EFFICIENCY INCREASE BY JET QUALITY IMPROVEMENT AND REDUCTION OF SPLASHING WATER IN THE CASING OF PELTON TURBINES T. Staubli, P. Weibel, C. Bissel, A. Karakolcu, U. Bleiker Abstract: In the course of refurbishment of the HPP Rothenbrunnen with three horizontal twin Pelton turbines the injectors were replaced and the internals within the casing were modified. The runners were not replaced. Thermodynamic efficiency measurements before and after refurbishment provided proof of an efficiency increase of up to 1.4 percent, an excellent result for such minimal modifications. In addition to efficiency measurements also flow visualizations were performed installing a camera and lightening system within the casing of the turbine. The visualizations clearly showed a reduction of the splashing water in the casing of the turbine and less jet dispersion.
1 Introduction / Hydro Power Plant Rothenbrunnen The hydro power plant of Rothenbrunnen, owned by the Kraftwerke Zervreila AG, is located in the Eastern Alps of Switzerland near the city of Thusis. The water is supplied from the intermediate compensation reservoir of Safien Platz with a gross head of approximately 670m. The power plant has been equipped with 3 double horizontal Pelton turbines in the mid of the 1950s by the Swiss company of Charmilles. Each turbine is equipped with two runners and one nozzle per runner. The power output of the turbine before refurbishment was 42MW. In 2007, Kraftwerke Zervreila AG appointed Andritz Hydro to carry out a study to improve the performances of the existing turbines and to increase the maximum power by 10 percent. Already in 2009 the refurbishment project was completed and efficiencies measured with the thermodynamic method showed that guaranteed efficiency and power increases were well reached.
2 Performance optimization and jet quality In the Alps area, the majority of the Pelton power plants were installed before 1970. In Switzerland, as an example, approximately 90% of the electricity based on Pelton turbines is produced in power plants that were installed between 1900 and 1970. After 1970 only a small number of new Pelton turbines with unit power > 5 MW were commissioned. In the past refurbishment of Pelton turbines concentrated primarily on runner replacement. The influence of other components, such as distributors, injectors, housings and internals, were often under-estimated in their contribution to the global performances of the turbine. Since 2002, Andritz Hydro is following now an intensive program of research & development dedicated to the refurbishment of Pelton turbines with a focus on the static components of the turbine.
Figure 1: Section view of the horizontal twin Pelton units of the HPP Rothenbrunnen before refurbishment [8]
Figure 2: Powerhouse of the HPP Rothenbrunnen and inside view of the casing of the Pelton turbines with camera support for visualization
The history of Swiss Pelton jet research dates back to 1930 with a first experimental campaign of Escher Wyss on prototypes, followed by a study of Oguey [1] in 1944. Later in 1972, important results regarding Pelton jet properties were published in the thesis of Villacorta [2]. This means that for the majority of Swiss Pelton turbines, the ones built before 1970, little fundamental knowledge on the jet, its disturbances and the resulting deficit in efficiency of the turbine was available, even if some experimental studies in model or prototype scale were at hand. This underlines the importance of a careful look at the static components when planning a refurbishment.
Already in 1996 Staubli et al. [3] presented a case study where they pointed out that the jet quality may have a major impact on efficiency and noise. In 1997 Matthias et al. [4] discussed the influence of the turbine casings. During the last decade, Andritz Hydro found more and more convincing facts of the importance of the jet characteristics in a series of experiments, numerical flow simulations and refurbishment projects of horizontal and vertical Pelton turbines. With experience the accuracy of the prediction of efficiency improvement increased. Due to a large variety of nozzle and distributor geometries in refurbishment projects the certain uncertainty of prediction remained and the need for a knowledge increase was identified. After completion of several internal jet-visualization projects (together with Sulzer Innotec in year 2000), the need for a more precise jet quality evaluation under prototype heads came up. In year 2006, Andritz Hydro launched an important 3-year collaboration program with Competence Centre Fluid Mechanics and Hydro Machines of the Hochschule Luzern in order to work on the issue of the "jet quality”. The program was supported by research funds from swisselectric research, a research fund fed by swiss electricity companies. The project encompassed jet observations in model and in prototype combined with thermodynamic efficiency tests, but also numerical flow simulations for jet prediction. The main goals of this collaboration were to extend the knowledge of the complex physical phenomena which govern the behaviour of the jet and to enhance the modelisation skills of the jet prediction based on analytical and numerical simulation tools. The jet quality depends on a large number of influencing factors. Not only local Froude, Weber and Reynolds numbers have influence on the jet properties, but also the entire upstream flow history. In the upstream flow, secondary flows due to bends, bifurcations and elements within the flow (e.g. internal or external servomotors for the needle operation within the injector) hardly decay due to the usually high Reynolds number observed in hydro power plants. These secondary flows influence the topology of the jet and lead to deformation (surface), deviation (jet axis) and rotation of the jet's deformations. Secondary flow also causes increased dispersion of the jet diameter. The closer the elements are to the nozzle exit, e.g. needle and mouth piece, the more important their influence on the jet development becomes. Old designs of injectors often show smaller needle and mouth piece angles compared to more recent designs, an indicator that the jet quality might not be optimum. Upstream unsteadiness of the flow reflects on the jet surface. Such unsteadiness may be of stochastic nature, e.g. due to the flow turbulence generated in the penstock or due to deterministic phenomena such as pressure wave propagations or periodic vortex formations at cavities, bifurcations or support structures (Kármán vortices). The jet dispersion is defined by the increase of the jet diameter with distance from the nozzle. Included in the definition of the jet diameter is the air water mixture surrounding the core of the jet. Thus, the increase of the measured jet boundaries is not equivalent to momentum decrease of the same order. The physical background is described in detail by Zhang [5]. The deviation of the jet is defined by the difference of the jet centre line from the theoretical axis. Once the jet has exited the nozzle, further effects may negatively influence the jet and its dispersion, such as: ventilated droplets or water centrifuged from the bucket cut outs, splashing water being diverted within the housing or from deflectors onto the jet. Also interference with other jets or injectors may occur.
In a prototype case study where splashing water was prevented from impinging the jet it was found that the measured jet dispersion reduced by this action. An other jet disturbing effect may be caused by pressurized air bubbles being enclosed in the oncoming water. When expanding after the nozzle exit this will lead to sudden expansions observed on the jet surface. During the starting process of Pelton turbines such enclosed air and the resulting air expansions are quite common. The effect of larger amounts of air can be easily heard outside of the housing during startups. During normal operation this noise normally disappears. In course of the research project a clear correlation of increased jet dispersion and decreased efficiency could be measured in a series of plants. Comparing upper and lower jets of horizontal two nozzle Pelton turbines also a correlation of the upstream bend angles and individual nozzle efficiency was found. The project findings are summarized in Staubli et al [6, 7].
3 Refurbishment of the units In the course of the study for the HPP Rothenbrunnen, Andritz Hydro performed in a first step a detailed analysis of each component of the existing turbine. This study led to three alternative solutions that were proposed to the client: A: install new turbines with two jets per runner instead of one jet per runner, including new distributor, new casing, new nozzles and new runner. B: install new injectors and new runner and improve the existing casing. C: limit the refurbishment to new injectors and improve the existing casing. For each solution, the expected performance increase and refurbishment costs were evaluated. Even if the complete refurbishment of the turbines (Alternative A) was clearly linked to the highest performance potential, the Kraftwerke Zervreila AG decided for economical reasons to choose a “light” refurbishment (Alternative C). The main reasons were: Shortest downtime of the turbines. Best cost effectiveness of solution C with interesting performance improvement. The runners were recently replaced and were in a good hydraulic and mechanical condition. Thus the chosen solution included replacement of the injectors with the servomotors, of the deflectors and also the entire turbine control system was modernized. The existing injectors were replaced by state of the art injectors in order to reduce the head losses but also considering optimized flow acceleration. Thus the jet quality could be considerably improved with the new design. Additionally, the casing flow was also analyzed in detail. Spots with harmful flow returns from the casing into the runner were detected and eliminated by modifying the casing locally.
4 Thermodynamic efficiency measurement Comparative thermodynamic efficiency tests were performed before and after refurbishment. The employed procedure corresponded to the direct method described in IEC 60041. For temperature measurement very stable Seabird thermometers were used (one in the high pressure section and three in the tail water). The temperature measuring principle bases on AC excited thermistors (accuracy: ± 1 mK, resolution: 0.25 mK, stability: < 1 mK in six months). Accordingly repeatability of the measured efficiencies was excellent and lay within a band of 0.2 percent.
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Figure 3: Results of thermodynamic efficiency measurement before and after refurbishment
The performed measurements confirmed that: An important efficiency improvement is measured over the full operating range of the turbine after refurbishment, see Fig. 3. The expected efficiency improvements were fully achieved and even exceed. A power increase of 10 percent was realized.
5 Flow visualization The camera housing and the stroboscopic lights were mounted within protecting housings in the shelter of the injector and the deflector, as shown in Fig. 4. All equipment could be operated from outside of the housing.
Figure 4: Jet visualization support with camera and strobe lights
For a quantitative utilization of the images the camera was calibrated with a dummy jet of known diameter in the laboratory. Unfortunately, an automated evaluation of the jet boundaries based on image processing tools was not possible due to the dispersed jet surface, splashing water, unbalanced background and brightness of the images. Therefore, the geometrical properties of the jet were measured manually importing the images into a CAD software, see Fig. 5. For each operating point at least 15 images were measured, converted into mm dimension and averaged. Like this the jet dimensions could be evaluated as a function of the distance from the nozzle exit and could be compared to the theoretical jet diameter.
Figure 5: Jet diameter measurement in the HPP Rothenbrunnen at 10.0MW
The visualizations performed before and after the refurbishment clearly demonstrate that: The new injectors lead to an evident improvement of the jet quality with less disturbances on its surface, see Fig. 6 and 7. The measured jet diameter is now closer to the theoretical jet diameter, demonstrating the reduced dispersion after refurbishment, see Fig. 8. Generally there the number of droplets seen on the images is reduced with the new internals in the turbine housing, Fig. 6. Less water is seen in the background flowing over the deflector and thus less water impinges on the back of the jet, Fig. 6.
10.0 MW 15.5 MW 26.2 MW 35.3 MW Figure 6: Jet images after (left, z/D0=1.61) and before refurbishment (right, z/D0=1.68)
7.0 MW 15.5 MW Figure 7: Time averaged jet images after (left, z/D0=1.61) and before refurbishment (right, z/D0=1.68)
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Power machine group [MW] Figure 8: Measurement of the jet dispersion before and after refurbishment
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6 Conclusions Replacing the injectors and modifying the internals within the casings of the turbines led to an efficiency improvement of 1.4 percent. Such an improvement was possible by selective measures based on knowledge on effects influencing jet quality and splashing water development in the casing, as well as on the experience gained from recent projects. The efficiency increase is clearly correlated with less disturbances seen on the jet surface, less jet dispersion and a smaller amount of splashing water. Once more, the project of Rothenbrunnen demonstrates the important influence of the jet and splashing water on the efficiency of Pelton turbines. Especially for refurbishement, the potential of performance increase related to this issue has to be carefully evaluated. Its prediction is rather complex and is based on analytical and numerical simulation tools validated with feed-back from prototype refurbishment projects. In general, the improvement potential of Pelton turbines in the Alpine space is considered to be high. In many cases efficiency improvement can be achieved with rather cost effective modifications, but every turbine has to be analysed individually. References [1]
[2]
Oguey, P. Etude théorique et expérimentale de la dispersion du jet dans la turbine Pelton. Société du bulletin technique de la Suisse Romande, Lausanne, Switzerland, 1944 Villacorta R., Theoretische und experimentelle Untersuchungen an Einlaufdüsen von Freistrahlturbinen, Dissertation. ETH Nr. 4678, 1972
[3]
Staubli T., Humm H.J., Neubacher G., Schlechte Strahlqualität als Lärmursache bei Peltonturbinen, 7tes Internationales Seminar Wasserkraftanlagen, Wien, 1996
[4]
Matthias H.B., Prost J., Rossegger Cc., Investigation of the Flow in Pelton Turbines and the Influence of the Casing, International Journal of Rotating Machinery, vol. 3, no. 4, 1997, pp. 239-247 Zhang Zh., Casey M., Experimental studies of the jet of a Pelton turbine, Proc IMechE, Vol.221, Part A, J. Power and Energy, 2007, pp. 1181-1192 Staubli T., Abgottspon A., Weibel P., Bissel C., Parkinson E., Leduc J., Leboeuf F., Jet quality and Pelton efficiency, Hydro 2009, Lyon, 2009 Staubli T., Die Auswirkung der Strahlqualität auf den Wirkungsgrad von Peltonturbinen, Wasser Energie Luft, 2009, 101. Jahrgang, Heft 3 Bovet Th., Bulletin technique de la Suisse Romande, 1959
[5] [6] [7] [8]
Authors Dr. Prof. Thomas STAUBLI & Pascal WEIBEL Hochschule Luzern CC Fluid Mechanics & Hydro Machines Technikumstrasse 21, CH-6048 Horw, Switzerland Phone: +41 41 349 35 52 E-mail:
[email protected]
Clause BISSEL & Adem KARAKOLCU ANDRITZ HYDRO AG Rue de Deux-Gares 6, CH-1800 Vevey, Switzerland Phone: +41 21 925 77 79 E-mail:
[email protected] Ueli BLEIKER Kraftwerke Zervreila AG CH-7405 Rothenbrunnen, Switzerland Phone: +41 81 650 11 33 E-mail:
[email protected]
Thomas Staubli graduated in Mechanical Engineering from the Swiss Federal Institute of Technology (ETH) in Zürich. After two years of post-doctoral research in the field of flow induced vibration at Lehigh University, Pennsylvania, he worked in experimental fluid mechanics at Sulzer Hydro (now Andritz Hydro) in Zürich. He then headed the Hydromachinery Laboratory at the ETH Zürich. During this period he directed research projects in the field of hydraulic machinery. Since 1996 he is professor for Fluid Mechanics and Hydro Machines at the Hochschule Luzern. Pascal Weibel graduated in mechanical engineering from Hochschule Luzern – Technik & Architektur. Since 2007 he is research assistant at the CC Fluid Mechanics and Hydro Machines. Claude Bissel graduated in Fluid Engineering at the Electricity and Mechanical Engineering school of Nancy (France) in 1991. He joined Andritz Hydro (Switzerland) in 2001 and now heads the hydraulic Research & Development for Pelton turbines within Andritz Hydro. Adem Karakolcu graduated in Mechanical Engineering in 2006 from Vienna University of Technology. He worked for three years as Development and Project Engineer for Pelton turbines focusing on cavitation prediction, loss analysis, hydraulic layout and hydraulic engineering for Andritz Hydro. He is now head of the R&D Pelton Team in Zurich. Ueli Bleiker is electrical engineering technician. He is project manager and heads the power plants of Zervreila AG. He is the responsible engineer for maintenance and refurbishment of the HPP Rothenbrunnen and Realta.
