I. Pipeline centrifugal pumps are widely used and the most common type of pump across various fields. As long as users are aware of the eight common misconceptions about pipeline centrifugal pumps and take steps to avoid them, they can significantly extend the pump’s service life. Some pipeline centrifugal pump users mistakenly believe that using a smaller pipe diameter 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 determined, the total head remains constant. Head loss primarily results from pipeline resistance; obviously, the smaller the pipe diameter, the greater the resistance and thus the greater the head loss. Therefore, reducing the pipe diameter not only fails to increase the actual head but actually decreases it, leading to a drop in pump efficiency. Similarly, when a small-diameter pump is connected to a larger-diameter pipe for water pumping, the actual head of the pump will not decrease. On the contrary, because the pipeline resistance is reduced, the head loss will diminish, thereby increasing the actual head. Some operators also assume that using a large-diameter pipe with a small-diameter pump will greatly increase the motor load. They reason that as the pipe diameter increases, the water pressure exerted on the pump impeller from the discharge pipe will rise, thus significantly boosting the motor load. However, this assumption is incorrect. The magnitude of liquid pressure depends solely on the head height and is independent of the cross-sectional area of the pipe. As long as the head remains constant and the impeller size stays unchanged, the pressure acting on the impeller will remain the same regardless of the pipe diameter. The only effect of increasing the pipe diameter is to reduce the flow resistance, which in turn slightly increases the flow rate and power consumption. Yet, as long as the pump operates within its rated head range, it can function normally even with an increased pipe diameter—and moreover, this approach can help reduce pipeline losses and improve pump efficiency.

　　II. High-lift pumps used for low-lift pumping
　　Some users of pipeline centrifugal pumps mistakenly believe that the lower the pump head, the lighter the motor load will be. Guided by this misconception, they often select pumps with excessively high heads when making their purchase. In fact, for centrifugal pumps, once the pump model is determined, the power consumption is directly proportional to the pump’s actual flow rate. As the pump head increases, the flow rate decreases; consequently, the higher the head, the smaller the flow rate, and the lower the power consumption. Conversely, the lower the head, the greater the flow rate, and the higher the power consumption. Therefore, to prevent motor overload, it is generally recommended that the actual pumping head used should not fall below 60% of the pump’s rated head. Thus, when a high-head pump is used for pumping at a much lower head than its rated capacity, the motor is prone to overload and overheating, and in severe cases, the motor could even burn out. If such a situation arises and an emergency use is necessary, it is essential to install a gate valve (or block the small outlet with wood or other materials) on the discharge pipe to regulate the water flow and thereby reduce the flow rate and avoid motor overload. Pay close attention to the motor’s temperature rise—if you notice the motor overheating, promptly reduce the flow rate at the outlet or shut down the pump immediately. This point is often misunderstood: some operators assume that blocking the outlet and forcibly reducing the flow rate would increase the motor load. In fact, the opposite is true. On the discharge pipes of standard high-power centrifugal pump irrigation units, there are always gate valves installed. To minimize the motor load during unit startup, one should first close the gate valve and then gradually open it after the motor has started—this is precisely the principle behind it.

　　3. The pipeline centrifugal pump has many elbows on the suction pipe line.
　　If many elbows are used in the inlet pipeline, it will increase local flow resistance. Moreover, elbows should be turned in the vertical direction and must not be turned horizontally to prevent air from accumulating.

　　4. When installing the inlet pipeline, ensure that the horizontal section is either level or slopes upward.
　　Doing so will cause air to accumulate inside the inlet pipe, reducing the vacuum level in both the pipe and the pump, thereby lowering the pump’s suction lift and decreasing the water output. The correct approach is to ensure that the horizontal section slopes slightly toward the water source—not only should it not be perfectly horizontal, but it must also never curve upward.

　　5. The inlet of the pipeline centrifugal pump is directly connected to the elbow.
　　This can cause uneven distribution of water flow as it passes through the bend and enters the impeller. When the diameter of the inlet pipe is larger than that of the pump’s suction port, an eccentric reducer should be installed. The flat section of the eccentric reducer must be mounted on top, while the sloped section should be placed at the bottom. Otherwise, air may accumulate, leading to reduced water output or even failure to draw water, accompanied by banging noises. If the diameter of the inlet pipe is the same as that of the pump’s suction port, a straight pipe should be added between the pump’s suction port and the bend; the length of this straight pipe must be no less than 2 to 3 times the diameter of the inlet pipe.

　　6. The outlet of the pipeline centrifugal pump should be located above the normal water level of the discharge pool.
　　If the outlet is located above the normal water level of the discharge pond, although this increases the pump head, it reduces the flow rate. If, due to site conditions, the outlet must be higher than the water level of the discharge pond, a bend and a short pipe should be installed at the pipe opening to convert the pipeline into a siphon system, thereby lowering the height of the outlet.

　　7. The inlet position of the water supply pipe is incorrect.
　　① The inlet of the water intake pipe is located less than the diameter of the inlet away from both the bottom and the walls of the intake basin. If there is sediment or other debris at the bottom of the basin, and the distance between the inlet and the basin bottom is less than 1.5 times the diameter of the inlet, this could result in poor water intake during pumping or even cause the inlet to become clogged with sediment and debris.
　　② If the intake depth of the water inlet pipe is insufficient, it can cause vortices to form around the inlet pipe, which will interfere with water intake and reduce the flow rate. The correct installation method is as follows: For small- and medium-sized pumps, the intake depth must be no less than 300–600 mm; for large pumps, it must be no less than 600–1000 mm.

　　8. When a bottom valve is installed, the lowest section of the inlet pipe 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 installed vertically. If vertical installation is not feasible due to site conditions, the angle between the pipe axis and the horizontal plane should be at least 60°.