The Research On The Dynamic Characteristics Of Crankshaft For Large-scale Reciprocating Compressor

Reciprocating compressor, as power equipment, plays a key role in petrochemical industry. The crankshaft forced by complex alternating load, with its magnitude and orientation, is key component of reciprocating compressor and occurs various vibrations, such as bending, torsioning, and combined bending and torsional vibrations. The crankshaft's vibration caused by high-speed rotation positions a key factor to trigger the vibration of compressors. crankshaft's dynamics characteristics affect not only the lifetime of crankshaft, but also safety and stability of compressor units, all of which has a negative impact on other components'lifetime and performance. The most obvious examples are crank bearing, main shaft bearing and gas valve. With the developing of chemical processes, the reciprocating compressor is now going towards the tendency of large-scale and multi crank. However, the increase of crank of crankshafts causes the decrease of torsional frequency of crankshaft. As a result, torsional resonance of crankshaft happens with increasing frequency. The torsional vibration has been a technical software for reciprocating compressor to develop towards multi crank and highly rotational speed. Also, research on shaft dynamics analysis of large-scale reciprocating compressor has received more and more great attention from scientists and technicians.Based on frequent fracture of crankshaft and crank bearing scuffing happened during development process of China's first 6M50 type reciprocating compressor and its subsequent products, the article conducts analysis and studies on the dynamics characteristics of large-scale reciprocating compressor crankshaft.The principal excitation loads, such as tangential force, normal force, harmonic load on crank pin from connecting rod and torsional moment borne by motor rotor, are obtained according to KOL heat-power analysis platform introduced from German BORSIG for special use of BX series compressors. Finite element method of statics is employed to make crankshaft statics analysis of large-scale reciprocating compressors and the static and fatigue strength of crankshaft under changing load are examined. Basic method that exerts inertial mass of piston, connecting rod, crosshead and drive motor rotor is forwarded to build dynamics analysis model which considers all inertial mass. Modal analysis of crankshafts is analyzed by dynamic finite element method (FEM). Basic concept of harmonic frequency load is presented so that basic methods to separate harmonic frequency load and steady-state load from harmonic load of crankshaft is proposed, which greatly simplifies the process to analyze harmonic response to shafting. Harmonic Response analysis of crankshafts forced harmonic frequency load is analyzed on basis of modal analysis. Transient dynamic analysis of crankshafts forced time history load is analyzed on basis of modal analysis, and the static and fatigue strength of crankshaft under changing load are examined. Add stress analysis of crankshafts is analyzed on basis the results of statics and transient dynamic analysis. Experimental study and structure optimization for those fracture'crankshafts are carried out by the analysis method, which is provided in this paper.The results obtained show that:the static strength problem can be identified as a result of checking of static and fatigue strength for the original crankshafts and the torsional resonance problem for the magnified crankshafts; Torsional resonance is only possible form of resonance, which can be clear that various kinds of bending and transverse modal don't have to be considered. The 2nd and more order torsional resonance can be negligible, which provides theoretical foundation to analyze and study only on 1st torsional natural frequency of large reciprocating compressors. After transient response analysis on crankshafts of different rotational speeds, it is further verified the origin of "beat" is an integration of the same vibration forms resulted from crankshaft load and inertial load. Through the research that compressor parameters affect on shafting dynamic performance, it turns out the smaller compressor stroke is, the better dynamic performance it will be. By structure optimization and tests contrast on 6M50 compressors with different main shaft diameters, test results are identical with theoretical analysis, proving the feasibility of that analytical method.The analysis methods of this paper can be applied to static, modal, harmonic response and transient response analysis of crankshafts for reciprocating compressors. Crankshaft static analysis has to be performed first during dynamic performance analysis and strength check for various reciprocating compressors. Modal analysis can only be executed once static analysis results meet the requirements. In accordance with standard of resonant vibration of API618, crankshaft resonance conditions can be determined by modal analysis results of crankshafts. Static analysis results can meet the design requirements if shafting speed is deviated from resonant zone. Dynamic response analysis for shafting is needed if shafting speed is within resonant zone. The application of research results can not only optimize the crankshaft structure of current reciprocating compressors, but also diagnose and handle faults for large reciprocating compressors, providing analysis basis on dynamics theory for the research and development of new multi crank reciprocating compressors.