*TB 43-0153
automatic dump valve. An increase in salinity (or TDS) above the normal range will activate an alarm
and signal the automatic dump valve to d ump the product water to the bilge. An increase in salinity (or
TDS) while operating in full strength seawater indicates that the product water may have been
contaminated as a result of seawater leakage or carryover. This product water may be disease-carrying
if the seawater feed is contaminated.
Carryover (also called priming) is the process in which salt- laden moisture is carried along with the
vapor as it passes through the moisture separators/demisters.
e. Operation in Fresh or Brackish Water. The salinity of fresh water and brackish water is much lower
than that of seawater. When a desalination unit is operated in these types of waters, the
microbiological and chemical purity of the product water cannot be assured. Since fresh and brackish
water contain so little salt, a small leak/carryover may not result in an excessive salinity reading. Thus,
a salinity alarm might not be received and the automatic dump valve might not dump the potentially
contaminated product water to the bilge. Under such conditions , the procedures in paragraph 12. f.
shall be followed.
f. Operation in Polluted Water. If it becomes necessary to operate the distilling plant/RO unit while in
polluted waters, the following steps will be taken to minimize the risk to the health of the ship's crew:
(1) Distilling plants shall be operated at or below rated capacity.
(2) Maintain evaporator shell water level low in the normal operating band to minimize the
possibility of carryover (submerged tube and vertical basket distilling plants).
(3) Maintain first-effect shell temperature at or above 165 F (submerged tube and vertical
basket distilling plants) at all times. This temperature is necessary in view of the fact that a
small leak/carryover may not result in a high salinity reading since fresh and brackish water
contain so little salt. Thus, under these conditions, a chloride content of less than 2.3 ppm is
not a reliable indication that the distillate is free from contamination.
When flash-type distilling units are opera ted in brackish water, contamination from
carryover is automatically avoided because the feedwater in the feedwater heater is
maintained at a temperature between 165 F and 170 F.
Heat recovery distilling units do not heat the feedwater to 165 F; however, most
units are equipped with a sterilizer that heats the distillate to between 210 F and
215 F.
The above temperature requirements guard against contamination of the distillate resulting
from carryover.
6
