Guide To Tank Weighing Solutions By METTLER
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Tank Weighing METTLER TOLEDO Guide To Tank Weighing Solutions By METTLER TOLEDO © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO How Much Is In The Tank ? To determine the actual amount of material in the tank for inventory purposes or production process control Measurement Technology Weight Sensing Flow Meter Probes Level Sensors Ultrasonic Tank Measurement Basics © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Flow Meter And Load Cell Comparisons Flow Meter Less Accurate Shorter Life Continuous Flow And Batching Processes Works Well With Liquid And Slurries © 1999, Mettler-Toledo (S.E.A.) Load Cell Better Accuracy Longer Life Batching Process Material Insensitive Works Well With Gases, Solids, Liquids and Slurries Tank Weighing METTLER TOLEDO Flow Meter And Load Cell Comparisons Flow Meter Subjected To Material Contamination And Corrosion Effects Not Easily Adaptable To Process Changes And Material Changes © 1999, Mettler-Toledo (S.E.A.) Load Cell Not Subjected To Material Contamination And Corrosion Effects Adaptable To Process Changes And Material Changes Tank Weighing METTLER TOLEDO Flow Meter And Load Cell Comparisons Flow Meter Separate Flow Meter Per Material And Able To Control Simultaneous Addition Of Materials Less Mechanical Integration © 1999, Mettler-Toledo (S.E.A.) Load Cell One Scale Can Weigh All Materials But Non- Simultaneous Addition Of Materials Require More Mechanical Integration Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing Weighing Performance A Weighing System’s Performance Is Measured By : Linearity Hysterisis Repeatability Creep System Resolution © 1999, Mettler-Toledo (S.E.A.) METTLER TOLEDO Tank Weighing METTLER TOLEDO Linearity is measured by a scale’s 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. 100 90 80 70 60 50 40 30 20 10 0 Counts Zero © 1999, Mettler-Toledo (S.E.A.) Linearity Error Ideal Actual Half Load Full Load Tank Weighing METTLER TOLEDO Hysterisis describes a scale’s ability to repeat the measurements for a linearity test when the scale is loaded and unloaded. 100 Counts 90 Hysterisis Error 80 70 60 50 40 30 20 10 0 Zero © 1999, Mettler-Toledo (S.E.A.) Half Load Ideal Actual (dec.) Actual (inc.) Full Load Tank Weighing METTLER TOLEDO Repeatability describes a scale’s ability to repeat the same reading when the same weight is applied and removed several times. Actual Weight Displayed Weight Repeatability Error 100 kg 100 kg 0.0 kg 100 kg 100.1kg + 0.1 kg 100 kg 99.9 kg - 0.1 kg 100 kg 100 kg © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Creep is the small change in the scale’s measured value of a constant load over a period of time due to continuous mechanical deformation of the counterforce material. Counts 100 98 96 94 92 90 88 86 84 82 80 Creep Error Ideal Actual Minutes 0 © 1999, Mettler-Toledo (S.E.A.) 15 30 Tank Weighing METTLER TOLEDO Resolution for a Legal for Trade scale is determined by the system capacity divided by the minimum approved increment size following a given accuracy standard. 2.5d Error Counts 1.5d 1.0d 0.5d 0 -0.5d 500d 2000d 4000d -1.0d -1.5d -2.5d Handbook-44 Acceptance Tolerance Chart Class III Accuracy © 1999, Mettler-Toledo (S.E.A.) Load Counts Tank Weighing METTLER TOLEDO Resolution for a Process Weighing scale 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 Displayed resolution can far exceed the Approved resolution or Accuracy of the scale. © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Ideally We Can Have A System Accuracy Equal To The Load Cell’s Accuracy However, The System Accuracy of a Tank Scale Is Influenced By These Factors Structural Integrity Installation Calibration Tank Design Environmental Effects Tank Weighing System Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 tank’s position Shield from the wind to minimize affects Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Wind or Seismic Force Tank Weighing METTLER TOLEDO 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 (Fabreeka® pads) to dampen the loading Use interior baffles and deflection cone to minimize the shock load Dropped Load Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Vibration Induces electrical noise on the load cell’s 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 tank’s support structure External Vibration Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Vibration Internal vibration caused by sloshing of liquid due to mixer’s agitation can be solved by using interior baffles Use isolation Fabreeka® pads 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 Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Moisture, Corrosion And Debris 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 Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Lightning And Surge Protection 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 Ground Rod Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Pressure Imbalance Rapid material flow in and out of tank causes an air displacement 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 Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO RFI And EMI Effects A strain gauge load cell’s 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 Analog Load Cell Analog Electrical Signal Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Structural Integrity Support structure deflection and non leveled supports can cause non-vertical loading leading to linearity, repeatability, and accuracy problems The tank’s structure when loaded should be able to resist deflection, especially tanks with A large diameter or long legs Minimize Deflection Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Live To Dead Connections The tank will not weigh accurately if there is any mechanical 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 Reaction Force Load Force Factors That Affect Tank Weighing Performance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Loading Force Criteria To produce an accurate tank weighing system we need to make sure that the load is always applied 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 Vertical Load Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Loading Force Problems Angular Loading Angular Load 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 A point other than its center line due to thermal movement Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Eccentric Load Tank Weighing METTLER TOLEDO Loading Force Problems 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 Side Force The twisting effect on A cell due to structural deflection, system dynamics and mounting misalignment of the cells Moment Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) End Force Tank Weighing METTLER TOLEDO Loading Force Problems 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 can be A solution Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Support Structure Design The support bracket and base support structure should not 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 Support Bracket 0.5 ° Support Beam Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Support Structure Design Using braces to strengthen the tank’s legs to avoid deflection Adding web stiffeners or gussets to the support beams can strengthen the beams Use the same support beam sizes to avoid nonuniform deflection which causes repeatability problems Leg Braces Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Support Structure Design Mounting load cells at mid-span of the support beams will 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 Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Grounded Vertical Beam Tank Weighing METTLER TOLEDO Support Structure Design Tanks sharing the same support structure will have an affect on each other’s 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 Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Safety Rods For Hanging Tanks Safety rods will be needed for tanks that are suspended in 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 Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Live To Dead Connections 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 Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Live To Dead Connections Rigid piping supports should be located as far away from the tank as possible Avoid rigid piping, use flexible connections along the pipe runs Maximum Distance Pipe Support Flexible Hose Flexible Joint Vertical Bellow Tank Scale Design Guidelines © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Tension Vs. Compression Load Cell Tension S-Type Load Cell For Hanging Tanks Compression Beam Load Cell For Ground Supported Tanks Types Of Weigh Modules © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Tension Vs Compression Load Cell Tension No Floor Space Required Lighter Loading Due To Overhead Structural Limitations No Thermal And Compression Effects © 1999, Mettler-Toledo (S.E.A.) Compression Floor Space Required Heavier Loading Possible With A More Rigid Foundation Thermal Expansion And Compression Effects Tank Weighing METTLER TOLEDO Tension Vs Compression Load Cell Tension Normally Indoor Heavy Wind Load Effects Bumpers And Safety Rods Required Less Surrounding Disturbances And Effects © 1999, Mettler-Toledo (S.E.A.) Compression Indoor And Outdoor Minimal Wind Load Effects Uplift Protection Required More Surrounding Disturbances And Effects Tank Weighing METTLER TOLEDO Number Of Weigh Modules The Number Of Weigh Modules is determined by the number of supports 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 (Tank’s 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Compression Weigh Module Mounting System The mounting hardware must always transmit the load vertically 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 the threads on the load cell and not on the sweet spot of the cell Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Load Pin Vertical Load Applied To A Single Point Tank Weighing METTLER TOLEDO Compression Weigh Module Mounting System The weigh module system should incorporate horizontal checking Full Floating Plate Allows Free Movement In Any Directions Semi Floating Plate Prevents Rotation Along The Fixed Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Fixed Pin Plate Anchors The System Tank Weighing METTLER TOLEDO Compression Weigh Module Mounting System The weigh module system should accommodate for shifting of the vertical load due to thermal expansion and contraction of the tank Top Plate Allows Movement Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Compression Weigh Module Mounting System Hold-down bolt provides anti-uplift protection Hold-down bolt eases installation and maintenance, especially when replacing a load cell Hold Down Bolt Serves As Uplift Protection And A Jack During Service Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Compression Weigh Module Mounting System Flexmount® Weigh 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 Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Tension Weigh Module Mounting System The mounting hardware should incorporate a bearing ball 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-up suspension system in case of failure and to ease load cell replacement Spherical Bearing Weigh Module Mounting System © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 cells instead if the weigh modules do not have jack up bolts Install the weigh modules where they can be easily accessible for maintenance © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Installation Cabling Properly shielded and insulated cabling is required RFI and EMI can easily affect the microvolt signals of the load cells causing errors 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) © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Installation Cabling Providing conduit for the cables will protect the cables from mechanical damage and moisture DO NOT cut or trim load cell cables as this will affect the strain gauge load cell’s resistance and output compensation Always check on the recommended size and distance for the home run cable to avoid signal losses © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Calibration Preparations Build brackets, evenly spaced around the tank to hang test weights Before calibration, each load cell’s signal output is measured to ensure an even load distribution If necessary, add shims for leveling of the modules; junction box potentiometer trimming will compensate for small differences in the outputs Calibration Procedures © 1999, Mettler-Toledo (S.E.A.) Mounting Brackets For Hanging Test Weights Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO 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 © 1999, Mettler-Toledo (S.E.A.) Tank Weighing METTLER TOLEDO Finish Thank You © 1999, Mettler-Toledo (S.E.A.)
