A Small Pipe can be used to transmit water from a Water Pump Station or Large Pipe. Small Pipes can transmit water up to 38 tiles from a Large Pipe or Water Pump Station. If the water pressure in the large pipe is low (i.e. the large pipe is too long), then the maximum range of the small pipes will decrease.
Pipes cannot be built through a Perimeter Wall (or Water), and building a Perimeter Wall over any existing pipe will remove it. It is not possible to build a small pipe over a large pipe. The large one has to be dismantled first.
Small Pipes offer a small boost in Tunnel digging speed, whereas large pipes offer a much larger boost. For this reason it is preferable to only have small pipes in your cells.