|1. Introduction :
use and importance of continuous weighing in industry
dosing setting error
Continuous weighing is one of the most important operations in dry bulk solids process industries. Continuous weighing systems are equipped with a control loop that allows to adjust the speed of the dosing equipment so that the mass flow (kg/h) is constant. Some applications for continuous dosing are listed below :
Continous feeders are complex equipment and very often process operators do not know where to start in order to solve the issue of an inaccurate loss in weight system. If the continuous weighing system is not accurate (which is to say, fluctuate around the setpoint, or just miss it outright), the resulting product may be out of spec leading to losses for the company.
This page is proposing to study the common root causes of continous dosing deviations, and proposes some solutions to have a better weighing control.
Figure 1 : example of continuous
weighing system, with loss in weight feeders dosing bulk solids
to a continuous mixer
A continuous dosing system is given a set point, a mass flow of material to dose, and must keep it all the time, within certain tolerance. If the system is exceeding these tolerances, quality defects may happen : incorrect metering of a flow to make a mass balance, incorrect mixing proportions...
Such a variability can be detected by studying the trends of weighing, or the drive command of the feeder. If the weight loss is not linear, or if the drive command is changing over large variation, it can indicate that the dosing is actually not well controlled. However it may not be very obvious that the weighing is not stable thus a test run of the dosing system, where the actual weight dosed by the feeder is checked by an independant scale, is usually required to determine the extent of the dosing variability.
A test of each feeder can be very tedious, thus it is important for factory operators to be able to prevent and quickly detect that the continuous weighing is out of tolerances. The following actions can be foreseen :
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When it is confirmed that the system is not accurate, the following can be checked :
Besides having too much variation during the continuous dosing, another common issue for feeders is to simply miss the mass flow target : for example the downstream process needs a feed of 1000 kg/h but the operator realizes that the feeder even at 100% is only delivering 800-900 kg/h. This kind of issues is actually usually coupled with a strong dosing variability.
Common root causes of a continuous weighing system missing its target are :
When it is confirmed that the system is not able to keep its setpoint, the following can be checked :