In the huge system of modern industry, centrifugal pumps are widely used in petroleum, chemical, electric power, metallurgy, food, pharmaceutical and many other fields, and are the key equipment to achieve fluid transport. In the petrochemical industry, centrifugal pumps are responsible for crude oil, refined oil products and a variety of chemical raw materials in different production links to ensure the continuity of the production process; in the electric power industry, it provides a stable feed water for the boiler to ensure the normal operation of power generation equipment; in the wastewater treatment plant, the centrifugal pump is responsible for lifting sewage, transported to the treatment unit of the important task of the role of environmental protection is indispensable.
However, like any mechanical equipment, centrifugal pumps in the long-term operation process, there will inevitably be a variety of faults, timely and accurate troubleshooting centrifugal pumps, and take effective solutions to protect the safety of industrial production, stability and efficient operation of the vital significance. Next, we will explore the centrifugal pump common failure types, causes and corresponding measures to solve the problem.
一.Common Faults and Strategies
Starting faults and solutions
Failure to start or high starting load
Centrifugal pumps can’t start or have a large starting load, which may be caused by a variety of factors:
Abnormal prime mover or power supply: This is one of the more common reasons. Power supply failures such as blackout, too low or too high voltage, line short circuit, disconnection, etc. may cause centrifugal pumps to fail to obtain normal power supply and thus fail to start. The prime mover failure, such as motor winding burnt, motor bearing damage, motor rotor stuck, etc., will also make the centrifugal pump can not start normally. When this kind of situation occurs, you need to carefully check the power supply and prime mover, use professional tools such as multimeter to check whether the power supply voltage is within the normal range, check whether the line is broken, short-circuit and other problems; for the prime mover, you need to check the insulation resistance of the motor windings, judge whether there is a burnt situation, check whether the motor bearings are rotating flexibly and whether there is any jamming phenomenon.
Pump jamming: damage to the pump parts, foreign objects into the pump body may lead to pump jamming. For example, solid particles entered between the impeller and the pump casing, or the key of the impeller is loose or falling off, resulting in relative movement between the impeller and the shaft, which may prevent the pump from rotating normally at startup. At this time, you can check the coupling by hand disc, if it feels difficult or impossible to disc, it is necessary to further disassemble and check to find out the failure points of the dynamic and static parts, remove foreign objects, repair or replace the damaged parts.
Packing too tight: packing seal is one of the commonly used sealing methods for centrifugal pumps, if the packing is pressed too tight, it will increase the friction between the pump shaft and the packing, resulting in increased starting load, or even unable to start. In this case, the packing should be appropriately relaxed so that the pump shaft can be easily rotated, and at the same time, pay attention to control the degree of tightness of the packing to ensure that it can play a good sealing role, but also will not cause excessive resistance to the pump startup and operation.
Discharge valve not closed: In the centrifugal pump start, if the discharge valve is in the open state, the pump needs to overcome the resistance of the piping system to discharge the liquid, which will lead to a significant increase in the starting load. Therefore, before starting the centrifugal pump, the discharge valve should be closed, and then gradually open the discharge valve to adjust the flow rate to the required value after the pump starts normally.
Balance tube is not smooth: the role of the balance tube is to balance the pressure on both sides of the impeller of the centrifugal pump to reduce the axial force. If the balance pipe is blocked or not smooth, it will lead to pressure imbalance on both sides of the impeller, increase the starting load, and even make the pump unable to start. In this case, it is necessary to unclog the balance pipe to ensure that it is unobstructed, so that the pressure on both sides of the impeller can maintain balance.
Motor can not start normally
When the centrifugal pump takes the electric motor as the prime mover, the motor can not start normally is also one of the common failures:
Starting capacitor failure or capacity reduction: for some single-phase motors with capacitor start, the starting capacitor is a key component to help the motor start. If the starting capacitor fails or capacity is reduced, the motor will not be able to get enough starting torque during starting, resulting in the motor not starting properly. At this time, you can use a capacitance tester to detect the capacity of the starting capacitor, if you find that the capacitance does not meet the requirements, you should promptly replace the same specifications of the starting capacitor.
Rotor is stuck: motor rotor is stuck may be due to bearing damage, motor internal foreign objects into the motor, the motor is not used for a long time resulting in rust and other reasons. Firstly, pluck the motor cooling fan by hand to check whether the rotation is flexible. If it does not turn, the rotor may be stuck, you need to dismantle the motor, clean the rust, add lubricating grease, or remove the foreign matter stuck rotor, so that the rotor can resume normal rotation.
Pump reverse rotation
When the centrifugal pump is used for the first time, sometimes the pump will rotate in the reverse direction, which may be caused by wiring error or other reasons:
Motor-driven centrifugal pumps: If an electric motor is used as the power source, simply switch the wiring of any two phases of the three-phase power supply to change the direction of rotation of the electric motor, thus correcting the direction of rotation of the water pump. When operating, be sure to ensure that the wiring is adjusted in the case of power failure to avoid electric shock accidents.
Diesel-powered centrifugal pumps: If the centrifugal pump is powered by a diesel engine, you need to consider the way the belt is connected. It may be that the belt is installed incorrectly or the pulley is not properly selected to cause the pump to reverse. At this time, it is necessary to re-check the installation of the belt to ensure that the belt is correctly connected to the diesel engine and the water pump pulley, and the belt tension is moderate; if it is an improper selection of the pulley, it should be based on the requirements of the diesel engine and the water pump’s rotational speed, select the appropriate diameter of the pulley to ensure that the water pump can rotate normally.
Operational Troubleshooting and Solutions
Non-discharge
Centrifugal pumps do not discharge liquid during operation, which will seriously affect their normal work, and there are various reasons for this failure:
Insufficient pump filling or gas in the pump is not discharged: centrifugal pumps need to be filled before starting to eliminate the air in the pump. If the pump filling is not enough, the pump is still residual air, due to the density of the air is much smaller than the liquid, the centrifugal force generated by the impeller rotation is not enough to suck the liquid into the pump, resulting in non-exhaustion of liquid. At this point, you need to re-fill the pump operation to ensure that the pump is full of liquid, and open the exhaust valve, the pump will be completely discharged.
Pump speed is too low: the speed of the pump directly affects its ability to discharge liquid, if the speed is too low, the centrifugal force generated by the impeller is insufficient to discharge the liquid effectively. The pump speed should be checked to see if it meets the design requirements, and a tachometer can be used to measure the actual speed of the pump. If the rotational speed is too low, it may be due to motor failure, belt slippage, unstable power supply frequency and other reasons. For motor failure, it is necessary to overhaul or replace the motor; for belt slippage, adjust the belt tension or replace the belt; for unstable power supply frequency, the power supply system should be checked to ensure that the power supply frequency is stable within the specified range.
Strainer clogging, bottom valve malfunction: The strainer and bottom valve at the suction port are important parts to prevent debris from entering the pump body, but if the strainer is clogged with debris or the bottom valve malfunctions, it will hinder the suction of the liquid, resulting in no liquid discharge. At this time, it is necessary to check the strainer, remove the clogged debris, and ensure that the strainer is clear; at the same time, check the working condition of the bottom valve, and if it is damaged, the bottom valve should be repaired or replaced in a timely manner.
Suction height is too high, or suction tank vacuum: centrifugal pump suction height is limited, if the suction height is too high, more than the pump’s allowable suction height, the liquid can not be sucked into the pump. In addition, if there is a vacuum in the suction tank, it will also result in the liquid not being able to enter the pump normally. In this case, the suction up height should be reduced to ensure that it is within the permissible range of the pump; at the same time, check the pressure in the suction tank and, if necessary, take measures to increase the pressure in the suction tank, e.g., by increasing the level of the liquid and improving ventilation.
Interruption after liquid discharge
Insufficient flow
Insufficient flow rate of centrifugal pump will affect the productivity, which is mainly due to the following reasons:
Increase in system static head: System static head refers to the difference in vertical height of the liquid from the suction level to the discharge level. If the system static head increase, such as the height of the transported liquid, the resistance of the discharge pipeline increases, etc., will lead to an increase in the actual head of the pump, while the flow rate decreases accordingly. At this time, it is necessary to check the height of the liquid and the system pressure, and analyse the reasons for the increase in the static head of the system. If the conveying height rises, consider increasing the head of the pump; if the resistance of the discharge pipeline increases, you should check whether the pipeline is clogged, whether the valve is not completely open and other issues, and timely cleaning and adjustment.
Increase in resistance loss: blockage of pipelines and check valves and other components, valve failure, scaling of the inner wall of the pipeline may lead to an increase in resistance loss, thus making the pump flow insufficient. It is necessary to check the pipeline and check valve and other parts, clear the blockage, repair or replace the faulty valve. For the case of serious scaling on the inner wall of the pipeline, chemical cleaning or physical cleaning can be used to remove the scaling and reduce the resistance of the pipeline.
Excessive wear of the casing and impeller wear ring: after long-term use of the casing and impeller wear ring, the gap will increase due to wear and tear, so that the liquid inside the pump leaks, thus reducing the flow of the pump. When it is found that the shell and impeller wear ring is worn out too much, the wear ring and impeller should be replaced or repaired in time to restore their normal clearance and performance. When replacing the wear-resistant ring and impeller, choose the appropriate materials and specifications to ensure that their quality and performance meet the requirements.
Leakage from other parts: Leakage from shaft seal and other parts will also lead to insufficient flow. The role of the shaft seal is to prevent the pump from leaking liquid, if the shaft seal is damaged or poorly sealed, the liquid will leak from the shaft seal, so that the flow of the pump is reduced. The sealing condition of the shaft seal and other parts should be checked, if the shaft seal is found damaged, the shaft seal should be replaced in time; for the case of lax sealing, the degree of compression of the shaft seal can be adjusted or the seal can be replaced.
Pump impeller clogging, wear and corrosion: impeller is the core component of centrifugal pump, if the impeller is clogged by debris, wear or corrosion, it will affect its normal work, resulting in insufficient flow. The impeller should be cleaned to remove the clogged debris; for the impeller that is badly worn or corroded, it should be inspected and exchanged. When cleaning the impeller, care should be taken to avoid damaging the blades of the impeller; when replacing the impeller, choose products with the same specifications and models as the original impeller.
Centrifugal pumps, as an indispensable equipment in industrial production, its normal operation is crucial for the stability and efficiency of the whole production system. We understand that failures are often the result of a combination of factors, including the structure and performance of the equipment itself, the norms of installation and operation, the stability of the operating environment and the timeliness of routine maintenance. For different types of failures, we also proposed a corresponding solution. In order to reduce the occurrence of failures, we also emphasise the importance of routine maintenance, operation norms and regular maintenance to prolong its service life.
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