This accumulated electrostatic charge will seek to balance with items at a different potential. The quantity of the electrostatic charge depends on the characteristics of the product, filling or emptying rate. In Europe/ROW, guidance for safe use of FIBCs in regards to hazardous zones/locations is defined in IEC TS 61340-4-4 and NFPA® 77 in USA and Canada.
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/cuboid 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 electrostatic 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. Electrostatic discharges exhibit effective energies of as much as several joules.
Where electrostatic charge accumulates on FIBCs or associated process equipment, the following four types of discharges can occur:
- Spark Discharge
- Brush Discharge
- Propagating Brush Discharge
- Bulking Brush Discharge
The risk of electrostatic discharges in FIBCs in potentially flammable atmospheres is well documented in best practice standards / recommended codes of practice such as IEC TS 61340-4-4, ‘Electrostatics – Part 4-4: Standard test methods for specific applications – Electrostatic classification of Flexible Intermediate Bulk Containers (FIBC)’ and NFPA 77 ‘Recommended Practice on Static Electricity’.
These standards/guidance documents specify that FIBC Type C bags must be grounded to the following maximum resistance levels: –
1 x 108 ohms (100 Meg-ohms) for IEC 61340-4-4 and NFPA 77
This provides Ex/HAZLOC engineers clear targets to aim for when protecting FIBC Type C bags against the accumulation of static electricity and is a critical fire and explosion prevention measure.
Types of FIBCs
IEC TS 61340-4-4, ‘Electrostatics – Part 4-4: Standard test methods for specific applications – Electrostatic classification of Flexible Intermediate Bulk Containers (FIBC)’, describes four (04) types of FIBCs, defined by the construction of the FIBCs, the nature of their intended operation and associated performance requirements: Type A, Type B, Type C and Type D.
Ex compliant bags carry identification tags which specify bag type along with any other relevant information specific to that bag. Only FIBC bags that have been qualified exactly in accordance with the procedures specified in IEC TS 61340-4-4 can legitimately be labelled as such.
Read the full article to learn about the different types of FIBC bags and why Type C FIBC should be specified for hazardous atmospheres.
