Provided in this paper is a case history that highlights use of advanced numerical analysis techniques to assess the Fitness-for-Service of cyclic pressure vessels using API 579-1/ASME FFS-1 (API 579) [1]. The components were subject to repeated thermal and pressure loadings which resulted in cracking of the base material of these components. A detailed level 3 FFS assessment was performed to assess potential for brittle fracture, determine critical flaw dimensions and evaluate the structural integrity of engineered repairs. The case study demonstrates the value of advanced numerical analysis to substantiate critical run-repair-replace decision making for pressure containing equipment. Detailed simulation of Weld Residual Stress (WRS) and local Post-Weld Heat Treatment (PWHT) showed that the local PWHT performed in the field was ineffective at reducing residual stress and highlights the need for more engineering of PWHT layouts and procedures, particularly for cases where stress corrosion cracking or brittle fracture are limiting design/operating considerations. The results also show how advanced analysis can be used to gain further insight into structural behavior in order to validate more simple methods that support practical decision-making related to fitness-for-service assessments going forward. Fatigue life predictions from a strain-life approach are also compared to ASME Section VIII, Division 2 [2] (S8D2). In addition, a brief comparison of these fatigue analysis methodologies is provided. The results are framed in terms of practical field experience obtained by periodic UT shear wave inspection of 16 identical vessels over a 10 year operating history.

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