Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSBecause the concentration of chloride ions isrelatively constant in sea water, the chloridecontent is used as a measure of the amount ofsolid matter that is derived through sea watercontamination. The results of the chloride testare used as one indication of the need for blowdown.Chloride content is expressed in equivalentsper million (emp).^Hardness is a property that water acquiresbecause of the presence of certain dissolvedsalts. Water in which soap does not readilyform a lather is said to be hard.Alkalinity is a property that the water acquiresbecause of the presence of certain impurities.On ships that make alkalinity tests,the results are expressed in epm.Some ships are required to determine the pHvalue , rather than the alkalinity, of the boilerwater. The pH unit does not measure alkalinitydirectly; however, it is related to alkalinity insuch a way that a pH number gives an indicationof the acidity or alkalinity of the water. ThepH scale of numbers runs from to 14. Onthis scale, pH 7 is the neutral point. Solutionshaving pH values above 7 are define das alkalinesolutions. Solutions having pH values below 7 aredefined as acid solutions.Boiler water that is treated with phosphatesmust be tested for phosphate content . Boilerwater that is treated with standard Navy boilercompound is not tested for phosphates. Phosphatecontent is expressed in parts per million(ppm). When the phosphate content of boilerwater is maintained within the specified limits,the hardness of the water shovild be zero. Therefore,hardness tests are not required for boilerwater when phosphate water treatment is used.The test for chloride content indicates somethingabout the amount of solid matter that ispresent in the boiler water, but it indicates onlythe solid matter that is there because of seawater contamination. It does not indicate anythingabout other solid matter that may be dissolvedin the boiler water. A more accurateindication of the total amount of dissolved solidsEquivalents per million can be defined as the numberof equivalent parts of a substance per million parts ofsome other substance. The word "equivalent" hererefers to the chemical equivalent weight of a substance.For example, if a substance has a chemical equivalentweight of 35.5, a solution containing 35.5 parts permillion is described as having a concentration of 1epm.can be obtained by measuring the electricalconductivity of the boiler water, since this isrelated to the total dissolved solid content . Allships are now furnished with special electricalconductivity meters for measuring the conductivityof the boiler water. The total dissolvedsolid content is expressed in micromhos, a unitof electrical conductivity.As a regular routine, the test for dissolvedoxygen is made only on feed water, althoughoccasional testing of water in other parts ofthe system is recommended. A chemical testfor dissolved oxygen is made aboard ship. Sincethis test cannot detect dissolved oxygen in concentrationsof less than 0.02 ppm, more sensitivelaboratory tests are sometimes made as acheck on the operation of the deaerating feedtanks.When tests of the boiler water show that thewater is not within the prescribed limits, chemicaltreatment and blowdown are instituted.Several methods of chemical treatment are nowauthorized. Each method is designed to completelyeliminate hardness and to maintain thealkalinity (or the pH value) within the prescribedlimits. The method of boiler water treatmentspecified for each ship is the method that willbest perform these two functions and, at thesame time, take account of the total concentrationof solids that can be tolerated in theparticular type of boiler. The type of watertreatment authorized for any particular ship isspecified by the Naval Ship Systems Command;it is not a matter of choice by ship's personnel.Chemical treatment of the boiler water increases,rather than decreases, the need forblowdown. The chemical treatment counteractsthe effects of many of the impurities in theboiler water, but at the same time it increasesthe total amount of solid matter in the boilerwater and thus increases the need for blowdown.Each steam boiler must be given a surface blowat least once a day, and more often if the watertests indicate the need. Bottom blows are givenat least once a week, usually about an hour afterthe boiler has been secured. Bottom blows mustnot be given while a boiler is steaming. Specialinstructions for boiler blowdown are issued tocertain categories of ships.COMBUSTION REQUIREMENTSCertain requirements must be met beforecombustion can occur in the boiler furnace. Thefuel must be heated to the temperature that will265