DSS063: Design Of Bulk Material Storage Systems With Keith McGuire

In this episode of the DustSafetyScience Podcast, we interview Keith McGuire, Director of Structural Engineering at CST Industries, which is headquartered out of Kansas City, Missouri. Keith has over 40 years’ experience with the company, which focuses on bolted tanks, domes, covers and other storage solutions for bulk materials and for liquid storage as well.

Dr. Chris Cloney was first introduced to CST when it entered into a partnership with Fike and environmental and safety consulting firm, CTI (Conversion Technology Inc). All three companies were combining their respective expertise to create dust explosion solutions that complied with NFPA 652. It’s a model that DustSafetyScience has replicated for the upcoming Digital Dust Safety Conference.

In this interview, Keith answers the following questions:

What are the different bulk storage solutions?
How is flow testing done?
What are the two types of flow?
When should dome structures be used?
What venting solutions are available for bulk storage systems?
What systems and materials are more prone to combustion?
How should bulk storage systems be maintained?

What Are The Different Bulk Storage Solutions?

Keith explained that the most common form of dry bulk storage consists of a cylindrical vertical silo or tank with a hopper. When a slide gate is opened, gravity causes the product to flow into the conveyor.

“Some products are not easily discharged by gravity,” he acknowledged. “They need to be encouraged a bit. You might have to put in some air slides or a vibrating discharge to persuade the product to come out in a gravity system.”

To determine what type of tank would best suit the needs of a customer, CST tests the flowability and combustibility of a product.

How Is Flow Testing Done?

Flow testing, which is basically friction testing, can be done in laboratories. Companies like CST also carry it out. After getting a product sample, they determine which coating should be applied to the interior of the customer’s silo to decrease friction and increase durability. Sometimes a 2B or other stainless steel finish is the best solution, while in other cases a combination approach is better: powder epoxy in the cylinder and 2B stainless in the hopper portion.

“By testing the material, you get these friction angles under various pressure,” Keith said. “There are calculations that we do to determine the slope of the hopper and the size of the opening. If it’s pressure-sensitive, you don’t want a very tall tank. We’ll work with the customer on the geometry of their tank: whether it’s tall and slender or larger diameter and shorter. All of those play a significant role in getting the flow of the product out now.”

What Are The Two Types Of Flow?

Keith identified two general theories on product flow.

First In, First Out: As the name suggests, the product closer to the opening is the first product to come out. It then flows uniformly from the bottom of the tank as it flows down. This mass flow is the most reliable type.
Last In, First Out: If the product is granular, uniform, and easy-flowing, the flow could be designed as a funnel type, which would be a shallower hopper. The product movement is last in, first out. It flows from the top down through the center of the tank, out of the outlet.

If a product is compressible, a taller tank can cause issues because the higher the product is piled up, the more pressure it exerts on itself. Once compressed, it may not want to flow properly.

“Frangibility of a product is also an issue,” Keith said. “If it’s frangible, that means it breaks up. We’ve sold tanks to cereal producers, and you don’t want to open your box of cereal and all of a sudden it’s just pieces of Cheerios. You want to see the Cheerio.”

He pointed out that there is a lot of science involved in something that should be a simple case of putting product in a tank, opening the slide gate, and getting the product out.

“In a perfect world, that would be nice. But it does require investigation, study and experience to get something that day in and day out is going to work for you.”

When Should Dome Structures Be Used?

While not as common as silos, dome structures are recommended when you have large volumes of product like sand, gravel, and other things like that you want to protect from getting wet.

The height of the dome allows the product its angular repose, so a large volume of product can be stored in a short structure. If you have enough real estate to support it, the dome could actually go all the way to the ground and eliminate the need for a tank.

What Venting Solutions Are Available For Bulk Storage Systems?

There are two types of venting explained in NFPA 68. Keith said that CST has two solutions for venting deflagration in a tank or silo:

Rupture or explosion panels that are calibrated to burst at a specific pressure in case of overpressure inside the tank, which could be caused by a deflagration. The tank can be used again after new panels are applied.
A weak-seam roof designed to lift up and vent the deflagration.

The weak-seam roof is the most economical option because smaller bolts are placed around the edge, where the roof attaches to the cylinder, creating a weak link in the tank. If an overpressure situation occurs, the side walls and hopper of the tank stay in place and the roof is free to lift off because of the undersized bolts. At the same time, they are strong enough to provide a seal so that product is protected from the elements.

The downside is that if an event occurs, the roof needs to be replaced and equipment on top of the tank, such as conveyors, dust collectors, and pressure relief valves, will be lost. You’re also limited to the amount of weight you can place on the roof and access has to be limited due to the potential danger.

Rupture panels are more scientific and dependable. The roof stays on and the panels burst, venting the deflagration safely away from the inside of the tank. Post-emergency repair is a lot less complicated: you purchase more panels, bolt them back on, and business can resume.

However, this solution is more expensive than a weak-seam roof. There also needs to be airspace. You have to limit your product capacity to below that area because the sensitive panels won’t resist the product pushing against them. You end up with a slightly taller tank than you need compared to the volume you can get with a weak-seam roof.

What Systems And Materials Are More Prone To Combustion?

The fire triangle, which has been discussed in earlier podcasts, involves fuel, oxygen, and heat.

With bulk storage systems, the heat would probably come from moving equipment, such as a conveyor bearing getting hot or a spark from a motor igniting a dust cloud.

“Some of our tanks, which were used to store coal, have had events which were successfully vented through a weak-seam roof,” Keith said. “The coal was not circulated or it sat too long inside the tank and actually spontaneously combusted.”

One tank was approximately 80 feet in diameter and 72 feet high, with a conical weak seam roof. It functioned properly: the roof separated and the fireball belched out. CST had designed the rafter system to be separate from the roof, so the trusses were undamaged. The owner only had to buy more sheet metal to build a new roof.

“I said, ‘You need to fix the flowability there in the tank. Fix those places where pockets of coal were allowed to accumulate and not flow out, so this doesn’t happen again,’” Keith recalled.

Biofuels are also a combustion risk, particularly if there is enough moisture present.

“We’ve done tanks for some of those industries,” Keith said. “We’ve installed explosion panels and weak-seam roofs because of spontaneous combustion possibilities.”

How Should Bulk Storage Systems Be Maintained?

Visual inspections are your first line of defence against tank deterioration. Periodic inspections can detect corrosion, water access points, or areas where product might be seeping out. A planned maintenance system (PM) can increase tank life and make working conditions safer.

Conclusion

Keith said that the key to maintaining a safe bulk storage system is being proactive instead of reactive.

“Bad things can happen if the good things don’t happen first. You need to take care of the equipment that you have, for sure.”

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Keith McGuire directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/keith-mcguire-pe-76148024/

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

DustSafetyScience
Combustible Dust Incident Database
DustSafetyScience Podcast
Questions from the Community
Digital Dust Safety Conference

Organizations
NFPA

Companies
CST Industries
CST Partnership

Ed Zurliene
Construction Project Manager
903 E 104th St. Suite 900,
Kansas City, MO 64131
Direct: (913) 748-4512
Mobile: (913) 481-9938
Fax: (913) 621-2145
Email: [email protected]

CST Industries – Combustible Dust
CST Industries – NFPA 652

Events
Educational Forum

Standards
NFPA 68
NFPA 652

Incidents
Coal silo collapse and explosion

Video
Grain bin silo collapse and explosion

Previous Podcasts
DSS 010: Biomass Fire & Explosion Hazards and UK regulations with Alan Tyldesley