Dust concept
Dust means solid particles suspended in the air. Traditionally there are many names for dust, such as dust, dust, smoke, dust, dust, powder, etc. These terms have no obvious boundaries. According to the International Organization for Standardization, a solid suspended particle with a particle size of less than 75 μm is defined as dust. The presence of dust in the atmosphere is one of the main reasons for maintaining the earth’s temperature. Too much or too little dust in the atmosphere will have a disastrous effect on the environment. However, in life and work, productive dust is a natural enemy of human health, and it is a major cause of many diseases.
2. The composition of dust
Inorganic dust (such as quartz, cement, metal), organic dust (such as grain, hair, explosives, etc.) and mixed dust.
3. Classification of dust
Dust can be divided by particle size:
(1) Visible dust (particle size greater than 10μm);
(2) Microscopic dust (its particle size is 0.25 to 10 μm);
(3) Ultramicro dust (its particle size is less than 0.25 μm).
4. Conditions for dust explosion
When organic dust reaches a certain concentration in air, it has a great contact surface with oxygen in the air. Once the accident happens, it spreads very quickly. As a result of intense combustion, a large amount of gas and heat are not instantaneously dispersed, causing local pressure to soar and explosion. If it is not done properly, there will be a continuous explosion.
Dust combustion and explosion must have four conditions at the same time:
(1) Flammable dust Only at a certain concentration, the distance between dust particles is so short that they can ignite each other, and the accumulation of heat is sufficient to cause an explosion. According to the data, when the flour is suspended in 1 to 3 cubic meters of air, it is most likely to explode. Especially for particles of about 10 μm, the risk is greatest when the concentration is 20 g/m 3 . This concentration is equivalent to obscure objects that look 2 meters away.
(2) The dust explosion must have sufficient oxygen. If the dust concentration is too high, the amount of oxygen will be relatively reduced, and insufficient oxygen will not cause an explosion. Therefore, the dust explosion concentration has a highest limit, ie, the concentration of dust in the air is 65g/m3. Dust concentrations above this limit generally do not pose a risk of explosion. Such as the accumulation of flour is not easy to burn and explode.
(3) There is a fire source factor that ignites the dust and oxygen mixture. These factors include mechanical friction, impact and other heat sources, electrical and other electrical appliances such as overheating, short circuit, lightning discharge, lightning and other sparks caused by lightning, and poor management caused the material to block spontaneous combustion, smoking and so on.
(4) There is a limited closed space. Combine moderately flammable dust with oxygen and ignite it. If there is not a limited capacity, there will be no significant pressure and dust will not explode.
5. How to prevent dust explosions:
(1) Anti-static and fireproof treatment of the working environment;
(2) Operate in a ventilated and open space, and do not operate in the environment where the material flows; 3. Seal the working pipeline and clear the job site.