ATEX Dust hazard assessment

Which are the most important characteristics to consider in dusts evaluation?

An ATEX zone is a space of a machine/plant or working area where there is a potential explosive atmosphere. The ATEX directive 2014/34/EU evaluates these zones as follows:

ATEX Zone 20: high probability of explosive atmosphere
ATEX Zone 21: average possibility of explosive atmosphere
ATEX Zone 22: low possibility of explosive atmosphere

In particular, TAMA AERNOVA is capable to manufacture products suitable for installation in ATEX zones 22 and products that process directly dusts which can generate a potential explosive atmosphere.

Each evaluation of the dust explosive risk has to base on the nature of the processed dust. This is because every dust has different innate characteristics that give them the explosiveness, regardless how they are processed. Among these characteristics, particles dimension is the one that affect the most this phenomenon and the reactivity of the sample. It is impossible to distinguish definitely explosive dusts and not explosive dusts according to them particles size. However, it is believed that particles with diameter greater than 500 μm have a low explosion tendency: this value is often assumed as limit beyond which a dust can’t explode. A decrease of the particles diameter entails an increase of the reactivity, a growth of the maximum pressure Pmax (Bar) and the growth of the pressure maximum speed Kst (Bar* m/s).

In particular, the Kst parameter is very important because it represents a constant that allows to categorise the explosion force in four risk classes:

Kst = 0 St.0 Weak explosion
0 < Kst < 200 St1 Moderate explosion
200 < Kst < 300 St2 Impetuous explosion
Kst > 300 St3 Dangerous explosion

It is good to remember that the Kst constant value depend on the following factors:
- Dust type;
- Source type;
- Dust particles size;
- Dust concentration/amount;
- Moisture.

For the calculation of the risk it is fundamental to also consider the minimum ignition energy (MIE) of the dust cloud; in particular:

MIE < 100 mJ indicates an ignition potential from an operator's electrostatic discharge;
MIE < 25 mJ indicates a potential trigger from an electrostatic discharge caused by dust bulking.