PRINCIPLES OF NAVAL ENGINEERING 



water after it has remained in the tanks for 

 some time. 



Just as distillate is diluted sea water, so 

 steam condensate is basically a diluted form of 

 boiler water. The amount of solid matter car- 

 ried over with the steam varies considerably, 

 depending upon the design of the boiler, tlie 

 condition of the boiler, the nature of the water 

 treatment, the manner in which the boiler is 

 operated, and other factors. In general, con- 

 densate contains from 1.7 to 3.5 ppm of solid 

 matter, or roughly 70 pounds per 20,000 to 

 10,000 tons. Condensate may pick up additional 

 contamination in various ways. Salt water leaks 

 in the condenser increase the amount of sea 

 salts present in the condensate. Oil leaks in 

 the fuel oil heaters may contaminate the con- 

 densate. Corrosion products from steam and 

 condensate lines may also be present in con- 

 densate. Under ideal conditions, condensate 

 should be no more contaminated than sea water 

 distillate; under many actual conditions, it is 

 more contaminated. 



The solid content of the water ( Boiler feed 

 water) in the system between the deaerating 

 feed tank and the boiler is essentially the same 

 as the solid content of the condensate. The main 

 difference between condensate and deaerated 

 boiler feed is that most of the dissolved gases 

 are removed from the water in the deaerating 

 feed tank. 



Practically all of the impurities that are 

 present in feed water, including those originally 

 present in the sea water distillate and those 

 that are picked up later, will eventually find 

 their way to the boiler. As steam is generated 

 and leaves the boiler, the concentration of im- 

 purities in the remaining boiler water becomes 

 greater and greater. In other words, the boiler 

 and the condenser together act as a sort of 

 distilling plant, redistilling the water received 

 from the ship's evaporators. In consequence, 

 the boiler water would become more and more 

 contaminated if steps were not taken to deal 

 with the increasing contamination. 



As an example, suppose that a boiler holds 

 10,000 pounds of water at steaming level, and 

 suppose that steam is being generated at the 

 rate of 50,000 pounds per hour. After an hour 

 of operation there would be approximately five 

 times as much solid matter in the boiler water 

 as there was in the entering feed water. Now if 

 we continued to steam this boiler for another 

 2000 to 4000 hours without using blowdown 

 and without using any kind of boiler water 



treatment, the boiler water would contain just 

 about the same concentration of sea salts as the 

 original sea water from which the distillate was 

 made. In addition, the boiler water would con- 

 tain increasingly large quantities of corrosion 

 products and other foreign matter picked up in 

 the steam and condensate systems. 



If we continued to steam the boiler with the 

 water in this condition, the boiler would dete- 

 riorate rapidly. To prevent such deterioration, 

 it is necessary to do the following things: 



1. Maintain the incoming feed water at the 

 highest possible level of purity and as free as 

 possible of dissolved oxygen. 



2. Use chemical treatment of the boiler 

 water to counteract the effects of some of the 

 impurities that are bound to be present. 



3. Use blowdown at regular intervals to re- 

 move some of the more heavily contaminated 

 water so that it may be replaced by purer feed 

 water. 



Although there are many sources of boiler 

 water contamination, the contaminating mate- 

 rials tend to produce three main problems when 

 they are concentrated or accumulated in the 

 boiler water. Therefore, boiler water treatment 

 is aimed at controlling the three problems of 

 (1) waterside deposits, (2) waterside corrosion, 

 and (3) carryover. 



Waterside deposits interfere with heat trans- 

 fer and thus cause overheating of the boiler 

 metal. The general manner in which a water- 

 side deposit causes overheating of a boiler tube 

 is shown in figure 10-27. In a boiler operating 

 at 600 psi, the temperature inside a generating 

 tube may be approximately 500° F and the tem- 

 perature of the outside of the tube may be ap- 

 proximately 100° F higher.'' Where a waterside 

 deposit exists, however, the tube cannot trans- 

 fer the heat as rapidly as it receives it. As 

 shown in figure 10-27, the inside of the tube 

 has reached a temperature of 800 ° F at the point 

 where the waterside deposit is thickest. The 

 tube metal is overheated to such an extent that 

 it becomes plastic and blows out into a bubble 

 or blister under boiler pressure. 



Waterside deposits that must be guarded 

 against include sludge, oil, scale, corrosion 



The temperatures used in this example do not apply 

 to all situations in which a boiler tube is overheated. 

 The exact temperatures of the inside andoutsideof the 

 tube would depend upon the operating pressure of the 

 boiler, the location of the tube in the boiler, the nature 

 of the deposit, and various other factors. 



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