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 motion, 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 vibrations that exceed acceptable levels 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, oil immersion or water exposure can reduce the stiffness of the foundation, causing the pump to develop another critical speed that is 180 degrees out of phase with the vibration frequency. This increases the vibration frequency of the pump, and 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 vibrations 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 shape of the coupling is not within acceptable limits, or if the coupling is not properly statically or dynamically balanced, 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 bolts used to connect the coupling varies 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 pool 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 vibration ； Boiler feed pumps that transport high-temperature water are prone to cavitation vibration ； Pressure fluctuations within the pump chamber are mainly caused by the sealing rings of the pump impeller; excessive gaps in these sealing rings lead to significant leakage losses and severe backflow, which in turn results in an unbalanced axial force on the rotor and pressure fluctuations, thereby increasing vibration.    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 changes 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: Loose structural components of the motor, loose bearing positioning devices, overly loose 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, broken bars in the squirrel-cage of a squirrel-cage motor 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. Due to quality issues in the installation process, the resistances between the phase windings of the motor stator winding become unbalanced. This results in an uneven magnetic field and unbalanced electromagnetic forces. These electromagnetic forces act as excitation forces, thereby causing vibrations. 6. Pump selection and operation under variable conditions Each pump has its own rated operating point, and whether the actual operating conditions match the designed ones has a significant impact on the dynamic stability of the pump. The water pump operates relatively stably under designed operating conditions; however, under variable operating conditions, vibrations increase due to the generation of radial forces within the impeller. This can be caused by improper selection of a single pump or by paralleling two pumps of incompatible models. All of these can cause vibration in multi-stage pumps. 7. Bearings and Lubrication A low stiffness of the bearings can 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 lubricants, lubricant degradation, excessive impurity content, or blocked lubrication pipelines can all lead to deteriorated operating conditions of bearings and subsequent vibrations. Additionally, the self-excitation of the oil film in the sliding bearings of electric motors can also generate vibrations. 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 forces are 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 channel 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 associated pipelines. 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 cylinder is excessive, and the gap of the sealing rings is not appropriate, which leads 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 pump impeller is eccentric due to poor quality control during its manufacturing process; for example, the casting quality and machining precision are not up to standard, or the liquid being transported is corrosive, which causes erosion of the impeller’s flow channels and leads to eccentricity. 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 mouth ring and the pump casing mouth ring, as well as between the inter-stage bushings and the partition bushings, starts as gentle contact friction but gradually turns into mechanical frictional wear; this wear tends to increase the vibration of the multi-stage pump.