David W. Spitzer

An insertion flowmeter is specified for a 14-inch pipe. What will be the approximate flow measurement error if the flowmeter is installed in an 18-inch pipe?

A. Increase flow by 40 percent

B. Increase flow by 20 percent

C. No effect

D. Decrease flow by 20 percent

E. Decrease flow by 40 percent

Commentary
Estimating the internal pipe diameters to be 14 and 18 inches, the fluid velocity will be approximately 40 percent (1-(142/182)) lower in the larger 18-inch pipe as compared to the 14-inch pipe. Therefore, the insertion flowmeter will tend to measure lower in the 18-inch pipe as compared to the 14-inch pipe, so Answers A, B and C are not correct.

The type of insertion flowmeter is not known. It could be a linear flowmeter using vortex-shedding, magnetic or thermal technology that is directly affected by fluid velocity. If so, Answer E is approximately correct. Inferential flowmeters, such as a Pitot tube are nonlinearly affected by fluid velocity and would exhibit an effect approximated by Answer D.

Additional Complicating Factors
For estimating purposes, the internal pipe diameters were assumed to be 14 and 18 inches. This is not correct because the inside diameter of these pipes is based not only on the pipe sizes but is also on the pipe schedules. A detailed calculation would entail using the inside diameter used for calibration purposes and the actual inside diameter of the pipe where the flowmeter is installed before applying the above relationship.

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.