Winter 2009

A Quarterly Newsletter of the Instrumentation Testing Association

ITA Enews

Spring 2011

Instrumentation Testing Association† (ITA)

 

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VELOCITY PROFILES

In many places, velocity profiles are measured and used as a comparator against the meter installed.† If this is undertaken on a single plane, the results may or may not be truly indicative of actual site conditions.†† In some companies the readings from insertion probes are used to adjust the installed meter factor.†† It is suggested this is poor practice and should be actively discouraged.††

Figure 7 shows a velocity profile taken in a DN900 pipe that runs straight for 5kms before the verification point. Point velocities were measured every 50mm across the pipe.

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It is quite apparent that the profile is not axisymmetric and that velocities vary considerably on opposite sides.† Standards say this should not occur.†† The mean velocity of 0.93 m/s on this vertical diameter was 5% different to the horizontally measured mean.† The profile therefore had significant 3-D effects even after 5000D.† The inside was possibly similar to that shown in Figure 6.†

 

More significantly the variations at each point were also measured and this appears in Figure 8 below.† It is noticeable that local velocity varies more in the centre than at either wall.†

Figure 7: DN900 pipe profile

Figure 8: Measured profile stability

The velocity profiles should be measured in more than one plane on any trunk main larger than DN300.† Australia has been a good example worldwide of a steady reduction in demand.† In such cases most meters are now running at lower-than-intended velocity and will therefore generally be over-recording the flow.† It is essential on any water balance that all key inlet and outlet meters are profiled.††† RPS verification methodology also uses clamp-on meters in conjunction with insertion methods. These are used in multi-path arrangements to assess the average velocity in two or four planes.†† A typical arrangement installed in a UK waterworks inlet is shown below in Figure 9.†† It allows profile asymmetry and bulk fluid swirl to be clearly identified and therefore can be used to correct meter readings in a meaningful way.

Figure 9: Profile assessment techniques

If insertion probes are used in series with clamp-on meters, the correlation coefficients obtained give very high confidence that fluid dynamic effects are present and thereby allow the in-situ meter readings to be fully understood and corrected.†