Pressure transient control: Part II - Simulation and design of a positive surge protection device for building drainage networks

Air pressure transient propagation within building drainage and vent systems may feature either, or both, positive and negative pressure changes in response to decelerating or accelerating flow conditions. Local venting, or the use of inwards relief air admittance valves, has traditionally suppressed negative transients. Positive transients have only been dealt with by the use of open roof terminations. This paper introduces an approach to positive air pressure transient suppression based upon the use of a flexible, variable volume containment vessel capable of reducing the rate of change of the entrained air following a system surcharge. The experimental programme to provide proof of concept, together with the simulation of the device utilizing the method of characteristics solution of the St Venant equations is detailed. Recommendations as to the suitability of bag materials and installation choices are presented, along with comparisons of surge relief efficiency. Practical application: The control of air pressure transien ts in building drainage systems has been limited due to the need to attenuate positive pressure propagation via an open termination at roof level - a poor solution as the transient will have affected all system trap seals before reaching the relief vent. The Positive Air Pressure Attenuator - an expandable bag that controls the rate of change of entrained airflow within the system - reduces the possibility of trap-seal Ioss due to positive transient propagation. Potentially this is a major contribution to vent system design that could revolutionize 150 years of design methodology. © The Chartered Institution of Building Services Engineers 2005.