Common faults and routine maintenance of centrifugal pumps

1. Centrifugal pump structure and working principle

1.1 Structure of the centrifugal pump

The structure of the centrifugal pump can be basically divided into horizontal centrifugal pump and vertical centrifugal pump according to the position of the shaft, and can be divided into a volute type and a vane type according to the type of the extrusion chamber and the suction method. 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

Taking a 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 the water at the inlet of the impeller decreases, lower than the atmospheric pressure, and the water along the radial direction of the impeller. The pressure is constantly rising, much higher than the atmospheric pressure, so that a certain suction is formed 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 a high place.

2. Common faults and repair suggestions for centrifugal pumps

There are various reasons for the failure of the centrifugal pump. Common equipment failures, installation failures, operational failures, 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 and so on. 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 does not start normally

If the motor is used as the motive device, firstly move the motor cooling fan by hand to see if the rotation is flexible: if it is flexible, it may be the starting capacitor failure or the capacity reduction. When replacing the same value of the starting capacitor; if it does not move, the rotor stuck, the iron rust after lubricating grease cleaning, foreign matter, or rotor clearance card.

2.1.2 Reverse rotation of the pump

In the case of such a situation, it should be used for the first time. 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 powered, the belt should be considered. Connection method.

2.1.3 No water after the centrifugal pump rotates

If the rotation is normal but there is no water, the possible causes 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, may leak from the weld, the pipe has blisters or cracks, and the joint 3) The gasket is not sealed properly; 3) The water absorption height is too high, which should be reduced 4) The impeller 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 to close the pump outlet regulating valve and open Circuit 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 Insufficient flow or stop

The possible reasons are: 1) the impeller or the inlet and outlet pipes are blocked, the impeller or pipeline should be cleaned; 2) the seal ring and the impeller are seriously worn, and the damaged seal 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 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 flooded enough to suck the air inside the pump; 6) 7) leakage of the suction pipe; 7) leakage of the packing; 8) wear of the sealing ring, replacement of the new sealing ring or rounding of the impeller, and with a thickened sealing ring; 9) serious impeller wear; 10) sand content in the water Large, should increase the filtration facilities or avoid booting.

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 bearing overheating

During operation, if the bearing is hot, the cause should be checked and processed 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; 3) bearing Severe wear; 4) the pump and the motor are not the same; 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, the impeller should be one by one Unblocking the balance hole

2.2.4 Pump power consumption is too large

If the current meter reading is abnormal during the operation of the pump and the motor is hot, there may be a pump running at a super power. Possible causes are: 1) friction occurs in the rotating part of the pump, such as impeller and seal ring, impeller and housing; 2) pump speed Too high; 3) the specific gravity or viscosity of the transport liquid exceeds the design value; 4) the packing is too tight or the water in the stuffing box does not enter the water; 5) the bearing is worn or damaged; 6) the shaft is bent or the axis is offset; 7) the pump is running Deviate from the design point to run at high flow rates.

3. Routine maintenance of the centrifugal pump

3.1 Precautions for use of centrifugal pumps

3.1.1 Preparation before starting up

In order to ensure the safe operation of the pump, the necessary checks should be made before starting the machine: firstly turn the coupling or pulley slowly, observe whether the pump is turning correctly, whether the rotation is flexible and stable, whether there is any debris in the pump, whether the bearing is running normally, the belt Whether the tightness is suitable; check whether all the screws are strong; check whether there are any debris around the unit that hinders the operation; check whether the suction pipe is deep enough; if there is a water outlet valve to close, reduce the starting load, and pay attention to open the valve in time after starting.

3.1.2 Inspection during operation

After starting up, check 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 stopping the machine, the water outlet valve should be closed before stopping to prevent water backflow and damage to the machine parts. After each stop, the oil stains of the pump body and pipeline should be cleaned in time to keep the appearance of the unit clean and timely to identify hidden dangers; The water should be drained immediately to prevent cracking of the pump body and internal parts; after the end of the season, the necessary maintenance is required.

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, that is, inspection in use, as described above; 2) monthly inspection, cleaning and minor repairs on the exterior of the equipment without disassembling parts, including Inspection of 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 Each component that comes into contact with the liquid damages the corrosion, and the like.

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