A brief history on bulk solids and powder flow shear cell testing

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The origins of bulk solids and powder flow properties testing were derived from soil mechanics. The original “shear cell” - or shear box, was design by Casagrande. This shear cell was rectangular, as knowledge in bulk solids and powder flow properties testing was limited at that time. Soil samples were easily prepared as rectangular as compared to other forms.

The rectangular shear cell was composed from a cell base, and a top-loading cover. The intention was a linear movement of the “loading cover” against the shear cell base, and the soil-sample shears in between the shear cell base and the loading cover. For soil samples, this method of shear test by Casagrande delivered acceptable results.

As knowledge and experience was gained, the Casagrande tester was introduced to determine bulk solids powder flow properties. However, unlike soil samples which have been consolidated for thousands of years, it was found that the samples of bulk solids and powders required to be consolidated in the shear cell, and the rectangular format was found to be less than ideal. This was due to the relatively unconsolidated state of powders and bulk solids compared to soils.

Experimentation and process requirements lead to a shear cell where powders can be more efficiently filled and consolidated. Early pioneer Jenike altered the square cell design of Casagrande, having a shear cell base ring and a loading cover. Operation of the Jenike shear cell remained a linear movement. In this revised design, the consolidation process occurred during the shearing process, until a stable shear stress was established. By experimentation, some powders too small in particle size were unable to reach a stable shear stress, making this design limited in effectiveness.

This lead to a need for shear cell tester design which would allow a longer shear strain until the shear stresses reaches a stable value

Walker then developed the ring shear tester which was of an annulus design. The Walker Shear Cell comprised of an upper and lower shear cell. The upper and lower shear cells included partitions. The assumption of a linear function of the stresses versus the radius was more realistic compared to previous shear tester designs. The linear function of stress would only be valid during the elastic deformation of the sample.

The above photo shows a Walker ring shear cell tester.

To overcome these shortfalls in shear cell design, Dr Ivan Peschl developed Peschl full area rotational split ring shear cell. The Peschl rotational shear cell combines the advantages of Jenike shear cell and Walker ring shear cell. In the rotational shear cell, the shear plane is located in the middle of the sample tested – well away from the surfaces of the upper and lower cells, with the ability to apply unlimited shear strain due to rotational movement. The Peschl Split Ring Shear Cell test is computer controlled, and produces a “shear profile” for any bulk solid or powder. As the Peschl Split Ring Shear Cell test continually shears the bulk solid or powder without any limit, it measures the bulk solid and powder flow properties at its plastic limit.

In all of the above bulk solids and powder flow properties shear cell testers, shearing occurs at various consolidation pressures, and most importantly, without aerating the bulk solid or powder.

The technology in bulk solids and powder flow test equipment is not new technology, but technology that has evolved over many decades. The Peschl Split ring shear cell test equipment also has the advantage of being compact in design, as compared to previous shear cell test equipment.

The Peschl Split ring shear cell test equipment is in fact the only shear cell test equipment that is easily transport to site to measure bulk solids and powder flow.

Dr Peschl also pioneered the computer controlled bulk solid and powder flow shear test about 30 years ago. This lead to the shear profile of a product, and needs to be a mandatory requirement when obtaining bulk solids and powder characterization results from a shear test.

Why measure powder flow properties?

The majority of the processing industries - minerals, food, pharmaceutical and mining require the bulk solids handling or powder handling equipment and bulk storage facilities. The efficient and profitable running of these industries is reliant on the product to be free flowing. However, many process plants experience bulk solids and powder flowability problems. These flowability problems can be the result of the supply of irregular or out of specification products, environmental operating conditions, affects of bulk storage (instantaneous and time consolidation), discharge, transfer, process requirements, and even change of product molecular structure or ingredients.

These bulk solids and powder flow problems can occur in storage silos, hoppers, transfer chutes, process equipment, feeders, dischargers and conveyors, where flow of bulk solids or powders is required, or where shear the bulk solid or powder is required - for example screw feeders, screw conveyors, belt feeders, bucket wheel reclaimers, mechanical type high shear mixers, etc.

The characteristics of bulk solids and powder flowability can vary immensely from one product to another. Even products of the same name can have significantly different handling characteristics - whether it is products from the same supplier or different supplier.

The characteristics of bulk solids and powder flowability can vary immensely from one product to another. Even products of the same name can have significantly different handling characteristics - whether it is products from the same supplier or different supplier. Bulk solids and powders can transfer shear stress, which is why an angle of repose is formed. Many products when consolidated (pressure applied due to storage or process), demonstrate cohesive or ideal plastic properties, and retain a shape under any particular consolidation pressure. It is important that the flow influences from the storage or process requirement do not exceed the limiting product pressure relations.

The density of the bulk solid or powder may vary considerably - consolidated bulk density - and this can be significant with certain products. The Peschl Split Ring Shear Cell test equipment automatically and accurately measures the consolidated bulk density for the applied consolidation load. Any changes in bulk density may have significant impact on the volumetric flow requirement of discharge equipment, including energy requirements.

Bulk solids and powder flow properties testing is a scientific test method of measuring and determining the handling characteristics flow properties of solids and powders. It is also referred to as shear testing. The test method mechanically shears the product under certain consolidation pressures, and values are recorded.

Some products have the tendency of loosing some of their characteristics when removed from site. Some of these effects include loss of moisture, gain in moisture and degradation or agglomeration due to transport and handling. On-site testing of bulk solids and powder flow properties has the advantage of carrying out flow testing which is true to use.

In fact, over the last 30 years many pharmaceutical companies in Europe and the USA have been using the Peschl Split Ring Shear Cell test equipment on site on a permanent basis to measure their powder flowability and fine tune their processing, tablet forming and packaging equipment as needed on a daily basis. The pharmaceutical companies are able to quickly powder flow test their product, to meet their process consolidation pressure requirements within 15 minutes! This has enabled research work to be quickly implemented into process, without any teething problems. Often process plants outsource various bulk solids and powders for process requirements. The bulk solids and powders are received, and generally only measured by mass (or volume) to ensure that the correct quantity is supplied. Generally, the bulk solid or powder characterization is not considered as a value item. The lack of bulk solids or powder flow characterization can lead to on going operational problems and maintenance issues in high critical process equipment is reliant on the product flow characteristics.

The design of silos and hoppers, storage stockpiles, storage facilities, silo discharge equipment feeders and conveyors can not be effectively designed without proper low testing and characterization of any bulk solid or powder.

The angle of a hopper wall, and transfer chute angle to achieve reliable product flow is dependant on wall friction testing at various consolidation pressures. Wall friction plays a very important role in determining silo wall pressures for structural design purposes as well. It is important that the bulk solids and powder flow test include results of wall friction testing at the various consolidation pressures.The shear profile for the wall friction test should always be requested to effectively evaluate bulk solids and powder flow properties.

Information available from bulk solids and powder flow characterization

The measurements of the bulk solid and powder flow test include:

The above measurements are obtained for instantaneous consolidation (No storage time), and also for time consolidation (Storage time).

The above measurements are obtained for instantaneous consolidation (No storage time), and also for time consolidation (Storage time).

The above graph shows a measurement of internal cohesion. These results, amongst others, are important for the design of silos, feeders and equipment, simply are not available generic type data.

A minimum of four tests is usually carried out to develop a family of yield loci. Once these values are obtained, we can then predict how the product will behave at the various consolidation pressures.

We then are easily able to:

Calculate the requirements for the design of silos and hoppers for flow regime.
Work out the maximum consolidated bin opening pressures for static and dynamic conditions.
Critical arching and bridge and rathole dimensions.
Check the unconfined compressive stress to vertical pressure in the silo.
Check for a cohesive arch in relation to the silo pressures.
Measure wall friction angle at the various consolidation pressures.
Carry out the design of stockpiles.
Transfer chute design.
Calculate the required pull out torque for feeders.
Accurately calculate energy requirements for conveyors and feeders.

Bulk solids and powder flow properties testing can also be used for quality assurance purposes – to ensure products meet purchase specifications for flow requirements and plant reliability. Companies in Europe and USA for example, use powder flow properties testing on plant on a continual basis to ensure their equipment operates at maximum efficiency.

The flowability and strength of a bulk solid or powder is defined by its Flow Function. The flow function is the relationship between the major consolidation stress, and the unconfined yield stress. The following table 1 provides a guide to flowability based on the flow function calculation.

The following Table 2 and Table 2a shows yield loci for various product types. These results can only be found after testing a product using a shear tester.

Table 2 - Yeild Loci comparisons.

Line 1 - Cohesionless bulk solid.
Line 2 - Cohesive bulk solid.
Line 3 - Ideal Plastic bulk solid.
Line 4 - Ideal Fluidised bulk solid.

Table 2a - Yield Loci comparisons - Expected product flow indicator.

An interesting point here is the industry in general applies the word cohesive to any products that are “sticky”. We would like to point out that another term needs to be considered is ideal plastic. The difference being: Cohesive bulk solids have a change of shear stress to the consolidating pressure. The do have very low flow functions, and will consolidate and agglomerate under compaction.

Without testing a product for flow characteristics, you risk the problems associated with safety issues, poor plant design, long commissioning times or potentially have to de-rate equipment. So if you are about to design a plant, or having problems with your product or plant operation, contact us and we will be happy to be of assistance.

Further reading on the principles bulk solids and powder flow properties testing can be found in the technical section of our website, and in the technical downloads section.