I have a question about stopper fittings, Mueller, bottom outs, "hot taps".
Welded on a Mueller 6" a few days ago and we had to do another one up the road, dual feed. Killed the one, killed the other. Cut out the no longer needed pipe and welded on a transition as it was going to poly.
A question that was raised and asked of me, "How long should we let it cool before we put gas back on it". My thoughts typically are, if you can lay your hand on it and not feel pain then it's probably good. More of a warm, not hot.
I have not looked thru my old API 1104 yet but will later on, but is there a guideline for this? 6010/5P was used on the transition as there was no gas on it, butt weld. From a metallurgical standpoint and my limited education of metallurgy cooling off to fast would be bad. But then again I was thinking if it is below the critical point as far as temperature has the structure, crystalline(?) already done it's thing and any cooling from the gas would be ineffective? There was a valve on the poly and it was shut off so there would be no gas flow except for the initial charge to fill the three feet or so of the transition.
Curious to hear thoughts on this and will dig around in my code book to see if I can locate anything on my own.
Shawn
For the most part, the critical cooling occurs from the austenitizing temperature, >1640 F, down through about 600 F. That being said, the chemistry of the base metal will determine what the lower critical temperature (Ms) is. The formation of martensite can continue well below 100 F and may not be complete until cryogenic temperatures have been reached (martensitic stainless steels).
Most line pipe is low carbon steel and while it can be made to be somewhat hard, the carbon content is low enough it isn't a major issue. If that was not the case, hot taps, made while fluid is flowing, would be much more difficult to do and would most likely be avoided.
Best regards - Al
That makes sense Al, didn't consider the hot taps but doing the hot taps we are also using 7018's, not 6010's. The difference being I guess one, the gas is continually flowing whereas the other was welded, 10-15 minutes pass and then gas turned on where it dead headed on a valve.
Maybe I'm thinking way to deep into this, lol!
Appendix B does a pretty good job of describing why low hydrogen electrodes are used for hot taps and in-service repairs. The rapid cooling experienced when welding a hot tap on a line that is in service is subject to rapid cooling, undesirable microstructure, and with the increased potential for delayed cold cracking if low hydrogen electrodes are not used.
Unfortunately, if my recollection of the discussion is correct, the discussion in the appendix does not do a good job of defining the need to keep the low hydrogen electrodes in a holding oven at an elevated temperature to ensure they are moisture free.
Best regards - Al
A large number of hot taps ( natural gas) have been made with 6010's and 6010 (root) -7010 (everything else) electrodes.
I know that the 7018 is recommended but past procedures and some currant ones specify xx10.
Floyd
I know there are a few out there that have been done with 6010's, I might have some personal knowledge of those, lol!
I think I remember reading somewhere, something from Mueller that suggested 7018's but also said 6010's could be used. I've actually done some here as per engineering on 3/4" high pressure lines using 6010's. 3/4 stopper fittings, tiny little things!
Forum
