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LUBE-OIL ANALYSIS

Gas turbine firing tempera-tures continue to increaseas OEMs relentlesslychase efficiency improve-

ments. This demands that lubricantmanufacturers formulate productscapable of withstanding the more

rigorous service conditions. Com-pounding the challenge are theowner/operators, who want turbineoils to last longer.

To meet these demands, tur-bine oils have undergone significantchanges over the last decade or so.For example, base oils have changedfrom Group I (solvent-refined) toGroup II (hydrotreated).Important to plant per-sonnel is that Group IIbase stocks, while moreoxidation-resistant, havea lower solvency, whichmeans varnish issues are areal possibility.

The antioxidants addedto protect the fluid also havechanged. Combinations ofantioxidants, referred to ascomplex additive packages,are used to take advantageof the synergies among thevarious constituents.

Fluid degradation.Critical to optimizing yourlube-oils life expectancy is under-standing how a lubricant degrades

(Fig 1) and what tests are availableto measure the amount of degrada-tion. Oxidation and/or heat are whatcause Group 1, Group 2, and syn-thetics to degrade.

Generally, oxidative degradationdoubles for every 18-deg-F increasein lube-oil temperature. Group I oilsare the most susceptible to this typeof degradation because they containthe highest levels of unsaturates andimpurities. The initial degradationproducts are aldehydes, ketones, andcarboxylic acids; they undergo fur-

ther reaction to form higher molecu-lar-weight products. As these byprod-ucts combine, sludge and varnish areformed.

Test methods

One of the first tests designed to mea-sure a turbine fluids ability to resistoxidation was the Rotating Pressure

Vessel Oxidation Test (RPVOT, ASTMD2272). In this test, the fluid is sub-

jected to pressure and heat in thepresence of water, oxygen, and a cop-per catalyst inside an enclosed vessel.Pressure drop indicates when the fluidis oxidizedthat is, once it absorbs(reacts with) the oxygen. The time ittakes for the pressure to drop by a pre-determined amount is the end point.

The older Group I fluids typi-

cally had RPVOT values of about 600minutes or less. Group II fluids with

sophisticated additive packages mayhave values in excess of 3000 min-utes. ASTM D4378 (Standard Prac-tice for In-Service Monitoring of Min-eral Turbine Oils for Steam and GasTurbines) states that oil should bechanged when its RPVOT falls below25% of the RPVOT for that same oilwhen it was new.

The universal validity of RPVOTtest results have been called intoquestion by some industry expertsrecently because of issues regard-ing repeatability and reproducibility

with at least some new oils. However,RPVOT is still listed routinely onmost oil manufacturers product datasheets and remains the most widely

used indicator of a fluids oxidativecondition.

Running the RPVOT test regular-ly helps in predicting the fluids use-ful life. ASTM D4378 recommendsthat RPVOT testing be done annu-ally, and more frequently as the fluid

ages. RPVOT does not necessarilydecrease linearly. A recent analysisof data from tests performed at regu-lar intervals on several oils foundthat the RPVOT results for some flu-ids decreased linearly while those forothers did not.

Fig 2 shows three possible waysin which a fluids RPVOT may

decrease. Keep in mind thatthese results are from testsrun under controlled labora-tory conditions. Under fieldconditions, the odds are evenhigher that RPVOT will notdecay linearly. Note, too,that RPVOT was not devel-oped as a means for compar-ing oilsthat is, for com-paring the absolute RPVOTvalues of alternative fluidsagainst each other. Rather,its value is in charting thedecrease in RPVOT for agiven fluid.

Two relatively newtests useful for assess-

ing a fluids oxidative health areQSA (Quantitative Spectrophoto-

metric Analysis, Analysts Inc) andRULER (Remaining Useful LifeEvaluation Routine, Fluitec Inter-national). Though neither of thesetests measures a fluids ability toresist oxidation under simulatedconditionsas RPVOT doestheydo provide useful information abouta fluids oxidative health.

Traditional oil analysis, while avaluable preventive maintenancetool, is unable to detect varnish orvarnish precursors. By contrast, QSAis designed to detect varnish precur-

sors. The amount of soft contami-nants present in a sample is quanti-fied using a spectrophotometer and isreported by use of a proprietary scale,

Fluid analysis critical to

maximizing lube-oil service lifeGene Wagenseller, CLS, OMA II, Analysts Inc

Hydrocarbon

Oxidativedegradation

Initialpolymerization

Varnish and sludge(oxygenatedinsolubles)

Thermaldegradation

Hot metalsurfaces/electrostatic

discharge

Carbon deposits

Air entrainment(adiabatic

compression)

Varnish fromcarbon insolubles

1. Understanding howhydrocarbons degrade isimportant for maximizinglube-oil life expectancy

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COMBINED CYCLE JOURNAL, Fourth Quarter 2009 61

developed by correlating lab resultswith known instances of varnish

occurrences in field studies.QSA has proven more reliable

for predicting varnish than alterna-tives such as the ultracentrifuge. Itis recognized by several OEMs asthe preferred method for varnishtesting. Fig 3 illustrates the VarnishPotential Rating (VPR) for a Frame 7engine at a combined-cycle plant.

RULER works on the principlethat some chemical speciessuch asantioxidantsare electrochemicallyactive. A compound is considered elec-trochemically active if, when a voltage

is passed through a solution contain-ing that compound, current flows.Different chemical species respondat different voltages. The amount of

antioxidant is directly related to theamount of current produced.

By increasing the applied voltageat a constant rate, one can develop agraph that reveals the antioxidantspresent. The quantity of antioxidantsremaining in the fluid can be calculat-ed as a percentage of the initial amountby comparing the area under the curvecreated for the oil in service to that forthe same oil when new (Fig 4).

Thus RULER measures the amountof antioxidants remaining in the fluid,not the oils useful life. This is impor-tant because some of the degradationproducts from the antioxidants may

themselves be antioxidants.RULER also requires a referencefluid to calculate the percentage ofadditives remaining, which may limit

its effectiveness in some situations.Heres why: Some gas-turbine fluids

may remain in service for 10 years,steam-turbine oils even longer. Overthe years, the original fluid may havebeen reformulated or topped off witha different product. The resultingfluid mixture could complicate inter-pretation of RULER results.

However, the test has severaladvantages, including: small samplesize, rapid analysis; plus, techniciansrequire little training to perform thetest.

The bottom line: RPVOT, RULER,and QSA each provide valuable infor-

mation regarding the health of yourturbine oil. Including all as part ofyour fluid-analysis program enablesthe most accurate decision-making.

Test A Test CTest B

RPVOT

1400

1200

1000

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200

00 500 1000 1500 2000

Time, min

2. Data from tests under controlled laboratoryconditions illustrate how RPVOT may decrease

72Abnormal

QSA patch

QSAnumber

100

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15

28

46

48

5864

58

72

4/19/05 10/19/05 4/19/06 10/19/06 4/19/07 10/19/07 4/19/08 10/19/08

3. Varnish Potential Rating for a Frame 7 engine increasedunchecked for years

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62 COMBINED CYCLE JOURNAL, Fourth Quarter 2009

Condition-basedmaintenance

Lubricating oil is changed by someusers on a fixed schedule. Thisapproach, while convenient, does notassure optimum machine and fluidlife. Oil changed prematurelyincreases the cost of lubrica-tionyou wind up buying morenew fluid than you need and payfor disposal of oil that may stillhave some life in it. By contrast,if fluid remains in service beyondits useful life, you place criticalrotating equipment at risk. If aforced outage results, revenue

loss could be significant.More cost-effective is sched-uling preventive maintenance on acondition-based approach. A condi-

tion-based lube program relies on theresults of fluid analyses to maximizethe use of your oil, thereby minimiz-ing the life-cycle cost of lubrication.

Decisions involving lube-oil recon-ditioning and replacement demandcareful consideration and planning. Toillustrate: Replacement requires thattime be budgeted for review of alter-native fluids, the purchasing process,delivery, etc. In addition, cleaning andflushing of the lube system may be nec-essary to maximize the effectiveness ofthe new oil and to return performanceto as new. Arrangements for safe dis-posal also must be made.

What follows are three examplesof how fluid analysis was used to

decide between lubricant replace-ment and conditioning.

Case history 1. A combined-cycleplant annually assessed the condi-

tion of its turbine fluids. After the2008 turbine lube assessment, plantpersonnel questioned whether thefluid would be suitable for continueduse until the next scheduled outagein fall 2009. Options included replac-ing the lube oil, removing existingvarnish and precursors with a provenclean-up technology and/or toppingoff the existing fluid.

Because the plant had been test-ing its fluids regularly to assess oxi-dative stability, meaningful datawere available for predicting the flu-ids expected condition a year hence(Fig 5). Data extrapolation indicatedthat the fluids RPVOT would bewell below the 25% minimum recom-

mended by ASTM D4378 and mostturbine manufacturers.

Additionally, RULER predictedonly 10% of the antioxidants would

remain by the fall 2009 outage.Based on this information, the plantopted to change the fluid ratherthan risk an unscheduled outagebecause of fluid failure.

Case history 2. A plant experi-enced varnish-related trips on bothof its Frame 7s, which had beencharged with new lube in 2007. QSAresults revealed that both lube-oil

systems had critical levels of var-nish present. Fig 6 presents testdata for one of the turbines.

Interestingly, both the RPVOT and

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0 5 10 15

600

400

200

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90

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4. Antioxidants remaining in lube oilcan be calculated as a percentage ofthe initial amount by comparing theareas under the curves for both sumpand new oil

5. Meaningful data can be extrapo-lated to estimate when new oil will beneeded

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COMBINED CYCLE JOURNAL, Fourth Quarter 2009 63

RULER values are within acceptablelimits. Recall that those tests focuson antioxidant levels, not the fluidstendency to form varnish, so theymight not provide adequate warn-ing for the user in situations suchas this.

The plant was remiss in notassessing the condition of its turbine

oils at least annually and personnelwere caught off-guard with the var-nish problem. At this plant, varnishbuild-up was such that it was diffi-cult to remove. Three different typesof varnish mitigation technologieswere tried without success.

In cases such as this, if varnish can-not be removed, the plant may have todrain the oil and possibly chemical-flush the system before rechargingwith fresh oil. Lesson learned: It ismuch better to deal with lubricantissues early, before the problem has

reached the critical stage.The first two scenarios illustrate

the difference between proactive andreactive maintenance philosophies.

At the first facility, proactive fluidanalysis enabled management toweigh benefits and risks and makean informed decision. The secondplant has lost production time andmay have to change the oil prema-turely.

Case history 3. An F-class com-bined-cycle facility became awareof varnish formation in lube oilsthrough the OEMs TIL-1528 andbegan performing the QSA test reg-ularly. A small QSA spike severalyears ago prompted investigation ofthe various varnish removal technol-ogies available and the plant optedto install an electrostatic process.

For several years test results havebeen very favorable. Recently, how-ever, the varnish potential increased(Fig 7). The plant has an outagescheduled for later this year; thefirst one after that will be in 2012. Sowhat to do this year questions areon the minds of plant supervisors.

Had the plant been performingRPVOT and RULER tests regularly,supervisors would be better equippedto determine the reason for the lat-est spikespecifically, if the increasein varnish potential was a result ofdiminished additive levels and insuf-

ficient oxidative reserve to providefluid protection. Only by having allthis information, can you make aninformed decision on fluid replace-ment now or in 2012.

Another lesson learned: Just hav-

ing a proactive approach to varnishcontrol is not enough. A completefluid-analysis program is neededto provide sufficient informationfor decision-making regarding fluidchange-out. ccj

120

100

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6. QSA test data revealsharp increase in varnishpotential (left)

7. First spike on the QSAplot prompted investiga-

tion of varnish removaltechnologies. An electro-static process was select-ed for this plant (right)

QSAnumber

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