Experiments on air-water interfaces in emptying pipes

In the US, there is a water main break every 2 minutes. During maintenance operations, pipelines often require emptying, and one key concern by waterworks is to minimize system downtime. However, fast pipeline emptying can lead to damaging negative pressures, which can sometimes damage the pipeline lining or the admission of external contaminants. Much is still unknown on how air-water interactions can develop. The near-horizontal interface that is often assumed to exist inside emptying conduits requires a certain combination of outflow rates and ventilation. This study presents the results of an experimental investigation on different types of air-water interfaces within closed conduits with limited ventilation being emptied. A wide range of discharge, slopes and ventilation conditions were tested, and five distinct flow patterns were observed during these tests. A simple numerical model indicated that the internal pressures could become very negative in extreme cases but that even a small amount of ventilation can mitigate negative peak pressures significantly. Future studies will combine these results with CFD modeling results to evaluate the ability of these tools to represent the types of flow patterns observed during these experiments.  

Keywords: Water pipelines, Emptying operations, Air-water interactions, Experimental studies, Numerical modeling. 

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