Failure analysis and creep remaining life of hydrogen reformer outlet pigtail tubes

A failure analysis and creep remaining life assessment on hydrogen reformer outlet pigtail tubes in a methanol plant was conducted. The tube material was manufactured to ASME SB-407 (UNS N08810) material specifications. The observed evolution and progression of grain boundary cavity nucleation, coalescence, microcrack formation and eventual cracks were indicative of long-term creep failure, which occurred before the design life of the tubes. The remaining life evaluation was performed based on metallurgical evaluation of the outer diameter (OD), middle of wall thickness, and inner diameter (ID) percentage creep cavities versus tube OD. Very high statistical correlation was achieved by fitting creep data points to the classic creep strain versus time curve by using third order polynomial equations. The onset of tertiary creep was found to occur at 9.3% increase in the strain of the tube OD. This work affirms that using percentage area of creep cavities and the correlation to increase in OD is a robust technique for the reliable and accurate determination of the creep remaining life of superalloy tubes.