b. Proper air ejector operation(1) Clean nozzle and strainer(2) Correct quality and quantity of steamc. Ample flow of circulating water(1) Clean strainer, pipeline, and tubes(2) Proper valve settings(3) Proper operation of the circulatingpumpd. Effective surface in the distilling condenser(1) No undue deposits inside the tubes(2) Proper venting of the condenser(3) Proper operation of the condensatepumpSteam PressureA distilling plant cannot maintain its full outputunless it is supplied with dry steam at the designedpressure. The orifices were constructed to pass theproper amount of steam plus about 5 psig pressure tosafely produce the designed plant output. Orificesshould be inspected annually. An orifice should bemeasured and the reading compared with the figurestamped on the plate. If necessary, the orifice should berenewed.If the steam pressure above the orifice varies, theexact source of trouble should be located and corrected.First the weight-loaded regulating valve and then thepressure-reducing valve (if installed) should be checkedto determine whether or not each valve is operatingproperly. If they are functioning properly and thepressure cannot be maintained above the orifice, youmay assume that an insufficient amount of steam isbeing supplied to the plant.The auxiliary exhaust steam supply for the distillingplants, after passing through the regulating valve, isusually slightly superheated because of the pressuredrop through the reducing valve and the orifice plate. Asmall amount of superheat has little or no effect on theplant operation or the prevention of scale formation.However, when live steam must be used, the installeddesuperheater spray connection should be used tocontrol the superheat. The water for desuperheatingmust be taken from the boiler feed system, preferablyfrom the first-effect tube nest drain pump. Water fordesuperheating must NEVER be taken directly from thefresh water distilled by the distilling plant.Fluctuations in the first-effect generating steampressure and temperature cause fluctuations of pressureand temperature throughout the entire plant. Withincreased salinity of the distillate, the fluctuations maycause priming, as well as erratic water levels in theshells. These fluctuations may he eliminated by properoperation of the automatic pressure regulators in thesteam supply line.First-Effect Tube Nest VacuumThe range of the pressure maintained in thefirst-effect tube must be between 16 inches of mercury(in.Hg), with clean tubes, to 1 to 2 in.Hg as scale forms.The output of a submerged-tube type of distilling plantis not greatly reduced until the deposits on the tubes havecaused the vacuum to drop to about atmosphericpressure. When the first-effect tube nest vacuum is lostentirely, the reduction in output becomes very great.Assuming the reduction in vacuum is due to scale andnot to improper operating conditions, the tubes must becleaned.Keeping the vacuum in the first-effect tube nest ashigh as possible reduces scale formation to a minimum,enabling the plant to’operate at full capacity.A vacuum reduction that results from any factorother than deposits on tube surfaces should be correctedto reduce deposits and greatly extend the intervalsbetween cleanings. The primary factors affecting thefirst-effect tube nest vacuum are air leakage, low waterlevels in the evaporator shells, improper venting of theevaporator shells, scale or other deposits on the tubes,and improper draining of the evaporator tube nests.Loss of vacuum resulting from deposits onevaporator tubes should be gradual. Under normalconditions, there will be no major loss of vacuum forany one day’s operation. Any sudden drop in vacuumcan be traced to causes other than scale deposits.The generating steam circuit operates undervacuum and is subject to air leaks. Leaks from the steamside of the first-effect tube nest to the first-effect shellspace cause losses of capacity and economy. Loss ofvacuum and loss of capacity may be due to air leaks. Theair leaks may he from the atmosphere into the generatingsteam line (downstream from the orifice plate); from thefirst-effect tube nest front header; or from the first-effecttube nest drain piping. Air leaks in this part of thedistilling plant may be less noticeable than air or waterleaks elsewhere, because the effect on the plant is similarto the scaling of the tube surfaces.8-3
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