Nuclear power plant main pump vibration problems what are the fundamental reasons

1 reactor main pump type and structure

The main reactor 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 reactor main pump special vibration problem

Since the commencement of commercial operation of the two power plants, the special vibration problems of the 100 main pump have long plagued the professional and technical personnel. These special vibration problems mainly show the following characteristics.

2.1 The main pump vibration level is significantly affected by the shaft seal water flow

April 8, 2003, Ling Ao No. 1 machine due to commissioning RCV charge pump discharge orifice plate, resulting in the main pump shaft seal No. 3 from 2.0m3 / h to 1.7m3 / h (system design requirements shaft seal flow control At 1.8m3 / h), the pump shaft vibration was also decreased from 200μm to 150μm. After that, the shaft seal flow was increased to 2.0m3 / h and the vibration level rose back to 190μm. It is worth noting that the response of the vibration of different main pumps to the change of shaft seal water flow is completely different. For example, the shaft seal water adjustment test of D1RCP001 / 002 / 003PO on February 22, 2000:

2.2 m 3 / h 160 μm 90 μm 200 μm

2.5 m 3 / h 155 μm 140 μm 180 μm

3.0 m 3 / h 155 μm 190 μm 160 μm

It can be seen from the above table: after the shaft sealing water flow increases from 2.2m3 / h to 3.0m3 / h, the vibration level of No.1 pump basically remains unchanged, the vibration of No.2 pump shaft increases significantly, while No.3 pump is obvious Decrease

2.2 The main pump vibration is constantly changing point

Such as: March 8, 1999, 105 after the overhaul of a loop temperature, the pressure rise process, recorded the vibration of D1RCP002PO vector changes in the cold start frequency component of 160μm / 8 °, heating, booster to heat When the reactor is shut down, the vibration vector changes to 154 μm / 104 °, which means the vibration direction changes by 96 °.

2.3 Repeated on-site balancing to reduce the vibration level

Whenever the vibration of the main pump shaft increases sharply, the main frequency components of the changes are the power frequency (accounting for 80% to 90% of the on-frequency vibration). The general treatment method is to reduce the vibration level on-site during thermal shutdown conditions . 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 main pump start-up sequence changes will also cause the main pump vibration state changes

Such as: April 17, 2003 109 overhaul later, the unit back to the thermal shutdown state, D1RCP002PO axis vibration is good, the lazy walk test while stopping the three main pump, followed by 1, 3, 2 in order to start the main pump , 002PO the original vibration at 90μm, then sharp rise to 200μm after the start, followed by the second lazy test, the three main pump outage in accordance with the order of 1, 2, 3 to start again, 002PO axis vibration level returned to 100μm good level. In the meantime there is no other action than changing the startup sequence.

3 Several Theoretical Explanations and Limitations of the Main Pump Vibration at Present

3.1 quality imbalance

Mass imbalance 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 crystallization caused by the inner surface of the bearing defects

In response to the vibration problem of the main pump, we issued a document to the EDF, hoping that the other party 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. However, it is regrettable that there is no consensus about the root cause. It is only a general reference to the defect that the solid boron crystallizes 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 The main pump shaft heat variable

For some time now, the thermal variation of the main shaft has been considered as the main cause of the main pump vibration problem. 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 and 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.
4 rotor in the dynamic characteristics of the sliding bearing is the root cause of vibration problems of the main pump

The dynamic theory of horizontal dynamic pressure sliding bearings that when the horizontal rotor stable operation, the rotor formed by the hydrodynamic fluid film has a bearing capacity of the rotor, the force and the rotor weight will be in a position balance, making the rotor The center of the shaft forms a stable angle with the center of the bearing. 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 toward the journal center O ', the force F is decomposed into the radial force Fe and the circumferential force FÏ„ of O'. Fe component from the role of support journals, equivalent to the elastic axis of rotation, component FÏ„ 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 if present.

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 fall 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 Apply a new theory to explain the vibration of the main pump

Knowing the dynamic characteristics of the vertical rotor can explain these "peculiar" vibrations of the main pump more completely.

5.1 changes in the shaft seal water flow changes the stiffness of the support and thus affect the rotor vibration

Study of the specific structure of the main pump is not difficult to find, to maintain a primary seal circuit seal 1,2,3 and the main pump water guide bearing unified constitute the main pump rotor radial support system. The thickness of the oil film (liquid film) is one of the factors that affect the stiffness of the sliding bearing. The thicker the oil film, the weaker the rigidity, and vice versa. When the shaft seal water flow changes in fact is to change the vertical support stiffness of the rotor, shaft vibration state changes are not hard to understand. On the other hand, during normal operation, the vertical rotor is in a state of weak equilibrium. Under the obvious disturbance of stiffness change, the original steady state is destroyed. Once the "unfortunate" position can not be found again, the eddy amplification, Vibration state can not be restored to the original level.

5.2 Changes in the working environment affect the working position of the rotor in the bearing

In fact, during the process of heating up and boosting of the first circuit after overhauling, the rotor system of the main pump rotor is constantly adapting to the working environment of the rotor, "finding" its best working position, and returning to the first circuit temperature and pressure in the state of thermal shutdown After the rotor journal in the final position in the sliding bearing will be determined. This process is reflected in the vibration phase, which manifests itself in the constantly changing vibrations.

5.3 Field Balancing is the balance of the rotor system synthesis factor

Through the analysis of the main pump shaft trajectory and time-domain waveform, we can find that when the main pump vibration is large, it shows most of the power frequency vibration, but in fact the vibration amplitude of this frequency is composed of two parts : First, the real mass imbalance of the rotor system, the absolute existence of this component, but the difference between the size; Second, and the frequency of the whirlpool frequency of the same frequency, the vortex is due to poor stability caused by vertical rotor , And continue to increase with centrifugal force. On-the-spot balancing is actually an additional counterweight to rotor thermal variables, hydraulic imbalances, mass imbalances, and specific conditions of the rotor system including stiffness and damping. This weight must make the rotor in sliding bearings to achieve a relatively stable state is considered successful, otherwise the balance will need to be carried out again. In the meantime, the obvious change in working condition (stiffness, damping) of the main shaft of the main pump during operation also leads to the failure of balancing efforts.

6 to improve the stability of vertical rotor method

6.1 practical means of operation

The issue of vertical rotor stability is fixed at the time of mechanical design, but that does not mean that the operating unit must do nothing. The stiffness of the support system, the clearance of the plain bearings, the precision of the mass balance, and many other factors can affect the stability of the rotor. In practice, if the main pump vibration larger in the repair process can focus on checking:

(1) bearings, seals and other support system bolts tightness. In the case of a certain rotor disturbance, the greater the stiffness, the smaller the response to vibration. Support system in the ideal design will help improve the stability of the rotor.

(2) the gap between the rotor and the sliding bearing. Sliding bearings with excessive clearance can cause small imbalances that can cause large vibrations.

(3) try to improve the balance of the rotor accuracy. According to the theory of vibration, unbalanced disturbance can exist on any rotor. Increasing the balance accuracy helps to reduce the disturbance to the rotor that is vulnerable to stability. Of course, whirlpool occupy the main components of the situation, to further improve the balance accuracy is very difficult, this time or should help to stabilize the rotor more efforts.

6.2 The ultimate solution to the main pump vibration theory discussion

In order to solve the problem of poor stability of the vertical rotor, some foreign technical documents suggest that an eccentricity should be preset when installing the back wheel of the vertical rotor, and the stability of the rotor can be improved by using the rotor eccentricity flexible force. Of course, this eccentricity The amount should be a strictly calculated value, it can not only improve the stability of the rotor but also to ensure that the shaft will not be too much eccentric equipment damage caused. May require the main pump manufacturers to carry out this work.


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