The salinity of distillate is monitored continuously by an electrical salinity monitoring system. A salinity cell (fig. 15-12) is placed in the

Figure 15-12.-Salinity indicator cell.

distillate line in constant contact with the distillate flowing out of the evaporator.

A salinity cell actually measures the ability of water to conduct electrical current (conductivity).

The higher the concentration of chemical impurities in the water, the higher the conductivity of the water. This measurement is indicated on a salinity monitoring panel that may be of the type shown in figure 15-13. The electrical reading from the cell is converted to either epm or ppm chloride on the meter face. The chloride reading is based on the assumption that all of the impurities in the distillate are from seawater; however, a salinity cell cannot differentiate between seawater or any other impurity. For this reason, the salinity cell readings are checked frequently by a chemical test for chloride. You must always believe a high salinity cell reading (such as an alarm or tripped valve situation) until it is proven absolutely false. In practically all cases, the measurement made by the distillate salinity cell will also activate the trip function of a three-way, solenoid-operated trip valve. If the salinity of the distillate reaches a preset value (2.3 ppm for some ships, 0.065 epm for others), the solenoid in the valve will be de-energized. This will allow spring pressure to disengage the latching linkage from the engaging lever, and the valve stem assembly will drop due to main spring pressure. When the stem assembly drops, the valve seat shuts off the water path of the distillate to the freshwater tanks or reserve feed system and opens a path to the bilge or waste drain system. This type of three-way valve is also used on the air ejector condenser drains and seawater heater drains of many distilling plants. These valves have locking pins that can be used to lock the main valve engaging lever in the reset position. The locking pins should NEVER be used without the specific permission of the engineer officer. Contaminated water can be put into clean tanks and systems at an excessive rate if these pins are used to prevent the trip valves from tripping.

Salinity cells are normally installed in other places in the evaporator system to monitor the salinity levels. Two examples are (1) the seawater heater drain pump discharge and (2) the loop seal between the first and second stages of the two-stage, flash-type distilling plant described earlier in this chapter. Cells installed in these places have a meter readout and an alarm, but do not actuate trip valves.

The temperature of the water affects the reading of the salinity cell. The higher the temperature of the water, the higher its conductivity. Salinity cells are designed to automatically compensate for changes in water temperature.