The problem of static electricity in hazardous atmospheres is ever present in many sectors of processing industries. This case study investigates the factors behind the ignition source of a static discharge during a powder processing operation.The problem of static electricity in hazardous atmospheres is ever present in many sectors of processing industries. This case study investigates the factors behind the ignition source of a static discharge during a powder processing operation.
Pneumatic transport systems represent the heart of any granular bulk material handling system throughout many industries today. Being an efficient method of transporting granular material, such systems allow for quick transportation of powders between processes enabling companies to keep up with an ever growing demand on production. However, such processes are not without their risks. When the product being processed is considered combustible and has an appreciable portion of fine material, the potential for having an explosion increases dramatically. Fine powders with low MIE (minimum ignition energies) will regularly reach the MEC (minimum explosive concentration) along the conveying system and may be at risk of combustion by several sources of ignition. One such ignition source is electrostatic discharge.
Pneumatic conveying systems have the ability to generate vast quantities of electrostatic charge via the movement of product through the plant equipment. The most common method of electrostatic charging on such process operations is due to tribo-electrification, which is simply the contact and separation of the powder with the walls of the processing equipment, the powder molecules itself or other factors that can cause charging, like surface contaminants.
In this incident a process operator working on a pneumatic conveying system heard a crackling noise when powdered material was being transported between the classifier and the loading hopper. During investigation of the noise, the operator came into contact with a section of the duct and received a significant static shock. Although the operator was unharmed, the severity of the incident warranted a full system shutdown to investigate how static charges had been able to accumulate on a particular section of ducting.
During the inspection the duct was examined and it was identified that the section of duct was not suitably grounded. When tested it was found the duct had a resistance path back to ground well in excess of 1011Ω, exceeding the recommended resistance of less than 10 Ω for metal plant items in good contact with ground, stated in IEC 60079-32-1:2013 Explosive atmospheres Part 32-1: Electrostatic hazards, guidance.
Further inspection found that the unusually high resistance was a result of a single grounding clip that had not been properly installed after a clean down operation. Consequently, the piping between the two ducts acted as an isolated conductor resulting in the generation and subsequent accumulation of charge. The lack of continuity to ground meant that the charge could not be dissipated, allowing an excessively high voltage potential to develop on the duct which eventually discharged onto the operator. Given the high rate of charge generation and spark discharge by a poorly fitted grounding clip; a review of grounding and bonding of all metallic parts was carried out. The inspection scrutinised the grounding and bonding integrity of all equipment units, all sections of ducts, bags and cages in the bag filters. As a result many deficiencies were found and swiftly rectified.