CREEP ESTIMATION OF GAS TURBINE ROTOR DISCS AT DIFFERENT AMBIENT CONDITIONS

Abstract

This paper discusses the usage of several creep models in combination with ANSYS MECHANICAL APDL finite element software and analytical calculations to predict creep strain in terms of axial and radial distortion for the typical gas turbine rotor disc model. Creep analysis of gas turbine disc using different creep models at offset design condition is the main consideration of this work. For many components of industrial gas turbine, the design for creep is conservative because of close control of dimensions required over the life, the maintenance of critical dimensions, such as clearance between rotating and static parts. Components operate for extensive periods of time under heavy loads in conditions of non-uniform temperature which go up to 5350C during the process. The stress-strain state also changes over time due to creep phenomena. These conditions make difficult to simulate the actual body by other means like finite element analysis; it is essential to establish creep properties over all ranges of temperatures, stresses and durations of time for the reliable prediction of creep deformation and stress rupture in gas turbine components. A typical gas turbine disc model is used in this present study to investigate the relationship between operating conditions and design parameters on the creep phenomena. Several ANSYS creep mathematical models are investigated to use for perfect fitting according to the conditions and available data. Time hardening and strain hardening creep model is further studied to analyze the creep of disk. While doing creep analysis, ambient temperature is set to standard 320C. But this standard process is not valid for every site of world so a different procedure has been set up to simulate the offset design conditions to capture site specific creep issues. Results show variations in axial and radial creep strain with respect to analysis done at constant average temperature for 160k hours. A good agreement is also found between the results obtained in this current study and the results available in the literature for strain hardening and time hardening model. It is found that stress relaxation after 160K hour using time hardening model is more than using strain hardening. Analysis with offset design condition shows that to capture the clearance correctly and more realistically, offset design condition needs to be simulated. Present work concludes that further study is required in the area of creep mathematical model and validation of simulated results with experimental data.