Principles of naval engineering - Historic Naval Ships Association

Chapter 15. -PUMPS AND FORCED DRAFT BLOWERSFigureSWINGING TYPEMOVING VANESCYLINDERROTOR15-23.— Moving vane pump.47.47may be required to operate at low capacity or inshutoff condition tor any length of time— are fittedwith recirculation lines from the discharge sideof the pump back to the source of suction supply.The main feed pump, for example, has a recirculatingline going back to the deaerating feed tank.An orifice allows the recirculation of the minimumamount of water required toprevent overheatingof the pump. On boiler feed pumps, therecirculating lines must be kept open wheneverthe pumps are in operation.On centrifugal pumps, there must always bea slight leakoff through the packing in the stuffingboxes, in order to keep the packing lubricatedand cooled. Stuffing boxes are used eitherto prevent the gross leakage of liquid from thepump or to prevent the entrance of air into thepump; the purpose served depends, of course,upon whether the pump is operating with a positivesuction head or is taking suction from a vacuum.If a centrifugal pump is operating with a positivesuction head, the pressure inside the pumpis sufficient to force a small amount of liquidthrough the packing when the packing gland isproperly set up on. On multistage pumps, it issometimes necessary to reduce the pressure onone or both of the stuffingboxes.Thisis accomplishedby using a bleedoff line which is tappedin to the stuffing box between the throat bushingand the packing.If a pump is taking suction at or below atmosphericpressure, a supply of sealing water mustbe furnished to the packing glands to ensure theexclusion of air. Some of this water mustbe allowed to leak off through the packing. Mostcentrifugal pumps use the pumped liquid as thelubricating, cooling, and sealing medium. However,an independent external sealing liquid isused on some pumps.There are several different designs of centrifugalpumps. The two types most commonlyused aboard ship are the volute pump and thevolute turbine pump.The volute pump is shown in figure 15-25. Inthis pump, the impeller discharges into a volute—that is, a gradually widening channel in the pumpcasing. As the liquid passes through the voluteand into the discharge nozzle, a greatpartof itskinetic energy is converted into potential energy.In the volute turbine pump (fig. 15-26) theleaving the impeller is first slowed downliquidby the stationary diffuser vanes which surroundthe impeller. The liquid is forced through graduallywidening passages in the diffuser ring (notshown) and into the volute. Since both the diffuserFigure147,11715-24.— Internal gear pump.23.18.0Figure 15-25.— Simple volute pump.409