Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGTOTAL HEADDIFFERENTIALPRODUCED BY PUMPFRICTION LOSS147.106Figure 15-4. — Pressure head (pump installedbelow surface of supply liquid).tlieCliquid to the pump— and C (friction loss) plusE ( static discharge head ) on the discharge side.Atmospheric pressure cancels out here, asbefore, except that in this case the atmosphericpressure at S^is required to lift the liquid to thepump.In both of the cases just described, we havedealt with systems which were open to the atmosphere.Now let us examine the head relationshipsin a closed system such as the one shown in figure15-6.In this system, a pump is being used to drivea work piston back and forth inside a cylinder. Wewill assume that the pump must develop a pressureequivalent to head H^ in order to drive thepiston back and forth against the resistance offered.Under this assumption, the total head differential,B^, must be produced by the pump afterthe system has begun to operate. Bis the sum ofthe friction head (or friction losses), C, and thestatic discharge head , D^. Since ^ is equal to H,D therefore produces the pressure required todo the work.Since the liquid returns to its original leveland the system is closed throughout, there willbe a siphon effect in the return pipe which willexactly balance the static suction lift, A.Therefore, atmospheric pressure plays a partin the operationof this system only when the systemis being started up, before the entire system^has been filled with liquid.At this point, we may pause and consider thevarious ways in which the term head has beenused, and attempt to formulate a definition.. Fromprevious discussion, we may infer that head is(1) measured in feet; (2) somehow related topressure; and (3) taken as some kind of a measureof energy. But what is it?NETSTATICHEADFRICTION LOSSSTATICDISCHARGEHEADi'/Ji I i cri_rTOTAL HEADDIFFERENTIALPRODUCED BYPUMPi U . i iAi i 11-147.107Figure 15-5.— Pressure head (pump installedabove surface of supply liquid).Figure147.10815-6.— Pressure head (closed system).394