Issue link: http://www.e-digitaleditions.com/i/796485
18 March 2017 | PBEI Call or Visit Our Website Today! 651.777.4780 tecweigh.com More an 100 Years of Material Handing Experience Gravimetric Feeders Flex Feed® Volumetric Feeders Sanitary Feeders Best Warranty in the Business 24/7 Customer Service We Are Superior By All Measure! Bulk Bag Unloaders Weigh Belt Feeders Hopper pressure errors can also be caused by other pro- cess factors, such as a clogged vent filter, a dust collection sys- tem connected to the hopper vent, or a nitrogen blanket applied to the hopper. In one installation, for example, a gravimetric feeder in a closed system was experiencing bumps in the mass flow signal after every refill. The chart in Figure 3 shows the feeder's pressure curve measurements (converted to grams) during three refill cycles, while the chart in Figure 4 zooms in on the first refill cycle. In the example, the feeder's setpoint was 150 kg/h, and the post-refill delay time was 5 seconds, which means that the gravimetric feeder controller was set to ignore irregulari- ties during the 5 seconds following the refill phase. The refill device above the feeder was a vacuum receiver with a powered discharge valve. The refill cycle consisted of the valve open- ing for 10 seconds with aeration pads operating during that time. The aeration pads pumped air into the vacuum receiver to help the material flow out. Material discharge from the re- ceiver into the feeder took approximately 3 seconds (t22 to t25 in Figure 4) and caused a large spike in feeder hopper pressure as air struggled to get out through the hopper's clogged vent filter or back up into the receiver. The aeration pads continued to operate for 7 more seconds (t25 to t32), then the refill valve closed. After the refill valve had closed, the pressure inside the hopper gradually dropped during the next 20 seconds (t35 to t55) before stabilizing at a slightly negative value. This slight negative pressure is caused by the continuous feeding out of material, which creates a slight vacuum inside the hopper since air can't enter freely to replace the discharged material. The feeding error occurred during this gradual 20-second pressure drop and ended once the pressure had stabilized. The gradual decrease of the internal hopper pressure caused an ap- parent decrease in feeder weight, which the controller inter- preted as an over-feeding condition. This became manifest in the mass flow display spiking after every refill. More impor- tantly, the apparent overfeeding caused the gravimetric con- trol to react by reducing the motor speed, so while the mass flow display appeared to show the feeder overfeeding after re- fills, the system was actually underfeeding during that time. Traditionally, feeding systems compensate for these troublesome pressure fluctuations using costly mechanical devices such as flexible bellows. However, factors such as me- chanical tolerances and misalignment or aging of flexible bellows can prevent these devices from fully compensating for the forces generated by changing pressures, making this solution deficient. Also, mechanical pressure equalization devices only equalize pressure, they don't measure pressure or indicate what is actually occurring in the feeder. Alternatively, you can use an electronic pressure compen- sation device (EPC) along with control algorithms to electroni- cally monitor and compensate for pressure fluctuations, as shown in Figure 5. The EPC device in the figure automatically detects pressure changes in the feeder and adjusts the weight signal to the controller accordingly. The device is connected to pressure sensors on both the feeder's hopper and material dis- charge to provide a detailed assessment of exactly what's hap- pening in the feeder to alter the feeder output. The control algorithm allows any feeder pressure changes to be identified as such and not misinterpreted as changes in mass flow from the feeder even during and after hopper refills. PBEI For further reading Find more information on this topic in articles listed under "Feeders" in Powder and Bulk Engineering/International's comprehensive article index at PBE/I's website, www.pbei nternational.com, and in books available in the website's Bookstore. Additional articles and other resources can be found in the Article Archive on Powder and Bulk Engineering's website, www.powderbulk.com. (All articles in PBE's Arti- cle Archive are available free to registered users.) Sharon Nowak, global business development manager Coperion K-Tron Sewell, NJ USA +1 656 589 0500 snowak@coperionktron.com www.coperionktron.com