Trap Seals Research Articles

Integrating the Design of Tall Building, Wastewater Drainage Systems into the Public Sewer Network: A Review of the Current State of the Art

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.

A Photogrammetric Technique for Developing Boundary Equations for Flexible Sheath Waterless Trap Seals as Used in Building Drainage Systems

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.

Significance of core analysis and gamma-ray trends in depositional facies interpretation and reservoir evaluation of Cenomanian sequence, Alam El-Shawish East Oil Field, North Western Desert, Egypt

The present paper aims to evaluate the sedimentological model of the study Cenomanain sequence in terms of electrofacies interpretation and depositional facies architecture to reveal the spatial distribution of the reservoir facies characterization. The influence of depositional lithofacies types on reservoir characterization of the Cenomanian Bahariya Formation and Abu Roash “G” Member was investigated using the integration of available core data and petrophysical evaluation from seven wells in Alam El-Shawish East Oil Field, North Western Desert, Egypt. Careful log interpretation and correlation enable differentiation of the study sequence into ten sedimentary cycles. The stacking of the recognized facies types in the available cored interval, form fifteen depositional facies associations from which fluvial, estuarine and shoreline-shallow shelf models were constructed. This facies information has been used to calibrate the log response, particularly the gamma-ray trends, as an aid to interpret the depositional environment of the non-cored intervals. However, the generalized log pattern shows micro-variations in log shape and trend based on the location relative to depositional environment and consequently major modified gamma-ray patterns were proposed depending on the cored facies architecture. Therefore, precautions must be taken into consideration in extrapolating the present patterns to any similar heterogeneous sequence due to the effects of variable diagenetic and facies type constraints on log response. It is indicated that hydrocarbons could be accumulated within shoreface and barrier tidal inlet complex near or within the depositional sequence boundary surfaces along the Upper Bahariya Member and the upper unit of Abu Roash “G” Member. The associated fine siliciclastics (mudstone and shale) of tidal lagoon and/or shoreface delta facies may be acted as local source rock and trap seals. Based on reservoir characterization data supplemented by well logs interpretation of the cored and non-cored intervals, further reservoir beds could be recommended as potential prospecting targets.

Modelling water trap seal boundary conditions in building drainage systems: Computational fluid dynamics analysis of unsteady friction to improve accuracy

The safe removal of disease-carrying human waste is the objective of all sanitation systems and the limiting of air pressure transients within the system remains a significant part of current codes and regulations. The water trap seal offers fundamental protection and is the system’s sole barrier between the public sewer network and habitable space inside a building. Modelling water trap seal responses to air pressure fluctuations offers an opportunity to analyse whole system performance, but the quality of the data depends on the accuracy of the modelling technique and that of the defining inputs. AIRNET, a 1D Method of Characteristics based model, enables rapid whole system testing; however, the present boundary condition for the water trap seal within the model is based solely on steady state conditions, ignoring system dynamics. Computational fluid dynamics offers an opportunity to numerically evaluate the flow patterns within the trap seal in response to applied air pressure transients. This research confirms the importance of the rate of rise, and hence frequency of air pressure transients incident on water trap seals and relates this to potential vulnerabilities of different device geometries, particularly the ratio between inner and outer wall length. The research led to the development of a dynamic velocity decrement model encapsulating unsteady friction and separation losses linked to device geometry for the first time. The development of a frequency-dependent internal energy term Δ v, suitable for inclusion in AIRNET provides the capability to predict more realistic water trap response to air pressure transients over a range of air pressure transient frequencies likely to cause problems: 1 Hz to 8 Hz.Practical application: Whole system modelling can greatly improve the ability of design engineers to fully simulate the operation of a building drainage system in a realistic way. The work described in this paper improves the accuracy of whole system models by evaluating water dynamic responses to air pressure transients using a range of techniques including computational fluid dynamics and more traditional 1D finite difference method of characteristics models. The work also paves the way for more robust evaluation of building drainage products through in-depth investigation of the fluid mechanics associated with their operation.

A data-processing approach for exploring water-use maneuver of high-rise building

Recognizing high-rise building water-use maneuver should be helpful for developing some practical strategies when indoor room space safety becomes a crucial feature in the building operation. This paper presents a data-processing approach for exploring the water-use maneuver of high-rise building. This approach combines the empirical mode decomposition and statistical analysis to process the mezzanine-floor air-pressure data measured in the drainage stack system of Li Ka-Shing building at PolyU of Hong Kong on 28th of April of 2008. Because the mezzanine-floor air-pressure signals in the building drainage system are recorded within about 10 hours with a recording rate of one signal per second, data re-sampling is obviously needed. Otherwise, the direct application of empirical mode decomposition should be unavailable because the mezzanine-floor air-pressure data are too massive. Statistical analysis is further encompassed after the empirical mode decomposition to seek the influences of re-sampling time interval and the empirical mode decomposition index so that the building water-use maneuver can be understood in detail. Practical application: From the viewpoint of Building Services Engineering, any occupied space should be safeguarded, because the depletions of the trap seals 1 and the bathroom floor drain traps 2 can result in cross-contamination via the drainage system. This suggests that it is crucial to investigate building water-use maneuver when indoor room space safety becomes urgent. To explore the water-use maneuver, an indirect way is by analyzing the mezzanine-floor air-pressure in the drainage stack system in terms of an appropriate approach. Since the mezzanine-floor air-pressure in the drainage stack system of an 18-floor building, i.e. Li Ka-Shing building at PolyU of Hong Kong has been recorded, 3 it can be used to propose the water-use maneuver approach which is helpful for developing some practical strategies for indoor room space safety when the safety should be carefully and urgently faced.

Automatic detection of depleted fixture trap seals using the reflected wave technique

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.

An assessment of, and response to, potential cross-contamination routes due to defective appliance water trap seals in building drainage systems

The outbreak of severe acute respiratory syndrome (SARS), centred on Southeast Asia in 2002 and 2003, sparked fears of an uncontrollable pandemic. The outbreak at one housing complex in Hong Kong resulted in a disproportionately large number of cases with 321 people contracting the virus, resulting in 42 fatalities. This outbreak led to an investigation by World Health Organisation, which firmly laid the blame for the spread of the virus in the housing complex on defects in the building drainage and vent system. The mode of viral transmission was identified as due to airborne aerosolised particles from virus-laden excreta emanating from the drainage system. The aerosolised particles were allowed to enter the habitable space through empty water trap seals which are the main protection against such ingress in a building drainage and vent system. This research reports on the defective trap identification system developed to provide a means of testing the seal afforded by the water trap seal inbuildings. This innovative sonar-like technology incorporates a means of identifying compromised water trap seals anywhere in a building in a repeatable, non-destructive manner. While the technology has been effectively validated by a combination of laboratory investigation, site testing and numerical simulation, the rationale for the need to monitor water trap seals is less obvious to professionals operating in this field. This article addresses this issue and presents such a rationale, identifying some of the harmful pathogens which could lead to cross-contamination from person to person via the building drainage system. Practical applications: The research carried out in this area has highlighted the lack of any real monitoring of water trap seal status in buildings and has put the issue firmly on the agenda for consideration by building owners, operators and regulators, for whom the building drainage and vent system has long been categorised as ‘fit and forget’. The inspiration for this study was the response to the SARS outbreak which began in 2002. The case to monitor this important seal is clearly made and whilst the predicted pandemic of the SARS virus failed to materialise on an extended time frame and the outbreak has largely been consigned to history, it should serve as a constant reminder to professionals in the industry that we ‘fit and forget’ at our peril.

Derivation of an empirical frequency-dependent friction factor for transient response analysis of water trap seals in building drainage systems

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. Practical application: The development of new empirically derived frictional expressions to predict the effects of air pressure transients on appliance trap seals within a method of characteristics model provides far superior prediction results to those currently available. These new expressions enhance the modelling capabilities of AIRNET and prove that high frequency air pressure waves do not contribute to water trap seal depletion. The non-destructive nature of short duration or high frequency air pressure wave oscillations has contributed to the development of a non-invasive methodology for detecting water trap seal defects in building drainage systems, and this research serves to prove conclusively that using low amplitude, short duration pulses or sinusoidal pressure waves with a frequency above 3 Hz is non-destructive to traditional water trap seals in building drainage systems.

Characteristics of air pressure fluctuations in high-rise drainage stacks

Maximum air pressure in a drainage stack can deplete the appliance water trap seals that prevent the ingress of foul gases and odors into a habitable space. This study investigates the air pressure fluctuation frequency, as well as the maximum and average air pressures with their respective standard deviations, in a 38 m high drainage stack of a full-scale experimental tower under steady flow conditions of flow rates 1 Ls −1 to 4 Ls −1 discharging at a height between 15 m to 33 m above the stack base. Mathematical expressions are proposed to correlate the maximum air pressure with the probability density function of the data measured. The average prediction and the maximum under-prediction of the absolute peak pressure were determined with the margin of error taken within certain confidence levels. It was demonstrated that water seal failure would likely be occurred at some heights below the discharge locations. The outcome enhances the understanding of the characteristics of air pressure fluctuation within a drainage stack of a high-rise building.

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.

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.

Surfactant effects on air pressure transients in building drainage, waste and ventilation (DWV) systems

Building drainage, waste and ventilation (DWV) systems isolate the habitable space from the waste material with water trap seals at various points in the network. Current design guides aim to produce systems which protect these trap seals from unwanted air pressures, but are based on steady state experiments utilising cold clean water as a test media, and include a large ‘safety factor’ with no formal basis. However, most ‘gray’ and ‘black’ water sources are dosed with detergent, and are often warm, which significantly alters the behaviour when compared to clean water. This paper describes the current status of the investigation of paramaters affected in systems operating with detergent-dosed waste water over typical temperature ranges, then uses an existing EPSRC-funded mathematical model (AIRNET) based on these findings to assess the impact of the effect of detergents on existing building codes. The results quantify the effects of detergency with temperature and indicate a general shortfall in current code-based predictions, and suggest that building codes may need revision.

Limits for use of vent elements in building drainage systems considering the risks of infection spread by means of water-seal behaviour and integrity: the case of Brazilian systems

Considering the water conservation programmes that promote a reduction in WC flush volume, and given the concern with the safe use of new concepts and techniques to avoid the transmission of serious disease via the drainage system as in the case of SARS, this paper discusses the influences of said reduction on the behaviour of water-based trap seals in drainage systems utilizing different types of ventilation, as allowed by Brazilian standards. The paper shows that the venting system capacity to limit the internal pressure in the drainage system, in order to maintain the integrity of the water seals, reduces the risk of infection spread in buildings.

Sealed building drainage and vent systems—an application of active air pressure transient control and suppression

The introduction of sealed building drainage and vent systems is considered a viable proposition for complex buildings due to the use of active pressure transient control and suppression in the form of air admittance valves and positive air pressure attenuators coupled with the interconnection of the network's vertical stacks. This paper presents a simulation based on a four-stack network that illustrates flow mechanisms within the pipework following both appliance discharge generated, and sewer imposed, transients. This simulation identifies the role of the active air pressure control devices in maintaining system pressures at levels that do not deplete trap seals. Further simulation exercises would be necessary to provide proof of concept, and it would be advantageous to parallel these with laboratory, and possibly site, trials for validation purposes. Despite this caution the initial results are highly encouraging and are sufficient to confirm the potential to provide definite benefits in terms of enhanced system security as well as increased reliability and reduced installation and material costs.

Pressure transient control: Part II—simulation and design of a positive surge protection device for building drainage networks

Air pressure transient propagation within building drainage and vent systems may feature either, or both, positive and negative pressure changes in response to decelerating or accelerating flow conditions. Local venting, or the use of inwards relief air admittance valves, has traditionally suppressed negative transients. Positive transients have only been dealt with by the use of open roof terminations. This paper introduces an approach to positive air pressure transient suppression based upon the use of a flexible, variable volume containment vessel capable of reducing the rate of change of the entrained air following a system surcharge. The experimental programme to provide proof of concept, together with the simulation of the device utilizing the method of characteristics solution of the St Venant equations is detailed. Recommendations as to the suitability of bag materials and installation choices are presented, along with comparisons of surge relief efficiency. Practical application: The control of air pressure transients in building drainage systems has been limited due to the need to attenuate positive pressure propagation via an open termination at roof level - a poor solution as the transient will have affected all system trap seals before reaching the relief vent. The Positive Air Pressure Attenuator - an expandable bag that controls the rate of change of entrained airflow within the system - reduces the possibility of trap-seal loss due to positive transient propagation. Potentially this is a major contribution to vent system design that could revolutionize 150 years of design methodology.

Transient identification of defective trap seals

The prevention of the ingress of contaminated air into habitable spaces has been a central concern in the design of building drainage and vent systems since the 1850s. Failure to provide the necessary protection, primarily through appliance water trap seals, was shown to have been a significant contributor to the Severe Acute Respiratory Syndrome (SARS) epidemic spread in Hong Kong, China, in 2003. Prevention depends upon both good design, to limit the air pressure transients propagated within the system, and good maintenance. Good practice has evolved to include wherever practical long radius bends to minimize the local air pressure changes. Similarly, modern usage of drainage networks includes the random discharge of disposable sanitary and hygiene products that may interact with undesirable rapid changes in flow direction and thereby contribute to local surcharge conditions. However, a maintenance regime within a large complex building network also requires a degree of prior knowledge of possible defective appliance trap seal locations. This paper proposes the use of air pressure transient simulation and transient response measurement to identify, during periods of system non-use, the location of depleted trap seals. The simulation of air pressure transient propagation within building drainage and vent systems is detailed, based on the proven capabilities of the method of characteristics solution of the St Venant equations. Validation of system transient response simulation is presented together with indications of the impact of a defective appliance trap seal on the system response to an applied low-amplitude, short-duration, pressure surge. Comparison of the defect free and defective system response is shown to be sufficiently discriminatory to allow the identification of an appliance trap seal in need of replenishment. Identification of persistent trap seal depletion would allow local installation of active control devices to prevent further failures – including the addition of either air admittance valves to limit trap seal loss to negative transients or variable volume containment positive air pressure attenuators to limit trap seal loss due to positive pressure transients.

Pressure transient control: Part I — criteria for transient analysis and control

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 large-scale pipeline and plant networks to the development of computing simulations to support system design. Pressure surge analysis is therefore a consideration in the design and operation of all fluid systems: the objective of such an analysis being the prediction, control and suppression of transients. This paper presents the background to the development of surge alleviation, from traditional pipeline protection to applications within building drainage and vent systems, thereby stressing that the system failure consequences of transient propagation are dependent on the particular system, independent of absolute surge pressure, and that system protection criteria may be developed that apply regardless of the system or the severity of the transient. Practical application: The control of air pressure transients in building drainage systems has been limited due to the need to attenuate positive pressure propagation via an open termination at roof level - a poor solution as the transient will have affected all system trap seals before reaching the relief vent. The Positive Air Pressure Attenuator - an expandable bag that controls the rate of change of entrained airflow within the system - reduces the possibility of trapseal loss due to positive transient propagation. Potentially this is a major contribution to vent system design that could revolutionize 150 years of design methodology.

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.

Control and suppression of air pressure transients in building drainage and vent systems

Air pressure transients propagate within building drainage and vent systems due to the unsteady nature of the network flows. These low-amplitude transients may deplete the appliance water trap seals that prevent ingress of foul gases and odours into habitable space. The simulation of transient propagation and trap seal response is achieved via the method of characteristics solution of the St. Venant equations. The application of transient control and suppression methodologies to control both positive and suction pressure trap seal loss is demonstrated and the importance of trap seal retention to prevent cross contamination via the drainage network is emphasised.

Detergents in drainage systems for buildings

Current soil–waste–vent (SWV) system design guides aim to produce ventilated drainage networks for buildings which protect attached water trap seals from unwanted air pressure effects. Such effects may cause trap seal failure that are caused by airflow induction during discharge. The ventilation design guides are based on steady-state experiments utilising cold clean water as a test media. However, most ‘grey’ and ‘black’ water sources are dosed with detergent, and are often warm, which significantly alters the behaviour when compared to clean water. Thus, current design standards provide only approximations of SWV system response. Results indicate that induced airflows in warm detergent-dosed water can be significantly higher than those measured in clean water systems, by a factor of more than 2. This paper quantifies the effect of detergents in SWV systems in terms of observed air entrainment rates and previously published research on the factors that contribute to air entrainment. Results of previous work are cast into dimensionless groupings suitable for inclusion into a mathematical simulation model based on a finite difference scheme. It utilises the method of characteristics as a solution technique to simulate drainage system operation via the equations that define unsteady partially filled or full bore pipe flows and the boundary conditions represented by water traps and other common system components.