In today’s episode of the Dust Safety Science podcast, we’re talking to Jeramy Slaunwhite, Chief Technical Officer of Explosion Safety at Rembe. He’s a frequent guest on the podcast, and is here to talk about the grounding and bonding of silos storing wood chips.
This was a question from a Canadian end user that came through the Dust Safety Science helpdesk system. They wanted to know whether they needed to test grounding resistance on a specific frequency or just do a visual inspection of the condition of the installation. Jeramy goes into detail on the subject here.
In Silos Storing Wood Chips, Should We Test Grounding Resistance at a Specific Frequency or Do a Visual Inspection of the Insulation’s Condition?
Jeramy explained that the standard hierarchy of NFPA evaluation and interpretation would start with NFPA 664, the Standard for Combustible Dust Safety in Wood Processing Facilities.
“Unfortunately, NFPA 664 is very light [on] static,” he says. “It basically says static accumulations must be managed or controlled. Air hoses are a big mention. It’s known that they can create higher static charge. But otherwise, it basically just says where equipment is subject to static charge, it should be controlled and considered – but nothing really concrete or black and white on what and where and how.”
The next step in that is the standard for the best practice on static electricity, which is NFPA 77, which includes a section on dust and powders and a set of controls. In addition, there is a section specifically on silos and how static charges affect them.
Static electricity control, or control of electrostatic discharge, is one method of controlling one of the viable ignition sources for combustible dust explosions and deflagrations. To get started, it’s important to understand a bit about the material and the ignitability of that dust cloud.
The ignitability of different materials will vary depending on their particle size, moisture content, and specific properties and parameters. In order to ignite a dust cloud in an ignitable concentration, how much energy must be applied? Following that, one can look at how much energy is created by static discharges – which aren’t all created equal. Different static discharges have different energies for different materials, so there are many variables in this equation.
“In a lot of cases, wood chips are coming into the process as green and undried high moisture content chips. With that, the dust may be quite hard to ignite, so to speak,” Jeramy says. “And even if it can still be ignited in a lab scenario, they’re using very high energy igniters to initiate that explosion.”
Knowing and evaluating the ignition hazard requires an understanding of the material aspect. The amount of millijoules required to ignite a wood dust cloud is typically in the thousands. If it’s wood flour, it’s going to be easier to ignite and require less millijoules.
“As far as the material conveying wood chips, imagine a belt conveyor dropping wood chips into the silo,” Jeramy explains. “You’ve got dust that’s being liberated as it’s falling, and then dropping, impacting, creating that cone of material filling. The fine dust is going to be liberated and moved and pushed out towards either a bin vent or an outlet filter. But a lot of it’s going to cling to the sides in the walls and inside of silos – it’s usually a layer that’s built up. It’s a layer like that where sampling is most effective.”
If that sample is lab-processed down to a low moisture content, it can be used to represent the worst case scenario, as it’s going to have the lowest ignition energy and be the most sensitive. In 2012, some of the worst explosions at Canadian sawmills occurred in the dead of winter, when it was cold and dry. They weren’t all initiated by electrostatic discharge, but it illustrates that the drier the dust is, the more ignition sensitive it becomes.
“I think about using my torch to light my charcoal grill. The charcoal can have glowing embers and I can have the flame in the torch, I can be holding it right over the ember, but it won’t reignite,” Jeramy explains. “And I’m thinking, “Well, it’s gas. It should be super ignition-sensitive.” But then, it comes down to concentration. If it’s being gradually, softly conveyed and filled, it’s going to create some dust cloud, but maybe not enough to create an ignitable concentration in the silo as opposed to fine material that’s being pneumatically conveyed, blown in. It’s going to inherently create a full, distributed, homogeneous dust cloud.”
He pointed out that when preparing explosion safety protection concepts for silos, one always has to be aware of the worst-case scenario because it is during those times that things almost always happen.
“It’s rarely during normal processing. It’s during those maintenance operations or power outages or bypasses that things go awry and wonky, and lead to a snowball of errors.”
Referring to the Canadian user’s original question, Jeramy clarified that ‘grounding’ referred to electrical potential being to the ground while bonded meant being at equal potential or connecting something to an adjacent connected piece of equipment so that the two potentials of those devices or that equipment are equal.
“That’s the real concern. You don’t want a differential in charge capacity at one item or equipment increasing in charge. As soon as it gets an opportunity, it’s going to want to discharge to the adjacent. And that’s where your spark jumps – because as the charge goes up, it jumps.”
A reliable way to ground would be a grounding wire cable, a braided cable, or another sort of mechanically attached, robust conductor. In NFPA 77, there’s a statement that says the size of the wire isn’t so much based on its current carrying capacity, but rather its structural integrity and reliability.
“We’re talking electrostatic current, so the conductor doesn’t need to be huge. It just needs to be reliable. Something that’s going to be visibly and securely attached to either ground or ground rod, something like a lightning rod or a system ground, or to another piece of the building that’s ground or equally connected to the rest of the network system.”
Silos often experience a phenomenon called cone discharge or bulking brush discharge. This can occur when material is fed into a silo from the center, making a cone, hence the cone discharge. As the silo fills up, the outer shell of the silo wall becomes conductive. The material is rolling and sliding on itself. If that material has a low resistivity, meaning it does not convey charge very well, it can accumulate by rubbing on itself. It gathers and collects charge.
By the time it reaches the outside grounded, bonded, conductive walls, it can discharge and create a chain reaction. There are reports stating that it can send metre-long lightning sparks up the side of the silo cone. The material discharges itself or builds on itself, and then discharges to the side walls.
“That being said, the amount of energy generated in this type of discharge is estimated to be around 20 millijoules, which isn’t a lot,” Jeramy says. “But some materials… do have ignition sensitivities in that range. So this is where it can be a concern in wood.”
In belt conveying and especially pneumatic conveying with higher velocity, static charge accumulation produces a lot more friction, resulting in a much higher energy output. It’s known as propagating brush discharge, and it can develop into hundreds or thousands of molecules that would ignite a wood dust cloud. This is why the material handling equipment is a lot more sensitive for static charge accumulation, and bonding and grounding becomes much more critical.
“A big question is ‘How do we prevent a bulking brush cone discharge if it’s possible? How do we stop it?’,” Jeramy explains. “The idea is that as the material flows down the cone and builds charge as it gets to the sidewalls, we want to interrupt that process so that the material is unable to build enough charge to arc and discharge out by the time it gets to the sides. We used to do that by putting metal rods vertically through the silo at intermediate spots, or rings, cones, some sort of conductive material that’s bonded back to the sidewalls to create an early path to our cone. It can even be done by chains that are hung from the top, rods up from the bottom or grids that are stacked through the silo that allow the material to dissipate and dump that charge as it rolls on itself before reaching the sidewalls.”
Conclusion
At the end of the discussion, Jeramy said, “The biggest takeaway for electrostatics is to understand the ignition sensitivity of the dust in the worst-case scenario and evaluate all the possible scenarios of ignition source development that might be relevant.” This level of understanding leads to action that can make a huge impact on safety levels in silos.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Jeramy Slaunwhite directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/jeramy-slaunwhite-04b30019/
Website: https://www.rembe.com/
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Dust Safety Share
Organizations
NFPA
Companies
Rembe
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