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In the previous two installments, we discussed the basics of volumetric and gravimetric feeding. Now that we have a foundation for understanding feeding equipment, we can concentrate on troubleshooting and improving feeder performance.

Because the loss-in-weight (LIW) feeder typically uses a volumetric screw feeder to meter material, many of the volumetric feeder problems and solutions already covered also apply to the LIW feeder. But since the LIW feeder’s operation is based on the weight-loss rate per unit time rather than the screw speed, the controller automatically compensates for material buildup on the screw or in the discharge tube, or a blockage in the hopper, by increasing the screw speed to maintain the setpoint. The controller continues to increase the screw speed until the feeder reaches an alarm condition, such as when the screw speed exceeds the recommended operating speed. If an alarm condition occurs in your LIW feeder, check first for material buildup on the screw or in the discharge tube or a blockage in the hopper. Understanding and properly configuring your controller’s alarm settings will allow the user to perform preventive maintenance.

If you find no material buildup or blockage, check the hopper to ensure that it has material in it. If the hopper is empty, you then need to check the upstream material delivery system for a blockage or other malfunction. The LIW feeder’s operation depends on accurate weight measurements of the material in the hopper and vibration can impose artificial forces on the feeder that cause weighing errors. Therefore, make sure the feeder and weight-sensing device are isolated from any external vibration created by other equipment in your process. This requires installing the feeder in such a way that the weight-sensing device is shielded from vibration effects. Do this by ensuring that the feeder mounting is stable, using flexible connections and shock mounts, and by eliminating strong air currents near the feeder.

Vibration problems can result later from installing new equipment near the feeder or improperly refitting the feeder’s flexible connections during maintenance. For example, if your LIW feeder has feed-rate problems that appear to correlate with the operation of newly installed machinery or that occur after feeder maintenance, external vibration is probably affecting the weight-sensing device. To solve these problems, you need first to make sure that the feeder and weight-sensing device are isolated from any vibration created by the newly installed equipment. Or, if the problems occur after maintenance, recheck the flexible connections to ensure that they’re properly connected to the feeder.

The weight-sensing device itself can cause performance problems if you don’t select it properly for your application. Carefully evaluate the weight-sensing device’s capabilities - such as resolution, stability, responsiveness, weight-signal integrity, vibration sensitivity, reliability, and data communications - before purchasing the LIW feeder. After installing your feeder, maintain its performance and find any problems such as drift (a gradual deviation from a set adjustment) as early as possible by regularly calibrating the weight-sensing device.

Other performance problems can result from a defective refill device or a leaky seal at the feeder’s discharge. If an automatic refill device loads material into the hopper, any leakage in the refill device at the hopper’s inlet will produce a feed-rate error because material will continue leaking into the hopper after the refilling process has stopped. This creates a weight-loss rate change; the controller senses that not enough material is being discharged from the hopper. To compensate for this, the controller increases the screw speed to meet the setpoint, discharging more material per unit time.

Also, if the LIW feeder discharges material to a non-ambient-pressure environment such as a pressurized or vacuum conveying line, a pressure pulse (air leaking from the downstream system through the feeder’s discharge tube to the weight-sensing device) can cause a feed-rate error. A pressure pulse affects the hopper’s instantaneous weight measurement by exerting a vertical force on the weight-sensing device opposite to the hopper’s downward force, in effect, slightly lifting it so that its weight reads lower. Traditionally the solution is a complex arrangement of pipes and flexible connections to compensate for known pressure differentials within the system. As an alternative to traditional mechanical compensation systems, instrumentation and control algorithms to electronically monitor and compensate for pressure influences can be supplied.

An innovative electronic pressure-compensation system, such as Coperion K-Tron’s Electronic Pressure Compensation (EPC), can be used to automatically detect changes in pressure within the feeder hopper or outlet tube and adjust the weight signal accordingly to compensate for any errors caused by pressure fluctuations (see Fig. 3).

If you're interested to read more about our Electronic Pressure Compensation you will find more information here: EPC-System

Feeder accuracy is an important factor in many processes. Proper installation, ideal weighing configurations, and the proper choice of feeder controls and instrumentation can prevent a variety of process problems. Understanding the significance of those factors is critical to achieving optimal performance. The strategies outlined in this series can improve the performance of loss-in-weight feeding.