In today’s episode of the Dust Safety Science podcast, Jeff Mycroft, Regional Sales Manager at Fike Canada, talks about ineffective dust explosion isolation using material chokes on screw conveyors. This is part one of a two-part series. In this part, we’ll be talking about screw conveyors. Next week, we’ll be talking about rotary valves and using material chokes above rotary valves for explosion isolation as well.
In the Dust Safety Academy, there was a post about explosion isolation using material chokes or product chokes. The poster had seen that a number of consultants had made recommendations on how these types of systems could be used globally. He’s worked in Australia and the Asia Pacific but they were no longer backed up by NFPA standards, at least not when it comes to screw conveyors parts. That’s what we’re talking about today, and we’ve broadened the scope of the question to include rotary valves as well.
In this episode, Jeff discusses the following:
- What a screw conveyor is
- The design concept for using a material choke to isolate a deflagration or an explosion.
- Reasons why specific testing with this type of equipment has been removed from the NFPA standards and how to fix it.
- Some alternative methods for deflagration and/or explosion protection that should be considered.
What is a Screw Conveyor?
And then, can you describe this now outdated concept of using a product choke or material choke for explosion isolation in these devices?
Jeff explained that a screw conveyor is a screw or auger encased in a housing and designed to rotate and drag product through the housing from one end to the other. It has a center shaft on which the outer fins of the auger rotate.
The idea of using a product choke came from a solid thought: that, if you had a plug of material from the top of the screw conveyor to the bottom and no air gap in between, then an explosion or deflagration could not pass through.
“They gave guidance on how to do this,” Jeff says. “They broke the screw conveyor, where they really just removed the section of the fins on the auger for a short section and put a metal plate in the top, so the top would be blocked entirely. The concept is where the screw is broken, the incoming product would push into that space, fill the entire screw housing, and then push the product through the bottom of the screw, and that would be the break. And on the other side of the screw, on the other side of the plate, the screw would start again and pick up that material coming through and allow it to continue to convey.”
When asked whether it was more common to not have the baffle plate with an inclined screw, Jeff replied that it was an outdated isolation method.
“It’s being removed in more facilities that I saw that it used to be – they had a product choke there,” he says. “I’ve seen them in both horizontal and elevated screws, and it really depends on the person designing it. A lot of people thought if it was an inclined screw, you’d get that product choke anyway. In the lower section of the screw, you would always have some sort of product – not necessarily the case and not an effective form of explosion isolation in a lot of cases. So it really is site-dependent, and it may or may not have been thought about or properly implemented as well.”
Why Were Screw Conveyors Removed From This Type of Isolation Technique?
Jeff said he didn’t have any hard data on when screw conveyors were removed from this form of isolation, but the committee found that several incidents had occurred, indicating that they didn’t work as designed.
“A lot of people I’ve talked to have an anecdotal story where they were working on-site and they had an explosion that propagated through a conveyor with a product choke. So it wasn’t uncommon for those stories to be occurring. I actually have been involved in a situation where a company was getting proper explosion isolation and removing these product chokes. They had an incident at a different facility, so they had firsthand experience that they didn’t work in that specific scenario.”
Fike Corporation has a large test facility, and Jeff has tested in screw conveyors.
“This particular test I’m referring to was a horizontal screw conveyor. We did this test and most people involved thought there would be enough material in the screw conveyor to stop the propagation. They were surprised to see that that was not the case, that the explosion did propagate from one vessel to the other. This was done using a functional hammer mill and malt. In this particular test, a lot of the factors that we saw that would have given us the impression that it wouldn’t have travelled through [but] we were wrong. So it’s not something that’s easily definable. And before you run an actual test, some of your assumptions being made might be incorrect.”
He said he had heard that testers found the functionality of broken screws to be subpar.
“They found that when they destroy the throughput of that screw, they cause blockages, there’s always increased maintenance, and increased wear that actually caused some fires in certain circumstances where the product was just sitting there and friction was rubbing on it for an extended period of time. From a production standpoint, I didn’t find anyone in maintenance who didn’t despise the choke itself even if it was working. But that’s beside the point. There are ways to defeat that choke even if it’s being done properly. And it’s not a good way of doing it.”
What Other Isolation Methods Can Be Used?
Jeff pointed out that explosions are more common in startup and shutdown and just after maintenance for a variety of reasons. But that’s what happens when a screw needs to be pulled apart for cleaning or maintenance and it’s empty when the system starts back up.
“Or maybe someone had left that auger going when it was supposed to shut off – the fill indicator was broken or there was some other issue,” he says. “These are all things that can make this isolation method unreliable. Even if everything’s working properly when a process upset occurs, there are just many, many ways of defeating this explosion isolation method.”
He said that in order to have a proper explosion barrier, there are a few options available, depending on throughput and need.
“You can put a rotary valve either before or after, or even both on the end of the screw – a properly NFPA-designed rotary valve that would provide that barrier to stop the explosion from propagating from one area to the other. There are also active explosion protection systems where you can detect the explosion and then inject a proven barrier in there so it lasts for the duration of that explosion and doesn’t allow it to propagate.”
Conclusion
Towards the end of the interview, Jeff said that Fike tries to steer clients away from material chokes, especially at the design stage. If such a system is already in place and the client has done an engineering study or real-world test on it and it has been proven effective, it’s possible that it is compliant and can be used.
“However, I have yet to find a company that has done that testing or has that depth of knowledge on their product. The cost to evaluate it and do that testing is always going to be more expensive than the various options that are available. We don’t even sell rotary valves. You actually can also use double dump valves as long as they can’t be open at the same time but to stop propagation. But they’re very, very effective and they’re cost-effective solutions as well.”
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
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