Cause analysis of centrifugal water pump not lifting water

Date of issue:[2020-08-17]     Click through rate:    

Centrifugal pump has become the most widely used pump in agriculture because of its simple structure, convenient use and maintenance, and high efficiency, but it is also troublesome because it can't lift water sometimes. This paper analyzes the cause of the failure.

There is air in the inlet pipe and pump body

(1) Some users didn't fill up enough water before starting the pump. Sometimes it seems that the filled water has overflowed from the vent hole, but the pump shaft is not rotated to discharge the air completely, resulting in a little air remaining in the water inlet pipe or pump body.

(2) The horizontal section of the water inlet pipe in contact with the water pump should have a downward gradient of more than 0.5% against the water flow direction. The end connecting the water pump inlet is the highest and should not be completely horizontal. If it is tilted upward, air will be left in the water inlet pipe, which will reduce the vacuum degree in the water pipe and water pump and affect the water absorption.

(3) The packing of the pump has been worn or the packing is too loose due to long-term use, resulting in a large amount of water ejected from the gap between the packing and the shaft sleeve of the pump. As a result, the external air enters the interior of the pump from these gaps, affecting the water lifting.

(4) Because of the long-term potential underwater, there are holes in the pipe wall corrosion of the water inlet pipe. After the water pump works, the water surface continuously drops. When these holes come out of the water surface, the air enters the civil water inlet pipe through the holes.

(5) Cracks at the elbow of the water inlet pipe and a small gap between the water inlet pipe and the water pump may cause air to enter the water inlet pipe.

Pump speed too low

(1) Human factors. There are quite a number of users due to the damage of the original motor, so they are randomly equipped with another motor to drive. As a result, the flow rate is small, the head is low and even no water is supplied.

(2) The drive belt is worn. There are many large centrifugal pumps using belt conveyor, because of long-term use, the transmission belt is worn and loose, out of the slip phenomenon, thus reducing the speed of the pump.

(3) Improper installation. If the center distance between the two pulleys is too small or the two shafts are not parallel, the belt is too close to the top, resulting in too small wrap angle, the calculation error of the diameter of the two pulleys and the large eccentricity of the two shafts of the coupling drive water pump, all of these will cause the change of the pump speed.

(4) Mechanical failure of water pump itself. Loose fastening nut of impeller and pump shaft or deformation and bending of pump shaft cause impeller moving more, direct friction with pump body, or bearing damage, which may reduce the speed of water pump.

(5) The power machine doesn't work well. The motor loses its excitation due to the burning of its winding. The change of winding turns, wire diameter and wiring method in the maintenance, or the failure is not completely eliminated in the maintenance, will also change the speed of the water pump.

The suction range is too large

Some of the water sources are deeper, and some of them are flat on the periphery, but the allowable suction head of the pump is ignored, resulting in little or no water absorption at all. It is necessary to know that the vacuum degree that can be established at the suction port of the pump is limited. The suction head of absolute vacuum is about 10 meters water column high, and it is impossible for the pump to establish absolute vacuum. Moreover, if the vacuum is too large, it is easy to make the water in the pump gasified, which is not conducive to the work of the pump. Therefore, each centrifugal pump has its maximum allowable suction, generally between 3-8.5 meters. When installing the water pump, it is not only convenient and simple.

The resistance loss in the inlet and outlet pipes of water flow is too large

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 less than the lift of the pump, the water is still small or can not be lifted. The reason is that the pipeline is too long and there are many bends in the pipe, and the resistance loss of water flow in the pipeline is too large. The reason is that the pipeline is too long and there are many bends in the pipe, and the resistance loss of water flow in the pipeline is too large. In general, the resistance of 90 degree elbow is greater than that of 120 degree elbow. The head loss of each 90 degree elbow is about 0.5-1 m, and the head loss of every 20 m pipe is about 1 m. In addition, some users also randomly pump in and out of the pipe diameter, which also has a certain impact on the lift.

Influence of other factors

(1) The bottom valve cannot be opened. Usually due to the pump shelving time is too long, the bottom valve gasket is stuck, no gasket bottom valve may rust.

(2) The filter net of bottom valve is blocked; or the filter screen is blocked in the sludge layer of bottom valve potential water.

(3) The impeller is seriously worn. The wear of impeller blade after long-term use affects the performance of water pump.

(4) The failure or blockage of the check valve of gate valve will reduce the flow rate and even fail to pump water.

(5) Leakage in the outlet pipe will also affect the water withdrawal.

Technology Department

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