We consider slowly evolving, i.e. ADIABATIC, operational regime within a
transmission level (continental scale) natural gas pipeline system. This allows
us to introduce a set of nodal equations of reduced complexity describing gas
transients in injection/consumption UNBALANCED (so-called line-pack) cases. We
discuss, in details, construction of the UNBALANCED ADIABATIC (UA)
approximation on the basic example of a single pipe. The UA approximation is
expected to play a significant "model reduction" role in solving control,
optimization and planning problems relevant for flawless functioning of modern
natural gas networks.Comment: 6 pages, 2 figure