Hello...

Just wanted to share my idea regarding this topic. I think this is somewhat related to elastic follow-up phenomena of piping operating at creep range where there are some unrelieved part of secondary thermal stress and converted to primary type stress. If I remember right, EN code considers 1/3 of thermal expansion stress to be added to primary sustained stress for piping at creep state. But I think the allowable given in ASME Code at high temperatures already cover this so there is already sufficient safety factor so as not to consider secondary as primary stress.