Affecting Component Lives - Steam Injection (July-August 2007)

7/31/2019 Affecting Component Lives - Steam Injection (July-August 2007)

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HUGH JIN

LIBURDI TURBINE SERVICES, INC.

Water or steam injection is a popularoption for augmenting gas turbine

power and controlling emissions.But this method affects cycle per-

formance and maintenance inspection inter-vals. For instance, a case study on anaeroderivative showed that a 3% injection ofsteam could reduce the first stage bucket life

by up to 20% (Figures 1, 2).Injecting water or steam into a com-

bustor affects dynamic pressure withinthe combustor, carbon monoxide emis-sions, combustion stability, and flame

blow out. Injecting it into the compressorinlet, inter-stage or discharge casingaffects the compressor aerodynamic sta-

bility, compressor surge margin and com-pressor degradation and fouling.

Aerodynamic instabilities in the axialcompressor and pressure oscillations inthe combustor can cause blade high-cyclefatigue and subsequent catastrophic fail-ure of the gas turbine. The injection hard-ware, engine control, auxiliaries and loadequipment must be modified and sizedappropriately as per the amount of wateror steam injection and supplemental

power output. The added water or steammust be of boiler feedwater quality to

prevent deposits and corrosion in the hotgas path downstream of the combustor.

Operators should tune their steam orwater injection methods so as to causeminimum impact on maintenance. Forinstance, in the case of aeroderivatives,which maintain a constant power turbineinlet temperature, the engine temperaturecan be reduced to minimize impact oncomponent lives.

Benefits of wet injectionNitrogen oxides are formed from the oxi-dation of free nitrogen in the combustionair or fuel. Thermal nitrogen oxide is

formed mainly as a function of the stoi-chiometric adiabatic flame temperature,which is the temperature reached by

burning a theoretically correct mixture offuel and air in an insulated vessel.

Thermal NOx production rises rapidlyas the stoichiometric flame temperature isreached. Away from this point, thermal

NOx production decreases rapidly. Thistheory provides the mechanism of thermal

NOx control. In a diffusion flame combus-tor, the primary way to control thermal

NOx is to reduce the flame temperature.Introducing water or steam into the flame

OPERATION & MAINTENANCE

Affecting component livesDESIGN WATER-STEAM INJECTION SYSTEMS TO MINIMIZE THE NEGATIVE IMPACT ON

OPERATING HOURS

July/August 2007 Turbomachinery International 29www.turbomachinerymag.com

Figure 1: Water or steam injection will

accelerate coating loss on stage 1 buckets due

to oxidation

Figure 2: Spray pattern in the water flow test

on the blade shows poor film cooling coverage


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zone achieves this effectively.NOx increases with fuel-to-air ratio, fir-

ing temperature, and residence time in theflame zone. It decreases exponentially withthe increase of water or steam injection.

Injecting water or steam into a gasturbine increases the mass flow and,therefore, the output. When water orsteam is injected for power augmenta-tion, it can be introduced into the com-

pressor discharge casing of the gas tur-bine, as well as the combustor. In com-bined-cycle operation, the cycle heat rateincreases with water or steam injectiondue to additional fuel required to heat thewater to combustor temperature. Water orsteam injection for power augmentationcan result in a power increase of 15% to18% by injection of up to a 5% mass flowof steam to the compression discharge.

For the relatively low levels of NOxreduction required in the 1970s, it was

found that injection of water or steaminto the combustion zone would producethe desired NOx reduction with minimaldetrimental impact to the gas turbinecycle performance or parts lives.Additionally, at the lower NOx reduc-tions, the other exhaust emissions weregenerally not affected adversely.

With the greater NOx reductionrequirements imposed during the 1980s,further reductions in NOx by increasedwater or steam injection began to causedetrimental effects on cycle performance,

parts lives and inspection criteria. Also,other exhaust emissions began to rise tomeasurable levels of concern. Moreexpensive technologies such as Dry LowEmission (DLE) and catalytic combus-tion were then introduced.

In a typical maintenance procedurerecommended by OEMs, a gas fuel unitoperating on continuous duty, with nowater or steam injection, is established asthe baseline condition. This conditionsets the maximum recommended mainte-nance intervals. For operation that differsfrom the baseline, factors are establishedthat determine the increased level of

maintenance required. For example, amaintenance factor of two would indicatea maintenance interval that is half of the

baseline interval. A maintenance factor ofthree or four may be required for water orsteam injection applications.

Impacting aeroderivativesThe impact of steam injection on the life ora hot section component of an aeroderiva-tive gas turbine has been studied. Stage 1

blades in the High Pressure (HP) turbinesection were selected as the most represen-tative hot section component in relation toexpected change in component tempera-ture, degradation rate and serviceable life

produced by the introduction and variationof the amount of steam injection.

The twin spool aeroderivative consistsof a five-stage Low Pressure (LP) com-

pressor, a 14-stage HP compressor, atwo-stage, air-cooled HP turbine, and afive-stage LP turbine. NOx suppression isachieved by the injection of water orsteam into the single annular combustorat a controlled rate. Depending upon theinjection rates, NOx emission of 100

ppmvd ~ 25 ppmvd (or lower) can beachieved. The gas turbine OEM permitscontinuous operation at 25ppm NOx.

During service, the hot section com-ponents undergo various types of time-dependent degradation due to exposurein the operating environment. Oxidation,hot corrosion, creep and thermalmechanical fatigue can all potentiallylead to component failure. The life ofturbine components and factors thatlimit the life vary significantly fromcomponent to component due to a com-

bination of design, material, coating andoperating condition. By identifying thespecific characteristics of degradationfor each component and predicting therate of damage occurred, economicalrepair, replacement and overhaul inter-

val can be established.A review of the operational history of

the aeroderivative gas turbine revealed thecoating losses on the leading edge of theHP turbine stage 1 blade. The standardmaterial for the HP turbine stage 1 bladesis Rene-142; a nickel-based, directionallysolidified superalloy. Metallurgical analy-sis identified the most likely root cause asoxidation spallation of the Thermal

Barrier Coating (TBC).The onset of the oxidation damage

occurring at 10,000 hours - 22,000 hourswas reported. Water-flow testing on ser-vice-run blades showed a poor film coolingcoverage on the suction side with the TBC.

TBC spallation causes increasedblade operating temperature and eventu-al oxidation of the base material. Sincethe effectiveness of the coating in pro-tecting the base metal from oxidationattack has been identified as the primarylife limiting factor a major decision

point in determining engine overhaulperiod this analysis is focused on theprediction of the impact on the oxidationlife of a stage 1 blade with steam injec-tion. The calculated life change is con-verted into an Equivalent OperatingHour (EOH) factor with reference to theengine design base condition.

Analysis and modelingTo simulate the blade operating environ-ment, an aerothermal performancemodel of the engine has been developed.The aerothermal model consists of acompressor stage-stacking program, tur-

bine mean line aerodynamic analysis,

and through-flow thermodynamic analy-sis with state-of-the-art loss calcula-tions. The aerothermal model calculatesthe average inter-stage temperature,

pressure and flow of the hot gas path.The calculated aerothermal values werethen used as the boundary condition forheat transfer analysis.

A composition-dependent gas proper-ty model was also developed for the cal-culation of gas-side mixing transport

properties. For example, thermal conduc-tivity of typical combustion gas specieswas first compiled as a function of tem-

perature.The thermal conductivity of water

increases significantly at elevated tem-peratures. The gas path mixture transportproperties were calculated. For air-cooledblades, heat transfer coefficients on bothsides of the blades have to be established

before a detailed heat transfer calculationcan be performed.

Heat transfer coefficients and gasstream properties were first established,and a detailed heat transfer calculationthroughout the blade was then performed.A steady-state energy balance approach

30 Turbomachinery International July/August 2007 www.turbomachinerymag.com

Figure 3: The thermal barrier coating (left) and

microstructure of a ceramic top coating (right)

from Electron Beam Physical Vapor Deposition

(EB-PVD)

Figure 4: Delamination on the ceramic top

coating is one of main courses of TBC failure


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was used to calculate heat conductionover a composite cylinder (e.g., TBC and

blade wall thickness). The internal cool-ing flow heat pick-up was calculatedalong the radial path. The model was cal-ibrated using estimations of metallurgicaltemperature.

Once the gas path properties wereestablished, the impact of different loadconditions was modeled. In the aeroderiv-ative under study, the external coating ofthe HP turbine blade was identified as a

TBC, with a ceramic top coating fromElectron Beam Physical Vapor Deposition(EB-PVD) and a platinum diffusion alu-minide as the bond coating (Figure 3).EB-PVD has excellent strain tolerancewith columnar microstructure. TBCs nor-mally fail by spallation due to delamina-tion of the ceramic layer along the vicini-ty of the Thermally Grown Oxide (TGO)and TBC interface (Figure 4).

The failure process involves severalmechanisms including oxidation of the

bond coat, thermal mechanical fatigue,sintering, and spallation of the TBC. Theformation of TGO and the correspondingcompressive growth stresses at the TBC-

bond coat interface remain one of maincauses of failure in TBCs. Activationenergies associated with the bond coatingoxides were used to determine the oxidegrowth rate as a function of temperature,and equivalent oxidation life was calcu-lated. This was then converted into anEquivalent Operating Hour (EOH) withreference to the design conditions speci-fied by the OEM.

For the aeroderivative turbine, withthe constant LP turbine inlet temperature

control, a 3% steam injection (25ppmNOx) increases the hot-gas-path thermalconductivity by 3.5% (Figure 5). Thiswould increase gas-side heat transfercoefficient by 6.2%. The internal heattransfer coefficient increased by 3.4%,which can be explained by the fact thatsteam injection increases the compressordischarge pressure and the cooling flowextraction. The overall heat transferresults increase the local metal tempera-ture by 12F (6.6C) resulting in a 20%

reduction in the part life.In an example given by GE for the

stage 1 buckets of the MS7001EA gasturbine, for constant firing temperaturecontrol, a 3% steam injection (25 ppm

NOx) increases the heat transfer coeffi-cient by 4%, which would result in a15F (8C) increase in the bucket metaltemperature and a 33% reduction in life.

The engine control for industrialframes is different from aeroderivative tur-

bines. The MS7001EA is controlled atconstant firing temperature, while theaeroderivative is controlled at constant LPturbine inlet temperature. It was found thatthe firing temperature of the aeroderivativeturbine decreases with steam injections.This would counter the effect of the high-er heat transfer on the gas side. Secondly,the criteria used for the MS7001EA maynot be oxidation life, which is used in theaeroderivative engine analysis.

Maintenance strategiesEngine maintenance intervals are affect-ed by the amount of water or steam injec-tion, the engine control, and how thewater or steam is injected. The impact of

water or steam injection on the mainte-nance interval can be optimized by care-ful planning in equipment selection,operation disposition and water or steaminjection design.

First, for relatively low levels ofNOx reduction, injection of water orsteam into the combustor is effectivewith minimal impact to the cycle per-formance or parts lives. Further reduc-

tions in NOx by increasing the amountof water or steam may have a negativeimpact on cycle performance, part livesand inspection intervals. Rather thangoing to Dry Low Emission (expensiveand less power), some operators usewater or steam injections to reduce

NOx to a certain level (e.g., 45ppm)and then use Selective CatalyticReduction (SCR) to do the finalcleanup. This could be a cost-effectiveoption that minimizes the impact onmaintenance intervals.

Secondly, impact on part life due towater or steam injection is related tothe way the turbine is controlled. Onmany industrial-frame gas turbinesoperating on base load, the control sys-tem automatically decreases the firingtemperature when water or steam isinjected. This counters the effect ofhigher heat transfer on the gas side, andresults in less or no impact on the livesof hot gas path parts.

However, in most of the aeroderiv-atives, the LP turbine or power turbineinlet temperature is maintained con-stant. Decreasing the engine tempera-ture manually during water or steam

injection will produce less power aug-mentation but reduce impact on com-

ponent lives.Lastly, most of the first stage buck-

ets, and first and second stage nozzlesare internally cooled by air. If water orsteam could be properly mixed with thecooling air before the air enters into theinternal cooling passages of the aircooled parts, it would increase the heattransfer coefficient on the coolant sideand reduce the metal temperature.

AuthorHugh Jin, is responsible for aerother-mal analysis, component upgrade andhot section parts life prediction inLiburdi TurbineServices Inc. He

provides a full scopeof engineering ser-vices, includingc o m m i s s i o n i n g ,auditing and perfor-mance testing ofcombined cycle

plants and pipelineturbocompressors.

TI

July/August 2007 Turbomachinery International 31www.turbomachinerymag.com

Figure 5: Comparison of life impact of steam injection on aeroderivative (left) and industrial frame

(right). The life of a stage 1 bucket on the High Pressure turbine could be reduced by up to 20%

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Affecting Component Lives - Steam Injection (July-August 2007)