Manual depressurization line

[DieguitoI]

In most of projects, isolation philosophy specify a vent valve between ball and globe valve in manual depressurization line, however sometimes there is not such valve. Can you explain purpose of such vent valve?

 

[LittleInch]

I suspect it's a function of the pressure and the isolation philosophy.

Top one is clearly higher pressure and has DBB on the prv inlets. Bottom one is 8 bar and only single valve isolation

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[mk3223]

In the bottom diagram, is the 2"-FG-024-3091 a continuous vent line with a FO?
If yes, a drain valve may not be needed as it's a sloping line.