10 reasons for excessive pump vibration

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Whether a pump vibrates abnormally is an important indicator for assessing the reliability of its quality. There are many reasons that can cause vibration in multi-stage pumps; factors such as water flow conditions, the complexity of fluid movement, dynamic and static balance, as well as high-speed rotating components, can all affect the stability of the pump. Below is a comprehensive analysis of the causes of pump vibration. 10 reasons for vibration: 1. Shaft – The pump shaft is quite long, and issues such as insufficient stiffness of the pump, excessive deflection, or poor alignment of the shaft system can lead to friction between the moving parts (the drive shaft) and the stationary parts (sliding bearings or thrust rings), thereby causing vibration in the pump. A too-long pump shaft is also subject to greater impact from the flowing water in the tank, which increases the vibration in the underwater portion of the multi-stage pump. Excessive clearance in the pump shaft’s balance disk, or improper adjustment of the axial movement, can also result in low-frequency shaft movement and thus vibration of the bearing shells. Eccentricity of the rotating shaft can likewise cause bending vibrations in the shaft. 2. Foundation and pump support: The method of contact fixation used between the drive unit frame and the foundation is inadequate; as a result, the foundation and the motor system have poor capabilities for absorbing, transmitting, and isolating vibrations. This leads to excessive vibration in both the foundation and the motor. The foundation of the water pump may become loose, or an elastic foundation may form during the installation process of the pump unit. Additionally, reduced foundation stiffness due to oil immersion or water exposure can cause the pump to develop another critical speed that is 180 degrees out of phase with the vibration frequency, thereby increasing the pump’s vibration frequency. If this increased frequency is close to or equal to the frequency of some external factor, it will result in an increase in the amplitude of vibration in the multi-stage pump. Furthermore, loose foundation anchor bolts, which reduce the restraining stiffness, can exacerbate motor vibration. 3. Coupling: Poor circumferential spacing of the coupling’s connection bolts leads to a loss of symmetry; eccentricity in the coupling’s extension sections generates centrifugal forces. If the conical accuracy of the coupling is not up to standard, or if the coupling lacks proper static or dynamic balance, the fit between the elastic pins and the coupling may be too tight, preventing the coupling from being properly aligned. An excessive clearance between the coupling and the shaft can also cause problems. Mechanical wear of the coupling’s sealing rings reduces their performance. Additionally, the quality of the transmission bolts used in the coupling may vary from one another. All these reasons can cause vibration in multi-stage pumps. 4. Factors related to the water pump itself: the asymmetric pressure field generated during the rotation of the impeller ； Vortices in the water intake tank and inlet pipe ； The formation and disappearance of vortices inside the impeller, as well as in the volute and guide vanes ； Vibrations caused by swirls resulting from the valve being partially open ； Uneven outlet pressure distribution due to the limited number of impeller blades ； Flow separation within the impeller; surge; pulsating pressure in the flow channel ； cavitation ； Water flows within the pump body, causing friction and impact on it; for example, the water strikes the partition ribs and the leading edges of the guide vanes, resulting in vibrations ； Boiler feed pumps that transport high-temperature water are prone to cavitation vibration ； Pressure fluctuations within the pump chamber are primarily caused by the sealing rings of the pump impeller and the pump casing; excessive gaps in these sealing rings lead to significant leakage losses and severe backflow, which in turn results in an imbalance of axial forces on the rotor and pressure fluctuations, thereby increasing vibrations. Furthermore, for stainless steel hot water pumps used to transport hot water, uneven preheating of the pump before startup, or abnormal operation of the pump’s sliding pin system, which leads to thermal expansion of the pump unit, can cause severe vibrations during startup ； If the internal stresses resulting from thermal expansion and other factors in the pump body cannot be relieved, it will cause a change in the stiffness of the shaft support system. Resonance occurs when the resulting stiffness is an integer multiple of the system’s angular frequency. 5. Motor: Looseness in the motor’s structural components, looseness in the bearing positioning devices, excessive looseness of the silicon steel sheets in the core, and reduced support 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 in turn results in an uneven magnetic field. This creates unbalanced electromagnetic forces, and such forces act as excitation forces that induce vibration. 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 pump’s dynamic stability. The water pump operates stably under design conditions, but when running under varying conditions, vibration increases due to the radial forces generated within the impeller. This can also occur if the pump selected is not appropriate, or if pumps of different models are connected in parallel. All of these can cause vibration in multi-stage pumps. 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 low wear resistance and inadequate fixation; excessive clearance in the bearing shells can also cause vibration. Meanwhile, wear in the thrust bearings and other rolling bearings exacerbates both longitudinal movement vibration and bending vibration of the shaft. Lubrication failures caused by improper selection of lubricating oil, oil deterioration, excessive impurity levels, or blocked lubrication pipelines can all lead to a deterioration in the operating conditions of bearings, resulting in vibration. Self-excitation of the oil film in the sliding bearings of electric motors can also cause vibration. 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, disrupting the alignment between the pump body and the motor. During installation, excessive force is applied to the pipes, leading to high internal stresses when the inlet and outlet pipes are connected to the pump. This results in loose connections between these pipes, a decrease in their restraining stiffness or even its complete loss. In some cases, the entire outlet passage breaks apart, with fragments getting stuck in the impeller, thereby obstructing flow. Issues such as air bubbles at the outlet, a malfunctioning outlet valve, or a valve that isn’t fully open, as well as air entering at the inlet, can all lead to uneven flow patterns and pressure fluctuations. All of these factors can directly or indirectly cause vibration in multi-stage pumps and their piping systems. 9. Fit between components: The concentricity between 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, but the concentricity of this coupling is also incorrect. The design tolerances between moving and stationary components (such as between the impeller ring and the seal ring) wear out over time. The gap between the intermediate bearing support and the pump casing is excessive, and the gap of the sealing rings is not appropriate, all of which lead to imbalance. Uneven gaps around the sealing rings can occur if, for example, the seal ring is not properly seated in its groove or if the partition plate is not in place. All these adverse factors can cause vibration in multi-stage pumps. 10. Impeller: The impeller of the pump is eccentric due to poor quality control during its manufacturing process; for example, the casting quality and machining precision are inadequate, or the liquid being transported is corrosive, leading to erosion of the impeller’s flow channels and thus causing eccentricity in the impeller. Whether the number of blades on the water pump 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. During operation, the wear that occurs between the impeller lip ring and the pump casing lip ring, as well as between the inter-stage bushings and the partition bushings, starts as intermittent contact friction but gradually turns into mechanical frictional wear; this wear tends to increase the vibration of the multi-stage pump.