WO2007131891A1

WO2007131891A1 - Lubricating oil composition

Info

Publication number: WO2007131891A1
Application number: PCT/EP2007/054321
Authority: WO
Inventors: Mitsuhiro Nagakari; Peter William Robert Smith
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2006-05-15
Filing date: 2007-05-03
Publication date: 2007-11-22
Also published as: JP2009537642A; EP2021440A1; US20090186789A1; CN101466818A; CN101466818B; EP2021440B1; JP5527758B2

Abstract

The present invention relates to a lubricating oil composition comprising lubricating oil base oil in combination with: (a) one or more substituted phenyl -naphthylamines; (b) one or more anti-wear additives, preferably ashless thiophosphates ; and (c) an additional base oil component selected from bright stock oil, polyalkylene glycols (PAGs) , alkyl naphtalenes, alkyl benzenes and natural and synthetic esters such as diesters and polyolesters, preferably bright stock oil. In another aspect the present invention provides the use of the lubricating oil composition to lubricate a combination of a gas turbine and a steam turbine.

Description

LUBRICATING OIL COMPOSITION

The present invention relates to lubricating oil compositions, more specifically to lubricating oil compositions for lubricating turbines and in particular, to lubricating oil compositions for lubricating a combination of a steam turbine and a gas turbine.

The use of a combination of a gas turbine and a steam turbine for power generation, has the advantage that it is more efficient than power generation with either type of turbine. The combination will hereinafter also be referred to as "combined cycle" .

A difficulty of lubricating a combined cycle resides in the large variety of different components which must be lubricated, i.e. bearings (both journal and thrust), gears, hydraulic control systems, flexible couplings and oil shaft seals. Although each component could be lubricated per se, it is advantageous to have a single, common lubricating system containing a single, common lubricant for all components .

The lubricating oil composition for use in the common system has to meet a rather outstanding combination of requirements in order to satisfactorily lubricate each component. These requirements comprise the rather severe thermal and oxidative stability requirements and stringent foaming levels for the gas turbine part, whereas the steam turbine part requires the oil to have good water shedding properties and excellent corrosion resistance.

High temperature oxidative stability means that the lubricating oil composition has a low tendency to form sludge, a low increase in viscosity and a low increase in total acid number at high temperature. Desirable characteristics are a viscosity increase of at most 20 %, a total acid number increase of at most 3.0 mg KOH/g and a sludge content of less than 300 mg/lOO ml after having been subjected to the oxidation stability test DIN 51394 performed according to the high temperature modifications set out in the General Electric specifications

GEK 32568 C and GEK 101941, preferably less than 250 tnl/g, more preferably less than 200 ml/g, most preferably less than 150 ml/g. In establishing the amount of sludge produced, the sludge must be removed carefully from all the equipment used in the test. The sludge is separated from the oil by filtration.

The tendency of a lubricating oil composition to form sludge is an important factor in whether it will be suitable for use in lubricating combined cycle equipment. A lubricating oil composition for use in combined cycle equipment must further have a certain viscosity index, pour point, filterability and anti-wear performance, while the composition should provide adequate lubrication over many years. Achieving this combination of complex lubricant properties allows the lubricating oil composition to be suitable for use in combined cycle equipment.

However, in addition to sludge formation, the degradation of lubricating oil compositions such as turbine oils in service can also result in lacquers and other deposits. Such lacquers and deposits also shorten the life of lubricating oil compositions such as turbine oils, thereby reducing the service interval of a turbine or resulting in expensive and unplanned turbine shutdowns .

Depending on turbine type and operating conditions, the typical lifetime of a turbine oil can be approximately 5 to 10 years or more. Thus, in addition to sludge reduction, it is important that the formation of lacquers and other deposits are also reduced. Although industrial standard oil performance tests are able to indicate the probability of a lubricating oil composition forming oxidative sludges, these tests are not able to predict accurately the likelihood of a lubricating oil composition forming lacquers in service.

Herein a predictive lacquer formation test for lubricating oil compositions such as turbine oils is disclosed which is a modified version of the Wolf Strip test as per ex. DIN 51392. EP-A- 0696636 describes a lubricating oil composition for use in internal combustion engines which comprises a combination of (A) a lubricating base oil, (B) one or more alkyl diphenylamines and/or one or more phenyl-α- naphthylamines and (C) oxymolybdenum sulfide dithiocarbamate and/or oxymolybdenum sulfide organophosphorodithioate .

In the examples of EP-A- 0696636, either a single alkyl diphenylamine or a single phenyl-α-naphthylamine is used. Oxymolybdenum sulfide organophosphorodithioate is present in only one of the examples according to the teaching. However, EP-A-0696636 is not concerned with reducing lacquer formation in turbines and does not disclose or teach any lubricating oil compositions for this purpose. GB-B-0990097 discloses a lubricating oil composition for use in aviation gas turbine engines which comprises blend of synthetic base oils and an aliphatic thiophosphate or a polymerised or sulphurised product thereof. Said composition may optionally comprise one or more anti-oxidant additives.

However, said document is not directed to the problem of reducing lacquer formation in combined cycle equipment and it does not disclose or teach the specific lubricating oil composition of the present invention. DE-A-1594405 describes lubricating oil compositions for steam turbines comprising mineral oil and (a) an aliphatic polycarboxylic acid containing at least 12 carbon atoms, (b) an alkylphenol, (c) an aromatic amine, and (d) a dialkyldithiophosphate .

DE-A-1594405 states that aromatic amines normally have a tendency to form sludge and have unsatisfactory light stability, whilst the lubricating oil compositions in accordance with DE-A-1594405 have better stability. The aromatic amine (c) in DE-A-1594405 may be a secondary aromatic amine such as diphenylamine , phenyl- alpha- or -beta-naphthylamine and alkyl substituted analogues such as p,p' -dioctyldiphenylamine .

WO-A-99/43770 discloses a lubricating oil composition for use in a combined cycle which comprises a hydrocarbon lubricant base oil in combination with (a) a phenyl-naphthylamine, (b) a thiophosphate and (c) a diphenyl amine .

In the examples of WO-A-99/43770, a combination of non-substituted phenyl-alpha-naphthylamine and an amine salt of a dialkyldithiophosphate are used in conjunction with additional amine antioxidant.

WO-A-99/43770 discloses a lubricating oil composition which exhibits an especially low tendency to form sludge at high temperatures. However, WO-A-99/43770 does not discuss the reducing lacquer formation in combined cycle equipment.

There has now been surprisingly found a lubricating oil composition comprising a specific combination of components, which composition not only meets the requirements for use in a combined cycle as disclosed in WO-A-99/43770 but which lubricating oil composition also exhibits low lacquer formation. Accordingly, the present invention provides a lubricating oil composition comprising lubricating oil base oil in combination with:

(a) one or more substituted phenyl-naphthylamines ; (b) one or more anti-wear additives; and

(c) an additional base oil component selected from bright stock oil, polyalkylene glycols (PAGs) , alkyl naphtalenes, alkyl benzenes and natural and synthetic esters such as diesters and polyolesters, preferably bright stock oil.

The one or more substituted phenyl-naphthylamines (a) which are used in the present invention, can be used as such or in the form of a salt. Examples of such substituted phenyl-naphthylamines are substituted phenyl- alpha-naphthylamines and substituted phenyl-beta- naphthylamines . The one or more substituted phenyl- naphthylamines are preferably substituted phenyl-alpha- naphthylamines .

Preferred substituted phenyl-naphthylamines are alkylated phenyl-alpha-naphthylamines and/or alkylated phenyl-beta-naphthylamines . More preferably, said substituted phenyl-naphthylamines are mono-alkylated phenyl-alpha-naphthylamines and/or mono-alkylated phenyl- beta-naphthylamines . Particularly preferred mono-alkylated phenyl-alpha- naphthylamines and mono-alkylated phenyl-beta- naphthylamines are those wherein the alkyl substituents are straight chain or branched alkyl groups having in the range of from 1 to 18 carbon atoms. Examples of substituted phenyl-naphthylamines that may be conveniently used include octylphenyl-beta- naphthylamine, t-octylphenyl-alpha-naphthylamine, p-octylphenyl-alpha-naphthylamine, 4-octylphenyl-l-octyl- beta-naphthylamine, n-t-dodecylphenyl-1-naphthylamine, N-hexylphenyl-2 -naphthylamine. A particularly preferred substituted phenyl-alpha-naphthylamine is a monooctylated phenyl alpha-naphthylamine .

Substituted phenyl-naphthylamines as commercially available can be used in the present invention. An example of a commercially available substituted phenyl- naphthylamine that may be conveniently used in the lubricating oil composition of the present invention is that available from Ciba Specialty Chemicals under the trade designation "IRGANOX L-06" . As anti-wear additives prefereably one or more ashless thiophosphates are used. Ashless thiophosphates are known in the art. These compounds are metal-free organic compounds .

The one or more ashless thiophosphates (b) present in the lubricating oil composition of the present invention may be esters and/or salts of thiophosphoric acids, substituted thiophosphoric acids. Preferably, the ashless thiophosphates are substituted by one or more hydrocarbyl groups which hydrocarbyl groups can optionally contain an acid, a hydroxy and/or an ester group. The hydrocarbyl moiety preferably is an alkyl containing up to 12 carbon atoms . The hydrocarbyl substituted thiophosphate preferably contains 2 or 3 hydrocarbyl groups, or is a mixture of thiophosphates containing 2 and 3 hydrocarbyl groups .

The ashless thiophosphates can contain any number of sulphur atoms directly linked to the phosphorus atom. Preferably, the thiophosphates are monothiophosphates and/or dithiophosphates . Examples of ashless thiophosphates which may be conveniently used in the lubricating oil composition of the present invention are described in EP-A-0375324 , US-A-5922657, US-A-4333841 and US-A-5093016 and may be conveniently made according to the methods described therein. _^

The one or more ashless thiophosphates (b) present in the lubricating oil composition of the present invention are preferably compounds according to formula (I)

(D

wherein R¹ and R² are each, independently, selected from hydrogen, branched or straight chain Ci to Ci₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms, C₅ to Ci₂ cycloalkyl groups, C₅ to C₉ cycloalkylmethyl groups, C₉ to Ci₀ bicycloalkylmethyl groups, C₉ to Ci₀ tricycloalkylmethyl groups, phenyl or C₇ to C₂4 alkylphenyl groups or R¹ and R² together may be a C₂ to Ci₂ alkylene group which may be interrupted by 1 or 2 oxygen atoms, R³ and R⁴ are each, independently, selected from hydrogen and branched or straight chain C₁ to C₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms, n is an integer from 1 to 5 and X is selected from -OH, -OR⁵ wherein R⁵ is selected from branched or straight chain alkyl groups having from 1 to 8 carbon atoms, -NR⁶R⁷ wherein R⁶ and R⁷ are independently selected from hydrogen or C₁ to Ci₈ alkyl groups .

Preferably, R¹ and R² are each, independently, selected from hydrogen, branched or straight chain C₃ to Ci₈ alkyl groups, C₅ to C_i2 cycloalkyl groups, C₅ to C₉ cycloalkylmethyl groups, C₉ to Ci₀ bicycloalkylmethyl groups, C₉ to Ci₀ tricycloalkylmethyl groups, phenyl or C₇ to C₂₄ alkylphenyl groups or R¹ and R² together may be a C₂ to C₁₂ alkylene group which may be interrupted by 1 or 2 oxygen atoms .

More preferably, R¹ and R² are each, independently, selected from hydrogen, branched or straight chain C₃ to C₁₈ alkyl groups selected from propyl, isopropyl, n- butyl, isobutyl, t-butyl, pentyl, isopentyl, hexyl, heptyl, 3-heptyl, octyl, 2-ethylhexyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, 2-ethylbutyl, 1- methylpentyl , 1 , 3-dimethylbutyl, 1,1,3,3- tetramethylbutyl, 1-methylhexyl, isoheptyl, 1- methylheptyl , 1, 1, 3-trimethylhexyl and 1-methylundecyl, phenyl and C₇ to C₂4 alkylphenyl groups selected from methylphenyl, dimethylphenyl , trimethylphenyl, ethylphenyl, isopropylphenyl , t-butylphenyl, di-t- butylphenyl and 2, 6-di-t-butyl-4 -methylphenyl .

With regard to the CR³R⁴ repeating unit in the compound of formula (I) , R³ and R⁴ groups in each repeating unit may be the same or different from R³ and R⁴ groups in other repeating units.

In a preferred embodiment, R³ and R⁴ are each selected, independently, from hydrogen, methyl and ethyl.

In a more preferred embodiment of the present invention, the one or more ashless thiophosphates (b) present in the lubricating oil composition of the present invention are preferably compounds according to formula (II)

:ii) wherein R⁸ and R⁹ are each, independently, selected from hydrogen, branched or straight chain Ci to Ci₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms, C₅ to C₁₂ cycloalkyl groups, C₅ to C₉ cycloalkylmethyl groups, C₉ to Ci₀ bicycloalkylmethyl groups, C₉ to Ci₀ tricycloalkylmethyl groups, phenyl or C₇ to C₂₄ alkylphenyl groups or R⁸ and R⁹ together may be a C₂ to Cχ₂ alkylene group which may be interrupted by 1 or 2 oxygen atoms, R¹⁰ is selected from hydrogen and branched or straight chain Ci to C₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms .

Preferably, R⁸ and R⁹ are each, independently, selected from hydrogen, branched or straight chain C₃ to Ci₈ alkyl groups, C₅ to C₁₂ cycloalkyl groups, C₅ to C₉ cycloalkylmethyl groups, C₉ to Ci₀ bicycloalkylmethyl groups, C₉ to Ci₀ tricycloalkylmethyl groups, phenyl or C₇ to C₂₄ alkylphenyl groups or R⁸ and R⁹ together may be a C₂ to Ci₂ alkylene group which may be interrupted by 1 or 2 oxygen atoms . More preferably, R⁸ and R⁹ are each, independently, selected from hydrogen, branched or straight chain C₃ to Cis alkyl groups selected from propyl, isopropyl, n- butyl, isobutyl, t-butyl, pentyl, isopentyl, hexyl, heptyl, 3-heptyl, octyl, 2-ethylhexyl , nonyl , decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, 2-ethylbutyl, 1- methylpentyl, 1, 3-dimethylbutyl, 1,1,3,3- tetramethylbutyl, 1-methylhexyl, isoheptyl, 1- methylheptyl , 1, 1, 3-trimethylhexyl and 1-methylundecyl, phenyl and C₇ to C₂₄ alkylphenyl groups selected from methylphenyl , dimethylphenyl, trimethylphenyl , ethylphenyl, isopropylphenyl, t-butylphenyl, di-t- butylphenyl and 2, 6-di-t-butyl-4 -methylphenyl .

In a preferred embodiment, R¹⁰ is selected from hydrogen, methyl and ethyl. Examples of commercially available ashless thiophosphates that may be conveniently used in the lubricating oil composition of the present invention include those available from Ciba Specialty Chemicals under the trade designations "IRGALUBE L-63" and

"IRGALUBE 353" and that available from Lubrizol under the trade designation "LZ 5125" .

The amount of the additives (a) and (b) present in the lubricating oil composition of the present invention, depends on the specific compounds used therein.

The one or more substituted phenyl-naphthylamines (a) are preferably present in the lubricating oil composition of the present invention in a total amount in the range of from 0.01 to 3.0 wt . %, more preferably in the range of from 0.01 to 1.0 wt . %, based on the total weight of the lubricating oil composition.

The one or more ashless thiophosphates (b) are preferably present in the lubricating oil composition of the present invention in a total amount in the range of from 0.01 to 1.0 wt. %, more preferably in the range of from 0.01 to 0.1 wt . %, based on the total weight of the lubricating oil composition.

The lubricating oil base oil in the lubricating oil composition of the present invention may be selected from mineral and/or synthetic lubricant base oils.

Said lubricating oil base oil is preferably present in an amount of at least 85 wt. %, more preferably at least 90 wt. %, based on the total weight of the lubricating oil composition. Mineral lubricant base oils that may be conveniently used include liquid petroleum oils and solvent treated or acid treated mineral lubricating oils of the paraffinic, naphthenic, or mixed paraffinic/naphthenic type which may be further refined by hydrocracking and hydrofinishing processes and/or dewaxing. Naphthenic base oils have low viscosity index (VI) (generally 40-80) and a low pour point. Such base oils are produced from feedstocks rich in naphthenes and low in wax content and are used mainly for lubricants in which colour and colour stability are important, and VI and oxidation stability are of secondary importance.

Paraffinic base oils have higher VI (generally >95) and a high pour point. Said base oils are produced from feedstocks rich in paraffins, and are used for lubricants in which VI and oxidation stability are important.

Fischer-Tropsch derived base oils may be conveniently used as the lubricating oil base oil in the lubricating oil composition of the present invention, for example, the Fischer-Tropsch derived base oils disclosed in EP-A-776959, EP-A-668342, WO-A-97/21788 , WO-00/15736, WO-00/14188, WO-00/14187, WO-00/14183, WO-00/14179, WO- 00/08115, WO-99/41332, EP-1029029, WO-01/18156 and WO- 01/57166.

Synthetic processes enable molecules to be built from simpler substances or to have their structures modified to give the precise properties required.

Synthetic lubricant base oils include hydrocarbon oils such as olefin oligomers (also known as polyalphaolefins (PAOs) ) . Synthetic hydrocarbon base oils sold by the Shell Group under the designation "XHVI" (trade mark) may be conveniently used.

Preferred lubricating oil base oils for use in the lubricating oil composition of the present invention are Group I, Group II or Group III base oils, polyalphaolefins, Fischer-Tropsch derived base oils and mixtures thereof .

By "Group I" base oil, "Group II" base oil and "Group III" base oil in the present invention are meant lubricating oil base oils according to the definitions of American Petroleum Institute (API) categories I, II and III. Such API categories are defined in API Publication 1509, 15^th Edition, Appendix E, April 2002.

Group I base oils contain less than 90 % saturates (according to ASTM D2007) and/or greater than 0.03 % sulphur (according to ASTM D2622, D4294, D4927 or D3120) and have a viscosity index of greater than or equal to 80 and less than 120 (according to ASTM D2270) .

Group II base oils contain greater than or equal to 90 % saturates and less than or equal to 0.03 % sulphur and have a viscosity index of greater than or equal to 80 and less than 120, according to the aforementioned ASTM methods .

Group III base oils contain greater than or equal to 90 % saturates and less than or equal to 0.03 % sulphur and have a viscosity index of greater than 120, according to the afore-mentioned ASTM methods.

As described in US-B1-6180575 and US-A-5602086 , polyalphaolefins and their manufacture are well known in the art. Preferred polyalphaolefins that may be used in lubricating oil compositions of the present invention may be derived from C₂ to C₃₂ alpha olefins.

Particularly preferred feedstocks for said polyalphaolefins are 1-octene, 1-decene, 1-dodecene and 1-tetradecene. Preferably, lubricating oil base oils that may be conveniently used in the lubricating oil compositions of the present invention have a kinematic viscosity at 100 ⁰C in the range of from 3 to 300 mm²/s, more preferably in the range of from 4 to 100 mm²/s. Preferably, the lubricating oil composition of the present invention has a kinematic viscosity in the range of from 32 to 150 mm²/s at 40 ⁰C, more preferably in the range of from 32 to 100 mm²/s, and most preferably in the range of from 32 to 68 mm²/s. Said additional base oil component is preferably present in the lubricating oil composition of the present invention in an amount in the range of from 0.01 to 10.0 wt. %, more preferably above 1.0 wt . %, even more preferred above 2.0 wt . % based on the total weight of the lubricating oil composition. It goes without saying that also two or more additional base oil components selected from bright stock oils, polyalkylene glycols (PAGs) , alkyl naphthalenes, alkyl benzenes and natural and synthetic esters such as diesters and polyol esters may be used, the combined amount then preferably being in the range of from 0.01 to 10.0 wt . % based on the total weight of the lubricating oil composition.

As an additional base oil component preferably bright stock oil is used.

Bright stock oil is known in the art and is described, for example in GB-A-1496045 and US-A-4592832. Bright stock oil is typically prepared by de-asphalting mineral-derived vacuum residue, for example, in the presence of propane. The resulting de-asphalted oil (DAO) then undergoes an upgrading process such as solvent extraction (or hydroprocessing) , for example with furfural or NMP, in order to extract de-asphalted cylinder oil (DACO) therefrom. The resulting bright stock waxy raffinate is then de-waxed utilising, for example, methyl ethyl ketone (MEK) and/or toluene to remove bright stock slack wax therefrom. The resulting bright stock oil may then undergo optional hydrofinishing.

Preferably, the bright stock oil has a kinematic viscosity at 100 ⁰C in the range of from 20 to 40 mm²/s, more preferably in the range of from 25 to 38 mm²/s, even more preferably in the range of from 30 to 34 mm²/s as determined according to ASTM D445. The lubricating oil composition of the present invention can further comprise one or more additives such as anti-oxidants, extreme pressure additives, other anti- wear additives than ashless thiophosphates, metal passivators such as copper passivators, corrosion inhibitors, foam inhibitors and/or demulsifiers .

In a preferred embodiment, the lubricating oil composition of the present invention comprises one or more diphenylamine antioxidants. The one or more diphenylamines can be substituted or non- substituted. It is preferred to use a hydrocarbyl substituted diphenylamine, more preferably an alkyl substituted diphenylamine. Preferred diphenylamines are monoalkyldiphenylamines, dialkyldiphenylamines and bis (dialkylphenyl) amines. The alkyl group preferably contains between 2 and 15 carbon atoms, more preferably between 5 and 12 carbon atoms.

Particularly preferred diphenylamines include dialkyldiphenylamines such as p,p' -dioctyl-diphenylamine, p,p' -di-α-methylbenzyl-diphenylamine and N-p-butylphenyl- N-p¹ -octylphenylamine, monoalkyldiphenylamines such as mono-t-butyldiphenylamine and mono-octyldiphenylamine, bis (dialkylphenyl) amines such as di-(2,4- diethylphenyl) amine and di (2-ethyl-4-nonylphenyl) amine . Examples of commercially available diphenylamines that may be conveniently used in the lubricating oil composition of the present invention include that available from Ciba Specialty Chemicals under the trade designation "IRGANOX L-57" . The one or more diphenylamines are preferably present in the lubricating oil composition of the present invention in a total amount in the range of from 0.01 to 3.0 wt. %, more preferably in the range of from 0.01 to 1.0 wt . %, based on the total weight of the lubricating oil composition. Anti-wear additives that may be conveniently used include zinc dithiophosphate compounds selected from zinc dialkyl-, diaryl- and/or alkylaryl- dithiophosphates, molybdenum-containing compounds, boron-containing compounds and ashless anti-wear additives such as substituted or unsubstituted thiophosphoric acids, and salts thereof..

Said anti-wear additives may each be conveniently added to the lubricating oil composition of the present invention in an amount in the range of from 0.1 to 3.0 wt. %, based on the total weight of lubricating oil composition.

Examples of such molybdenum-containing compounds may conveniently include molybdenum dithiocarbamates , trinuclear molybdenum compounds, for example as described in WO-A-98/26030, sulphides of molybdenum and molybdenum dithiophosphate .

Boron-containing compounds that may be conveniently used include borate esters, borated fatty amines, borated epoxides, alkali metal (or mixed alkali metal or alkaline earth metal) borates and borated overbased metal salts.

Compounds such as alkenyl succinic acid or ester moieties thereof, benzotriazole-based compounds and thiodiazole-based compounds may be conveniently used in the lubricating oil composition of the present invention as corrosion inhibitors.

Compounds such as polysiloxanes, dimethyl polycyclohexane and polyacrylates may be conveniently used in the lubricating oil composition of the present invention as foam inhibitors .

Demulsifiers which may be conveniently used in the lubricating oil composition of the present invention are polyalkylene glycol ethers or amines. - -

The amount of said additives to be present in the lubricating composition, depends on the specific compounds used.

The lubricating oil composition of the present invention preferably comprises in the range of from 0.01 to 3.0 wt . %, more preferably in the range of from 0.01 to 1.0 wt . % of one or more substituted phenyl- naphthylamines (a), in the range of from 0.01 to 1.0 wt. %, more preferably in the range of from 0.01 to 0.1 wt. %, of one or more ashless thiophosphates (b) , at least 85 wt. % by weight of lubricating oil base oil, preferably mineral oil base oil; and optionally, wherein said lubricating oil base oil comprises an amount in the range of from 0.01 to 10.0 wt . % of an additional base oil component selected from bright stock oils, polyalkylene glycols (PAGs) , alkyl naphthalenes, alkyl benzenes and natural and synthetic esters such as diesters and polyol esters, all based on total weight of the lubricating oil composition. The present invention further relates to the use of the lubricating oil composition according to the invention to lubricate a combination of a gas turbine and a steam turbine, more preferably such combination utilising a pressurised-steam generator. Further, the present invention relates to a method of lubricating a combination of a gas turbine and a steam turbine utilising a pressurised-steam generator by employing the lubricating oil composition of the present invention. In addition, the present invention further provides the use of the lubricating oil composition of the present invention in order to reduce lacquer formation in a turbine, preferably in a combination of a gas turbine and a steam turbine and a method of reducing lacquer formation in a turbine, preferably in a combination of a - -

gas turbine and a steam turbine, by lubricating said turbine (s) with a lubricating oil composition according to the present invention.

The lubricating composition according to the present invention can be conveniently prepared by blending together one or more substituted phenyl-naphthylamines (a) , one or more ashless thiophosphates (b) , the lubricating oil base oil and, optionally an additional base oil component as hereinbefore described and/or one or more additives.

The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the invention in any way. EXAMPLES Formulations were blended using the base oils and additives specified in Tables 1 and 2.

The amounts in Tables 1 and 2 are in wt . %, based on the total weight of the formulations.

The substituted phenyl-naphthylamine (a) used in the formulations of Tables 1 and 2 was that available from Ciba Specialty Chemicals under the trade designation "IRGANOX L- 06" .

The ashless thiophosphates (b) used in the formulations of Tables 1 and 2 are designated herein as "Ashless thiophosphate (b) -1" and "Ashless thiophosphate (b)-2". "Ashless thiophosphate (b) -1" and "Ashless thiophosphate (b) -2" are available from Ciba Specialty Chemicals under the trade designation "IRGALUBE 353" and from Lubrizol under the trade designation "LZ 5125" , respectively.

The diphenylamine antioxidant used in the formulations of Tables 1 and 2 was that available from Ciba Specialty Chemicals under the trade designation "IRGANOX L-57" . The formulations of Tables 1 and 2 also comprised an additive combination of conventional additives in conventional amounts to act as rust inhibitors, copper passivators, demulsifiers and foam inhibitors. The lubricating oil base oil used in the formulations of Tables 1 and 2 was a Group II mineral oil and the additional base oil component was bright stock oil.

Examples 1 to 3 are according to the present invention whilst the remaining Examples in Table 1 are comparative in nature.

In Table 2, Example 4 is according to the present invention whilst the remaining Example in Table 2 is comparative in nature. The formulations in Tables 1 and 2 were tested for lacquer formation in a version of the Wolf Strip Test (formerly known as DIN 51392) performed according to the modifications set out below.

DIN 51392 formerly measured the tendency of engine oils to produce thermal/oxidative deposits using an inclined metal plate over which the test engine oils were pumped. The volume of oil, pumping rate, plate temperature and test duration specified in said test were 200 ml, 50 ml/hour, 250 ⁰C and 12 hours, respectively. In order to improve the test repeatability and to more closely simulate the type of thermal/oxidative stress that a lubricating oil composition would experience under full hydrodynamic film conditions in a turbine, the test conditions used in the Examples of Tables 1 and 2 were modified to an oil volume of 150 ml, a pumping rate of 50 ml/hour, a plate temperature of 205 ⁰C and a test duration of 24 hours.

The formulations in Table 2 were also subjected to the Oxidation Stability of Steam Turbine Oils test (TOST) (1000 hours, ASTM D4310-03) to measure sludge and the Cincinnati Machine Test Procedure A (ASTM D 2070-91) to measure sludge.

The methodology used for the TOST (1000 hours) (ASTM D 4310-03) was to heat 300 ml of the test lubricating oil composition with 60 ml of water to 95 ⁰C, in the presence of a copper and a steel coil catalyst, blowing the test mixture with oxygen for 1000 hours. After 1000 hours, the weight of insoluble material was determined by filtering the lubricating oil composition through a 5 micron filter.

The methodology used for the Cincinnati Machine Test Procedure A (ASTM D 2070-91) was to heat 200 ml of the test lubricating oil composition in a beaker, in the presence of a copper and a steel rod, to 135 ⁰C in an oven for 168 hours. After 168 hours, the appearance of the copper and steel rods was visually rated and the weight of insoluble material was determined by filtering the lubricating oil composition through 20 and 8 micron filters . It is apparent from Tables 1 and 2 that the formulations in accordance with the present invention not only exhibit good sludge deposit control, but also display outstanding performance in reducing lacquer formation as measured in the modified Wolf Strip test outlined above.

After repeating the Experiments of Examples 1 to 3 , whilst replacing the Group II mineral base oil by a Fischer-Tropsch derived Group III base oil, it was shown that the addition of bright stock oil resulted in reduced lacquer formation as measured in the modified Wolf Strip test outlined above. TABLE 1

K*

O

nm = not measured. * Commercial turbine oil 1. ** Commercial turbine oil 2. *** Commercial turbine oil 3.

TABLE 2

It is apparent from the results of Table 2 that the formulation of Example 4 shows greatly reduced deposit formation tendency compared to the formulation of Comparative Example 1.

Claims

C L A I M S

1. Lubricating oil composition comprising lubricating oil base oil in combination with:

(a) one or more substituted phenyl-naphthylamines;

(b) one or more anti-wear additives; and (c) an additional base oil component selected from bright stock oil, polyalkylene glycols (PAGs) , alkyl naphtalenes, alkyl benzenes and natural and synthetic esters such as diesters and polyolesters, preferably bright stock oil.

2. Lubricating oil composition according to Claim 1, wherein said one or more substituted phenyl- naphthylamines (a) are selected from alkylated phenyl- alpha-naphthylamines and/or alkylated phenyl-beta- naphthylamines .

3. Lubricating composition according to Claim 1 or 2 , wherein said one or more anti-wear additives are selected from thiophosphates, preferably ashless thiophosphates .

4. Lubricating composition according to any one of Claim 1 to 3 , wherein said one or more ashless thiophosphates (b) are compounds according to formula (I)

O

(D wherein R¹ and R² are each, independently, selected from hydrogen, branched or straight chain Ci to Ci₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms, C₅ to Ci₂ cycloalkyl groups, C₅ to C₉ cycloalkylmethyl groups, C₉ to Ci₀ bicycloalkylmethyl groups, C₉ to C_xo tricycloalkylmethyl groups, phenyl or C₇ to C₂₄ - -

alkylphenyl groups or R¹ and R² together may be a C₂ to C₁₂ alkylene group which may be interrupted by 1 or 2 oxygen atoms, R³ and R⁴ are each, independently, selected from hydrogen and branched or straight chain Ci to C₈ alkyl groups which may be interrupted by 1 or 2 oxygen atoms, n is an integer from 1 to 5 and X is selected from -OH, -OR⁵ wherein R⁵ is selected from branched or straight chain alkyl groups having from 1 to 8 carbon atoms, -NR⁶R⁷ wherein R⁶ and R⁷ are independently selected from hydrogen or Ci to Ci₈ alkyl groups.

5. Lubricating oil composition according to any one of Claims 1 to 4 , wherein said one or more substituted phenyl-naphthylamines (a) are present in a total amount in the range of from 0.01 to 3.0 wt . %, based on the total weight of the lubricating oil composition.

6. Lubricating oil composition according to any one of Claims 3 to 5, wherein said one or more ashless thiophosphates (b) are present in a total amount in the range of from 0.01 to 1.0 wt . %, based on the total weight of the lubricating oil composition.

7. Lubricating oil composition according to any one of Claim 1 to 6 , wherein said additional base oil component is present in an amount in the range of from 0.01 to 10.0 wt. %, based on the total weight of the lubricating oil composition.

8. Lubricating oil composition according to any one of Claims 1 to 7 , wherein said lubricating oil composition further comprises one or more additives selected from anti-oxidants, extreme pressure additives, metal passivators, corrosion inhibitors, foam inhibitors and/or demulsifiers .

9. Use of a lubricating oil composition according to any one of Claims 1 to 8 to lubricate a combination of a gas turbine and a steam turbine. - -

10. Use of a lubricating oil composition according to any one of Claims 1 to 8 in order to reduce lacquer formation in a turbine, preferably in a combination of a gas turbine and a steam turbine .

11. Method of reducing lacquer formation in a turbine, preferably in a combination of a gas turbine and a steam turbine, by lubricating said turbine (s) with a lubricating oil composition according to any one of Claims 1 to 8.