Principles of naval engineering - Historic Naval Ships Association

Chapter 12- PROPULSION STEAM TURBINEStherefore, we are actually concerned with theconstruction or arrangement of the nozzleblocks in which the openings occur. In mostmodern turbines, the nozzle blocks are arrangedso that the nozzle openings occur ingroups, with each group being controlled bya separate nozzle control valve. The quantityof steam delivered to the first stage of theturbine is thus a function of the number ofnozzles in use and the pressure differentialacross the nozzles.On some auxiliary ships, hand controllednozzle valves are used in conjunction with athrottle valve to admit steam to the turbine.Any throttling of the inlet steam will reduceefficiency. To avoid throttling losses, all nozzlecontrol valves in use are opened fully beforeany additional valve is opened. Minor variationsin speed within any one nozzle control valvecombination are taken care of by the throttle.On modern combatant ships, the nozzle controlvalve arrangement shown in figure 12-16is employed. The throttle valve is omitted andsteam enters the turbine through nozzle controlvalves. Speed control is effected by varyingthe number of nozzle valves that are opened.The variation in the number of nozzle valvesis accomplished through the operation of alifting beam mechanism. The lifting beam mechanismconsists of a steel beam drilled withholes which fit over the nozzle valve stems.The valve stems are of varying lengths andare fitted with shoulders at the upper ends.When the beam is lowered, all valves restupon their seats. When the beam is raised.NOZZLES38.80XFigure 12-13.— Pressure-compoundedimpulse turbine (Rateau turbine).the valves open in succession, depending upontheir stem length— the shorter ones open first,then the longer ones.Nozzle DiaphragmsNozzle diaphragms are installed as part ofeach stage of a pressure- compounded impulseturbine. The diaphragm serves to hold thenozzles of the stage. Figure 12-17 shows atypical nozzle diaphragm. The nozzle walls aremachined, ground, and polished. The nozzles arefitted into a steel plate inner ring. An outerring fits over the outside of the nozzles. Theentire assembly is then welded together. Inorder to seal against steam leakage, labyrinthpacking (discussed later in this chapter) isused between the inner bore of the diaphragmand the rotor.Turbine RotorsThe turbine rotor carries the moving bladeswhich receive the steam. In some older turbines,the rotors were forged separately, machined,shrunk or pressed onto the shaft, and keyed tothe shaft. In most modern turbines, particularlylarge ones such as those used for ship propulsion,the rotors are forged integrally withthe shaft. Figure 12-18 shows an integrallyforged turbine rotor to which the blades havenot yet been attached.Turbine BladesThe purpose and function of turbine bladinghas already been discussed. At this point, it ismerely necessary to note that the moving bladesare fastened securely and rigidly to the turbinerotor. Figure 12-19 shows several ways offastening blades to the turbine rotor wheels.Turbine BearingsTurbine rotors are supported and kept inposition by bea rings .^ The bearings whichserve to maintain the correct radial clearancebetween the rotor and the casing are calledradial bearings. Those which serve to limitthe axial (longitudinal) movement of the rotorare called thrust bearings.Propulsion turbines have one radial bearingon each end of the rotor. These bearings are ofthe type generally known as journal bearings orsleeve bearings. The two metallic surfaces areBearings are discussed in chapter 5 of this text.329