Regarding the installation of eccentric large and small heads on the inlet pipe of a  axial split case pump  (that is, the pump inlet is a horizontal inlet), in the textbook on pump piping standards, it is stipulated as follows: The eccentric reducer diameter orientation on the horizontal pipe (horizontal part Up or down), determined by whether a liquid bag or air bag appears (divided into two situations):

When the medium enters the pump from bottom to top, it is installed with a flat and eccentric head to avoid air bags

In summary, it is generally "the top is flat when going up, and the bottom is flat when going down."

To understand this reason, we must first understand a knowledge point: cavitation phenomenon. Of course, if you are just doing industrial installation, you don’t need to know so much, because each pump manufacturer’s instructions have complete and correct installation methods, and you only need to follow the drawings.

Cavitation Phenomenon

The working principle of the axial split case pump: the driver drives the impeller to rotate through the pump shaft to generate centrifugal force. Under the action of centrifugal force, the liquid is thrown toward the impeller outlet along the blade flow path, and the liquid is collected through the volute and sent to the discharge pipe. The liquid obtains energy from the impeller, which increases both pressure energy and speed energy, and relies on this energy to transport the liquid to the work site. While the liquid is thrown toward 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 center of the impeller. Under the action of this pressure difference, the liquid in the suction tank continues to The ground enters the impeller through the suction pipe and the suction chamber of the pump.

According to the working principle of the axial split case pump, the liquid flow flows into the impeller under the pressure difference (Pa-Pk) formed between the suction tank pressure Pa and the minimum pressure Pk at the impeller inlet. The lower the pressure Pk at the impeller inlet, the lower the suction capacity. The bigger. However, if Pk decreases to a certain limit value (currently, the saturated vapor pressure Pt of the liquid at the delivery temperature is often used as the critical value of the liquid vaporization pressure), cavitation phenomenon will occur.

Serious consequences caused by cavitation:

(1) Produces vibration and noise.

(2) Impact on the working performance of the pump: When cavitation develops to a certain extent, a large number of bubbles will be generated, which will block the flow channel and significantly reduce the flow rate, lift and efficiency of the pump.

(3) There will be damage to the material of the flow channel: mainly fatigue erosion of the metal near the blade inlet.

Methods to improve the anti-cavitation performance of axial split case pumps include:

A. Improving the pump structure and reducing Δhr is a pump design issue.

B. Increase the effective NPSH in the device: The most important and commonly used method is to use a perfusion 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 use a pipe with a larger diameter, a shorter length, fewer elbows and valves, and the temperature of the transported liquid as low as possible. These measures can improve the effective cavitation margin of the device.

Okay, after understanding the above knowledge points, let’s take a look. What is the function of the big and small heads? It is used to change the size of the pipe diameter. So what is the use of changing the diameter of the pipe at the inlet and outlet of the pump? The main purpose is to reduce the flow velocity of the medium in the pipeline and reduce the flow velocity to reduce the friction when the medium flows in the pipeline.

The horizontal eccentric reducer at the inlet of the centrifugal pump generally adopts a top-flat arrangement, but when the reducer is directly connected to an upwardly bent elbow, a bottom-flat arrangement can be used. The reducer should be close to the pump inlet. The reason for the flat top installation is to prevent gas from accumulating in the eccentric reducer and entering the centrifugal pump, causing cavitation damage to the pump. When the inlet diameter of a axial split case pump is changed, the pipe entering the pump from bottom to top should be installed with a top level, and the pipe entering the pump from top to bottom should be installed with a bottom level (to prevent gas from accumulating at the pump port, a top level installation is also used). The medium contains impurities. And if the suction speed is lower than the impurity settling speed, it should be installed flat on the bottom. The principle is not to form air pockets.