A Pauper's Powder Testing Kit

by Lyn Bates


Measuring the physical properties of bulk materials for flow characterisation is generally thought to be an expensive process involving skilled technicians, specialised equipment and demanding an expert evaluation of the results. As a rigorous process, this is true and no one should decry the work and effectiveness of experts in this field. However, there is little to stop anyone with a rudimentary knowledge of mechanics and stress conditions from deriving rough values of key design parameters that will serve as an indication of the degree of difficulty than may be expected in handling a product and securing measured values that can be used in a conservative design for non-critical applications. The sizes and material of construction the items listed for this kit are not ‘cast in stone’, but are suggested as typical to show the principles involved and their practicality for bench scale testing.

The main features of interest are: -

• Bulk density, in stated conditions of preparation.
• Wall Friction, against a given contact surface.
• Bulk strength, in various states of consolidation.
• Any propensity to segregate may be of interest.
• Time effects, in ambient conditions to be experienced.


An important first step is to secure a sample or samples of the bulk material that reflect the worst or range of conditions that have to be accommodated. It should be noted that ‘worst’ condition for some aspects of handling, e.g. potential for arching, is the best to minimise loads on feeders, and visa versa, so the objective of testing can influence the choice of sample condition. Attention should be given to possible differences from multi-source origin or consistency of stock, the affect of season, climate or plant ambient conditions, moisture variations, residence times in process and exceptional circumstances that may arise in the working lifetime of the plant.

A two litre sample is usually adequate for a normal series of tests.

It should first be noted that a particulate solid does not have a unique bulk properties that can be expressed by a single value, although some materials vary considerably more than others with fine products being more variable. The measurement should reflect the value with the material in the plant condition of interest, e.g. loose fill from a state of free fall or pneumatic conveyor delivery, agitated in a mixer or screw conveyor or settled in storage.

Bulk Density Basic requirements for bulk density and settling tests.

Item 1 Plastic One Litre measuring cylinder. (from chemist or home brewing shop).

Item 2 Watch with second hand. (stop watch or wrist watch)

Item 3 Weighing device to One Kilogram. (Kitchen scale).

Basic Test Half fill the litre measuring cylinder with weighed sample, cover top and shake vigorously, then place on a table and record the immediate level, then note rate of settling and time taken to attain a stable value.

Settled Density Test Measure loose settled volume when condition stabilises.

Tapped Density Test Raise the measuring cylinder nom 25 mm and drop it onto hard surface 20 times.

The Final volume/loose settled volume = the Hausner Ratio, a crude measure of ‘flowability’, indicating that the material will compact and gain strength with consolidation under load. A value greater than 1.25 is generally taken as an indication of poor flow prospects.

Compaction tests Place a loose fitting disc over the measuring cylinder contents and apply a load with a length of bar of a weight that replicates the compacting conditions of interest. The Load must be applied slowly, not dropped onto the sample. Check the change in volume from the normal settled condition. Increasing loads can be applied for a load/compaction graph to be produced.

Results Sample weight/volume = Bulk density in measured condition.

Fluidity Test Plot the volume change over time. Ensure the sample is at the temperature of the situation under consideration. If initial behaviour is fluid-like, the void pressure is hydrostatic so the gas pressure gradient can be calculated. The change from initial to totally settled volume shows the change in void volume, hence the total volume of excess air that has escaped.

Wall Friction Basic requirements for wall friction tests

Item 4 Wood or similar flat piece approx. 150 x 20 x 600 mm long.

Item 5 10 swg Hot rolled sheet of Mild Steel approx. 150 x 20 x 600 mm long.

Item 6 10 swg 2B finish 304 quality, Stainless Steel approx. 150 x 20 x 600 mm long.

Item 7 6 or 10 mm thick UHMDPE sheet approx. 150 x 20 x 600 mm long.

Item 8 Protractor.

Item 9 Nom 15 mm length of 50 dia. Plastic drain pipe.

Item 10 Nom 55 mm length of 100 dia. Plastic drain pipe

Item 11Various weights


Wall Friction Test

This is the most common performance-influencing feature of a bulk material and applies to every application involving the use of loose solids. This should be a routine test on every project as, apart from its design use, it is an important reference factor in the categorisation of the bulk material. Bulk material is required to slip on many static and dynamic surfaces, such as the walls of hoppers, chutes, screw conveyor and feeder flights, mixer blades and many container boundary surfaces in both flooded and unconfined conditions. Contact friction is a prime design parameter for mass flow hopper design, chutes and self-clearance wall inclinations in bins and the like and its measurement highlights any tendency for surface cohesion and sticking to surfaces and in corners. The wall frictional forces that resist slip reduce the weight of the upper hopper contents compacting the bulk solid below; the higher the wall friction value, the lower are the compacting stresses that act on the stored product. These generally attain a limiting value at a bed depth of about four vessel diameters, in line with the Janssen equation.

Test Fill one of the short plastic cylinders with the sample and rest on one end of the contact surface to be used that is backed up by the flat wood. Slowly raise the end of the wood until the sample commences to slip down the slope. Reduce the slope slightly and find and inclination at which the sample just sustains slip when promoted to move. Compare this angle with that necessary to initiate slip, for values of static and dynamic wall friction. Repeat with different weights placed on the sample, not on the plastic cylinder.

Plot the results of the total load that is acting at 90 degrees to the slope, against the angle of slip to show the angle of wall friction.

An intersect showing a positive resistance to slip at zero applied load indicates a cohesive attraction between the material and the contact surface.

Repeat for alternative materials of contact to establish the optimum material of construction. (Note that differing finishes of stainless steel may give improved slip values. Various polishing grits, from dull polish to mirror finish may be used, as well as mill finish and electro polish.

Results Select appropriate contact surface for the application. Use the value to optimise the surface inclination of the equipment under consideration.

Caution It should be noted that slip taking place on contact surfaces over extended service conditions are likely to cause wear and change in the condition of the surface. Whereas this is often favourable in circumstances where the initial condition is rougher or soft, as with a painted finish, some prudence is advised when considering special coatings; especially when they are soft and thin, such as PTFE, or in situations that are sensitive to value changes.


Bulk Strength

A bulk material has to deform to flow and the ability of a bulk material to oppose deformation is determined by its bulk strength, which is measured by its resistance to shear. This property is highly variable, according to both how firmly it is compacted and by the loading conditions to which it is subjected. For example, a loose, unconfined powder can be poured to adopt the shape of the receiving vessel because the mass is sufficient dilate to shear with minimum resistance. However, the same product settled in a large silo may bridge over a wide outlet opening because the particles have assumed a close-packed structure and are compacted by the superimposed mass to resists their bulk expansion or relative motion.

In order to establish a meaningful value of bulk strength it is necessary to take account of both the current stresses that are acting on the bulk and the stress history that lead to their present structural formation and closeness of packing. In practice, the prevailing state of density reflects the relevant consequences of the bulk’s historical conditioning, so an accurate measure of this value is first needed to set against a value of its bulk strength. The second stage is to measure what force is required to shear material in this condition when it is either unconfined, or is confined under a given normal load, such as that created by the overburden of material in a bulk storage facility.

The unconfined failure conditions are highly relevant to the ability of the bulk material to sustain an ‘arch’ over the outlet of a hopper, because the underside of such an arch is not confined. If the forces acting on the surface of the arch exceed the unconfined failure stress, the arch will collapse and the hopper contents flow out to discharge. In a similar manner, the inner wall of a ‘rathole’ has an unconfined surface, so the stresses acting in this location must exceed those of the unconfined failure strength for the stability of the rathole to break down.

Bulk Strength Basic requirements for bulk strength tests (Items 12 & 13, or 14 & 15 used)


Item 11 Various weights.

Item 12Nom 150 mm length of 50 dia. Plastic drain pipe.

Items 13 One 150 mm and one 50 mm length of bar that slide in 50 mm dia. pipe.

Item 14 Nom 250 mm length of 100 dia. Plastic drain pipe.

Items 15 One 250 mm and one 100 mm length of bar that slide in 100 mm dia. pipe.


Unconfined Failure Test

Depending whether the test is to measure a fine powder or a coarse and perhaps damp product, the longer of items 13 or 15 is fixed at end to stand vertically and item 12 or 14 placed on top, to overlap the top 20 mm of the bar. (The tube to be held in position by a prop to the bench or a peg through the upstanding bar).

A weighed sample is filled in the tube to occupy about 110 mm depth of the 50 dia. tube, or 220 mm depth of the 100 dia. Tube. The short bars should be marked to show an insertion of 30 mm in the tube. Place the short bar in the top end of the tube and apply a weight to compact the tube contents. If the bar does not enter to the marked position, remove and add an extra small, weighed quantity to the tube and repeat until the mark is just covered.

(For a more refined value, the value of the applied weights should be reduced by the weight of the tube and its contents and the supporting prop removed so that the lower bas can exert an end force equal to that which was developed at the top end of the tube.)

Remove the weight and top bar, then remove prop or peg supporting of tube and slide the tube down over the lower bar to fully expose the tube contents. If the column of material is self-supporting, the bulk material has cohesive strength and Item 1, the one litre measuring jar, should be balanced on the top of the column and sand slowly added to the measuring jar until the column collapses.

The total weight required to collapse the column gives a measure of the bulk shear strength in the known density condition of consolidation.

This uniaxial unconfined failure test is conducted over a range of consolidation stresses
and the flow function of the material, (ff), is constructed by plotting the unconfined failure strength versus the consolidation stress.

The powder flow function, ff, is defined as:

Major Principal Consolidation Stress σ1
Unconfined Failure Strength Fc


The value of the flow function is a measure of the bulk material’s ‘flowability’, the greater the ratio of the flow factor (ff), the more free-flowing is the powder.

Broad classification: -

ff<1 Non flowing, without special assistance.
1<ff<2 very cohesive, will prove difficult to promote flow
2<ff<4 cohesive, will normally require some assistance to secure flow
4<ff<10 easy flowing, is generally easy to handle.
10<ff free flowing, probably has a tendency for fractions to segregate.

Jenike Bull 123 [2] indicates the wall inclinations for mass flow of cones and wedge shaped hoppers and also how the value of unconfined failure strength pertains to the opening size needed to prevent arching.

Segregation Basic requirements for segregation tests.

Item 16 Two clear, rigid plastic A4 size sheets and two 25 mm wood pieces 210 mm long. The wood to be glued or screwed between each long end of the plastic sheets to form and open channel between the wood of nom 25 x 250 mm cross section.

Place item on its side on a flat surface.

Test Slowly, loose-pour the sample material into the top of one end of the channel to allow a repose condition to build up to the top surface of the chamber.

Tip the apparatus at an angle and withdraw to allow the contents to spread out over the bench.

Isolate a layer from the inclined surface and compare composition of regions from 10% from each end.

A virtue of hands-on powder testing is that a ‘feel’ is developed for the powder behaviour. A few hours spent ‘playing’ with the material can show massive savings at the commissioning stage of a project, by highlighting potential problems at the design stage.

There are many factors to take into account when selecting hopper shapes and types of feeders, but a major consideration is the behaviour characteristics of the material to be handled. These cannot be reliably secured from data bases or written description of the bulk material and quantified values of the specific product to be handled are essential for optimum design.

References:

1. "Guide to the Specification of Bulk Materials for Storage and Handling Applications", Bulk Materials Handling Committee. I.Mech.E.

2. "Storage and Flow of Solids", Bull. 123 Univ. of Utah. Exp. Station. Vol. 53, No. 26 1964. Revised 1980.

3. "On powder Flowability", James Prescott and Roger Barnham - (A Jenike and Johanson publication).

4. "Characterisation of bulk materials, Industrial practice", Paper available from lyn@ajax.co.uk




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