Principles of naval engineering - Historic Naval Ships Association

CHAPTER 11BOILER FITTINGS AND CONTROLSThe fittings, instruments, and controls usedon naval boilers are sufficiently numerous andimportant to warrant separate discussion. Theterm boiler fittings is used to describe a numberof attachments which are installed in orclosely connected to the boiler and which arerequired for the operation of the boiler. Boilerfittings are generally divided into two classes.Internal fittings (also called internals ) are thoseinstalled inside the steam and water spaces ofthe boiler; external fittings are those installedoutside the steam and water spaces. Boilerinstruments such as pressure gages and temperaturegages are usually regarded as externalbqiler fittings. Boiler controls are special systemswhich automatically control the fuel oil,combustion air, and feed water inputs in orderto regulate the steam output of the boiler.ESTTERNAL FITTINGSThe internal fittings installed in the steamdrum usually include equipment for distributingthe incoming feed water, for giving surfaceblows, and for directing the flow of steam andwater within the steam drum. In addition, boilerswhich do not have controlled superheat havedesuperheaters for desuperheating steam neededfor auxiliary purposes; the desuperheater ismost commonly installed in the steam drum,but is installed in the water drum in some ofthe newer boilers. Internal fittings in someboilers also include equipment for injectingchemicals for boiler water treatment.The specific design and arrangement ofboiler internal fittings varies somewhat fromone type of boiler (and from one boiler manufacturer)to another. The arrangement of internalsin several boilers is therefore describedhere.Figure 11-1 illustrates atypical arrangementof internal fittings installed in the steam drumof a header-type boiler. This illustration alsoshows many of the external connections. Feedwater enters through the feed inlet (A) and flowsto the internal feed pipe (B), The feed pipe iscapped at one end. The horizontal part of thefeed pipe runs about 80 percent of the length ofthe drum, well below the normal water level.The feed pipe is perforated along the upper sideso that the feed water will be evenly distributedalong the length of the pipe.The dry pipe (C) is suspended near the topof the steam drum, along the centerline of thedrum. Both ends of the dry pipe are closed.Steam enters by way of perforations in theupper surface of the dry pipe. Thus the steammust change direction in order to enter the drypipe. Since some moisture is lost wheneversteam changes direction, the dry pipe acts asa device to separate steam and moisture. Steamleaves the dry pipe through the main steam outlet(D) and from there goes to the superheater.A few perforations in the bottom of the dry pipeallow water droplets to drain back down to thewater in the steam drum.A longitudinal baffle (N) also helps to separatemoisture from the steam before the steamenters the dry pipe. The baffle is installed insuch a way as to allow steam to flow to the drypipe but to keep moisture (and any solid matterthat might be carried over with the moisture)from entering the dry pipe.The surface blow-off pipe (E) is used to removegrease, scum, and light solids from theboiler water and to reduce the salinity of theboiler water while the boiler is steaming. Thesurface blow-off pipe is installed near the centerof the steam drum, with the upper surfaceof the pipe slightly below the normal water levelof the drum. The pipe runs almost the entirelength of the drum. Holes are drilled along thetop centerline of the pipe. One end of the pipeis blanked off. The other end is connected through276