Position: Question
Which is better, using three or four load cells on a belt scale? How many load cells should be used in belt scale ?
Answer
In theory any number of load cells or weighing transmitters can be used to support a vessel. The number actually used is dependent
on one or more of the following:
· The vessel’s geometry (shape and number of supports).
· The vessel’s gross weight (both live and dead weight).
· The vessel’s structural strength.
· The environment the vessel is located in.
· What’s available structurally to provide a stable load bearing support.
· The characteristics of the material being weighed.
INSTALLATONS USING THREE LOAD CELLS:
For short, upright cylindrical vessels in a compression installation, three load points spaced at
120-degree intervals provide the most convenient support. Three-leg weighing systems balance like a tripod, with load distribution being virtually automatic, and they only
require minor balancing at installation. All the load cells or weighing transmitters must be installed in the same plane within 3
degrees of each other.
Cylindrical vessels suspended symmetrically in tension and using three load points provide the advantage of the load being equally distributed among the load cells.
What’s available structurally has a big say in this situation. A vessel in tension can be hung in a
corner where there are two supporting structures at right angles. All that’s required is a 45-degree
cross brace to provide support for the third load point. Of course, the support beams must be sufficiently
strong and stiff to support not only the fully loaded vessel, but also other vessels that may be supported from
the same structure, and any changes in the structural load such as an accumulation of snow, water or ice.
INSTALLATIONS USING FOUR LOAD CELLS:
Vessels that are square, rectangular, horizontal cylindrical, tall cylindrical, or require greater stability,should use at least four load points, one in each corner. Vessels subject to fluid sloshing, material agitation or mixing, violent internal chemical reactions, high winds or seismic effects all require greater stability against tipping.
A four-leg weighing system adds structural strength but requires more care in the installation process in order to balance the loading on the four legs. With this type of support system, it is necessary to equalize (level the base) to spread the load evenly among the four cells. For belt scales with accuracy requirements equal or greater than 0.1%, the base plate support surfaces must be within 0.4 degrees (0.08mm/100mm). If one cell is mounted on a lightweight crossbeam that has a high deflection, it can sag and throw the load on to the two adjoining cells, possibly overloading them.
It is a simple but critical process to shim the load cells or, if equipped, fine tune the cell mounting hardware during installation to balance the four legs. Proper load sharing should see only a difference of plus or minus 0.5mV between load points. This can be accomplished by measuring the DC mV signal between each of the load sensors, plus and minus signal wires with handheld meters, or through the weighing instrumentation if that feature is available.
Larger differences between load cells due to motors hanging off one side of the vessel or excessive or low flexing piping should not exceed plus or minus 2.0 mV between the highest and lowest reading. Four legs also offer a larger area to support the vessel, provide for equal load sharing among the legs, and help to keep the vessel from tipping over.
Long horizontal tanks with saddles symmetrically positioned in from the ends should also use four load points.
If the material the tanks contain is self-leveling, if there are no partitions in the vessel, and low accuracies of 0.5% or greater are acceptable, using one or two load points at one end and flexures at the other is an acceptable set-up. The load fraction seen by the load points must stay the same, no matter what the level in the vessel.
Very large capacity or heavy vessels in excess of one million pounds require more than four load points. Since the wall thickness and supporting structure of the vessel increases as the number of supports decreases, the vessel’s dead weight and therefore installation costs increase dramatically. These vessels are usually designed for six or eight load points. Generally no more than eight load points should be used. It becomes proportionally more difficult to get even weight distribution (and therefore better accuracy) on vessels with more than four load points.
Which belt scale system to choose?
As for which belt scale system is better, there is no correct answer. The precision can be equal on one, three, four, or more compression or tension load point systems. The road to higher accuracy is mostly in the care taken during system design and installation. Taking into account level footings, balanced load points, absence of binding, flexible piping, environmental characteristics such as wind and temperature, and the load’s center of gravity within the footprint of the belt scale all make for a good belt scale installation. All of Hardy weighing controllers’ Rockwell Encompass? approved weight controllers and PLC weigh modules support multiple load cell installations and provide features for easy calibration, verification and maintainability.
