In my previous article, I stated that in mass flow, all the material in the bin moves while any is being withdrawn. This means that the walls of the hopper are steep and smooth enough to overcome wall friction and allow material to slide. The opening of the bin must also be large enough to prevent arching. Just because material slides at the walls does not necessarily mean it will flow without arching. But, just how large does the hopper opening need to be to prevent arching?

I previously discussed the first of two types of arch that can occur with bulk solids, namely, particle interlocking; however, a cohesive arch is more difficult to deal with.

As the name implies, a cohesive arch occurs because of cohesion between the particles of a bulk solid. Imagine reaching into a box of detergent and gently lifting your hand out. The detergent will flow through your fingers. Now, reach in again and squeeze the detergent. A "snowball" forms, arching or bridging over your fingers. You have applied consolidation pressure capable of causing an arch to occur. This can also happen in a bin simply due to the weight of material in the bin.

Because of cohesive strength, an arch (bridge, dome, etc.) can form which is strong enough to support the entire contents of the bin above. An arch is a stable obstruction to flow that forms over the point of narrowest cross-section of the bin (usually, the outlet). Consider the size of the structural members used to form a bridge spanning a certain width river.

If, without changing the structural members, you use that same bridge to span a river that is twice as wide, the bridge will collapse under the stresses applied to it. While we do not want this to happen to a bridge over a river, we do want this to happen to a bridge over an outlet.

So, we need to be able to determine the cohesive strength of our bulk solid, and also the stresses we need to apply to it to make it collapse.

Cohesive strength can be measured using a bench scale laboratory testing device such as a direct shear tester (Jenike Shear Tester). This device allows us to determine a material's "Flow Function" (strength, pressure relationship).

Additionally, we know that as we increase in hopper span, we apply more stress to the solid. Analysis of the test results allows us to determine the point at which the stress applied (by the hopper span or opening) exceeds the cohesive strength of our material. This way, we can predict the opening size required to overcome arching and cause material to flow.

In my next article, I will briefly describe how these tests are run and what kind of information can be obtained from them.