The nature of a powder processing operation means that the generation of static electricity is to be expected in all parts of the system because of the movement of the particles through equipment. Therefore, regular maintenance is required to stop material from clogging up the machinery. Regular disassembly for cleaning and maintenance can result in bonding connections being missed or not made correctly when the equipment is reassembled. Vibration and corrosion may also degrade assembly connections so it is imperative to ensure that no parts in the assembly become isolated from a true earth ground.
Fortunately in this particular scenario, a larger scale incident was avoided by the luck of an operator. If the isolated ducting had not been found the outcome could have been very different. A static discharge in the right place through a combustible atmosphere along any section of ducting could have resulted in a significant ignition incident, putting the lives of employees and plant assets at risk.
The most effective way of ensuring complex equipment used in powder processing operations cannot accumulate static electricity is to provide a dedicated static grounding solution that is capable of monitoring the ground connection to components at risk of isolation. Such a solution should also be able to prevent the flow of product and alert personnel to a potential hazard should a component lose its connection to ground. This is especially important if the ground connection point to the equipment is not readily visible or isn’t easily accessible for example, the grounding clips mentioned in this scenario.
What actions could have been taken to prevent this incident?
It is highly plausible that charge accumulation on an isolated ducting section had resulted from a misplaced grounding clip multiple times during previous operations without a visible electrostatic ignition incident ever occurring. Without a flammable/combustible atmosphere being present in the spark gap when such a discharge occurs, frequent discharges could have regularly gone unnoticed. This is a common feature of process operations that have suffered from the consequences of a fire or explosion caused by static electricity.
The first place to start is to determine why electrostatic charge was “permitted” to accumulate on the section of ducting. In this case electrostatic charge had been allowed to accumulate because the loss of continuity resulted in the section being electrically isolated from the general mass of ground. Had the section been connected to a true ground, charge would not have been accumulated on its surface. Instead excess electrostatic charges would simply have found their way to ground. So in accordance with industry guidelines like NFPA 77 and IEC 60079-32-1, the isolated section (in this case the length of ducting) should have had continuity through to a verified ground with a resistance of 10 Ohms or less.
In this example, the system is configured to ground 7 sections of ducting (1 – 7) and a drum (8). Each channel is individually monitored back to ground to a resistance of less than 10 ohms and interlocked with the control equipment responsible for the flow of product. All monitored channels including the drum (1 – 8) need to have a path to ground (via bonding straps or grounding clamp) before the system will go permissive allowing the operation and flow of product to commence.
Powder processing equipment presents more of a challenge compared to standard applications as there are metal parts that can make up larger assemblies that can be electrically isolated from each other. The risk of removable sections becoming isolated conductors will occur if:
- Each section does not have a sufficiently low path to ground to safely dissipate charge.
- The correct reassembly of equipment after cleaning between operations and regular examination of bonding straps between the metal pipework and duct sections by plant personnel is not routinely carried out.
The Earth-Rite® MULTIPOINT II system solves these problems by ensuring that all parts of the equipment have continuity through to ground with a resistance connection of less than 10 Ohms. The reliance on human intervention to perform regular resistance checks is alleviated due to the monitoring and interlocking capabilities of the Earth-Rite MULTIPOINT II. If the loss of continuity between one of the ducting sections to the verified earth ever occurs, the system will go non-permissive and the operation will cease to continue. The system only permits the product transfer process when the ground loop resistance of each utilised channel is less than 10 Ohms, as recommended in the various international standards for the control of undesirable static electricity. The system has cCSAus, ATEX and IECEx approval for use in hazardous atmospheres and meets all current EC directives.
This example demonstrates the Earth-Rite MULTIPOINT II system grounding and monitoring multiple parts of the conveying system which could be at risk of isolation
If you would like to learn more about the Earth-Rite MULTIPOINT II follow this link to the product webpage.
Please note this case study is referenced from a third party source and is not in any way linked to the operations of Newson Gale customers.
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.