Chapter 12- PROPULSION STEAM TURBINES 



CURVED STEAM LINE 



DEEP FLEXIBLE 

 I-BEAM 



f^^^'y^^ 



AFT 



FORWARD 



47.12X 

 Figure 12-15.— Foundation forpropulsion 

 turbine . 



flow of steam out of the turbine or to entirely 

 prevent the flow of air into the turbine. For 

 this reason, gland sealing steam^ is brought 

 into the shaft gland in the manner shown in 

 figure 12-20. In this illustration, the gland seal- 

 ing steam enters a space between the labyrinth 

 packing and the carbon packing. In more recent 

 installations, the sealing steam enters between 

 the segments of the labyrinth packing. The 

 sealing steam enters at a pressure of about 2 

 psig (17 psia). This pressure is, of course, 

 slightly greater than the atmospheric pressure 

 in the engineroom. 



When the pressure of the gland sealing steam 

 is greater than the pressure inside the turbine 

 casing, the sealing steam flows both into the 

 casing and into a line leading to the gland ex- 

 haust condenser,"^ excluding all air from the 

 turbine in the process. When a high pressure 

 turbine is operating at high speed, the pressure 



of the steam leaking through the shaft gland 

 packing may be slightly higher than the pressure 

 of the gland sealing steam. When this situation 

 prevails, it causes a reversal in the direction 

 of flow of the gland sealing steam. At such 

 times, the gland seal line is closed and the 

 excess steam is led through gland leak-off 

 connections to a later stage of the turbine, to 

 the gland exhaust condenser, or to other glands 

 to be used as gland seal steam. In the illustra- 

 tion (fig. 12-20) the excess steam leaking past 

 the labyrinth packing is being led back into the 

 eighth and twelfth stages of the high pressure 

 turbine. 



Dummy Pistons and Cylinders 



The steam passing through a multistage im- 

 pulse turbine does not impart any appreciable 

 axial thrust to the rotor, since the pressure 

 drop actually takes place in the nozzles. ^ in a 

 reaction turbine, however, considerable axial 

 thrust does result from the drop in steam pres- 

 sure, since a pressure drop occurs in the 

 moving blades as well as in the stationary 

 blades. 



In single-flow reaction turbines, this axial 

 thrust is partially counterbalanced by the use of 

 a dummy piston and cylinder arrangement such 

 as that shown in figure 12-21. Space "A" sur- 

 rounds the inlet area of the turbine rotor and 

 is connected by an equalizing pipe to space "B" 

 which surrounds the outlet area of the rotor. 

 The shoulder on the rotor, shown in figure 12- 

 21, is under full inlet steam pressure, while 

 the corresponding area on the other side of the 

 dummy piston is under exhaust pressure. This 

 difference in pressure causes a thrust toward 

 the high pressure end of the turbine which 

 partially counterbalances the thrust in the op- 

 posite direction caused by the pressure drop 

 through the turbine. 



Dummy pistons and cylinders are not re- 

 quired in double-flow reaction turbines, since 

 the axial thrust caused by the pressure differ- 

 ential across one-half of the turbine is counter- 

 balanced by the equal and opposite axial thrust 

 in the other half of the turbine. 



Gland seal and gland exhaust systems are dis- 

 cussed in chapter 9 of this text. 



The gland exhaust condenser is discussed in chapter 

 13 of this text. 



An equalizing hole drilled axially through each rotor 

 wheel also helps to minimize thrust in an impulse 

 turbine. 



331 



