Electrostatic ignition hazards from flexible intermediate bulk containers (FIBCs) with materials of minimum ignition energies down to 0.12 milliJoules

The use of flexible intermediate bulk containers (FIBCs) or ´big bags´ as they are commonly known is becoming widespread in a variety of industries including chemical, pharmaceutical and foodstuffs. Typically, FIBCs, roughly cubic in shape and constructed from woven polypropylene, are used to store and transport powdered or granular material in loads of between 300 and 1000 kg. In many cases, FIBCs are used in the vicinity of sensitive flammable atmospheres which may arise from the presence of dispersed combustible dusts or solvent vapours. Over the years, there have been a number of serious fires and explosions (particularly during the discharging of the bag contents) which have been attributed to static electricity. As a result various ´antistatic´ FIBC designs have been produced containing intrinsic features aimed at minimising the risk of electrostatic ignition. This paper presents the results of a rigorous assessment of a variety of FIBC designs with regard to electrostatic ignition hazard in flammable atmospheres of minimum ignition energy 0.12, 0.25 and 1.5 mJ (the minimum ignition energy of methanol, a common industrial solvent, is 0.14 mJ). The study involved setting up a test rig to enable the FIBCs to be filled and emptied under controlled conditions of relative humidity. During these operations the electrostatic activity in terms of surface potentials, electric field and electrostatic discharge energy was carefully monitored. Incendiary discharges were quantified by using a calibrated propane/oxygen/nitrogen gas probe. The findings of this study indicate which of the designs can be safely used in various industrial processes without risk of ignition.