Treatment of eccentric reducer at the inlet of corrosion-resistant centrifugal pump

As for the installation of the eccentric reducer on the inlet pipe of the end suction centrifugal pump (i.e. the pump inlet is a horizontal inlet), in the teaching materials related to the pump piping standards, it is stipulated that the eccentric reducing pipe diameter orientation on the horizontal pipe (whether the horizontal part is upward or downward) is determined by the absence of liquid bags or air bags (in two cases):

1. When the medium enters the pump from top to bottom, the bottom flat eccentric reducer is used for installation, and there is no liquid bag;

2. When the medium enters the pump from bottom to top, the top flat eccentric reducer is used for installation, and there is no air bag;

Generally speaking, it is "from top to bottom, from bottom to top" To understand this 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 are complete and correct installation methods in the instructions of each pump manufacturer, and you only need to construct according to the drawings

1、 Cavitation phenomenon

Working principle of centrifugal pump: the driving machine drives the impeller to rotate through the pump shaft to generate centrifugal force. Under the action of centrifugal force, the liquid is thrown to the impeller outlet along the blade channel, and the liquid is collected through the volute and sent to the discharge pipe. The liquid gets energy from the impeller, which increases the pressure energy and speed energy, and relies on this energy to transport the liquid to the working place. 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 action 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 flow flows into the impeller under the action of 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 vapor pressure pt of the liquid at the transmission temperature is mostly taken as the critical value of the liquid vaporization pressure), cavitation will occur.

2、 Methods to improve the cavitation resistance of centrifugal pumps include:

A. Improve the pump structure and reduce Δ HR, it is a pump design problem.

B. Improve the effective NPSH in the device: the main common method is to use the filling head suction device.

In addition, try to reduce the resistance loss of the suction pipeline and reduce the saturated vapor pressure of the liquid, that is, when designing the suction pipeline, try to choose measures such as larger pipe diameter, shorter length, fewer elbows and valves, and lower temperature of the transmitted liquid, which can improve the effective cavitation allowance of the device.

Well, after understanding the above knowledge points, let's take a look. What is the role of big and small heads? It is used to change the size of pipe diameter. So what is the use of changing the size of the pipe diameter at the inlet and outlet of the pump? It is mainly to reduce the flow velocity of the medium in the pipe, so as to reduce the friction when the medium flows in the pipe.
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