Explosion Severity Test and St Class : Modified Hartmann vs 20 Litre (St 0H/1H/2H vs St 1/2/3)

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It has recently been told to me by one of the clients’ that their explosion severity test states the results as St 1H in place of St 1/2/3. His question was simply that should I go ahead and provide an explosion vent corresponding to St 1? Now, this is a tricky one.

St 0H/1H/2H are the results of an explosion screening analysis which is carried out using Modified Hartmann Tube Apparatus. The Modified Hartmann Tube apparatus consists of a 1.2-Liter vertical tube mounted onto a dust dispersion system. Dust samples of various quantities are dispersed in the tube and attempts are made to ignite the resulting dust cloud by an approximately 10 Joule constant arc ignition source. On top of the glass tube, a hinged cover allows for pressure relief in the case of a dust fire or explosion in the tube. If a flame propagates with or without lifting, then a material is classified as explosible, otherwise marked as non-ignitable.


The opening of the lid is monitored and gives an indication of “0” = no opening or lift of the cover, “1” = small lift or opening of the cover or “2” = complete opening of the cover. This is what is defined as the result of explosion screening. Powders that are explosible and for which a maximum indication of “0” or “1” is observed over a wide range of concentrations belong to usually belong to dust explosion class “St 1” (Kst < 200 bar.m/s). However, nothing can be said for indication of 2 with certainty.

If the material fails to ignite in the Modified Hartmann Tube apparatus, the testing is continued in a 20-Litre Sphere apparatus (or a 1 cubic meter sphere) where the dust cloud is exposed to a 10,000 Joule (or 10kJ) ignition source. 20 Litre sphere is primarily used for measuring explosibility characteristics such as Explosion Severity, LOC and MEC. 

Under explosion severity, it gives Pmax (Maximum overpressure) and (dP/dt)max (maximum rate of pressure rise) which when used in cubic law gives Kst. Powders can be divided into following categories:

1. St 1 - Kst value of 0-200 bar.m/s
2. St 2 - Kst value of 200-300 bar.m/s
3. St 3 - Kst value of > 300 bar.m/s

The level of explosion violence increases with the class number. Even a St-1 dust generates sufficient power to cause a flash fire, compromise containment on a piece of equipment, or blow out the walls of a building.

So, it is imperative to know the right test parameter before we move on to designing of explosion protection. Modified Hartmann can always be used for explosion screening. But one should not use the data from Modified Hartmann for designing of explosion protection measures. Explosion severity data should come out of 20 Litre or 1 cubic meter sphere.

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