Experimental study on the effect of metal protrusions inside silos on electrostatic discharges

Abstract

This paper experimentally introduces the electrostatic discharges generated from metal protrusions inside a silo during the loading of powders. An industrial-scale pneumatic powder transport facility including a silo and approximately 400 kg of polypropylene powder (PP, 2–3 mm in size) were used for this test. The PP powder loading speed was approximately 0.38 kg/s. Six different diameters (D: 1 cm to 6.3 cm) of metal protrusions were used. The metal protrusions were attached to the silo wall using a 30 cm support rod that was electrically grounded. The height from the accumulated PP powder to the protrusion in the silo varied from 10 cm to 50 cm. An image-intensifier unit was used in order to observe the electrostatic discharges generated from the metal protrusions and a current probe attached to an oscilloscope was used in order to measure the current of the discharges. As for the results, electrostatic discharges (such as brush discharges) from protrusions during the loading of PP powders were clearly observed. Specifically, the electrostatic discharge was the strongest in this paper when the diameter of the protrusion was 4 cm and the height from the accumulated powder was 30 cm. As an important finding, the value of the discharge amount was affected to a greater extent by the maximum discharge current. The maximum charge amount of electrostatic discharges generating from the metal protrusions in all of the test conditions was −122 nC, which is twice as high as the threshold for electrostatic risk assessment in the liquid painting process. These results suggest that, in the chemical process, sensitive powders with MIEs of less than 4 mJ, powders wetted with solvents, and the solvents themselves must be carefully handled, as these kinds of powders may be ignited by brush discharges. In order to prevent and reduce electrostatic accidents, it is important to make sure that no external material enters the silo.