Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGwater after it has remained in the tanks forsome time.Just as distillate is diluted sea water, sosteam condensate is basically a diluted form ofboiler water. The amount of solid matter carriedover with the steam varies considerably,depending upon the design of the boiler, tliecondition of the boiler, the nature of the watertreatment, the manner in which the boiler isoperated, and other factors. In general, condensatecontains from 1.7 to 3.5 ppm of solidmatter, or roughly 70 pounds per 20,000 to10,000 tons. Condensate may pick up additionalcontamination in various ways. Salt water leaksin the condenser increase the amount of seasalts present in the condensate. Oil leaks inthe fuel oil heaters may contaminate the condensate.Corrosion products from steam andcondensate lines may also be present in condensate.Under ideal conditions, condensateshould be no more contaminated than sea waterdistillate; under many actual conditions, it ismore contaminated.The solid content of the water (Boiler feedwater) in the system between the deaeratingfeed tank and the boiler is essentially the sameas the solid content of the condensate. The maindifference between condensate and deaeratedboiler feed is that most of the dissolved gasesare removed from the water in the deaeratingfeed tank.Practically all of the impurities that arepresent in feed water, including those originallypresent in the sea water distillate and thosethat are picked up later, will eventually findtheir way to the boiler. As steam is generatedand leaves the boiler, the concentration of impuritiesin the remaining boiler water becomesgreater and greater. In other words, the boilerand the condenser together act as a sort ofdistilling plant, redistilling the water receivedfrom the ship's evaporators. In consequence,the boiler water would become more and morecontaminated if steps were not taken to dealwith the increasing contamination.As an example, suppose that a boiler holds10,000 pounds of water at steaming level, andsuppose that steam is being generated at therate of 50,000 pounds per hour. After an hourof operation there would be approximately fivetimes as much solid matter in the boiler wateras there was in the entering feed water. Now ifwe continued to steam this boiler for another2000 to 4000 hours without using blowdownand without using any kind of boiler watertreatment, the boiler water would contain justabout the same concentration of sea salts as theoriginal sea water from which the distillate wasmade. In addition, the boiler water would containincreasingly large quantities of corrosionproducts and other foreign matter picked up inthe steam and condensate systems.If we continued to steam the boiler with thewater in this condition, the boiler would deterioraterapidly. To prevent such deterioration,it is necessary to do the following things:1. Maintain the incoming feed water at thehighest possible level of purity and as free aspossible of dissolved oxygen.2. Use chemical treatment of the boilerwater to counteract the effects of some of theimpurities that are bound to be present.3. Use blowdown at regular intervals to removesome of the more heavily contaminatedwater so that it may be replaced by purer feedwater.Although there aremany sources of boilerwater contamination, the contaminating materialstend to produce three main problems whenthey are concentrated or accumulated in theboiler water. Therefore, boiler water treatmentis aimed at controlling the three problems of(1) waterside deposits, (2) waterside corrosion,and (3) carryover.Waterside deposits interfere with heat transferand thus cause overheating of the boilermetal. The general manner in which a watersidedeposit causes overheating of a boiler tubeis shown in figure 10-27. In a boiler operatingat 600 psi, the temperature inside a generatingtube may be approximately 500° F and the temperatureof the outside of the tube may be approximately100° F higher.'' Where a watersidedeposit exists, however, the tube cannot transferthe heat as rapidly as it receives it. Asshown in figure 10-27, the inside of the tubehas reached a temperature of 800 ° F at the pointwhere the waterside deposit is thickest. Thetube metal is overheated to such an extent thatit becomes plastic and blows out into a bubbleor blister under boiler pressure.Waterside deposits that must be guardedagainst include sludge, oil, scale, corrosionThe temperatures used in this example do not applyto all situations in which a boiler tube is overheated.The exact temperatures of the inside andoutsideof thetube would depend upon the operating pressure of theboiler, the location of the tube in the boiler, the natureof the deposit, and various other factors.262