PRINCIPLES OF NAVAL ENGINEERING 



11.318 

 Figure 14-15.— Butterfly valve. 



and closes when the pressure on the outlet side 

 is greater. All check valves open and close 

 automatically. 



A swing- check valve is illustrated in figure 

 14-16. Figures 14-17 and 14-18 show alift-check 

 valve. As may be seen, the disk of the swing- 

 check valve moves through an arc, while the 

 disk of a lift-check valve moves up and down in 

 response to changes in the pressure of the incom- 

 ing fluid. A good example of a ball-check valve 

 is the discharge valve in the bimetallic steam 

 trap, described and illustrated earlier in this 

 chapter. 



Stop-Check Valves 



As we have seen, most valves can be classi- 

 fied as being either stop valves or checkvalves. 

 Some valves, however, function either as stop 



11.319X 

 Figure 14-16.— Swing-check valve. 



valves or as check valves, depending upon the 

 position of the valve stem. These valves are 

 known as stop-check valves . 



Stop-check valves are shown in cross section 

 in figure 14-19. As may be seen, this type of 

 valve looks very much like a lift- check valve. 

 However, the valve stem is long enough so that 

 when it is screwed all the way down it holds 

 the disk firmly against the seat, thus preventing 

 any flow of fluid. In this position, the valve acts 

 as a stop valve. When the stem is raised, the 

 side can be opened by pressure on the inlet 

 side. In this position, the valve acts as a check 

 valve, allowing the flow of fluid in only one 

 direction. The maximum lift of the disk is con- 

 trolled by the position of the valve stem. There- 

 fore, the position of the valve stem limits the 

 amount of fluid passing through the valve even 

 when the valve is operating as a check valve. 



Stop -check valves are widely used through- 

 out the engineering plant. One of the best ex- 

 amples is the so-called boiler feed check valve, 

 which is actually a stop- check valve rather 

 than a true check valve. Stop- check valves are 

 used in many drain lines; on the discharge side 

 of many pumps; and as exhaust valves on aux- 

 iliary machinery. 



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