Chapter 7—AUXILIARY MACHINERYgage glass will also result in false level indicationsin the gage glass.Once the distilling plant is in operation, thefeeding must be maintained at a steady rate. Sud-den rising of the water levels or too high a waterlevel will cause carryover of small particles ofbrine with the vapor (priming). The level of waterin the shell must be carried at the highest level thatcan be held and still prevent the carrying over ofsaltwater particles with the freshwater vapor,otherwise scale will form rapidly on exposed tubesurfaces.The pressure differential between the first andsecond effects permits the second-effect feed tobe discharged into the second-effect shell. A par-tial or total loss of pressure differential indicatesthat air leaks have occurred between the first andsecond-effect shells in the two-effect distillingplants. Large air leaks between the first effect andsecond effects can be readily detected because thevacuum gage for the first effect will read approx-imately the same as the vacuum gage for the sec-ond effect. Large air leaks of this type will disruptthe operation of the plant and must be locatedand repaired before the plant will operateproperly.Improper Venting of EvaporatorTube NestsImproper venting of the evaporator tube nestscauses either an accumulation of air in the tubes,with a loss of capacity, or an excessive loss of tubenest steam to the distilling condenser, with lossof economy. Troubles of this type usually resultfrom improper operation rather than frommaterial failures.Scale Deposits onEvaporator TubesUntil 1958, scale deposits on evaporator tubeshad been one of the more serious causes ofoperating difficulties. In 1958, a new compoundwas authorized for treatment of evaporatorfeedwater. The new compound PD-8 evaporatortreatment is far superior to the cornstarch-boilercompound formerly used. For details on PD-8and its use, refer to the applicable chapter inEngineman 3 & 2, NAVEDTRA 10541 (currentedition).Last-Effect Shell VacuumMost manuafacturers’ technical manualsindicate that a vacuum of approximately26 inches of mercury should be obtained in thelast-effect shell when the temperature of theseawater is 85 °F, and that the vacuum should behigher when the seawater is colder. Failure to ob-tain a vacuum of 26 inches of mercury, or more,can generally be traced to one of the followingfactors: air leaks, improper operation of air ejec-tors, insufficient flow of seawater, and ineffec-tive use of heat transfer surface in the distillingcondenser.Testing for Air LeaksThe importance of eliminating air leaks can-not be overemphasized. Many distilling planttroubles are direct results of air leaks. Air leaksin the shells of distilling plants cause a loss ofvacuum and capacity. Extreme care must be takenin making up joints and in keeping them tight.Joints should be periodically tested under pressurefor leaks.There are several methods by which tests canbe made for air leaks in tube nests, heatexchangers, shells, and the piping systems of thedistilling plant. When the plant is in operation,a candleflame can be used to test all joints andparts under vacuum. With the plant secured, airpressure tests or a soapsuds test can be used onthe various component parts of the distilling plant.The manufacturer’s technical manual describeshow the various parts of the plant can be isolatedand placed under air pressure.Air leakage may also be detected byhydrostatically testing the various parts of theplant. When performing air tests or hydrostatictests, precautions should be taken not to exceedthe maximum limit of the test pressures specifiedby the manufacturer.Testing for Saltwater LeaksIf a leak is detected in a heat exchanger, thedefective tube(s) should be located by means of7-25
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