Experiments concerning the dust explosion risk for mixers

Status:

completed 10/2006

Aims:

The project's aim is to determine whether ploughshare mixers filled with potentially explosive dusts can be operated explosion-proof at filling degrees of 70 vol.-% and 50 vol.-%. For this, a high ignition initial of 10 kJ of chemical igniters is to be provided. If the dust/air mixture does not ignite in the running mixer, it can be presumed that, on the present conditions (high concentration and turbulence), there is no potentially explosive atmosphere, so that the example "mixer", as in no. 3.3.5, column 3 d in the explosion directive's collection of examples remains valid.

Activities/Methods:

Experimental set-up: Within the scope of the tests, a 300 l ploughshare mixer was equipped with the following metrology:

- Two pressure transducers
- Two temperature transducers

These served for the continuous recording of both pressure and temperature during the test. The mixer's filling port was sealed with a PE-foil as bursting foil. The tests were conducted at six predetermined ignition places. Four ignition spaces were above, two were below the shaft. After the ignition of the dust/air mixture, the mixer was equipped with a new bursting foil, was, if necessary, refilled, and equipped with new igniters. During the emptying process it was visually tested that the product had not altered due to the ignitions. In all tests, the filling degree was 70%. The procedure was video captured. Dusts used: To cover a spectrum of different dusts as wide as possible for the various relevant industries, the tests were conducted using the following dusts:

- Antioxidant
- Corn starch
- Brown coal

Results:

The tests have shown that, on the predefined testing conditions, all three dusts caused the dust/air mixture to ignite. The dust/air mixtures ignited exclusively above the shaft. In the two ignition spaces below the shaft, five ignition attempts with each of all the three dusts did not bring about an ignition. It can be assumed that the concentration's dispersal in the mixer is not homogeneous. Above the shaft, the concentration of the dust/air mixtures is within the explosion limits and can be ignited. Below the shaft, the dust concentration is likely to be so high that the dust/air mixture is not potentially explosive. With view to the tested mixer it has become clear that, above the shaft, there are potentially explosive mixtures, so that, at least on the given conditions, the concentration in the mixer with a filling degree of 70% cannot be expected everywhere to be above the upper explosion limit during operation. The test results do not say whether these ignitions do also occur as a result of lower ignition energies presented, for example, by hot surfaces. Further tests could make possible an assessment on the extent to which existing installations with such mixers effectively are at risk. This means that only after further tests it can be assessed whether and in how far the explosion directive's collection of examples no. 3.3.5, column 3 d needs to be questioned and changed.
Since the first tests with a filling degree of 70 vol.-% resulted in the ignition of the dust/air mixture, it can be presumed that a filling degree of 50% with the remaining operational parameters staying the same would only increase the probability of an ignition in the predetermined ignition spaces. For this reason, no further tests were conducted with this filling degree.

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