外文原文：
Sealed building drainage and vent systems
—an application of active air pressure transient control and suppression Abstract
The introduction of aled building drainage and vent systems is considered a viable proposition for complex buildings due to the u 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 prents a simulation bad on a four-stack network that illustrates flow mechanisms within the pipework following both appliance discharge generated, and wer impod, transients. This simulation identifies the role of the active air pressure control devices in maintaining system pressures at levels that do not deplete trap als.
Further simulation exercis would be necessary to provide proof of concept, and it would be advantageous to parallel the with laboratory, and possibly site, trials for validation purpos. Despite this caution the initial results are highly encouraging and are sufficient to confirm the potential to provid
e definite benefits in terms of enhanced system curity as well as incread reliability and reduced installation and material costs.
Keywords: Active control; Trap retention; Transient propagation
Nomenclature
C+-——characteristic equations
c——wave speed, m/s
D——branch or stack diameter, m
f——friction factor, UK definition via Darcy Δh=4fLu2/2Dg
g——acceleration due to gravity, m/s2
K——loss coefficient
L——pipe length, m
p——air pressure, N/m2
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t——time, s
u——mean air velocity, m/s
x——distance, m
γ——ratio specific heats
Δh——head loss, m
Δp——pressure difference, N/m2
Δt——time step, s
Δx——internodal length, m
ρ——density, kg/m3
Article Outline
Nomenclature
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1. Introduction—air pressure transient control and suppressionwin10自带录屏
2. Mathematical basis 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 aled building drainage and vent system
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5. Simulation sign conventions
6. Water discharge to the network
7. Surcharge at ba of stack 1
8. Sewer impod transients
9. Trap al oscillation and retention
10. Conclusion—viability of a aled building drainage and vent system
1.Air pressure transients generated within building drainage and vent systems as a natural conquence of system operation may be responsible for trap al depletion and cross contamination of habitable space [1]. Traditional modes of trap al protection, bad on the Victorian engineer's obssion with odour exclusion [2], [3] and [4], depend predominantly on passive solutions where reliance is placed on cross connections and vertical stacks vented to atmosphere [5] and [6]. This approach, while both proven and traditional, has inherent weakness, 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. 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 time dependent water-flow conditions within the system. AAVs reprent 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 al [9].
However, 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 wer system, possibly by some surcharge event downstream—including heavy rainfall in combined wer 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 transient control and suppression to be introduced into the design of building drainage and vent systems, for both ‘standard’ buildings and tho requiring particular attention to be paid to the curity implications of multiple roof level open stack terminations. The positive air pressure attenuator (PAPA) 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 u of AAVs the introduction of the PAPA device allows consideration of a fully aled building drainage and vent system.
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Fig. 1 illustrates both AA V and PAPA devices, note that the waterless sheath trap acts as an AA V under negative line pressure.
男人喜欢的电影Fig. 1. Active air pressure transient suppression devices to control both positive and negative surges.
Active air pressure transient suppression and control therefore allows for localized intervention to protect trap als 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 from a vent open to atmosphere is removed and the effect of the transient on all the other system traps pasd 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 within 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 propagation 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 the transients at the system boundaries, including open terminations, connections 女生的个性签名
to the wer, appliance trap als and both AAV and PAPA active 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 0.001 s and 300 mm. In addition, the simulation replicates local appliance trap al oscillations and the operation of active control devices, thereby yielding data on network airflows and identifying system failures and conquences. While the simulation has been extensively validated [10], its u to independently confirm the mechanism of SARS virus spread within the Amoy Gardens outbreak in 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 positive 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 variation 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 t
he lower wet stack the pressure recovers to above atmospheric due to the traction forces exerted on the airflow prior to falling across the water curtain at the stack ba.
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 entrained air. Extensive experimental data allowed the definition of a ‘pudo-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 pressure transients in building drainage and vent systems is defined by the St Venant equations of continuity and momentum [9],
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(2)
The quasi-linear hyperbolic partial differential equations are amenable to finite difference solution once transformed via the Method of Characteristics into finite difference relationships, Eqs. (3)–(6), t
hat 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. Note , .
For the C+ characteristic:
(3)
when
(4)
and the C- characteristic:
(5)
when
(6)
where the wave speed c is given by
