Effect of humidity on a simplified method for assessing the risk of spray electrification

Abstract

Spray electrification, the electrostatic charging of liquids during ejection from nozzles, is known to depend strongly on the liquid type and nozzle material. The author previously proposed a simplified method to evaluate the hazard of spray electrification for various liquid-material combinations. This method has proven to be useful for reproducing spray electrification and assessing associated hazards. However, the experimental conditions used in those studies were provisional and not necessarily optimal. In particular, ambient humidity during measurement may significantly affect the amount of charge generated and could potentially lead to misjudgment of electrostatic hazards. This study examines the effect of relative humidity on charge generation for eight liquids (kerosene, mineral spirits, butyl acetate, ethyl acetate, methyl acetate, acetone, ethanol, and tap water) and four materials (SUS304, PTFE, MC nylon, alumina). In most cases, higher humidity significantly reduced the charge amount. However, some combinations showed increased charging at higher humidity. These results indicate that humidity has a substantial impact on spray electrification and must be carefully considered in hazard assessments using the proposed method.