Principles of naval engineering - Historic Naval Ships Association

CHAPTER 16AUXILIARY STEAM TURBINESAuxiliary steam turbines are used to drivemany auxiliary machinery units aboard steamdrivenships. Turbine-driven auxiliaries locatedin the engineering spaces include ship's servicegenerators, forced draft blowers, air compressors,and a number of pumps such as main condensatepumps, main condenser circulatingpumps, main feed pumps, feed booster pumps,fuel oil service pumps, and lubricating oil servicepumps.In many cases, the turbine-driven auxiliariesare duplicated by electrically driven units for inportor cruising use. Although the motor-drivenunits have a comparatively high efficiency, theircapacity is not sufficient (on some ships, at least)to meet the demands of the engineering plant athigh speeds. A further advantage of auxiliary turbinesis their greater reliability; in general,there is greater possibility of interruption orloss of electric power supply than of steam supply.In addition, the use of auxiliary turbines improvesthe overall plant efficiency because exhauststeam from the auxiliary turbines can beutilized in various ways throughout the plant.The basic principles of steam turbine design,classification, and construction discussed inchapter 12 of this text apply in general to auxiliaryturbines as well as to propulsion turbines,except for specific differences noted in the remainderof this chapter.TYPES OF AUXILIARY TURBINESMany auxiliary turbines are of the impulsetype. Reduction gears are used with most auxiliaryturbines^ to increase efficiency. Sincespace requirements frequently demand relativ-Direct drive, rather than geared drive, units includeforced draft blowers, high speed centrifugal pumps,and some recent ship's service turbogenerators.ely small units, auxiliary turbines are usuallydesigned with comparatively few stages— oftenonly one. This means a large pressure drop anda high steam velocity in each stage. To obtainmaximum efficiency, the blade speed must alsobe high. With auxiliary turbines, as with propulsionturbines, reduction gears serve to reconcilethe conflicting speed requirements of the drivingand the driven units.Until about 1950, many generator turbineswere designed and installed in such a way thatthey could be operated on steam from either superheatedor saturated steam lines at full boilerpressure. Most of the other auxiliary turbines atthis time operated on saturated steam at fullboiler pressure. During the early 1950's, a fewships were built in which all auxiliary turbineswere designed to operate on steam at full superheatand full boiler pressure. On most oil-firedships built since 1953, steam at full superheatand full boiler pressure is supplied to the auxiliaryturbines for generators, main feed pumps,and forced draft blowers; the other auxiliary turbineson these ships usually operate on steam atreduced temperature and pressure (about 50° Fof superheat and 600 psig). On nuclear ships, allturbines (propulsion and auxiliary) are designedto operate on wet steam— usually, steam whichcontains about 1 percent moisture. The auxiliaryturbines on nuclear ships operate over a widepressure range which varies according to thetype of nuclear propulsion plant. For more recentnuclear submarines, the pressure range is from285 to 750 psig; on the nuclear carrier USS Enterprise,the pressure range is from approximately585 to 1025 psig under normal operatingconditions. The generator turbines usually exhaustto their own separate auxiliary condensers;on recent submarines, however, they exhaust tothe main condenser. Most other auxiliary turbinesexhaust to the auxiliary exhaust system.The auxiliary exhaust system imposes a back422