Guide To Tank Weighing Solutions By METTLER TOLEDO.ppt

8/10/2019 Guide To Tank Weighing Solutions By METTLER TOLEDO.ppt

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Tank Weighing METTLER TOLEDO

1999, Mettler-Toledo (S.E.A.)

Guide To Tank Weighing SolutionsBy METTLER TOLEDO


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How Much Is In The Tank ?

To determine the actual amount of material in the tank for inventory

purposes or production process control

Tank Measurement Basics

Measurement Technology

Weight Sensing

Flow Meter

Probes

Level Sensors

Ultrasonic


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Flow Meter

Less Accurate

Shorter Life

Continuous Flow And Batching

Processes

Works Well With Liquid And

Slurries

Load Cell

Better Accuracy

Longer Life

Batching Process

Material Insensitive Works Well

With Gases, Solids, Liquids and

Slurries

Flow Meter And Load Cell Comparisons


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Flow Meter

Subjected To Material

Contamination And Corrosion

Effects

Not Easily Adaptable To Process

Changes And Material Changes

Load Cell

Not Subjected To Material

Contamination And Corrosion

Effects

Adaptable To Process Changes And

Material Changes



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Flow Meter

Separate Flow Meter Per Material

And Able To Control Simultaneous

Addition Of Materials

Less Mechanical Integration

Load Cell

One Scale Can Weigh All Materials

But Non- Simultaneous Addition Of

Materials

Require More Mechanical

Integration



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Learning About Tank Scales

Introduction To Tank Weighing

Weighing performance of a tank scale

Environmental considerations that affect the weighing performance

Tank scale design guidelines

Weigh module selection

Installation and calibration


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Weighing Performance

A Weighing Systems Performance Is Measured By :

Linearity

Hysterisis

Repeatability

Creep

System Resolution


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0

10

2030

40

50

60

7080

90

100

Zero Half Load Full Load

Counts

Ideal

Actual

Linearity Error

Linearityis measured by a scales ability to correctly follow the linear relationship between the

weights applied and the displayed value. Linearity error is the maximum difference between the ideal

straight line and the actual curve at a given weight.


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0

1020

30

40

50

60

70

80

90

100

Zero Half Load Full Load

Counts

Ideal

Actual (dec.)

Actual (inc.)

Hysterisis Error

Hysterisisdescribes a scales ability to repeat the measurements for a linearity test when the scale is

loaded and unloaded.


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100 kg

100 kg

Repeatabilitydescribes a scales ability to repeat the same reading when the same weight is applied

and removed several times.

Repeatability Error

0.0 kg

+ 0.1 kg

- 0.1 kg

Actual Weight

100 kg

100 kg

100 kg

Displayed Weight

100 kg

100.1kg

99.9 kg


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Creepis the small change in the scales measured value of a constant load over a period of time due

to continuous mechanical deformation of the counterforce material.

80

82

84

86

88

90

92

94

96

98

100

0 15 30

Minutes

Counts

Ideal

Actual

Creep Error


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500d 2000d 4000d Load

Counts

0.5d

1.0d

1.5d

2.5d

0

-0.5d

-1.0d

-1.5d

-2.5d

Error

Counts

Resolutionfor a Legal for Tradescale is determined by the system capacity divided by the

minimum approved increment size following a given accuracy standard.

Handbook-44 Acceptance Tolerance Chart Class III Accuracy


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Resolutionfor a Process Weighingscale is only limited by the system capacity (Size

of Load Cells) and the sensitivity of the analog to digital converter in the weighing

terminal. The Displayedresolution can far exceed theApprovedresolution or

Accuracyof the scale.


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Environmental Effects

Ideally We Can Have A

System Accuracy Equal To

The Load Cells Accuracy

Tank Design

Structural Integrity

Calibration

Installation

Tank Weighing System Performance

However, The System Accuracy

of a Tank Scale Is Influenced By

These Factors


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Wind or

SeismicForce

Wind or Seismic Loading

Wind and seismic forces can have a great affect on

tank scales, especially on tall narrow tanks

Upward, downward, and shear forces are exerted on

the tank and load cells

Use larger capacity load cells to accommodate for the

added loads

Mechanical restraints can be applied to maintain the

tanks position

Shield from the wind to minimize affects

Factors That Affect Tank Weighing Performance


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Shock Loading

Due to load dropped on the scale from A high point

exert strong forces that can damage the load cells or

tilt the tank

Using larger capacity load cells to accommodate for

huge shock loads

Use shock absorbing materials (Fabreekapads) to

dampen the loading

Use interior baffles and deflection cone to minimize

the shock load


Dropped Load


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Vibration

Induces electrical noise on the load cells signal

causing unstable and inaccurate reading

External vibration is caused by the surrounding

environment and structure is transmitted to the tank

Separate the surrounding structure from the tanks

support structure


External

Vibration


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Vibration

Internal vibration caused by sloshing of liquid due to

mixers agitation can be solved by using interior baffles

Use isolation Fabreekapads between the load cells

and structure to minimize vibration effects

The use of electronic filtering systems in the weight

indicators and terminals to reject the vibration



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Temperature Effects

Temperature can cause the tank to expand and

contract leading to A shifted and non vertical loading

on the load cell

It can also damage the strain gauges and cause

inaccuracy if used beyond the specifications of the cells

Insulation and low thermal conductive material

(Acetal/PEI) can be used in between the cells and the

tank



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Moisture and corrosive substances can damage the

cells physically and by shorting out its electronics

Cables and junction boxes must also be protected from

moisture

Debris collected on the load cells and tank will cause

weighing errors by mechanically binding the scale and

giving non-zero return errors

Proper cleaning and protection will prevent A failure


Moisture, Corrosion And Debris


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Electrical surges can cause permanent damage to the

load cells

Electrical surges may be caused by lightning, large

electrical machines or welding.

Providing single point grounding and surge protection

devices to the system can solve surge problems


Lightning And Surge Protection

GroundRod


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Rapid material flow in and out of tank causes an airdisplacement and thus a pressure imbalance in the

tank

Weighing errors will be registered due to this air

displacement

Tanks should be built with vents or pressure regulators


Pressure Imbalance


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A strain gauge load cells signal output is in millivolts

Any small change in the signal will cause a change in weight reading

RFI and EMI can induce electrical noise in the signal causing errors in the weighing

The effects can be reduced by using shielded and insulated cables which are properly grounded


RFI And EMI Effects

Analog Load Cell Analog Electrical Signal


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Support structure deflection and non leveled supports can cause non-vertical loading leading tolinearity, repeatability, and accuracy problems

The tanks structure when loaded should be able to resist deflection, especially tanks with A large

diameter or long legs


Structural Integrity

Minimize

Deflection


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The tank will not weigh accurately if there is anymechanical binding to the tank from any component

that is not supported by the load cells

Piping connections to the tank will exert unwanted

forces that bind the tank when it is being loaded


Live To Dead Connections

Load Force

Reaction Force


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To produce an accurate tank weighing system we need to make sure that the load is alwaysapplied vertically and distributed evenly to all the load cells

To achieve this, the tank and its support need to be designed as level, rigid, straight and parallel

as possible

Tank Scale Design Guidelines

Loading Force Criteria

Vertical Load


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Loading Force Problems

Angular Loading

The load cell is subjected to an angular force

loading instead of A vertical force

Eccentric Loading

The load force is applied to the load cell at Apoint other than its center line due to thermal

movement

Angular

Load

Eccentric

Load


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Side And End Loading

This is due to horizontal forces that are caused

by dynamic loading, mounting misalignment of

the cells and thermal movement

Torsional Loading

The twisting effect on A cell due to structural

deflection, system dynamics and mounting

misalignment of the cells

Side Force End Force

Moment


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Unevenly Distributed Load

Due to unleveled foundation supports and structural

deflection will cause linearity and repeatability errors

Support structures and foundation base should be level

(+/-0.5 degree) and in the same plane; Shimming canbe A solution


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Support Structure Design

The support bracket and base support structure shouldnot deflect more than 0.5 degree out of level

Top support brackets and bottom support structure

should be aligned and leveled

The center line of the load on A load cell should align

to the center line of the support beam to prevent

deflection of the structure due to heavy loads

0.5

SupportBracket

Support Beam


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Using braces to strengthen the tanks legs to avoiddeflection

Adding web stiffeners or gussets to the support beams

can strengthen the beams

Use the same support beam sizes to avoid non-

uniform deflection which causes repeatability problems

Leg Braces


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Mounting load cells at mid-span of the support beamswill cause the most deflection on the beams at high

loads

It is better to mount load cells nearer to grounded

vertical columns

Reinforcement to the support beams is another

recommended solution to minimizing deflection

Grounded Vertical

Beam


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Tanks sharing the same support structure will have anaffect on each others weighing performance

Any movement and disturbances from one tank is

easily transferred to the next tank sharing the same

support structure

Tanks having separated and rigid support structures

will minimize tank interaction


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Check Rods

Excessive horizontal shear and uplift forces can

tip or rotate the tank out of alignment and cause

A non-vertical loading and also unsafe operation

Check rods and bumpers can be added to limit

this horizontal swaying and tipping motion on

the tank

Bumper

Bolts


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Safety Rods For Hanging Tanks

Safety rods will be needed for tanks that are suspendedin case the tension load cells suspension system fails

A safety rod is installed next to each tension load cell

and will have to be strong enough to support the tank

Safety Rod


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Piping connections need to be designed to exert minimal forces on the tank

Long horizontal pipe runs will exert less vertical force on the tank than short rigid pipe runs

A 90 degree bend on horizontal pipes will also make the piping more flexible



Horizontal Pipe Run


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Rigid piping supports should be located as far away from the tank as possible

Avoid rigid piping, use flexible connections along the pipe runs



Vertical Bellow

Flexible Hose

Flexible

Joint

Pipe Support

Maximum Distance


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Weigh Modules

Weigh Modules consist of a load cell and mounting hardware that converts the

tank to a scale and safely supports the tank

Weigh Module Selection Considerations Tension or compression load cell

Number of weigh modules

Load cell metrological performance and size

Environmental protection

Weigh module mounting system


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Types Of Weigh Modules

Tension Vs. Compression Load Cell

Tension S-Type Load

Cell For Hanging

Tanks

Compression Beam Load Cell

For Ground Supported Tanks


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Tension

No Floor Space Required

Lighter Loading Due To OverheadStructural Limitations

No Thermal And Compression

Effects

Compression

Floor Space Required

Heavier Loading Possible With AMore Rigid Foundation

Thermal Expansion And

Compression Effects

Tension Vs Compression Load Cell


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Tension

Normally Indoor

Heavy Wind Load Effects

Bumpers And Safety Rods Required

Less Surrounding Disturbances And

Effects

Compression

Indoor And Outdoor

Minimal Wind Load Effects

Uplift Protection Required

More Surrounding Disturbances And

Effects

Tension Vs Compression Load Cell


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Number Of Weigh Modules

The Number Of Weigh Modulesis determined by the number ofsupports on the tank

A 3-point support is ideal for tank weighing since the load is more evenly

distributed to the load cells, will not rock across corners

Four-module systems provide added stability under wind or seismic conditions

Weighing systems can consist of up to eight (8) modules

Level Detection systems use a combination of live and dead modules, lower

cost, lower accuracy


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Weigh Modules

Load Cell Metrological Performance

Important factors to consider are the linearity, hysterisis, repeatability, resolution

of the load cell to determine the accuracy

Typical Load Cell Performance

0.01 % R.C. Non-Linearity

0.02 % R.C. Hysterisis

0.03 % R.C. Combined Error


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Weigh Modules

Load Cell Sizing

Sometimes The Load Cell Size Is Increased To Compensate For Wind And Shock

Load Effects

Tank And Its Attachments Weight = A

Net Weight (Tanks Content)= B

Total Gross Weight = A + B = C

Number Of Cells = D

Load Cell Size = 1.25 C

D

Note: The 1.25 factor is for low weight estimates and uneven

load distribution


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Weigh Modules

Environmental Protection

The load cells and mounting hardware should have proper protection against

moisture, corrosion, extreme temperatures, shock loading or mechanical abuse

Hermetically sealed stainless steel load cells are better to prevent corrosion and

moisture effects

Load cells normally have an overload protection of 150% of full capacity for

shock loading effects

Always check load cells for their safe operation and compensated ranges of

temperature


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Weigh Module Mounting System

Compression Weigh Module Mounting System

The mounting hardware must always transmit the loadvertically from the tank support to a single point on

the load cell

To allow for thermal movement, the load pin should

not be bolted or fastened into the load cell

By bolting the load pin, the load is supported by thethreads on the load cell and not on the sweet spot of

the cell

Load PinVertical

Load Applied To A

Single Point


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The weigh module system should incorporatehorizontal checking

Semi Floating Plate

Prevents Rotation Along

The Fixed

Fixed Pin Plate

Anchors The System

Full Floating PlateAllows Free

Movement In Any Directions


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The weigh module system should accommodate for shifting of the vertical load due to thermalexpansion and contraction of the tank

Top PlateAllows

Movement


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Hold-down bolt provides anti-uplift protection

Hold-down bolt eases installation and maintenance,

especially when replacing a load cell

Hold Down BoltServes

As Uplift Protection

And A Jack During

Service


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FlexmountWeigh Modules Stainless Steel Mild Steel Mounting Plates

Accommodates thermal expansion and contraction

Easy mounting for any type of vessel

Self checking, no check rods required

Hermetically Sealed Stainless Steel Load Cell

Washdown safe

Meets 5000 counts approved resolution

Hold Down Bolt Feature

Serves as A jack to support tank for installation

As an uplift protection




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Tension Weigh Module Mounting System

The mounting hardware should incorporate a bearingball joint to prevent misalignment and keep the

loading vertical

Check rods or bumpers may be required to prevent

the tank from swaying

Secondary safety rods are needed as a back-upsuspension system in case of failure and to ease load

cell replacement

Spherical

Bearing


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Installation

Mechanical Installation

Preparations for foundation to be level, rigid and in the same plane

The tank should not be immediately lowered onto the load cells, use dummy cellsinstead if the weigh modules do not have jack up bolts

Install the weigh modules where they can be easily accessible for maintenance


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Installation

Mechanical Installation

Shimming is sometimes necessary to ensure the load is evenly supported by the

load cells The top and bottom mounting plates of the weigh module must be aligned to

avoid non-vertical loading

Never weld near the weigh modules without removing the load cells first, or the

welding current might pass through to the cells


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Installation

Cabling

Properly shielded and insulated cabling is required

RFI and EMI can easily affect the microvolt signals of the load cells causingerrors

Properly grounded cables in combination with a ferrite ring will minimize RFI

and EMI

Isolate power cables from the load cell cables

Provide junction boxes with protection against moisture and corrosion (NEMA

4X / IP56)

k h


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Installation

Cabling

Providing conduit for the cables will protect the cables from

mechanical damage and moistureDO NOT cut or trim load cell cables as this will affect the strain

gauge load cells resistance and output compensation

Always check on the recommended size and distance for the home

run cable to avoid signal losses

k h R O DO


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Calibration Preparations

Build brackets, evenly spaced around the tank to hangtest weights

Before calibration, each load cells signal output is

measured to ensure an even load distribution

If necessary, add shims for leveling of the modules;

junction box potentiometer trimming will compensatefor small differences in the outputs

Mounting Brackets For Hanging TestWeights

Calibration Procedures

T k W i hi METTLER TOLEDO


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Calibration Methods

Test Weights To Full Capacity (High Accuracy)

Possible to load test weights to full capacity on smaller tanks

Test Weight With Substitution (Good Accuracy)

10 -20 % test weights are used to perform the initial calibration and water is

used as substitution to be added with the weights to further test the scale to

full capacity



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Calibration Methods

Material Transfer (Medium Accuracy)

A material is weighed on A separate scale and transferred to the tank scale

and used as calibration weight

Electronic Calibration (Low Accuracy)

The use of an accurate simulator to produce the similar output produced by

the load cells at full capacity



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Maintenance

Periodically check and clean to avoid collection of debris on the tank, load cells

and junction box

Make sure that there is no mechanical binding on the tank

Any mechanical changes to the tank may affect the previous calibration

Inspect check rods and bumpers for proper gaps to avoid mechanical binding



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Finish

Thank You

Documents

Guide To Tank Weighing Solutions By METTLER TOLEDO.ppt