Low-pressure submerged-tube distilling plants differ from ship to ship, but the operating conditions and the maintenance procedures are basically the same. In almost all instances, the personnel who stand watches on the distilling plants are also responsible for the maintenance of the plants. When operating problems do occur, it is the responsibility of the EN2, ENl, or ENC on duty to locate the trouble and to make the necessary adjustments or repairs.

Distilling plant reliability and consistent operating conditions are essential for satisfactory results. Except under emergency conditions, no plant should be forced beyond its rated capacity. Requirements for higher steam pressures result in higher temperatures, which will cause more rapid scaling of the evaporator tubes. During operation, the various elements of any plant depend on the heat and fluid balances throughout the plant. Adjustment of any one control can produce widespread changes to these balances. For example, an increase in the feed to the first effect will raise the liquid level in the first effect. More heat will be required to raise the feed to the boiling point, so that less heat will be available for evaporation in the first-effect shell and a smaller amount of heat will flow to the second-effect tube nest. These changes produce a new balanced condition, and other adjustments would be required to make the new balance satisfactory. Under such circumstances, overcontrolling could require many readjustments. The operator will always find it better to make small adjustments, one at a time. This will allow enough time between each adjustment for all the conditions to become steady.

Causes of Low Plant Output

Failure to obtain full rated capacity is one of the most frequent problems encountered during the operation of a distilling plant. The problem may be very difficult to remedy since it may result from a combination of things. Adecrease in the distilling output efficiency may result if any of these factors are not met. Full output requires the following:

1. Proper steam pressure above the orifice

a. Ample steam supply

b. Proper operation of reducing valves

2. Highest possible vacuum in the first-effect tube nest

a. No air leaks

b. Proper water levels in the evaporator shells

c. Continuously vented evaporator tube nests

d. Reasonably clean evaporator tube nests

(1) Continuous feed treatment

(2) Mechanically cleaned tubes

Density of brine overboard not over 1.5/32

(1) Reasonably clean overboard piping

(2) Proper valve settings

(3) Proper operation of brine pump (clean piping and strainers, proper speed and direction of rotation, properly vented pump, properly packed and sealed gland, and no air leaks in the piping) Properly drained tube nests

(1) Proper operation of all drain regulators

(2) Proper operation of the tube nest drain pump

3. Highest possible vacuum in the last-effect shell

a. No air leaks

b. Proper air ejector operation

(1) Clean nozzle and strainer

(2) Correct quality and quantity of steam

c. Ample flow of circulating water

(1) Clean strainer, pipeline, and tubes

(2) Proper valve settings

(3) Proper operation of the circulating pump

d. Effective surface in the distilling condenser

(1) No undue deposits inside the tubes

(2) Proper venting of the condenser

(3) Proper operation of the condensate pump

Steam Pressure

A distilling plant cannot maintain its full output unless it is supplied with dry steam at the designed pressure. The orifices were constructed to pass the proper amount of steam plus about 5 psig pressure to safely produce the designed plant output. Orifices should be inspected annually. An orifice should be measured and the reading compared with the figure stamped on the plate. If necessary, the orifice should be renewed.

If the steam pressure above the orifice varies, the exact source of trouble should be located and corrected. First the weight-loaded regulating valve and then the pressure-reducing valve (if installed) should be checked to determine whether or not each valve is operating properly. If they are functioning properly and the pressure cannot be maintained above the orifice, you may assume that an insufficient amount of steam is being supplied to the plant.

The auxiliary exhaust steam supply for the distilling plants, after passing through the regulating valve, is usually slightly superheated because of the pressure drop through the reducing valve and the orifice plate. A small amount of superheat has little or no effect on the plant operation or the prevention of scale formation. However, when live steam must be used, the installed desuperheater spray connection should be used to control the superheat. The water for desuperheating must be taken from the boiler feed system, preferably from the first-effect tube nest drain pump. Water for desuperheating must NEVER be taken directly from the fresh water distilled by the distilling plant.

Fluctuations in the first-effect generating steam pressure and temperature cause fluctuations of pressure and temperature throughout the entire plant. With increased salinity of the distillate, the fluctuations may cause priming, as well as erratic water levels in the shells. These fluctuations may he eliminated by proper operation of the automatic pressure regulators in the steam supply line.