In today’s episode of the Dust Safety Science podcast is the second in a two-part series about ineffective dust explosion isolation using material chokes. Last week Jeff Mycroft, Regional Sales Manager at Fike Canada, talked about screw conveyors in explosion isolation. Today, the conversation focuses on rotary valves, which are an accepted isolation method in NFPA 69. We’ll answer the following questions:
- What is a rotary valve?
- What are the two options for isolating an explosion?
- What are some of the different design considerations for these types of systems?
- What are the best practices for maintaining these systems?
What is a Rotary Valve?
Rotary valves are actually a common process tool. As the valve turns, the product from above collects in the built-in veins and discharged below into a secondary chamber space vessel. A rotary valve can be an effective explosion isolation device, but there are specifications it has to conform to.
What Are the Two Options for Isolating an Explosion?
There are two types of valves:
- A close clearance valve where the distance between the fingers or the veins of the valve and the outer casing is a very, very close clearance: around 0.008 of an inch.
- A material blocking valve where that gap can be above 008 of an inch. There needs to be a certain amount of material on top of that rotary valve in order to make sure that it isn’t pushed through and propagation can occur.
When asked why there were two different methods, Jeff explained he wasn’t sure about why they were developed, but added that it was a generally accepted fact that if you had a certain amount of material above the rotating valve, even if it didn’t have a close clearance, the explosion wouldn’t be able to travel past it.
“I think it is a flawed way of protecting your vessel because there are many circumstances where there won’t be material: maintenance cleaning, someone leaves the rotary valve on and all the product goes past it,” he says. “Level sensors fail quite often. So it’s an incomplete method because there are ways of defeating that explosion isolation device. Whereas the close clearance valves [are] very, very effective.”
What are Some of the Different Design Considerations for These Types of Systems?
Jeff warned that even with close clearance valves, the fingers/veins on the outer service can wear, so they require routine maintenance. Sometimes there are replaceable edges to the fingers. Other times, reconstruction of those veins or replacement might need to be done.
“When you’re shredding wood, you get fines and you get some of the larger stuff in pellet industries and things like that, and the fibrous nature of it actually gums up and plugs form above it. So the rotary can be rotating for hours and not be pulling anything through. Even worse, it can get jammed up and that excess friction can sometimes cause fires. And then that’s not even including, if it’s outside, the heat of the summer and the cool of the winter. The gap is supposed to be 0.2 millimetres: when winter comes around, the valve may not even rotate because it cooled enough. In the summer, it can expand and the gap is bigger.”
He also cautioned against using rotary valves for faster operations.
“Rotary valves are not designed for speed of discharge from a vessel or ingress into another vessel. You need exceptionally large valves in order to get large amounts of product in it. So sometimes you just might not want to use these devices for one reason or another. But if you are using them, you do need to make sure it’s a close clearance.”
Why Do The Deflagration Characteristics and Volume of the Material Matter?
Jeff explained that Kst is how quickly an explosion happens. Generally speaking, it’s related to volume. Pmax is the maximum pressure that the dust is capable of producing. In conjunction with the volume of the vessel, these characteristics can have huge effects on how much product would be needed on top of the material blocking the rotary valve.
“If it’s a short, small explosion, less material would be needed in order to provide that barrier that’s permanent. In a large silo, that piston is going to be much, much bigger and much, much longer. If the Kst is a certain number, it’s going to produce a lot more pressure a lot quicker, and it’s going to last a lot longer depending on the scenarios that are involved – depending on the gas, depending on the volume of the vessel, and various other factors. So all of these things need to be factored in. The valve itself needs to be able to withstand whatever the maximum pressure that’s going to be generated in this event – that’s clear.”
He pointed out that how much product needs to be above any material blocking rotary valve is the big question.
“If you have icing sugar- a really fine, dry, light dust in a large silo- you’re going to need, let’s say, multiple feet on top of this rotary valve. Whereas if you have something heavy like corn or metal dust, you would need less of that there. It depends on how it moves as well. But you may need less of that because it would stay there. It’s not going to move as easily. The larger particulates are not going to pass through the veins. I’m not making this very clear, but it’s one of those situations where this question would need to be determined in order to ensure that you have a proper explosion isolation method when you’re using material blocking.”
What Are Some of the Best Practices for Maintaining These Systems?
Jeff stated that best maintenance practices depend largely on the process and the technology being used.
“With close clearance valves, the biggest thing you have to worry about is clearance.If you have an abrasive product, that can wear down things extremely, extremely quickly. So again, it might not be the best form of protection. If it is being used, you just need to check that clearance is being sustained over time. If you’re using material blocking, then there’s a whole bunch of things that need to be maintained. You probably should have redundant level sensors because they do tend to fail quite regularly. But if not, you need to make sure that not only the level sensor but the material level that’s on there is being maintained.”
When people clean or work on the system and they drop product out, they need to make sure they re-establish that level of material above and depending on how it works, below the rotary valve in order to maintain that barrier.
Conclusion
As the interview came to a close, Jeff reminded everyone of the importance of training when it comes to dust explosion isolation systems. No matter how sophisticated a system may be, its effectiveness is hampered when it isn’t used or maintained properly.
“I think you could probably do an episode just on training alone. It’s necessary because if someone hasn’t been trained properly, you will lose that protection.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Jeff Mycroft directly:
LinkedIn: https://www.linkedin.com/in/jeff-mycroft-66b013a/
Website: https://www.fike.com/
Email: [email protected]
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
Companies
Fike Canada
Organizations
NFPA
Standards
NFPA 69
Previous Episodes
DSS177: Ineffective Dust Explosion Isolation Using Material Chokes – Part 1 – Screw Conveyors with Jeff Mycroft
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