Dust explosion related knowledge
2019-10-10 17:05:03
Dust explosion is that the dust is within the explosion limit. When it encounters a heat source (open flame or temperature), the flame instantaneously propagates through the entire mixed dust space. The chemical reaction speed is extremely fast, and a large amount of heat is released, forming a high temperature and a large pressure. The energy of the system is converted into mechanical work as well as light and heat radiation, which has a strong destructive power.
   1, dust explosion
Dust explosion refers to the explosion phenomenon when combustible dust suspended in the air touches an ignition source such as an open flame or an electric spark.
   2, the explosion conditions
   The dust generated in the production process of some factories and mines, especially the dust generated in the processing of some organic materials, may explode and burn under certain conditions. Research results at home and abroad show that there are generally three of these conditions:
   (1) The flammable dust is suspended in the air at an appropriate concentration to form a dust cloud as is often said;
   (2) There is sufficient air and oxidant;
   (3) There is a source of fire or strong vibration and friction.
   It is generally believed that as long as the explosive dust meets the conditions (1) and (2), it means that there is a possibility that an accident may occur.
   3, explosive dust
Generally, dusts that are more prone to explosion accidents include aluminum powder, zinc powder, ferrosilicon powder, magnesium powder, iron powder, aluminum processing abrasive powder, various plastic powders, intermediates of organic synthetic drugs, wheat flour, sugar, wood chips, Dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal dust, plant fiber dust, etc. The dust of these materials is prone to explosive combustion because there are strong reducing agents such as H, C, N, S, etc. When they coexist with peroxide and explosive dust, they decompose and are produced by oxidation reaction. A large amount of gas, or a small amount of gas, releases a large amount of heat of combustion. For example, aluminum powder is at risk of explosion as long as it is in a carbon dioxide atmosphere.
The difficulty of dust explosion is related to the physical, chemical and environmental conditions of the dust. It is generally believed that substances with a higher combustion heat are more likely to explode, such as coal dust, carbon, sulfur, and the like. Substances with fast oxidation rate are prone to explosion, such as magnesium powder, aluminum powder, ferrous oxide, dyes, and the like. Dust that is easily charged can also cause explosions, such as synthetic resin powder, fiber dust, and starch. These poorly conductive substances accumulate due to static electricity generated by friction with the machine or the air. When a certain amount is reached, an electric spark is generated to form an explosion source.
   Dust that is not easily explosive is soil, sand, iron oxide, abrasive materials, cement, quartz dust, and dust similar to burning. The dust chemical properties of these materials are relatively stable and therefore not easy to burn.
   However, if such dust is generated in oil mist and combustible gases such as CO, CH 4 and gas, it is also prone to explosion.
Smoking and open fire operations are strictly prohibited if effective ventilation and dust removal measures are used. A pressure relief valve or other device is provided in the outer casing of the device, and an explosion containment system or the like is used. For workshops with dust explosion hazard, it must be designed in strict accordance with the explosion-proof technical grade, and separate ventilation and dust removal systems. Always clean the floor and equipment in the workshop to prevent dust from flying and gathering. Ensure that the system has good airtightness. If necessary, fill the flammable dust in the closed container or pipeline with nitrogen, carbon dioxide and other gases to reduce the oxygen content and suppress the explosion of dust.
   4, the explosion process
The explosion of dust can be regarded as the following three steps: the first step is that the suspended dust is quickly retorted or vaporized by the heat source to produce a combustible gas; the second step is that the combustible gas is mixed with air to burn; The three steps are the heat released by the dust combustion, which is transmitted to the nearby suspended or blown dust by heat conduction and flame radiation. The dust is heated and vaporized to make the combustion cycle. As each cycle progresses, the reaction speed gradually increases, and through intense combustion, an explosion is formed. This explosion reaction, as well as the speed of the explosion flame, the speed of the explosion wave, the pressure of the explosion, etc., will continue to accelerate and rise, and will leap forward.
   5. Influencing factors
   (1) Physicochemical properties
The greater the heat of combustion of the substance, the greater the risk of explosion of the dust, such as coal, carbon, sulfur, etc.; the more easily oxidized, the more explosive the dust, such as magnesium, ferrous oxide, dyes, etc.; The more easily charged dust, the easier it is to explode. In the production process, dust is not easily lost due to collision, friction, etc., causing static electricity to accumulate. When a certain value is reached, electrostatic discharge occurs. Electrostatic discharge sparks can cause fires and explosions. Dust explosions are also related to the volatiles they contain. If the volatile matter in the pulverized coal is less than 10%, no explosion will occur, and thus the coke dust has no risk of explosion.
   (2) particle size
The surface of the dust adsorbs oxygen in the air. The finer the particles, the more oxygen is adsorbed, so the more likely the explosion occurs, and the lower the ignition point, the lower the lower explosion limit. As the diameter of the dust particles is reduced, not only the chemical activity is increased, but also the static electricity is easily charged.
   (3) Dust concentration
Similar to flammable gases, dust explosions also have a certain concentration range, and there are also upper and lower limits. However, most of the general information only lists the lower explosion limit of dust because the upper limit of dust explosion is higher.
   6, explosion characteristics
   (1) Multiple explosions are the biggest feature of dust explosions.
   (2) The minimum ignition energy required for dust explosion is high, generally above tens of millijoules.
   (3) Compared with the explosion of flammable gas, the dust explosion pressure rises slowly, the higher pressure lasts for a long time, the energy released is large, and the destructive power is strong.
   7. Explosion hazards
   (1) It is extremely destructive. Dust explosions cover a wide range of sectors, including coal, chemical, pharmaceutical processing, wood processing, food and feed processing. For example, between 1952 and 1979, there were 209 kinds of dust explosion accidents in Japan, and there were 546 casualties. Among them, the crushing and milling project and the dust separation project were more prominent, each with 46 cases. In the Federal Republic of Germany, there were 768 dust explosion accidents from 1965 to 1980. The most serious ones were wood powder and wood products dust and food and feed explosion accidents, accounting for 32% and 25% respectively. In recent years, the frequency of dust explosions in China is: local explosions 150-300 times, system explosions 1-3 times, and a growing trend. These dust explosions in our country, especially the system explosion, caused serious losses. In 1987, the linen dust explosion accident of Harbin Flax Factory killed 58 people, 177 people were light and serious, and the direct economic loss was 8.82 million yuan.
   (2) It is easy to produce a secondary explosion. The first explosion of air waves blows the dust deposited on the equipment or the ground. In the short time after the explosion, the central area of ​​the explosion will form a negative pressure, and the surrounding fresh air will be filled in from the outside to the inside, forming a so-called "return". The wind, mixed with the raised dust, caused a second explosion under the igniting of the first explosion. In the second explosion, the dust concentration is generally much higher than in an explosion, so the power of the second explosion is much larger than the first time. For example, in a sulphur powder plant, an internal explosion occurs in the grinder, and the blast wave diffuses from the grinder to the cyclone along the gas pipe. A secondary explosion occurs in the cyclone, and the blast wave is generated on the cyclone after passing the explosion. The crack spread to the workshop, sulphur dust settled on buildings and process equipment, and an explosion occurred.
   (3) It can produce toxic gases. One is carbon monoxide; the other is a toxic gas that is decomposed by explosives (such as plastic). The generation of poisonous gas often causes a large number of human and animal poisoning casualties after the explosion, and must be given full attention.
   8, dust removal system
   (1) Causes of fire and explosion in the dust removal system
In order to prevent dust pollution and dust explosion, it is necessary to use dust removal system to discharge dust in the working environment in time and reduce the dust concentration in the workshop. However, for flammable dust, dust removal equipment must also take fire prevention and explosion-proof measures, otherwise it will cause an explosion fire accident if it is used improperly. The reason for the explosion and fire of the dust removal system is mainly to satisfy the conditions (1) and conditions (2). Since the general dust collector is inhaled and then discharged, the oxygen supply in the dust removal system is sufficient. According to the survey, the main possibility of causing fire in the dust removal system is static spark, self-ignition fire, impact friction and open flame. The static spark is caused by the charging of the nylon filter cloth of the dust collector and the charging of the pipeline. The self-ignition fire is caused by the heat of oxidation accumulated by the collected dust slowly oxidizing in the bag or ash hopper. Impact friction is generally considered to be the collision of iron pieces inhaled by the dust removal system, or the occurrence of Mars caused by collision when the hammer head vibrates the dust attached to the ash or bag chamber during the cleaning process. Open flames are caused by gas welding flames and electric welding sparks during maintenance.
   (2) Explosion-proof measures for various parts of the dust removal system
Dust explosion accidents often occur in crushers, pulverizers, sieving machines, dryers, feeders and storage bins. In the dust removal system, there is generally no explosion accident in the dust hood at the dust inlet, because the dust concentration is only 0.1 to 10 g/m 3 here , and this value is safe. However, the dust hood can suck in the spark as a source of ignition, so attention should be paid. For example, when a grinder is working, it generates a lot of sparks. If it is very dangerous to share a dust collector with other equipment (source of flammable dust), isolation measures should be taken.
There are many instances where the air duct of the dust removal equipment explodes. This is because the dust is deposited in the tube. When it is subjected to some impact, the flammable dust flies again, forming a high concentration dust cloud in an instant. If it is exposed to fire, it is easy to explode. . The main reason for the deposition of dust in the duct is that the conveying wind speed is too small or there is air leakage. Therefore, in order to prevent explosion, the dust removal pipeline of flammable dust should be as short as possible, and the dust collector of the dust collector of the same system (the dust source capable of forming dust during production and processing) should be no more than four. For elbows, reducers, etc. in the system, the radius of curvature of the elbow should be 1.5 times or more of the pipe diameter D, and the expansion angle of the reducer should be 15 degrees or less to reduce the resistance. According to foreign experience, if the particle size of flammable dust is less than 15μm and the sedimentation velocity is about 0.1m/s, the wind speed in the pipe should be 100-200 times of the sedimentation velocity. When the dust concentration increases, the conveying wind speed should also increase accordingly. In the bag room and the ash hopper, movable doors should be installed for easy maintenance and cleaning. When the joints are connected by inserts, they should be carefully constructed so that there are no gaps or liners in the pipes, no dust and gas leakage. phenomenon. For the dust collector, the ash bucket is the place with the worst working conditions, and it is also the most prone to fire and explosion. Because part of the dust in the suction duct falls directly into the hopper, most of it is filtered in the filter bag. After the dust forms a layer on the surface of the filter bag, it will fall off by gravity or be sprayed, shaken and then detached. At this time, it is possible to generate a high concentration dust cloud, or pile up on the ribs in the ash hopper and on the steel plate weld. . If the dust is caused to fly twice for some reason, the conditions are appropriate and the fire source will be very dangerous.
Therefore, for the fire prevention and explosion prevention measures of the dust source (processing equipment) and the dust removal system, the first is to remove the fire source. For mechanical friction and sparks generated by impact, measures should be taken to remove Mars, and a suitable metal mesh should be installed at the dust hood to prevent the iron sheets, screws and the like from being sucked into the pipeline and causing sparks to cause combustion.
Preventing static sparks is an important issue. Especially for dust removal equipment that handles flammable dust, the dust collectors, pipes, fans and other facilities of the dust removal system should be connected and grounded. You can also use an anti-static filter cloth or put the bag of the dust collector with an iron clip and ground it.
For metal dust, such as lead, zinc, ferrous oxide, zirconium, etc., the slow oxidation reaction occurs in the ash hopper of the dust removal system, and the plastic synthetic resin, rubber and the like still maintain the friction heat of the product processing, Continuous ash removal should be adopted. Do not allow excessive dust accumulation in the ash bucket. Always observe the temperature inside the hopper and bag room, or install a temperature sensor to control the temperature inside the device at any time to prevent accumulation of heat and induce fire. explosion.
The oil mist and ventilating and exhausting facilities containing flammable gases in the production workshop must not be connected to the flammable dust dedusting system or the ventilating equipment operated by organic solvents or share the fan. In particular, bag-type dust collectors for iron-making blast furnaces, although the entire system has corresponding explosion-proof and fire-proof measures, it is necessary to strictly prevent gas explosion when the blast furnace is off. For this reason, in the absence of steam in the blast furnace, all the discharge valves and the deashing valves of the gas system must be quickly opened to collide with the outside of the tank to drive the gas out of the precipitator in a short time. For ventilation systems that cannot be separated in principle, in addition to the above measures, the residual gas in the entire dust removal system should be removed before and after the dust collector is operated and stopped.
   (3) Fire prevention measures in the dust removal system
Although the dust removal system can prevent explosion accidents after taking the above measures, there is still a possibility of fire and burning in the dust collector, which cannot be ignored. For example, in the bag dust collector for treating flammable dust, when the flue gas cooling device or the spray nozzle fails, the filter cloth may be burned by the high temperature flue gas or even cause a fire. Dry dust removal equipment generally uses the following fire protection measures.
   1 In the large-scale dust removal device of outdoor equipment, if it is found that there is a fire phenomenon, when the dust collector fan is turned off, in order to prevent the flue gas from flowing back into the workshop through the pipeline, the fan should be turned off after the fire damper or air volume switch is turned off. The filter part of the electrostatic precipitator and bag filter is usually a sealed structure. Without special circumstances, the combustion of the filter bag will not spread to other parts. For small dust-removing devices set up indoors, the fan should be turned off immediately when it encounters fire in the bag room.
   2 Once the dust accumulated in the ash hopper is on fire, it can be extinguished with a fire extinguisher, and then the dust inside is discharged. The discharged dust should be carefully treated to prevent re-ignition and to avoid the occurrence of deflagration accidents during the cleaning process. Clean the ash bucket or bag room and replace it with a new one.
   3 After the fire extinguishing, in order to prevent the lack of oxygen and carbon monoxide poisoning in the bag, the personnel should be fully ventilated before entering, and the safety and visibility can be entered before viewing and operating.
   4 When controlling dust with strong flammability and friction, in principle, the fan should be installed behind the bag filter. If placed in front, the dust adheres to the fan blades, and the fouling layer formed after the moisture will cause the impeller to lose balance, resulting in excessive eccentric torque during operation. The bearing heats up, vibrates or breaks the blade, damaging the casing, and generating sparks. The source of the dust. In order to avoid such an accident, a vibration switch can be added to automatically control the fan to turn off when an abnormality occurs in the fan.
   (4) Precautions for setting the dust removal equipment
Compared with the wet dust removal method, the dry dust removal method is convenient for the recovery and utilization of dust, and does not need to consider the antifreeze measures and sewage treatment problems in winter, and the cost and operation cost are low. In particular, bag filters have been widely used in flue gas and dust control in various industrial sectors. However, the following points should be noted in the design and construction of dust removal systems and buildings for the treatment of flammable dust.
1 The dust collector should maintain an appropriate safety distance with other production equipment, and the pressure-resistant wall should be placed around.
2 Workshop buildings with flammable dust should adopt fireproof structure, and fire doors, shutters and vents should be provided at the joints with other workshops to disperse the explosive gas.
3 In the dust collectors installed in workshops containing flammable and explosive gases, explosion-proof holes are generally designed. The size of the hole should be 5 to 6% of the area of ​​the bag or 0.16 m 2 /m 3 of the volume of the bag.
4 The number of air changes in the building and the proportion of the vents are designed to prevent dust explosions.
5 To prevent flammable dust from leaking and leaking, there should be no dust accumulation in the beams, supports, ceilings, etc. in the workshop.
   9, prevention
(1) Strengthen the strong wind and control the dust concentration below the lower explosion limit.
(2) In the case of explosive dust, avoid high temperature, open flame, electric spark and impact.
(3) For some dust that may cause spontaneous combustion or explosion when in contact with water, contact with water or liquids that may react may be avoided.
(4) Pay attention to the type and nature of dust, and avoid mixing or contact with dust that may react with each other.
   1, dust explosion
Dust explosion refers to the explosion phenomenon when combustible dust suspended in the air touches an ignition source such as an open flame or an electric spark.
   2, the explosion conditions
   The dust generated in the production process of some factories and mines, especially the dust generated in the processing of some organic materials, may explode and burn under certain conditions. Research results at home and abroad show that there are generally three of these conditions:
   (1) The flammable dust is suspended in the air at an appropriate concentration to form a dust cloud as is often said;
   (2) There is sufficient air and oxidant;
   (3) There is a source of fire or strong vibration and friction.
   It is generally believed that as long as the explosive dust meets the conditions (1) and (2), it means that there is a possibility that an accident may occur.
   3, explosive dust
Generally, dusts that are more prone to explosion accidents include aluminum powder, zinc powder, ferrosilicon powder, magnesium powder, iron powder, aluminum processing abrasive powder, various plastic powders, intermediates of organic synthetic drugs, wheat flour, sugar, wood chips, Dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal dust, plant fiber dust, etc. The dust of these materials is prone to explosive combustion because there are strong reducing agents such as H, C, N, S, etc. When they coexist with peroxide and explosive dust, they decompose and are produced by oxidation reaction. A large amount of gas, or a small amount of gas, releases a large amount of heat of combustion. For example, aluminum powder is at risk of explosion as long as it is in a carbon dioxide atmosphere.
The difficulty of dust explosion is related to the physical, chemical and environmental conditions of the dust. It is generally believed that substances with a higher combustion heat are more likely to explode, such as coal dust, carbon, sulfur, and the like. Substances with fast oxidation rate are prone to explosion, such as magnesium powder, aluminum powder, ferrous oxide, dyes, and the like. Dust that is easily charged can also cause explosions, such as synthetic resin powder, fiber dust, and starch. These poorly conductive substances accumulate due to static electricity generated by friction with the machine or the air. When a certain amount is reached, an electric spark is generated to form an explosion source.
   Dust that is not easily explosive is soil, sand, iron oxide, abrasive materials, cement, quartz dust, and dust similar to burning. The dust chemical properties of these materials are relatively stable and therefore not easy to burn.
   However, if such dust is generated in oil mist and combustible gases such as CO, CH 4 and gas, it is also prone to explosion.
Smoking and open fire operations are strictly prohibited if effective ventilation and dust removal measures are used. A pressure relief valve or other device is provided in the outer casing of the device, and an explosion containment system or the like is used. For workshops with dust explosion hazard, it must be designed in strict accordance with the explosion-proof technical grade, and separate ventilation and dust removal systems. Always clean the floor and equipment in the workshop to prevent dust from flying and gathering. Ensure that the system has good airtightness. If necessary, fill the flammable dust in the closed container or pipeline with nitrogen, carbon dioxide and other gases to reduce the oxygen content and suppress the explosion of dust.
   4, the explosion process
The explosion of dust can be regarded as the following three steps: the first step is that the suspended dust is quickly retorted or vaporized by the heat source to produce a combustible gas; the second step is that the combustible gas is mixed with air to burn; The three steps are the heat released by the dust combustion, which is transmitted to the nearby suspended or blown dust by heat conduction and flame radiation. The dust is heated and vaporized to make the combustion cycle. As each cycle progresses, the reaction speed gradually increases, and through intense combustion, an explosion is formed. This explosion reaction, as well as the speed of the explosion flame, the speed of the explosion wave, the pressure of the explosion, etc., will continue to accelerate and rise, and will leap forward.
   5. Influencing factors
   (1) Physicochemical properties
The greater the heat of combustion of the substance, the greater the risk of explosion of the dust, such as coal, carbon, sulfur, etc.; the more easily oxidized, the more explosive the dust, such as magnesium, ferrous oxide, dyes, etc.; The more easily charged dust, the easier it is to explode. In the production process, dust is not easily lost due to collision, friction, etc., causing static electricity to accumulate. When a certain value is reached, electrostatic discharge occurs. Electrostatic discharge sparks can cause fires and explosions. Dust explosions are also related to the volatiles they contain. If the volatile matter in the pulverized coal is less than 10%, no explosion will occur, and thus the coke dust has no risk of explosion.
   (2) particle size
The surface of the dust adsorbs oxygen in the air. The finer the particles, the more oxygen is adsorbed, so the more likely the explosion occurs, and the lower the ignition point, the lower the lower explosion limit. As the diameter of the dust particles is reduced, not only the chemical activity is increased, but also the static electricity is easily charged.
   (3) Dust concentration
Similar to flammable gases, dust explosions also have a certain concentration range, and there are also upper and lower limits. However, most of the general information only lists the lower explosion limit of dust because the upper limit of dust explosion is higher.
   6, explosion characteristics
   (1) Multiple explosions are the biggest feature of dust explosions.
   (2) The minimum ignition energy required for dust explosion is high, generally above tens of millijoules.
   (3) Compared with the explosion of flammable gas, the dust explosion pressure rises slowly, the higher pressure lasts for a long time, the energy released is large, and the destructive power is strong.
   7. Explosion hazards
   (1) It is extremely destructive. Dust explosions cover a wide range of sectors, including coal, chemical, pharmaceutical processing, wood processing, food and feed processing. For example, between 1952 and 1979, there were 209 kinds of dust explosion accidents in Japan, and there were 546 casualties. Among them, the crushing and milling project and the dust separation project were more prominent, each with 46 cases. In the Federal Republic of Germany, there were 768 dust explosion accidents from 1965 to 1980. The most serious ones were wood powder and wood products dust and food and feed explosion accidents, accounting for 32% and 25% respectively. In recent years, the frequency of dust explosions in China is: local explosions 150-300 times, system explosions 1-3 times, and a growing trend. These dust explosions in our country, especially the system explosion, caused serious losses. In 1987, the linen dust explosion accident of Harbin Flax Factory killed 58 people, 177 people were light and serious, and the direct economic loss was 8.82 million yuan.
   (2) It is easy to produce a secondary explosion. The first explosion of air waves blows the dust deposited on the equipment or the ground. In the short time after the explosion, the central area of ​​the explosion will form a negative pressure, and the surrounding fresh air will be filled in from the outside to the inside, forming a so-called "return". The wind, mixed with the raised dust, caused a second explosion under the igniting of the first explosion. In the second explosion, the dust concentration is generally much higher than in an explosion, so the power of the second explosion is much larger than the first time. For example, in a sulphur powder plant, an internal explosion occurs in the grinder, and the blast wave diffuses from the grinder to the cyclone along the gas pipe. A secondary explosion occurs in the cyclone, and the blast wave is generated on the cyclone after passing the explosion. The crack spread to the workshop, sulphur dust settled on buildings and process equipment, and an explosion occurred.
   (3) It can produce toxic gases. One is carbon monoxide; the other is a toxic gas that is decomposed by explosives (such as plastic). The generation of poisonous gas often causes a large number of human and animal poisoning casualties after the explosion, and must be given full attention.
   8, dust removal system
   (1) Causes of fire and explosion in the dust removal system
In order to prevent dust pollution and dust explosion, it is necessary to use dust removal system to discharge dust in the working environment in time and reduce the dust concentration in the workshop. However, for flammable dust, dust removal equipment must also take fire prevention and explosion-proof measures, otherwise it will cause an explosion fire accident if it is used improperly. The reason for the explosion and fire of the dust removal system is mainly to satisfy the conditions (1) and conditions (2). Since the general dust collector is inhaled and then discharged, the oxygen supply in the dust removal system is sufficient. According to the survey, the main possibility of causing fire in the dust removal system is static spark, self-ignition fire, impact friction and open flame. The static spark is caused by the charging of the nylon filter cloth of the dust collector and the charging of the pipeline. The self-ignition fire is caused by the heat of oxidation accumulated by the collected dust slowly oxidizing in the bag or ash hopper. Impact friction is generally considered to be the collision of iron pieces inhaled by the dust removal system, or the occurrence of Mars caused by collision when the hammer head vibrates the dust attached to the ash or bag chamber during the cleaning process. Open flames are caused by gas welding flames and electric welding sparks during maintenance.
   (2) Explosion-proof measures for various parts of the dust removal system
Dust explosion accidents often occur in crushers, pulverizers, sieving machines, dryers, feeders and storage bins. In the dust removal system, there is generally no explosion accident in the dust hood at the dust inlet, because the dust concentration is only 0.1 to 10 g/m 3 here , and this value is safe. However, the dust hood can suck in the spark as a source of ignition, so attention should be paid. For example, when a grinder is working, it generates a lot of sparks. If it is very dangerous to share a dust collector with other equipment (source of flammable dust), isolation measures should be taken.
There are many instances where the air duct of the dust removal equipment explodes. This is because the dust is deposited in the tube. When it is subjected to some impact, the flammable dust flies again, forming a high concentration dust cloud in an instant. If it is exposed to fire, it is easy to explode. . The main reason for the deposition of dust in the duct is that the conveying wind speed is too small or there is air leakage. Therefore, in order to prevent explosion, the dust removal pipeline of flammable dust should be as short as possible, and the dust collector of the dust collector of the same system (the dust source capable of forming dust during production and processing) should be no more than four. For elbows, reducers, etc. in the system, the radius of curvature of the elbow should be 1.5 times or more of the pipe diameter D, and the expansion angle of the reducer should be 15 degrees or less to reduce the resistance. According to foreign experience, if the particle size of flammable dust is less than 15μm and the sedimentation velocity is about 0.1m/s, the wind speed in the pipe should be 100-200 times of the sedimentation velocity. When the dust concentration increases, the conveying wind speed should also increase accordingly. In the bag room and the ash hopper, movable doors should be installed for easy maintenance and cleaning. When the joints are connected by inserts, they should be carefully constructed so that there are no gaps or liners in the pipes, no dust and gas leakage. phenomenon. For the dust collector, the ash bucket is the place with the worst working conditions, and it is also the most prone to fire and explosion. Because part of the dust in the suction duct falls directly into the hopper, most of it is filtered in the filter bag. After the dust forms a layer on the surface of the filter bag, it will fall off by gravity or be sprayed, shaken and then detached. At this time, it is possible to generate a high concentration dust cloud, or pile up on the ribs in the ash hopper and on the steel plate weld. . If the dust is caused to fly twice for some reason, the conditions are appropriate and the fire source will be very dangerous.
Therefore, for the fire prevention and explosion prevention measures of the dust source (processing equipment) and the dust removal system, the first is to remove the fire source. For mechanical friction and sparks generated by impact, measures should be taken to remove Mars, and a suitable metal mesh should be installed at the dust hood to prevent the iron sheets, screws and the like from being sucked into the pipeline and causing sparks to cause combustion.
Preventing static sparks is an important issue. Especially for dust removal equipment that handles flammable dust, the dust collectors, pipes, fans and other facilities of the dust removal system should be connected and grounded. You can also use an anti-static filter cloth or put the bag of the dust collector with an iron clip and ground it.
For metal dust, such as lead, zinc, ferrous oxide, zirconium, etc., the slow oxidation reaction occurs in the ash hopper of the dust removal system, and the plastic synthetic resin, rubber and the like still maintain the friction heat of the product processing, Continuous ash removal should be adopted. Do not allow excessive dust accumulation in the ash bucket. Always observe the temperature inside the hopper and bag room, or install a temperature sensor to control the temperature inside the device at any time to prevent accumulation of heat and induce fire. explosion.
The oil mist and ventilating and exhausting facilities containing flammable gases in the production workshop must not be connected to the flammable dust dedusting system or the ventilating equipment operated by organic solvents or share the fan. In particular, bag-type dust collectors for iron-making blast furnaces, although the entire system has corresponding explosion-proof and fire-proof measures, it is necessary to strictly prevent gas explosion when the blast furnace is off. For this reason, in the absence of steam in the blast furnace, all the discharge valves and the deashing valves of the gas system must be quickly opened to collide with the outside of the tank to drive the gas out of the precipitator in a short time. For ventilation systems that cannot be separated in principle, in addition to the above measures, the residual gas in the entire dust removal system should be removed before and after the dust collector is operated and stopped.
   (3) Fire prevention measures in the dust removal system
Although the dust removal system can prevent explosion accidents after taking the above measures, there is still a possibility of fire and burning in the dust collector, which cannot be ignored. For example, in the bag dust collector for treating flammable dust, when the flue gas cooling device or the spray nozzle fails, the filter cloth may be burned by the high temperature flue gas or even cause a fire. Dry dust removal equipment generally uses the following fire protection measures.
   1 In the large-scale dust removal device of outdoor equipment, if it is found that there is a fire phenomenon, when the dust collector fan is turned off, in order to prevent the flue gas from flowing back into the workshop through the pipeline, the fan should be turned off after the fire damper or air volume switch is turned off. The filter part of the electrostatic precipitator and bag filter is usually a sealed structure. Without special circumstances, the combustion of the filter bag will not spread to other parts. For small dust-removing devices set up indoors, the fan should be turned off immediately when it encounters fire in the bag room.
   2 Once the dust accumulated in the ash hopper is on fire, it can be extinguished with a fire extinguisher, and then the dust inside is discharged. The discharged dust should be carefully treated to prevent re-ignition and to avoid the occurrence of deflagration accidents during the cleaning process. Clean the ash bucket or bag room and replace it with a new one.
   3 After the fire extinguishing, in order to prevent the lack of oxygen and carbon monoxide poisoning in the bag, the personnel should be fully ventilated before entering, and the safety and visibility can be entered before viewing and operating.
   4 When controlling dust with strong flammability and friction, in principle, the fan should be installed behind the bag filter. If placed in front, the dust adheres to the fan blades, and the fouling layer formed after the moisture will cause the impeller to lose balance, resulting in excessive eccentric torque during operation. The bearing heats up, vibrates or breaks the blade, damaging the casing, and generating sparks. The source of the dust. In order to avoid such an accident, a vibration switch can be added to automatically control the fan to turn off when an abnormality occurs in the fan.
   (4) Precautions for setting the dust removal equipment
Compared with the wet dust removal method, the dry dust removal method is convenient for the recovery and utilization of dust, and does not need to consider the antifreeze measures and sewage treatment problems in winter, and the cost and operation cost are low. In particular, bag filters have been widely used in flue gas and dust control in various industrial sectors. However, the following points should be noted in the design and construction of dust removal systems and buildings for the treatment of flammable dust.
1 The dust collector should maintain an appropriate safety distance with other production equipment, and the pressure-resistant wall should be placed around.
2 Workshop buildings with flammable dust should adopt fireproof structure, and fire doors, shutters and vents should be provided at the joints with other workshops to disperse the explosive gas.
3 In the dust collectors installed in workshops containing flammable and explosive gases, explosion-proof holes are generally designed. The size of the hole should be 5 to 6% of the area of ​​the bag or 0.16 m 2 /m 3 of the volume of the bag.
4 The number of air changes in the building and the proportion of the vents are designed to prevent dust explosions.
5 To prevent flammable dust from leaking and leaking, there should be no dust accumulation in the beams, supports, ceilings, etc. in the workshop.
   9, prevention
(1) Strengthen the strong wind and control the dust concentration below the lower explosion limit.
(2) In the case of explosive dust, avoid high temperature, open flame, electric spark and impact.
(3) For some dust that may cause spontaneous combustion or explosion when in contact with water, contact with water or liquids that may react may be avoided.
(4) Pay attention to the type and nature of dust, and avoid mixing or contact with dust that may react with each other.
Joint Rehabilitation Tape
I. Description
Joint Rehabilitation Tape is Cold applied tape coating system for anti corrosion of field joints,fittings, and specifically piping. The adhesive is a key in ground performance characteristic coupled with the low and high density polyethylene backing.
The Joint Wrap Tape shall be applied by hand or with a wrapping machine.
II. Structure
The specification of the tape consists of two layers, adhesive layer and film backing
Adhesive: butyl rubber
Outer layer: Special blend of stabilized polyethylene
III .Features:
Heavy duty adhesive
Resistant to UV
Can be applied over a wide temperature range
Good conformability and consistent uniform thickness
Easily applied with no special equipment
Conformable to irregular shapes.
Meets ASTM D 1000 Standard
Cold applied
â…£ .Technical datas:
Joint Rehabilitation Tape
Joint Rehabilitation Tape, Pipe Joint Tape, Pipeline Joint Rehabilitation Tape, PVC Pipe Wrap Tape
Jining Xunda Pipe Coating Materials Co.,Ltd , http://www.pipe-wrap.com