1. Structure and working principle of centrifugal pump 1.1 Structure of centrifugal pump The structure of centrifugal pump can be divided into horizontal centrifugal pump and vertical centrifugal pump according to the position of the shaft. At the same time, according to the type of extrusion chamber and suction method. Divided into volute and vane. The centrifugal pump is relatively simple in composition and consists of four parts: the prime mover, the impeller, the pump casing and the shaft seal. The prime mover is the power device of the centrifugal pump, which is usually connected to the pump body through a coupling transmission or other transmission mode to provide kinetic energy; the impeller generally has 6-12 pieces of curved blades, and its main function is to drive the prime mover. The mechanical energy is transmitted to the liquid to be transported; the pump casing, also known as the volute, is a transducing device that collects the liquid thrown by the impeller; the shaft sealing device is a seal between the pump shaft and the pump casing. Its function is to prevent high pressure liquid from leaking from the inside of the pump casing along the circumference of the shaft or outside air to enter the pump casing inward.
1.2 Working principle of centrifugal pump Take common water pump as an example. Before starting, the pump casing should be filled with liquid first. After starting, the impeller rotates at high speed under the motor. When the impeller rotates, the pressure of water at the inlet of the impeller decreases, lower than atmospheric pressure. The pressure of the water along the radial direction of the impeller increases continuously, much higher than the atmospheric pressure, thus forming a certain suction force in the inlet pipe. Under the atmospheric pressure of the outside world, the low water pushes the inlet valve, enters the pump casing along the inlet pipe, and is sucked into the outlet pipe by the impeller. Such low water can be continuously pumped to high places [3].
2. Common Faults and Repair Suggestions for Centrifugal Pumps There are many reasons for centrifugal pump failures. Common equipment faults, installation faults, operational faults, and selection errors. Such as: the pump can not start normally, the pump does not produce water or flow, pump vibration and noise, bearing heat, pump super power, cavitation, etc. [4]. When judging the failure of the centrifugal pump, it should be combined with the basic indicators of equipment status and rich maintenance experience for diagnosis. The following describes some common faults.
2.1 Startup failure 2.1.1
The motor can not start normally. If the motor is used as the prime mover, firstly move the motor cooling fan by hand to see if the rotation is flexible: if it is flexible, it may be the start capacitor failure or capacity reduction. When replacing the same value of the start capacitor; Move, indicating that the rotor is stuck, add grease when cleaning rust, or remove foreign matter from the card rotor.
2.1.2
When the reverse rotation of the pump encounters such a situation, it occurs in the first use. At this time, it should be stopped immediately. If it is a motor, any two phases of the three-phase power supply should be exchanged to change the direction of rotation of the pump. If the diesel engine is used as the power source, The connection of the belt should be considered.
2.1.3
After the centrifugal pump rotates, there is no water, such as normal rotation but no water. Possible reasons are: 1) the suction port is blocked by debris, and the filter device should be installed after cleaning; 2) the suction pipe or the instrument leaks, and the pipe may leak. There are sand holes or cracks, poor sealing of the joint gasket, etc.; 3) the water absorption height is too high, it should be reduced 4) cavitation cavitation occurs; 5) the amount of water injected into the pump is not enough; 6) there is air in the pump, and the emptying method is closed Pump outlet regulating valve, open the loop valve; 7) The water discharge resistance is too large, the length of the water pipe should be checked or the outlet pipe should be cleaned; 8) If the pump speed is not enough, the pump speed should be increased.
2.2 Operation failure 2.2.1
The reasons for insufficient or stop flow are: 1) impeller or inlet and outlet pipes are blocked, the impeller or pipeline should be cleaned; 2) the seal ring and impeller are seriously worn, and the damaged seal ring or impeller should be replaced; 3) the pump shaft speed is low At the specified value, the pump speed should be adjusted to the specified value; 4) the bottom valve is not open enough or the check valve is blocked, the bottom valve should be opened or the parking check valve should be cleaned; 5) the suction pipe is not deep enough to inhale the pump. Air; 6) suction pipe leakage; 7) packing leakage; 8) seal ring wear, should replace the new seal ring or round the impeller, and with a thickened seal ring; 9) the impeller wear is severe; 10) underwater If the sand content is too large, increase the filtration facilities or avoid starting the machine.
2.2.2
Abnormal sound or excessive vibration When the pump is in normal operation, the whole unit should be stable and the sound should be normal. If the unit has noise or abnormal vibration, it is often a precursor to the failure of the pump. It should be stopped immediately to check for hidden dangers. The reasons for the vibration of the pump unit are very complicated. From the perspective of the cause of vibration, there are mainly mechanical, hydraulic, electrical and other aspects. From the mechanism of vibration, there are mainly excessive vibration force, insufficient stiffness, and resonance. The reasons may be:
Mechanical aspects: 1) Impeller balance is not calibrated, when it is corrected immediately; 2) Pump shaft and motor shaft are not concentric, when corrected; 3) Foundation is not strong, arm bracket is not strong, or anchor bolt is loose; 4) Pump or motor The rotor rotates unbalanced.
Hydraulic aspects: 1) The suction stroke is too large, and the impeller inlet generates cavitation; when the water flows through the impeller, bubbles appear in the low pressure zone, and the bubble in the high pressure zone collapses, causing vibration caused by the impact. At this time, the installation height of the pump should be reduced; 2) The pump runs at a non-design point. If the flow is too large or too small, it will cause pressure change or pressure pulsation of the pump. 3) If the pump sucks in foreign matter, blocks or damages the impeller, it should be shut down for cleaning. 4) The shape of the inlet pool is unreasonable. When the several pumps are operated in parallel, the inlet pipe is improperly arranged, and a vortex appears to make the suction condition of the pump deteriorate. The vibration caused by resonance is mainly caused when the natural frequency of the rotor and the rotational speed of the pump are consistent. The above faults should be determined and the corresponding measures should be taken.
2.2.3
When the bearing is overheated, if the bearing is hot, the cause should be checked and dealt with from the following aspects: 1) insufficient lubricant or poor oil circulation; 2) poor quality of the lubricant, impurities make the bearing rust, wear and rotation inflexible; The bearing is seriously worn; 4) the pump is not concentric with the motor; 5) the bearing inner ring and the pump shaft journal are too loose or too tight; 6) the belt is too tight when driven by the belt; 7) the axial thrust is too large, should The dredging of the balance holes on the impellers one by one [3, 5, 6].
2.2.4
Excessive power consumption of the pump If the current meter reading is abnormal and the motor is hot, there may be a pump running at a super power. Possible causes: 1) The rotating part of the pump is rubbed, such as the impeller and the seal ring, the impeller and the shell. 2) pump speed is too high; 3) the specific gravity or viscosity of the transport liquid exceeds the design value; 4) the packing is too tight or the filler does not enter the water; 5) the bearing is worn or damaged; 6) the shaft is bent or the axis is biased Move; 7) The pump runs away from the design point and runs at high flow.
3. Routine maintenance of centrifugal pump 3.1 Precautions for use of centrifugal pump 3.1.1 Preparation before starting to ensure the safe operation of the pump, the necessary inspection should be done before starting the machine: first slowly rotate the coupling or pulley by hand to observe whether the pump is turning Correct, whether the rotation is flexible and stable, whether there is any debris in the pump, whether the bearing is running normally, whether the belt is tight or not; check whether all the screws are strong; check whether there are any debris around the unit that hinders the operation; check whether the submerged depth of the suction pipe is sufficient; The outlet valve should be closed to reduce the starting load, and the valve should be opened in time after starting.
3.1.2 Inspection during operation After starting up, it should be checked whether the various instruments are working properly and stable, and the current should not exceed the rated value. The gauge pointer should be in the design range; check whether the water output of the pump is normal, check whether the parts of the unit are leaking; check the packing tightness. Under normal circumstances, there should be a small amount of leakage at the packing (no more than 10~20 drops per minute), mechanical The leakage of the seal should not be greater than 10 ml / hour (about 3 drops per minute); the rolling bearing temperature should not be higher than 75 ° C; the sliding bearing temperature should not be higher than 70 ° C. And pay attention to the presence or absence of abnormal noise, abnormal vibration, water output reduction; timely adjustment of the intrusion depth of the inlet pipe; often clean the floating debris on the trash rack; through the belt drive, but also pay attention to whether the belt slips.
3.1.3
Precautions after shutdown and shutdown Before shutting down, the water outlet valve should be closed before stopping to prevent water backflow and damage to the machine parts. After each shutdown, the oil stains of the pump body and pipeline should be cleaned in time to keep the unit clean and timely. Hidden dangers are discovered; after winter shutdown, the water should be drained immediately to prevent cracking of the pump body and internal parts; after the end of the season, necessary maintenance is required [7].
3.2 Periodic inspection of centrifugal pumps The periodic inspection of centrifugal pumps can be generally divided into the following three types: 1) daily inspection, ie inspection in use, as described above; 2) monthly inspection, equipment without disassembly of parts The surface is cleaned and minorly repaired, including inspections on bearing temperature, shaft seal leakage and motor insulation; 3) regular maintenance. Including replacement of shaft seal lubricant, check pump and motor alignment, check bushing wear, check coupling rubber ring damage, clean mechanical seal, coolant filter and pump filter, check sliding parts wear, check Corrosion of various parts in contact with liquid [8] and so on.
4. Conclusion The above is a brief summary of the common faults and troubleshooting methods of the centrifugal pump in actual use, and provides the precautions for the centrifugal pump in daily use, hoping to provide practical convenience for everyone's work.