Proper  Water  Levels Scale  Deposits  on  Evaporator  Tubes 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 plants, 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 the second or  third  effects  is  due  to  air  leakage  or  heavy  scale deposits  in  the  feed  lines  between  the  effects.  It  is important that you keep the gauge glass and the gauge glass fittings free from scale and air leaks. Air leaks or scale will result in false water level indication readings. Once the distilling plant is in operation, the feeding must be maintained at a steady rate. A sudden rise of the water   levels   or   too   high   a   water   level   will   cause carryover of small particles of brine within the vapor (priming). Maintain the level of water in the shell at the highest  level  that  can  be  held  and  still  prevent  the carrying   over   of   saltwater   particles   within   the freshwater  vapor.  If  this  constant  water  level  is  not maintained, scales will form rapidly on the exposed tube surfaces. The   pressure   differential   between   the   first   and second  effects  permits  the  second-effect  feed  to  be discharged into the second-effect shell. A partial or total loss  of  pressure  differential  indicates  that  air  leaks  have occurred   between   the   first-effect   and   second-effect shells in the two-effect distilling plants. Large air leaks between the first effect and second effect can be readily detected, because the vacuum gauge for the first effect will read approximately the same as the vacuum gauge for the second effect. Large air leaks of this type will disrupt the operation of the plant and must be located and repaired before the plant will operate properly. Improper Venting of Evaporator Tube Nests Improper venting of the evaporator tube nests can cause either an accumulation of air in the tubes or an excessive  loss  of  tube  nest  steam  to  the  distilling condenser. A loss of tube nest air or steam results in a loss of capacity or a loss of economy. Problems of this type usually result from improper operations, rather than from  material  failures. Scale deposits on evaporator tube nests have been a serious  cause  of  operating  difficulties.  The  rate  of  scale formation is affected by the density of the brine and by the  types  of  solids  present  in  the  feed.  Although  the major  constituents  of  seawater  (sodium  chloride, magnesium  chloride,  and  others)  do  not  form  scale under normal plant operating conditions, they may do so when the last-effect brine density exceeds 1.5/32. The primary scale-fomling constituent of seawater, calcium carbonate,  will  form  scale  even  under  normal  plant conditions. But, the rate of scaling depends on the brine density.   For   this   reason,   you   must   maintain   the last-effect  brine  density  at  1.5/32. Another method to control scale formation is by the use of scale preventive compound. This material helps retard scale formation and foaming in distilling plants. The only authorized distiller scale preventive compound for  surface  ships  is  DOD-D-24577  (SH),  Distiller Scale Preventive  Treatment  Formulutions,  available  from  the Navy  Supply  System  under  National  Stock  Number (NSN)  9G6850-00-173-7243.  Ships  that  were  not originally equipped with chemical injection equipment conforming to MIL-P-21397, Chemical (For Distilling Plants   Naval   Shipboard   Use)   Proportioning   Unit, should install such equipment through a ship alteration (SHIPALT). Note that all plants require 24 gallons of solution regardless of plant capacity. You will use 1 pint of scale preventive compound for each 4,000 gallons per day of distilling plant capacity. You must combine the total  amount  of  scale  preventive  compound  in  the mixing tank with enough fresh water to make 24 gallons of solution. WARNING Concentrated  scale  preventive  compound is strongly alkaline. Avoid contact of the liquid with skin or eyes. Wash hands thoroughly after using. In case of contact with eyes, flush with fresh water for at least 15 minutes and report to sick  bay  immediately. Last-Effect  Shell  Vacuum A  vacuum  of  approximately  26  in.Hg  should  be obtained in the last-effect shell when the temperature of seawater is 85°F. The vacuum should be higher when the seawater is colder. Failure to obtain a vacuum of 26 in.Hg, or more, can generally be traced to one of several factors or a combination of these factors. It could be air 8-4