Stephen Hall discusses the golden rules for design
WEIGHT is often used for process monitoring and control, especially for storage tanks, bins, silos, mixing vessels, and batch reactors. Typically, three or four load cells are mounted beneath the vessel, and a summing instrument transmits the combined measured weight to the process control system. The load cells are typically installed under the tank legs or between side-mounted lugs and the steel support structure.
Accurate weight measurements are obtained when extraneous forces are eliminated or compensated for. Extraneous forces may be generated from lateral loads, thermal expansion or contraction, or bending moments from attached components and piping. The extra forces might be compensatable through the taring step, where the load cell system is zeroed prior to adding the measured load. But for this, the forces would have to remain constant over the duration of the measurement. Generally, the design should anticipate and mitigate spurious forces.
Accurate: Tank on load cells with calibration weights and digital display
Here are some tips for load cell design and installation:
- Use compression load cells for tanks, not shear beams. Compression load cells are relatively unaffected by non-vertical loads because they float beneath the tank. If a tank experiences thermal expansion, its supporting surface slides across a compression load cell and therefore maintains a vertical force. Shear beams are bolted to the tank; thermal expansion results in a bending moment that the load cell interprets as added weight. Use or specify mounting kits for the load cells and ensure that they are installed to absorb side forces.
- Provide a rigid support structure. The load cell support and vessel system should be designed to eliminate flexing that would affect the readings. If the tank sits on long legs, the load cells should be installed near the top of the legs, not at the bottom. Provide adequate stiffening and cross-bracing. If multiple load cells are used, ensure they carry approximately equal load.
- Isolate the vessel system from its surroundings. Ensure that appurtenances float with the tank; avoid fixed connections to external structures. For instance, ladders should be attached to the tank and not the support structure, or attached to the support structure with a small gap between the ladder and tank.
- Ensure that piping and conduit connections are flexible. Although many engineers routinely specify flex connections, such as hoses or bellows, for this purpose, adequate flexibility is usually obtainable with careful routing and support of the piping. The goal is to eliminate extra horizontal or vertical forces from the piping. Taking potential thermal expansion and contraction of the vessel into account, piping bending moments can be calculated between the connection point and the closest rigid pipe support. The bending moments translate into vertical and horizontal forces imposed on the tank; their magnitude determines the potential effect on the load cell reading and may be inconsequential when taken as a percentage of the total mass of the vessel system.
- For tanks, provide attachment points for hanging weights when the load cells are calibrated. Alternatively, ensure that standard weights can be placed on top of the tank. If the standard weights weigh less than the maximum load, then the load cells are calibrated in steps. The weights are placed onto an empty tank to calibrate the first point. The weights are removed, and water is added to bring the display to the first calibrated point. The weights are again placed onto the tank and the second point is calibrated. This procedure continues until the maximum weight is calibrated. Because the weight of each water addition is subject to the accuracy of the load cells, this stepwise calibration method introduces error into the overall calibration.
You can calculate the force that is exerted by connected piping, using Equation 1.