Chapter 7—AUXILIARY MACHINERYThe purpose of this arrangement is to enablethe single-stage ejector to produce the high degreeof vacuum required in stages four and five. Anejector discharging into a vacuum is able toachieve a higher degree of vacuum than onedischarging to atmosphere. A vacuum of 28inches of mercury is required in stage five.The two-stage ejector draws noncondensablesfrom the saltwater heater and the first threeevaporator stages and, since the noncondensablesfrom stages four and five are directed back intostage three, the two-stage ejector actually handlesall noncondensables within the unit.The suction chamber of the first stage of thesecond ejector (first two-stage) is flanged to thenoncondensables outlet of the precooler throughwhich the gases pass before entrainment in the airejector steam. The two-stage ejectors use ship’ssteam and produce a vacuum in the precoolerslightly greater than in the first evaporator stage.Orifice plates of varying size are flanged intothe piping from the evaporator stages and thesaltwater heater leading to the air ejectors. Theseplates prevent the air ejectors from withdrawingany undue amount of steam from the evaporatoralong with the noncondensables.The discharge of the first stage of the secondejector is flanged to the suction chamber of thethird (second two-stage) ejector. The discharge ofthe third ejector is flanged to piping, containinga check valve, which runs diagonally across thetop of the evaporator shell to the air ejector steaminlet of the preheater shell near the front waterbox.The pressure of ship’s steam piped to the ejec-tors is indicated on the independently mountedpressure gage panel. Line pressure to the air ejec-tors must be maintained at or above 135 poundsper square inch gage (psig), as a lower pressurewill cause unstable operation of the ejector andwill affect the vacuum in the evaporator.A DUPLEX STRAINER, located in the ship’sfeedwater inlet piping, removes solid matter fromseawater by filtering through one of two per-forated and screened bronze baskets. Basket wellsare located in the body or housing of the straineron either side of the centrally located flanged in-let and outlet.A lever handle between the wells directs thefeedwater into the left- or right-hand well. Whenone basket becomes clogged, flow is switched tothe other and the clogged basket is ready to beremoved and cleaned.An inlet and outlet angle-type RELIEFVALVE is flanged into the feedwater inlet be-tween the feedwater pump and the air ejectorprecooler. The valve is set to open at 75 psig toprevent pressure buildup from an obstruction inthe feedwater lines or accidental operation of thefeedwater pump with the feedwater control valveclosed.Two FLOWRATORS are mounted on theunit to measure the amount of feedwater andcooling water pumped into the system. Since theamount of fluid to be measured in both coolingand feedwater lines is large, the flowrators aremounted in bypass piping arrangements, measur-ing a small portion of the actual main stream flowand providing a reading on the graduated scaleof the cylinder for the entire flow. Main streamand range orifices are provided for each flowrator.The flowrators serve as manometers. Thepressure drop across the manometer is equal tothe pressure drop created by the constriction ofthe main stream orifice. The range orifice at theinlet of the flowrator constricts the bypass flowso that a maximum main stream flow will registera maximum reading on the flowrator scale.It is, therefore, essential that main stream andrange orifices be in good condition and ofproper bore diameter, if correct readings are tobe obtained on the flowrators. The size of theorifice bore should be checked regularly. Whencleaning orifice plates and checking bore diameter(stamped on the plates), be careful not to damagethe metering edge (the upstream edge). It mustbe square and sharp, free of either burrs orrounding so that the corner does not reflect lightwhen viewed with magnification. Piping shouldalso be inspected to see that scale deposits havenot decreased the inside diameter.Maintenance of FlashType UnitsMany maintenance procedures for a flash typedistilling plant are similar to the maintenance pro-cedures required for a submerged tube plant. Bothtypes of plants are subject to air leakage, saltwaterleakage, and malfunctioning of pumps and other7-31
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