|David W. Spitzer, P.E.|
Sometimes we “know” things that are not true. For example, we used to “know” Mother Earth was flat. Columbus “knew” he could sail eastward to reach the Far East, but did not “know” that a few continents would lie in his path. We used to “know” the Sun was in orbit around Earth, until we found out it was the other way around. It is in this light that we consider what we’ve “known” about straight-run requirements for flowmeters in relatively large pipes.
Based upon experimental test results, we “know” the flowmeter will measure accurately if a minimum straight-run is installed upstream of the flowmeter. For example, we used to “know” that flowmeters required 10 pipe diameters of upstream straight-run and five pipe diameters of downstream straight-run. We now “know” that the minimum upstream requirement is often much more than this 10-pipe diameters that we used to “know” we needed.
By inference, we “know” the number of pipe diameters required is independent of pipe size. For example, two-inch and 48-inch flowmeters requiring six pipe diameters of upstream straight-run would require one- and 24-feet of upstream straight-run respectively. Stated differently, in this example, one- and 24-feet of upstream straight-run dissipate velocity profile distortions to the extent that they are small enough to not affect the flow measurement.
However, recent flowmeter data taken on four continents shows this dissipation does not occur as quickly in larger pipes as it does in smaller pipes. This appears to be because the fluid in larger pipes does not have as extensive contact with the pipe wall as do fluids in smaller pipes, as well as the large amount of momentum present in the fluid flow. As a result, more pipe diameters of straight-run are required to dissipate flow profile distortions in larger pipes.
One can conceptualize this phenomenon by imagining a large pipe with swirl in its center that has little or no contact with the pipe wall. It is not likely that this swirl, involving tons of fluid per second, will be sufficiently dissipated in (say) six-pipe diameters. In other words, flowmeter performance can be adversely affected in this application and the effect will likely be large due to the large pipe size and large flowrate involved.
This is something that I “know” (at least for now).
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.