Transient Airflow in Building Drainage Systems

John Swaffield

• Publisher: Routledge
• Year: 2010
• eBook ISBN: 9781134006960
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Giving you the first comprehensive presentation of the ground breaking research undertaken at Heriot Watt University, with Research Council and industrial funding, this book brings a new perspective to the design of building drainage and vent systems. It provides the building services community with clear and verifiable design methods that will be robust enough to meet challenges such as climate change and water conservation; population migration to the mega cities of the developing world, and the consequent pressures of user concentration; the rise of the prestige building and the introduction of new appliances and control strategies. These all combine to make traditional codified design guidance insufficient. Many assumptions in existing codes defining the entrained airflows within building drainage vent systems cannot be theoretically supported, so designers concerned with these systems need analysis and simulation capabilities which are at least as reliable as those enjoyed by other building services practitioners.

The Method of Characteristics solution techniques which are well established in the pressure surge field are now used to provide solutions for drainage designers. The material is applied to a whole range of abstract scenarios then to a series of real world applications including the forensic modelling of the SARS virus spread within Amoy Gardens in 2003 and the refurbishment of the O2 Dome. Applications to specialised services, including underground station drainage and highly infectious disease treatment facilities are discussed and demonstrated, alongside the use of design and simulation techniques in support of product development.

Aimed at both professional and academic users, this book serves both as a design aid and as a core text for specialist masters courses in public health and building services engineering.

1
Building drainage and vent systems, a traditional building service requiring an engineering analysis makeover?

The provision of reliable sanitation is a prerequisite of a developed society; this is as true for the mega-cities of the twenty-first century as it was for the emerging Greek civilisations around Homer’s ‘wine dark sea’. The pressures of population and climate change present new challenges to the design engineer that may not be met merely by reference to past experience or to codified guidance. Drainage and vent system design for the highly complex structures now being developed requires that the engineer has access to proven and fundamental analysis techniques that will, at the very least, provide the same level of confidence as that ascribed to other building services simulations.

In order to provide that understanding it is necessary to redefine the operation of building drainage and vent systems in terms of their fundamental operational characteristics and then draw upon the accumulated knowledge of fluid mechanics to provide analysis and simulation techniques that can contribute to improved system design.

While the perceived objective of a drainage system may be the removal of waste to the sewer, fundamentally an unsteady free surface liquid flow operation, the prevention of cross-contamination is an absolute necessity to protect against both infection spread and odour ingress. Therefore the analysis of building drainage and vent system operation must emphasise the mechanisms of air entrainment that accompany the unsteady liquid flows through the network and determine both the air pressure regime within the system and the probability of appliance trap seal depletion and cross-contamination. That this is an issue worthy of fundamental consideration was evidenced by the fatalities during the SARS epidemic in Hong Kong in 2003; an infection spread identified by WHO (2003) as exacerbated by poor drainage design and trap seal depletion.

The movement of entrained air within a building drainage and vent system is readily identified as a two-phase fluid flow phenomenon driven by the shear forces between the appliance water discharges and the air within the network, initially quiescent and at atmospheric pressure. The unsteady nature of the water flows inevitably result in an unsteady entrained airflow where the changes in airflow demand, as a result of the random discharge of the system appliances, are communicated through the propagation of low amplitude air pressure transients.

The pivotal role of transient propagation in establishing the air pressure regime within the network immediately links the analysis of building drainage and vent systems to the much wider and well-established field of pressure surge analysis. Developed over 100 years through a truly international research effort, pressure surge analysis is now an established component of fluid system design with industry standard analysis techniques based around the application of the mathematical Method of Characteristics and computer simulation. While failure conditions in pressure surge analysis may involve pressure surge predictions measured in atmospheres, the analogous failure conditions in drainage and vent system applications may only be 10s of mm of water gauge – sufficient to deplete essential appliance trap seals and lead to fatalities through cross-contamination. However the techniques employed to analyse pressure surge in a whole range of flow situations, from water supply or long distance oil or gas distribution to in-flight refuelling, are wholly transferable to the study of air pressure transient propagation in drainage networks; the fluid properties, the pipeline materials and the wave speed magnitudes may change but the defining St Venant equations of unsteady flow continuity and momentum remain identical. Many of the boundary conditions that will be met in drainage applications, including junction representation, relief valves and air chambers, mechanical interfaces to fans, pumps, variable setting valves, the reflections at system terminations and the treatment of unsteady fluid friction, all have direct analogues in pressure surge analysis. Similarly the more specialised applications of air pressure transient analysis to such areas as train and elevator motion within tunnels and shafts are directly related to the analysis techniques that will be developed.

This book will therefore take as its main objective the reassessment of the analysis of entrained air movement within building drainage and vent systems. The historical development of drainage and vent systems will be briefly presented together with a development of the essential relationships linking the water and airflows, including the methodology required to estimate system water throughflow as a result of random appliance discharge.

The fundamental concepts of pressure surge analysis will be presented together with a full development of the Method of Characteristics predictive and simulation techniques, drawn from the wider pressure surge research area. Examples of the application of these techniques will be presented to demonstrate the operational characteristics of a drainage network in the abstract. This will allow a better understanding of the overall conditions that determine the air pressure regime and the possibility of trap seal depletion. The theoretical aspects of transient analysis will be informed by drawing on a twenty-year research-led development of these analysis methods at Heriot Watt University where the Drainage Research Group has been continuously funded by government and international industry to develop these methodologies.

Applications to current building design processes, as well as forensic investigations aided by the analysis methods introduced, will be presented along with the theoretical base for a series of innovative products and system monitoring methodologies. The concept of Active Control to provide pressure surge control and suppression will be discussed and demonstrated, along with the basis for the sealed building design methodology.

The book will also identify areas where the national codes that have grown up over decades based on experiential knowledge, as opposed to the laws of physics that do not recognise national preferences, should take note of the predictive capability of the simulation techniques presented as a means of providing international standards.

Building drainage and vent system design and analysis may be recognised as a branch of unsteady fluid mechanics in every way amenable to the application of the predictive and simulation techniques already established within pressure surge analysis. This book aims to extend those methods to this undervalued branch of building services engineering.

1.1 The requirement for drainage and vent systems

The role of a building drainage system is to transport fluid, faecal and other waste from the appliance to the sewer connection with no possibility of cross-contamination, leakage or interruption to system availability. In addition, under the current conditions of climate change, the process should be undertaken with a minimum of water usage and the installed system should wherever possible use the minimum of resources.

The basic requirement for an efficient building drainage system, mirroring the provision of safe drinking water supplies, has been a prerequisite of civilisation over several thousands of years, from early and well-documented provision in the Eastern Mediterranean through the Roman era and the eventual reintroduction of sanitary knowledge to Europe, following the re-conquest of Spain, to the challenges posed by the development of the modern city, both as a result of the Industrial Revolution and now the worldwide migration of rural populations into the mega-cities of the developing world. While the development of modern sanitation is well documented, it will be helpful to highlight several key events as a basis for discussing the continuing design challenges to the successful provision of building drainage.

1.2 Basic operational mechanisms within drainage and vent system

The operation of a building drainage and vent system involves the unsteady flow of water, where the time dependency depends upon the random operation of the appliances connected to the network, as well as the time dependent discharge profiles characterising each appliance, and the attenuation of any discharge wave during its passage through the network. In parallel a time dependent entrained airflow is established, where the time dependency arises as a result of the shear forces ...

Table of contents

• Contents
• Figures
• Tables
• Preface
• Acknowledgements
• Abbreviations and notation Variables
• 1 Building drainage and vent systems, a traditional building service requiring an engineering analysis makeover?
• 2 Pressure transient propagation in building drainage and vent systems
• 3 Mathematical basis for the simulation of low amplitude air pressure transients in vent systems
• 4 Simulation of the basic mechanisms of low amplitude air pressure transient propagation – AIRNET applications
• 5 Pressure surge as a source of system failure, leading to the development of control and suppression strategies
• 6 Application of the Method of Characteristics based simulation to a series of case studies
• 7 Airflow applications of the Method of Characteristics simulation techniques
• 8 The role of national codes in drainage and vent system design
• Afterword
• References
• Author and organisation index
• Topic index