Re: Historical Overview Of The Indicizers And Jenike Shear Cell
Posted on October 29, 1999 at 09:27:03:
In Reply to: Historical Overview of the Indicizers and Jenike Shear Cell posted by Jerry R. Johanson, Ph.D. on July 22, 1999 at 12:46:05:
Dr. Johanson’s critique of the Jenike cell highlights some of the sensitive features of this testing procedure and the impediments to establishing a uniform and consistent stress distribution in the sample. There are many other factors which place this instrument as a tool for experts, as many industrialists and non-experts have discovered, and left the devices to gather dust as they moved on to less awkward subjects than powder testing. Dr. Johanson is uniquely placed to understand the virtues and shortcomings of this test method. It is by virtue of his contribution to the technology that it has been possible for this test to achieve its current standing. As a result of his pioneering work with Andrew Jenike, the method has proved it’s worth over the last thirty years. In combination with the theory and design method developed in association with the instrument, despite its drawbacks, many hundreds if not thousands of successful applications have been installed, without a single reported failure. This must say something about the degree of safety inherent in the predictions, but nevertheless this process established hopper design as a science rather than a black art and initiated a coherent interest in bulk technology.
The Indicizer clearly offers a totally different approach, and the reasons for its introduction are laudable. However, the stress relationship of sample preparation and failure is co-axial, whereas it is transverse in the Jenike cell. The test represents incipient failure of a uniaxially compacted mass, whereas the Jenike cell seeks to reflect the critical state conditions prevailing during solids flow conditions. The fact that one test is carried out with the Indicizer is hardly proof of improved consistency as there is little opportunity for variation to appear in the results, and a ‘black box’ solution precludes a critical theoretical analysis being made of the underlying theory. Inspection of the various patents leading up to the final Indicizer version shows the degree of development involved in achieving the current construction, but there remain geometrical aspects that may be questioned on fundamental grounds. These details are however completely irrelevant to whether the Johanson or the Jenike techniques are useful or not. The two devices are chalk and cheese and should not be compared, or attempts made to reconcile their measurements other than perhaps to evaluate the suitability of the results. Each approaches the assessment of strength of a powder compact in different ways and has different interpretations and conclusions. The proof of the pudding is in the eating and there is ample time and opportunities for the effectiveness of the new method to be assessed.
There is no doubt that industry desperately needs better, simpler, and cheaper tools to evaluate solids flow problems. Anyone starting to examine stress conditions in loose solids faces a substantial task because of the many interacting physical, chemical and operational factors and the need to accommodate stress history, ambient fluid behaviour during changes of voidage, time effects, and many other influences. The nature of particulate bulk strength mechanisms is complex and ultimately a subject for experts. Fundamentally, a simple test gives a simple result. Much experience is required to understand the limitations of an elementary method, unless they are clearly defined. It is seductively attractive to set aside complications of the technology, until being made painfully aware of the shortcomings of a limited evaluation.
General engineers and industrialists would be well a advised to understand the basic principles of flow channel geometry, the importance of wall friction and the relative virtues of differing discharge devices. The determination of a critical size of opening to guarantee reliable flow for a cohesive product is not simple. For free flowing materials, provided that ‘free flowing’ is a consistent quality and not a transient property, the orifice size is usually much easier to establish. In general it is much easier to initiate flow than it is to generate wall slip if the design is not adequate, hence a primary interest should rest in choosing the most appropriate form and wall inclination for a bulk storage vessel. The orifice size may be a problem, but any difficulty it causes is usually more readily overcome than rectifying the specification of an incorrect wall angle.
My advice regarding testing is to measure density and wall friction with some, even if possibly misplaced, confidence but be careful about measuring and interpreting shear strength unless you really know what you are doing. If it is vital to secure an answer of impeachable reliability, then the Jenike cell in the hands of specialists seems to be the odds-on current favourite. For a quick and simple assessment of flow related properties, quality control and comparative evaluations, the Johanson Indicizer appears to be more appropriate method, if financially viable. Wall friction tests appear to be much neglected for general design work, hence it would seem that there is much to be done in the way of specialised industrial education in the field. ■
Re: Historical Overview Of The Indicizers And Jenike Shear Cell
Posted by Dr. Harald Wilms, Zeppelin Systems USA, Inc. on July 26, 1999 at 21:35:17:
In Reply to: Historical Overview of the Indicizers and Jenike Shear Cell posted by Jerry R. Johanson, Ph.D. on July 22, 1999 at 12:46:05:
With interest I followed the contribution to thius issue. Being a delegate to the European committee cited by Dr. Johanson (he probably refers to the EFCE-WPMPS), I would like to share some additional insight into the testing this committee has done. More than 20 labs participated in a round-robin test with a very fine calcite powder. The results on the yield locus and the flow function (Unconfined yield) showed a lot of scatter. i don't recall that a vale of 400% ever was deterined or stated. However, the committee then asked on how these tests had been performed and found diferences in the procedure. A more detailed procedure was given and more consistent results were obtained. This method was repeated until a very detailed testing procedure was defined and later published as the EFCE 'Standard Shear Testing Procedure for Particulate Solids Using the Jenike Shear Cell'. The basic concept with only some marginal change has recently become an ASTM Standard, the first for testing flow properties for powders.
At a later stage of the European project, the procedure was even more fine-tuned and then used by the European Commission Bureu of Reference Materials (BCR)for certification of limestone powder flow properties. The results from Jenike shear tests were so consistent and reproducible that they even matched the very stringent criteria of BCR. The best person to contact for additional information on this BCR Project in UK is Dr. Richard Akers at Loughborough University.
Having worked on both committes, it is my strong belief that the Jenike tester is best suitable for all fine powder testing applications and design purposes. For comparative and qulity control purposes, other testers may be suitable as well. The rotational shear tester developed and marketed by Schulze in Braunschweig is an alternative with some advantages regarding training and the experience level of the lab technicians.
Numerous Jenike testers are available in UK at various research institutes, universities, and at consultants, such as Dr. Harold Wright. ■
Re: Historical Overview Of The Indicizers And Jenike Shear Cell
I would not dispute the success of shear cell tests in the field of hopper design.
But I would like to add the comment that these tests all apply to the quasi-static flow regime, as encountered in storage hoppers. One can see the reference to hoppers in Dr. Johanson's message above.
Sadly, techniques so carefully developed for the quasi-static flow regime have been leaked across to the very different situation of high velocity dense granular flows, such as encountered in transfer chutes, without any distinction being made.
Let us be clear, tests measured under quasi-static conditions on samples of fine material do not describe flow under high velocity dense granular or 'inertial' flow regimes such as one sees in conveyor transfer chutes where we might have rocks of 250 mm diameter travelling at more than 10 m/s. ■
Expert Attitudes
"The nature of particulate bulk strength mechanisms is complex and ultimately a subject for experts."
This is precisely the nature of the business. Either the experts know or they don't know. Which is it?
Fluid handling has its ups and downs: not because the fluid misbehaved but because the design was lacking in detail, vents and drains etc.
Fluids can change phase without great cause for concern. Why is this? Because fluids are chemically consistent. Sadly, particulates are very inconsistent and that is the end of the story for the body of dry bulk practitioners. For simplicity ignore gases (hot air) and then ask why: just because we achieve gravity flow with liquids we expect gravity flow from particulates? Both liquids and solids have to be transported upwards. Liquids descend of their own accord, under gravity, but why should partly constrained solids do the same? Sometimes they do and sometimes they don't fall out of the bottom of their world. In a situation like that any practitioner of the craft must choose a worst case scenario and make the stuff move. That is why there are things like Louise paddle extractors and vibrators. Brute force is a noble thing and particulate solids must obey and so they do obey.
In i977 I told some important guys at the Chamber of Mines of South Africa that none of us really knew what we were doing as we tried to improve mechanised stope working at 12,800 feet underground. They reluctantly agreed but we still carried on. That is our nature. We were experts who didn't know. Nothing has changed. ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com
Re: Historical Overview Of The Indicizers And Jenike Shear Cell
Hi John,
Interesting but the work Peter has done and we have done on granular flow in the dense granular phase suggests we are coming to grips with the way solids flow. The first observation is they don't flow uniformly, i.e. the larger particles separate from the smaller particles and this is more pronounced if water is present. Secondly if we focus on the extremes rather than the averages we can come to grips with what our designs need to be to avoid the pitfalls of blockages or excessive wear. I also think we can now pretty accurately calculate material trajectories, at least that is our experience as we always compare our results in the field to what we calculated theoretically and we are getting excellent correlations. Add to this some interesting physical properties we have ascertained in our work and we are well on the way to developing quite accurate flow models and from this design outcomes that work extremely well. This is most definitely been our recent experience. As Peter says I think the models of the past have been too prepared to take what has been researched and accepted in the quasi-static regime into the dense granular flow regime while we have seen very little correlation frankly.
Containment and physical constraints are not the answer as it inevitably leads to maintenance issues and in many cases poor reliability. Where we are now is getting the materials to work for us rather than us having to work for them
Cheers
Colin Benjamin
Gulf Conveyor Systems Pty Ltd
www.conveyorsystemstechnology.com ■
Historical Overview of the Indicizers and Jenike Shear Cell
Posted on July 22, 1999 at 12:46:05:
[In reply to Dick Adams]
I apologize for entering this dialog at such a late date; however, I would like to provide you with a historical overview of both the Jenike Shear Cell and the Johanson Indicizers.
As a graduate student at the University of Utah, my first assignment with Jenike in 1959 was to develop shear cell techniques to make the shear tests more consistent. After much experimentation with the number and angle of twists, varying offset of the top disk and the top shear ring to try to even out the highly non-uniform stresses applied to the sample, varying initial offset between the top and bottom rings and various attempts to compensate for the lack of consistency in steady state values (including prorating the failure results), there emerged a test procedure that a practiced technician could generally produce results within a plus or minus 10 percent deviation in actual shear stress points consistently. Unfortunately, plus or minus 10 percent in Shear Cell values does not produce the same accuracy in the unconfined yield stress interpreted from the test points. This was evident when the European committee for evaluating shear testing Sent a uniform sample our to various test facilities for a Jenike Shear test and the results on the unconfined yield strength varied by as much as 400% from one extreme to the other. While I was still working at Jenike & Johanson, I developed an algorithm to provide a consistently conservative interpretation of the Jenike Shear test for unconfined yield strength. This often meant ignoring some data points completely with the justification that the only valid shear points will lie on the yield locus between the tangent to the unconfined yield strength circle and the tangent to the steady flow circle. This poses a difficult problem of interpreting the time effect points because the time yield locus does not touch the steady state stress circle. By assuring that the slope of the yield locus remained constant with time (i.e , the angle of internal friction, is constant), the interpretation became consistent.
Others have run enough multiple points to fit a least squire’s curve through the points. I’m not sure that either of these data interpretation methods is correct, or that either will produce the same test results with different technicians or even the same technician at different times using a true blind test (not knowing they are repeating the test with the same sample).
The specimen factors that affect the consistency of the results include particle size variation, moisture variations, preconditioning and number of preformed soft lumps in the various samples used to develop the required data points for one unconfined yield strength determination. Technician variances are also numerous and include a heavy or light hand while twisting the sample, bumping or vibrating the specimen either during steady state or failure shear, non-uniform sample distribution in the Shear Cell, particle size segregation imposed by taking the sample from the container to the Shear Cell, the amount of gap between the top ring and the push pin on the top disk bracket (This is affected not only by the placement of the top disk by the technician, but also by the change in adjustment caused by dropping the top or even from an unusually heavy load during the test), and finally the data interpretation. I once found a technician that consistently produced yield loci data always on a straight line, only to find out that he reran the points until he had three data points that lay on a straight line. All other runs were discarded.
In February 1985, after leaving Jenike & Johanson and forming JR Johanson Inc., I decided that there must be a better way to measure bulk solids flow properties. First, the test needed to produce an unconfined yield strength using one specimen so we could eliminate all the sample-to-sample variations introduced by the Jenike Shear tests. Second, the test needed to be controllable so that the strength could be measured at a predetermined consolidation pressure and allow a single test to produce a critical arching dimension. Third, the test needed to take into account the initial consolidation of material being dropped into an empty hopper or onto a bed of material near the point where an obstruction may form. Fourth, the test need to produce repeatable results when the solids was removed from the test cell and rerun. Fifth, the test must be quickly run under conditions totally independent of the technician, which meant all loads needed to be applied with controlled duplicated rates incapable or being altered by the technician. The technician’s sole responsibility needed to be loading the test cell and emptying it when the test was done.
After a series of learning experiences, we finally developed a tester in the early 1990s that could produce repeatable results that duplicated experiences with actual hoppers. Three years ago, I had a young civil engineering student working at JR Johanson Inc. who needed a senior project and approached me with the idea of comparing the Indicizer results with a standard triaxial soil test machine using density as the criterion for measuring “sameness” in compaction. The agreement between the Indicizer and traixial tests was excellent (about ± 10 percent maximum deviation on the unconfined yield strength results).
I suggest you try both a Jenike Shear Cell tester and a Johanson Indicizer before making any judgments. You can try the Indicizers at Copley Instruments, Private Road No. 7, Colwick Industrial Estate, Nottingham NG4 2ER England. Telephone 44-11-961-6229. I don’t know where you can find a convenient Jenike Shear Cell as there are not as many of them available.
Jerry R. Johanson, Ph.D.
Added by Administrator as an example only:
Johanson Indicizer ■