Principles of naval engineering - Historic Naval Ships Association

Chapter 16. -AUXILIARY STEAM TURBINESpressure which is approximately 15 psig on oilfiredships and somewhat higher on nuclearships.Most auxiliary turbines are axial flow unitswhich are quitesimilar (except for size and numberof stages) to the axial-flow propulsion turbinesdescribed in chapter 12. However, someauxiliary turbines are designed for helical flowand some for radialflow— types of flow which areseldom if ever used for propulsion turbines.A helical-flow auxiliary turbine is shown infigure 16-1. In a turbine of this type, steamenters at a tangent to the periphery of the rotorand impinges upon the moving blades. Theseblades, which . consist of semicircular slotsmilled obliquely in the wheel periphery, arecalled buckets . The buckets are shaped in such away that the direction of steam flow is reversedin each bucket, and the steam is directed into aredirecting bucket or reversing chambermounted on the inner cylindrical surface of thecasing. The direction of the steam is again reversedin the reversing chamber, and the continuousreversal of the direction of flow keepsthe steam moving helically.Several nozzles are usually installed in thistype of turbine, and for each nozzle there is anaccompanying set of redirecting buckets or reversingchambers. Thus the reversal of steamflow is repeated several times for each nozzleand set of reversing chambers.Now let us consider the classification of ahelical-flow turbine with respect to staging andcompounding, as discussed in chapter 12. It isa single-stage turbine because it has only oneset of nozzles and therefore only one pressuredrop. It is a velocity-compound turbine becausethe steam passes through the moving blades(buckets) more than once, and the velocity of thesteam is therefore utilized more than once. Thehelical-flow turbine shown in figure 16-1 mightbe said to correspond roughly to a turbine inwhich velocity- compounding is achieved by theuse of four rows of moving blades.Helical-flow auxiliary turbines are used fordriving some pumps and forced draft blowers.The arrangement of nozzles and blading thatprovides radial flow in a turbine is shown in figure16-2. Turbines of this type are sometimesused for driving auxiliary units such as pumps.As discussed in chapter 12, turbines may beclassified as single- entry or re-entry turbines,depending upon the number of times the steamenters the blading. All multistage (and hence all33.45X 38.81XFigure 16-1.— Helical-flow turbine. Figure 16-2.— Radial flow.423