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Ask an Expert
Reducing elbow wear in pneumatic conveying
Q: My pneumatic conveying system, which transports an abrasive mineral, suffers from extreme bend wear problems. We're never done welding up the elbows—some fail every week or two—and the worst always seem to be the ones at line's end and the silo's top, which present a safety hazard to access. Would it help us to change to a longer bend radius?
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Mike Bradley, of
The Wolfson Centre for Bulk Solids Handling Technology, says: Anywhere that you have hard, angular particles, there's potential for them to cause bend wear due to the particles' impact on the bend’s inner radius. It's tempting to think that a longer bend radius will allow the particles to follow the flow more easily and will reduce collisions with the wall. However, this is a fallacy—even in a very long radius bend, the particles still travel in virtually a straight line from the inlet to the impact point. A longer radius bend will give some extension to wear life because the impact zone is longer in respect to the arc’s length. Tests in our laboratory have shown roughly a doubling in bend life when the ratio of radius to diameter increases from 6 to 12. However, if you have a serious wear problem, you need much more than a doubling of bend life!
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Mike Bradley is the director of The Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich.
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Feature
Using crushers for coarse grinding
Rob Voorhees, Hosokawa Micron Powder Systems
Size reduction equipment for dry bulk materials can yield coarse, medium, fine, or ultrafine particles. In coarse grinding, the particles produced are usually in the range of 150 microns (0.75 millimeter) to 0.5 inch (12.70 millimeters) or larger.
Coarse grinding mills for dry bulk materials operate based on one or more of four size reduction principles: shearing, attrition, compression, and impact. In shearing, a particle is cut or cleaved in two with a sharp blade. In attrition (also called autogenous grinding), a rubbing action in opposite parallel planes between two particles or between a particle and a surface abrades each. In compression, a particle is pressed between two solid surfaces to reduce it. In impact, a solid object moving at high velocity strikes a particle and breaks it. This article will discuss crushers, which employ the final kind of grinding (impact) to achieve the finest possible particles, especially with hard crystalline and amorphous particles.
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Read It Here First:
Gravimetric feeder controls
Sharon Nowak, K-Tron
Gravimetric feeders are commonly used to feed bulk solids in the chemical, pharmaceutical, food, and plastics industries. Unlike a volumetric feeder (such as a screw feeder), which feeds material by volume, a gravimetric feeder feeds material by weight. In both batch and continuous operations, the gravimetric feeder uses a load cell measurement to feed material into a process at a constant weight per unit of time. Integrating the load cell measurement with the feeder's control algorithms is critical to the feeder's performance.
Today most bulk solids gravimetric feeders are highly automated, and their combination of advanced load cells and control algorithms achieves feeding accuracies as high as ±0.25 percent of the desired feedrate, based on load cell resolutions as high as 1 in 4,000,000. Today's control technologies allow gravimetric feeders to be customized to deliver high feedrate accuracy even in tough plant environments.
Simply put, gravimetric feeder controllers automate feeding by comparing key measured variables with their required setpoints. These comparisons generate an output signal that the controller uses to adjust a feeder control parameter, such as the feeding device's motor speed.
The gravimetric feeder measures the actual material discharge rate by taking consecutive weight samples at short time intervals (for example, 1 second, 5 seconds, or 15 seconds) during feeder operation. To control the feedrate, the controller must then compare the actual discharge rate to the feedrate setpoint and calculate and apply any required corrective motor-speed adjustment for the feeding device. The controller also evaluates weight noise (ambient vibration) and adjusts its weight filter (software that filters weight noise from the weight signal) and control parameters accordingly.
To learn more about advanced control technologies and how the interact with gravimetric feeders, you can read "Gravimetric feeders: Advanced control technologies for precise feeding" by Sharon Nowak, K-Tron, in the December issue of Powder and Bulk Engineering.
To read it today, click here.
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Copyright 2011, CSC Publishing Inc. All rights reserved.
PBE-News provides information updates about the dry bulk materials processing and handling industry twice a month. The opt-out link below will allow you to opt out of future PBE-News. This will in no way affect our contact methods regarding your subscription or communication. Thank you.
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