Discharge Measurements Using the Classic Gibson Method With Instrumentation Installed Inside a Pipeline

Abstract

The Gibson method (pressure-time method) is one of the basic methods of discharge (flow rate) measurement applied in hydropower plants. Flow rate is determined by integrating the recorded variation of pressure difference between two measuring (hydrometric) sections in a pipeline (penstock). The Gibson method in its classic version consists in direct measurement of pressure difference variation between two hydrometric sections of a pipeline. Particular difficulties, related to application of the method in its various versions, arise in conditions of no access to the hydrometric sections from the outside of a pipeline. In such cases, it is necessary to install dedicated measuring instrumentation inside the pipeline. Such instrumentation has been implemented for the purpose of efficiency tests of two Francis turbines (upgraded and not upgraded) fed from a common penstock of 10 m diameter. The hydrometric sections were furnished with pressure taps connected by means of small copper tubes (impulse tubes) and hermetic manifolds to the differential pressure transducer. The transducer was installed in a hermetic housing and its electric signal was sent from the inside of the penstock to a computer data acquisition system. Using this method, the efficiency characteristics of the tested hydraulic turbines were determined. According to the authors’ knowledge, the pressure-time method has not been used in such an application so far. The method under consideration requires transmitting pressure signals from both penstock sections to the differential pressure transducer by means of impulse tubes. This raises the question on the influence exerted by dynamic properties of the connecting pipes / transducer system on the discharge measurement results. The previously developed computational method incorporating dynamic models of the piping and the transducer has been applied in order to determine this influence. In result of calculations conducted, the piezometric tubes / transducer system has been found to exert a negligible influence on the discharge measurement results.