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Modern Technological Developments in the Storage and Handling of Bulk Solid

A. W. Roberts

Director, School of Engineering, Director TUNRA Ltd,

The University of Newcastle, NSW, Australia

Acknowledgements : Bionic Research Institute ‐ Chute Design Conference 1992

SUMMARY

This paper presents an overview of some modern developments in the technology of bulk solids handling. An overv

of storage system design including bins and gravity reclaim stockpiles is presented and aspects of feeder performanc

given. The importance of wall or boundary friction in hopper and chute design is discussed and the associated adhes

and wear characteristics are outlined in relation to the selection of appropriate lining materials. Problems due to f

instabilities during discharge from coal bins are reviewed; these flow pulsations may give rise to severe dynamic loads

the bin structure.

1. INTRODUCTION

Throughout the world bulk materials handling operations perform a key function in a great number and variety

industries. While the nature of the handling tasks and scale of operation vary from one industry to another and, on

international scene, from one country to another according to the industrial and economic base, the relative cost

storing, handling and transporting bulk materials are, in the majority of cases, very significant. It is important, theref

that handling systems be designed and operated with a view to achieving maximum efficiency and reliability .Dire

related to these objectives is the ongoing need for engineers and those involved in the operation of handling plant

be kept informed of the latest research and technological developments relevant to their industry and, at the sa

time, contributing to these developments and to the dissemination of information in the light of their own experience

The theme embodied in the foregoing remarks is of particularly relevance to Australia in view of the heavy depende

on bulk handling operations. While these operations range across the broad spectrum of industries, of prime importa

are the mining and mineral processing industries which handle coal and mineral ores in large tonnages. These indust

make a major contribution to Australia's export earnings and economic growth.

Over the past three decades much progress has been made in the theory and practice of bulk solids handling. Relia

test procedures for determining the strength and flow properties of bulk solids have been developed and analyt

methods have been established to aid the design of bulk solids storage and discharge equipment. There has been w

acceptance by industry of these tests and design procedures and, as a result, there are numerous examples through

Australia of modem industrial bulk solids handling installations which reflect the technological developments that h

taken place.

Notwithstanding the current situation, the level of sophistication required by industry demands, in many cases, a be

understanding of the behaviour of bulk solids and the associated performance criteria for handling plant des

Experience indicates that the solution one problem which leads to an improvement in plant performance often expo

other problems which need to be solved. It becomes progressively clearer that there are many gaps in the present st

of knowledge where further research is necessary.

The purpose of this paper is to briefly highlight the present state of knowledge associated with bulk handling

indicate where further work is necessary. The material presented is based on the research conducted by Tunra B

Solids Handling Research Associates of the University of Newcastle. This research group has been involved in b

handling research and industrial consulting for some considerable time and in recent years has been .supported

research grants obtained from AMIRA.



2. HANDLING PLANT DESIGN ‐ BASIC CONCEPTS

2. 1 General Remarks

The design of handling plant, such as storage bins, gravity reclaim stockpiles, feeders and chutes is basically a four s

process:

i. Determination of the strength and flow properties of the bulk solids for the worst likely flow conditi

expected to occur in practice.

ii. Determination of the bin, stockpile, feeder or chute geometry to give the desired capacity, to provide a f

pattern with acceptable characteristics and to ensure that discharge is reliable and predictable.

iii. Estimation of the loading on the bin and hopper walls and on the feeders and chutes under opera

conditions.

iv. Design and detailing of the handling plant including the structure and equipment.

The general theory pertaining to gravity flow of bulk solids and associated design procedures are fully documented [1

For the purpose of the present discussion, the salient aspects of the general philosophy are briefly reviewed.

2.2 Modes of Flow in Bins of Symmetrical Geometry

As is now well established, there are two basic modes of flow, namely, mass‐flow and funnel‐flow. These are illustra

in Figure 1.

In mass‐flow, the bulk solid is in motion at every point within the bin whenever material is drawn from the outlet. Th

is flow of bulk solid long the walls of the cylinder (the upper parallel section of the bin) and the hopper (the lo

tapered section of the bin). Mass‐flow guarantees complete discharge of the bin contents at predictable flow rates.

as a first‐in, first‐out flow pattern; when properly designed, a mass‐flow bin can re‐mix the bulk solid during discha

should the solid become segregated upon filing of the bin. Mass‐flow requires steep, smooth hopper surfaces and

abrupt transitions or in‐flowing valleys.

Mass‐flow bins are classified according to the hopper shape and associated flow pattern. The two main hopper types

conical hoppers which operate with axi‐symrnetric flow and wedged‐shaped or chisel‐shaped hoppers in which plaflow occurs. In plane‐flow bins, the hopper half ‐angle a will usually be, on average, approximately 8 to 10 larger than

corresponding value for axi‐symmetric bins with conical hoppers.

Figure 1. Modes of Flow

Therefore, they offer larger storage capacity for the same head room than the axi‐symmetric bin, but this advantag

somewhat offset by the long slotted opening which can give rise to feeding problems. The transition hopper, which

plane‐flow sides and conical ends, offers a more acceptable opening slot length. Pyramid shaped hoppers, while sim



to manufacture, are undesirable in view of build‐up of material that is likely to occur in the sharp corners or in‐flow

valleys. This may be overcome by fitting triangular‐shaped gusset plates in the valleys.

Funnel‐flow occurs when the hopper is not steeply sloped and the walls of the hopper are not smooth enough. In

case, the bulk solid sloughs off the top surface and falls through the vertical flow channel that forms above the open

Flow is generally erratic and gives rise to segregation problems. Flow will continue until the level of the bulk solid in

bin drops an amount HD equal to the draw‐down. At this level, the bulk strength of the contained material is suffic

to sustain a stable rat‐hole of diameter Df as illustrated in Figure 1(b). Once the level defined by HD is reached, ther

no further flow and the material below this level represents 'dead' storage. This is a major disadvantage of funnel‐fl

For complete discharge, the bin opening needs to be at least equal to the critical rathole dimension determined at

bottom of the bin corresponding to the bulk strength at this level. However, for many cohesive bulk solids and for

normal consolidation heads occurring in practice, rat‐holes measuring several meters are often determined. This ma

funnel‐flow impracticable. Funnel‐flow has the advantage of providing wear protection of the bin walls, since

material flows against stationary material. However it is a 'first‐in last‐out' flow pattern which is unsatisfactory for b

solids that degrade with time. It is also unsatisfactory for fine bulk solids of low permeability. Such materials may aer

during discharge through the flow channel and this can give rise to flooding problems or uncontrolled discharge.

The disadvantages of funnel‐flow are overcome by the use of expanded‐flow, as illustrated in Figure 2. This combi

the wall protection of funnel‐flow with the reliable discharge of mass‐flow. Expanded‐flow is ideal where large tonna

of bulk solid are to be stored. For complete discharge, the dimension at the transition of the funnel‐flow and mass‐f

sections must be at least equal to the critical rathole dimension at that level. Expanded‐flow bins are particulsuitable for storing large quantities of bulk solids while maintaining acceptable head heights. The concept of expand

flow may be used to advantage in the case of bins or bunkers with multiple outlets.

Figure 2. Expanded Flow

Generally speaking, symmetric bin shapes provide the best performance. Asymmetric shapes often lead to segregat

problems with free flowing materials of different particle sizes and makes the prediction of wall loads very much m

difficult.

2.3 Mass‐Flow and Funnel‐Flow Limits for Symmetrical Bins

(a) Established Theory due to Jenike

The mass‐flow and funnel‐flow limits have been defined by Jenike on the assumption that a radial stress field exist

the hopper [1,2]. These limits are well known and have been used extensively and successfully in bin design. The lim

for axi‐symmetric or conical hoppers and hoppers of plane‐symmetry depend on the hopper half ‐angle α, the effec

angle of internal friction 8 and the wall friction angle Φ. Once the wall friction angle and effective angle of inte

friction δ have been determined by laboratory tests, the hopper half ‐angle may be determined. In functional form

α = ( Φ,δ ) ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (1)

The bounds for conical and plane‐flow hoppers are plotted for three values of δ in Figure 3. In the case of conical or

symmetric hoppers, it is recommended that the half ‐angle be chosen to be 3 less than the limiting value. For plane‐fl



the bounds between mass and funnel‐flow are much less critical than for conical hoppers. In plane‐flow hoppers, m

larger hopper half angles are possible which means that the discharging bulk solid will undergo a significant chang

direction as it moves from the cylinder to the hopper.

For plane‐flow, the design limit may be selected; if the transition of the hopper and cylinder is sufficiently radiused

that the possibility for material to build‐up by adhesion is significantly reduced, then a half ‐angle 3 to 4 larger than

limit may be chosen.

Figure 3. Limits for Mass‐flow for Conical and Plane‐Flow Channels

(b) Modification to Mass‐Flow Limits ‐More Recent Research

Since in the work of Jenike, flow in a hopper is based on the radial stress field theory, no account is taken of

influence of the surcharge head due to the cylinder on the flow pattern developed, particularly in the region of

transition. It is been known for some time that complete mass‐flow in a hopper is influenced by the cylinder surcha

head. For instance, there is a minimum level Hcr which is required to enforce mass‐flow in the hopper [5]. For the m

flow bin of Figure 1(a), this height ranges from approximately O.75D to 1.0 D.

More recent research has shown that the mass‐flow and funnel‐flow limits require further explanation and refinem

For instance, Jenike [6] published a new theory to improve the prediction of funnel‐flow; this led to new limits

funnel‐flow which give rise to larger values of the hopper half ‐angle than previously predicted, particularly for hvalues of the wall friction angle. In the earlier theory, the boundary between mass‐flow and funnel‐flow was based

the condition that the stresses along the centre line of the hopper became zero. In the revised theory the flow bound

is based on the condition that the velocity becomes zero at the wall.

In a comprehensive study of flow in silos, Benink [7] has identified three flow regimes, mass‐flow, funnel‐flow and

intermediate flow as illustrated in Figure 4. Whereas the radial stress theory ignores the surcharge head, Benink

shown that the surcharge head has a significant influence on the flow pattern generated. He derived a fundame

relationship for Hcr in terms of the various bulk solid and hopper geometrical parameters, notably the H/D ratio of

cylinder and the effective angle of internal friction δ. Benink developed a new theory, namely the arc theory, to quan

the boundaries for the three flow regimes. This theory predicts the critical height Hcr at which the flow changes.

Figure 4. Flow Regimes for Plane‐Flow Hopper defined by Benink [7]



2.4 Bin Geometry for Mass‐Flow

(a) Basic Considerations

Basically the aim in mass‐flow design is to determine the hopper geometry to give reliable flow at all times at

required discharge rate. Primarily, the requirement is to determine the hopper half angle α, which defines the slope w

respect to the vertical, and opening dimension B.

Undisturbed storage time and changes in moisture content can significantly influence the unconfined yield strength

the bulk solids. By way of illustration, the critical hopper opening dimension B for three Hunter Valley coals plotted

function of moisture content are shown in Figure 5. This figure shows three coal samples, Sample (1) being a Raw O

Cut Coal, Sample (2) a washed version of (1) and Sample (3), a blend of (2). The high strength of the raw, unwashed c

is clearly evident. Experience has shown that the peak bulk strength of coal may occur at a moisture content somewh

between 70% and 90% of the saturation limit.

(b) Influence of Wall Friction

Since an increase in both the normal wall pressure and consolidation pressure accompany an increase in hopper sp

then the corresponding decrease in wall friction angle will permit the hopper half angles to be increased. Hence

possible to calculate a hopper half angle α as a function of hopper span or opening dimension as indicated in Figure 6

shown, the half hopper angles for both wedge and conical hoppers tend towards limiting values as the opendimension increases.

2.5 FUNNEL‐FLOW AND EXPANDED‐FLOW GEOMETRY

As previously discussed, it is necessary to compute the critical or minimum diameter Df for an unstable pipe or "

hole" from which the minimum bin opening for funnel‐flow or the transition dimension for expanded‐flow

determined. The transition dimension for expanded‐flow refers to the transition of the mass‐flow hopper with the up

funnel‐flow section of the bin.

Figure 5 Critical Opening Dimension BCR as a Function of Moisture Content for Three Coal Samples ‐Stainless Stee1304‐2B Linin

Figure 6 Variation of Hopper Half ‐Angle with Span for Coal on 304‐2B Stainless Steel



3. BIN WALL LOADS

In bin design, the prediction of bin wall loads continues to be a subject of some considerable complexity. In view o

obvious importance it is a subject that has, in recent years, attracted a good deal of research effort. Currently, there

several research groups in various countries of the world directing their attention to the study of bin wall loads usin

range of analytical and numerical techniques such as those involving finite element analysis. Despite the widely vary

approaches to the analysis of bin wall loads, it is clear that the loads are directly related to the flow pattern develope

the bin.

The flow pattern which a mass‐flow bin exhibits is reasonably easy to predict and is reproducible. However, in fun

flow bins the flow pattern is more difficult to ascertain, especially if the bin has multiple outlet points, the loading of

bin is not central and/or the bulk solid is prone to segregation. Unless there are compelling reasons to do otherwise,

shapes should be kept simple and symmetric.

3.1 Wall Pressures in Mass‐Flow Bins

In mass‐ flow bins, the pressures acting normal to bin walls vary from the static or filling conditions to the dynami

flow conditions. The pressure distributions are well defined and, using current theories [3‐4], may be predicted w

confidence.

It is to be noted that in the flow situation a high switch stress occurs at the transition. The magnitude of this sw

stress is several times the corresponding static value. Further, it may also be noted that the wall pressures acting in

cylindrical section during flow may be higher than the static values. For a perfectly parallel cylinder, the wall pressu

during flow would be the same as the static values. However, when imperfections such as weld projections or p

shrinkage give rise to flow convergences, peak stresses occur. The stresses are taken into account by computing

locus of all such possible peak pressures.

3.2 Wall Pressures in Funnel Flow Bins

While for design purposes wall pressures in symmetrical funnel‐flow bins may be determined with a high degree

confidence, the wall loading in bins with multiple outlets and eccentric discharge points are far more difficult

estimate. Under eccentric discharge, the walls are subject to bending moments and hence, bending stresses in additto hoop stresses [8]. In the case of tall grain silos, the use of anti‐dynamic tubes offers significant advantage

controlling the wall pressures, both in the case of symmetrical funnel‐flow silos as well as silos with eccentric load

points [9‐10].

3.3 Australian Standard for Loads in Bulk Solid Containers

In recent years there has been considerable activity in several countries of the world in the development of new

revised codes for bin wall loads. Of particular note is the preparation of the new Australian Standard "AS‐89138 Lo

for Bulk Solids Containers " [11], which represents a major milestone. This publication presents a very comprehen

review of the loads acting in bin and silo walls under a the full range of operating conditions likely to occur in practice

an example, Figure 7 shows the wall loadings determined on the basis of this new Standard for a large coal bin havseven outlets; the pressure profiles correspond to one possible mode of discharge involving the operation one eccen

outlet only.



Figure 7. Circumferential Pressure Variation due to Operation of One Eccentric Outlet

4. FEEDING OF BULK SOLIDS

4.1 Use of Feeders to Control Discharge

In general, a feeder is a device used to control the flow of bulk solids from a bin. While there are several types of feed

commonly used, it is essential that they be selected to suit the particular bulk solid and the range of feed rates requi

It is particularly important that the hopper and feeder be designed as an integral unit so as to ensure that the flow fr

the hopper is fully developed with uniform draw of material from the entire hopper outlet. For example, in the case screw feeder, this is achieved by using selected combinations of variable pitch, variable diameter and variable core

shaft diameter.

In the case of a belt or apron feeder, a tapered opening is required as illustrated in Figure 8. The use of vert

triangular plates in the hopper bottom are an effective way to achieve the required taper. The gate on the front of

feeder is used only for flow trimming and not for controlling the flow rate. The height of the gate is adjusted to give

required release angle Ψ to achieve uniform draw along the slot. Once correctly adjusted, the gate is then fixed

position and the feed rate is controlled by varying the speed of the feeder.

Figure 8. Belt and Apron Feeder

In the case of vibratory feeders, there is a tendency for feed to occur preferentially from the front. To overcome

problem, it is recommended that the slope angle of the front face of the hopper be increased by 5 to 8 as illustrate



Figure 9. Alternatively, the lining surface of the front face in the region of the outlet may selected so as to have a hig

friction angle than the other faces.

Figure 9. Vibratory Feeder

4.2 Determination of Feeder Loads and Power

The determination of feeder loads and drive powers requires a knowledge of the stress fields generated in the hop

during the initial filling condition and during discharge. Under filling conditions, a peaked stress field is generathroughout the entire bin as illustrated in Figure 10. Once flow is initiated, an arched stress field is generated in

hopper and a much greater proportion of the bin load is supported by the hopper walls. Consequently, the load ac

on the feeder substantially reduces as shown in Figure 10.

Figure 10. Load Variations on a Feeder

It is quite common for the load acting on the feeder under flow conditions to be in the order of 20% of the initial lo

The arched stress field is quite stable and is maintained even if the flow is stopped. This means that once flow is initia

and then the feeder is stopped while the bin is still full, the arched stress field is retained and the load on the fee

remains at the reduced value. The subject of feeder loads is discussed in some detail in Refs. [12‐15]. The loads

feeders and the torque during start‐up may be controlled by ensuring that an arched stress field fully or partially ex

in the hopper just I prior to starting. This may be achieved by such procedures as:

Cushioning in the hopper, that is leaving a quantity of material in the hopper as buffer storage.

Starting the feeder under the empty hopper before filling commences.



Raising the feeder up against the hopper bottom during filling and then lowering the feeder to the operating condit

prior to starting. In this way an arched stress field may be partially established.

5. GRAVITY RECLAIM STOCKPILES

5.1 Draw‐Down Performance Considerations

Gravity reclaim stockpiles when properly designed, operate under expanded‐flow, as illustrated in Figure 11. This sho

discharge through a single opening in a stockpile. Discharge will take place by funnel‐flow in the main body of

stockpile, with the flow expanded through the mass‐flow hopper. In this way, reliable flow to the feeder is assured. F

will continue until the draw‐down head hD is reached; flow then ceases as a stable pipe or rathole is formed. The dr

down is consistent with critical rathole dimension Df which forms at that level. The shape of the rathole depends on

consolidation conditions within the stockpile, the particle or lump size range of the stored bulk solid and the moist

content.

Figure 11. Draw‐Down in Stockpile

Complete draw‐down, as illustrated in Figure 11, corresponds to the critical rathole dimension Dfm at the base of

stockpile. For complete draw‐down to occur, it is necessary for the diagonal dimension of the hopper transition to b

least equal to Dfm Since values of Dfm may be several meters, often it is not practical or economical to employ a la

enough hopper to achieve complete draw‐down. For this reason, the design of stockpile reclaim hopper and fee

systems requires a full consideration of the various options available with a view to optimizing the reclaim performa

within specified practical and economic limits.

5.2 Use of Multiple Hoppers

The use of multiple hopper systems which allows for intersection of the flow channels to occur, as illustrated in Fig

12, provides for good reclaim performance to be achieved. By varying the separation distance X, an optimum spac

can be established as illustrated in Figure 13.

5.3 Live Capacity versus Moisture Content

In a programme of research conducted at the University of Newcastle 16,17], studies have been performed usin

conical stockpile model which allowed different feeder configurations to be examined. Although the scale of the mo

relative to actual stockpiles is very small (a factor of 1/50 in one case of an iron ore stockpile), the predic

performance base on the model studies were surprisingly good. The modeling process involves scaling the particle

and adjusting the moisture content of the bulk solid to reproduce, as close as possible, the same arching characteris

in the model feed hoppers as would occur in the full scale stockpile.



Figure 12. Improved Reclaim Performance using Double Reclaim System

Figure 13. Live Capacity versus Feeder Separation

By way of illustration, Figure 14 shows the reclaim performance for a double hopper system for five different c

moisture contents. Several tests were conducted over a range of hopper separation lengths. The separation of

hopper is measured by the distance S between the inner edges of the two hoppers which are equi‐distant on each s

of the stockpile centreline.

The data in Figure 14 show the reduction in live capacity with increase in moisture. This is to be expected because

strength of the bulk solid increases with moisture content. The results also show that there is an optimal separat

length for the two‐hopper system where maximum reclaim of material can be expected. This optimal distance be

dependent on the moisture content of the bulk material.



Figure 14. Double Hopper Stockpile Live Capacity for Model Stockpile using Coal. Hopper length at transition = 12Omm, width

100mm

5.4 Loads on Reclaim Hoppers and Feeders

The loads on reclaim hoppers and feeders and the corresponding power to drive the feeders varies from the "initial

the flow condition as discussed in Section 4. The loads are illustrated in Figure 15. The initial load will correspond to

case when the stockpile or crater above the feeder is filled. The surcharge load Qs will depend on the consolidat

condition of the bulk solid in the stockpile. The worst case corresponds to the hydrostatic pressure. However, if a rath

has been pre‐formed, then the surcharge load will be reduced. When an arched or flow stressed field has been formwithin the mass‐flow reclaim hopper, the load on the feeder will be greatly reduced. Confirmation of the load conditi

acting on reclaim hoppers has been obtained from the model stockpile tests.

Figure 15. Loads on Stockpile Feeders

6. SURFACE OR WALL FRICTION

6. 1 Selection of Lining Materials

Of the various parameters affecting the performance of hoppers, feeders and chutes, the friction at the bound

surface has, in most cases, the major influence. Judicious choice of lining material to achieve low friction and wear is

important consideration.

There are a great many lining materials and surface coatings on the market, some common linings being illustrate

Table III. Also shown are the bulk materials for which the lining material is commonly used.

TABLE III ‐ SOME COMMON LINING MATERIALS

Lining Material Remarks

Carbon Steel Cheap ‐ suitable for most bulk materials ‐ corrosion a problem. High friction often a limiting

factor.

Stainless Steel

304

‐

2B

Excellent material for bulk materials which are not too abrasive. Very suitable for black

coal. Very poor performance for brown coal. Low friction.

Stainless Steel

3Cr12

Similar application to 304‐2B stainless. Is cheaper and lower chrome content than 304‐2B.

Low friction.

Ultra High

Polyethylene

Excellent for bulk materials which are not too abrasive. Fixing must be by mechanical

fasteners. Very good performance for both black and brown coal.

Bisalloy 360

Domite Ni Hard

For more arduous applications with Domite being quite expensive. Suitable for such bulk

material as Bauxite, Iron Ore, Copper Ore, Copper Ore, Lead Ore, Zinc Ore. Generally high

friction.

Epoxy Coated

Surfaces

Good performance for bulk materials such as coal where abrasive wear is not a major

problem. Relatively low friction.



While cost is a significant consideration, it is most important that the lining material be selected on the basis of serv

life and performance. Factors to be considered include:

Surface friction and adhesion

Resistance to impact, if appropriate

Method of attachment

Installation cost and maintenance

Resistance to abrasive wear

Resistance to corrosion

Initial cost

It is recommended that appropriate tests be conducted to determine the relevant flow properties of the bulk solid

the proposed lining surface. In view of the importance of surface or wall friction, some salient aspects are now review

6.2 Surface Friction and Adhesion

The adhesion of bulk solid particles to hoppers and chutes is a result of the interaction between the bulk solid and

boundary or wall surface[18‐21]. While adhesion and/or cohesion are difficult to measure directly, an indication of th

parameters may be gleaned from bulk solid and wall surface friction measurements using a direct shear test appara

The parameters of interest are defined in Figure 16. 1.

Figure 16. Wall or Surface Yield Locus (WYL)

The surface friction characteristics are displayed by the wall yield locus W .Y .L.; the surface friction angle Φ is define

follows:

Φ = tan‐1

/ ح) σw) ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (2)

where ح = shear stress at the wall

σw = corresponding normal stress at the surface.

As indicated by Figure 16, the friction angle Φ between the bulk solid and boundary surface decreases as the nor

pressure increases. This effect is illustrated in Figure 17.



Figure 17. Characteristic Surface or Wall Friction Variation with Normal Pressure

6.3 Interaction Characteristics

Bulk solid/boundary surface friction and adhesion depend on the interaction between three groups of variables, th

relating to the bulk solid, those relating to the wall or boundary surface and those which arise from loading

environmental conditions. This interaction is shown, diagrammatically in Figure 18.

Figure 18. Bulk Solid/Boundary Surface Friction Interactions

The relevant properties in each of the three groups are summarized as follows :

(i) Bulk Solid Characteristics ‐

Particle size and size distribution

Panicle shape, hardness and density

Moisture content

Bulk density

Chemical composition

(ii) Wall Surface Characteristics ‐

Surface roughness

Hardness

Chemical composition

(iii) Loading and Environmental Factors ‐

Pressure between bulk solid and wall surface

Relative rubbing or sliding velocity

Temperature and humidity or moisture conditions

Wall vibrations



Undisturbed contact time

6.4 Wall Friction versus Normal Pressure Characteristics

As an illustration of some of the factors influencing wall or surface friction, a set of wall yield loci graphs are shown in Figure 19 [

The bulk solid in this case is coal. Figure 19(a) shows the variation of wall friction for black coal at 10% moisture cont

on three surfaces, namely, stainless steel type 304 with 2B finish, mild steel polished and mild steel rusted all at

instantaneous or zero storage time condition. In the case of the 120 polished mild steel surface the wall friction was a

determined after 72 hours undisturbed contact or storage time is also shown; the high friction in this case is q

considerable with corrosion and adhesion or bonding of coal particles to the steel surface.

Figure 19(b) compares the Wall Yield Loci for black coal at 19.7% moisture content and brown coal at 65% moist

content on two surfaces, stainless steel type 304 with 2B finish and Tivar 88, an ultra high molecular we

polyethylene material. While in absolute terms the moisture contents of the two coals are significantly different

relative terms, taking account of their composition and saturated moisture conditions, they are comparable. For

black coal the wall friction angle for stainless steel and Tivar are similar, both exhibiting low friction. This is also the c

for brown coal on the Tivar surface. However, the brown coal has abnormally high friction on the stainless steel, des

the smoothness of the surface; the stainless steel is entirely unsuitable for brown coal.

(a) Wall Yield Loci for Black Coal (b) Wall Yield Loci for Black and Coals

Figure 19. Typical Wall Yield Loci

6.5 Surface Roughness

(a) Roughness Parameters and Effect on Wall Friction

Surface roughness is an important factor in terms of its influence on wall friction. Yet the specification of surf

roughness in terms of appropriate parameters which adequately describe the surface is a complex matter requi

careful and detailed consideration.

It is common to simplify surface characteristics in terms of height parameters, such as the centre line arithmetic aver

roughness (CLA or Ra Number) or RMS roughness. Such parameters are useful for comparison purposes but do

adequately provide an assessment of the interaction between panicle size and surface profile. For this reason, Ooms

Robens [18] have found it useful to specify the surface characteristics in terms of Surface Spectoral Density. This disp

the RMS roughness amplitude as a function of frequency where the frequency is the inverse of the roughness w

length.

(b) Roughness Classification of Surfaces used in Bulk Solids Handling

The new Australian Standard on Loads in Bulk Solids Containers [11] designates four surface roughness characteristic

to D4. In order that these surface classifications may be quantified in some way, Ooms (Ref. [21]) proposed that the f



categories of surfaces be grouped in terms of roughness bands based on the Mean Centreline Roughness or Ra num

Surfaces commonly used have been classified according to this procedure and presented in Figure 20.

Figure 20. Ooms Chan for Lining Surface Roughness Classification

From the discussion in the previous Section, it is apparent that the proposed form of classification is somew

restrictive in terms of the limited information conveyed by the Ra number. Also the wall roughness may not necessa

remain constant and should be considered as a variable. For instance, a polished or lightly rusted carbon steel surf

may become deeply pitted and change from group D2 to group D3. An aluminum surface is easily scored and m

change from group Dl to group D2. On the other hand, some stainless steel surfaces will polish during service and m

change from group D2 to D 1.

6.5 Influence of Vibrations

Roberts et al [22‐24], have shown that the application of vibrations to a wall surface can significantly reduce wall frictand therefore promote flow. Vibrations can also reduce bulk strength, further assisting in promoting gravity flow.

evidence indicates that the best results are achieved by using frequencies of 100 Hertz or higher, and low amplitude.

7. ADHESION OF BULK SOLIDS ON WALLS OR SURFACES

7. 1 Adhesion of Bulk Solids in Chutes

The characteristics of surface or wall friction discussed in the previous section indicate that, for most bulk solids

lining materials, the Wall Yield Loci (WYL) tend to be convex upward in shape. Furthermore the WYL often intersect

shear stress axis corresponding to zero normal pressure indicating an adhesion/cohesion effect as depicted in Figure

Problems due to high wall friction, cohesion and adhesion, which are associated with low pressure conditions, of

occur in chutes and standpipes. Cohesion and adhesion can cause serious flow blockage problems when corro

bonding occurs, such as when moist coal is in contact with carbon steel surfaces. The bonding action can occur a

relatively short contact times. Impurities such as clay in coal can also seriously aggravate the behaviour due to adhes

and cohesion.

Transfer chutes should be designed to ensure that satisfactory flow is obtained without flow blockages. Yet despite t

apparent simplicity, the flow patterns developed in chutes often not fully appreciated. Occasions have arisen in prac

where costly flow interruptions have occurred due to incorrect chute design arising from a lack of understanding of

bulk solid and chute surface friction characteristics.



The design of transfer chutes is discussed in References [25‐28]. Consideration should be given to the flow propertie

the bulk solid and the characteristics of the chute lining material. Moist coal can adhere to vertical, as well as incli

faces of steel chutes eventually causing blockages. This has been found to occur in practice after only a few hour

operation. Problems of this type have occurred, for example, in conveyor feed chutes in coal screening operation

illustrated in Figure 21. The momentum of the coal particles falling from the screen is usually not sufficient to ca

scouring of the chute surfaces and, as a result, build‐up and complete blockages have been known to occur.

Figure 21. Build‐Up of Cohesive Bulk Solid such as Coal on Screen House Conveyor Feed Chute

When determining chute slope angles, account must be taken of the variation of friction angle with change

consolidation pressure, or more particularly, with change in bed depth. Figure 22 shows, for a typical coal, the variat

of wall friction angle with bed depth. As indicated, high friction angles can occur at low bed depths, the decreas

friction angle being significant as the bed depth increases.

Figure 22 Wall friction angle versus bed depth for bulk solid on chute

The slope θ of the chute should be at least 5 larger than angle of equivalent friction [4].

That is :

θmin = tan‐1 [tanΦ (1 + Kv Ho / B)] + 5o

where Φ = Wall friction angle corresponding to HO

Ho = Bed depth

B = Chute width

Kv = Ratio lateral to normal pressure

Kv will depend on bulk solid properties Normally Kv = 0.5 to 1.0. In the absence of information Kv may be taken to be 0



Often moist bulk solids will adhere initially to a chute surface, but as the bed depth increases, the correspond

decrease in friction angle will cause flow to be initiated. In some cases flow commences with a block‐like motion of

bulk solid as depicted in Figure 23.

Figure 23 Block‐like Flow Down Chute

7.2 Adhesion in Vertical Chutes or Standpipes

Bulk solids, such as coal with high clay contents and at high moisture contents, may adhere to walls of vertical pipechutes leading to progressive build‐up and flow choking. Problems of this type have been known to occur in the c

handling plant of power stations, as depicted schematically in Figure 24.

Figure 24 Schematic Arrangement of Coal Handling Plant of Typical Power Station

When blockages occur in feed‐pipes to the feeder and mill, a boiler may "flame out" in the space of a few minuBlockages are initiated by the coal adhering to the pipe wall and then growing inwardly, this action often occurring a

only a few tonnes of coal have passed through the system. Often such problems occur when unwashed coal is store

open stockpiles prior to use. The weathering process can cause the clays to be dispersed, rendering them more likel

adhere to chute and pipe walls. The adhesion process may be aggravated in this case due to the temperature of the

and standpipe above the mill.

It is important that the pipe or chute diameter be sufficiently large to cause the bulk solid to fall away from the wall

proposed simplified methodology is presented. Referring to Figure 25, assuming the weight of bulk solid is just suffic

to cause slip along the wall, the required pipe or chute diameter D is given by

D ≥ 4ţ / ŷ (1 ‐ C2

) ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ (4)



where :

C = d / D such that C ≥ 0.8

ţ = shear stress at wall corresponding to normal pressure σ

ŷ = ρ g = bulk specific weight.

Figure 25 Build‐Up of Bulk Solid in Vertical Chute or Standpipe

It is also wise to check whether the pipe diameter is sufficient to prevent a cohesive arch forming. For this analysis,

methods presented in Refs. [3,4] may be used.

8. WEAR IN BULK HANDLING PLANT

Wear in bulk handling plant may result from impact or abrasion or, as is often the case, a combination of both

addition, deterioration of metal surfaces can occur as a result of corrosion.

8.1 Impact

Erosive type wear due to impact consists of a combination of plastic deformation and cutting wear. Such wear,

example, occurs in pipe bends of pneumatic conveying systems where impact velocities are normally relatively high

where several impacts and rebounds may take place. Normally the particle size is small in this case.

Impact wear also occurs at discharge points of belt conveyors and at entry points to transfer chutes. Velocities of imp

are normally relatively low whereas particle size range can be quite wide with large lumps being present.

Impact wear depends on several factors, the relative hardness of the particles and the surface having a signific

influence. For impact on hard, brittle materials, the greatest amount of damage occurs when particles impringe at an

of approximately 90. On the other hand, for ductile materials, the greatest amount of erosive wear occurs wparticles strike the surface at low angles of attack, usually in the range 15 to 30. Erosive wear due to impact is norm

composed of two types, deformation wear and cutting wear.

8.2 Abrasive or Rubbing Wear

This occurs in storage bins and silos particularly in hoppers under mass‐flow conditions. Under mass‐flow the pressu

in a hopper will vary significantly over the hopper surface, with the maximum pressure occurring at the transition,

pressure decreasing towards the outlet. The velocity of the bulk solid adjacent to the wall increases non‐linearly fr

the transition to the hopper outlet. While the magnitude of the velocity at particular point on the hopper wall depe

on the bin discharge rate, normally the bulk solid velocities are quite low with pure sliding taking place.



Abrasive wear also occurs in transfer chutes, the flow being characterised by lower pressures and higher velocities t

those occurring in hoppers. There are several other areas where abrasive wear is experienced such as in feeders,

conveyors, vibratory conveyors and screw conveyors. Any mechanical device which involves the motion of bulk so

relative to surfaces will experience wear problems.

8.3 Abrasive Wear Parameters

The concepts of a non‐dimensional Relative Wear Number NWR has been introduced [20] in order to per

comparisons to be made between different bin and chute geometries, is defined as :

NWR = [(σw / ŷB) (Vs / Vo) tan Φ

Where :

σw = Normal pressure at boundary

ŷ = Bulk specific weight

B = Characteristic dimension, B = outlet dimension in case of hopper; B = chute width in case of chute

Vs = Velocity of sliding at wall

Vo = Sliding velocity at reference location.

For hopper, Vo is defined at transition of cylinder and hopper

For chute, Vo is normally defined at point of entry to chute

Φ = Wall friction angle

8.4 Wear in Mass‐Flow Bins

The application of the foregoing to the assessment of relative wear in mass‐flow bins has been discussed in Ref. [20]

way of illustration, the relative wear profiles for axi‐symmetric (or conical) and plane‐flow bins having the same open

dimension and hopper half angle respectively are illustrated in Figure 26. In the case of the axi‐symmetric bins,

maximum relative wear occurs at the outlet, while for the plane‐flow bins the maximum relative wear occurs at

transition. In the latter case the wear at the transition is likely to be less than as indicated in Figure 26 owing to

possible build‐up of material at the transition. Also, the normal wall pressure occurring at the transition is difficul

predict precisely and is likely to be lower than as indicated.

Some bins are constructed with a variable hopper slope and with the hopper section having different surface textu

Such a bin is discussed in Ref. [29]. The bin in question is axi‐symmetric with a capacity of 2400 tonnes. The hopper

lined with 3 mm type 304‐2B stainless steel. Examination of the lining after approximately 5 million tonnes of coal passed through the bin showed that the maximum wear of the stainless steel was around 1mm.



Figure 26 Relative wear profiles for axi‐symmetric and plane‐flow mass‐flow bins

B = 1.0 m, α = 22, Ф (cylinder) = 30, Ф (hopper) = 20

8.5 Avoidance of Wear Problems due to Eccentric Funnel‐Flow

Serious wear problems will occur during funnel‐flow where the flow channel or pipe is not fully contained in the b

solid itself but may incorporate part of the hopper or bin wall. Problems of this nature may occur when bins w

eccentric discharge are used, particularly when the bin opening is located near aside wall. On other occasions a ba

designed feeder may cause material to pipe adjacent to the hopper wall. Flow channels of this nature give rise to hvelocity flow against the wall resulting in accelerated wear.

Often side delivery chutes are incorporated in bins for the purpose of off ‐loading bulk materials. Side delivery chu

create undesirable flow patterns in bins, leading to accelerated wear of the bin wall in the region of the chute intake

well as in the plates above the chute. This wear is caused by both abrasion and impact Abrasive wear results from

high velocity of the materials during chute discharge, the flow velocity of the materials during chute discharge, the f

following a funnel‐flow pattern, as Indicated in Figure 27. The eccentric discharge induces a non‐uniform press

distribution, as shown; bending is induced and the bin shell is deformed as indicated by the dotted curve.

Figure 27 Eccentric discharge due to use of side delivery chutes

Impact wear can occur on filling the bin after discharging from the side delivery chute. The surface is left in a ri

condition as indicated in Figure 27. When filling commences, lumps of bulk material may bounce off the rilled surf

and impact the wall in the weakened area above the chute.

It should be noted that despite the fact that side delivery chutes may only be used intermittently, the wear rate du

operation is considerable. It is therefore most desirable that side delivery chutes be avoided and incorporate any

loading via a transfer conveyor operating from the main bin discharge. If side delivery chutes are used, such as



existing installation, it is essential that the bins be lined with wear plates in the region of the chute intakes as wel

above the chutes.

8.6 Wear in Transfer Chutes

Abrasive wear in transfer chutes has been discussed in Ref. [18‐21]. In the case of straight inclined chutes of const

cross‐sectional geometry, the wear is constant along the chute. For chutes of constant curvature, it has been shown t

the wear varies along the chute as depicted in Figure 28 reaching a maximum at a particular chute angle and t

decreasing. However, the wear is virtually independent of chute radius.

Figure 28. Wear factor for circular curve chutes [20]

Q = 30 tonnes/hr, Vo = 0.2 m/s, ρ= 1000 kg/m3, b = 0.5 m, E = 0.6, Ф = 30.

8.8 Abrasive Wear Tests

In order to evaluate lining materials for wear resistance, a linear wear tester, as proposed by Roberts [30,31], has b

developed jointly at The University of Twente, The Netherlands, and The University of Newcastle, Australia. The tes

which is shown in Figure 29, incorporates the following features:

i.

Provision for a continuous supply of "fresh" bulk solid. ii. Provision for the bulk solid normal pressure on the test surface to be varied over the specific range.

iii. Provision for the sliding or rubbing velocity to be varied over the specific range.

Figure 29. Abrasive Wear Tester



During tests, the friction coefficient, surface roughness, weight and thickness loss are progressively monitored. The t

samples used in the linear wear tests are of similar size to those used in the Jenike Direct Shear Test. This allows cr

checking of the wall or boundary friction at various stages throughout the tests. A typical set of test results

illustrated in figure 30.

Figure 30. Comparative Wear Rates for Three Common Lining Materials

9. PULSATING LOADS IN BINS ‐'SILO QUAKING'

9.1 General Discussion

As is often the case, the solution of one problem which leads to an improvement in plant performance exposes ot

problems which require further research and development, This applies particularly to gravity flow in storage bins

silos where the application of known theories for reliable discharge, such as by mass‐flow, can give rise to dynami

pulsating flow effects. These effects are normally imperceptible as far as bin discharge is concerned having

detrimental effect on the plant operation. However, the pulsating flow can have a significant influence on the lo

acting on bin walls by imposing severe dynamic loads. The phenomenon is often described as 'silo quaking'; it may

linked with the critical head Hcr for mass‐flow as discussed in Section 2.

The discussion that follows provides a qualitative view of the 'silo quaking' problem as it r relates to mass‐flow, fun

flow and expanded‐flow bins.

(a) Velocity Profiles and Pressure Distribution (b) Variable Density and Dilation

Figure 31 Mass‐Flow Bin

Referring to the mass‐flow bin depicted in Figure 31; as the material flows, it dilates leading to variations in density fr

the static condition. This is depicted pictorially in Figure 31(b). With H > Hcr, the flow in the cylinder is uniform or 'p

like' over the cross‐section, with flow along the walls. In the region of the transition, the flow starts to converge due

the influence of the hopper and the velocity profile is no longer uniform. The velocity profile is further developed in

hopper as shown. As the flow pressures generate in the hopper the further dilation of the bulk solid occurs. As a re

of the dilation, it is possible that the vertical supporting pressures decrease slightly reducing the support given to



plug of bulk solid in the cylinder. This causes the plug to drop momentarily giving rise to a load pulse. The cycle is t

repeated.

Studies of the phenomenon of pulsating loads in bins and silos are presently in progress at the University of Newcas

Australia. In this work, a pilot scale mass‐flow, steel silo 1.2m diameter by 3.5 m high and fitted with a stainless s

hopper is being used. The silo is fitted with 14 load cells designed by Prof. V. Askegaard of the Technical Universit

Denmark; these cells are capable of measuring both normal pressure and wall shear stress. An example of a w

pressure and shear stress records depicting the pulsating load in the cylinder are shown in Figure 32.

Figure 32. Load Cell Records depicting Pulsating Loads in Mass‐Flow Bin

A similar action to that described above for mass‐flow bins may occur in tall funnel‐flow bins or silos where the effec

transition intersects the wall in the lower region of the silo. As a result, there is flow along the walls of a substantial m

of bulk solid above the effective transition.

During funnel‐flow in bins of squat proportions, where there is no flow along the walls, as depicted in Figure 33, dilat

of the bulk solid occurs as it expands in the flow channel. As a result some reduction in the radial support given to

stationary material may occur. If the hopper is fairly steeply sloped, say [θ ≥ δ], then the stationary mass may

momentarily causing the pressure in the flow channel to increase as a result of the 'squeezing' action. The cycle t

repeats.

Figure 33 Funnel Flow Bin



A similar behaviour may occur in expanded flow bins, such as the bin depicted in Figure 2 .Pulsating loads can occu

such bins, particularly if the slope angle e of the transition is too steep. Owing to segregation on filling, larger

particles are more likely to be located adjacent to the sloping surface at the lower end of the funnel‐flow section. S

particles tend to roll as well as slide, aggravating the load slipping problem and giving rise to load pulsations. Proble

of this type have been experienced in large coal bins.

7.2 Multi‐Outlet Coal Bins

Silo‐quaking problems have been known to occur in bins with multiple outlets. By way of illustration, consider the la

coal bin shown in Figure 34. The bin has seven outlets, six around an outer pitch circle and one located centrally.

hopper geometries provide for reliable flow permitting complete discharge of the bin contents. Coal was discharged

means of seven vibratory feeders onto a centrally located conveyor belt. When the bin was full or near full, severe sh

loads were observed at approximately 3 second intervals during discharge. The discharge rate from each feeder wa

the order of 300 t/h. When the level in the bin had dropped to approximately half the height, the shock loads

diminished significantly. With all the outlets operating, the effective transition was well

Figure 34 Multi‐Outlet Coal Bin

down towards the bottom of the bin walls and the critical head Hm was of the same order as the bin diameter

greater than DF. Substantial flow occurred along the walls, and since the reclaim hoppers were at a critical slope for m

and funnel‐flow as determined by flow property tests, the conditions were right for severe 'silo quaking' to occur.

Confirmation of the mechanism of silo quaking was obtained in field trials conducted on the bin. In one series of te

the three feeders along the centre line parallel with the reclaim conveyor were operated, while the four outer feed

were not operated. This induced funnel‐flow in a wedged‐shaped pattern as indicated in Figure 34, with the effec

transition occurring well up the bin walls, that is Hm < Hcr (= DF ) or Hm << D. The same was true when only the cen

feeder (Fdr. 1) was operated; in this case the stationary material in the bin formed a conical shape. Under th

conditions, the motion down the walls was greatly restricted and, as a result, the load pulsations were ba

perceptible.

In a second set of trials, the three central feeders were left stationary, while the four outer feeders were operated. T

gave rise to the triangular prism shaped dead region in the central region, with substantial mass‐flow along the wThe load pulsations were just as severe in this case as was the case with all feeders operating. Dynamic st

measurements were made using strain gauges mounted on selected support columns. When the bin was full (or n

full), the measured dynamic strains with Hm Hcr were in the order of 4 times the strains measured when the f

pattern was controlled so that Hm < Hcr.

10. CONCLUDING REMARKS

In this paper an overview of some salient aspects of the storage, flow and handling of bulk solids has been presente

is quite clear that, in recent years, significant advances have been made in research and development associated w

bulk handling systems. It is gratifying to acknowledge the increasing industrial awareness and acceptance through

the world and particularly in Australia of modern bulk materials handling testing and plant design procedures. Th



procedures are now well proven, and while much of the industrial development has, and still is, centred aro

remedial action to correct unsatisfactory design features of existing systems, it is heartening that in many new indust

operations the appropriate design analysis and assessment is being performed prior to plant construction

installation. It is most important that this trend continues.

The paper has indicated, by way of example, the ongoing need for research and development which is necessary

industrial plant and processes become more sophisticated, the demands for better quality control become m

stringent and both national and international competition requires more efficient and cost‐effective performance.

11. ACKNOWLEDGEMENTS

Much of the work presented in this paper is based on a current research grant from AMIRA. The support of AMIRA

the sponsoring companies is gratefully acknowledged.

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