Flow Properties: The Missing Database

Dr. Andrew Jenike pioneered an approach for the design of silos. His method relies on material flow testing as the starting point to achieve reliable discharge. Through the measurement of properties including cohesive strength, wall friction, and compressibility of a given material a hopper can be engineered to achieve the desired performance.

It takes a lot of testing work to get to that point though. In our information rich age, there is a yearning for a simpler and quicker way. If only, instead of testing, we had a library of values from which we could reference.

The first challenge is that it would take a lot of work to create such a database. Our material world extends far further than those elements whose initials fill the periodic table. Some general properties, such as loose bulk density, have been compiled elsewhere (more on this later) for a number of common particulate materials, but not for flowability. The first great challenge would be the work involved to simply go through the testing on each and every bulk solid. This list would exclude proprietary, new, or intermediary materials. Those handling such materials would therefore be unable to rely on this theoretical library of properties. Our problems, however, do not stop there.

Bulk solids are typically identified by general names based on their makeup. Hydrated lime is one such material. In a recent project, an equipment supplier experienced problems with a system handling hydrated lime. They had in the past supplied numerous identical and successful systems to handle other hydrated limes. This version of the material, as found through testing after the system’s installation, had more cohesion than prior products with the same name and visual appearance.

Other problems were encountered for an application with a cementitious product. The earlier mentioned catalogue of bulk densities was employed when sizing a bucket elevator. This led to an unloading rate for railcars more than double the planned time. Testing here revealed that this product’s true bulk density was significantly less than that listed for a product with the same name in this reference literature.

Standardization of bulk materials for some industries seems to have made significant advancements for many factors but not always flowability. Flowability seems to be more than the sum of the interparticle forces at play. The many phenomena that drive particle interactions intersect, overlap, and cascade creating behavior that is hard to arrive at simply by knowing something about the particles involved. Also, different particles make up the whole of the material meaning particle properties will not be constant throughout the population.

Bulk solids are also aware of their surroundings. Their behavior is not constant as it relates to how it will come out of a silo. Both environment and operation, such as time at rest, can change how the material flows. This means the same material, under two different handling conditions can have significantly different flowability. As an example, look to those handling the same prilled sulfur in a Texan summer versus a Michigan autumn. A single entry in our theoretical database of flow properties would need to have cohesion entries not only for a specific time at rest, but also for the same time at rest for every environmental condition of consideration.

In light of these challenges, we find that a database of flow properties does not lay easily within our grasp. If it did it would in some cases lead to problems where we use the wrong values or lull ourselves with a false sense of security, believing our material was something it was not. Materials vary from one another, and a single material can vary as a function of handling conditions.

There are times when we may reference the flowability of a similar material, but such comparisons should be made with the utmost caution. It is said that history does not repeat itself, but it often rhymes. Likewise similar bulk solids often show adjacent trends, but do not flow the same. So, we still must get actual material and conduct actual tests. For silo design, the most appropriate approach still remains the one Dr. Jenike first laid out, where material flow testing is the basis for design.

Scott Miller has been providing consulting services for Solids Handling Technologies Inc. since 2016. For more information, visit www.solidshandlingtech.com.