Dust explosion risks — Characterization

Dust explosions result from the ignition of organic or mineral flammable materials in a divided state, suspended in the air.

When dust ignites in a confined space, such as an enclosure, it is accompanied by mechanical effects that depend on the overpressure induced by the explosion and the rate of pressure rise.

While the risk of gas explosions and condensation phase explosions is well understood and accepted, the risk of dust explosions is more difficult to grasp and understand. It is less frequently encountered in everyday life. It is often necessary to insist and use persuasion to make people accept the idea that dust handled in a plant presents, due to its technical characteristics, a high risk of dust explosion.

That's why we've listed the industries and activities exposed to this risk, and quoted some accident statistics.

When flammable dust is present in sufficient concentration in a plant, the likelihood of a dust explosion depends on the sensitivity of the dust to different sources of ignition, and whether an ignition source is present permanently, frequently or occasionally.

The severity of a dust explosion depends on its P _max and K _St characteristics.

The wide range of tests available to characterize the sensitivity of dusts to different types of ignition source is described. A distinction is made between tests characterizing dust deposited in layers and tests characterizing airborne dust.

With regard to the latter, determining the minimum ignition energy (MIE) and minimum ignition temperature (MIT) of suspended dusts is important for characterizing dust sensitivity.

Explosion characteristics are measured in spherical enclosures. Thus, the classification of dusts according to the violence of the explosion is based on the K _St measured in a spherical enclosure. For practical reasons, we use the 20-liter Kühner sphere.

All these determinations enable us to draw up a summary of the risk of dust explosion, which serves as a basis for the choice of prevention and protection measures for industrial plants.

There are many possible means of prevention and protection:

• the classification of zones where a flammable atmosphere is present and the choice of equipment adapted to these zones, in accordance with ATEX regulations;

• prevention of ignition sources ;

• inerting of equipment ;

• protection...