The Dynamic Characteristics Study Of An Integrally Geared Centrifugal Compressor

In recent years, integrally geared centrifugal compressors are developing rapidly in the world. At the same time, in our country, the demand of this type of compressor is large, and is widely used in petroleum, chemical, metallurgy, pharmaceuticals and etc. At present, though the domestic manufacturers can create some simple types of gear compressors, but the high-end products are still mainly relied on import. The gear-rotor-bearing system of integrally geared centrifugal compressor is a typical coupling shafts system, but some large foreign compressor manufacturers separate the coupling shafts system and design the single shaft respectively when design the integrally geared centrifugal compressor. The dynamic characteristics of a gear-rotor-bearing system are more complex, can couple to new modal frequencies and generate new critical speeds and vibration peaks due to the engagement of the coupling gears. It is necessary to set a gear-rotor-bearing system as the research object in the study of this kind of compressor and analize the dynamic characteristics of coupling shafts system in the processes of acceleration and lifting load.Therefore, this paper sets a four axis and six stages integrally geared centrifugal compressor as the reseach object and studies the following three aspects of the coupling system under the range of 20% load to 100% load:(1) Firstly, compute the dynamic characteristics of the coupling system in flexible supports during the acceleration process.The study found that coupling system modal frequencies are mainly consist of uniaxial bending frequency, uniaxial bending and torsion coupling frequency, multi-axial bending and torsion coupling frequency. The study also found that, the bearings unbalance response of the coupling system emerge new peaks, and the critical speeds of the bearings and impellers change.(2) Secondly, analyze the dynamic characteristics of the coupling system at operating speed during the process of lifting load. The study found that as the load is increasing, the coupling system frequencies increase.The bending and torsional coupling modal frequencies of the low and middle speed shafts intersect at 75% load with their respective excitation frequencies. At 80% load, the vibration peaks occured in the unbalance response of the low speed shaft and middle speed shaft and these explain the critical load phenomenon during increases the load in the factory theoretically. With the load incrasing, the corresponding logarithmic decrement rates of the uniaxial bending mode frequencies decrease. The corresponding logarithmic decrement rates of the uniaxial bending and torsion coupling frequencies portion decrase, the others increase.The corresponding logarithmic decrement rates of the multi-axial bending and torsion coupling frequencies increase.(3)Finally, this paper investigated the impacts of dynamic characteristics on the gear-rotor-bearing system with different bearing parameters. The study found that the pads arrangement in the form of tilting pad bearing has significant impact on the vibration characteristics of the coupling system. The arrangement in the form of load between pads is superior to the load on the pads. With the increasing numbers of tilting pad, the bearing vibration amplitude of the coupling system decrease, the amount of vibration of the impeller increase. From the point of vibration amplitude, we know that why four and five pads of the tilting pad bearing are often chosen in engineering as the supports of an integrally geared centrifugal compressor. With the bearing pre-load increasing, the bearing vibration of the coupling shafts increase and the impeller vibration reduce.