Principles of naval engineering - Historic Naval Ships Association

-PRINCIPLES OF NAVAL ENGINEERINGHOLLOW ARMSLIDE PINSUCTION-•^DISCHARGEDRIVE SHAFTDISCHARGE PORTROTATING PLUNGERECCENTRICFigure 15-22.— Rotating plunger pump.47.46and the sizes of the waterways will be too largeor too small for efficient operation. Therefore, acentrifugal pump cannot operate satisfactorilyover long periods of time at excess capacity andlow discharge pressure or at reduced capacityand high discharge pressure.It should be noted that centrifugal pumps arenot self-priming. The casing must be floodedbefore a pump of this type will function. For thisreason, most centrifugal pumps are locatedbelow the level from which suction is to be taken.Priming can also be effected by using anotherpump to supply liquid to the pump suction— as, forexample, the feed booster pump supplies suctionpressure for the main feed pump. Some centrifugalpumps have special priming pumps, airejectors, or other devices for priming.Where two or more centrifugal pumps are installedto operate in parallel, it is particularlyimportant to avoid operating the pumps at verylow capacity, since it is possible that a unit havinga slightly lower discharge pressure might bepushed off the line and thus forced into a shutoffposition.Because of the danger of overheating, centrifugalpumps can operate at zero capacity foronly short periods of time. The length of timevaries. For example, a fire pump might be ableto operate for as long as 15 to 30 minutes beforelosing suction, but a main feed pump would overheatin a matter of a few seconds if operated atzero capacity.Most centrifugal pumps— and particularlyboiler feed pumps, fire pumps, and others which408