Validation of Creep Crack Growth Life Estimation Methodology/Hot Reheat Steam Pipes

Abstract

This paper documents a validation study of a creep crack growth life estimation methodology developed by Riedel and Rice, Saxena, and Bassani which has been implemented into a Babcock & Wilcox computer code. This computer code called PCCREEP was developed for estimation of remaining lives of fossil power plant components with both postulated and in-service determined defects. The initial validation was performed through comparisons with other computer codes. However, these comparisons were limited to continuous operating conditions (steady-state creep) and bulk creep deformation properties for secondary creep only. Even with the independent comparison with other available life estimation codes, the question still remained of how well can the creep crack growth methodology estimate actual field lives? This paper presents the results of a study of a life estimation scheme for hot reheat steam pipes having defects found in service. The initial survey was conducted by the Electric Power Research Institute (EPRI) of U.S. electric utility problems with hot reheat steam pipes. The primary reason for that study was three catastrophic failures of hot reheat steam pipes in 1979, 1985, and 1986. These pipes were seam-welded 1-1/4 Cr-1/2 Mo and 2-1/4 Cr-1 Mo. All these ruptures initiated from flaws (near the weld fusion line) which grew by a creep mechanism. This EPRI data base consisted of twelve pipes with flaws found from inspection. While several assumptions relating to material properties and operating conditions were required (due to insufficient information), predictions demonstrated that life estimation is possible provided material property data representative of the material condition for the component in question is available. Discussion is given regarding the significance of the postulated flaw location, constraint (stress triaxiality) effects for buried flaws, and the importance of weld metal, heat-affected zone, and base metal properties to the accuracy of the life estimations.