Life assessment methodologies for high temperature branch pieces

Abstract

Branch pieces in high temperature steam circuits are a common feature of power generating plants both conventional and nuclear. A simple inverse code methodology based on BS1113 [1-3] exists for estimating base rupture life in cylinder to cylinder configurations (branches) under constant pressure and temperature. This does not cover the complex issue of estimating the mixed creep-fatigue effects of cycling which can have a significant influence on damage especially under the current practice of multi-shifting as utilities follow fluctuating energy markets. The current work is primarily aimed at extending the inverse code methodology for base rupture to include cycled loading due to startups and shutdowns. This is achieved under the guidelines of the R5 assessment code by the use of an analytical expression for metastable thermal stresses [4,5] and mapped thermal stress response. System loads are not considered in this work other than by the simple method suggested using the inverse code method. Only 90° non protruding branches are considered in the current work. Examples of 90° branches are examined showing the significance of cyclic loading on a variety of branch configurations. It was observed that base rupture dominates most configurations up to a "cliff edge" in thermal ramp rate followed by rapid accumulation of creep-fatigue damage at higher rates. The results are a useful aid when assessing the optimal operating conditions for individual power stations. © 2009, Australian Institute for Non-Destructive Testing (AINDT)