Analyzing the reasons why centrifugal pumps cannot lift water

Centrifugal pump is a pumping machine that utilizes the centrifugal motion of water. Due to its simple structure, convenient use and maintenance, and high efficiency, it has become the most widely used water pump in agriculture, but it is also troublesome due to sometimes not being able to lift water. So, what are the reasons for the failure of the centrifugal pump to lift water?
1. There is air in the inlet pipe and pump body
(1) Some users do not fill up enough water before starting the water pump; Sometimes it appears that the filled water has overflowed from the vent hole, but the air has not been completely discharged by rotating the pump shaft, resulting in a small amount of air remaining in the inlet pipe or pump body.
(2) The horizontal section of the inlet pipe in contact with the water pump should have a downward slope of more than 0.5% in the opposite direction of the water flow, and the end connecting the water pump inlet should be the highest, not completely horizontal. If it is tilted upwards, air will remain in the inlet pipe, reducing the vacuum in the water pipe and pump, and affecting water absorption.
(3) The packing of the water pump has been worn out or pressed too loose due to long-term use, causing a large amount of water to spray out from the gap between the packing and the pump shaft sleeve. As a result, external air enters the interior of the water pump through these gaps, affecting water extraction.
(4) Due to long-term potential underwater corrosion, holes appear on the pipe wall of the inlet pipe. After the water pump operates, the water surface continuously drops. When these holes are exposed to the water surface, air enters the inlet pipe through the holes.
(5) Cracks appear at the bend of the inlet pipe and small gaps appear at the connection between the inlet pipe and the water pump, which may allow air to enter the inlet pipe.
2. The water pump speed is too low
(1) Human factors. A considerable number of users, due to damage to the original motor, randomly install another motor to drive, resulting in low flow rate, low head, and even inability to pump water.
(2) The drive belt is worn. Many large water pumps use belt transmission, but due to long-term use, the transmission belt is worn and loose, resulting in slipping and reducing the speed of the water pump.
(3) Improper installation. The center distance between the two pulleys is too small or the two shafts are not too parallel, the tight edge of the transmission belt is installed on top, resulting in a small wrap angle, incorrect calculation of the diameter of the two pulleys, and a large eccentricity between the two shafts of the water pump in the coupling drive, all of which can cause changes in the speed of the water pump.
(4) Mechanical failure of the water pump itself. The loosening of the fastening nut between the impeller and the pump shaft or the deformation and bending of the pump shaft can cause excessive displacement of the impeller, direct friction with the pump body, or damage to the bearings, which may reduce the speed of the water pump.
(5) Power machine maintenance is not recorded. The motor loses excitation due to the burning of the winding, and changes in winding turns, wire diameter, and wiring methods during maintenance, or factors that do not completely eliminate faults during maintenance, can also cause changes in the speed of the water pump.
3. Suction lift too large
Some water sources are deeper, and the surrounding terrain of some water sources is relatively flat, neglecting the allowable suction head of the water pump, resulting in less or no water absorption at all. It should be noted that there is a limit to the degree of vacuum that can be established at the suction port of the water pump. The suction head under absolute vacuum is about 10 meters water column high, while the water pump cannot establish absolute vacuum. Moreover, excessive vacuum can easily vaporize the water inside the pump, which is detrimental to the operation of the water pump. So each centrifugal pump has its maximum allowable suction head, which is generally between 3 and 8.5 meters. When installing the water pump, it is not only convenient and simple.
4. Excessive resistance loss in the inlet and outlet pipes of water flow
Some users have measured that although the vertical distance from the reservoir or water tower to the water surface of the water source is slightly smaller than the pump head, the water lift is still small or cannot be lifted. The reason is often that the pipeline is too long, there are many bends in the water pipe, and the resistance loss of water flow in the pipeline is too large. The reason is often that the pipeline is too long, there are many bends in the water pipe, and the resistance loss of water flow in the pipeline is too large. In general, a 90 degree bend has greater resistance than a 120 degree bend, with a head loss of approximately 0.5-1 meter per 90 degree bend, and a head loss of approximately 1 meter per 20 meter of pipeline resistance. In addition, some users also choose the diameter of the inlet and outlet pipes of the water pump at will, which also has a certain impact on the head.
5. The impact of other factors
(1) The bottom valve cannot be opened. Usually, due to the prolonged storage time of the water pump, the bottom valve gasket is stuck, and the bottom valve without gasket may rust.
(2) The bottom valve filter mesh is blocked; Or the bottom valve may cause clogging of the filter screen in the sludge layer of the water.
(3) The impeller is severely worn. The impeller blades wear out after long-term use, which affects the performance of the water pump.
(4) Failure or blockage of gate or check valves can result in a decrease in flow rate or even inability to pump water.
(5) The leakage of the outlet pipeline can also affect the water extraction capacity.