preventing solids flow problems

8/22/2019 Preventing Solids Flow Problems

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PREVENTING SOLIDS FLOW PROBLEMS IN

FEEDERS, BINS, HOPPERS AND STOCKPILESJerry R Johanson, JR Johanson Inc

2975 Hawk Hill Lane, San Luis Obispo, CA 95405

ABSTRACTThis presentation shows how to use basic properties and the indices derived from them to solve flow

problems inbins, hoppers, stockpiles and feeders and how to use these valuable tools to select and size equipment

for trouble free plant operation. Practical guidelines and suggestions are provided for avoiding solids handling

problems in bins and stockpiles.

Flow Properties effecting feeders, bins, hoppers and stockpile design and selectionAny quantitative discussion of bin and feeder selection must relate the flow properties of the ore

concentrate or mineral being handled and processed. These basic properties are listed in table 1 along with the

factors that affect each (1.2.3).

Table 1. Basic Bulk Solids Properties That Affect bins and feeder selection and sizing

Name Symbol Unit

Factors usually increasing

magnitude

Unconfined yield strength fc Force per unit

area

Increased solids contact

pressure, increased moisture

content, increased time at rest,

and reduced particle size

Bulk specific weight Weight per unit

volume

Increased solid contact pressure,

time at rest, particle size and

decreased moisture

Kinematic surface friction

angle

= Degrees

Decreased solids contact

pressure, increased moisture,

increased surface roughness,

increased time at rest

Angle of internal friction Degrees Decreased unconfined yield

strength

Effective angle of internal

friction

Degrees Increased unconfined yield

strength

Steady state rough plate

angle of slide

Degrees

Sin()= Sin()/Cos( - )

For coheasionless solids,

===angle of repose

Permeability (incipient Increased moisture, decreased


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fluidization velocity) k Velocity solids contact pressure and

increased particle size

Each of these properties is a function of the level of consolidation pressure and consequently the behavior

of the material in a bin or feeder is a function of the size of the bin or feeder. Recognizing this allows one tosimplify the flow properties to a series of indices where the indices basis is related to the size of the bin and feeder.

For a conical bin, this size is characterized by the hopper outlet diameter d through which the material must flow on

the feeder of related size and the bin top diameter D. In a practical sense the questions of hopper and feederselection are summarized by: How large must the outlet be to insure that the material does not arch in the hopper?

What is the maximum sustained flow rate that one can expect from the hopper outlet? How steep must the hopper

be to provide cleanout? What size of flow channel must one provide in a flat bottom bin or stockpile to cause the

sides to cave in at a reasonable angle of repose? What density does one use in calculating the gravimetric capacity

of a given bin or pile? What density can one expect at the feeder for calculating the required feeder material cross-

section to accommodate the required gravimetric rate? And will the material; stick onto feeder pans and chutes thus

requiring special design considerations. These seven questions are answered directly by the indices illustrated infigure 1. Each of the indices is calculated from the basic flow properties of the solid handled as a function of the

indices basis as shown in table 2. Figure 2 shows the basic properties of the corn meal used in the photographs of

basic flow patterns.


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Figure 2. Corn meal consolidation pressure

vs , ,, Strength, Density and Permeability

0

5

10

15

20

25

30

35

40

45

0 25 50 75 100 125 150Consolidation Pressure, psf

FrictionAngles(deg.),strength(psf),

Density(lb/cuft)

0

0

0

0

0

DELTA Strength Surface friction 304-2B SS Density gamma Permeability K

36.17 0.787852 0.031804 37.8 0.252949 4.870417

=(1+/) =(/)

Density Parameters Permeability Parameters


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Figure 1

Physical Meaning of Indices

ARCHING INDEX( A I ) i s t h erecommended outletdiameter to ensurearch collapse in aconical bin.

RATHOLING INDEX

( R I ) i s t h er e c o m m e n d e dcircular flow channeldiameter to ensurerathole failure andcleanout in a funnel-flow bin.

HOPPER INDEX(HI)is the recommendedconical hopper half-angle to ensure flowat the walls.

FLOW RATE INDEX

(FRI) is the maximumsolids flow ratee x p e c t e d a f t e r deaeration of apowder in the bin.

DENSITY INDICES

(FDI and BDI) are thedensities in the feederand bin respectively.

CHUTE INDEX(CI) is therecommended chuteslope angle at impactpoints.

SPRINGBACK INDEX

(SBI) is the percentage


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TABLE 2Johanson Indices as They Relate to Basic Flow Properties

d = Hopper outlet diameter indices basis

D = Bin diameter indices basis

Name Symbol Units Relation to Basic Bulk Properties

ArchingIndex

AI Linear AI = 2.2 fc / where fc and are measured at a solidscontact pressure equal to 3 d/2.

RatholingIndex

RI Linear RI = 3.5 fc / where fc and are measured at a solidscontact pressure equal to D.

Flow Rate

Index

FRI Weight

per unittime

FRI =( 2 B+(B**2 4 A C)**0.5) /(2 A)

Where: A = 4 Tan K / (g d3 / 4)

B = 1 - / BDI

C = - K d2 / 4where K and are measured at a solids contactpressure of d / 2, is the conical hopper anglemeasured from the vertical, and g is thegravitational constant dependent on units of.

Feed

DensityIndex

FDI Weight

per unitvolume

FDI = measured at a solids contact pressure equal to

d/2.

BinDensityIndex

BDI Weightper unitvolume

BDI = measured at a solids contact pressured equal toD.

HopperIndex

HI Degrees HI = 42-= where= is measured at a solids contactpressure equal tod/2, or if= is larger at a higher pressure,then use= at a pressure ofD.

ChuteIndex

CI Degrees CI = ASC + 10 where ASC is the angle of slide on a flatsurface after the powder sample has been pressed againsta surface at a solids contact pressure of 4700 N/m2 (100

psf), then released before the surface is tilted to determineASC.

Note that HI and CI are both specific to the surface the material is sliding on and consequently are not constant forany given bulk solid but have a dual dependency on solids and surface material and surface conditioning.

The indices thus defined for a conical hopper provide the answers to each question. The minimum outlet

for prevent arching [3] at the conical hopper outlet is AI. The minimum flow channel diagonal to prevent ratholing

[8] and thus cause the sides of the channel to cave in at a reasonable angle of repose is RI. The maximum flow rate

that one can expect from the outlet is FRI. The density at the feeder is FDI. The density in the bin is BDI. The


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hopper angle measured from the vertical to insure flow at the walls is HI. Sticking on chutes and feeders is very

likely wheneverCI exceeds 70 degrees. Vibrating feeders should not be used with CI larger then 80 degrees.

So far these indices have been related only to conical hoppers. As various hopper shapes are discussedthese indices will be related to the particular hopper shape usually by a simple multiplier. In this way the

effectiveness of various shapes can be expressed relative to the common conical hopper.

Stockpile hopper selectionStockpiles are essentially flat-bottom bins. The basic flow patterns in flat bottom bins are photographed in

a model using corn meal, a relatively cohesive material for the model size. The indices for the meal in the model are

given in table 3 as calculated from the basic properties given for the meal in figure 2. The table also shows the

indices for a fine wet coal in a large bin (example 2 and in a pile worked with tractors (example3). The ratios of

AI/d and RI/D for the various materials and conditions provided a measure of relative cohesiveness. The meal in themodel is much like the coal in the large bin and consequently the flow patterns photographed in the model will

closely resemble those observed with the coal. The flow rate divided by the density and the area of the outlet givingthe velocity through the outlet is somewhat higher in the large bin. The compressibility as given by the BDI/FDI

ration is much larger for the large coal bin.

Table 3 Indices

Indices name or ratio Example 1Corn meal

D=0.833 ftd=0.0125 ft

Example 2Fine wet coal

D=40 ftD=3 ft

Example 3Fine wet coal

D= 30 ftd= 30 ft

Arching AI (ft) 0.025 5.34 13.4

AI / d 2 1.78 0.46

Ratholing RI (ft) 0.75 26.0 ---

RI / D 0.9 0.67 ---

Flow rate FRI (lb/ min) 37.5 41,600 ---

FRI / (FDI x Pi x d**2 /4)

(ft/min)

101 150 ---

Feeder density FDI (lb/cu ft) 37.8 40.0 ---

Bin density BDI (lb/cu ft) 40.8 61.0 59.0Compressibility ratio

BDI/FDI

1.078 1.496 ---

Hopper HI (degrees) 30* 16** ---

Chute CI (degrees) 22* 90** ---

*On Plexiglas **on aged carbon steel

The corn meal of example1 was used in the photographs of the 10-inch diameter model. Example 2 is for a

large 40-foot diameter bin with a 3-foot diameter or square outlet. This also applies to a bottom tunnel reclaim pile

40 feet tall that was initially formed by having the feeder running at least intermittently during the filling until the

pile height reached 10 feet. Example 3 is for the top of a hopper under a pile worked with tractors that compact the

solids to 1700 psf. It is also for the top of a hopper under a bottom tunnel reclaim pile that was fill initially from a

flat surface at the top of the hopper to a height of 30 feet without running the feeder.


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The relationship between the indices and the flow patterns in bins is evident in the photographs of the

model flat bottom bin.

Photo 1 shows flow from a small outlet 1.5 by 1.5 inches square. An arch initially developed over the outlet. This

was expected since the AI for the corn meal in the 10-inch diameter bin with a 1.5-inch circular outlet is 0.25 feet or

3 inches. When this arch is broken by prodding from the bottom, a stable rathole forms as is expected with an RI of.75 feet Since the flow channel that develops is essentially equal to the side of the outlet or about 1.5 inches (0.125

feet). Since the ratio of RI /flow diameter is 6, the rathole is very stable. Photo2 shows the result of flow from a 4-

inch square outlet. No arch forms as would be expected since AI = 3 inches. However there is still a stable rathole

of about 4 inches in diameter (the side of the outlet). With a free flowing material the flow channel will expand to

the diagonal of the outlet, however since AI/d is almost 1 (indicating a very cohesive condition) the material archedat the corners of the square causing a circular flow channel diameter of dimension equal to the side of the square.

This rathole is still very stable since the flow channel is about half of the RI.

Photo 3 shows the results with a 1.5 by 4 inch outlet. There is only a slight hesitation to flow. Any arch is easily

broken with a slight tapping. Once broken the flow continues as long as there is no major collapse of material onto

the exposed outlet. There is no arch even though the width of the slot is less than AI. This is because of the material


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is required to converge in one dimension only above the long slot. For a one dimensional convergence channel, the

critical arching dimension is AI/2. Note that at the ends of the slot, there is a tendency to arch and thus neck down

the flow channel to about 3 inches in diameter. For a slot to be considered one dimensional as far an arching is

concerned, the length must be 2.5 to 3 times the width or greater. Photo 4 shows the result when the slot length isincreased to 7 inches. There is still a tendency to close in the ends of the slot, however the diameter of the flow

channel is increased to about 6 inches. This channel diameter is very close to the RI and as a result, the rathole ismuch less stable. This is evident by the expansion of the channel almost to the bin walls.

Photo 5 shows the flow when the slot length equals the diameter of the bin. The rathole collapses

completely to the bin walls. The material flows freely from the flat bottom bin. The reclaim angle is about 60

degrees from the horizontal at the center of the slot. The support from the bin walls tends to make the angle slightlysteeper at the ends of the slot. This is likely caused from the arching tendency at the slot ends.

The general conclusions emerging from these photos are:

1. Arching occurs whenever AI is less than the side of a square or the diameter of a circular outlet.2. Arching over a one dimensional convergence long slot does not occur when the slot width exceeds AI/2.3. The flow channel in a flat bottom bin forms equal to the diagonal of the outlet provided arching is eliminated at

the ends of the slot.

4. A rathole above an outlet is stable whenever the diagonal of the outlet is less than RI.

5. Extending the outlet to the full diameter of the bin eliminates any ratholing.

Indices basis for stockpilesApplying these general principals to stockpiles begs the question, what is the rathole indices basis D for a

stockpile where there is no defining diameter. D for a pile is the height of the pile above the flow channel. Forexample consider photo 6 with a.15 by10 inch slot under a 7-inch tall pile. The height at the center is 7 inches

where as the height at the ends of the slot is only 3 inches. The RI are respectively 6.5 inches and 3.3 inches. Since

the slot length and hence the flow channel diameter is 6 inches we expect the ends of the slot to be unstable and the

rathole caves in. Notice that the reduced head at the ends of the slot has eliminated the arching tendency at the endsof the slot. The d for determining AI when initially filling the pile on the flat bottom without running the feeder is

given by half the pile height. This is a very unusual case however and in most cases the d is given by the outlet

width at the flat bottom of the hopper outlet diameter in the case of a hopper. When tractors are working the pile the

indices basis must be modifies to account for the compaction given by the tractor wheels or tracks and the material.

At the level or the flat bottom d is equal to half the pressure from the tractor divided by the bulk density of the

material. At the bottom of the hopper d is the hopper outlet diameter.

Stockpile layout


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Using the above general principles, suggests the following pile outlet parameters: For a single outlet under

a conical pile as shown in figure 3 the length of the slot l must be greater than RI and the width greater than AI/2 for

a long slot. The dashed line circles indicate the region of influence of the flow above each slot.

P an V ew

L

W

L>RI

W>AI/2, if L>3W

FIGURE3 BASIC SINGLE OUTLET PERAMETERS

FLOW CHANNEL

FROM SLOT OUTLET

EDGE OF PILE

The elongated pile in figure 4 is serviced by a series of single slots each designed to break its associatedrathole.


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P an V ew

L

W

LL

AS CLOSE AS FEASABLE EDGE OF PILE

FUGURE 4 INDEPENDENT SINGLE SLOTS

The single slot design is not practical when RI is very large. Arranging multiple outlets as shown in figure5 reduce the required slot length by causing adjoining flow patterns to intersect. This increases the effective

diameter to about 2.14 times the slot length as illustrated in the figure. The multiple direction feeder arrangementallows room for feeder head and tale pulleys.

P an V ew

L

W>AI/2

DO=2.14 L > RI

FIGURE 5 LAYOUT FOR EXPANDED

EXTENT OF COMBINEDFLOW PATTERN


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P an V ew

LW

EDGE OF PILE

FIGURE 6 MULTIPLE ADJIONING FLOW PATTERNS

The multiple parallel slots shown in figure 6 extends this multiple intersecting flow patterns to a series of

slots arranged to form a combined slot essentially the full length of the elongated pile. Any combined flow pattern

must produce an effective length greater than RI or extent the flow to the bin walls or full pile extent.

Flat bottom binsAny of the flow patterns discussed for stockpiles can be used effectively in flat bottom bins to produce a

flow pattern that extends the full bin diameter. Photo 5 shows the results when a1.5-inch wide slot is extended

across the entire diameter of the bin. The 60 degree angle of reclaim (measured from the horizontal) is much steeper

than the angle of repose in a conical pile of about 42 degrees and is much steeper than a hopper angle would need to

be for cleanout. A figure 5 arrangement when used in a flat bottom bin minimizes the dead regions.Hopper selection

Hoppers under stockpiles and flat-bottom bins serve to reduce the size and consequently the cost of the

feeders required and to prevent potential arching from initial pile loading or the use of tractors on the pile. This last

consideration requires that the hopper inlet be sufficiently wide to break up the over compacted solids before they

reach the outlet just above the feeder.


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L > 2.5 W top

End View

P an V ew

Side View

W bottom > AI / 2 for hopper conditionsMATERIAL PROFILE ON BELT

FIGURE 7 TYPICAL BELT FEEDER HOPPER

W top

tapered transition to belt

>HI >HI+10

Corner Plate typical

W top > AI for tractor worked pile or forinitial fill pile height before feeder is run

Figure 7 shows a rectangular hopper and belt feeder. The width of the hopper can be as small as AI/2 for the

appropriate pile condition. The length however must be at least 2.5 times the width for the arch breaking to beeffective. Consider the wet coal and tractor worked pile condition of table 3 example 3. W= 6.9 feet and L= 17.3

feet. W at the bottom is taken from example 2 at 2.7 feet. The end and sidewalls are HI and HI+10 degreesrespectively or 16 and 26 in the coal example. The sharp corners will build up with solids unless corner plates are

provided as indicated. A tapered transition to t belt feeder is essential as shown to reduce feeder power requirements

and to eliminate bridging from cohesive material packed by the belt against the front end of the hopper. This type of

hopper design tends to reduce headroom requirements but maximizes the feeder length feeder width and feeder

power required. The belt feeder in the coal example would be about 20 feet long and 4 feet wide. Using the hopper

concept in figure 8 can minimize feeder requirements.

W top

W bottom > AI / 2 for hopper

Side View End View

P an V ew

> HI+10>HI

Typical corner plate

W top

W top > AI for tractor worked pile or forinitial fill pile height before feeder is run

Vertical outer skirts

FIGURE 8 TYPICAL APRON FEEDER HOPPER

This concept requires more headroom than in figure 7 because the hopper width at the top is equal; to the

length and consequently must be equal to AI instead of AI/2 as in figure 7. When the feeder cost is a major factor as


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in the case of an apron feeder, the extra headroom required may easily pay for itself. In the coal example 3, W=13.8

feet. The length of the feeder is reduced to about 10 feet. The figure also shows a typical transition to an apron

feeder to minimize the width of feeder required. The vertical skirts prevent spillage and allow the material to

occupy the full feeder width. This reduces the feeder width to about 3 feet. The hopper angles are as is figure 7.The corner filler plates are very important in this case. It is important to note that in both figures 7 and 8, the

assumption is that the cohesive arching of the material dominates the hopper design, as is the case with example 3where the tractors destroy any rathole. Ratholing may still be a serious problem without tractors so these hoppersmust be used to form long slots as indicated in the pile layouts of figures 3, 4, 5 and 6. One very effective way of

forming long slots in given in figure 9.

Side View End View

Plan View

HI+15

3 x AI 2AI

Side View End View

HOPPER DETAIL

Belt

Gate

Fi ure 9. One-dimensional conver in ho ers for lon slot

Figure 9 uses a series of one dimensional convergence hoppers [4,5] to deposit material directly on the

conveyor belt. This can totally eliminate any feeders. The hopper angles are Hi + 15 degrees and consequently are

flatter than any of the previous hoppers. The size of outlet required to prevent arching is AI, the same as for a long

slot. This configuration was used successfully to retrofit a covered limestone pile that had severe ratholing

problems. In this case there were already a series of short vibratory feeders feeding from square outlets onto a

collecting belt.

Erratic flow from hoppers and feeders

Errat ic f low with co hesive sol ids

Sequential arching and ratholing may cause pulsating or erratic flow. As an arch forms and vibration or

other flow aid devices are activated, the arch breaks and flow continues until the rathole collapses. The impact of

the collapsing solids compacts material at the hopper outlet, causing it to arch. The cycle then repeats itself. This

sequence occurs when material is cohesive and bins walls are not steep enough or the right shape to prevent

ratholing. While the frequency and duration of non-flow conditions can often be controlled with the judicious use of

flow aid devices, the wear and tear of the solids impact and flow aid agitation on bin walls may still cause hopper

wall failure.

Sever flow rate limitations may occur with cohesive solids that have strain rate sensitive strength

properties. Viscous solids such as oil sand and wet clay require special tests to determine the strength as a function

of the strain rate occurring in converging hoppers. In effect the AI is a function of the feed rate. The limiting rate is

reached when the arching tendency associated with the rate is equal to the hopper outlet size.


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Er ratic fl ow of powders

With respect to erratic flow, fine, relatively non-cohesive, dry powders such as lime added to coal or fluedust from a mineral processing unit, introduce an entirely different problem. One time a powder may flow like a

fluid through belt feeders, screw feeders, rotary air locks or even hopper bolt holes. At other times, a powder may

come out of a hopper so slowly that the feeder, even though active, appears empty. This is explained in reference

[1]. This limiting rate is given by the Flow Rate Index (FRI) for the powder (see Table 1). The FRI is directly

proportional to the air permeability at the hopper outlet, and inversely proportional to the density change that the

powder experiences in the hopper. Uncontrolled powder flow is usually associated with a source of air such as a

rathole collapse, air introduced to purge feeder bearings, air injected to aid flow such as from air cannons or pulse

jets and leakage past rotary valves. The type of feeder under the hopper also influences the limiting flow rate.

Er rati c powder fl ow controlled with feeders

Consider the condition where a powder is quickly loaded into a bin so there is little time for the air to escape.

Before the gate is opened, pressure at the gate is positive and the powder tries to escape from any available hole.

When the gate is opened, air pressure forces the powder out of the hopper. This forced flow may overcome the

feeders resistance and the powder flushes. If flow occurs at the hopper walls, the powder flow may cause sufficient

expansion to eliminate the positive pressure and given sufficient time, will reach the vacuum-limiting rate. If the

powder flows in a limited diameter, central flow channel as shown in photo 1, the fresh pressurized powder may

reach the hopper outlet and flushing may continue for several minutes. In some cases, the bin may totally empty as

powder sloughing in the top of the rathole continually refills the rathole with freshly fluidized powder.

F low at the hopper wall s controls err atic flow

The lack of flow along hopper walls is the most likely cause of powder flushing. Photo 7 shows four oscillating

plates in the corners of a pyramid shaped 45-degree hopper [6]. The plates are pivoted about a pin near the top and

activated with a slight cross movement near the bottom [6]. This activation keeps the rathole full of deaerated

powder, moves the powder along the hopper walls, prevents aeration of the material and allows the hopper to be

emptied without flushing [9].


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F lushing when refi ll ing the hopper

When hoppers are refilled after being almost emptied a large air pressure often develops in the powder as the

entrained air is compressed. A tight closing gate is essential and the material must be allowed to stand until it isdeaerated. The FRI, FDI, indices basis d and the height of the bin provide a guide for the time t to deaerated.

t =FDI x H x (pi/4) x d x d / FRI (1)

If this time is too short for the operation, then activating the oscillating plates during filling can significantly shorten

the deaeration time.

Sealing screw feeders [7] can eliminate flooding and flushing. The sealing screw (Photo 8) is equipped with a

pressure plate at the end that forces powder to make a plug. When deaeration and powder compaction is needed, avent pocket allows compressed air to escape. If only a seal is required, the vent usually can be eliminated even

though it serves as a safety valve to maintain the end seal. This feeder can contain the positive pressure created in

the initial bin fill. If a rotary valve is required to isolate a high-pressure system from the bin, it should be placed at

the discharge end of a sealing screw so that air leakage can be vented at the discharge end of the screw and not be

allowed to penetrate the hopper.


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Belt and vibrating feeders do not have the same holdback characteristics of a screw feeder even with well-designed

leveling boxes. The retention time in leveling boxes is usually too short for effective deaeration to occur. Equation

1 provides some guide as to their effectiveness where H is the height of the box.

Using air permeation to control errati c powder f low

Since the cause of limiting rates is the vacuum that forms in the material, the obvious solution is to introduce air at

key points in the hopper and eliminate the vacuum. This is accomplished with an air permeation unit APU. Air

permeation can increase powder flow rates as much as several hundred times beyond that allowed without air

injection. The objective of an APU is to increase the allowable flow rate to or beyond that required to fill the feeder

below the hopper, and thereby allow the feeder to control the feed rate. In the case of free-falling powders, its

possible to adjust the APU setting to keep rates within certain limits. Air permeation systems may be required

whenever the following occur: (1) The fine, dusty powder has sufficient bin residence time to lose most of its

entrained air; (2) the required instantaneous process flow rate exceeds the measured Johanson Flow Rate Index; or

(3) the air pressure at the hopper outlet is sufficiently large, relative to the air pressure within the bin, to retard the

powder flow rate. An APU may also help to a limited extent with viscous rate limits, however the air pressure

required is usually several psi. Consequently the air tends to blow holes in the cohesive material allowing the air to

escape. This results in wildly surging flow rates that may effect down stream operation.

Eliminating hopper flow problems

Hopper designed using the indices to size the outlet and the wall slopes to achieve flow at the walls can eliminate

most hopper flow problems. The major hopper hang-up problems, their causes, relation to the indices and likely

solutions are summarized in table 4.

Table 4. Eliminating Hopper flow problems

Observations Likely causes Flow Properties Likely Solutions

Nothing comes out ofthe hopper when thefeeder is started rightafter initial hopperloading.

The solids are cohesiveenough instantaneously toarch over the hopperoutlet.

AI is greater thanthe conicalhopper outletdiameter.

The outlet size must be increased or made moreeffective. Increasing the size means a new,larger, and more expensive feeder. The existingfeeder may be used by replacing the lower portionof the hopper with a one-dimensionalconvergence racetrack- shaped cross sectionDiamondback Hopper. This will increase theoutlet's effective diameter by a factor of two.Vibration should not be used in this case as it willlikely over compact the solid and make arching

worse.

Solids initially flowfrom the hopper butnothing comes fromthe hopper after thesolids sit at rest forsome time.

The solids are cakingsufficiently to causearching after time at rest.

Instantaneous AIis smaller thanthe conicalhopper outlet.However, the AI,after sitting at

Vibration or air cannons placed near the outletcan be effective in initiating solids flow providedthe conical hopper outlet diameter is greater than1.5 times the instantaneous AI. If this is notsatisfied, then the hopper and/or feeder will needto be modified to make the effective outlet 1.5


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some time. rest, is greaterthan the conicalhopper outlet.

times the instantaneous AI.

Flow from hopperoccurs for several

minutes then stopscompletely

There is likely a rathole inthe hopper, and the walls

are not steep enough tocause flow at them. Thehopper is likely cone orpyramid shaped.

HI is less than thesteepest hopper

angle measuredfrom the vertical.RI is greater thanthe diagonal ofthe hopper outlet.

Vibrators or air cannons may be effective if placedso as to open up the flow channel diameter

enough to exceed RI. However, the impact of thefalling rathole may cause an arch in the hopper, orin extreme cases, break the hopper from the bin.

A safer solution is to replace the lower part of thehopper with one that causes flow at the walls upto a diameter of RI or greater.

No flow occurs at thefront of the feeder.This may beobserved at thefeeder or at the top ofthe bin.

This is caused by solidsbuildup against the frontedge of the hopper. Thisindicates that either thefeeder interface to thehopper is improper or thatthe feeder is not designed

to draw from the entireoutlet cross-section.

Instantaneous AIis less than theconical outletdiameter, butinstantaneous RIis large. Thisindicates that the

solid is pressure-sensitive.

The simplest way to avoid buildup is to provide avertical section between the hopper outlet and thefeeder so that the buildup does not affect hopperflow. In cases where the buildup is too severe, itwill be necessary to modify the feeder interface orfeeder to provide flow along the entire outletcross-section.

Eliminating feeder problems

Feeder problems often start with the selection of the feeder type and size. In general the feeder must besized based not only of the feeder rated capacity but also on the arching and ratholing capability of the material

being handled. If the opening to the feeder is less than AI for a square or slot opening less than 2.5 times the width,

there will be arching problems at least some or the time. Using a small one dimensional convergence hopper [2, 3,4] can reduce this requirement to AI/2. Using oscillating plates [6] in conjunction with the one dimensional

convergence hopper can reduce the requirement to AI/4. Even meeting the arching requirement does not guarantee

feeder performance if the hopper is not designed to produce at least intermittent flow at the hopper walls and thus

eliminate ratholing. Collapsing the rathole with flow aid devices such as vibrators or air cannons will likely produceerratic flow that is independent of feeder operation. Making the feeder inlet long enough to eliminate ratholing

(length =RI) may be feasible with belt feeders apron feeders and specially designed screw feeders provided the

feeder interface is properly designed. This approach is almost imposable with vibratory feeders. Table 5 lists some

of the common feeder problems and their solutions.

Table 5. Eliminating Feeder problems

Observations Likely Causes Flow Properties Likely solutions

Any feeder typecannot start after thebin is initially filledwithout overloadingit.

The likely cause after allof the obviousimpediments to feederoperation such asimproper wiring,wrenches, hard hats,rocks, or other foreign

Large FDI andlarge HI increasethe load problem.

Running the feeder when the bin is initiallyfilled and then maintaining a heel in thebin of at least three hopper outletdiameters above the hopper outleteliminates the excessive initial loads. Ifthe process makes this impossible, thenthe feeder could be suspended from the


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material in the feeder areruled out, is excessivesolids pressure from theinitial bin loading. Thesepressures can be 2.5 to3.0 times the normalrunning pressures.

hopper on springs or high loads should bedesigned for and a high starting torquefeeder motor should be provided.

Any feeder typesuddenly feeds veryinefficiently eventhough it remainsactive. The feedrate reacts to feederspeed but at alesser amount.

There is likely a blockageof the outlet either fromforeign material or fromchunks of solids likelydislodged from thehopper walls.

Large time RI anda small H,possibly causedby temperaturecycles over a timeperiod, contributeto the problem.

A grizzly at the top of the bin will keep outmost foreign materials and will limit lumpsize into the bin. The best way toeliminate lumps formed inside of the bin isto design the hopper for flow at the walls.

Any feeder typegradually losesfeeding capacity

over time.

This is an indication thatthere is solids buildup onthe belt, screw shaft,

vibrating pan or rotatingvane pockets.

Large CI, largetime AI and RI areindications of this

problem.

While better belt scrapers, vibration of thefeeder and rotating vane blow-throughmay help; the problem is the interaction of

the solid and the feeder surface asreflected by the high CI. Using stainlesssteel shafts vanes and pockets, or coatingthem with materials with a low CI will workin some cases. Other times there may beno solution except to eliminate the use ofthe offending feeder types (most often avibrating feeder).

A screw feederinitially starts thensoon stops with anoverload.

The likely cause is apinch point in the screwflights where the screwflight spacing in the

direction of solids flowdecreases significantly.This is usually amanufacturing toleranceproblem but may becaused by the feederrunning against a closedgate and thus bendingthe final flight.

Less than 5%differencebetween FDI andBDI indicate a

moderatelyincompressiblesolid that is likelyto have problemswith typical screwconveyortolerances.

If the pinch point occurs in a region wherethere is no solids flow into the screw fromthe hopper, eliminating the shroud at thepinch point and at all points down stream

reduce the overload. If the pinch pointoccurs under the hopper, or if the screw isin a barrel, it is necessary to rework thescrew to eliminate pinching to within atolerance of half the percentage differencebetween FDI and BDI.

A belt feeder initiallystarts then suddenlystops with anoverload.

The most likely cause isan improperly designedbelt interface that doesnot provide an increased

capacity in the directionof feed. The next likelycause is belt saggingbetween pulleys.

Large FDI and asmall percentagedifferencebetween FDI and

BDI contribute tothe overload.

The belt interface must be designed sothat the gap between the belt and theinterface edge increases at least3 of aninch per foot in the direction of solids

movement. Belt sagging can be reducedby placing idlers as close together aspossible, or by using a slide plate insteadof idlers.


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REFERENCES

[1] Troubleshooting Bins, Hoppers and Feeders Chemical Engineering Progress CEP April 2002 pp100-112

[2] Eliminating arching hang-ups in hoppers. Diamondback Technology News. Vol. 1, No.1, Summer 1997,

pp. 1-3.

[3] Johnson, J. R. Making Solids Flow in Hoppers Using Passive Activation.bulk solids handling, Vol. 20,No. 1, January/March 2000, pp. 9-15.

[4] Johanson, J. R. Arch breaking Hopper for Bulk Solids. U. S. Patent 6,055,781. May 2, 2000.

Johanson, J. R. Modular Mass-Flow Bin, European Patent 0477219, March 3, 1996.

Johanson, J.R. Modular Mass-Flow Bin (a.k.a. Diamondback Hopper). Canadian Patent 2058942. May

16, 1995.

Johanson, J. R. Modular Mass Flow Bin (a.k.a. Diamondback Hopper). Australian Patent 640933.

January 17, 1994.

Johanson, J.R. Modular Mass-Flow Bin (a.k.a. Diamondback Hopper). Canadian Patent 2058942. May

16, 1995.

Johanson, J. R. Modular Mass Flow Bin (a.k.a. "Diamondback Hopper"). Australian Patent 640933.

January 17, 1994.

Johanson, J. R. Modular Mass Flow Bin (a.k.a. Diamond-back Hopper).U.S. Patent 4,958,741.

September 25, 1990.

[5] Johanson, J. R. Combination Hopper (a.k.a Diamo-Cone). U. S. Patent 5,361,945. November 8, 1994.

Johanson, J. R. Combination Hopper (a.k.a. Diamo-Cone). Canadian Patent 2,161,521. March 30, 1994.

Johanson, J. R. Combination Hopper (a.k.a Diamo-Cone). U. S. Patent 5,361,945. November 8, 1994.

Johanson, J. R. Combination Hopper (a.k.a. Diamo-Cone). Canadian Patent 2,161,521. March 30, 1994.

[6] Johanson, J. R. Hoppers with Directionally Applied Relative Motion to Promote Solids Flow . U. S.

Patent 6,086,307. July 11, 2000.

[7] Johanson, J. R. Compacting Screw Feeder. U. S. Patent 5052874. October 1, 1991.


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Johanson, J. R. Compacting Screw Feeder. Canadian Patent 2058942. May 16, 1995.

[8] Understanding and eliminating costly rathole. Diamondback Technology News. Vol. 1, No. 2, Fall 1997,pp. 1-3.

[9] Preventing powder flooding and flushing. Diamondback Technology News. Vol. 2, No.1, Spring 1998,pp. 1-3.

preventing solids flow problems

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