Should You Re-Purpose That Silo?
Recently I was in Houston teaching an AIChE course on flow of solids in bins, silos and hoppers. During our discussions, repurposing of plant equipment, such as using an existing pump for a new process line, a liquid storage tank to store a different liquid, or moving a conveyor from an abandoned line to a new part of the plant. That brought up the question of whether it’s possible or wise to repurpose a silo. The answer is………. maybe, but proceed with caution.
A silo should be designed to perform functionally, as well as structurally. The functional design of a silo must ensure that solids flow can be controlled. This means that the solids flow continuously and at a controlled rate when the gate is open and the feeder is on, and that solids stop flowing when the feeder stops. In addition to controlled flow, certain processes may require that particle segregation be minimized, particle attrition prevented, residence time controlled, etc. Functional design of a silo must be based on certain properties of the material to be handled, including cohesive strength, wall friction, bulk density, segregation tendencies, attrition potential, and more. Therefore, if a silo is to be repurposed and it is expected to perform functionally, the existing design must be reviewed against the flow properties of the new material to be handled. If, for example, the new material is more cohesive, bridging and ratholing may occur, causing flow stoppages.
The structural design of a silo must also be based on the properties of the material to be handled. One of the most critical properties that will impact the structural design is the bulk density of the material. Placing a material with a higher density than what the silo was originally designed for can overload the silo and result in a structural failure, possibly catastrophic, endangering life and property. However, to ensure a silo can safely be repurposed requires more than just comparing the new material’s bulk density to the design basis. For example, a change in wall friction could also have a significant impact. Higher wall friction between the bulk solid and the walls of the silo will result in higher shear loads in the cylinder section. A lower wall friction between the particles and the hopper surface may alter the flow pattern in the silo. Both of these conditions can impose much different loads on the structure than the original design basis, again possibly resulting in structural failures.
Changing the operation of a silo, whether as a part of a repurposing effort or not, can be risky. For example, adding a side discharge to feed an additional stream (another reactor, packaging line, truck loading, etc.) can cause local stresses that exceed the design basis. While buckles on a silo cylinder may seem unrelated to a new side discharge, in fact they are often caused by asymmetric solids flow inside.
These are just a few considerations in deciding whether or not an existing silo is fit for the new use. When repurposing a liquid storage tank, a review, inspection and recertification are often required by applicable codes. The same level of rigor should be applied to repurposing a silo. Before a new material is place inside a silo, or the operation of a silo is changed, a thorough design review should be conducted by a solids handling professional. This will ensure reliable solids flow that will keep your process running, and prevent structural failures that can threaten plant personnel and property. If you are considering repurposing a silo, remember, “An ounce of prevention is worth a pound of cure.”