Book - School of Science and Technology

208 Pumps and other auxiliary equipmentFigure 8.6 Centrifugal pumps. Submerged (canned) rotor type (Brefco)these two methods. Most manufacturers now advise that pumps of this type should in nocircumstances be mounted with the shaft in a vertical plane.Centrifugal pumps for condensateA separation of condensate pumps from those handling water circulation may seemartificial but there are several important differences in the present context. First is thematter of pressure differential in that condensate is most usually pumped from suctionat a near-to-atmospheric condition to discharge at a much higher potential. Second isthe difficulty of quantifying the temperature/pressure relationship at the pump suctionconnection where condensate may be less `stable' than is the case for water circulated ina closed system subjected overall to pressure above ebullition point. Third is thequestion of performance, since practice is to select a condensate pump having a capacityseveral times the calculated mean demand, in the realisation that performance mustmeet peaks well in excess of that level. Lastly, since condensate pumps are in most casesinstalled adjacent to plant items producing a much higher noise level, this will affectchoice.In consequence of these circumstances, the centrifugal pump most commonly used tohandle condensate is, in comparison with those used for circulation purposes, designed toproduce an output against a significant pressure differential, with good access for cleaningand without much regard for noise production: close coupled drive is the type mostusually adopted and the volute is often split at right-angles to the shaft as an aid tocleaning. Reference will be made later in this chapter to factory made condensate receiversets incorporating such pumps.System characteristicsThe operation of a centrifugal pump cannot be considered in isolation from the distributionsystem with which it is associated, and which system is made up of a pipeworkreticulation and various single items such as boilers and terminal equipment. As waterflows through such a system, surface friction and other parallel effects will cause a loss ofpressure which, for practical purposes, may be considered as varying in proportion to the