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 85F. 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 leaks, improper operation of air ejectors, insufficient flow of seawater, or ineffective use of heat transfer surface in the distilling condenser.

Air Leaks

Many distilling plant troubles are direct results of air leaks. Air leaks in the shells of distilling plants cause a loss of vacuum and capacity. You must take extreme care when making up joints, for they must be kept tight. Periodically test the joints under pressure for leaks. When the plant is in operation, use a candle flame to test all joints and parts under vacuum. When the plant is secured, you can use air pressure or soapsuds for testing. Air leakage may also be detected by hydrostatically testing the various parts of the plant. You should take the necessary precautions not to exceed the maximum limit of the test pressure specified by the manufacturer.

Saltwater Leaks

Defective tube(s) on the heat exchangers can be located by means of an air or a hydrostatic test. You should follow the recommended procedure according to the manufacturers instructions.

Air Ejector

The steam pressure at the nozzle inlet of the air ejector must not be less than that for which the ejector is designed (stamped on the nameplate). Pressures at the air ejector nozzles may be 10 to 15 psig higher than the minimum specified by the manufacturer.

The primary causes of air ejector problems are low steam pressure, wet steam, an obstructed nozzle, or a clogged steam strainer. Problems are usually indicated by a failure to obtain or to maintain the required vacuum. If a problem is due to low steam pressure or wet steam, you should increase the steam pressure, install a drainage trap, or devise a manual solution. A clogged nozzle or strainer must be removed and cleaned. You should use special reamers to clean the air ejector nozzles. You should NEVER use a sharpedged tool to clean nozzles! Improper tools will damage the nozzle surfaces and impair the efficiency of the air ejecter. Procedures for testing air ejectors can be found in the manufacturers technical manual. In general, the same maintenance procedures should be followed for distilling plant air ejectors as for air ejectors for the main condensers.

You should inspect the air ejector strainer according to the PMS. Failure to keep the strainer clean will cause a reduced or fluctuating vacuum. When a strainer or a nozzle becomes damaged, you should replace it.