David W. Spitzer, P.E.

Insertion flowmeters typically measure the flowrate at one or more strategic locations in the pipe in order to determine the total flow in the entire pipe. They are often used in large pipes to obtain economical flow measurements. However, locating the sensor incorrectly can cause the flow measurement to be in error by as much as 10 to 30 percent or more. At which location (expressed as a percentage of pipe radius from the pipe wall) will the sensor register the maximum flowrate?

A. 20 percent
B. 30 percent
C. 50 percent
D. 100 percent
E. Any of the above

Commentary
Fully developed flow in the pipe will exhibit a velocity profile that is not flat, that is, the fluid velocity at different locations in the pipe will be different. Therefore, Answer E is not correct.

The velocity profile has the boundary condition that the fluid velocity at the pipe wall is zero. The fluid velocity will increase as the sensor is moved away from the pipe wall. The maximum velocity will occur at the center of the pipe where the effects of the pipe wall are low. Therefore, the location where the fluid velocity is at its maximum is the center of the pipe, that is 100 percent of the pipe radius from the pipe wall. Answer D is correct.