LIFE ASSESSMENT OF SUPERHEATER REHEATER TUBES IN FOSSIL BOILERS/

Creep rupture failure of superheater (SH)/reheater (RH) tubes is a maj or cause of forced outages of power boilers. A methodology developed r ecently by EPRI and its contractors has helped utilities make more inf ormed run/replace decisions for tubes by judiciously combining calcula tional, nondestructive, and destructive evaluations. In this methodolo gy, the tubes/tube assemblies at risk are identified by ultrasonically measuring the thickest steamside oxide scale and thinnest wall thickn ess in the tubes. The remaining life of each tube/tube assembly is pre dicted using a computer code known as TUBELIFE, thus achieving a furth er level of focus on the tubes/assemblies in the ''highest risk'' cate gory. Sacrificial tube samples are then removed from the select locati ons and subjected to laboratory metallurgical evaluation and isostress rupture testing to refine the remaining life estimates. Research has further refined this methodology by validating the ultrasonic techniqu e for scale measurement, identifying the appropriate stress formula an d oxide growth laws and evaluating the limitations of creep damage sum mation rules and isostress rupture test procedures. This paper provide s an overview of the research in the field, and establishes a road map for assessing the remaining life of SH/RH tubes.