My first recommendation is to first try with a much simpler model so you can gain experience. You said you don’t want to change the geometry, but is there a reason why you cannot extend the pipes all the way to below z_min? It is unlikely that they are physically cut-off at that plane. Would some of them intersect the large bent pipe?
If you cannot extend them, create a small rectangular block, that intersect the tubes, say one cell above your XY plane, assign a NSW BC to then. Then create a Pressure outlet along the top plane of each box.
Again, my suggestion is to try with a simpler model, say one tube so you see how it works.