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Why Measuring Flow is a Difficult Task: Profile Effects

By David W. Spitzer and Walt Boyes

E-Zine July 2014

Click here to read "Introduction"
Click here to read "Damping the Signal" (Part 1 of 2)
Click here to read "Damping the Signal" (Part 2 of 2)

Pipe hydraulic problems are among the leading causes of flowmeter problems. Flowmeters are designed to operate in piping where the flow entering the flowmeter exhibits a homogeneous and symmetrical velocity profile.

While there are some flowmeters that are immune to effects of velocity profile, the performance of many flowmeters can and are significantly affected by distortion of the velocity profile that enters the flowmeter. To ensure an adequate velocity profile, many flowmeters require straight run upstream and downstream of the flowmeter. The rule of thumb that flowmeters need 10 diameters upstream and 5 diameters downstream is a myth. Some flowmeters may need upwards of 40 diameters upstream to ensure that the velocity profile entering the flowmeter is adequate. Other flowmeters may need less than 5 diameters. These values are themselves not correct in all locations. Studies have shown velocity profile distortion caused by piping restrictions located over 100 diameters upstream.

Control valves and fittings create turbulence that distorts the velocity profile in the pipe. They should be located downstream of the flowmeter. For similar reasons, thermowells are generally located downstream of the flowmeter, even though a slight temperature measurement error can occur.

Click here to read "Physical Properties"

From Flow Control (August 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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