Do I need a “weep hole” at my PSV discharge if I have a “rain cap” on the end of my vent lines/discharge piping?

Yes, we still need to consider drainage at the low point of our discharge piping, even with a “rain cap”. While a rain cap is designed to deflect direct vertical precipitation from entering the discharge lines/piping, it is not a hermetic seal. Over time, moisture can still enter the discharge piping through condensation, wind-driven rain, or melting frost/snow.

Looks look at ASME Section VIII for what is required at a minimum:

NOTE: I also discuss API 520, Part II in this post.