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92-GT-102

The LM6000 Gas Turbine as a MechanicalDrive Power Source

R. L. CASPER and R. B. SPECTOR

GE Marine & Industrial EnginesGeneral Electric Company

Evendale, Ohio

Historical BackgroundABSTRACT

The LM6000 Gas Turbine was formallyintroduced at the IGTI Gas Turbine Conference inBrussels in 1990. It was immediately acceptedfor power generation/cogeneration applications;however, inquiries were received concerning the useof the LM6000 gas turbine for mechanical driveapplications. These inquiries included the amountof power available with decreasing gas turbinespeed, breakaway torque capability and resonantfree operation over a broad speed range.

This paper discusses the engineering studiesperformed to ensure that the LM6000 will be asuitable power source for mechanical driveapplications in the 30-40 MW power range.

Introduction

The LM6000 will begin commercial operation in1992 as the power source for 60 Hz turbinegenerator sets. In these applications the LM6000,which has been designed for peak efficiency of over40% at 3600 rpm, is directly coupled to theelectric generator. LM6000 powered 50 Hz electricgenerator sets are scheduled for startup in 1993and will utilize a speed reducing gearbox betweenthe turbine and generator.

The twin shaft LM6000 also has potential formechanical drive applications providing power forpipeline and process plant compressors. This paperdiscusses the features of the LM6000 and itsadaptability for compressor drive applications.

The mechanical drive industry has contributedgreatly to the promotion and development of gasturbines and have employed the largest number ofgas turbines in pipeline service. Pipelines havebeen particularly suited to take advantage of thebest features of gas turbines - adjustable speed,low operating costs, high reliability and lowmaintenance costs. In the 1950's, the large gastransmission lines in the United States began usinglarger combustion gas turbines for base loadcompressor drives. In addition, gas turbines havebeen applied to crude oil pipelines, gas storagefields, and secondary oil recovery schemes, such asgas repressurization and waterflood. Ratings ofgas turbines applied for these type services havealso grown rapidly. Today the ratings of units inservice range from below 1,000 hp to over 35,000 hpand include heavy frames (Type H) and aeroderiva-tive (Type G) gas turbines in single and multishaft versions and in simple, regenerative andcombined cycle arrangements.

In the past, there has been a reluctance toinstall a pipeline station with only one gasturbine, since shutdown of that single turbo-com-pressor unit due to equipment malfunction wouldresult in limited line through-put and a resultingreduction in revenue. In recent years, however,documented data shows that gas turbines, ingeneral, have compiled a remarkable record ofreliability, with gas turbine caused shutdowns ofless than one-half of one percent. In addition,the newer larger gas turbines provide bettereconomy of operation in terms of fuel consumptioncoupled with lower initial cost per installedhorsepower. Another factor favoring the use of asmaller number of units is in the overall design ofa transmission system whereby the remainingequipment can compensate for a partial or full lossat another station.

Presented at the International Gas Turbine and Aeroengine Congress and ExpositionCologne, Germany June 1-4, 1992

Copyright © 1992 by ASME

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In addition, aircraft derivative gas turbinesoffer structural and hot gas path componentsdesigned to the standards developed from aircraftgas turbine design. The considerable derate fromflight conditions results in extended periods ofoperation between shutdowns coupled with an "oncondition" maintenance concept where maintenance isapplied only as required rather than on a predeter-mined maintenance schedule based purely on operat-ing hours. Due to the modular design concept ofthe aircraft derivative unit, maintenance can beaccomplished "on site" or, at the option of theuser, the complete unit can be replaced in a shortinterval, with any required repairs accomplished atan off-site facility.

The majority of mechanical drive applicationsutilize gas turbines for natural gas compression.Gas turbines have been used extensively both onshore and for off-shore platforms. For off-shoreapplications, the aircraft derivative gas turbineoffers the highest power to weight ratio of anyprime mover available together with a smallfootprint.

Compressor Selection

When selecting a gas turbine-centrifugalcompressor arrangement to perform a specific duty,it appears logical to first determine the bestcompressor design and speed. If, however, thisoptimum compressor speed does not coincide with therated speed of an existing gas turbine, thecompressor is normally designed to match the ratedspeed of the gas turbine selected since it providesthe most economical gas turbine and gas compressorcombination for high reliability.

For some very high head applications (i.e.,gas injection) operation without speed increasinggears is either aerodynamically unacceptable orvery expensive. For medium to high head applica-tions, there can be an economic choice betweenusing gears or direct drive.

For medium to low head applications such asfor pipeline stations, direct drive compressors canbe used. Previous studies have indicated that forlow head operation, a low speed compressor designresults in a higher efficiency than a high speedcompressor. For medium or high head requirements alow speed compressor may require one more impellerthan a high speed compressor. Even though anadditional impeller may be required, this turbinecombination may be the best economical choice ifthe low speed gas turbine has a high thermalefficiency.

The GE LM6000 is a large aeroderivative gasturbine which can deliver in excess of 53,500 shafthorsepower (40 MW) at ISO conditions. A centri-fugal gas compressor driven by the LM6000 mustoperate over a band of rotating speeds with highefficiency and maintain the impeller tip speedwithin acceptable limits. For gas compressors ofthis size, an operating speed of approximately3500-4000 rpm would be appropriate to provide anacceptable impeller tip speed.

The LM6000

The LM6000 (Ref. Figure 1) is based upon theCF6-80C2 high bypass commercial aircraft engine.The entire CF6-80C2 high pressure core (highpressure compressor, high pressure turbine andcombustor) as well as the low pressure turbine,which powers both the low pressure compressor andthe load, is utilized with only minor modification(Ref. Figure 2). The low pressure compressor isthe same as that used on the LM5000 gas turbineexcept for modifications to facilitate the frontend drive output shaft, and was derived from thefan section of the CF6-50 commercial aircraftengine.

TURBINECOMPFESSOR REARFRAME E AUSTBEAR FRAME

\ COMBUSTOR /

OUTPUT-^.1'POWER -SHAFT

U U UJUP HP LP

COMPRESSOR COMPRESSOR TURBINE TURBINE

Figure 1 LM6000 Major Components

LM6000

Figure 2 CF6-80C2/LM6000 Comparison

POWER/ SHAFT

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NW12

25% -1600

0'

New rotor support frames were developed forthe LM6000. The front frame was designed tofacilitate a fixed mount configuration forindustrial applications as well as to increase theflow area of the variable bleed valves (VBV) whichare used to bleed low pressure compressor dischargeair during start-up and part power operation forfixed speed turbine/generator set applications. Anew turbine rear frame was designed to facilitatethe incorporation of a balanced piston to controlloading of the low pressure rotor thrust bearingand to increase the available exhaust flow area.Additionally, a variable inlet guide vane (VIGV)assembly was added at the inlet to the low pressurecompressor. The VIGV's restrict inlet air flow atlow power and are scheduled in conjunction with thefront frame VBV's to match low pressure compressorand high pressure compressor airflow during start-up and part power operation in constant speedapplications. For mechanical drive applications, amodified or fixed position VIGV schedule is moreappropriate since low pressure compressor airflowwill vary with low pressure rotor speed. (Table 1summarizes the design heritage of the LM6000).

TABLE 1LM6000 ENGINE DESCRIPTION

LPC

DERIVED FROM: LM5000/CF6-50NORMAL OPERATING SPEED 3600 RPMSTAGES-ROTOR 5

- STATOR 5 + VIGV

HPC

COMMON TO: CF6-80C2STAGES-ROTOR 14

- STATOR 14 (6 VARIABLE)

HPT

COMMON TO: CF6-80C2STAGES-ROTOR 2

-STATOR 2

LPT

COMMON TO: CF6-80C2/CF6-80E1STAGES-ROTOR 5

-STATOR 5

OVERALL LENGTH(FWD. COUPLING TO REAR COUPLING) 188.0 IN.TOTAL WEIGHT (ESTIMATED) 13,270 LBS.

Studies conducted using computer modeling topredict performance in mechanical drive applica-tions indicate that at ISO day conditions with 4inches of water loss at the inlet and exhaust, theLM6000 can deliver in excess of 55,000 shafthorsepower at 3600 rpm at a thermal efficiency of42%. At 100°F, the LM6000 will produce 37,500shaft horsepower at 3600 rpm. At 100°F, a maximumoutput capability of approximately 41,500 shafthorsepower at 3200 rpm is produced. Figure 3 showsthe maximum predicted output capability and effi-ciency of the LM6000 at ISO day conditions over thespeed range from 1600 to 3750 rpm.

ii - _

1600 2000 2400 2800 3200 3600 4000

Output Shaft Speed, RPM

45%

Max Power

40%

35%

I 30%

2000 2400 2800 3200 3600 4000

Output Shaft Speed, RPM

Figure 3

Estimated Mechanical Drive Performance

By coincidence, the design speed of fansections and low pressure turbines for the GE CF6family of aircraft engines is approximately 3600rpm. As a result, peak power and operatingefficiency of the LM6000 is achieved also at 3600rpm. Aircraft engine design requirements includethe capability to operate reliably and economicallyover a wide range of power settings. Since 60-80%of the output power on a high bypass aircraftengine is derived from the fan, the low pressurerotor components are designed to operate over awide range of speeds. The LM6000 benefits fromthese design features.

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Gas Turbine Factory Testing

For compressor drive applications, powerrequirements vary as a function of shaft speed. Asthrough-flow requirements of a centrifugalcompressor vary, the speed of drive train isvaried. This requires a gas turbine design whichis free of design limitations which would prohibitsteady state operation over the entire speedrange. Rotor dynamics are typically matched bydesigning the coupling shafts and mountingstructure for the gas turbine and driven equipmentto avoid critical vibration modes in the operatingrange. Airfoils used in aircraft engines are alsodesigned to avoid vibrational excitation modeswhich could be detrimental over the engineoperating speed range. The reaction of airfoilexcitation modes can be influenced by changes ingas turbine rotor support structures such as thefront and rear frames. To validate the suitabilityof the LM6000 for variable speed operation, the lowpressure compressor and low pressure turbineairfoils of the LM6000 factory development testunit shown in Figure 4 have been instrumented sothat stress levels can be measured over the entireoperating range.

Figure 4

LM6000 Test Set-Up

The test facility utilized for both thefactory development testing (and final acceptancetesting of production gas turbines) utilizes anelectric generator and a series of resistor loadbanks which dissipate the output energy from thegenerator. This arrangement allows the gas turbineto be tested at both the constant speed/variablepower conditions typical of that needed forelectric generator drive applications and also atvariable speed/variable load conditions which aresimilar to those of compressor drive applications.

Factory development testing was initiated inthe fourth quarter of 1991, using an LM6000equipped with over 900 pieces of instrumentation.This instrumentation was applied to measureinternal pressures, temperatures, stresses andflows needed to verify mechanical design calcu-lations and to measure gas turbine performance.This instrumentation includes strain gages onrotating and stationary airfoils and structuralcomponents in both the low pressure compressor andlow pressure turbine. Testing which will continueinto 1992, is planned to verify analyticalpredictions for both fixed speed generator driveapplications as well as for mechanical drive. Gasturbine starts will be made with/without thegenerator rotor jacking oil system activated in aneffort to evaluate the breakaway torque capabilityof the LM6000.

Component Tests

Component testing has always been used to gainconfidence in a design prior to use on a gasturbine. Development work on the LM6000 hasfollowed this philosophy. This section describesseveral of the component tests that were completedin advance of the detail design and factory testing.

Scale Model Inlet Distortion Testing. Aquarter-scale flow test model of the radial inletvolute was created and flow tested for design andoff-design conditions. There were two primarypurposes for the model tests: 1) to confirm ductpressure losses, and 2) to measure distortionlevels at the VIGV inlet plane, given differentamounts of distortion at the volute inlet plane.

VIGV Single Vane vs. Trailing Flap CascadeTests. Back-to-back comparative cascade testswere performed on single and articulating variable-inlet guide vane designs. Six different vanesettings (-15, 0, 15, 30, 45, and 60 deg) wereevaluated for both designs at nominal exit machnumbers of 0.4, 0.5, and 0.6. Testing includingflow visualization was carried out using a titaniumoxide emulsion in conjunction with oil and oleicacid to determine when flow separation had occurred.

VBV Door Flow Testing. A half-scale modelof the front frame flowpath and VBV doors wascreated and flow tested in order to verify the flowfunction of the +83 percent flow area VBV doors andlarge collector manifold. Data was obtained for anumber of door angles, flowpath mach number, andbleed door pressure ratios. Based on this testing,the VBV bypass system flow capability andperformance is expected to exceed LM6000requirements.

Combustor Component Testing. Germaine tothe LM6000 application is the flame radial profileand pattern factor (circumferential variation) forthe CF6-80C2 combustor when operating on naturalgas, particularly when using steam injection foremissions reduction. A full-size combustor rigtest with 30 fuel/steam nozzles was run at GE'sCombustion Laboratory. The rig was run at engineflow velocities, temperatures, and fuel-to-airratios, with pressures and weight flows scaled toone atmosphere.

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Results for natural gas operation showed theflame profile to be within the statistical varia-tion measured for -80C2 tests for distillate fuel,with a reduced pattern factor. When the steam-to-fuel ratio is increased from 0 to 3.0, the outerprofile becomes cooler and the inner profile hotterrelative to the radial average, yet the overalltemperature magnitude becomes cooler due to thesteam's lower temperature.

Water Wash Flow Visualization. Key tosuccessful continued operation of gas turbineengines is the ability to remove deposits from thecompressor airfoils. For the LM6000, both cranksoak and on-line water wash systems have beendesigned. A full scale flow model of the radialinlet and water wash system was completed todemonstrate successful placement of the water/cleaning solution injection nozzles.

Traditional AeroderivativeApproach to Industrialization

Fan

LP

L-i

compressor

HP Lpcanpressor Hp turbine

urbineParent aircrall

engine

Common er ^Unigue—^

ae=nd"ulna

.nvalive HP

HP turbine LP Generamr orcompressor turbine P_

-.Pressor

J turbine

compressor

LM6000 Approach to AircraftEngine Industrialization

Fan

LPcompressor

HP LPconvessr HP turbine

turbineParent aircran

engine

Common

LM60DOaeroderivalive I L_-- HPHP turbine ^Ma

compressor Turbine conPeesor

LPcompressor

Figure 5

Traditional and New Approach ForAeroderivative Engines

Summary

As previously stated, the LM6000 is a largeaeroderivative gas turbine which can deliver inexcess of 53,500 shaft horsepower (40 MW) at ISOconditions. The two shaft design concept of theLM6000 gives it a maximum power output versus speedcharacteristic which matches closely with the cubicload characteristics of a typical gas compressor.By eliminating the free turbine normally appliedwhen adapting on aircraft engines for industrialapplications, Ref. Figure 5, the LM6000 is able toproduce power more efficiently than other gasturbines of comparable size.

Figure 3 shows the predicted performancecharacteristics of the LM6000 for mechanical driveapplications. Verification of these performancepredictions, as well as verification of mechanicaldesign capability, is planned as part of the LM6000factory demonstration testing.

References

Gas Turbine in the Pipeline Industry, G.E. Walker,Sawyer's Gas Turbine Engineering Handbook, SecondEdition, Volume II, Chapter 9.

GE Aeroderivatives for Pipeline Applications,T.R. McCormack, GE Document AE 3168 LM Aeroderiva-tive Gas Turbine Applications Information Series,General Electric Co.

Oganowski, G., "GE LM6000, Development of the First40% Thermal Efficiency Gas Turbine", GE Marine andIndustrial Engines

Haaser, F.G. and Casper, R.L., "Development of theLM6000, The World's Most Efficient Simple Cycle GasTurbine", ASME Cogen-Turbo V, Sept. 3-5, 1991

Casper, R.L., "GE LM6000, Development of the First40% Thermal Efficient Gas Turbine", American PowerConference, Chicago, IL, April 28-May 1, 1991

Casper, R.L. and Millhaem, W.H., "The LM6000,Efficient and Adaptable Gas Turbine Power",International Joint Power Conference, San Diego,CA, October 6-10, 1991

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