1.the characteristics of dust explosion
(1) Dust explosion is more complex than flammable substances and flammable gas. Generally, combustible dust is suspended in the air to form a dust cloud within the explosive concentration range. Under the action of the ignition source, part of the dust in contact with the ignition source is first ignited and formed. A small fireball. Under the action of the heat emitted by the combustion of this small fireball, the nearby dust is heated, the temperature rises, and the fire burning phenomenon occurs, so that the fireball will rapidly expand and form a dust explosion.
(2) After a dust explosion occurs, secondary explosions tend to occur. This is due to the fact that during the first explosion, a lot of dust was deposited together, and its concentration exceeded the upper limit of dust explosion and could not be exploded. However, when the blast wave or air wave formed by the first explosion rejuvenates the deposition dust and mixes with the air in the air, and the concentration is within the dust explosion range, it may be followed by a secondary explosion. The disaster caused by the second explosion is often much more serious than the first explosion.
(3) Mechanism of Dust Explosion Combustible dust releases energy when it is burned in the air. The well produces a large amount of gas. The rate of energy released is the speed of combustion, which is related to the area of the powder exposed to the air. Therefore, for the same solid material powder, the smaller the particle size, the greater the specific surface area, the faster the combustion and diffusion. If the solid particle size is fine. Even if it can be suspended, once there is an ignition source to make it ignite, it can release a large amount of energy in a very short time. This energy is too late to dissipate into the surrounding environment, causing the gas in the space to be heated and adiabatically expanded. On the other hand, when the powder burns, a large amount of gas is generated, which will cause the system to form a local high pressure, resulting in explosion and propagation. This is usually Called a dust explosion.
(4) Differences between dust explosion and combustion The combustion of large solid combustibles is carried out in parallel to the interior of the parallel layers, such as the combustion of coal. This release of combustion energy is slow. The heat and gases generated can quickly dissipate. Combustible dust is combusted in a well-ventilated form and burns in an open flame, in the event of poor ventilation. It can form smokeless or flame flammable.
(5) Combustible dust classified powders can be classified into two categories based on their flammability: one is flammable; the other is non-flammable. Classification methods and standards for combustible powders vary in different countries.
2. dust concentration and particle impact on the explosion
(1) Dust Concentration Combustible dust explosion also has upper and lower limits of dust concentration. This value is affected by various factors such as ignition energy, oxygen concentration, powder particle size, powder type, and moisture. Using the simplified formula, the explosion limit can be estimated. In general, the lower concentration of dust explosion is 20-60g/m3, and the upper limit is between 2-6kg/m3. The upper limit is affected by many factors. Its value is not easily determined by the following limits, and it is usually not easy to reach the upper limit. Therefore, the lower limit value is more important and useful.
(2) When the powder particle size of the powder is more than 400um, the formed dust cloud no longer has explosive property. However, ultrafine powders have a greater risk when their particle size is below 10um. It should be noted that sometimes even if the average particle size of the powder is greater than 400um, it often contains finer powders, which also have explosive properties.
3.dust explosion technical measures
Combustion reactions require flammable substances and oxygen, as well as ignition sources with a certain amount of energy. For dust explosions, there should be three elements: ignition source; combustible fine dust; dust suspended in the air, forming a dust cloud within the explosive concentration range. The explosion of these three elements will occur at the same time. Therefore, the explosion can be prevented as long as one of the conditions is eliminated. The following technical measures are often used in baghouses.
(1) Explosion-proof structural design measures In the special design of the body structure, in order to prevent the accumulation of dust from combustible dust inside the dust collector, all beams, partition plates, etc. shall be provided with a dust-proof plate, and the dust-proof plate shall have a slope of less than 70 degrees. . The slip angle of the hopper is greater than 70 degrees. In order to prevent the ash from being too small between the two bucket walls, the two adjacent side plates should be welded with slide plates to eliminate the deposition of dust, taking into account the abnormal operation and dust humidity. When the ash bucket collapses and coldness occurs during large time, when the ash bucket is designed, steam tube insulation or tubular electric heaters are added to the high temperature dust collector on the ash wall. In order to prevent ash buckets, a hopper shaker or air cannon is required for each ash bucket.
(2) The use of anti-static filter bags in the interior of the dust collector, due to the high concentration of dust rubbing against each other during the flow, the dust and the filter cloth also have friction between each other can generate static electricity, static electricity accumulation will produce sparks and cause combustion. For the pulse cleaning method, the filter bag is made of polyester needle-punched felt. In order to eliminate the static deficiency of the polyester needle-punched felt, the conductive wire or carbon fiber is spun into the filter bag fabric. When the filter bag is installed, the filter bag passes through the steel frame. Connected to the perforated plate and connected to the plant grounding grid through the housing. For anti-bleeding filter bags, MP922 and other anti-static products have been developed. The use effect is very good.
(3) It is important to set the safety hole (valve) to limit the explosion to the inside of the bag filter without extending it to other areas. It is important to set up the safety hole and the essential fire-fighting equipment. The purpose of setting up safety holes is not to prevent the safety holes from exploding, but to use it to limit the scope of explosion and reduce the number of explosions. Most dust collectors that deal with explosive dust are operated with safety holes. Because of this, the design of the safety hole should ensure that if an explosion accident occurs, it can effectively play a role; in the meantime, it is necessary to strengthen the maintenance and management of the safety hole. 1 Explosion-proof panel Explosion-proof panel is an emergency pressure relief device driven by pressure difference and not automatically closed. It is mainly used for pipelines or dust removal equipment, so that they can avoid damage caused by overpressure or vacuum. Compared with the safety valve, the rupture disc has the advantages of large drainage area, sensitive movement, high precision, corrosion resistance, and not easy to block. Bursting discs can be used individually or in combination with safety valves. 2 explosion-proof valve design Safety explosion-proof valve design There are two main types: one is explosion-proof plate; the other is heavy hammer type explosion-proof valve. After the former one needs to replace the new board, the production must be interrupted. When the negative pressure is high, it is easy to be blank and difficult to keep warm. The latter is more advanced than the former, and is closed by the weight and the tightness is poor. The above two methods are not suitable for high-pressure pulse cleaning. In order to solve the problem of tightness, an explosion-proof safety lock can be designed on the heavy hammer type explosion valve. Its characteristic is: When closing, the safety door is mainly through this lock, in the event of an explosion can be automatically opened to release, and its release force (safety) can be adjusted through the spring. In order to balance the force of the security door, generally 4 to 6 locks need to be set according to the area of the security door. In order to make the explosion-proof door tightly leak-proof, it can be designed into the double structure of explosion-proof plate and safety lock.
(4) Testing and Fire Fighting Measures In order to prevent them from happening, necessary fire protection measures can be taken on the dust removal system. 1 Firefighting facilities. Mainly water, CO2 and inert fire extinguishing agents. For cement plants, CO2 is mainly used for dumping, while steel plants can use nitrogen. 2 temperature detection. In order to understand the change of the temperature of the dust collector and control the ignition point, generally a number of thermometers are installed on the hopper at the entrance of the dust collector. 3CO detection. For large dust removal equipment due to the large size, the installation of the thermometer is very limited, and sometimes the phenomenon of combustion occurs farther away from the measuring point of the thermometer, which is difficult to reflect from the thermometer. A CO detection device can be installed at the outlet of the precipitator to assist in the detection. As long as combustion occurs anywhere in the precipitator, the CO in the flue gas will rise. At this time, the CO concentration alarm and dust removal system will be increased. Control the joint sales to stop the operation of the system dust collector in time.
(5) Equipment Grounding Measures Explosion-proof dust collectors are often deployed outdoors due to operational safety requirements. Even if it is arranged on the tall steel structure in the open air, according to the equipment grounding requirements, grounding of the equipment is a necessary measure. However, the lightning rod is generally not provided with a lightning rod.
(6) Explosion-proof parts The selection of explosion-proof parts in the explosion protection measures of the dust remover is essential. Explosion-proof dust collectors avoid dust in operating conditions and induce explosion hazards within the electrical load. When the dust collector operates, the electrical load and components are in contact with the current, and even when conducting, it will inevitably lead to the occurrence of electric shock cremation. It is easy to cause the explosion of the dust source gas that exceeds the limit concentration by the discharge spark, and the electric load components must be all explosion-proof. Parts, to prevent the explosion induced factors. Ensure equipment operation and operation safety. For example, the impulse valve of the pulse dust collector and the solenoid valve of the poppet valve should all be explosion-proof.
(7) Preventing Mars from mixing measures In baghouses that handle exhaust gas from sawdust boilers, rice hull boilers, aluminum regenerators, and smelting furnaces, the burnt dust in the furnace may enter the tank with the air flow from the duct, causing the accumulation of dust. Dust on the filter cloth caught fire and caused an accident. In order to prevent Mars from entering the bag filter, the following measures should be taken: 1 Set the pre-dust collector and cooling pipe. The picture shows a cyclone or inert dust collector as a pre-dust collector to trap coarse dust and Mars. Particle Mars, which is too fine in this way, is not easy to trap. In most cases, particulate dust can burn out before entering the dust collector. Set the cooling pipe after the pre-dust collector and control the flow rate in the pipe to make it as low as possible. This is a relatively reliable technical measure that allows the gas to have a sufficient residence time in the pipe. 2 Ling spray tower. Gas cooling method with direct water spray. In order to ensure the safety of dust-containing gas in the bag filter, the cooling water supply is controlled. Most of the burning dust can be cooled upon contact with the fine water droplets, but the water droplets are easily gasified. In order to allow the combustion dust not yet in contact with the water droplets to cool, there should be necessary space and residence time. Under special circumstances, the combined use of a spray tower, a cooling pipe, and a pre-dust collector will completely prevent the mixing of Mars. 3 The Mars trap is shown in the figure. Installing a Mars trap on the pipeline is a simple and feasible method. There are also moments when Mars passes through the catcher, which can send an electrical signal to make an alarm. At the same time, stop the operation or change the gas circuit.
(8) Control of inlet dust concentration and addition of non-combustible powder bag filter During operation, its internal concentration distribution will inevitably cause a part to be within the explosion limit. To improve safety, avoid dust in the pipe. The concentration between the upper and lower explosion limits. For example, in the dust collection work such as pneumatic conveying and crushing and classifying, it is necessary to take care when designing to make it operate at a high concentration that exceeds the upper limit; in the case of local collection, the dust concentration must be maintained in the piping. Low concentration below the lower limit.