I couldn't find anything in ASME VIII that says you must re-hydrotest after stud welding attachments. It does say attachments which could not be welded prior to hydrotest do not require re-hydrotest. I believe you would have to re-hydrotest whenever you violate min wall thickness, but not in your case. You would however have to re-do any required post weld heat treatment, if any. in that case, you would have to re-hydrotest. Check wit your AI. He/she will be able to tell you for sure.

Thanks to all who replied by post and e-mail...
I found through your help and from my AI that while small insulation pin "stud welding" may not seem like a big deal, it falls under stringent stud welding rules in the ASME code.
For instance, even though a Procedure Qualification is not necessary, a WPS and subsequent WPQ's are mandatory. As a matter of fact, the Welding Operators have to re-qualify at the beginning of each shift and certified records have to be kept.
The only way to have avoided re-hydro would be to file a R-1 form along with the U1-A and have a Technician certified to SNT-TC-1A NDT each attachment.
I think we'll resort to the glue-on type of pins....

Thanks again,
Tim Gary

Back in my days of erector engineer, I insulated a lot of ASME VIII Section I vessels (deaerators, storage tanks, etc.) without welding a single pin whatsoever. Important: the vessels should be preferably horizontal.
If you're still interested, give me an advise and I'll tell you how I did.
Giovanni S. Crisi
Sao Paulo - Brazil