Life Assessment of Superheater/Reheater Tubes in Fossil Boilers

Creep rupture failure of superheater (SH)/reheater(RH) tubes is a major cause of forced outages of power boilers. A methodology developed recently by EPRI and its contractors has helped utilities make more informed run/replace decisions for tubes by judiciously combining calculational, nondestructive, and destructive evaluations. In this methodology, the tubes/tube assemblies at risk are identified by ultrasonically measuring the thickest steamside oxide scale and thinnest wall thickness in the tubes. The remaining life of each tube/tube assembly is predicted using a computer code known as TUBELIFE, thus achieving a further level of focus on the tubes/assemblies in the “highest risk” category. Sacrificial tube samples are then removed from the select locations 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 technique for scale measurement, identifying the appropriate stress formula and oxide growth laws and evaluating the limitations of creep damage summation rules and isostress rupture test procedures. This paper provides an overview of the research in the field, and establishes a road map for assessing the remaining life of SH/RH tubes.