What causes pump vibration? How to eliminate it?

1. Causes of vibration

1. Motor

The motor structural parts are loose, the bearing positioning device is loose, the iron core silicon steel sheet is too loose, and the support stiffness of the bearing is reduced due to wear, which will cause vibration. Mass eccentricity, uneven rotor mass distribution caused by rotor bending or mass distribution problems cause static and dynamic balance quantities to exceed standards.

In addition, the squirrel-cage cage bars of the rotor of the squirrel-cage motor are broken, causing an imbalance between the magnetic field force on the rotor and the rotational inertia force of the rotor, causing vibration. Phase loss of the motor, unbalanced power supply of each phase and other reasons can also cause vibration.

In the motor stator winding, due to quality problems during the installation process, the resistance between the phase windings is unbalanced, resulting in an uneven magnetic field and an unbalanced electromagnetic force. This electromagnetic force becomes the excitation force and causes vibration.

2. Foundation and pump bracket

The contact fixation between the drive device frame and the foundation is not good, and the foundation and motor system have poor vibration absorption, transmission, and isolation capabilities, resulting in excessive vibrations of both the foundation and the motor.

If the foundation of the water pump is loose, or the water pump unit forms an elastic foundation during the installation process, or the foundation stiffness is weakened due to oil-immersed water bubbles, the water pump will generate another critical speed with a phase difference of 1800 from the vibration, thereby increasing the vibration frequency of the water pump. If the increased frequency is close to or equal to the frequency of an external factor, the amplitude of the water pump will increase.

In addition, loose foundation anchor bolts will reduce the restraint stiffness and intensify the vibration of the motor.

3. Coupling

The circumferential spacing of the connecting bolts of the coupling is poor, and the symmetry is destroyed; the extension section of the coupling is eccentric, which will generate eccentric force; the taper of the coupling is out of tolerance; the static balance or dynamic balance of the coupling is not good; the fit between the elastic pin and the coupling is too tight, causing the elastic column to The pin loses its elastic adjustment function, causing the coupling to not be well aligned; the matching gap between the coupling and the shaft is too large; the mechanical wear of the coupling rubber ring causes the matching performance of the coupling rubber ring to decrease; the quality of the transmission bolts used on the coupling is not equal to each other.

These reasons all cause vibration.

4. Impeller

(1) Impeller mass eccentricity. The quality control during the impeller manufacturing process is not good, for example, the casting quality and machining accuracy are unqualified; or the transported liquid is corrosive, and the impeller flow channel is eroded and corroded, causing the impeller to become eccentric.

(2) Whether the number of blades, outlet angle, wrapping angle of the impeller, the radial distance between the throat partition tongue and the impeller outlet edge are appropriate, etc.

(3) During use, the initial friction between the impeller orifice ring and the pump body orifice ring, and between the interstage bushing and the separator bushing, gradually turns into mechanical friction and wear, which will aggravate the vibration of the pump.

5. Drive shaft and its auxiliary parts

Pumps with long shafts are prone to insufficient shaft stiffness, excessive deflection, and poor linearity of the shaft system, causing friction between the moving parts (drive shaft) and static parts (sliding bearings or mouth rings), resulting in vibration.

In addition, the pump shaft is too long and is greatly affected by the impact of flowing water in the pool, which increases the vibration of the underwater part of the pump. If the balance plate gap at the shaft end is too large, or the axial working movement is improperly adjusted, it will cause the shaft to move at low frequency and cause the bearing bush to vibrate. The eccentricity of the rotating shaft will cause bending vibration of the shaft.

6. Pump selection and variable operating conditions

Each pump has its own rated operating point. Whether the actual operating conditions are consistent with the designed operating conditions has an important impact on the dynamic stability of the pump.

The water pump operates relatively stably under the design working conditions, but when running under variable working conditions, the vibration increases due to the radial force generated in the impeller; a single pump is improperly selected, or two mismatched pump models are connected in parallel. These will cause vibration in the pump.

7. Bearings and Lubrication

If the stiffness of the bearing is too low, it will cause the first critical speed to decrease and cause vibration. In addition, poor performance of the guide bearing results in poor wear resistance, poor fixation, and excessive bearing clearance, which can easily cause vibration; while the wear of the thrust bearing and other rolling bearings will intensify the longitudinal scurrying vibration and bending vibration of the shaft at the same time.

Improper selection of lubricating oil, deterioration, excessive impurity content and lubrication failure caused by poor lubrication pipelines will cause the bearing working conditions to deteriorate and cause vibration. The self-excitation of the oil film of the motor sliding bearing will also produce vibration.

8. Pipes and their installation and fixation

The outlet pipe support of the pump is not stiff enough and deforms too much, causing the pipe to press down on the pump body, destroying the neutrality of the pump body and the motor; the pipe is too strong during the installation process, and the internal stress is large when the inlet and outlet pipes are connected to the pump; the inlet and outlet pipelines are loose, and the restraint stiffness decreases or even fails; the outlet flow channel is completely broken, and debris gets stuck in the impeller.

The pipeline is not smooth, for example, there is an air bag at the water outlet; the water outlet valve has fallen off or is not open; there is air intake at the water inlet, the flow field is uneven, and the pressure fluctuates. These reasons will directly or indirectly cause vibration of the pump and pipeline.

9. Fit between parts

The concentricity of the motor shaft and the pump shaft is out of tolerance; a coupling is used at the connection between the motor and the transmission shaft, and the concentricity of the coupling is out of tolerance; the wear of the design gap between the dynamic and static parts (such as between the impeller hub and the mouth ring) becomes larger; the gap between the intermediate bearing bracket and the pump cylinder exceeds the standard; the gap between the sealing ring is inappropriate, causing imbalance; the gap around the sealing ring is uneven, such as the mouth ring is not grooved or the partition is not grooved, this will happen. These adverse factors can cause vibration.

10. Factors of the water pump itself

The asymmetric pressure field generated when the impeller rotates; the vibration caused by the vortex caused by the half-open valve; water flows in the pump body, which will have friction and impact on the pump body, such as the water flow hitting the front edge of the diaphragm tongue and guide vane, causing vibration; boiler feed water pumps that transport high-temperature water are prone to cavitation vibration;

The pressure pulsation in the pump body is mainly caused by the pump impeller sealing ring. The gap of the pump body sealing ring is too large, causing large leakage losses and serious backflow in the pump body, which in turn causes the imbalance of the rotor axial force and pressure pulsation, which will enhance vibration.

In addition, for pumps transporting hot water, if the preheating of the pump is uneven before starting, or the sliding pin system of the water pump does not work properly, causing thermal expansion of the pump unit, it will induce severe vibration during the starting stage; the internal stress of the pump body from thermal expansion and other aspects cannot be released, which will cause the stiffness of the rotating shaft support system to change. When the changed stiffness is an integral multiple of the angular frequency of the system, resonance will occur.

2. Measures to reduce vibration

1. Eliminate vibration from installation and maintenance processes

(1) Shafts and Axis Systems

Before installation, check whether the water pump shaft, motor shaft, and transmission shaft have bending deformation or mass eccentricity. If so, they must be corrected or further processed; check whether the transmission shaft in contact with the guide bearing is in contact with the bearing bushing or bushing due to bending and causing itself to be stimulated.

If monitoring shows that the shaft is actually bent, straighten the pump shaft. At the same time, check the end clearance value of the shaft. If the value is too large, it indicates that the bearing is worn and needs to be replaced.

(2) Impeller

Whether the dynamic and static balance is qualified.

(3) coupling

Whether the bolt spacing is good; the combination of the elastic pin and the elastic ring cannot be too tight; whether the fit between the inner hole of the coupling and the shaft is too loose. If it is too loose, methods such as spraying can be used to reduce the inner diameter of the coupling until it reaches the size required for transition fit, and then fix the coupling on the shaft.

(4) Sliding Bearing

Whether the clearance value meets the standard; whether the lubrication everywhere is good; improve the level of the pump's bearing maintenance process, strictly follow the cycle procedure of first scraping, then grinding, and then scraping to ensure that the contact area between the bearing and journal reaches the specified standard:

① The clearance value between the pump journal and the bearing must be qualified by replacing the front and rear bearings, grinding, scraping, and adjusting.

② The clearance value between the pump bearing body and the spherical top of the bearing box is qualified.

③ Contact points and contact angles between the lower shoe of the pump shaft bearing and the pump shaft journal: The standard stipulates that the contact area between the lower shoe back and the bearing seat should be above 60%, the density of contact points on the sliding contact surface at the journal should be maintained at 2 - 4 points per square centimeter, and the contact angle should be maintained at "60 - 90".

(5) Bracket and Base Plate

Timely detect the fatigue of vibrating supports to prevent the natural frequency from decreasing due to reduced strength and stiffness.

(6) Clearance and consumable parts

Ensure that the motor bearing clearance is appropriate; properly adjust the gap between the impeller and the volute; regularly inspect and replace impeller mouth rings, pump body mouth rings, interstage bushings, partition bushings and other easily worn parts.

2. Eliminate vibrations caused by improper pump selection and operation

Two pumps connected in parallel should ensure the same pump performance. The pump performance curve should be a slow-down type without a hump. When using, pay attention to: eliminate factors that cause the water pump to be overloaded, such as blockage of the flow channel;

Appropriately extend the starting time of the pump, reduce the disturbance to the transmission shaft, reduce the collision and friction between rotating parts and stationary parts, and the resulting thermal deformation; for water-lubricated sliding bearings, sufficient pre-lubricating water should be added during the starting process to avoid dry starting, and stop water injection until the water pump comes out; regularly add an appropriate amount of oil to the bearings that need oil injection;

For long-axis submersible centrifugal pumps, because there is torsional vibration in the shaft system, if thrust pads are used, the thrust pads will mainly be damaged. In this case, the viscosity of the lubricating oil can be appropriately increased to prevent damage to the hydrodynamic lubricating film.

Finally, in order to prevent the amplitude of the pump from being too large, measurement and analysis of vibration conditions can also be used to determine the optimal operating parameters of the water pump.

Source: Public account "Gongke"

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