Which Flowmeter Type Requires the Least Straight Run? 

David W. Spitzer, P.E.

Space for straight run of pipe upstream and downstream of the flowmeter is often not available in many installations. Which of the following flowmeter technologies have the shortest straight-run requirements?

A. Coriolis
B. Magnetic
C. Ultrasonic
D. Vortex shedding

It is often difficult to generalize this type of question across an entire technology. For example, most Coriolis mass flowmeters do not have upstream or downstream straight-run requirements. However, the installation instructions for one Coriolis mass flowmeter require that the flowmeter be installed with 20 diameters and 10 diameters of straight pipe upstream and downstream of the flowmeter respectively. Another Coriolis mass flowmeter requires symmetrical supports 6-20 diameters upstream and downstream of the flowmeter.
Magnetic flowmeters generally require 3-5 diameters upstream and 2-3 diameters downstream, whereas ultrasonic flowmeters typically require 10-15 diameters upstream and five diameters downstream of the flowmeter. Vortex-shedding flowmeters have straight-run requirements that are similar to those of ultrasonic flowmeters.
Given this analysis, there can be no definitive determination of which flowmeter technology has the shortest straight-run requirements. However, a number of Coriolis mass flowmeter technology designs can exhibit the shortest straight-run requirements of the listed technologies. Answer A is a practical answer.
Additional Complicating Factors
There are other flowmeters and variants of these technologies. For example, some smaller ultrasonic flowmeters (typically under one inch) do not require straight run. The same is true for some small vortex-shedding flowmeters. Small magnetic flowmeters often have their entire straight-run requirements satisfied within the face-to-face length of the flowmeter.

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.




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