|1. Introduction :
importance of solids weighing
|3. Weighing speed
issues (too long cycle time)
Many bulk solids processes include one or several steps of weighing. It is especially the case for a dry mixing process where several components must be weighed in order to reach a composition desirable for the client, product and regulation.
Weighing issues can quickly lead to losses for companies : losses of ingredients (for example through over dosing), losses of final product (if the composition is not compliant), losses in productivity (if the weighing is taking much longer than expected). Engineers have then from time to time to perform a troubleshooting of a batch weighing system (which can be either Gain In Weight, Loss In Weight, or a mix of both technologies) in order to solve weighing accuracy or weighing speed issues.
Figure 1 : example of batch weighing system with a loss in weight dosing (left) and a gain in weight dosing (right)
The accuracy of weighing system is actually quite complex as it includes variability coming from the load cells themselves and the weighing hopper. To relate more to industrial problem, what is designated here as accuracy problem is the issue of setting a dosing weight in the control system, and getting another weight in reality (overdose or underdose). From there, 2 cases are possible : either it is possible to detect that the dosing is not accurate, because the dosing system is giving an alarm, or the dosing system "thinks" the weighing was accurate but it is not the case in reality.
The case in which the weighing problems go undetected because the control system is "seeing" the dosing right is the most difficult to tackle. Dosing issues are sometimes realized way down the process flow, and sometimes at customer. It is however possible to act in terms of prevention and detection :
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When the control system detects that the dosing is out of tolerance, the following can be checked :
Besides missing the dosing target, another issue that can impact a factory having a batch weighing system of bulk solids is to take too long to perform the weighing within the tolerance of accuracy. Dosing slowly then impacts the batching system cycle time and in turn the line productivity and profitability. Factory operators must then always dose as fast as possible with the weighing tolerances. Here are some typical root causes of a dosing system taking too long to weigh :