ENGINEMAN 1 & Con the plate. If necessary, the orifice should berenewed.If the steam pressure above the orifice varies,the source of trouble should be located and cor-rected. First the weight-loaded regulating valveand then the pressure reducing valve (if installed)should be checked to determine whether or noteach valve is operating properly. If they are func-tioning properly and the pressure cannot be main-tained above the orifice, you may assume that aninsufficient amount of steam is being supplied tothe plant.The auxiliary exhaust steam supply for thedistilling plants, after passing through theregulating valve, is usually slightly superheatedbecause of the pressure drop through the reduc-ing valve and orifice plate. A small amount ofsuperheat has little or no effect on the operationor the scale formation; however, when live steammust be used, the installed desuperheater sprayconnection should be used to control thesuperheat. The water for desuperheating must betaken from the boiler feed system, preferablyfrom the first-effect tube nest drain pump. Waterfor desuperheating must never be taken directlyfrom the freshwater distilled by the distilling plant.Fluctuations in the first effect generatingsteam pressure and temperature cause fluctuationsof pressure and temperature throughout theentire plant. With increased salinity of thedistillate, the fluctuations may cause priming, aswell as erratic water levels in the shells. These fluc-tuations may be eliminated by proper operationof automatic pressure regulators in the steamsupply line.First-Effect Tube Nest VacuumThe range of the pressure maintained in thefirst-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 submergedtube type distilling plant is not greatly reduceduntil the deposits on the tubes have caused thevacuum to drop to about atmospheric pressure.When the first-effect tube nest vacuum is lostentirely, the reduction in output becomes verygreat. Assuming the reduction in vacuum is dueto scale and not to improper operating conditions,the tubes must be cleaned.Keeping the vacuum in the first-effect tubenest as high as possible reduces scale formationto a minimum, enabling the plant to operate atfull capacity.A vacuum reduction which results from anyfactor other than deposits on tube surfaces shouldbe corrected to reduce deposits and greatly pro-long the interval of time between cleanings. Theprimary factors affecting the first-effect tube nestvacuum are air leakage, low water level in theevaporator shells, improper venting of theevaporator shells, scale or other deposits on thetubes, and improper draining of the evaporatortube nests.Loss of vacuum resulting from deposits onevaporator tubes should be gradual. Under nor-mal conditions, there will be no large change ofvacuum for any one day’s operation. Any sud-den drop in vacuum can be traced to causes otherthan scale deposits.The generating steam circuit operates undervacuum and is subject to air leaks. Leaks fromthe steam side of the first-effect tube nest to thefirst-effect shell space cause losses of capacity andeconomy. Losses of vacuum and capacity may bedue to air leaks from the atmosphere into thegenerating steam line (downstream from theorifice plate), from the first-effect tube nest frontheader, and from the first-effect tube nest drainpiping. Air leaks in this part of the distilling plantmay be less noticeable than air or water leakselsewhere because the effect on the plant is similarto the scaling of the tube surfaces.Proper Water LevelsA reduced first-effect tube nest vacuum canresult from low water level in any evaporator shell.On older plants, the water levels are controlledby manually regulating the feed valves. On newerships, the water levels are automatically controlledby weir type feed regulators. Inability to feed thefirst effect is usually due either to scale depositsin the seawater sides of the air ejector condenserand the vapor feed heater, or to obstructions inthe feed line. Inability to feed second or thirdeffects is due to air leakage or heavy scale depositsin the feed lines between the effects. It is impor-tant that the gage glass and the gage glass fittingsbe kept free of scale, otherwise false water levelindications will be given. Air leaks around the7-24
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