David W. Spitzer

Which of the following constraints are likely violated by a typical two-inch vortex shedding flowmeter application flowing water at 10-100 gallons per minute?

A. Flowmeter size

B. Reynolds number

C. Minimum flow

D. Minimum linear flow

E. Maximum flow

The fluid velocities and Reynolds numbers at 10-100 gallons per minute are approximately 0.95-9.5 feet per second and 15,800-158,000 respectively.

Most suppliers offer a two-inch flowmeter (Answer A), so flowmeter size is not a problem. The maximum velocity (Answer E) is well within the constraints of most vortex-shedding flowmeters.

In this application, Reynolds number will vary from approximately 15,800 to 158,000. Operating at a Reynolds number as low as 15,800 (Answer B) may cause some vortex-shedding flowmeters to operate non-linearly (Answer D). However, operating at 0.95 feet per second (Answer C) will likely cause the typical vortex-shedding flowmeter to turn off and measure zero flow.
It should be noted that the velocity and Reynolds number constraints are interrelated. The flowrate directly affects the fluid velocity and its operating Reynolds number – both of which are constraints that must be satisfied for the vortex-shedding flowmeter to operate properly.

Additional Complicating Factors
Analyzing flowmeter operation with a liquid exhibiting a higher viscosity that varies with temperature would further complicate 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.