Principles of naval engineering - Historic Naval Ships Association

Chapter 9-MACHINERY ARRANGEMENT AND PLANT LAYOUTthe relationships between the auxiliary exhaustsystem and the main and auxiliary steamsystems.STEAM ESCAPE PIPINGSteam escape piping is installed to providean unobstructed passage for the escape of steamfrom boiler safety valves and from the reliefvalves installed on steam-driven auxiliaries. Aline is also provided from the auxiliary exhaustsystem to the escape piping to allow the auxiliaryexhaust to unload to atmosphere if thepressure becomes excessively high. Steam escapepiping is usually shown on the same plansor drawings as the ones that show the auxiliaryexhaust piping.GLAND SEAL AND GLANDEXHAUST SYSTEMSGland sealing steam is supplied to the shaftglands^ of propulsion turbines and turbogeneratorturbines to seal the shaft glands againsttwo kinds of leakage: (1) air leakage into theturbine casings, and (2) steam leakage out ofthe turbine casings. These two kinds of leakagemay seem contradictory; however, each kindof leakage could occur under some operatingconditions if the shaft glands were not sealed.Pressures in the gland seal system are low,ranging from about 3/4 psig to 2psig, dependingupon the conditions of operation. Gland exhaustpiping carries the steam and air from the turbineshaft glands to the gland exhaust condenser,where the steam is condensed and returned tothe condensate system.On most ships, gland sealing steam is suppliedfrom the auxiliary exhaust system, althoughon some ships it is supplied from the 150-psiauxiliary steam system. In either case, thesteam is supplied through reducing valves orreducing stations. Figure 9-19 illustrates atypical gland seal and gland exhaust system forpropulsion turbines on an older type of destroyer.CONDENSATE AND FEED SYSTEMSCondensate and feed systems include all thepiping that carries water from the condensers6shaft glands are devices for holding various kindsof packing at the point where the shaft extends throughthe turbine casing. Shaft glands and shaft gland packingare discussed in chapter 12 of this text.to the boilers and from the feed tanks to theboilers. The condensate system includes themain and auxiliary condensers, the condensatepumps, and the piping. The boiler feed systemincludes the feed booster pump, the main feedpump, and the piping required to carry waterfrom the deaerating feed tank to the boilers.Together, the condensate and feed systemsbegin at the condenser and end at the economizerof the boiler.It is a little hard to say whether the deaeratingfeed tank is part of the condensate systemor part of the boiler feed system, since thetank is generally taken as the dividing line betweenthe two systems. The water is calledcondensate between the condenser and thedeaerating feed tank. It is called feed water orboiler feed between the deaerating feed tankand the economizer of the boiler. Since thecondensate and feed systems actually formone continuous system, the terms feed systemand feed water system are quite commonlyused to include both the condensate systemand the boiler feed system.Four main types of feed systems have beenused on naval ships: (1) the open feed system,(2) the semiclosed feed system, (3) the vacuumclosedsystem, and (4) the pressure-closed system.The development of these systems, in thesequence listed, has gone along with the developmentof boilers. As boilers have been designedfor higher operating pressures andtemperatures, the removal of dissolved oxygenfrom the feed water has become increasinglyinportant, since the higher pressures and temperaturesaccelerate the corrosive effects ofdissolved oxygen. Each new type of feed systemrepresents an improvement over the one beforein reducing the amount of oxygen dissolved orsuspended in the feed water.Since practically all modern naval shipshave pressure- closed feed systems, this isthe only type discussed here. Pressure- closedsystems are used on all naval ships havingboilers operating at 600-psi and above; theyare also used on some ships that have lowerboiler operating pressures.In a pressure- closed system, all condensateand feed lines throughout the system (exceptfor the very short line between the condenserand the suction side of the condensate pump)are under positive pressure. The system isclosed to prevent the entrance of air. A pressure-closedsystem is shown in figure 9-20.213