Lynne Barbara Jack

Developments in the definition of fluid traction forces within building drainage vent systems

With developments in the understanding of the pressure regime generated within single-stack building drainage systems, analysis by numerical modelling of complex and detailed systems serving a wide range of sanitary appliances has progressed notably. Computer-based finite-difference simulations incorporate both the complex unsteady fluid flow principles inherent within such systems and the specific design details of the drainage network, thereby allowing the time-dependent pressure response of the system to be assessed and the appliance trap seal retention or depletion to be predicted. This paper develops a new approach to the way in which the fluid flows present within the system are modelled, by providing an enhanced analysis of the waste water and entrained air flow interaction. It also extends the applicability of empirical data and examines resultant friction factor data. This approach can be applied to both single and multiple inlet vertical stack flow and can significantly enhance the designers ability to predict system response. Combined with unsteady free surface horizontal pipe flow theory, this enables the complete drainage network, from appliance trap through branch connections, vertical stacks and the sewer connection, to be analysed.

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.

Numerical simulation of pressure and airflow response of building drainage ventilation systems

Identification of the under-performance of the building drainage and ventilation system as a significant contributor to the transmission of the SARS virus in the ‘Amoy Gardens’ outbreak has prompted a re-examination of the methods adopted to ensure appropriate network design, implementation and maintenance. The physical separation between the miasma present within drainage pipe work and the habitable space occupied by the building user is achieved primarily through the use of the (commonly water-based) appliance trap seal. Systems must therefore be designed such that the integrity of this seal is sustained throughout all user or system-imposed operating conditions. This paper focuses on the work of the authors in defining key simulation model components that facilitate the prediction of the pressure response of building drainage systems and that thus allow an assessment of trap seal integrity to reduce the risk of infection spread. The paper draws upon the empirically defined network characteristics and extensive site data that have, so far, been established independently by researchers in the UK, Japan and Taiwan. The paper identifies the congruency of resultant data, and highlights the potential benefits of pooling research resources such that the contribution of simulation techniques to forensic analysis of virus spread via building drainage systems is confirmed.

Pressure transient identification of depleted appliance trap seals: a pressure pulse technique

The appliance trap seal plays a vital role in safeguarding occupied space from ingress of foul sewer gases driven by the barrage of pressure transients generated within the system during normal appliance discharge. The health risks related to depleted trap seals can be severe. In 2003, the rapid spread of the SARS virus at the Amoy Gardens housing complex in Hong Kong was attributed to depleted bathroom floor-drain traps. This paper presents a technique whereby depleted trap seals can be located remotely by monitoring the system response to an applied single pressure pulse. A Method of Characteristic based numerical model allows the system pressure response to be predicted while laboratory and site test results are shown to validate this proposed technique. Practical application: Appliance trap seal depletion poses a serious health risk by providing a route for cross-contamination and infection spread. Implementing a routine and regular maintenance regime would help to ensure that the water level within the trap seal remains above the critical level. However, current methods rely on visual inspections which are highly impractical in large complex buildings. A technique allowing the status of all connected trap seals to be quickly determined would be an invaluable tool for facility managers by improving operational efficiency and by indicating persistent failures, thus, highlighting areas requiring modification to ensure performance compliance.

Property-based rainwater drainage design and the impacts of climate change

Property-based rainwater drainage provision comprises a number of components broadly categorised as roof, surface or underground drainage. These systems are relied upon to prevent water ingress to the building and to avoid localised ponding or flooding. Recognition of the inherently unsteady response of the integrated network, in part driven by a particular rainfall event but also due to the transient nature of the fluid flows therein, is important in allowing an understanding of system performance under both current and future climate conditions. Codes and standards suggest that roof drainage systems, conventional or siphonic, are designed using a single rainfall intensity figure, representative of the most intense part of a longer storm, whereas runoff to surface and underground drainage is typically based on a single-peaked rainfall profile. Together with the difference in event duration typically adopted as part of the design process, this means that, unless a straightforward uplift factor is applied, then understanding the potential impacts of climate change on overall system performance can be difficult. Using numerical modelling techniques to analyse the performance of a case study site located in Edinburgh, this research identifies an appropriate common rainfall event from which a system-specific exceedance threshold is identified. Specification of this rainfall intensity facilitates an exploration of the impacts of climate change undertaken within the context of UKCP09 projections. Using gutter overtopping as an indicator of failure, this paper explores possible changes in the frequency of system under-capacity under varying future climate change scenarios. Practical application: Engineers are encouraged, in relevant codes and standards, to take account of the potential impacts of climate change when designing rainwater drainage systems for buildings. However, little guidance is offered therein on how to achieve this. This paper presents an example of UKCP09 climate change projections, specifically Weather Generator precipitation data, applied to a case study site that integrates roof, surface and underground drainage networks. Results illustrate the possible change in frequency of system failure, identified in this case as gutter overtopping.

Pressure transient identification of depleted appliance trap seals: a sinusoidal wave technique

Depleted appliance trap seals were shown to be a causal factor in the spread of the SARS virus at the Amoy Gardens housing complex in Hong Kong in 2003. This serious health risk has emphasised the requirement for an effective maintenance regime to ensure trap seals do not again facilitate the spread of infection and disease. This paper introduces a remote and non-invasive technique to identify depleted trap seals through analysis of the system response to an applied sinusoidal pressure wave. The pressure signal will be analysed in both the time domain and the frequency domain to determine system status. Results obtained from laboratory experiments will be used to confirm the practicality of this technique while a Method of Characteristic based numerical model will validate the methodology. Practical application: The transients generated within the building drainage system as a result of normal system operation continue to pose a threat to the integrity of the appliance trap seal. The technique outlined in this paper will help to quickly identify defective trap seals to ensure that cross-contamination is minimised. However, it is important that this transient-based technique does not itself introduce an additional risk to trap integrity. A sinusoidal pressure wave offers a completely non-invasive testing option that will not only be of great advantage to facility managers but also to the building users due to the reduced risk of cross-contamination.

Positive air pressure transient propagation in building drainage and vent systems

A major objective of drainage research over the past 100 years has been a reduction in the complexity of building drainage vent systems associated with the retention of appliance trap seals. Further simplification requires that the system operation is recognized as time dependent, where changes in water flow conditions result in the propagation of air pressure transients. Negative pressure transients that reduce trap seal levels by induced siphonage are well understood. More problematic is the propagation of positive air pressure transients, generated by stack or branch surcharge. The paper identifies the sources of positive air pressure transient propagation and demonstrates that such transients may be described by the St Venant equations of unsteady flow. Solutions are provided based on the proven finite difference methods and the method of characteristics and an understanding of the boundary conditions that represent the constituent components of a building drainage and vent system.

Overview of investigations addressing the issue of low water use sanitary appliances and associated drainage network design

With the development of water conservation as a key factor in both social and economic issues, the use of low water consumption appliances has significantly increased. A good deal of research has been carried out in order that such fixtures can be utilised efficiently and with confidence on the part of the designer, installer and end user. This paper introduces some of the basic principles of the drainage network and will attempt to collate and thereby complement associated research findings. Developments are detailed and parallel or synonymous findings are highlighted, bearing in mind that the overall aim of the research is to ensure that low water use appliances effectively and efficiently remove waste from the fixture, and that transport through the connected network is maximised. A comprehensive list of references is given.

The impact of detritus accumulation on the performance of siphonic rainwater outlets

This paper presents the findings of an 18-month study of detritus accumulation at two siphonic rainwater drainage systems installed in a building in Edinburgh. Findings are based on an analysis of data recorded from the site, and are positioned within the context of enhancing representation of the outlet loss coefficient used within both steady-state design calculations and finite-difference-based unsteady flow modelling techniques. The scope of findings reported herein is extensive but, in the main, shows how detritus accumulation not only builds relatively rapidly but also that ‘wash-through’ or ‘displacement’ occurs. Further, data suggest that detritus accumulation in the gutter, that is either wind-blown or is introduced from roof runoff, can, and does, build around the outlet with only relatively low intensity rainfall but that high-intensity rainfall events do seem to result in a direct and significant increase in blockage. An indication of the impact upon performance, assessed using simulation software, is also presented. Practical application: This paper reports patterns of detritus accumulation at on-site siphonic rainwater outlets. Changes in detritus ‘categories’ are mapped to corresponding weather data and conclusions drawn on influencing factors. The paper therefore yields key information for building owners/operators on the extent of detritus accumulation at siphonic system outlets. Outcomes also inform designers of the potential shift in loss coefficient and flow performance post-installation.