1. What is dust explosion prevention

What is dust explosion prevention? It means that when dust is within the explosion limit and encounters a heat source (open flame or temperature), the flame instantly spreads throughout the entire mixed dust space. The chemical reaction speed is extremely fast, and a large amount of heat is released, forming high temperature and pressure. The energy of the system is converted into mechanical work and radiation of light and heat, which has strong destructive power.

2. Basic Definition

Any solid substance in a fine powder state is called dust. Dust that can burn and explode is called combustible dust; Dust floating in the air is called suspended dust; The dust that settles on solid walls is called sedimentary dust. The following seven types of substances have been found to have explosive dust: metals (such as magnesium powder and aluminum powder); Coal; Grain (such as wheat, starch); Feed (such as blood meal, fish meal); Agricultural and sideline products (such as cotton, tobacco); Forest products (such as paper powder, wood powder); Synthetic materials (such as plastics, dyes). The dust generated during the production process of certain factories and mines, especially the dust generated during the processing of organic matter, can cause explosion and combustion accidents under certain specific conditions. Research results both domestically and internationally indicate that there are generally three types of these conditions:

(1) Combustible dust is suspended in the air at an appropriate concentration, forming what is commonly known as a dust cloud;

(2) Having sufficient air and oxidants;

(3) There is a fire source or strong vibration and friction.

It is generally believed that as long as explosive dust meets conditions (1) and (2), it means that there are signs of potential accidents.

3. Explosion process

The explosion of dust can be regarded as the formation of the following three steps:

The first step is the rapid dry distillation or gasification of suspended dust under the action of a heat source to produce combustible gases;

The second step is to mix combustible gas with air and burn it;

The third step is to transfer the heat released by the combustion of dust to nearby suspended or blown dust through thermal conduction and flame radiation. These dust particles are heated and vaporized, causing the combustion cycle to continue. As each cycle progresses, the reaction rate gradually increases, and through intense combustion, an explosion is finally formed. This explosion reaction, as well as the speed of the explosion flame, explosion wave, explosion pressure, etc., will continue to accelerate and increase, and develop in a leapfrog manner.

4. Explosion characteristics

(1) Multiple explosions are the biggest characteristic of dust explosions;

(2) The minimum ignition energy required for dust explosion is relatively high, generally above tens of millijoules.

(3) Compared with combustible gas explosions, dust explosions have a slower increase in pressure, longer duration of higher pressure, greater energy release, and stronger destructive power.

5. Main hazards

(1) Has extremely strong destructive power.

Dust explosions involve a wide range of sectors, including coal, chemical, pharmaceutical processing, wood processing, grain and feed processing, etc. Between 1952 and 1979, there were 209 dust explosion accidents in Japan, resulting in a total of 546 casualties. Among them, the crushing and powder engineering and vacuum separation engineering were more prominent, with 46 incidents each. From 1965 to 1980, there were 768 dust explosion accidents in the Federal Republic of Germany, among which the more serious ones were dust explosions caused by wood powder and wood products, as well as grain and feed, accounting for 32% and 25% respectively. In recent years, the frequency of dust explosions in China has been 150-300 local explosions and 1-3 system explosions per year, with an increasing trend. The dust explosions, especially system explosions, that have occurred in our country have caused serious losses. In 1987, the flax dust explosion accident at Harbin Linen Factory resulted in 58 deaths, 177 minor injuries, and a direct economic loss of 8.82 million yuan.

(2) Easy to cause secondary explosions.

The first explosion wave blows up the dust deposited on the equipment or ground. In a short period of time after the explosion, negative pressure is formed in the central area of the explosion, and fresh air around fills in from the outside to the inside, forming a so-called "return wind". It mixes with the raised dust and ignites the second explosion under the residual fire of 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. For example, in a sulfur powder plant, an explosion occurred inside the grinder, and the explosion wave spread along the gas pipeline from the grinder to the cyclone separator. A secondary explosion occurred in the cyclone separator, and the explosion wave spread to the workshop through the cracks generated on the cyclone separator after the explosion, lifting up sulfur dust that had settled on buildings and process equipment, and causing another explosion. (3) Can produce 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 a large number of casualties and injuries to humans and animals after explosions, which must be fully taken seriously.

As can be seen from this, it is becoming increasingly important to prevent dust explosion accidents. To eliminate safety hazards, it is necessary to prevent the occurrence of explosions from the source. In terms of industrial automation control scope, the use of electromagnetic valves in dust environments requires the use of dust explosion-proof electromagnetic valves.