This is difficult to give a definitive answer, but I have some thoughts.
For a vertical wall, the minimum thickness is determined by your nozzle size, typically 0.4mm. If you are able to print entirely in a vertical orientation (a pipe standing on end), you may consider using "vase mode" with no top or bottom to print. This can help simplify the CAD modeling, as you don't have to model the hollow interior and wall thickness.
For a horizontal wall, in theory this could be a single layer height, about 0.1mm depending on the printer and settings. Realistically, you will probably need 2 to 4 layers to get something "solid". However, this is not likely to have good results for a hollow pipe due to performance of the printer in bridging and overhangs. A thin, round pipe laying on its side is not likely to be successfully printable.
For printing in a horizontal orientation, I'd recommend a non-circular shape; the specific shape would depend on your overall design and goals. A "teardrop" or "flat teardrop shape can help with horizontal holes through parts.
Also, rather than individual pipes, perhaps you could design a block with hollow passages. This could allow thicker walls, make it much stronger, reduce assembly needs, allow for extremely complex routing and internal connections, and may even allow for functional parts like valves or sensors to be installed.
Finally, I would not expect an FDM-printed part to be airtight. There will most likely be some small gaps between layers and where layers start and stop. As an example, I tried making an ice mold, but almost all the water seeped through before it froze.
Another poster recommended an acetone vapor bath, which should help seal any pores. This only works with ABS. Another idea might be to coat the inner surfaces of the pipes with a resin that can seal everything up.