Investigating hydraulic removal of air from water pipelines

Abstract

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.

On elongated air pockets in downward sloping pipes

“…The air pocket head loss reduces at increased tailwater pressure heads. The transition from regime 2b to 3 depends on the clearing (subscript c) flow number F c ; Q wc /[A D (gD) 1/2 ] (Gandenberger 1957, Kent 1952, Wisner et al 1975, Montes 1997, Escarameia 2007.…”

“…Figure 3 shows the clearing flow numbers for u ¼108 as a function of Eo. The curve labelled 'Various, 108' combines the data of Bendiksen (1984), Gandenberger (1957), Kent (1952) and Escarameia (2007). Kent did not consider u ¼ 108, but an extrapolation to u ¼ 108 seems reasonable by combining Kent's and Escarameia's data.…”

“…Kalinske and Robertson (1943) first identified the blow-back flow regime in air-water flow in downward sloping pipes. Kent (1952) and Escarameia (2007) investigated the required water velocity to keep air pockets of a certain size stable in a downwardly inclined pipe. Escarameia (2007) derived an expression for air discharge as a function of pipe angle and water discharge.…”

“…Kent (1952) and Escarameia (2007) investigated the required water velocity to keep air pockets of a certain size stable in a downwardly inclined pipe. Escarameia (2007) derived an expression for air discharge as a function of pipe angle and water discharge. The effect of air pocket length and slope on the net air discharge is considerable, yet this was investigated only recently .…”

Study on Bubble Velocity under Bubble Flow Scenario in the Horizontal Water Transmission Pipeline

“…In the inclined upward section of the water transmission pipeline, the air could be easily taken away due to buoyancy. In the horizontal pipe, Benjamin and Bakopoulos proposed a dimensionless number of 0.54 as the minimum flow rate for the bubbles starting to move; while Corcos suggested a dimensionless number of 0.484 for small diameter tubes [5]; and according to Escarameia's experiment [6], the critical velocity for bubble motion in the horizontal pipe was about 0.8 m/s, which was multiplied by a safety factor of 1.1. Therefore, the air could not be easily taken away when the water velocity is too small.…”