Dust explosion
Combustible dust within the explosive limit range, when exposed to a heat source (open flame or high temperature), the flame instantly spreads to the entire mixed dust space; The chemical reaction speed is extremely fast, while releasing a large amount of heat, forming high temperature and high pressure. The energy of the system is converted into mechanical energy and radiation of light and heat, which is extremely destructive.
Dust explosions often occur in production and processing sites accompanied by aluminum powder, zinc powder, aluminum processing grinding powder, various plastic powders, intermediates of organic synthetic drugs, wheat flour, sugar, sawdust, dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal dust, plant fiber dust, etc.
The hazards of dust explosion
1. Has extremely strong destructive power. Dust explosions involve a wide range of areas, including coal, chemical, pharmaceutical processing, wood processing, grain and feed processing, etc.
2. Easy to cause secondary explosions. A blast wave lifts up dust deposited on equipment or the ground, creating negative pressure in the central area of the explosion shortly after the explosion. Fresh air from the surrounding area fills in from the outside to the inside, mixes with the raised dust, and ignites a second explosion with the residual fire from the first explosion. During the second explosion, the dust concentration is generally much higher than during the first explosion, so the power of the second explosion is much greater than that of the first one.
3. Generate toxic gases. One is carbon monoxide; Another type is the toxic gas produced by the decomposition of explosives (such as plastics) themselves. The production of toxic gases often causes poisoning and casualties in humans and animals after explosions, and must be fully taken seriously.
The conditions for the generation of dust explosions
1. Dust itself is flammable, and combustible dust refers to dust that undergoes violent oxidation reactions with gaseous oxidants (mainly air) under certain conditions.
2. In a limited space, dust is suspended in the air to reach an explosive concentration (the lowest concentration of explosion is called the lower explosive limit, and the highest concentration is the upper explosive limit. Due to the large upper explosive limit of dust, it cannot be reached in most situations, so it is rarely used).
3. The initial energy sufficient to cause dust explosions can manifest in various forms such as flames, electric sparks, etc.
Explosive parameters of dust
The explosive parameters of dust are important basis for conducting explosion hazard assessment and explosion protection. These parameters are related to the composition, particle size distribution, and humidity of dust. Mainly includes:
1. Maximum explosion pressure Pmax: The maximum value of all explosion pressure peaks corresponding to different concentration values within the range of combustible dust concentration in a dust cloud under specified volume and ignition energy.
2. Explosion index Kst: In a closed container, the product of the maximum explosion pressure rise rate and the cube root of the container volume in a dust explosion test is a constant, which is called the dust explosion index. The dust explosion index is expressed as Kst value, unit: MPa · m/s.
3. Lower explosive limit (LEL): refers to the minimum concentration at which a mixture of flammable vapor, gas, or dust with air can explode upon contact with a source of fire (concentration of flammable vapor or gas, calculated by volume ratio)
4. Minimum ignition energy MIE: The minimum value of ignition source energy required to ignite a dust cloud at its most easily ignited concentration;
5. Minimum ignition temperature MIT-L of dust layer: the lowest thermal surface temperature at which a certain thickness of dust layer on a specific hot surface ignites under atmospheric pressure and room temperature conditions.
6. Limit oxygen concentration LOC: is a characteristic indicator used to evaluate and classify the flammability of liquid or solid materials. It is suitable for evaluating the flammability or flame retardancy of materials such as plastics, rubber, resins, fabrics, wood, insulating liquids, etc.
7. Maximum explosion pressure Pmax: The maximum value of the peak explosion pressure corresponding to different concentrations of dust clouds under specified volume and ignition energy, measured by experiments, unit: MPa；
The maximum explosion pressure rise rate and explosion index Kst are important parameters that reflect the intensity of the explosion, and are used for explosion pressure relief, explosion suppression, explosion isolation, and blast resistance design.
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