Transactions

496 TRANSACTIONS OF T H E A.S.M.E. AUGUST, 1941
but it has only been during recent years that it has been an item
of great importance. As the newer oils are more resistant to oxidation,
compounds that protect the metal from corroding are
slower in forming; and corrosion will occur, where previously it
was not experienced. Also, the newer oils will dissolve films
that have been coated upon metal surfaces and thus allow the
water to contact the bare metal and cause corrosion. Previously,
the corrosion products formed were principally ferric oxide
(Fe2Oa), which, when carried in the oil stream, did not cause
severe abrasion. The product formed with the newer oils is magnetic
iron oxide (Fe30 4), which when formed in the presence of
oil appears to be small, dense, hard crystals, and very abrasive.
When corrosion occurred in the high-pressure unit at Springdale,
it was impractical at that time to remove the unit from
service for a prolonged period and means for preventing the
corrosion had to be devised. Laboratory tests showed that the
addition of used oil which had become partially oxidized would
prevent corrosion. This remedy was applied to the oil in the
unit, and corrosion was completely stopped. Before adding the
used oil, moisture to the extent of 0.1 per cent would cause corrosion;
after the addition of the used oil, the unit was operated
for 3 weeks with 3 per cent of water in the oil, and no corrosion
occurred. Since these results have been obtained at
Springdale, it has become a general and successful practice to
stop corrosion in turbine oil systems by the addition of used oil
to the new oil.
We differ from the author in our theory as to why the addition
of the old oil prevents corrosion. Laboratory experimental
work and service records indicate that, when water is added to a
used oil, it makes a coating of a very minute film on the metal
surface. The first water added causes the formation of this film,
but in so doing it also removes the film-forming properties from
the oil. So until the oil is further oxidized any other additions
of water can cause corrosion of new metal that was not in the
system when the protective film was formed. Several experiences
with corrosion on changed metal parts has lead us to believe
that the inhibition was imparted to the metal and was not
permanently given to the oil, by having imparted to the oil an
increased wetting action.
The addition of the old oil to the new oil in a turbine system is
ideal for corrosion prevention but is not to be desired when considering
the service life of the new oil. The used oil apparently
serves as a catalyst to start oxidation in the new oil and reduce
its service life. Laboratory tests have shown that the life may
be reduced to as much as 20 to 30 per cent of the life expected
if the used oil had not been added.
Turbine-lubrication problems and their solutions can be arrived
at only by the cooperation of the turbine designers, the
oil refiners, and the users of the oil. Committees composed of
these three groups have been formed, and the results of their
cooperative studies will be very valuable.
D. L. B a r b o u b . 8 Operators of geared turbines would be interested
in the author’s comments concerning the desirability of
using for this service, as contrasted with direct-connected units,
an oil having a low viscosity index, i.e., a relatively steep temperature-viscosity
curve.
For a geared unit, it is general practice to employ a common
oiling system to serve the bearings of the turbine, reduction
gear, and generator or other driven machine and also the oil
sprays for gear-tooth lubrication. The operating conditions
of the bearings, particularly those of the turbine and high-speed
pinion, are the same as on a direct-connected unit. The viscosity
range for an oil most suitable for the bearings is as recommended
8 Chief Turbine Engineer, Elliott Company, Jeannette, Pa. Mem.
A.S.M.E.
in the first column in Table 1 of the paper. On the other hand,
an oil of much higher viscosity is preferred for gear-tooth lubrication.
This necessarily results in a compromise viscosity, intermediate
between the two, as recommended in the second and
third columns of Table 1.
There is also a considerable difference in oil-temperature rise
in the bearings and in the gear teeth. While the bearing-outlet
oil temperature is generally between 140 and 160 F with 120 F
inlet, the oil leaving the gear teeth may be at only approximately
125 F.
Assuming (1) an oil with a viscosity index of 100 and 49 sec
S.U.V. at 210 F and (2) an oil with zero viscosity index and the
same S.U.V. at 210 F, the temperature-viscosity curves are as
shown by the solid lines in Fig. 11 of this discussion. For the
100 V.I. oil, the viscosities at a bearing-outlet temperature of
150 F and the gear-tooth oil temperature of 125 F are, respectively,
86 and 132 S.U.V. The corresponding viscosities at the
same operating temperatures for the zero V.I. oil are respectively
100 and 180 S.U.V. Or similarly, as shown by the broken line,
a zero V.I. oil with the same viscosity as the 100 V.I. oil, at the
bearing operating temperature of 150 F, would have 146 S.U.V.
at the gear-tooth oil temperature of 125 F, compared with 132
S.U.V. for the 100 V.I. oil at this temperature.
By taking advantage of the steeper viscosity-temperature curve
of the zero or low-index oil, a somewhat better compromise can
be realized for the desired different viscosities for the bearings
and gear teeth at their respective operating temperatures. It
is questionable, however, whether this advantage is of sufficient
importance to justify the recommendation of low-index oil for
geared units.
By way of corroborative operating experience in connection
with the corrosion-inhibitive value of used turbine oil, as compared
with 100 per cent new oil of the same grade, the writer’s
company has had several cases of corrosion of steel parts during
the first few weeks of operation of new units with all fresh high-
grade oil. In each instance there were traces of water in suspension
in the oil, probably from gland leakage, although the
leakage and the clearance in the oil baffles were not excessive.
It has been found in most cases that, with no change in operating