I believe you are talking about F1+F4 force generated (see attachment).
In established flow condition, the force F1 generated (Primarily considered as momentum component of API 520 eqn in most companies, see http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=28012&page=2) is balanced especially in short runs as the force quickly cancels each other and the +P*A component acts at exit of vent pipe.
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=21003#Post21003
"...we haven’t a realistic model for the steady state flow, exactly here, in the PSV body, where the flow is not one-dimensional. Probably is not a significant value and exactly for this reason, you cannot see, in API520 part II or B31.1 figures, this force as horizontally applied on the PSV body, but for some safety reasons, the stress tradition asks to count as this force exists, and I would say is more a way to be conservative rather to be realistic…. of course, this is just my opinion!
...."
and C2TR says,
"this is the thrust load that acts on the elbow just before the pipe opening into the atmosphere"
In case of hard piped, the thrust would be at Vent exit point rather than at relief valve piping.
Which is F1+F4 in the attached image.