For silo design it is recommended to characterize the powder with a shear tester, i.e. a quantitative tester like the Jenike Shear Tester or the Schulze Ring Shear Tester. You will find more detailed information on the principles of powder testing, on silo design and on appropriate equipment for powder testing on my homepage www.dietmar-schulze.de.

Why is it recommended to run shear tests? Because a powder cannot be characterized only by its name or particle size. The flow properties of a powder, i.e. internal friction, bulk density, unconfined yield strength, and time consolidation (= caking), are dependent on several parameters. Therefore, whatever you guess without testing may result in an inappropriate silo design.

Best regards

Dietmar Schulze ■

Silo design involves many different factors and Dietmar lists several important measurements that must be taken for materials that offer difficult flow characteristics. In practice a lot of common products do not present great flow problems but, unless the designer has enough practical experience with the specific material, it is not readily apparent what problems may arise.

In any event, a primary decision is whether the silo is to be of mass flow or non-mass flow construction. This choice may be dictated by the unstable nature of the product over time, the segregating characteristics of the material or by the flow benefits that are offered by this mode of discharge. Wall friction tests are essential in these circumstances and these, for their relatively small cost, are useful as a check on the slip behaviour for self-clearing in non-mass flow silos.

Bulk measurement values are also relatively simple and should similarly reflect the conditions of application. Bulk density is a key parameter for stimulating flow, apart from its relation to storage capacity, so controlled measurements in relevant conditions of preparation are also needed.

Aspects of caking, sintering and similar bonding mechanisms are usually determined independently, as these can negate any prospect of flow and demand special attention.

There is a large pragmatic aspect to bulk flow considerations. The usual industrial dilemma is assessing the shear strength potential of an unknown bulk material because few users can justify the cost of the equipment and training for sparse use and that of independent tests are not insignificant. Although testing costs are nothing like the cost of being wrong, they are an expense and it is worth considering if and when then may be required.

Many users find to their cost that basing a silo or hopper design on the movement of powder in a bag bears no relation to how it behaves under confined compaction. A hand ‘snowball’ test can often provide an early warning of potential difficulties. A more refined, but simple, evaluation can be conducted by loading a sample for a period and seeing whether it holds together strongly. Going a bit further, a load/compaction graph or a vertical shear test will quickly give a measurement of this resistance to deformation or shear in compacted conditions. These tests are no substitute for Jenike silo design parameters but, with limited experience and low cost, can give a clear indication of the degree to which further testing may be necessary. They will also direct attention to the fundamentals of silo design and focus interest to the hazards to be avoided. 'Simple' testing needs to be approached with care as simple tests can only produce simple answers. This does not mean that the answers are necessarily not important.

A budding silo designer also needs parallel education in the benefits of differing container geometry and discharge mechanisms. A design chart indicating the various key decision steps and choices in hopper and silo design is given on our web site, www.ajax.co.uk, together with hopper geometry’s, capacities, weights, typical bulk densities and other relevant data, hints and technical articles on flow and feeding problems. There is a host of information available on the subject. The most common mistakes are to assume that flow is not a problem and that simple construction or low capital cost fabrication means low tech.

Lyn Bates ■

Dear Mr. Stekke,

You thesis is extremely interested and I will try to help you.

I send you a literature list, where you can find the newest developments in the silo technology and the characterization of powder by a shear test. I also adwize you yo follow the course “QUALITY CONTROL AND POWDER TECHNOLOGY which will be given in November 2003. shortly itthe program and application form will be on our website

WWW.IPT-ONLINE.COM

CALL ME IF ANY HELP NEEDED

BEST REGARDS

Dr. Ivan Peschl

Enclosure: 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 Schäfer

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. Qualitätskontrolle 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 ■