Chapter 15. -PUMPS AND FORCED DRAFT BLOWERS 



SWINGING TYPE 

 MOVING VANES 



CYLINDER 



ROTOR 



47.47 



Figure 15-23.— Moving vane pump. 



may be required to operate at low capacity or in 

 shutoff condition tor any length of time— are fitted 

 with recirculation lines from the discharge side 

 of the pump back to the source of suction supply. 

 The main feed pump, for example, has a recircu- 

 lating line going back to the deaerating feed tank. 

 An orifice allows the recirculation of the min- 

 imum amount of water required toprevent over- 

 heating of the pump. On boiler feed pumps, the 

 recirculating lines must be kept open whenever 

 the pumps are in operation. 



On centrifugal pumps, there must always be 

 a slight leakoff through the packing in the stuf- 

 fing boxes, in order to keep the packing lubri- 

 cated and cooled. Stuffing boxes are used either 

 to prevent the gross leakage of liquid from the 



pump or to prevent the entrance of air into the 

 pump; the purpose served depends, of course, 

 upon whether the pump is operating with a posi- 

 tive suction head or is taking suction from a va- 

 cuum. 



If a centrifugal pump is operating with a pos- 

 itive suction head, the pressure inside the pump 

 is sufficient to force a small amount of liquid 

 through the packing when the packing gland is 

 properly set up on. On multistage pumps, it is 

 sometimes necessary to reduce the pressure on 

 one or both of the stuffingboxes.Thisis accom- 

 plished by using a bleedoff line which is tapped 

 in to the stuffing box between the throat bushing 

 and the packing. 



If a pump is taking suction at or below atmos- 

 pheric pressure, a supply of sealing water must 

 be furnished to the packing glands to ensure the 

 exclusion of air. Some of this water must 

 be allowed to leak off through the packing. Most 

 centrifugal pumps use the pumped liquid as the 

 lubricating, cooling, and sealing medium. How- 

 ever, an independent external sealing liquid is 

 used on some pumps. 



There are several different designs of cen- 

 trifugal pumps. The two types most commonly 

 used aboard ship are the volute pump and the 

 volute turbine pump. 



The volute pump is shown in figure 15-25. In 

 this pump, the impeller discharges into a volute— 

 that is, a gradually widening channel in the pump 

 casing. As the liquid passes through the volute 

 and into the discharge nozzle, a greatpartof its 

 kinetic energy is converted into potential energy. 



In the volute turbine pump (fig. 15-26) the 

 liquid leaving the impeller is first slowed down 

 by the stationary diffuser vanes which surround 

 the impeller. The liquid is forced through grad- 

 ually widening passages in the diffuser ring (not 

 shown) and into the volute. Since both the diffuser 



147,117 

 Figure 15-24.— Internal gear pump. 



23.18.0 



Figure 15-25.— Simple volute pump. 



409 



