The greatest concern posed by static electricity in a flammable or combustible atmosphere is the risk of an explosion due to an electrostatic discharge. With the right approach electrostatic ignition hazards can be identified and controlled. This case study explores the factors behind the ignition source of a static discharge during an FIBC unloading operation.
Flexible Intermediate Bulk Containers (FIBCs) have long been considered a great innovation in the transportation of dry flowable material since their introduction in the sixties. Commonly referred to as a “bulk bag”, “big bag” or “tote”, FIBCs have a body made of flexible woven material, typically the high strength thermoplastic, polypropylene, along with a linear insert. FIBCs are efficient, and transportation of dry bulk goods such as sand, fertilisers, plastic granules, seeds, resin and powder coatings, to name a few, can be unloaded at a fast rate, with anywhere between 300 – 500kg typically in 30 seconds or less. Used in agricultural, chemical, food and pharmaceutical industries, FIBCs have proven to be simple to use, cost-effective and strong, and are more convenient than rigid IBCs for powder transfers because they can be collapsed after use and stored away. However, the use of FIBCs are not without their risks and when filling and emptying FIBCs in hazardous areas, electrostatic charge can accumulate on both the contents (product) and the fabric of the material itself. It is common under these circumstances for the rates at which static electricity charges are generated to exceed the rates at which the charges can relax, allowing the accumulation of a static electric charge to develop within the process.
The danger to plant personnel and the surrounding environment is if the charge is released in the presence of a flammable atmosphere, an ignition can occur. Since many products are combustible, the inherent electrostatic discharge hazard from the material cannot be overlooked. Eliminating* the potential risk of an electrostatic ignition is of paramount importance.
In the first of two incidents, an operator suffered a singed head, a burn to the back of their neck and a second-degree burn on their right arm. The second incident led to second and third-degree burns to their stomach and face too. As a result of the second static incident, the employee made the decision to leave his role, citing his apprehensive nature towards the job.
In the first incident an FIBC Type C bag was used to transfer resin to a 6,000 gallon 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 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.
The tank lid was open allowing solvent vapour 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 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. Materials with low MIE will regularly reach the minimum explosive concentration (MEC) in an FIBC emptying operation such as the one described due to the flow-rate 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.
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.