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Rainer Kurz is Manager of Systems

Analysis and Field Testing for Solar Turbi nes

Incorporated, in San Diego, California. His

organization is responsible for predicting

gas compressor and gas turbine perfor mance,

for conducting appli cation studies, and for

fi eld performance tests on gas compressor

and generator packages. He has authored

numerous publications in the f ield of

turbomachinery and fluid dynamics. He won

the ASME-IGTI Oil & Gas Application Committee Best Paper

awards in 1998, 2000, and 2003 for his work on gas turbine testing,

degradation, and application considerations.

Dr. Kurz attended the Uni versity of the Federal Armed Forces, in

Hamburg, Germany, where he received the degree of a Dipl.-Ing.,

and, in 1991, the degree of a Dr.-Ing. He is an ASME Fellow, and

a member of the Turbomachinery Symposium Advisory Committee.

Klaus Brun is currently a Principal

Engineer at Southwest Research Institute,

in San Antonio, Texas. Hi s experi ence

includes positi ons in business development,

project management, engineering, and

marketing at Solar Turbines, General

Electri c, and ALSTOM Power. D r. Brun is

the i nventor of the Single Wheel Radial

Fl ow Gas Turbine, and co-inventor of the

Planetary Gear Mounted Auxili ary Power

Turbine. He has authored over 30 papers on turbomachinery (and

related topics), gi ven numerous technical lectures and tutor ial s,

and published a textbook on gas turbine theory. He is the Chair of

the ASME-IGTI Oil & Gas Applications Committee, a member of

the Gas Turbine Users Symposium Advisory Committee, and a pastmember of the Electri c Power and Coal- Gen Steeri ng Committees.

Dr. Br un received his Ph.D. and M.Sc. degrees (Mechanical and

Aerospace Engineeri ng, 1995, 1992) from the University of

Virginia, and a B.Sc. degree (Aerospace Engineering, 1990) from

the University of Florida.

INTRODUCTION

Proper maintenance and operating practices can significantlyaffect the level of performance degradation and thus time betweenrepairs or overhauls of agas turbine. Proactivecondition monitoring

will allow the gas turbine operator to make intelligent servicedecisions based on the actual condition of the gas turbine ratherthanonfixedandcalendar basedmaintenanceintervals. Maintaininginlet air, fuel, and lube oil quality will further reduce gas turbinedegradation anddeterioration.This tutorial provides a discussion onhow degradation develops and affects the performance of the gasturbine. Recommendations are provided on how the operator canlimit this degradation and any deterioration of the gas turbinesthrough proper maintenancepractices. The effects of water-washingandbestwashing practicesarediscussed. Emphasis is onmonitoringof gas turbines performanceparameters to establishcondition-basedmaintenancepractices.

GAS TURBINES

Gas turbines are widely used in industrial applications. Theyusually areeither single shaft ortwo shaft designs.Either design canuse a standard, diffusion flame combustor, or a dry-low-emissionscombustor. Most mechanical drive applications for gas turbinesemploy two-shaft gas turbines, while applications for powergeneration either employ singleshaft or two shaft concepts.

A two-shaft gas turbine(Figure1) consistsof anair compressor,a combustor, a gas generator turbine, anda power turbine. (Someengines are configured as multispool engines. In this case, thegas generator has a low-pressure [LP] compressor driven by alow-pressureturbine and a high-pressure [HP] compressor drivenby a high-pressure turbine. For this configuration, the shaftconnecting the LP compressor andturbine rotates insidethe shaftconnecting the HP compressor and turbine. In general, all theoperating characteristics described above also apply to theseengines.) The air compressor generates air at a high pressure,which is fed into the combustor, where the fuel is burned. Thecombustion products and excess air leave the combustor at highpressure and high temperature. This gas is expanded in the gasgenerator turbine, which has the sole task of providing power to

turntheair compressor.After leavingthegasgeneratorturbine, thegas still has a high pressure and a high temperature. It is nowfurther expanded in the power turbine. The power turbine isconnected to the driven equipment (compressor, pump, or in somecases a generator). It must be noted at this point that the powerturbine(together with the driven equipment) can and will run at aspeed that is independentof thespeed of thegas generator portionof the gas turbine (i.e., the air compressor and the gas generatorturbine). Thegas generatoris controlled by theamountof fuel thatis suppliedtothecombustor. Itsoperatingconstraintsarethefiringtemperature (turbine inlet temperature [TIT]: temperature of thegas upstream of the first stage turbine nozzle; turbine rotor inlet

173

GASTURBINETUTORIALMAINTENANCEANDOPERATINGPRACTICES

EFFECTSONDEGRADATIONANDLIFE

by

Rainer Kurz

Manager ofSystemsAnalysisandFieldTesting

SolarTurbinesIncorporated

SanDiego,California

and

KlausBrun

Principal Engineer

SouthwestResearchInstitute

SanAntonio,Texas

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temperature [TRIT]: temperature of the gas upstream of the firststage turbine rotor) and the maximum gas generator speed. If thefuel flow is increased, thefiringtemperatureandthegas generatorspeed increase, until oneof theoperatinglimits is reached. (At thematch temperature of the engine, both limits are reached at thesametime.At ambient temperaturesbelow thematchtemperature,thespeedlimit is reachedfirst[in twin spool engines, this could beeither theLP spool speed or theHP spool speed]. At ambient tem-peratures above the match temperature, the firing temperaturebecomes the limiting factor.) Often, variable geometry either for

the stators of engine compressor or the power turbine nozzles isused to optimize the gas generator operating points for differentloads or ambient conditions. The fuel to air ratio varies greatlybetween full load and idle, because the fuel flow at part load isreduced at a higher rate than the airflow. The fuel to air ratio isthusleaner at part load than at full load. Gas generator speed andfiring temperature also impact the pressure in the combustor:

The combustor pressure increases with gas generator speed andfiringtemperature.

Figure 1. Typical Industri al Two-Shaft Gas Turbine.

Engineswithemissionscontrol systems(dry-low-NOx [DLN])require that thefuel toair ratio is keptwithin anarrow window atall loads. Otherwise, the nitrous oxide(NOx), carbon monoxide(CO), and unburned hydrocarbon (UHC) emissions cannot bekeptwithin therequiredlimits. Theenginethushas toemploy anadditional meansof control. Intwo-shaft engines,this couldmeanthe bleeding of combustion air, or the use of staged combustion.While the combustor exit temperature at part load dropssignificantly for engines with standard combustion, it stays highfor DLN engines.

In a single shaft engine, the air compressor and the turbinesection operateon the same shaft, thus at the same speed. For thepurposes of this paper, the authors will consider only single shaftmachines that are operated at constant speeds, as is commonpractice in power generation applications. Since the generatorspeed has to be kept constant in order to avoid frequency shifts, itwill be controlled to run at constant mechanical speed. The gasgenerator is controlled by theamountof fuel that is suppliedtothecombustor. Its operating constraint is the firingtemperature (TIT,

TRIT). If the fuel flow is increased, the firing temperature

increasesuntil theoperatinglimit is reached.Thefiringtemperaturealso impacts thepressurein the combustor.

Dueto the constant speed, the airflow through the engine willnot vary greatly between full load andpart load. This meansthatthe fuel to air ratio drops significantly at part load and thecombustor exit temperature drops signif icantly from full loadto part load. Therefore, most single shaft engines with DLNcombustors use variable stator vanes on the engine compressor tovary the airflow, and thus keep the fuel to air ratio relativelyconstant. For single shaft engines as described above the partload efficiencies of gas turbines with DLN combustion andconventional combustion arevery similar.

GAS TURBINE DEGRADATION

Any primemover exhibits theeffectsof wear andtear over time.The problem of predicting the effects of wear and tear on theperformance of any engine is still a matter of discussion. Becausethe function of a gas turbine is the result of the fine-tunedcooperation of many different components, the emphasis of thispaper is on the entire gas turbine as a system, rather than onisolated components.Treating thegas turbineas asystemrevealstheeffectsof degradation on thematchof thecomponents.

The function of a gas turbine (Figure 1) is the result of the

fine-tuned cooperation of many different components: The gasgenerator consists of the air compressor that is driven by the gasgenerator turbine. The air compressor provides compressed air tothe combustor where the fuel is burned. The hot, pressurizedexhaust gases power the gas generator turbine, and subsequentlythepower turbine. I n multispool gas turbines, the air compressorandthegas producer turbinemay consistof several sections, whilein a single shaft gas turbine, the power turbine sits on the sameshaft asthegas generator.

In this system, the airflow (and thus the compressor operatingpoint) is determined by the turbine flow capacities, while thecompressor absorbed power has to be balanced by the powerproduced by the gas generator turbine. The compressor dischargepressureis dictated by theenginefiringtemperature, which in turnimpacts theengine power.

Any of these parts can show wear and tear over the lifetime ofthe gas turbine, andthuscan adversely affect the operation of thesystem. In particular the aerodynamic components, such as theengine compressor, the gas generator turbine, and the powerturbine have to operate in an environment that will invariablydegradetheir performance. The understandingof the mechanismsthat causedegradation, aswell astheeffectsthatthedegradation ofcertain components can cause for the overall system are a matterof interest.

Several mechanisms causethedegradation of engines:

Fouling

Fouling is caused by the adherence of particles to airfoils andannulus surfaces. The adherence is caused by oil or water mists.

The result is a build-up of material that causes increased surface

roughness and to some degree changes the shape of the airfoil.Particles that cause fouling aretypically smaller than 2 to 10m.Smoke, oil mists, carbon, and sea salts are common examples.Plugging of turbine blade cooling holes, caused by submicronparticles, promotes damage from overheating. Air contaminationcan be significantly reduced by the use of efficient air filtrationsystems, especially those that reduce the quantity of smallerparticles ingested. Therefore, it is essential that the air filtrationsystem be optimized to remove small particles, while also beingcapable of protecting against larger contaminants. Some filtercompanies have developed media, using micro and nanofibertechnologies, to capture moreparticles in the submicron to 2 msize range, while still offering satisfactory filtration of largerparticles and acceptable service life. Another issue may be thechange in pressure drop and filtration efficiency if the filter

mediumis wet.Air filtrationsystemshaveto balancetheconflictingrequirements for particle removal, pressure loss, cost, andservicelife, for varioustypes of contaminants.

The filtration system should be optimized for the specific sitecontamination and environmental conditions (Figure 2). A marinefiltrationsystemwill bevery different froma filtrationsystemin adesert application. A marinefiltration system will be designed toavoid theingestion of salt ladenwater droplets, for exampleby useof combinations of vane separators and coalescing filters. Thefiltration system also has to protect the engine from dry dustparticles, which may becoatedby seasalt. Beyondtheair filtrationsystem, it is also prudent to optimize the package installation:

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Ingestion of oil mist, or exhaust gases fromthe gas turbine, otherengines, flares, or other contaminant producing processes duringthe prevailing main wind direction should be avoided. In offshoreapplications, the inlet should bearranged to minimize the impactof spray water for the prevailing wind direction. Some types offiltration media only achieve their best filtration efficiency afterbecoming dirty, so in areas without large dust loads, it may takesignificant time until the filter reaches the design efficiencyespecially for very small particles. Certain typesof filtrationmediacan avoid this problem. Particles below about 0.3 m are usually

capable of following the air even throughthe strong accelerations,decelerations, and centrifugal forces in the gas turbine flow pathand are therefore not a significant problem for the compressorsection, unless they coagulate to larger particles (Dring, et al.,1979; Kurz, 1991a).

Figure 2. Air Inlet System wi th Filter and Silencer for a Gas

Turbi ne I nstallati on.

Becauseof very high massflows, even very low levels of ingestedforeign material can produce a substantial input of contaminantsthrough gas turbine engines. For example, 1 ppmw of impuritiesentering theengineresults in over 1100lbs of ingested material for asmall 3.5 MW industrial engine over an 8000 hr operating period.Even after several stages of state-of-the-artfiltrationof ambient inletair, deposits commonly form in gas turbine compressors due toingested particulate so that compressor washing is periodicallyneeded torestoreefficiency byremoving deposits.

Fouling can be reversed to a large degree by regular offlinewashing or a combination of online and offline water washing.Offline(or on-crank) washing(Figure3)requiresthegasturbinetobeshut down.After awaitingperiodtolettheenginecool down(orcrank cooling it), large amounts of cleaning solution (about 120liters [31.7 gallons] for a 10,000hpgas turbine, plus another 120liters [31.7 gallons] of water for rinsing) are flowed through theenginecompressor while it is operated at crank speed.

Figure 3. Performance Recovery (Compressor Pressure Ratio, Ai r

Fl ow, Compressor Effi ciency, Power, Thermal Effi ciency) from on

Crank Wash.

Becausethe engine has to be shut down for on-crank cleaning,online cleaning methods, where the engine stays online, are veryattractiveto operators. Here, acleaningsolutionis sprayed intotherunning engine through spray nozzles in the engine inlet. Thenozzles haveto providea spray pattern that covers the entireinletarea of the engine, with droplet sizes that are large enough toprovide some cleaning action, but small enough to avoid bladeerosion. Thekey limitationsof onlinecleaningaretwofold:

Because the air temperature in the compressor increases fromstage to stage, eventually the cleaning detergent will evaporate.

Therefore, onlinecleaning will only clean the first few stages.

Dirt particlesdissolvedinthecleaningdetergentcanberedepositedatthelater stagesof thecompressor, orwill betransportedintothehotsection of theturbine. The latter effect is in particular a probleminsalt laden atmospheres, because sodium and potassium are keyingredients for hot section hot corrosion.

Online cleaning is, nevertheless, a valuable practice to extendtheoperating hoursbetweenon-crankcleaningevents.Thesuccessof either type of cleaning is particularly dependent on a regularapplication. Severely fouled compressor blades may yield onlylimited performance recovery from cleaning. It has become goodpracticetoschedule onlineandofflinecleaningnotcalendar based,but performance based. Using condition monitoring systems,cleaning can be initiated whenever the engine shows a certainamount of power degradation, or reduction of compressordischarge pressure or airflow. Since all these parameters arevarying significantly depending on load and ambient conditions,methodsof datacorrectionstodatumconditionshavetoemployed.

This will be discussed later in this paper.

Hot Corrosion

Hot corrosion is the loss or deterioration of material fromflowpath components caused by chemical reactions between thecomponent andcertain contaminants, such as salts, mineral acids,or reactive gases (Figure 4). The products of these chemicalreactions may adhere to the aero components as scale. Hightemperature oxidation, on the other hand, is the chemical reactionbetween the components metal atoms and oxygen from thesurrounding hot gaseous environment. The protection through anoxidescale will in turnbereducedby any mechanical damagesuchas crackingor spalling, for exampleduring thermal cycles.

Figure 4. Hot Corrosion on a Turbine Rotor.

Corrosion

Corrosion is caused both by inlet air contaminants and by fueland combustion derived contaminants. Fuel side corrosion istypically morenoted andseverewithheavy fuel oils anddistillatesthan with natural gas because of impurities and additives in theliquid fuels that leave aggressive deposits after combustion.Corrosion is also acceleratedby impurities presentin theair duetothe combustion of fuel in the engine. Natural gas is a generic



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term applied to a mixture of hydrocarbon gases, with methaneasthe predominant constituent. In most cases, pipeline natural gascontains at least some impurities. Quality specifications, which inthe US aredetailed in an interstatepipelinestariff, areestablishedtoprotect thepipelineandcompression equipment againstphysicaldamage and performance degradation. When tolerances forimpurities are exceeded, decisions must bemadeas to how much,if any, gas of substandard quality can beaccepted, or if filtrationsolutions are required to gain or maintain acceptable cleanliness.For compressor stations operating on substandard field gas,

gas cleanup is generally essential. Technology is available toaccomplishthis task.Corrosion is often produced by salts such as sodium and

potassium, but lead and vanadium are also common contributors.Sincemany oil andgas gas turbines arelocated near thesea, seasalt(sodiumchloride) is often apotential offender.Cold enginepartsareattackedbysodiumchloride, whereashot partsaresubject tosodiumsulfate (sulf idization) corrosion. Sodium sulfate is producedfromthecombinationof sulfurin thefuel andsodiumchloridein theair. As with erosion and fouling, corrosion can be controlled withfiltration, however the filtration solution must becarefully thoughtout, especially in terms of filter media selection and droplet andmoisture control. Manufacturers specify limits on the amount ofcontaminants in the fuel, combustion air, and the water used forvariouspurposes(e.g., water orsteaminjection, evaporativecooling,fogging and overspraying, and detergent washing). Corrosion onengines that are not operated for extended periods is an issue thatneeds to beaddressedby preservation methods.

It is important to recognize that corrosion processes are oftenself-propagating, and will continue even if the source is removedor abated.

Thefusionof particlesonhot surfacesleadsto another sourceofproblems. While dry, 2 to10msizeparticlescould passthrougholder engines,causinglittleornodamage, theseparticlescancauseproblems in new generation, hotter running engines. If the fusiontemperature of the particles is lower than the turbine operatingtemperature, theparticleswill melt andstick tohot metal surfaces.

Thiscancausesevereproblemssincetheresultantmoltenmasscanblock coolingpassages, alter surface shape, andseverely interferewith heat transfer, often leading to thermal fatigue. Affectedsurfaces are usually permanently disfigured and will eventually

needreplacement.Consequently, gas turbine materials need to be protected from

the service atmosphere by coating, and in some cases it is thecoating integrity that limits the lifetime of the component.Manufacturers successfully usecoatings in gas turbines to combatoxidation, corrosion, and erosion, and as thermal barriers. Somecoatedpartscanbeaddedduringmaintenance, if economically andtechnically warranted. The use of coated parts, however, does notmitigatetherequirementfor goodfiltration.

Erosion

Erosion is theabrasiveremoval of material fromtheflow pathbyhard or incompressible particlesimpingingonflow surfaces.Theseparticles typically have to be larger than 10 m in diameter tocauseerosion by impact. Erosion is morea problemfor aero engine

applications, because state-of-the-art filtration systems used forindustrial applications will typically eliminatethebulk of thelargerparticles.Erosioncanalso becomeaproblemfor drivencompressorsor pumpswherethe processgas or fluid carriessolid materials.

Damage

Damage is often caused by large foreign objects striking theflowpathcomponents.Theseobjectsmay enter theenginewith theinletair, orthegascompressorwiththegasstream, oraretheresultof broken off pieces of the engineitself. Pieces of icebreaking offtheinlet,orcarbonbuildupbreakingoff fromfuel nozzlescanalsocausedamage.

Abrasion

Abrasion is caused when a rotating surface rubs ona stationarysurface. Many engines use abradable surfaces, where a certainamount of rubbing is allowed during the run-in of the engine, inorder to establish proper clearances. The material removal willtypically increaseseal or tip gaps.

While some of these effects can be reversed by cleaning orwashing the engine, others require the adjustment, repair, orreplacement of components (Diakunchak, 1991). It is thuscommon to distinguish betweenrecoverableandnonrecoverable

degradation. Any degradation mechanisms that can be reversedby online and offline water washing are considered recoverabledegradation. Degradationmechanismsthat require thereplacementof parts are considered nonrecoverable, because they usuallyrequire an engine overhaul. There are some grey areas, becausesome degradation effects can be recovered by control systemadjustments(that arehowever difficult to performin the field dueto limited capabilities to measure mass flow and performance).Another area where not all authors agree is regarding theperformance recovery at overhaul. While many manufacturersoffer overhauls to the same performance specifications as newengines, there are some cases where full performance recovery isdeemed impossible.

It should be noted that the determination of the exact amountof performance degradation in the field is rather difficult. Test

uncertainties are typically signif icant, especially if packageinstrumentation as opposed to a calibrated test facility is used.Eventrending involves someuncertainties, becausein all cases theengine performance has to becorrected fromdatumconditions toa reference condition.

DEGRADATION OF COMPONENTS

Airfoils

In order to judge thedegradation of aerodynamic components,the authors will first evaluate the effect of fouling, erosion,deposits, corrosion, and other damage on the individual airfoil.Fouling, andto some extent erosion generatea blade surface withincreased roughness. Any increased roughness can increase thefriction losses. It also may cause early transition fromlaminar toturbulent boundary layers, which increases loss production.Erosion, deposits, or damages to the airfoil change the geometricshape of the airfoil. On a well-designed airfoil, optimized for theapplication, this will always reducethe performanceof the airfoil.

Bammert, et al. (1965), Bammert and Sonnenschein (1967),Kind, et al. (1996), Kurz (1991a, 1991b, 1995), and Boelcs andSari (1988) have reported investigations on thechangeof turbineblade perfor mancedue to alterations of the blade geometry due toerosion, corrosion, and fouling. The deterioration of the turbineblades is accompanied by changes in exit angles (thus reducedwork) and increased losses. If the blade operates at or neartransonic velocities, depositsor addedroughness(withtheassociatedgrowth in boundary layer thickness) will also reduce the possibleflow throughtheblade row. Thicker boundary layers onthe bladesand sidewalls reduce the flow capacity, especially near chokingconditions. Ontheother hand, if thetrailingedgeerodes, thethroat

width of the blade is increased, thusallowingmore flow, but withless work extraction. Schmidt (1992) describes the effect of adeliberatereduction of the chordlength in a power turbine nozzlethat significantly reduced thework outputof said turbine.

Erosion typically has the most significant effects on the bladeleading edges.This can significantly affectthe location andextentof thetransitionfromlaminar toturbulent boundary layers(Pinsonand Wang, 1994). Because the heat transfer characteristics of aboundary layer depend, besides its thickness, on its state (i.e.,laminar, turbulent, transitional, separated), leading edge erosioncan influence the heat balanceof the blade.

Milsch(1971) reportsincreases in profilelosses from2 percent


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at ks/s=0.3103 to10 percent at ks/s=5 10

3 for NACA65-(12)06 compressor cascades, as well as significantly reducedturning. This reduction in performance is caused by significantlyincreased boundary layer growth, prematuretransition to turbulentflow, andprematureflow separation.

In general, all these influences will create higher losses andless turning. This means that the following row of airfoils willsee different incidence angles, higher temperatures, lower (forcompressors) or higher (for turbines) pressures and densities.

Clearances

Clearances between stationary and rotating parts (i.e., betweenstationary blades and the rotating hub or between rotating bladesand the stationary casing) have a tendency to open up during theaging process of the equipment. This results in higher leakageflows.Theseleakageflowsreducethepossible headcapability andthe efficiency of the components (Shabbir, et al., 1997; Khalid, etal., 1998; Frith, 1992). Because a signif icant amount of the lossproductionin anaxial compressor occurs in thetip endwall region,an increaseof therotor tip clearancefrom1percentof bladechordto3.5percentof bladechordreduces thepressureratio of thestageby up to 15 percent.

The loss production is due to intensive mixing of leakage flowswith the main flow, thus producing losses and reducing theeffective through-flow area (blockage). A simulation by Singh, et

al. (1996), onvariouscompressor stagessuggeststhat theeffectsoftip clearance and added roughness after ingestion of sand lead toabout thesamemagnitudeof performancedeterioration. However,the degradation mechanismby sandingestion is morerelevant foraeroapplications thanfor industrial applications.

Gas Turbine Compressor

Three major effects determine the performance deterioration ofthe gas turbinecompressor:

Increased tip clearances

Changesin airfoil geometry

Changes in airfoil surfacequality

While the first two effects typically lead to nonrecoverabledegradation, the latter effect can be at least partially reversed bywashing the compressor (Stalder, 1998). Typically, a degradedcompressor also will have a reduced surge or stall margin(Spakovszki, et al., 1999; Brun, et al., 2005). While the reducedsurge margin may not directly affect the steady-state operation, itmay reduce transient capabilities (e.g., block loads or droppedloads for generator sets), andcould causedamages if other actionsare taken that further reduce the surge margin. Examples are theuseof overspraying (i.e., the sprayingwater in the engine inlet toreducethe engine inlet temperature, to the extent that some of thewaterdoesnotcompletely evaporatebeforeenteringthecompressor)for performance enhancement, or if fuels with a very highcontentof dilutantsareused (Brun, et al., 2005).

Millsaps, et al. (2004), show in their simulationof afouled (as aresult of increased airfoil roughness) three stageaxial compressorthat the fouled first stage has a higher impact on the overallcompressor performance(in termsof pressureratio, efficiency, andmassflow) than a similarly fouledlater stage.

Syverud, et al. (2005), and Syverud and Bakken (2005, 2006)performed tests on a gas turbine, where they deteriorated thecompressor performancebyspraying salt water in theengineinlet.

Thedeposits causeincreased surfaceroughness on thecompressorairfoils.They found that themajority of deposits occur onthefirststage, andbecome insignificant after about the fourth compressorstage. One interesting finding is that the deterioration shifts theequilibrium operating line to both a lower flow rate and a lowerpressure ratio. The datashow further that the degradation not only

leads to reduced stage performance, but also to additional lossesbecause individual stages no longer operate at their design flowcoefficients.

In fact, the operating points of the deteriorated engine wereconsistently at lower flow coefficients than for the clean engine.

This also leads to additional efficiency reductions due to themovement of the stage operating points away from the stagedesign point.

Syverud, et al. (2005), areable to show the direct impact of theblade surface roughness on added profile losses, andthe increase

of sidewall boundary layers dueto the deposits onthe deterioratedcompressor performance. However, their data also show that thecompressor conditioncannot beseparated form theturbinesectionof the engine, since the turbine flow capacity determines theoperating point of thecompressor.

Graf, et al. (1998), show data for an axial compressor, whereincreased clearances caused reduced surge margin and reducedefficiency. In this case, the clearance was increased from 2.9percent (design value) to 4.3 percent. This lead to an increase insurge flow coefficient of about 20 percent, a reduction in designpressure coefficient of 12 percent and a loss of design pointefficiency of 2.5 points. Notably, the loss in pressure coefficientbecamenegligiblecloser tochokeflow. Similarresults arereportedby Smith andCumpsty (1984) where an increaseof theclearancefrom 1 percent to 3.5 percent reduced the pressure coefficientby 9 percent.

Frith (1992) tested a helicopter gas turbine with compressorbladescropped to simulateincreased clearances. A 3 percent croponthe axial compressor stages reduced airflow by 4.6percentandpressureratio by 3 percent.Thecompressor dischargetemperatureis reported to remain unchanged, which indicates a reduction incompressor efficiency by about 2.5 percent.

For agivenspeedof adegradedcompressor,eachsubsequentstagewill seelower Machnumbers(becauseof thehigher temperature), andlesspressureratio (duetothereduced work), thusan increased axialvelocity component. This meansin particular that stagedegradationhas a cumulative effect, becauseeach subsequent stagewill operatefurther away from its design point. While in the new machine allstages were working at their optimum efficiency point at designsurge margins, the degradation will force all stages after the firstone to work at off-optimum surge margins and lower than design

efficiency. This will not only lower the overall efficiency and thepressure ratio than can be achieved, but also the operating range.Further more, increased tip clearances will effectively reduce theflow capacity of the compressor. It should benoted that the reducedmass flow alone will not automatically lead to changes in thelocal axial flow velocities at midspan, because the reduced massflow is largely due to increased blockage at the endwalls. Carefulreadjusting variable geometry where available could be used tocounteract someof themismatching effects of degradation.

Combustion System

The combustion system is not likely to be the direct cause forperformancedeterioration.Thecombustionefficiency will usuallynot decrease, except for severe cases of combustor distress.However, deterioration could potentially lead to a variation in the

combustor exit temperatureprofile. Theproblems with a distortedexit temperaturedistributionarethreefold:

1. Local temperaturepeakscan damagetheturbinesection

2. The altered temperature profile will increase secondary flowactivity, thusreducingthe turbineefficiency.

3. Because the control temperature is measured at discretecircumferential points theaverageof thesemeasured temperaturesis not thesame as thetruethermodynamic average temperatureinthis plane.



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Therefore, in the factory test, the correlation between themeasured average and the true thermodynamic average isestablished. If the temperaturefield is altered due to 1) or 2), thiscorrelation is no longer correct. The engine could therefore beoverfired (thus producing more power, but shortening the life)or underfired, thus additionally losing power. On a two-shaftengine that uses the power turbine inlet temperature as ameans to indirectly control the firing temperature (i.e., the firingtemperature is inferred from the power turbine inlet temperature),degradation of the gas producer turbine (i.e., reduction of

gas power turbine efficiency) will lead to a reduction in firingtemperature. This is actually oneof thepositivesideeffectsof thiscontrol modethe engine is not driven into a more damagingoverfiringsituation.

Since many engines bleed air from the compressor dischargedirectly into the exhaust, either for surge avoidance during startupor for emission control purposes, it should be mentioned thatleaks in the bleed valves havea significant impact on the engineperformance. Leakingvalvescan berelatively easily detected, and,sincethey areusually external to theengine, easily repaired.

Turbine Section

Theturbinesectionofagasturbineissubjectedtoanenvironmentof very high temperatures. The gas properties are significantlydifferent from the inlet air, as they contain the combustion

products. A significant amount of problems in the turbine sectionarises fromthetypeandqualityof thefuel, or interactionsbetweenthe fuel and contaminants in the combustion air. Gaseous fuelscan vary frompoor quality wellhead gas to highquality consumeror pipeline gas. In many systems, the gas composition andquality may besubject to variations. Typically, themajor sourcesof contaminants within these fuels are: Solids, water, heavyhydrocarbons present as liquids, oils typical of compressor lubeoils, hydrogen sulf ide (H2S), hydrogen (H2) carbon monoxide(CO), carbon dioxide(CO2), and siloxanes. Water in the gas maycombinewithother small molecules to produceahydratea solidwith an ice-like appearance. Hydrate production is influenced, inturn, by gas composition, gas temperature, gas pressure, andpressuredrops in thegas fuel system. Liquid water in thepresenceof H2S or CO2will form acids that can attack fuel supply linesand components. Free water can also cause turbine flameoutsor operating instability if ingested in the combustor or fuelcontrol components.

Heavy hydrocarbon gases presentasliquids providemanytimesthe heating valueper unit volume than they would as a gas. Sinceturbine fuel systems meter the fuel based on the fuel being a gas,this creates a safety problem, especially during the enginestart-upsequence when the supply line to the turbine still may be cold.Hydrocarbonliquids can cause:

Turbine overfueling, which can cause an explosion or severeturbinedamagedueto highlocal temperatures.

Fuel control stability problems, because the system gain willvary as liquid slugs or dropletsmovethroughthecontrol system.

Combustorhotstreaksandsubsequentenginehotsection damage.

Overfuelingof thebottomsectionof thecombustorwhenliquidsgravitatetoward thebottomof themanifold.

Internal injector blockage over time, when trapped liquidspyrolyze in the hot gas passages.

Liquid carryover is a known causefor rapid degradation of thehot gas path components in a turbine. The condition of thecombustor components also has a strong influence, and fuelnozzles that have accumulated pipeline contaminants that blockinternal passageways will probably be more likely to missdesiredperformanceor emissiontargets. Thus, it followsthat more

maintenanceattentionmay benecessary to assurethat combustioncomponents are in premium condition. This may require that fuelnozzles beinspected andcleaned at moreregular intervals or thatimproved fuel filtration components be installed. A gas analysisalone may not be entirely sufficient for the detection of heavyhydrocarbons, because it may only include the gases, but notthe liquids in the stream. Also, it is common practice to lump allhydrocarbons from Hexane and heavier into one number. Whilethis is perfectly acceptable for the calculation of the lower heatingvalue as long as the hexaneandheavier hydrocarbons constitutea

minutefraction of the gas, it will lead to a wrong estimateof thedew point. Tetradecane (C14H30), even in parts-per-millionamounts has a significant impact on the dew point of the gasmixture, as will beshown later. Certainly a gas analysis has to beused in the project stage to allow for equipment sizing. Also, fuelsystemsusually limit thegassupply temperatureduetotemperaturelimits of its components. If the necessary superheat temperatureexceeds the fuel system temperature limits, additional gastreatment may benecessary.

Industrial gas turbines can also burn liquid fuels. Thesourceofliquid fuels is usually petroleumliquids, but liquid petroleumgas,natural gas liquids, and alcohols have also been used. Petroleumliquids originate from processed crude oil. They may be truedistillates (diesel, kerosene) or ash-forming fuels. Natural gasliquids(NGL ) areparaffin hydrocarbonsother thanmethanefound

innatural gas, whichcanbeextractedand subsequently handled asliquid fuels. They can also include liquid petroleum gas (LPG).LPG fuels areprimarily mixtures of propaneandbutane.

The combustion of liquid fuels in gas turbines is dependent oneffectivefuel atomization to increasethe specific surface area ofthefuel, thusresultingin sufficientfuel-air mixingandevaporationrates. Ignition performance is affected by two factors: the fuelvolatility, as indicated by the Reid vapor pressure or the ASTMevaporatedtemperature, andthetotal surfaceareaof thefuel spray,which is directlyrelatedtothespray Sauter meandiameter (SMD),and hencethefuel viscosity.

One of the main differences in the combustion between liquidand gas fuel is the presenceof free carbon particles (soot), whichdetermine the degree of luminosity in the flame. The presence ofcarbon particles at high temperatures is highly significant and

luminous radiation from the liquid fuel dominates, whereas thenonluminous radiation of gas fuels is less important. The amountof radiation impacts the combustor liner temperature, and theheavier the fuel, the greater the emissivity and the resulting walltemperatures become.

Further issues involve the storage and transport of the liquidfuel: Low viscosity fuel (such as LPG) requires special pumps toavoid problems due to poor lubrication of thepump. Foaming andformation of solids and waxes have to be avoided (Reid vaporpressure, cloud point). The fuel has to bewarmenough to be stillcapable of flowing under gravity (pour point), but nottoo warmtoexceed is flashpoint temperature(flashpoint). Thefuel also has tobe evaluated for its capability to corrode components of the fuelsystem(copper strip corrosion). Theashcontent has to be limitedto avoid fuel system erosion. Excessive content of olefins anddiolefins can cause fuel decomposition and plugging of fuelsystem components. If the carbon residue of the distillate is toohigh, carbon deposits may form in the combustion system. Solidsin the fuel can causeclogging of the fuel system, in particular thevery fineflow passages in injectors.

Lastly, certain contaminants that can cause corrosion of the hotsection of the engine have to be limited. These contaminantsinclude sodium, potassium, calcium, vanadium, lead, and sulfur,and in general the ash content. Sodium is often found in liquidfuels that aretransported in barges, due to the contamination withsalt water.

Trace metal contaminants of concern include: sodium,potassium, calcium, lead, vanadium, and magnesium. At elevated


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temperatures, vanadium, sodium, potassium, and lead arecorrosiveto theturbine buckets, particularly when present in amountsabovespecification limits. All of thesematerials, plus calcium, can alsoform hard deposits, which are difficult to remove with a normalturbine wash system. These deposits can cause plugging andreduced output.

Table1 lists the trace metals of interest, their specified limits inthe fuel supply, their effect on the turbine, the meansof treatmentto reduce harmful effects, and typical gas turbine permissiblelimits in fuel supplied to theturbine.

Table 1. Trace Metals Specif icati ons and Effects.

The quality of the gas available at each installation determinesthe gas filtration requirements, if any, needed for a given site.Keepingfuels freeof contaminants suchas water, compressor oils,anddirt areimportant to efficientoperation andlow maintenance.Coalescer technology for liquid/gas separation is available.Coalescer technology is also available for liquid/liquid separationanddisposable filter elements caneffectively control particulateinliquid fuel systems.

Another issue that may cause problems is the clogging ofcooling passages due to insufficient filtration of the inlet air.

Further, and this may beaproblemif it is insufficiently addressed,theinjection of water or steamin thecombustor sectionwill causea change in the heat transfer capability of the exhaust gas, whichcan cause accelerated degradation of turbine blades and nozzlesdue to overheating.

Just asfor thecompressor section, theturbine sectionexperiencesthe following issues as aresult of degradation:

Increased tip clearances

Changesin airfoil geometry

Changes in airfoil surfacequality

Maintenance of tip clearances is in particular a problemin theturbine section, due to the extreme changes in temperatures

between a cold engine and an engine acceleratingto full load. Inmany designs the stationary components expand at a differentrate than rotating components. Many new turbine designs useabradable seals to minimize these clearances. However, the mostsevere case, which is usually after a hot restart, will determinethe minimum clearance for the engine. Clearance losses areapproximately linear with the clearance (Traupel, 1988) andthereis anindicationthat theselosses increasewiththeincreaseof flow.In addition to a reduced efficiency, added clearances will alsoincrease the axial flow blockage, and thus will cause reducedthrough-flow, and increased velocities in the main flow. Radtkeand Dibelius (1980) report a reduction in efficiency of a

multistage turbine by 0.6 percent when they increased the radialclearances from0.5 percent of the blade height at the rotors and0.4 percent of the blade height at the stators to 0.8percent of theblade heightat rotors andstators.

Corrosion tendstoaltertheflowpathin two regards: It increasesthesurfaceroughness, but it may also removematerial, in particularat theleadingedges andthetrailingedgeof theairfoils. Especiallythe turbine nozzles, operating at or near choked conditions, arevery sensitivetochanges in flowarea. Furthermore, changes in theflow capacity of the turbine section will subsequently alter the

operating points for theenginecompressor.Increased surface roughness causes thicker boundary layers onthe blades and sidewalls, and thus may reduce the flow capacity,especially near choking conditions. Boyle (1994) found for atwo-stage turbine efficiency losses of 2.5 percent for a 10.2 msurfaceroughnesswhencomparedtosmoothblades. He alsofoundthat the most pronounced differences appear at the optimumoperating point at the turbine, whereas the far off-optimumefficiency was almost the same for rough and smooth blades. Itshould benoted that the losses due to clearances werein the sameorder of magnitudeas the profile losses.

However, if the degradation of the turbine section leads tomaterial removal, especially in the nozzle area, the oppositeeffect is seen:Theflow capacity increases for any given pressureratio. Because the flow capacity of any nozzle is limited by theeffectivethroat area, erosion of thetrailing edgecausesthethroatarea to increase and the exit flow angle to become more. Thismeans a reduction of turning in the stator and the rotor, whichwill lead to reduced work extraction for this stage and to anincreased flow capacity. Since the turbine nozzles constitute aflow restriction, any change in the flow capacity of the turbinesection will also impact the operating points that the enginecompressor will see.

EFFECT OF DEGRADATION ON THE GAS TURBINE

Degradation of engine components has acompounded effect onengineperformance, becausethechangeincomponentperformancecharacteristics leads to a mismatch of these components on theengine level. The impact of individual component degradation isalso influenced by thecontrol systemandthecontrol modes of theengine. Single shaft engines, operatingatconstantspeed, will showdifferent degradation behavior than two-shaft engines. Theimpactof degradation on two-shaft engines depends on thecontrol modethey are in, i.e., whether the gas generator speed or the firingtemperature are the limitingfactors. Additionally, the method andlocation of measuring the firing temperature will determine thebehavior of theengine in degraded conditions.

In the following comparisons, it should be noted that theauthors separate effects that normally occur together: Compressordegradationwill impactpressureratio, efficiency, andflowcapacity,albeit to variousdegrees, depending onthetypeof degradation.

First, theauthors will review thecaseof an enginewithreducedcompressor efficiencydue to fouling.

Compressor degradation, in theoverall engine environment willyield different results for single andtwo-shaft engines. Due to thefixed speed of the single shaft machine, in combination with a

usually choked turbine nozzle, loss of compressor efficiency willmostly affect compressor pressureratio, but only to a very limiteddegreetheflow throughthemachine(Figures5and6). A two-shaftengine with a compressor with reduced efficiency will exhibitsignificant changes in flow and pressure ratio. The reduction inflow is observed by numerous authors (Syverud, et al., 2005;Millsaps, et al., 2004) Figure 7 reflects the same findings asSpakovszky, et al. (1999) (Figure 8): The pressure ratio-flowrelationship of the gas turbine compressor remains unchanged(because it is determined by the turbine section), but the enginewill have to run faster, and the compressor will consume morepower for any point onthe pressure-flow lineonceit deteriorates.



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Figure 5. Impact of Loss of Compressor Efficiency Due to

Degradation on Gas Turbine Heat Rate.

Figure 6. Impact of Loss of Compressor Efficiency Due to

Degradation on Gas Turbine Power Output.

Figure 7. Gas Generator Compressor Operating Line for a New

and Cl ean, and a Degraded Engi ne Compressor. (Singl e Shaft L eft,

Two Shaft Right.)

Figure 8. Comparison of a New and Clean (Solid Line) with a

Degraded (Broken Line) Engine Compressor Map, and the

Respective Operating Points of the Compressor. (Courtesy of

Spakovszky, et al., 1999, ASME)

It seems that the power output of a single shaft engine is lesssusceptible to the effects of degradation, while the heat rate ofsingle and two-shaft engines is affected at about the same rate.BothTarabrin, et al. (1998), andMeher-Homji andBromley(2004)also find that twin spool and three spool engines seem to beparticularly susceptibleto performancedeterioration. For all types,the relativedegradation in power is far morepronounced than thedegradation in heat ratefor atwo-shaft engine.

Next, theauthorsconsider theimpactofreduced compressor flowcapacity, whichcanbetheresult of fouling, orincreasedclearances.

Again, the finding is that the same level of compressor flowblockage leads to more power degradation in a two-shaft enginethan in asingle shaft engine. It is interestingthat neither for singleshaft, nor for two-shaft enginesthereis asignif icant impactonheatrate at higher ambient temperatures. Only for temperatures below50F, the heat ratestartsto increaseslightly at about the same rateof single shaft andtwo-shaft machines.

For two-shaft engines, the effectsof degradation depend alsoon the control mode the engine is in. If the engine is operatingat ambient temperatures belowthematchtemperature, it will bein speed topping mode, i.e., the gas generator output is limitedby thegas generator speed. If theengine is operating at ambienttemperatures above the match temperature, it will bein turbineinlet temperature topping mode, i.e., the gas generator outputis limited by the firing temperature (so will any singleshaft engine).

Tarabrin, et al. (1998), point out the effect of degradation istypically more severe for two-shaft engines than for single shaftengines. However, since compressor degradation on thetwo-shaftengine leads to a drop in gas generator speed at high ambienttemperatures, then, if variable geometry is available (in the

Tarabrin, et al. [1998] example adjustable PT nozzles, in theexample cited by Kurz and Brun [2001] adjustable compressorinlet guide vanes (IGVs) and stators), this drop in speed can beavoided by readjusting the settings, and the effects of compressordegradation are closer between singleand two-shaft engines.

Another point worth mentioning is the part load behavior.An operating point at part load (i.e., below maximum firingtemperature and below maximum NGP) can still be maintainedwith adegraded engine, albeit at a higher firingtemperatureandahigher gas generatorspeed thanfor thenew condition. Therelative

loss in efficiency is significantly lower than for an engineat fullload for thesame amount of degradation.

Compressor deterioration by itself will usually cause higherpower losses than losses in heat rate, because higher compressorexit temperature (due to lower efficiency) at a fixed firingtemperature will reduce the possible fuel flow. MacLeod, et al.(1991), who investigated the effect of component deterioration onoverall engine performance on a single shaft turboprop engine,found that an increasein GP nozzle through-flow areaof 6 percentduetoerosion, cracking, bowing, andcorrosioncausedanincreasein heat rateby 3.55 percent but virtually no loss in power.

Rather severe degradation of the gas generator turbine nozzleoccurs when material is removed due to erosion or corrosion. I nthis case, the pressure ratio over the turbineandits efficiency willdrop. In a two-shaft engine, this will lead to a reduction in gas

generator speed. The effect on the overall engine performancedepends on the speed characteristic of the compressor, but if it isassumed that the compressor efficiency does not change withspeed, then a signif icant reductionin engine outputandefficiencyis seen, in particular at higher ambient temperatures. For somecompressor designs, thedesignoperatingpoint is at slightly higherspeedsthantheoptimumefficiencypoint of thecompressor. Inthiscase, the increasein compressor efficiency at reduced speeds canto some extent counteract the loss of engine efficiency (Kurz andBrun, 2001).

For a single shaft engine, operating at constant gas generatorspeed, anincreaseingasgeneratorflowcapacity will leadtoadrop


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in compressor discharge pressure (that is, the compressor gainsin surge margin), while the engine flow stays the same. This isaccompanied by aloss in power andefficiency.

Yet another effect isintroduced by thefact that inmost enginestheturbineinlet temperatureis not measured directly. Rather, thepower turbine inlet temperature (PTI T) or (on single shaftengines) the exhaust gas temperature (EGT) are measured, and

TIT is calculated. The measured PTIT or EGT is not thethermodynamic average temperature, but thearithmetic averageof several point measurements. Degradation of engines can lead

to a shift in the true ratio between TIT and PTIT, thus causingthe engine to over or underfire. This shift can be due toseveral reasons:

Change in flame patterns, with resultingchange in temperaturepatterns

Change in gas generator turbine efficiency or flowcapacity

The second cause in the list has in fact a pronounced effect,mainly on the engine output, but only a small effect on engineefficiency. It should benoted that theselosses can berecovered byreadjustingthe control system. It can beshown that a reductionofgasgeneratorturbineefficiency oranincreaseinflowcapacity willlead to a underfiringof the engineunder these conditions. This isactually one of the positivesideeffects of this control modethe

engineis not driven into a moredamaging overfiringsituation.Degradation also affects the optimum power turbine speed,albeit not very much. If a lower engine compressor ratio ordeterioration at thegas generatorturbineitself leadsto alower gasgenerator turbine pressure ratio, then the actual flow through thepower turbine will increase slightly, thus increasing the optimumspeed. However, the effect on the engine output at fixed powerturbine speed is rather small (less then 0.1 percent) for modernpower turbines with relatively wide operating range.

Figure 9 shows the result of a variety of measures to reverse theeffects of degradation. In the example, the engine was returned tothefactory after several thousandhoursof operation. Theinitial runof the engine, without any adjustments whatsoever andat the IGVandT5 topping temperature from the original factory test, showedthe TIT was 40F (22C) below design T3and the gas generatorspeed was 3 percent below design speed. After adjusting the guidevanes to get back to thedesired designT3 and gas generator speed,theengineimproved power andreduced heat rate. Then, theenginewas detergentwashedand continuedto improveperformance. Nexttheindividual stages of compressor variable vanes wereadjusted tothe factory settings improving power. After all of the adjustmentsthe engine, compared to the factory testing when the engine wasnew, had lost 2.5 percent in power and 1.2 percent in heat rate.

Theseresultsshow thatasignificantamountof apparentdegradationcan bereversed bywashingand adjustments.

Figure 9. Performance Recovery from Sequentiall y Applied

Methods: Adjustment of IGVs, Detergent Wash, Indivi dual

Adjustment of IGVs. (Courtesy of Kurz and Brun, 2001, ASME)

In another example, anindustrial gas turbine was operated 3500hours without a detergent wash. The environment was laden with

jet engine exhaust and salt laden air. Borescope inspections hadshown deposits on the compressor and turbine. A detergent washwas recommended. Control systemdatawere recorded at full loadbefore and after detergent washing and are displayed in Figure 9.

Thedataweretakenwiththeambient temperaturebelow thedesignmatch temperature.

The improvement in compressor pressure ratio, compressorefficiency, power, and heat rate are as expected, because the

washing was reported to bevery black and dirty. The engine wasalso underfired, because one of the effects of degradation is thereduction in theT3/T5 ratio. These improvementsexplain the verysubstantial 9.7 percent increase in output power. After washing,andgiventhetestuncertainties, thisengineappearstobeperformingessentially the same as when new. The model predicts exactly thisbehavior: if oneuses a2.1 percentloss in compressor efficiency, a5 percentreductionin airflow andpressureratio, anda0.5percentreductionin gas generator turbine efficiency, one sees a reductionin power of 8.6 percent, the efficiency dropsby 3.5 percent, whilethe engine speed stays almost the same, and T3 drops due to areduction in T3/T5 ratio (Figure10).

Figure 10. Salt Deposits on the Compressor Blades, Before andAfter

Onli ne Washing. ( Courtesy of Syverud and Bakken, 2005, ASME)

Interestingly, while Tarabrin, et al. (1998), indicate that smallerengines may bemoresusceptible to degradation, the study of AkerandSaravanamuttoo(1988) reachesexactly theoppositeconclusion.It is suspected that the intricateinteractions that define the amountof degradation as a function of a certain level of ingested material,or of a certain amount of material removal arevery engine specific,anddonot lend themselves to simplifiedrules of thumb.

RECOVERABLE ANDNONRECOVERABLE DEGRADATION

The distinction between recoverable and nonrecoverable

degradation is somewhat misleading. The majority of degradationis recoverable, however the effort is very different depending onthe type of degradation. The recovery effort may be as small aswater or detergentonlinewashing, or detergenton-crank washing.

The degradation recovery by any means of washing is usuallyreferred to as recoverable degradation. However, a significantamount of degradation can be recovered by engine adjustments(such as resetting variable geometry). Last, but not least, variousdegrees of component replacement in overhaul can bring thesystemperformanceback to as-new conditions.

Therearefew publications indicating the rate of degradationon gas turbines. It seems that, in many instances, the initial



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degradation on a new engine is seen as more rapid than thedegradation after several thousand hours of operation. Onecausefor this mightliein differentperformancetestpractices ofdifferent manufacturers. If the engine, as a part of the factorytest process, is subjected to a hot restart (i.e., the engine is shutdown, and almost immediately restarted. This will normallycause the largest possible clearances. Therefore, during lateroperation of the engine, clearances will not open up further)prior to the performance test, the performance datawill alreadyreflect the clearances that will likely not deteriorate during the

later engine operation. In this case, the early sharp drop indegradation may be reduced. Another design feature that canslow degradation is the effort to thermally match stationary androtating parts of theengine. This meansthat thethermal growthof components is matched such that the running clearanceremains constant during thermal cycles (i.e., change of load orstart and stop).

DatagatheredbyVeer, et al. (2004), indicatethat bothrecoverable(fouling) and nonrecoverable degradation follow a logarithmicpattern, i.e., a large rate of initial degradation that is reducedgradually. Thenonrecoverable degradation of the power output intheir dataindicated alossof power of 3.5 percent in thefirst 5000hours, followed by only an additional 0.5 percent over the next5000 hours. However, this is not universally confirmed forother applications.

Obviously, the dominant degradation mechanisms for aircraftengines and industrial engines are different. Aircraft engines areoperated without the benefit of an inlet air filtration system, and,therefore, erosion is oneof thekey contributors.Industrial engines,assuming an appropriate air filtration system is installed, areprobably moresubject to fouling caused by smaller particles (andpossibly lube oil).

PROTECTION AGAINST DEGRADATION

While engine degradation cannot entirely be avoided, certainprecautions can clearly slow the effects down. These precautionsinclude the careful selection and maintenance of the air filtrationequipment, and the careful treatment of fuel, steam, or water thatare injected into the combustion process. It also includes obeyingmanufacturers recommendationsregardingshutdownandrestartingprocedures. For thedriven equipmentsurgeavoidance, process gasfree of solids and liquids, and operation within the design limitsneed to bementioned. With regard to steam injection, it must benotedthat therequirementsfor contaminantlimits for agasturbineare,duetothehigherprocesstemperatures, morestringentthanforasteamturbine.

The site location and environment conditions, which dictateairbornecontaminants, their size, concentration, andcomposition,need to be considered in the selection of air filtration.Atmospheric conditions, such as humidity, smog, precipitation,mist, fog, dust, oil fumes, industrial exhausts, will primarilyaffect the engine compressor. Fuel quality will impact the hotsection.Thecleanlinessof theprocessgas,entrainedparticles, orliquids will affect the driven equipment performance. Given allthese variables, the rate of degradation is impossible to predictwith reasonable accuracy.

While the rate of deterioration is slowed by frequent onlinewashing, thorough on-crank washing can yield a moresignif icantrecovery. Online washing will usually only clean the f irst fewstages of thecompressor, because theincrease in air temperatureduring compression will evaporate the washing detergent. Theonline washingprocessthereforecan transport contaminants fromthe front stages of the compressor to rear stages or the turbinesection. No matter how good the washing, the rear stages of thecompressor will not get cleaned. If the compressor blades can beaccessedwith moderateeffort (for example, when the compressorcasing is horizontally split), hand cleaning of the blades can bevery effective.

WATER WASHING

Syverud and Bakken (2005) performed investigations on theeffectiveness of ingestive online cleaning on the performancerecovery of a gas turbine compressor that had been deteriorateddue to salt water ingestion. Online water washing has theadvantagethat engine shutdown can beavoided. They found thatsomeof thesalt will redeposit ontheaft stages, but statethat if thewater flow is high enough, most redeposition can be avoided.

They recommend that a high water flow rate produces the leastredeposition in the aft stages. It should be noted however that the

compressor they wroteabouthas only apressureratio of 6.5, whichis significantly less than what is found in modern industrial gasturbines. In higher pressureratio machines, redeposition of dirt inthelater stagesisalmostunavoidable,becausetheair temperatureinthe compressor increases with pressure. Further findings include(Syverud, et al., 2005;Tarabrin, et al., 1998)that thevastmajority ofinitial deposits (50 to 60 percent) occur on the first stage, withanother20to25percentonthesecondcompressorstage(Figure10).

Online washing reduces fouling in the compressor only if thecleaningis performed in very frequentintervals. The duration canbe very short becausemost of the cleaning occurs within the first20 seconds after turning on the spray. An even spray pattern iscritically important. Online cleaning will only extend offlinecleaning intervals and does not eliminate the need to performoffline compressor cleaning. Redeposition of the fouling in

downstream stages of the compressor cannot be avoided, but anincreased water-to-air ratio reduces the level of redeposit.However, too muchwater (and large liquid droplet sizes) will leadto surface erosion and possible aerodynamically induced highcycle fatigueif operatedfor extendedperiods. Also, althoughsomemanufacturers claim that specialized liquids will improve thecleaning efficiency and reduce fouling redeposition, test resultsshowthat thetypeof liquid beinguseddoesnot signif icantly affecteither one. Clean, deionizedwater works as well as any liquid butwill not introduceother undesirabledeposits.

INLET SYSTEM

Industrial gas turbines usually employ an air inlet system thatincludes some sort of filtration system, a silencer, and ductwork(Figure 2). The inlet system of the gas turbine needs to be

mentioned for tworeasons:

It is one of the key devices that protects the engine from thedeterioratingeffects of airbornecontaminants.

The pressure loss created by the inlet system increases over time,therefore reducing gas turbine power and efficiency (Figure 11). Itshould be noted that the exhaust system can similarly increase thepressurelossovertime, especially if wasteheatrecovery is employed.

Figur e 11. Impact of Inl et System Pressure L osses on Power and

Heat Rate of a Typical Two-Shaft G as Turbine.


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Previous conventional thinking was that a well designed airfiltration system for a given application would perform well inalmost any environment with equal results. This thinking has beenproved tobefaulty andeconomically costly. Many companiesofferawiderangeof filter houses,filtrationsystems,andair filter elementsfor gas turbineengines tohelp themperformsuccessfully in specificoperating environments. Modular construction means custom andstandard designsareavailableinself-cleaning, reverse-pulse, barrier,upflow, crossflow, and downflow models. Theseunits are generallyengineered for fast andeasyonsiteerection, initially andfor retrofit,

and can be configured to meet many application requirements,including multistage f ilters with extremely low-pressure drop. Asmentioned earlier, there is a tradeoff between cost, size, pressuredrop, andfiltrationefficiency that has tobebalanced.

In terms of gas turbine f ilter cartridges for pulse filterhouses, the trend has been moving steadily toward improvedself-cleaningpulsefilter systemsthat caneasily bepneumaticallycleaned by compressed air. Manufacturers have been focusing onnew media blends (impregnated to be moisture resistant) andspunbond synthetics. These synthetics, which were designed toreplace older less efficient cellulose fiber type media, oftenprovide better release of dust particles.

Different sources of contaminants produce particles of vastlydifferent sizes: Diesel soot smoke, andsmogare typically smallerthan 1m (and can be as small as 0.01m), while coal dust canrange from 1 m to 10 m. Dust in industrialized areas hasdiametersbetween 2 and over 10m(Bammert, et al., 1965).

Pulsefilter housingconstructionhas also advanced. Most filtersare available with corrosion-resistant materials, including zinccoatings andstainless steel metals. Pulsefilter manufacturers alsohave added strength supports to their filters and upgradedadhesives that seal better and prevent bypassinto theair stream.

Offshore applications (or applications close to shore) areparticularly challenging due to airborne salt. Airborne saltconcentrations vary widely, as shown in Figure 12. Airborne saltscan simultaneously exist as aerosols, sprays, or crystals. Ofimportanceis that aerosols typically range in sizefrom2 micronsto 20 microns, while spray generates much larger droplet sizesin the range of 150 to 300 microns. Sea salt crystals aretypically below 2 microns in size, and absorb moisture under therelativehumidity conditions found in typical offshoreapplications

(Meher-Homji and Bromley, 2004). A dditionally, there are othercontaminants present on offshore platforms, such as carbon orsulphur from flares, drilling cements, and other dusts. Poorlypositioned flare stacks, or sudden changes in wind direction cancause problems. Filter systems that combine vanes (for moistureremoval) with coalescingfilters (to coalescesmall droplets andtoremovedust particles) can bevery effectivein this environment.

Figure 12. Salt Content in the Air in PPM as a Function of Wind

Speed. (Courtesy of Cleaver,1988, BHRA)

Theeconomicimpact of faulty filtrationis oftenunder appreciated.Poor filtration can lead to degraded power output, accelerateddamage to components leading to premature servicing andexpensive parts refurbishment or replacement, and unscheduleddowntime. Properly conditioned inlet air is critical for keepinggas turbines and other rotating machinery operating at peakperformance. Since gas turbines are installed in all types ofenvironments, the air filtration system for a particular applicationmust be designed for specific environmental conditions. Highquality air is required to prevent fouling, erosion, corrosion, and

particle fusing, regardless of site conditions. The air filtrationsystemmust providelow flow resistance(low-pressuredrop), longservice life, and ease of maintenance. It is important to comparefiltrationsystems not only onthe basis of filtrationefficiency, butalso onthebasisof thepressuredropnecessary toachieveacertainfiltration efficiency at a given airflow. Also, the efficiency of thefilter in anew and clean condition (wherethepressuredrop mightbe low, but the filtration efficiency may notyet be at its peak) andin a dirty condition (where the pressure drop will increase, butpossibly also the filtration efficiency, especially for very smallparticles) needto beevaluated.

CONDITION MONITORINGAND DATA CORRECTION

Determining the degradation of a gas turbine in operation

requires a certain level of instrumentation and a methodologyto compare data taken under various ambient and operatingconditions. The most accurate way of correcting the operation ofgasturbinestodatumconditionsisby using manufacturer softwarethat models the gas turbineor manufacturer-suppliedperformancecurves. This is especially important if part-load operating pointsare used to monitor the engine efficiency. The general correctionprocedureis as follows:

1. The control system can calculate full load power and heat ratefor the recorded, prevailing ambient conditions and a definedpower turbine speed based on the packagesensor data. It can alsorecord compressor discharge pressure, and possibly inlet systempressure losses.

2. Use the maps or the performance program to calculate theperformanceof areference engine(usually nominal) under theconditionsabove.

3. Calculate the percent difference between the test results forpower and heat ratein (1) andthereferenceresults in (2) above.

The same procedure will also apply to part-load conditions.The idea is now to monitor the percent difference fromthe reference (nominal) performance. This allows detectionperformance changes in the engine. Good practice is to excludedata taken under transient conditions. Engine compressordischargepressureisagoodindicator for compressorfouling, andthus for compressor cleaning decisions. Some parties suggestusing inlet air flowmeasurements for this purpose(usually basedon a drop in static pressure when the air is accelerated in the

engine bellmouth). The authors find the compressor dischargepressure measurement simpler and more reliable. While drivengenerators are an excellent means of measuring engine poweroutput, a driven compressor will yield data with significanttest uncertainties, unless the driven compressor instrumentationis signif icantly upgraded (and regularly calibrated). Powermeasurements using torquemeters are an alternative, but tendto be rather expensive. If the gas turbine drives a pump,measurementsof absorbed pump power are virtually impossible.Reliable fuel flow metering is state-of-the-art for most modernpackages (Coriolis flow meters, turbine flow meters, or floworifice metering runs areaccurateandreliable).



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The reason for the described method is that gas turbineperformance is very sensitive to ambient conditions (pressureandtemperature) andpower turbinespeedand, inaddition, theheat rateof thegas turbineis sensitivetopart load. Theresults can be quitedifferent for the different loads, particularly for gas turbines thatbleed air at part-load operations (such as for emissions control).

Therefore, theinformation about thepower producedis useless forcondition monitoring if it is not corrected for the prevailingoperating conditions.

Other parameters, suchasalarms,vibrations, bearingtemperatures,

oil temperatures, etc., need to be recorded as well, as they cangive valuable correlations in the search for causes of performancedegradation, in particular if thedegradationis unexplained andrapid.

CONCLUSIONS

The paper covered in detail degradation mechanisms and theimpactof componentdegradationonoverall gasturbineperformance.Proper design and selection of inlet systems and fuel treatmentsystems, togetherwithproper maintenanceand operatingpracticescan signif icantly affect the level of performance degradation andthustimebetweenrepairsoroverhauls of agasturbine. Theauthorshave avoided presenting figures about the speed of degradation,because it is subject to a variety of operational anddesignfactorsthat typically cannot becontrolled entirely.

Proactive condition monitoring will allow the gas turbine

operator to make intelligent service decisions based on the actualconditionof thegasturbinerather thanonfixedandcalendar basedmaintenanceintervals. Recommendations areprovidedonhow theoperatorcan limit this degradationandany deteriorationof thegasturbines through proper maintenance practices. The effects ofdetergent washingand best washing practicesare discussed.

Maintenance and overhaul decisions are ultimately based oneconomic and safety considerations. Understanding performancedegradation, as well as factorsthat influencedegradation, can helpin these decisions.

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