QUIZ CORNER: What’s the Purpose of Straight Run Pipe in Flowmeter Installations?

It’s a common best practice to adhere to recommended straight-run requirements in flow measurement applications, but do you know why?

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David W. Spitzer

What is the purpose of providing straight run upstream and downstream of flowmeter installations?

A.    Provide a good velocity profile for the flowmeter
B.    Ensure the fluid is homogeneous
C.    Ensure the flow is turbulent
D.    Ensure the flow is laminar
E.    All of the above

As I wrote in last month’s Quiz Corner—failure to provide a good velocity profile immediately upstream of many flowmeters can degrade flowmeter performance. Answer A is correct.
The fluid flowing through the flowmeter should be homogeneous for accurate measurement. However, straight run does not necessarily make the fluid homogenous. To the contrary, piping fittings and obstructions tend to mix the fluid and create greater homogeneity. Long straight runs of (say) an oil/water mixture can cause a phase separation upstream of the flowmeter. Answer B is not correct.  
Turbulent and laminar refer to the flow regime as typically determined by calculating Reynolds number. Reynolds number is a function of the flowrate, fluid density, fluid viscosity, and pipe diameter. Straight run alters none of these variables, so Answer C and Answer D are not correct.

Additional Complicating Factors
Straight run is only one method that can be used to effectively eliminate the effects of velocity profile distortion on the flowmeter such that it can measure accurately. Flow conditioners can also be used to develop a good velocity profile when sufficient space for straight run is not available.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.

Mr. Spitzer can be reached at 845 623-1830 or 
www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.

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