Winter 2009

A Quarterly Newsletter of the Instrumentation Testing Association

ITA Enews

Spring 2011

Instrumentation Testing Association  (ITA)

 

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FLUID DYNAMIC CONSIDERATIONS

Users have been told that at high Reynolds numbers, the velocity profile is axisymmetric, fully developed and swirl-free.   This may not be the case at all.  Further, standards and suppliers’ documents use fixed upstream lengths (5D, 10D, 20D, etc.) depending on the measurement technology involved as guidance for the installation of meters.   Recent research has shown that as diameter increases, disturbances travel further, so these recommended lengths become increasingly insufficient (Furness 2008b).   Also as pipes ‘age’, tuberculation and sedimentation cause disturbances to travel further and for swirl to form even in long straight lengths. Such knowledge and experience is not documented in existing texts and ‘Best practice’ standards.

Consider the two cases shown below in Figure 5.  In both cases the volumetric flowrate is the same.  The left hand figure represents the profile when the meter is first calibrated (ideal flow) and the right hand figure is the velocity profile measured downstream of a bend (real flow).  As the fluid exits the bend, the highest velocity will occur on the opposite wall to the inside radius (Stauss 2004).  This abnormal profile will try to rectify itself into the left hand picture but for diameters greater than DN300 it will take longer than 5D.  Measurements in pipes larger than DN1200 show the length needed is far longer than recommended.

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It is clear therefore that the velocity profile must be examined close to the in-situ meter in order that its installed uncertainty can be assessed.   It is also clear from Figure 5 that measuring the profile on a single diameter may also not reveal the true extent of any problems.  Consider a traverse on the vertical and horizontal diameters in the right figure above. These will give quite different answers.  

 

However there is another factor almost never included in uncertainty assessments and that is age.   Over time, the pipe wall roughness will change. This will alter the velocity distribution even if the profile was originally acceptable.  It also causes swirl to strengthen and grow (even in straight pipes).  Figure 6 shows the pattern observed in a long straight pipe DN1200 in the USA. Similar observations have been made in Brazil, India, South Africa and the UK.  Roughness also changes the coefficients of flowmeters (Hutton 1954) causing them to over-record. If age is not included in the analysis, then uncertainty of measurement can be large.

The swirl pattern shown above (fossilized over time onto the walls of the water pipe in Figure 5) is clear.  Accepted standards say this does not occur but field data confirms it does – and in the majority of cases.  Measurements also confirm that as pipe diameter increases the period of rotation increases.  In the case above it is 20D. For a pipe of DN2000 it is more than 50D.  This seriously calls into question the standards’ recommendation of 5 or 10D for larger meters.  Often tests are performed in small sized lines and the results are extrapolated to larger sizes with no additional increases in installation length. Verification work suggests this introduces significant errors.   

 

For the top down approach to leakage assessment it was stated that accuracy is the cornerstone.  If the uncertainty analysis is incomplete, then the leakage assessment will be flawed, with real losses not known.

Figure 5: Ideal (L) and non-ideal profiles (R)

Pipe swirls

Figure 6: Swirl pattern observed in straight pipe