Physical-chemical properties of the combustible dust and explosiveness parameters
These are the main parameters that define the dust level of danger: Kst, Pmax, MIE, MIT, LEL and UEL. Let’s analyse them:
Combustible dust Kst values and Pmax
The Kst index, together with the Pmax index (maximum explosion pressure of a given powder – its unit of measurement is bar), is the date that allows the risk of explosion connected to a plant or a process to be evaluated and allows us to understand how destructive a given powder is in case of an explosion. Different classification systems have been adopted, in order to compare the explosive violence of dust. One of the most famous systems refers to the Bartknecht classification, which divides the combustible dusts in four danger classes, according to the Kst index:
|Explosiveness class||Kst [bar⋅m/s] with primer from 10 kJ||Kst [bar⋅m/s] with primer from 10 J||Type of explosion|
|St 3||>300||>200||Very strong|
Kst and Pmax indexes are the main parameters to be used in the safety panels‘ assessment or in their elimination systems. As a matter of fact, as soon as the pressure suddenly increases, you will have to use wider panels or to create a more resistant pressure filter.
Limits of explosion (LEL – UEL)
The LEL (Lower Explosion Limit) and the UEL (Upper Explosion Limit) represent the explosiveness limits and they are respectively the concentration of flammable material in the air under and above the explosion range. These limits are measured mixed with air.
Minimum Ignition Energy (MIE)
The MIE (Minimum Ignition Energy) is the energy that is able to ignite the tested dust, under specific test conditions. When we speak about the explosive dust we must consider that the MIE is connected to the particle size: the fine particles usually have lower ignition energies than the rough ones.
Minimum ignition temperature (MIT)
The MIT (Minimum Ignition Temperature) represents the minimum temperature in which the dust can ignite. In this case, we must specify that the dust, unlike gas and mists, have two ignition temperatures: one is related to the dust cloud and the other refers to the dust level ignition (LIT)