Prevent dust explosion technical measures
1, the characteristics of dust explosion
Dust explosion means that the dust particles in the air are in full contact with the oxygen in the air, and the oxidation reaction instantaneously completes under certain conditions, and a large amount of heat is released during the reaction, thereby generating high temperature and high pressure. Any dust explosion must have these three conditions: ignition source; combustible fine dust; dust suspended in the air and reaches the explosive concentration limit range.
(1) Dust explosion is more complex than flammable substances and flammable gases. 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 that comes into 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.
The difficulty and severity of dust explosion are closely related to the physical and chemical properties of dust and the surrounding air conditions. In general, the higher the combustion heat, the finer the particles, and the higher the activity of dust, the greater the risk of explosion; lighter suspensions have a greater risk of explosion; combustibles have a higher risk of explosion; when the oxygen content in the air is high, the dust is easily ignited. The explosion is also more intense. Since moisture has the effect of suppressing explosion, the drier the dust and gas, the greater the risk of explosion.
(2) After a dust explosion, a secondary explosion often occurs. 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 the concentration 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 range of the dust explosion, it may be followed by a secondary explosion. The disaster caused by the second explosion is often much more serious than the first explosion.
The direct cause of the dust explosion in 1963 at a domestic aluminum product factory was caused by the friction between the impeller of the exhaust fan and the end face of the suction inlet. Fans at the suction inlet of the fan and the crotch trousers have poor air flow and are prone to dust accumulation. In particular, dust is more likely to be trapped during shutdown. Once activated, the deposited dust is lifted and quickly reaches the lower explosion limit, causing dust 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. It can be suspended, and once it is ignited by an ignition source, 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 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 with the inside of the parallel layers, such as coal combustion. 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 open flames without good ventilation. It can form smokeless or flame flammable.
Combustion dust has several stages of combustion: in the first stage, the surface powder is also heated; in the second stage, the surface layer is gasified and overflows the volatiles; in the third stage, volatile matter is generated in the gas phase.
Explosion of ultra-fine powder is also a more complicated process. Because the dust cloud is generally small in scale, and the flame propagation speed is fast, several hundred meters per second, so the fire ignition occurs in the dust center, in less than 0.1s. In time, it can burn all over the dust cloud. During this process, if the dust has burned out, the highest pressure is generated; if not, a lower pressure is generated. Whether combustible particles can be burned depends on the size of the particles and the depth of combustion.
(5) Combustible dust classification Powder according to its flammability can be divided into two categories: one is flammable; one is non-flammable. Classification methods and standards for combustible powders vary in different countries.
The United States classifies flammable powders as class II dangerous goods, while including metal powder, carbon dust, and grain dust in different groups. The classification method developed by the United States is to classify the powders to be tested according to the rate of pressure increase that occurs when a dust explosion occurs in a standard test device and classify them into three grades. China has not yet seen ready-made standards for the classification of combustible dust.
2, dust concentration and particle impact on the explosion
(1) Dust concentration The 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 a variety of factors, and 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.
From a physical point of view, the upper and lower limits of the dust concentration reflect the influence of the distance between particles on the flame propagation of the particles. If the distance between the particles is such that the combustion flame cannot extend to adjacent particles, the combustion cannot continue (propagation). The explosion will not happen; the dust concentration is lower than the lower explosion concentration. If the distance between the particles is too small, the oxygen between the particles is not enough to provide sufficient combustion conditions, it will not be able to form an explosion. At this time, the particle concentration is higher than the upper limit value.
(2) Particle size When the combustible powder particles are larger than 400um, the formed dust cloud no longer has explosive properties. 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.
Although the particle size of the powder does not affect the regularity of the explosion performance, the smaller the size of the powder, the larger the specific surface area, the faster the combustion, and the pressure increase rate increases linearly. Under certain conditions, the maximum pressure changes little, because it depends on the total energy emitted when burning, and there is no obvious relationship with the speed of releasing energy.
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 slope of the dust-proof plate shall be less than 70 degrees. . The slip angle of the hopper is greater than 70 degrees. To prevent the angle between the two bucket walls being too small, 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.
One set of dust collectors may have at least 2 to 3 ash buckets, and as many as 5 to 8 airbags. When used, there will be deflection due to uneven air flow. The amount of pulverized coal in each ash bucket is uneven, and the amount of ash behind H is large.
In order to solve the problem of non-uniform air volume, the following measures can be taken in the structure: 1 Add a wind deflector to the inclined partition of the air duct, as shown in Figure 5-168. The size of the baffle should be determined according to the principle of equal wind volume and equivalent wind pressure; 2 and then taking into account changes in the actual situation at the scene, adjustable between the lift valve stem and the valve plate, so that the export high h is the change value to further correct 3 Air flow control valve is provided in the air intake branch. After the equipment is in operation, the air volume of each compartment is adjusted. Make the difference between the air volume of each box controlled within 5%.
(2) The use of anti-static filter bag In the interior of the dust collector, due to the high concentration of dust rubbing with each other during the flow, the dust and the filter cloth can also generate friction, which can generate static electricity.
The accumulation of electricity creates sparks that 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 filter wire or carbon fiber is spun into the filter bag fabric. When the filter bag is installed, the dust bag passes through the steel. The skeleton is connected to the perforated plate and is connected to the workshop 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) Setting safety holes (valves) It is important to set the safety holes and essential fire-fighting equipment so as to confine the explosion to the interior of the bag filter without extending it to other areas. 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; it is necessary to strengthen the maintenance and management of the safety hole at ordinary times.
Cracked plate safety holes are made of ordinary thin metal plates. Because the bag filter housing cannot withstand much pressure, the strength of the design of the rupture plate should be such that the plate is destroyed at a lower pressure. Sometimes, due to the long-term stress of the box, the aluminum plate is fatigued and deformed, resulting in cracking. Even if it is normal, it is not allowed to replace the high-strength plate.
Spring type safety holes are used to adjust the opening pressure by increasing or decreasing the spring tension. In order to ensure that the door hole can play a safe role in accidents, it must be periodically tested on its operation.
The area of the safety hole should be determined according to the relationship between the maximum pressure at the time of dust explosion, the rate of pressure increase, and the compressive strength of the tank, but there is no exact data yet. According to the form and structure of the bag filter, the size of the safety hole area is determined. We think that the ratio of the safety hole of the small-sized dust collector to the volume of the dust collector is 1/10 to 1/30, and the ratio of the large and medium-sized dust remover is 1. 1/30 to 1/60 are suitable. When encountering difficulties, it is necessary to properly refer to the actual determination of safety explosion-proof holes reserved for other devices.
1 Explosion-proof plates Explosion-proof plates are emergency pressure relief devices that are actuated by a pressure difference and are not automatically closed. They are mainly used for pipes or dust removal equipment to prevent them from being destroyed 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.
Explosion-proof plate device is composed of two parts: bursting disc and holder. The holder is made of Q235, 16Mn or OCr13. Its role is to clamp and protect explosion-proof plate to ensure stable burst pressure. Explosion-proof plates are made of aluminum, nickel, stainless steel or graphite and have different shapes: the concave surface of the arched explosion-proof plate faces the pressure side, and the tensile or shearing failure occurs at the time of blasting; the convex surface of the reverse-arched explosion-proof plate faces compression On the side, due to instability, the blast suddenly turns over and is cut by the edge of the blade or tears along the slot; the explosion of the plane-shaped explosion-proof plate also occurs tensile or shear damage.
The pressure resistance of the dust-precipitator selection explosion-proof plate should be based on the working pressure of the dust collector. Because the dust collector body pressure requirements 8000 ~ 18000Pa according to the set pressure requirements to check the information to determine the burst pressure diaphragm rupture pressure.
2 explosion-proof valve design There are two kinds of safety explosion-proof valve design: one is the explosion-proof plate; the other is the heavy hammer 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 it is in the closed state by 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-fighting measures may 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 and 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 an essential measure, but the dust collector is generally not equipped with lightning rods.
All the connecting flanges of the precipitator are provided with conductors with better conductivity, and the conductor form can be made into a card type. It can also be made into a line style. Line-type conductors are shown in the figure. Card-type conductors are shown in the figure. Regardless of which form of conductor is used, the connection must be solid, and it must be buried at the surface with a certain corrosion resistance. Otherwise it will affect the equipment grounding lightning protection effect.
(6) Explosion-proof of supporting components It is indispensable to choose explosion-proof components in the explosion-proof measures of the dust remover. 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 also very easy for discharge spark to induce dust gas explosion exceeding the limit concentration, and the electric load components must all be selected explosion-proof type. 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 burned dust in the furnace may enter the tank with the airflow from the duct and cause 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 pre-precipitator 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 more 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 easy to gasify. 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.
In special cases, spray towers, cooling pipes and pre
