Flat Belt Bulk Material Feeder Accuracy:How to achieve it, how to sustain it, how to prove it

By:

Kevin A. Alexeff, P.E.Manager, Mechanical DesignStock Equipment Co.Cleveland, Ohio

William E. DownsVice President, EngineeringStock Equipment Co.Cleveland, Ohio

Presented at Power-Gen International Conference and ExhibitionDallas, Texas, U.S.A.

December 9 - 11, 1997

i

Contents

Page

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Flat Belt Bulk Material Feeder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2What is accuracy? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Factors that affect the accuracy of Flat Belt Bulk Material Feeders . . . . . . . . . . . . . . . . . . . . 4

How to achieve accuracy in Flat Belt Bulk Material Feeders . . . . . . . . . . . . . . . . . . . . . . . . . 4Gravimetric feeder belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4System geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

How to sustain accuracy in Flat Belt Bulk Material Feeders . . . . . . . . . . . . . . . . . . . . . . . . . . 9Environmental factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

How to prove accuracy in Flat Belt Bulk Material Feeders . . . . . . . . . . . . . . . . . . . . . . . . . . 11Material Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Chain Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1

Summary

A Flat Belt Bulk Material Feeder is a metering device designed to accurately control continuousflow of a bulk material into a downstream process. In simple terms, it is a flat belt conveyor withfeedback control of the amount of material conveyed. While much has been written on th efactors that affect the accuracy of other types of bulk metering devices, very little has bee nwritten on Flat Belt Bulk Material Feeders. This study focuses on the factors which affect theaccuracy of the modern gravimetric coal feeder. The factors that affect the accuracy of thes especial flat belt feeders are examined in detail. Then, specific aspects of feeder design ar epresented which allow one to achieve accurate control of material flow, sustain it for extendedperiods, and prove it easily.

Special attention is given to three critical areas. First, proper belt design is critical to achievingaccurate control of material flow. Second, resistance to environmental factors is critical t osustaining accurate control of material flow. Third, a design that is conducive to an accurat echain test is critical to easily proving accurate control of material flow.

A highly flexible belt is not required for most bulk material conveying applications. However,belt flexibility is critical to accuracy on gravimetric flat belt feeders. Using a single-ply belt canreduce the effect of the belt on weighing the material by up to 75 percent versus a two-ply belt.Both the belting material and the splice must be designed for minimum impact on the weighingelements.

The environment inside a gravimetric coal feeder is arguably one of the worst environment simaginable for accurate operation of a precision instrument -- hot, wet, corrosive and dust-laden.Yet the measuring components of the feeder must operate continuously in this environment forsometimes up to a year without recalibration. All of the linkage points in the weighing systemmust be designed to be resistant to corrosion and dust build-up.

To prove that a bulk material feeder is accurate, it must be compared with a measuremen tstandard. The only commonly accepted standard for verifying the accuracy of a Flat Belt BulkMaterial Feeder has been to run a material test -- diverting the material into a portable receiver,then weighing the material on a certified scale. Most flat bel t feeder installations, however, makematerial tests difficult and expensive. Simulating material load using chains has been used withvarying results as a way to check calibration data between material tests. But, when the tes tchains and the feeder are designed together, it is possible to obtain results with chain comparableto a material test.

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Figure 1Typical Flat Belt Coal Feeder

Introduction

Flat Belt Bulk Material Feeder

A Flat Belt Bulk Material Feeder is a metering device designed to accurately control continuousflow of a bulk material into a downstream process. In simple terms, it is a flat belt conveyor withfeedback control of the amount of material conveyed.

There are several ways to achieve control of the amount of material conveyed. One method isto ensure a constant volume of material on the belt and vary the speed of the conveyor in linearresponse to the amount of material required. These feeders are referred to as "volumetric. "Another method is to insure a constant weight of material on the belt and vary the speed of theconveyor in linear response to the amount of material required. Early mechanical weighin gfeeders used this type of control.

A more sophisticated approach to this method is to measure both the weight of the material onthe belt and the speed of the conveyor. These two values are electronically integrated to producethe flow rate of the material. The speed of the conveyor is varied throug h a feedback control loopto maintain the desired flow rate. These feeders are referred to as "gravimetric."1

The factors that affect the accuracy of both types of flat belt feeders are similar in many respects.Gravimetric feeders are, however, by nature much more complex. There are many more factors

3

Figure 2 - Precision versus Bias

that affect the accuracy of gravimetric feeders. So why use gravimetric feeders? Many bul kmaterials have a bulk density that varies considerably over relatively short periods of time. Coalis a perfect example. The bulk density of coal taken from an open pile can easily vary up to 20or 30 percent. Since a volumetric feeder assumes constant density, the weight basis feed rat efrom a volumetric feeder will vary along with the density. 2

While much has been written about the factors that affect the accuracy of other types of bul kweighing devices, especially troughed-belt conveyor scales, very little has been written3 4,5,6,7,8

about Flat Belt Bulk Material Feeders. Colijn, Abbott, Fristedt, and others have presente d9,10

theoretical models that attempt to quantify likely sources of error, focusing mainly on idle rmisalignment. The study by Abbott et al. is one of the few attempts to verify these model sexperimentally. However, except for an early investigation into speed sensing, their focus has11

been on troughed-belt conveyors. Flat belt feeders have been treated as a special case, or subset,of troughed-belt conveyor scales. In fact, flat belt feeders have a unique set of factors that affectaccuracy. This study focuses on the factors which affect t he accuracy of the modern gravimetric,flat belt coal feeder (shown in Figure 1). Much of the information presented has been gaine dduring a comprehensive three-year study, recently completed, at Stock Equipment Company inChagrin Falls, Ohio.

What is accuracy?

Before discussing the factors that affect the accuracy of Flat Belt Bulk Material Feeders, it i snecessary to understand what accuracy is. Accuracy has two components: bias and precision.Bias is the degree of agreement between a measured property and an accepted reference valuefor that property. Precision isthe degree of agreementbetween two or moremeasurements of the sameproperty. Precision, in turn,can be broken down into twocomponents: repeatabilitya n d reproducibility .Repeatability is the degree ofagreement between individualmeasurements taken by thesame operator under identical conditions within a relatively short period. Reproducibility is thedegree of agreement between groups of measurements taken by different operators, or unde rdifferent conditions, or at different times.

Error is a term used to describe the relative magnitude of the difference between two values. Weuse different expressions of error to describe the different components of accuracy. Offset erroris often used to describe bias. Random error is often used to describe repeatability, and operatorerror is often used to describe reproducibility.

P n Q Lcos

± ( 2 T DL

24 E I DL 3

)

e%

( 2 T DL

24 E I DL 3

)

n Q Lcos

×100%

4

Factors that affect the accuracy of Flat Belt Bulk Material Feeders

How can the factors that affect the accuracy of Flat Belt Bulk Material Feeders be identified? Itis useful to start with a theoretical model. However, a theoretical model is only useful i nidentifying where to look for sources of error. It cannot predict the accuracy of a system. Thisis where physical testing takes over. Experiments designed to isolate the effects of certai nvariables, and determine their influence on the rest of the system, are necessary to identify thefactors that affect accuracy. Then, ideally, these factors are ver ified under actual field conditions.

How to achieve accuracy in Flat Belt Bulk Material Feeders

Gravimetric feeder belt

A model of the factors associated with the belt and their effect on weighing accuracy, which canbe used for a Flat Belt Bulk Material Feeder, was developed by Hyer. This model assumes that12

the weigh system is symetrical. That is the approach angle of the belt into the weigh system isthe same as the retreat angle of the belt out of the weigh system.

where: P = Force on scale, poundsn = Number of weighing idlersQ = Belt load, pounds per inchL = Idler spacing, inches

= Angle of conveyor incline, degreesT = Belt tension, poundsD = Idler misalignment, inchesE = Modulus of elasticity of belt, pounds per square inchI = Moment of inertia of belt cross section, inches to fourth power

It is convenient to recognize that the force sensed by the scale (P) is the actual weight of materialon the belt plus (or minus) an error caused by the operating parameters of the conveyor belt. Theerror term can be expressed as a percentage of the actual weight of material, dividing it by theactual weight and multiplying by 100 percent:

e%100 cos

n Q( 2 T D

L 2

24 E I DL 4

)

5

Figure 3

Figure 4

Rearranging yields:

Here, the first term in the parentheses relates to the error in weigh ing imposed on the system frombelt tension (T). The second term in the parentheses relates to the error in weighing imposed onthe system from belt stiffness (E·I). In troughed-belt conveyor scale applications, the effect ofbelt stiffness is minimized by using anidler spacing that is as large as possible,usually 36 inches or more. Since theerror imposed on the system from beltstiffness is inversely proportional to thefourth power of idler spacing, beltstiffness is of little concern. A coalfeeder, though, has extreme constraintson the idler spacing possible due to theenvelope requirements of mostinstallations. At a typical idler spacingof only 18 inches, the model suggestsbelt stiffness is a significant factor infeeder accuracy. Experimentalevidence supports this. A typical two-ply belt represents about 10 percent ofthe total weigh system loading. Tests show that even the best feeder belts can vary in apparentweight (the weight sensed by the weigh system at any given time) by up to 70 percent (see Figure3).

Historically, Flat Belt Bulk Material Feeders have used two-ply belts for a combination o fdurability and accuracy. A "continuous" construction, or vulcanized splic e, was used to minimizethe effect of the belt on the weighingsystem. The two plies, though, act13

like an I-beam for the moment of inertia(I) in the model above. StructuralEngineers know that an I-beam has anextremely high strength-to-weight ratioin bending when compared with arectangular bar of equal cross sectionalarea. That is why floors are supportedby I-beams. The two plies of belt actlike the flanges of an I-beam, increasingthe stiffness (see Figure 4).

F P ± Ml

(1 d br b

)

6

Figure 5

Recent developments in feeder beltconstruction allow a single-ply,mechanically spliced belt with the sametensile strength and durability of a two-ply continuous belt. Tests show that thevariation in apparent weight of thesingle-ply belt was less than 25 percentof the variation of the two-ply belt (seeFigure 5).

System geometry

The other significant factor suggested by the model above is idler misalignment (D). Since belttension (T) in the weighing area of a flat belt feeder, ve ry much unlike a troughed-belt conveyor,is determined by the force required to shear material out of the inlet, there is little that can b edone to reduce belt tension. However, the idler misalignment can be controlled. Since idle rmisalignment is a function of both variation in initial roller placement and load cell deflection,it can never be completely eliminated. Initial roller placement can be controlled by a carefullydesigned calibration procedure. Load cell deflection can be minimized in the design of the loadcell itself. Most commercial load cells used in feeder applications are not designed to minimizedeflection over full load. Load cells should be carefully selected for a designed deflection overfull load of 0.004" or less. Using a weigh system designed with two load cells, instead of one,will cut deflection in half. When the weigh system is properly designed, with approximatel yequal tare weight and material load, both totaling very nearly the capacity of the load cells ,deflection (D) is minimized.

Hyer's model, above, only partially addresses the forces that affect the weigh system of short -centered flat belt feeders. Since nothing has been previously published on the subject, we havedeveloped our own theoretical models. The derivation of these models is too complex for14,15,16

the scope of this paper. However, the most significant contribution is duplicated here:

where:F = Force transmitted to load cell, poundsP = Force on scale, poundsM = Moment resulting from bearing

friction, inch-poundsl = Drag link length, inchesd = Drag link anchor elevation, inchesr = Weighing idler radius, inchesb = Bearing effective radius, inches

e%100 M

P l(1 d b

r b)

7

Figure 7Typical Flat Belt Feeder Control

Again, it is convenient to express the error term as a percentage of the actual force (P):

This model suggests that in order to minimize the effects of transferring the force on the scale tothe load cells through an idler roll, the bearing friction, and thus the resulting moment (M), mustbe minimized. The resultant moment on the weighing system must be counterbalanced with along drag link (l). And, the ideal location for the anchor point on the drag link is directly in linewith the top of the weighing idler roll (d = r). The other models point out the importance o fmaintaining the alignment in the linkages that transfer the force through the load cell. Some ofthese factors are minimal in a typical troughed-belt conveyor scale application and can b eignored. But all must be taken into consideration in a short-centered flat belt feeder.

Electronics

The typical Flat Belt Bulk MaterialFeeder uses an electronic controller,based on digital microprocessortechnology, to perform the calculationsnecessary to integrate the weight of thematerial and the speed of the conveyorinto the flow rate. Like the mechanicalcomponents, above, the electroniccontroller must be designed to minimizeerror. The system that measures the weight and the system that measures the speed are bot hpotential sources of error for the electronic controller.

Weight Measurement. An electrical force transducer, or load cell, is commonly used to measurethe force transmitted through the weighing idler. A load cell is a device that produces a nelectrical signal proportional to the applied force. Strain gages are used to convert deflection ofthe cell into the electrical signal. Precision strain gage load cells are suitable for use in a flat beltfeeder as long as they are sealed from the harsh environment and barometrically insensitive.

Strain gage load cells are powered by an excitation voltage and have an output signal usuall yrated as millivolts per volt of excitation. For example, a load cell with 10 VDC applied as theexcitation voltage and a nominal output rating of 3 mV/V will have a full scale output signal of30 mV. With such a small output signal, care must be taken to accurately measure the voltage.Even with the most accurate voltage measurement, a commercial load cell must be chosen thatmaximizes this output signal.

The accuracy of the weight reading is directly dependant on the stability of the strain gage bridgeexcitation voltage. Any change in the excitation power supply will have a subsequent change17

in the output signal of the load cell, which is interpreted by the controller as a change in weight.

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Of course, it is the excitation voltage directly at the load cell that determines the output voltage.Since cable resistance varies with temperature, the voltage directly at the load cell will var yslightly from the power supply voltage over any given temperature range. These mino rtemperature effects, inherent in the electronics and load c ell cabling system, can be compensatedfor by using a six-wire load cell cable and an excitation voltage sensing system. This allows thecontroller to make a true ratiometric measurement of the change in output voltage relative to thechange in excitation voltage. This change in output represents only the weight change, and notthe effects from temperature and power supply changes.

The small analog signal output from the load cell is converted to a digital signal by an analog-to-digital converter for input to the microprocessor. The analog-to-digital converter is anothe rpotential source of error that must be minimized. Resolution is importa nt to accuracy, to a certainpoint. But resolution means nothing if the analog-to-digital converter is not linear within ±0.05percent over the normal operating range of the system. Also, the electronic controller shoul dperform a calibration of the analog-to-digital convertor chip to el iminate the zero-scale and span-scale errors. This calibration must be performed often to assure that temperature drift effects areremoved.

Speed Measurement. There are many methods, and difficulties, with accurately measuring thebelt speed. A speed sensor is used to provide a rotational sp eed proportional to belt speed. Sincebelt tension varies considerably at various locations around the conveyor, the ideal location tomeasure the belt speed is at the weighing idler. Belt speed measurement at the weighing idler isnot practical, however, since it will introduce a reaction force in the weighing system an dadversely effect the weighing accuracy. As alternatives, the speed sensor can be mounted at thetail pulley, at an idler roll, or at the driven head pulley. The driven head pulley gives a nadvantage in a Flat Belt Bulk Material Feeder, having close proximity to the weigh system andtherefore the closest belt tension and speed. The advantage of not having the speed sensormounted the driven head pulley is that the effects of belt slippage or breakage are minimized .This could be an important consideration in long troughed-belt conveyor scale applications. Itis less of an advantage in gravimetric flat belt feeder applications where belt tension is alwaysreletively low.

An accurate and reliable method for sensing belt speed is to measure the belt drive motor speedand directly correlate this to belt speed. The correlation can be made by directly measuring beltspeed, with the use of precision optical switches placed over the weigh plane, during calibrationof the gravimetric feeder. This method virtually eliminates the usual problems involved i nmeasuring belt speed at a pulley or roller (ie: material accumulation aroun d the tail pulley or idlerrolls causes nonrepeatability of the belt speed measurement).

A tachometer is mechanically coupled directly to the output of the drive motor. This keeps thebelt measurement instrumentation outside the harsh environment of th e feeder, thereby increasingthe reliability of the device. The tachometer signal can be measured to an accuracy of withi n0.05% through the use of a stable crystal oscillator as a reference. This allows the belt spee dsignal, in total, to be accurate to within ±0.10 percent of true speed.

9

There is one last design consideration for the electronic controller of a gravimetric flat belt feeder,which holds true for any microprocessor based instrument where accuracy is demanded. Th estability of the electrical inputs and outputs are dependant upon a stringent grounding schem ewhere the analog and digital ground circuits are separated. This separation minimizes an yelectrical noise coupling which the load cell cables and speed sensor wiring may induce. Filtersmay also be used for the six wire load cell cable, including the excitation sensing lines, to shuntany induced RFI to the ground circuit before the noise is injected into the microprocessor circuits.

How to sustain accuracy in Flat Belt Bulk Material Feeders

Environmental factors

The environment inside a gravimetric coal feeder is arguably one of the worst environment simaginable for accurate operation of a precision instrument -- hot, wet, corrosive and dust-laden.The ambient temperature near the boiler on the feeder floor of a power plant can be well ove r100 F. The inside of the feeder can be exposed to continuous temperatures of up to 140 F.o o

Steam inerting, periodically flooding the environment in side the feeder with 250 F steam, is usedo

to reduce the risk of fires and explosions. And moisture from the steam, or from the coal itself,reacts with sulfur in coal dust to form acidic condensation. The p resence of moisture encouragescoal dust to build up on every exposed surface. Yet the measuring components of the feede rmust operate continuously in this environment for sometimes up to a year without recalibration.How can the weigh system of a feeder sustain better than ±1/2 percent accuracy in thi senvironment?

All of the linkage points in the weighing system must be designed to be resistant to changes intemperature, corrosion and dust build-up. While many sources of error described above ar ebiases, and may be calibrated out of the total error, changes in temperature, corrosion, and dustbuild-up affect repeatability. These environmental factors can greatly affect the accuracy of thefeedrate over time.

To minimize dust build-up, horizontal flat surfaces should be eliminated to the greatest exten tpossible. Pivots should be designed to transfer only the force on the scale to the load cell and beresistant to corrosion. Early mechanical weighing feeders used knife-edge pivots for this purpose.But knife-edge pivots only isolate the weigh system about one axis. The load cells in a nelectronic gravimetric feeder really must be isolated about every axis perpendicular to the forceon the scale. For this, a point-contact pivot is required. Spherical bearings simulate a point-contact pivot, but they have large surface contact and freeze when subjected to corrosion an ddust. A pin-and-ring pivot provides the ideal isolation for the load cell. Two curved surfaces ofdifferent radius provide theoretical point-contact, insuring high surface pressure, which helps tokeep the pivot free of dust and corrosion. For the drag link anchor, a flexure is the ideal pivot --it has zero stiction and is impervious to dust and corrosion.

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A free-hanging load cell that is not constrained in any direction, except in the direction the forceis applied to the scale, also protects the weigh system from outside forces caused b yenvironmental factors such as change in temperature, foreign objects, and vibration. Thes eoutside forces affect the accuracy of the weigh system by introducing random error.

Maintenance

The owner of a Flat Belt Bulk Material Feeder can have sig nificant impact on the accuracy of thefeeder in two ways: preventive maintenance and calibration. The importance of preventiv emaintenance is obvious, and the manufacturer's guidelines for care and cleaning should b efollowed explicitly. These guidelines should include steps to prevent the environmental effects,above, from overwhelming the weigh system's capability to overcome them. Perhaps mor eimportant, though, is a properly designed and implemented calibration procedure. The feede rmust be designed with a means to accurately calibrate the weigh system.

The error imposed on the system from belt tension is well understood. If the weighing idler istoo high, tension on the belt pushes down on the roll adding to the material weight. If th eweighing idler is too low, tension on the belt holds up a portion of the material subtracting fromthe material weight. As stated earlier, a carefully designed calibration procedure can help t ocontrol idler misalignment and, therefore, reduce the total system error. A typical flat belt feedermust have the weighing idler positioned accurately on the order of ±0.001 inch.

The first step is to align the weighing idler with the fixed idlers. This must be done with a loadvery near to the normal material weight on the weighing idler. To simulate the load, a test weightis applied to the weigh system. Then, the elevation of the weighing idler is adjusted to align itwith the fixed idlers. To do this within ±0.001 inch requires a very accurate straightedge an dsufficient resolution on the adjustment of the idler elevation. This is difficult to achieve i npractice.

Even if the weighing idler could be positioned with the required precision, it still would not bein the correct location. A test weight applied to the weigh system does not increase the bel ttension as a material load does. As a result, the belt effects are not properly accounted for. Tocorrect this, a belt perturbation procedure, similar to that proposed by Colijn and Hyer fo rtroughed-belt conveyor scales, can be added to the calibration. Belt perturbation alternatel y18

raises and lowers the belt tension. The weighing idler elevation is adjusted after each tensio nchange until the ideal location is reached where the belt tension has no effect on the weig hsystem.

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How to prove accuracy in Flat Belt Bulk Material Feeders

Material Test

NIST Handbook 44 (formerly National Bureau of Standards, NBS Handbook 44) states that “Anofficial test of a belt-conveyor scale system shall be a materials test.” Simulated load tests may19

be used to monitor the system’s performance between tests, but shall not be used for officia lcertification. A material test requires diverting the material discharged from the feeder into aportable receiver and weighing the material on a certified scale. Handbook 44 gives importantguidelines for properly conducting a material test. Implicit in the description of a material test,but not explicitly stated, is the fact that the test must be started and finished with the belt empty.Most coal feeder applications, though, make a material test extremely difficult and costly, if notimpossible, to perform. In most cases, when a material test is performed on a coal feeder, the testitself is flawed and the error attributed to the coal feeder.

Chain Test

How, then, can the owner of a Flat Belt Bulk Material Feeder verify the accuracy of his feeders?We first examine the reasons why, historically, test chains have not been accepted as a meansof verifying accuracy:

1. Test chains are not repeatable, because longitudinal positioning error produces differentscale readings.

2. Test chains have an inherent bias, because the chains do not create the same tension in thebelt as a full load of material does (an exception would be a horizontal conveyor wherethe chains extend all the way from the loading point through the discharge point).

3. Test chains are not homogeneous and the pound per foot rating is not easily determined.Therefore, longitudinal positioning affects scale loading and calibration of the chains isdifficult.

4. Test chains can affect belt tracking.

5. Test chains are made up of many unbalanced rollers that spin at high speed and bounceover imperfections in the belt or the splice. This causes “noise” on the weigh system thatmay be integrated as a bias error.

6. Wear and corrosion, in extreme cases, can noticeably decrease the pound per foot ratingof the chains.

7. Test chains can be very expensive, especially in large troughed-belt conveyo rapplications.

12

Colijn and Hyer identified these potential problems with test chains and provided the key tosolving them for a short-centered Flat Belt Bulk Material Feeder. They note: “The significanceof chain position depends on both the pitch and uniformity of the chain, and on the design andlength of the scale suspension system. Ideally, the pitch [of the chain] should be as small a spossible, and it should divide into the idler spacing exactly.” The significance of this has been20

unappreciated for many years.

We have been able to prove that, on a properly designed short-centered Flat Belt Bulk MaterialFeeder, a precision chain whose pitch divides evenly into the idler spacing of the weigh systemcan repeatably represent a material load on the belt independent of chain position (for example,chains with a 3-inch pitch are used on a feeder with an idler spacing of 18 inches). Further, if thechains are applied properly, a chain test can accurately simulate the results of a material test withless than a 0.2 percent bias. Hundreds of tests, under laboratory and field conditions, confir mthis.

What does this mean to the owner of a Flat Belt Bulk Material Feeder trying to verify th eaccuracy of his feeders? A chain test can be used for calibration and verification of Flat Bel tBulk Material Feeder accuracy, because:

1. Test chains are repeatable if the weigh system and chains are both designed to have anintegral pitch, and thus be independent of longitudinal position.

2. The chains can be designed to fit the feeder exactly. Short-centered Flat Belt Bul kMaterial Feeders are typically horizontal and the chains can easily e xtend all the way fromthe loading point through the discharge point.

3. The chains can be designed to be extremely homogeneous over the short weigh systemin a Flat Belt Bulk Material Feeder.

4. Test chains do not adversely affect belt tracking in a properly designed Flat Belt Bul kMaterial Feeder.

5. The chains can be designed to reduce “noise” on the weigh system of a Flat Belt Bul kMaterial Feeder. This is especially easy if the belt and splice are designed to minimizeimpact on the weighing system (as suggested above).

6. The chains can be designed to be extremely resistant to wear and corrosion, especially inFlat Belt Feeder Applications, where calibration of the feeder is infrequent.

7. The short chains required for Flat Belt Bulk Material Feeders are relatively inexpensivewhen compared with typical troughed-belt conveyor applications.

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Conclusions

There are many factors that affect the accuracy of gravimetric Flat Belt Bulk Material Feeders.This study focuses on our work with a typical coal feeder, but much of the information is generalin nature and can be applied to any flat belt feeder handling almost any bulk material. It i simportant to realize that while a flat belt feeder is similar to a troughed-belt conveyor scale i nsome respects, there are many unique considerations created by the operational requirements formost applications and by the constraints in the envelope for most installations. Design of a naccurate Flat Belt Bulk Material Feeder (no bias, high precision) must be a pproached from a bodyof knowledge specific to short-centered flat belt feeders, and n ot as a scaled-down, self-containedbelt conveyor scale.

All of the factors presented must be attended to in some degree in order to meet an error budgetof ±1/2 percent or less, and to maintain that accuracy for extended periods of continuou soperation without calibration. We have found that one of the most trivialized components of aflat belt feeder, the belt, is one of the most critical factors in Flat Belt Bulk Material accuracy.A highly flexible belt is not required for most bulk material conveying applications. However,belt flexibility is critical to the accuracy of gravimetric flat belt feeders. The stiffness of the beltin bending causes a bias in the weight sensed by the weigh system. This bias can not b ecalibrated away using the conventional method of calibrating with a test weight. Using a single-ply belt can reduce the effect of the belt on weighing the material by up to 75 percent versus atwo-ply belt. Both the belting material and the splice must be designed for minimum impact onthe weighing elements.

Environmental factors, including changes in temperature, corrosion, dust build-up, foreig nmaterials, and vibration, affect the accuracy of Flat Belt Bulk Material Feeders by introducingrandom error. They can greatly affect the feedrate over time and can not be calibrated out of thetotal error. To combat these factors, all of the linkage points in the weighing system must b edesigned to resist corrosion and dust build-up. A free-hanging load cell that is not constrainedin any direction, except in the direction the force i s applied to the scale, is critical to the accuracyof gravimetric flat belt feeders.

The conventional method of calibrating a Flat Belt Bulk Material Feeder with a test weight canleave a considerable component of the bias error in the system -- the belt factors. Adding aperturbation procedure can greatly improve the calibration. However, the calibration is stil lsubject to operator error. To prove that a bulk material feeder is accurate, it must be comparedwith a measurement standard. The only commonly accep ted standard for verifying the accuracyof a Flat Belt Bulk Material Feeder has been to run a material test. Simulating material loa dusing chains has been used with varying results as a way to check calibration data betwee nmaterial tests. But, when the test chains and the feeder are designed together, it is possible t oobtain results with chain comparable to a material test, at a fraction of the cost.

The checklist on the following page sumarizes the specific aspects of feeder design which allowone to achieve accurate control of material flow, sustain it for extended periods, and prove i teasily.

Checklist for Flat Belt Bulk Material Feeder Accuracy

The weigh system is symetrical. That is the approach angle of the belt into the weig hsystem is the same as the retreat angle of the belt out of the weigh system.

The feeder inlet is designed to have no influence on the weighing system and to providea consistent flow of material onto the belt.

The belt is single-ply, mechanically fastened, and specifically designed for gravimetricfeeder applications to minimize impact on the weighing system.

Load cells are applied such that the full range of material loading causes a deflection ofless that 0.002 inches.

Idler rolls have a total runout of less than 0.010 inch.

Idler bearings and seals are designed to minimize friction on the weighing idler.

A long drag link is used to counterbalance moments on the weighing idler, and the draglink anchor point is located directly in line with the top of the weighing idler roll.

The weigh system linkage is designed to be impervious to outside forces.

All of the linkage points in the weighing system are designed to be resistant to corrosionand dust build-up.

Load cell excitation voltage and nominal output rating are selected to maximize the fullscale output signal.

The analog-to-digital converter is linear within ±0.05 percent over a temperature rangeof 0 C to 65 C.o o

The analog-to-digital converter reference voltage is frequently calibrated to the load cellexitation voltage to eliminate voltage variation error.

The belt speed signal is accurate to within ±0.10 percent of true speed.

The electronics are configured and packaged to maintain a high degree of isolation fromexternal electrical noise.

Calibration procedure allows for the weighing idler to be aligned to the fixed idlers within±0.001 inch.

Feeder design allows for a perturbation procedure during calibration to eliminate the beltfactors.

The weighing idler, ideally, is centered between the fixed idlers within ±0.005 inch.

1. Colijn, Hendrik. Weighing and Proportioning of Bulk Solids, 1st ed. (Clausthal,Germany: Trans Tech Publications, 1975), pp. 266 - 270.

2. Bennett, A. and Hardgrove, R. “Coal Flow From Bunker to Pulverizer or Cyclone,”Paper presented at the South Eastern Electrical Exchange, Production SectionMeeting, Hot Springs, Arkansas, 1970.

3. Colijn, Hendrik. Weighing and Proportioning of Bulk Solids, 1st ed. (Clausthal,Germany: Trans Tech Publications, 1975), pp. 157 - 158, 185, 227 - 229.

4. Colijn, Hendrik. "Effect of Belt Conveyor Parameters on Belt Scale Accuracy," Paperpresented at the 18th Annual ISA Conference, Chicago, 1963.

5. Abbott, J. A. et al. "Belt Weighing Test Facility at Warren Spring Laboratory," BulkSolids Handling, Volume 1, Number 2, May 1981, pp. 239 - 243.

6. Abbott, J. A. "The effect of Idler Misalignment and Belt Stiffness on Belt WeighingErrors," Bulk Solids Handling, Volume 6, Number 1, February 1986, pp. 121 - 128.

7. Fristedt, K. "Belt Weighing Errors -- From Where Do They Origin?" Bulk SolidsHandling, Volume 6, Number 5, October 1986, pp. 963 - 968.

8. Abbott, J. A. "The Effect of Conveyor Belt Mistracking on Beltweighing Errors,"Bulk Solids Handling, Volume 9, Number 1, February 1989, pp. 107 - 117.

9. Bateson, R. N. and Grader, J. E. "Controlling the Flow Rate of Dry Solids,"Control Engineering, March 1968, pp. 60 - 64.

10. Rebucci, Gene. "Gravimetric Feeder Technology Designed to 'Pour On The Coal',"Combustion, April 1978, pp. 14 - 17.

11. Jones, R. J. and Laws, K. G. "Speed Sensing in Belt Weighing," Warren SpringLaboratory Report No. LR 267 (MH), 1979.

12. Hyer, F. S. "A Scientific Approach to Conveyor Weighing," Masters Thesis,University of Wisconsin, 1967.

13. Stock, Arthur J. "The Use of Endless Belts and Spliced Belts in Stock Feeders,"1975.

14. Homer, J. C. "Drag Link Analysis," Stock Equipment Company FeederAccuracy Program, notes to file dated March 31, 1994.

15. Homer, J. C. "Weigh Roll Misalignment Error," Stock Equipment CompanyFeeder Accuracy Program, notes to file dated July 12, 1994.

16. Homer, J. C. "Study the Effect of Vertical Misalignment of Weigh Linkage,"Stock Equipment Company Feeder Accuracy Program, notes to file dated

References:

September 12, 1996.

17. Norden, K. Elis. Electronic Weighing in Industrial Processes, 1st ed. (GranadaPublishing, London, 1984), pp. 86 - 88.

18. Colijn, H. and Hyer, F. S. “Belt Scale Calibration - Test Weight or Test Chain,”Paper presented at the ISA Conference, Chicago, October 4 - 7, 1971, p. 5.

19. “Specifications, Tolerances, and Other Technical Requirements for Weighingand Measuring Devices,” NIST Handbook 44, 1996 ed. (National Institute ofStandards and Technology, Office of Weights & Measures, Gaithersburg, MD,January 1996), Sec. 2.21, Belt-Conveyor Scale Systems, par. N.1.1, p. 2-36.

20. Colijn, H. and Hyer, F. S. “Belt Scale Calibration - Test Weight or Test Chain,”Paper presented at the ISA Conference, Chicago, October 4 - 7, 1971.