The Relationship Between Pump Head
and Inlet/Outlet Water
1. High-Head Pumps Operating at Low Load
Many people believe that the lower the pump head, the lower the motor load. Misled by this misconception, they often select pumps with very high heads when purchasing them. In fact, for centrifugal pumps, once the pump model is determined, its power consumption is directly proportional to the actual flow rate. The flow rate decreases as the head increases; therefore, the higher the head, the lower the flow rate, and the lower the power consumption. Conversely, the lower the head, the higher the flow rate, and the higher the power consumption.
Therefore, to prevent motor overload, the actual pump head should generally not be less than 60% of the rated head. When a high-head pump is used for pumping at too low a head, the motor will overload and overheat, potentially burning out the motor. In emergencies, a gate valve to regulate the flow rate must be installed on the outlet pipe to reduce the flow and prevent motor overload.
Pay attention to motor temperature rise. If the motor overheats, immediately reduce the outlet flow or shut down the pump. This point is also easily misunderstood. Some people believe that blocking the outlet and forcibly reducing the flow rate will increase the motor load. Actually, the opposite is true. High-power centrifugal pump irrigation units are equipped with gate valves on their outlet pipes. To reduce the motor load during startup, the gate valve should be closed first, and then gradually opened after the motor starts. This is the principle.
2. Large-diameter pumps with small pipes
Many users believe this increases the actual head. However, the actual head of a pump = total head - head loss. Once the pump model is determined, the total head is fixed. Head loss mainly comes from pipe resistance. The smaller the pipe diameter, the greater the resistance, and therefore the greater the head loss. Therefore, reducing the pipe diameter will not increase the actual head of the pump; in fact, it will decrease it, leading to a decrease in pump efficiency.
Similarly, when a small-diameter pump is used with a large pipe, the actual head of the pump will not decrease. Instead, the reduced pipe resistance will decrease the head loss, thus increasing the actual head. Some users believe that using a large water pipe with a small-diameter pump will significantly increase the motor load. They argue that a larger pipe diameter increases the pressure of the water on the pump impeller, thus greatly increasing the motor load.
However, liquid pressure is only related to the pump head, not the pipe cross-sectional area. As long as the head is constant and the impeller size remains the same, the pressure acting on the impeller is constant regardless of the pipe diameter. Increasing the pipe diameter reduces flow resistance, leading to a slight increase in flow rate and power consumption. However, within the rated head range, the pump can operate normally regardless of the pipe diameter, and it can even reduce pipeline losses and improve pump efficiency.
3. The horizontal section of the inlet pipe is too straight or tilted upwards
This causes air to accumulate in the inlet pipe, reducing the vacuum in the pipe and pump, lowering the pump's suction head, and decreasing the output. The correct approach is that the horizontal section should be slightly inclined towards the water source, not completely horizontal, and certainly not tilted upwards.
4. Too Many Bends in the Inlet Pipe
Too many bends in the inlet pipe will increase local water flow resistance. Bends should be perpendicular, not horizontal, to prevent air accumulation.
5. Direct Connection Between the Pump Inlet and a Bend
This will cause uneven water distribution as it passes through the bend and enters the impeller. When the inlet pipe diameter is larger than the pump inlet diameter, an eccentric reducer should be installed. The flat section of the reducer should be on top, and the beveled section on the bottom. Otherwise, air will accumulate, reducing water output or preventing pumping, and causing knocking noises. If the inlet pipe diameter is the same as the pump inlet diameter, a straight pipe should be added between the pump inlet and the bend. The length of the straight pipe should not be less than 2-3 times the pipe diameter.
6. Inlet Pipe with Foot Valve Not Perpendicular
This installation will prevent the valve from closing automatically, causing leakage. Correct installation method: The bottom section of the inlet pipe with a foot valve should ideally be vertical. If vertical installation is not possible due to terrain limitations, the angle between the water pipe axis and the horizontal plane should be greater than 60°.
7. Incorrect Inlet Position of the Inlet Pipe
The distance between the inlet of the inlet pipe and the bottom and wall of the inlet pool is less than the inlet diameter. If there is mud or other debris at the bottom of the pool, and the distance between the inlet and the bottom is less than 1.5 times the diameter, it will cause poor water intake or draw in mud and debris, clogging the inlet.
Insufficient immersion depth of the inlet pipe will cause eddies on the water surface around the inlet pipe, affecting water intake and reducing output. The correct installation method is: the immersion depth of small and medium-sized water pumps should not be less than 300-600mm, and that of large water pumps should not be less than 600-1000mm.
8. Outlet Above the Normal Water Level in the Outlet Pool
If the outlet is above the normal water level in the outlet pool, although it increases the pump head, it reduces the flow rate. If the outlet must be higher than the water level in the pool due to terrain limitations, an elbow and a short pipe should be installed at the pipe opening to make the water pipe a siphon type and reduce the height of the outlet.
