|David W. Spitzer|
In previous columns, we discussed a vortex-shedding flowmeter that was specified for a liquid with an operating-specific gravity, temperature, and viscosity of 1.2, 110 C, and 140 cSt respectively in a 14-inch line (Feb. 2011, page 14; Mar. 2011, page 12). The flowmeter measured correctly after its configuration was changed to reflect the physical properties of the actual liquid and the 16-inch pipe in which it was installed. It was then discovered that the flow measurement was unreliable and dropped out because its minimum Reynolds number constraint was violated. What can be done to remedy these problems?
Recognizing that the minimum Reynolds number constraint has been violated, the goal should be to increase Reynolds number such that the Reynolds number remains above its constraint at all times, lest the flowmeter drop out and measure zero flow. It should be noted that many (and often all) of these remedies may not be viable, leaving the user to replace the existing flowmeter with another flowmeter that has different constraints and/or that uses a different technology.
Decreasing pipe size will increase the velocity through the flowmeter and increase Reynolds number. This can increase Reynolds number dramatically for small pipe sizes, however its impact can be relatively small in larger pipe sizes. This remedy is often not viable because changing pipe size can be expensive after installation.
Increasing the temperature of the liquid will decrease its viscosity and increase Reynolds number. This approach is sometimes viable, especially when the flowmeter can be relocated to a different part of the process where the liquid is already warmer.
Changing liquid composition, such as diluting the liquid, can reduce its viscosity and increase Reynolds number. Direct dilution of the liquid is typically not viable, however it may be possible to relocate the flowmeter to a different part of the process where the liquid is already more diluted and less viscous.
Can you think of another approach to remedy this problem?
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.