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Impeller not only refers to the disc with moving blades, which is an integral part of the impulse turbine rotor, but also refers to the general term of the disc and the rotating blades installed on it. Impellers can be classified according to shape and opening and closing conditions.
Turbine impeller is generally composed of rim, wheel face and hub. The impeller is used to install blades and transmit the torque generated by steam flow force on the cascade to the main shaft. The stress of the impeller during operation is very complex. In addition to the huge centrifugal force caused by the impeller itself and the quality of blade parts, there are also thermal stresses caused by temperature difference, tangential and axial forces caused by moving blades, steam pressure difference on both sides of the impeller, and alternating stresses caused by blade and impeller vibration.
The balance hole is opened on the impeller to reduce the steam pressure difference on both sides of the impeller and reduce the excessive axial force generated by the rotor. However, in the regulating stage and the last stage, and the second stage of the low-pressure part with a large reaction and heavy load, the balance hole is generally not opened, so as not to weaken the strength of the impeller and reduce the steam leakage loss.
An odd number of balance holes are set on each impeller, which can avoid setting two balance holes on the same radial section so that the strength of the impeller section will not be excessively weakened. Generally, 5 or 7 holes are opened.
Centrifugal impeller type accelerates the fluid by rotating centrifugal force. The greater the centrifugal force, the greater the speed of the fluid. The process of acceleration is the process of pressurization. To pressurize, directly increasing the speed of the centrifugal impeller can play the role of pressurization. If the speed cannot be increased, another way is to increase the diameter of the centrifugal impeller (that is, replace the centrifugal impeller with a larger diameter) under a certain speed. Principle: Centrifugal force is in direct proportion to rotating speed and diameter.
The impeller of a centrifugal pump mainly has the following four forms: (a) closed type; (b) Front half-open type; (c) Rear half-open type; (d) Open. 1 – Impeller; 2 – Rear cover plate; 3 – Hub; 4 – Front cover plate; 5 – Impeller seal ring; 6 – Stiffener
Closed impeller: composed of blades and front and rear cover plates. The closed impeller has high efficiency and is difficult to manufacture, so it is widely used in centrifugal pumps. It is suitable for conveying clean water, solution, and other clean liquids with low viscosity and without particles.
Semi-open impeller: generally, there are two structures: the first is the front semi-open type, which is composed of the rear cover plate and the blade. This structure has low efficiency. In order to improve efficiency, an adjustable clearance seal ring is required; The other is the back half open type, which is composed of the front cover plate and the blade. Because the same sealing ring as the closed impeller can be used, the efficiency is basically the same as that of the closed impeller. Besides conveying liquid, the blade also has a sealing function (back blade or auxiliary impeller). The semi-open impeller is suitable for conveying liquid containing suspended solids such as solid particles and fibers. The semi-open impeller is less difficult to manufacture, has lower cost, and has strong adaptability. It is increasingly used in centrifugal pumps for oil refining and the chemical industry and is used to transport clean water and liquid similar to clean water.
Open impeller: impeller with only blade and blade stiffener and no front and rear cover plate (the number of open impeller blades is less 2-5). The impeller has low efficiency and few applications. It is mainly used to transport liquid with high viscosity and slurry liquid.
The blades of the centrifugal pump impeller are generally backward curved blades. There are two types of blades: cylindrical and twisted. The application of twisted blades can reduce the load on the blades, improve the suction performance of the centrifugal pump, and improve the cavitation resistance, but it is difficult to manufacture and expensive.
Damage causes 1. There are friction marks on the side of the impeller due to axial movement; 2. Mechanical damage, impeller quality or manufacturing problems; 3. The impeller expands due to high temperature; 4. The clearance is not adjusted properly, causing wall rubbing with the pump casing; 5. The impeller is damaged and deformed due to foreign matters in the pump body; 6. When the pump is started up, water suddenly enters the pump, causing a collision between the impeller and the pump casing’s peripheral surface and end face.
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