Note: Descriptions are shown in the official language in which they were submitted.
CA 02276920 1999-07-06
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TURBINE AND R&O OILS
CONTAINING NEUTRAL RUST INHIBITORS
The present invention relates to turbine and rust and
oxidation (R&O) oils (hereinafter "turbine oils") having
improved wet filterability without detriment to hydrolytic
stability.
Steam and gas turbine oils are top-quality rust- and
oxidation-inhibited oils. Steam turbines employ steam that
enters the turbine at high temperature and pressure and
expands across both rotating and fixed blades. Only the
highest-quality lubricants are able to withstand the wet
conditions, high temperatures and long periods of service
associated with steam turbine operation. In gas turbines,
they must withstand contact with very hot surfaces, often
with intermittent operation and periods of nonuse.
Therefore, to be effective, both types of oil must have, in
addition to good corrosion protection and demulsibility,
outstanding resistance to oxidation, which includes a
minimum tendency to form deposits in critical areas of the
system.
To achieve these desired properties, it is necessary
to formulate these oils from specially refined base stocks
of the highest quality plus a carefully balanced additive
package. The nature of these fluids makes them very
CA 02276920 1999-07-06
susceptible r_o contamination, particularly rrom other
lubricants and additives. A relatively small degree of
contamination can markedly affect the properties and
expected service life of these lubricants. Further, to
maintain effective operating conditions and to avoid
damaging the equipment in which they are used, turbine oils
should be kept meticulously clean and free of contaminants.
Contamination is minimized by filtration of the turbine
oils. To ensure that the turbine oils are substantially
free of contaminants very fine filters are used.
Due to the requirements of turbine oils, only a few
classes of additives, relative to other types of
lubricating compositions, are combined with the base oils.
Generally, a finished turbine oil will contain only the
base oil, antioxidants, rust inhibitors, demulsifiers,
corrosion inhibitors and diluents, if necessary.
Prior art turbine oils contain acidic rust inhibitors.
For example, acidic rust inhibitors, of the type taught in
U.S. Patent No. 4,101,429, have been used in turbine oils.
Although, turbine oils containing acidic rust inhibitors
exhibit satisfactory rust performance, they tend to
interact with, for example, water and metal detergents
present as contaminants producing particulates,
precipitates and/or sludge. Acidic rust inhibitors thus
create problems with deposit formation and filterability
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upon exposure to contaminants such as water and/or metal
detergents. The resulting filterability problems and
deposit formation are expensive and highly undesirable.
It is an object of this invention to provide turbine
oils that exhibit good rust performance as well as good wet
filterability and good thermal stability upon exposure to
contaminants such as water and/or detergents. This
objective is obtained, in one embodiment of the present
invention, by the use of neutral rust inhibitors, in place
of acidic rust inhibitors, in preparing the finished
turbine oils. Accordingly, the present invention provides
a composition suitable for use as a turbine or rust and
oxidation (R & 0) oil comprising a major portion of a base
oii and (A) at least one neutral rust inhibitor, wherein
the base oil has a viscosity index of greater than 80, a
saturates content of greater than 90 wt,, and a sulfur
content of 0.5 wt's or less. Those skilled in the art will
appreciate that such oils are commonly referred to as Group
II, Group III or Group IV oils. The viscosity index is
assessed in accordance with IP 226. The saturates and
sulfur content are assessed by mass spectroanalysis.
The term "neutral rust inhibitors", in the present
specification, means rust inhibitors that are essentially
free of a -COOH functional group.
The term "major portion" means that the composition
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contains at least 50'.'. by weight base oil.
In another embodiment of the present invention, a
combination of neutral rust inhibitors) and a compound (B)
of formula:
//O //O
Z-HC -C~ Z-HC -C -NH2
/ NH
HZC- C\ H2C-C ~ NH2
O O
in which Z is a group R,R~CH-, in which R, and R., are each
independently hydrocarbon groups containing from 1 to 34
carbon atoms, the total number of carbon atoms in the
groups R; and R~ being from 11 to 35, are added to the base
oil in order to provide a turbine oil which ensures good
rust performance, good wet filterability and good
performance in thermal stability tests where water and/or
metal detergents are present, e.g. the ASTM D 2619 and ASTM
D 4310 tests.
Unless otherwise stated all hydrocarbyl groups and
moieties may be straight- or branched-chain.
In another embodiment of the present invention,
turbine oils are produced which are substantially free of
acidic rust inhibitors and/or metal detergents. For
purposes of the present invention, the term "substantially
free" means that no acidic rust inhibitors or metal
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detergents are purposefully added to the finished oil
although there may be some present due to contamination or
as an impurity.
Preferably, the at least one neutral rust inhibitor is
a hydrocarbyl ester of formula R (COOR')~, in which R and
R' are each independently hydrocarbyl groups, or
hydroxyhydrocarbyl groups, containing upto about 40 carbon
atoms, preferably 8 to 20 carbon atoms, and n is 1 upto
about 5.
It will be appreciated that the maximum number of
groups COOR' which are present on the hydrocarbyl or
hydroxyhydrocarbyl group R will vary depending on the
number of carbon atoms in R. For example, if R is a
hydrocarbyl group containing only one carbon atom, the
maximum possible value of n will be 3. When R is a
hydroxyhydrocarbyl group containing one carbon atom the
maximum value of n will be 2.
The esters contain at least one, and preferably from 1
to 5 hydroxy groups in the molecule. The hydroxy groups
may all be attached to the group R or to the group R' or
they may be attached to R and R' in varying proportions.
Further, the hydroxy groups can be at any position or
positions along the chain of R or R'.
The hydrocarbyl esters can be prepared by conventional
esterification procedures from a suitable alcohol and an
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acid, acid halide, acid anhydride or mixtures thereof.
Also, the esters of the invention can be prepared by
conventional methods of transesterification. By
"essentially free", it is meant that the starting acids,
acid halides, acid anhydrides or mixtures thereof used in
preparing the neutral rust inhibitors are reacted with an
amount of alcohol sufficient to theoretically convert all
of the -COOH groups to esters. Typically, the neutral rust
inhibitor will have a TAN of less than lOmgKOH/g. Preferred
esters include, but are not limited to, octyloleyl malate,
dioleylmalate, pentaerythritol monooleate and glycerol
monooleate.
Another class of preferred neutral rust inhibitors
includes aspartic acid diesters of 1-(2-hydroxyethyl)-2-
heptadecenyl imidazoline. This imidazoline is primarily a
mixture of diester of L-aspartic acid and an imidazoline
based on the reaction between oleic acid and
aminoethanolamine. Esters of this type are commercially
available from Mona Industries, Inc. as Monacor'N 39.
In compound (B) the radical Z may be, for example, 1-
methylpentadecyl, 1-propyltridecenyl, 1-pentyltridecenyl,
1-tridecenylpentadecenyl or 1-tetradecyleicosenyl.
Preferably, the number of carbon atoms in the groups R, and
RZ is from 16 to 28 and more commonly 18 to 24. It is
especially preferred that the total number of carbon atoms
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in R, and R is about 20 to 22. The preferred compound (B)
is 3-C18_~4 alkenyl-2, 5-pyrrolidindione, i . a . a compound in
which the average number of carbon atoms in the alkenyl
group is from 18 to 24.
In one aspect of the invention, the compound (B) has a
titratable acid number (T.AN) of about 80 to about 140
mgKOH/g, preferably about 110mgKOH/g. The TAN is
determined in accordance with ASTM D 664.
The compounds (B) are commercially available or may be
made by the application or adaptation of known techniques
(see for example EP-A-0389237).
Lubricating oils contemplated for use in this
invention include natural lubricating oils, synthetic
lubricating oils and mixtures thereof. Suitable
lubricating oils also include basestocks obtained by
isomerization of synthetic wax and slack wax, as well as
basestocks produced by hydrocracking (rather than solvent
extracting) the aromatic and polar components of crude oil.
In general, both the natural and synthetic lubricating oils
will each have a kinematic viscosity ranging from about 1 x
10-'~ m' / s to about 4 0 x 10-'~ m-/ s ( about 1 to about 4 0 cS t ) at
100° C, although typical applications will require each oil
to have a viscosity ranging from about 2 x 10-b m2/s to about
8 x 10-b ml/s (about 2 to about 8 cSt) at 100° C.
Natural base oils include animal oils, vegetable oils
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_g_
(e. g., castor oil and lard oil), petroleum oils, mineral
oils, and oils derived from coal or shale. The preferred
natural base oil is mineral oil.
The mineral oils useful in this invention include all
common mineral oil base stocks. This would include oils
that are naphthenic or paraffinic in chemical structure.
Oils that are refined by conventional methodology using
acid, alkali, and clay or other agents such as aluminum
chloride, or they may be extracted oils produced, for
example, by solvent extraction with solvents such as
phenol, sulfur dioxide, furfural, dichlordiethyl ether,
etc. They may be hydrotreated or hydro-refined, dewaxed by
chilling or catalytic dewaxing processes, or hydrocracked.
The. mineral oil may be produced from natural crude sources
or be composed of isomerized wax materials or residues of
other refining processes.
Typically the mineral oils will have kinematic
viscosities of from 2 x 10-'' m'/s to 12 x 10-' m-/s (2 cSt to
12 cSt) at 100°C. The preferred mineral oils have
kinematic viscosities of from 3 x 10- m=/s to 10 x 10-' m=/s
(3 to 10 cSt), and most preferred are those mineral oils
with viscosities of 5 x 10-6 m2/s to 9 x 10-b m'/s (5 to 9
cSt) at 100°C.
Synthetic lubricating oils useful in this invention
include hydrocarbon oils and halo-substituted hydrocarbon
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_q_
oils such as oligomerized, polymerized, and
interpolymerized olefins [e. g., polybutylenes,
polypropylenes, propylene, isobutylene copolymers,
chlorinated polylactenes, poly(1-hexenes), poly(1-octenes),
and mixtures thereof]; alkylbenzenes [e. g., polybutylenes,
polypropylenes, propylene, isobutylene copolymers,
chlorinated polylactenes, poly(1-hexenes), poly (1-octenes)
and mixtures thereof]; alkylbenzenes [e. g., dodecyl-
benzenes, tetradecylbenzenes, dinonyl-benzenes and di(2-
ethylhexyl)benzene]; polyphenyls [e. g., biphenyls,
terphenyls, alkylated polyphenyls]; and alkylated diphenyl
ethers, alkylated diphenyl sulfides, as well as their
derivatives, analogs, and homologs thereof, and the like.
The preferred synthetic oils are oligomers of a-olefins,
particularly oligomers of 1-decene, also known as polyalpha
olefins or PAO' s .
Synthetic lubricating oils also include alkylene oxide
polymers, interpolymers, copolymers, and derivatives
thereof where the terminal hydroxyl groups have been
modified by esterification or etherification. This class
of synthetic oils is exemplified by: polyoxyalkylene
polymers prepared by polymerization of ethylene oxide or
propylene oxide; the alkyl and aryl ethers of these
polyoxyalkylene polymers (e. g., methyl-polyisopropylene
glycol ether having an average molecular weight of 1000,
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diphenyl ether of polypropylene glycol having a molecular
weight of 100-1500); and mono- and poly-carboxylic esters
thereof (e. g., the acetic acid esters, mixed C,-Cefatty acid
esters, and C,~oxo acid diester of tetraethylene glycol).
Another suitable class of synthetic lubricating oils
comprises the esters of dicarboxylic acids (e. g., phthalic
acid, succinic acid, alkyl succinic acids and alkenyl
succinic acids, malefic acid, azelaic acid, suberic acid,
sebacic acid, fumaric acid, adipic acid, linoleic acid
dimer, malonic acid, alkylmalonic acids and alkenyl malonic
acids) with a variety of alcohols (e. g., butyl alcohol,
hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol,
ethylene glycol, diethylene glycol monoethers and propylene
glycol). Specific examples of these esters include dibutyl
adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate,
dioctyl sebacate, diisooctyl azelate, diisodecyl azelate,
dioctyl isophthalate, didecyl phthalate, dieicosyl
sebacate, the 2-ethylhexyl diester of linoleic acid dimer,
and the complex ester formed by reacting one mole of
sebacic acid with two moles of tetraethylene glycol and two
moles of 2-ethyl-hexanoic acid. A preferred type of oil
from this class of synthetic oils are adipates of C9 to C12
alcohols.
Esters useful as synthetic lubricating oils also
include those made from CS to Cl~ monocarboxylic acids and
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polyols and polyol ethers such as neopentyl glycol,
trimethylolpropane pentaerythritol, dipentaerythritol and
tripentaerythritol.
Silicon-based oils (such as the polyalkyl-, polyaryl-,
polyalkoxy-, or polyaryloxy-siloxane oils and silicate
oils) comprise another useful class of synthetic
lubricating oils. These oils include tetra-ethyl silicate,
tetraisopropyl silicate, tetra-(2-ethylhexyl) silicate,
tetra-(4-methyl-2-ethylhexyl) silicate, tetra-(p-tert-
butylphenyl) silicate, hexa-(4-methyl-2-pentoxy)-
disiloxane, poly(dimethyl)-siloxanes and poly
(methylphenyl) siloxanes. Other synthetic lubricating oils
include liquid esters of phosphorus containing acids (e. g.,
tricresyl phosphate, trioctylphosphate, and diethyl ester
of decylphosphonic acid), polymeric tetra-hydrofurans and
poly-a-olefins.
The lubricating base oils may be derived from refined,
re-refined oils, or mixtures thereof. Unrefined oils are
obtained directly from a natural source or synthetic source
(e. g., coal, shale, or tar sands bitumen) without further
purification or treatment. Examples of unrefined oils
include a shale oil obtained directly from a retorting
operation, a petroleum oil obtained directly from
distillation, or an ester oil obtained directly from an
esterification process, each of which is then used without
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further treatment. Refined oils are similar to the
unrefined oils except that refined oils have been treated
in one or more purification steps to improve one or more
properties. Suitable purification techniques include
distillation, hydrotreating, dewaxing, solvent extraction,
acid or base extraction, filtration, and percolation, all
of which are known to those skilled in the art. Re-refined
oils are obtained by treating used oils in processes
similar to those used to obtain the refined oils. These
re-refined oils are also known as reclaimed or reprocessed
oils and are often additionally processed by techniques for
removal of spent additives and oils breakdown products.
White oils, as taught in U.S. 5,736,490 may also be used as
the base oil for the turbine and R&0 oil.
The base oils have a viscosity index (VI) of greater
than 80, a saturates content of greater than 90 wt~ and
sulfur content of 0.5 wt's, or less. In a preferred
embodiment the oils have a sulfur content of 0.3 wt, or
less, more preferably 0.1 wt'~ or less. The preferred base
oils for use in the present invention are the
hydroprocessed and/or iso-dewaxed mineral oil, synthetic
oils and mixtures thereof.
The turbine and R&O oils of the present invention may
be prepared by simple blending of the various components
with a suitable base oil.
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For the sake of convenience, and in a preferred
embodiment of the present invention, the additive
components) used in practice of this invention may be
provided as a concentrate for formulation into a turbine or
R&O oil ready for use. Concentrates of the present
invention, containing neutral rust inhibitor(s), but no
compound (B), are typically added to the base oil at a
treat rate of 0.7 to about 2o by weight based on the weight
of the finished oil. Concentrates of the present invention
containing both neutral rust inhibitors) and compound (B)
tend to impart greater rust protection in the presence of
sea water (ASTM D665B) and are typically added to the base
oil at a treat rate of from about 0.3 to about 2-: by weight
based on the weight of the finished oil.
When the neutral rust inhibitors) are used, without
the addition of compound (B), they are generally present in
the additive concentrate in an amount of from about 10 to
about 60 percent by weight, based on the total weight of
the concentrate. When used in combination with compound
(B), the neutral rust inhibitors) are generally present in
the additive concentrate in an amount of from 10 to 60
percent by weight, based on the total weight of the
concentrate, while compound (B) is generally present in the
additive concentrate in an amount of from about 1 to 15
percent by weight. The concentrate may comprise, in
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addition to the fluid components, a solvent or diluent for
the fluid components. The solvent or diluent should be
miscible with and/or capable of dissolving in the turbine
base oil to which the concentrate is to be added. Suitable
solvents and diluents are well known. The solvent or
diluent may be the turbine base oil itself. The
concentrate may suitably include any of the conventional
additives used in turbine oils. The proportions of each
component of the concentrate are controlled by the intended
degree of dilution, though top treatment of the formulated
fluid is possible.
Whether added directly tc the base oil, or in the form
of a concentrate, the neutral rust inhibitors) should be
present in the finished oil in an amount of at least about
0.10, and preferably from 0.10 to about 0.45 percent by
weight. Whether added directly to the base oil, or in the
form of a concentrate, compound (B), if used, should be
present in the finished oil in an amount of about 0.008 to
about 0.25 percent by weight.
The additive concentrates and finished oils of the
present invention may further contain additional additives
such as phosphorus-containing additives and sulfurized
esters. Preferred phosphorus containing additives include
amine salts of acid phosphates and phosphorus and sulfur
containing compounds.
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Other additives commonly used in turbine and R&0 oils
may be included in the turbine and R&O oils of the present
invention. These include antioxidants, demulsifiers and
corrosion inhibitors. These additives, when present, are
used in amounts conventionally used in turbine oil
packages.
The invention will now be illustrated by the following
Examples that are not intended to limit the scope of the
invention in any way.
EXAMPLES
In Table l, the formulations for various turbine oil
concentrates are set forth. Turbine Oil Concentrates 1-4
represent formulations within the scope of the present
invention, i.e., they contain neutral rust inhibitors and
are substantially free of acidic rust inhibitor. Turbine
Oil Concentrate 5 represents an additive concentrate
outside of the scope of the present invention in that it
contains an acidic rust inhibitor. All of the samples used
similar conventional additives (e. g., antioxidants,
demulsifiers and corrosion inhibitors) in similar amounts.
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Table 1: Turbine Oil Concentrates
Turbine Turbine Turbine Turbine Turbine
Oil 1 Oil 2 Oil 3 Oil 4 Oil 5
(TO1) (T02) (T03) (T04) (T05)
( Com-
parative)
Rust Inhibitor 21.25
V
Rust Inhibitor 0.50
W
Rust Inhibitor 22.50 22.50
X
Rust Inhibitor 20.00
Y
Rust Inhibitor 12.00
Z
Compound B 5.00
RI V: Glycerol monooleate neutral rust inhibitor.
RI W: Ashless sulfonate neutral rust inhibitor.
RI X: Pentaerythritol monooleate neutral rust
inhibitor.
RI Y: Octyloleyl malate neutral rust inhibitor.
RI Z: An acidic rust inhibitor comprising the reaction
product of oleic acid, triethylene
tetramine and malefic anhydride substituted by a
C12 alkenyl group of the kind described in U.S.
Patent No. 4,101,429.
Comp. B: 3-C,a_~4 alkenyl-2, 5-pyrrolidindione.
Turbine oils are prepared by adding additive
concentrates as described above to base oils of various
viscosities at a treat rate of 0.8 percent by weight. The
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base oil used was a hydro-processed (HP) mineral oils
having a VI of at~least 98, a saturates level of at least
98o and a sulfur content of less than 0.01 wt~. A solvent
refined (SR) base oil was also used. The finished oils
were tested for wet filterability using the Shell
Filtration Test and for rust performance using the ASTM D
665B rust test.
The Shell Filtration Test is intended to evaluate the
filterability characteristics of oil based hydraulic fluids
with and without calcium and/or water contamination. The
fluids as blended and the contaminated fluids are each
tested in duplicate as follows. After pre-treatment at 70
°C, 300 ml of test oil are filtered through a 1.2 micron
Millipore membrane using a 650 mm Hg vacuum. The fluid
temperature is not controlled but should be in the range of
19 to 26 "C. The times for each successive 100 ml of fluid
to filter, or for the filter membrane to block, are noted.
In the following Tables the results of the Shell Filtration
Test are indicated as either PASS, meaning that all 300 ml
of oil passed through the filter, or FAIL, meaning that the
filter became blocked.
The results of the Shell Filtration Test and the ASTM
D 665B rust test are set forth below in Table 2.
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Table 2: Shell Filtration and ASTM D 6658 Rust Test
results
Additive Base Oil Shell Filtration D665B rust test
Concentrate - ISO # Test
T04 HP - 32 PASS PASS
T05 SR - 32 FAIL PASS
T05 SR - 46 FAIL PASS
T04 HP - 68 PASS PASS
T05 SR - 68 FAIL PASS
T04 HP - 100 PASS PASS
It is clear from the above Table that compositions of
the present invention exhibit both passing Shell Filtration
results and ASTM D 6658 results. Further, it is clear from
the above Table 2 that turbine oils (T05) containing
sufficient amounts of acidic rust inhibitor to pass the
ASTM D 6658 rust test fail the Shell Filtration Test.
In handling, i.e., storing and transporting, of
turbine oils, the turbine oil often comes into contact with
residual lubricating fluids containing acidic rust
inhibitors and/or metal detergents or turbine oils
containing acidic rust inhibitors. The turbine oils of the
present invention enable passing Shell Filtration Test
results upon contamination with these sources of acidic
rust inhibitors and/or metal detergents.
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For some applications, oxidation performance in the
presence of water. is desired along with acceptable rust
test performance. Turbine oils containing components (A)
and (B) of the present invention have been found to exhibit
excellent thermal stability in tests with water present and
passing rust test performance. ASTM D 4310 is used to
determine the tendency of inhibited mineral oils,
especially turbine oils, to form sludge during oxidation in
the presence of oxygen, water, and copper and iron metals
at an elevated temperature. In this test, an oil sample is
reacted with oxygen in the presence of water and an iron-
copper catalyst coil for 1000 hours. The oil is then
analysed to determine the total acid number (TAN), the
weight of sludge and loss of copper and iron from the
catalyst. Table 3 shows the hydrolytic stability in the
presence of water of turbine oils containing the
combination of additives (A) and (B) of the present
invention.
The formulated oil included pentaerythritol monooleate
at a concentration of 0. 18 wt 'r and 3-C,.__; alkenyl-2, 5-
pyrrolidindione at a concentration of 0.04 wt«.. The base
oil was a hydro-processed basestock having a viscosity
index of 99, a saturates content of 99.5 wt o and a sulfur
content of 0.02 wt o.
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Table 3
Sludge (mg) Copper wt. Iron wt. TAN
change (mg) change (mg) (mg/KOH g)
12.5 I 0.5 I 0.75 I 0.425
The results represent the average of 2 runs.
The additive combinations of the present invention are
especially effective in hydro-processed mineral oils. Table
4 demonstrates the exceptional properties of the additive
systems of the present invention in these hydro-processed
mineral oils. The following formulated oils contained
identical additive concentrates (T04). The solvent refined
mineral oils (SR-32 and SR-68) contained 0.82 wto of T04,
while the hydro-processed mineral oils (HP-32, HP-68 and HP-
100) contained 0.80 wt'~ of T04. The formulated oils were
tested in the Rotating Bomb Oxidation Test (RBOT) defined in
ASTM D-2272. The RBOT is a test for estimating the
oxidation stability of turbine oils. The test oil, water
and copper catalyst coil, contained in a covered glass
container, are placed in a bomb equipped with a recording
pressure gauge. The bomb is charged with oxygen to a
pressure of 620 kPa, placed in a constant-temperature oil
bath set at 150 "C, and rotated axially at 100 rpm at an
angle of 30" from the horizontal. The number of minutes
required to reach a specific drop in gauge pressure is the
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oxidation stability of the test sample.
The formulated oils containing solvent refined
basestocks were also tested in the Lifetime Turbine Oil
Oxidation Test (Life TOST) as defined in ASTM D-943. The
Life TOST is used to evaluate the oxidation stability of
inhibited steam turbine cils. In the Life TOST, the oil
sample is reacted with oxygen in the presence of water and
an iron-copper catalyst at 95 "C. The test continues until
the measured total acid number of the oil is 2.0 mg KOH/g.
The number of test hours required for the oil to reach 2.0
mg KOH/g is the ~~oxidation lifetime". The RBOT for base oil
alone would be in the region of 15-30 minutes.
Table 4
Base oil RBOT (minutes) Life TOST (hours)
SR-32 602.5 6188
(avg. of 2 runs)
HP-32 1199
(avg. of 3 runs)
SR-68 707.5 6032
(avg. of 2 runs)
HP-68 1292
(avg. of 3 runs)
HP-100 1253
(av . of 3 runs)
It is clear from Table 4 that the turbine oils prepared
from hydro-processed mineral oils exhibit superior oxidation
CA 02276920 1999-07-06
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stability, compared to solvent refined mineral oils, as
evidenced by the increased (nearly doubled) length of the
RBOT for the turbine oils prepared from hydro-processed
mineral oils. As is readily apparent from Table 4, the Life
TOST for the turbine oils prepared from solvent refined
mineral oils was over 250 days. Due to the extremely long
test time for these oils the RBOT may be used as a tool to
predict Life TOST results. In the paper Mookken, R.T. et
al., Dependence of Oxidation Stability of Steam Turbine Oil
on base Oil Composition, Lubrication Engineering, October
1997, pages 19-24, it was shown that the RBOT can be used as
a screening test to get an indication of the TOST life of
the blended turbine oil. The study shows that the RBOT and
Life TOST are directly proportional. Thus, it is clear from
Table 4 that the turbine oils prepared from hydro-processed
mineral oils will exhibit superior oxidation stability as
determined by Life TOST in view of their significantly
longer RBOT life.
