Philip van der Meer
Turbine OEM’s use finite element modeling (FEM) to model stress and creep life exhaustion in high pressure turbine discs with complicated geometries. These calculations include many safety factors and lead to premature replacement of expensive components. On pressure parts, calculations are used for sensitivity analysis only as scatter in material properties and other drivers (temperature and stress) can easily make a 300% difference in calculated life. Under certain conditions that prevail on Power Station systems, actual creep ageing is assessed from replication to give a much more realistic life estimate. To follow the same approach on turbine discs, a plug sampling and friction hydro pillar processing repair procedure was developed for Eskom's Turbine Fleet. Life assessment based on evaluation of replicas from the sample revealed severe over-conservatism of FEM based life calculation models.
Creep modeling is complex due to high peak stresses on turbine components and further microstructural characterization using state of the art scanning electron – and transmission electron microscopy can be used to evaluate high temperature - and creep degradation of this material in the high stress areas. This could be enhanced by creep testing which simulates loading / geometry / stress fields in these materials. Improved understanding of the creep behaviour (including creep notch sensitivity) of the various turbine steels and geometries will improve Life Management strategies significantly.