Chapter 12- PROPULSION STEAM TURBINES 



(Curtis) stage at the high pressure end of the 

 turbine, followed by impulse staging and then 

 by reaction blading. The impulse blading effects 

 large pressure and temperature drops in the 

 beginning, with a high initial utilization of 

 thermal energy. The reaction blading is more 

 efficient at the low pressure end of the turbine. 

 Hence the combination impulse and reaction 

 turbine is a highly efficient machine that utilizes 

 the advantages of both impulse and reaction 

 blading. Combination impulse and reaction tur- 

 bines are very commonly used as propulsion 

 turbines. 



Mode of Steam Flow 



NOZZIE 



Turbines may be further classified accord- 

 ing to the manner in which steam flows through 

 the turbine. The three aspects of steam flow 

 considered here are (1) the direction of flow, 

 (2) the repetition of flow, and (3) the division of 

 flow. 



DIRECTION OF STEAM FLOW.-The di- 

 rection of steam flow through a turbine may be 

 axial, radial, or helical. In general, the direc- 

 tion of flow is determined by the relative posi- 

 tions of nozzles, diaphragms, moving blades, 

 and fixed blades. 



Most turbines are of the axial flow type— that 

 is, the steam flows in a direction approximately 

 parallel to the long axis of the turbine shaft. 

 As we have seen, the blades in an axial-flow 

 turbine project outward from the periphery of 

 the rotor. 



In a radial-flow turbine, the blades are 

 mounted on the side of the rotor near the 

 periphery. The steam enters in such a way 

 that it flows radially toward the long axis of 

 the shaft. Radial flow is not used for propul- 

 sion turbines, but is used for some auxiliary 

 turbines. 



In a helical-flow turbine, the steam enters 

 at a tangent to the periphery of the rotor and 

 impinges upon the moving blades. The blades 

 are shaped in such a way that the direction of 

 steam flow is reversed in each blade. Helical 

 flow is not used for propulsion turbines, but is 

 used for some auxiliary turbines. 



REPETITION OF STEAM FLOW. -Turbines 

 are classified as single-entry turbines or re- 

 entry turbines, depending on the number of 

 times the steam enters the blades. If the steam 



38.78X 

 Figure 12-11.— Simple impulse turbine 

 (Rateau stage). 



passes through the blades only once, the turbine 

 is called a single-entry turbine. All multistage 

 turbines are of the single-entry type. 



Re-entry turbines are those in which the 

 steam passes more than once through the blades. 

 Re-entry turbines are used to drive some pumps 

 and forced draft blowers, but are not used as 

 propulsion units, 



DIVISION OF STEAM FLOW. -Turbines are 

 classified as single-flow or double-flow, depend- 

 ing upon whether the steam flows in one direc- 

 tion or two. In a single-flow turbine, the steam 

 enters at the inlet or throttle end, flows once 

 through the blading in a more or less axial 

 direction, and emerges at the exhaust end of 

 the turbine. A double-flow turbine consists 

 essentially of two single-flow units mounted on 

 one shaft, in the same casing. The steam enters 

 at the center, between the two units, and flows 

 from the center toward each end of the shaft. 

 The main advantages of the double-flow arrange- 

 ment are (1) the blades can be shorter than 

 they would have to be in a single-flow turbine 

 of equal capacity, and (2) axial thrust is avoided 

 by having the steam flow in opposite directions. 

 This second point applies primarily to reaction 

 turbines, since impulse turbines develop rela- 

 tively little axial thrust in any case. 



The turbines shown in figures 12-11, 12-12, 

 and 12-13 are single-flow turbines. A double - 

 flow reaction turbine of the type used as the low 

 pressure turbine in some propulsion plants is 

 shown in figure 12-14. 



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