Conditions of Explosion

If combustible material is mixed homogeneously with the oxidizing agent (e.g., to create a homogenous dust cloud), the burning rate increases significantly, and the combustion process can be completed in a 10-millisecond order. This phenomenon is responsible for the significant difference between burning a piece of coal and the rapid deflagration of a coal dust cloud. If the third condition – the ignition source – is also provided, then all of the conditions exist for the rapid release of the stored energy, i.e. the dust explosion. In a closed vessel, under certain conditions, this rapid reaction can usually cause a pressure rise of 10-12 barg (although some powders can reach higher pressures). By definition, an explosion is an exothermic chemical process that occurs in a finite volume by a sudden and significant increase in pressure.

So, the explosion properties of dust significantly differ from the original materials from which are formed. The main cause of this phenomenon is the following: the same material has larger order of magnitude specific surface after dust formation. In this bigger area, the intensity of oxidation is much higher, and after glowing of one particle, a chain reaction maybe start.

Figure 2.: Conditions of explosion

The minimum condition of an explosion of dust-air and gas-air systems are the simultaneous presence of an explosive substance, and an oxidizing atmosphere, and an ignition source. An additional requirement is a sufficiently small particle size of the hazardous substance in an appropriate concentration. The two endpoints of this concentration range are the lower and upper explosion limits, within which the substance is explosive. Due to the existence of these five conditions at the same time and place, an explosion can occur. The common name of the system of conditions is the "explosion pentagon" (Figure 2).

The condition of "adequate concentration" already refers to the differences in the physicochemical properties of given dust depending on whether it is present in a deposited layer or forms a homogeneous dust cloud. The deposited dust layer can glow or decompose on a surface with a suitable temperature, while a dust cloud can be ignited by contact with the glowing layer or a hot surface. The primary risk of explosion arises in the case of suspended dust in the air, because in this case, a mixture of combustible material and air may be present. Sedimented dust poses a huge secondary hazard in addition: cleaning, sudden air movement, or an explosion can also stir up it.

In most cases, the dust cloud is present in tanks, silos, or other operating equipment in which a sudden increase in pressure occurs during a possible explosion. If these dust treatment facilities are not adequately protected, an explosion could cause serious damage to them.

The danger posed by the explosion itself depends on many factors:

- Physical and chemical properties of combustible material.

- The size of the enclosed explosive atmosphere.

- Physical properties and the current state of the environment.

- Strength of structures, supports, and equipment.

Clothing and protective equipment is worn by vulnerable persons.

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