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Explosion proof dust collector

Author: admin Source: This station Time: 10:43, July 6, 2015
1. Characteristics of dust explosion
Dust explosion is a kind of oxidation reaction which is completed instantaneously under specific conditions when the dust particles of suspended solids in the air are in full contact with oxygen in the air. A lot of heat is released in the reaction, and then high temperature and high pressure are produced. Any dust explosion must have the following three conditions: ignition source; combustible fine dust; dust suspended in the air and reaching the explosion concentration limit range.
(1) Dust explosion is more complex than combustible materials and combustible gases. Generally, combustible dust is suspended in the air to form a dust cloud within the explosion concentration range. Under the action of ignition source, part of the dust contacting with the ignition source is ignited and forms a small fire ball. Under the action of the heat released by the combustion of this small fireball, the surrounding dust will be heated, the temperature will rise, and the phenomenon of ignition and combustion will occur. In 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. In general, the greater the combustion heat, the finer the particles, and the higher the activity of dust, the greater the risk of explosion; the higher the concentration of oxygen in the air, the dust is easy to ignite, and the explosion is more severe. Because moisture can inhibit explosion, the more dry the dust and gas, the greater the risk of explosion.
(2) After the occurrence of dust explosion, secondary explosion often occurs. This is because during the first explosion, a lot of dust is deposited together, and its concentration exceeds the upper limit of dust explosion and cannot explode. However, when the shock wave or air wave formed by the first explosion lifts up the deposited dust again, when it is mixed with air in the air and the concentration is within the range of dust explosion, a secondary explosion may occur immediately. The disaster caused by the second explosion is often much more serious than that caused by the first explosion.
The direct cause of dust explosion accident in a domestic aluminum production plant in 1963 was the fire caused by friction between exhaust fan impeller and suction end face. The airflow of shrimp bend and underpants tee at the fan inlet is not smooth, which is easy to accumulate dust. In particular, it is easier to retain dust when the machine is shut down. Once started, the deposited dust is lifted up and quickly reaches the lower explosion limit, causing dust explosion.
(3) The amount of energy released from the gas in the well is related to the amount of energy released by the combustion gas in the air. Therefore, for the same kind of solid powder, the smaller the particle size is, the larger the specific surface area is, and the faster the combustion diffusion is. If the particle size of this solid is very fine. So it can be suspended. Once there is a ignition source to ignite it, a large amount of energy can be released in a very short time. This energy has no time to dissipate to the surrounding environment, causing the gas in the space to be heated and adiabatic expanded. On the other hand, a large amount of gas generated during the combustion of powder will make the system form local high pressure, resulting in explosion and propagation, which is commonly known as dust explosion.
(4) The difference between dust explosion and combustion the combustion of large solid combustibles, such as coal combustion, is pushed inward in a nearly parallel layer. The release of this combustion energy is relatively slow. The heat and gas produced can be quickly dissipated. The pile combustion of combustible dust forms open fire in the case of good ventilation, but in the case of poor ventilation. It can form smokeless or flame ignition.
There are several stages in the combustion of combustible dust: in the first stage, the surface powder is also heated; in the second stage, the surface layer is vaporized and the volatile matter is overflowed; in the third stage, the volatile gas phase combustion is carried out.
The explosion of superfine powder is also a more complex process. Because the size of dust cloud is generally small, and the flame propagation speed is fast, hundreds of meters per second, so the ignition of fire source occurs in the dust center, which can burn the whole dust cloud in less than 0.1s. In this process, if the pressure is lower, the pressure will be lower. Whether the combustible particles can burn out depends on the particle size and combustion depth.
(5) Classification of combustible dust powder according to its flammability can be divided into two categories: one is combustible, the other is non combustible. The classification method and standard of combustible powder are different in different countries.
In the United States, combustible powder is classified as class II dangerous goods, while metal powder, carbon dust and grain dust are classified into different groups. According to the classification method developed by the United States, the powder is classified according to the pressure rising speed obtained when the powder under test occurs dust explosion in the standard test device, and it is divided into three grades. At present, there is no ready-made standard of combustible dust classification in China.
2. Influence of dust concentration and particles on Explosion
(1) Dust concentration combustible dust explosion also has the upper and lower limits of dust concentration. This value is affected by many factors such as ignition energy, oxygen concentration, powder particle size, powder type, moisture and so on. Generally speaking, the lower limit 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, and its value is not as easy to determine as the lower limit, and usually it is not easy to reach the upper limit concentration. Therefore, the lower limit is more important.
In physical sense, the upper and lower limits of dust concentration reflect the influence of the distance between particles on the flame propagation of particle combustion. If the distance between particles reaches such that the combustion flame cannot extend to adjacent particles, the combustion cannot continue (propagation), and the explosion will not occur; at this time, the dust concentration is lower than the lower limit concentration value of explosion. If the distance between the particles is too small, the oxygen between the particles is not enough to provide sufficient combustion conditions, and the explosion can not be formed. At this time, the particle concentration is higher than the upper limit.
(2) When the particle size of combustible powder is more than 400 um, the dust cloud is no longer explosive. But for the ultra-fine powder, when the particle size is less than 10um, it is dangerous. It should be noted that sometimes even if the average particle size of the powder is greater than 400 um, it often contains fine powder, which is also explosive.
Although the influence of the particle size of the powder on the explosion performance is not strong, the smaller the size of the powder is, the larger the specific surface is, the faster the combustion is, and the increasing speed of pressure 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 with certain energy. For dust explosion, there should be three elements: ignition source; combustible fine dust; dust suspended in the air, forming dust cloud within the range of explosion concentration. Only when these three elements exist at the same time can the explosion occur. Therefore, as long as one of the conditions is eliminated, the explosion can be prevented. The following technical measures are often used in bag filter.
(1) In the special design of the body structure, in order to prevent the dust accumulation of combustible dust in the internal components of the dust remover, all beams and partition plates should be equipped with dust-proof plates, and the slope of dust-proof plates should be less than 70 degrees. The slip angle of the ash hopper is greater than 70 degrees. In order to prevent dust accumulation due to the small angle between the two bucket walls, the two adjacent side plates should be welded with slide plates to eliminate dust deposition. Considering that the ash hopper is dewed and blocked due to abnormal operation and high dust humidity, steam pipe insulation or tubular electric heater shall be added to the high temperature dust collector on the ash hopper wall panel when designing the ash hopper. In order to prevent the hopper from spreading, each hopper should also be equipped with a bin arm vibrator or an air gun.
One dust collector has 2-3 ash hoppers at least and 5-8 ash hoppers at most. When it is used, the deflection caused by uneven air volume will be produced. The amount of pulverized coal in each ash hopper is uneven, and the ash volume behind h is large.
In order to solve the problem of uneven air flow, the following measures can be taken in the structure: 1. Add wind deflector on the inclined baffle of air duct, as shown in Fig. 5-168. The size of baffle plate shall be determined according to the principle of equal air volume and equal air pressure; ② considering the change of actual situation on site, adjustable valve rod and valve plate shall be adopted between lifting valve rod and valve plate, so that the outlet height h is the variable value for further correction; ③ air volume regulating valve shall be set in the air inlet branch pipe to regulate the air volume of each box room after the equipment runs. The air volume difference of each box room is controlled within 5%.
(2) Using anti-static filter bag in the dust collector, due to high concentration dust friction with each other in the flow process, dust and filter cloth also have mutual friction, can produce static electricity, the accumulation of static electricity will produce sparks and cause combustion. For the pulse dust cleaning method, the filter bag is made of polyester needle felt. In order to eliminate the electrostatic deficiency of the polyester needle felt, conductive metal wire or carbon fiber is spun into the fabric of the filter bag. When installing the filter bag, the filter bag is connected with the porous plate through the steel framework, and connected to the workshop grounding grid through the shell. For the filter bag of reverse blowing dust cleaning, a variety of anti-static products such as mp922 have been developed. The effect is very good.
(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 necessary fire extinguishing equipment. The purpose of safety hole is not to prevent explosion, but to limit the explosion range and reduce the number of explosions. Most dust collectors for explosive dust are operated 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 effectively play a role; at ordinary times, the maintenance and management of safety hole should be strengthened.
The safety hole is made of thin metal plate. Because the box of bag filter can't bear a lot of pressure, so the strength of rupture plate should be designed so that the plate will be destroyed under lower pressure. Sometimes, because of the long-term compression of the box, the aluminum plate will produce fatigue deformation and fracture, even if this is normal, it is not allowed to replace the thick plate with high strength.
Spring door type safety hole is to adjust the opening pressure by increasing or decreasing the spring tension. In order to ensure the safe operation of the door regularly.
The area of safety hole should be determined according to the relationship between the maximum pressure of dust explosion, the speed of pressure increase and the compressive strength of the box. However, there is no exact data. The size of safety hole area should be determined according to the form and structure of bag filter. We think that the ratio of safety hole to dust collector volume is 1 / 10 ~ 1 / 30 for medium and small dust collector, and 1 / 30 ~ 1 / 60 for large and medium dust collector. In case of difficulties, it is necessary to refer to the actual situation of other devices to reserve safety explosion-proof holes.
① The explosion-proof plate is an emergency pressure relief device driven by pressure difference and not automatically closed. It is mainly used for pipeline or dust removal equipment to avoid damage caused by overpressure or vacuum. Compared with the safety valve, the bursting disc has the advantages of large discharge area, sensitive action, high precision, corrosion resistance and not easy to block. The bursting disc can be used alone or combined with safety valve.
The explosion-proof plate device is composed of bursting disc and holder. The holder is made of Q235, 16Mn or OCr13 and other materials. Its function is to clamp and protect the explosion-proof plate to ensure the stability of blasting pressure. The explosion-proof plate is made of aluminum, nickel, stainless steel or graphite and has different shapes: the concave surface of the arched explosion-proof plate faces the pressure side, and tensile or shear failure occurs during blasting; the convex surface of the reverse arched explosion-proof plate is facing the pressure side, which is cut by the blade or along the slot due to sudden overturning due to instability during blasting; the tensile or shear failure also occurs when the plane shaped explosion-proof plate is blasted.
The pressure resistance of the explosion-proof plate should be based on the working pressure of the dust collector. Because the pressure resistance requirement of dust collector body is 8000 ~ 18000pa, the rupture pressure of explosion relief valve membrane is determined according to the set pressure resistance requirements.
② There are two kinds of safety explosion-proof valve design: one is explosion-proof plate; the other is heavy hammer type explosion-proof valve. In case of high negative pressure, it is easy to billet and not easy to keep warm. The latter one is more advanced than the former one, which is pressed by heavy hammer in the closing state, and has poor tightness. The two methods mentioned above are not suitable for high voltage pulse cleaning. In order to solve the problem of tightness, the explosion-proof safety lock can be designed on the heavy hammer type fat explosion valve. Its characteristics are: when closing, the safety door is mainly locked through this lock, which can be automatically opened and released in case of explosion, and its release force (safety force) can be adjusted by spring. In order to balance the force of the safety door, 4-6 locks should be set according to the area of the safety door. In order to make the explosion-proof door tight without air leakage, the double structure of explosion-proof plate and safety lock can be designed.
(4) Detection and fire protection measures in order to prevent in advance, the necessary fire control measures can be taken in the dust removal system.
① Fire fighting facilities. It mainly includes water, CO2 and inert extinguishing agent. For cement plant, CO2 and nitrogen can be used in steel plant.
② Temperature detection. In order to eliminate the temperature change of the dust collector and control the ignition point, several thermometers are installed on the ash hopper at the inlet of the dust collector.
③ CO detection. For large dust removal equipment, the installation of thermometer is very limited because of its large volume. Sometimes combustion occurs in the distance of the thermometer measuring point, which is difficult to reflect from the thermometer. A CO detection device can be installed at the outlet of the dust remover to help detect. As long as combustion occurs anywhere in the dust collector, the CO in the flue gas will rise. At this time, the alarm of elevated CO concentration and the control of the dust removal system will be linked to stop the operation of the system dust remover in time.
(5) Equipment grounding measures explosion proof dust remover is often arranged in the open air due to safe operation. Even in the open air on the tall steel structure, according to the equipment grounding requirements, equipment grounding lightning protection has become an essential measure, but the dust collector generally does not set lightning rod.
The conductor with good conductivity is added between all connecting flanges of dust collector, and the conductor form can be made into card type. It can also be made into line type. The line conductor is shown in the figure. The card type conductor is shown in the figure. No matter which type of conductor is used, the connection must be firm and buried on the surface, which has certain corrosion resistance function. Otherwise, it will affect the lightning protection effect of equipment grounding.
(6) It is necessary to select the explosion-proof parts in the dust remover explosion-proof measures. Explosion proof dust remover taboo operating conditions of dust into the electrical load induced by the induction of explosion risk. When the dust remover is running, the electrical load and components will inevitably produce electric shock and cremation even during the current transmission and contact, and it is also very easy to cause the explosion of dust source gas exceeding the limit concentration induced by discharge spark. All electrical load components must be explosion-proof components to prevent explosion induced factors. Ensure the safety of equipment operation and operation. For example, the pulse valve of pulse dust collector and the solenoid valve for lifting valve should be explosion-proof products.
(7) Measures to prevent Mars from mixing into the bag filter used to treat the waste gas from sawdust boiler, rice husk boiler, aluminum regeneration furnace and smelting furnace, the burned dust in the furnace may enter into the box body with the air flow of the air duct, which may cause the dust accumulated on the filter cloth to ignite and cause accidents.
In order to prevent Mars from entering the bag filter, the following measures should be taken:
① Pre precipitator and piping. The picture shows a cyclone or inert dust collector as a pre dust collector to capture coarse dust and sparks. In this way, the particles which are too fine are not easy to capture, and in most cases, the dust can be burned out before entering the dust collector. After the pre dust remover, the cooling pipe is set, and the flow rate in the pipe is controlled to make it as low as possible. This is a relatively reliable technical measure, which can make the gas have sufficient residence time in the pipe.
② Lingque spray tower. The gas cooling method of direct water spray in advance. In order to ensure the safety and fire prevention of dust containing gas in bag filter, the cooling water supply is controlled. Most of the burning dust can be cooled as soon as it contacts with micro water droplets, but the water droplets are easy to vaporize. In order to cool the combustion dust which has not been in contact with water drops, necessary space and residence time should be provided.
In special cases, spray tower, cooling pipe and pre dust collector are combined to prevent Mars from mixing.
③ The Mars capture device is shown in the figure. It is a simple and feasible method to install Mars capture device on the pipeline. In addition, when Mars passes through the catcher, it can send out electrical signals and alarm. At the same time, stop the operation or change the gas circuit.
The design requirements of Mars catcher are as follows:
a. When the spark catcher is used to capture the spark particles in high-temperature flue gas, the main material of the device is usually 15Mo3 or 16mo, and Q235 is used for beam, column and platform ladder. When the spark catcher is used as the flue gas pre separator, in addition to the rotating blade, other materials can be Q235;
b. The speed of equipment import and export is generally between 18 and 25m / s;
c. The separation effect of dust should be considered. The blade should be wear-resistant measures and appropriate rotation angle;
d. Equipment structure design should consider the deformation caused by high temperature.
(8) When the dust concentration at the inlet is controlled and the non combustible powder is added into the bag filter, the internal concentration distribution will inevitably make a certain part within the explosion limit. In order to improve the safety, avoid the concentration between the upper and lower limits of dust explosion in the pipeline. When the dust concentration is higher than the lower limit, it is necessary to keep the dust concentration in the middle and lower limit when the dust concentration is higher than the lower limit.
An example of using dilution method to prevent fire. When collecting explosive dust, the dust is diluted with air due to the installation of dust hood, and the concentration in the pipeline is far lower than the lower explosion limit. Non combustible powder, such as clay and bentonite, is continuously provided to the pipeline from the middle of the system, and the explosive dust is diluted in the dust collector to prevent the risk of explosion and fire.
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