Which of the following piping configurations requires the longest upstream straight run?
A. Concentric reducer
B. Single 90-degree elbow
C. Two 90-degree elbows in perpendicular plane
D. Two close-coupled 90-degree elbows in perpendicular plane
E. All of the above require the same straight run
Commentary
Straight run is one method used to effectively eliminate the effects of velocity profile distortion on the flowmeter such that it can measure accurately. Piping configurations that are more tortuous generally result in more velocity profile distortion. Reducing a larger amount of velocity profile distortion generally requires longer straight run.
The piping configurations above are listed in order of the amount of velocity profile distortion that they generate. Therefore, one would expect that the piping configuration with the largest velocity profile distortion (Answer D) would require the longest upstream straight run. This is reflected in the AGA-3 standard that requires 7D, 30D, 44D and 95D for an orifice plate with a b ratio of 0.50 for piping configurations A thru D, respectively.
Additional complicating factors
As in the above discussion, there is a tendency to associate straight run with orifice plate flowmeters. However, some flowmeters have upstream straight-run requirements that are independent of the upstream piping configuration. For example, magnetic flowmeters typically require 3-5 diameters of upstream straight run, while positive displacement, Coriolis mass, proprietary magnetic, and proprietary differential-pressure flowmeters do not have any straight-run requirements. Notwithstanding the discussion above, a case could be made that for certain flowmeters all of the configurations require the same straight run (Answer E).
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.
David can be reached at 845-623-1830 or spitzerandboyes.com. Click on the "Products" tab to find his "Consumer Guides" to various flow and level measurement technologies.
