Chapter 12- PROPULSION STEAM TURBINES 



therefore, we are actually concerned with the 

 construction or arrangement of the nozzle 

 blocks in which the openings occur. In most 

 modern turbines, the nozzle blocks are ar- 

 ranged so that the nozzle openings occur in 

 groups, with each group being controlled by 

 a separate nozzle control valve. The quantity 

 of steam delivered to the first stage of the 

 turbine is thus a function of the number of 

 nozzles in use and the pressure differential 

 across the nozzles. 



On some auxiliary ships, hand controlled 

 nozzle valves are used in conjunction with a 

 throttle valve to admit steam to the turbine. 

 Any throttling of the inlet steam will reduce 

 efficiency. To avoid throttling losses, all nozzle 

 control valves in use are opened fully before 

 any additional valve is opened. Minor variations 

 in speed within any one nozzle control valve 

 combination are taken care of by the throttle. 



On modern combatant ships, the nozzle con- 

 trol valve arrangement shown in figure 12-16 

 is employed. The throttle valve is omitted and 

 steam enters the turbine through nozzle con- 

 trol valves. Speed control is effected by vary- 

 ing the number of nozzle valves that are opened. 

 The variation in the number of nozzle valves 

 is accomplished through the operation of a 

 lifting beam mechanism. The lifting beam mech- 

 anism consists of a steel beam drilled with 

 holes which fit over the nozzle valve stems. 

 The valve stems are of varying lengths and 

 are fitted with shoulders at the upper ends. 

 When the beam is lowered, all valves rest 

 upon their seats. When the beam is raised. 



NOZZLES 



38.80X 

 Figure 12-13.— Pressure-compounded 

 impulse turbine (Rateau turbine). 



the valves open in succession, depending upon 

 their stem length— the shorter ones open first, 

 then the longer ones. 



Nozzle Diaphragms 



Nozzle diaphragms are installed as part of 

 each stage of a pressure- compounded impulse 

 turbine. The diaphragm serves to hold the 

 nozzles of the stage. Figure 12-17 shows a 

 typical nozzle diaphragm. The nozzle walls are 

 machined, ground, and polished. The nozzles are 

 fitted into a steel plate inner ring. An outer 

 ring fits over the outside of the nozzles. The 

 entire assembly is then welded together. In 

 order to seal against steam leakage, labyrinth 

 packing (discussed later in this chapter) is 

 used between the inner bore of the diaphragm 

 and the rotor. 



Turbine Rotors 



The turbine rotor carries the moving blades 

 which receive the steam. In some older turbines, 

 the rotors were forged separately, machined, 

 shrunk or pressed onto the shaft, and keyed to 

 the shaft. In most modern turbines, particularly 

 large ones such as those used for ship pro- 

 pulsion, the rotors are forged integrally with 

 the shaft. Figure 12-18 shows an integrally 

 forged turbine rotor to which the blades have 

 not yet been attached. 



Turbine Blades 



The purpose and function of turbine blading 

 has already been discussed. At this point, it is 

 merely necessary to note that the moving blades 

 are fastened securely and rigidly to the turbine 

 rotor. Figure 12-19 shows several ways of 

 fastening blades to the turbine rotor wheels. 



Turbine Bearings 



Turbine rotors are supported and kept in 

 position by bea rings .^ The bearings which 

 serve to maintain the correct radial clearance 

 between the rotor and the casing are called 

 radial bearings. Those which serve to limit 

 the axial (longitudinal) movement of the rotor 

 are called thrust bearings. 



Propulsion turbines have one radial bearing 

 on each end of the rotor. These bearings are of 

 the type generally known as journal bearings or 

 sleeve bearings. The two metallic surfaces are 



Bearings are discussed in chapter 5 of this text. 



329 



