Chapter  7—AUXILIARY  MACHINERY The  purpose  of  this  arrangement  is  to  enable the single-stage ejector to produce the high degree of vacuum required in stages four and five. An ejector  discharging  into  a  vacuum  is  able  to achieve  a  higher  degree  of  vacuum  than  one discharging  to  atmosphere.  A  vacuum  of  28 inches of mercury is required in stage five. The two-stage ejector draws noncondensables from  the  saltwater  heater  and  the  first  three evaporator stages and, since the noncondensables from stages four and five are directed back into stage three, the two-stage ejector actually handles all noncondensables within the unit. The suction chamber of the first stage of the second ejector (first two-stage) is flanged to the noncondensables  outlet  of  the  precooler  through which the gases pass before entrainment in the air ejector steam. The two-stage ejectors use ship’s steam  and  produce  a  vacuum  in  the  precooler slightly greater than in the first evaporator stage. Orifice plates of varying size are flanged into the  piping  from  the  evaporator  stages  and  the saltwater heater leading to the air ejectors. These plates  prevent  the  air  ejectors  from  withdrawing any  undue  amount  of  steam  from  the  evaporator along with the noncondensables. The discharge of the first stage of the second ejector is flanged to the suction chamber of the third (second two-stage) ejector. The discharge of the third ejector is flanged to piping, containing a check valve, which runs diagonally across the top of the evaporator shell to the air ejector steam inlet of the preheater shell near the front water box. The pressure of ship’s steam piped to the ejec- tors is indicated on the independently mounted pressure gage panel. Line pressure to the air ejec- tors  must  be  maintained  at  or  above  135  pounds per square inch gage (psig), as a lower pressure will cause unstable operation of the ejector and will affect the vacuum in the evaporator. A DUPLEX STRAINER, located in the ship’s feedwater inlet piping, removes solid matter from seawater  by  filtering  through  one  of  two  per- forated  and  screened  bronze  baskets.  Basket  wells are located in the body or housing of the strainer on either side of the centrally located flanged in- let and outlet. A lever handle between the wells directs the feedwater into the left- or right-hand well. When one basket becomes clogged, flow is switched to the other and the clogged basket is ready to be removed and cleaned. An   inlet   and   outlet   angle-type   RELIEF VALVE is flanged into the feedwater inlet be- tween  the  feedwater  pump  and  the  air  ejector precooler. The valve is set to open at 75 psig to prevent pressure buildup from an obstruction in the feedwater lines or accidental operation of the feedwater pump with the feedwater control valve closed. Two  FLOWRATORS  are  mounted  on  the unit  to  measure  the  amount  of  feedwater  and cooling water pumped into the system. Since the amount of fluid to be measured in both cooling and feedwater lines is large, the flowrators are mounted in bypass piping arrangements, measur- ing a small portion of the actual main stream flow and providing a reading on the graduated scale of the cylinder for the entire flow. Main stream and range orifices are provided for each flowrator. The  flowrators  serve  as  manometers.  The pressure drop across the manometer is equal to the pressure drop created by the constriction of the main stream orifice. The range orifice at the inlet of the flowrator constricts the bypass flow so that a maximum main stream flow will register a maximum reading on the flowrator scale. It is, therefore, essential that main stream and range   orifices   be   in   good   condition   and   of proper bore diameter, if correct readings are to be obtained on the flowrators. The size of the orifice bore should be checked regularly. When cleaning orifice plates and checking bore diameter (stamped on the plates), be careful not to damage the metering edge (the upstream edge). It must be  square  and  sharp,  free  of  either  burrs  or rounding so that the corner does not reflect light when  viewed  with  magnification.  Piping  should also be inspected to see that scale deposits have not decreased the inside diameter. Maintenance  of  Flash Type Units Many maintenance procedures for a flash type distilling plant are similar to the maintenance pro- cedures required for a submerged tube plant. Both types of plants are subject to air leakage, saltwater leakage,  and  malfunctioning  of  pumps  and  other 7-31