Figure 9 - uploaded by Michael Gormley
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The air admittance valve operates to allow an air inflow when the local system pressure falls to below a preset subatmospheric level. The valve closes as the local pressure recovers
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Pressure transient propagation is a wholly natural consequence of any change in operating conditions for a fluid carrying system. Rapid changes in flow conditions generate surge conditions that may result in system failure. The analysis of these phenomena has progressed over the past 100 years from empirical research aimed at the protection of larg...
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... These components require to be sized, oriented and suitably assembled to achieve an acceptable level of performance at minimal cost. Active components such as air admittance valves (AAVs) or pressure suppression devices (e.g., PAPA devices [22]) may be added to enhance this design at a relatively low additional expense. The passive components affect the general nature of response to discharges (response to continuous flows and transient flows), while the active components will specifically target response to transient events. ...
... Wind shear at the top of the stack [22] (which can give rise to suction pressure and encourage air updraft); • Discharges of solids into the stack [34] (leading to brief interruptions in the air supply); • Discharges of surfactants within the waste stream (leading to creation of foams at air-water interfaces); • ...
Diameters for drainage stacks and vent lines within high-rise building drainage systems are determined by consulting building standard agencies’ design codes. While these are critical design decisions, codes are based upon dated research (1940s to 1970s), which has numerous inherent limitations, and the methodologies employed within the codes are unclear. Thus, a new methodology is presented which is based upon an analogy with other forms of multiphase flow transport systems. This methodology assumes, as a pre-condition, that flows of air and the flow of water within the stack are reasonably steady over time. Component diameters must then be chosen which ensure an acceptably large air supply or air–water flow ratio, and an acceptably small pressure excursion within the stack. Two ways to implement this methodology are presented: an ‘explicit approach’, in which component diameters are directly calculated using empirical correlations, and an ‘implicit approach’, in which component diameters are determined by iteration, using a hydraulic model. The methodology pre-conditions of the approach are then discussed. The physical geometry of the stack and branches tends to promote steady water flow but to render air flow very susceptible to temporary interruptions. A need to maintain the air pathway within high-rise drainage systems using components to supplement the air feed drawn in through the roof vent as required is highlighted.
... Thereafter, some scholars proposed the concept of pipe filling rate. Swaffield et al. ( , 2005aSwaffield et al. ( , 2005b and Campbell & MacLeod (2001) conducted a large number of theoretical studies and experiments in full-scale buildings and achieved significant results, as well as proposed a series of improvements on the applicability of the mathematical model. The researchers measured the transient pressure in the drainage stack. ...
... The results suggested that the main factors affecting air flow include the shear stress caused by the velocity gradient between the water film and surface of the air column, and the drag effect of free water droplets. Based on the tests of the drainage system in low-rise buildings, Heriot-Watt University (Swaffield et al. , 2005a(Swaffield et al. , 2005bKelly & Gormley 2014) also conducted a series of studies on the relationship between the internal pressure fluctuations of the system and the water seal loss of water appliances. Continuity equations, momentum equations and quadratic partial differential equations were adopted to establish mathematical models that could be used to predict internal pressure fluctuations and water seal losses in the drainage system. ...
The final velocity was put forward to study the water flow characteristics inside the building drainage system; however, it is more suitable for low-rise and multi-storey buildings, not for high-rise buildings. This study revealed the drainage transient characteristics of a double stack drainage system in high-rise residential buildings. Based on the final velocity, the air-water interaction mechanism and two-phase flow conditions in high-rise residential drainage stacks were discussed. An influence model of drainage system flow rate on pressure fluctuation under the change of state parameters such as ventilation rate, pipe wall roughness and building height was established. The pressure limit and flow rate data were obtained through full-scale experiments. The pressure limit and flow rate model were simplified to . After the data were verified, the fitting coefficients A, B and C were linear to the floor height. HIGHLIGHTS
This study puts forward a model of drainage system flow rate on pressure fluctuation under the change of state parameters in double stack drainage system in high-rise residential buildings.;
The pressure limit and flow rate data were obtained through full-scale experiments in high-rise experiment tower.;
The fitting coefficient A, B and C of pressure fluctuation model were linear to the floor height.;
... In particular, the need to focus on pressure transient phenomena within the fluid services associated with building operation has increased with the complexity of the built environment [9]. Different studies have shown that the effect of these air pressure (positive and negative) transients on the system can be devastating and lead to the ingress of foul air into the habitable space [10,11]. ...
... It is worthy of mention that most of the solutions for public health protection are available inside the building, and the main reason is to minimise, or as much as possible control the pressure fluctuations occurring as a result of appliances' random discharges that contribute to the time dependent water-flow conditions within the system to generate negative pressure as a result of increased entrained air. In addition, this occurs as a result of system surcharge at the base, at offsets, or at discharging branches that can cut off the air path down the stack, which generates positive pressure, and finally, the external factors that were mentioned before, such as a wind, sewer surcharge, and pump selection [10,11]. ...
... Passive solutions include providing a traditional vent that will hold the pressure transient excursion to a minimum, but the growth in building heights has led to an increased need to innovate new techniques that are cost effective and applied as close as possible to the source of the transient, using methods such as the air admittance valve (AAV), waterless trap, and positive air pressure transient attenuator (PAPA). The control device must be positioned between the source of the transient and the site to be protected from pressure [10]. ...
The design of above ground building drainage systems follows codes and standards that only give cursory recognition to the fact that this system connects, in the majority of cases, directly to a vast network of sewer pipes leading to a wastewater treatment plant. At the same time, for underground systems, airflow within as well as in and out of sewers is often neglected during the design of sewers, which depend on these building installed systems for pressure relief and venting. There is clearly an interaction between the two systems, yet this is not reflected in the design guidance, particularly inside buildings where air pressure fluctuations can lead to the destruction of water trap seals and the ingress of foul air containing sewer gases and potentially harmful pathogens. In this systematic review of historical research and design practice for both above and belowground drainage systems, we present the current state of the art and make recommendations for advancements that recognise the interaction between systems and present a view on how design could be advanced in a more holistic way.
... American, European and British regulation have historically been written to avoid the loss of trap seals due to siphonage or blowout (Swaffield et al., 2005a); evaporation is often afforded less attention (CIBSE, 2014: Guide G;BS EN 12056-2, 2000), or ignored (Department of Health (UK), 2013). Evaporation is not mentioned in the main text of BS:EN 12056, however the National Annex cites the risk of evaporation specifically from floor gullies, suggesting that they should only be sited where they would be adequately replenished. ...
... There are also now waterless traps, typically consisting of a silicone sheath which opens under the weight of wastewater, or in response to negative air pressure in the drainage system (Swaffield et al., 2005a); these have found extensive use in practice (Gormley and Beattie, 2010;Gormley et al., 2017). However, their function is not regulated by any standard, which may decrease confidence in their adoption; they are also vulnerable to blockage by solid matter (CIBSE, 2014). ...
... The risk of trap blowout due to transient pressure waves caused by the sudden interruption of air flows, such as by backup, water curtain formation, or branch discharge into the stack, can be mitigated by attenuating the pressure waves. (Swaffield et al., 2005a, Swaffield et al., 2005b) developed a positive air pressure transient attenuator (PAPA) for this purpose, the use of which has been demonstrated experimentally and in the field. Kelly et al. (2008) demonstrated the use of pressure waves as relatively lowamplitude vibrations to identify vacant trap seals on a BDS. ...
There is emerging evidence of the transmission of SARS-CoV-2 via the sanitary plumbing wastewater system, a known transmission pathway of SARS-CoV-1. These events can no longer be dismissed as isolated cases, yet a lack of awareness and of basic research makes it impossible to say just how widespread this mode of transmission might be. Virus is transmitted within wastewater systems by the aerosolisation of wastewater and subsequent transport of bioaerosols on naturally occurring airflows within the piped network. Central to the debate around risk to building occupants from SARS-CoV-2 spread via wastewater plumbing systems is the question of infectivity of faeces, urine and associated aerosols. This paper presents an examination of the processes which underlie this mode of transmission, and the existing epidemiological evidence, as well as existing mitigation strategies; significant gaps in the state of the knowledge are also identified. It is hoped that this review will cultivate a wider awareness and understanding of this most overlooked of threats, and to facilitate the selection and adoption of appropriate mitigation strategies. Key gaps in the knowledge span the rate of generation of bioaerosols within the building drainage system, their composition and transport properties, and the viability and infectivity of virions and other pathogens which they carry. While much of this work will be conducted in the laboratory, we also identify a dearth of field observations, without which it is impossible to truly grasp the scale of this problem, its character, or its solution.
... While these fears about possible cross-contamination routes exist and efforts to limit the effects of pressure transients on system performance have been developed [15][16][17][18] there is an alternative which has been available for nearly 20 years; it is, however, not as prevalent in buildings as the simple water trap seal. ...
The water trap seal is still the main method of protecting building inhabitants from the ingress of foul contaminated air and noxious gases from the sewer. This seal can become compromised when water is lost in the trap by processes including evaporation and siphonage from excessive system suction pressures. A recent innovation is the waterless trap seal, which uses flexible sheaths, typically made from silicone rubber to form the seal. The sheath opens in response to a sub-atmospheric air pressure and will shut tightly under a supra-atmospheric pressure in order to form a seal. Full system numerical modelling of building drainage systems has offered insight into system responses to pressure transients and has opened up the evaluation of building wastewater systems to predictive modelling which has assisted in producing improvements to public health. A requirement of any predictive model is a mathematical representation of the physical characteristics of the system. This research develops a technique for developing boundary equations so that predictive modelling is possible. We combine photographic and pressure data analysed by Fourier analysis to develop the model. The technique is applicable to any device were the fluid structure interaction plays a significant role in its operation.
... Gravity is the only driving force, and all the water, air, and air-to-water frictional forces balance the gravity force, together producing the familiar pressure recovery curve shown in Figure 1, which also depicts the pressure effects on the water-trap seals. However, some studies [7,23], as well as anecdotal evidence, have shown special cases of anomalous results in the building drainage systems of tall buildings, which are difficult to account for. All pressure phenomena in the air propagate at the local speed of sound, adequately approximated to 340 ms −1 . ...
National design guides provide essential guidance for the design of building drainage systems, which primarily ensure the basic objectives of preventing odor ingress and cross-transmission of disease through water-trap seal retention. Current building drainage system design guides only extend to buildings of 30 floors, while modern tall buildings frequently extend to over 100 floors, exceeding the predictive capability of current design guides in terms of operating system conditions. However, the same design guides are being used for tall buildings as would be used for low-rise buildings. A complicating factor is the historic roots of current design guides and standards (including the interpretation of the governing fluid mechanics principles and margins of safety), causing many design differences to exist for the same conditions internationally, such as minimum trap seal retention requirements, stack-to-vent cross-vent spacing, and even stack diameter. The design guides also differ in the size and scale of the systems they cover, and most make no allowance for the specific building drainage system requirements of tall buildings. This paper assesses the limitations of applying current building drainage system design guides when applied to the case of tall buildings. Primarily, the assessments used in this research are based on codes from Europe, the USA and Australia/New Zealand as representative of the most common approaches and from which many other codes and standards are derived. The numerical simulation model, AIRNET, was used as the analysis tool. Our findings confirm that current design guides, which have been out of date for a number of decades, are now in urgent need of updating as code-compliant systems have been shown to be susceptible to water-trap seal depletion, a risk to cross-transmission of disease, which is a major public health concern, particularly in view of the current COVID-19 pandemic.
... After the SARS outbreak of 2002 to 2003, efforts to formally regulate wastewater systems-similar to regulation of water supply systems to control such pathogens as Legionellafailed to gain traction. Other innovations developed at that time to deal with air pressure surges, a common cause of empty U-traps in high-rise buildings, are currently used in some locales but are not common practice (9). In another development, a method was invented to determine whether a system is sealed, but this likewise has not been used widely (10). ...
... Excessive negative pressures generated by discharging appliances or adjacent appliances, can lead to trap depletion by self-siphonage or inducedsiphonage, respectively. The propagation of excessive positive pressures within the system can be large enough to completely displace the water seal into the appliance [26,27,28], leaving the trap either wholly or partially depleted. In some cases, even though the trap is not noticeably depleted, positive pressures can push air from the sanitary plumbing system through the water seal as air bubbles and into the building. ...
... Limiting pressure fluctuations forms the basis of much of the design of a sanitary plumbing network and while great efforts are made to reduce the likelihood of this, transient generation and propagation is inevitable (particularly in larger systems where overload is possible). Air pressure surges in excess of 50 mm (both negative and positive) are not uncommon and while Pathogen cross-transmission via plumbing systems efforts are made to limit these by introduce air admittance valves and P.A.P.A's into the system, localised pressure surges and hence depleted U-traps, are inevitable on large systems [5,26,27,28]. The likelihood of a building occupant becoming infected through transmission from the sanitary plumbing system is dependent on four main variables: (i) the presence of a depleted U-trap; (ii) the presence of infectious pathogens within the sanitary plumbing system; (iii) adequate system airflows to transmit the infectious pathogens; and (iv) the susceptibility of the occupant to those infectious pathogens, heightened due to immunity suppression. ...
... Air flow rates (ls -1 ) equate to; low (<20), medium(20)(21)(22)(23)(24)(25)(26)(27) and high (>27) B = bottom wall, L = left wall, T = top wall, R = right wall, F = front wall, TP = Test Point Blank cells indicate where testing was not carried out or sample locations were unavailable # Passive sampler splashed by flush-count not possible. doi:10.1371/journal.pone.0171556.t001 ...
The WHO Consensus Document on the epidemiology of the SARS epidemic in 2003, included a report on a concentrated outbreak in one Hong Kong housing block which was considered a ‘super-spreading event’. The WHO report conjectured that the sanitary plumbing system was one transmission route for the virus. Empty U-traps allowed the aerosolised virus to enter households from the sewerage system. No biological evidence was presented. This research reports evidence that pathogens can be aerosolised and transported on airstreams within sanitary plumbing systems and enter buildings via empty U-traps. A sanitary plumbing system was built, representing two floors of a building, with simulated toilet flushes on the lower floor and a sterile chamber with extractor fan on the floor above. Cultures of a model organism, Pseudomonas putida at 10⁶–10⁹ cfu ml⁻¹ in 0·85% NaCl were flushed into the system in volumes of 6 to 20 litres to represent single or multiple toilet flushes. Air and surface samples were cultured on agar plates and assessed qualitatively and semi-quantitatively. Flushing from a toilet into a sanitary plumbing system generated enough turbulence to aerosolise pathogens. Typical sanitary plumbing system airflows (between 20–30 ls⁻¹) were sufficient to carry aerosolised pathogens between different floors of a building. Empty U-traps allowed aerosolised pathogens to enter the chamber, encouraging cross-transmission. All parts of the system were found to be contaminated post-flush. Empty U-traps have been observed in many buildings and a risk assessment indicates the potential for high risk cross-transmission under defect conditions in buildings with high pathogen loading such as hospitals. Under defective conditions (which are not uncommon) aerosolised pathogens can be carried on the airflows within sanitary plumbing systems. Our findings show that greater consideration should be given to this mode of pathogen transmission.
... The program yields air pressure and velocity within bounded conduits subjected to air pressure transient propagation. [17][18][19] The method of characteristics provides a flexible mathematical model which can deal well with the representation of pressure transients in complex pipe and duct networks, and has become the standard solution technique applied to their analysis throughout the field of pressure surge analysis and prediction. Figure 3 shows the rectangular grid representation of the scheme used for the calculation of the propagation of pressure transients along a pipe. ...
The loss of fixture trap seals presents a potential cross-contamination route for sewer-borne pathogens. The defective trap identification method was developed to assess the status of trap seals using a non-destructive ‘sonar-like’ test based on the reflected wave technique. System diagnosis, proven over several years of laboratory and sited-based validation, primarily depended on manual interpretation of system pressure responses to identify the trap location from the reflected wave return time. This paper advances the technique by introducing the developed ‘TRACER’ program which includes a time series change detection algorithm allowing automatic system diagnosis. Outputs were validated against simulations using the AIRNET numerical model which, together with the ‘PROBE’ Method, was used to determine trap pipe periods, and therefore the location of defective trap seals, with improved accuracy over those calculated theoretically. This technique thus provides a reliable and automated approach to monitoring the protective seal between habitable space and the sewer network.
Practical application: Once installed, the building drainage system is often afforded little consideration in terms of regular maintenance, due partly to the problem of the ‘out of sight, out of mind’ mentality which is so often associated with this fundamental system, but also due to the practicalities of executing such an onerous task. Armed with a greater appreciation of the health risks associated with fixture trap seal loss, due to understanding its role in the transmission of the severe acute respiratory syndrome virus in 2003, building owners, operators and regulators must take action to safeguard the integrity of this important protective seal.
... Many innovations in drainage system design have been incorporated with the specific aim of protecting the integrity of the water trap seal since this is the last line of defence between the habitable space and the sewerage system. 4,5 The prediction of water trap seal operation is therefore of great importance in the overall prediction of system performance. ...
... The method is used to yield air pressure and velocity within a bounded duct system subjected to air pressure transient propagation. 1,2,4 The method of characteristics provides a flexible mathematical model, which can deal well with the representation of pressure transients in complex pipe and duct networks, and has become the standard solution technique applied to their analysis throughout the field of pressure surge analysis and prediction. ...
... This frictional term is the focus of this research. The new empirically derived friction factor expression will be applied to Equation (4). Term 4, the mass time acceleration term may be expressed as: ...
Flows in building drainage systems (BDS) are inherently unsteady. The changing nature of these flows produce pressure transients that can be harmful to water trap seals, the last line of defence against sewer gas ingress into habitable spaces. Air pressure transients propagate throughout BDS until they are diminished by either a relief valve, an open termination or, under certain circumstances, produce their own ‘relief valve’ through a compromised water trap seal. The spread of the SARS virus through empty water traps in Hong Kong in 2003 highlighted the worst possible consequences of water trap seal compromise. These air pressure transients can be simulated and their effects predicted. Only one numerical model exists for modelling airflow and the associated air pressure regime in BDS—the method of characteristics-based AIRNET. The current water trap boundary condition within the AIRNET model consists of a steady laminar frictional flow relationship, which does not replicate the unsteady pressure regime in the building drainage system. An improvement on this frictional representation is due to Carsten—Zilke, a methodology that allows a moving frictional term for water to trap surface interaction. This research proposes an alternative, empirically derived friction factor, and represents a significant simplification of the calculation process, leading to a robust, dynamic prediction methodology for water trap seal response to applied air pressure waves.
