Fitness for Service and Remaining Life Assessment for 1-1/4Cr-1/2Mo Reactor System Piping on a Catalytic Reformer Unit

Fitness for service and remaining life assessment were performed on high temperature reactor piping to verify mechanical integrity for a desired remaining life. The NPS 20 1-1/4Cr-1/2Mo piping, which is the subject of this paper, was built in 1968 to the 1966 edition of ASA B31.3 for design conditions 465 psi at 950 °F / 365 psi at 1000 °F (3.16 MPa at 510 °C / 2.48 MPa at 538 °C) for hydrogen and naphtha service. The actual operating conditions are 400 psi at 900 °F / 330 psi at 950 °F (2.72 MPa at 482 °C / 2.24 MPa at 510 °C). Due to numerous reported failures in the industry in the 1980’s, the ASME codes for piping and pressure vessels lowered the allowable stresses for low alloy steels operating in the creep range, mainly above 900 °F (482 °C). Piping systems designed prior to changing the allowable stress do not satisfy today’s codes. The operating stresses which can lead to failure from potential damage mechanisms, e.g. creep, reheat cracking and general corrosion, are defined and their impact on fitness-for-service and remaining life evaluated. Acceptance criteria for different types of defects were established prior to the unit maintenance turnaround by: 1. Finite element modeling of assumed different degrees of weld peaking and pipe out-of-roundness for longitudinally welded pipe. 2. Piping flexibility / stress analysis to identify areas with the highest operating stresses. 3. Stresses from 1 and 2 above were used to calculate the creep life based on Larson-Miller parameter (API 530). Acceptable flaw sizes were limited by the desired remaining life. Inspection plans were developed to inspect for reheat cracking, creep damage, peaking, out of roundness, as well as general corrosion.