Assessment of electrostatic ignition hazards during water spray cleaning processes above 50 MPa

Abstract

Pump pressures of a few tens to several hundred MPa are used for vessel cleaning processes with water. Where an explosive atmosphere is present, the pump pressure used must be limited to 50 MPa in order to avoid electrostatic ignition hazards due to brush discharges from charged droplet clouds to earthed installed parts. Pressures above 50 MPa are not considered in electrostatic regulations such as technical specification IEC TS 60079-32-1. This pressure limit originates from the performance limits of the cleaning technologies investigated in the past. The pump pressure above which an electrostatic ignition hazard exists is, however, unknown. This limitation works to impair the effectiveness of the cleaning processes. To satisfy the technical state of the art, tests were conducted in vessels of up to 44 m³ and pump pressures of up to 250 MPa. Based on the outcome of these extensive tests and on relevant references, an assessment method is described that takes account of the space charge densities present in the vessel when water is sprayed into it. The derived flow chart enables manufacturers and operators to safely carry out assessments of cleaning processes with water up to 250 MPa in explosive atmospheres, factoring in the vessel dimensions, cleaning technology parameters, and water type. This opens new possibilities for more economical cleaning processes and better-quality cleaning results while maintaining the same level of safety.