Then, what kind of dust is likely to cause an explosion accident, and how should the dust explosion accident be prevented?
Explosion can occur in a variety of dusts. Dust explosions are caused by the high pressure created by the combustion of combustible dust suspended in the air. Dust is a fine particle of a solid substance, and its surface area is much larger than that of a bulk material of the same weight, so that it is easy to catch fire. If it is suspended in the air and reaches a certain concentration, an explosive mixture is formed. Once you encounter Mars, it can cause rapid burning or even explosion.
Dust fire and explosion accidents occur in coal mines, flour mills, sugar mills, textile mills, sulfur plants, feed, plastics, metal processing plants and grain storage companies. This is related to the conditions required for a dust explosion. The dust explosion itself is a special type of combustion phenomenon, which also requires three conditions of combustibles, combustion aids and ignition sources. First of all, the dust itself is flammable dust. Combustible dust is classified into two types: organic dust and inorganic dust. Organic dust such as flour, wood flour, chemical fiber dust, etc., inorganic dust includes metal dust and a part of mineral dust (such as coal, sulfur, etc.). The most common combustible dusts are coal dust, corn dust, potato dust, aluminum dust, zinc dust, magnesium dust, sulfur dust and so on. Secondly, the dust must be suspended in a combustion-supporting gas (such as air) and mixed to reach the explosion limit of the dust concentration. The suspension of dust in the combustion-supporting gas is caused by mechanical actions such as pulverization, grinding, transportation, and ventilation. Large-diameter dust generally settles as a deposition dust with only combustion ability, and only a small particle size dust can be suspended in the combustion-supporting gas. At the same time, the danger of explosive dust is represented by the lower limit of the concentration explosion limit, generally 20-60 g/m3. Below this concentration, it is difficult to form continuous combustion, let alone explosion. Third, there is an ignition source sufficient to cause a dust explosion. Dust has a small auto-ignition point and minimum ignition energy, as long as the external energy exceeds the minimum ignition energy (more than 10 mJ-100 mJ) or the temperature exceeds its auto-ignition point (more than 400 °C -500 °C). In addition, equipment that is prone to static electricity is not properly grounded or sparks are generated at electrical and wiring connections. In particular, the feed of the pulverizer is not screened, causing iron to mix in and causing collisional sparks, which can cause dust explosion.
It should be pointed out that dust explosions are also highly likely to cause more destructive secondary explosions. When the dust is suspended in an oxygen environment containing enough to maintain combustion, and there is a suitable ignition source, the initial explosion may occur and cause disturbance to the surrounding environment, so that the dust deposited on the ground and equipment will diffuse to form a dust cloud. The fire source formed a catastrophic second explosion. In addition, after the first explosion, at the point of explosion of dust, due to the expansion of air, the density becomes smaller, and the explosion point is rapidly formed (referred to as “returning windâ€). In the case of dust clouds and thermal energy, a second explosion will occur. .
Scientific prevention is the key
Due to the special nature of dust explosions, rescue methods are also different from ordinary fire explosions. The effective fire extinguishing agent for the rescue of dust explosion accidents is water, especially misty water. It can extinguish the combustion, humidify the unburned dust, disperse and eliminate the suspended dust, and reduce the concentration. Avoid using DC spray water and foam, and it is not advisable to use impact dry powder, carbon dioxide and 1211 fire extinguishing agent to prevent the deposition dust from being suspended due to impact and causing a secondary explosion.
For some metal dust (water-repellent substances) such as aluminum, magnesium powder, etc., the reaction with water will make the combustion more severe, so it is forbidden to use water to save. Dry sand, lime, etc. (non-impactable) can be used; dust accumulated such as flour, cotton and hemp powder, etc., after the fire is extinguished, the interior may still be smoldering, and should also be paid enough attention; for dust fires in workshops with large area and long distance, Pay attention to taking effective measures to prevent the fire from spreading along the deposition dust or causing a chain explosion.
Because dust explosion accidents are extremely difficult to fight, it is especially important to do preventive work. The main preventive measures are as follows:
The first is to eliminate the source of dust. The use of good dust removal facilities to control the dust in the plant is the primary. The available measures are closed equipment, ventilation and dust extraction, dust extraction or wetting and dust reduction. The fan of the dust removal equipment should be installed on the side of the clean air. It should be noted that flammable dust cannot be used for electric dust removal equipment, and metal dust cannot be used for wet dust removal equipment. When the equipment starts, the dust removal equipment should be turned on first, then the main machine should be turned on; when it is stopped, the opposite is true to prevent the dust from flying. All parts of the dust workshop should be smooth, and try to avoid setting up some unrelated facilities (such as window curtains, curtains, etc.). Pipelines should not go through the dust workshop as much as possible. It should be laid in the wall to prevent dust accumulation. In addition, under the conditions, spray water in the dust workshop, adding moisture to the crushed material can also promote dust deposition and prevent the formation of dust clouds. . In addition, cleaning work in the workshop and timely manual cleaning is also a good way to eliminate the dust source.
The second is to strictly control the ignition source. Eliminating the ignition source is the most practical and effective measure to prevent dust explosion. Among the common ignition sources, electric sparks, static electricity, friction sparks, open flames, surfaces of high-temperature objects, and welding cutting sparks are the main causes of dust explosion. Therefore, it should be highly valued. The electrical equipment of the site shall be designed and installed in strict accordance with the "Code for Design of Electrical Installations for Explosive and Fire Hazardous Environments" to achieve the overall explosion-proof requirements. Try not to install or install mechanical equipment that is prone to static electricity and is prone to sparks, and adopt electrostatic grounding protection. Measures. The pulverized material must be subjected to strict screening, stone removal and iron absorption to prevent impurities from entering the pulverizer to generate sparks.
The third is to take reliable and effective protective measures. For smaller comminution devices, the strength can be increased and it is considered to prevent the explosion flame from propagating outward through the joint. In order to reduce the destructiveness of the explosion, a pressure relief device may be provided, such as using a lightweight roof, a wall or adding doors and windows to the workshop. However, it should be noted that the pressure relief device should be close to the part that is prone to explosion, not to the place where people are concentrated and the main traffic routes; to reduce the content of combustion gas, add inert gas (such as nitrogen) to the mixture of dust and combustion-supporting gas to reduce Oxygen content is also one of the feasible methods.
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