The most common reason for inaccurate flow measurement of flow meter reading is the incorrect installation of flow meter. Every ultrasonic flowmeter works on the basic assumption that the pipe is completely full when the water is flowing through the pipe network. Flowmeter measures the velocity accurately by analysing the to and fro acoustic signals captured by in-build transducers and uses the cross-sectional area of the flow meter to estimate the flow rate of the water in the pipe.
Flow rate (Q) = V*AFM
Where, V = estimated velocity, AFM = cross-sectional area of flow meter = cross-sectional area of pipe
Now, since AFM corresponds to the cross-sectional area of the pipe, therefore, the pipe should be flowing full for accurate measurement. However, there may be a partial flow condition due to many reasons swirl, valves, bends, negative pressure, motor capacity and availability of water in the bore well resulting in flow profile distortion. Obstructions upstream and near the flowmeter can cause errors ranging beyond 50% and the flow meter showing a higher reading. This is shown in the table below:
|
Y (Depth of flow) |
AActual |
Error (%) |
|
0.5D |
0.5(AFM) |
50 |
|
0.6D |
0.626(AFM) |
37.4 |
|
0.75D |
0.804(AFM) |
19.6 |
|
0.8D |
0.858(AFM) |
14.2 |
|
0.9D |
0.948(AFM) |
5.2 |
|
D |
AFM |
0 |
The inaccuracy in flow rate measurement by flow meter can result in flow meter showing higher reading leading to overestimation of discharge through pipe impacting groundwater utilization. Therefore, for an ultrasonic flow meter, it is essential to eliminate flow profile distortion and partial flow conditions near the flow meter for better accuracy.
Also Read: Top 9 Water Measurement Essentials Explained
A very simple and effective way to stabilize a flow profile is to provide adequate lengths of straight pipe ahead of (and behind) the flow meter. Ideally, the flow meter should be installed with a straight length of tube on either side for a distance of 10 pipe diameters upstream and 5 pipe diameters downstream of the flow meter.
A U-bend, as shown in sketch below, is one such setup that is widely accepted and used by industrial practitioners to avoid inaccuracies caused by partial flow conditions. Under those conditions, the U-bend shall eliminate partial flow in the pipe and ensure that the pipe is completely full by utilizing gravity and creating a positive pressure system. As a result, the additional error caused by partial flow will be eliminated. Furthermore, with other technical aspects to deal with, such as turbulence and a wide range of other disturbances, it is preferable to install the U-bend setup according to the dimensions specified for accurate flow measurement.
