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TC1018

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faced in typical process environments. But given the need to discern exceedingly small weight changes reliably in hostile process surroundings, these design measures sometimes prove insufficient. Trend analysis can reveal process influences that make it past a feeder's defenses, allowing you to identify and solve a problem. Figure 2 Locations and causes of feeding problems Poor connection between display and control module If manual, operator input error; if remote, compromised signal integrity None, internally computed None, internally computed Arching or bridging; running empty; poor screw fill; material buildup in metering zone Significant change in characteristics of the material; problems with supply or refill system Same as "Weight loss per revolution" above; faulty wiring or speed sensing Same as "Weight loss per revolution" above Faulty weight sensor(s) or data connections Errors introduced from shock, vibration, process connections, refill and venting systems Prime indicator of underlying problem; requires diagnosis Prime indicator of underlying problem; requires diagnosis None, internally computed None, internally computed Desired weight loss (interval) Desired mass flow (setpoint) Simplified LIW feeder control loop Typical locations and causes of feeding problems Weight loss per revolution Weight loss per revolution adjustment Feeder speed Feeder speed adjustment Measured weight loss (interval) Measured mass flow (interval) Mass flow error (interval) In the feeder In the process mance problem. However, the trend for weight measure- ment is most helpful for narrowing the diagnostic possi- bilities. After you have checked the feeder and eliminated it as a problem's cause, you must then turn your attention to the surrounding process environment. The first alert to a feeding problem will likely be an alarm from the feeder's control system. Properly used, alarms are your first tool for detecting and diagnosing a problem, whether inside or outside the feeder. When an event such as crossing a limit triggers an alarm, the alarm's cause may be a condition that lingers long enough for diagnosis or it may be an isolated, momentary condi- tion that passes before you can identify it. By analyzing trends through time, you can often correlate events and conditions inside the feeder with events and conditions in the external process environment, even in the absence of a triggered alarm. Analyzing trends While analyzing the trends can reveal much about problems inside the feeder, the most useful parameter for locating causes outside the feeder is measured weight. Most equipment manufacturers design modern LIW feed- ing systems to combat performance-eroding influences Tablets & Capsules October 2018 39 Figure 1 LIW feeder operating principle Weight Time Refill complete Refill request Δ Time Δ Weight Mass flow = -Δ Weight/Δ Time Refill duration Feeding cycle time

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