Life Assessment of Superheater/Reheater Tubes in Fossil Boilers

Abstract

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.

A review of non-destructive techniques for the detection of creep damage in power plant steels

“…are very difficult to take into account, as recognised for example in [24], and small uncertainties on the values of stress intensity can translate into large errors in the computed creep life [25]. Nevertheless, the collected data can help identify areas that require special attention during periodic inspection [26] great care must be taken when extrapolating the data of laboratory creep tests, as they cannot always reproduce the same conditions of temperature and stress at which the actual components will operate.…”

Comprobación de la uniformidad de resultados de un equipo de ensayos simultáneos de termofluencia

“…De esta manera se cotejaron entre si las curvas de ensayo respectivas y los parámetros característicos obtenidos. Al mismo tiempo se realizó un contraste de los sistemas de medición y control, haciendo uso de equipos certificados por laboratorios acreditados [1,2]. Se obtuvieron resultados que indican la uniformidad del funcionamiento de las tres máquinas y su concordancia no sólo entre sí, sino con datos obtenidos en la literatura actual y presentados por otros autores.…”

“…La caracterización de este fenómeno se debe efectuar con máquinas de ensayos apropiadas, las cuales suelen ser de tecnología dependiente de patentes costosas [1].…”

Limit Load Analysis of Pressure Structures Containing Pitting Defects Subjected to High Temperature Creep

“…Research on design and prediction of remaining strength of thin/thick walled cylinders under creep condition has been discussed [10][11][12] . Theoretical derivation and experimental procedures are covered as well as numerical analysis, including the determination of plastic loads pipes with internal uid pressure.…”