To find out the reason, first understand a knowledge point: cavitation phenomenon. Of course, if you only do industrial installation, you don't need to know so much, because there is a complete and correct installation method in the manual of each pump manufacturer, just according to the drawing construction.
Working principle of centrifugal pump: the driving machine drives the impeller to rotate through the centrifugal pump shaft to generate centrifugal force. Under the centrifugal force, the liquid is thrown to the impeller outlet along the blade passage, and the liquid is collected by the volute and sent to the discharge pipe. The liquid obtains the energy from the impeller to increase the pressure energy and speed energy, and the liquid is transported to the working place by this energy. When the liquid is thrown to the impeller outlet, a low pressure is formed at the center of the impeller inlet, and a pressure difference is generated between the liquid in the suction tank and the liquid in 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 centrifugal pump.
According to the working principle of centrifugal pump, the liquid flows into the impeller under the pressure difference (PA PK) formed between the suction tank pressure PA and the lowest pressure PK at the impeller inlet. The lower the pressure PK at the impeller inlet, the greater the suction capacity. However, if PK is reduced to a certain limit value (at present, the saturated steam pressure Pt at the delivery temperature is the critical value of liquid vaporization pressure), cavitation will occur.
2. Serious consequences caused by cavitation:
(1) Produce vibration and noise.
(2) It has an impact on the working performance of the centrifugal pump: when the cavitation develops to a certain extent, a large number of bubbles will be generated, which will block the flow channel, and make the flow, head, efficiency of the centrifugal pump significantly reduced.
(3) The material of the runner will be damaged: mainly the fatigue erosion of the metal near the blade inlet.
3. The methods to improve the anti cavitation performance of centrifugal pump are as follows:
A. It is a problem of centrifugal pump design to improve pump structure and reduce Δ hr.
B. Improve the effective NPSH in the device: the main and most commonly used method is to use the filling head suction device.
In addition, the effective cavitation allowance of the device can be increased by minimizing the resistance loss of the suction pipeline and reducing the saturated vapor pressure of the liquid, i.e. selecting measures such as larger pipe diameter, shorter length, fewer elbows and valves, and lower temperature of liquid delivery when designing the suction pipeline.
OK, let's take a look at the above knowledge points. What's the function of big and small heads? It is used to change the size of pipe diameter of centrifugal pump. So what's the use of changing the pipe diameter at the pump inlet and outlet? The main purpose is to reduce the flow velocity of the medium in the pipeline and reduce the flow velocity so as to reduce the friction when the medium flows in the pipeline.
The horizontal eccentric reducer at the inlet of the centrifugal pump is generally arranged in the top plane, but in the case that the reducer is directly connected with the upward curved elbow, the bottom plane arrangement can be adopted. The reducer should be close to the pump inlet. The reason of top flat installation is to prevent the accumulation of gas in the eccentric reducer from entering the centrifugal pump and causing cavitation damage. When the inlet diameter of horizontal suction centrifugal pump is changed, the top flat installation shall be adopted for the pipeline from bottom to top, and the bottom flat installation shall be adopted for the pump from top to bottom (in order to prevent the gas from accumulating at the pump inlet, the top flat installation shall also be adopted), and the bottom flat installation shall be adopted for the medium with impurities and the suction speed is lower than the settlement speed of impurities. The principle is not to form air pockets.
Generally, it is "the top enters the bottom flat, the bottom enters the top flat"