Static Electricity Hazard Assessment and Control

Static ignition is a recurring cause of fire and explosion in liquid and powder handling, and it is a hazard that ordinary earthing practice does not address. Charge is generated by flow, by splash filling, by pneumatic conveying and by the movement of people and plant, and whether it becomes an ignition source depends on the charge relaxation of the material, the geometry of the accumulation and the minimum ignition energy of the atmosphere it discharges into. Our assessment works through the operations where charge is generated, establishes the conductivity and relaxation behaviour of the materials handled, identifies the discharge types that are credible in each geometry, from spark through brush and propagating brush to the cone discharges that occur in silos, and compares the energy available against the minimum ignition energy of the vapour or dust present. The controls that follow are mostly not equipment. They are bonding and earthing arrangements, fill rate limits, dip pipe and splash filling changes, material and liner selection, and operating practice on drum filling, sampling and manual charging, which is where the human element makes the ignition path.