John Alexander McDougall

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.

The influence of flush volume and branch drain cross-section on deformable solid transport in attenuating flows

The satisfactory removal of faecal and other waste by w.c. flushing and solid transport within a branch drain is a prerequisite of building drainage system design. The increasing importance of water conservation has led to renewed interest in the possibilities for reductions in overall building use through reduced w.c. flush volume operation. Current UK and international legislation has led to the 6 L flush being regarded as the upper acceptable limit, with consideration being given to further reductions to 4 L and below. Maintaining the operational integrity of the drainage system will require an understanding of both w.c. design and transport mechanisms to ensure solid clearance and the avoidance of solid deposition and the consequent disruption to the building user. It is necessary to regard the w.c. and the branch drain as a system. This paper analyses a major data set that includes a wide range of w.c. flush volumes as well as consideration of branch drain cross-sectional area and shape. Data for low flush volume operation with a parabolic cross-section branch drain is presented. Conclusions are drawn that indicate that low flush volume operation is feasible provided drain slope and cross-sectional area and shape are fully considered. The degree of slope enhancement required as flush volume is decreased is demonstrated.

Pressure surge in building utility services exacerbated by the presence of trapped or entrained air

Pressure transient propagation within building services utility systems is an inevitable consequence of any change in the system operating condition. In common with a wide range of pressure surge applications, these conditions may be analysed and modelled by solving the applicable St Venant equations numerically via the method of characteristics. This paper presents the underlying basis for such models and presents applications within building services utility systems that feature trapped or entrained air as a major modifier of either the transient propagation or the system boundary conditions. In particular, surge pressures exacerbated by the effect of trapped or entrained air are considered, including dry riser and sprinkler applications. The role of entrained air in the occurrence of physical injury following the violent fracturing of a w.c. bowl is discussed and explained in terms of a transient analysis.

Drainage flow and solid transport simulation in defective building drainage networks

The flow conditions within multi-storey building drainage networks may be characterised as unsteady free surface flows and as such may be analysed and simulated by the use of the method of characteristics to solve numerically the governing equations of continuity and momentum. However the application of such simulations must take into account the possibility of defective system installation and must be able to predict the wave attenuation following appliance discharge and the consequent effects upon solid transport due to system defects caused by faulty installation or the development of obstructions to the flow. This paper presents both a summary of a detailed survey of defects in installed building drainage networks and the development of appropriate boundary conditions that allow the method of characteristics based computer simulation to accurately predict the effect of defects on system performance. The introduction of drainage simulations capable of providing design advice under the real conditions represented by this study is particularly timely in view of governmental pressure for water conservation and the introduction in 1998 of the new Water Regulations to replace the Water Bye-Laws that will introduce reduced and dual flush w.c. operation, together with reduced shower and other appliance flows.