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| Treatment of eccentric reducer at inlet of corrosion resistant centrifugal pump
With regard to the installation of eccentric reducer on the inlet pipe of the end suction centrifugal pump (i.e. the pump inlet is a horizontal inlet), in the textbooks related to the pump piping standards, it is stipulated that the diameter orientation of the eccentric reducer on the horizontal pipe (the horizontal part is upward or downward) is determined according to the absence of a liquid bag or air bag (in two cases):
1. When the medium is fed into the pump from the top to the bottom, the bottom flat eccentric reducer is used for installation, and there is no liquid bag;
2. When the medium is fed into the pump from bottom to top, the top flat eccentric reducer is used for installation, without air bag;
Generally speaking, it is "flat at the top and flat at the bottom" To find out the reason, first understand one knowledge point: cavitation phenomenon. Of course, if you only do industrial installation, you don't need to know so much, because there are complete and correct installation methods in the instructions of each pump manufacturer, just follow the drawings
1? Cavitation phenomenon
Working principle of centrifugal pump: the driver drives the impeller to rotate through the pump shaft to generate centrifugal force. Under the effect of centrifugal force, the liquid is thrown to the impeller outlet along the blade flow path, and the liquid is collected by the volute and sent to the discharge pipe. The liquid gets energy from the impeller to increase both the pressure energy and the velocity energy, and the liquid is transported to the work site by this energy. When the liquid is thrown to the impeller outlet, a low pressure is formed at the center of the impeller inlet, which creates a pressure difference between the liquid in the suction tank and the impeller center. Under the effect of this pressure difference, the liquid in the suction tank continuously enters the impeller through the suction pipeline and the suction chamber of the pump.
According to the working principle of the centrifugal pump, the liquid flows into the impeller under the pressure difference (Pa Pk) formed between the suction tank pressure Pa and the low pressure Pk at the impeller inlet. The lower the pressure Pk at the impeller inlet, the greater the suction capacity. However, if Pk decreases to a certain limit value (at present, the saturated steam pressure Pt of liquid at the transportation temperature is the critical value of liquid vaporization pressure), cavitation will occur.
2? The methods to improve the cavitation resistance of centrifugal pump are:
A. Improve pump structure and reduce ? Hr, belongs to the pump design problem.
B. To improve the effective NPSH in the device: the main common method is to use the injection head suction device.
In addition, the resistance loss of suction pipeline shall be reduced as much as possible, and the saturated vapor pressure of liquid shall be reduced, that is, the suction pipeline shall be designed with larger pipe diameter, shorter length, fewer elbows and valves, and lower temperature of liquid delivery, which can improve the effective cavitation allowance of the device.
Well, after understanding the above knowledge, let's have a look. What is the role of the reducer? It is used to change the size of pipe diameter. So what's the use of changing the pipe diameter at the pump inlet and outlet? It is mainly used to reduce the flow velocity of the medium in the pipeline, so as to reduce the friction force when the medium flows in the pipeline.
3? Serious consequences caused by cavitation:
(1) Generates vibration and noise.
(2) The working performance of the pump is affected: when the cavitation develops to a certain extent, a large number of bubbles will be generated, which will block the flow passage, making the flow, head, efficiency, etc. of the pump drop significantly.
(3) The material of the runner will be damaged, mainly due to the fatigue erosion of the metal near the blade inlet.
The horizontal eccentric reducer at the inlet of the centrifugal pump generally adopts the top flat layout, but when the reducer is directly connected with the upward curved elbow, the bottom flat layout can be adopted. The reducer shall be close to the pump inlet. The reason for flat top installation is to prevent gas from accumulating in the eccentric reducer and entering the centrifugal pump to cause cavitation damage to the pump. When the inlet diameter of the horizontal suction centrifugal pump is changed, the top flat installation is adopted for the pipeline to enter the pump from bottom to top, and the bottom flat installation is adopted for the pipeline to enter the pump from top to bottom (the top flat installation is also adopted to prevent the gas from accumulating at the pump inlet). The bottom flat installation is adopted for the medium with impurities and the suction speed is lower than the impurity settlement speed. The principle is not to form air bags.
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Treatment of eccentric reducer at inlet of corrosion resistant centrifugal pump
