DUST EXPLOSIBILITY TEST (HARTMANN)
[VDI-Guideline-2263, Part 1]
A dust explosion occurs if in a mixture of raised dust and air flame propagation occurs after local ignition. In a closed container this can lead to an increase in pressure.
The test substance is dried and ground or sieved such that a fine fraction fulfilling the standards can be used for the experiment. (Sieving is allowed only for homogeneous substances).
The powder is filled into the Hartmann tube (a vertical glass tube with a volume of 1,2-ℓ).
A raised dust cloud is produced by pushing air through a nozzle in the bottom of the tube. At the same time, a permanent electric spark (10 kV, 4 mm spark gap, energy approx. 10 J) is fired in the center of the tube.
A dust explosion in the tube is detected by an electronic sensor coupled to the movable lid on top of the tube. In addition it is visually observed, if a dust fire (i.e. combustion without pressure build-up) occurs.
The test is carried out with samples of 120, 240 and 600 mg (100, 200 and 500 g/m3).
Three trials are made with each concentration. If these nine trials show no explosion a further series is made with 1200 mg (1000 g/m3).
The overall rating of the test is POSITIVE, if in at least one trial either an explosion was detected electronically or a dust fire was observed. In such a case the dust is considered as explosive (on the screening level).
A final assessment of the explosivity according to international standards require test in the 20-ℓ-Apparatus.
If the rating in the Hartmann tube is NEGATIVE, the minimum ignition energy of the powder is > 1 J.