Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGblades are mounted on the revolving rotor,they are called moving blades .Fixed or stationary blades of the same shapeas the moving blades are fastened to the casingin which the rotor revolves; these fixed bladesare installed between successive rows of themoving blades. The fixed blades guide thesteam into the moving blade system and, sincethey are also shaped and mounted in such away as to provide nozzle- shaped spaces betweenthe blades, the fixed blades also act asnozzles. The general arrangement of the fixedand moving blades, together with the pressureand absolute velocity relationships in a reactionturbine, are shown in figure 12-9. Figure 12-10shows a section of a reaction turbine rotor withone row of moving blades and one row of fixedblades.A reaction turbine is moved by three mainforces: (1) the reactive force produced on themoving blades as the steam increases in velocityas it expands through the nozzle- shapedspaces between the blades; (2) the reactiveforce produced on the moving blades when thesteam changes direction; and (3) the push or"impulse" of the steam impinging upon theblades. Thus, as previously noted, a reactionturbine is moved primarily by reactive forcebut also to some extent by direct impulse.From what we have already learned aboutthe function of nozzles, it will be apparent thatthermal energy is converted into mechanicalkinetic energy in the blading of a reactionturbine. The second required energy transformation—thatis, from kinetic energy to work—also occurs in the blading. A velocity diagramsuch as was used to analyze the work done onimpulse blading may be similarly used to analyzethe work done on reaction blading; however, theangles and velocities are different in the twotypes of blading.Since the velocity of the steam is increasedin the expansion through the moving blades, theinitial velocity of the entering steam (V^) mustbe lower in a reaction turbine than it would bein an impulse turbine with the same blade speed(V^); or, alternatively, the reaction turbine mustrun at a higher speed than a comparable impulseturbine in order to operate at approximatelythe same efficiency.TURBINE CLASSIFICATIONAs we have seen, turbines are divided intotwo general groups or classes— impulse turbinesand reaction turbines— according to the way inwhich the steam causes the rotor to move.Turbines may be further classified accordingto (1) the manner of staging and compounding,and (2) the mode of steam flow through theturbine.Staging and CompoundingPRESSUREABSOLUTEVELOCITYNOZZLE-^BLADES38.76XFigure 12-6. — Nozzle position and pressurevelocityrelationships in an impulse turbine.Thus far in this chapter, we have more orless assumed that an impulse turbine had oneset of nozzles and one row of blading on therotor, and that a reaction turbine had one rowof fixed blades and one row of moving blades.In reality, however, propulsion steam turbinesare not this simple. Instead, they use severalrows of blading, arranged in various ways.It has been shown that the amount of thermalenergy which can be utilized in a turbinedepends upon the relationship between the velocityof the entering steam (Vi) and the bladespeed (Vb). It might seem reasonable, therefore,to think that the work output of the turbinecould only be increased by increasingVi and Vb in the proper ratio. However,mechanical considerations and problems concerningstrength of materials impose certainlimits on blade speed. In modern naval ships,the amount of available energy per pound ofsteam is so great that there is no practicableway of utilizing the major portion of it in onerow of blades. When several rows of blades are324