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Solving Downstream Pressure Problems with LIW Feeders |
June 28, 2006 |
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Posted by Delmar Schmidt at June 28, 2006 12:26 AM |
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We all know that gravimetric feeders cannot weigh accurately when feeding into even a slight downstream pressure. However many applications, especially the polyolefin resin producers, will have the feeders feed directly into a purged blending screw, or even worse a poorly vented rotary airlock. Everyone complains “but it’s only 1 inch of water purge, so why is the scale weight always erratic?” And no, our PLC-based retrofit control system will not fix a pressure problem.
The CauseThe classic feeder discharge method is a vertical elbow welded solidly to the feed tube. A flexible connector is then clamped to the bottom of the stub and the downstream inlet. Let’s do the calculations on a theoretical feeding application:
You have a LIW feeder running at 60 lbs/hour. The downstream flexible connector is 6” diameter. Looking at the 28 square inch cross-sectional area of a connector exposed to the 1” purge, there will be a resultant lifting force of one approximately one pound (yes I picked easy measurements for my example). Whenever the purge is rapidly applied or removed from the process, like when the operator opens an inspection hatch, you just negated a full minutes’ worth of material loss from the scale. Double the purge pressure, or cut the feed rate in half, and the problem gets twice as bad.
Exaggerating the pressure problem are two unexpected forces:
1. The flex connector will balloon under pressure, increasing the cross-sectional area exposed to the purge with a corresponding increase in the resultant lifting force.
2. When you add an extended helix and nozzle to the feeder, the lever arm is increased with a corresponding increase in the resultant lifting force.
The “Melfi Elbow” SolutionThe obvious solution would be to put the flex connector in its side. Most good scales will have horizontal flexures or linkages to minimize effect of side loading. While a horizontal flex connector will not negate all the effects of downstream pressure, it will greatly improve the weighing accuracy when the feeder is exposed to unpredictable changes in the pressure level.
The horizontal design will require a slightly custom flex connector design, but most suppliers should be able to supply a reducing sleeve. Email me for the name of a local connector company we use. Alternately you could weld on a tube blank with a matching OD of the elbow stub and use a straight connector.
Regards, Delmar Schmidt
