This paper considers the risk of ignition of a flammable atmosphere within a chemical cargo tank by an electrical discharge from a water slug originating from a washing machine. Simple safety criteria are identified on the basis of new and existing measurements of space potential, assessed via an existing computational model which has been revised to account for other generally present charged objects. These criteria specify limitations for tank size, nozzle throughput and nozzle number for application regardless of residue type. The limitations roughly coincide with the largest tank sizes and water throughputs characteristic of chemical tankers and their washing operations. They are consistent with the good safety record of such operations as currently performed. However, the findings indicate that the situation has been reached where the proponents of changes in current practice need to demonstrate clearly the inherent safety of such changes.