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Manual Powder Processing Ignites Dust Cloud

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hes aug 18 13Manual Powder Processing Ignites Dust Cloud

A huge range of industrial processes are susceptible to electrostatic charge generation, creating a risk of fires and explosions in hazardous areas.

In one case a manual powder processing operation resulted in the ignition of a dust cloud, caused by the accumulation of static electricity, as evidenced by the results of a thorough investigation, summarised here. (Read More)

 

In this case study, a process operator was tasked with manually tipping approximately 18kg (40lbs) of powder, which had a minimum ignition energy of 12 milli-joules, from a plastic drum into a metal process vessel. A metal chime was positioned around the circumference of the top of the plastic polyethylene drum to provide it with impact protection from daily usage in the plant.

The operator tipped the powder into the process vessel, resting the drum on the edge of the vessel, but as he removed the drum when the powder was fully deposited, there was an ignition of the dust cloud that had formed at the top of the vessel. It was postulated that the accumulation of an electrostatic charge resulted in a static spark discharge from the chime as it came into close proximity with the vessel when the drum was removed. The vessel itself was grounded through its own fixed connection to the plant, but other equipment used in this process was overlooked.

In order to verify this theory, an experiment was conducted to determine how much electrostatic charge could have been generated by the movement of the powder.

18kg (40lbs) of the same powder was tipped from a similar drum into a Faraday cage, from which electrostatic charge measurements were taken.

In this case the powder was charged due to the friction caused between the powder and the plastic drum as the powder slid down the inside surface. A field meter reading of 500KV/m (the maximum voltage the meter was capable of measuring) was recorded on an isolated area of the polyethylene drum, which would have had the effect of charging the metal chime by induction.

Given the high rate of charge generation caused by frictional charging, the amount of electrostatic charge that could have been induced on the chime would have been limited by its surface area.

If the total quantity of electrostatic charge created by the movement of the powder was induced on the chime, this would have exceeded the maximum charge density any surface can hold in air.

It can be assumed that the maximum charge density, i.e. the total possible amount of charge that could be held on the chime, was achieved through the simple and rapid act of tipping the powder from the drum into the vessel.

Given the minimum ignition energy of the powder dispersed in air, and the circumstances of the process being proven to generate a significant electrostatic charge on the equipment, as well as all other sources of detonation being eliminated, the conclusion reached was that a static spark caused the ignition of the dust cloud that formed around the grounded process vessel.

Both IEC 60079-32-1 (13.4.1) and NFPA 77 (7.4.1.6) & (7.4.1.4) state:

“Temporary connections can be made using bolts, pressure-type earth (ground) clamps, or other special clamps. Pressure-type clamps should have sufficient pressure to penetrate any protective coating, rust, or spilled material to ensure contact with the base metal with an interface resistance of less than 10 Ω.”  

Codes of practice like IEC 60079-32-1 and NFPA 77 outline what practical measures can be taken to minimise the risk of a fire or explosion caused by discharges of static electricity. The majority of hazards can be controlled through the installation and proactive use of static grounding devices. Devices ranging from basic grounding clamps through to ground status indicators with output contacts for interlocking with processes can be specified for a wide range of processes.

For the actual mathematical calculations and further insight into this experiment, contact Newson Gale to receive the full case study.

Visit: www.newson-gale.co.uk