Siphonic Roof Drainage Systems Research Articles

Analysis of Siphonic Roof Drainage Systems with EPANET

Analysis and design of siphonic roof drainage systems are usually performed with specific software developed by the system’s manufacturers. An experimental study was carried out to verify if a general software for the analysis of hydraulic pressurized pipe networks can be used to analyze a siphonic roof drainage system. A test model was built and tests were conducted to compare the prototype results with simulation runs in EPANET. A new EPANET data model was developed to overcome software limitations for the siphonic drainage systems analysis. Considering the results, less than 5% of average error was observed between the measures in the real test model and the simulation results, which can be attributed to measurement error. To validate the EPANET data model, it was compared with a specific software that analyzes siphonic roof drainage systems. It can be concluded that EPANET is a software that can be used to analyze and, therefore, to design, siphonic roof drainage systems in buildings.

Alternative low-cost overflows for siphonic roof drainage systems: Proof of concept

This paper describes an experimental investigation of alternative overflow systems for roof structures that are drained siphonically. Many buildings that have siphonic roof drainage systems currently incorporate a separate secondary overflow system, which is quite often also siphonic. An alternative low-cost technique that is explored in this study is to connect each overflow outlet to a single, vertical downpipe. Seven different overflow configurations, each with five different downpipe lengths, are investigated in terms of maximum flowrate and corresponding water depth in the gutter. The results of this study are significant in that they do not support the common theoretical assumption that there is a limiting length of a siphonic downpipe over which pipe-full flow may occur before gravity causes the water to accelerate enough for it to no longer occupy the full cross-sectional area. Instead, this study has found that once a downpipe is flowing full, it is possible for pipe-full flow conditions to be maintained over the entire pipe length.

Estimating flow rates through individual outlets of siphonic roof drainage systems

An experimental investigation is used to quantify the flow rates through individual outlets of multi-outlet siphonic roof drainage systems under both full-pipe and partially filled pipe flow conditions. A pressure transducer and a propeller-type current meter were installed within each tailpipe to measure water depth and flow velocity. The study tests the hypothesis that it may be possible to predict accurately the flow rates through individual outlets of multi-outlet siphonic roof drainage systems. A new technique of estimating flow rates is trialled by comparing instantaneous pressure transducer and current meter readings with previously calibrated flow data to find the best data match. The study results were very positive and clearly demonstrate that the underlying methodology was appropriate and that it may be possible to model numerically the individual outlet flow rates in multi-outlet siphonic roof drainage systems.

Air water flows in building drainage systems

This paper investigates the mechanisms of two-phase flows that occur in commercial and industrial roof drainage systems. Both traditional gravity-fed and siphonic roof drainage systems are examined.Air entrainment plays a fundamental role in the performance of siphonic roof drainage systems. In particular, air entrainment has a significant effect on:maximum system flowrate capacity;pipe friction losses;operational system pressures;operational gutter water depths; andability of the system to prime.However, experimental results presented here demonstrate that the reduction in system capacity is not directly proportional to the increase in air content. One possible explanation for this is that water and air will be affected differently by sub-atmospheric pressures. The effect air entrainment has on roof drainage performance is investigated. The ways in which air is introduced into the system are identified and the effects of different air/water ratios are quantified. Finally methods for reducing air entrainment are described.

Novel modulated flow, self configuring, siphonic roof drainage system

Siphonic roof drainage systems are an efficient method of removing rainwater rapidly from roofs. Siphonic roof drainage systems are designed to run full-bore, resulting in sub-atmospheric system pressures, higher driving heads and higher system flow velocities. Hence, siphonic roof drainage systems normally require far fewer downpipes, and the depressurised conditions also mean that much of the collection pipework can be routed at high level, thus reducing the extent of any underground pipework. But, they work properly at only one roof run-off rate and therefore suffer from sizing and operational problems that limit their performance. Climate change is creating situations where normal ranges of rainfall intensity are being frequently exceeded, and this may have an impact on the performance of siphonic roof drainage systems. A multiple parallel pipe siphonic roof drainage system appears to offer benefits and avoids sizing problems associated with current siphonic roof drainage systems. A movable cap covering the inlet to a small bank of parallel pipes has the potential to avoid noise associated with making and breaking siphonic action through flow modulation. Laboratory scale tests demonstrate the basic feasibility of the multiple parallel pipe system and indicate that handover of flow between pipes occurs smoothly and that the flow modulation cap functions reliably. This technology includes moving parts and smaller diameters (19 mm ID) than are currently accepted (32 mm ID) in the British Standards and product development will be required. Practical application: Original research detailing an innovative development of a multi-pipe siphonic roof drainage system. This provides a wide design flow rate range and flow modulation caps reduce noise significantly by avoiding priming failure. Potentially extends the technique to a smaller building size than with current siphonic systems and offers climate change resilience potential.

Investigation of a Flow Modulation System for Siphonic Roof Drainage Systems

Siphonic roof drainage systems (SRDS’s) have been widespread used now for approximately 40 years and are an efficient method of removing rainwater rapidly from roofs. SRDS’s are designed to run full-bore, resulting in sub-atmospheric system pressures with high hydraulic driving heads and higher system flow velocities than conventionally guttered systems. Hence, SRDS’s normally require far fewer downpipes, and the depressurised conditions also mean that much of the collection pipework can be routed at a high level, thus reducing the extent of any underground pipework. But, they work properly at only one roof run-off rate and therefore suffer from sizing and operational problems including noise and vibration which limit their performance and adoption rate. Climate change is creating situations where normal ranges of rainfall intensity are being frequently exceeded, so the typical:storm ratios (rTS) are large increasing. Current SRDS’s typically operate within a small rTS range of 2. This may have an impact on the future uptake of SRDS’s. This paper describes the development of a novel SRDS which includes a small mobile cap at the roof of outlet appears to offer benefits and avoids sizing problems associated with current SRDS’s. The cap has the potential to avoid noise associated with making and breaking siphonic action through flow modulation. Laboratory scale tests demonstrate the basic feasibility of the cap system and indicate that the cap functions reliably. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Basic on sizing and design optimiza-tion factors are suggested. The rTS range is increased from approximately 2 to approximately 6.

Measuring flows in partially-filled pipes in siphonic roof drainage systems

While a variety of flow measurement devices are available to measure the flow of water through closed pipe systems, these devices generally only function correctly when the pipes are completely full of water. Accurate measurement of water flows in partially-filled pipes is extremely difficult. In siphonic drainage systems, this problem is further compounded by the unsteady flow conditions that occur in the pipework during the priming process. This has been a major obstacle to understanding the performance of these systems in practice. In order to accurately model the priming process in multioutlet siphonic roof drainage systems, a method of estimating the instantaneous flows through the partially-filled individual pipes needs to be developed. This paper describes an experimental method of determining flows in partially-filled pipes using a propeller-type current meter to measure flow velocity and a pressure transducer to measure water depth and a modified version of the continuity equation. A computational model is presented which estimates the unsteady flows passing through partially-filled pipework. Overall, the experimental results are promising and correspond well with the model. The results of this study will ultimately be used to develop an unsteady flow model of the priming process in multi-outlet siphonic roof drainage systems.

Plastic pipe pressures in siphonic roof drainage systems

Siphonic roof drainage is a highly efficient type of drainage system that is particularly suitable for large buildings and other structures over approximately 4 m in height. Although siphonic roof drainage systems are enjoying ever-increasing popularity with architects, there is still some uncertainty regarding the minimum pipe pressure class required for siphonic pipework, especially in tall buildings. This is particularly the case in warmer countries since higher temperatures can drastically decrease the strength of the pipework material used in siphonic systems – typically unplasticized polyvinylchloride (PVC-U) and high-density polyethylene (HDPE). However, there is very limited information available on how plastic pipes behave under the sub-atmospheric pressures that occur under operating conditions in siphonic systems. This paper describes experiments conducted to investigate sub-atmospheric pressures in siphonic systems and how they may be controlled by injecting air into vertical downpipes. Recommendations for minimum pipework pressure classes are provided together with methods for limiting the minimum pressures without significantly decreasing the system capacity. This paper will help engineers design siphonic systems with more confidence so that such systems will continue to perform adequately over their intended design life. L’évacuation siphonique des eaux de toit est un type de système d’évacuation des eaux hautement performant, qui est particulièrement adapté aux grands immeubles et autres structures dépassant 4 m de hauteur environ. Bien que les systèmes d’évacuation siphonique des eaux de toit bénéficient d’une popularité sans cesse croissante auprès des architectes, il existe encore quelque incertitude quant à la classe de pression minimale des tuyaux nécessaire pour les tuyauteries siphoniques, surtout dans les grands immeubles. Ceci est tout particulièrement le cas dans les pays plus chauds, dans la mesure où des températures plus élevées peuvent considérablement réduire la résistance du matériau utilisé pour les tuyauteries dans les systèmes siphoniques – généralement du polychlorure de vinyle non plastifié (PVC-U) et du polyéthylène haute densité (PEHD). Les informations disponibles sont cependant très limitées concernant la manière dont se comportent les tuyaux en plastique soumis aux pressions sub-atmosphériques qui se rencontrent dans les conditions de fonctionnement des systèmes siphoniques. Cet article décrit des expériences conduites afin d’étudier les pressions sub-atmosphériques dans les systèmes siphoniques et la façon dont elles peuvent être contrôlées en injectant de l’air dans les tuyaux de descente des eaux pluviales. Des recommandations concernant les classes de pression minimales des tuyauteries sont fournies, ainsi que des méthodes pour limiter les pressions minimales sans abaissement significatif de la capacité des systèmes. Cet article aidera les ingénieurs à concevoir avec plus d’assurance des systèmes siphoniques de façon à ce que ces systèmes puissent continuer à fonctionner convenablement tout au long de leur durée de vie nominale prévue. Mots clés: pression critique de flambage, pression négative, évacuation siphonique des eaux de toit

Aeration and gutter water levels in siphonic roof drainage systems

An investigation is presented into how negative system pressures, the degree of aeration, and gutter water levels are affected by the number of outlets in a siphonic roof drainage system. The experimental results show that system pressures decrease if outlets are blocked. It is also shown that the depth of water above an outlet is strongly influenced by the negative pressure acting at the outlet. As the suction effect at the outlet increases, due to lower system pressures, more water and air will be drawn into the outlets and this will result in a corresponding increase in overall gutter water depth. The results also show that there is often built-in redundancy in multiple outlet siphonic systems experiencing lower-intensity rainfall events, and if one or two outlets were blocked, the system would still operate satisfactorily. A new outlet suppressor was trialled and this was shown to reduce gutter water levels by up to 58% at some positions in the gutter. Il est présenté une enquête sur la manière dont les pressions négatives dans le système concerné, le degré d'aération et les niveaux d'eau dans les chéneaux sont affectés par le nombre d'orifices de sortie dans un système d'écoulement des eaux de toit par siphonnage. Les résultats expérimentaux montrent que les pressions dans le système diminuent si des orifices de sortie sont obturés. Il est également montré que la profondeur de l'eau au-dessus d'un orifice de sortie est fortement influencée par la pression négative s'exerçant à l'orifice de sortie. Plus l'effet de succion à l'orifice de sortie augmente, en raison des pressions plus faibles dans le système, et plus les quantités d'air et d'eau entraînées dans les orifices de sortie seront importantes, et cela aura pour résultat une augmentation correspondante de la profondeur générale de l'eau dans les chéneaux. Les résultats montrent également qu'il existe souvent une redondance intégrée dans les systèmes à siphons comportant des orifices de sortie multiples et soumis à des épisodes pluvieux de faible intensité, et que si un ou deux orifices de sortie sont obturés, le système fonctionne encore de manière satisfaisante. Un nouvel obturateur d'orifice de sortie a été essayé, ce qui a permis de montrer que celui-ci réduit dans une proportion allant jusqu'à 58 % en certains points les niveaux d'eau dans les chéneaux. Mots clés: aération, niveaux d'eau dans les chéneaux, pressions négatives, écoulement des eaux de toit par siphonnage

Capacity loss in siphonic roof drainage systems due to aeration

An experimental investigation into air entrainment in siphonic roof drainage systems is presented. By injecting known quantities of air into a 4.7 m high experimental test rig, the effect of aeration on flow velocity and flow capacity in the siphonic system was determined. A preliminary relationship between air/water content and a decrease in system capacity was formulated. This indicated that system capacity can decrease by up to 40% for air/water ratios approaching 45%. At typical operational air/water ratios of approximately 10%, system capacity decreased by 16% from the assumed no-air-design situation. In addition, average velocities in the pipework decreased by 13% for air/water ratios of 35%. Friction and form losses were also measured and compared with theoretical values. Une étude expérimentale sur l'entraînement d'air dans les systèmes d'évacuation des eaux de toiture par effet siphoïde est présentée. En injectant des quantités connues d'air dans un banc d'essai expérimental de 4,70 m de hauteur, l'effet de l'aération sur la vitesse d'écoulement et le débit dans le système à siphon a été déterminé. Il a été formulé une corrélation préliminaire entre le rapport air/eau et une diminution du débit du système. Celle-ci indiquait que le débit du système peut enregistrer jusqu'à 40% de diminution pour des rapports air/eau approchant les 45%. A des rapports air/eau opérationnels types d'environ 10%, le débit du système a diminué de 16% par rapport à la situation supposée d'utilisation d'un système fonctionnant sans air. En outre, les vitesses moyennes dans la tuyauterie ont diminué de 13% pour des rapports air/eau de 35%. Les pertes de charge et de forme ont également été mesurées et comparées aux valeurs théoriques. Mots clés: aération systèmes d'évacuation des eaux pertes de forme frottement dans la tuyauterie toitures système d'évacuation des eaux de toiture par effet siphoïde écoulement diphasique

Cavitation, aeration and negative pressures in siphonic roof drainage systems

The main purpose of this study was to examine negative pressures in siphonic downpipes and to see whether these can be controlled through breaking the siphonic action by injecting air at certain points in a vertical downpipe. The study also investigated cavitation and its propagation through a prototype scale siphonic system. The paper presents methods for limiting the minimum pressure in siphonic downpipes in tall buildings. A distinctive drop in pressure between approximately —70 kN/m2 and —96 kN/m2 occurred in a 80 mm diameter downpipe and it was not possible to sustain pressures between these limits. The pressure drop to -96 kN/m2 caused the formation of a distinctive cavitation zone. Any increase in downpipe length, beyond the cavitation zone, produced a proportional increase in the cavitation zone length. Practical application: Cavitation is perceived to be a serious problem for designers of siphonic drainage systems to be installed in tall buildings. Using full-scale testing this paper provides evidence that in practice cavitation in plastic downpipes is unlikely to cause damage. The paper also provides guidelines for maximum spacing of air inlets to limit the minimum pressure to ∼70 kN/m 2 below atmospheric pressure without significantly decreasing the system capacity.

Siphonic roof drainage systems—priming focused design

This paper represents the most thorough examination of siphonic system priming to date. Data is presented which has been obtained from a number of systems. The text details the priming of siphonic roof drainage systems, and outlines how knowledge of the priming process, if considered at the design stage, may be used to improve the operational system performance. The role of different system components in improving “ability to prime”, estimating priming time and reducing ongoing maintenance costs is considered and used as the basis of the “ Priming Focused Design” philosophy proposed in this paper.

Recent and future advances in roof drainage design and performance

The past 10 years have witnessed significant changes in the way roof drainage systems are understood and designed. In particular, there has been a stepchange in the confidence with which siphonic roof drainage systems may be specified and expected to perform. These changes have occurred whilst urban drainage design in general has been revolutionized by wider acceptance of Sustainable Urban Drainage Systems and greater public concern regarding pluvial flooding within the context of climate change. This text considers, in detail, both how roof drainage systems are designed and how they should be expected to perform. Particular attention is drawn to weaknesses in accepted design methods. Consideration is also given to ‘innovative’ roof drainage related approaches such as green roofs and rainwater harvesting. Practical application: Over the past few years there have been many changes in how roof drainage systems are specified and designed. On large buildings, technologies such as ‘siphonic roof drainage’ are now commonplace and there is an ever increasing demand for ‘green roofs’ to be specified due to their potential to ‘green’ developments. Based on ongoing research, this paper details how these different types of roof drainage solutions can be efficiently designed and what levels of performance can be expected.

Numerical simulation of the dynamic operation of multi-outlet siphonic roof drainage systems

Although siphonic roof drainage systems have been installed in Europe since the 1970s, the design of such infrastructure is still based on steady state theory. Such approaches are only truly applicable at the design condition, under specific rainfall conditions, and cannot be used to assess the response of siphonic systems to alternative rainfall conditions and/or operational scenarios. Previous research at Heriot-Watt University led to the development of a numerical model capable of accurately simulating the performance of single outlet systems under a range of different conditions. The work reported herein details the logical extension to this work; i.e., the development of a numerical model to simulate the performance of multi-outlet siphonic roof drainage systems. The experimental work undertaken to assist in the development of the new model boundary conditions is briefly described, and relevant results are illustrated. The development of the numerical model is then detailed, and model output is presented. Finally, conclusions are drawn regarding the developed model, and plans for future work are outlined.

Operational performance of siphonic roof drainage systems

Siphonic roof drainage systems have been in existence for approximately 30 years, and are becoming an increasingly common element of urban drainage infrastructure. In that time, only limited data have been published relating to their performance, and what does exist relates to laboratory test facilities. This text focuses on the analysis of a large data set obtained from installed siphonic roof drainage systems. Particular attention is given to the ability of the studied systems to prime and meet self-cleansing criteria during sub-design criteria events with return periods of less than one year. Conclusions are drawn regarding the performance characteristics of multi-outlet siphonic roof drainage systems, and plans for future work are outlined.

The performance characteristics of multi-outlet siphonic roof drainage systems

Siphonic roof drainage systems have been in existence for approximately 30 years, and are becoming an increasingly common element of urban drainage infrastructure. In that time, the construction sectorin most developed countries have been gradually persuaded of the benefits that these systems offer when compared to conventional roof drainage technologies. The work reported herein details anongoing UK government-funded research programme to investigate the performance characteristics of multi-outlet siphonic roof drainage systems. The experimental aims, apparatus and procedures are described, and results are illustrated. In addition, ‘real’ data obtained from three installed siphonic roof drainage systems are discussed. Conclusions are drawn regarding the performance characteristics of multi-outlet systems, and plans for future work are outlined.

The performance characteristics of multi-outlet siphonic roof drainage systems

Siphonic roof drainage systems have been in existence for approximately 30 years, and are becoming an increasingly common element of urban drainage infrastructure. In that time, the construction sectorin most developed countries have been gradually persuaded of the benefits that these systems offer when compared to conventional roof drainage technologies. The work reported herein details anongoing UK government-funded research programme to investigate the performance characteristics of multi-outlet siphonic roof drainage systems. The experimental aims, apparatus and procedures are described, and results are illustrated. In addition, ‘real’ data obtained from three installed siphonic roof drainage systems are discussed. Conclusions are drawn regarding the performance characteristics of multi-outlet systems, and plans for future work are outlined.

Siphonic roof drainage: current understanding

Over the past five years there has been an ever increasing amount of independent research directed at understanding how siphonic roof drainage systems perform hydraulically. Since 1995, there has been a substantial amount of time and money invested in researching these systems by both government and industry. This increased amount of research activity means that, increasingly, publications are entering the public domain – the main source of this work being UK-based. This text aims to collate this work, and give an overview of what the current level of understanding is. Furthermore, it will analyse what this research means for existing systems, and recommend areas for future research in this field.