22. Measuring Discharges From Pipes With Current Meters

Propeller-type current meters have been used with limited success to measure rates of flow discharging from full pipelines (Rohwer, 1942). Measurements are made by traversing the pipe outlet with the meter to obtain an average velocity and then multiplying this velocity by the pipe cross-sectional area and by a correction coefficient. This coefficient has a value less than 1.0 because the meter traverses do not adequately measure flows close to the pipe walls and give a velocity measurement higher than the true average.

Accuracies within +/-5 percent can be obtained when the velocity of flow is enough to operate the meter but is less than 9 or 10 ft/s, provided the flow occurs without significant spiral flow, the discharge pipe is long enough to produce relatively uniform distributed flow, and the inside diameter of the pipe can be measured accurately. Velocities that are too high or too low, swirling flows, velocity concentrations, and pipelines not flowing full or carrying air reduce the accuracy obtainable. Also, the presence of the meter in the pipeline exit partially obstructs the flow. This obstruction reduces the rate of flow and increases the head in the pipeline. The effect is relatively small, but a correction factor is necessary to obtain best accuracies. In general, the method gives quick, comparative results but is not recommended where accurate flow measurements are needed. A simple, low, flat-crested, long-throated measurement structure (chapter 8) would be an accurate method to measure flow from a partially full pipe.