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It is explained in the form of a water pump with a flow rate of 8m3/h and a head of 60m.: one: Reasons for specific rotation number From the perspective of water pump design, the first-stage impeller lift of a single-stage impeller water pump (horizontal end-suction pump) is 60m, and the calculated specific rotation number is about 23. In the case of a multi-stage impeller water pump, calculated based on a 7-stage impeller, the single-stage impeller lift is about 9m, and the specific rotation number is about 96. The specific speed of 23 is a low specific speed. This kind of water pump is usually not designed because the low specific speed is inefficient and the curve is prone to humps, that is, there are two flow points under one head, as shown in the figure below. The specific rotation number of 96 is a medium specific rotation number, which belongs to the normal water pump design range. The performance curve decreases monotonically, which is a conventional design pump type. two: Efficiency reasons: The efficiency curve in the water pump performance curve is usually a parabola with a hump, and the hump is located on the right side of the middle of the curve. When a single-stage pump is selected, the operating point is to the left, resulting in serious efficiency loss. When a multi-stage pump is selected, the operating point is in the middle, and the efficiency is higher. three: When selecting a single-stage pump due to the location of the operating point: The working point is to the left. When the water pump is running, only a small amount of water flows out. A large amount of water circulates in the pump body and rubs against the impeller and pump body. As the saying goes, the pump is boring, which will cause friction and heat, and the mechanical seal will be damaged after the pump head overheats. When using multi-stage pump selection: The working point is in the middle and the water flow is normal, which can take away the heat in the pump body, ensure the water temperature at the mechanical seal position, and extend the mechanical seal life. Four: When selecting a single-stage pump due to motor power reasons: The operating point is located at the left end of the curve, and a large number of curves are still retained at the right end. The flow range of the water pump exceeds the design parameters by being too large, resulting in excessive motor power. It may be necessary to adapt to a 5.5kw or even 7.5kw motor. When using multi-stage pump selection: The working point is located in the middle of the curve, the right end of the curve drops rapidly, and the water pump motor power is relatively low. Only needs about 3kw motor. five: Performance Adjustment Reasons When Sizing a Single-Stage Pump: When the curve has a hump, it is difficult to automatically adjust the pump operating status through valve opening or sensors, because the monitoring of the pump operating status is usually done by pressure or pressure difference, which will lead to double solutions in the automatic control logic. When the curve decreases monotonically, the operating performance point must work at the far left end of the curve, and the left end curve tends to be flat. The pressure is not sensitive to flow changes, so the system flow cannot be accurately controlled by pressure or pressure difference. When using multi-stage pump selection: The curve decreases monotonically, and the operating point is located in the middle of the curve. It is sensitive to changes in flow and pressure, and the water pump flow can be accurately adjusted through pressure or pressure difference.