Chapter 10- PROPULSION BOILERS 



boilers. A pressurized-furnace boiler may re- 

 quire only about 2000 pounds of refractory, as 

 against the 21,000 pounds or more usually re- 

 quired in a conventional boiler of equal capacity. 

 Increased efficiency, a substantial saving 

 in space and weight requirements, a substantial 

 reduction in ship's force maintenance require- 

 ments, shorter boiler start-up time, and better 

 maneuverability and control are the major ad- 

 vantages of the pressurized-furnace boiler. Al- 

 though some operational and maintenance prob- 

 lems do exist with this boiler, it appears likely 

 that most of them can eventually be solved by 

 increased training of personnel, increased pre- 

 cision in the erection of the boilers, and perhaps 

 continued refinements of design and construc- 

 tion. 



BOILER WATER REQUIREMENTS 



Modern naval boilers cannot be operated 

 safely and efficiently without careful control of 

 boiler water quality. If boiler water conditions 

 are not just precisely right, the high operating 

 pressures and temperatures of modern boilers 

 will lead to rapid deterioration of the boiler 

 metal, with the possibility of serious casualties 

 to boiler pressure parts. 



Although our ultimate concern is with the 

 water actually in the boiler, we cannot consider 

 boiler water alone. We must also consider the 

 water in the rest of the system, since we are 

 dealing with a closed cycle in which water is 

 heated, steam is generated, steam is condensed, 

 and water is returned to the boiler. Because the 

 cycle is continuous and closed, the same water 

 remains in the system except for the water that 

 is lost by boiler blowdown^ and the very small 

 amount of water that escapes, either as steam 

 or as water, and is replaced by makeup feed. 



There are two kinds of boiler blowdown: surface 

 blowdown and bottom blowdown. Surface blowdown is 

 used to remove foam and other light contaminants 

 from the surface of the water in the steam drum. 

 Bottom blowdown is used to remove sludge and other 

 material that tends to settle in the lower parts of the 

 boiler. Both surface blowdown and bottom blowdown 

 may be used to remove a portion of the boiler water 

 so that it can be replaced with purer makeup feed, 

 thereby lowering the chloride content of the boiler 

 water. Surface blows may be given while the boiler is 

 steaming; bottom blows must not be given until some 

 time after the boiler has been secured. The valves and 

 piping used for making surface and bottom blows are 

 discussed in chapter 11 of this text. 



Although we must remember the continuous 

 or cyclical nature of the shipboard steam plant, 

 we must also distinguish between the water at 

 different points in the system. This distinction 

 is necessary because different standards are 

 prescribed for the water at different points. To 

 identify the water at various points in the steam - 

 water cycle, the following terms are used: 



Distillate or sea water distillate is the fresh 

 water that is discharged from the ship's distill- 

 ing plants. This water is stored in fresh water or 

 feed water tanks. All water in the steam - water 

 cycle begins originally as distillate. 



Makeup feed is distillate used as replacement 

 for any water that is lost or removed from the 

 closed steam - water cycle. 



Condensate is the water that results from the 

 condensation of steam in the main and auxiliary 

 condensers. This water is called condensate until 

 it reaches the deaerating feed tank. 



Boiler feed or feed water is the water in the 

 system between the deaerating feed tank and the 

 boiler. 



Deaerated feed water is feed water that has 

 passed through deaerating feed tank and has had 

 the dissolved or entrained oxygen removed from 

 it. 



Boiler water is the water actually contained 

 within a boiler at any given moment. 



Sea water , the source of practically all fresh 

 water used aboard ship, contains about 35,000 

 parts per million (ppm) of sea salts. This is 

 equivalent to roughly 70 pounds of sea salts per 

 ton of water. When sea water is evaporated and 

 the vapor is condensed in the distilling plant, the 

 resulting distillate contains about 1.75 ppm of 

 sea salts, or roughly 70 pounds per 20,000 tons. 

 In other words, distillate is actually diluted sea 

 water— sea water that is diluted to about 1/20,000 

 of its original concentration. It is not "pure 

 water," In considering water problems and 

 water treatment, it is essential to remember that 

 the basic impurity of sea water distillate would 

 make water treatment necessary even if no other 

 impurities entered the water from other sources. 

 The salts that are present in sea water— and, 

 therefore, to a lesser extent in distillate— are 

 chiefly compounds of sodium, calcium, and mag- 

 nesium. 



Although makeup feed enters the tanks as 

 distillate, the makeup feed usually contains a 

 slightly higher proportion of impurities than the 

 distillate. The difference is accounted for by 

 slight seepage or other contamination of the 



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