Gravimetric level control is one of the most accurate inventory control methods available. Its high degree of accuracy makes it particularly helpful when measuring high-value liquids and sometimes even gases. Due to the fact that load cells or weigh modules are placed outside of the tank or silo, gravimetric level control is perfect for measuring aggressive, hot, frozen, non-free-flowing, and non-self-leveling materials.
Tank/silo weighing is preferred in many applications for other reasons as well. Because of the system’s inherent design, operators never need to handle materials being weighed. This makes it almost impossible to contaminate stored materials. Also, results are independent of tank shape, measured materials, or process parameters such as temperature and material viscosity.
A well-designed, properly installed system has very low maintenance requirements and promises long life. On a typical one-year maintenance and calibration cycle, a moderately used tank scale provides decades of reliable inventory control and weighing.
1. Advantages of gravimetric level control
Highly effective inventory management can be obtained using gravimetric level control. This is primarily because tank-based gravimetric inventory control is independent of many influences that other technologies must mitigate. Tank design can be optimized for the material that will be measured. Therefore, gravimetric inventory control offers a great deal of production flexibility. Materials and other process parameters, such as temperature, viscosity, humidity, density, and pressure, can be changed without recalibration or sensor adjustment.
The gravimetric method provides the best accuracy combined with high flexibility for inventory control over a long time. Even simple systems achieve accuracy levels of 1 percent or better. Accuracy can be as good as to 0.1 percent or lower of the applied load in optimized systems. No other inventory control technology for tanks comes close to this level of optimized material use.
Single load cells and weigh modules are available from a few kilograms to several hundred tons. Multiple cells or modules can be arranged under one tank for a net capacity of several thousand tons.
In addition, load cells can be integrated into complete weigh modules with factory-adjusted overload protection. Dedicated mechanisms protect the load cells from damage due to under load coming from the effects of wind forces. The tank can even be protected from tipping over in case of earthquakes or very strong wind forces. Stabilizers protect the load cells from torsion effects, which result from mixers inside of the tank.
Gravimetric inventory control also requires no direct product contact with the sensor. No special access to the silo or direct contact with material is required as with other technologies. All maintenance is possible without contact with the material or the inside of the tank. Thus, the tank can be optimized for hygienic aspects and low contamination thread.
Furthermore, manufacturers have the ability to choose the most suitable tank material and surface quality without compromising accuracy. This simplifies the design phase and construction of food-grade tanks.
Flexible use, accurate results
Tank weighing provides a high level of flexibility to a weighing process. Results are independent of:
- Tank shape: Gravimetric inventory control works with all tank shapes, which allows material flow to be optimized without compromising sensor requirements.
- Measured material: Gravimetric inventory control is independent of material. It works with liquids, high-viscosity liquids, and foam, for example. Change of material does not require recalibration or sensor changes. It is possible to measure input of different materials in batching or mixing processes.
- Process parameters: Changes of temperature, pressure, density, and viscosity don’t influence the result. Gravimetric level control always works.
Furthermore, one common level or mass-control technology for the entire process is possible. This reduces training needs for operators and maintenance to understand functionality, calibration, and limits of many different sensor technologies.
Gravimetric sensors, or their so-called load cells, are low-maintenance parts. They require less calibration than many other inventory control technologies. Depending on environmental conditions, proper installation, and protection measures, load cells can last for decades with no performance loss.
2. Tank scale design
Weigh modules help easily convert almost any tank, hopper or silo into a scale. They contain all features required for load-cell accuracy, as well as for safe operation, including the ability to operate under the effects of various temperatures.
There are two basic types of weigh modules:
• Compression weigh modules are designed so that a tank or other structure can be mounted on top of the weigh modules.
• Tension weigh modules are designed so that a tank or other structure can hang from the weigh modules.
Whether compression or tension weigh modules are used depends on the specific application. The table below provides an overview of general design considerations affecting the choice of weigh modules.
|Design consideration||Compression weigh module||Tension weigh module|
|Floor Space||Requires enough floor space to accommodate tank size. Might require buffer space around tank.||Requires no floor space and can be suspended to allow free movement beneath tank.|
|Structural Restrictions||Weak floors might require additional construction or a special installation to accommodate weight of filled tank.||Weak overhead supports/ceilings might require additional construction or special installation to accommodate weight of filled tank.|
|Weight Limit||Generally unlimited. Even load distribution is inherent with three vessel supports, and is increasingly difficult to achieve as the number of weigh modules grows beyond four.||Tension weigh modules are available up to 10 tons. (20,000 lb). This and structure considerations limit tension-system capacity.|
|Load-Cell Alignment||Designs may vary and must consider floor deflection, available support beams, and tank size, shape and condition.||Cell alignment will not vary significantly because tension rods and other support equipment tend to accommodate most deflections.|
A tank scale’s support structure should deflect as little as possible, and any deflection should be uniform at all support points. Excessive deflection can cause inlet and outlet piping to bind, creating linearity errors. When deflection is not uniform, it can cause repeatability, linearity and zero-return errors due to creep. In extreme cases, it can result in overloading the load cells that are placed on a more rigid part of the support structure.
For an existing installation, the number of supports determines the number of weigh modules. If a tank has four legs, four weigh modules are required. For new installations, a three-point support system is preferred. Using a three-point system, correct load distribution on the weigh modules is assured. However, if wind, fluid sloshing or seismic loading factor into operations, the tank might require four or more supports for additional stability and protection against tipping.
The method of calibration has to be determined before designing the unit because there are three different methods that influence design of tanks and silos. A full-capacity calibration with certified weights traceable to the national standard ensures the best accuracy and reduces risks. This method is applied for expensive material or if it is not possible to fill tanks to full capacity with material that is stored on water.
A step calibration is applied in using weights of 20 percent of full capacity only. After the first step of calibration, the weights are taken off and substituted by water or any other suitable substitute with the weight of 20 percent of full capacity. In the second calibration step, the weights are added again to reach a total of 40 percent. In the third step, weights are substituted by water again. Then the weights are added again to reach 60 percent. This procedure is repeated until 100 percent of full capacity is reached.
The Mettler Toledo CalFree method ensures reasonable accuracy without using weights on site. The actual calibration of the load cells is performed on production. The deviation from the traceable test weight is recorded and listed. Each individual load cell, which can be used with CalFree, is delivered with its deviation values. A dedicated weighing electronic is configured with these values on site to substitute calibration with weights.
The full capacity and the step calibration require fixtures to hang or place weights with an equal distribution on the tank. The application of the CalFree method requires weight transmitters or weighing terminals with the capability to input the deviation values of the load cells.
3. Tank scale installation
A tank scale’s accuracy and safety depend on correct installation and calibration. It is advised to use dummy load cells or dead stands. Mettler Toledo’s SWB505 MultiMount and SWC515 PinMount can be installed without a load cell and can be used as a dead stand to save money without compromising safety.
Dummy load cells or dead stands used during welding on the support structure and while mounting the tank help installers avoid damage from overload or flow of electric currents through the load cell. These dummy load cells should also be used while concreting the base plates for the weigh modules.
Those weigh modules are equipped with SafeLock, which protect the load cell from accidental overload during shipment and installation and ensure proper mounting alignment for the best possible performance and reliability. The SafeLock protects the load cell against shock loads and keeps the weigh module aligned during shipping and installation.
An initial calibration with at least 20 percent of the full load in using certified weights that are traceable to the national standard is recommended. A step or even full load calibration should be performed if expensive material is stored or a high risk is related in case of inaccurate results.
It is recommended that technicians with experience in weighing technology handle initial installation and calibration to achieve optimum accuracy and safety. The initial calibration should be certified with a document issued by a notified body. Such a document confirms correct installation and calibration right from the beginning of operation.
4. Weighing terminals and transmitters
Simple weighing terminals display the weight. Transmitters just transmit the weight via serial interface or Fieldbus to a PLC or ERP. However, advanced weighing terminals don’t only display and transmit weight. They also play an important role in providing accurate results if they have advanced filter technology to eliminate effects from sloshing liquids, environmental vibrations, or mixers. A predictive maintenance function warns the operator of a potential failure. The terminal with Mettler Toledo CalFree enables calibration without weights. This can save installation and repair costs for applications with low accuracy and no traceability requirements.
Regular maintenance of weighing equipment is recommended. A one-year interval at 20 percent of full load is adequate for many applications. However, a maintenance schedule should always be based on a thorough risk assessment that takes into account impacts of incorrect results, minimum weight, and raw material costs.
Mettler Toledo uses proven methods for determining optimal calibration. During regular maintenance visits, experts check the status of load cells, connection boxes, cables, and terminals. They will also perform load tests with certified weights, which are traceable to the national standard and will recalibrate the modules if necessary. With this minimum amount of maintenance effort, a tank scale typically works well for decades. Advanced weighing terminals and transmitters offer predictive maintenance functions and constantly check the signals of the load cells for abnormalities. In such situations, they inform operators. In extreme cases, they can substitute a defective load cell electronically until replacement.
Gravimetric level control using tank scales is one of the most accurate methods to determine inventory level. It provides maximum freedom to optimize tanks to material and process while also allowing a high degree of flexibility surrounding materials being managed.
Pristine hygiene and process flexibility are inherent in tank and silo weighing. Parameters, such as material temperature, density and viscosity, do not affect results, so recalibration and sensor changes are not required when changing materials or recipe formulations. This flexibility also enhances the method’s cost-effectiveness. The choices of compression versus tension weigh modules, as well as the number of weigh modules required, is determined by application and current use considerations. For new systems, a three-point weigh module design is preferred for balanced load distribution. More may be considered when other process parameters come into play.
Maintenance requirements of tank/silo weigh modules are few. A well-designed system working in moderate environmental conditions only requires testing and calibration once a year and provides decades of reliable use.
With gravimetric measuring, one technology can be used for all tanks and many other applications on the production line. Mettler Toledo can help with the complete product range, weigh modules, installations, and maintenance consulting.
Comprehensive engineering documents can be downloaded here.