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5/13/2018 Modern Technological Developments in the Storage and Handling of Bulk Solids_Edit - slidepdf.com
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
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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
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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]
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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.
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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
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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.
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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.
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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.
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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)
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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.
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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.
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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
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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
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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
5/13/2018 Modern Technological Developments in the Storage and Handling of Bulk Solids_Edit - slidepdf.com
7. Benink, E.J. "Flow and Stress Analysis of Cohesionless Bulk Materials in Silos Related to Codes". Doct
Thesis, The University of Twente, Enschede, The Netherlands. 1989.
8. Roberts, A.W. and Ooms, M. "Wall Loads in Large Steel and Concrete Bins and Silos due to Eccen
Draw‐Down and Other Factors". Proc. 2nd Inti. Conference on 'Design of Silos for Strength and Flo
Powder Advisory Centre, U.K., 1983, (ppI51‐170).
9. Ooms, M. and Roberts, A. W ."The Reduction and Control of Flow Pressures in Cracked Grain Silos". BSolids Handling, Vol. 5, No.5, Oct. 1985. (pp.1009‐1016).
10. Roberts, A. W. "Some Aspects of Grain Silo Wall Pressure Research ‐Influence of Moisture Content
Loads Generated and Control of Pressures in Tall Multi‐Outiet Silos". Proc. 13th Inti. Powder and B
Solids Conf., Chicago, USA, May 1988. (pp.II‐24).
11. Australian Standard AS89138 "Loads on Bulk Solids Containers"
12. Roberts A. W ., Ooms M and Manjunath K.S., "Feeder Loads‐ and Power Requirements in the Contro
Gravity Flow of Bulk Solids from Mass‐Flow Bins" Trans. I.E.Aust., Mechanical Engineering, V.ME9, N
April 1984.
13. Manjunath,K.S. and Roberts, A.W., 'tWall Pressure‐Feeder Load Interactions in Mass‐F
Hopper/Feeder Combinations". Part I. IntI. Jnl. of Bulk Solids Handling, Vol. 6, No.4, Aug. 1986.
14. Manjunath, K.S. and Roberts, A.W., "Wall Pressure‐Feeder Load Interactions in Mass‐FHopper/Feeder Combinations". Part II. Inti. Jnl. of Bulk Solids Handling, Vol. 6, No.5, Oct. 1986.
15. Rademacher, F.J.C., "Reclaim Power and Geometry of Bin Interfaces in Belt and Apron Feeders". IntI.
of Bulk Solids Handling, Vol. 2, No.2, June 1982.
16. Roberts, A. W. and Teo, L.H., "Performance Characteristics of Gravity Reclaim Stockpiles of Con
Form", Trans. of Mechanical Engineering, The Instn. of Engrs. Australia, Vol. ME 14, No.2, 1989, pp
102.
17. Roberts, A. W .and Teo, L.H., "Design Considerations for Maximum Reclaim Capacity of Con
19. Robens, A.W., Ooms, M. and Scott, O.J. "Surface Friction and Wear in the Storage, Gravity Flow
Handling of Bulk Solids". Proc. Conf. 'War on Wear', Wear in the Mining and Mineral Extraction Indus
Instn. of Mech. Engnrs, Nottingham U.K., 1984. (pp.123‐134).
20. Robens, A. W. "Friction, Adhesion and Wear in Bulk Materials Handling". Proc., AntiWear 88, The Ro
Soc. London. 1988. Inst. of Metals, I.Mech. E. .
21. Roberts. A.W., Ooms, M. and Wiche, S.J. "Concepts of Boundary Friction, Adhesion and Wear in B
Solids Handling Operations". IntI. Jnl. of Bulk Solids Handling, Vol.10, No.2, May 1988. :
22. Robens, A.W."Vibrations of Powders and Bulk Solids". Chapter 6, Handbook on Powder Science
Technology. (1984) Van Nostrand.
23. Robens, A.W., Ooms, M. and Scott, O.1. "Influence of Vibrations on the Strength and Boundary FrictCharacteristics of Bulk Solids and the Effect on Bin Design". Inti. Jnl. of Bulk Solids Handling, Vol.6, N
1986. (pp.161‐169).
24. Robens, A.W. and Rademacher, F.J.C. "Induced Gravity Flow by Mechanical Vibrations". To appea
Inti. Jnl. of Bulk Solids Storage in Silos, UK.
25. Robens A. W. "An Investigation into the Gravity Flow of Non‐Cohesive Granular Materials Thro
Discharge Chutes". Trans. A.S.M.E., Jnl. for Engng. in ,Industry, Vol. 91, Series B, No.2, May 1969.
373‐381). ,
26. Robens A. W. and Scott O.1. "Flow of Bulk Solids Through Transfer Chutes of J Variable Geometry