PRINCIPLES OF NAVAL ENGINEERING 



this artificial limitation upon combustion in the 

 boiler furnace is the factor that would cause the 

 end point for combustion to occur before either 

 of the other two end points. 



End Point for Moisture 

 Carryover 



The rate of steam generation should never 

 be increased to the point at which an excessive 

 amount of moisture is carried over in the steam. 

 In general, naval specifications limit the allow- 

 able moisture content of steam leaving the 

 saturated steam outlet to 1/4 of 1 percent. 



As you know, excessive carryover can be 

 extremely damaging to piping, valves, and tur- 

 bines, as well as to the superheater of the 

 boiler. It is not only the moisture itself that 

 is damaging but also the insoluble matter that 

 may be carried in the moisture. This insoluble 

 matter can form scale on superheater tubes, 

 turbine blades, piping and fittings; in some 

 cases, it may be sufficient to cause unbalance 

 of rotating parts. 



As the evaporation rate is increased, the 

 amount of moisture carryover tends to increase 

 also, due to the increased release of steam 

 bubbles. Because modern naval boilers are 

 designed for high evaporation rates, steam 

 separators and various baffle arrangements 

 are used in the steam drum to separate mois- 

 ture from the steam. 



End Point for Water 

 Circulation 



In natural circulation boilers, circulation is 

 dependent upon the difference between the density 

 of the ascending mixture of hot water and steam 

 and the density of the descending body of rela- 

 tively cool water. As the firing rate is increased, 

 the amount of heat transferred to the tubes is 

 also increased. A greater number of tubes carry 

 the upward flow of water and steam, and fewer 

 tubes are left for the downward flow of water. 

 Without downcomers to ensure a downward flow 

 of water, a point would eventually be reached 

 at which the downward flow would be insufficient 

 to balance the upward flow of water and steam, 

 and some tubes would become overheated and 

 burn out. This condition would determine the end 

 point for water circulation. 



The use of downcomers ensures that the 

 end point for water circulation will not be reached 

 merely because the firing rate is increased. 



Other factors that influence the circulation in a 

 natural circulation boiler are the location of 

 the burners, the arrangement of baffles in the 

 tube banks, and the arrangement of tubes in the 

 tube banks. 



Full-power and overload ratings for the 

 boilers in each ship are specified in the manu- 

 facturer's technical manual. The total quantity 

 of steam required to develop contract shaft 

 horsepower of the ship, divided by the number 

 of boilers installed, gives boiler full-power 

 capacity. Boiler overload capacity is usually 

 120 percent of boiler full-power capacity. For 

 some boilers, a specific assigned maximum fir- 

 ing rate is designated. 



A boiler should not be forced beyond full- 

 power capacity— that is, it should not be steamed 

 at a rate greater than that required to obtain 

 full-power speed with all the ship's boilers in 

 use. A boiler should never be steamed beyond 

 its overload capacity, or fired beyond the as- 

 signed maximum firing rate, except in dire 

 emergency. 



Checking Boiler Efficiency 



In order to check on boiler efficiency it is 

 necessary to compare the amount of fuel actu- 

 ally burned in a boiler with the amount that 

 should be burned. This check is usually made 

 during economy runs and during full-power 

 runs. As a rule, 4 hours are allowed for each 

 run. During the run, fuel consumption is meas- 

 ured at intervals of precisely 1 hour. This 

 measure, when corrected for meter error and 

 verified by tank soundings, gives the amount of 

 fuel that is actually used. 



The amount of fuel that should be used under 

 specified conditions may be taken from tables 

 or curves supplied in the manufacturer's tech- 

 nical manual for the boilers or from the ship's 

 fuel performance tables. Since these two sources 

 give different figures for the amount of oil that 

 should be burned under various conditions, it is 

 necessary to make a clear distinction between 

 them. The differences, incidentally, arise from 

 the fact that there are two basic approaches to 

 the problem of checking on fuel consumption. 

 When you are concerned only with boiler per- 

 formance, you use the tables and charts from the 

 manufacturer's technical manual; when you are 

 concerned with plant performance with respect 

 to fuel consumption, you use the ship's fuel 

 performance tables. 



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