The Horizontal Steam Turbine For Stationary Plants 1920
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sometimes occurs. This condition should
become normal after a few hours’ operation
and will not have any detrimental effect on
the service of the circulating oil. Continued
excessive foaming is evidence that air is
being drawn into the suction pipe by the oil
pump. Air leaks should not be allowed to
exist.
DEPOSITS
As has been shown the oil used in hori-
zontal steam turbines is subjected to severe
strain. The general belief among engineers
in the past has been that petroleum lubri-
cating oils are indestructible. This is very
nearly true in practice, where the older
methods of lubrication are employed,. as in
the case of drop feed oilers for reciprocating
engines.
However, where the turbine is lubricated
by means of an oil circulating system, in
which the oil is foræd to the bearings under
pressure, then collected, filtered, cooled
and repeatedly returned to undergo the same
severe service, deposits may form, due to
the breaking down of some portions of the
oil from the following causes:
Air and heat
Solid impurities
Water
Broken down o il
Foreign substances
Adding new o il
Air and Heat
The circulating oil always contains more
or less air, and when the temperature of the
oil is above normal—say more than 140
degrees F.—the air has a strong tendency
to oxidize the oil. This will be realized when
it is remembered that the oil film in the
bearings is very thin and that the air is
present in very fine bubbles, which are inti-
mately mixed with the oil.
The result is that the oil darkens consid-
erably in color. In extreme cases, a very
dangerous black carbon deposit develops,
which may choke the oil inlets to the bear-
ings and cause the oil-worked piston in the
governor gear to stick.
Solid Impurities
Due to the high temperature at which the
oil passes through the circulating system,
the oxidizing effect of impurities—such as
iron oxides, dust and dirt, etc.—is very con-
siderable, particularly where ordinary oils
are in use.
The effects produced are the quick dark-
ening of the color of the oil, a considerable
increase in viscosity, the production of a
large percentage of petroleum acids and the
breaking down of the oil from oxidation.
The oil in this condition smells bumt and
throws down a slimy deposit which lodges
in the system, particularly in the oil cooler.
In addition, if there is even a slight leak-
age of water into the system, the oil will
emulsify badly, owing to its al ready weak-
ened condition from the oxidizing effect of
the air and solid impurities.
This will explain why, when starting up a
new turbine for the first time, emulsification
of the oil may occur as a result of the com-
bined effects of the water, air and dirt pres-
ent, even though the oil may be of high
quality.
Water
Water has an emulsifying effect on the oil,
particularly if the water contains impurities.
Where considerable quantities of water
leak into the system and emulsification takes
place, the mixture becomes yellow or brown-
ish-yellow in color. If a sample is taken out
and heated, it will separate into clean oil at
the top (Fig. 11), more or less milky water
at the bottom and a spongy sludge separ-
ating the two.
The clean oil will be founcl darker in color
and heavier in body than the original and
will have a strong characteristic odor.
When subjected to rapid circulation, espe-
cially in small systems, the oil and water
forms an objectionable mixture, which clogs
the screens, forms a slime on the cooling
coils, and accumulates in the clearance
spaces throughout the system.
This mixture will generally become hard,
and in some instances cause partial stoppage
in the oil pipes. In such cases, the increasing
temperature of the bearings indicates insuffi-
cient supply of oil; and this in turn demands
an immediate shutdown of the turbine to
thoroughly cleanse the system.
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