ATEX: Understanding the Kst

We have already dealt with the MIE. Beside MIE, a further important parameter to determine the dust explosion hazard is the Kst.

MIE describes the likelihood that a mixture of air and dust ignites, so it explains how likely it is for a given kind of dust to explode. However there are other parameters assessing the attitude for the dust to ignite such as: LEL (Lower Explosion Limit), LOC (Limit Oxigen Concentration) and MIT (Minimum Ignition Temperature).

The minimum limits set by the above mentioned parameters mean that underneath certain conditions a potentially explosive ignition may not occur.

On the other hand Kst, along with P_max (Maximum explosion overPressure) assesses the destructive potential of the dust cloud in case the ignition starts.

It is the equivalent of the explosion class given to the highest value describing the pressure rise in a closed vessel which is determined under specific test conditions, and expressed in bar. m/sec. The class value is usually provided in deflagration tests in spherical closed vessels of 20 L or greater volume.

If on the one hand, in parameters like the MIE, the most unfavourable test conditions are due to the potentially explosive atmosphere of a given dust (unbalanced combustible/comburent ratio, use of a dry product, specifically of the dust fraction), for the Kst class the used vessel dimension is not relevant, as it is always based on the standard vessel of 1 m3. In few words, Kst assesses the speed of pressure rise in case of explosion of a given dust cloud. In addition, Kst assesses the speed of the expansion of the fire caused by the explosion.

According to Kst there are three dust explosion classes ranging from 1 to 3 describing the hazard level.

Kst ist the main parameter to apply to the measurement of the explosion vents or for the explosion suppression. Actually, a rapid rise of pressure makes it compulsory to adopt larger vents or to manufacture a vessel with a higher resistance to the rise of pressure.

The fire generated by a product bearing a higher Kst will be longer and faster. For this reason the fire will be more difficult to suppress and contain and for the flames to vent externally much grater safety distances will have to be considered.

In case of pipe explosion, a higher Kst will induce a faster pressure rise on the inside so the subsequent stage of detonation will be reached very soon. This explains the achievement of the supersonic combustion whose dangerousness is much superior.