The simulation of air pressure propagation in building drainage and vent systems

Trap seal loss due to induced siphonage and back pressure effects may be predicted within complex building and drainage vent systems by the recognition of these phenomena as examples of transient propagation. As such the pressure level within complex pipe networks may be analysed and predicted by application of the method of characteristics and finite difference techniques, provided that the source for the transient can be identified and modelled. This paper will demonstrate the relationship between appliance discharge to a building drainage network, the resulting entrained air flow and the subsequent propagation of air pressure transients within both the system vertical stack and the associated vent network. Trap seal oscillations as a result of either positive or negative air pressure transient excursions will be modelled and retention predicted. The role of air admittance valves in limiting air pressure transient levels will be discussed.