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Analysis Of The Structure And Operating Characteristics Of High-Head Francis Turbine

July 8, 2020

 

The so-called high-head Francis turbine refers to a Francis turbine with a rated head above 250m. Compared with foreign countries, the application of domestic high-head Francis turbines is relatively late, coupled with the limitations of design and manufacturing technology, resulting in certain impacts on its application. However, with the development of science and technology, the application of such Francis turbines has gradually deepened. Today, there are many built in China, and they have been put into operation.

1. Structure of high-head Francis turbine

1.1 The runner has auxiliary blades

In high-head Francis turbines, the length of the runner blade is very long, and the difference between the inlet radius and the outlet radius is relatively large. In the runner design, if the conventional design method is adopted, then the radius of the inlet of the blade is It will be very large, much wider than the exit radius. The amplitude flow component is an important part of the runner water wheel. Between the two blades, the water flow is affected by the Coriolis force and will show an uneven distribution. In this way, during the operation, There will be the problem of zero water flow velocity in some parts, which has a relatively large impact on the operation efficiency of the turbine, and in serious cases will cause damage to the normal operation of the unit. Based on this, in order to avoid the problem, when the width of the inlet is reduced, the width of the outlet must not change, and at the same time, a blade needs to be added between the two blades, the distance between the blade and the inlet is relatively short, and the length To be short [1]. The increase of auxiliary blades effectively improves the hydraulic efficiency, avoids unnecessary losses, improves the operating efficiency of the turbine, and ensures its stable operation.

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1.2 Eccentric structure of guide vane

The height of the guide vane of the high-head Francis turbine is not high. Therefore, the water leakage of the facade is relatively small. However, the water pressure is much higher than that of the ordinary Francis turbine, and the top cover deformation is easy to occur, which leads to the guide. The gap between the end surfaces of the blades continues to increase, resulting in serious water leakage. In order to avoid this problem, the structure of the guide vanes needs to be adjusted.

Nowadays, the guide vane of the turbine adopts an eccentric structure, the end face is relatively large, and has wings. At the same time, a seal ring is installed at the journal of the guide vane, which effectively reduces the damage [2]. In the guide vane of the eccentric structure , The wing is added, the installation position is the upper and lower ends of the positive pressure surface, and the diameter of the journal has also been increased, so as to effectively avoid excessive water leakage and keep the turbine in operation in colleges and universities .

1.3 Welding of stainless steel

In the guide vane, the gap between the two end surfaces is relatively small, such as the top cover and the anti-wear plate. When the two are connected, they are realized by screws. During the operation of the high head, the anti-wear plate is affected by its The effect of the action will be deformed at a local position, so that it is very difficult to maintain a small gap on the end surface. In order to improve this situation and ensure a small gap between the end faces, when connecting the anti-wear plate and the top cover, a welded stainless steel layer is used instead, so as to avoid the deformation of the anti-wear plate to the greatest extent.

1.4 Spindle seal

According to the maintenance record of the high-head Francis turbine, one of the most prone to problems is the main shaft. The main shaft of the hydraulic turbine needs to be sealed, and the sealing structure adopted is a non-contact seal. This sealing method significantly improves the service life of the main shaft. In the non-contact seal structure, it consists of three parts, one is the gap seal, the other is the pump plate at the crown of the runner, and the third is the top cover drain pipe [3]. During the operation of the unit, the pump plate rotating device The start of the pump needs to be driven by the rotation of the runner. After the pump plate rotates, the water in the inner cavity will also rotate, and the pressure difference will enter the water collection tank. The water in the water collection tank has two main purposes. One is used to cool the unit, and the other is discharged to the draft tube. In the sealing area of the main shaft, the hydraulic unit requires a relatively high speed, and the non-contact seal structure can fully meet the speed requirements. At the same time, there is almost no wear during operation, which effectively reduces the number of main shaft inspections and extends The service life of the spindle.

1.5 Taking water from the top cover

In the process of running the upper labyrinth ring of the runner, the problem of water leakage may occur. The top cover is mainly to avoid the occurrence of water leakage. Install the pump plate at the upper crown of the runner and remove the water under the action of the pump plate At the same time, the extracted water is used to cool the unit. When the hydropower unit is supplying water, the water pressure and water volume have certain requirements. In order to fully meet the requirements, it is necessary to use a crown without pressure relief holes, and finally take out the water at the top cover. At the same time, the radial vane pump structure is also installed on the upper crown, so that the flowing water of the upper labyrinth ring will enter two places, one is the inner wall of the top cover, and the other is the gap between the runner and the top cover on.

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2. Operating characteristics of high-head Francis turbine

2.1 General operating characteristics

At the beginning of the application of high-head Francis turbines in domestic hydropower stations, the structure of the turbine was a conventional structure. During the operation of the turbine, problems such as leakage of the guide vane, sand wear and noise, and noise continued to occur, and they all brought serious problems. As a result, overhaul and repair welding are required almost every year, especially the runner and guide vane components. Generally speaking, when the turbine is in operation for about 1 to 2 years, it must be overhauled. This has a very big impact on the long-term safe and stable operation of the unit, which in turn affects the operation efficiency of the hydropower station. In order to change this situation, domestic hydropower stations using high-head Francis turbines have optimized their structure, and the runner has added auxiliary blades. In this way, the operation status of the turbine unit has been significantly improved. The leakage of guide vanes and other issues have also been significantly reduced, and the period of overhaul has also been extended, which also enables the long-term safe operation of the turbine unit. This type of conventional operating characteristics has been relatively well practiced in Lushuihe Hydropower Station and Yuzixi I Hydropower Station.

2.2 Operating characteristics of the new structure

With the continuous progress of society and science and technology, the turbine technology has also been well developed. In this process, the high-head Francis turbine technology was introduced into the country and has been widely used in various hydropower stations. In order to improve the application effect, China began to increase research on high-head Francis turbine technology, and achieved good research results in hydraulic design, structural design, processing and manufacturing. From this, domestic high-head The application effect of the Francis turbine has been significantly improved, and its operating characteristics and performance have also been greatly improved [5]. In the South-to-North Water Diversion Project, the high-head Francis turbine was applied to the Danjiangkou Dam of the central line After the heightening of the dam, the height of the dam was increased from 162 meters to 176.6 meters. At the same time, the maximum water level was increased from 157 meters to 170 meters, and the maximum operating head was 79 meters. These changes provide good convenience for the application of hydraulic turbines, and promote the development of the South-to-North Water Diversion Project through the safe and stable operation of hydraulic turbines. After applying the new structure to the high-head Francis turbine, the abrasion of the sediment becomes negligible. Even if the sand content of the machine mud is very high, the abrasion is very small during the operation, and there are very few places where the abrasion occurs The depth of wear is also within 1mm. Between the top cover and the anti-wear plate and the two ends of the guide vane, there will be no cavitation erosion and water erosion.

 

Compared with the medium and low head Francis turbine, the structure of the high head Francis turbine has very unique characteristics. During its operation, the rated head and rated speed are relatively high, which can effectively improve the operating efficiency of the unit. Although the new structure has better performance and operating characteristics, the new structure's high-head Francis turbine is imported from abroad. Based on this, China should increase research efforts to independently produce new structural components of the turbine, which is promoting At the same time of the deep application of the turbine, it promotes the development of the hydraulic design of the turbine.