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Thread: Flow Properties

  1. #1
    brian Guest

    Flow properties

    Hi all,
    Can anyone tell me in detail how to find the following flow properties for a typical bulk solid:
    - Instantaneous yield loci.
    - Kinematics angle of internal friction vs consolidation.
    - A wall yield locus.
    Thanks in advanced.

  2. Hello Brian,

    you can obtain all these data using a shear tester. On my website www.dietmar-schulze.de you will find some essays on the fundamentals of powder technology and powder testing. We also offer Ring Shear Testers for the measurement of the flow properties of powders and bulk solids.

    Best regards

    Dietmar

  3. Shear Data

    The correct technical procedure is given by shear testing of the powder and applying a formula by Andrew Jenike. First published as a working method in 1964 in Bull. 123 of the experimental station of the University of Utah, the technique has been published in a more digestible form by Roberts, Arnold and McClean of TUNRA, the New South Wales Research Associates, in 1982, as Bulk Solids: Storage Flow and Handling ISBM O 7259 03031.

    The testing process is described in a document prepared by the Working Party for the Mechanics of Particulate Solids of the European Federation of Chemical Engineers and published by the Institution of Chemical Engineers as A Standard Shear Cell Testing Technique.

    Lyn Bates

  4. #4
    venky Guest

    Material properties

    Hello all,

    Could someone tell me what are the values for the following :

    Wall friction angles: (Phi)

    1) For Starch and Mild steel
    2) For Stucco (Ground Dry Gypsum) and Mild steel

    Effective angle of internal friction: (Delta)

    1) for Stucco
    2) for Starch

    Thanks a lot in advance

  5. Wall Friction and Internal Angle of Friction

    The value of wall friction depends on the specific nature of the bulk solid, (a generic name is not sufficient as many factors can influence how the material slips on a contact surface), and also the particular material against which it is in contact. The material and its surface finish are important.

    Likewise, it is dangerous to rely on material property data based on a named product, particularly if they are hygrosopic or available in different particle size range distributions.

    Wall friction is relatively easy to measure and there should not be any excuse for not securing this value on representative samples. The value has powerful uses for comparing different contact materials, specifying optimum materials of construction for hoppers, chutes and any equipment involving contact surface slip.

    Shear strength measurements demand more experise and understanding for correct interpretation. Whether to conduct a full series of tests or to employ a single, 'simple' test depends largely on the purpose for which the information is required. Ajax Equipment has some notes on basic powder testing that may be of interest. See www.ajax.co.uk or contact Eddie McGee at tech@ajax.co.uk

  6. #6
    Peter Brown Guest
    I must register my TOTAL agreement with Lyn Bates that "it is dangerous to rely on material property data based on a named product".
    This can be route to disaster as the properties of two bulk materials given the same name can be TOTALLY DIFFERENT.

  7. #7
    venky Guest

    Flow data/problems

    Dear Lyn,

    I understand that relying on a named product can be dangerous. I hence have the material characteristics laid out below. We also have had problems with the Stucco (Dry ground Gypsum)metering feeding our Gypsum wallboard line. This is a weigh screw feeder device with metering data obtained from load cells. There is variation in the stucco flow.

    The Stucco feed bin is actually designed for funnel flow whereas it needs to be mass flow(the bin was changed quite sometime back). Could this however affect the metering ? The feed bin actually feeds to a headbox via a constant speed scalping screw that in-turn feeds a conditioning screw. The conditioning screw is controlled to maintain a constant level in the headbox. A variable speed metering screw draws stucco from the headbox to feed the weigh screw which runs at fixed speed. The weigh screw finally conveys stucco to the forming line mixer. As it is the metering and weigh screws that measure and deliver the stucco there should be no metering problems as long as the headbox level is maintained constant ? Could you clarify this please.

    I) Bindex 420 Starch:

    Specifications:

    Density = 40 Ibs/cu ft (Range 38-56)
    Particle size = 0%/30 mesh, 80%/100 mesh
    Cohesiveness = Slightly cohesive
    Moisture = 10%
    Temperature = 70 degF (ambient)

    REQUIRED INFO:
    1)Effective angle of Internal friction (delta)
    2)Wall friction angle (phi, between Starch and Mild steel/Low carbon
    steel)

    II) Ground Gypsum:

    Specifications:

    Density = 70 Ibs/cu ft
    Particle size = 2.5%/50 mesh
    Cohesiveness = It does bridge and rathole
    Moisture = 0.2%
    Temperature = 70 degF (ambient)

    REQUIRED INFO:

    1)Effective angle of Internal friction (delta)
    2)Wall friction angle (phi, between Ground Gypsum and Mild steel/Low
    carbon steel)

    Regards

  8. Flow properties

    Dear Venky,

    The two most common types of weigh screw feeders are loss-in-weight and continuous weighing sections that feed back control to a metering device. You description indicates the second type, whereby corrections to the in-transit screw are regulated by the variable speed metering screw. If this yields uneven output it is either because the control system is hunting, which is unlikely with a proprietary control system, or the bulk density of the material entrained in the metering screw varies.

    The design of the metering screw system is therefore suspect. This is said to be of non-mass flow operation, which raises questions as to whether the bin or the screw could be improved in construction. There is also a query about the design and operation of the conditioning screw mentioned, as this is obviously not doing its job properly. A constant head of product does not necessarily mean consistent discharge density, as the flow channel in a non-mass flow hopper can be very erratic and unstable. I would have to see detailed drawing of the system to examine how the situation is best improved.

    Regarding the given properties of the bulk material, these should be measured with representative samples of the specific product being used as generic values would not necessarily reflect the condition in the materials actually handled. Neither material would be classed as free flowing, but it would be interesting to learn why there is interest in these two values. Wall friction is a predominant determinant for the inclination of a hopper wall to generate mass flow, so has a useful, independent value. It is suspected that the problem in this instance may rest more with the feed screw design than the hopper shape and, in any event the screw/hopper interface is crucial to stable discharge conditions. Based on the particular geometry of the hoppers concerned there may also be some advantage in considering liners for the walls, in which case the wall friction of other reference materials should be considered if the current construction offers marginal conditions. Flow, as such, does not appear to be at issue; only the density at which the product emerges. A flow audit appears called for to, to evaluate the present behaviour and requirements for securing better results.

    Lyn Bates

  9. shear testers

    There are a number of shear testers for industrial application. I suggest you to look to our website www.ipt-online.com . We also organise every year course of powder techology - contact us for next course. There, you can learn about the shar test application in theoretical and practical classes. contact us for literature.

    regards

    dr Ivan Peschl

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