A previously-developed computer program [1] features the least-squares analysis for finding the best fitting model’s regression coefficients: The analysis is acted on points (test temperature, ultimate strength at 100000 hours) available in literature for any creep-sensitive material. The automatic procedure gives time to rupture at operation temperature directly through the model chosen for the curve Larson-Miller parameter (LMP) – stress. It applies the first-choice method in the Section II of the guideline [2] to the Italian technical procedure for creep-operated pressure vessel components [3].
Aim of the present work is to compare the results of the Italian code with those predicted by the API 579-1 standard [4]. Comparison is on data gathered by one author in his previous work [5]. That investigated a high energy pipeline after 230000 hours of service and calculated critical components’ life through an API 579-1 Level-3 assessment. Present study uses stresses from standard analysis based on B31.1 code [6] and from a finite element analysis (FEA, elastic and plastic): it finds out the residual life of three components, a 90-LR bend and two tees. Besides, it carries out creep assessment according to the Italian procedure and API 579-1 (Level 1 to 3); the former is LMP-approach based whereas the latter uses both LMP and the Omega methods.
For the Level-1 assessment (B31.1 stresses), the Italian procedure and the API 579-1 return consistent results. Acting a Level-2 assessment through an elastic FEA, results are consistent only when the API 579-1 analysis adopts the Omega Method. The LMP-approach method (minimum stress to rupture data) shows instead more conservative results. API 579-1 Level-3 assessment incorporates creep, plasticity and relaxation: When compared with the Italian procedure it predicts longer creep lives (up to a factor of ten).