flow within pipes is almost always full, as otherwise airlocks occur. incidentally even with a pressurised flow one can run gas through the pipe at the same time. This has been attempted for gas rich oil fields, but it almost always leads to grief. The aim in pieplines is to achieve a linear flow with no trubulence, such that the reynolds number is below 1000. this is why they run fast. (do you want the equations for reynolds numbers and pressure drop across a pipe?)