Electrostatic Ignition Hazards with Flammable Liquids

The hazard of spark ignition of flammable vapor/air mixtures is recognized and receives much attention in the petroleum and chemical industries. Bonding and grounding procedures, described in various consensus standards, prevent hazardous differences of potential between conductive objects. These techniques are not adequate, however, to prevent hazardous accumulations of charges in poorly conductive liquids, for which additional precautions are necessary. A lack of consensus concerning the definition of ``static-prone'' liquids has hampered progress in this area, and has frequently led to the development of costly protective measures to meet arbitrary requirements. The results of experiments that measure the charge accumulation in liquids during agitation as a function of liquid conductivity are presented. Data are included for the particularly severe static charging that accompanies the mixing of powders in poorly conductive solvents. The degree of static hazard is determined by comparing the measured value of charge accumulation to a calculated value that is sufficient to cause sparking. Techniques for keeping the charge accumulation to safe values are described. Special static hazards of nonconductive containers are discussed.