David W. Spitzer, P.E.

Insertion flowmeters are often used to avoid the expense and difficulty of installing a full-bore flowmeter in large pipes. Which of the following factors can affect the performance of insertion flowmeters?

A. Velocity profile
B. Reynolds number
C. Pipe tolerances
D. All of the above
E. None of the above

Commentary
The velocity profile of the flowing stream (Answer A) can have a profound impact on the performance of most flowmeters. Insertion flowmeters are small devices relative to the pipe size that measure the flow at one location in the flowing stream that is often strategically positioned to measure the flow at a location where the flowrate is equal to the average velocity in the entire pipe, from which the flow in the entire pipe can be inferred. The location of this measurement is predicated on the assumption that the velocity profile in the pipe is uniform and fully developed. If this is not the case such as when the flowmeter is installed with short straight run, the insertion flow measurement may not be representative of the average velocity and the total flow in the pipe cannot be accurately measured. Flow conditioners and/or multi-point insertion flowmeters are often used where velocity profile distortions are known or suspected to occur.
It should be noted that Reynolds number changes (Answer B) as flowrate changes. This changes the curvature of the velocity profile Reynolds number that has a minor impact on the location at which the average flowrate should be measured. However, for installations where the centerline velocity is measured, Reynolds number changes can affect the factor used to calculate average velocity. Because large pipe sizes are involved, a change of a few percent can be significant in actual flow units.
The flow rate is calculated by multiplying the cross-sectional area of the pipe by the average velocity measurement. Pipe tolerances (Answer C) can affect the flow measurement because the tolerances describe the accuracy to which the cross-sectional area is known. The pipe diameter can be measured in lieu of tolerance information.
Thus, Answer D is correct.

Additional Complicating Factors
The appropriateness of an insertion flowmeter technology for a particular application is dependent upon the fluid properties of the fluid in conjunction with the advantages and disadvantages of the insertion flowmeter technology. Fluids with difficult fluid properties may limit the choice(s) of insertion flowmeter technology.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… 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 spitzer@spitzerandboyes.com or 845 623-1830.

www.spitzerandboyes.com