Is there any advice on designing low-pressure manual vents (used for purging in/out of service (<0.5 barg) venting horizontally rather than vertically directed away from a building?

Low-pressure vents at mostly low hydrogen purity are not as large safety risk as high-pressure pure hydrogen vents. These vents should still go to a vent stack, but it will probably be small in diameter and thus the tee vent at the top can be small.

If the purge requires high flow, if purging horizontally, the reaction forces of the flow exiting and the hydrogen cloud should be modeled based on NFPA 2 to ensure the safety of the surrounding area.