This electrostatic charge can accumulate on both the contents (product) and the fabric of the material itself. If this occurs there is a danger to plant personnel and the surrounding environment because release of the charge in the presence of a flammable atmosphere causes an ignition and potentially and explosion.
“A recent incident illustrates the dangers associated with FIBCs,” explains James Grimshaw, Marketing Manager at Newson Gale. “The operator was transferring resin to a 6,000 gallon mixing tank to make lacquer for can coatings. The mixing tank was equipped with thin conductive wires running lengthwise through the spout and connected to a bare stranded aluminium wire and alligator clip. The FIBC was hoisted above the tank using a fork lift and the resin was dumped through a circular port on a hinged tank cover. There was no independent venting of displaced vapour and the tank lid was not gas tight. Despite the operator reporting that the ground wire was missing from the FIBC, it did not stop him proceeding to unload the container regardless.”
Although the operator himself was not grounded, the nature of the operation involved making a lacquer, meaning that static dissipative footwear would probably have been ineffective as there was possibility of a film of lacquer on the floor around the tank.
The investigation into the incident made the assessment that a spark discharge had occurred from the ungrounded FIBC during emptying. The lack of continuity to ground meant that charge could not be dissipated. Charge on an insulated object is retained because of the resistance of the material itself. For a conductor, such as the FIBC to remain charged, it has to be isolated from earth. As it was known that the resin had low minimum ignition energy (MIE), it was assumed that flammable vapour was a significant factor in the ignition process reaching well in excess of an acceptable level.
Helpful Tips for correctly grounding Type C FIBC bags:
- Ensure that the grounding system selected can check and continuously monitor the full range of resistance through the bag.
- Ensure the grounding system not only check the condition of the bag’s static dissipative threads, but also ensure that the ground circuit includes a direct monitored connection to a verified True Earth grounding post.
- Ensure the grounding system does not monitor a limited percentage of the permitted range of resistance as they may pass faulty bags and reject acceptable bags.
- Ensure Type C bags are manufactured in accordance with the electrostatic recommendations of IEC 61340-4-4 / NFPA 77 or CLC/TR: 50404
James adds: “In this incident electrostatic charge had been allowed to accumulate because the FIBC was isolated from ground, whether this was through the negligent actions of the plant operator or inconclusive grounding methods.”Had grounding been accomplished via a Type C bag with either passive (single pole clamp and cable) or through active means (monitoring systems), connection to a true earth ground would have been verified and the charge subsequently dissipated. In accordance with industry guidelines such as NFPA 77 “Recommended Practice on Static Electricity” and IEC 61340-4-4 “Electrostatics – Part 4-4: Standard test methods for specific applications – Electrostatic classification of flexible intermediate bulk containers (FIBC)” the resistance through the bag should be less than 1 x 107 ohms (10 meg-ohm).
“Our Earth-Rite FIBC system validates and monitors the resistance of Type C bags ensuring that conductive elements of the bag are capable of dissipating charges in compliance with the necessary guidelines,” explains James.
Type C bags are designed to dissipate static electricity through static dissipative threads that are interwoven through the bag material. Grounding tabs located on the bags are points where grounding systems can be connected to ensure static electricity does not accumulate on the bag. Once the connection of two grounding clamps has been made onto the grounding tabs, the FIBC system will identify if the bag is operating in accordance with the relevant standard. This is achieved by sending an intrinsically safe signal through the bag. The system verifies the grounding of the bag by ensuring the signal returns via a verified true earth ground (static ground NOT verified by the FIBC). Should any charge have accumulated on the bag, it will leave via the static dissipative threads to the verified ground.
The static dissipative loop system check continuously monitors the resistance of the bag so that if it rises above 1 x 107 ohms, a red LED on the remote indicator station illuminates to tell the operator the system has gone non permissive.
“It is easy to assume that the use of simple clamps will automatically eliminate the risk posed by static electricity. However, the complexity of dissipating static effectively requires careful planning and a sound approach to risk management,” concludes James, who goes on to say: “The correct bag and grounding system can always be negated by plant personnel that purposely or inadvertently circumvent safety procedure as seen above. Regular static hazard awareness training combined with grounding equipment that displays compliance with industry guidelines will go a long way to eliminating the risk of fires or explosions caused by static electricity.”
View our latest video to discover more about the dangers of electrostatic discharge: https://bit.ly/2MTm3Io or call: 0115 940 7500.
Incident A – Reference Britton, L (1983). Static Hazards Using Flexible Intermediate Bulk Containers for Powder Handling.Click here for related articles online
Sign up for Explosion Protection & Process Safety E-Updates
Recent Explosion Protection and Process Safety e-newsletters
Read our last “Explosion Protection and Process Safety” e-newsletter campaigns here: