The effect of the steam temperature fluctuations during steady state operation on the remnant life of the superheater header

Ligament cracking in superheater headers is generally understood to be associated with thermal transients that occur during starts. However, recent analysis has shown that under some circumstances stress changes due to the steam temperature fluctuations during continued operation also contribute to crack growth. The correlation of the stress range due to these temperature fluctuations has been established by carrying out a transient thermo-mechanical FE analysis. The contribution to fatigue crack propagation of existing defects has been quantified. Frequent thermal fluctuations at steady state operation were identified as the most influential factor for the remnant life of the header.