Physically examining flow and level measurement installations can be quite interesting because the range of issues found can widely vary — from incorrect tubing, inappropriate materials of construction, attack from the environment, improper calibration techniques and more. It is rewarding to sort things out and solve clients’ problems. However, there is one situation that makes me stop in my tracks and check everything — and I mean everything.
Most flowmeters require a good velocity profile upstream of the flowmeter to achieve accurate measurement. The velocity profile downstream of a control valve is distorted due to the fluid passing through a relatively small and often tortuous flow path that varies with valve position. There is also the possibility of flashing and cavitation in some applications. In other words, the velocity profile downstream of a control valve is the antithesis of a good velocity profile.
Knowing this, why would anyone install a flowmeter downstream of a control valve? In 45 years of practice, I have never run into an application where a flowmeter should be installed downstream of a control valve. Maybe there are a handful of applications somewhere with this requirement but finding them has been elusive.
Seeing a flowmeter installed or designed to be installed downstream of a control valve prompts me to stop all work and investigate everything (and I mean everything) because such installations are indicative of one or more voids in knowledge at some point in the design cycle. Whoever did this or was party to it did not understand what they were doing. No one having stopped them is a likely indication that there are additional (hidden) issues elsewhere.
Stop and tread carefully when you stumble upon a telltale sign.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.