Estudo teórico e experimental de um método de medição de vazão baseado em transientes hidráulicos

Abstract

The measurement of fluid flow in forced conduits is a great challenge for engineers in charge with hydroelectric plants and pumping units. The Gibson method provides the steady-state flow rate based on the analysis of the transients generated by closing a downstream valve. During the transient, a pressure difference is developed between any two positions in the conduit, upstream from the valve. The integration of the curve representing the variation of the pressure, will yield the initial flow rate prevailing when the valve was fully open. The flow rate calculated through thís method can be very accurate, depending mainly on the precision of pressures measurements. In our study, pressure transducers installed in the conduit and connected to a data acquisition system are used to acquire the pressure variation. We present in thís work the main characteristics of the transducers and the calibration procedure. In the first stage, the governing equations for the hydraulic transient has been obtained. From these equations, an algorithm was implemented to obtain the steady-state flow rate. Another algorithm was implemented to obtain the pressure-time record during the hydraulic transient. This algorithm is based on the Method of Characteristics (MOC) and it is adopted to compare the results of the Gibson Method. The MOC algorithm has also been used to design the experimental setup components, which has been used to validate the Gibson Method. The experimental results obtained through the Gibson method were also compared with the flow rate measured with a direct method. In order to verify the validity of the procedures adopted in this work, statistical analysis were applied to the results obtained through direct measurements and by Gibson method. It was verified that the Gibson method is quite precise, versatile and competitive.