This simple mod uses nodeboxes to supply a complete set of 3D flanged 
pipes.  There are enough nodes defined here to bend from any axis 
(X/Y/Z) to any other, or to join multiple pipes together from any or all 
axes.  There are 10 unique nodes defined, times two versions for each 
(for a total of 20).  Getting them into the right orientation is handled 
with the usual facedir parameter.

One version bears one or more dark windows on each pipe, suggesting 
they're empty, while the other version bears green-tinted windows, as if 
full (the two colors should also be easy to select if you want to change 
them in a paint program).

This mod requires a recent git pull or build of Minetest dated June 17, 
2012 or later.

There are no crafting recipes, however, you can use the usual /give 
commands with names such as pipes:vertical, pipes:crossing_xy, and so 
on, if you want to add them to your world for decorative purposes.  See 
init.lua for more details.

The overall format of the code is borrowed from the game and written by 
me.  16x16 textures by me also.

This mod is intended to be used as a basis or at least as sort of a 
model for something else to build on (perhaps a nicer-looking oil mod?), 
and does not provide any of the code necessary to cause the pipes to 
rotate around as they're placed.  I may add such code later, but not 
right now.