ENGINEMAN 1 & Cthe reed in the neutral position, the fluid pressureis relieved to chamber C through the nozzles anda pressure drop allows the centering springs toreturn the spool valve to a central position; in thisposition, the valve blocks the pressure and returnline, creating a hydraulic lock in chambers E andD. By energizing the left-hand solenoid, themagnetic reed will move to the left and the entireprocess will be reversed.The position of the spool valve can be adjustedby using the centering screw. Fixed orifices areused so that the pressure drop in the hydraulicservovalve will not create a pressure drop in theopposite nozzle which is closed. Note that the ser-vovalve is basically a sliding spool valve. This typeof valve has many other applications in hydraulicsystems. For example, servovalves are used in theguidance systems of missiles and in the controlsystems of aircraft.DISTILLING PLANTSThis section will deal with the operation,troubleshooting, and repairing of the submergedtube and the flash type distilling plants that areused by the Navy. For additional and more de-tailed information than is provided by this train-ing manual, consult the manufacturer’s technicalmanual for the type of distilling plant installedon your ship.Distilling plants in naval ships are of threegeneral types: (1) vapor compression, (2) low-pressure steam, and (3) heat recovery. Themajor differences between the three types are thekinds of energy used to operate the units and thepressure under which distillation takes place.Vapor compression units use electrical energy (forheaters and compressors). Low-pressure steamdistilling units use low-pressure steam from eitherthe auxiliary exhaust steam systems or the aux-iliary steam system. Heat recovery distilling unitsuse diesel engine jacket water instead of steam asthe heat source. Vapor compression units boil thefeedwater at a pressure slightly above atmosphericpressure. Low-pressure steam and heat recoveryunits depend on a relatively high vacuum foroperation.Vapor compression type distilling units areused in submarines and small diesel-driven sur-face craft where the daily requirements do notexceed 4000 gallons per day (gpd). Since the vaporcompression type found on surface crafts isbeing replaced with the heat recovery distillingunits, vapor compression distilling units will notbe covered in this manual. Chapter 531 (9580-II)of Naval Ships’ Technical Manual contains infor-mation on these plants.The low-pressure steam distilling unit is usedin all steam-driven surface ships and nuclear sub-marines. Enginemen usually share responsibilitywith Machinist’s Mates for the maintenance andoperation of the low-pressure steam distillingplants.There are two reasons why low-pressure steamdistilling units are considered “low pressure”: (1)they use low-pressure steam as the source ofenergy, and (2) their operating shell pressure isless than atmospheric pressure.The three major types of low-pressure steamdistilling units are submerged tube, flash type, andvertical basket.In this section of the chapter we will bediscussing only two of these distilling units—thesubmerged tube and the flash type.SUBMERGED TUBE PLANTSLow pressure, submerged tube plants differfrom ship to ship, but the operating conditionsand the maintenance procedures are basically thesame. In almost all instances, the personnel whostand watches on distilling plants are also respon-sible for the maintenance of the plants. This givesthem ample opportunity to detect abnormaloperating conditions before such conditions reachadvanced stages. When operating troubles dooccur, it is the responsibility of the EN1 or ENCon duty to locate the trouble and to make thenecessary adjustments or repairs.Steady operating conditions are essential forsatisfactory results. Except under emergency con-ditions, no plant should be forced beyond its ratedcapacity, because higher steam pressures will berequired and the resulting higher temperatures willcause more rapid scaling of the evaporator tubes.During operation, the various elements of anyplant are interdependent due to the heat and fluidbalances throughout the plant. Adjustment of anyone control can produce widespread effects onthese balances. For example, an increase in the7-22
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