DAVID W SPITZER'S E-ZINE (September 2025)

Some materials have electrically properties that can vary widely during operation and/or be below the electrical conductivity limits of the available sensors. In these applications, level measurement systems are available that measure the electrical parameter and compensate for its changes.

Coating on the sensor can effectively change the capacitance value at a given level. These coatings can be temporary, but they more likely build up over time. To avoid periodic maintenance, some capacitance level system designs are available with features that reduce the effect of coating on the measurement. One technique is to measure the admittance of the probe whereby the resistance and capacitance of the probe are used to identify and subtract the effects of coating. Another technique is to use an additional active shield to quantify the effects of coating. High frequency excitation of the capacitance probe can also be used to reject the effects of some coatings.

Capacitance level measurement systems for solids applications are similar to those for liquids except that the sensor may be different. Solids applications pose the additional complication in that variations in the material bulk density can affect its electrical properties and hence the level measurement.

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

Once upon a time, there was a shutdown.  During this shutdown, changes were made to the raw material pipe upstream of the catalyst recirculation flowmeter.  The Venturi flow element should not have been touched and this was "confirmed".  During startup, the operator could not get sufficient recirculation flow --- despite having the catalyst recirculation control valve wide open.  The operation asked me to investigate because he knew that I had worked on the recirculation control valve. 

Examination of the Venturi flow element quickly revealed that it was installed backwards (despite not being touched).  In this installation, the Venturi flow element presented the flow stream with a different geometry that caused the pressure drop to be lower than expected at a given flow rate.  Even opening the control valve completely could not generate enough differential pressure to obtain 20 percent flow, so the operator suspected a problem.  In addition, the reactor system did not have the right "feel" under these conditions because the flow seemed to be approximately double its normal operating flow --- even though the flowmeter measured only about 20 percent of flow. 

At this stage in the startup, the decision was made to not shut down to reverse the Venturi flow element.  To operate the plant, the flow measurement obtained with the control valve open in manual at its normal operating position was used as the setpoint for the flow controller.  The reactor operated successfully for about 6 months with this setpoint. 

More next month...  

 This article originally appeared in Flow Control magazine.

In liquid service, the taps should be oriented so that the impulse tubing remains completely filled with liquid in order to maintain a hydraulic connection between the flow element taps and the differential pressure transmitter.  Failure to maintain a liquid hydraulic connection between the flow element taps and the differential pressure transmitter can adversely affect the accuracy of the flow measurement system. 

Locating the taps on the top of the pipe (Answer A) is not recommended because non-condensable gas such as air can enter the impulse tubing, accumulate, and cause the flow measurement to be inaccurate.  Locating the taps 45 degrees above the centerline of the pipe (Answer B) is better than locating the taps on the top of the pipe (Answer A) but is still not recommended for the same reason.  Locating the taps on the bottom of the pipe (Answer D) can allow dirt and other solids to enter and potentially plug the impulse tubing thereby adversely affecting the flow measurement. 

Locating the taps on the side of the pipe on the centerline (Answer C) is generally recommended because it mitigates the issues associated with Answers A, B and D.

Additional Complicating Factors

Note that simply locating the taps on the side of the pipe on the centerline (Answer C) does not ensure that the impulse tubing will be full of liquid.  The impulse tubing between the taps and the differential pressure transmitter should be sloped so as to allow non-condensable gas such as air to naturally rise and exit the impulse tubing into the flow stream.

This article originally appeared in Flow Control magazine.