Steve Davis explores some of the issues plaguing silos in the bulk handling industry, and what can be done to solve them.
Collapses are a commonly reported in many of the bulk handling industries. Other silo problems are also common but get less attention as the results are not as spectacular. I have detailed some of my experiences, but I am certain there are other stories.
If you are considering silos for your installation, or have problems with existing units, find an engineer who understands the stored material, it’s flow properties and the structural design rules that apply. The following example shows the result of lacking knowledge, and apparently not considering that some knowledge could be a good thing.
I reviewed the design of a silo, and this combined several silo issues seen over the years. The silo in question had failed in service after a relatively short period of time.
The failure showed as holing through just above the conical discharge outlet and was unexpected. Other problems were also apparent but had been accepted as something the operator could live with. The outlet did not always flow well, and evidence of hammer rash showed the cure.
Silo quaking was observed but the operator had no concept of what this could be and accepted this. At times, much of the content hung up in the silo and was mechanically prodded from above to start flow.
We believe that collapse of this hang up under gravity, after being undercut by some flow, and before consolidating to the prodding stage, is the cause of quaking. The silo was relatively small, holding a few hundred tonnes of material. Let’s explore the problem.
The silo is used as a buffer in production. Production is around the clock, but the onward transport of the bulk product occurs only for a single shift.
The bulk material accumulates through the night and is released in batches through an open/closed slide gate. Occasionally, the bulk remains in the silo for several days.
The designer knew little about bulk material flow, or the bulk material. No material flow testing had been completed. The standard suite of bulk material flow tests would likely not reveal the hidden properties but would certainly have identified flow angles and outlet diameter and the like.
This bulk material is deceptive in that it flows freely when at ambient and production temperatures. When in storage at production temperature it cools very slowly due to low thermal conductivity. When hot and under consolidation pressure, the material can under certain conditions bond together into a single body.
Bonding is tenuous and broken easily with some mechanical effort. Laboratory testing, if completed at the storage temperature, would possibly have identified this issue.
Testing at usual laboratory temperature would not show any problems. In addition, water sprays were used to suppress dust during conveying to the silo. This bulk material, when hot and wet, generates small quantities of highly corrosive liquids. Liquid flow to the bottom of the silo.
The silo design was not noted as being specifically bulk or mass flow, and no reason for selection of the cone angle or outlet diameter was given or the overall geometry.
The design assumed a single bulk density at the maximum in the silo, when production data clearly showed a range between this and lower density.
The silo could not store its full nominal capacity as the average bulk density was seven per cent lower than the design. The angle of repose was used both for the silo and as the conveyor surcharge angle and the angle used was incorrect for either.
Conveyor edge distance is notable reduced from best practice as the actual surcharge is 15-degrees lower than that used for calculation.
