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Gypsum and Shale Rock Front-Loaded Bin

Gypsum and Shale Rock Front-Loaded Bin

 

This case study is an example of a retrofit of a front-loaded metering bin handling rock gypsum and limestone shale rock. This installation when into service in 2018.

The Challenge

The cement plant uses front end-loaders to unload barges. The plant has a front-loaded metering bin at the wharf that discharges onto a conveyor belt that carries material to either the plant’s covered storage or to silos feeding the plant’s kiln. Prior to the retrofit, the front-loaded metering bin consisted of a pyramid hopper that was discharged via a small 1-foot by 3-foot opening straight onto the conveyor belt below. The discharge opening had a hydraulic flap that was tasked with metering material onto the conveyor belt. Unfortunately, the front-loaded bin never worked. If the plant put any head of material into the bin, it would plug. To compensate, the front-end loaders would spoon feeder material into the hopper, reducing the pyramid hopper to a chute.

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Why the Pyramid Hopper and Belt Feeder Were Suffering from Chronic Plugging and Inconsistent Discharge

The existing pyramid hopper was suffering from chronic plugging for three reasons:

 

  1. Poor bin geometry. The sloping walls and discharge opening of the existing hopper above the belt conveyor were insufficient to produce a reliable mass flow or first-in, first-out discharge pattern. First, the sloping walls were too shallow. As a result, material could not slide along the hopper walls, but would instead try to slip within itself in a funnel flow, or first-in, last-out discharge pattern. Second, the discharge opening was to narrow. The narrower the opening, the less strength a material requires to form a stable arch that gravity cannot reliably break.
  2. The conveying action of the belt conveyor below the pyramid hopper, which is shearing material from the storage bin, was compacting the gypsum and limestone against the front of the bin wall. As the stored materials compacted, they would gains significant shear strength. With enough compaction, the stored material would gain sufficient shear strength to bridge over the hopper discharge opening.
  3. The belt conveyor had a propensity to withdraw material from the rear of the pyramid hopper. This selective withdrawal of material, in addition to the shallow sloping walls, induced a funnel flow, or first-in, last-out discharge pattern in the bin. In principle, there is nothing wrong with funnel flow as long as the effective opening of the bin exceeds the bulk solid’s piping dimension (or distance over which the bulk solid can form a stable rat-hole). Unfortunately, the piping dimension for gypsum and limestone is quite large – in fact, it is larger than the metering bin. To handle gypsum and limestone reliably without hang-ups it must be discharged in mass flow, or a first-in, first-out discharge pattern. To do so, the Feeder must withdraw material evenly from the hopper’s full discharge outlet.

Kamengo’s Solution

The solution to fix this front-loaded metering bin was to replace the storage hopper with a hopper with plane flow shape, steeper hopper walls and a wide and long opening. Unlike a pyramid hopper, which converges into two planes, a plane flow hopper only converges in one plane at a time. A plane flow bin shape is the most conversative bin shape and is often necessary when designing for a difficult flowing material. Further, the steeper hopper walls were required to promote a mass flow discharge, where material slides along the hopper walls. The angle for the sloping walls were determined via material flow testing. Finally, the wide and long discharge opening is required to ensure the stored materials cannot bridge over the Feeder. By correcting the geometry of the storage bin, if one were to remove the feeder, the hopper would now self-empty with gravity in a mass flow or first-in, first-out discharge pattern.

 

The second half of the solution is to ensure the entire hopper and feeder arrangement will discharge in a mass flow or first-in, first-out discharge. To do so, Kamengo paired the new hopper with a Kamengo Feeder. The value and necessity of the Kamengo Feeder is that it withdraws material evenly from the entire discharge opening of the storage hopper. By definition, to achieve mass flow, the bulk solid must descend the storage bin as a single body with all the stored material in motion, and the only way to achieve this is for the feeder to withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Cement Industry. 

Limestone Surge Bin Retrofit

Limestone Surge Bin Retrofit 

This is an example of a limestone bin that is suffering from chronic bridging. It is a good example of an instance where the behavior of the Feeder is inducing rat-holing and bridging despite the fact that the bin is constructed with correct geometry.

The Existing Storage and Feed Arrangement and Why it is Suffering from Chronic Plugging and Inconsistent Discharge

The existing bin consists of several stacked chisel hoppers, and is discharged using a slide gate that opens from the middle outwards. To control the discharge onto the conveyor below, the slide gate opening is incrementally adjusted. Unfortunately, the system suffers from chronic rat-holing and bridging.

 

In summary, the bin is suffering from chronic plugging because when the slide gate is only partially open it induces a funnel flow discharge pattern despite the fact that the geometry of the storage bin is correct, where if the slide gate were removed, the bin would self-empty in a mass flow or first-in, first-out discharge pattern. Funnel flow (which is a first-in, last-out discharge pattern) can be made to work with a large discharge outlet. However, when the discharge outlet is small, gravity is insufficient to overcome the strength of the bulk solid at the discharge outlet, and hence chronic bridging and rat-holing is expected.

 

As noted, the storage bin is designed to discharge in a mass flow, or first-in, first-out discharge pattern. The definition of mass flow is that during discharge, the entire mass of stored material comes down as a single body (single mass). To achieve this, material must discharge evenly from the entire discharge outlet of the storage bin. The tell-tale sign that you have mass flow is that material is sliding down the bin walls. In contrast, with funnel flow, material is stagnant along the hopper walls. When the slide gate is only partially open, then material is not permitted to withdraw evenly from the entire discharge outlet of the storage bin because the slide gate has limited the “live” opening. The result is stagnant material along the bin walls and a funnel flow discharge pattern.

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Kamengo’s Solution

The solution to fixing this problem bin is simple. Given that the bin geometry is correct, such that if the Feeder were removed, the entire bin would self-empty with gravity in a mass flow or first-in, first-out discharge pattern, only the behavior of the Feeder needs to be fixed. In this case, the feeder must be fully effective, which means that it must withdraw material evenly from its entire opening. The attached solution pairs the bin with a Kamengo Feeder. The reason for doing so is that the Kamengo Feeder withdraws material evenly from its entire opening, which by definition is necessary to actually achieve a mass flow discharge pattern in the bin.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.

Hot Lime Silo Retrofit

Hot Lime Silo Retrofit

This is an example of a hot lime silo that is suffering from chronic bridging. It is a good example of poor bin geometry combined with uneven discharge that is resulting in chronic bridging.

Why the Existing Storage and Feed Arrangement is Suffering from Chronic Plugging and Inconsistent Discharge

The existing bin consists of a 25-foot diameter silo and 55-deg cone that converges to a 2-foot diameter opening that is fitted with a basket gate.

 

In summary, the silo is suffering from chronic plugging because the shallow cone and the behavior of the basket gate are inducing a funnel flow discharge pattern. Funnel flow (which is a first-in, last-out discharge pattern) can be made to work with a large discharge outlet. However, when the discharge outlet is small, gravity is insufficient to overcome the strength of the bulk solid at the discharge outlet, and hence chronic bridging and rat-holing is expected.

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Kamengo’s Solution

The solution to fixing this problem storage and feed system has two parts. The first half of the solution is to replace the bottom half of the cone with a mass flow chisel hopper with a wide and long opening. The result is an expanded flow bin, with the lower half of the storage silo emptying in mass flow and upper portion emptying in funnel flow. As long as the funnel flow portion is emptied completely on a regular basis the storage bin geometry will be reliable.

 

Mass flow is a first-in, first-out discharge pattern. The definition of mass flow is that during discharge, the entire mass of stored material comes down as a single body (single mass). The tell-tale sign that you have mass flow is that material is sliding down the bin walls. To achieve this, material must discharge evenly from the entire discharge outlet of the storage bin. This requirement leads to the second half of the solution.

 

The second half of the solution is to pair the plane flow chisel hopper with a fully-effective feeder – that is a feeder that withdraws material evenly from its entire infeed opening. Again, by definition, to achieve mass flow, where the stored material comes down as a single body, the feeder must withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.

 

A great example of a fully-effective feeder is the Kamengo Feeder. In addition to being fully-effective, the Feeder offers consistent metering, and can be made as wide as needed and as long as wanted. As a result, the Kamengo Feeder offers valuable advantages when designing for a difficult flowing material.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.

Lime Silo Retrofit

Lime Silo Retrofit

This is an example of a solution to fix a lime bin that is suffering from chronic bridging.

Why the Existing Silo is Suffering From Chronic Plugging and Inconsistent Discharge

The existing bin is a silo with a steep cone that converges to a vibratory feeder. The vibratory feeder employs a shallow cone and a small outlet.

 

In summary, the silo is suffering from chronic plugging because the shallow cone of the vibratory feeder is inducing a funnel flow discharge pattern, while constricting the material discharge to too small an opening. Funnel flow (which is a first-in, last-out discharge pattern) can be made to work with a large discharge outlet. However, when the discharge outlet is small, gravity is insufficient to overcome the strength of the bulk solid at the discharge outlet, and hence chronic bridging and rat-holing is expected.

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Kamengo’s Solution

The solution to fixing this problem bin has two parts. The first half of the solution is to rip out the funnel flow cones, and replace them with a steeper mass flow cone and mass flow chisel hopper. Doing so fixes the geometry of the silo such that if the Feeder were removed, the entire bin would self-empty with gravity in a mass flow or first-in, first-out discharge pattern. The second half of the solution is to pair the new cone and hopper with a Kamengo Feeder. The reason for doing so is that the Kamengo Feeder withdraws material evenly from its entire opening, which by definition is necessary to actually achieve a mass flow discharge pattern in the silo. If the Feeder instead withdrew material unevenly, a funnel flow discharge pattern would ensue (regardless of the bin geometry), and rat-holing would occur.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.