Book - School of Science and Technology

Water treatment 593Cold waterTemperaturerelief valveHot waterExpansionvesselPressurereducingvalveNon-returnvalveWater mainDrainExpansionvalveThermostatimmersion heaterenergy cut-outPrimary connections(feed and expansionnot shown)ThermostatEnergy cut-outBoilerFigure 20.27 Unvented hot water secondary supply system. A temperature and pressure operated relief valve, factory set to open at 90±95 C. Theoutlet from this valve must be to a tun-dish and piped thence to a point where dischargeof very hot water will not be dangerous. The discharge pipework sizes, related to lengthof pipe run and number of bends, are indicated in Section G3 of the BuildingRegulations.Functional. A pressure controller at the connection to the service pipe is required to deal withfluctuations in the supply and to determine the system operatingpressure. Goodcontrol characteristics at low flow rates and the ability to accept a wide range of inletpressures are necessary. Either a pressure reducingvalve or a pressure limitingvalvemay be used, factory set.. A non-return valve in the feed to the storage vessel is necessary in order to avoid backflowof hot water into the cold water supply.. A diaphragm type expansion vessel to cater for the expected increase in volume(nominally 4%).. An expansion relief valve which should not open duringnormal system operation. Theoutlet from the valve must be visible and piped to a convenient point.Water treatmentA short discussion was included in Chapter 8 to cover the incidence of corrosion inheatingsystems which have the advantage of reusingand re-circulatingthe same watercontent. In the present context of hot water supply, the secondary water content inindirect systems and the whole content of direct systems is subject to continual change.In general terms, the problem in this case relates more usually to the formation of scalerather than to the effects of corrosion. In this respect it is of interest to note that publicwater supplies to the mainland of the British Isles, south and east of a line drawn betweenHull and Bristol (with local exceptions), all fall into either the `hard' or `very hard'categories.