Chapter 12- PROPULSION STEAM TURBINES 





■ »al 



'A 



BRAKE LOCK SCREW 



47.35 



Figure 12-22.— After end of main reduction gear, showing 

 turning gear and propeller locking mechanism. 



axially without bypassing any stages (there is 

 partial bypassing of the first row of blades at 

 high power), and then is exhausted to the LP 

 turbine through a crossover pipe. 



3. Type II-B (EXTERNALbypassunit).-The 

 Type II-B propulsion unit is similar to the Type 

 II-A, except that provision is made for bypass- 

 ing of steam around the first stage or first 

 several stages of the HP turbine at powers above 

 the most economical point of operation. Bypass 

 valves are located in the HP turbine steam chest, 

 with the nozzle control valves. 



4. Type II-C (INTERNAL bypass unit).-The 

 Type II-C is similar to the Type II-A, except 

 that provision is made for bypassing steam from 

 the first-stage shell aroundthenext several (one 

 or more) stages of the HP turbine at powers 

 above the most economical point of operation. 

 Bypass valves and steam connections are usually 

 integral with the HP turbine casing; however, 



some installations have the valves separate, but 

 bolted directly to the casing, with suitable con- 

 necting piping between the first- stage shell and 

 valve to the bypass belt. 



5. Type III (series-parallelunit).— The Type 

 III propulsion unit consists of three ahead ele- 

 ments, known as the HP element, intermediate 

 pressure (IP) element, and LP element. The 

 HP and IP elements are combined in a single 

 casing, and known as the HP-IP turbine. Steam 

 is admitted to the HP-IP turbine and exhausted 

 to the LP turbine through a crossover pipe. 

 For powers up to the most economical point of 

 operation, only the HP element receives inlet 

 steam, with the IP element being supplied in 

 series with steam from the HP element ex- 

 haust. At powers above this point of operation, 

 both elements receive inlet steam in a manner 

 similar to that in a double-flow turbine. During 

 ahead operation no ahead blading is bypassed. 



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