Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSAs may be seen, classifying boilers by operatingpressure is not very precise, since actualoperating pressures may vary widely within onegroup. Also, any classification based on operatingpressure may easily become obsolete. Whatis called a high pressure boiler today might wellbe called a low pressure boiler tomorrow.BOILER COMPONENTSMost propulsion boilers now used by theNavy have essentially the same components:steam and water drums, generating and circulatingtubes, superheaters, economizers, fueloil burners, furnaces, casings, supports, and anumber of accessories and fittings required forboiler operation and control. The basic componentsof boilers are described here. In latersections of this chapter we will see how thecomponents are arranged to form various commontypes of naval propulsion boilers.Drums and HeadersDrum-type boilers are installed in the shipin such a way that the long axis of the boilerdrums will run fore and aft rather than athwartships,so that the water will not surge from oneend of the drum to the other as the ship rolls.The steam drum is located at the top of theboiler. It is cylindrical in shape, except that onsome boilers, it may be slightly flattened alongits lower curved surface. The steam drum receivesfeed water and serves as a place for theaccumulation of the saturated steam that is generatedin the tubes. The tubes enter the steamdrum below the normal water level of the drum.The steam and water mixture from the tubesgoes through separators which separate thewater from the steam.Figure 10-3 shows the way in which a steamdrum is constructed. Two sheets of steel arerolled or bent to the required semicircularshape and then welded together. The upper sheetis called the wrapper sheet; the lower sheet iscalled the tube sheet. Notice that the tube sheetis thicker than the wrapper sheet. The extrathickness is required in the tube sheet to ensureadequate strength of the tube sheet after theholes for the generating tubes have been drilled.The ends of the drum are enclosed with drumheadswhich are welded to the shell, as shown infigure 10-4. One drumhead contains a manholewhich permits access to the drum for inspection,cleaning, and repair.WRAPPERSHEETTUBEFigure 10-3.— Boiler steam drum.SHEET38.19The steam drum either contains or is connectedto many of the important fittings andinstruments required for the operation andcontrol of the boiler. These fittings and controlsare discussed separately in chapter 11 of thistext.Water drums and water headers equalize thedistribution of water to the generating tubes andprovide a place for the accumulation of loosescale and other solid matter that may be presentin the boiler water. In drum-type boilers, thewater drums and water headers are at theDRUMHEAD38.20Figure 10-4.—Drumhead secured to steam drumshell.235