|David W. Spitzer|
Last month, I described an email exchange whereby a large (over 30-inch) orifice-plate flowmeter was being installed with 10 diameters of upstream straight run instead of 20 diameters, as prescribed by ISO 5167. The originator wanted to know if the flowmeter accuracy would be within about 5 percent and if it would be reliable and repeatable. The first response was to consider an insertion flowmeter, but this was not possible because the orifice-plate flowmeter was already purchased. A second response contained multiple thoughts as follows. My commentary is provided under each.
The measurement should be repeatable if the flow profile is constant.
• This may sound good, but I am not sure what the responder means.
• In the general case, even a fully developed flow profile is not constant because the flow profile is dependent upon Reynolds number (and hence flowrate).
• I doubt that the responder performed any calculations, but Reynolds number in this particular application is likely so high that the effect of Reynolds number on the hydraulics is probably negligible. However, the upstream piping will likely cause flow profile distortion that varies with flowrate.
• I suggest that if the resulting (distorted) velocity profile is repeatable as a function of flow, then the flow measurement should also be repeatable. However, the assumption of repeatability is questionable without verification in a flow laboratory or in the field.
Bigger issues include questions asking whether the pipe is full and what turndown is acceptable.
• These are legitimate questions.
What kind of differential pressure will you see across such a large orifice?
• I presume the responder is (indirectly) asking for the differential pressure generated by the orifice plate—another legitimate question.
It gets better; more next month…
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.