1. The cause of abnormal vibration or abnormal sound of marine centrifugal pump
(1) Vibration caused by mechanical reasons
1 Vibration caused by unbalanced rotor or uneven mass. It is characterized by large vibrations occurring at the two bearings. When the centrifugal pump speed rises to near the rated speed, its amplitude increases greatly. Trouble-shooting: Re-dismantling the centrifugal pump will damage the rotating parts and perform dynamic balance test on the rotor. After correcting the installation, the vibration caused by the cause can be eliminated.
2 The marine pump is poorly lubricated. Features: When the vibration occurs, the machine has abnormal shaking and noise. Troubleshoot Check the lubricant condition of the pump, the bearing clearance, and replace if there is any problem.
3 The vibration caused by the misalignment between the marine pump shaft and the motor shaft set center. It is characterized by an increase in vibration as the pump load increases, and an instantaneous change in vibration occurs from time to time. Troubleshooting: Check the alignment of the pump set and re-align the motor if necessary.
4 Long-term operation causes vibration caused by the displacement of the pump. Its characteristics: After a long period of operation, the vibration of the pump may cause the anchor bolt to loosen, the copper pad of the pump body to slip or the center of the pump shaft to shift, causing vibration during operation. Trouble-shooting: Check the pre-tightening of the anchor bolts, the displacement of the pump body, and re-tighten or align if there is any problem.
5 Vibration caused by defective basic components (such as insufficient stiffness and strength of the base and bracket). The characteristic is that the vibration frequency of the pump and the non-frequency of the supporting component cause the vibration of the pump to be moderate, and the vibration frequency of the pump and the frequency of the supporting component may cause resonance, resulting in more serious consequences. Trouble-shooting: Pay attention to the quality of the product, increase the number of supporting parts and increase the rigidity and strength, and increase the weight of the base to make it have better firmness.
(2) Vibration caused by hydraulic causes
1 Vibration caused by cavitation. When the centrifugal pump is working, its suction vacuum must be less than the allowable vacuum of the pump. Otherwise, the liquid may be vaporized in the pump, and “cavitation” may occur. Before the liquid sucked by the marine centrifugal sea water pump flows from the suction port of the pump to the inlet of the blade to increase the energy, the flow velocity is increased due to the flow area after the impeller, and the flow velocity is uneven, and the flow resistance loss is added to further the pressure. decline. If the pressure of the water drops to a saturated vapor pressure or lower, it will vaporize, and the gas originally dissolved in the water will escape, which will generate many bubbles. These bubbles flow with the liquid to the high pressure zone where the vapor rapidly condenses and the gas re-dissolves into the water, creating a partial vacuum at many points. At this point, the surrounding liquid quickly replenishes this vacuum during the mutual impact of the liquid particles, causing local pressures of up to tens of MPa. The greater the bubble, the greater the local water hammer pressure. These collapses on the metal surface of the impeller, causing the liquid particle to impact the metal surface at a high speed. The frequency of the hydraulic shock velocity is as high as 600 Hz to 2500 H, the flow head and efficiency of the pump are reduced, and the metal surface is peeled off due to fatigue. Since the gas bubbles are also mixed with other active gases (such as oxygen), the heat released during the condensation of the bubbles causes the local temperature to rise to 200 ° C - 300 ° C, causing electrochemical corrosion of the metal and speeding up the destruction of the metal. This phenomenon is called "cavitation". Cavitation creates noise and causes strong vibrations in the pump.
2 The vibration of the centrifugal pump flow is too small. During the operation of the centrifugal pump, the pressure and flow rate will change, and the surge will be emitted. This is because the centrifugal pump works below the minimum design flow. The internal pressure of the liquid is reduced, the gas is vaporized, and the unstable condition is generated. The intense fluctuations caused severe vibration of the pipeline. Therefore, in order to avoid such failures, the flow rate of the centrifugal pump during normal operation should not be lower than the minimum flow specified by the design.
2. Treatment method and preventive measures for abnormal vibration of marine centrifugal pump
(1) Strict control of pump design, installation, pipeline construction, commissioning, etc.
(2) Calculate the critical speed value of the centrifugal pump to avoid running at the critical speed and resonance.
(3) By performing dynamic and static equilibrium experiments on the rotor of the centrifugal pump, the pump is kept in equilibrium during operation.
(4) Longer pipelines should be pumped with positive pressure as much as possible to avoid cavitation of the pump due to low pressure vaporization.
(5) Pay attention to the problem of thermal expansion and contraction of the pipeline when installing the centrifugal pump, and eliminate the thermal stress by adding a thermal compensator to the pipeline.
(6) Ensure that the base and bracket of the pump have sufficient rigidity and strength to ensure good stability of the pump during operation.
(7) Regularly check the tightness of the anchor bolts, check the corrosion or blockage of the impeller regularly, periodically check the gap size of the bearing, the bending of the bearing, the packing sealing condition, the conveying pressure, the connection of the pipeline, and the lubrication. Check the oil level and quality.
(8) After the new installation of the centrifugal pump or after a long period of operation, check the coaxial alignment of the coupling of the motor shaft and the pump shaft.
