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Thread: Flow of Bulk Solids

  1. #1
    borkowski Guest

    flow of bulk solids

    My company produse ingredients for the food industries and we have some problems with the flow of our products.

    We have looked into different methods for analyse of these flow properties and we are slightly confused by some aspects. We have been told by consultants that the only way to analyse flow is by using a shear cell because it is the only way to create uniform stresses througout the sample. But recently other academics have shown this not to be the case, and have heard of repeatability problems due to sample being filled into the cells differently. So does this mean that the indicizor method or another would be better?? Our other concern is that this methods are for engineering of silos, we would not be prepared to do this length of analysis for simple quality control of powder flows, but still need to measure the flow behaviour under different conditions.

    What have others used for quality control in thier factories?

    Andrzej
    andrzej_borkowski@hotmail.com

  2. Flow of Bulk Solids

    The three main parameters that determine how a bulk solid will flow are: - Bulk density, wall friction and shear strength of the product. For a given product it is usually the bulk strength that is important and shear cell measurments are the only way to determine this feature in the dynamic conditions of failure that pertain in flow situations. The procedure for measuring this value is technically demanding and requires expertise beyond normal industrial practice. Nevertheless, guides to this behavior can be secured by more simple methods, such as measuring tensile strength, cohesion or static shear strength will a 'simple' cell. It is crucial to recognise the scope and limitations of these methods, guidence on which is not readily available.

    Much publicity has been given in recent years to phenomenological devices that produce a behavioural measument. The great danger of this approrach is that the measurements cannot be related to a fundamental property or a quanitfied design process, whereas a measurement of a physical property has a meaningful value. Ajax Equipment has some notes on powder testing that explores these aspects and makes powder testers for measuring these values for evaluation and comparative purposes. See www.ajax.co.uk

  3. Shear Testers

    Dear Mr Borkowski,

    your asking about test equipment like shear testers and indicizers, and you seem to have concerns regarding their reproducibility and applicability.

    First, I would say that different types of shear testers have been developed, so one cannot say generally that a shear tester is good or not. Let me concentrate on two popular types of testers:

    The Jenike shear tester was introduced about 40 years ago and is used since that time for research and for silo design. It is not easy to operate and requires some skill, because there can be some influence of the operator, but it is fact that with this testers thousands of silos in the world have been designed for mass flow and proper flow (no arching, no pping). I have used this tester in the past for consultancy, and all of the silos, which we designed on basis of the results of this tester, worked well. And as a consultant you have to guarantee proper operation. Hence, this tester cannot be so bad.

    In the past 40 years a lot of research has been done regarding shear testers. The Working Party on the Mechanics of Particulate Solids (part of the EFCE) did comparative tests with the Jenike tester and provided the "Standard Shear Testing Technique" where it is described how the Jenike tester has to be operated in order to obtain reliable results.

    The ring shear principle also came up in the sixties of the last century. A newer version was designed by me about 10 years ago. Also regarding the ring shear tester (also known as annular shear tester) much research has been done in the last 35 years. For example, at the Univ. of Karlsruhe at least two dissertations were made on the ring shear tester (Mnz, Gebhard).

    If you regard the shear zone of the shear cells of the Jenike tester or the ring shear tester, where the powder is mainly deformed and where its properties are measured, the mean stress is exactly known (this follows from simple mechanics: force and momentum equilibrium). So, even if the stresses in the shear zone are showing some deviations from the mean stress, e.g. at the wall of the shear cell, we do know the mean stress. And this is the advantage of the mentioned shear testers compared to so-called simpler testers, e.g. the Indicizer you mentioned in your posting: In many simple testers we do not know the stresses, neither the exact local stress distribution nor the mean stress.

    You mentioned some researchers which are concerned about the homogeneity of stresses in shear cells. I have seen such work where the density was investigated with X-ray, or where the stresses at the bottom (!) of the shear cell were measured. But it is not so important to know what is happening at the bottom of the shear cell, because we measure in the shear zone (see above). And it is not new that the stress in the shear zone can locally differ somewhat from the mean stress. However, since shear tester are the standard testers in powder technology, you can attract more attention by showing that a shear tester is not perfect than by showing the opposite. An another question should be allowed regarding the investigations on shear testers: Why didn't the researchers look into the stress distribution of other test equipment, e.g. the indicizer?

    If you look around the world, the majority of researchers and industrial users (e.g. in the chemical industry) prefer shear testers because they trust in the results.

    Regarding reproducibility I recommend to regard an IFPRI study on the comparison of shear testers and some other powder testers (e.g. indicizer), where the reproducibility of the automated ring shear tester was found to be the best, better than other shear testers, and better than the indicizer.

    Regarding my customers, who mostly work with automized ring shear testers, I have to say that they are in the position to find out differences between different powders or mixtures, even if the differences are very small. This is because of the good reproducibility you can obtain with this device. Most of my customers use the ring shear tester for product characterization, development and quality control, of course some also for silo design, but there are much more people interested in flow properties for product characterization etc. than working on silo design.

    I invite you to visit me and to try out our tester with your products! More information you will find on my website www.dietmar-schulze.com.

    With kind regards

    Dietmar Schulze

  4. shear tester

    Thema: quality control - shear tester
    Datum: 05.06.03 00:27:16 (MEZ) - Mitteleurop. Sommerzeit
    Von: IPTVADUZ
    An: andrzej_borkowski@hotmail.com
    Datei: LITERATUR LIST.ZIP (6158100 Bytes) DL Zeit (64000 Bit/s): < 27 Minuten



    dear nr Borkovsky,

    It is true that the best method to characterize the powders is the shear test. These tests have to be based on soil mechanics in order to receive the right parameters for further calculation of handling equipment. The best test method is the Peschl shear tester, because this method approach the best the theoretic requirements of soil mechanic. You mentioned Indicizor, but this method don't deliver any results to be recognized in soil mechanic. The results depending from the way of filling can be connected to the Jenike shear tester. The Ring Shear Tester, have a big variation of vertical load across the sample surface in the shear plane and measure direct under the loading cove, thus a mixture or internal friction and friction with metal loading cover.

    Please open our website WWW.IPT-ONLINE, WHERE OUR SHEAR TESTERS ARE DESCRIBED.

    We send you an literature list of publications written by dr. Peschl. Results with this shear testers are reproducible and independint of operaror.

    In October we will have a course "Quality Control and Powder Technology" at university Delft - Netherlands . Please send us your full address and we will send you the Course Program and the registration form.

    We would like also to inform you about the possibility to lease a shear tester. you are welcomme for a test to our labratory in Switzerland too.

    we tried to send you an article too, but the mail come bag as to big. For your first orientation, please reed the article nr 2, where the different types of shear testers are described, and the article nr 11, where the performanmce of shear test is describes.


    We hope to hear from you soon.

    Best regards

    Dr. Peschl

    Attachment : literature list.


    List of recent publications
    written by Dr. Ir. I A S Z Peschl

    1. Mechanical properties of powders
    bulk solids handling
    vol 8, number 5, October 1988

    2. Equipment for measurement of mechanical properties of powders
    powder handling and processing
    volume 1, number 1 march 1989

    ERRATA: in original publication the Mohr-diagram of fig 3. and fig 4. is exchanged. In our copy we have already correct it.

    3. Measurement and evaluation of mechanical properties of powders
    powder handling and processing

    volume 1, number 2, June 1989

    4. Quality control of powders for industrial application
    powder handling and processing
    volume 1, number 4, November 1989

    5. Flowability test of powders
    PMI - powder metallurgy international
    pmi 22 (1990) [2}


    6. Ideal flow silo
    Advances in feed technology, Verlag Moritz Schfer
    number 1, 1989

    7. Universal blender
    a blending method and mixing system for cohesive and free flowing powders.
    bulk solids handling
    volume 6, number 3, June 1986

    8. New silo theory
    based on the development of expansion zone.
    powder handling and processing
    volume 3, number 1, march 1991

    ERRATA: in original publication change:
    page 25; D - diameter of silo, page 30;K=LAM*MU*Z/D,

    9. Qualittskontrolle von pulvern in der industriellen Praxis
    Keramische Zeitschrift
    47.Jahrgang - Nr 11 - 1995, Seite 7-10

    10 Shear Test for Process Control and Engineering
    powder handling and processing Vol. 11 no 1 January/march 1999

    11 Principals of Soil Mechanics for Characterisation of Industrial Powders
    Powder Handling & Processing No.1 /2001

    12 Universal Mixer
    Powder Handling & Processing No. 3 2001

    13 Arching and Ratholing in Silos
    Powder Handling & Processing No. 4 2001

  5. Shear Testers

    Dear Mr Peschl,

    your statement on the ring shear tester is very interesting, but, I am sorry that I must say this, not correct. I understand that it is your interest to promote your testers, but in fact there is no problem with slip at the lower surface of the lid of the ring shear tester. Due to the bars at the lid, we imitate a number of Jenike cells behind each other, and after shear you can clearly see the shear plane which is located below the bars.

    In my last posting in this forum I mentioned the good reproducibility of the ring shear tester, which was found in an independent comparative test series (PhD.-Thesis of A. Verlinden, Bradford, UK, 2000) clearly better than that of the Peschl tester. I think that this would be not the case if we had problems like you mentioned. And, in addition, there were so many investigations on ring shear testers, including those who investigated the particle movement, why did no one found what you are trying to critisize? The ring shear tester principle is not only applied in powder technology since nearly 40 years, but also in soil mechanics since the 1930's (e.g. Hvorslev).

    But, we can discuss other effects: The friction between the rings of the shear cell in your tester (a problem which does not occur with the ring shear tester), or the variation of shear deformation with the radius of the cell with circular cross-section, where in the centre (radius = 0) no shear deformation can be, because the shear deformation is proportional to the angular velocity times the radius.

    I think we can discuss advantages or disadvantages of testers, but, please, on the basis of the laws of mechanics/physics.

    With kind regards

    Dietmar Schulze

  6. SHEAR TESTERS

    DEAR MR. SCHULZE,

    THANK YOU FOR YOUR REPLY. HOWEVER I DONT AGREE WITH YOUR STATEMENTS AT ALL. WILL DISCUSS WITH YOU THE ON BASIS OF MECHANIC AND SOIL MECHANIC AND INVITE YOU TO REED MY ARTICLES 1-3 AND 11 WHERE JENIKES, MINE AND RING SHEAR TESTER ARE ANALYZED ON BASE OF MECHANIC AND SOIL MECHANIC. I INVITE YOU ALSO TO REED MY ARTICLE NR 11 ON THE LIST, WHERE A NEW SOIL MECHANIC IS DEVEPOPED VALID FOR INDUSTRIAL POWDERS. IF YOU HAVE SOME COMMENTS ON STATEMENTS POSTULATED IN THIS ARTICLES THEN I WOULD BE HAPPY TO DISCUS IT WITH SOMEBODY WHO UNDERSTAND THE MECHANIC AND SOIL MECHANIC. UNTIL THIS TIME I STAY WITH THE COMMENTS I MADE.

    YOU MENTION THE PHD. THESIS OF MISS VERLINDEN. SHE WAS IN MY LAB FOR TWO WEEKS AND HAVE GOT GOOD RESULTS. YOUR RESULTS ARE CALCULATED ON THE OLD AND WRONG JENIKE METHOD AND THEREFORE MOT COMPARABLE WITH MY TEST RESULTS.

    I DISAGREE WITH YOUR COMMENT ABOUT THE MISSING OF THE STRESS IN THE CENTER OF THE SHEAR CELL. THE PROCESS IN THE WHOLE CROSS SECTION IS DESCRIBED IN MY ARTICLE MR.2 EQUIPMENT FOR MEASUREMENT OF MECHANICAL PROPERTIES OF BULK MATERIALS PAGE 78 79 AND FIG 9, WHERE THE WHOLE DEVELOPMENT OF SHEAR STRESSES FOR AN MEASUREMENT POINT IS DESCRIBED. IF YOU WOULD REED THIS YOU COULD SEND ME YOUR COMMENT BASED ON MECHANIC AND SOIL MECHANIC, BUT NOT MADE SUCH COMMENTS AS IN YOUR REACTION . YOU WOULD SEE THAT THE SHEAR STRESSES IN STAGE OF PLASTIC DEFORMATION ARE NOT DEPENDING OF THE RADIUS. OTHER EFFECTS PLEASE REED IN THE ARTICLE.

    YOUR COMMENT ABOUT THE FRICTION BETWEEN THE RING IS NOT TRUE. THIS QUESTION IS ALREADY MANY TIMES DISCUSS BY JENIKE SHEAR TESTER. IN JENIKE AND MY SHEAR TESTER THE MATERIAL EXPAND DURING THE BEGIN OF SHEAR, THE RING WILL TAKEN A LITTLE UPWARDS AND THEREFORE IT NO FRICTION BETWEEN RING AND THE CELL-BOTTOM.

    IN THE RING SHEAR TESTER MANNY PROBLEMS OCCUR JUST BECAUSE IT HAS NO RING.
    1. THERE IS A FRICTION BETWEEN THE CELL WALLS AND MATERIAL ABOVE THE SHEAR PLANE
    2. THE POWDER COME OUT OF THE CELL TROUGH THE ORIFICE BETWEEN THE CELL-WALL AND THE LOADING COVER. THIS DISTURBS THE SHEAR STRESS AND THE VERTICAL LOAD IN THIS REGION.
    3. NOBODY KNOWS WHERE EXACTLY THE SHEAR PLANE DEVELOP, BECAUSE OF THE FORM OF THE LOADING COVER.

    REGARDING ANGULAR VELOCITY AND HOW THE SHEAR STRESS DEVELOP REED MENTIONED ARTICLE AND LOOK TO FIG 9.

    I WOULD BE VERY GLAD TO HEAR YOUR COMMENTS BASED ON MECHANIC AND SOIL MECHANIC

    BEST REGARDS

    DR. IVAN PESCHL

  7. Never ending story?

    Dear Mr. Peschl,

    thank you for your answer. Unfortunately, it seems that there is some misunderstanding, so I have to answer, although I think that this discussion, which you have started, will not help the visitors of this forum. This is especially true for statements which lack of background and seem to be made for promotion purposes than for education and information.

    However, let me answer as follows:

    1. Concerning the comparative tests of A. Verlinden: I mentioned the shear stresses (at steady-state flow and at shear to failure), which are much better reproducible with the ring shear tester than with your tester. The shear stresses do not have anything to do with the method of calculation ("wrong Jenike method") as stated in your answer. The ring shear tester values are just more reproducible (and reproducibility is a very important point for the users of shear testers). I assume that this is one reason for the fact that the ring shear tester has become a very popular powder tester in the last years, other reasons might be its easy operation, high reproducibility, and the control software based on experience with many different powders and bulk solids. Not to forget the attractive price.

    2. I know that you think that the Jenike method is wrong. We have discussed this years ago at the POWTECH exhibition. But the majority of experts from industry as well as scientists do accept the Jenike method, because it works very well, which has been shown by more than 40 years of successful application of this method.

    3. You state that I have written about the missing stress in the centre of your shear cell. This is not true. I have written that there is no strain, because the circumferential velocity of a rotating disk is zero in the rotationaL axis. Very simple. So the strain rate varies from zero (in the centre) to a maximum value (at the outer edge of the shear cell). This is the reason for us using an annular cell with a much smaller difference between the minimum and maximum relative strain rate. In the past it has been shown that the ratio of the inner diameter to the outer diameter of the annular powder sample should not be smaller than 0.5. Otherwise the shear stresses measrued at failure are too small.

    4. Friction between the rings of your tester: I just mentioned that this has to be discussed. Following the Standard Shear Testing Technique for the Jenike tester (published by the EFCE), the upper ring of the Jenike tester is slightly lifted prior to the test. This results in smoother shear force curves and better reproducibility. Since in your tester the rings are in contact with each other, the question about friction must be allowed. At least at steady-state flow, where the material does not dilate, it is to expect that there can be friction, and also a vertical force can be transferred from ring to ring, so that the mean vertical stress in the powder is not exactly known.

    5. Your statememts on the ring shear tester are interesting, but without sufficient background. Statements 1 and 2 do not play a role (otherwise we would not be in the advantageous situation to measure values as with the Jenike shear tester, which is the standard in powder technology). Statement 3 is in contradiction to your last posting, where you stated to know that the material slips at the lid. However, we know that the shear plane is close below the vanes at the lid of the ring shear tester. This can be easily seen, and can be easily demonstrated with vertical layers of coloured powder.

    However, I recommend that you should visit international scientific conferences on powder handling. There you can present your view, meet many experts and critically discuss your ideas and the ideas of others. Unfortunately, I haven't met you at such a conference for years. At a recent event was a very interesting paper showing why the Peschl tester measures much smaller values of the unconfined yield strength than the ring shear tester. And with the ring shear tester you can measure values comparable to the Jenike tester (a tester which you do not accept, but which is still accepted as a standard in the community of powder tevchnology experts and scientists.).

    I think that a conference with many experts is a better forum for discussions like ours. Here in this forum people, who are not so deep involved in powder flow theories, want to get (and should get) simple and applicable answers.

    With kind regards

    Dietmar Schulze, Prof. Dr.-Ing.

  8. I end the discussion

    Dear Mr. Schulze

    I agree that this is not the forum where this should be discuss. However, you are talking such misleading and not true arguments that I dont like that this get un answered.

    1. In report of Verlinden are not such conclusion the ring shear tester has better reproducibility than the mine shear tester. In mine literature is reported about accuracy and reproducibility which is not reached by any other shear testers. But, you have not react to mine suggestion in my last answer. Ring shear tester is absolutely not easy to handle. Your comments has nothing to doe with discussion, bat it is only a cheap advertising. For a bad equipment the lowest price can not be attractive.
    2. The most of people are users and not experts which do not understand the equipment or the background. I dont blame Jenike because at the tine he wrote his theory he solved some contradictions by taking assumptions which cover the measurement of his shear tester, which was far from the ideal equipment. I blame also not all de users, but well so called experts and scientists which have slept 40 year long without critical treatment according the new shear testers like the mine which deliver more and better results then Jenike equipment. It can not be called successful but blind application of the method.
    3. The inventor of the annular shear tester Mr. Walker, 40 years ago, made a mistake, by mixing the strain in elastic and plastic deformation. If he would analyze it better, than he would found, that ring shear tester was not necessary. In this case I blame the experts that they have slept 40 year long. I advise you again reed the article Nr.11 you could find there an answer to many questions.
    4. Before any steady state flow (plastic shear stress), was an elastic deformation and in this period the ring get lifted. Also, friction is out of the question.
    5. My statement 1 and 2 is related to ring shear tester and has nothing to do with the Jenike tester. By the way Jenike tester was newer standard in powder technology, tri-axial tester could be closer to this. I have many times reported on conferences and I have written a lot of articles where I present new theories an reports from practice where the theories are approved. I invite every body to discuss the postulated.

    Best regards

    Peschl

  9. Dear Dr. Peschl,

    as stated in my last posting, this boring discussion makes no sense. I understand that the success of the ring shear tester might be a problem for you, because today the ring shear tester has become very popular and is used by many companies and research institutions. People who are using it are happy with it. They are able to obtain similar results as with the Jenike tester, which is still regarded as a standard in powder technology, and it is very easy to operate.

    You are writing that my statements are not correct, e.g. your point #1 regarding reproducibility. But everyone can read chapter 6.2 "Reproducibility" of the thesis of A. Verlinden, where one will find tables showing the standard deviation in percent. For example, for limestone powder the shear stresses measured with the ring shear tester have a standard deviation smaller than 1 %, the corresponding values of your tester show values up to 3.9%. So why are you writing that it is not true what I wrote? Is it "cheap advertisment" to list facts? Or isn't it much more "cheap advertisment" to state that facts are not true, just because they could be regarded as an argument against your tester?

    You wrote, that the majority of scientists slept 40 years, but you did better! Very nice. Very good argument.

    With kind regards

    Dietmar Schulze

  10. end

    IT IS A CHEAP ADVERTISEMENT , MOT TO ANSWER TO CRITIC, BUT ADVERTISE YOU OWN PRODUCT (WHICH YOU HAVE COPIED FROM WALKER RING SHEAR TESTER).
    THE DATAS FROM VERLINDEN ARE NOT PROVED BY ME AND I DONT BELIEVE THEY ARE TRUE. READ MY ARTICLES WHERE ARE DESCRIBED RESEARCH RESULTS WHICH SHOWS THE HIGH ACCURACY AND REPRODUCIBILITY BETTER THAN 0.6 %, WHICH IS THE LIMIT FOR ACCEPTANCE FOR MEASUREMENT OF YIELD LOCI.
    I AND MANY OTHERS DID NOT SLEEP 40 YEARS LONG, BUT HAVE DEVELOPED A SHEAR TESTER ACCEPTABLE FOR INDUSTRIAL REQUIREMENTS, DISCHARGE EQUIPMENT, MIXERS AND DEVELOPED THE THEORY THERE WHERE JENIKE STOPPED SUCH AS SILO THEORY, BRIDGING, SOIL MECHANIC FOR INDUSTRIAL USE AND MANY OTHERS.
    THIS YOU COULD REED IN MANY OF MINE ARTICLES AND HAVE HEARD ON MANY OF MY LECTURES ON CONGRESSES. I SUPPOSE YOU HAVE NOT READ AND PROBABLY HEAR BUT NOT UNDERSTOOD. AT ANY TIME, I AM WILLING TO DISCUSS ANY OF QUESTIONS BUT NOT ON THE LEVEL LIKE WE DID HERE. I THIN WE DO GOOD TO STOP THIS.

    BEST REGARDS
    DR. IVAN PESCHL

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