Hydraulic design of Three Gorges right bank powerhouse turbine for improvement of hydraulic stability

Transcription

1 IOP Conference Series: Earth and Environmental Science Hydraulic design of Three Gorges right bank powerhouse turbine for improvement of hydraulic stability To cite this article: Q Shi 2010 IOP Conf. Ser.: Earth Environ. Sci Related content - Hydraulic design development of Xiluodu Francis turbine Y L Wang, G Y Li, Q H Shi et al. - Development of low head Kaplan turbine for power station rehabilitation project S M Lim, N Ohtake, S Kurosawa et al. - Design of large Francis turbine using optimal methods E Flores, L Bornard, L Tomas et al. View the article online for updates and enhancements. Recent citations - Characteristic analysis of the efficiency hill chart of Francis turbine for different water heads Pengcheng Guo et al This content was downloaded from IP address on 22/09/2018 at 23:40

2 Hydraulic design of Three Gorges right bank powerhouse turbine for improvement of hydraulic stability 1. Introduction Q Shi 1 1 Dong Fang Electrical Machinery Co., Ltd., DEC 188, Huanghe West Road, Deyang, , China qhshi@dfem.com.cn Abstract. This paper presents the hydraulic design of Three Gorges Right Bank Powerhouse turbine for improvement of hydraulic stability. The technical challenges faced in the hydraulic design of the turbine are given. The method of hydraulic design for improving the hydraulic stability and particularly for eliminating the upper part load pressure pulsations is clarified. The final hydraulic design results of Three Gorges Right Bank Powerhouse turbine based on modern hydraulic design techniques are presented. Three Gorges turbine features with its giant size, large power capacity, large head variation and extraordinary technical challenges. The head variations of the initial stage and final stage of Three Gorges Project construction are respectively 33m and 42m, which are one of the largest head variations all over the world. Furthermore, for the sake of flood control, the turbines will, for 10 months in one year, operate in low head for flood period and in high head for non-flood period. Therefore, the turbines will operate for long time in off-design conditions. The hydraulic stability of Three Gorges turbines, operating both at high head and small wicket gate opening and at low head and large discharge, must be optimized in the hydraulic development of the turbine. The large power output, giant size, large head variation and excellent hydraulic stability are the main technical challenges faced in the hydraulic design of the turbine. The efficiency and cavitation etc. characteristics of Three Gorges Left Bank Powerhouse turbines represented were most excellent at that time. However, the model tests of Three Gorges Left Bank Powerhouse turbines showed that the hydraulic stability of the turbines did not meet the contract requirement and was not satisfactory to the customer. Especially, the upper part load pressure pulsation of Three Gorges Left Bank Powerhouse turbines may have a harmful impact on the smooth operation of the turbines. 2. Technical Challenges to be faced in Hydraulic Design of Three Gorges Right Bank Powerhouse Turbine Table 1 shows the main parameters of Three Gorges turbines. The overcritical technical requirements of Three Gorges Right Bank Powerhouse turbine impose particular challenges for the hydraulic design of the turbine. The main technical challenges and key points are summarized as the following aspects. Table 1 Main parameters of Three Gorges turbines Parameters Initial stage Final stage Maximum head (m) Minimum head (m) Rated head (m) Rated power (MW) Complete Elimination of Upper Part Load Pressure Pulsations The model tests of Three Gorges Left Bank Powerhouse turbine on different hydraulic testing rigs show that the model turbine of Three Gorges Left Bank Powerhouse has definitely an upper part load pressure pulsation c 2010 Ltd 1

3 operating zone (Shi [1]). The upper part load pressure pulsation operating zone of Three Gorges Left Bank Powerhouse model turbine is a narrow operating zone with a range of wicket gate opening difference of about 1.5 degrees, as shown in Fig. 1. Relative fluctuating amplitude H/H(%) Upper part load pressure pulsation operating zone at draft tube cone at vaneless zone at spiral case inlet Power output P(MW) Fig. 1 Typical upper part load pressure pulsation operating zone of Three Gorges Left Bank Powerhouse model turbine Because the turbines of Three Gorges Left Bank Powerhouse have an upper part pressure pulsation operating zone, the biggest challenge in hydraulic design of Three Gorges Right Bank Powerhouse turbine is to eliminate this upper part pressure pulsation operating zone. In the technical specification of call-for-bid document, Three Gorges customer claimed explicitly that they only accepted those model turbines without upper part pressure pulsation operating zone. 2.2 Minimizing Pressure Pulsations at Different Locations of Flow Passageway Draft tube vortex rope is an intrinsic attribute of fixed blade Francis turbine operating at off-design conditions, and reducing pressure pulsations induced by draft tube vortex rope continues to be an industrial task. Due to the huge size and large head variation of Three Gorges Right Bank Powerhouse turbine, minimizing pressure pulsations induced by draft tube vortex rope is extremely important to guarantee the smooth operation. This is because huge size of the turbine means reduced component stiffness and large head variation implies extended operating range away from the design condition. If the draft tube pressure pulsations can not be limited to a reasonable level in whole operating range, the turbine components will subject to extraordinary alternating stresses. Moreover, an adequate safety margin for separating surging frequency and component natural frequency needs to be given for avoiding resonance. Inter-blade vortices in the turbine runner operating at low load and/or part load conditions are unstable flow phenomena which need to be taken into account in the hydraulic design of Three Gorges Right Bank Powerhouse turbine. The large head variation makes the inter-blade vortices at high head and small guide vane openings very harmful to smooth operation of the turbine. Also the runner blade with large area and long stream span builds a very long trajectory of the inter-blade vortices. Therefore, the hydraulic design of runner blade for Three Gorges Right Bank Powerhouse turbine should, as much as possible, push inception and development of inter-blade vortices toward lower load, particularly in the high head operating range. Furthermore, the pressure surging induced by flow separation, wicket gate and runner interaction, Von Karman vortices at runner blade trailing edge and stay vane outlet must be taken into account and minimized in the hydraulic design of the turbine. Particularly, the visible Von Karman vortices observed in model turbine testing must be removed completely to avoid hydraulic elastic resonance. 2.3 No Cavitation All Around Operating Range At normal and extreme tail water level and plant sigma of Three Gorges Project, any cavitation in the turbine 2

4 is not permitted, even including cavitation incipience. For a Francis turbine with very large head variation such as Three Gorges turbines, this technical requirement is very challenging to hydraulic designer, particularly for runner blade inlet cavitation due to flow separation at off-design operating conditions. Generally, while Francis turbines operate at high head, the blade inlet cavitation occurs on the blade back due to positive flow incidence. And while Francis turbines operate at low head, the inlet cavitation occurs on the blade face owing to negative flow incidence. Usually, it is easy to pushing the blade face inlet cavitation out of low head operating range, but it is difficult to pushing the blade back inlet cavitation out of high head operating range due to sensitivity of blade inlet back cavitation to head variation. The relative amount of head variation can represent the difficulty of removing runner blade inlet cavitation in whole operating head range. The very large head variation of Three Gorges turbine imposes a big challenge for removing the blade inlet cavitation, particularly the blade back inlet cavitation. The hydraulic parameters of best efficiency point need to be selected carefully and special hydraulic design measures need to be taken. 2.4 High Efficiency at High Head Range and Large Power at Low Head Range The first major function of Three Gorges Project is flood protection to the middle-stream and downstream area of Yangtz River. The unique operating features of Three Gorges reservoir result in a long time operation of the turbine at off-design conditions. For the sake of flood control, the turbines will, for four months (from April to September), operate in low heads below 80m for flood period. And the turbine will, for six months (from November to April of next year), operate in high heads above 90m for non-flood period. In April of 2007, the head water level of Three Gorges reservoir had been up to 156m above sea level. At the same time, the probability of turbine operation below the rated head of 85m was respectively 34.4% in a low head range of 65 75m and 47.2% in a high head range of 85-94m. When the head water level of the reservoir was up to 175m about in the end of 2009, the probability of turbine operation was respectively 30.2% in a low head range of m and 64.8% in a high head range of m. It can be seen that the turbines of Three Gorges Right Bank Powerhouse have to operate, for a long time, at low head for flood period and at high head for non-flood period. As a result, the turbines developed for Three Gorges Right Bank Powerhouse must, as much as possible have high efficiencies at the high head range of more than 90m, and have large power capacities at the low head range of less than 75m so as to reduce the discarded water during the flood period as much as possible. 3. Hydraulic Design of Three Gorges Right Bank Powerhouse Turbine As mentioned previously, the main technical requirements of Three Gorges Right Bank Powerhouse turbine are the better hydraulic stability, excellent cavitation behavior and good performance characteristics at off-design operating conditions. Due to the fact that the turbines of Three Gorges Left Bank Powerhouse have an upper part pressure pulsation operating zone and larger pressure pulsation at different water passageway locations, the biggest challenge and the major design target in hydraulic development of Three Gorges Right Bank Powerhouse turbine are to eliminate this upper part pressure pulsation operating zone and to reduce the pressure pulsations as much as possible for the best hydraulic stability improvement. Since upper part load pressure pulsations are strongly connected with the hydraulic design of Francis runners (Shi [1]), the emphasis was put on hydraulic design optimization of the runner in hydraulic development of Three Gorges Right Bank Powerhouse turbine. Through a lot of CFD computation and comprehensive model testing of Three Gorges Left Bank Powerhouse, many different runner designs were developed for Three Gorges Right Bank Powerhouse. All these runners were tested and compared respectively in author s hydraulic laboratory for investigating the effect of runner design on upper part load pressure pulsations. Finally, the hydraulic design methodology of the runner for eliminating the upper part load pressure pulsations was built, and an excellent runner without upper part load pressure pulsations was successfully developed for Three Gorges Right Bank Powerhouse. For the elimination of upper part load pressure pulsations and the reduction of pressure fluctuations at different water passageway locations, also for obtaining excellent efficiency and cavitation characteristics, a new Francis runner was innovated (Chinese patent No.: ZL ). The new Francis runner is considerably different from conventinal runner, as shown in Fig. 2. 3

5 (a) New innovated runner (b) Conventional runner Fig. 2 Comparison of new innovated Francis runner with conventional runner The new Francis runner developed for Three Gorges Right Bank Powerhouse is of excellent performance characteristics at off-design conditions and makes the efficiency curve flatter compared with a conventional runner. When the operating conditions are away from the best efficiency point, the efficiency increase with changing the head and output is significant, as shown in Fig. 3. Additionally, the new Francis runner designed for Three Gorges Right Bank Powerhouse has a better cavitation behavior than conventional runner, and help especially remove the blade inlet back cavitation due to positive flow incidence at high head conditions, as shown in Fig. 4. Fig. 3 Efficiency curve comparison between new Francis runner and conventional runner New runner ---Traditional Fig. 4 Blade inlet back cavitation removed by new runner developed for Three Gorges Right Bank Powerhouse 4

6 The three-dimensional geometry of runner blade has an important influence on upper part load pressure pulsations. The design of blade profile both near blade leading edge and near trailing edge is critical to eliminate upper part load pressure pulsations. In some cases, a very small design modification of blade profiles both near blade leading edge and near trailing edge has a crucial effect on upper part load pressure pulsations. Additionally, the combination of blade profiles from runner crown to band is also important. Figure 5 shows, in conformal plane of stream surface, the comparison of runner blade profiles of Three Gorges Right Bank Powerhouse turbine with Left Bank Powerhouse. Left Bank Left Bank Right Bank Powerhouse Left Bank Right Bank Powerhouse 399 Right Bank Left Bank Right Bank (a) On crown near leading edge (b) On band near leading edge (c) On crown near trailing edge (d) On band near trailing edge Fig. 5 Comparison of runner blade profiles of Three Gorges Right Bank Powerhouse turbine with Left Bank Powerhouse in the conformal plane of stream surface Figure 6 shows the comparison of runners between Three Gorges Right Bank Powerhouse and Left Bank Powerhouse. The difference of runner geometry, especially the blade shape, can be clearly seen. The improvement in runner blade design can optimize the flow pattern in runner. The optimized internal flow in runner is essential for removing the upper part load pressure pulsations. Especially, the improvement of runner flow patterns at high head and upper part load conditions is critical to remove upper part load pressure pulsations and to reduce the pressure pulsations induced by draft tube vortex rope. Figure 7 demonstrates the comparison of flow patterns in runners between Three Gorges Right Bank Powerhouse and Left Bank Powerhouse. It can be found that the flow pattern in runner of Three Gorges Right Bank Powerhouse is significantly improved compared with Left Bank Powerhouse. (a) Runner for Left Bank Powerhouse (b) Runner for Right Bank Powerhouse Fig. 6 Comparison of runners between Three Gorges Right Bank Powerhouse and Left Bank Powerhouse 5

7 (a) Flow pattern in Left Bank Powerhouse runner (b) Flow pattern in Right Bank Powerhouse runner Fig. 7 Comparison of flow patterns in runners between Three Gorges Right Ban Powerhouse and Left Bank Powerhouse at high head and part load operating condition (H=113m, P=480MW) In order to remove the Von Karman vortices shedding from runner blade outlet, a special shape of blade trailing edge was adopted to avoid resonance. Model test showed that the visible Von Karman vortices were removed completely at the same operating conditions by employing this blade trailing edge shape, as shown in Fig. 8. The same measures were taken for the discharging edge of stay vanes. (a) Before blade outlet modification (b) After blade outlet modification Fig. 8 Model testing observation of Von Karman vortices shedding from runner blade outlet at the same operating conditions The main geometric parameters of Three Gorges turbine runners are shown in Table 2. It can be noted that the blade thickness, area and weight of Three Gorges Right Bank Powerhouse runner are larger than Three Gorges Right Bank Powerhouse. As a result, the runner of Three Gorges Right Bank Powerhouse is more robust and is of better mechanical property. Table 2 Main geometric parameters of Three Gorges turbine runners Geometric parameters of runner Left Bank Powerhouse Right Bank Powerhouse Guide vane relative height Model runner nominal diameter (mm) Prototype runner nominal diameter (mm) Blade number Prototype runner blade trailing edge (mm) Surface area of single prototype blade (m 2 ) Weight of single prototype blade (t) Prototype runner weight (t)

8 The design of spiral case was to optimize its hydraulic outline to reduce the hydraulic loss of case itself as much as possible and to provide adequate entrance flow angel for stay vanes. The hydraulic optimization of distributor was to build the reasonable matching relation between stay vans and wicket gates, so as to obtain minimum hydraulic losses throughout operating range of Three Gorges Right Bank Powerhouse turbine. The design of draft tube was to optimize its hydraulic outline to reduce the hydraulic loss of draft tube itself, as well as to optimize the cross-sectional area distribution from inlet to outlet of the draft tube. The hydraulic design of draft tube has an important influence on the pressure pulsations induced by the draft tube rope. Moreover, the adequate combination of the runner to the draft tube design is extremely critical to reduce the hydraulic loss and to improve the hydraulic stability. The final hydraulic design of spiral case, distributor and draft tube for Three Gorges Right Bank Powerhouse is shown in Fig. 9. The flow patterns in spiral case, distributor and draft tube at a typical high head and part load condition are shown in Fig. 10. It can be seen that all the wetted components were successfully optimized. (a) Spiral case (b) Distributor (c) Draft tube Fig. 9 Final hydraulic design of spiral case, distributor and draft tube for Three Gorges Right Bank Powerhouse (a) In spiral case (b) In distributor (c) In draft tube 4. Model Turbine Test Results Fig. 10 Flow patterns at a typical high head and part load condition The model turbine test results show that Three Gorges Right Bank Powerhouse turbine is of excellent efficiency and cavitation characteristics. The hydraulic stability of Three Gorges Right Bank Powerhouse turbine is largely improved. The amplitudes of pressure pulsations at every locations of Right Bank Powerhouse model turbine are significantly decreased in comparison with Left Bank Powerhouse model turbine. As an example, the comparison of pressure pulsation amplitudes at the draft tube between Three Gorges Right Bank Powerhouse turbine and Three Gorges Left Bank Powerhouse turbine is demonstrated in Table 3. Figure 11 shows the comparison of typical pressure pulsation distribution over the operating range between Three Gorges Right Bank Powerhouse turbine and Three Gorges Left Bank Powerhouse turbine. It can be clearly seen that no upper part load pressure pulsation operating zone exists throughout the operating range of Three Gorges turbines. 7

9 Table 3 Relative amplitudes of pressure pulsations at draft tube Operating range H=61m - 71m P=70% - 100% of max. output H=71m - 85m P=70% -100% of max. output H=85m - 94m P=497MW - 767MW H=94m - 113m P=497MW - 767MW H=88.88m - 113m P=767MW - 852MW H=61m - 85m P=0-70% of max. output H=85m - 113m P=0-70% of max. output Relative amplitude for Left Bank Powerhouse turbine Relative amplitude for Right Bank Powerhouse turbine 9% 6% 8% 3% 3% 2% 10% 5% 1% 1% 12% 8% 8% 7% Upper part load pressure pulsation operating zone (a) Left Bank Powerhouse turbine (b) Right Bank Powerhouse turbine Fig. 11 Comparison of typical pressure pulsation distribution over the operating rang From model test results, the hydraulic characteristics of Three Gorges Right Bank Powerhouse turbine are summarized in the following points: No upper part load pressure pulsation operating zone exists all around the operating range. The pressure pulsations at different locations of turbine water passageway were minimized. The inception and development of inter-blade vortices in the turbine runner operating at low loads were completely pushed out of normal operating range. Runner blade inlet cavitation due to flow separation at off-design operating conditions was completely eliminated. No cavitation occurs all over the operating range. No visible Von Karman vortices appeare both at the runner blade outlet and at stay vane outlet. The excellent efficiency at high heads and large power at low heads were obtained to be suited to the large head variation. In addition, a larger power output margin below the rated head of 85m causes the turbine to drink more discarded water during flood period of Three Gorges Project. The turbine runner is more robust and is of better mechanical property. By competitive model tests, the contract for four turbine-generator units in Three Gorges Right Bank Powerhouse was awarded to Dong Fang Electrical Machinery Co., Ltd. In May of 2004, the turbine model acceptance test was carried out in author s hydraulic laboratory. The hydraulic characteristics were verified and compared with the contract guarantees. All the hydraulic characteristics met and even exceeded the contract guarantees. 8

10 Figure 12 shows the prototype runner with a maximum diameter of 10m and a weight of 473t for Three Gorges Right Bank Powerhouse, and Fig. 13 shows that the giant runner was being transported to Three Gorges Project site. Fig. 12 Prototype runner for Three Gorges Right Bank Powerhouse 5. Concluding Remarks Fig. 13 Prototype runner being transported to Three Gorges Project site Up to October of 2008, all four prototype turbines (Units 15-18) supplied by Dong Fang Electrical Machinery Co., Ltd. for Three Gorges Right Bank Powerhouse were put into operation successfully, one year earlier than schedule. The turbines have experienced operation at all head and load conditions. A series of site tests were performed at different head and load conditions. The test results on the prototype and site operating experience show that all four turbines have a smooth operation all over the operating range. No upper part load pressure pulsations have been detected. It needs to be mentioned that in comparison with the turbines provided by other suppliers, the four turbines supplied by Dong Fang Electrical Machinery Co., Ltd. have a power output margin of about 80MW in the low head operating range of less than 75m during the flood period of every year. So, this performance feature is specially welcomed by the customer. The successful development of the turbines for Three Gorges Project is a great industrial achievement. A lot of technical advances have been made during the design, manufacturing, installing and operating of the turbines. The technological know-how and experiences obtained from Three Gorges Project are being widely applied to other large-scale hydro projects. References [1] Shi Q H 2008 Experimental Investigation of Upper Part Load Pressure Pulsations for Three Gorges Model Turbine Proc. of the 24 th IAHR Symp. on Hydraulic Machinery and Systems (Foz do Iguassu, Brazil) 9