Capacity loss in siphonic roof drainage systems due to aeration

An experimental investigation into air entrainment in siphonic roof drainage systems is presented. By injecting known quantities of air into a 4.7m high experimental test rig, the effect of aeration on flow velocity and flow capacity in the siphonic system was determined. A preliminary relationship between air/water content and a decrease in system capacity was formulated. This indicated that system capacity can decrease by up to 40% for air/water ratios approaching 45%. At typical operational air/water ratios of approximately 10%, system capacity decreased by 16% from the assumed no-air-design situation. In addition, average velocities in the pipework decreased by 13% for air/water ratios of 35%. Friction and form losses were also measured and compared with theoretical values.