Unwrapping the explosion pentagon to reduce risk
This article sets out an approach to reducing the risk of dust explosions, using a rigorous assessment procedure. This procedure gains its rigour from the study of the unwrapped explosion pentagon that leads to a step-by-step route map that combines the replace, reduce and refine approach to the reduction of the inventory of dangerous substances with hazardous area classification, a review of sources of ignition, and consideration of the severity and consequences of an explosion.
An explosive dust atmosphere is defined in the following way by EN 60079-10-2 :
“a mixture with air, under atmospheric conditions of flammable substances in the form of dust, fibres or flyings which, after ignition, permits self-sustaining flame propagation.”
It is also important to distinguish between an explosion and a flash fire as discussed below. Both of these are covered by the definition above.
The five fundamental requirements for a dust explosion are shown on the pentagon in Figure 1 and discussed in detail below. Furthermore, it is important to note that this risk assessment only applies to deflagrations (where the flame propagation velocity is sub-sonic), and not detonations.
For a dust explosion, the fuel is any combustible dust or flying, where a combustible dust is defined by EN 60079-10-2 as:
“finely divided solid particles, 500 µm or less in nominal size, which may form an explosive mixture with air at atmospheric pressure and normal temperatures.”
and combustible flyings are defined as:
“solid particles including fibres, greater than 500 µm in nominal size, which may form an explosive mixture with air at atmospheric pressure and normal temperatures.”
The oxidant is normally atmospheric air at close to atmospheric pressure. However, air with a reduced oxygen content can still allow an explosion to take place. The limiting oxygen concentration for a dust explosion is typically in the range 5–15% v/v.
This risk assessment recognises that dust explosions can occur in air at pressures considerably greater than atmospheric pressure, as well as below atmospheric, if there is still sufficient oxygen. Also that for some applications the oxidant can be a chemical other than oxygen, eg chlorine.
A combustible dust will not explode unless it is dispersed into a cloud. However, layers of dust can catch on fire acting as a source of ignition and/or forming an inventory of dust that has the potential to be dispersed.
An ignition source is a spark or hot spot that is powerful enough to be incendive. Electrical and non-electrical (mechanical equipment) that is certified under the ATEX 100a Directive 2014/34/EU address some of the ignition sources but not those created by the process, cleaning or maintenance. This article covers all sources of ignition.
Confinement could be a totally enclosed container, a vented container or a constricted area. Confinement leads to an accelerating flame which in turn creates more turbulence that leads to a rise in pressure. In the case of a pressure-tight, unvented vessel the pressure can rise to Pmax (see step 6, later).
With no confinement, a flash fire occurs which is still dangerous but the pressure does not rise above atmospheric pressure.