Complex interactions of water, air and its controlled removal during pipeline filling operations

Removing air from pipelines during filling operations is a complex fluid design and modelling challenge. In order to explore the influence of air valves on the transient response of a pipe system during rapid filling, the conventional air valve boundary condition is extended to account for the main features of the two-phase flow established in the downstream piping. A shock-fitting numerical model simultaneously tracks the filling water column, the response of the air valve and the possibility of column separation. The extended air valve boundary condition is shown both to permit a more realistic representation of the filling hydraulics and to fix a flaw with the conventional air valve boundary condition that sometimes causes it to converge to an incorrect steady state condition. It is also confirmed that the presence of an air-vacuum valve, while effectively preventing negative pressure and local liquid vaporization, can still permit column separation and that the rapid expulsion of the admitted air can induce severe secondary transient pressures. However, the secondary transient pressures captured by the conventional air valve boundary conditions are shown to be overly sensitive to the shape of pipeline profile. Finally it is demonstrated that the proposed air valve boundary condition nicely represents air binding at the high point and the associated overall reduction in the conveyance capacity of a pipe system.