A remaining life assessment of a cracked attemperator steam line

Abstract

The present paper attempts to describe a series of detailed remaining life assessments of a cracked reducer weld location in a attemperator steam line in a 120 MW fossil-fired unit. Crack growth occurred by either pure fatigue or by creep-fatigue depending upon the combinations of stress and crack depth, i.e. maximum stress intensity level, K MAX. From consideration of certain fatigue and creep threshold details and measured fatigue striation spacings on the failure surface a stress of 80 MPa was estimated to be driving the cracking process. Also high temperature fatigue crack growth data was treated in terms of a series of constant probability crack growth curves which allowed real probability values to be assigned to any particular remaining life assessment. It was shown that for probability levels of 10 −4 and an estimated active stress of around 80 MPa the remaining life of initial 4 mm and 7 mm deep cracks was 231 and 87 starts respectively.