Rapid Flow Startup in Filled Horizontal Pipelines

Maintenance or repair of water mains may require the interruption of flow. Re-initiation of flow may present opportunities for the interaction of air and the accelerating water, resulting in undesirable pressure transients. This study focuses on air intrusion during flow startup in an initially filled horizontal pipeline supplied by a constant-head reservoir. Flow was initiated by opening the downstream end, allowing a free discharge at that point. During the initial phases of flow startup, an air cavity intruded in the upper portion of the pipe cross section while water exited the pipeline through the lower portion. The initial reservoir head controlled the rate of water velocity increase and thus the air intrusion advance. Air intrusion occurred until the water velocity was sufficient to arrest the intrusion. Higher reservoir heads resulted in the sub sequent expulsion of the intrusion from the pipeline. However, for lower reservoir heads, the intrusion was not expelled, but re-advanced to a final position that was controlled by shear forces. Criteria are developed for the flow conditions necessary to arrest the air intrusion in a horizontal pipeline as well as the intrusion lengths to be expected.