Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGpropulsion) turbines are of the single- entrytype.However, some auxiliary turbines are ofthe re-entry type.Re-entry turbines are those in which thesteam passes more than once through the blading.Hence the helical-flow turbine just discussed isa re-entry turbine. A different kind of re-entryturbine is shown in figure 16-3. This turbine issimilar in principle to the helical-flow turbine,38.82XFigure 16-3.— Re-entry turbine with one reversingchamber.but it has one large reversing chamber insteadof a number of redirecting chambers. Re-entryturbines are sometimes made with two reversingchambers instead of one.The auxiliary turbine shown in figure 16-4is used to drive main condensate pumps, feedbooster pumps, and lubricating oil service pumpson many older destroyers. Note that this is aradial-flow turbine. This same design of turbineis used on some newer ships, but with improvedmetals designed to withstand higher pressures,higher temperatures, and high-impact (HI)shock.Another kind of auxiliary turbine is shown infigure 16-5. This turbine, which is usedtodrivemain condenser circulating pumps, is a verticallymounted, axial-flow, velocity- compoundedimpulse turbine. Although this type of turbine isbecoming obsolete, it is still in operation onsome older types of ships. The turbine shaft issecured to the vertical shaft of the pump. A thrustbearing, mounted integrally with tlie upper radialbearing, carries the weight of the rotating elementand absorbs any downward thrust. A throttlevalve and a double- seated balanced inlet valve(normally held wide open by the governor mechanism)admit steam to the turbine.Figure 16-6 shows an auxiliary turbine usedto drive a 400-kilowatt a-c, 50-kilowatt d-cship's service turbogenerator. The turbine is anaxial-flow, pressure- compounded unit. It exhauststo a separate auxiliary condenser whichhas its own circulating pump, condensate pump,and air ejectors. Cooling water for the condenseris provided by the auxiliary circulating pump,through separate injection and overboard valves.In case of casualty to the auxiliary condenser,the turbine can exhaust to the main condenserwhen the main plant is in operation.The turbogenerator turbine shown in figure16-6 is so designed that it can operate on eithersaturated steam or superheated steam. Provisionis made for supplying steam to the turbineeither from the main steam line (superheated)while under way or from the auxiliary steam line(saturated) during in-port operation when thepropulsion turbines and the main steam systemare secured. The steam is admitted to the turbinethrough a throttle trip valve to the steam chest,the speed being regulated by a number of nozzlecontrol valves under the control of a governor.Because the ship's service generator mustsupply electricity at a constant voltage and frequency,the turbine must run at a constant speedeven though the load varies greatly. Constantspeed is maintained through the use of a constantspeedgovernor (discussed later in this chapter).As may be seen in figure 16-6, the shaftglands of the ship's service generator turbine aresupplied with gland sealing steam. The systemis much the same as that provided for propulsionturbines. Other auxiliary turbines in naval usedo not require an external source of gland sealingsteam since they exhaust to pressures aboveatmospheric pressure.Generator turbines vary greatly, and are notall like the one shown in figure 16-6. Forexample, one recent type of turbogenerator consistsof seven stages— one Curtis stage and sixRateau stages. This turbine is direct drive,rather thangeared; the turbine operates at 12,000rpm and so does the generator.AUXILIARY TURBINE LUBRICATIONAuxiliary turbines designed to Navy specificationshave pressure lubrication systems tolubricate the radial bearings, reduction gears.424