A fitness-for-service (FFS) evaluation was performed on a cogeneration power plant high pressure steam line. The piping system included a Gr 91 to Gr 22 dissimilar metal weld (DMW) between an SA-335 Gr P91 pipe and an SA-335 Gr P22 reducer. This unit had accumulated more than 100,000 service hours. The FFS project included as-built and as-found piping stress analyses, redistribution of creep stresses, weld residual stress estimates, review of the nondestructive examination (NDE) results, evaluation of the plant information data, and a fracture mechanics evaluation.
Due to tapering of the Gr P22 pipe to match the wall thickness of the Gr P91 pipe at the DMW interface, the Gr P22 pipe wall thickness was not in compliance with the ASME B31.1-2000 Code minimum wall thickness (MWT). In addition, an inspection of the single groove girth DMW revealed ultrasonic indications along the fusion face of the weld. The lack of side wall fusion (LSWF) can be considered as a planar and crack-like indication, so a fracture mechanics evaluation was performed.
The piping stress analysis revealed that the DMW was subject to low axial stresses. A creep life fraction evaluation was performed for the material that does not have the LSWF indication, considering conservative creep rupture data, weld performance degradation, redistributed creep stresses, and multiaxial stresses in the DMW. Due to the low stresses, the life fraction analysis estimated more than 50 years of remaining creep life.
Failure Assessment Diagrams (FADs) were developed to evaluate the component integrity for various combinations of crack depths and lengths. The reported indications should not compromise the integrity of the component subject to the typical plant operation during the next 12 years and the component is fit for continued operation.
The crack propagation evaluation determined that the DMW has a predicted minimum safe operating life of at least 12 years if operated as in the past. Because the wall thickness of the tapered reducer is below the ASME B31.1 Code allowable MWT, it was recommended that the DMW be examined during a scheduled outage within 5 years and the component be replaced with Gr P91 material during a major outage within 8 years.