Great mod!!!!!
However, after playing around with it for a while,
i noticed that the ducts are usually either 100% full or 100% empty,
since there is only force pump in or force pump out.
I did not find a good way to make a fluid storage for example somewhere along the duct line.
Setting both in and outlets to a fluid tank buffer, will just cycle the fluid around at 100% speed,
leaving the duct itself completely empty.
A solution would be to make it possible to read with wires how much fluid is inside the duct, so it only outputs into storage tanks if it gets too full, but that is not possible since you can not connect them.
A both way soft valve would solve this, or maybe there are other ways like enable to read the current fluid amount in the duct with wires.

Flow Control mod seems to have good solutions to it,
but it would still be neat if there was something else.

I will look if Krastor have time to make the sprites for a not returing valve for ducts

Another thing that came to mind, was how i was automatically assuming while playing, that the pipes would be locked to a 2x2 grid like rails are built. When placing pipe to grounds, sometimes i missaligned the pipe by a one tile offset, strangely still making a connection. https://imgur.com/TlveCXc
Locking the pipes to a 2x2 grid might make pipe autoconnect features or other things easier too, if it is even possible.
Would be perfect if there were only "duct-to-ground" and "duct" like there is "pipe-to-ground" and "pipe" in vanilla!

I don't think is possible lock rails like vanilla, maybe with scripting, not from prototypes. Anyway I think the "duct-to-ground" is more "free" like hpw is know

Rails have building_grid_bit_shift = 1, not sure if this is the grid building setting, tried setting this for the ducts out of curiousity, but the value seems to get reset to 0.

Asking in threads, building_grid_bit_shift variable is read-only unsigned integer variable, so no way to use it