10 Reasons for Excessive Pump Vibration

Abnormal vibration of the pump is an important indicator to evaluate whether the quality of the pump is reliable. There are many reasons for the vibration of multi-stage pumps, such as the influence of water flow conditions, the complexity of fluid motion, dynamic and static balance, high-speed rotating parts, etc., which may affect the stability of the pump. Let's give you a comprehensive analysis of the reasons for the vibration of the pump.

10 Causes of Vibration:

1. Shaft
The pump shaft is very long, and it is easy to cause friction between the moving part (transmission shaft) and the static part (sliding bearing or mouth ring) due to insufficient rigidity, too large deflection, poor straightness of the shaft system, etc. of the pump, resulting in vibration of the pump. The pump shaft is too long, and is greatly affected by the impact of the flowing water in the pool, which increases the vibration of the underwater part of the multi-stage pump, too large gap of the pump shaft balance disc, or the pump shaft, it will also cause low-frequency movement of the shaft, resulting in bearing vibration, and the eccentricity of the rotating shaft will also cause bending vibration of the shaft.

2. Foundation and pump support
The contact and fixing form adopted between the driving device frame and the foundation is not good, and the vibration absorption, transmission and isolation ability of the foundation and the motor system are poor, resulting in the vibration of the foundation and the motor exceeding the standard, the foundation of the pump is loose, or the elastic foundation is formed during the installation of the pump unit, or the rigidity of the foundation is weakened due to oil-immersed blisters, the pump will generate another critical speed 1800 with the vibration phase difference, thus increasing frequency, if the increased frequency is close to or equal to the frequency of an external factor, the amplitude of the multi-stage pump will increase. In addition, the foundation anchor bolts are loose, resulting in reduced restraint stiffness, which will increase the vibration of the motor.

 

3. Coupling

The axial spacing of the coupling connecting bolts is poor, the symmetry is destroyed, the coupling extension section is eccentric, eccentric force will be generated, the coupling cone degree is out of tolerance, the coupling static balance or dynamic balance is not good, the elastic pin and the coupling are too tight, the elastic pin loses the elastic adjustment function, the coupling cannot be aligned well, the coupling clearance between the coupling and the shaft is too large, and the mechanical wear of the coupling rubber ring will cause the coupling rubber ring fitting performance to decline, the quality of the drive bolts used on the couplings is not equal to each other. These reasons will cause multi-stage pump vibration.

 

4. The pump's own factors

The asymmetric pressure field generated when the impeller rotates; The vortex of the suction pool and the inlet pipe; The inside of the impeller and the volute lead to the occurrence and disappearance of the vortex of the blade; The vibration caused by the vortex caused by the half-open valve; Uneven distribution of outlet pressure due to the limited number of impeller blades; Deflow in the impeller; Surge; Pulsating pressure in the flow passage; Cavitation; Water flowing in the pump body will have friction and impact on the pump body, for example, the water flow hits the leading edge of the spacer tongue and the guide vane, causing vibration; Cavitation vibration occurs in the boiler feed pump conveying high temperature water; The pressure pulsation in the pump body is mainly the sealing ring of the pump impeller. The clearance of the sealing ring of the pump body is too large, resulting in large leakage loss in the pump body and serious reflux, which in turn causes imbalance and pressure pulsation of the axial force of the rotor and will enhance vibration.

In addition, for the stainless steel hot water pump conveying hot water, if the preheating of the pump before starting is uneven, or the sliding pin shaft system of the pump does not work properly, the thermal expansion of the pump group will induce severe vibration during the starting phase; The internal stress of the pump body from thermal expansion and other aspects cannot be released, which will cause the stiffness of the shaft support system to change.

 

5. Motor

The motor structural parts are loose, the bearing positioning device is loose, the iron core silicon steel sheet is too loose, and the bearing will cause the support stiffness to drop due to wear, which will cause vibration. Mass eccentricity, rotor bending or mass distribution problems lead to uneven distribution of rotor mass, resulting in excessive static and dynamic balance.

In addition, the squirrel cage bar of the squirrel cage motor rotor is broken, which causes the magnetic force of the rotor and the rotation inertia force of the rotor to be unbalanced and causes vibration, the motor is short of phase, and the power supply of each phase is unbalanced and other reasons can also cause vibration. The stator winding of the motor, due to the operation quality problem of the installation process, causes the resistance imbalance between the phase windings, which leads to the uneven magnetic field and produces unbalanced electromagnetic force, which becomes the excitation force and causes vibration.

 

6. Pump selection and variable operating conditions

Each pump has its own rated operating point, and whether the actual operating conditions are in line with the design conditions has an important impact on the dynamic stability of the pump. The pump is relatively stable under the design conditions, but when it is running under variable conditions, the vibration is increased due to the radial force generated in the impeller, the selection of a single pump is improper, or two types of pumps are not matched in parallel. These will cause the vibration of the multi-stage pump.

 

7. Bearing and lubrication

If the stiffness of the bearing is too low, the first critical speed will be reduced and vibration will be caused. In addition, poor performance of the guide bearing leads to poor wear resistance, poor fixation, and excessive bearing clearance, which can easily cause vibration. The wear of thrust bearings and other rolling bearings will cause longitudinal vibration and bending vibration of the shaft. Intensified at the same time. Lubrication failure caused by improper selection of lubricating oil, deterioration, excessive impurity content and poor lubrication pipeline will cause deterioration of bearing working conditions and cause vibration. Self-excitation of motor sliding bearing oil film will also cause vibration.

 

8. Pipeline and its installation and fixation

The rigidity of the outlet pipe support of the pump is not enough and the deformation is too large, causing the pipe to press down on the pump body, causing the neutral of the pump body and the motor to be damaged. The pipe is too strong during installation, the internal stress is large when the inlet and outlet pipes are connected with the pump, the inlet and outlet pipes are loose, the restraint rigidity decreases or even fails, the outlet flow passage part is completely broken, the debris is stuck in the impeller, and the pipeline is not smooth, or not open, the inlet has air intake, uneven flow field, pressure fluctuations, these reasons will directly or simply lead to multi-stage pump and pipeline vibration.

 

9. Coordination between parts

The concentricity of the motor shaft and the pump shaft is out of tolerance, the coupling is used at the connection between the motor and the transmission shaft, the concentricity of the coupling is out of tolerance, the design clearance between the dynamic and static parts (such as between the impeller and the mouth ring) becomes larger, the clearance between the intermediate bearing bracket and the pump cylinder exceeds the standard, and the clearance of the sealing ring is inappropriate, resulting in imbalance and uneven clearance around the sealing ring, such as the mouth ring not entering the groove, that's what happens. These unfavorable factors can cause the multistage pump to vibrate.

 

10. Impeller

The quality of the pump impeller is eccentric, and the quality control of the impeller is not good in the manufacturing process. For example, the casting quality and machining accuracy are unqualified, or the liquid in the conveying place is corrosive, and the impeller runner is eroded and corroded, resulting in the impeller eccentricity. Pump impeller blade number, outlet angle, wrap angle, throat tongue and impeller outlet edge of the radial distance is appropriate. In use, between the impeller mouth ring and the pump body mouth ring of the multi-stage pump, and between the interstage bushing and the partition bushing, the initial rubbing gradually becomes mechanical friction and wear, which will aggravate the vibration of the multi-stage pump.