Vacuum pump plays an important role in condensing steam turbine. Before start-up, the vacuum pump extracts the gas in the condenser to establish the vacuum; during start-up and operation, the vacuum pump extracts the non condensable gas which leaks or the working medium carries into the condenser, so as to strengthen the condensation effect of steam. In many forms of vacuum pump, water ring vacuum pump has the characteristics of simple structure, economic operation, wide range of application and so on, which has been widely used in power plants.
The water ring vacuum pump is composed of impeller, pump body, suction and exhaust port, auxiliary pipeline, etc. the specific structure is shown in Figure 1. The impeller is eccentrically installed in the pump body. When the impeller rotates, under the action of centrifugal force, water is thrown around, forming a closed water ring similar to the shape of the pump cavity. The outer surface of the water ring is in contact with the pump cavity, the inner surface is in contact with the blade, and a number of crescent shaped cavities are formed between the impeller hub and the water ring. When the impeller rotates clockwise, due to the influence of eccentric arrangement, the volume of a single small chamber increases from small to large, and the pressure in the chamber decreases continuously. When the pressure is lower than the gas pressure in the pumped container, the gas enters the small chamber from the suction port, which is the suction process. As the impeller continues to rotate, the volume of the small chamber decreases and the pressure increases until the pressure of the gas is greater than the exhaust pressure, the compressed gas is discharged from the exhaust port. In this way, during the continuous operation of the pump, the gas is continuously sucked in and discharged along the axial direction of the impeller, so as to achieve the purpose of continuous air extraction.
Due to the high vacuum requirements of the condenser, the suction pressure of the vacuum pump is easy to approach the limit vacuum (the saturation pressure of the working fluid), which leads to the working water in the pump being gasified and cavitation, which affects the safe operation of the pump. The specially designed cavitation protection pipe leads a way of air from the exhaust to the high true cavity in the pump to prevent cavitation. At the same time, a layer of gas protection film is formed on the surface of the impeller to prevent the impeller from being damaged when cavitation occurs. After accurate calculation, the size of the cavitation protection pipe will not affect the working pressure of the vacuum pump.
The working water of vacuum pump will be consumed in operation. The make-up water enters the pump body through the working fluid interface under the suction generated by the rotation of the impeller and makes up the water in time. Generally, the make-up water will be cooled by the cooler in advance, and cavitation can be avoided to some extent.
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