Principles of naval engineering - Historic Naval Ships Association

5PRINCIPLES OF NAVAL ENGINEERINGASTERN STEAM INLETAHEAD STEAM FROMH.P. TURBINEASTERN STEAM INLETASTERNNOZZLEBLOCKFigure 12-14.— Double-flow reaction turbine.47,8Xseparated only by a fluid film of oil. Theeffectiveness of oil-film lubrication dependsupon a number of factors, including propertiesof the lubricant (cohesion, adhesion, viscosity,temperature, etc.) and the clearances, alignment,and surface condition of the bearing andthe journal. Except for the momentary metalto-metalcontact when the turbine is started,the metallic surfaces of the bearing and thejournal are constantly separated by a thin filmof oil.As previously noted, impulse turbines do not,in theory, develop end thrust. In reality, however,a small amount of end thrust is developedwhich must be absorbed in some way. Kingsburyor pivoted-shoe thrust bearings are usually usedon propulsion turbines.Shaft GlandsShaft glands are used to minimize steamleakage from the turbine casing (or air leakageinto the casing) at the points where the shaftextends through the casing. Two types of packing,carbon packing and labyrinth packing, are usedin shaft glands.Carbon packing is suitable only for relativelylow pressures and temperatures. When bothtypes of packing are used in one gland, therefore,as shown in figure 12-20, the labyrinthpacking is used at the initial high pressurearea and the carbon packing is used at thelower pressure area. Since most modem shipsutilize relatively high pressures and temperatures,most modern propulsion turbines areonly labyrinth packing.Labyrinth packing consists of rows of metallicstrips or fins. These strips are fastenedto the gland liner in such a way as to make avery small clearance between the strips and theshaft. As the steam from the turbine leaksthrough the small spaces between the packingstrips and the shaft, the steam pressure isgradually reduced.Where carbon packing rings are used, theyrestrict the passage of steam along the shaftin much the same manner as do the labyrinthpacking strips. Carbon packing rings aremounted around the shaft and are held in placeby springs. As a rule, three or four carbonrings are used in each gland; each ring isfitted into a separate compartment of the glandhousing.Gland Sealing SystemsLubricants and the oil-film theory of lubrication arediscussed in chapter 6 of this text.On propulsion turbines, the shaft glandpacking is not sufficient to entirely stop the330