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Problems with WeightometersProblems with Weightometers

S A G

R O M

W 1

W 2

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A Recommended Weightometer Setup

A Recommended Weightometer Setup

W 1

W 2

S am p lin gT o w er

B elt # 1

B elt # 2

B elt # 3

S to c kp ile

Les s than 70 m eters

Les s than 70 m eters

A b o ut 12 m eters

A b o ut 12 m eters

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Three Elements for a Successful ProgramThree Elements for a Successful Program

The accurate tonnage determination entering the plant is critically important.

Weightometers are notorious sources of reconciliation problems.

The accuracy of many weightometers is doubtful because of the location they are installed and the way they are cleaned, maintained, and calibrated.

The accuracy of weightometers depends on many factors.

Furthermore, field experience proves to be far less optimistic than manufacturer’s guarantees.

Nevertheless, if a few precautions are taken, weighing accurately with weightometers is possible.

Basically, you need three elements to implement a successful program:

The initial installation of the weightometer needs to be

correct.

A logical test program needs to be implemented.

Monitor deviations on Relative Difference Plots.

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The Weightometer Installation Needs to

be Correct. [1]

The Weightometer Installation Needs to

be Correct. [1]

The weightometer and the conveyor do not exist independently: It is necessary for the engineering firm to build them together as an integrated system.

Retrofitting a weightometer to a conveyor belt after it has been built is not likely to give satisfactory results.

The location selected to install the weightometer is critical, as it must be placed where the tension of the belt is least.

The scale location should be more than 7 meters but less than 17 meters from the loading point.

Skirting and training idlers should not be located closer than 7 meters from the scale location.

The weightometer must be installed where the conveyor is horizontal: It does not belong in an area where the conveyor is inclined. Horizontal conveyors are preferred over inclined conveyors because in horizontal conveyors the belt tension is much lower and it is more predictable.

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The Weightometer Installation Needs to

be Correct. [2]

The Weightometer Installation Needs to

be Correct. [2]

The conveyor belt should be relatively short, no more than 70 meters long.

The weightometer must be plumb and level, which is an unsolvable problem if it is installed on a slope.

For the weightometer to work well, run out of idler rolls must be minimized.

The angle of troughing idlers must be very accurate.

Deflection of conveyor stringers must be minimized.

The ambient temperature must be kept as constant as possible: In other words, the weightometer does not belong outdoors.

Calibration chains or weights should be selected to permit calibration at about 75% of scale capacity.

The layout of the weighing system should be such that a quantity of the material that passed or is to be passed over the belt can be weighed if required in a separate weigh hopper or in a railroad car.

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The Weightometer

Vulnerability

The Weightometer

Vulnerability

The only purpose of showing the following formula is to show the weakness of weighing systems. The force P exerted on the scale in pounds normal to the belt is as follows:

 P = [nQLcosθ/I2] + [2TD/L] + [24EID/L3]

 Where:

n is the number of conveyor mounted idlers,

Q is the normal conveyor belt loading in pounds per foot of length,

L is the spacing between idlers in inches,

T is the tension in conveyor belt at the scale location in pounds,

E is the modulus of elasticity of belt carcass material in psi,

I is the moment of inertia of carcass cross-section in (in4),

D is the misalignment between scale idlers and adjacent idlers in inches,

θ is the angle of conveyor incline in degrees.

In theory, on a normal day, only the factor Q should change: it is the one that is measured. But, if T is no longer predictable, everything falls apart. Also, E, I, and D are subject to slight changes with time: Remember, these systems are submitted to tremendous vibration and frequent temperature and wind changes.  

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A Logical Test Program

A Logical Test Program

There are three methods of calibration:

Using material of a known weight run,

Using chain tests, and

Using static weight tests.

Common to any of these methods is the zero balancing step.

The so-called “electronic zero calibration” is useful only to check the electrical circuits, and completely ignores the mechanical aspects of the weighing system, which are actually the most vulnerable parts.

Calibration begins with a check of scale alignment, cleanliness, and general condition of the installation. The conveyor belt should be operated about 20 minutes to be sure that the conveyor itself approximates operating conditions.

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A Logical Test Program:

Zero Balancing

A Logical Test Program:

Zero Balancing

The zero balancing should fulfill the following requirements:

Include an integral number of belt revolutions. This assures zeroing which comprehends variations in belt thickness.

Include at least 3 integral revolutions or 10 minutes whichever is greater. This reduces the problem of observation accuracy.

If possible, begin and end zero balancing with the conveyor running.

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A Logical Test Program:

Chain Requirements

A Logical Test Program:

Chain Requirements

Chain calibration is much better than static weight calibration. The chain should be selected to meet the following requirements:

The chain should have a nominal-per-foot rating equal to the desired conveyor loading, and be marked as such to within 0.1% accuracy of total weight.

The uniformity of the chain on its pounds-per-foot rating shall be within 0.25%, and maintained.

The chain should be long enough to span beyond the second fixed idler to either side of the scale, as shown in the next slide.

The chain should be of the free roller or wheel type and have a pitch that is no longer than 6 inches and divides the idler spacing as evenly as possible.

The chain must be kept dry and under dust cover.

The chain mounting hardware must be such that precise positioning of the chain can be attained separately.

For practical reasons, the chain weight is limited to about 250 pounds/foot and this is insufficient for the calibration of large belt scale systems (e.g., 10000 tons/hour and over).

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Typical Calibration Systems

Typical Calibration Systems

W eight R eading

C ha in S p a n = nL

W eight R eading

S ta nd a rd W e ight

C h a in T es t

S ta tic W eigh t T es t

# 2 fixed id ler

# 2 fixed id ler # 2 fixed id ler

L