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Varnish and related deposits occur in a range of lubricantapplications where the oil is exposed to high temper-atures. It can give rise to a number of issues such asaccelerated bearing wear, sticking and faulty operation ofcontrol valves, blockage of oil filters and impeded heattransfer. In an operational gas or steam turbine, under suchcircumstances, control systems will shut down a turbine toprevent any permanent and potentially hazardous damage,resulting in what is often termed a ‘unit trip’. A non-operationalturbine suffering these issues may not be able to be broughtonline. The unscheduled downtime arising from these unit tripsand failed starts can be extremely costly for an end user. Forexample, an energy utility company which fails to provideelectrical power on an agreed day is contractually obliged tomake up the deficit by purchasing power from the market oftenat highly inflated rates. Costs in excess of $1 million for a singleunit trip are common. Moreover, varnish-related failures aremost prevalent for peaking and cycling units, which are broughtonline intermittently to provide power when grid demand is atthe highest. Inability to provide power when most neededtherefore has consequences not only for the utility company butfor the entire grid.

Several factors have been identified as contributing to varnishformation in turbines. Perhaps the prime reason is the thermaldegradation and oxidation of oils by the heat emanating fromthe superheated steam in a steam turbine or the combustiongases in a gas turbine. Degradation can also occur by theadiabatic compression of entrained air bubbles in the oil(microdieseling), and electrostatic spark discharge leading tolocalised hotspots in the oil and therefore decomposition. Thedecomposition products are often insoluble in oil and thereforeprecipitate out as sludge or plate out on surfaces as varnish. Thechoice of base oil is also influential as solvent extracted,

hydrocracked and hydrotreated oils can have very differentabilities to solubilise additives as well as additive decompositionproducts. Design features of the turbine itself, such asarrangement of the lubrication and hydraulic circuits, proximityof bearings to high temperature zones, and the maintenancepractices applied also have a significant impact on problems dueto varnish and deposit formation.

Some solutions for varnish issues have emerged. A largeamount of effort has been put into developing superiorcondition monitoring technologies. Traditionally, the conditionof an oil and propensity for forming deposits was predictedeasily by tracking oxidation-related parameters such as acidnumber and viscosity of the in-service oil. However, with theincreased availability of hydrofinished base oils, which enableoxidation lifetimes up to an order of magnitude higher thanpossible with solvent extracted base oils, deposit formation canoccur before any noticeable changes in the viscosity and acid-number profile. Novel condition monitoring methods such asMembrane Patch Colorimetry ASTM D7843 offer the ability topredict varnish formation reliably. Improved maintenanceprocedures have also been advocated. Heat-tracing of pipeworkto minimise sharp temperature drops and therefore minimisingprecipitation by sudden cooling, use of advanced filtrationsystems with optimised filter materials, use of electrostaticprecipitators and ion exchange technologies in conjunction withappropriate condition monitoring have met with success inreducing varnish-related downtime for turbine end users. Asformulators of additive technology for turbine oils, AftonChemical sought through this study to investigate the effect ofthe oil formulation, in other words the combination of additivechemistry comprising of antioxidants, metal deactivators,corrosion inhibitors, etc. with base oils, on the potential to formvarnish and related deposits.

Formulating Low VarnishTurbine Technology –lubricant additives with improveddeposit control

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Afton Chemical obtained samples of varnish or varnish-cloggedparts from three turbines which had been experiencing varnish-related issues, namely a plugged servo valve from a gas turbine(Figure 1), the varnish from a blocked filter in the Inlet GuideVane valve unit from a gas turbine and an electrostatic precip-itator which had accumulated a large amount of varnish.

Afton Chemical also obtained samples of the new oil brandswhich had been used in each of the turbines, Oil A, B and Crespectively. After isolating the varnish from the parts, we ran extensive chemical analysis of the varnish as well as the new oilin order to investigate any connections between new oiladditive chemistry and material present in varnish. A range oftechniques including ICP, FTIR, LCMS, HPLC, GCMS, UV/Visspectroscopy, GC/NPD, phosphorus NMR, SEM and EDX wereused. In some cases, virtually identical chemistry was detected inthe new oil and the varnish, corresponding for example tounreacted antioxidants (Figure 2).

The varnish samples were found to contain both unreactedphenolic and aminic antioxidants, as well as their decompositionproducts, together with triazole metal deactivators, unreactedtriarylphosphate antiwear along with base oil oxidationproducts. Notably, the Inlet Guide Vane varnish containedPANA, which was absent in the new oil B used in that turbine. It is possible that the PANA appears due to inadequate systemflushing when this oil was first introduced into the system (Table 1) on page 24.

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Figure 1. Servo valve containing varnish

Figure 2. LCMS comparison of varnish on bushing of servo valve with antioxidants present in new oil

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Interestingly, all of these varnish samples were soluble in polarorganic solvents such as THF, which differentiates it from thevarnish deposits found, for example, on pistons in an internalcombustion engine. Typically piston deposits contain largeamounts of sooty carbonaceous material and insoluble polyaro-matics on account of the oil being exposed to higher temper-atures and coming into direct contact with combustion gases,which does not occur in a gas or steam turbine.

Around the same time as the investigation of varnish samplesfrom the field, we set out to develop a rapid screener test inorder to simulate varnish formation in the field under laboratoryconditions. Approximately 50ml of oil is aged in a glass beakerin the presence of a copper and steel catalyst coil at a settemperature for a set duration. The oil is then filtered, sludgeweight recorded, and viscosity, acid number and colour of theend of test oil is measured. We found that when the test is runfor 7 days at 150C, on oils A, B and C, the filtered sludge waschemically very similar to the varnish from the field and

therefore showed potential for being an effective, experimentallysimple, high throughput bench test to mimic varnish depositsfrom turbine. Moreover, along with the sludge weight, a quantitative indicator of varnish forming potential, several otherdata points emerge from each test that give an indication of aformulation’s oxidation stability made the test a useful tool forfurther formulation studies.

Armed with an understanding of problematic chemistries fromthe field varnish study, and a varnish simulation bench test,Afton Chemical then initiated several matrix studies to identifylow varnish formulations. Two Design of Experiment (DoE)matrices investigating multiple phenolic and aminic antioxidants,metal deactivators in several base oils were tested in a numberof deposit formation and oxidation tests such as the ASTMD2272 RPVOT, D4310 1000h TOST, D943 life TOST as well asAfton’s varnish screener test and other tests specified by turbineOEMs. Statistical analysis was carried out of the results werecarried out in order to determine the main effects of each

Table 1. Chemistry detected in varnish samples and new oils

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additive component and interactions between them. Additionaltest matrices were put together to identify the effect of morephenolic as well as phosphorus and sulphur based antioxidants.

The combined learnings of the varnish sample analysis and thematrix studies were used to assemble further formulationswhich were run in further oxidation tests including the ‘DryTOST’ test. Afton ran the Dry TOST essentially as a modifiedD943 TOST, but in the absence of water at 120°C instead of thestandard 95°C with water present as in the D943. It is possibleto run the test on multiple tubes of the same oil sample, witheach tube running to a set duration. When each tube completesits test duration, the end of test oil is decanted, the residualRPVOT measured (relative to fresh oil) and the oil is filtered toobtain the sludge deposit weight. The residual RPVOT can beplotted on a graph against test duration to give anunderstanding of the oxidation stability of the oil, and thesludge can be plotted against time or residual RPVOT to showthe propensity for deposit formation as the antioxidants getdepleted and the oil ages. Afton Chemical has been partici-pating in an ASTM working group to derive a formal testmethod for the Dry TOST.

Initially we ran the Dry TOST on the better performingformulations from the preceding matrix studies, and API Group II base oil, by using two tubes per sample, set to500h and 700h. We had set ourselves a limit of maximumsludge of 100mg/kg and a minimum time to 25% residualRPVOT of 500h (Figure 3).

Although sludge control of these early formulations waspromising, we sought to improve the oxidation stability and somodified the levels of antioxidants and other components, whileusing a commercially available turbine oil as a reference (Figure4). The results were encouraging, with consistently low sludgeand much greater oxidation stability (higher residual RPVOTafter given number of hours).

Efforts to increase the oxidation stability far beyond the limit byincreasing the level of antioxidants by 50-100% of originalshowed that sludge formation also increased. We thereforeinvestigated a range of solubilisers and diluents to lower thesludge while having an oxidation life beyond well beyondrequirements. Dispersants, various aromatic hydrocarbons,napthenic base oils and polyoxygenated compounds wereinvestigated, in some cases with success but also with side-effects on other performance areas such as water separation,rust and oxidation life, for example in Figure 5.

Figure 3. Dry TOST at 500h and 700h on initial candidates

Figure 4. Improved oxidation stability

Figure 5. Example of solubilisers with negative effect on sludge and RPVOToxidation stability

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After further optimisation of antioxidant and metal deactivatorlevels, we evaluated a number of neutral and acidic rustinhibitors as well as combined systems in these formulations forrobust performance in the ASTM D665B rust test. We thenwent on to optimise the water separation performance byinvestigation of a range of demulsifier systems in the D1401water demulsibility and IP 19 steam demulsibility tests. Inparticular, we ran the testing on both fresh and thermally agedoils in order to simulate an in-service oil in a turbine. Newturbine oils frequently show excellent water separationproperties but after a few months in service the waterseparation can deteriorate rapidly as demulsifiers decompose ordrop out of the oil. Effective water separation even after athermal ageing simulation therefore displays the potential ofstrong performance in actual turbines (Figure 6).

With the optimised formulation defined as additive HiTEC® 2505,Afton Chemical went on to generate full datasets against keygas and steam turbine OEM specifications and industrystandards. Stringent requirements on all aspects ofperformance, including oxidation, air release, waterseparation, rust prevention, foam control, filterability, compatibility with trace calcium and magnesium contaminants,yellow metal corrosion and elastomer compatibility were metconsistently in Group II and Group III base oils. We then wenton to test these formulations in the full Dry TOST test, runningsix to eight tubes. In one of the Group II base oils investigated,the test was run in duplicate and took approximately 900hours to reach 25% residual RPVOT (Figure 7). Sludge wasconsistently below our limit of 100mg/kg, at 50-60mg/kg at25% residual RPVOT (Figure 8).

Figure 6. Strong water separation for fresh and thermally aged oils

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Having demonstrated strong performance in group II base oils,our attention turned to Group III base oils. We ran 6-8 tubes ofour optimised HiTEC®2505 additive in two different Group IIIbase oils to durations well over 1000h, allowing for theexpected increase in oxidation stability for Group III versusGroup II. Both oils took approx. 1200h to reach 25% residualRPVOT (Figure 9) and gave sludge values of approximately40mg/kg at 25% residual RPVOT (Figure 10).

Note that the sludge is still very low even when the residualRPVOT is well below 10% residual RPVOT. In other words, evenwhen the antioxidants are almost completely consumed, thesludge formation in the oil is still low. This has potentiallybeneficial ramifications in a turbine where localised extensivedegradation of oil, for example in hotspots still does not lead todeposit formation.

Understanding the composition of base oils, in particular theproportions of various types of hydrocarbons (aromatics, n-paraffins, isoparaffins, polycycloparaffins) can enable us topredict performance in oxidation tests and select additive treatrates accordingly.

We are involved in on-going work to understand the applicationof this additive technology to a broader range of base oils, andalso to investigate further the compatibility with industrysolutions in the field such as electrostatic filtration, ionexchange oil purification and also new condition monitoringmethods. Afton Chemical has several decades of experience informulating turbine oil additive technology which has beenhighly successful in the field. Our efforts will now focus onacquiring similar field performance for our newest technologies.In conclusion, an extensive study on the chemistry of formationof varnish and related deposits in turbines has been carried outas well as development of laboratory methods to simulatedeposit formation in operation. This knowledge is then appliedtogether with an understanding of base oil chemistry toformulating oils which can perform well in challenging industrytests and exceed the more stringent turbine manufacturers’specifications whilst controlling not only sludge and varnishdeposit formation but also air and water separation, foam andcorrosion. A synergistic combination of suitable additive andbase oil technology therefore yields turbine oil formulationswhich are capable of overcoming the challenges which modernturbine lubricants face.

Shyam Prasad, Afton Chemical LtdSenior Development Chemist and Formulator of Additive Technology for Turbine Lubricants

LINKwww.aftonchemical.com

Figure 7. Duplicate dry TOST on ISO 32VG formulation using Group II base oil - oil life

Figure 8. Duplicate dry TOST on ISO 32VG formulation using Group II base oil -sludge control

Figure 9. Dry TOST on ISO 32VG using Group III base oils - oil life

Figure 10. Dry TOST on ISO 32VG using group III base oils - sludge control

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