Metallurgical Coal Railcar Load-Out
This is an example of a met coal railcar loadout bin that is suffering from chronic bridging and rat-holing. It is a good example of poor bin geometry combined with uneven discharge that is resulting in inconsistent discharge.
The Existing Storage and Feed Arrangement and Why it is Suffering from Chronic Plugging and Inconsistent Discharge
The existing bin consists of of twin funnel flow (first-in, last-out) pyramid hoppers leading to small square discharge openings. One discharge opening is metered with a screw feeder and the other is metered using a clamshell gate.
The combination of the small openings and multiple shallow walls as well as the behavior of the feeders results in a hopper that cannot reliably discharged with only gravity. Because the discharge openings are small one would expect a tendency for stable rat-holes to form over the openings, and for the bins to suffer from erratic discharge. Finally, a consequence of having a first-in, last-out discharge is that stagnant material is left along the silo walls. Stagnant material will gain strength over time, further creating the conditions for plugging and erratic discharge.
The solution to fixing this problem storage and feed system has two parts. The first half of the solution is to reconfigure the hopper to include steep sloped walls, and move away from pyramid style hoppers to more conservative plane flow hoppers with a wide and long discharge opening. The wide and long discharge opening is required to overcome the bridging dimension of met coal. Overcoming the bridging dimension is necessary to ensure that the material cannot bridge over the feeder.
Changing the bin geometry will promote a mass flow discharge. 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 each plane flow 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.
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 Mining Industry.