PRINCIPLES OF NAVAL ENGINEERING 



FRICTION LOSS C 



TOTAL HEAD 

 DIFFERENTIAL 

 PRODUCED BY PUMP 



147.106 

 Figure 15-4. — Pressure head (pump installed 

 below surface of supply liquid). 



tlie liquid to the pump— and C ( friction loss) plus 

 E ( static discharge head ) on the discharge side. 

 Atmospheric pressure cancels out here, as 

 before, except that in this case the atmospheric 

 pressure at S^is required to lift the liquid to the 

 pump. 



In both of the cases just described, we have 

 dealt with systems which were open to the atmos- 

 phere. Now let us examine the head relationships 



in a closed system such as the one shown in fig- 

 ure 15-6. 



In this system, a pump is being used to drive 

 a work piston back and forth inside a cylinder. We 

 will assume that the pump must develop a pres- 

 sure equivalent to head H^ in order to drive the 

 piston back and forth against the resistance of- 

 fered. Under this assumption, the total head dif- 

 ferential, B^, must be produced by the pump after 

 the system has begun to operate. Bis the sum of 

 the friction head (or friction losses), C, and the 

 static discharge head , D^. Since ^ is equal to H, 

 D therefore produces the pressure required to 

 do the work. 



Since the liquid returns to its original level 

 and the system is closed throughout, there will 

 be a siphon effect in the return pipe which will 

 exactly balance the static suction lift, A. 

 Therefore, atmospheric pressure plays a part 

 in the operationof this system only when the sys- 

 tem is being started up, before the entire system^ 

 has been filled with liquid. 



At this point, we may pause and consider the 

 various ways in which the term head has been 

 used, and attempt to formulate a definition.. From 

 previous discussion, we may infer that head is 

 (1) measured in feet; (2) somehow related to 

 pressure; and (3) taken as some kind of a meas- 

 ure of energy. But what is it? 



FRICTION LOSS 



NET 

 STATIC 

 HEAD 



STATIC 



DISCHARGE 



HEAD 



i'/J 



TOTAL HEAD 



DIFFERENTIAL 



PRODUCED BY 



PUMP 



i U . i i 



A 



i I i c 



ri_r 



i i 1 1- 



147.107 

 Figure 15-5.— Pressure head (pump installed 

 above surface of supply liquid). 



147.108 

 Figure 15-6.— Pressure head (closed system). 



394 



