David W. Spitzer

Which of the following flowmeter technologies is potentially the most susceptible to downstream disturbances?
A. Differential-Pressure
B. Magnetic
C. Thermal
D. Ultrasonic

Commentary
All of these flowmeters can be affected by downstream disturbances. To determine the relative effects of downstream disturbances on the performance of each technology, one could analyze the downstream straight-run requirements for each technology. Investigation might conclude that (say) magnetic flowmeters require less downstream straight run than (say) an orifice plate, so the orifice plate is more sensitive to downstream disturbances.

However, the noise generated by a control valve downstream of an ultrasonic flowmeter can be so overwhelming that the ultrasonic flowmeter may simply cease to operate. Therefore, it would be prudent to carefully investigate control valves located downstream of ultrasonic flowmeters to determine whether the noise generated by the control valve occurs at the same frequency as the ultrasonic signal. If it does, the control valve should be relocated farther from the ultrasonic flowmeter and/or carefully selected to incorporate special trim that can shift the frequency of the valve noise away from the frequency used by the ultrasonic flowmeter.

Additional Complicating Factors
Paying special attention to these details can avoid costly piping rework. In some installations, the ultrasonic flowmeter can be relatively easily relocated in the field with clamp-on sensors. However, installations with wetted sensors may be required to be relocated at great expenditure of time and money — especially in custody-transfer applications.

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.