Explosibility/dust explosion class
Definition
Dust explosibility exists if a flame spreads after ignition in a dust/air mixture, causing a pressure increase in a closed vessel.
Categories, into which dusts
are classified on the basis of their KSt values:
| Dust explosion class | KSt value
in bar · m · s-1 |
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Determination
Explosibility is preferably determined in closed apparatuses according to the described methods. While the same ignition source is used in the 1 m3 vessel as in the test to determine the characteristics pmax and KSt (EZ = 10 kJ), the ignition energy to determine dust explosibility in the 20 l sphere is only 1 kJ to 2 kJ.
If the described method produces no pressure build-up (Dp <0.5 bar above the initial process-related pressure), the dust in the tested form (composition, particle size, moisture content) is regarded as non-dust explosible. If a pressure value exceeding 0.5 bar above the initial process-related pressure occurs, the dust is regarded as in mixture with air explosible (dust explosible).
The modified Hartmann tube can also indicate to a certain extent explosibility and dust explosion class. The modified Hartmann tube consists of a vertically arranged glass tube with a volume of about 1.2 l, which is closed at the top by a hinged lid. The dust to be tested is placed on the bottom of the apparatus and distributed by a defined air blast guided via a mushroom-shaped atomiser from a preceding compressed-air reservoir (V = 50 ml, p = 8 bar). The dust quantity introduced is varied over a wide range in each of three test series. A continuous spark gap serves as ignition source. In the event of non-ignition, the test is repeated with a stronger ignition source in the form of a glowing coil.
Depending on the reaction of the dust/air mixture, the lid is opened to a varying extent and the opening angle displayed digitally in three stages via inductive transmitters. The maximum reactions are evaluated according to the following method:
Indication "0" (without visually perceptible independent flame propagation) merely means that no ignition occurs under these test conditions. However classification of the dust particles as non-dust explosible is not justified. More extensive tests in closed vessels are necessary.
"1" indicates that the dust in the mixture with air is explosible and should be classified in the dust explosion class St 1. Visually perceptible independent flame propagation, even without deflection of the hinged lid, is equal to "1".
"2" also indicates that dust explosibility is present, but does not permit classification in a dust explosion class. More extensive tests in closed equipment are required.
Remarks
The information "yes" to dust explosibility refers to tests which could not be carried out in the whole scale of determining the characteristics. But they showed clearly that the tested material is dust explosible.
The information "no" (non-dust explosible) is only given if the results of tests with fine dusts in a closed apparatus produced "no ignition" and the result of the combustion test is "BZ 1". Data on the dust explosion classes is usually derived from the respective KSt value. Insofar as only results of explosion tests in the modified Hartmann tube are available, classification only in the dust explosion class St 1 is regarded as certain and is shown without reservation. Other results from the modified Hartmann tube, on the other hand, appear in brackets, e.g (St 2), and, apart from indicating the presence of a dust-explosible substance, only act as a rough guideline.
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