Principles of naval engineering - Historic Naval Ships Association

-Chapter 10- PROPULSION BOILERSA pressurized-furnace boiler may re-boilers.quire only about 2000 pounds of refractory, asagainst the 21,000 pounds or more usually requiredin a conventional boiler of equal capacity.Increased efficiency, a substantial savingin space and weight requirements, a substantialreduction in ship's force maintenance requirements,shorter boiler start-up time, and bettermaneuverability and control are the major advantagesof the pressurized-furnace boiler. Althoughsome operational and maintenance problemsdo exist with this boiler, it appears likelythat most of them can eventually be solved byincreased training of personnel, increased precisionin the erection of the boilers, and perhapscontinued refinements of design and construction.BOILER WATER REQUIREMENTSModern naval boilers cannot be operatedsafely and efficiently without careful control ofboiler water quality. If boiler water conditionsare not just precisely right, the high operatingpressures and temperatures of modern boilerswill lead to rapid deterioration of the boilermetal, with the possibility of serious casualtiesto boiler pressure parts.Although our ultimate concern is with thewater actually in the boiler, we cannot considerboiler water alone. We must also consider thewater in the rest of the system, since we aredealing with a closed cycle in which water isheated, steam is generated, steam is condensed,and water is returned to the boiler. Because thecycle is continuous and closed, the same waterremains in the system except for the water thatis lost by boiler blowdown^ and the very smallamount of water that escapes, either as steamor as water, and is replaced by makeup feed.There are two kinds of boiler blowdown: surfaceblowdown and bottom blowdown. Surface blowdown isused to remove foam and other light contaminantsfrom the surface of the water in the steam drum.Bottom blowdown is used to remove sludge and othermaterial that tends to settle in the lower parts of theboiler. Both surface blowdown and bottom blowdownmay be used to remove a portion of the boiler waterso that it can be replaced with purer makeup feed,thereby lowering the chloride content of the boilerwater. Surface blows may be given while the boiler issteaming; bottom blows must not be given until sometime after the boiler has been secured. The valves andpiping used for making surface and bottom blows arediscussed in chapter 11 of this text.Although we must remember the continuousor cyclical nature of the shipboard steam plant,we must also distinguish between the water atdifferent points in the system. This distinctionis necessary because different standards areprescribed for the water at different points. Toidentify the water at various points in the steamwater cycle, the following terms are used:Distillate or sea water distillate is the freshwater that is discharged from the ship's distillingplants. This water is stored in fresh water orfeed water tanks. All water in the steam - watercycle begins originally as distillate.Makeup feed is distillate used as replacementfor any water that is lost or removed from theclosed steam - water cycle.Condensate is the water that results from thecondensation of steam in the main and auxiliarycondensers. This water is called condensate untilit reaches the deaerating feed tank.Boiler feed or feed water is the water in thesystem between the deaerating feed tank and theboiler.Deaerated feed water is feed water that haspassed through deaerating feed tank and has hadthe dissolved or entrained oxygen removed fromit.Boiler water is the water actually containedwithin a boiler at any given moment.Sea water , the source of practically all freshwater used aboard ship, contains about 35,000parts per million (ppm) of sea salts. This isequivalent to roughly 70 pounds of sea salts perton of water. When sea water is evaporated andthe vapor is condensed in the distilling plant, theresulting distillate contains about 1.75 ppm ofsea salts, or roughly 70 pounds per 20,000 tons.In other words, distillate is actually diluted seawater— sea water that is diluted to about 1/20,000of its original concentration. It is not "purewater," In considering water problems andwater treatment, it is essential to remember thatthe basic impurity of sea water distillate wouldmake water treatment necessary even if no otherimpurities entered the water from other sources.The salts that are present in sea water— and,therefore, to a lesser extent in distillate— arechiefly compounds of sodium, calcium, and magnesium.Although makeup feed enters the tanks asdistillate, the makeup feed usually contains aslightly higher proportion of impurities than thedistillate. The difference is accounted for byslight seepage or other contamination of the261