Session: MF-05-02 Fitness-For-Service and Failure Assessment-2

Paper Number: 124319

124319 - Remaining Useful Creep Life Evaluations of Three Main Steam Piping Systems With Girth Weld Cracks 

Safety and reliability are the preeminent concerns in the design, operation, and maintenance of power piping systems.  Main steam (MS) piping systems in power piping plants are subject to severe operating conditions including creep range temperatures, high pressures, high stresses, cycling, and malfunctioning supports.  Over time, these factors lead to material degradation which can significantly reduce the long term integrity and safety of the piping system.  The evaluation of remaining useful creep lives of critical components is vital to ensuring the efficacy of these piping systems and is useful to make informed maintenance decisions.

Conventional designs of MS piping systems in the United States comply with the American Society of Mechanical Engineers (ASME) B31.1 Power Piping Code.  Code compliant thermal expansion stresses are below the yield stress and gradually redistribute over time.  However, the ASME B31.1 Code does not consider the evaluations of time-dependent thermal stress redistribution, multiaxial stress effects, through-wall stress redistribution, residual welding stresses, and time-dependent creep life consumption due to material creep degradation. 

The MS piping system is one of the most critical high energy piping systems.  Girth weld failures are typically due to a combination of high operating temperatures and unintended high stresses due to malfunctioning supports.  Industry best practices include:

·         The evaluation of historical operating conditions.

·         Hot and cold walkdowns to document the field piping system conditions and anomalies.

·         Examinations of critical weldments to reveal detailed geometry data, actual material chemical compositions, nondestructive examination (NDE) indications, and other microstructural material damage.

·         An as-designed piping system stress analysis to provide baseline information.

·         An as-found piping system stress analysis to simulate significant field displacements anomalies.

·         Recommended support corrective actions.

·         Creep life consumption evaluations at critical girth welds considering the effective multiaxial weldment stresses.

·         Remaining useful life (RUL) contour plots to illustrate the range of girth weld RUL estimated values.

·         Recommended schedule for critical weld examinations.

Over the past two decades, a creep life consumption methodology has been used to predict material creep damage in MS piping system girth welds considering the most current site information.  This methodology has been used to prioritize the most likely girth welds to have creep damage and provide the circumferential locations of maximum creep damage.  Three MS piping systems with service-related girth weld creep cracks are evaluated in this paper to fine-tune the creep life consumption methodology, including the time-dependent stress redistribution curves, cumulative life consumption curves, and illustrations of color-coded piping system RUL contour plots.  The piping system RUL contour plots provide a visual representation of the girth weld remaining creep lives, which facilitate prioritized inspection decisions.

Presenting Author: Marvin Cohn Intertek

Presenting Author Biography: Marvin Cohn is the director of HEP at Intertek-AIM. Over the past 40 years, Mr. Cohn has written more than 170 reports and published more than 60 technical papers, including 8 professional journal papers. Many of these technical papers regarding high energy piping systems subject to creep/fatigue damage have discussed the process of detailed and well-characterized data gathering, simulation of existing field conditions, evaluation of applicable creep/fatigue stresses, creep rupture life consumption in base material and weldments, and ranking of the most critical examination locations.
Throughout the past 25 years, he has participated in the American Society of Mechanical Engineers (ASME) Pressure Vessel and Piping Conferences where he has presented papers, developed and chaired technical sessions, and edited Proceedings. During the last 20 years, he has also actively participated in the ASME B31.1 Power Piping Code Committee. He has particularly contributed to the ASME B31.1 recommended guidelines for post-construction power piping activities.
Mr. Cohn has professional engineering licenses in the United States and Canada. Mr. Cohn is also an ASME Fellow.

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