Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGFigure11.319X14-16.— Swing-check valve.valves or as check valves, depending upon theposition of the valve stem. These valves areknown as stop-check valves .11.318Figure 14-15.— Butterfly valve.and closes when the pressure on the outlet sideis greater. All check valves open and closeautomatically.A swing- check valve is illustrated in figure14-16. Figures 14-17 and 14-18 show alift-checkvalve. As may be seen, the disk of the swingcheckvalve moves through an arc, while thedisk of a lift-check valve moves up and down inresponse to changes in the pressure of the incomingfluid. A good example of a ball-check valveis the discharge valve in the bimetallic steamtrap, described and illustrated earlier in thischapter.Stop-Check ValvesAs we have seen, most valves can be classifiedas being either stop valves or checkvalves.Some valves, however, function either as stopStop-check valves are shown in cross sectionin figure 14-19. As may be seen, this type ofvalve looks very much like a lift- check valve.However, the valve stem is long enough so thatwhen it is screwed all the way down it holdsthe disk firmly against the seat, thus preventingany flow of fluid. In this position, the valve actsas a stop valve. When the stem is raised, theside can be opened by pressure on the inletside. In this position, the valve acts as a checkvalve, allowing the flow of fluid in only oneThe maximum lift of the disk is con-direction.trolled by the position of the valve stem. Therefore,the position of the valve stem limits theamount of fluid passing through the valve evenwhen the valve is operating as a check valve.Stop -check valves are widely used throughoutthe engineering plant. One of the best examplesis the so-called boiler feed check valve,which is actually a stop- check valve ratherthan a true check valve. Stop- check valves areused in many drain lines; on the discharge sideof many pumps; and as exhaust valves on auxiliarymachinery.376