The Flexible Intermediate Bulk Container (FIBC) causes an electrostatic charge of equal intensity, on the inner surface, but with an opposite polarity on the outer surface. This accumulated electrostatic charge always seeks a conductive path to ground. The quantity of the electrostatic charge depends on the characteristics of the product, filling or emptying rate.
The risk of electrostatic discharges in FIBCs in potentially flammable atmospheres is well documented in International Standards and Recommended Practises such as IEC 61340-4-4:2018, ‘Electrostatics – Part 4-4: Standard test methods for specific applications – Electrostatic classification of Flexible Intermediate Bulk
Containers (FIBC)’ and NFPA 77® 2019 ‘Recommended Practice on Static Electricity’.
FIBCs are very large fabric bags supported in a frame. They are more convenient than rigid IBCs (Intermediate Bulk Containers) for powder transfers because they can be fully collapsed after use, taking up little storage space. The fabric used is usually polypropylene and is sewn to form a three-dimensional cube or rectangle with lifting straps. An FIBC can be filled with a powder or granular material and moved about with conventional handling equipment (forklifts, etc.).
An advantage of FIBCs is that they can be unloaded quickly, typically 300kg to 500kg in 30 seconds or less. It is common under these circumstances for the rates at which static electric charges are generated to exceed the rates at which the charges can relax, accumulation of a static electric charge is developed during the process. If the accumulated charge is strong enough and is released in the presence of a combustible atmosphere, ignition can occur. Static charges can be generated both during the filling and emptying of FIBCs. These discharges exhibit effective energies of as much as several Joules.
Where a static electric charge accumulates on FIBCs or associated process equipment, the following four types of discharges can occur:
1) Spark Discharge
2) Brush Discharge
3) Propagating Brush Discharge
4) Bulking Brush Discharge
Type C FIBC bags are designed to dissipate static electricity through static dissipative or conductive threads that are interwoven through the bag’s material. The Type C FIBC bags usually have conductive grounding tags located on both the top section and bottom section of the bag.