ENGINEMAN 1 & C
on the plate. If necessary, the orifice should be
renewed.
If the steam pressure above the orifice varies,
the source of trouble should be located and cor-
rected. 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 func-
tioning properly and the pressure cannot be main-
tained 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 reduc-
ing valve and orifice plate. A small amount of
superheat has little or no effect on the operation
or the 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 freshwater 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 fluc-
tuations may be eliminated by proper operation
of automatic pressure regulators in the steam
supply line.
First-Effect Tube Nest Vacuum
The range of the pressure maintained in the
first-effect tube must be between 16 inches of mer-
cury, with clean tubes, to 1 to 2 inches of mer-
cury as scale forms. The output of a submerged
tube type distilling plant is not greatly reduced
until the deposits on the tubes have caused the
vacuum to drop to about atmospheric pressure.
When the first-effect tube nest vacuum is lost
entirely, the reduction in output becomes very
great. Assuming the reduction in vacuum is due
to scale and not to improper operating conditions,
the tubes must be cleaned.
Keeping the vacuum in the first-effect tube
nest as high as possible reduces scale formation
to a minimum, enabling the plant to operate at
full capacity.
A vacuum reduction which results from any
factor other than deposits on tube surfaces should
be corrected to reduce deposits and greatly pro-
long the interval of time between cleanings. The
primary factors affecting the first-effect tube nest
vacuum are air leakage, low water level in the
evaporator shells, improper venting of the
evaporator shells, scale or other deposits on the
tubes, and improper draining of the evaporator
tube nests.
Loss of vacuum resulting from deposits on
evaporator tubes should be gradual. Under nor-
mal conditions, there will be no large change of
vacuum for any one days operation. Any sud-
den drop in vacuum can be traced to causes other
than scale deposits.
The generating steam circuit operates under
vacuum and is subject to air leaks. Leaks from
the steam side of the first-effect tube nest to the
first-effect shell space cause losses of capacity and
economy. Losses of vacuum and capacity may be
due to air leaks from the atmosphere into the
generating steam line (downstream from the
orifice plate), from the first-effect tube nest front
header, and from the first-effect tube nest drain
piping. Air leaks in this part of the distilling plant
may be less noticeable than air or water leaks
elsewhere because the effect on the plant is similar
to the scaling of the tube surfaces.
Proper Water Levels
A reduced first-effect tube nest vacuum can
result from low water level in any evaporator shell.
On older plants, the water levels are controlled
by manually regulating the feed valves. On newer
ships, the water levels are automatically controlled
by weir type feed regulators. Inability to feed the
first effect is usually due either to scale deposits
in the seawater sides of the air ejector condenser
and the vapor feed heater, or to obstructions in
the feed line. Inability to feed second or third
effects is due to air leakage or heavy scale deposits
in the feed lines between the effects. It is impor-
tant that the gage glass and the gage glass fittings
be kept free of scale, otherwise false water level
indications will be given. Air leaks around the
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