Creep damage and long term life of steam line components:Case X20

Hot steam lines operate at high temperatures and are designed against creep. In service they will consume the creep life at the highest rate in locations with the most adverse combination of material strength (weakness) and high stress (e.g. system loads). Adverse effects in stress state are promoted by features of geometry and discontinuities in the materials properties. In practice it is essential to find the areas of maximum damage, as these will determine the locations and timing to inspect and finally to repair or replace before failures or unplanned outages. The inspection experience from power plant steam systems have provided the established views on the expected locations of the early creep damage, and rules on timing the next inspection. The experience has also shown that optimal timing is materials dependent, and that the steel grade X20CrMoV11-1 (X20, 11% Cr steel) performs particularly well in the inspection statistics. This paper describes a case with observed creep cavitation and cracking in a branch weld of a X20 steam header that was replaced after 135 000 service hours. The characteristics of the observed damage and its evolution are discussed