Chapter 14. -PIPING. FITTINGS, AND VALVES 



11.320 

 Figure 14- 1 7. — Cutaway view of lift- check 

 valve. 



Pressure-Control Valves 



Pressure- control valves are used to re- 

 lieve or prevent excessive pressure, to reduce 

 pressure, and to control or regulate pressure. 

 Several common types of pressure-control 

 valves are described in the following para- 

 graphs. ^ 



RELIEF VALVES .-Relief valves are de- 

 signed to open automaticially when the pressure 

 in the line or in the machinery unit becomes 

 too high. There are several different types of 

 relief valves, but most of them have a disk or 

 a ball which acts against a coil spring. The spring 

 pushes downward against the disk or ball and 

 so tends to keep the valve closed. When the 

 pressure in the line or in the unit is great 

 enough to overcome the resistance of the spring, 

 the disk or ball is forced upward and the valve 

 is thereby opened. After the pressure has been 

 relieved by the escape of fluid through the relief 



■^Boiler safety valves, discussed in chapter 11 of this 

 text, might also be considered as a special kind of 

 pressure-control valves. 



38.119 

 Figure 14-18.— Cross-sectional view of 

 lift- check valve. 



valve, the spring again exerts enough force to 

 close the valve. 



Relief valves are installed in steam, water, 

 oil, and air lines, and on various units of aux- 

 iliary machinery. One or more adjusting nuts 

 at the top of the valve provide a means by which 

 the relief valve setting may be changed when 

 necessary. However, unauthorized changes to 

 relief valve settings must never be permitted. 



SENTINEL VALVES. -Small spring-loaded 

 sentinel valves are sometimes attached to the 

 inlet chamber of a relief valve, to give warning 

 of dangerous pressures. Sentinel valves operate 

 on the same general principles as relief valves. 



REDUCING VALVES. -Reducing valves are 

 automatic valves used to reduce the supply pres- 

 sure to a specified lower discharge pressure. 

 A reducing valve can be set for any desired 

 discharge pressure, within the design limits 

 of the valve. After the valve has been set, the 

 reduced pressure will be maintained regardless 

 of changes in the supply pressure (as long as 



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