centreline flow measurements

When using the Quadrina insertion flowmeter, centreline measurements are quite feasible and frequently used. Ideally the point of mean flow is the best position at which to place the flowmeter. This point usually remains substantially constant even when the velocity profile changes its shape, as it will do when the fluid velocity alters. However, the exact location of the mean flow point can be a difficult place to position the flowmeter head due to the high slope of the velocity profile curve in this region. A small misplacement of the flowmeter head can make a measurably large error in the flow readings obtained.

Positioning the flowmeter head at the pipe centreline is much easier, mainly because the slope of the velocity profile curve is, in most cases, extremely low. Therefore, a small positional error of the head will, in general, produce a small, often negligible measurement error when using the centreline for measuremens.

Some additional care is necessary to ensure that readings obtained, when using the centreline, are properly corrected for the ratio of mean flow to centreline flow which in turn must be corrected for blockage caused by the flowmeter in the pipeline. Unfortunately the ratio is not a constant, it changes with flowrate, because it is largely reynolds number dependent (as Re increases so does Vmean to Vcentreline ratio), but in most cases a reasonable average value can be chosen to cater for the range of flows anticipated. To obtain a close tolerance ratio the Jestim Velocity Profile programme is ideal. This inexpensive and simple to use package runs on a laptop computer and enables full information about flow measurement location to be determined, complete with all blockage data.

RSP - Last Modified 21-07-06 - Jestim Limited, All Rights Reserved