E10) Active air pressure suppression of drainage systems - from research to the marketplace

Transcription

1 E) Active air pressure suppression of drainage systems - from research to the marketplace S. White steve@studor.net Studor Limited, Studor House, 13 Sheridan Terrace, Hove, East Sussex, BN3 5AE, United Kingdom Abstract An insight into the post-development, and operational issues related to, of active air pressure suppression is the utilisation of Air Admittance Valves (AAVs) together with the Positive Air Pressure Attenuator (P.A.P.A.) to provide full protection to a building s drainage system. They provide protection to the water trap seals within the drainage system by dealing with the negative and positive transient pressures at source so they no longer become harmful to the trap seals. Negative and positive transient pressures are routinely generated within building drainage systems and their consequent harmful effects are well documented. Continuous research in this area has resulted in the P.A.P.A. and how it works alongside AAVs to provide active air pressure suppression. This paper focuses on the device s acceptance into the marketplace and what the accepted solutions were throughout the world before the development of active air pressure suppression. This paper also considers the inherent dangers associated with an ad-hoc approach to the design of high-rise buildings in the absence of a workable standard. Keywords Standards, Vent System, Air Admittance Valves, Positive Air Pressure Attenuator, Active Air Pressure Suppression 1 Introduction This paper focuses on the acceptance in the market place of active air pressure transient control and suppression; how it can provide superior protection to trap seals within high-rise buildings and the role that continuous research provides to the industry with design solutions that provide designers and contractors with up-to-date research, enabling engineers to design safe and effective solutions for their drainage systems. The current design practices utilised for the design of high-rise building drainage and vent 393

2 systems tend to fall outside many regional standards or codes and rely on the engineers to adapt the standard or code for their designs. The general understanding of the requirement for transient suppression in the industry tends to be limited, with the codes and standards not providing sufficient information for transient relief in the system. This leads to a number of designs being adapted with an ad-hoc approach and, in some cases, to a less efficient transient relief; thereby resulting in less protection to the trap seals within the system that is the only barrier between the drainage system and the living space. There is also concern that some of these designs are becoming standard practice and are then adopted as the basic standard or code for the region and, in some cases, becoming enforced by inspectors within the region. The market is generally traditional and change is sometimes hard to accept even though the research provides strong evidence that current practice is unsafe. This is mainly due to the designers poor education and/or misunderstanding of the requirements of their system designs enforced by the standards or codes that they follow. 2 Active air pressure transient control The research of this concept is an element of several papers submitted over the last few years, with the concept being put together in "The active control and suppression of air pressure transients within the building drainage and vent system" paper submitted by Prof. J.A. Swaffield, Dr. L.B. Jack and Dr. D.P. Campbell. This paper gave the industry a basis to design a high-rise drainage system with active control. Active control uses a single stack design by utilising Air Admittance Valves (AAVs) to deal with the negative pressures and the Positive Air Pressure Attenuator (P.A.P.A.) to attenuate any positive transients generated within the system. Further research carried out by the drainage research group of Heriot-Watt University using AIRNET for a -storey building produced some surprising results, which are illustrated in Figures 1 and 2, especially considering that the conventional system analysed is very typical of high-rise drainage designs. Trap response to pressure fluctuations Trap response to pressure fluctuations Stack A Stack B Stack C Figure 1 - -storey building with 1mm wet stacks and 0mm vent pipe network appliance side system side - Trap blown appliance side system side appliance side system side Water Retained = 46 mm 2 = 44.5 mm = 0 mm Figure 2 - Results of the trap seals when subjected to a discharge of 12.4l/s over a 35 second period has siphoned at 8 seconds, at which point the system has approximately 4.5l/s in the system. This simulation has demonstrated that although the drainage system design is fully vented with a 0mm relief vent pipe and 0mm cross vents with a 1mm 394

3 wet stack the trap seal at the lowest point of the building is subjected to negative transients that have depleted the trap. It can be assumed that due to the height of the building in the simulation the height of the building has a major impact in the communication times for the system to respond to the pressure needs of the system. Further research has to be carried out to fully understand why the trap at the lower part of the building has siphoned even though the design is typical of many high-rise building designs. Figures 3 and 4 illustrate the results when the same -storey building is vented using AAVs. It can be seen in Figure 4 that when AAVs are installed at the point of need (throughout the system) pressure relief is provided throughout the system. The reason that the system in the simulation now provides protection to the trap seals is due to the fact that AAVs installed on each floor respond typically at around -80Pa to the pressure in the system to relieve the negative pressure and keep the system within -1Pa. This is well below the point that traps will siphon from -400Pa to 0Pa and return the system back to atmospheric pressure. Trap response to pressure fluctuations Stack A Stack B Stack C Vertical Stack diameter 1 mm Active Zone A Active Zone B Figure 3 - -story building with 1mm wet vent stack and AAVs placed at the point of need in the system Trap response to pressure fluctuations appliance side system side appliance side system side Water Retained 1 = 46.5 mm 2 = 42.6 mm 3 = 48.5 mm Figure 4 - Results of the trap seals when subjected to a discharge of 12.4l/s over a 35 second period Figures 5 and 6 below illustrate the results when the same -story building is simulated by AIRNET for positive transients with half the hydraulic loading of 6.5l/s. It can be seen by the simulation results at trap C has depleted due to positive transients. This indicates that the 0mm relief vent which is in the design and commonly used is insufficient in diameter to divert the positive transient that is moving at 0m/s away from the trap seals in the system. Further research is required to determine why a commonly sized venting system in high-rise buildings and its code does not provide the protection for which it is designed. When the -storey building is designed as an active controlled system it can be seen that protection is provided throughout the system, as illustrated in Figures 7 and 8 below. By using AAVs and P.A.P.A. placed throughout the system the simulation results provided by AIRNET show the provision of the trap seals throughout the system with protection from negative and positive pressures. It is the concept of using AAVs and P.A.P.A. together that keeps the system pressure below -1Pa and thus the trap seals within the system are not subjected to the harmful pressures of over Pa. 395

4 Trap response to pressure fluctuations Trap response to pressure fluctuations Stack A Stack B Stack C Figure 5 - -storey building with 1mm we stacks and 0mm vent pipe network appliance side system side appliance side system side appliance side system side Water Retained = 46 mm 2 = 44.5 mm = 0 mm Figure 6 - Results of the trap seals when subjected to a discharge of 6.5l/s over a 35 sec. period and a surcharge placed downstream after a sec. period with the load at that point of 3.5l/s Case study 2 Distributed venting using AAVs Stack A 1 mm Vertical stack Surcharge simulated here Active Zone A P.A.P.A. TM Stack B Stack C Active Zone B 1 mm collection drain Active Zone C Figure 7 - -storey buildings with 1mm stack with a loading of 6.5l/s utilising AAVs and P.A.P.A Trap response to pressure fluctuations appliance side system side appliance side system side appliance side system side Water Retained = 46 mm 2 = 42 mm = 43 mm Figure 8 - Results of the trap seals when subjected to a discharge of 6.5l/s over a 35 sec. period and a surcharge placed down stream after a sec. period The involvement of research has demonstrated two major factors; firstly that high-rise buildings designed conventionally can be affected by negative and positive transients; and secondly that working with the industry there is a safe and practical solution to designing a high-rise building drainage and vent system. 3 Current high-rise design practice and standard/code requirements The main standards that are followed through experience around the world are: EN : The design guide for Europe, also commonly used in the Middle East and Asia. AS/NZS : Used in Australia, New Zealand, also in the Middle East and Asia. BS 5572: Can still be found in being used in Asia and the Middle East, despite its withdrawal as a British Standard in 2000 when it was replaced in the building regulations by the EN

5 International Plumbing Code: A private code for the USA adopted by states. Unified Plumbing Code: A private code for USA adopted by 12 states in the USA and also recently adopted by the Indian Plumbing Association as the IUPC. The UPC is also followed in Vietnam and the Philippines. There are a number of other plumbing standards but from experience in the market place these listed seem to be the main standards that are adopted. 3.1 EN drainage venting requirements It can be seen by the recommendations of the standard show in the below Tables 1 to 3 that engineers have been given the vent requirements for the size of pipes that they use in their system. Table 1 Branch loadings with required branch and vent sizing Table 2 - Secondary stack and vent requirements commonly used in highrise designs Table 3 - Limitations The EN was developed for buildings up to 20 floors, although there is no maximum height specified in the standard. Buildings in the UK and across Europe are commonly being built well above twenty floors, especially in main city areas. It can be seen in Table 3, that if a 200 DN stack is being used, the secondary vent should be sized at 0 DN; % smaller than the waste carrying stack. It can also be seen that a 1 DN pipe (which is the most commonly used pipe used in high-rise buildings) requires a secondary vent of 80 DN; 47% smaller than the waste carrying pipe. If a design of the building is above twenty floors and in some cases above floors it can be seen by the results in Figures 2 and 6 that there is a potential for the system to fail due to positive and negative transients in the system, even with the vent sizing at 70% of the discharge stacks. 397

6 3.2 AU/NZS drainage vent requirements As shown in Tables 4 and 5 below, the AU/NZS 30 (that must be followed by engineers in Australia and New Zealand) has a maximum branch vent size of 40 DN, even for a 0 DN discharge pipe typically connected to a 125 DN relief vent on 1 stack for a -storey building; with the relief vent being 83% of the stack size. Table 4 Branch vent sizing Size of fixture Trap DN Size of Trap-Vent DN Table 5 Size of relief vents and stack vents It is becoming common practice in Australia that the venting of high-rise designs becomes a one-to-one vented system, where the relief vent is the same size as the stack, but with the requirement for the branch vent to be a maximum size of 40 DN. The resistance of this small pipe diameter can lead to restriction of communication for pressure relief of the branches in high-rise buildings and thus lead to the possibility that the traps seals could be depleted. 3.3 Main USA codes The IPC is the most commonly adopted code within the USA followed by the UPC and is more of a rule book, than a code or guide, which is enforced by local inspectors who are in the mainly retired plumbers. This raises separate issues as they generally have good interpretation and understanding of the code book, but have not undergone degreelevel engineering required to design drainage systems in high-rise buildings. This leads to two issues: Firstly, the inspector becomes the dominant factor in the design of the system and if the building system is not to the code it will not be accepted (red flagged); and secondly, the design engineer becomes accustomed to designing to the code and can therefore forget the principles of engineering and understanding of the requirements of 398

7 the system. If the code is wrong then the design is wrong. The question that needs to be addressed is the code suitable for high-rise buildings. Section of the code relating to vent pipe sizing states: "Size of stack vents and vent stacks. The minimum required diameter of stack vents and vent stacks shall be determined from the developed length and the total drainage fixture units connected thereto in accordance with [Table 6], but in no case shall the diameter be less than onehalf the diameter of the drain served for less than 1 ¼ inches (32mm). The average DFU per a bathroom group in the USA is under 6 DFU. Table 6 Size guide for IPC code Table 7 illustrates the sizing of vents within the Uniform Plumbing Code. Table 7 Sizing of vents from the UPC 399

8 3.4 International code discussion Tables 2, 3 and 7 provide the main guidance available to engineers for their system designs. However, it can be seen in Figures 4 and 6 that a conservative design can fail even though the sizing and loading tables are followed. This is because the guidelines or rules in the standards are not applicable to high-rise buildings and it should be questioned whether they should be enforced or even used for the design of high-rise buildings. The research carried out at Heriot-Watt University, as well as other leading technical institutions, can and should assist code and standards originations in providing technical solutions for the design engineers to design systems that are safe and practical for the needs of high-rise buildings. Introducing a new concept that is not part of the guide tables within the standards and challenges the guide sizing and system designs that are available can only be achieved by having the latest research that meets the requirements of high-rise drainage systems. It has also been discovered that due to the lack of guidance provided by the standards and codes, engineers are adapting the information from the standards to meet their design requirements by either increasing the sizing of the pipes used or inventing their own system designs leading to a poor design because the understanding of parameters of the drainage system is not freely available to them. 3.5 United Arab Emirates design Within the UAE, where most high-rise buildings in the world are being built with the average height of over floors, the original designs were based on the BS modified one pipe system. As the building heights were becoming increasingly higher the recommendations within the BS where failing the design of the high-rise buildings due to loss of trap seals. The obvious reason was the height and the increased loading and therefore greater chance of increased transient pressure within the system of the higher buildings. To overcome the loading factor in the modified one pipe system the local authority then moved to a three pipe system to separate the WC discharge into its own soil pipe and the other discharges into a waste pipe line with a shared venting system, reducing the load within the stack. Typical sizing of the stacks for waste and soil lines are 1 DN to 200 DN with a venting system being 0 DN shared venting. By enforcing this design they have solved a problem in buildings of a certain height. However, with the limits being pushed in the new proposed design many of the buildings being over 0 floors in height, will the same principles work and will the drainage system function correctly and provide a safe system by ensuring the trap seals are maintained? The three pipe system still has the limitations for the relief of negative and positive transient pressures because the relief points are at the top of the building and therefore time taken for the communication of the transient pressure is increased. Figures, 11 and 12 are all examples of designs that are commonly used for high-rise drainage designs. The common factor between the different systems is the vent pipe which is 35% to % smaller in dimension than the waste carrying pipes and therefore 400

9 can be subjected to negative and positive pressure as seen if Figures 2 and 6; pressures that can affect the trap seals - the only barrier between the drainage system and the living space. Figure - Typical three pipe system UAE Figure 11 - Two pipe systems, fully vented UK Figure 12 - Example of a fully vented modified one pipe system, most commonly used for high-rise designs 4 Do these designs fail? The real test to prove the effectiveness of active control is to solve transient related problems in existing buildings. The example shown in Figure 13 below is of a recently finished government-owned building in Hong Kong designed to the code of Hong Kong. This building passed all pre-inspection tests, but as soon as the building was occupied by residents it started to become affected by positive pressure problems on the lower floors. In this case, the solution found was to open-vent the system at the base of the stacks, as illustrated in picture 2. There was a real health issue because the vents were near a doorway and also located close to an air conditioning system. The worrying factor is that a state which was subjected to the SARS outbreak is still allowing systems to be designed and built to a code that is not meeting the requirement of the system design. The issue is known and active control has been utilised to solve similar problems in buildings using the same design, as can be seen in Figure 14. Due to the large amount of positive transients generated in the standard building design (typically a 1 DN stack with a 0 DN riser and DN group venting with the restrictions as seen in Figure 13), active control was placed between the source of the positive transients and the trap seals that needed to be protected with great success; making a failing system work by adding active control to a passive vented system. 401

10 1 2 3 Figure 13 - Example of base of stack connection in Hong Kong on a 56-floor building suffering from positive pressure problems Figure 14 - Positive Air Pressure Attenuators (P.A.P.A.) There are a number of such cases to lesser degree in high-rise building around the world; one that is being assessed at the time of writing this paper is the building Taipei 1, which has been designed as a three pipe system (as in Figure ) with a one-to-one sized system using 200 DN pipes. The problem occurred when the occupancy of Taipei 1 reached 70%. The toilets on the ninth floor (first office floors) were subjected to bubbling and self siphonage. The solution will be similar to the fitting of active control devices such as the P.A.P.A. and AAVs between the source of the problem and the trap seals that need to be protected due to the strong possibility that the system is being subjected to positive transients greater than mm WG. 5 Active control - is it a solution? This paper has demonstrated that there is sufficient data for multiple-flush situations where active control provides superior protection to the system, as seen in the results of the simulations in Figures 4 and 8 from the -story Heriot-Watt study and that active control has been used to rectify problem systems by adding AAVs and P.A.P.A. to deal with positive and negative pressures in the system. Taking the two factors of the scientific research carried out in active control and the fact that utilising AAVs and P.A.P.A. can problem solve existing systems, it is logical to design systems as fully active ventilated drainage systems from the start to provide the system with: reduced system complexity; reduced time of installation and labour; reduced material used in the system, bringing sustainability to the design; increased predictability of system operation; 402

11 ability to place suppression between transient source and appliance trap seals to be protected; interception of transients prior to propagation throughout the network and impact on all connected appliance trap seals. The drainage system becomes a single stack system that can vent buildings from floors to over 0 floors in height and keeps the system pressures in the region of +- 1Pa; well below the Pa that affect the trap seals in the system designed in Figure 15. Figure 15 - Single stack active vented system 6 Conclusion Over the last decade there has been an unprecedented increase in high-rise buildings around the world requiring engineering disciplines to meet the requirements in structural and system operation to these types of buildings. In regards to drainage, has this been met? This paper highlighted and asked the question if the standards and codes meet these demands for high-rise building designs. The leading research carried by Heriot-Watt University has demonstrated that the current standard and code requirements may not be sufficient in their recommendations to provide effective guidance to the engineering community to design workable safe systems. There are many cases where the recommendations in the standards have been modified by the engineers as they have experienced problems in pervious designs and wish to engineer out problems in future buildings. Sometimes this has been restricted by the regulators within different states due to their insistence that the code is followed as it is written, and therefore engineers are unwilling to deviate from this even if it means that the system may become susceptible to fail. Drainage is a relatively easy system to understand for any type of building design: Hydraulic loading for the sizing of pipes; and a venting system that keeps the pressures below Pa throughout the system. If the pressures are kept well below this, in the region of Pa, there is less stress placed on the trap seals within the system. With a passive system there is a single limitation in that the only way to relieve the transient pressures is through a network of ventilation pipes that terminate at the top of the building. Is the sizing of these ventilation pipes sufficient for the demands of a highrise building? More research is required to demonstrate that the guidance in the codes 403

12 and standards currently meet the requirements for high-rise buildings. Or do they need to be modified so that a pipe network can provide sufficient relief to transient pressures generated in the system and who is qualified to provide this information? There is also a lack of education within the industry as to how the drainage system operates and more is needed to improve this by providing up-to-date research and improvement in the education of the engineers. Active control of drainage systems has been thoroughly researched and with over high-rise buildings operating to this principle without issues, this system meets the demands of high-rise drainage ventilation. At present it is the only system that is proven to do this and this could only be achieved by the cooperation of researchers and industry working together to provide a solution that meets the demands of the buildings being built. 8 References 1. BSI British Standard Institution (2004). European Standard BS EN : International Code Council (2003). US Standard International Plumbing Code-IPC 3. International Association of Plumbing and Mechanical Officials (2003). US Standard Uniformed Plumbing Code-UPC 4. Standard Australia (2003). Australian Standard AS/NZS Wang Xuejiao (2007). Study on flux capacity of single stack drainage system: Non-Roof, Penetration Self Circulating Drain System Stack Tongji University 6. Prof J.A. Swaffield, Dr L.B. Jack, Dr D.P. Campbell (2006). The Active Control and Suppression of Air Pressure Transients within the Building Drainage System. Studor commissioned report. 7. Prof J.A. Swaffield, Dr M. Gormley (2006). Comparisons of Venting Options under positive and negative pressure transients for a storey building using numerical model AIRNET. Studor commissioned report. 8. Web page: 9 Presentation of Author Steven White is Technical Manager for Studor Limited. His responsibility is the development of new markets and codes in the Middle East, Europe and Asia as well as supporting code issues for Studor in the USA. Steven was project manager for the Studor P.A.P.A. and worked closely with Heriot-Watt University developing the product. 404