First, the conditions
the flammable dust to the appropriate concentration of suspended in the
air, forming what people often say that the dust cloud;
Any powdery state of the solid material are called dust. Can be burned and exploded dust is called combustible dust; floating in the air dust is called suspended dust; settlement in the solid wall of the dust is called deposition dust.
(Such as wheat, starch); feed (such as blood meal, fish meal); agricultural products (such as cotton, tobacco); forest products (such as paper powder, wood); Powder); synthetic materials (such as plastics, dyes).
Some factories and mines in the production process of dust, in particular, some of the organic processing of dust generated under certain conditions, the occurrence of explosive combustion accidents.
(3) there is a strong source of fire or vibration and friction.
Second, the explosion principle
Generally more likely to have an explosion of dust dust generally have aluminum powder, zinc powder, ferrosilicon powder, magnesium powder, iron powder, aluminum processing grinding powder, all kinds of plastic powder, organic synthetic medicine intermediates, wheat flour, sugar, Dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal dust, plant fiber dust and so on. The dust of these materials is easy to occur because of the explosive combustion of strong reducing agent H, C, N, S and other elements exist, when they coexist with peroxide and explosive dust, decomposition occurs by the oxidation reaction A large amount of gas, or a small amount of gas, but a large amount of combustion heat. For example, aluminum powder in carbon dioxide atmosphere as long as the risk of explosion.
The difficulty of dust explosion and dust of the physical, chemical properties and environmental conditions. Generally believed that the greater the burning heat of the material more likely to explode, such as coal dust, carbon, sulfur and so on. Fast oxidation of the material easy to explode, such as magnesium, aluminum, ferrous oxide, dyes and so on. Easy to charge dust is also very easy to cause an explosion, such as synthetic resin powder, fiber dust, starch and so on. These poor conductive material due to friction with the machine or air generated by the accumulation of static electricity, when a certain amount, it will discharge electrical spark, constitute the source of the explosion.
Dust, sand, iron oxide, abrasive materials, cement, quartz dust, dust, and the like, which are less likely to cause explosions. Dust chemical properties of these substances is relatively stable, it is not easy to burn. However, if such dust generated in the oil mist and CO, CH4, gas and other flammable gases, but also prone to explosion.
The explosion of dust can be regarded as the development of the following three steps: the first step is to quickly suspend or vaporize the suspended dust under the action of the heat source to produce combustible gas; the second step is combustible gas and air mixture; The three steps are the heat released by the combustion of the dust, which is transferred to nearby suspended or blown dust by means of heat conduction and flame radiation. The dust is vaporized and the combustion is cycled. As each cycle progresses, the reaction rate gradually accelerates, through intense combustion, and finally the formation of an explosion. This explosion reaction and the explosion speed of the flame, the speed of the blast wave, the explosion pressure will continue to accelerate and increase, and was leaping development.
Third, the characteristics of hazards
3.1 Influencing factors
Physical and chemical properties
Such as coal, carbon, sulfur dust, etc .; the more easily oxidized substances, the dust more easily exploded, such as magnesium, ferrous oxide, dyes, and so on; the greater the heat, The more easily charged the dust more easily from the explosion. Dust in the production process, due to mutual impact, friction and other effects, the resulting static dissipation is not easy, resulting in electrostatic accumulation, when a certain value, it will appear electrostatic discharge. Electrostatic discharge sparks can cause fire and explosion accidents. Dust explosions are also associated with volatiles. Such as coal when the volatile matter less than 10%, no longer an explosion, so there is no explosion of coke dust.
Dust surface adsorption of oxygen in the air, the finer particles, the more oxygen adsorption, which is more prone to explosion, and the lower the ignition point, the lower the lower the explosion. As the diameter of the dust particles decreases, not only the chemical activity increases but also the static electricity is liable to be brought.
With the combustible gas phase, the dust explosion also has a certain concentration range, there are upper and lower limits of the points. However, most of the general information only lists the dust explosion lower limit, because the dust explosion upper limit is higher.
3.2 Explosion characteristics
(1) multiple explosion is the biggest characteristic of dust explosion.
The first explosion of air waves, will be deposited on the equipment or dust blowing on the ground in the explosion after a short time the center of the explosion will be the formation of negative pressure, the surrounding fresh air will be filled in from outside to inside, and raised Dust mixed, causing a secondary explosion. Secondary explosion, the dust concentration will be higher.
(2) the minimum ignition energy required for dust explosion is high, usually tens of mJ above.
(3) compared with the flammable gas explosion, the dust explosion pressure rise more slowly, higher pressure for a long time, the release of energy, destructive power.
3.3 Major hazards
(1) is extremely destructive. Dust explosion involves a wide range of coal, chemicals, pharmaceutical processing, wood processing, food and feed processing and other departments have occurred.
(2) prone to secondary explosion. The first explosion of air waves to the deposition of dust on the equipment or the ground blowing in the short period of time after the explosion will form a negative pressure in the central area, the surrounding fresh air from outside to inside to fill in the formation of the so-called "return Wind ", mixed with the dust raised, the first explosion in the fire caused by a second explosion. Second explosion, the dust concentration is generally much higher than an explosion, so the secondary explosion power is much larger than the first time. For example, a sulfur powder mill, the internal grinding mill explosion, the explosion wave along the gas pipeline from the grinder to the cyclone separator, the cyclone secondary explosion occurred, the explosion wave through the cyclone after the explosion produced Of the gap spread to the workshop, raised the settlement in the buildings and process equipment on the sulfur dust, and the explosion occurred.
(3) can produce toxic gases. One is carbon monoxide; the other is explosives (such as plastic) itself decomposition of toxic gases. Gas production is often caused after the explosion of a large number of human and animal poisoning casualties, must pay full attention.
Fourth, preventive measures
Smoking and open fire are strictly prohibited if dust and ventilation are used effectively. To do a good job in the early "preventive measures" in preparation for the workshop to detect the dust g / h content, the use of appropriate dust collector in a timely manner to the collection. This is the preferred, that is, to protect the environment, but also to provide a good working environment for workers to help improve work efficiency. Second choice, strengthen the ventilation, so that the dust concentration within the workshop to combustible and explosive concentrations below. But also the use of time sprinkler operations, so that dust in the humid environment to the settlement, but the wet environment of the equipment and electrical parts of corrosion and short-circuit hidden dangers, the operation should be careful not to the effect.