Abstract
The cyclic nature of intermittent supply subjects the network to frequent (partial) draining and filling. During filling, water displaces air in the pipes, and much of that air is discharged through user connections and water meters, causing over-reading and reliability issues. Important laboratory experiments have measured air discharge through meters during filling, but have not yet quantified or modeled the role of air compression. This paper revisits and augments these pipe-filing experiments, proposing, calibrating and interpreting a numerical model for the compressible flow of air during pipe filling. The rapid filling of a distribution-scale (1500 L) pipe compressed the air by 1.3 -2.5 fold, broke two-thirds of the tested water meters, and caused water meters to register 33-63% of the volumetric air flow as if it were water (500-1000 L). This partial registration was primarily caused by compression. Our results quantitatively relate air discharge to water meter over-registration, emphasizing the importance of gradual filling in intermittent systems. For meter protection, we recommend limiting filling speeds to less than the nominal flow rate of all downstream water meters.