Brief analysis of common faults and routine maintenance of centrifugal pumps

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1. Structure and working principle of centrifugal pump 1.1 Structure of centrifugal pump The structure of centrifugal pump can basically be divided into two categories: horizontal type and vertical type according to the position of the shaft. At the same time, it can be divided into volute type and guide vane type according to the type of pressure chamber and suction method. The centrifugal pump is relatively simple and mainly consists of four parts.: Prime mover, impeller, pump casing and shaft seal device. The prime mover is the power device of the centrifugal pump. It is generally connected to the pump body through coupling transmission or other transmission methods to provide kinetic energy. ； There are generally 6-12 backward curved blades in the impeller. Its main function is to transfer the mechanical energy of the prime mover to the liquid being transported. ； The pump casing, also known as the volute, is an energy-transfer device that collects the liquid thrown by the impeller. ； The shaft seal device is the seal between the pump shaft and the pump casing. Its function is to prevent high-pressure liquid from leaking from the pump casing along the periphery of the shaft or outside air from entering the pump casing inwardly. 1.2 The working principle of a centrifugal pump takes a common water pump as an example. Before starting, the pump casing should be filled with liquid. After starting, the impeller rotates at a high speed driven by the motor. When the impeller rotates, the pressure of the water at the entrance of the impeller decreases and is lower than the atmospheric pressure, while the pressure of the water along the radius of the impeller continues to increase, much higher than the atmospheric pressure. In this way, a certain suction force is formed in the water inlet pipe. Under the external atmospheric pressure, the water at the lower level pushes open the water inlet valve, enters the pump casing along the water inlet pipe, and is thrown in and out of the water pipe by the impeller. In this way, water from lower areas can be continuously pumped to higher areas. 2. Common faults and repair suggestions of centrifugal pumps There are various reasons for centrifugal pump faults. Common ones include inherent faults of equipment, installation faults, operating faults and selection errors. like: The pump cannot start normally, the pump does not discharge water or has insufficient flow, pump vibration and noise, bearing heating, pump overpower, cavitation, etc. When judging centrifugal pump faults, basic indicators of equipment status and rich maintenance experience should be used for diagnosis. The following introduces some common faults. 2.1 Startup failure 2.1.1 The motor cannot start normally. If the motor is used as the prime mover, first turn the motor cooling fan by hand to see if the rotation is flexible.: If it is flexible, it may be that the starting capacitor has failed or its capacity has been reduced. When replacing the starting capacitor with the same value ； If it cannot rotate, it means that the rotor is stuck. After cleaning the rust, add lubricating grease or remove the foreign matter stuck in the rotor. 2.1.2 When the water pump reversely rotates, this kind of situation usually occurs when it is used for the first time. At this time, it should be stopped immediately. If it is an electric motor, any two phases of the three-phase power supply should be exchanged to change the rotation direction of the water pump. If a diesel engine is used as the power, the connection method of the belt should be considered. 2.1.3 No water comes out after the centrifugal pump rotates. If the rotation is normal but no water comes out, the possible reasons are: 1) The suction inlet is blocked by debris, which should be removed and a filter device installed. ； 2) The suction pipe or instrument is leaking, which may be caused by welding, blisters or cracks in the pipe, poor gasket sealing at the joint, etc. ； 3) The water absorption height is too high and should be reduced. 4) Cavitation occurs in the impeller. ； 5) The amount of water injected into the pump is not enough ； 6) If there is air in the pump, the emptying method is to close the pump outlet regulating valve and open the circuit valve. ； 7) The water outlet resistance is too large. You should check the length of the water pipe or clean the water outlet pipe. ； 8) The water pump speed is not enough, the water pump speed should be increased. 2.2 Operation failure 2.2.1 The possible reasons for insufficient flow or stop are:: 1) The impeller or the inlet and outlet pipes are blocked. The impeller or pipes should be cleaned. ； 2) The sealing ring and impeller are seriously worn. The damaged sealing ring or impeller should be replaced. ； 3) The pump shaft speed is lower than the specified value. The pump speed should be adjusted to the specified value. ； 4) The bottom valve is not opened enough or the check valve is clogged. You should open the bottom valve or stop the car to clean the check valve. ； 5) The suction pipe is not submerged deep enough, causing air to be sucked into the pump. ； 6) Air leakage in suction pipe ； 7) The packing is leaking ； 8) The sealing ring is worn. It should be replaced with a new sealing ring or the impeller should be rounded and equipped with a thickened sealing ring. ； 9) The impeller is seriously worn ； 10) If the sand content in the water is too large, filtering facilities should be added or the machine should be avoided. 2.2.2 Abnormal sound or excessive vibration When the water pump is operating normally, the entire unit should be stable and the sound should be normal. If the unit has noise or abnormal vibration, it is often a precursor to water pump failure, and it should be stopped immediately for inspection to eliminate hidden dangers. The causes of vibration of water pump units are very complex. From the perspective of the causes of vibration, there are mainly mechanical, hydraulic, electrical and other aspects. From the perspective of the vibration mechanism, the main factors include excessive vibration force, insufficient stiffness, and resonance. The reasons may include mechanical aspects: 1) The impeller balance is not calibrated, please correct it immediately ； 2) The pump shaft and the motor shaft are not concentric. When correcting ； 3) The foundation is not strong, the arm bracket is not strong, or the anchor bolts are loose. ； 4) The rotor of the pump or motor rotates unbalanced. Hydraulic aspects: 1) The suction lift is too large and cavitation occurs at the impeller inlet. ； When the water flows through the impeller, bubbles appear in the low-pressure area. The bubbles collapse in the high-pressure area, causing impact and vibration. At this time, the installation height of the pump should be lowered. ； 2) If the pump operates at non-design points and the flow rate is too large or too small, it will cause pressure changes or pressure pulsations in the pump. ； 3) If the pump absorbs foreign matter and blocks or damages the impeller, it should be shut down and cleaned. 4) The shape of the inlet pool is unreasonable, especially when several water pumps are running in parallel, the water inlet pipeline is improperly arranged, and a vortex appears, which deteriorates the suction conditions of the water pump. Vibration caused by resonance is mainly generated when the natural frequency of the rotor is consistent with the speed of the water pump. The above fault causes should be judged and corresponding measures should be taken to solve them. 2.2.3 When the bearing is overheated, if the bearing is hot to the touch, the cause should be investigated and dealt with from the following aspects:: 1) Insufficient lubricating oil or poor oil circulation ； 2) The quality of lubricating oil is poor, and impurities make the bearings rust, wear and rotate inflexibly. ； 3) Bearing wear is serious ； 4) The pump and motor are not concentric ； 5) The fit between the inner ring of the bearing and the journal of the pump shaft is too loose or too tight. ； 6) When using belt transmission, the belt is too tight ； 7) If the axial thrust is too great, the balance holes on the impeller should be cleared one by one. 2.2.4 The pump consumes too much power. If the ammeter reading is abnormal and the motor is hot during pump operation, it may be that the pump is running at overpower. Possible reasons: 1) Friction occurs in the rotating parts of the pump, such as the impeller and the sealing ring, the impeller and the casing ； 2) Pump speed is too high ； 3) The specific gravity or viscosity of the conveyed liquid exceeds the design value ； 4) The packing is pressed too tightly or water does not enter the stuffing box. ； 5) Bearing wear or damage ； 6) Shaft bending or axis offset ； 7) The pump operation deviates from the design point and operates at a large flow rate. 3. Routine maintenance of centrifugal pumps 3.1 Precautions for using centrifugal pumps 3.1.1 Preparation before starting up To ensure the safe operation of the water pump, necessary inspections should be made before starting up: First, slowly turn the coupling or pulley by hand to observe whether the water pump is turning correctly, whether the rotation is flexible and stable, whether there are any debris in the pump, whether the bearings are operating normally, and whether the belt is tight and appropriate. ； Check that all screws are secure ； Check whether there are any debris around the unit that may hinder its operation. ； Check whether the submersion depth of the suction pipe is sufficient ； If there is a water outlet valve, it should be closed to reduce the starting load, and attention should be paid to opening the valve in time after starting. 3.1.2 Inspections during operation After starting up, check whether various instruments are working normally and stably, and the current should not exceed the rated value. The pressure gauge pointer should be within the design range ； Check whether the water output of the water pump is normal and whether there is water leakage in all parts of the unit ； Check the tightness of the packing. Normally there should be a small amount of leakage at the packing (no more than 10 to 20 drops per minute), and the leakage of the mechanical seal should not be greater than 10 ml/hour (about 3 drops per minute). ； The rolling bearing temperature should not be higher than 75℃ ； The sliding bearing temperature should not be higher than 70℃. And pay attention to whether there are abnormal noises, abnormal vibrations, or reduced water output. ； Timely adjust the submersion depth of the water inlet pipe ； Frequently clean floating objects on the sewage rack ； If it is driven by a belt, pay attention to whether the belt is slipping. 3.1.3 Precautions for shutdown and after shutdown. Before shutting down, the water outlet valve should be closed before shutting down to prevent water from flowing back and damaging the machine parts. ； After each shutdown, the oil stains on the pump body and pipelines should be wiped off in time to keep the unit's appearance clean and detect hidden dangers in time. ； After shutting down in winter, the water should be drained immediately to prevent the pump body and internal parts from freezing and cracking. ； After the use season is over, necessary maintenance must be performed. 3.2 Periodic inspection of centrifugal pumps Periodic inspections of centrifugal pumps can generally be divided into the following three types:: 1) Routine inspections, i.e. in-use inspections, as described above ； 2) Monthly inspections, cleaning and minor repairs on the appearance of the equipment without disassembling parts, including inspections of bearing temperature, causes of shaft seal leakage, and motor insulation conditions, etc. ； 3) Regular maintenance. This includes replacing the shaft seal lubricating oil, checking the alignment of the pump and motor, checking the wear of the shaft sleeve, checking the damage of the coupling rubber ring, cleaning the mechanical seal, coolant filter and pump filter, checking the wear of the sliding parts, checking the damage and corrosion of the components in contact with the liquid, etc. 4. Conclusion The above is only a brief summary of the common faults and troubleshooting methods of centrifugal pumps in actual use, and provides precautions for daily use of centrifugal pumps, hoping to provide practical convenience for everyone's work.