It seems you are interested in the implementation of these loads in CAESAR static analysis.
Mr. Loren Brown has a post where he detailed the procedure:
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=2148

About flow dynamics you've mentioned. IMO the gas that exits the relief valve already lost most of its pressure due to frictional losses in PSV. Is not a rule the fluid will accelerate so much in the initial length of the discharge piping, it will accelerate in the end of system where the boundary conditions offers conditions for such acceleration to critical speed. As you said, some software is available for steady state calculation. About the transient calculation for gases exiting the PSV- well... this is an endless discussion.