The fact that you are referring to multiple, numbered pipes makes this an interesting question, with little precedent to build off of.
As FumbleFingers said, the usual preposition for leaky pipes is in, but we don't usually talk about numbered pipes in the context of diagnostic systems, so you have a bit more wiggle room.
The leak is located in Pipe #1.
You can't go wrong with that one.
The leak is located on Pipe #1.
In my mind, that could work. One meaning of on is on the surface of (which is why we hang pictures on the wall, or have lacerations on our arm).
The leak is located at Pipe #1.
It's not a conventional way to say it, but given a diagnostic system with several possible locations for a leak, I think at could be used – particularly if the leak could appear in several different location types and you wanted to use the same preposition in each message:
- The leak is located at Pipe #23.
- The leak is located at Connector D6.
- The leak is located at Valve V-16
I have to say I've never thought about it, but you're not wrong for tripping up on this. It's kind of weird when you think about it. As far as in implies direction and location it logically makes as much sense to say "out pipe 1" really, but that sounds super back-woods, Southern.
– sas08 Mar 05 '16 at 00:21