As you know most of closed drain vessel has a vapor line connected to flare with a locked open valve.
This vessel has several feed from closed drain network.
During early stage of project, we need to have good estimation for this line (vent to flare).
Rigorous calculation may need detail gas blowby calculation from control valve and orifice connected to closed drain drum. However, detail information of such orifice and control valve is only available after significant development in the project.
Is there some simple rule for such estimation.