Abnormal vibration of pumps is a key indicator for assessing their reliability. Multiple factors can cause multi-stage pump vibrations, including water flow conditions, fluid motion complexity, dynamic-static balance, and high-speed rotating components—all of which may compromise pump stability. Below is a comprehensive analysis of the causes of pump vibrations.
Pump shafts are excessively long, making them prone to dynamic friction between moving components (driving shaft) and stationary parts (sliding bearings or mouth rings) due to insufficient pump stiffness, excessive deflection, or poor shaft alignment. This friction causes pump vibration. The extended shaft length also amplifies vibrations in the submerged section of multi-stage pumps when exposed to water flow impacts. Additionally, excessive clearance in the shaft balance disc or improper adjustment of axial movement can induce low-frequency shaft oscillations, resulting in bearing vibration and rotational shaft eccentricity, which may further lead to shaft bending vibrations.
The contact fixation method between the drive unit frame and foundation is suboptimal, resulting in inadequate vibration absorption, transmission, and isolation capabilities of both the foundation and motor system. This leads to excessive vibration levels in both components, causing the pump foundation to loosen. During installation, the pump unit may form an elastic foundation or experience reduced foundation stiffness due to oil immersion cavitation, triggering a critical rotational speed with a 180-degree phase difference from the vibration. This increases the pump's vibration frequency, and if the increased frequency aligns with an external factor's frequency, it amplifies the amplitude of the multistage pump. Additionally, loose foundation anchor bolts decrease restraint stiffness, exacerbating motor vibration.
Improper circumferential spacing of coupling bolts, compromised symmetry, eccentricity in the coupling's extension section, excessive taper tolerance, poor static or dynamic balance, overly tight elastic pin coupling, loss of elastic pin's self-adjusting function causing misalignment, excessive shaft coupling clearance, mechanical wear of the coupling rubber ring leading to reduced sealing performance, and inconsistent quality of transmission bolts used in the coupling—all these factors can cause vibration in multi-stage pumps.
The asymmetric pressure field generated during impeller rotation; vortex formation in suction tanks and intake pipes; vortex generation and dissipation within the impeller, volute, and guide vanes; valve half-open-induced vortex-induced vibration; uneven outlet pressure distribution due to limited impeller blade count; flow separation within the impeller; surge; pulsating pressure in flow channels; cavitation; water flow in the pump body causing friction and impact, such as water impacting the tongue and leading edges of guide vanes, resulting in vibration; boiler feed pumps handling high-temperature water are prone to cavitation-induced vibration; pressure pulsations in the pump body, primarily caused by excessive clearance between the impeller seal ring and pump body seal ring, leading to significant internal leakage, severe backflow, and subsequent unbalanced axial force on the rotor and pressure pulsations, which intensify vibration.
Furthermore, for stainless steel hot water pumps used in hot water delivery systems, uneven preheating prior to startup or malfunctioning sliding pin systems can cause thermal expansion in the pump assembly, triggering severe vibrations during the startup phase. If internal stresses from thermal expansion cannot be released, this may alter the stiffness of the shaft support system. When the modified stiffness becomes a multiple of the system's angular frequency, resonance occurs.
Loose motor structural components, loose bearing positioning devices, excessively loose silicon steel sheets in the iron core, and reduced bearing support stiffness due to wear can all cause vibrations. Eccentric mass distribution, rotor bending, or uneven mass distribution resulting from quality issues may lead to excessive static and dynamic balance deviations.
Additionally, broken squirrel-cage bars in the rotor of squirrel-cage motors can cause vibrations due to an imbalance between the magnetic force acting on the rotor and its rotational inertia. Other contributing factors include phase loss in the motor and power supply imbalance across phases. Regarding the stator windings, poor installation quality may lead to resistance imbalance between phases, resulting in uneven magnetic field distribution. This creates unbalanced electromagnetic forces that act as excitation forces, ultimately triggering vibrations.
Every pump has its own rated operating point. Whether the actual operating conditions match the design specifications significantly impacts the pump's dynamic stability. While pumps operate more stably under design conditions, variable operating conditions can cause increased vibration due to radial forces generated in the impeller. Factors such as improper single-pump selection or parallel operation of mismatched pump models may all contribute to vibration in multi-stage pumps.
Insufficient bearing stiffness reduces the first critical speed, leading to vibration. Poor performance of guide bearings, such as inadequate wear resistance, improper fixation, or excessive bearing bush clearance, can also cause vibrations. Additionally, wear in thrust bearings and other rolling bearings may intensify both axial movement and bending vibrations. Lubrication failures—such as improper lubricant selection, degraded oil, excessive impurities, or clogged lubrication lines—can worsen bearing conditions and trigger vibrations. Self-excited vibrations in motor sliding bearing oil films may also contribute to operational instability.
The pump's outlet pipeline support lacks sufficient rigidity, causing excessive deformation that presses the pipeline against the pump body. This results in misalignment damage between the pump body and motor. During installation, the pipeline experiences excessive force, leading to high internal stress when connecting the inlet and outlet pipes to the pump. Loose connections in the inlet and outlet pipelines reduce or even nullify the restraint rigidity, causing partial or complete fracture of the outlet flow channel. Broken fragments may get lodged in the impeller, obstructing the pipeline. Issues such as air pockets at the outlet, missing or improperly opened water discharge valves, air intake at the inlet, uneven flow fields, and pressure fluctuations can directly or indirectly cause vibrations in the multistage pump and its pipelines.
The motor shaft and pump shaft exhibit concentricity deviations. A coupling is used at the motor-pump shaft connection, but its concentricity is out of specification. This causes increased wear on the designed clearance between moving and stationary components (e.g., between the impeller hub and the mouth ring). Additionally, the clearance between the intermediate bearing bracket and the pump cylinder exceeds the standard, while the sealing ring clearance is improperly adjusted. These factors collectively create imbalance, resulting in uneven clearance around the sealing ring. Issues like the mouth ring not fitting into the groove or the partition plate not aligning with the groove can lead to such problems. All these adverse factors contribute to the vibration of the multistage pump.
The pump impeller's eccentricity stems from inadequate quality control during manufacturing, such as casting defects or insufficient machining precision. When handling corrosive liquids, the impeller's flow channels may be eroded, causing misalignment. Key factors include proper blade count, optimal outlet angle, appropriate wrap angle, and correct radial spacing between the throat tongue and impeller outlet edge. During operation, initial contact between the impeller's mouth ring and the pump body's mouth ring, along with friction between stage bushings and partition bushings, evolves from initial contact to mechanical wear, ultimately exacerbating the pump's vibration.
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