Principles of naval engineering - Historic Naval Ships Association

Chapter 15. -PUMPS AND FORCED DRAFT BLOWERSFigure 15-2. — Pressure in liquid is not affected by the shape of the vessel.147.105When the pump is installed below this level,as shown in figure 15-4, a certain amountof energy in the form of gravity head will alreadybe available when the liquid enters the pump. Inother words, there is a static pressure head , A,on the suction side of the pump. This head is partof the total input head necessary to produce theoutput head, F, that is required to raise theliquid to the top, T, of the discharge reservoir.liFORCE 1=20 LBS-PISTON 12 SO. IN.10 LBS. PER SO. INCHPISTON 220 SO. INFORCE 2 =200 LBSFttW?"rttf f t::\LLLLLL14.7Figure 15-3. — Relationship of force, pressure,and area in a simple hydraulic system.The action of the pump produces the total headdifferential . B, which can be broken down intofriction loss , C , and net static dischargehead , D. Since D_ is the vertical distance fromthe surface of the supply liquid to the surface ofthe liquid in the discharge reservoir, it is clearthat our previous definition of head would applyonly to ID. E, the total static discharge head, isthe vertical distance from the center of the pumpto the surface of the liquid in the discharge reservoir;thus, ^ is equal to D plus A.As may be seen in figure 15-4, atmosphericpressure is acting upon the free surface of thesupply liquid and upon the free surface of theliquid in the discharge reservoir. Since atmosphericpressure is exerted equally on both sidesof the pump, in this system, the two heads createdby atmospheric pressure cancel out.Now consider the case of a pump that is installeda vertical distance^ above the free surface^ofthe supply liquid (fig. 15-5). In this case,energy must be supplied merely to get the liquidinto the pump (static suction lift . A). In addition,energy must be supplied to produce the staticdischarge head , E, if the liquid is to be raisedto the top of the discharge reservoir, T. B, thetotal head differential produced by pump action,is here the total energy input. It is divided intoA on the suction side— the head required to raise393