Batch weighing Troubleshooting

1. Introduction : importance of solids weighing

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)

2. Weighing accuracy issues

How to solve weighing accuracy problems ?

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 :

• Prevention : the equipment on load cells (typically a hopper) must be checked on a regular basis by using calibrated weight. For example, at the start of every shift, the operator puts a defined mass on the hopper, only if the load cells indication is correct, the process can start. A large part of this prevention is based on Standard Operating Procedure which are more and more integrated in computer systems to make sure they are applied.

• Detection : it is required to controlled the dosing load cells by other means, for example by performing a mass balance in the consumption of materials. This method is however efficient only to detect relatively big variations.

When the control system detects that the dosing is out of tolerance, the following can be checked :

• Start by checking the weighing hopper with calibrated weights, especially if it has not been done for a long time
• Verify all flexible connections : on the process pipes but also electrical and pneumatic cables to make sure they don't push or pull on the weighing hopper
• Check the filters : a pressure problem can influence the weight but also disturb the flow of materials
• Verify that the dosing parameters have not been changed
• Check that the material flows freely to the dosing equipment
• Verify that there is no excessive fluidization of the powder that could lead to it flowing too much "almost like a liquid" and generating overdosing.
• Check the adequacy of the dosing equipment design

3. Weighing speed issues (too long cycle time)

How to solve dosing speed problems ?

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 :

• Incorrect dosing parameters. Most batch dosing system are following a 2 speed pattern : dose as fast as possible then switch to fine dosing when getting close to the dosing targets. If the switch to fine dosing is too early (too much material left to dose) or if the dosing speed of either coarse or fine is too low, then the dosing time can be strongly impacted. Check the dosing parameters and modify if they are too conservative. This may require also to re-check the accuracy.
• Bad flow of materials. Dosing speed will be fast if the dosing instrument, for example a screw conveyor, can be filled with materials. Check that the flow of solids in the supply hopper is good, if require re-set or add the discharging aids.
• Undersized dosing equipment (for example a screw conveyor). This can happen for example when having to dose a material with a lower density. The dosing equipment needs to supply higher volumes for a same mass result, it may appear that is undersized, leading to the need to replace it (still having in mind the accuracy, do not oversize either).