HRSG HEADER CREEP-ASSESSMENT THROUGH A PROCEDURE FOR THE ITALIAN CODE APPLICATION AND COMPARISON WITH THE AMERICAN STANDARD

摘要

Lower headers of bottom-supported heat-recovery steam generators (HRSG) may be critical because of their longitudinal dimensions, thermal expansions and external loading (the harp's weight): Present work considers the creep analysis of the high-temperature-section (superheater /reheater) headers: they may be critical because of the long continued service (175000 hours or twenty years), larger dimensions and the external loads, including a negligible steam-drum weight fraction. The aim of the work is to compare life results from the Italian creep code with those predicted by the American standard API 579-1. This work also checks the compatibility of results coming from the two polynomial models in both Italian and API 579-1 procedures. Classical methods, applied using both ASME and Italian pressure formulae, show that, as for the evaporator-section header, the pressure contribution to longitudinal stress may be greater than bending alone; considering now the increased header's weight, the stress ratio is also comparable to the evaporator's. Consistency of results from numerical-model stress analysis (elastic) is good, confirming the pressure contribution is greatest. For the Level-1 assessment (B31.1 stresses), the Italian procedure and the API 579-1 return consistent creep life results, though the API 579-1 results appear more conservative than the Italian-procedure's. Level-1 assessment, acted through an elastic finite element analysis (FEA), uses Larson-Miller parameter (LMP)-approach method with minimum stress-to-rupture data: the Italian procedure and API 579-1 return consistent creep life results when evaluated on the tube-hole branch side, Italian-procedure's appearing little more conservative than the API 579-1's. For the Level-2 assessment (FEA stresses), again the Italian procedure and the API 579-1 return consistent creep life results with the Italian-procedure ones again a little more conservative than the API 579-1's for both sides of the intersection. Level-3 assessment (incorporating creep, plasticity and relaxation) shows (short) creep lives similar to Italian-procedure's.