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Figure 10 shows a trace pattern signaling an abrupt change in the material's density or handling characteris- tics. This condition could arise from any of several causes, ranging from a different material supplier to changes in storage or transport practices that mistakenly introduce the wrong material. The figure shows the con- dition where the density of the material abruptly falls to a slightly lower-than-expected value. The feeder's speed increases in a step-like fashion to adjust for the sensed reduction in weight loss per revolution. To compensate for an increase in material density, the opposite would occur. The figure shows that the feedrate and hopper weight remain on target, but if the change in material properties or handling characteristics is too great, the feeder may not be able to accommodate it, and an alarm will sound. Summary Proper refill algorithms, the ideal weighing configura- tions, and the proper choice of feeder controls and instrumentation can help to avoid many feeding prob- lems, but feeder performance problems may still occur. Using the trending capabilities that your feeder displays can provide valuable clues to some of the more elusive causes of feedrate disturbances, allowing you to keep your process running smoothly. T&C Sharon Nowak is the global business development manager for the food and pharmaceutical industries at Coperion K-Tron (856 256 3119, www.coperionktron.com). She holds a BS in chemical engineering from Rutgers University, New Bruns- wick, NJ, and has more than 25 years of experience in develop- ing process equipment for the pharmaceutical and other process industries. This article originally appeared in T&C's Solid Dose Digest on February 26, 2018. flow problems to return or new ones to emerge. While such problems will almost certainly cause one or more alarm conditions, monitoring a feeder's performance variables through trends can identify and diagnose emerging concerns. Figure 8 shows a trendline pattern typical of material suddenly becoming hung up due to arching, bridging, or some other form of blockage in the hopper. After the feeder empties the material below the blockage, the fee- drate quickly falls to zero; the hopper's net weight remains constant; the feeder's speed maxes out in its futile attempt to dose material that is no longer available; and weight loss per revolution drops to nil. Figure 9 shows a trendline pattern typical of material buildup on the feed screw. In this case, the weight loss per screw revolution declines more than expected over time as material builds up on the metering element(s). In response, the feeder's speed increases to compensate for the reduction in the screw's efficiency. If buildup stabi- lizes and is not severe, feedrate and hopper weight remain on track. However, too much buildup will eventu- ally trigger an alarm related to the feeder's speed or a vio- lation of the limits for weight loss per revolution. 42 October 2018 Tablets & Capsules Figure 8 Material blockage (0) Gravimetric feedrate Hopper net weight Feeder speed Weight loss per revolution Blockage onset (0) (constant) (max) Figure 9 Buildup on feed screw and/or screw tube Buildup stabilized Gravimetric feedrate Hopper net weight Feeder speed Weight loss per revolution Onset of buildup Figure 10 Material-property variation Material change Gravimetric feedrate Hopper net weight Feeder speed Weight loss per revolution