|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
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.
Additional Complicating Factors
The effects of a distorted velocity profile can be profound — even when the sensor is properly located. Locating the sensor without sufficient upstream and downstream straight run can distort the velocity profile and cause the flow measurement to exhibit exorbitant errors. Locating the sensor in a velocity profile that is jetting due to a partially open valve or similar restriction poses particular problems for insertion flowmeters because only one or (at best) a few measurements are used to infer relatively large flowrates.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.