【正文】 ns will be distributed and the network interconnection will continue to provide venting routes. These concepts will be demonstrated by a multistack network.4. Simulation of the operation of a multistack sealed building drainage and vent systemFig. 3 illustrates a fourstack 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 that 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 stacktosewer route to act as a relief route for the 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 and from stack base to stack base of approximately . In order to simplify the output from the simulation no local trap seal protection is included—for 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 provided to atmosphere, however a sealed building would dispense with this venting arrangement. 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 the sewer network also acts to aid system self venting. In a plex 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 increased in diameter downstream. Other connections to the network would in all probability be from buildings that included the more traditional open vent system design so that a further level of diversity is added to offset any downstream sewer surcharge events of long duration. Similar considerations led to the current design guidance for dwellings. It is stressed that the network illustrated is representative of plex building drainage networks. The simulation will allow a range of appliance discharge and sewer imposed transient conditions to be investigated. The following appliance discharges and imposed sewer transients are considered: 1. . discharge to stacks 1–3 over a period 1–6s and a separate . discharge to stack 4 between 2 and 7s.2. A minimum water flow in each stack continues throughout the simulation, set at , to represent trailing water following earlier multiple appliance discharges.3. A 1s duration stack base surcharge event is assumed to occur in stack 1 at .4. Sequential sewer transients imposed at the base of each stack in turn for from 12 to 18s.The simulation will demonstrate the efficacy of both the concept of active surge control and interstack venting in enabling the system to be sealed, . to have no high level roof penetrations and no vent stacks open to atmosphere outside the building envelope. The imposed water flows within the network are based on ‘real’ system values, being representative of current . discharge characteristics in terms of peak flow, 2l/s, overall volume, 6l, and duration, 6s. The sewer transients at 30mm water gauge are representative but not excessive. Table 1 defines the . discharge and sewer pressure profiles assumed. Table1. . discharge and imposed sewer pressure characteristics . discharge characteristicImposed sewer transient at stack baseTimeDischarge flowTimePressureSecondsl/sSecondsWater gauge (mm)Start timeStart time+2++4++6+5. Simulation conventionsIt should be noted that heights for the system stacks are measured positive upwards from the stack base in each case. This implies that entrained airflow towards the stack base is negative. Airflow entering the network from any AAVs installed will therefore be indicated as negative. Airflow exiting the network to the sewer connection will be negative. Airflow entering the network from the sewer connection or induced to flow up any stack will be positive. Water downflow in a vertical is however regarded as positive. Observing these conventions will allow the following simulation to be better understood. 6. Water discharge to the networkTable 1 illustrates the . discharges described above, simultaneous from 1s to stacks 1–3 and from 2s to stack 4. A base of stack surcharge is assumed in stack 1 from to 3s. As a result it will be seen from Fig. 4 that entrained air downflows are established in pipes 1, 6 and 14 as expected. However, the entrained airflow in pipe 19 is into the network from the sewer. Initially, as there is only a trickle water flow in pipe 19, the entrained airflow in pipe 19 due to the . discharges already being carried by pipes 1, 6 and 14, is reversed, . up the stack, and contributes to the entrained airflow demand in pipes 1, 6 and 14. The AAV on pipe 12 also contributes but initially this is a small proportion of the required airflow and the AAV flutters in response to local pressure conditions. airflows during appliance discharge.Following the . discharge to stack 4 that establishes a water downflow in pipe 19 from 2s onwards, the AAV on pipe 12 opens fully and an increased airflow from this source may be identified. The flutter stage is replaced by a fully open period from to s into the simulation. The air pressure in stack 4 demonstrates a pressure gradient patible with the reversed airflow mentioned above. The air pressure profile in stack 1 is typical for a stack carrying an annular water downflow and demonstrates the establishment of a positive backpressur