Brief Analysis of Common Faults in Centrifugal Pumps and Daily Maintenance

Thread Content

1. Structure and working principle of centrifugal pumps 1.1 Structure of centrifugal pumps The structure of centrifugal pumps can generally be divided into two main categories based on the position of the shaft: horizontal centrifugal pumps and vertical centrifugal pumps. Additionally, they can be classified as volute-type or vane-type depending on the design of the discharge chamber and the suction method. The structure of a centrifugal pump is relatively simple, consisting mainly of four parts: the prime mover, the impeller, the pump casing, and the shaft sealing device. The prime mover is the power unit of a centrifugal pump; it is usually connected to the pump body through a coupling or some other transmission mechanism in order to provide kinetic energy ； An impeller typically has 6–12 backward-curving blades, whose primary 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 a device for converting energy, and it collects the liquid ejected by the impeller ； The shaft seal device provides sealing between the pump shaft and the pump casing. Its function is to prevent high-pressure liquid from leaking out of the pump casing along the shaft, as well as to prevent outside air from entering the pump casing. 1.2 Working principle of centrifugal pumps: Taking a common water pump as an example, before starting, the pump casing must first be filled with liquid. After startup, the impeller rotates at high speed driven by the electric motor. As the impeller rotates, the pressure of the water at its inlet decreases, falling below atmospheric pressure, while the pressure of the water along the radius of the impeller increases significantly, far above atmospheric pressure. This creates a certain suction force in the inlet pipe. Under the external atmospheric pressure, the water at the lower level pushes open the inlet valve, enters the pump casing through the inlet pipe, and is then thrown out through the outlet pipe by the impeller. In this way, water from lower areas can be continuously pumped to higher areas. 2. Common faults of centrifugal pumps and repair suggestions. There are various reasons for the failure of centrifugal pumps; common ones include inherent defects in the equipment, installation issues, operational problems, and incorrect selection of the pump model. Examples include: the pump failing to start properly, not pumping water or having insufficient flow, vibration and noise from the pump, bearing overheating, the pump operating above its rated power, and cavitation. When diagnosing faults in centrifugal pumps, it is necessary to combine the basic indicators of the equipment’s condition with extensive maintenance experience; the following outlines some common faults. 2.1 Starting failures 2.1.1 Motor fails to start properly If the motor is used as the driving mechanism, first manually operate the motor’s cooling fan to check whether it rotates smoothly; if it does, the problem may lie in a failed or depleted starting capacitor, and in such cases a starting capacitor with the same specifications should be replaced ； If it cannot rotate, it means the rotor is stuck; clean off the rust, apply lubricant, or remove any foreign objects that are causing the rotor to get stuck. 2.1.2 Reverse rotation of the pump: This situation often occurs during the first use of the pump. In such cases, the pump should be stopped immediately. If it is an electric motor, swapping any two of the three phase powers can change the direction of rotation of the pump; if it is powered by a diesel engine, then the belt connection method needs to be considered. 2.1.3 If no water flows out when the centrifugal pump is turned on, and it rotates properly but no water is discharged, possible reasons include 1) the suction inlet being blocked by debris; this should be cleared and a filtering device installed ； 2) Air leakage from the intake pipe or instruments may be caused by weld leaks, sand holes or cracks in the pipes, or poor sealing of the gaskets at the joints ； 3) The water absorption height is too high; it should be reduced. 4) Cavitation occurs in the impeller ； 5) Insufficient water flow from the injection pump ； 6) There is air in the pump; to remove it, close the pump outlet control valve and open the circuit valve ； 7) The outlet resistance is too high; check the length of the water pipe or clean the outlet pipe ； 8) The pump speed is insufficient; the pump speed should be increased. 2.2 Operational faults 2.2.1 Insufficient flow or stoppage may be caused by: 1) Blockage in the impeller or inlet/outlet pipes; the impeller or pipes should be cleaned ； 2) The sealing ring and impeller are severely worn; the damaged sealing ring or impeller should be replaced ； 3) If the pump shaft speed is below the specified value, the pump speed should be adjusted to the specified value ； 4) If the bottom valve is not opened enough or the check valve is blocked, the bottom valve should be opened or the machine should be stopped to clean the check valve ； 5) The submersion depth of the suction pipe is insufficient, allowing air to be drawn into the pump ； 6) Air leakage in the suction pipe ； 7) Packing air leakage ； 8) If the sealing ring is worn, it should be replaced with a new one, or the impeller should be rounded off and equipped with a thicker sealing ring ； 9) Severe wear of the impeller ； 10) If the sand content in the water is too high, filtering facilities should be added or operation should be avoided. 2.2.2 Abnormal noise or excessive vibration: When the pump is operating normally, the entire unit should be stable, and the noise level should be normal. If the unit produces noise or abnormal vibrations, it is often a sign of a problem with the water pump; the machine should be stopped immediately for inspection in order to eliminate potential hazards. The causes of vibration in water pump units are complex. In terms of the factors that trigger vibration, these mainly include mechanical, hydraulic, and electrical aspects; as for the mechanisms behind vibration, they primarily involve excessive excitation forces, insufficient stiffness, and resonance. The possible reasons are: Mechanical aspects: 1) The impeller balance is not calibrated; it should be corrected immediately ； 2) The pump shaft is not aligned with the motor shaft; correction is required when… ； 3) The foundation is not solid, the arm support is not secure, or the anchor bolts are loose ； 4) The rotor of the pump or motor is unbalanced in rotation. Hydraulic aspects: 1) Excessive suction lift, causing cavitation at the impeller inlet ； As the water flow passes through the impeller, bubbles form in the low-pressure area; these bubbles burst in the high-pressure area, resulting in shocks that cause vibration. In such cases, the installation height of the pump should be reduced ； 2) When the pump operates outside its designed conditions, with flow rates that are too high or too low, it can cause pressure changes or pressure fluctuations in the pump ； 3) If foreign objects are drawn into the pump, causing the impeller to get blocked or damaged, the machine should be stopped for cleaning. 4) The shape of the water inlet tank is unreasonable; especially when several pumps operate in parallel, an improper layout of the inlet pipes leads to vortices that deteriorate the suction conditions for the pumps. Vibrations caused by resonance occur mainly when the natural frequency of the rotor matches the speed of the pump. It is necessary to identify the causes of such faults and take appropriate measures to resolve them. 2.2.3 Bearing overheating: During operation, if the bearing is hot to the touch, the cause should be investigated and addressed from the following aspects: 1) Insufficient amount of lubricating oil, or poor oil circulation ； 2) Poor quality of lubricating oil; impurities cause rusting, wear, and poor flexibility in the bearings ； 3) Severe bearing wear ； 4) The pump and motor are not aligned ； 5) The fit between the inner ring of the bearing and the pump shaft journal is either too loose or too tight ； 6) The belt is too tight when using belt drive ； 7) Due to excessive axial thrust, the balance holes on each impeller should be cleared one by one. 2.2.4 Excessive power consumption by the pump: If the ammeter reading is abnormally high or the motor heats up during operation, it is possible that the pump is operating beyond its rated capacity. Possible reasons include: 1) Friction occurring in the rotating parts of the pump, such as between the impeller and the seal ring, or between the impeller and the casing ； 2) Pump speed is too high ； 3) The specific gravity or viscosity of the liquid being transported exceeds the design values ； 4) The packing is compressed too tightly or no water enters the stuffing box ； 5) Bearing wear or damage ； 6) Shaft bending or axis deviation ； 7) The pump operates away from its design point and runs at high flow rates. 3. Daily maintenance of centrifugal pumps 3.1 Precautions for using centrifugal pumps 3.1.1 Preparations before starting up To ensure the safe operation of the pump, necessary checks should be carried out before starting it: first, manually rotate the coupling or pulley slowly to check whether the pump rotates in the correct direction, whether its rotation is smooth and steady, whether there are any foreign objects inside the pump, whether the bearings are functioning properly, and whether the belt is tight enough ； Check that all screws are tight ； Check for any debris around the unit that could interfere with its operation ； Check whether the submersion depth of the suction pipe is sufficient ； If there are outlet valves, they should be closed to reduce the starting load, and care should be taken to open them promptly after startup. 3.1.2 Checks during operation: After startup, it is necessary to check whether all instruments are functioning properly and stably, and that the current does not exceed the rated value. The pressure gauge pointer should be within the designed range ； Check whether the water output of the pump is normal, and check if there is any leakage in various parts of the unit ； Check the degree of packing compression; under normal circumstances, there should be a slight amount of leakage at the packing site (no more than 10–20 drops per minute), while the leakage rate for mechanical seals should not exceed 10 milliliters per hour (about 3 drops per minute) ； The temperature of rolling bearings should not exceed 75℃ ； The temperature of the sliding bearing should not exceed 70°C. Also pay attention to any unusual noises, abnormal vibrations, and a decrease in water output ； Adjust the submersion depth of the water inlet in a timely manner ； Regularly clean the debris from the trash racks ； For those driven by belts, it is also necessary to pay attention to whether the belt is slipping. 3.1.3 Shutdown and precautions after shutdown: Before shutting down the machine, the outlet valve should be closed first to prevent water from flowing back and damaging the components ； After each shutdown, the oil stains on the pump body and pipelines should be wiped clean promptly to keep the exterior of the unit clean and to identify potential problems early ； After shutting down the system in winter, the water should be drained immediately to prevent the pump body and its internal components from cracking due to freezing ； After the end of the usage season, necessary maintenance must be carried out. 3.2 Periodic inspection of centrifugal pumps The periodic inspection of centrifugal pumps can generally be divided into the following three types: 1) Routine inspections, that is, inspections during operation, as mentioned above ； 2) Monthly inspection: Cleaning and minor repairs are carried out on the exterior of the equipment without removing any components, including checks on bearing temperatures, the causes of shaft seal leaks, and the insulation condition of the motor ； 3) Regular maintenance. This includes replacing the lubricating oil in the shaft seal, checking the alignment of the pump and motor, inspecting the wear on the shaft sleeves, examining any damage to the rubber rings of the couplings, cleaning the mechanical seal, coolant filter, and pump filter, checking the wear on the sliding components, and assessing any damage or corrosion on all parts that come into contact with liquids. 4. Conclusion The above content provides a brief summary of the common faults that occur in centrifugal pumps during actual use and the methods for troubleshooting them. It also offers guidelines for proper use of centrifugal pumps in daily operations, with the aim of offering practical assistance to those working with such pumps.