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Title: The Effect Of In-Service Velocity Profiles On Flow Measurement Systems Of Several Types
Author: Herb Estrada
Source: 2002 International Symposium on Fluid Flow Measurement
Year Published: 2002
Abstract: There are over 25 flow measurement systems of the chordal ultrasonic transit time type currently in service in nuclear plant feedwater systems. These systems base their flow measurements on the numerical integration of the fluid velocities projected onto 4 parallel chords that are located in accordance with the requirements of K.F. Gauss numerical integration method. The data collected by chordal meters provide not only a flow measurement but also quantitative data on the axial velocity profile and the swirl present at the meter location. Some of these chordal systems are equipped with velocity profile alarms, to alert a user to potentially significant changes in the flow field. Several recent alarms on changes in profile led to a comprehensive survey of the hydraulic configurations and operating data for 18 of these chordal systems. The data from these systems show that dynamic changes in profile are common. The survey data also indicate that profiles in locations complying with commonly accepted guidance on the installation of flow measurement systems do not conform to conventional expectations. Specifically, 10 to 15 diameters downstream of a bend, the profiles are often flatter than are those for fully developed flow in smooth pipe. Additionally, a swirl having a tangential velocity in the order of 10% of the axial velocity is often present at these locations, even though an additional distance of more than 10 diameters separates the closest non-planar feature from the upstream bend. Although the shapes of the velocity profiles may differ from conventional expectations and although they are dynamic in nature, experimental data and semi-empirical analyses demonstrate that the errors in 4 chord transit time systems produced by these effects are small-less than 0.1%. The character and variability of the velocity profiles imply more significant uncertainties for externally mounted ultrasonic systems. Because these systems are constrained by Snells Law to operate on diametral paths, their calibrations are determined by the relationship between the axial fluid velocity projected onto the diametral path and the axial velocity averaged over the pipe cross section. Since the profiles are often flatter those for fully developed flow in smooth pipe, an incorrectly assumed profile can lead to an error of 1% or more. Furthermore, the variability of the profiles can lead to time-varying calibration errors the data suggest these errors are in the 0.7% range. The presence of a large and varying swirl at locations far distant from the hydraulic features that produced it can cause significant errors in nozzle-based flow measurements. The data suggest that nozzles without flow straighteners can experience errors in the 0.7% to 2% range, depending on the beta ratio of the nozzle.

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