Question: Where and for which purposes is the ring shear tester (RST-01.pc, or RST-XS.s) used?
Answer: Ring shear testers are used in industry and research. It is found, beside others, in the chemical industry, the pharmaceutical and food industry, environmental industry, and at plant and equipment manufacturers. Bulk solids, which are tested with the ring shear tester, are nearly all materials of these industries, e.g. pharmaceutical powders and mixtures, plastics pellets and platics powders, instant soup, filter dust, zeolites, carbon black, additives, washing powder, and so on.
In research at universities the ring shear tester is employed mainly in process engineering and chemical engineering, but also in other fields like agricultural engineering or geophysics.
Question: Are there ring shear testers used outside of Germany?
Answer: The ring shear testers are distributed world-wide. Up to now, more than 50% of our ring shear testers have been delivered to companies or organisations outside of Germany, e.g. in the USA, UK, France, Switzerland, the Netherlands, Spain, Poland, Turkey, Denmark, Australia, Brazil, Mexico, Singapore and India.
Question: Since many years the Jenike shear tester has been the standard test device in bulk solids technology. Why shall I use a ring shear tester now? What are the advantages of the ring shear tester?
Answer: The Jenike shear tester is used successfully since the 1960s. I have tested a few hundreds bulk solids with this device. However, the Jenike tester has some disadvantages, which have been accepted due to the lack of better alternatives, e.g.: (1) the time-consuming measuring procedure: for every measurement point (of a yield locus) a new specimen has to be filled into a shear cell and and preconsolidated, and the optimal degree of preconsolidation has to be determined by preliminary tests; (2) possible influence of the operator on the results; (3) reliable results can only be obtained with well-trained personnel; (4) limited application with respect to different bulk solids: too elastic products (e.g. fine-grained PE-powder), very bad flowing bulk solids (moist fine-grained products like clay) or too coarse materials are hardly to test; (5) limited application with respect to the range of realizable stress levels, e.g. tests at consolidation stresses smaller than 3000 Pa are hardly to realize. Many of these limitations are known from the literature.
Also ring shear testers (annular shear testers) are known since many years,. For example, in soil mechanics ring shear testers are used since the 1930s, and for bulk solids ring shear testers are applied sind the 1960s. Unfortunately, the ring shear testers used in the past had some disadvantages, so that they played a minor role besides the Jenike shear tester. Some of these disadvantages are, for example: (1) Too heavy construction, which was disadvantageous for tests at low stresses; (2) stiff bearing of the lid of the shear cell, what created a non-homogeneous stress distribution in the shear cell, if the shear cell was not filled absolutely homogeneous; (3) no possibility to remove the shear cell with the bulk solid sample from the tester in order to perform time consolidation tests apart from the ring shear tester; (4) no possibility to perform wall friction tests.
With respect to the experience with the Jenike shear tester and the problems with ring shear testers used in the past, first the ring shear tester RST-01.01 was developed. Later teh automated testers RST-01.pc and RST-XS.s (a smaller tester with smaller specimen volumes thus limited to fine powders). The aim of the development was to obtain a shear tester, which has neither the disadvantages of the older ring shear testers nor those of the Jenike shear tester, but which makes possible not only flowability tests, but also all tests required for silo design. The following list of some capabilities of the ring shear tester shows, that this goal has been attained:
Question: What equipment is required for comparative tests and flowability tests?
Answer: For the computer-controlled testing you need the computer-controlled ring shear tester RST-01.pc or RST-XS.s without further accessories, just a PC has to be provided by the user. This tester has to be connected to the PC via the serial communication port.
Question: Can I perform all tests, which are required for silo design, with the ring shear tester?
Answer: Yes. Besides yield loci you can measure wall yield loci (with the optional accessory "wall friction cell") and time consolidation. For the time consolidation tests additional shear cells and so-called time-consolidation benches are available. With this additional equipment you can set up several shear cells with bulk solid samples for time consolidation and store them apart from the tester, so that the ring shear tester can be used for other tests.
Question: Is it difficult or complicated to evaluate the shear test results?
Answer: The computer-controlled ring shear testers perform the tests and evaluate the results autiomatically. With the manually operated tester the measured shear forces are plotted with a chart recorder (or an alternative data acquisition instrument). After the test, some characteristic values have to be taken from the chart and entered into a computer using the evaluation software RSV 95, which is delivered with the ring shear tester.
Question: Is the angle of repose a measure of the flowability?
Answer: The flowability is calculated from the
compressive strength of a bulk solid. The compressive strength is dependent
on the consolidation stress which has been exerted on the bulk solid before.
The more a bulk solid gains strength at a certain consolidation stress, the
more difficult it is to initiate flow and, hence, the worse is the flowability.
An angle of repose results on properties at the surface of a (often conical)
heap, i.e. it is a function of interparticle forces (adhesive forces) and
the shape of the particles. Therefore, the angle of repose gives only limited
information on the flowability as defined above.
The reproducable determination of the angle of repose is not easy (e.g. at fluidizable bulk solid it is of influence, how the material is handled and poured on the heap. In addition, the range of the typical measured angles of repose is relatively small, so that a clear distinction between powders of similar (but different) flow behaviour is not possible.
Question: We usually determine the flowability by measuring the time a powder needs to flow out of a small test hopper. Sometimes the results are contradisctionary. What could be the reason?
Answer: One can find several reasons for contradictionary results: (1) The filling procedure can have an influence, because the bulk material can become more or less fluidized or loosen up (especially at easy fluidizable bulk solids). (2) At discharge of the hopper the conditions are similar to a silos' hopper, where the stresses decrease in the downwards direction. This leads to the dilation of the bulk solid (decrease of the bulk density) and, hence, to an increase of the voids between the particles. This results in a decrease of the gas pressure in the voids and, thus, a gas countercurrent from the outlet opening into the bulk solid within the hopper. This influences/reduces the discharge mass flow rate. (3) Also the flow regime (mass flow and funnel flow) plays a role: At mass flow all the bulk solid within the hopper moves downwards simultaneously, whereby at funnel flow only the bulk solid in the flow zone moves downwards. The flow zone is a region above the outlet, confined by stagnant bulk solid (the stagnant zone) which sticks at the hopper wall and does not flow downwards even at discharge. Often funnel flow emerges in those hoppers used for flowability tests. Since the form of the flow zone is unpredictable, but of influnce on the mass flow rate, it is a possible reason for contradictionary or not reliable results. (4) The solid density is of influence on the mass flow rate, because the force of gravity is responsible for the discharge process. Therefore a product with a larger solid density will flow out faster than a product with identical properties, but a smaller solid density.