ABSTRACT

An overview is given of current knowledge and understanding of environment induced cracking of steam turbine discs and blades. There has been extensive research on this theme, though our ability to predict the impact of environmental variables on service life is still constrained as early research had focused on environments that were not always representative of condensates formed in service. More recent research has yielded insight into the evolution of damage from pits and has provided more detailed long crack growth rates but the growth of cracks in the short crack regime is largely undetermined. For aging coal-fired plants, the advent of two-shifting (switching on and off-load on a daily basis) will lead to a reduction in remnant life due to transients in stress and environment. Since the number of cycles is small the impact of load cycling is predicted to be modest. However, there may be an effect of the transient exposure to oxygen off-load on the subsequent crack growth rate on-load and this also needs to be accounted for in life assessment.

INTRODUCTION

Maintaining the availability of operating power plant has become increasingly important in response to increasing demand for reliable energy supply. The conversion of steam to electricity underpins conventional fossil fuel and nuclear power generation and ensuring the integrity of steam turbine systems is paramount. However, system reliability can be undermined by environment induced cracking (EIC) of the turbine blades and discs and this becomes potentially more significant with increased age of the plant and with the associated advent, for coal-fired plant, of two-shifting. Although failure of the turbine system is rare, compared with that of boiler tubes in steam generation for example, the potential consequences in terms of system damage are much more severe, with downtimes often lengthy. It is essential to have relevant data and a predictive framework combined with understanding of the damage process to enable an informed perspective on life assessment and life extension and also to reduce the time to market of advanced materials for next generation plant and for retrofit.

Environment induced cracking of steam turbine disc (typically 3NiCrMoV) and blade steels (11-13Cr) has been studied for more than 30 years, as summarised in various overviews1-9. However, the environments tested were sometimes poorly related to service condensate chemistry and, occasionally, the data generated showed inconsistency for nominally the same conditions. The consequence has been a clouding of our understanding of the impact of system variables, compounded by the complexity and transient nature of service conditions and constraints in their detailed characterisation, e.g. the chemistry of the condensate formed on the turbine steel surface. To advance this situation, two reviews were undertaken in 2003. One review7 focused on establishing best understanding of service condensate chemistry based on literature evaluation but, importantly, supported by detailed discussion with power plant chemists.

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