A flowmeter needs to be installed within a 10 diameter straight run. How much of the straight run should be located upstream/downstream of the flowmeter?
A. 0 D / 10 D
B. 2 D / 8 D
C. 5 D / 5 D
D. 8 D / 2 D
E. 10 D / 0 D
A poor velocity profile at the flowmeter inlet tends to degrade the performance of most flowmeters. One way to develop a good velocity profile is to install the flowmeter with sufficient straight run upstream to allow velocity profile distortions to dissipate. Therefore, Answers A, B and C would appear not to be correct.
Answer E provides the longest straight run upstream of the flowmeter and would appear to be correct. However, the downstream piping can affect the performance of some flowmeters—albeit the effects are smaller than those upstream. In addition, installing the flowmeter with no downstream straight run may not be practical in some installations.
Given generic flowmeter constraints, Answer D is the most pragmatic answer. It is by no means an exact answer.
Additional Complicating Factors
Different flowmeters have different upstream/downstream requirements. For example, positive-displacement and Coriolis mass flowmeters have no straight-run requirements and could be located virtually anywhere in the run. Differential-pressure flowmeters generally require more than 10 diameters upstream (plus a few downstream) so performance in any location will likely be compromised. As such, the availability of straight run can have a profound influence on flowmeter selection.
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.