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Nuclear power plant main pump vibration problems what are the fundamental reasons
1. The type and structure of the main reactor of the reactor The main pump of the reactor is the main rotating equipment of the primary circuit of the PWR nuclear power station and takes on the important functions of compensating the pressure drop of the coolant in the primary circuit and promoting the coolant circulation. Daya Bay and Ling Ao Nuclear Power Plant operated by Daya Bay Nuclear Power Operation Company are equipped with 12 main pumps (3 units per unit) manufactured by JEUMONT-INDUSTRIES, France. Each main pump is an air-cooled, three-phase induction motor driven single-stage shaft seal unit. The machine is a vertical component, as shown in Figure 1, from top to bottom by the motor, seal components and hydraulic components of the pump, cascading arrangement of the three-stage shaft seal by the pump shaft leakage. Sealing water supplied by the fluid control system is injected between the pump bearings and the seals to prevent the reactor coolant from flowing upward while cooling the shaft seals and pump bearings. The electric pump is assembled with three radial bearings and one thrust bearing, of which two radial bearings and one thrust bearing are used to support the motor rotor and the other radial bearing is to form a pump bearing, which is a water-lubricated bearing, Alloy surfacing stainless steel journals and graphite ring sleeve composition. 2. Special vibrations of the main pump of the reactor Since the commencement of commercial operation of the two hydropower stations, the special vibrations of the 100-type main pump have long plagued the professional and technical personnel. These special vibrations mainly show the following characteristics. 2.1 The main pump vibration level was significantly affected by the shaft seal water flow April 8, 2003, Ling O 1 due to commissioning RCV charge pump discharge orifice plate, resulting in the No. 3 main shaft seal flow from the 2.0m3 / h reduced to 1.7m3 / h (system design requirements shaft seal flow control at 1.8m3 / h), the pump shaft vibration from 200μm down to 150μm, then increase the shaft seal flow to 2.0m3 / h, the vibration level and rise To 190 μm. It is worth noting that the response of different main pumps to shaft seal water flow changes is completely different. For example, the shaft sealing water adjustment test of D1RCP001 / 002 / 003PO on February 22, 2000: 2.2m3 / h160μm90μm200μm 2.5 m3 / h155μm140μm180μm 3.0m3 / h155μm190μm160μm It can be seen from the above table that after the shaft sealing water flow rate increases from 2.2m3 / h to 3.0m3 / h, the vibration level of No.1 pump basically keeps unchanged and the vibration of No.2 pump shaft increases significantly , While 3 pump is significantly reduced. 2.2 The main pump of the high vibration point is constantly changing, such as: March 8, 1999, 105 after the overhaul of a circuit temperature, step-up process, recorded D1RCP002PO vibration vector changes in the cold start frequency component 160μm / 8 °. The vibration vector changes to 154μm / 104 ° when the temperature is increased and the pressure is increased to the thermal shutdown level. That is to say, the vibration direction changes by 96 °. 2.3 need to be repeated on-site balancing to reduce the vibration level Whenever the main pump shaft vibration increases sharply, the main frequency components of the change are the power frequency (accounting for 80% to 90% of the pass frequency vibration), the general approach is to stop Heap conditions for on-site balancing to reduce vibration levels. What is strange, however, is that the balanced main pump goes through a simple start-stop process and again exhibits obvious unbalance characteristics (ie, the frequency vibration rises again) and has to be balanced again. Such as: March 5, 2000, D1RCP003PO 106 after the start of repair shaft vibration level in the thermal shutdown condition reached 230μm, on-site dynamic balancing test shaft vibration reduced to 140μm, followed by lazy test, the pump after a simple Start and stop the process, once again started the original well-balanced state of good vibration repeated, was forced to the second balance. 2.4 The change of main pump start-up sequence will also cause drastic change of the vibration state of the main pump. For example, after the overhaul of the main pump on April 17, 2003, the unit returned to the state of thermal shutdown, the D1RCP002PO shaft vibrated well, Main pump, followed by 1, 3, 2 in order to start the main pump, 002PO original vibration at 90μm, then sharp rise to 200μm after the start, followed by the second lazy test, the three main pump outage after the same in accordance with the 1,2,3 order to start again, 002PO axis vibration level returned to a good level of 100μm. In the meantime there is no other action than changing the startup sequence. 3. Current Several Theoretical Explanations and Limitations on Main Pump Vibrations 3.1 Unbalanced Quality Unbalanced quality is one of the main causes of vibration problems in rotating machinery. According to the general theory of vibration analysis, the main frequency spectrum characteristic when the rotor is in mass imbalance is that the frequency component accounts for more than 80% of the total vibration level. This should be consistent with the spectrum we collected. In addition, every time the main pump vibration problem is solved through on-site dynamic balancing, some people think that there must be mass balance problems on the main pump rotor that need constant correction. In fact the rotor mass imbalance, there is a basic feature, that is, the phase is basically stable, which is obviously inconsistent with the fact that the main pump vibration vector changes, while dealing with the rotor mass imbalance, in a short period of time generally not There will be situations that require frequent correction. Obviously, the vibration problem of the main reactor of the reactor is not caused entirely by the imbalance of mass. 3.2 solid boron crystals in the bearing surface defects caused For the existence of the main pump vibration problems, we sent a letter to the EDF (EDF), I hope each other can take advantage of its strong technical strength and rich operating experience to help us. EDF admits that their power plant has the same problem in the palindrome and is also solved by balancing. Unfortunately, it is regrettable that there is a lack of agreement on the root cause. It is only a general reference that the defects caused by solid boron crystals on the inner surface of the bearing may be caused Vibration reasons, and so on. We do not agree with this explanation. First of all, equipment bearing vibration defects can not be resolved by balance; Second, the bearing defects caused by high vibration equipment can not be adjusted by adjusting the shaft seal water flow to be eased. It can not be argued that the explanation provided by EDF simply can not explain the vibration we are facing. 3.3 Main Pump Shaft Thermal Variables The thermal variables of the main pump shaft system have been considered as the main cause of the main pump vibration for some time. This explanation argues that the main pump shaft temperature, the temperature difference between the various components lead to the existence of a heat pump rotor variable, it is precisely because of the existence of this thermal variable and changes in the main shaft vibration caused particularity. Each time the field balancing test is to balance the heat variable. It should be said that the theory of thermal variables can basically explain the problems listed above, such as: the existence of the rotor thermal variables change with the conditions will cause the rotor vibration vector changes, to some extent, the thermal variables can indeed be corrected by on-site balancing ,and many more. However, the explanation of the heat variable does not fully explain how the change of shaft sealing water flow obviously affects the rotor shaft vibration. At the same time two simple start-stop process will lead to dramatic changes in shaft vibration with the theory can not be perfectly explained. Accordingly, it can be said that the thermal variable should be one of the factors leading to the vibration of the main pump, but it is not the root cause. The dynamic characteristics of the rotor in the sliding bearing is the root cause of the main pump vibration The dynamic theory of the horizontal dynamic pressure sliding bearing that when the horizontal rotor stable operation, the rotor formed by the rotation of the hydrodynamic fluid film on the rotor A bearing capacity, this force and the rotor weight will be in a position balance, making the rotor axis and the center of the bearing to form a stable angle. Centrifugal force due to eccentric rotor mass disturbance, the rotor center will be in this fixed position to do a small bow-eddy, its frequency and rotor frequency synchronization. Unlike horizontal rotors, which have different dynamic characteristics in plain bearings, vertical rotors lack the load of rotor gravity in plain bearings. Can be seen, the bearing center O, journal center O 'due to imbalance and deviation O, the journal and bearing clearance along the circumferential uneven. Lubricating fluid is driven by the journal, along the direction of rotation from a wide gap into the narrower gap to form a liquid wedge on the journal squeeze effect. Assuming that the total pressure F of the entire liquid film of the bearing against the rotor journal is located on the crushed side and toward the journal center O ', the force F is decomposed into the radial force Fe and the circumferential force Fτ of O'. Fe component from the role of support journals, equivalent to the elastic axis of rotation, component Fτ perpendicular to the radius of O 'and along the direction of rotation, so O' speed is to make the rotor eddy force. Intuitively, it looks as if the "push" shaft of the liquid film makes orbit around the bearing. For designs with better rotor stability, this vortex will be confined to a smaller area, but for vertical shafts that lack gravity load, the stability is poor and the vortex will appear to diverge more easily once it appears. This large-scale vortex caused by the poor stability of the vertical rotor can be clearly found on the shaft orbit and time-domain waveforms of the shaft vibration collected in the field. Normally stable rotor due to the existence of the role of centrifugal force, the axis trajectory will show an approximate ellipse, and constrained in a smaller range, such as Ling Ao No. 2 machine No. 3 main pump in the first fuel cycle axis vibration level About 50μm. The major pump of Ling'ao No.1 Unit No.3 maintained a high level of 200μm for most of the time of the first fuel cycle. The axial trajectory of the main pump diverged significantly and showed obvious "8" shape, indicating that the vibration signal In addition to the main frequency there is a larger disturbance there. Then observe the waveform of the time domain, the main pump motor operating frequency (25Hz) is very clear, but each of the sine wave generated due to centrifugal force are loaded with a component of the same frequency with the operating frequency, the two peak amplitude The summation basically coincides with the frequency component on the spectrum. It can be seen, the main frequency component of the pump spectrum is actually composed of several components with the same frequency (including mass eccentricity, rotor eddy, etc.), to significantly reduce the power frequency vibration balance of mass eccentricity is Not enough, to help stabilize the rotor, eliminate the same frequency disturbance axis vibration will drop to the desired level. It can be further considered that the vortices generated by instability of the vertical rotor will initially be confined to a small area due to the rigidity of the sliding bearing liquid film. However, the longer running time undoubtedly increases the centrifugal force and the centrifugal force increases the vortex force, Need to increase the stiffness to constrain a wide range of eddy, bearing fluid film will be more and more thinner, the final vibration amplitude exceeds the bearing clearance, making static and dynamic friction, destruction of equipment. In fact, Daya Bay No. 1 machine No. 3 main pump in the ninth fuel cycle axis vibration level up to 220μm, in the axis of the trajectory shows signs of touch mill, overhaul check the pump's water guide bearing has been found to wear Pit. 5. The use of new theory to explain the vibration of the main pump Knowing the dynamic characteristics of the vertical rotor can be more fully explain the main pump appear these "peculiar" vibration phenomenon. 5.1 changes in the shaft seal water flow changes the stiffness of the support and influence