What causes pump vibration? How to eliminate it?

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Vibration is an important indicator for evaluating the operational reliability of pump units. The main hazards of excessive vibration include: vibration preventing the pump unit from operating properly; causing vibration in the motor and pipelines, which can lead to equipment damage and injuries; damaging components such as bearings; causing loose connections, cracks in the foundation, or damage to the motor; causing looseness or damage to the pipes and valves connected to the water pump; and generating vibration noise. There are various reasons that cause pump vibration. The shaft of the pump is generally connected directly to the shaft of the drive motor, which causes the dynamic performance of the pump to interact with that of the motor; there are many high-speed rotating components, and it can be difficult to achieve satisfactory dynamic and static balance; the components that come into contact with the fluid are greatly affected by the flow conditions; the complexity of fluid motion itself is also a factor that limits the stability of the pump’s dynamic performance. 1 Analysis of the causes of pump vibration 　　1.1 Motor Looseness of the motor’s structural components, looseness of the bearing positioning devices, excessive looseness of the silicon steel sheets in the core, and a decrease in the supporting stiffness of the bearings due to wear can all cause vibration. Mass eccentricity, rotor bending, or uneven mass distribution due to issues with the mass distribution of the rotor result in static and dynamic balance values that exceed the specified limits. Additionally, breaks in the cage bars of the squirrel-cage motor rotor can cause an imbalance between the magnetic force acting on the rotor and the rotor’s rotational inertial force, leading to vibration. Vibration can also be caused by issues such as a missing phase in the motor or an imbalance in the power supply across the various phases. In the motor stator windings, quality issues in the installation process lead to uneven resistance among the various phase windings, which results in an uneven magnetic field. This in turn generates unbalanced electromagnetic forces, and such forces act as excitation forces that cause vibration. 　　1.2 Foundation and pump supports: The method of contact fixation between the drive unit frame and the foundation is inadequate; the foundation and motor system have poor capabilities in absorbing, transmitting, and isolating vibrations. As a result, the vibration levels of both the foundation and the motor exceed the permissible limits. If the foundation of the water pump is loose, or if an elastic foundation is formed during the installation of the pump unit, or if the stiffness of the foundation is reduced due to oil immersion or water accumulation, the pump will develop another critical speed that is 180 degrees out of phase with the vibration frequency. This results in an increase in the pump’s vibration frequency; if this increased frequency is close to or equal to the frequency of some external factor, it will lead to an increase in the pump’s amplitude of vibration. Additionally, loose foundation anchor bolts lead to a reduction in constraint stiffness, which intensifies the vibration of the motor. 　　1.3 Couplings: The circumferential spacing of the coupling connection bolts is poor, resulting in a loss of symmetry; if the extension sections of the coupling are eccentric, it will generate eccentric forces; the conical accuracy of the coupling is out of spec; the coupling is not properly statically or dynamically balanced; the fit between the elastic pins and the coupling is too tight, preventing the elastic pins from performing their elastic adjustment function and thus preventing proper alignment of the coupling; the clearance between the coupling and the shaft is too large; mechanical wear of the coupling’s sealing rings leads to a decline in their fitting performance; the quality of the transmission bolts used in the coupling varies. All these reasons can cause vibration. 　　1.4 Impeller ① The impeller mass is eccentric. Poor quality control during the impeller manufacturing process can occur, such as inadequate casting quality or machining precision; or the liquid being transported may be corrosive, leading to erosion of the impeller’s flow channels and thus causing eccentricity in the impeller. ②Whether the number of blades on the impeller, the outlet angle, the wrap angle, the throat partition, and the radial distance from it to the impeller’s outlet edge are appropriate, etc. ③During operation, the wear that occurs between the impeller mouth ring and the pump body mouth ring, as well as between the stage bushings and the partition bushings, evolves from initial impact friction to mechanical frictional wear; this wear tends to increase the vibration of the pump. 　　1.5 Drive shaft and its accessories: In pumps with very long shafts, it is easy to encounter issues such as insufficient shaft stiffness, excessive deflection, and poor straightness of the shaft assembly. This leads to friction between the moving parts (the drive shaft) and the stationary parts (sliding bearings or mouth rings), resulting in vibration. Additionally, the pump shaft is too long, making it highly susceptible to the impact of flowing water in the pool, which increases the vibration of the submerged portion of the pump. An excessive clearance in the balance disc at the shaft end, or improper adjustment of the axial movement amount, can cause low-frequency movement of the shaft, leading to vibration of the shaft bearings. The eccentricity of the rotating shaft can cause bending vibrations of the shaft. 1.6 Pump selection and operation under variable conditions Each pump has its own rated operating point, and whether the actual operating conditions match those designed has a significant impact on the dynamic stability of the pump. The water pump operates stably under design conditions, but when operating under varying conditions, vibration increases due to the radial forces generated within the impeller; this can occur if the pump selected is not appropriate, or if pumps of different models are connected in parallel. All of these will cause vibration in the pump. 　　1.7 Bearings and Lubrication 　　 If the stiffness of the bearings is too low, it will result in a decrease in the first critical speed, thereby causing vibration. Furthermore, poor performance of the guide bearings leads to poor wear resistance and inadequate fixation, as well as excessive clearance in the bearing shells, all of which can easily cause vibration. Wear in the thrust bearings and other rolling bearings, on the other hand, exacerbates both longitudinal movement vibrations and bending vibrations of the shaft. Lubrication failures caused by improper selection of lubricating oil, oil degradation, excessive impurity levels, or blocked lubrication lines can all lead to deterioration in the bearing conditions and trigger vibration. Self-excitation of the oil film in the sliding bearings of electric motors can also cause vibrations. 　　1.8 Pipes and their installation fixation: The stiffness of the supports for the pump’s outlet pipes is insufficient, resulting in excessive deformation; this causes the pipes to press down on the pump body, thereby disrupting the alignment between the pump body and the motor. Excessive force is applied during the installation process, leading to high internal stresses when the inlet and outlet pipes are connected to the pump. The inlet and outlet pipes become loose, reducing their restraining stiffness or even causing it to fail. The outlet duct may break completely, with fragments getting stuck in the impeller. There may also be blockages in the pipes, such as air bubbles at the outlet. The outlet valve may not be properly seated or may not be opened at all. Air may enter at the inlet, leading to an uneven flow pattern and pressure fluctuations. These reasons all lead to vibration in the pumps and pipelines, directly or indirectly. 　　1.9 Fit between components: The concentricity of the motor shaft and the pump shaft is out of spec; a coupling is used at the connection between the motor and the drive shaft, and the concentricity of this coupling is also out of spec; the design tolerances between moving and stationary components (such as between the impeller and the seal ring) become larger due to wear; the gap between the intermediate bearing support and the pump casing exceeds the specified limits; the gap of the sealing ring is not appropriate, resulting in imbalance; uneven gaps around the sealing ring occur when, for example, the seal ring is not seated properly or the partition is not in place. All these adverse factors can cause vibration. 　　1.10 Factors related to the pump itself: The asymmetric pressure field generated as the impeller rotates; vortices in the suction chamber and inlet pipes; the formation and disappearance of vortices inside the impeller as well as in the volute and guide vanes; vibrations caused by vortices resulting from valves being partially open; uneven outlet pressure distribution due to the limited number of impeller blades; flow separation within the impeller; surge; pulsating pressures in the flow channels; cavitation; as water flows through the pump, friction and impacts occur, such as when the water hits the partitions and the leading edges of the guide vanes, causing vibrations; boiler feed pumps used for transporting hot water are prone to cavitation-induced vibrations; pressure fluctuations within the pump are caused mainly by issues with the impeller seal rings – if the gaps around these seals are too large, it leads to significant leakage losses and severe backflow, which in turn results in unbalanced axial forces on the rotor and pressure fluctuations, thereby increasing vibrations. Furthermore, for pumps used to transport hot water, uneven preheating of the pump before startup, or abnormal operation of the pump’s sliding pin system, can lead to thermal expansion of the pump unit, thereby causing severe vibrations during startup. If the internal stresses resulting from thermal expansion and other factors in the pump body cannot be relieved, it will lead to changes in the stiffness of the shaft support system. Resonance occurs when the altered stiffness is an integer multiple of the system’s angular frequency. 　　2 Measures to reduce vibration 　　 2.1 Eliminating vibration at the design and manufacturing stages 　　2.1.1 Points to consider in mechanical structure design 1) Shaft design. Increase the number of support bearings for the drive shaft, reduce the spacing between these bearings, shorten the shaft length within an appropriate range, increase the diameter of the shaft to enhance its stiffness. When the speed of the pump shaft increases and approaches or becomes an integer multiple of the natural vibration frequency of the pump rotor, the pump begins to vibrate violently. Therefore, during design, the natural frequency of the drive shaft should be kept away from the angular frequency of the motor rotor. Improve the quality of manufacturing the shaft to prevent mass eccentricity and excessive geometric tolerances. 　　 2) Selection of sliding bearings. Lubrication-free sliding bearings are used; in chemical pumps that handle liquid hydrocarbons, the materials for such bearings should have good self-lubricating properties, such as polytetrafluoroethylene. In deep well hot water pumps, the guide bushings are made of materials filled with polytetrafluoroethylene, graphite, and copper powder, and their structure is designed carefully to ensure reliable fixation of the sliding bearings. Friction pairs with a low coefficient of friction, such as M20lK graphite material combined with steel, are used at the impeller seal rings and the pump body seal rings. The maximum operating speed is limited, and the load-bearing capacity of the bearing shells as well as the stiffness of the bearing housings are improved. 　　 3) Use a stress relief system. For pumps used to convey hot water, during design, it is necessary to allow for the release of structural stresses between the connecting components resulting from deformation of the pump body; for example, bolt sleeves can be added to the pump body’s foot bolts to prevent the pump body from coming into direct contact with a foundation that has high stiffness. 　　2.12 Considerations for the hydraulic design of water pumps 1) The pump impeller and flow channels should be designed appropriately to minimize cavitation and flow separation within the impeller; parameters such as the number of blades, blade exit angle, blade width, and blade exit crowding factor should be selected carefully to eliminate peaks in the head curve. Regarding the distance between the impeller outlet and the volute baffle, it is believed that when this distance is one-tenth of the impeller’s outer diameter, the pulsating pressure is at its minimum; the exit edge of the blades should also be given an inclination angle (for example, 20°). left and right), to reduce shock; maintain the gap between the impeller and the volute; and improve the pump’s efficiency. At the same time, the outlet flow channels of the pump and other related flow channels are optimized to reduce vibrations caused by hydraulic losses. Properly designing the suction chamber at the inlet section of various pumps, as well as the mechanical structure of the compression stages, to reduce pressure pulses can ensure flow field stability, improve the pump’s efficiency, minimize energy losses, and also enhance the stability of the pump’s vibration dynamics. 2) Cavitation vibration is a very important part of pump vibration. Cavitation accompanied by severe vibration occurs when the inlet pressure of the pump is lower than the saturation pressure at the corresponding water temperature. Measures to reduce cavitation include: when determining the installation height of the water pump, ensuring that the effective NPSH of the system is greater than the minimum required NPSH for the pump; appropriately increasing the diameter of the inlet pipe, shortening its length, reducing the number of piping accessories, minimizing changes in the cross-sectional area along the flow path, and increasing the roughness of the pipe walls; reducing the number of elbows and increasing the angle at which the pipes turn; lowering the operating speed of the pump; using materials resistant to cavitation, such as stainless steel, or applying epoxy resin to areas prone to cavitation; designing the water inlet channel in a way that ensures smoothness, so as to achieve a uniform flow velocity and pressure distribution in the water entering the impeller, thereby avoiding localized low-pressure zones; improving the quality of manufacturing and processing to prevent excessive local flow velocities and high pressure drops caused by inaccurate blade profiles; enhancing the cavitation resistance of the pump system, including installing hydraulic boosters at the pump’s inlet, whose design helps increase the suction head of the pump and thus raises its NPSH; increasing the geometric backflow height; minimizing head losses in the inlet piping; and using double-suction pumps. To ensure that no air accumulates in the suction pipe or pressure pipe, no part of the suction pipe may be higher than the inlet of the water pump. To reduce pressure pulsations at the inlet where water meets the pump, the diameter of the suction pipeline should be one order of magnitude larger than that of the pump inlet, so that the water flow contracts slightly at the pump inlet and the flow velocity distribution becomes more uniform. Additionally, there should be a straight section of pipe in front of the pump inlet, with a length that is at least 10 times the diameter of the pipeline. Pay attention to creating favorable conditions for water inflow; the water flow in the inlet tank should be steady and uniform in order to eliminate vibrations associated with Karman vortices. 3) Design of the foundation. The weight of the foundation should be more than three times the total weight of the machinery such as the pump and motor; the foundation of the water tank must have sufficient strength; it is advisable for the motor mount and the foundation to be integrated or in surface contact; vibration isolation pads or isolators should be installed between the pump and the mount. Furthermore, using vibration-damping materials to connect the pipelines and simplifying the pipeline layout can eliminate vibrations caused by elastic contact and hydraulic losses. 　　 2.2 Eliminating vibration from installation and maintenance processes 　　 1) Shafts and shafting systems. Before installation, check whether the water pump shaft, motor shaft, and drive shaft are bent or deformed, or if there is any eccentricity in their shape. If so, they must be corrected or further processed. Also, examine the drive shaft that comes into contact with the guide bearings to determine whether bending is causing friction against the bearing shells or bushings, which in turn generates forces on the shaft. If monitoring indicates that the shaft has actually bent, correct the pump shaft. At the same time, check the end clearance value of the shaft; if this value is too large, it indicates that the bearing is worn out and needs to be replaced. 2) Impeller. Whether the dynamic and static balance are satisfactory. 3) Coupling. Check whether the bolt spacing is appropriate; the connection between the elastic pin and the elastic sleeve should not be too tight; verify whether the fit between the inner hole of the coupling and the shaft is too loose. If it is, methods such as spraying can be used to reduce the inner diameter of the coupling until it reaches the size required for a proper fit, after which the coupling can be fixed to the shaft. 4) Sliding bearing. Check whether the clearance values meet the standards; ensure proper lubrication in all areas. Improve the quality of the maintenance procedures for the pump’s bearing shells by strictly following the cycle of first scraping the shells, then grinding them, and scraping them again, so as to ensure that the contact area between the bearing shells and the shaft journals meets the specified standards. The clearance value between the pump shaft journal and the bearings should be brought within acceptable limits through methods such as replacing the bearings, grinding, scraping, and adjustment. ②The clearance value between the pump bearing housing and the spherical top of the bearing box is within the acceptable range. ③Contact point and contact angle between the lower bearing shell of the pump shaft and the pump shaft journal: According to standards, the contact area between the back of the lower bearing shell and the bearing housing should be over 60%, the density of contact points on the sliding surface at the journal should be 2 to 4 points per square centimeter, and the contact angle should range from 60° to 90°. 5) Bracket and base plate. Detect fatigue in vibrating supports in a timely manner to prevent a decrease in natural frequency due to reduced strength and stiffness. 6) Gaps and vulnerable parts. Ensure that the bearing clearance of the motor is appropriate; adjust the clearance between the impeller and the volute as needed; regularly inspect and replace wear-prone components such as the impeller collar, pump casing collar, inter-stage bushings, and partition bushings. 　　2.3 Eliminating vibration caused by improper pump selection and operation: When two pumps are operated in parallel, it is necessary to ensure that they have identical performance. The pump performance curve should be of a gradually declining type, with no humps. When in use, it is important to eliminate factors that cause overload of the water pump, such as blockages in the flow channels. The pump should be started for a longer period of time to reduce disturbances to the drive shaft, as well as collisions and friction between rotating and stationary parts, which can lead to thermal deformation. For water-lubricated sliding bearings, sufficient pre-lubricating water should be supplied during startup to avoid dry starting, and the supply of water should be stopped only after water begins to flow out of the pump. Oil should be applied regularly to the bearings that require lubrication. In the case of long-shaft submersible centrifugal pumps, since torsional vibrations occur in the shaft system, if thrust bearings are used, it is these bearings that are most likely to be damaged. In such situations, the viscosity of the lubricating oil can be increased slightly to prevent the breakdown of the hydrodynamic lubrication film. Finally, to prevent excessive pump amplitude, measuring and analyzing the vibration conditions can also be used to determine the optimal operating parameters of the water pump. 　　3 Conclusion The causes of pump vibration include mechanical, hydraulic, and electrical factors. Vibration control takes into account mechanical machining techniques, the skill level of mechanical installation personnel, the competence of water pump operators, the functions of hydraulic design software, the performance characteristics of various materials, and the performance of monitoring instruments. In practical work, eliminating vibration requires a combination of experience and theoretical analysis, integrating the analysis of vibration mechanisms with the data obtained from actual testing instruments. Many vibrations can be eliminated by improving design and installation quality, enhancing operational standards, and strengthening routine maintenance. With the development of new material technologies and new manufacturing processes, as well as advances in computer technology, numerical methods, and the fundamental theories of fluid dynamics, coupled with the emergence and growth of vibration and noise diagnosis techniques, the design, operation, and maintenance of water pumps will continue to improve, their performance will become increasingly optimal, and their dynamic characteristics will become more stable.