Investigating hydraulic removal of air from water pipelines

The presence of air in pipe systems can result in problems such as loss of carrying capacity, disruption of the flow, reduced pump and turbine efficiency, effects on pipe materials and pipeline structure; it can also change the fluid properties and create environmental concerns at the point of discharge. Considerable costs are incurred in providing air release valves and chambers, and in deepening pipe trenches so as to provide the minimum gradients thought necessary to enable air bubbles and pockets to move towards the valves. If the accumulated air can be moved hydraulically out of the pipe system, then potentially large cost savings are possible. At present there are a number of often contradictory recommendations for assessing how air may be moved through a pipe system. In order to assess the viability of hydraulically removing air that accumulates in pipes, an experimental investigation was undertaken of the movement of air pockets in a pipeline with different slopes, from horizontal to around 22°. From the test results a prediction equation was developed for determining the critical velocity required to remove air from a water or sewerage carrying pipe. This study, which was part of a wider project aiming at providing practical guidelines, confirmed the results of previous researchers and extends current understanding of air movement, particularly in horizontal and near horizontal pipes.