PRINCIPLES OF NAVAL ENGINEERING 



HOLLOW ARM 



SUCTION- 



DRIVE SHAFT - 



ROTATING PLUNGER 



SLIDE PIN 



•^ DISCHARGE 



DISCHARGE PORT 



ECCENTRIC 



47.46 



Figure 15-22.— Rotating plunger pump. 



and the sizes of the waterways will be too large 

 or too small for efficient operation. Therefore, a 

 centrifugal pump cannot operate satisfactorily 

 over long periods of time at excess capacity and 

 low discharge pressure or at reduced capacity 

 and high discharge pressure. 



It should be noted that centrifugal pumps are 

 not self-priming. The casing must be flooded 

 before a pump of this type will function. For this 

 reason, most centrifugal pumps are located 

 below the level from which suction is to be taken. 

 Priming can also be effected by using another 

 pump to supply liquid to the pump suction— as, for 

 example, the feed booster pump supplies suction 

 pressure for the main feed pump. Some centrif- 

 ugal pumps have special priming pumps, air 

 ejectors, or other devices for priming. 



Where two or more centrifugal pumps are in- 

 stalled to operate in parallel, it is particularly 

 important to avoid operating the pumps at very 

 low capacity, since it is possible that a unit hav- 

 ing a slightly lower discharge pressure might be 

 pushed off the line and thus forced into a shutoff 

 position. 



Because of the danger of overheating, cen- 

 trifugal pumps can operate at zero capacity for 

 only short periods of time. The length of time 

 varies. For example, a fire pump might be able 

 to operate for as long as 15 to 30 minutes before 

 losing suction, but a main feed pump would over- 

 heat in a matter of a few seconds if operated at 

 zero capacity. 



Most centrifugal pumps— and particularly 

 boiler feed pumps, fire pumps, and others which 



408 



