Principles of naval engineering - Historic Naval Ships Association

Chapter 11-BOILER FITTINGS AND CONTROLSOUTLET TOSTEAM LINEa^:iiINLETFROM BOILERDRAIN CONNECTIONSFigure 11-17.— Cross-sectional view of main steam boiler stop valve.38.53Xupon this larger area overcomes the resistanceof the spring, and the valve "pops"— that is, itopens quickly and fully. Because of the largerarea now presented, the valve cannot reseatuntil the pressure has become considerablysmaller than the pressure which caused thesafety valve to open.A steam drum safety valve of the huddlingchamber type is shown in figure 11-18. As thestatic pressure of the steam in the steam drumcauses the valve to open, the huddling chamber(which is formed by the position of the adjustingring) fills with steam. The steam in thehuddling chamber builds up a static pressurethat acts upon the extra area provided by theprojecting lip of the feather. The resulting increasein force overcomes the resistance of thespring, and the valve pops. After the specifiedblowdown has occurred, the valve closes cleanly,with a slight snap. The amount of tensionon the spring determines the pressure at whichthe valve will pop. The position of the adjustingring determines the shape of the huddlingchamber and thereby determines the amount ofblowdown that must occur before the valve willreseat.A steam drum safety valve of the nozzlereaction type is shown in figure 11-19. Theinitial lift of the valve occurs when the staticpressure of the steam in the drum acts uponthe disk insert with force sufficient to overcomethe tension of the spring. As the disk insertlifts, the escaping steam strikes the nozzlering and changes direction. The resulting forceof reaction causes the disk to lift higher,up to above 60 percent of rated capacity.Full capacity is reached as the result of asecondary, progressively increasing lift whichoccurs as an upper adjusting ring is exposed.The ring deflects the steam downward, and theresulting force of reaction causes the disk tolift still higher. Blowdown adjustment in this291