FEATURE HAZARDOUS AREAS
Static electricity is often perceived as an invisible risk but the consequences, as illustrated in this case file, are very real. Here, James Grimshaw, marketing manager at Newson Gale, looks at how to mitigate the risks during FIBC operations
T
he movement of product during filling or emptying of Flexible Intermediate Bulk
Containers (FIBC) can generate a large amount of electrostatic charge. FIBCs, which are made from polypropylene, generate 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 being handled as well as the filling or emptying rate. Discharges of static electricity from FIBCs during
loading and unloading are capable of igniting sensitive, flammable atmospheres causing an explosion. This electrostatic charge can accumulate on both the contents (product) and the fabric of the FIBC material itself.
CASE FILE: ELECTROSTATIC IGNITION WHEN DISCHARGING AN FIBC An 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. Even though the operator himself not being
grounded, the nature of the operation, which involved making a lacquer, meant that static dissipative footwear would probably have been ineffective as there was the possibility of a film of lacquer on the floor around the tank. Following the incident an investigation 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. It was concluded that electrostatic charge had been allowed to
8 SPRING 2020 | INDUSTRIAL COMPLIANCE
THE DANGERS OF ELECTROSTATIC IGNITION WHEN HANDLING FIBCS
accumulate because the FIBC was isolated from ground, whether this was through the negligent actions of the plant operator or inconclusive grounding methods. If grounding had 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). System such as Newson Gale’s Earth-Rite FIBC
that continuously validates and monitors the resistance of Type C bags ensure that conductive elements are capable of dissipating charges in compliance with the necessary guidelines. 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. 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.
THE ANSWER 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. The correct bag and grounding system can always be negated by plant personnel that purposely or inadvertently circumvent safety procedure as we have seen in the case file. 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.
Newson Gale
www.newson-gale.co.uk DO NOT BE DAZZLED IN SMALLER CONFINED SPACES
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