PRINCIPLES OF NAVAL ENGINEERING 



CONTROLLED PRESSURE' 



AIR SUPPLY 



l^TO DIAPHRAGM 



Control valve 



38.121X 



Figure 14-23,— Air-operated control pilot. 



air pressure going from the pilot to the diaphram 

 control valve. A reverse-acting pilot has a lever 

 which reverses the pilot action. In a reverse- 

 acting pilot, therefore, an increase in controlling 

 pressure produces a decrease in operating air 

 pressure. 



In the diaphragm control valve, operating air 

 from the pilot acts on the valve diaphragm. The 

 superstructure which contains the diaphragm is 

 direct acting in some valves and reverse acting 

 in others. If the superstructure is direct acting, 

 the operating air pressure from the control pilot 

 is applied to the top of the valve diaphragm. If 

 the superstructure is reverse acting, the operat- 

 ing air pressure from the pilot is applied to the 

 underside of the valve diaphragm. 



Figure 14-24 shows a very simple type of 

 direct-acting diaphragm control valve, with 

 operating air pressure from the control pilot ap- 

 plied to the topof the valve diaphragm. Since this 

 is a downward seating valve, any increase in 

 operating air pressure pushes the valve stem 

 down and tends to close the valve. 



Now let us look at figure 14-25. This is also 

 a direct-acting valve, with operating air pres- 

 sure from the control pilot applied to the top of 

 the valve diaphragm. But the valve shown in 

 figure 14-25 is more complicated than the one 

 shown in figure 14-24. The valve shown in figure 

 14-25 is an upward seating valve, rather than a 

 downward seating valve. Therefore, any increase 



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