Influence from bends on a pressure-time measurement

Abstract

One of the criteria for using the pressure-time method is that there is a straight pipe between the measurement cross sections. If the physical influences from a bend were known the method could be expanded for use over bends and hence on more power plants. At the Norwegian University of Science and Technology (NTNU) a test rig suited for experiments on the pressure-time method has been built. In addition to earlier experiments performed on straight pipe and double 90 degree bend, experiments on double 45 degree bend has been performed. This is done in order to try to find out if there is a clear relationship between bend geometry and its influence on the pressure time curve. To further develop the knowledge a full scale test in the field has been conducted on a 5 MW 58 meter head machine with a vertical penstock and a 90 degree bend. In the laboratory a double 90 degree bend caused an underestimation of the flow of about 1 % compared to a straight pipe, while for a double 45 degree bend a conclusion can not be found as the spread around the mean and uncertainty bands is within the range for the straight pipe. The field measurements with a 90 degree bend showed that the underestimation became 8.5%. When measuring pressure difference over bends the resulting calculation will give an underestimation of the flow. This should imply that the measured transient pressure is damped. (Regardless if this is the correct term it is here chosen to call this damping.) The question is if the damping is caused by friction or kinetic effects. The rapid deceleration of the water masses will give transient friction. How transient friction behaves is a topic of interest in hydraulic research. Bends will also cause skewed velocity and pressure profile on inlet and outlet of the bend as well as secondary flows. How this affects the transient pressure is also an interesting parameter.