Principles of naval engineering - Historic Naval Ships Association

IPRINCIPLES OF NAVAL ENGINEERINGDRUMVALVETOATMOSPHEREACTUATINGVALVESUPERHEATERUNLOADING(ACTUATED)VALVEFigure 11-20.— Pressure pilot-operated superheater outlet safety valve assembly.98.81valve causes that valve to open fully, so thatsteam is discharged from the unloading valveto atmosphere.Many 1200-psi single-furnace boilers, andalso some 600-psi single-furnace ones, areequipped with Crosby two-valve superheateroutlet safety valve assemblies of the type shownin figure 11-21. In an assembly of this type,both of the valves are spring loaded. Thepilot valve on the steam drum and the superheatervalve at the superheater outlet areconnected by a pressure transmitting line thatruns from thedischarge side of the drum pilotvalve to the underside of the piston that isattached to the spindle of the superheater safetyvalve. The superheater valve is set to pop at apressure about 2 percent higher than the pressurewhich causes the drum pilot valve to pop.When the drum pilot valve pops, the steam pressureis transmitted immediately through thepressure line to the piston, and the superheatervalve is thus actuated. If for any reason thedrum pilot valve should fail to open, the superheatervalve would open at a slightly higherpressure.The Consolidated three-valve superheateroutlet safety valve assembly shown in figure11-22 is used on a number of 1200-psi294