Centrifugal pump, as a general-purpose machinery, plays a vital role in many fields such as petroleum, chemical, electric power, metallurgy, mining, textile, shipping and urban water supply and drainage. It relies on the centrifugal force generated by the high-speed rotation of the impeller to transfer mechanical energy to the liquid, so that the liquid obtains energy and realises transport. However, in the process of long-term operation, centrifugal pumps will inevitably have a variety of faults, affecting its normal work, and even lead to production interruption. Therefore, understanding the common faults of centrifugal pumps and mastering effective routine maintenance methods are of great significance to ensure the stable and efficient operation of centrifugal pumps.
一.Common faults of centrifugal pumps
1.Pump does not discharge water
Failure performance: After the centrifugal pump is started, there is no liquid outflow in the pump body and pipeline, and there is no change in the pressure gauge pointer or the pointer swings violently.
Cause analysis
There is air in the pump or the suction line: it may be that the pump is not fully filled before starting, or the suction line is not sealed properly, resulting in air entering the pump. When there is air in the pump, the liquid cannot form a continuous flow path, and thus cannot be transported normally.
Suction line blockage: there may be debris, sediment, fibres and other blockages in the suction line, which impede the suction of liquid. For example, when taking water from some water sources with poor water quality, impurities tend to accumulate in the suction pipe.
The installation height of the pump is too high: centrifugal pumps have a certain permissible suction vacuum height, if the actual installation height exceeds the value, the pressure at the entrance of the pump will be too low, resulting in the vaporisation of the liquid at the entrance of the pump, forming a cavitation phenomenon, which affects the normal suction of the pump.
Impeller damage or reversal: the impeller is the key component of the centrifugal pump, if the impeller is damaged due to long-term operational wear, corrosion or mechanical impact, it will affect its ability to do work on the liquid. In addition, the motor wiring error leads to the reversal of the impeller, which will also prevent the pump from discharging water normally.
2.Insufficient flow
Failure performance: the actual flow rate of the centrifugal pump is lower than the design flow rate, which cannot meet the production demand.
Cause analysis
Insufficient pump speed: The flow rate of the pump is directly proportional to the rotational speed. If the motor speed decreases, it may be due to low power supply voltage, motor failure or slippage of the belt drive device, resulting in the speed of the pump not reaching the rated value, thus reducing the flow rate.
Impeller wear: the impeller is subject to liquid scouring, corrosion and impurity wear during long-term operation, and its blades will be thinned, deformed or even nicked. This will reduce the impeller’s ability to do work on the liquid, so that the pump’s flow and head drop.
Seal ring wear: seal ring is located in the impeller inlet, its role is to reduce the pump high pressure liquid leakage to the low pressure area. When the sealing ring is worn seriously, the leakage in the pump increases, and part of the liquid circulates in the pump, resulting in a reduction of the actual discharge flow.
Pipeline resistance increases: with the increase in the use of time, the inner wall of the pipeline may be scaling, rust, or the pipeline installed too many valves, elbows, these will increase the resistance of the pipeline. According to the performance curve of the pump, when the pipeline resistance increases, the working point of the pump will change and the flow rate will decrease accordingly.
3.Insufficient head
Failure performance: the output liquid pressure of the centrifugal pump does not reach the design head, and the liquid cannot be transported to the specified height.
Cause analysis
Insufficient pump speed: similar to insufficient flow, insufficient speed will lead to a reduction in the centrifugal force exerted by the impeller on the liquid, thus reducing the head of the pump.
Damaged impeller: Damage to the impeller not only affects the flow rate, but also has an effect on the head. When the impeller blades are severely damaged, their ability to do work on the liquid is drastically reduced and they are unable to lift the liquid to the desired height.
Serious internal leakage of the pump: in addition to the internal leakage caused by the wear of the sealing ring, the damage of the sealing gasket between the pump body and the pump cover, and the failure of the sealing device between the pump shaft and the pump body may lead to the leakage of high-pressure liquid in the pump to the low-pressure area, so that the effective head of the pump is reduced.
Change in the weight of the liquid: If the actual weight of the liquid conveyed is different from the weight of the liquid under the design condition, according to the formula for calculating the head of the pump, the head will also change. For example, when the density of the conveyed liquid is less than the design value, the head of the pump will be reduced under the same impeller speed and flow rate.
4.Excessive pump vibration
Failure performance: centrifugal pumps in the operation process, the pump body and pipelines appear obvious vibration, serious cases may lead to loose ground bolts, connecting parts damage, and even affect the stability of the entire unit.
Cause analysis
The pump shaft is not concentric with the motor shaft: when installing the centrifugal pump, if the centre line of the pump shaft and the motor shaft are not aligned, it will cause the pump to generate periodic alternating forces during operation, thus causing vibration. The greater the degree of non-centrifugal, the more violent the vibration.
Bearing damage: the bearing is an important part of the centrifugal pump to support the rotation of the pump shaft. During long-term operation, the bearing may be damaged due to poor lubrication, overload, fatigue and other reasons. When the bearing is damaged, the friction between its rolling body and raceway increases, which will produce abnormal vibration and noise.
Impeller imbalance: The impeller may have uneven quality distribution during the manufacturing process, or the local quality of the impeller may change during operation due to corrosion, wear and tear, which may cause the impeller to produce unbalanced force. This unbalance force will cause the pump vibration, and the vibration frequency is related to the speed of the pump.
The foundation is not solid: If the foundation of the centrifugal pump is not constructed according to the design requirements, there are problems such as insufficient strength and loose ground bolts, which cannot provide stable support for the pump. When the pump is running, the vibration of the foundation will be transferred to the pump body, aggravating the vibration of the pump.
Cavitation phenomenon: when the pressure of the liquid in the pump is lower than its saturated vapour pressure, the liquid will vaporize to form bubbles. These bubbles in the liquid flow to the high-pressure area, will quickly rupture, resulting in local high-frequency water pressure. This cavitation phenomenon will not only damage the impeller, but also cause the pump vibration and noise.
5.Bearing heat
Failure performance: After the centrifugal pump has been running for a period of time, the temperature of the bearing part rises significantly, exceeding the normal operating temperature range, which may lead to bearing burnout in serious cases.
Cause analysis
Poor lubrication: the normal operation of the bearing requires good lubrication. If the amount of lubricant is insufficient, the quality of oil is poor (such as mixed impurities, moisture, etc.) or too much grease, it will affect the lubrication effect of the bearings, so that the bearing friction increases, generating excessive heat, resulting in increased temperature.
Improper installation of bearings: When installing bearings, if the bearings are installed too tightly, it will increase the radial and axial load of the bearings, making the bearings run inflexibly and friction heat. On the contrary, if the bearings are installed too loosely, it will lead to the displacement of the bearings on the shaft, generating bias wear, which will also cause the bearings to heat up.
The pump shaft and motor shaft are not concentric: the pump shaft and motor shaft that are not concentric will make the bearings bear extra radial and axial forces, which will aggravate the wear and heat of the bearings.
Bearing damage: As mentioned above, after the bearing is damaged for various reasons, the friction between the rolling body and the raceway inside it increases, which generates a large amount of heat and leads to an increase in the temperature of the bearing.
二.Routine maintenance of centrifugal pumps
1.Inspection before starting
Check whether the pump body and suction and discharge pipelines are damaged or leaking. Check whether the pipeline connections are firm and whether the ground bolts are tightened. Clean up the sundries around the pump body and pipelines to ensure a clean operating environment.
For centrifugal pumps, pump filling operation must be carried out before starting to ensure that the pump and suction pipeline are full of liquid and exhaust air. You can exhaust through the exhaust valve on the pump body, when there is continuous liquid outflow, indicating that the pump is full of liquid.
Check whether the lubricant oil level in the bearing box is within the normal range, and it should generally be kept at 1/2 – 2/3 of the oil mark. Check whether the oil quality of the lubricant is good, if you find that the oil quality is poor, impurity or emulsification phenomenon, you should replace the lubricant in time.
Check whether the wiring of the motor is correct and firm, and whether the insulation of the motor is good. Check whether the belt tension of the belt drive device is appropriate.
For the coupling drive device, check whether the connection bolts of the coupling are loose and whether the concentricity of the coupling meets the requirements.
2.Monitoring in operation
Through the flow meter, pressure gauge and ammeter installed in the pipeline, real-time monitoring of the pump’s flow, pressure and current values. Compare these parameters with the design parameters of the pump to determine whether the pump is running under normal operating conditions.
Regularly touch the pump body and bearing parts by hand to feel the vibration of the pump. You can also use the vibration meter to quantitatively measure the vibration of the pump, and the general vibration value of the centrifugal pump should be controlled within the specified range. At the same time, pay attention to listen to the sound of the pump running, under normal circumstances, the pump running sound should be smooth and uniform. If you hear abnormal noise, such as friction sound, impact sound, vapour erosion sound, etc., you should stop the machine immediately for inspection.
Monitor the temperature of the bearings: Use thermometer or temperature sensor to measure the temperature of the bearings, generally the normal working temperature of the rolling bearings should not exceed 70℃, and the normal working temperature of the sliding bearings should not exceed 65℃. If the temperature of the bearing is too high, you should check the lubrication, whether the bearing is installed correctly and whether the pump shaft and the motor shaft are concentric, etc., and take corresponding measures to cool down the temperature in time.
3.Maintenance after shutdown
When shutting down, the outlet valve of the pump should be closed first, and then the inlet valve should be closed. This will prevent the liquid in the pipeline from flowing backward and causing damage to the pump.
After shutdown, clean the pump body, pipeline, motor and transmission device, etc., to remove surface dirt, dust and oil. Supplementary lubrication or replacement of grease for bearings, and timely replacement of worn parts. At the same time, check whether the connecting parts of the pump are loose, if loose, it should be tightened in time.
According to the use of the centrifugal pump and the manufacturer’s recommendations, make a regular maintenance and overhaul plan. Regularly carry out comprehensive inspection and maintenance of the pump, including inspection and replacement of key parts such as impeller, sealing ring, bearing, shaft, etc., and test and adjust the performance of the pump to ensure that the pump is always in good running condition.
Centrifugal pumps as an indispensable equipment in industrial production, the timely diagnosis and effective treatment of its common failures as well as the conscientious implementation of the daily maintenance work is of great significance to ensure the continuity of production, improve production efficiency and reduce production costs. By analysing the common failures of centrifugal pumps, we can strengthen monitoring in daily operation, discover potential problems in time, and take corresponding maintenance measures to extend the service life of centrifugal pumps and ensure their safe, stable and efficient operation. At the same time, operators and maintenance personnel should continue to improve their professional knowledge and skill level, and accumulate experience in order to better cope with the various problems arising during the operation of centrifugal pumps.
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