The fatigue behavior in steam turbine valve is generally considered to be associated with thermal transients during start and stop phase. However, the recent analysis has shown that steam parameter fluctuations under in-service steady state operation could induce significant increase in the damage. In this paper, the effect of in-service steam parameter fluctuations on stress-strain behavior in the valve was analyzed based on FE analysis under in-service data. The rain-flow cycle counting method was applied to get effective stress-strain cycle numbers and cycle amplitudes to classify the types of fatigue cycle based on in-service data. The creep-fatigue damage during steady state operation of the valve was estimated by using R5 (Volume 2/3) high temperature assessment procedure together with the FE results. Frequent steam pressure fluctuations at steady state operation were identified as the most influential factor for the fatigue life of the steam turbine valve.

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