Why Can't Centrifugal Pumps Lift Water

Jan 15, 2025

There is air in the inlet pipe and pump body

(1) Some users do not fill enough water before starting the water pump; Sometimes it appears that the injected water has overflowed from the vent hole, but the pump shaft has not been rotated to completely discharge the air, 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 descending slope of at least 0.5% against the direction of water flow. The end connected to the inlet of the water pump should be the highest and not completely horizontal. If lifted upwards, air will remain in the inlet pipe, reducing the vacuum level in the water pipe and pump, and affecting water absorption.

(3) The packing of the centrifugal pump has worn out or become 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 pump through these gaps, affecting water lifting.

(4) The inlet pipe has holes due to long-term potential underwater corrosion on the pipe wall. After the water pump works, the water level continuously drops. When these holes are exposed to the water surface, air enters the inlet pipe through the holes.

(5) Cracks at the bend of the inlet pipe and small gaps at the connection between the inlet pipe and the water pump may allow air to enter the inlet pipe.

The speed of the water pump is too low

(1) Human factors. A considerable number of users, due to the damage of the original motor, randomly install another motor to drive it, 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 may become loose and slip, thereby 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 very parallel, the tight edge of the transmission belt is installed on top, resulting in a small wrap angle, calculation errors in the diameter of the two pulleys, and a large eccentricity between the two shafts of the water pump in the coupled transmission, all of which can cause changes in the speed of the water pump.

(4) Mechanical failure of the water pump itself. Loosening of the fastening nut between the impeller and the pump shaft or deformation and bending of the pump shaft can cause excessive movement of the impeller, direct friction with the pump body, or damage to the bearings, all of which may reduce the speed of the water pump.

(5) Improper maintenance of the power machine. The motor loses magnetism due to burnt windings, and changes in winding turns, wire diameter, and wiring methods during maintenance, or incomplete troubleshooting during maintenance, can also cause changes in the speed of the water pump.

The suction stroke of the water pump is too large

Some water sources are deeper, while others have flatter terrain around them, neglecting the allowable suction range of the water pump, resulting in less or no water absorption. It should be noted that there is a limit to the vacuum that can be established at the suction port of a water pump. The suction range under absolute vacuum is about 10 meters of water column, and a water pump cannot establish absolute vacuum. Moreover, excessive vacuum can cause the water inside the pump to vaporize, which is detrimental to the operation of the water pump. So all kinds of centrifugal pumps have their maximum allowable suction range, generally between 3 and 8.5 meters. When installing the water pump, it is important not to just focus on convenience and simplicity.

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 source surface is slightly less than the pump head, the amount of water they can lift is still small or they cannot lift water. 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, the resistance of a 90 degree bend is greater than that of a 120 degree bend. The head loss of each 90 degree bend is about 0.5-1 meter, and the resistance of every 20 meters of pipeline can cause a head loss of about 1 meter. In addition, some users also arbitrarily choose the diameter of the inlet and outlet pipes of the water pump, which also has a certain impact on the head.

The influence of other factors that prevent centrifugal pumps from lifting water

1) The bottom valve cannot be opened. Usually, it is due to the water pump being left idle for too long, causing the gasket of the bottom valve to stick, and the bottom valve without a gasket may rust.

2) The bottom valve filter is blocked; Or the bottom valve may cause filter blockage in the potential sludge layer in the water.

3) The impeller is severely worn. The wear and tear of impeller blades over long-term use has affected the performance of the water pump.

4) Malfunctions or blockages in gate valves or check valves can result in reduced flow or even inability to pump water.

5) Leakage in the outlet pipeline can also affect the amount of water extracted.