Proper Water LevelsScale Deposits on Evaporator TubesA reduced first-effect tube nest vacuum can resultfrom low water level in any evaporator shell. On olderplants, the water levels are controlled by manuallyregulating the feed valves. On newer plants, the waterlevels are automatically controlled by weir-type feedregulators. Inability to feed the first effect is usually dueeither to scale deposits in the seawater sides of the airejector condenser and the vapor feed heater or toobstructions in the feed line. Inability to feed the secondor third effects is due to air leakage or heavy scaledeposits in the feed lines between the effects. It isimportant that you keep the gauge glass and the gaugeglass fittings free from scale and air leaks. Air leaks orscale will result in false water level indication readings.Once the distilling plant is in operation, the feedingmust be maintained at a steady rate. A sudden rise of thewater levels or too high a water level will causecarryover of small particles of brine within the vapor(priming). Maintain the level of water in the shell at thehighest level that can be held and still prevent thecarrying over of saltwater particles within thefreshwater vapor. If this constant water level is notmaintained, scales will form rapidly on the exposed tubesurfaces.The pressure differential between the first andsecond effects permits the second-effect feed to bedischarged into the second-effect shell. A partial or totalloss of pressure differential indicates that air leaks haveoccurred between the first-effect and second-effectshells in the two-effect distilling plants. Large air leaksbetween the first effect and second effect can be readilydetected, because the vacuum gauge for the first effectwill read approximately the same as the vacuum gaugefor the second effect. Large air leaks of this type willdisrupt the operation of the plant and must be locatedand repaired before the plant will operate properly.Improper Venting of Evaporator Tube NestsImproper venting of the evaporator tube nests cancause either an accumulation of air in the tubes or anexcessive loss of tube nest steam to the distillingcondenser. A loss of tube nest air or steam results in aloss of capacity or a loss of economy. Problems of thistype usually result from improper operations, rather thanfrom material failures.Scale deposits on evaporator tube nests have been aserious cause of operating difficulties. The rate of scaleformation is affected by the density of the brine and bythe types of solids present in the feed. Although themajor constituents of seawater (sodium chloride,magnesium chloride, and others) do not form scaleunder normal plant operating conditions, they may doso when the last-effect brine density exceeds 1.5/32. Theprimary scale-fomling constituent of seawater, calciumcarbonate, will form scale even under normal plantconditions. But, the rate of scaling depends on the brinedensity. For this reason, you must maintain thelast-effect brine density at 1.5/32.Another method to control scale formation is by theuse of scale preventive compound. This material helpsretard scale formation and foaming in distilling plants.The only authorized distiller scale preventive compoundfor surface ships is DOD-D-24577 (SH), Distiller ScalePreventive Treatment Formulutions, available from theNavy Supply System under National Stock Number(NSN) 9G6850-00-173-7243. Ships that were notoriginally equipped with chemical injection equipmentconforming to MIL-P-21397, Chemical (For DistillingPlants Naval Shipboard Use) Proportioning Unit,should install such equipment through a ship alteration(SHIPALT). Note that all plants require 24 gallons ofsolution regardless of plant capacity. You will use 1 pintof scale preventive compound for each 4,000 gallons perday of distilling plant capacity. You must combine thetotal amount of scale preventive compound in themixing tank with enough fresh water to make 24 gallonsof solution.WARNINGConcentrated scale preventive compoundis strongly alkaline. Avoid contact of the liquidwith skin or eyes. Wash hands thoroughly afterusing. In case of contact with eyes, flush withfresh water for at least 15 minutes and report tosick bay immediately.Last-Effect Shell VacuumA vacuum of approximately 26 in.Hg should beobtained in the last-effect shell when the temperature ofseawater is 85°F. The vacuum should be higher whenthe seawater is colder. Failure to obtain a vacuum of 26in.Hg, or more, can generally be traced to one of severalfactors or a combination of these factors. It could be air8-4
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