给排水工程专业外文翻译

1 Embedding sustainability in the design of water supply and drainage systems for buildings Abstract In addressing sustainability issues for the built environment focus is often directed towards minimising energy consumption and material use Often forgotten however is the potential for the integration of sustainable solutions when designing water and waste management systems for buildings The fundamental functions of such systems are clearly recognised but traditional design principles often constrain opportunities for performance enhancement and for water and pipework economies To an extent this is unsurprising given the basic premise that steady state analysis of flows underpins many of the codes and guidelines used worldwide However advances in simulation methods mean that system performance resulting from the use of new techniques and from the integration of innovative and more sustainable design approaches can now be fully assessed This paper provides an overview of the watersupply and drainage systems for buildings whose performance has been assessed through the development at Heriot Watt University of a suite of numerical simulation models These models accurately predict using appropriate forms of the St Venant equations the pressure and flow regime within such systems by applying the Method of Characteristics finite difference technique The paper provides three different examples of application where the focus of each is on embedding sustainability in design 1 Introduction In providing watersupply and waste management systems for buildings it is essential that performance is assured Key functions encompass the provision of potable water and that required for basic hygiene the removal of water that has been contaminated with waste products and the provision of a physical barrier between the potentially harmful miasma present in drain pipes and sewers and the habitable space It is also important that the building uses to best benefit any impinging rainwater as well as any resultant wastewater thus reducing unnecessary wastage and limiting the loading on sewer and drainage networks and or collection systems Sustainability should underpin design theory in each of these aspects through limiting watersupply and consumption and through reducing material use cost and environmental impact Watersupply and drainage systems for buildings therefore provide a number of 2 opportunities for the integration of sustainable solutions however these must be achieved without compromising performance and thus the response of systems during use must be fully understood Often the approach adopted for the design of water and wastewater systems is based upon the application of steady state principles in order to determine for example flow loading or pressure response Although such methods facilitate system specification in a somewhat deterministic fashion they seldom provide the opportunity to assess the time dependent response of systems information that can readily inform key design decisions The following text will therefore illustrate how an understanding of the dynamic response of systems coupled with the development at Heriot Watt of a suite of numerical simulation models has facilitated the effective and efficient design and analysis of watersupply and drainage for buildings thereby enabling a comprehensive assessment of the potential for integration of innovative and sustainable design solutions It is worth noting at this point that throughout this paper the term watersupply will be presented within the context of water use within the building that indirectly dictates supply from large scale networks Each component model contributing to the suite developed at Heriot Watt utilises the Method of Characteristics technique This technique was first used by Massau in 1900 to analyse open channel flow and then by Lamoen in 1947 to analyse water hammer and transforms the appropriate forms of the St Venant equations of continuity and momentum into a pair of total differential equations solvable by finite difference methods These equations are termed the C and C characteristics and define the conditions at a node one time step in the future in terms of current conditions at adjacent upstream and downstream nodes The finite difference grid is defined using the independent variables distance x and time t linked with dependent variables either u and c fluid velocity and propagation wave speed for air or u and h fluid velocity and depth for free surface water It will be appreciated that at system boundaries an additional equation is required to complete the finite difference solution Equations are therefore defined at these locations and provide information on the static or dynamic behaviour as appropriate of the boundary The theoretical and empirical definition of these boundary condition equations has formed the focus of both past and present research at Heriot Watt and has facilitated the development of the three component models referred to in this text DRAINET AIRNET and ROOFNET All three are based on the Method of Characteristics technique described and each has been successfully used to enhance the design approach for relevant systems DRAINET deals with the transient analysis of partially filled i e free surface pipe flow predominantly addressing the 3 performance of internal buildingdrainage systems Its application has recently been extended to encompass local external drainage systems where the flow regime may still be characterised by wave attenuation AIRNET examines the transient response of drainage ventilation systems by predicting the pressure and airflow that influences the integrity of the water based appliance trap seal whereas ROOFNET assesses the performance of both conventional and siphonic rainwater drainage systems for buildings It will be appreciated that to an extent ROOFNET and DRAINET can be operated jointly to facilitate the prediction of rainwater conveyance from roof surfaces through to local drainage systems This paper will illustrate through the use of examples how these model components may be applied to integrate and embed sustainability in the design of watersupply and drainage systems for buildings 2 Potable water use and the impact of reducing w c flush volumes The definition of sustainability may and often does differ depending upon the context within which it is set For many developed countries sustainability focuses on reducing or optimising the use of for example energy or materials whereas in other regions sustainability is more about the stable provision of basic needs Within the latter context and set against the UN s Millennium Development Goals one key aim conveyed in the UN s Task Force on Water and Sanitation is to halve by 2015 the proportion of people without sustainable access to safe drinking water and basic sanitation 1 It therefore seems counterintuitive that in many countries a significant proportion of the potablewatersupply to buildings is used for w c flushing The direct cost savings associated with any reduction in w c flush volume that arise from treatment processes alone are clearly significant and when coupled with indirect savings facilitated by a reduction in pipe size for both supply and drainage systems increase yet further Proposals to introduce any significant reduction in w c flush volume however are often met with concerns over the efficiency of the removal of waste and other products from sanitary appliances and of their conveyance through associated drainage networks In the UK a flush volume of 40 l had been recognised as excessive as early as around 1900 however following a dramatic reduction to 9 1 l 2 gallons it then took a number of decades before any further significant reductions were imposed Statutory regulations implemented by 2001 2 now stipulate for installation a maximum flush volume of 6 l and a reduced flush volume not greater than two thirds of the maximum thereby targeting the seemingly disproportionate level of around one third of domestic watersupply currently used for w c flushing 3 4 Assuming any waste products are either organic or that they comply with accepted flushability criteria the focus therefore shifts to the performance of the pipework that conveys this waste to a downstream drain or sewer The flow regime in the pipework serving the sanitary appliance is inherently unsteady and there has been a substantial body of work undertaken at Heriot Watt and elsewhere with the aim of predicting the impact of design changes and or changes in water consumption upon the drainline carry of discrete solids Being able to predict the location of solid deposition and being able to take preventative action clearly avoids the propensity for blockage The following text presents a straightforward example of how the performance of pipework when subjected to variable w c discharge volumes can be assessed using DRAINET In this case the discharge volume from the appliance shown connected to Pipe 2 Fig 1 has been varied between 9 6 4 5 and 3 l Each of the four flush volumes selected was represented using a profile of the type also shown in Fig 1 In this example the time at which the solid leaves the appliance was varied appropriately to ensure that in all cases this preceded the point at which peak flow discharge occurs This is important as it is well known that in addition to discharge profile solid parameters and pipe slope diameter roughness and base flow the solid discharge time relative to the overall flush duration i e thereby defining the trailing volume has a significant influence on drainline carry where early solid removal ensures a greater travel distance 4 In this simulation a second w c with a flush volume of 3 l is connected 5 m downstream but initially was not operated All downstream pipework was specified as 100 mm diameter set at a slope of 1 in 100 Through the use of DRAINET it is possible to simulate both the free surface attenuation of the discharge flow from the w c and the location at which any deposition of a discrete solid will occur This requires the inclusion of pre defined boundary condition equations that link flow depth flow rate and or time and that determine conditions at the locations of components of the physical system for example pipe junctions and hydraulic jumps Equations defining the flow 5 conditions at the location of any discrete solids are also required to facilitate the simulation of both single and multiple solid deposition Fig 2 shows how for a flush volume of 9 and 6 l the point of deposition of the solid lies beyond the network modelled i e beyond 9 m thereby indicating that the solid has successfully been conveyed to an appropriate downstream connection For flush volumes of 4 5 and 3 l the travel distances are 7 9 m and 5 9 m respectively thus indicating in both cases deposition in Pipe 3 This scenario is often perceived as a failure of the system to be remedied by an increase in flush volume however by adjusting the pipe diameter to 75 mm it can be shown Fig 2 that the drainline carry can be extended to beyond 9 m and 7 8 m for the 4 5 and 3 l flush respectively It will be appreciated that for a downstream connection point located 9 m from the appliance the discharge from the 3 l flush remains insufficient in terms of conveyance However through the simulation of a subsequent 3 l flush from Pipe 1 representing for example discharge from an adjacent property or room at a simulation time of 30 s flow conditions can extend the travel distance for this solid beyond the minimum required Fig 2 Comparison of solid deposition as predicted by DRAINET arising from w c discharge with variable water consumption 6 This example illustrates how a reduction in flush volume need not be accompanied by a reduction in the drainline carry performance of the network In this case conveyance was facilitated in the main by a reduction in pipe diameter A similar improvement can be implemented through an adjustment of pipe slope or by assessing combined or sequential discharge flows It will be appreciated that although the example presented herein is based upon the use of only three pipes DRAINET is clearly capable of simulating any number of pipes representative of a typical building or a small cluster of buildings and can therefore readily provide information on the best approach when implementing water conservation policies or when pursuing sustainability in the design of watersupply and drainage systems 3 Maintaining a physical separation between the habitable space and the drainage pipework It will be appreciated that any unsteady flow from a discharging appliance will naturally generate pressure changes within a pipe network This is particularly true when vertical pipes are subjected to discharge flow that forms a water annulus and where an associated airflow is entrained from system vent locations Any pressure change within the drainage pipe network will clearly have an effect upon the overall response of the system however it is predominantly the transient nature of pressure excursions that induces the potential for depletion of trap seal water Typically the water based trap seal provides the physical barrier between the habitable space and the miasma present in the pipework that serves the building and provides a conduit to the sewer system and it is therefore important that any pressure changes that might displace this water thereby compromising the integrity of the barrier are minimised Air pressure transients are most commonly generated within drainage and ventilation pipework when there is a relatively rapid change in the rate of the discharge flow from one or more appliances Fig 3 illustrates how the formation of the annulus within the vertical or stack pipe entrains though the principle of no slip an airflow that in most cases is drawn from the upper stack termination Fig 3 also shows how a change in the discharge flow rate at a given point is communicated through the system by means of a change in air pressure and how when ventilation is provided by the upper stack termination results in an imposed change in pressure at all connected traps encountered en route 7 Fig 3 Annular flow in the stack pipe where a change in discharge flow rate results in an air pressure transient for which relief is provided by the upper stack termination View thumbnail images The Heriot Watt developed numerical model AIRNET similarly uses the Method of Characteristics technique to facilitate the prediction of whole system pressure and airflow response Boundary conditions again require definition to enable system simulation and a significant component of the work undertaken at Heriot Watt has focussed on the characterisation of appropriate theoretical and empirically derived descriptive algorithms representing both the system drivers and components 5 The model also encompasses an advanced approach to the simulation of the water air interface shown in Fig 4 that through the integration of dimensionless velocity differential terms releases the model from the constraints of single discharge flow simulation to enable an analysis of multiple branch inlet flows 6 Flexibility of system specification as input data coupled with appliance discharge patterns thus allows the prediction of transient airflow and pressures and trap seal retention levels thereby providing an important step forward in the ability to assess system performance in response to 8 changes initiated in the pursuit of sustainability Fig 4 An illustration of how control devices provide effective ventilation View thumbnail images Water conservation can clearly have an impact upon the flow regime within this drainage pipework Generally the effect of a reduction in flow volume is characterised by an overall reduction in the terminal water velocity within the stack hence resulting in a corresponding reduction in air entrainment and system pressure It will be appreciated however that reduced volume appliance discharge profiles remain notably time dependent and the impact of transient pressures must therefore continue to be assessed in order to ensure trap seal integrity The ability of the simulation model to accurately predict system pressures not only introduces 9 the potential for a significant reduction in water consumption through the use of low flush w c s but also for the integration of design solutions that yield pipework economies The benefits of reduced pipework costs are clear and these are enhanced yet further when installation maintenance and space costs as well as environmental impact factors are taken into account A brief review of the historical development of drainage ventilation systems shows how across the past century systems in the UK and Europe have progressed from the overly cumbersome two pipe system comprising four vertical downpipes through the one pipe system with two vertical pipes and onto for buildings up to a height of around 30 storeys the single stack system with only one vertical downpipe In some regions of the world the use of the single stack system has been avoided due to concerns related mainly to the potential for the generation of excessive pressure excursions however the ability to understand the origin and nature of the inherently unsteady flow conditions that characterise such systems should allay such concerns Not only can it be shown using numerical simulation techniques that the single stack is viable performs well and reduces the need for pipework AIRNET also facilitates an assessment of overall system performance under pre defined conditions and or the installation of innovative and sustainable design solutions 4 Concluding comments This paper has presented a brief overview of how developments in drainage system flow modelling undertaken at Heriot Watt University have not only enhanced knowledge of system performance but have also facilitated the timely exploration of the suitability of a range