The majority of the calculations found in the Relief Load Synthesis processor were developed in ASME Publication 75-WA/FE-23 "Steam Flow through Safety Valve Vent Pipes" by H. E. Brandmaier and M. E. Knebel.

You may see the paper in this post:
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=20567&page=2

Hope this can help you to understand the theoretical basis.

Best regards.