1、外文原文：Sealed building drainage and vent systemsan application of active air pressure transient control and suppressionAbstractThe 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
2、suppression in the form of air admittance valves and positive air pressure attenuators coupled with the interconnection of the networks vertical stacks. This paper presents a simulation based on a four-stack network that illustrates flow mechanisms within the pipework following both appliance discha
3、rge 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 advantag
4、eous 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 an
5、d reduced installation and material costs. Keywords: Active control; Trap retention; Transient propagation NomenclatureC+-characteristic equations cwave speed, m/s Dbranch or stack diameter, m ffriction factor, UK definition via Darcy h=4fLu2/2Dggacceleration due to gravity, m/s2 Kloss coefficient L
6、pipe length, m pair pressure, N/m2 ttime, s umean air velocity, m/s xdistance, mratio specific heats hhead loss, m ppressure difference, N/m2 ttime step, s xinternodal length, m density, kg/m3Article OutlineNomenclature 1. Introductionair pressure transient control and suppression2. Mathematical bas
7、is for the simulation of transient propagation in multi-stack building drainage networks 3. Role of diversity in system operation 4. Simulation of the operation of a multi-stack sealed building drainage and vent system 5. Simulation sign conventions 6. Water discharge to the network 7. Surcharge at
8、base of stack 1 8. Sewer imposed transients 9. Trap seal oscillation and retention 10. Conclusionviability of a sealed building drainage and vent system1.Air pressure transients generated within building drainage and vent systems as a natural consequence of system operation may be responsible for tr
9、ap seal depletion and cross contamination of habitable space 1. Traditional modes of trap seal protection, based on the Victorian engineers obsession with odour exclusion 2, 3 and 4, depend predominantly on passive solutions where reliance is placed on cross connections and vertical stacks vented to
10、 atmosphere 5 and 6. This approach, while both proven and traditional, has inherent weaknesses, including the remoteness of the vent terminations 7, leading to delays in the arrival of relieving reflections, and the multiplicity of open roof level stack terminations inherent within complex buildings
11、. The complexity of the vent system required also has significant cost and space implications 8. The development of air admittance valves (AAVs) over the past two decades provides the designer with a means of alleviating negative transients generated as random appliance discharges contribute to the
12、time dependent water-flow conditions within the system. AAVs represent an active control solution as they respond directly to the local pressure conditions, opening as pressure falls to allow a relief air inflow and hence limit the pressure excursions experienced by the appliance trap seal 9. Howeve
13、r, AAVs do not address the problems of positive air pressure transient propagation within building drainage and vent systems as a result of intermittent closure of the free airpath through the network or the arrival of positive transients generated remotely within the sewer system, possibly by some
14、surcharge event downstreamincluding heavy rainfall in combined sewer applications. The development of variable volume containment attenuators 10 that are designed to absorb airflow driven by positive air pressure transients completes the necessary device provision to allow active air pressure transi
15、ent control and suppression to be introduced into the design of building drainage and vent systems, for both standard buildings and those requiring particular attention to be paid to the security implications of multiple roof level open stack terminations. The positive air pressure attenuator (PAPA)
16、 consists of a variable volume bag that expands under the influence of a positive transient and therefore allows system airflows to attenuate gradually, therefore reducing the level of positive transients generated. Together with the use of AAVs the introduction of the PAPA device allows considerati
17、on of a fully sealed building drainage and vent system. Fig. 1 illustrates both AAV and PAPA devices, note that the waterless sheath trap acts as an AAV under negative line pressure.Fig. 1. Active air pressure transient suppression devices to control both positive and negative surges.Active air pres
18、sure transient suppression and control therefore allows for localized intervention to protect trap seals from both positive and negative pressure excursions. This has distinct advantages over the traditional passive approach. The time delay inherent in awaiting the return of a relieving reflection f
19、rom a vent open to atmosphere is removed and the effect of the transient on all the other system traps passed during its propagation is avoided. 2.Mathematical basis for the simulation of transient propagation in multi-stack building drainage networks.The propagation of air pressure transients withi
20、n building drainage and vent systems belongs to a well understood family of unsteady flow conditions defined by the St Venant equations of continuity and momentum, and solvable via a finite difference scheme utilizing the method of characteristics technique. Air pressure transient generation and pro
21、pagation within the system as a result of air entrainment by the falling annular water in the system vertical stacks and the reflection and transmission of these transients at the system boundaries, including open terminations, connections to the sewer, appliance trap seals and both AAV and PAPA act
22、ive control devices, may be simulated with proven accuracy. The simulation 11 provides local air pressure, velocity and wave speed information throughout a network at time and distance intervals as short as 0.001s and 300mm. In addition, the simulation replicates local appliance trap seal oscillatio
23、ns and the operation of active control devices, thereby yielding data on network airflows and identifying system failures and consequences. While the simulation has been extensively validated 10, its use to independently confirm the mechanism of SARS virus spread within the Amoy Gardens outbreak in
24、2003 has provided further confidence in its predictions 12. Air pressure transient propagation depends upon the rate of change of the system conditions. Increasing annular downflow generates an enhanced entrained airflow and lowers the system pressure. Retarding the entrained airflow generates posit
25、ive transients. External events may also propagate both positive and negative transients into the network. The annular water flow in the wet stack entrains an airflow due to the condition of no slip established between the annular water and air core surfaces and generates the expected pressure varia
26、tion down a vertical stack. Pressure falls from atmospheric above the stack entry due to friction and the effects of drawing air through the water curtains formed at discharging branch junctions. In the lower wet stack the pressure recovers to above atmospheric due to the traction forces exerted on
27、the airflow prior to falling across the water curtain at the stack base. The application of the method of characteristics to the modelling of unsteady flows was first recognized in the 1960s 13. The relationships defined by Jack 14 allows the simulation to model the traction force exerted on the ent
28、rained air. Extensive experimental data allowed the definition of a pseudo-friction factor applicable in the wet stack and operable across the water annular flow/entrained air core interface to allow combined discharge flows and their effect on air entrainment to be modelled. The propagation of air
29、pressure transients in building drainage and vent systems is defined by the St Venant equations of continuity and momentum 9,(1)(2)These quasi-linear hyperbolic partial differential equations are amenable to finite difference solution once transformed via the Method of Characteristics into finite di
30、fference relationships, Eqs. (3)(6), that link conditions at a node one time step in the future to current conditions at adjacent upstream and downstream nodes, Fig. 2.Fig.2. St Venant equations of continuity and momentum allow airflow velocity and wave speed to be predicted on an x-t grid as shown.
31、 Note , . For the C+ characteristic:(3)when(4)and the C- characteristic:(5)when(6)where the wave speed c is given byc=(p/)0.5.(7)These equations involve the air mean flow velocity, u, and the local wave speed, c, due to the interdependence of air pressure and density. Local pressure is calculated as
32、(8)Suitable equations link local pressure to airflow or to the interface oscillation of trap seals.The case of the appliance trap seal is of particular importance. The trap seal water column oscillates under the action of the applied pressure differential between the transients in the network and th
33、e room air pressure. The equation of motion for the U-bend trap seal water column may be written at any time as(9)It should be recognized that while the water column may rise on the appliance side, conversely on the system side it can never exceed a datum level drawn at the branch connection.In prac
34、tical terms trap seals are set at 75 or 50mm in the UK and other international standards dependent upon appliance type. Trap seal retention is therefore defined as a depth less than the initial value. Many standards, recognizing the transient nature of trap seal depletion and the opportunity that ex
35、ists for re-charge on appliance discharge allow 25% depletion. The boundary equation may also be determined by local conditions: the AAV opening and subsequent loss coefficient depends on the local line pressure prediction. Empirical data identifies the AAV opening pressure, its loss coefficient dur
36、ing opening and at the fully open condition. Appliance trap seal oscillation is treated as a boundary condition dependent on local pressure. Deflection of the trap seal to allow an airpath to,or from, the appliance or displacement leading to oscillation alone may both be modelled. Reductions in trap
37、 seal water mass during the transient interaction must also be included. 3. Role of diversity in system operationIn complex building drainage networks the operation of the system appliances to discharge water to the network, and hence provide the conditions necessary for air entrainment and pressure
38、 transient propagation, is entirely random. No two systems will be identical in terms of their usage at any time. This diversity of operation implies that inter-stack venting paths will be established if the individual stacks within a complex building network are themselves interconnected. It is pro
39、posed that this diversity is utilized to provide venting and to allow serious consideration to be given to sealed drainage systems. In order to fully implement a sealed building drainage and vent system it would be necessary for the negative transients to be alleviated by drawing air into the networ
40、k from a secure space and not from the external atmosphere. This may be achieved by the use of air admittance valves or at a predetermined location within the building, for example an accessible loft space. Similarly, it would be necessary to attenuate positive air pressure transients by means of PA
41、PA devices. Initially it might be considered that this would be problematic as positive pressure could build within the PAPA installations and therefore negate their ability to absorb transient airflows. This may again be avoided by linking the vertical stacks in a complex building and utilizing the
42、 diversity of use inherent in building drainage systems as this will ensure that PAPA pressures are themselves alleviated by allowing trapped air to vent through the interconnected stacks to the sewer network. Diversity also protects the proposed sealed system from sewer driven overpressure and posi
43、tive transients. A complex building will be interconnected to the main sewer network via a number of connecting smaller bore drains. Adverse pressure conditions will be distributed and the network interconnection will continue to provide venting routes. These concepts will be demonstrated by a multi
44、-stack network.4. Simulation of the operation of a multi-stack sealed building drainage and vent systemFig. 3 illustrates a four-stack network. The four stacks are linked at high level by a manifold leading to a PAPA and AAV installation. Water downflows in any stack generate negative transients tha
45、t deflate the PAPA and open the AAV to provide an airflow into the network and out to the sewer system. Positive pressure generated by either stack surcharge or sewer transients are attenuated by the PAPA and by the diversity of use that allows one stack-to-sewer route to act as a relief route for t
46、he other stacks.The network illustrated has an overall height of 12m. Pressure transients generated within the network will propagate at the acoustic velocity in air . This implies pipe periods, from stack base to PAPA of approximately 0.08s and from stack base to stack base of approximately 0.15s.
47、In order to simplify the output from the simulation no local trap seal protection is includedfor example the traps could be fitted with either or both an AAV and PAPA as examples of active control. Traditional networks would of course include passive venting where separate vent stacks would be provi
48、ded to atmosphere, however a sealed building would dispense with this venting arrangement.Fig.3.Four stack building drainage and vent system to demonstrate the viability of a sealed building system.Ideally the four sewer connections shown should be to separate collection drains so that diversity in
49、the sewer network also acts to aid system self venting. In a complex building this requirement would not be arduous and would in all probability be the norm. It is envisaged that the stack connections to the sewer network would be distributed and would be to a below ground drainage network that increa
陕公网安备 61072602000132号 违法和不良信息举报：0916-4228922