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HOW DO WE GET WRONG LIQUID LEVELS?

Andrew W. Sloley

VECO USA, Inc. (current affiliation)
1313 Bay St
Bellingham, WA 98229 360-676-1500

Published in
Hydrocarbon Online: Tech Talk
1 December 2000

Abstract copyright Andrew W. Sloley
2000
All rights reserved

Poor liquid level control reduces the effectiveness of advanced control systems. Decoupled control systems depend upon using liquid inventory changes to dynamically compensate for control disturbances while minimizing regulatory changes to the system. High liquid levels damage equipment. Liquid backing into reboiler return lines damages trays. Entering reboiler vapors push the liquid up the tower. Tray damage results. Damaged trays do not work.

Many different devices measure the vessel level. Choices include displacement floats, buoyancy floats, thermal measurements, thermocouple arrays, radiation methods, sight glasses with optical sensors, and differential pressure cells. Due to the low cost and wide applicability, differential pressure (DP) cells are the most common.

Control calculations use an assumed liquid density to determine the liquid level inside the vessel. Obviously, if the lassumed density is the same density as the liquid in the vessel, the reading is accurate. Problems begin when the vessel boot density does not equal the assumed density.

6 pages.
Electronic version available in Adobe Acrobat PDF format file 089.PDF 250k.

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This page updated 19 January 2005.
© 2005 Andrew W. Sloley. All rights reserved.