David W. Spitzer

Which of the following upstream piping configurations causes the greatest velocity profile distortion?
A. Elbow
B. Reducer
C. Two elbows
D. Two elbows out of plane
E. Two close-coupled elbows out of plane

Most flowmeters require a good velocity profile upstream of the flowmeter in order to measure accurately. A number of techniques can be used to develop a good velocity profile upstream of the flowmeter, including the installation of straight pipe immediately upstream of the flowmeter and/or installing a flow conditioner to remove the distortion present. Another alternative is to select a flowmeter that is designed to tolerate the distorted velocity profile.

Standards published by organizations such as AGA, API, ASME, and ISO specify the upstream and downstream straight-run requirements for various flowmeters. The diameters of straight run required to mitigate the effects of the above piping configurations tend to progressively increase from piping configuration A thru piping configuration E. Therefore, the correct answer is Answer E.

Additional Complicating Factors
In many installations, the actual straight run is less (and often much less) than the published straight run requirement for a given piping configuration. When piping changes cannot be made to have the straight run in conformance with its requirement, the user may choose to make other modifications such as installing a flow conditioner and/or accepting a less than accurate flowmeter installation.

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.