In today’s episode of the Dust Safety Science podcast, Matt Williamson, Director of Engineering at ADF Engineering based in Dayton, Ohio, returns to talk about dust hazard control for groups of concrete silos.
Matt previously appeared in Episode #138, where he presented dust hazard control strategies for grain truck load-out bays. His insights on these subjects are especially valuable and relevant because he has over 30 years of experience in process engineering and carried out several dust hazard analyses and incident investigations across multiple industries.
Concrete Silo Groups: Their Layout and Function
Concrete silos are primarily used in agricultural and green business sectors. The typical process will start with the unloading pit. A truck dumps its grain into the pit or onto a railcar. A conveyor will go through that pit and feed the bucket elevator, which then feeds a series of conveyors running along the top of the silos.
These interconnected silos are made of a single concrete system that shares walls between them. There is usually a large concrete deck roof covering them, and conveyors mounted on top of that roof are feeding them. Underneath, there are typically slide gates that lead to conveyors that, in turn, feed another elevator that takes the material away. The grains might also be conveyed to an incline conveyor for further processing downstream, such as at a corn mill or an oilseed extraction plant. If it is a remote grain elevator, they will transport it back to a truck load-out facility.
A bank of concrete silos can consist of as few as four and as many as hundreds. A system of that size can be difficult to manage, especially when interstitial spaces between the circle-shaped structures are often used as bins that can be hard to reach.
Common Dust Hazard Control Concerns With Concrete Silos
As Matt explains, concrete silos present one of the greatest challenges in terms of explosion protection because they are typically very large and concrete itself cannot support explosion panels. On a similar note, deflagrations can’t be fought with chemical suppression because the size, space and headspace are too large.
“In these systems, if one incident occurs, it tends to spread to the next one, to the next, to the next,” Matt says. “It causes a chain reaction of events. When you look at photos of combustible dust explosions in concrete silos, you can see that the entire roof area has been destroyed in some of these systems.”
Silos can be designed with weak roofs, but when equipment is piled on afterward, it can defeat the purpose. Adding to the danger is the fact that there may always be someone up there at any given time doing maintenance on the conveyors.
“One other thing to consider is detectability,” Matt explains. “When you’re trying to suppress an event that occurs in a smaller system such as a dust collector, you have a spark detector or pressure sensor. Such detectors would not be sufficient in a concrete silo. The source is usually going to be hotspots that occur deep in the silo, making them nearly impossible to detect. Many times charred grain is not discovered until a long time after material has been extracted from these silos.”
Industry Response to Dust Hazards in Concrete Silos
A chemical suppression system on the bucket elevator outlet is typically used to isolate the silos from an event in the bucket elevator. But when there are hundreds of silos, they are not isolated from each other or from the discharge drag conveyors that run across the tops of these things.
Drag conveyors always require an open silo at the far end since they can’t be blocked. There are also slide gates (usually manual) under the rest, which means that at any given time, at least two silos are open up through the conveyors themselves. Isolation from upstream is therefore important.
“Some silos, especially the smaller ones, are connected via a dust collector or a drag conveyor,” Matt explains. “The drag conveyor will be connected to a dust collector. If you are attempting to aspirate the conveyors feeding your silos or even the headspace of the silos, you should never mount that dust collector on the roof. If the dust collector has a problem, it will blow out its own explosion panels and launch a fireball down the concrete silos. That’s the last place you need to see a fireball, because the roofs of those silos often have dust and grain piled up if not properly cleaned. That’s something you absolutely need to avoid. The dust collector needs to be on the ground, adjacent to the silos.”
He emphasized that regular material movement was essential to minimizing the risk of smouldering combustion.
“if your material is moving at a reasonable pace- in other words, if you’re turning over your silo at least daily- then these problems are unlikely to ever occur. But if you’re leaving your material in your silos for a week or a month, then the probability is suddenly very high that it’s going to start to decompose, especially if the material is put in there wet.”
Smoke and other signs of smouldering require an immediate response, he added.
“If you detect a smoulder or start to see smoke coming out of the vent on the vent port, you need to empty out that silo. You need to get that material as a priority onto your discharge conveyor and take it away, get it to a storage pile on the ground or just get it out of the silo. That needs to be done as quickly as it can practically be done.”
Conclusion
“Keep in mind these concrete silos are meant for incoming whole grain storage,” Matt says. “They are not meant for finished meal feeds. That kind of material, which is much dustier, needs to be kept in a different way, whether it’s in a smaller steel bin or a bunker. The concrete silos are only designed to be used for whole corn, whole soybeans and whole grains, not for the finished product.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Matt Williamson directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/matthew-williamson-6776309/
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
Companies
ADF Engineering
Previous Podcast Episodes
DSS138: Dust Hazard Control for Grain Truck Load-Out Bays with Matt Williamson
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