Cavitation, aeration and negative pressures in siphonic roof drainage systems

Abstract

The main purpose of this study was to examine negative pressures in siphonic downpipes and to see whether these can be controlled through breaking the siphonic action by injecting air at certain points in a vertical downpipe. The study also investigated cavitation and its propagation through a prototype scale siphonic system. The paper presents methods for limiting the minimum pressure in siphonic downpipes in tall buildings. A distinctive drop in pressure between approximately —70 kN/m2 and —96 kN/m2 occurred in a 80 mm diameter downpipe and it was not possible to sustain pressures between these limits. The pressure drop to -96 kN/m2 caused the formation of a distinctive cavitation zone. Any increase in downpipe length, beyond the cavitation zone, produced a proportional increase in the cavitation zone length. Practical application: Cavitation is perceived to be a serious problem for designers of siphonic drainage systems to be installed in tall buildings. Using full-scale testing this paper provides evidence that in practice cavitation in plastic downpipes is unlikely to cause damage. The paper also provides guidelines for maximum spacing of air inlets to limit the minimum pressure to ∼70 kN/m 2 below atmospheric pressure without significantly decreasing the system capacity.