Continuous weighing / dosing Troubleshooting guide

1. Introduction : use and importance of continuous weighing in industry

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. Such operation is often performed by Loss in Weight feeders. Some applications for continuous dosing are listed below :

• Regulating the flow of bulk solids to a crusher or grinder
• Accurate dosing to a mixing equipment : loss in weight feeders are used to dose material to a mixer in order to produce a finish product ; in this case several continuous systems have to be  controlled at the same time and all must perfectly work.

Continuous 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

2. Dosing variability issues

How to solve weighing stability problems ?

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 :

• Prevention : the equipment on load cells (typically a hopper, or a weigh belt) 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.

When it is confirmed that the system is not accurate, the following can be checked :

• Start by checking the load cells 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
• Verify the support of the feeders and make sure that no external vibration is transmitted to the feeder
• Check the filters : a pressure problem can influence the weight but also disturb the flow of materials
• In the case of a loss in weight feeder using a screw conveyor as dosing equipment, make sure that the outlet is connected to a pressure compensator in order to avoid that back pressure from the downstream process is influencing the weighing system.
• For small continuous feeder, check that the equipment is not submitted to any air draft that could push on the hopper and change the weight reading
• Verify that the dosing parameters have not been changed - continuous dosing systems have quite sophisticated control algorithms, if the setting is not correct it can make them correct too slowly or too quickly a deviation from the setpoint
• Check that the material flows freely to the dosing equipment
• A loss in weight feeder equipped with a hopper on load cells must be refilled from time to time : during refill, the weight loss due to feeder cannot be measured as the weight in exceeds the weight out which leads the feeder to freeze its command during the refill : if the refill is too long, or if refill are too frequent, this will lead to dosing inaccuracy. Make sure that such refill are as fast as possible, and as less frequent as possible.
• Check the adequacy of the dosing equipment design

3. Continuous dosing setting error

How to solve dosing capacity problems ?

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 :

• Undersized dosing equipment : sometimes, factory operators simply ask too much to their dosing system. For example after a process change (debottlenecking), a material change (lower bulk density) or when trying to speed up the line to make up for a previous problem (time lost). Operators should have a capacity reference for each of their feeder and shouldn't consider that it can run consistenly at 100% of speed (drive command) ; the capacity at which the feeder is operated should be in the 75-80% of max drive command.
• 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.

When it is confirmed that the system is not able to keep its setpoint, the following can be checked :

• If the feeder is equipped with a screw conveyor, it can be possible to change it for a different pitch or a larger diameter
• If the issue is a feeding of the dosing equipment due to a bad flow of product, then the design of the hopper can be modified, implementing an agitator, or some fluidizing equipment.