Failure analysis of Rotating Blades of Lowpressure Steam Turbine Rotors and Possibility of Prediction Corrosion-fatigue Ruptures

Josef Kasl,Miroslava Matějová,Jakub Mrštík,S Pantelakis,S Koubias

doi:10.1051/matecconf/201818804020

Josef Kasl, Miroslava Matějová + Show 3 more

Open Access

https://doi.org/10.1051/matecconf/201818804020

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Abstract

Over the last few years, corrosion fatigue fractures initiating from corrosion pits have been a root cause of failures of rotating blades of the third stage of the low-pressure parts in several 200 MW turbines in ČEZ a.s. power stations. This contribution deals with the analysis of several of these failures. Metallurgical investigation of the blades showed that the cause of the failures was the initiation and growth of a fatigue crack from a corrosion pit. ČEZ, a.s. has developed, based on the knowledge obtained by EPRI (Electric Power Research Institute), a methodology which can, in real conditions during checks of turbines, reliably detect the parameters of corrosion pits and predict the possibility of development of fatigue damage from detected pits. The process of methodology and its uncertainties (influence of filling of pits with oxides, cyclic stress calculations, and the selection of the geometric factor Y) are summarized.

Highlights

Quite a large portion of the forced idle times of the blocks of thermal and nuclear power stations is connected with defects in their turbines
Over the last few years, corrosion fatigue fractures initiating from corrosion pits have been a root cause of failures in several 200 MW turbines in ČEZ a.s. power stations
Chemical and phase compositions of the material in the corrosion pit were analysed by ED microanalysis and X-ray diffraction

Summary

Introduction

Quite a large portion of the forced idle times of the blocks of thermal and nuclear power stations is connected with defects in their turbines. Based on the membership of ČEZ a.s. in the P65 programme in EPRI (since 2014), a methodological procedure enabling the evaluation of fatigue crack development in the rotating blades of low-pressure components of steam turbines when cracks are initiated from corrosion pits on the surface of blades was obtained. One of its three principal objectives is to implement a diagnostic procedure which will make monitoring and evaluating the influence of pit corrosion on the surface of the rotating blades of the LP components of steam turbines in thermal and nuclear power stations possible with respect to the potential development of fatigue damage of these blades. This contribution deals firstly with an example of analysis of the last failed blade in the power station in Počerady and secondly with the current state of application of the methodology of evaluation of the corrosion pits on the blades of the steam turbines from point of view of the susceptibility of fatigue failure

Failure of the blade in the power station in Počerady

Principle of methodology

Application of methodology

Uncertainties of the applied methodology

Summary

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