Research On Service Life Of Large Turbine Rotors

Life problem of large steam turbine rotor is of great important to ensure the safe andeconomic operation of the power units. It is related with many theoretical and technicalityproblems including high-temperature low-cycle fatigue mechanism, numerical method, heattransferring analysis, mechanical analysis, probabilistic method, operation optimization andthe conditional maintenance of the rotors. In this paper, some works are presented on new method to assess the life damage oflarge turbine rotor, the online stress analysis model, endurance limit of damage based on thereliability and nonlinear damage accumulate rule, and development of the professional lifeassessment software system of the rotors, etc. 1. The incremental finite element program for emulating the internal stress field iscarried out on the bases of elasto-plastic constitution equation. Damage model is also beendiscussed with the theoretical framework of the equivalent plastic energy; thus it can be usedto evaluate the damage for high temperature low cycle loading. This model can reflect thebehaviors of cyclic softening, yield surface changing, which is consistent with experimentaldata. A numerical based fatigue experiment method has also been raised, which can be usedto evaluate the loop loading process, this method can also be used to evaluate the damage ofthe rotors directly. 2. A high precision one-dimensional online model has been developed on the base ofthe elasto-plastic theory, which is widely used in monitoring system for calculating the stresson the critical location of the rotors. Method of finite difference has been applied in thedevelopment, which makes the model having a better adaptability to computing hardware.This model also has the ability of considering temperature-changing material parameters,which increases the calculating precision. Correcting method of stress concentration incondition of plasticity has been discussed in the paper. With this calculating model andcorrecting measure, monitoring system will have a power ability to balance the requirmentfor precision and calculation speed. 3. A serious of peak load cyclic experiments on a 125MW power unit is reported,including brief introduction of experiments. Stresses spectra of dangerous points and thedamage of rotor's service life during the peak load operation are given with the full-sizerotor finite element analysis。Through detailed numerical emulation for these experiments,three ways of how to decrease the damage of service life without increasing the time forstart-up and shutdown process has been discussed. The research results show: Increasinginitial temperature, decreasing initial pressure of steam, as well as decreasing the operationtime in low load, can obviously control the cold shock to rotors, thus will extraordinarydecrease the life damage of rotors. Controlling steam parameters smoothly can avoid thestress wave-motion, which can reduce the life damage of rotors. Decreasing raise rate ofload under low load states can also control the life consumption of the turbine rotors. 4. Endurance limit of cumulative damage of steam turbine rotors, which based on thereliability estimation method and nonlinear damage cumulating theory, has been investigated.The endurance limit can reflect the low-cycle fatigue life damage and its cumulative effectsbetter than the traditional damage criterion quantity in some situation. The logarithmicnormal distribution law has been used in this paper for its coincidence with the actualstatistic distribution of the rotor's failure. Using this technique and nonlinear cumulativedamage theory, the internal relationship between the crack initiation life damage and thecrack propagation life damage is investigated under any selected reliability, and thedetecting schedule of the rotors all through its service time. This research shows that thenonlinear theory will get more conservative results. 5. Aiming at the characteristic of heat transferri...