Chapter  7—AUXILIARY  MACHINERY distiller   condenser   vacuum.   The   vacuum   is dependent upon the temperature and quantity of the circulating water, and the proper operation of   the   air   ejectors.   Too   low   an   overboard discharge temperature of the distiller condenser circulating water is accompanied by efficiency losses  in  the  distilling  plant.  The  overboard discharge temperature should be kept as high as possible,  without  exceeding  the  desired  20°F temperature  rise  through  the  distiller  condenser. In addition, limiting the quantity of circulating water tends to prolong the service life of the tubes and tube sheets. When troubles occur which are not  caused  by  improper  operating  procedures,  an inspection should be made of the condenser cir- culating water system to determine the cause of faulty  operation. Preventive maintenance procedures should be carried out to ensure that the circulating water pump is maintained in good material condition. The maintenance and repair procedures for this pump are similar to those for the other pumps of the plant. Routine procedures should be carried out to ensure the proper setting and maintenance of the back-pressure regulating valve. If this valve is not functioning    properly, the  valve  should  be disassembled, the valve parts replaced, and the necessary repairs to the valve made, before its faulty operation interferes with the operation of the distilling plant. To ensure that the condenser circulating water system is clean and free from scale and foreign matter,  the  piping  should  be  inspected  at  regular intervals.  The  operators  of  the  distilling  plant should inspect and clean the strainers, in accord- ance with the PMS, to prevent accumulations of foreign   matter   from   interfering   with   the proper  operation  of  the  plant. Improper Drainage Failure  of  the  distilling  plant  to  produce designed  output  when  the  pressure  above  the orifice  is  5  psig  and  the  first-effect  tube  nest vacuum  is  several  inches  of  mercury  always indicates improper drainage of the distiller con- denser  or  of  one  of  the  evaporator  tube  nests subsequent  to  the  first  effect.  Complete  flooding of the flash chamber gage glass is also a positive indication of improper draining of the condenser, but the fact that the level appears to be in the gage glass or below is not necessarily an indication of improper drainage because air leaks at the gage glass fittings may indicate a false liquid level. A temperature difference of more than 5 °F to 10°F between the last-effect shell temperature and   the   temperature   of   the   distillate   at   the distillate  cooler  inlet  is  another  indication  of improper  drainage;  however,  the  fact  that  the temperature difference is within the proper range does not necessarily indicate proper drainage. Scale  deposits  are  unlikely  to  form  in  the distilling  condenser  tubes  if  the  plant  is  properly operated and a full flow of circulating water is maintained. However, if scale deposits do occur, the tubes must be cleaned. Venting of the vapor side of the distiller con- denser is continuously accomplished by the air ejector. Venting of the saltwater side of this and other units of the distilling plant need not be con- tinuous.  While  starting  the  plant,  and  once  every watch thereafter, the vents on all saltwater heads should be opened until all air is expelled and a solid  stream  of  water  appears,  then  the  vents should be closed. Constant  Brine  Density The concentration of brine in the evaporators, to a certain extent, has a direct bearing on the quality  of  the  distillate,  and  since  varying  quan- tities of brine discharged overboard may affect the operating conditions, the quantity of brine discharged and the brine density must be kept as constant as possible. If the brine concentration is too low, there will be a loss in capacity and economy. If the brine concentration is too high, there will be an increase in the rate of scaling of the evaporator heating surfaces, and the quality of the distillate will be impaired. The brine density, which should never exceed 1.5/32, is dependent mainly on the quantity of brine   pumped   overboard   and   the   amount   of freshwater  being  produced.  The  density  must  be checked   frequently   during   each   watch   and adjusted to the required density. On older distill- ing plants, the brine density is adjusted by means of a hand-controlled valve located in the discharge 7-27