Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGthis artificial limitation upon combustion in theboiler furnace is the factor that would cause theend point for combustion to occur before eitherof the other two end points.End Point for MoistureCarryoverThe rate of steam generation should neverbe increased to the point at which an excessiveamount of moisture is carried over in the steam.In general, naval specifications limit the allowablemoisture content of steam leaving thesaturated steam outlet to 1/4 of 1 percent.As you know, excessive carryover can beextremely damaging to piping, valves, and turbines,as well as to the superheater of theboiler. It is not only the moisture itself thatis damaging but also the insoluble matter thatmay be carried in the moisture. This insolublematter can form scale on superheater tubes,turbine blades, piping and fittings; in somecases, it may be sufficient to cause unbalanceof rotating parts.As the evaporation rate is increased, theamount of moisture carryover tends to increasealso, due to the increased release of steambubbles. Because modern naval boilers aredesigned for high evaporation rates, steamseparators and various baffle arrangementsare used in the steam drum to separate moisturefrom the steam.End Point for WaterCirculationIn natural circulation boilers, circulation isdependent upon the difference between the densityof the ascending mixture of hot water and steamand the density of the descending body of relativelycool water. As the firing rate is increased,the amount of heat transferred to the tubes isalso increased. A greater number of tubes carrythe upward flow of water and steam, and fewertubes are left for the downward flow of water.Without downcomers to ensure a downward flowof water, a point would eventually be reachedat which the downward flow would be insufficientto balance the upward flow of water and steam,and some tubes would become overheated andburn out. This condition would determine the endpoint for water circulation.The use of downcomers ensures that theend point for water circulation will not be reachedmerely because the firing rate is increased.Other factors that influence the circulation in anatural circulation boiler are the location ofthe burners, the arrangement of baffles in thetube banks, and the arrangement of tubes in thetube banks.Full-power and overload ratings for theboilers in each ship are specified in the manufacturer'stechnical manual. The total quantityof steam required to develop contract shafthorsepower of the ship, divided by the numberof boilers installed, gives boiler full-powercapacity. Boiler overload capacity is usually120 percent of boiler full-power capacity. Forsome boilers, a specific assigned maximum firingrate is designated.A boiler should not be forced beyond fullpowercapacity—that is, it should not be steamedat a rate greater than that required to obtainfull-power speed with all the ship's boilers inuse. A boiler should never be steamed beyondits overload capacity, or fired beyond the assignedmaximum firing rate, except in direemergency.Checking Boiler EfficiencyIn order to check on boiler efficiency it isnecessary to compare the amount of fuel actuallyburned in a boiler with the amount thatshould be burned. This check is usually madeduring economy runs and during full-powerruns. As a rule, 4 hours are allowed for eachrun. During the run, fuel consumption is measuredat intervals of precisely 1 hour. Thismeasure, when corrected for meter error andverified by tank soundings, gives the amount offuel that is actually used.The amount of fuel that should be used underspecified conditions may be taken from tablesor curves supplied in the manufacturer's technicalmanual for the boilers or from the ship'sfuel performance tables. Since these two sourcesgive different figures for the amount of oil thatshould be burned under various conditions, it isnecessary to make a clear distinction betweenthem. The differences, incidentally, arise fromthe fact that there are two basic approaches tothe problem of checking on fuel consumption.When you are concerned only with boiler performance,you use the tables and charts from themanufacturer's technical manual; when you areconcerned with plant performance with respectto fuel consumption, you use the ship's fuelperformance tables.270