Static electricity in a flammable or combustible atmosphere can result in an explosion due to electrostatic discharge. This article will discuss how to mitigate the risks during FIBC operations.
Flexible Intermediate Bulk Containers (FIBCs) are now widely used for the transportation of dry bulk goods such as fertilizers, plastic granules, seeds, resin, and powders, making them suitable for use in a range of industries. However, they are susceptible to generation of electrostatic charge. This occurs when powders and other granular materials contact each other, rub together, and separate - a process known as is triboelectrification. It is now recognized that discharges of static electricity from ungrounded FIBC bags during loading and unloading can ignite sensitive, flammable atmospheres causing an explosion. This electrostatic charge can accumulate on both the contents (product) and the fabric of the material itself.
Since many products are combustible, the inherent electrostatic discharge hazard from the material cannot be overlooked. In these situations, eliminating* the potential risk of an electrostatic ignition is of paramount importance.
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Fortunately, there is now an effective way of monitoring the resistance of Type C FIBC bags to ensure that the static dissipative elements can conduct electrostatic charges through the FIBC in compliance with IEC 61340-4-4 and NFPA 77. Before we consider these standards in more detail, we look at two case studies that illustrate the dangers of failing to dissipate electrostatic charge when using FIBCs.
Case File: The Dangers of Electrostatic Discharge
In this incident, the tank lid was open allowing solvent vapor to readily escape into the operating area. Although it was not categorically determined whether the fire occurred immediately or after the FIBC was nearly completely empty, as the operator was standing within the vicinity of the tanker during the operation, he turned away when he observed the flash. An operator typically stands within close proximity of the FIBC during emptying, first to untie the strings and later to shake out residual powder. In this scenario an ignition occurred, and the operator was caught in the flash-fire zone and severely burned in the process.
The operator was using FIBC Type C bags to transfer resin to a 6,000-gal mixing tank. This operation involved making lacquer for can coatings. The mixing tank was equipped with thin conductive wires running lengthwise through the spout and connected to a bare stranded aluminum wire and alligator clip. The FIBC was hoisted above the tank using a forklift and the resin was dumped through a circular port on a hinged tank cover. There was no independent venting of displaced vapor 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. Common in processes where coatings are prevalent, a build-up on the sole of the shoe regularly occurs. A cleaner sole will typically give off a lower resistance. Despite this, he was not considered a likely source of ignition.
The investigation into incident A 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 must be isolated from earth. As it was known that the resin had low minimum ignition energy (MIE), it was assumed that flammable vapor was a significant factor in the ignition process reaching well in excess of an acceptable level. Materials with low MIE will regularly reach the minimum explosive concentration (MEC) in FIBC emptying operations such as the one described due to the flowrate and ability to charge and may be at risk of combustion by several sources of ignition. In this incident electrostatic discharge was the ignition source.
Unfortunately, a second incident involving the same operator occurred and was not dissimilar to the first. The main difference being the FIBC was designed with an internal conductive aluminum liner bonded to the polypropylene in the spout. This was connected to an external grounding tab to which a grounding clamp was to be connected by the operator. The FIBC was suspended over the tank as before, and after applying the grounding clamp the discharge spout was pushed through the port in the tank manway so that it is extended 10-12 in. inside the tank. The draw cord was then cut to open the spout and release resin into the tank.
The FIBC was not opened at the top to vent the contents and prevent drawing vapor into the FIBC. On this occasion flow was delayed and the operator “puffed” the FIBC to free the flow. Within 10 seconds of flow, a flash occurred. Failure to vent the FIBC was not believed to be a contributing factor as there was no fire or explosion inside it.
The operator once again was in the vicinity of the FIBC but not touching it. As a result, he received second- and third-degree burns. The sprinkler system installed above the tank did not emit water; however, pallets of resin bags were singed at 20–30 ft from the tank. Although the hinged lid was closed there was again no provision for venting either the purge gas or the air entrained into the tank by the powder flow. A significant displacement of flammable vapor therefore took place into the operating area.
Unlike the first incident, in Incident B it was reported – but not conclusively determined - that a grounding connection had been properly made ensuring continuity to earth to dissipate static charge. However, it was not possible to completely ascertain whether this was the case as the grounding clamp was unavailable for examination. As a result, an FIBC operational error causing loss of continuity could not be ruled out since the FIBC involved was destroyed in the fire. If we are to generalize failures for FIBC operations, these typically occur due to manufacturing defects, operator error or disabling continuity to a verified earth via a grounding clamp.
What actions could have been taken to prevent these incidents? In these incidents 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’ve been verified and 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).
Given the magnitude of charge that can build up on bags, an active grounding system is the recommended and safer choice. This is because the system can determine whether or not the bag’s construction complies with the relevant standards and will ensure the bag is grounded for the duration of the filling/emptying operation.
The 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. Type C bags are designed to dissipate static electricity through static dissipative threads that are interwoven through the bag’s 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.
There is no doubt that ignition of explosive atmospheres is a serious hazard when handling ungrounded FIBCs. The most effective way of avoiding this problem is by using Type C FIBC bags because they have conductive material woven into the fabric and seams and are then grounded via a cable attached to the FIBC. Type C bags are compliant with IEC 61340-4-4 and NFPA 77.
However, 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. However, as documented in Incident A and B, the effects far outweigh the time it takes to perform the necessary checks and conclusively confirm visually that (a) the operator has clamped on, and (b) the system has confirmed a resistance to ground of 1 x 107 ohms or less.
James Grimshaw is marketing manager at Newson Gale. For more information, call: 0115 940 7500 or visit www.newson-gale.co.uk or.
* “For the avoidance of doubt. ‘eliminate’ shall have the meaning that the likelihood of electrostatic discharge will be eliminated or mitigated to a low level where the risk and harm will be eliminated, in line with the internationally recognized recommended best practice. We would like to clearly outline that static electricity as such can be never completely eliminated.”
Incident A** and B** - Reference Britton, L (1983). Static Hazards Using Flexible Intermediate Bulk Containers for Powder Handling.
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