I'm not updated with the last versions, in the past Relief Load Synthesis was based on 75-WA/FE-23 "Steam Flow through Safety Valve Vent Pipes" by H. E. Brandmaier and M. E. Knebel.
It is posted in forum, you can find a "better" copy on internet, someone posted it on files.engineering.com, scribd, etc
(just Google Steam_Flow_through_Safety_Valve_Vent_pipes)
I made some comments on that article, you can find them in http://forums.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=45529
For me B31.1 model makes sense, the article- no. I posted my understanding on B31.1 model in http://forums.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=70993