Discharge Measurement in a Short Intake with a Converging Section – a Case Study

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In the context of a planned retrofit and upgrade of the hydropower plant Wettingen, operated by the Elektrizitätswerke Zürich, three intakes were equipped with acoustic transit time flow measuring devices. Since the flow meters are installed in a converging section, downstream of a pier, it became necessary to perform numerical flow simulations in order to weight the acoustic path velocities for a quantitative flow rate evaluation. With the flow rate available and performing additional site measurements, cam test could be performed with the turbines. While one turbine operated at optimum cam, the other two turbines operated off-cam. Accordingly a substantial improvement potential could be identified.

Introduction
The hydropower plant (HPP) Wettingen, operated by the Elektrizitätswerke Zürich (EWZ) since 1933, is located at the river Limmat downstream of Zurich. The HPP Wettingen houses three Kaplan units with a total power of 24 MW. The head is normally
between 21.6 and 22.8 m. In the context of a planned renovation the possibilities of discharge and efficiency measurements
were evaluated. Goal of such measurements was to quantify the benefit of the refurbishment works and also to be later on able to measure the efficiency improvement of new turbines.

Preferable one of the methods described in IEC Standard 60 041 [1] should have been chosen for the measurements, but it was recognized after the first site visits, that none of the proposed methods (current meter, pressure time, tracer methods) was
applicable in this plant. It was then decided to use an unconventional installation on the basis of acoustic transit time flow measurement.

Also this method is described in principle in the appendix of IEC Standard 60 041, but it is not foreseen to use the method for converging sections and not directly downstream of a pier. In the HPP Wettingen, however, the only possible location for installation was identified to be in the convergent section, downstream of a central pier in the inlet (Figure 1).

From previous, good experience with measurements in converging sections as reported by Staubli et al. [2] and also from comparative field tests in the intake of the Kootenay Canal power plant described by Almquist [3], it was decided to venture these measurements in the HPP of Wettingen. The involved risk with respect to the repeatability of the measurements before and after refurbishment was low, however a prediction of the measuring uncertainty was only a guess based on experience at this early stage of the project.

The EWZ decided to grant the contract for installation to Hydrovision GmbH. In each of the three intakes of the power plant 8-path acoustic transit time flow meters (ATT) were installed as depicted in Figures 1 and 2.

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