Air is introduced into water transmission lines through vortices at intakes, pumps, vents, and other causes. This results in loss of capacity, surges and blow backs, difficulties in filtering operations, corrosion of lines, and reduction of pump efficiency to name a few. The best solution to air problems is to prevent the introduction of air at the intake. Other means of air removal are air relief valves and by the inertia of the flowing water. No conventional similitude exist to study the time of clearance of isolated air pockets and results of Froudian scale model tests are very conservative. For pipe sizes below 1 ft. (0.30m) the clearance of isolated pockets may require higher than practical velocities.
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Wisner et al. (1975) studied this question.