It is a sleeve to extend the stud for high temperature service.
Every time I try to do the theory I seem to come up with a different answer. Example: one time I try to maintain constant gasket stress by holding gasket strain between ambient and operating (bolts colder than flange); another I will try to ensure that the gasket does not unload in shut-down. Every time I come up with different answers, some of which do not even seem realistic.
However, sleeve nuts have been used countless times, so where am I going wrong???
In the 1990s I had a book called 'Theory and Design of Pressure Vessels' by Harvey, which had the theory behind this (about page 435 - I used to catalog pages when I was young and keen!). Lent it to my Client's engineer-weasel and never saw it again - Thank You Shanghai Chemical! - Hope you enjoy it.
In a similar vein, I am trying to work out whether Belleville washers make sleeve bolts redundant (another subject I think addressed by Harvey).
Tough for me to ask for a freebie on this, but with the downturn in O&G I cannot really face buying another copy of Harvey just for this one question.
Thanks for your time.