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Thread: Answers to 10 Common Questions about Storage Hoppers

  1. #1
    Author Guest

    Answers to 10 Common Questions about Storage Hoppers

    This is the discussion on the article
    Answers to 10 Common Questions about Storage Hoppers
    by Lyn Bates
    in the bulk-online column Ask Lyn...

  2. #2
    Henrik Larsson Guest
    Hi Lyn,

    I am very new to the topic of bulk-solids and was delighted to find this web portal, which is a great aid in increasing my knowledge.

    What lead me here is that one of the pieces of equipment I'm now required to operate is a Johanson Hang-up indicizer, so rather than just be a pair of hands I thought it best to find out as much as I could about its capabilities.

    Which is why I was surprised when I read the "Answers to 10 Common Questions About Storage Hoppers" that no mention was made at all about it, particularly in the section below.

    There is three parts to this design process. The first is to determine the orifice size needed for the outlet, second, fix the inclination of the mass flow wall angle and finally calculate the transition width and self clearing angle for the upper region.

    It is necessary to conduct a series of shear and wall friction tests on the bulk material and follow a design procedure, as originally set out by Andrew Jenike, to establish the required orifice size. The calculations take account of the shape of the opening and the geometry of the hopper. Details of this method is given in a publication by the Institution of Chemical Engineers entitled Standard Shear Testing Technique for Particulate Solids using the Jenike Shear Cell, (SSTT), and also in the ASTM Standard D 6128-00. You should be aware that the testing and interpretation is demanding of technical skills, therefore this full work normally rests in the domain of specialists. Wall friction measurements themselves are relatively easy to conduct and these measurements are invaluable for selecting an optimum contact surface, determining a reliable self-clearing angle for chutes and hopper walls and giving a good guide to the slope of walls needed to generate mass flow. No similar short cuts are available to predict a proven orifice size that will guarantee reliable flow. Some consolation is that a range of retrofit actions may be taken to stimulate outflow, but the situation is much more difficult to correct if the wall angle is wrong.
    This reply really peaked my interest. In all the literature I've read to date on the Johanson Hang-Up Indicizer it seems to suggest that is ideal for figuring out outlet orifice size and that the Jenike Shear test has limitations that the Johanson Hang-Up Indicizer addresses or overcomes.

    I'm interested to know what someone outside the products' literature has to say on this.

    Thanks for your time.

    Henke.

  3. #3
    Join Date
    Mar 03
    Posts
    1,641
    "In all the literature I've read to date on the Johanson Hang-Up Indicizer it seems to suggest that is ideal for figuring out outlet orifice size"

    If the literature is from the seller then to use the famous quotation - "They would say that wouldn't they"

  4. #4
    RPD - Invista (UK) Ltd., U.K. Guest
    There is a great deal of nervousness amongst many silo designers about trusting the indicizer. My understanding is that this is on the basis that how it works and the theory behind it is has never been properly explained.

    Quoting from one comparitive review of different testers (http://www.imvt.tu-bs.de/files/Ifpri...er%20jenike%22):

    "Since the state of stress in these testers is not exactly known, at least not in the second step of a shear test, the results of those tests cannot be used without further assumptions to get a flow function and thus their results cannot be recommended for silo design."

    Academics like to understand things, practioners like to find something which is simple to use. People understand the Jenike design basis, although I have never used one I am led to believe that it is much easier to use.

    Which one you feel most comfortable with probably depends whether your personal tendancy is towards the academic or practitioner side.

    As far as relative accuracy of the systems is concerned, I will leave that to others who have actually done the comparisons.

  5. #5
    RPD - Invista (UK) Ltd., U.K. Guest
    Sorry, the reply below should have said:

    People understand the Jenike design basis, although I have never used one I am led to believe that the Indicizer is much easier to use.

  6. Johanson Indicizer

    I have the very greatest respect for Jerry Johanson as one of the principle pioneers of bulk technology. His introduction of the indicizer is based on many years of experience in this field and a fundamental understanding of the theory of particulate solids in relation to hopper design. He has seen to heed for a simpler industrial tool to evaluate the flow properties of a bulk material and uses a technique that has a quite separate approach to the Jenike method, and hence does not give a comparable answer.

    It must be remembered that the Jenike calculations are based on a developed flow stress field, that is, the conditions pertaining when flow has already taken place and then stopped. From this situation, the strength of the Jenike test procedure is that it can specify the conditions that are required to guarantee flow in respect of orifice size and hopper wall angle based on the yield loci in a specifically prepared state of density that reflects the operating condition of interest.

    An inherent weakness of the basic method is that it cannot relate to initial fill conditions and features such as impact surcharges, this is why considerable expertise is needed by specialists to provide the interpretation and reliability in important installations.

    The Johanson device essentially measures the shear strength of a prepared compact that, in effect, is similar to the mechanics of a consolidated bulk material over an orifice. The situation there is that self weight will induce collapse if the opening size is large enough, whatever the strength of the product, because the vertical stress above the orifice boundary increases linearly with diameter. Unfortunately for clarity, the Johanson apparatus is linked to a 'black box and unsubstantiated formulae for the production of predictions, and is therefore not well accepted by the scientific community. This test is however easy to conduct and produce results, so has become a useful tool for industry. A simpler piece of kit that adopts this form of test and also publishes that basis on which it is founded is the Ajax vertical shear test. See www.ajax.co.uk for a description and the fundamental calculations. As a 'cheap and cheerful' tool, this later device is invaluable for an initial evaluation of a products potential to arch and rathole and, within the limitations that are readily apparent, will provide a guide to the size of opening necessary to stimulate initial flow from a settled bed. It is not however a substitute or comparable method with the Jenke test, but reflects a different situation that may well be of less or more interest.

    A deep and sensitive understanding is required to recognise the implications of the assumptions, equipment limitations and weaknesses and myriad variations in particle composition and packing structures in relation to powder testing equipment. I have considered this subject for many years and designed interesting devices for measuring tensile and cohesive strength. It has to be accepted that solids flow is a complex phenonenon and take these tests with the limitations they have. The proof of the pudding is in the eating and, if it works, stick with it and your experience will grow. If it's important and your inexperienced, consult an expert.

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