Air-Water Interface Flows in Pipe Emptying

Abstract

Pipe emptying is an important component of water supply operations. Pipes can be drained as a scheduled operation preceding maintenance, flushed for health reasons or they may empty as a result of an accidental break causing flooding to the surroundings. The critical parameters to consider when a pipe is drained are the sub-atmospheric pressure variations and flow rate. Sub-atmospheric pressure can cause damage on the pipeline installations as well as the pipeline itself, while flow rates can cause flooding if not adequately controlled. This paper presents laboratory experiments that were carried out to observe the air-water interfaces as well as record the pressure changes and outflow rates. These were generated by systematic variations in slope, ventilation and discharge port. The results are compared with those of Zukoski (1966), Benjamin (1968), Baines (1991) and the orifice equation. The interfaces can generally be divided into five major groups, gulping, near horizontal, gravity current, dual gravity current and combination of gravity current and near horizontal. The initiation of discharge resulted in sub-atmospheric pressure upstream. The combination of no ventilation, maximum discharge port and steepest slope produced the largest sub-atmospheric pressure. Maximum flow rate was produced from steepest slope, maximum discharge port and largest ventilation. A Runge-Kutta 4th order model utilizing the rigid column theory is also presented to assess the ability of such simplifying modeling approaches to explain the pressure variations and flow rates.