|David W. Spitzer|
In the last two months (links provided below), I described an e-mail 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. The second response implied that the measurement would be repeatable if the velocity profile was repeatable. It also asked for turndown requirements, whether the pipe was full, and the differential pressure generated. The remaining points in the second response are provided here, along with my commentary under each.
A flow conditioner would improve flowmeter performance.
• I agree.
The 20 diameter upstream straight run is required to achieve a laminar flow profile upstream of the flowmeter.
• This may sound good to the casual observer, but it does not make sense.
• A laminar flow profile is exhibited when the flow is in the laminar flow regime with Reynolds number of less than approximately 2,000.
• Orifice plate flowmeters are designed to operate in the turbulent flow regime. Why the responder would want to achieve a laminar flow profile upstream of the flowmeter is not understood.
• Notwithstanding the previous point, the large pipe size implies that the flowmeter is likely operating at Reynolds numbers of millions or tens of millions. Achieving a laminar flow profile at such a high Reynolds number is not possible. Suggesting that straight run will do this is preposterous.
• As a general comment, the installation of straight run is only one technique that can be used to develop a good velocity profile upstream of the flowmeter.
The answer to the original question is to refer to ISO/TR 12767, entitled “Measurement of fluid flow by means of pressure differential devices—Guidelines on the effect of departure from the specifications and operating conditions given in ISO 5167.”
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.