Over the past couple of months I described the operation of a horizontal extractor whose outlet flow of “clean” water at one end of the extractor was controlled using the interface level on the opposite end of the extractor. This did not seem logical because there was a significant distance and, hence, time delay between the interface level measurement and the control valve. A newly refurbished extractor would be installed in parallel with an existing extractor that would also be refurbished.
Various interface level measurement technologies were considered to implement the new control strategy of measuring and controlling the interface level above the “clean” water outlet. The existing displacer level controller had worked for decades so a differential-pressure interface measurement was possible, but its small range could pose calibration issues, especially outdoors in the Northeastern United States. A capacitance level transmitter was an option, but it required new nozzles that had to be carefully placed to avoid obstructions and the inlet/outlet ports in the separation zone. A guided-wave level transmitter could be installed, but it too required a new nozzle and was relatively expensive at the time.
The good news was that there were multiple interface level technologies from which to choose. The bad news is that none stood out as being ideal. In the end, a new nozzle was carefully located and installed on the extractor for a capacitance interface level probe and transmitter. Capacitance technology was selected because the large conductivity difference between the clean water and the solvent made the interface easy to detect. The extractor was filled with solvent to calibrate zero and then filled with water to calibrate span.
This work was done some years ago. If done today, it is likely that a guided-wave level transmitter would be selected due to its lower cost. It would still require carefully locating a new nozzle, but its metal probe would not be as susceptible to damage as a similar-length coated capacitance probe.
David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting, and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
