|Useful Tips | Centrifugal Pump Operation and Troubleshooting Methods

Thread Content

Operation of centrifugal pumps: 1. Before starting the pump, check the condition of the equipment and its components, ensure that the bolts are properly tightened, verify the lubrication of the bearings (add oil if there is a lack of it, and replace it if it has deteriorated), and check for any leaks in the sealing devices. Fill the pump chamber with liquid and close the outlet valve. 2. While driving, one should always pay attention to and check the operation of the pump, regularly checking: the temperature changes of the bearings, whether there are any noises or vibrations, whether the motor is overheating (not exceeding 80°C), whether the motor load is normal, and any leaks at the sealing surfaces. If any problems are detected, the machine should be stopped immediately for maintenance. 3. Before shutting down, the outlet valve should be closed first, and then the machine should be stopped. If it is winter, the water inside the pump and the pipes should be drained after shutdown to prevent the centrifugal pump from being damaged by freezing. 4. Regular maintenance. Generally, centrifugal pumps should be thoroughly overhauled every 6 to 12 months (this can be done in conjunction with major equipment repairs). During maintenance, the pump is disassembled, and the impeller, bearings, shafts and shaft sleeves, water seal tubes, stuffing boxes, pump casing, etc. are thoroughly cleaned. After cleaning the bearing, if wear, cracks, looseness, or pitting on the balls are detected, a new bearing should be installed; if it is in good condition, new lubricant should be applied ； If the shafts and bushings are worn, they can be repaired by welding followed by machining ； If the shaft is bent, it should be straightened ； Worn or corroded impellers can be repaired or replaced (a rebalancing test must be conducted after repair) ； Replace the packing in the seal box. After regular maintenance and assembly, a restart of the trial operation is required. 5. Conditions for emergency pump shutdown: ① Sudden intense vibration or a clear sound of metal friction inside the pump. ② When the bearing smokes or its temperature rises sharply to 75 degrees, and refilling oil does not help. ③ When a high-pressure pipeline ruptures and endangers the safety of people or equipment. ④ The motor is smoking or on fire. ⑤ When vaporization or water loss occurs suddenly, the outlet pressure and current drop sharply, resulting in large fluctuations. Repair of centrifugal pumps 1. Repair of the rotor The rotor of a centrifugal pump is mainly composed of the impeller, main shaft, and shaft sleeve, etc. Before repair, the rotor must be disassembled, cleaned, and inspected; careful attention should be paid to the wear of the impeller, the radial runout (not exceeding 0.05 mm), and the degree of eccentricity ； Wear and bending of shafts and bushings, etc. ① Repair of the main shaft: The pump’s main shaft may become bent or cracked due to vibrations that occur during rotation or as a result of sudden impacts ； The surface of the shaft develops severe dents and wear due to friction or corrosion caused by the conveyed medium, resulting in radial runout or a decrease in strength, which affects its usability and necessitates repair. Slight surface damage can be smoothed out by polishing with oil-soaked sandpaper. The shaft needs to be straightened when it is bent ； Axes with very severe damage can be repaired using methods such as welding, spraying, electroplating followed by grinding. After repair, the geometric accuracy and radial runout need to be retested (not exceeding ±0.05mm). ② Repair of the shaft sleeve: During operation of the pump, friction at the area where the shaft sleeve comes into contact with the packing causes wear and radial runout. For less severe wear, repair can be carried out by welding and then turning; however, shaft sleeves with severe wear need to be replaced. ③ Repair of the impeller: The impeller is one of the vulnerable components of a pump. As it rotates, friction and corrosion can cause severe damage, leading to changes in its size and shape that affect the pump’s performance; in such cases, it is necessary to repair or replace it promptly. For impellers with less severe damage, they can be repaired by patch welding. During repair, the impeller is first heated to 600°C, then rewelded, with care being taken to cool it slowly after welding to prevent the formation of new cracks. Cool it down and then turn it to the original size. If a loose key is detected, the keyway can be widened or re-cut if possible (at 90° or 180° to the original keyway), and a new key fitted. If the impeller has severe sand holes or is heavily damaged, it needs to be replaced. The repaired impeller needs to be re-balanced. For the repaired impeller, the end-face deflection shall not exceed 0.2 mm, and the radial runout at the outer edge of the impeller and the seal ring shall also not exceed 0.05 mm. 2. Repair of balance devices: Most multi-stage centrifugal pumps are equipped with thrust balance devices, which are used to counteract the axial thrust resulting from uneven pressures on both sides of each impeller, thereby reducing the axial movement of the rotor. During operation, the balance disc and balance ring (usually made of bronze) wear out due to frequent contact; if there are solid impurities such as sediment in the medium, the wear is even more severe. If the balance disc and balance ring are worn too much (usually over 2mm), they should be repaired. For those with less severe wear, the worn grooves can first be planed flat, then ground and reassembled. During assembly, shims of the appropriate thickness can be added behind the balance ring to restore it to the original thickness. Asbestos gaskets can be used, but the thickness of the gaskets must be uniform and even. If the wear is excessive, a new part needs to be replaced. During assembly, it is required that the surfaces of the balance disc and balance ring make tight contact (with an axial clearance of 0.1–0.25 mm), and the misalignment between their working end faces relative to the pump shaft’s centerline should not exceed 0.03 mm. Additionally, if the sealing ring on the pump casing is severely worn, it should be replaced promptly. Any damage to the sealing device should be repaired promptly to prevent liquid leakage and affect the pump’s performance. Common faults of centrifugal pumps and their troubleshooting methods: Some faults often occur in centrifugal pumps during use and need to be resolved. To troubleshoot a fault, it is first necessary to know the cause of the fault, and then take appropriate action to address it. 1. No water is pumped out after startup① Air is still present inside the pump. Liquid can be slowly poured in through the filling hole on the pump cover (to facilitate air expulsion). ② If there is leakage at the suction pipe fittings or flanges, it can be fixed by tightening them properly. ③ If the pump’s speed is low, check the motor speed or whether the voltage is too low. ④ The suction height of the pump is too high, exceeding the pump’s suction range, or the suction pipe inlet is not submerged in the water tank. ⑤ The motor reverses direction. ⑥ The pump’s impeller is moving and cannot function. ⑦ The water-absorbing mesh is blocked and cannot absorb water. 2. Insufficient water output from the pump after startup ① The impeller is blocked; the pump should be disassembled for cleaning. ② Air enters the pump body, and the vent valve can be opened to release air. ③ The pump’s speed is low. ④ The gap between the impeller and the seal ring inside the pump is too large, or the impeller is worn or loose, and the packing gland seal is not proper. ⑤ The outlet valve is not opened enough. ⑥ There is a problem with the design. 3. Insufficient water discharge volume: Air is trapped in the pump; check whether the suction pipe and stuffing box are leaking. 4. The pump stops delivering water suddenly after starting up. ① The suction pipe connection becomes loose, allowing air to enter, or there is an air bubble in the suction pipe. ② Water was poured into the pump too quickly, and the air was not exhausted. 5. Excessive motor load ① Improper assembly and installation of the pump (such as overly tight packing, misalignment of the axial centerlines of the pump and motor, friction between the impeller and the casing, etc.). ② The pump’s feed water height is too high or the flow rate is too high. ③ The pump’s rotational speed is too high. 6. Others ① Vibration during pump operation may be caused by the pump’s axis not aligning with that of the motor, bending of the main shaft, loose fastening bolts, or eccentricity of the impeller. ② The bearing is overheating; it may be due to a lack of oil or misalignment with the motor shaft. ③ Abnormal sound. It may be due to air entering or vibrations, as well as misalignment between the pump shaft and the motor shaft. Diagnosis and Remedial Measures for Pump Failures – Diagnosis and Remedial Measures for Centrifugal Pumps 1. No liquid available, insufficient liquid supply, or low pressure. 2. The pump stops operating after running for a short time. 3. Excessive power consumption by the pump. 4. Severe leakage from the pump’s packing gland. 5. Excessively high bearing temperature. 6. Overheating of the packing gland. 7. Difficult rotation of the rotating components or friction present