John Swaffield

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

Abstract 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.

Numerical modelling of air pressure transient propagation in building drainage systems, including the influence of mechanical boundary conditions

Abstract Air pressure transients within building drainage systems belong to a family of unsteady flow phenomena amenable to numerical modelling via the method of characteristics. The defining equations are solved for a range of transient conditions, including trap seal retention prediction and air admittance valve stack termination. An air admittance valve boundary condition is investigated, both numerically and experimentally. The resultant boundary equation is validated and the method of characteristic model is shown to be capable of predicting the valves operational performance. The model is shown to be capable of development as a basis for air admittance valve design and acceptance criteria.

Air pressure transient propagation in building drainage vent systems, an application of unsteady flow analysis

Abstract Air pressure variations within building drainage systems as a result of appliance discharge may result in the loss of trap seal protection by induced syphonage and/or overpressure. Air pressure transient propagation may be described by the quasilinear hyperbolic partial differential equations of continuity and momentum, solvable numerically by the method of characteristics. This paper develops these governing equations and illustrates the numerical model validation for a drainage system and vent network, including a wide range of commonly found boundary conditions. Trap seal response and retention are modelled and the technique shown to be applicable to both code and standard preparation and systems design.

Siphonic roof drainage system analysis utilising unsteady flow theory

Abstract Over the past three years a UK EPSRC research programme has been underway at Heriot-Watt University investigating siphonic roof rainwater systems. This text aims to report the principle findings of the project to date. A brief description of experimental and numerical aims is given. The priming procedure which occurs in an idealised system is documented. The test procedures employed are described, and experimental results are illustrated. The framework employed to numerically model the ambient hydraulics is described in some detailed. Conclusions are drawn regarding the operational characteristics of siphonic roof rainwater systems as a whole.

Siphonic roof drainage: Current understanding

Abstract Over the past five years there has been an ever increasing amount of independent research directed at understanding how siphonic roof drainage systems perform hydraulically. Since 1995, there has been a substantial amount of time and money invested in researching these systems by both government and industry. This increased amount of research activity means that, increasingly, publications are entering the public domain – the main source of this work being UK-based. This text aims to collate this work, and give an overview of what the current level of understanding is. Furthermore, it will analyse what this research means for existing systems, and recommend areas for future research in this field.

The influence of water conservation on drain sizing for building drainage systems

Water conservation is a prerequisite to a sustainable built environment. The effect of reductions in water throughflow on the operation of building drainage systems must be understood and incorporated into the design of building drainage networks. The water consumption represented by water closet (w.c.) usage identifies es w.c. flushing as the major contributor of waste water from domestic and commercial buildings and hence the water closet becomes the defining appliance in terms of identifying appropriate drain sizing techniques. Within the building envelope w.c. discharge flow is subject to wave attenuation which has a predominant role to play in the final solid transport distance expected following w.c. operation. This paper discusses the interaction of the parameters governing solid transport and illustrates the application of mathematical modelling techniques to predict transport distance under water conservation criteria.

Modelling solid transport in building drainage systems

The transient flow conditions within a building drainage system may be simulated by the numerical solution of the defining equations of momentum and continuity, coupled to a knowledge of the boundary conditions representing either appliances discharging to the network or particular network terminations. While the fundamental mathematics has long been available, it is the availability of fast, affordable and accessible computing that has allowed the development of the simulations presented in this paper. A drainage system model for unsteady partially filled pipeflow will be presented in this paper. The model is capable of predicting flow depth and rate, and solid velocity, throughout a complex network. The ability of such models to assist in the decision making and design processes will be shown, particularly in such areas as appliance design and water conservation.

Simulation of building drainage system operation under water conservation design criteria

The Water Supply (Water Fittings) Regulations 1999, introduced to encourage innovation and water conservation, will profoundly affect building drainage design. Reduced shower flows and washing machine volumes and 6/3 litre dual flush wc operation will lower through flows, while drop valve wc flushing will affect system operation. Design for water conservation will necessitate the prediction of wave attenuation and its impact on solids transport and, without the safety factor provided by higher throughflows, simulations will ensure efficient design for minimal solid deposition. A simulation employing the method of characteristics solution of the St Venant equations and empirical relationships between solids velocity and the surrounding flow conditions is presented to predict solids transport performance within networks specified by slope, diameter, material, and appliance distribution and discharge characteristics.

Drainage vent systems: Investigation and analysis of air pressure regime:

Knowledge of the performance of building drainage vent systems and associated appliance trap seal loss and retention has developed in recent years due to extensive research into the generation and propagation of air pressure transients within drainage networks. This paper demonstrates how data gathered from experimental testing of several single stack systems, considering the influence of a wide range of parameters including stack diameter, roughness, height and applied water flow rate, can be used to provide generally applicable mathematical expressions which assist in determining the pressure regime present at critical points within such systems.

Simulation and analysis of airborne cross-contamination routes due to the operation of building drainage and vent systems

Recent concerns about the role of the drainage and vent systems installed in high-rise and other buildings in the possible spread of airborne contamination have highlighted the need for simulations capable of predicting system response when subject to multiple and random events. Such simulations would allow designers to predict the possible contamination routes established as a result of failures of the system, e.g. trap seal loss and/or the influence of dried-out traps. In addition, the simulations proposed would provide diagnostic tools in the event of cross-contamination. Mathematical solutions are used to simulate the system unsteady water and entrained airflows, and the application of these simulations to predict likely contamination routes. The AIRNET simulation is employed to represent the unsteady air and water down-flow conditions in the network, the associated propagation of air pressure transients and trap response. The simulation can provide a design and standards development tool as well as a forensic and diagnostic tool for the investigation of suspected cross-contamination. The simulations confirm that cross-contamination routes result from normal operation and random failure conditions, including system surcharge and trap depletion due to, for example, poor maintenance. It suggests that simulation predictions have an important role in ascertaining potential hazards, as well as a forensic role.