【正文】 ow an airpath to,or from, the appliance or displacement leading to oscillation alone may both be modelled. Reductions in trap seal water mass during the transient interaction must also be included. 3. Role of diversity in system operationIn plex 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 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 interstack venting paths will be established if the individual stacks within a plex building network are themselves interconnected. It is proposed 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 network 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 PAPA 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 plex building and utilizing the 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 positive transients. A plex 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 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