant structures: explosion pressure resistance and explosion impact pressure resistance. The explosion pressure resistance design should comply with the pressure vessel design and manufacturing specifications, while the explosion impact pressure resistance can be appropriately relaxed. Generally: the explosion resistance pressure is 1.5 times the explosion pressure resistance.
   Explosion suppression is a measure to extinguish the explosion of combustible materials in equipment at the initial stage to avoid excessive explosion pressure. A successful explosion suppression system can operate when the explosion pressure is 1×104Pa. After suppression, the maximum pressure in the equipment is lower than 1×105Pa. There are two types of explosion suppression. One is passive, such as water bags and flame-retardant powder devices. This type of device automatically breaks and throws out water or flame-retardant powder when the explosion pressure rises to a certain range; the other is an intelligent explosion suppression device consisting of an initial explosion detection sensor, a control unit and a high-speed spray (HRD) explosion suppressor with high-voltage drive.
   Explosion isolation is to prevent an explosion that occurs in one container from propagating to subsequent containers along the connecting pipe, causing the system to explode and causing greater losses. However, in practice, the most harmful explosion is because the explosion isolation device cannot work or is not designed. It can generally install a flame probe on the pipeline to trigger the corresponding mechanism to achieve explosion interruption.
Dust explosions will generate pressures as high as 0.7-1MPa, and explosion relief measures can limit the rising pressure within the range that the component material strength can withstand. Dust explosions have the characteristics shown in Figure 1. The curve a represents the explosion in a closed container, Pmax represents the maximum explosion pressure, and Pn represents the maximum explosion pressure rise rate; curve b represents the situation where explosion leakage occurs, Pred represents the maximum explosion pressure, and Pn represents the maximum explosion pressure rise rate. It can be seen that explosion venting plays an extremely important role in reducing explosion hazards. 

3 Explosion-proof measures for bag dust collectors

 1 Control the dust accumulation in the bag dust collector
     The dangerous concentration of industrial combustible dust is generally: 20-6000g/m3. Usually the bag dust collector is the last part of the process system. The dust-containing gas is sent to the bag dust collector through the pipeline and is collected to form a dust layer. The dust is cleared by pulse backflushing and falls into the ash hopper. During these processes, the concentration of dust in the baghouse is likely to be within the above dangerous concentration range. Therefore, it is necessary to strengthen the ventilation of the system, especially the dust cleaning in time, so that the dust concentration in the bag dust collector and pipes is lower than the lower limit of the dangerous range. Timely removal of the dust from the hopper is crucial to the safe and normal operation of the bag dust collector. Most of the dust handled by the bag dust collector is flammable dust. If the dust from the hopper is not removed in time, heat will accumulate in the dust and it is easy for the dust to spontaneously ignite. The accumulated dust is not a solid whole. There is air in it. The accumulated dust is actually a mixed colloid of air and dust. This will become the source of secondary dust explosions and lay hidden dangers for safety. The use of double-layer pneumatic ash discharge valve can ensure timely ash discharge, reduce air leakage rate, avoid dust spillage in loose places, and prevent secondary dust pollution.

  2 Remove ignition sources
    The ignition sources in the bag dust collector are mainly the following: ordinary ignition sources, cremation caused by impact or friction, electrostatic cremation and shell temperature, etc. 
① Common ignition sources are mainly brought in directly from external fire sources, especially gas cutting flames and welding sparks. Since bag dust collectors are generally welded parts, gas cutting flames and welding sparks are mainly generated when repairing instruments. Therefore, they can be controlled by strengthening management and improving workers' awareness of explosion prevention. When repairing instruments, the dust around the repair parts must also be removed.
② Sparks generated by impact or friction are usually caused by the collision of metal objects such as nuts or iron blocks being sucked into the bag filter. The main elimination methods are: setting up appropriate metal mesh and electromagnetic iron removal devices at the dust collection hood to prevent metal from entering the dust collection pipe and bag filter. After maintenance, metal materials falling into the pipe should be removed in time. Secondly, it is best to arrange the ventilator on the clean air side after the bag dust collector to prevent metal foreign objects from colliding with the high-speed rotating blades of the fan to produce sparks, and to prevent flammable and explosive dust from friction, heat and combustion with the high-speed rotating blades. Finally, the wind speed in the pipe network must be reasonable. Excessive wind speed can cause dust to accelerate wear on the pipes. Tests have shown that the wear rate has a cubic relationship with the wind speed, thus bringing more metal substances inside the dust collector.
③The mechanism of static electricity generation has not yet been clarified, but necessary measures can be taken to prevent static electricity. Select substances that are similar to the charged sequence, or combine them with substances that are opposite to the charged sequence to minimize the contact potential difference, so that the generation of static electricity can be suppressed to the greatest extent. The positive and negative charges they carry neutralize themselves to achieve the purpose of eliminating static electricity. Secondly, grounding is one of the most important and common leakage measures. Equipment, containers, pipes, etc. should be maintained at equal potentials. Depending on the actual situation, use a decentralized or centralized grounding system for reliable grounding, and the grounding resistance is required to be within the range of 1-2Ω. In addition, to increase conductivity, use conductors or conductive substances instead of highly insulating substances. Especially the anti-static filter material is used. This is mainly because tests show that the filter material is the most concentrated part of static electricity. In addition, the dust layer accumulated on the filter bag increases the intensity of the spatial electric field, which may break down the air and cause spark discharge. Finally, you can use the method of increasing the ambient humidity to leak static electricity, or you can directly add water during the production process.
④Keep the temperature of the dust collector shell not to be too high. Since a large amount of dust is adsorbed by the inner wall of the shell, the surface of the dust is heated when the temperature of the shell is too high. After obtaining energy, it melts and vaporizes to burst out hot tiny proton particles or sparks, which also form an ignition source for the dust. Therefore, control the ambient temperature and do not place the bag dust collector outside the house.

 3 Install the explosion vent membrane
     For bag dust collectors, the most effective and economical explosion-proof measure is to install an explosion-proof membrane. The key to installing an explosion-proof membrane lies in the calculation of the explosion-proof area. If the explosion-proof area is too small, it will not work, and if it is too large, it will increase the cost. The most commonly used calculation methods for explosion venting areas in the world are the Kst Nomo diagram method and the St Nomo diagram method. The explosion venting position of the bag dust collector is also very important. It is mainly close to the explosive occurrence point, so the explosion venting effect is better. As shown in Figure 2, the structural diagram of the bag dust collector is shown. 
 

 4 Explosion proof and explosion suppression
      The explosion-proof device can use an emergency shut-off valve, which is realized by quickly actuating a pneumatic spring valve by an infrared flame sensor. It can trigger the emergency shut-off valve installed far enough away from the sensor to prevent flames or explosions and explosives from spreading to other places to form secondary explosions, thereby controlling the specific area of ​​the explosion accident and avoiding the deterioration of the situation and more serious consequences. If it is a small bag dust collector, it is easy to use a passive pressurized water bag or a flame retardant powder device. If the dust is hydrophilic, it is easy to use a pressurized water bag, and for other things, a flame retardant powder device is used. If it is a large bag dust collector, it is easy to use an intelligent high-pressure spray device.

 5 others
    Most dust explosions are caused by personnel negligence. Strengthening personnel management is the most fundamental and important. ① Provide theoretical and practical knowledge training to operators to make them aware of the dangers of dust explosions and how to effectively prevent dust and explosions; ② Strengthen labor discipline and pay attention to the operation of the bag dust collector at all times during work; ③ It is absolutely prohibited to perform operations such as electric welding and gas cutting on the bag dust collector during working hours, and to allow fire to enter or approach the work area.