One of our bundles in our HRSG has had a couple of failures on the top return bends. These are 2 1/4 Chrome tubes, .105 wall, with a design temperature and pressure of 700ºF at 790 psig. As it turns out, these tube to return bend welds were not PWHT during fabrication. The bundle is about 2 years old. The code does not require PWHT as far as I can tell, but after some analysis on the failed weld, the hardness came back twice as high as our standards allow. So now I would like to pursue doing a PWHT in place. The contractor came up there with me to look. Doing the circumferential band around each weld (what the code recommends if doing a PWHT in sections) on each return band would be a logistical nightmare. Access is also extremely limited to the welds themselves due to tube supports. I estimate about 900 welds that we'd have to stress relieve. The contractor suggested stuffing insulation down below the welds and laying the beads directly on top of the return bends and heating down towards the welds. The code doesn't explicitly forbid this but I'm kind of skeptical about how effective his idea is. However, I can't see any other way to do it. The other problem I have is if we're heating to 1200+ degrees, how would that affect the hanger supports? These are carbon steel and they butt up right near the return bends. We probably wouldn't do all the tubes at once; we'd mostly likely do them in 6 or 7 sections, depending on what the contractor can manage. Anybody ever do anything like this before, or have any insight?
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