Oscillation is an important objective to evaluate the operation reliability of pump units. The harm of oscillation overspending mainly includes: the pump unit cannot operate normally due to oscillation; The vibration of the motor and pipeline is caused, causing machine damage and human injury; Damage to components such as bearings; The connecting parts are loose, the foundation cracks or the motor is damaged; The pipe fittings or valves connecting with the water pump are loose or damaged; Constitute oscillation noise.

There are many reasons for the pump oscillation. The rotating shaft of the pump is generally directly connected with the driving motor shaft, so that the dynamic function of the pump and the dynamic function of the motor interfere with each other; There are many high-speed rotating parts, and the dynamic and static balance baths can meet the requirements; Components with fluid effect are greatly affected by water flow; The complexity of fluid movement itself is also a factor limiting the stability of pump dynamic function. Alfa Laval

Analysis of causes of pump oscillation

electric machinery

The structural parts of the motor are loose, the bearing positioning equipment 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 oscillation. The rotor mass is unevenly distributed due to mass bias, rotor winding or mass dispersion problems, which constitutes an excess of static and dynamic balance. In addition, the squirrel cage bars of the rotor of the squirrel cage motor are cracked, and the magnetic force that forms the rotor and the rotating inertia force of the rotor are unbalanced, causing oscillation. The motor is out of phase, and the power supply of each phase is unbalanced, and other reasons can also cause oscillation. For the motor stator winding, due to the operation quality problem of the equipment process, the resistance between the windings of each phase is unbalanced, which leads to the uneven magnetic field and unbalanced electromagnetic force, which becomes the excitation force and causes oscillation. IMO agent

Foundation and pump support

The touch fixing form selected between the drive equipment frame and the foundation is not good, and the foundation and the motor system are poor in absorbing, transmitting and blocking oscillation, resulting in the foundation and motor oscillation being overspent. The pump foundation is loose, perhaps the pump unit forms an elastic foundation during the equipment process, or because the stiffness of the foundation formed by the oil soaked bubbles is weakened, the pump will produce another critical speed with a phase difference of 1800 from the oscillation, and then increase the pump oscillation frequency. If the added frequency is close to or equal to the frequency of an external element, the amplitude of the pump will increase. In addition, the loosening of the foundation bolts will reduce the binding stiffness, which will increase the vibration of the motor.

coupling

The circumferential distance of coupling connecting bolts is poor, and the symmetry is damaged; The coupling extension will have a pain, which will produce a pain force; The taper degree of the coupling is out of tolerance; The static balance or dynamic balance of the coupling is poor; The cooperation between the elastic pin and the coupling is too tight, so that the elastic pin loses its elastic adjustment function, which makes the coupling can not be well aligned; The cooperation gap between coupling and shaft is too large; The mechanical wear of coupling rubber ring leads to the decline of coupling rubber ring cooperation function; The quality of drive bolts used on the coupling is not equal to each other. All these reasons will form oscillation.

impeller

① Impeller quality is very bad. The quality control during the impeller manufacturing process is not good, for example, the casting quality and machining accuracy are unqualified; Perhaps the liquid conveyed is corrosive, and the fan channel of the impeller is eroded and corroded, causing the impeller to have a preference.

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

③ During operation, the initial friction between the impeller mouth ring and the pump body mouth ring, and between the interstage bushing and the diaphragm bushing gradually turns into mechanical collision wear, which will aggravate the pump oscillation.

Pipe and equipment fixation

The rigidity of the outlet pipe support of the pump is not enough and the deformation is too large, which makes the pipe press down on the pump body, causing the neutral damage of the pump body and the motor; The pipeline is too stiff in the process of equipment, and the internal stress is large when the inlet and outlet pipelines are connected with the pump; The inlet and outlet pipelines are loose, and the binding stiffness decreases or even fails; All the outlet passages cracked, and debris stuck in the impeller; The pipeline is blocked, such as air bag at the water outlet; The outlet valve falls off or is not opened; There is air inlet at the water inlet, the flow field is uneven, and the pressure fluctuates. These reasons will directly or indirectly lead to the oscillation of pump and pipeline. IMO screw pump

Bearing and lubrication

If the stiffness of the bearing is too low, the critical speed will drop at the top of the list, causing oscillation. In addition, poor closing of the guide bearing leads to poor wear resistance, poor fixation, and excessive bearing clearance, which also simply constitutes oscillation; The wear of thrust bearing and other rolling bearings will aggravate the longitudinal displacement oscillation and zigzag oscillation of shaft together. Lubrication problems caused by improper selection of lubricating oil, deterioration, excessive impurity content and blocked lubrication pipeline will all result in deterioration of bearing conditions and vibration. The self-excited oil film of motor sliding bearing will also produce oscillation.

Ways to reduce oscillation

Eliminate oscillation from planning and production

1) Axis planning. Add the number of support bearings of the transmission shaft, reduce the support distance, reduce the shaft length within the appropriate range, properly increase the diameter of the shaft, and add the stiffness of the shaft; When the pump shaft speed gradually increases and is close to or integral multiple of the natural oscillation frequency of the pump rotor, the pump will oscillate violently. Therefore, during planning, the natural frequency of the drive shaft should avoid the angular frequency of the motor rotor; Improve the manufacturing quality of the shaft to prevent quality bias and excessive geometric tolerance.

2) Selection of sliding bearings. Select the sliding bearing without lubrication; In chemical pumps such as liquid hydrocarbon, sliding bearing materials should be materials with good self-lubricating function, such as polytetrafluoroethylene; In the deep well hot water pump, the guide bushing is made of materials filled with PTFE, graphite and copper powder, and its structure is reasonably planned to make the sliding bearing fixed reliably; The impeller seal ring and the pump body seal ring shall select the conflict pair with small conflict factor, such as M20lK graphite material - steel; Limit the maximum speed; Improve bearing capacity of bearing bush and rigidity of bearing seat.

3) Apply the stress release system. For the pump delivering hot water, the structural stress between the connectors caused by the deformation of the pump body should be released during the planning. For example, bolt sleeves should be added on the foundation bolts of the pump body to prevent the pump body from directly touching the foundation with high stiffness.

Hydraulic Planning Considerations for Pumps

1) Reasonably plan the pump impeller and flow passage to reduce cavitation and flow separation in the impeller; Reasonably select the parameters such as the number of blades, blade outlet angle, blade width, and blade outlet crowding coefficient to eliminate the hump of the head curve; The distance between the outlet of the pump impeller and the volute tongue is considered to be one tenth of the outer diameter of the impeller, and the pulsating pressure is the smallest; Tilt the outlet edge of the blade (for example, about 20.) to reduce the impact; Ensure the clearance between impeller and volute; Improve the working efficiency of the pump. Together, the pump outlet channel and other related channels are optimized to reduce the oscillation caused by hydraulic loss. Reasonably planning the suction chamber at the inlet section of various pumps, as well as the mechanical structure of the compression stage, and reducing the pressure pulse can ensure the stability of the flow field, improve the working efficiency of the pump, reduce energy loss, and improve the stability of the pump's oscillating dynamic function.

2) Cavitation oscillation is a very important part of pump oscillation. When the population pressure of the pump is lower than the sum pressure under the corresponding water temperature, cavitation accompanied by severe oscillation will occur. The methods to reduce cavitation include: when confirming the equipment height of the pump, make the useful NPSH of the equipment greater than the minimum equipment NPSH of the pump; Properly increase the diameter of the water inlet pipe, shorten the length of the water inlet pipe, reduce the pipeline accessories, minimize the change rate of the section of the flow passage, and improve the roughness of the pipe wall; Reduce the number of bends and increase the pipe turning viewpoint; Lower the working speed of water pump; Select cavitation resistant materials, such as stainless steel, or apply epoxy resin on the parts where cavitation may occur; The intake channel shall be planned reasonably and smoothly to ensure uniform distribution of water flow velocity and pressure into the impeller and prevent some low-pressure areas; Improve the manufacturing and processing quality to prevent excessive flow rate and pressure drop due to inaccurate blade profile; The anti cavitation function of the progressive pump equipment includes setting a hydraulic booster at the pump inlet, the structure of the booster, improving the suction head of the pump, and then improving the NPSH of the pump equipment; Add a few inverted heights; Try to reduce the loss of water head in the inlet pipe; Double suction pump is selected.