Approximately how much straight run is required for an orifice-plate flowmeter installation when the upstream piping has two close-coupled elbows out of plane? Select the best answer.
A. 10 diameters upstream and 5 diameters downstream
B. 100 diameters upstream and 5 diameters downstream
C. 5 diameters upstream and 10 diameters downstream
D. 5 diameters upstream and 100 diameters downstream
E. No straight run is required
Failure to provide a good velocity profile immediately upstream of many flowmeters (including orifice-plate flowmeters) can degrade flowmeter performance. In addition, flowmeter performance can be affected (to a lesser extent) by piping influences downstream of the flowmeter.
Installing flowmeters in a straight run is one way to generate a good velocity profile at the inlet of the flowmeter and reduce the effect of piping influences downstream. Orifice-plate flowmeters are affected by velocity profile, so Answer E is not correct.
Upstream straight run is almost always longer than the downstream straight run because it tends to have a much greater effect on flowmeter performance. Therefore, Answers C and D are not correct.
Many years ago, I was taught that flowmeters require straight run with 10 diameters upstream and 5 diameters downstream. Since then, I discovered that most flowmeters have other straight-run requirements, many of which (including orifice-plate flowmeters) are much longer. Answer A is not correct.
There is no correct answer, but the best answer is Answer B.
Additional Complicating Factors
Straight run is just one technique that can be used to generate a good velocity profile upstream of the flowmeter. Installing a flow conditioner upstream of the flowmeter can reduce straight-run requirements to the extent that Answer A could be the best answer.
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.