Assessment of explosion risks caused by cone discharges when filling silos and containers with pellets

Abstract

It is well known that during the filling of silos and containers with bulk material, so-called cone discharges can occur because of electrostatic charges. Whether or not cone discharges occur at all depends on whether the breakdown field strength of air under atmospheric conditions of 3 MV/m is reached at the silo and container inner wall. This in turn depends on the charge to mass ratio of the bulk, the bulk resistivity, the bulk density, the relative permittivity of the bulk material, the silo or container diameter and the filling rate. If cone discharges can't be avoided, the energy of cone discharges can be estimated according to the equation given in the relevant guidelines TRGS 727 (2016) and IEC/TS 60079-32-1 (2013). Therefore, the coarse fraction must be considered. As soon as the energy of the cone discharge is greater than or equal to the minimum ignition energy of the bulk material introduced, there is a risk of dust explosion. Here the fine fraction of the bulk material is relevant.The investigations described are a practical example how computer models can be used to assess the occurrence of cone discharges. It is calculated for which silo and container diameters and filling rates the critical field strength of 3 MV/m is reached. In these calculations the charge relaxation during pneumatic filling with bulk material is taken into account. The results of the computational modelling together with operational boundary conditions serve as a decision basis whether exclusion of incendive ignition sources is an adequate safety measure or whether further explosion protection measures must be considered. Finally, a brief overview of other possible explosion protection measures is given.