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Causes of water pump vibration and treatment methods

Causes of water pump vibration and treatment methods

2020-07-28

The vibration of the various components of the water pump unit, from the hydraulic and mechanical structure design of the pump, to the installation, operation, and maintenance of the pump, has proposed several measures to reduce the pump vibration. The results show that it is ensured that the structural size and accuracy of the water pump components are compatible with the hydraulic characteristics such as the non-overload performance of the pump; the actual operating conditions of the pump are consistent with the design operating points of the pump; the consistency of the processing accuracy and the design accuracy is ensured; Ensure the consistency of the installation quality of the parts and its operation requirements; ensure the consistency of the maintenance quality and the wear rules of the parts, which can reduce the vibration of the pump. The main hazards that can be caused by excessive vibration are: causing the pump unit to not operate normally; causing vibration of the motor and pipeline, causing damage to the machine, causing damage to the bearing and other parts; causing loose connection parts, basic cracks or motor damage; causing The pipes or valves connected to the water pump are loose or damaged, causing vibration and noise. There are many reasons for pump vibration. The shaft of the pump is generally directly connected with the shaft of the drive motor, so that the dynamic performance of the pump and the dynamic performance of the motor interfere with each other; there are many high-speed rotating parts, and the dynamic and static balance can meet the requirements; the parts that interact with the fluid are greatly affected by the water flow; The complexity of fluid movement itself is also a factor that limits the stability of pump dynamic performance.


1. Analysis of the causes of water pump vibration


1.1 Motor

The motor structural parts are loose, the bearing positioning device is loose, the iron core silicon steel sheet is too loose, and the bearing stiffness decreases due to wear, which will cause vibration. Mass eccentricity, uneven rotor mass distribution caused by rotor bending or mass distribution problems, resulting in excessive static and dynamic balance. In addition, the squirrel-cage motor rotor's squirrel cage bar is broken, causing the magnetic field force of the rotor and the rotor's rotational inertia force to be unbalanced and causing vibration. The lack of phase of the motor and the unbalanced power of each phase can also cause vibration. In the stator winding of the motor, due to the operation quality problem of the installation process, the resistance between the phase windings is unbalanced, which results in the uneven magnetic field and unbalanced electromagnetic force. This electromagnetic force becomes an exciting force and causes vibration.


1.2 Foundation and water pump bracket

The contact and fixing form used between the drive device frame and the foundation is not good, and the foundation and the motor system have poor ability to absorb, transmit, and isolate vibration, resulting in excessive vibration of the foundation and the motor. 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-soaked water bubbles, the water pump will produce another critical speed with a phase difference of 1800 from the vibration, which will increase the vibration frequency of the water pump. The frequency is close to or equal to the frequency of an external factor, which will increase the amplitude of the pump. In addition, the looseness of the foundation anchor bolts leads to a decrease in the restraint rigidity and aggravates the vibration of the motor.


1.3 Coupling

The circumferential spacing of the coupling bolts is poor, and the symmetry is destroyed; the coupling extension section is eccentric, which will produce eccentric force; the coupling taper is out of tolerance; the coupling static or dynamic balance is not good; elasticity The fit between the pin and the coupling is too tight, which makes the elastic pin lose its elastic adjustment function, causing the coupling to not be well aligned; the coupling gap between the coupling and the shaft is too large; the mechanical wear of the coupling rubber ring The coupling performance of the rubber ring is reduced; the quality of the transmission bolts used on the coupling is different from each other. These reasons will cause vibration.


1.4 Centrifugal pump impeller 

① The mass of the impeller of the centrifugal pump is eccentric. In the impeller manufacturing process, the quality control is not good, for example, the casting quality and machining accuracy are unqualified; or the conveyed liquid is corrosive, and the impeller flow path is eroded and corroded, resulting in eccentricity of the impeller.


②Whether the number of blades, outlet angle, wrap angle of the centrifugal pump impeller and the radial distance between the throat tongue and the impeller outlet edge are appropriate.


③In use, between the impeller mouth ring and the pump body mouth ring of the centrifugal pump, between the interstage bushing and the diaphragm bushing, the friction and wear from the initial impact will gradually become mechanical friction and wear, which will aggravate the centrifugal pump vibration.


1.5 Drive shaft and water pump auxiliary parts

Pumps with very long shafts are prone to insufficient shaft rigidity, too much deflection, and poor shaft straightness, causing friction between the moving part (drive shaft) and the static part (sliding bearing or mouth ring), resulting in vibration. In addition, 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 pump. If the balance disc gap at the shaft end is too large, or the adjustment of the axial working movement is improper, 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.


1.6 Water pump selection and variable working condition operation

Each pump has its own rated operating conditions. 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 runs relatively stable under design conditions, but when operating under variable conditions, the vibration is increased due to the radial force generated in the impeller; the single pump is improperly selected, or the two types of pumps do not match in parallel. These will cause pump vibration.


1.7 Bearing and lubrication

The rigidity of the bearing is too low, which will cause the * critical speed to be reduced and cause vibration. In addition, poor performance of the guide bearing leads to poor wear resistance, poor fixation, excessive bearing gaps, and vibrations. The wear of thrust bearings and other rolling bearings will aggravate the longitudinal movement vibration and bending vibration of the shaft. . Lubrication failures caused by improper selection of lubricants, deterioration, excessive impurity content and poor lubrication pipelines will cause deterioration of bearing working conditions and cause vibration. The self-excitation of the oil film of the motor sliding bearing also produces vibration.


1.8 Piping and water pump installation

The pump outlet pipe support is not rigid enough, and the deformation is too large, causing the pipe to press down on the pump body, causing the pump body and the motor to be neutralized; the pipe is too strong during the installation process, and the inlet and outlet pipes are connected to the pump. The stress is large; the inlet and outlet pipelines are loose, and the restraint stiffness decreases or even fails; the outlet flow channel is completely broken, and the fragments are stuck in the impeller; the pipeline is not smooth, such as the air bag at the outlet; the outlet valve is off or not open; the inlet has Intake, uneven flow field, pressure fluctuation. These reasons will directly or indirectly lead to pump and pipeline vibration.


1.9 Fit between parts

The concentricity of the motor shaft and the pump shaft is out of tolerance; the coupling between the motor and the drive shaft is used, and the concentricity of the coupling is out of tolerance; the design between moving and static parts (such as between the impeller and the mouth ring) The abrasion of the gap becomes larger; the gap between the intermediate bearing bracket and the pump barrel exceeds the standard; the gap of the seal ring is not suitable, resulting in an imbalance; the gap around the seal ring is uneven, such as the mouth ring groove or the partition groove. This happened. These unfavorable factors can cause vibration.


1.10 Factors of the water pump itself

The asymmetric pressure field generated when the impeller rotates; the vortex of the suction pool and the inlet pipe; the occurrence and disappearance of the vortex inside the impeller and the volute and the guide vane; the vibration caused by the vortex caused by the half-opening of the valve; due to the limited number of impeller blades Uneven outlet pressure distribution; outflow in the impeller; surge; pulsating pressure in the flow channel; cavitation; water flowing in the pump body will cause friction and impact on the pump body, such as water flow hitting the front of the diaphragm and guide vane Boiler feed water pumps that deliver high-temperature water are prone to cavitation vibration; pressure pulsation in the pump body is mainly caused by the seal ring of the pump impeller. The gap of the seal ring of the pump body is too large, causing large leakage losses in the pump body and serious backflow. The imbalance of the rotor axial force and pressure pulsation will increase the vibration. In addition, for hot water pumps that deliver hot water, if the preheating of the pump is uneven before starting, or the sliding pin system of the water pump is not working properly, thermal expansion of the pump set will be induced, which will induce violent vibration in the starting stage; the pump body comes from thermal expansion, etc. If the internal stress cannot be released, it will cause changes in the rigidity of the shaft support system. When the changed rigidity becomes an integral multiple of the angular frequency of the system, resonance occurs.


2. Methods to eliminate water pump vibration


2.1 Eliminate vibration from design and manufacturing links


2.1.1 Attention in mechanical structure design

1) The design of the shaft. Increase the number of support bearings of the drive shaft, reduce the support spacing, reduce the shaft length within an appropriate range, appropriately increase the diameter of the shaft, and increase the stiffness of the shaft; when the speed of the pump shaft gradually increases and is close to or an integer multiple of the inherent pump rotor At the vibration frequency, the pump will vibrate violently. Therefore, in the design, the natural frequency of the drive shaft should avoid the angular frequency of the motor rotor; the shaft manufacturing quality should be improved to prevent quality eccentricity and excessive geometric tolerances.


2) Selection of sliding bearings. Use sliding bearings that do not require lubrication; in chemical pumps such as liquid hydrocarbons, the sliding bearing material should be made of materials with good self-lubricating properties, such as polytetrafluoroethylene; in deep well hot water pumps, the diversion bushing should be filled with polytetrafluoroethylene , Graphite and copper powder materials, and reasonable design of the structure, so that the sliding bearing is fixed and reliable; the impeller seal ring and the pump body seal ring adopt a friction pair with a small friction factor, such as M20lK graphite material and steel; limit the maximum speed; Improve bearing capacity and stiffness of bearing seat.


3) Use a stress relief system. For pumps that deliver hot water, the structural stress between the connecting parts caused by the deformation of the pump body should be released during the design, such as adding bolt sleeves on the pump body anchor bolts to avoid direct and rigid pump bodies. Basic contact.


2. Matters needing attention in hydraulic design of water pump


1. Reasonably design the pump impeller and flow channel to reduce cavitation and flow out of the impeller; reasonably select the number of blades, blade outlet angle, blade width, blade outlet extrusion coefficient and other parameters to eliminate the head curve hump; pump impeller outlet and The distance between the tongues of the volute is considered to be one-tenth of the outer diameter of the impeller, and the pulsating pressure is the smallest; make the exit edge of the blade an inclination (for example, about 20.) to reduce the impact; Ensure the gap between the impeller and the volute; improve the working efficiency of the pump. At the same time, optimize the design of related flow passages such as the water outlet flow passage of the pump to reduce vibration caused by hydraulic loss. Reasonably design the suction chamber at the inlet section of various pumps and the mechanical structure of the compression stage to reduce pressure pulses, ensure stable flow field, improve pump efficiency, reduce energy loss, and improve pump vibration dynamic performance The stability.


2. Cavitation vibration is a very important part of pump vibration. When the population pressure of the pump is lower than the sum pressure at the corresponding water temperature, cavitation accompanied by severe vibration will occur. Measures to reduce cavitation include: when determining the installation height of the pump, make the effective cavitation margin of the device larger than the minimum device cavitation margin of the pump; appropriately increase the diameter of the inlet pipe, shorten the length of the inlet pipe, and reduce pipeline accessories , The cross-section change rate of the flow part strives to be the smallest, to improve the roughness of the pipe wall; reduce the number of elbows and increase the pipe turning angle; reduce the working speed of the pump; use anti-cavitation and cavitation materials, such as stainless steel, or Epoxy resin should be applied to the part where cavitation occurs; the design of the inlet flow channel should be reasonable, and strive to be smooth, so that the water flow speed and pressure distribution into the impeller are uniform, avoid local low pressure areas; improve the quality of manufacturing and processing, and avoid the inaccurate blade profile. The local flow rate is too large and the pressure drop is too much; to improve the anti-cavitation performance of the pump device, including setting a hydraulic energizer at the inlet of the pump, the structure of the energizer, increasing the suction head of the pump, thereby increasing the steam of the pump device Erosion allowance; increase the geometric backflow height; minimize the head loss of the water inlet pipe; adopt a double-suction pump.


In order to ensure that there is no air accumulation in the suction pipe or the pressurized water pipe, no part of the suction pipe can be higher than the inlet of the water pump. In order to reduce the pressure pulsation at the water inlet, the diameter of the suction pipe should be an order of magnitude larger than the diameter of the pump port, so that the water flow at the pump port has a certain degree of contraction, so that the flow velocity distribution is more uniform, and there should be a section before the pump port Straight pipe, the length of the straight pipe is not less than 10 times the pipe diameter.


Pay attention to creating good water inlet conditions, and the water flow in the inlet pool should be stable and even to eliminate the vibration accompanying the Karmen vortex.


3. Basic design. The weight of the foundation should be more than three times the total weight of the pump and the motor; the foundation of the tank should have considerable strength; the motor support and the foundation should be integrated or surface contact; set between the pump and the support Vibration isolation pads or vibration isolators. In addition, the damping material is used to connect the pipelines to reduce the pipeline layout and eliminate vibration caused by elastic contact and hydraulic loss.


2.2 From the installation and maintenance process as a method to eliminate pump vibration


1. Shaft and shafting. Before installation, check whether the water pump shaft, motor shaft, and transmission shaft are bent, deformed, or eccentric in quality. If so, they must be corrected or further processed; check whether the transmission shaft contacting the guide bearing is caused by bending and rubbing against the bearing bush or bushing Encourage yourself. If the monitoring shows that the shaft has actually been bent, correct 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 are qualified.


3. Coupling. Whether the bolt spacing is good; the combination of elastic pin and 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, use methods such as spraying to reduce the inner diameter of the coupling until it reaches The required size of the transition fit, and then fix the coupling on the shaft.


4. Sliding bearing. Whether the clearance value meets the standard; whether the lubrication is good everywhere; improve the maintenance level of the bearing bush of the pump, strictly follow the cycle procedure of scraping, grinding, and scraping the bush to ensure that the contact area between the bearing bush and the journal meets the specified standard:

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

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

③Pump shaft bearing lower bush and pump shaft journal contact point and contact angle: the standard stipulates that the contact area between the lower bush back and the bearing seat should be more than 60%, and the contact point density on the sliding contact surface at the journal should be maintained at 2 per square centimeter One 4 points, the contact angle is maintained at 60"-90".


5. Bracket and bottom plate. Find out the fatigue of the vibrating support in time to prevent the natural frequency from falling due to the decrease in strength and rigidity.


6. Gap and wearing parts. Ensure that the motor bearing clearance is appropriate; properly adjust the clearance between the impeller and the volute; regularly check and replace the impeller orifice ring, the pump body orifice ring, the interstage bushing, the diaphragm bushing and other easily worn parts.


2.3 Vibration caused by improper selection and operation of centrifugal pump

Two pumps in parallel should ensure the same pump performance. The pump performance curve should be a slow-down type, and there should be no hump. Pay attention when using: Eliminate the factors that cause the pump to overload, such as blockage of the flow passage; appropriately extend the start time of the pump, reduce the disturbance to the drive shaft, reduce the collision and friction between rotating parts and stationary parts, and the resulting For water-lubricated sliding bearings, sufficient pre-lubricated water should be added during the starting process to avoid dry starting until the water pump is discharged and then stop water injection; regular oil injection into the bearings that need oil; for long-axis submerged centrifugal pumps, Because there is torsional vibration in the shafting, if thrust pads are used, the thrust pads will be damaged. At this time, the viscosity of the lubricating oil can be appropriately increased to prevent the damage of the hydrodynamic lubricating film. After zui, in order to prevent the pump's amplitude from being too large, measurement and analysis of vibration conditions can also be used to determine the best working parameters of the pump.


3. Conclusion

The causes of water pump vibration include mechanical, hydraulic and electrical causes.

Vibration control comprehensively reflects the mechanical processing technology, the operation level of the mechanical installer, the quality of the pump operator, the function of the hydraulic design software, the performance of each part of the material, and the performance of the monitoring instrument. In actual work, the elimination of vibration should be combined with experience and theoretical analysis, and the vibration mechanism analysis and the data obtained by the actual testing instrument should be combined. Many vibrations can be eliminated by improving the design and installation quality, improving the operation level, and strengthening daily maintenance. With the development of new material technology and the emergence of new processes, as well as the advancement of electronic computer technology and numerical methods and basic theories of fluid mechanics, coupled with the rise and development of vibration and noise diagnosis technology, the design, use and maintenance of water pumps are bound to flourish. , The performance will definitely be optimized day by day, and the dynamic performance will be more stable day by day.


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