Propagating Brush Discharges are the most energetic electrostatic discharges encountered in industrial situations. They arise when a thin insulator (<10 mm thick) acquires exceptionally high surface charge. They are capable of igniting a wide range of flammable materials. For this reason, in a section on silos and containers for flammable powders CLC/TR 60079-32-1 states: "Insulating containers should in general be avoided due to the risk of propagating brush discharges".
In the work reported here plastic silos were being used to store wheat flour. The powder was pneumatically conveyed into the silos from tankers, thereby providing likely high levels of charge. Even though the silos had been used in this service for some time without incident, the published guidance meant the margin of safety had been questioned.
The only way of assessing the margin of safety would be to determine the electric fields inside the silo during filling. Unfortunately, direct measurements in this location are virtually impossible.
The authors have been developing the use of field modelling software in combination with experimental measurements to answer similar questions where charge generation is in liquids. Here, though, are the first results of combining measurements of electric fields outside silos with computer modelling to determine the electric field in critical locations inside.
This paper presents the principles of the approach, the experimental detail, how the field modelling software was used, and the results. The conclusion that, although these silos would not be expected to produce propagating brush discharges, the margin of safety was not large enough to warrant dismissing their occurrence altogether, is explained and justified.
Further work is proposed to improve confidence in the results even where modest or low margins of safety are indicated by this method.