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.