1. High-head pumps are used for low-head pumping.

Many pump operators believe that the lower the head, the lighter the motor load. Misled by this misconception, they often select pumps with a very high rated head when making their purchase. In fact, for centrifugal pumps, once the pump model is fixed, the power consumption is directly proportional to the actual flow rate. As the head increases, the flow rate decreases; therefore, the higher the head, the lower the flow, and the lower the power consumption. Conversely, the lower the head, the higher the flow, and the greater the power consumption. To prevent motor overload, it is generally required that the actual operating head of the pump should not be less than 60% of the rated head. Consequently, if a high-head pump is used to deliver water at a much lower head, the motor is prone to overload and overheating, and in severe cases, the motor may even burn out. If such emergency operation is unavoidable, a gate valve must be installed on the discharge pipe to regulate the flow (or the discharge outlet can be partially blocked with wood or other materials) to reduce the flow and prevent motor overload. Pay close attention to the motor temperature rise; if the motor becomes excessively hot, immediately reduce the discharge flow or shut down the pump. This point is also often misunderstood: some operators assume that blocking the discharge outlet to forcibly reduce the flow will increase the motor load. In fact, the opposite is true. Standard high-power centrifugal pump irrigation units are equipped with gate valves on the discharge pipes; to minimize the motor load during startup, the gate valve should be closed first, and only after the motor has started should the valve be gradually opened.

2. Using a large-diameter pump with a small-diameter water pipe for pumping

Many pump operators believe that doing so can increase the actual head; in fact, the actual head of a pump is equal to the total head minus the head loss. Once the pump model is selected, the total head remains constant; head loss is primarily due to pipeline resistance, and the smaller the pipe diameter, the greater the resistance and the larger the head loss. Therefore, reducing the pipe diameter does not increase the pump’s actual head—it actually decreases it, leading to a drop in pump efficiency. Conversely, when a small-diameter-pipe pump is used with a large-diameter pipe, the actual head does not decrease; on the contrary, the reduced pipeline resistance lowers the head loss, thereby increasing the actual head. Some operators also assume that using a large-diameter pipe with a small-diameter-pipe pump will significantly increase the motor load, reasoning that a larger pipe diameter results in greater water pressure on the pump impeller, which in turn places a heavier load on the motor. However, the magnitude of fluid pressure depends solely on the head, not on the cross-sectional area of the pipe. As long as the head remains constant and the impeller size stays unchanged, the pressure exerted on the impeller will be the same regardless of the pipe diameter. The only effect of increasing the pipe diameter is to reduce flow resistance, which allows for a higher flow rate and a corresponding increase in power consumption. Nevertheless, as long as the operating conditions remain within the pump’s rated head range, the pump can function normally even with a larger pipe diameter, while also reducing pipeline losses and improving pump efficiency.

3. When installing the inlet piping, ensure that the horizontal sections are either level or sloped upward.

Doing so will cause air to accumulate in the suction pipe, reducing the vacuum in both the pipe and the pump, thereby lowering the pump’s suction lift and decreasing the flow rate. The correct practice is to slope the horizontal section slightly toward the water source; it should not be horizontal, nor should it curve upward.

4. There are many elbows used in the water inlet pipeline.

Using too many elbows in the inlet piping will increase local flow resistance. Moreover, elbows should be installed to change direction vertically; horizontal bends are not permitted to prevent air from accumulating.

5. The pump inlet is directly connected to the elbow.

This will result in uneven distribution of the water flow as it passes through the bend and enters the impeller. When the diameter of the suction pipe is larger than that of the pump inlet, an eccentric reducer should be installed. The flat side of the reducer must be positioned on top, with the sloped side on the bottom; otherwise air will accumulate, reducing the flow rate or preventing priming altogether, and causing knocking noises. If the suction pipe and the pump inlet have the same diameter, a straight section of pipe should be added between the pump inlet and the bend, with a length no less than 2 to 3 times the pipe diameter.

6. The lowest section of the suction pipe equipped with a foot valve is not vertical.

If installed in this manner, the valve will fail to close automatically, resulting in water leakage. The correct installation method is as follows: for the inlet pipe equipped with a foot valve, the lowest section should ideally be vertical. If vertical installation is not feasible due to site constraints, the angle between the pipe axis and the horizontal plane must be at least 60°.

7. The inlet of the water supply pipe is improperly positioned.

(1) The inlet of the water intake pipe is located at a distance from the bottom and walls of the intake basin that is less than the diameter of the inlet. If there is sediment or other debris on the basin floor, and the distance between the inlet and the floor is less than 1.5 times the inlet diameter, this can lead to poor water intake during pumping or result in the suction of sediment and foreign matter, which may clog the inlet.
(2) If the intake of the suction pipe is not submerged deep enough, vortices will form on the water surface around the pipe, which can impede water intake and reduce the discharge rate. The correct installation procedure is as follows: for small and medium-sized pumps, the immersion depth of the suction pipe must be no less than 300–600 mm; for large pumps, it must be no less than 600–1000 mm.

8. The outlet pipe shall be positioned above the normal water level of the discharge basin.

If the outlet is located above the normal water level of the discharge basin, although the pump head will increase, the flow rate will decrease. If, due to site topography, the outlet must be higher than the basin water level, a bend and a short pipe should be installed at the pipe outlet to convert the pipeline into a siphon configuration, thereby reducing the outlet elevation.