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The results of velocity testing (Figure 3) show that indications of velocity profile distortion extend throughout the entire 200-diameter section without any reduction. The magnitude of the velocity errors was between -4% and +4%, depending on position along the pipe section. Also, the velocity error actually increased beyond 100 diameters!
Figure 3. Flow velocity errors in a 48-in. straight pipe.
These results are explained by the presence of rotational distortion (swirl) that doesn’t readily attenuate in large pipes. Flow testing indicates that swirl is attenuated in relatively small pipes because that swirl is in relatively close contact with the pipe wall, so the effect of wall friction influences all parts of the flow area. However, in large pipes, the swirl has far less contact with the pipe wall. In one of our studies of a large pipe (over 4 meters in diameter), the pipe wall had a negligible influence in the main body of the flow. By way of explanation, the momentum inherent in the swirl increases with the mass flow (in three dimensions), whereas the pipe wall surface increases with its area (in two dimensions). Therefore, the ability of the pipe wall to attenuate swirl should decrease as pipe diameter increases.
A large differential pressure flowmeter in North America was installed with its recommended 5 diameters of upstream straight run. Operators reported differences between flow measurements made on opposite sides of the flowmeter, which we subsequently confirmed by more detailed experimentation. One would expect that pressures on both sides of the flowmeter to be identical. However, the pressure difference between taps on opposite sides of the pipe were quite different and oscillated (Figure 4). The operators also found that introducing a flow upstream of the bends affected the flow measurement by as much as 10%. Please note that this observation is anecdotal and hasn’t been tested rigorously.
Figure 4. Measured pressure difference between opposite sides of pipe.
In this installation, the flowmeter was installed downstream of two large bends located within 2 diameters of one other. A general rule of thumb is that fittings within 5 diameters of one other generally create swirl. Bends in different planes, complex valves and pumps are similarly troublesome, and generate different profiles in different planes at the same position along a pipe. The differences in the static wall pressure at the flowmeter inlet, plus the oscillating pressure difference between upstream and throat taps on opposite sides of the pipe, is an indication of the presence of bulk swirl in the line.
The differences between flow measurements taken on opposite sides of the flowmeter also oscillated with different periods under high and low flow conditions (Figures 5 and 6). In particular, doubling the flow reduced the period of oscillation by about half, while the flow measurement bias errors varied from –1.5% to +1%. This shows that swirl depends on flow rate.
Figure 5. Calculated flow differences – high flow.
Furthermore, physical data also is available to support the premise that swirl propagates down large pipes more than just a few pipe diameters. Consider the photo of the interior of a 100-year-old water pipe installed as part of a water supply system in North America (Figure 7).
Figure 6. Calculated flow differences – low flow.
The pattern of encrustation on the pipe wall follows the average motion of the fluid over the century and reflects the presence of swirl in the flowing stream. The flowmeter in this particular installation exhibited flow measurement errors that equate to millions of dollars of revenue. Theory developed in the 1950s in the U.K. hinted this may be the case, and now we’ve observed it.
Figure 7. Swirl encrustation “fossilized” onto straight pipe wall.
The extent to which swirl can be present in large pipes calls into question what we “know” about the upstream requirements for flowmeters in large pipes. The magnitude of the potential flow measurement errors caused by these results can be significant—regardless of flowmeter technology. Further, the upstream straight run requirements in flowmeter standards and manufacturers’ recommendations may be woefully inadequate to reduce the influence of swirl in large pipes effectively. Further investigation is necessary (and is on-going) to quantify these effects and/or develop standardized techniques to detect and mitigate the effects of swirl in large pipes. We plan to report the findings as the data is gathered and analyzed.
David W. Spitzer, Spitzer and Boyes, LLC
845-623-1830 or www.spitzerandboyes.com
Richard A. Furness, PhD, CEng, JDF & Associates Ltd
44-145-278-0893 or www.jdfandassociates.com
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