山东建筑大学毕业设计外文文献及译文
It will also be seen, Fig. 6, that the predicted pressure at the base of pipes 1, 6 and 14, in the absence of surcharge, conform to that normally expected, namely a small positive back pressure as the entrained air is forced through the water curtain at the base of the stack and into the sewer. In the case of stack 4, pipe 19, the reversed airflow drawn into the stack demonstrates a pressure drop as it traverses the water curtain present at that stack base.
The simulation allows the air pressure profiles up stack 1 to be modelled during,and following, the surcharge illustrated in Fig. 6. Fig. 7(a) and (b) illustrate the air pressure profiles in the stack from 2.0 to 3.0 s, the increasing and decreasing phases of the transient propagation being presented sequentially. The traces illustrate the propagation of the positive transient up the stack as well as the pressure oscillations derived from the reflection of the transient at the stack termination at the AAV/PAPA junction at the upper end of pipe 11.
Fig.7.(a) Sequential air pressure profiles in stack 1 during initial phase of stack base surcharge. (b) Sequential air pressure profiles in stack 1 during final phase of stack base surcharge.
8. Sewer imposed transients
Table 2 illustrates the imposition of a series of sequential sewer transients at the base of each
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山东建筑大学毕业设计外文文献及译文
stack. Fig. 8 demonstrates a pattern that indicates the operation of both the PAPA installed on pipe 13 and the self-venting provided by stack interconnection.
Fig.8.Entraind airflows as a result of sewer imposed pressure transients.
As the positive pressure is imposed at the base of pipe 1 at 12 s, airflow is driven up stack 1 towards the PAPA connection. However, as the base of the other stacks have not a yet had positive sewer pressure levels imposed, a secondary airflow path is established downwards to the sewer connection in each of stacks 2–4, as shown by the negative airflows in Fig. 8.
As the imposed transient abates so the reversed flow reduces and the PAPA discharges air to the network, again demonstrated by the simulation, Fig. 8. This pattern repeats as each of the stacks is subjected to a sewer transient.
Fig. 9 illustrates typical air pressure profiles in stacks 1 and 2. The pressure gradient in stack 2 confirms the airflow direction up the stack towards the AAV/PAPA junction. It will be seen that pressure continues to decrease down stack 1 until it recovers, pipes 1 and 3, due to the effect of the continuing waterflow in those pipes.
The PAPA installation reacts to the sewer transients by absorbing airflow, Fig. 10. The PAPA will expand until the accumulated air inflow reaches its assumed 40 l volume. At that point the PAPA will pressurize and will assist the airflow out of the network via the stacks unaffected by the imposed positive sewer transient. Note that as the sewer transient is applied sequentially from stacks 1–4 this pattern is repeated. The volume of the high level PAPA, together with any others introduced into a more complex network, could be adapted to ensure that no system pressurization occurred.
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山东建筑大学毕业设计外文文献及译文
Fig.9.Air pressure profile in stack 1 and 2 during the sewer imposed transient in stack 2, 15s into the simulation.
Fig.10.PAPA volume and AAV throughflow during simulation.
The effect of sequential transients at each of the stacks is identifiable as the PAPA volume decreases between transients due to the entrained airflow maintained by the residual water flows in each stack.
9. Trap seal oscillation and retention
The appliance traps connected to the network monitor and respond to the local branch air pressures. The model provides a simulation of trap seal deflection, as well as final retention. Fig. 11(a,b) present the trap seal oscillations for one trap on each of the stacks 1 and 2, respectively. As the air pressure falls in the network, the water column in the trap is displaced so that the appliance side water level falls. However, the system side level is governed by the level of the branch entry connection so that water is lost to the network. This effect is illustrated in both Fig. 11(a) and (b).
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山东建筑大学毕业设计外文文献及译文
Transient conditions in the network result in trap seal oscillation, however at the end of the event the trap seal will have lost water that can only be replenished by the next appliance usage. If the transient effects are severe than the trap may become totally depleted allowing a potential cross contamination route from the network to habitable space. Fig. 11(a) and (b) illustrate the trap seal retention at the end of the imposed network transients.
Fig.11.(a) Trap seal oscillation, trap 2. (b) Trap seal oscillation, trap 7.
Fig. 11(a), representing the trap on pipe 2, illustrates the expected induced siphonage of trap seal water into the network as the stack pressure falls. The surcharge event in stack 1 interrupts this process at 2s. The trap oscillations abate following the cessation of water downflow in stack 1. The imposition of a sewer transient is apparent at 12s by the water surface level rising in the appliance side of the trap. A more severe transient could have resulted in ‘bubbling through’ at this stage if the trap system side water surface level fell to the lowest point of the U-bend.
The trap seal oscillations for traps on pipes 7, Fig. 11(b) and 15, are identical to each other until the sequential imposition of sewer transients at 14 and 16s. Note that the
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