Experimental study on electrostatic discharges from a metal protrusion inside a silo during continuous loading of polypropylene powder

Abstract In this study, the electrostatic discharges from a metal protrusion existing inside a conical-cylindrical silo during continuous loading of polypropylene powders (PP, 3 to 4 mm) were experimentally investigated. A pilot-scale silo connected to a pneumatic powder transport facility was used while all parts were electrically grounded. The metal protrusion used in this study had a diameter of 4 cm. The silo was continuously loaded with PP powders at a mass flow rate of 0.68 kg/s to a total mass of approximately 800 kg. An image intensifier unit was used to observe the electrostatic discharges generated from the metal protrusion, and a current probe attached to an oscilloscope was used to measure the discharge amount (Q [nC]). As for the results, a multiplicity of small electrostatic discharges (brush discharges) from the protrusion was observed. The electrostatic discharge from the protrusion during PP powder loading clearly developed at the 391 s mark, when approximately 270 kg of the powder had been loaded. In addition, a difference was observed in the electrostatic discharges before and after the protrusion was embedded in the accumulated powder. That is to say, after the metal protrusion was embedded in the accumulated powder, the electrostatic discharges were stronger than when the protrusion was not embedded. As well, the Q after the immersion of the metal protrusion in the accumulated powder was several times larger than that when the protrusion was out. Strong broad bulk surface discharges clearly appeared after the protrusion was fully immersed in the powder.