1. the speed of multistage centrifugal pump is too low
(1)artificial factors. Some users, because of the damage of the original motor, are randomly equipped with another motor to drive, resulting in small flow rate, low head or even no water.
(2)Mechanical failure of multistage centrifugal pump itself. The loose unfastening of impeller and the fastening nut of pump shaft or the deformation and bending of pump shaft cause the impeller to move more directly to the pump body or the bearing is damaged, which may reduce the rotational speed of the pump.
(3)The power engine is not working well. The speed of the pump will be changed because of the loss of magnetic field due to the burnout of the winding, the change of the connection method of the winding turns in the maintenance, or the failure of thorough elimination of the fault in the maintenance.
2. Air in inlet and pump body of multistage centrifugal pump
(1) the multistage centrifugal pump is not filled with enough water before it is started. Sometimes it appears that the water filled has overflowed from the vent, but the pump shaft has not been rotated completely, resulting in a little air remaining in the intake pipe or pump body.
(2) the inlet pipe contacted by multistage centrifugal pump has a descending slope of more than 0.5% in the reverse flow direction of the horizontal section, and the inlet end of the multistage centrifugal pump is the highest, but not completely horizontal. If up, the intake pipe will retain air, reducing the vacuum in the water pipe and pump, affecting water absorption.
(3) the packing of the multistage centrifugal pump has been worn out for a long time or the packing pressure is too loose, resulting in a large amount of water being ejected from the gap between the packing and the shaft sleeve of the pump. As a result, the external air enters the water pump from these gaps into the inside of the pump. It affects the lifting of water.
(4) because of the long-term potential underwater, holes appear in the pipe wall corrosion, and the water surface of the multistage centrifugal pump continuously drops after the work. When these holes are exposed to the water surface, the air enters the intake pipe from the holes.
(5) Cracks appear at the elbow of intake pipe and tiny gap between inlet pipe and pump, which may make air enter the inlet pipe.
3. The resistance loss in the inlet and outlet pipes of multistage centrifugal pump is too large
Some users have measured that although the vertical distance from the reservoir or water tower to the water surface is slightly smaller than that of the centrifugal pump head, the amount of water raised is still small or unable to lift water. The reason is that the pipe is too long, the pipe has many bends, and the resistance of the flow in the pipe is too large. The reason is that the pipe is too long, the pipe has many bends, and the resistance of the flow in the pipe is too large. In general, the resistance of 90 degree bend is greater than that of 120 degree bend. The head loss of every 90 degree bend is about 0.5-1 meters, and the resistance of every 20 meters pipe can cause the head loss to be about 11 meters. In addition, some users also pump in and out of the pipe diameter, these also have a certain impact on the head.
4.The suction range of multistage centrifugal pump is too big
Some water sources are deeper and some water sources are flat on the periphery, and the allowable suction distance of centrifugal pump is ignored, which results in less water absorption or no water absorption at all. It is necessary to know that there is a limit to the vacuum degree that can be established at the suction point of the pump. The suction distance of the absolute vacuum is about 10 meters high, while the centrifugal pump cannot establish the absolute vacuum. And the vacuum degree is too big, easy to make the water vapor in the pump, to the pump work unfavorable. So each centrifugal pump has its maximum allowable suction, generally between 3-85 meters. Water pump installation should not only be convenient and simple.