ENGINEMAN  1  &  C on the plate. If necessary, the orifice should be renewed. If the steam pressure above the orifice varies, the source of trouble should be located and cor- rected. First the weight-loaded regulating valve and then the pressure reducing valve (if installed) should be checked to determine whether or not each valve is operating properly. If they are func- tioning properly and the pressure cannot be main- tained above the orifice, you may assume that an insufficient amount of steam is being supplied to the plant. The auxiliary exhaust steam supply for the distilling   plants,   after   passing   through   the regulating  valve,  is  usually  slightly  superheated because of the pressure drop through the reduc- ing valve and orifice plate. A small amount of superheat  has  little  or  no  effect  on  the  operation or the scale formation; however, when live steam must be used, the installed desuperheater spray connection   should   be   used   to   control   the superheat. The water for desuperheating must be taken  from  the  boiler  feed  system,  preferably from the first-effect tube nest drain pump. Water for  desuperheating  must  never  be  taken  directly from the freshwater distilled by the distilling plant. Fluctuations  in  the  first  effect  generating steam pressure and temperature cause fluctuations of   pressure   and   temperature   throughout   the entire   plant.   With   increased   salinity   of   the distillate,  the  fluctuations  may  cause  priming,  as well as erratic water levels in the shells. These fluc- tuations may be eliminated by proper operation of  automatic  pressure  regulators  in  the  steam supply  line. First-Effect  Tube  Nest  Vacuum The range of the pressure maintained in the first-effect tube must be between 16 inches of mer- cury, with clean tubes, to 1 to 2 inches of mer- cury as scale forms. The output of a submerged tube type distilling plant is not greatly reduced until the deposits on the tubes have caused the vacuum to drop to about atmospheric pressure. When the first-effect tube nest vacuum is lost entirely, the reduction in output becomes very great.  Assuming  the  reduction  in  vacuum  is  due to scale and not to improper operating conditions, the tubes must be cleaned. Keeping the vacuum in the first-effect tube nest  as  high  as  possible  reduces  scale  formation to a minimum, enabling the plant to operate at full  capacity. A vacuum reduction which results from any factor other than deposits on tube surfaces should be corrected to reduce deposits and greatly pro- long the interval of time between cleanings. The primary factors affecting the first-effect tube nest vacuum are air leakage, low water level in the evaporator  shells,  improper  venting  of  the evaporator shells, scale or other deposits on the tubes, and improper draining of the evaporator tube nests. Loss of vacuum resulting from deposits on evaporator tubes should be gradual. Under nor- mal conditions, there will be no large change of vacuum for any one day’s operation. Any sud- den drop in vacuum can be traced to causes other than scale deposits. The generating steam circuit operates under vacuum and is subject to air leaks. Leaks from the steam side of the first-effect tube nest to the first-effect shell space cause losses of capacity and economy. Losses of vacuum and capacity may be due  to  air  leaks  from  the  atmosphere  into  the generating  steam  line  (downstream  from  the orifice plate), from the first-effect tube nest front header, and from the first-effect tube nest drain piping. Air leaks in this part of the distilling plant may be less noticeable than air or water leaks elsewhere because the effect on the plant is similar to the scaling of the tube surfaces. Proper Water Levels A reduced first-effect tube nest vacuum can result from low water level in any evaporator shell. On older plants, the water levels are controlled by manually regulating the feed valves. On newer ships, the water levels are automatically controlled by weir type feed regulators. Inability to feed the first effect is usually due either to scale deposits in the seawater sides of the air ejector condenser and the vapor feed heater, or to obstructions in the feed line. Inability to feed second or third effects is due to air leakage or heavy scale deposits in the feed lines between the effects. It is impor- tant that the gage glass and the gage glass fittings be kept free of scale, otherwise false water level indications will be given. Air leaks around the 7-24