Product Application Guidelines. Weigh Modules

Transcription

1 Product Application Guidelines Weigh Modules

2 Table of Contents Preface... 3 Introduction... 3 Weigh module terminology... 4 Weigh module selection... 6 Scale location... 6 Location... 6 Vibration... 6 Outdoors... 6 Scale considerations... 7 Load cells... 7 Vessel design... 8 Wind loading Seismic loading Vessel restraints Shock loading Temperature Cleaning Filling and discharging Lightning protection Scale verification Maintenance and modifications Maintenance Maintenance precautions: Material build-up Material spills Product selection Load cell selection Load cell construction Self-aligning assemblies, bearings and guide elements Weigh modules

3 Preface This guide is intended to be used before purchasing parts for a weigh module. In addition to properly sizing a weigh module system, environmental factors affecting weigh module performance must be taken into account. If environmental factors are not properly managed or eliminated in the application, weigh module performance can be adversely affected. When installed and applied according to guidelines, weigh measurements are more accurate. This guide describes factors that will lead to poor scale performance and recommends strategies for proper application of weigh modules under specific environmental conditions. Please refer to the appropriate load cell or mounting unit instruction manual for full specifications and installation and calibration procedures. Manuals can be downloaded from the Siemens website at Note: All diagrams are specific to bins, hoppers, bags or vessels. When applying this guideline to other weigh module applications, use diagrams as examples only. Introduction Weigh modules can be used for: Shipping and receiving verification or indication Inventory control Batching or dosing of liquids or solids Custody transfer Filling containers (bags or sacks) Production monitoring A weigh module is a weighing solution that combines two components: Load cell Mounting unit Load cells deflect under the weight of the material in the vessel. Mounting units consist of a bottom plate, for securing the module to the floor or support, a top mount, which can be a plate or bearing, and in some cases, alignment guides or check rods to reduce horizontal or vertical movement. A complete scale also includes a terminal which collects the weight data from the scale and outputs. Weigh modules, as a minimum, must be arranged in equally spaced groups of three to fully support the vessel. Three modules are ideal if the vessel is still in the design stage. Arrangements of four or more can also be supplied as required, based on the geometry of the vessel and if wind, material movement, or seismic loading are factors. Compression style load cells are suitable for most weighing applications supporting the vessel from below or above. Tension style load cells can also be used if the bag or vessel is suspended from above. Generally, tension style load cells are used in filling applications or bagging. 3

4 Weigh module terminology Bin: an open top storage vessel for liquids. Hopper: an open top style vessel for bulk solid material. Tank: generally, a closed container used to store liquids which may or may not be pressurized. Silo: a closed container that stores bulk solid material. Silos are usually large and constructed from cement. Bottle: a very small closed container that stores liquids. Bag: a small to medium size storage container for bulk solid materials. Sack: a medium to large size storage container for bulk solid materials. Guide element: a check rod assembly supplied with the mounting unit as part of the weigh module. For the purposes of clarity, bin, hopper, tank, bottle, or silo will be referred to as a vessel. 4

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6 Weigh module selection Choose a load cell and mounting unit combination that best suits your application, based on the following criteria: Maximum weight Vessel size Environmental influences Wash-down conditions Structural restrictions (Support necessary for filled vessel) Use your application criteria to find appropriate weigh modules. Then, confirm your choice by checking the scale location and application considerations below. Scale location Location Select a location that is flat within 1/8 (3mm), solid, level, and that fully supports the weight of the vessel plus the load that will be applied to it. Avoid areas where the scale might receive damaging shock loads from falling objects. Avoid areas where water or chemical spills could damage a scale that is not meant for a wash-down environment. Vibration If a scale vibrates constantly, the load cells will never come to a steady reading long enough to ensure an accurate weight. Normal vibration can be controlled with calibrations if the frequency does not deviate from the time it was captured. Shock or vibration pads can also help reduce the influence that vibration has on the weighing components. Reduce or eliminate the source of the vibration to ensure optimum scale performance. If the vibration originates from inside the vessel due to agitation or natural material movement during filling or discharging the installation of internal baffles can reduce the vibration. Equipment such as crushers, vibratory feeding equipment, bins subject to hammering, and hammer mills should be avoided. Outdoors Weigh modules or any kind of scale installed outdoors must be able to withstand the natural elements. Weigh modules supporting a vessel need to be installed on heavy duty concrete pads to ensure they can support the large loading that will be applied to them. Protection for weigh modules must be in place to ensure that moisture and debris do not collect on or around the load cells. Smooth and level mounting floors are required. The area must not collect water above or below the weigh modules; drains should be in place to ensure proper runoff of rain or snowmelt. Sun shields should also be considered if certain load cells will be subject to high and direct sunlight during the course of the day. Outdoor scales can be subject to harsh environmental conditions including lightning strikes. Proper installation and maintenance are critical. Optimum accuracy is required for load out vessels used in trade applications; therefore, special attention must be paid to the manufacturer s recommendations. 6

7 Scale considerations Load cells Load cell requirements will vary depending on vessel type and capacity. Small vessels that only require one load cell can use a single point, tension, bending beam, ring torsion or shear beam load cell. Larger vessels with 3 or more supports can use shear beam, bending beam, tension, compression or ring torsion style load cells. In cases where a single point load cell is used, the capacity should not exceed 1800 lb (800 kg). The load cell should be mounted so that the center of the cell is aligned to the center of the vessel. All other types of load cells can be loaded directly where the vessel mounts to the weigh module. The chart below shows different styles of load cells and the capacity ranges for each. Single point 0.3 to 500 kg (0.7 to 1100 lb) Bending beam 10 to 350 kg (22 to 770 lb) Shear beam 0.5 to 5 t (0.5 to 5 Lt) Tension 0.05 to 10 t (0.05 to 10 Lt) Compression 2 to 500 t (2 to 500 Lt) Ring torsion 0.06 to 60 t (0.06 to 60 Lt) 7

8 Load cells use strain gauges (or other means) which are extremely sensitive to any change in weight. To ensure optimum scale accuracy, force on the load cell must be consistent and controlled. Loads must only be applied vertically. Non-vertical loads will create a mechanical stress concentration that can damage the load cell or the mounting unit and also create inaccuracies. This effect can be created by thermal expansion or poor load cell installation. Angular forces will have a horizontal and vertical component. The load cell will deflect under the vertical component and will therefore be offset to the proper deflection. Leverage or pulling forces can damage the load cell and also create a potentially dangerous situation. Mechanical restraints must be installed to control external forces that produce leverage or pulling. Torsional or rotational forces occur when structural changes in the vessel create a misalignment. These forces will reduce a scale s accuracy and could potentially damage the load cells. Proper structural support and installation will help compensate for vessel movement. Ensure that any scale has the right load cell for capacity and environmental conditions. Multiple load cell scales should use self-aligning mounts to ensure that no mechanical binding occurs during deflection under load. Vessel design The design of the vessel must be suitable for the chemical composition of the material as well as the amount of force the material will exert on it. If the vessel deflects or changes its shape as it is loaded or unloaded, scale accuracy may be affected. Once the weigh modules are added to the vessel, the entire assembly becomes a scale. Expansion and contraction with connections to infeed and discharge pipes can cause mechanical binding or de-formation. These adverse effects can introduce forces that will act against the load cell and create inaccuracies or damage. For this reason, many high-capacity load cells feature a rocker pin as part of their design or the design of their mounting unit. This allows for a minimal amount of misalignment of the vessel and the weigh module. It is also because of this design that a minimum of three weigh modules are required for large vessels, to ensure proper contact and support. The supports of the vessel must also allow for the entire mass of the scale to be supported by the weigh modules. The supports must not be outside the confines of the mass of the scale. 8

9 Mounting the weigh module on a rough surface can create stress concentrations on the base plate and potentially damage the mounting unit, or lead to poor scale performance. Using support structures that deflect under the load of the vessel and its contents will lead to inaccuracy and potential mechanical failure. Foundations that cannot support the weight of the vessel and its contents present a very dangerous situation. Proper mechanical design and review must be done to avoid this hazard. The weigh modules must be installed on a sturdy, horizontal support structure. The vessel structure must also be rigid and secured to the weigh modules to ensure that no horizontal movement can occur beyond what is allowed for thermal expansion. Vessels on elevated supports should be braced to ensure that deformation under load does not occur. The entire surface area of both the top and bottom of the mounting unit must contact the support surfaces of the vessel and foundation. The vessel supports should be centered directly above the load point on the weigh modules to avoid buckling or deformation of the support structure. The number of supports for a vessel should not exceed eight because even distribution and weigh module alignment becomes very difficult with more than eight assemblies. When multiple vessels are mounted on the same foundation scale, performance in one vessel can be affected by changes in an adjacent vessel. Concrete foundations offer the best isolation; however, if steel support is being used, ensure the weigh modules are mounted as close to vertical support members as possible. 9

10 Wind loading Outdoor vessels are subject to horizontal wind, which can lead to increased forces on the weigh modules and in extreme cases vessel failure. Scale accuracy can also be affected because changes in vessel loading occur with wind. The weigh modules must have the proper mechanical support components to ensure that the vessel can resist the forces from the wind in any direction. Generally, vessels installed outdoors are mounted on an elevated structural frame. The wind effect is not only in the direction the wind is moving, but it also in other locations; for example, suction forces on the opposite side of the vessel. The wind force and suction force combine, making the sum of them greater than each individually. Suction forces on the vessel sides are also created; however, they are opposite and equal so they have no effect on the stability of the vessel. Ensure that the load cell capacity is not exceeded after wind loads are taken into account. The maximum allowable horizontal force must not exceed the corresponding load cell or mounting unit specification. The wind effect can be minimized if the weigh modules are mounted closer to the center of gravity of the vessel. Vessel restraints must be used if the wind forces could potentially knock over a vessel. Ensure that the vessel construction is in accordance with the local governing code and certified by a professional design engineering body. 10

11 Seismic loading Forces from earthquakes can be extremely strong. Earth quakes can create both horizontal and vertical forces on a vessel. Ensure that the vessel construction is in accordance with the local governing code and certified by a professional design engineering body. Relevant codes can be found below: Standard Description Governance NZSEE Guideline for the Seismic Design of Storage Tanks NZSEE D100 Welded Carbon Steel Tanks for Water Storage AWWA D103 Factory-Coated Bolted Steel Tanks for Water Storage AWWA EN Eurocode 8: Design of Structures for Earthquake Resistance Part 4: Silos, Tanks & Pipelines CEN ACI Seismic Design of Liquid-Containing concrete structures & Commentary ACI 11

12 Vessel restraints Some mounting units from Siemens feature restraints; however, additional restraint systems are often required. Check rods are typically used because they can prevent a vessel from horizontal and vertical movement and also allow for deflection under load to maintain scale accuracy. Check rod supports: Ball pin guidance can also be used because the spherical ends of the pins allow for minimal deflection and maintain the vessel position relative to the foundation. 12

13 The following illustrations depict ideal restraint configurations: NOTE: Only three restraints are required. A fourth restraint would introduce mechanical binding under thermal expansion conditions. With four guide elements, there is a risk of the guide elements mutually tensioning, which introduces weighing errors. If four restraints are to be used, the guide elements must be installed with a sufficient amount of play. Restraints are critical in conjunction with elastomeric bearings. Guide elements must be installed exactly horizontally so that they do not affect weighing accuracy. Weighing accuracy cannot be guaranteed across the weighing range with soft elastomeric bearings. Ensure that the guide elements to be used comply with weighing best practice.. In applications with vessels or bags hung on a tension style load cell, a safety rod is also recommended. In case of load cell failure, this safety rod holds the vessel in place. Lift locks or stops must be installed to counteract wind or seismic forces and to prevent assembly tipping The stops should not interfere with the normal movement of the vessel. 13

14 Check rods should be used to ensure the vessel does not move or tip over due to wind or seismic forces. Check rods should be positioned at or above the center of gravity of the vessel when it is full. Check rods must be installed so that they limit the movement of the vessel but also allow for deflection to the scale. Load cells generally only deflect [ mm]. Check rods on a round take should be mounted tangentially for best support. Shock loading Shock loads can be a natural part of the process or an accidental occurrence. Shock loading should always be considered as part of the load cell and mounting unit capacity calculation. Typically, an additional 20% of capacity will cover any unplanned loading during a fill cycle or accidental discharge. Otherwise, the loading must be controlled to ensure that damage does not occur to the vessel or the scale components. The use of elastomer bearings and mounts can reduce the amount of shock load transferred to the load cell; however, only applications with lower capacities can use elastomer bearings and mounts. Loading conditions or accidental upset scenarios should be considered when selecting the capacity of the load cell and corresponding mounting unit. Temperature A change in ambient or material temperature can significantly affect scale accuracy. Temperature change can cause a scale to become inaccurate. Thermal expansion is the biggest concern due to temperature change. Thermal fluctuations can create mechanical binding of the weighing modules or external horizontal or rotational forces, which affect accuracy. Choose temperature compensated load cells to reduce the effects of daily changes in temperature. Thermal isolation pads can also be installed between the vessel and the load cells to reduce the influence from material temperature changes. Matching the weigh module construction with the vessel can help with thermal expansion and mechanical binding during temperature changes. Cleaning In applications where the weigh modules will be cleaned frequently, proper conduit, glands and materials of construction must be considered. Stainless steel components are ideal for wash-down requirements. Ensure that mounting unit and load cell construction is stainless steel. Load cells should be IP67 rated, as a minimum. 14

15 Filling and discharging Because the vessel deflects under load with the weigh modules, the connections for the infeed and discharge must be considered. Mechanical binding can occur if these connections are not flexible in nature. This binding can introduce scale performance issues. The non-linear relationship to rigid connections of pipes or chutes to the vessel makes electronic calibration difficult because the resultant force varies depending on the amount of material inside. The total vertical force of the piping connected to the vessel should follow the guideline below for optimum scale accuracy: Total vertical force generated from the deflection of all piping should not exceed a percentage of the maximum load. Scale accuracy Total vertical force 0.1% <1.0% 0.25% <2.5% 0.5% <5.0% 1.0% <10.0% Ensure the vessel deflects as little as possible when full. Minimize the amount of fill and discharge connections; valves or connections should be made before entering the vessel and not with multiple entries. Piping should be run from the tank in the horizontal plane; only after a pipe support member is installed, should the change to a vertical be pipe run be made. Pipe supports should be as far away from the vessel as possible. Pipes should be small in diameter and at the lowest gauge possible to support 15

16 the material flow. Flexible piping joints or nested connections should be used to minimize mechanical binding. Do not support piping on structures which can deflect independently of the vessel. Lightning protection With outdoor applications the possibility of a lighting strike on a steel vessel must be considered. Lightning protection should be installed to protect the weigh modules. The grounding cable represents an electrical bypass via the load cell and mounting unit. High weld currents can destroy the load cell and the electronics so protection is always recommended. The electronics can also be affected by voltage sparkovers of static discharges. Use grounding straps that are designed to keep the current produced by lightning from reaching the load cells. A single point grounding system is ideal. Scale verification The best means of verifying a scale s performance is with certified weights. However, for applications with a high capacity rating, the use of weights is typically not practical. Generally, 20% of the material loading is ideal for calibration with weights. A weighed material sample can be use as the best alternative. The application of calibration weights should be considered prior to vessel design to accommodate them. Calibration test weights are a good choice for scale verification. Calibration weights should be used to meet the needs of the application not exceed it. The accuracy of the scale should be taken into consideration as well. 16

17 Maintenance and modifications Maintenance Once the scale is installed, it requires little to no maintenance. Maintenance precautions: When welding near the scale, do not allow current to pass through the load cells. On weigh modules with aligning threads, lock the top plate down when performing vessel maintenance to reduce the probability of damage to the load cell. Material build-up Keep the scale and associated equipment as clean as possible so that the scale measures only the loads intended and not the added load due to material sticking to the scale. To remove materials that stick to the scale equipment, use a vacuum to remove solids or washer for cleaning. Although scales can be frequently and automatically recalibrated at no load (zero), it is not a good practice to allow material buildup to remain on the scale. Material spills General good housekeeping is always important. Material spillage results in lost production and can also adversely affect scale operation when spilled material wedges between dynamic parts preventing proper scale deflection. In addition, the build-up affects the zero balance of the scale. Do not overload the scale. As a precaution, install deflectors to keep spills from reaching the scale. Product selection Load cell selection The capacity of the load cell is the first factor in selecting the proper type of load cell. Generally, single point styles do not exceed 500kg [1100 lb], while compression style cells are not available below 1t [1Lt]. To determine the capacity of the load cell, use the following equation: Load cell capacity = Live load(max amount of material in vessel) + Dead load (mass of assembly on scale) # of vessel supports Generally, a 20% safety factor is applied and the next largest load cell capacity is selected. Single point Should only be used in applications when one load cell can support a small vessel. load cells: Tension load cells: Bending beam load cells: Shear beam load cells: Are ideal for applications that are suspended. One or more can be used to support the vessel. Can also be used for small, suspended vessels. Multiple bending beam load cells can be used to support a vessel from below. The use of elastomer bearings is suggested for proper alignment during deflection or when thermal expansion needs to be managed. Offer a compact high capacity solution for medium sized vessels. The use of a self-aligning mounting unit allows the load to be directly applied to the cell. Elastomer bearings can also be used to suppress oscillations and shock loading. 17

18 Compression load cells: Ring torsion load cells: Offer the highest range of capacity and also feature a spherical load button that requires support of a mounting unit for proper alignment of the load from the vessel. Self-aligning bearing plates can also be used. Provide the highest potential accuracy for a vessel weighing application and a large range of capacities to choose from. Self-aligning bearing plates and elastomer bearings can be used to reduce both shock damage to the load cell and vibration. Self-aligning units provide support for lateral forces during vessel movement. Load cell construction Load cells can be constructed from a variety of materials using different finishing and sealing techniques. The following materials are used: Aluminum- Generally used in single point load cell designs for low capacity applications. Aluminum load cells will deflect more than other steel designs due to the softness of the material. Aluminum load cells are the most cost effective solution. Alloy tool steel- Most load cells can be constructed from alloy steel from low to high capacity styles. Tool steel load cells are typically nickel-plated for additional protection against corrosion. The cost performance ratio of tool steel vs. aluminum and stainless steel make it a popular choice. Stainless steel- Stainless steel load cells are the most expensive type available but also offer the highest performance and longevity. The advantage of corrosion resistance, as well as the ability to protect the internal electrical components through hermetic sealing, makes them the most popular choice for weigh modules. Strain gauge protection- There are multiple methods for protecting the sensitive components of the load cell, including potting internally or externally using a silicone based chemical. Welded seals are often used in single point of bending beam cells and hermetic seals are used in the most critical or wash-down applications to ensure the highest level of protection. 18

19 Self-aligning assemblies, bearings and guide elements Self-aligning mounting units offer the best performance over the widest variety of applications. The alignment components ensure that the installation is ideal and that the entire weighing module is aligned with the vessel for optimum performance. They also include overload and lifting protection for the load cell. Guide elements or stabilizers as part of the weighing module are an easy way to combine restraint of the vessel and the weighing module into one. The following applications will often require the addition of a guide element or stabilizer: Vessel with a high shear agitator or mixer mixers are motor driven and ensure that materials do not separate or bridge inside the vessel. Horizontal batch mixing vessels similar to a mixer or agitator, only horizontal. These vessels generally have a screw or paddles to help avoid material separation or bridging. If the state of the vessel is static or the capacity is low, then guide elements are generally not required. Elastomer bearings can reduce the amount of vibration transferred from the vessel to the load cells and also allow for a small amount of misalignment during deflection. Thermal expansion is also managed well with elastomer bearings. 19

20 Weigh modules There are many different styles of weigh modules available: compact mounting units, elastomer bearing supports, integral guide elements and the different load cell options. Generally, there are three factors to consider when selecting parts for a weigh module: Application needs drive the specific selection of models and options. The environment will also play a part--the scale should be stainless or plated tool steel in corrosive areas or near sea ports. If the location is considered hazardous, an approved scale will be needed. Price point influences the selection of the type of scale. Featureless scales are less expensive, but if the application is not suited for them, they should not be used. Scale sizing Limitations based on application needs and product features or performance will narrow down the selection of the product. Load size and load cell optimization are considered when a weigh module is engineered. 20

21 Siemens AG Industry Sector 1954 Technology Drive P.O. Box 4225 Peterborough, ON Canada K9J 7B1 For more information