Charge relaxation in powder beds

Many plastic powders become highly charged during mixing or transport operations. When such powders are stored in hoppers or silos, strong electric fields are established which can lead to hazardous dust-igniting electrostatic discharges. The most serious hazard is apt to occur during or shortly after filling operations. Thus, realistic risk assessment of ignnition probabilities requires solution of a transient electroquasistatic field problem, where both filling of the vessel with charged powder and simultaneous charge relaxation and/or brush discharge to the vessel walls are taken into account. A one-dimensional model of a filling silo with continuous charge relaxation is proposed. The problem is solved numerically using finite difference equations in time and space. The temporal solution is divided into the filling period and the relaxation period (after filling stops). The solutions for electrostatic field and surface charge demonstrate how various parameters influence the electrostatic discharge hazard. The ratio of filling time to charge relaxation time T is the most important of these paramweters. For T≫1.0, the hazard is decreased because the charge relaxes to the conducting walls as soon as the powder is deposited in the vessel.