Book - School of Science and Technology

Condensate return pumping unitsSteam pressure reduction 223In instances where either distance or site levels make it impossible for condensate to bereturned to the boiler plant by gravity flow, it may be delivered either piecemeal or inquantity to some convenient central point where a built-up pumping unit of the type shownin Figure 8.21 is sited. Such condensate receiver sets are made to a range of manufacturers'ratings and consist of a cylinder fitted with a float operated switch which is mounted abovea pump or pumps. The cylinder is sized to accommodate about one and a half to twice themaximum quantity of condensate returned per minute and each pump is rated to empty thecylinder in about two minutes. In order to avoid any possibility of pressure build up, thecylinder must be provided with an open vent to atmosphere, terminating in what is knownas an exhaust head from which a drain pipe must be run to a safe position.Super-lifting steam trapsWhere it is difficult to provide for a pipe run from isolated items of equipment to conveycondensate back to a condensate receiver set, use may be made of a super-lifting trapwhich is, in effect, a steam-pressure-operated pump, Figure 8.22. A small vented receiveris required to collect incoming condensate which then feeds the trap, in preparation forintermittent discharge.Steam pressure reductionWhere, for some industrial process, it may have been necessary to generate steam at apressure quite unsuitable for heating purposes, some means of reduction will be necessary.For this purpose, pressure-reducing valves are available, as in Figure 8.23. It has beenexplained earlier, on pp. 162±63, that dry-saturated steam entering a valve of this type willproduce a super-heated output. However, if there were some level of wetness in the enteringstream, as is probable at the end of a long pipe run, and some losses following pressurereduction then the condition at the reduced pressure may approach that of saturation.As in the case of steam traps, the provision of a pressure reducing valve involvesinstallation of a number of auxiliary components as shown in Figure 8.24, the completearray being known as a pressure reducing set.Condense inlet Vent/exhaust Level controlControlsPumpdischargeDuplicatepumpsTo suittemperatureFigure 8.21 Condensate pump and receiver unit