Principles of naval engineering - Historic Naval Ships Association

-PRINCIPLES OF NAVAL ENGINEERINGUPPER BEARINGOIL GUARDOIL TOUPPER BEARINGpump of the type shown in figure 15-36; but theseblowers, like the other newer ones, have a completelyexternal oil supply to the bearings insteadof a hollow-shaft arrangement.One feature of some of the newer verticalblowers is an anti-rotation device on the shaftof the lubricating pump. This device preventswindmilling of the blower in a reverse directionin the event of leakage through the automaticshutters. The anti-rotation device is continuouslylubricated through a series of passagewayswhich trap some of the leakage from the thrustbearing.Control of Blower SpeedCENTRIFUGALPUMP IMPELLERPRESSUREGAGETACHOMETER^OILRESERVOIRTHERMOMETER38.116Figure 15-37. — Lubrication system with hollowshaftoil supply and external oil supply forvertical forced draft blower.pressure than is needed for lubrication, and theexcessive pressure would tend to cause floodingof the bearings and loss of oil from thelubrication system. The viscosity pump, whichis nothing more than a shallow helical threador groove on the lower part of the shaft, helpsto assure sufficient lubrication at low speedsand to prevent the development of excessive oilpressures at high speeds.The hollow-shaft type of lubrication systemjust described is still found on some verticalforced draft blowers. The newest verticalblowers, however, do not use a hollow shaft tosupply oil to the upper bearings. Instead, oil ispumped to the bearings through an externalsupply line, passing through an oil filter and anoil cooler on the way to the bearings. Most ofthese newer vertical blowers have a gear pumpand an additional hand pump that is used toestablish initial lubrication when the blower isbeing started. Some of the newer verticalblowers have a centrifugal pump and a viscosityForced draft blowers are manually controlledon all naval ships except those that have automaticcombustion control systems for the boilers.Speed-limiting governors (discussed inchapter 16 of this text) are fitted to all forceddraft blowers, but they function merely as safetydevices to prevent the turbine from exceedingthe maximum safe operating speed; the speedlimitinggovernors do not have any control overthe turbine at ordinary operating speeds.Manual control of blower speed is achievedby a valve arrangement that controls the amountof steam admitted to the turbines. In someblowers a full head of steam is admitted to thesteam chest; steam is then admitted to the turbineby means of a manually operated lever orhandwheel that controls four nozzle valves. Thelever or handwheel may be connected by linkagefor remote operation. The four valves areso arranged that they open in sequence, ratherthan all at the same time. The position of themanually operated lever or handwheel determinesthe number of valves that will open, andthus controls the amount of steam that will beadmitted to the driving turbine. The steam chestnozzle valve shafts of all blowers serving oneboiler are mechanically coupled so as to providefor synchronized operation of the blowers.If only one blower is to be operated, the rootvalve of the nonoperating blower must remainclosed so that steam will not be admitted to theline.In other installations, a throttle valve is usedto control the admission of steam to the steamchest. From the steam chest, the steam entersthe turbine casing through fixed nozzles ratherthan through nozzle valves. Varying the openingof the throttle valve varies the steam pressureto the steam chest and thus varies the speed of418