Thomas Staubli

Methods for discharge measurements in low head hydro power plants are presented with a series of field studies. The most classical method is the multipoint velocity measurement with propeller type current meters. For discharge evaluation, the integration of the difference between measured velocities and a known velocity distribution is recommended, thus avoiding integration in the outer zones with exponential velocity decay. The newest and best suited method for measurements in intakes is the acoustic scintillation method, a cross-correlation method. For accurate results the scintillation method must be installed downstream of trash racks. The method of acoustic transit measurement can also be adopted to measurements in low head, but new procedures for weighting the individual path velocities have to be applied due to varying shape of the flow cross sections.

The lowest point of the underground hydroelectric power station Kopswerk II of the Vorarl-berger Illwerke, Austria, lies 60m below the tailwater level. For emergency draining even in the case of complete power failure a water jet pump fed by the headwater was designed. The challenge of this jet pump development lies in the high feeding pressure of 83 bar. Jet pumps with such high driving pressure tend to suffer from severe cavitation. Focus during the devel-opment process was therefore on the prevention of cavitation. On one hand the jet pump was designed following experimental experience from several sources. On the other hand numerical flow simulation was performed. The wall contour of the mixing chamber was changed iteratively in order to avoid low pressure zones. Efficiency of the pump could be optimized by adjusting the nozzle position of the ejector. The global pump data obtained by numerical flow computation agreed well with the experimental data published in literature.

Wear of turbine parts due to abrasive particles in the water of hydroelectric power plants (HPP) is of special importance for Pelton turbines where extreme redirection and deceleration of the flow occurs. In an on-going research project at HPP Fieschertal, Switzerland, wear on coated runner buckets is measured with a 3D optical scanner and a thickness gauge, turbine efficiency is periodically evaluated by “sliding needle” index tests and suspended sediment is monitored using various devices. 3D digitizations (measurements) of selected Pelton buckets allow quantifying material losses of the main splitter or of the cut-out section due to turbine operation during a sediment season. In the sediment season 2012 the splitter height decreased in the range of 3 to 5 mm, i.e. approximately 0.5 to 0.8 percent of the inner bucket width of 650 mm. The erosion on the splitter is influenced by the initial geometric condition of the splitter, the particle load and the operating hours. The history ...

La Rance was the first tidal power plant worldwide with a capacity of 240 MW supplied by 24 bulb units working in both flow directions. Many tests had been undertaken in laboratory and on the prototype of Saint-Malo, but no real commissioning tests had been carried out until now. Measuring discharge at La Rance is indeed complex because of a very low and changing head from 2 m to 11 m depending on the tides, large gate bays of 8.7 m on 8.8 m, and high flow rate variations from 75 m 3 /s to 280 m 3 /s. It was decided to use current meters on a moving frame in the bulkhead slot for flow rate measurement. Further challenge was to maintain an almost constant head of 4 m during data acquisition. Full exploration of the velocity field was completed in both directions in 6 vertical positions. Additionally cam tests were performed, using index measurement with the frame in the middle position.

Geometrical changes and material loss of Pelton turbine runners as well as changes in turbine efficiency have been measured at HPP Fieschertal in Valais, Switzerland since 2012. The HPP is equipped with two horizontal axis Pelton units, with each 32 MW nominal power, 7.5 m3/s design discharge, 515 m head and two injectors. The nozzles and the buckets are hard-coated. Due to the relatively high hydro-abrasive erosion during the summer (sediment transport season), the splitters and cut-outs of the runner buckets are grinded and re-coated on-site usually every winter. Turbine erosion was quantified based on repeated measurements on two runner buckets using 3d-scanning and a coating thickness meter. The detailed geometrical models showed amongst others that the splitter width distributions are similar for buckets of one runner, but differ considerably between runners. Changes in turbine efficiency were measured by the sliding needle procedure and continuously monitored based on operatin...

The Francis turbine runners of the Grimsel 2 pump storage power plant showed repeatedly cracks during the last decade. It is assumed that these cracks were caused by flow induced forces acting on blades and eventual resonant runner vibrations lead to high stresses in the blade root areas. The eigenfrequencies of the runner were simulated in water using acoustic elements and compared to experimental data. Unsteady blades pressure distribution determined by a transient CFD simulation of the turbine were coupled to a FEM simulation. The FEM simulation enabled analyzing the stresses in the runner and the eigenmodes of the runner vibrations. For a part-load operating point, transient CFD simulations of the entire turbine, including the spiral case, the runner and the draft tube were carried out. The most significant loads on the turbine runner resulted from the centrifugal forces and the fluid forces. Such forces effect temporally invariant runner blades loads, in contrast rotor stator interaction or draft tube instabilities induce pressure fluctuations which cause the temporally variable forces. The blades pressure distribution resulting from the flow simulation was coupled by unidirectional-harmonic FEM simulation. The dominant transient blade pressure distribution of the CFD simulation were Fourier transformed, and the static and harmonic portion assigned to the blade surfaces in the FEM model. The evaluation of the FEM simulation showed that the simulated part load operating point do not cause critical stress peaks in the crack zones. The pressure amplitudes and frequencies are very small and interact only locally with the runner blades. As the frequencies are far below the modal frequencies of the turbine runner, resonant vibrations obviously are not excited.

Hydro-abrasive erosion of hydraulic turbines is an economically important issue due to maintenance costs and production losses. The topic is of particular importance at highand medium-head run-of-river hydropower plants (HPPs). To address this issue, a comprehensive research study has been conducted at the case study HPP Fieschertal, in the Alps of Switzerland since 2012. The HPP is equipped with two horizontal-axis Pelton units, each with 32 MW nominal power. The injectors and the buckets are hard-coated. The suspended sediment concentration (SSC) and particle size distribution (PSD) in the penstock of the study HPP are continuously measured using an innovative combination of several techniques. In this way, the variability of SSC and PSD is captured and the sediment load is reliably determined. Geometrical changes and material loss on the Pelton runners are quantified on the site based on repeated measurements on two buckets using structured-light 3D optical scanning. Efficiency c...

According to the appendix of the IEC 60041standard the volume flux Q in a conduit can be determined with the acoustic discharge measurement method by integrating averaged individual path velocity readings. This is done by applying a simplified Gauss-Jacobi integration method, where the individual path readings are weighted and added up. This integration method is then summarized for circular and rectangular sections. The IEC 60041 has the following limitations: First, by assuming a uniform velocity profile, the method cannot cope well with truly turbulent velocity profiles, which fall off at the boundary layers near the wall. To overcome this limitation Voser [3] proposed a modified integration method called OWICS (Optimal Weighted Integration for Circular Sections) with slightly modified optimum sensor positions and weighting coefficients, thus reducing the integration error by 0.1 up to 0.2 percent. Second, the method uses fixed weighting of the averaged path velocities and thus t...

In order to determine the flow rate through a pipe with the acoustic transit time method, integration of the measured mean projected velocities on the acoustic paths is needed. Adaptive integration methods may increase considerably accuracy of the integrated flow rate. Since velocity can be mapped onto discharge with an integral operator, the discrete output of an ADM (Acoustic Discharge Measurement) must be integrated numerically. One common method to do this for pipe flows is called OWICS method. OWICS uses a fixed parameter to describe the flow field, which might not be best for any flow situation. Thus, this paper introduces a dynamic OWICS method, which adaptively determines the flow parameter needed for integration. The presented examples show that discharge calculation can be improved with an adaptive scheme by one fifth up to one half. However, the magnitude of improvement depends strongly on how the velocity profile is disturbed. The adaptive scheme can handle disturbances,...

The uncertainty of an acoustic transit time (ATT) flow measurement can be estimated on the basis of computational fluid dynamics (CFD) simulations in advance of an installation. The paper illustrates an example of a flow simulation in the hydro power plant (HPP) Aratiatia, NZ, for heavily disturbed velocity distributions. In this power plant three turbines are fed from a common surge tank and are operated in parallel or individually. Details on the geometrical and numerical model are given, as well as the physical boundary conditions. The OWICS (Optimal Weighted Integration for Circular Sections) discharge integration method at Gauss-Jacobi positions is used to calculate the flow rates in the circular measuring sections immediately upstream of the spiral casing for different operating conditions. The procedures employed for extraction of the relevant data from the simulated flow fields needed for integration of the flow rates are described. A comparison of the axial and cross veloci...

Hydro-abrasive erosion in hydraulic turbines is a weil known problem, but comprehensive data sets from hydropower plants are still rare. Therefore, the sediment load, erosion depths and efficiency changes have been measured and evaluated since 2012 at a high-head run-of-river hydro plant in the Swiss Alps. An erosion model for hard-coated Pelton buckets was calibrated based on the data obtained. lt is demonstrated that hydro plant shutdowns in periods of exceptionally high erosion potential are profitable. A procedure to estimate a site-specific threshold value of suspended sediment concentration for hydro plant shutdowns is proposed.

Corrective terms to improve the accuracy of the thermodynamic efficiency measurements are proposed in section 14.6 of the IEC standard 60041 – 1991. The following contribution focuses on the corrective term for the specific mechanical energy. The physical background of this correction will be discussed, the equations will be derived and the range of validity of empirical numbers will be given. On behalf of examples it will be demonstrates how the corrective terms can be determined. The used additional instrumentation such as an anemometer and a hygrometer will be specified. According to the IEC standard, the arithmetical sum of the corrective terms should not exceed 2 percent of the specific mechanical energy, otherwise the measurement shall not be considered valid. The largest correction evaluated in the example of the measurements recently performed in the hydro power plant of Oberems, Valais, Switzerland, was due to direct exchange with the ambient air and amounted to 13.2J/kg. T...

The paper focuses on the comparison of index measurements of a Francis turbine with steady state measuring points and measurements under quasi-steady conditions. This second method is called “sliding gate method” and was introduced by Almquist [2]. It is shown that in spite of large fluctuations of the data an excellent repeatability of the curves and perfect agreement with the steady state measurements can be achieved. These measurements have been performed twice, since the Kraftwerke Zervreila AG wanted to quantify the effects of their repair works on the efficiency level. Efficiency differences of 2.3% at partial load and 1.8% at full load were found. This improvement is mainly due to the reduction of the leakage flow (revision of labyrinth seal). During both tests, before and after maintenance, the results of the trends from the sliding gate method agree perfectly with the steady state point efficiencies. The result of the sliding gate method is a continuous efficiency curve ins...

Hydropower plants operating in fragile mountainous regions face severe hydro-abrasive erosion of hydraulic components, which results in drop in efficiency, frequent interruptions in power generation and downtime for repair/replacement. Due to high flow velocity in components like nozzles and buckets, a Pelton turbine is highly susceptible to erosion. The effect of erosion is observed as cumulative loss of material and study of its gradual progress required frequent measurement. In this study, in-situ images of Pelton buckets were captured throughout a sediment period at a hydropower plant located in Himalayan region, India and analysed for characteristics of progressive erosion damages. A multi-layered paint method was used to visualize the initiation of erosion in different parts of bucket. Removal of colours due to erosion provided the information on erosion initiation and various erosion zones in bucket. The erosion of outer side of bucket was analysed with single layer paint rem...

Abstract Hydro-abrasive erosion, a severe issue in hydraulic turbines, is caused by sediment laden flow leading to shutdowns and losses. In this study, a simplified analytical model is developed which describes the relationship of hydro-abrasive erosion in Pelton buckets with sediment properties, erosion velocity, turbine material property and duration of erosion. This model has been validated by comparison with both experimental measurements and literature, and shows strong agreement for most of the parameters. Further, mechanism of erosion in various parts of the buckets made of 6 types of material (3 types of steel, 2 coatings and bronze) were analysed from scanning electron microscope (SEM) images. The mechanism of erosion in different parts of Pelton buckets was found to be strongly related with the flow pattern in that part. The metallic turbine buckets showed plastic deformation whereas coatings showed crack formation initiating material removal. The study shall be useful for design and material experts working in the field of hydraulic machinery.

The flow field at the inlet of a small low head hydroelectric power plant was measured upstream of a trash rack using an acoustic Doppler probe. By integration of the multi-point measurements the time averaged discharge was calculated. Knowledge of the velocity and flow angle distributions upstream of the trash rack allowed to estimate local and integral energy losses. Furthermore, the flow field at the intake was statistically analyzed and turbulence intensity distributions were evaluated. Influences of upstream disturbances on the flow field, turbulence and vorticity distributions were quantified. Generally, knowledge of details of the inlet flow field to the turbines will be of interest if numerical flow computations for optimized turbine performance are planned in course of e.g. rehabilitation projects. In such cases, local acoustic velocity measurements provide accurate information with comparably little installation and measurement effort. 1. Acoustic Doppler Velocimeter (ADV)...