Chapter 11-BOILER FITTINGS AND CONTROLS 



OUTLET TO 

 STEAM LINE 



a^:ii 



INLET 

 FROM BOILER 



DRAIN CONNECTIONS 



Figure 11-17.— Cross-sectional view of main steam boiler stop valve. 



38.53X 



upon this larger area overcomes the resistance 

 of the spring, and the valve "pops"— that is, it 

 opens quickly and fully. Because of the larger 

 area now presented, the valve cannot reseat 

 until the pressure has become considerably 

 smaller than the pressure which caused the 

 safety valve to open. 



A steam drum safety valve of the huddling 

 chamber type is shown in figure 11-18. As the 

 static pressure of the steam in the steam drum 

 causes the valve to open, the huddling chamber 

 (which is formed by the position of the adjust- 

 ing ring) fills with steam. The steam in the 

 huddling chamber builds up a static pressure 

 that acts upon the extra area provided by the 

 projecting lip of the feather. The resulting in- 

 crease in force overcomes the resistance of the 

 spring, and the valve pops. After the specified 

 blowdown has occurred, the valve closes clean- 

 ly, with a slight snap. The amount of tension 

 on the spring determines the pressure at which 



the valve will pop. The position of the adjust- 

 ing ring determines the shape of the huddling 

 chamber and thereby determines the amount of 

 blowdown that must occur before the valve will 

 reseat. 



A steam drum safety valve of the nozzle 

 reaction type is shown in figure 11-19. The 

 initial lift of the valve occurs when the static 

 pressure of the steam in the drum acts upon 

 the disk insert with force sufficient to overcome 

 the tension of the spring. As the disk insert 

 lifts, the escaping steam strikes the nozzle 

 ring and changes direction. The resulting force 

 of reaction causes the disk to lift higher, 

 up to above 60 percent of rated capacity. 

 Full capacity is reached as the result of a 

 secondary, progressively increasing lift which 

 occurs as an upper adjusting ring is exposed. 

 The ring deflects the steam downward, and the 

 resulting force of reaction causes the disk to 

 lift still higher. Blowdown adjustment in this 



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