CPD PROGRAMME | WATER SUPPLY Zone 3 at a maximum owrate limited by the water suppliers, but must not allow backow into the mains. However, in larger systems, the booster pump typically draws water indirectly from an intermediate or break water store. Such stores will be referred to as cisterns in this article, and may include one-piece cisterns, sectional tanks or any other vessel open to atmospheric pressure but sufciently sealed to prevent airborne contamination. The stored water provides a physical break from the main supply, so preventing backow while also maintaining water supply to outlets under peak ow demands, possibly when the supply from the mains supply is insufcient. Depending on application, this can provide storage sufcient for, typically, up to 24 hours consumption (as detailed in CIBSE uide section 2.3.2). As described by CIBSE uide , pumping will provide exibility in the positioning of storage cisterns, as well as ensuring that there is sufcient remaining operating pressure at the outlets to deliver the required ow at times of peak simultaneous demand. Pump booster sets may be used to move water from smaller low-level break cisterns to larger high-level cold-water storage cisterns, so that water can then be supplied using a gravity ow system. The pump set can move water to the higher-level cisterns (or multiple cisterns at intermediate levels serving groups of oors) at a rate that is signicantly lower than the peak water usage, making use of the buffer provided by the distribution cisterns. This combination of pumped and gravity feeds would continue to provide water in the event of a failure of the pump. Depending on the application, such a system may also require pumping (from the high-level tank) to the nal outlets on the top oors of the building in order to meet the pressure requirements of ttings that cannot be fully met by the available static head. Booster sets may also be used to pressurise both the hot- and cold-water WATER PRESS RE Common units: 1 bar = 100kPa 10m head. Static head is because of the height difference, z (m), between the water source and the point of use. Static pressure = gz where =density of water ( 1,000kg.m-3), g = acceleration as a result of gravity (9.81m.s-2). Distribution dynamic pressure loss is because of flow losses through pipe length, valves and fittings, and is normally calculated for the index run of pipework. Required dynamic outlet pressure is typically between 0.2 and 4 bar, depending on the type of fitting. systems directly, drawing from a break cistern without any further storage, as in the example shown in Figure 1. This is the most popular arrangement6 for contemporary commercial multiZone 2 storey buildings and reduces the risk of contamination by removing the need for distributed storage cisterns. Where this is used in high-rise commercial developments, the pressure Figure 1: Pressure zoning in a delivered (see Water pressure panel) high-rise building will typically be zoned with a separate (Source: CIBSE Guide G) pump set and riser pipe for each pressure zone, as in the high-rise building Zone 1 illustrated in Figure 1. This reduces the Water risk of developing excessively high pump mains pressure within system pipework and ttings in the lower levels of the building. CIBSE uide provides guidance for Break tank the appropriate delivery pressure to the initial oor in a group ranging from 00kPa for groups of six oors up to 1,000kPa for a 12-oor group. There may be a need for pressure-reducing valves on the lower oors of the groups to moderate the pressure for some ttings. For low-rise buildings such as small apartment blocks (up to six oors) and light commercial applications, a simple variable speed pump set can be used, such as that shown in Figure 2. The pump normally has a ooded suction (that is, the inlet is below the water level in the break cistern), and delivers water to each tap and outlet to meet demand. For applications such as single dwellings, an automatic pressurised pumping system supplying water direct from the water supply to all taps and outlets can be used but must be carefully selected and operated, otherwise it might cause signicant issues in the systems. The water regulations allows up to 12 litres per minute to be drawn from the water main by such an arrangement without the requirement for water authority consultation. Fixed speed booster sets, by virtue of their on-off control, create varying pressures that lead to uctuating ows at the system outlets and pressure surges within the system, with consequent noise and potential damage. Despite the application of buffer vessels, switchgear and contactors can wear quickly as a result of the high number of starts and stops, particularly when demand is low. In tall buildings or large system layouts, uctuating water demand can lead to pumps regularly stopping and restarting, causing pressure surges throughout the system. ariable speed pumps deliver a more smoothly controlled water system and, because they employ soft start and stop, the hydraulic and electrical stresses are reduced. Electrical energy consumption is less than when using xed speed pumps, since they can be operated to meet the load at best operating efciency more closely. As with any pump selection for building services, the pump should be selected to operate at high efciencies for the range of expected pump speeds (typically 25 through to 100 ). So, for example, in the variable speed pump characteristics in Figure 3, the dark blue section is where the pump should normally operate and, ideally, as close to its most efcient operating point as marked by the orange curve. This is possible with carefully considered pump staging and control. Figure 2 shows a small combined break cistern and Figure 2: Example of an all-in-one tank and booster set 20 October 2019 www.cibsejournal.com CIBSE Oct19 pp19-22 CPD 153 Supp v2.indd 20 20/09/2019 13:50