1 rotor system finite element model

1.1 5-watt tilting pad bearing simplification

The dynamic characteristics of the oil film of the sliding bearing reflect the corresponding variation of the oil film force when the journal deviates from the static equilibrium position and moves around this position and usually plays the role of nonlinear elastic force and damping force. However, in most cases, it is usually possible to reduce the oil film force to a linear elastic force and a damping force because only a small amplitude is involved. Large centrifugal compressor rotor system to use more than 5 watts tilting pad bearings.

1.2 diaphragm coupling simplified

In flexural vibration analysis, the flexural rigidity of the coupling must be taken into account. Therefore, the necessary simplification is required according to its actual dimensions. The mass and moment of inertia of the coupling must be the same, which simplifies to a solid shaft where the coupling and the shaft fit together.

1.3 rotor system finite element model

In this paper, the beam element is used to simulate the shaft. Taking into account the actual rotor system axial deformation relative to the bending deformation and torsional deformation is small, can be ignored, therefore, the displacement of the two ends of the node as a generalized unit coordinates us.

Considering the influence of diaphragm coupling and 5-watt tilting pad bearing, the finite element model of the rotor bearing system is established.

2 Unbalanced response analysis

According to the relevant design standards, the location and size of the imbalance of each cylinder rotor system.

3 different bearing parameters of the system under the unbalanced response simulation

Under different aspect ratios (L / D = 0.5, 0.7, 1.0), the horizontal and vertical composite amplitudes of the journal at the left bearing of the low-pressure cylinder are calculated as a function of speed. As the aspect ratio increases , The resonance peak value will decrease when it passes through the critical speed, while its amplitude will be 5.58, 5.16 and 4.59m respectively when the working speed is near. The large aspect ratio will help to reduce the vibration of the journal However, taking into account the large aspect ratio, easy to cause oil discharge difficulties, which may result in increased oil temperature is not conducive to the stable operation of the rotor. If you choose the aspect ratio of 0.5, will cause over-resonance peak, the vibration is too large. So for the system to meet the design requirements under the conditions of the selected aspect ratio of 0.7. Take L / D = 0.7, the remaining parameters are the same as Figure 5. At different clearance ratios (cp / R = 0.20%, 0.25%, 0.30 %), The composite amplitude of the journal at the left bearing of the low-pressure cylinder is calculated as a function of rotational speed. As the clearance ratio increases, the critical speed will decrease when the critical speed is passed, but the resonance peak will increase , While in the working speed range of its amplitude were 3.19,4.64,5.56 m. This shows that increasing the clearance ratio, will result in reduced critical speed, increased resonance peak and increased neck vibration.

Take cp / R = 0.30. In the case of different lubricant types (No. 32, No. 46 and No. 68), ie under different lubricant viscosities, calculate the combined amplitude of the journal at the left bearing of the low pressure cylinder as a function of speed , Different types of lubricants have a great influence on the vibration characteristics of the system. The vibration of the system using 32 lubricating oil is obviously greater than the other two, while the other two are not very different. The amplitudes are 13.5, 5.56 , 5.29m. It can be seen from the above analysis, the viscosity of the lubricating oil is helpful to reduce the vibration of the journal, for engineering practice, usually choose No. 46 lubricating oil.

Through the above analysis, it can be found that the large aspect ratio can reduce the resonance peak and reduce the vibration of the journal. The small clearance ratio will result in the increase of the critical speed, the decrease of the resonance peak and the vibration of the journal. The change of the lubricant type Lubricant viscosity, can reduce journal vibration.

4 test and theoretical analysis of the comparative analysis

A large centrifugal compressor on-site test-drive coupling through the coupling of each cylinder, by adjusting the control system can control the speed of the lifting speed, the vibration test of the measuring point for the left and right bearings of each cylinder, the acquisition of the bearing during the speed up and vertical Vibration data in the straight direction. The parameters selected for the bearing test are as follows: the aspect ratio L / D = 0.7, the clearance ratio cp / R = 0.3% and the lubricating oil type No. 46.

Still taking the low-pressure cylinder left bearing as an example, the imbalance response of the theory and experiment in the horizontal direction and the vertical direction is compared. The critical speeds are basically the same, but the difference between the resonance peaks is quite large. However, the amplitude is relatively close to the working speed range, but the actual amplitude is still slightly larger than the calculated value. This may be due to the fact that the basic rigidity of the test car is small, Or choose a larger damping option.

5 Conclusion

(1) By studying the influence of different bearing parameters on the vibration of the journal, it is found that the large aspect ratio can reduce the formant and reduce the vibration of the journal. The small clearance ratio will result in the increase of the critical speed, the decrease of the resonance peak and the reduction of the shaft Neck vibration; change the lubricant type, increase the viscosity of lubricating oil, can reduce the vibration of the journal.

(2) by comparing the theoretical calculation and the experimental analysis shows that the theoretical calculation of actual shaft vibration is slightly larger than that, considering the influence of various factors may be due to the small stiffness test, test gas medium or damping due to the larger choice.