At this point, we need to mention the problem of distilling plant CARRYOVER. Practically all cases of high salinity (salt content) in the distillate (fresh water) output of a distilling unit will be caused by internal seawater leakage (from a tube) or by carryover. Carryover actually consists of molecules of seawater that have not been filtered out of the vapor before the vapor is condensed into distillate. All types of distilling plants have some type of device to prevent carryover. These devices may be called moisture separators, vapor separators, or demister pads. If the plant is operated improperly, these separators will not function properly. Some seawater will pass through them and show up as high salinity in the distillate. This may occur when you attempt to in-crease the output of the plant beyond its rated ca-pacity. (NOTE: A common cause of carryover in distilling plants that use demister pads is leakage around the pads due to improper sealing of the edges of the pads. Therefore, when the pads are removed for any reason, extreme care must be taken for prevention of damage to the sealing surfaces.)

Salinity, which is caused by chemical salts in seawater, is undesirable. Chemical salts in boiler feedwater will cause corrosion of the tubes. In addition, the normal operating temperature of a naval distilling plant may not be high enough to completely sterilize the distillate. Therefore, any carryover (or leakage) of seawater is a potential health hazard. Many types of microorganisms (primarily coliform bacteria) may be present. For these reasons, restrictions are placed on the opera-tion of distilling plants while operating in either contaminated seawater or fresh water. Freshwater carryover may not have enough salinity to be detected by either the operator or the salinity indicating system. Restrictions for operation under these conditions are found in chapter 531 (9580) of the Naval Ships Technical Manual. You have read that the distillate must meet specific standards of chloride content. For those standards to be met, the distillate must be moni-tored continuously by the salinity indicator and periodically tested for concentration levels of chemical impurities. The results of distillate tests for chloride are expressed in terms of either parts per million (ppm) or equivalents per million (epm).

Parts per million is a weight-per-weight unit denoting the number of parts of a specified substance in a million parts of water. For example, 58.5 pounds of salt in 1,000,000 pounds of water represents a concentration of 58.5 ppm. Note also that 58.5 ounces of salt dissolved in 1,000,000 ounces of water or 58.5 tons of salt dissolved in 1,000,000 tons of water represents the same concentration-58.5 ppm.

Equivalents per million can be defined as the number of equivalent parts of a substance per million parts of water. (The word equivalent refers to the chemical equivalent weight of a substance.) The chemical equivalent weight is different for each element or compound. The chemical equivalent weight of sodium chloride (common table salt) is 58.5. A solution containing 58.5 ppm of this salt is said to contain 1 epm. Chloride has an equivalent weight of 35.5. Therefore, a solution containing 35.5 ppm of chloride has a concentration of 1 epm of chloride.