Should the eccentric flange at the inlet of a centrifugal pump be installed with its flat surface facing up or down? Why?

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Regarding the installation of eccentric reducers on the inlet pipeline of end-suction centrifugal pumps (i.e., pumps with horizontal inlets), relevant standards for pump piping specify that the orientation of the eccentric reducer in a horizontal pipe – whether the horizontal section faces upward or downward – should be determined to prevent the formation of liquid pockets or air bubbles (two scenarios exist): 1. When the fluid flows into the pump from top to bottom, an eccentric reducer with a flat bottom should be used to avoid the formation of liquid pockets； 2. When the medium flows into the pump from bottom to top, a top-flattened eccentric reducer should be used to prevent the formation of air pockets ； In general, it is characterized by a flat bottom at the upper part and a flat top at the lower part. To understand why this is the case, one first needs to know a concept: cavitation. Of course, if you are only involved in industrial installation, you don’t need to know that much, as each pump manufacturer’s manual provides complete and accurate installation instructions – you just need to follow them. 1. Cavitation phenomenon and the working principle of centrifugal pumps: The drive unit rotates the impeller through the pump shaft, generating centrifugal force. Under this force, the liquid is forced along the blade channels toward the outlet of the impeller; the liquid is then collected by the volute and sent into the discharge pipe. The liquid gains energy from the impeller, resulting in an increase in both pressure energy and kinetic energy; it is this energy that is used to transport the liquid to the point of use. As the liquid is thrown toward the outlet of the impeller, a low pressure is created at the center of the impeller inlet. This creates a pressure difference between the liquid in the suction tank and that at the center of the impeller; under this pressure difference, the liquid in the suction tank continuously flows into the impeller through the suction pipeline and the pump’s suction chamber. According to the working principle of centrifugal pumps, the fluid flow enters the impeller under the effect of the pressure difference (Pa-Pk) generated between the pressure Pa in the suction tank and the lowest pressure Pk at the impeller inlet; thus, the lower the pressure Pk at the impeller inlet, the greater the suction capacity. However, when Pk drops to a certain limit value (which is currently often the saturated vapor pressure Pt of the liquid at its transport temperature, taken as the critical value for the liquid’s vaporization pressure), cavitation occurs. 2. Severe consequences caused by cavitation: (1) Generation of vibration and noise. (2) It affects the operating performance of the pump: When cavitation reaches a certain level, a large number of bubbles are generated, which can block the flow channels and cause significant drops in the pump’s flow rate, head, efficiency, etc. (3) The material of the flow channel can be damaged: mainly fatigue erosion of the metal near the blade inlet. 3. Methods to improve the cavitation resistance of centrifugal pumps include: A. Improving the pump structure to reduce Δhr, which is a matter related to pump design. B. Increasing the effective net positive suction head inside the device: The most important and commonly used method is to use a priming head suction device. Furthermore, minimizing the resistance losses in the suction pipeline, reducing the saturated vapor pressure of the liquid – by using pipes with larger diameters, shorter lengths, fewer bends and valves during the design of the suction pipeline, and keeping the temperature of the liquid as low as possible – can all help increase the effective net positive suction head available in the system. Well, after understanding the above concepts, let’s take a look at what the function of the large and small ends is It is used to change the diameter of the pipe. So what’s the use of changing the diameter of the pipes at the pump’s inlet and outlet? It is mainly to reduce the flow velocity of the medium within the pipeline; by lowering the flow velocity, the frictional force associated with the flow of the medium in the pipeline can be reduced. The horizontal eccentric reducer at the inlet of a centrifugal pump is generally arranged with its top aligned, but when the reducer is directly connected to an elbow that bends upward, it can be arranged with its bottom aligned. The reducing pipe should be located near the pump inlet. The reason for top-level installation is to prevent gas from accumulating inside the eccentric reducer and entering the centrifugal pump, thereby causing cavitation damage to the pump. When the inlet diameter of a horizontal suction centrifugal pump is changed, if the fluid enters the pump from below, it should be installed with its top at the same level as the surrounding structure; if it enters from above, it should be installed with its bottom at the same level (top-level installation is also used to prevent gas from accumulating at the pump inlet). When the fluid contains impurities and the suction speed is lower than the settling speed of those impurities, bottom-level installation is employed. The principle is to avoid the formation of air pockets.