CPD PROGRAMME | FOUL WATER DRAINAGE the appropriate sizes of sanitary pipework. Larger installations, such as those used in high-rise buildings, are likely to require the more extensive data that is developed in BSEN 12056-2. The stack is typically sized using discharge units (relating to expected flowrates from appliances and fixtures) and a frequency factor that depends on the type of use for example, house, school, gym, hospital or commercial kitchen. The procedure and calculation method are explained in BS EN 12056-2. According to Jack,4 the UK has witnessed a consistent simplification of vent system design from the use of the two-pipe system in the early 1900s through to the use of the one-pipe and modified one-pipe systems in the mid-1900s, and the development of the single-stack system (commonly used since around 1970). The one-pipe system is known formally as a primary ventilated stack system. The flow down a stack is normally considered as annular a ring of soil water spiralling down the inside surface of the soil stack. As shown by Jack,4 in reality this is not likely to be a simple smooth ring of soil water but, instead, can change shape significantly asit is affected by transients when water enters the stack pipe from the various branches (as illustrated in Figure 1). As the water flows and swirls down the stack, air is entrained, being drawn down from the top of the stack vent (which is open to atmosphere) and being joined by the air entering as part of the flow from the branches. The flow of fluid from the branches will intermittently interrupt the free flow of ventilating air in the stack, so exacerbating transient pressure variations throughout the stack. If the pressure is sufficiently low, caused Figure 1: Section through a branch entry to the stack, indicating the disturbance to the downward flow and potentially transient blocking of the air path down the stack Figure 2: A simple secondary ventilated stack system in a high-rise building with separate dry ventilating stack (Source: Geberit) by the free flow of ventilating air in the stack being interrupted by branch flows or changes in the direction of the stack, the water seal can be siphoned from the traps. A positive pressure at the base of the stack is caused by the entrained airflow being impeded by a water curtain formed at the point of separation of the water downflow from the pipe inner radius, and by restrictions in the downstream system. A positive pressure at the base of the stack may compromise (and even blow out) the trap for branches close to the base, so BS EN 12056-2 limits how closely a branch waste pipe can connect to the main discharge stack, relative to the base of the stack. A significant element of the system design process is to ensure the integrity of all water seals, and so prevent foul gases entering the occupied space. Although trap seal loss by suction pressure can be avoided by strategic positioning of mechanical devices such as automatic air valves (AAVs) and waterless traps, these effectively act as closed ends when subject to positive pressures and, therefore, are not able to relieve these positive transients. As suggested by Jack,4 these devices have the potential to make the magnitude of the positive transient worse, as they can increase the local airflow velocity halted by the water curtain at the base of the stack. Transients are particular challenges for multi-storey installations where there can be significantly variable flows from branches, particularly in high-rise buildings where the pressure variations are made more extreme by the numerous and diverse branch connections. To help overcome this, the two-pipe system formally known as a secondary ventilated stack system is often employed, where one pipe is a dry ventilating stack used to supply air at intermediate points to the soil stack (as illustrated in Figure 2). Detail and further descriptions of these systems, and variants, are provided in CIBSE Guide G and BS EN 12056-2. Despite the tried and tested nature of such systems, however, they require significant amounts of pipework and space, as shown in the relatively simple example of Figure 3. Proprietary systems have been developed that allow branch flows into the main stack without impacting the main downward flow or interrupting the air path. An example of such an enhanced geometry branch fitting is shown in Figure 4. As the downward flow enters the fitting, a flow divider breaks the annular flow so that, while the flow changes direction in the curve of the fitting, a clear air path is maintained. The smoothly shaped internal profile reinstates the swirl as the water leaves the fitting, and so annular flow passes down the stack. The flow divider allows the main downward flow to continue without being adversely affected by the incoming branch flow, and the rotating movement allows the water to flow around and down the pipe wall, allowing a continuous column of air. The separation of the branch connection from the main flow of the stack maintains airflow, and avoids the negative pressures that may adversely affect water seals. A manufacturer reports that this effect increases the discharge rate of a 100mm fitting potentially by more than 70% (from 6.8Ls-1 to 12Ls-1). Such branch fittings may be applied in UK installations 50 August 2019 www.cibsejournal.com CIBSE Aug19 pp49-52 CPD v5.indd 50 19/07/2019 15:05