Floating ball valves are bidirectional.The right-hand seat is identical to the left-hand seat so going to the left is just the same as going to the right.  In very low dP applications I prefer vertical down to vertical up, but I have installed hundreds of small floating ball valves in vertical-up vent service with less than 3 psid that have worked very well with leaks being rare so this is probably more "fear and superstition" than a real data point.
In my interpretation of the OSHA definition of double-block-and-bleed, a trunnion ball valve satisfies the requirements.  I do work for several companies that adamantly disagree with my interpretation.  At the end of the day, company policy determines whether a trunnion ball valve satisfies their risk tolerance for DBB.

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