Ulf Sjoemar, Manager of Mechanical Group Skandinaviska Raffinaderi AB, Lysekil, Sweden Georg Wahl, General Manager, Engineering, Voith Turbo GmbH & Co. KG, Crailsheim, Germany Wolfgang Sautter, Project Manager, Variable Speed Drives, Voith Turbo GmbH & Co. KG, Crailsheim, Germany David Pell, Manager of Application Engineering and Sales, Voith Transmissions, Inc., York, PA. USA Retrofit of steam turbine with fixed speed induction motor and variable planetary gear presented at ASME-Conference Baton Rouge/USA, February 1999
Transcript
Ulf Sjoemar, Manager of Mechanical Group Skandinaviska Raffinaderi AB, Lysekil, Sweden
Georg Wahl, General Manager, Engineering, Voith Turbo GmbH & Co. KG, Crailsheim, Germany
David Pell, Manager of Application Engineering and Sales, Voith Transmissions, Inc., York, PA. USA
Retrofit of steam turbine with fixed speed inductionmotor and variable planetary gearpresented at ASME-Conference Baton Rouge/USA, February 1999
Wolfgang Sautter is the ProjectManager in the Variable Speed DriveGroup at Voith Turbo PowerTransmission in Crailsheim, Germany.He joined Voith in 1990 and hasgathered experience in engineering,project management, applicationengineering, marketing and sales.From October 1996 to October 1997Wolfgang Sautter worked in theVariable Speed Drive Group at VoithTransmissions in York, Pennsylvania.He graduated from BerufsakademieHeidenheim, Germany with a Dipl.-Ing. (BA).
David Pell is the Manager ofApplication Engineering and Sales inthe Variable Speed Drive Group atVoith Transmissions in York,Pennsylvania. He has 15 years ofexperience in applying Voith variablespeed drive technology to variousrotating equipment. Prior to joiningVoith in 1982, David Pell worked forIngersoll Rand ReciprocatingCompressor Division in India. Hegraduated from the University ofMadras, India in 1977 with a Master’sDegree in Chemical Engineering.
Abstract
The retrofit of a speed-controlledcompressor drive is presented. Thesteam turbine used previously isreplaced by an induction motor withconstant speed and a variableplanetary gear. To justify the projectthere are several points consideredbased on a pay back period belowtwo years. Two different electric motor drivingsystem are evaluated. A variablefrequency drive and a constant speedinduction motor driving through avariable planetary gear drive. Totalcost comparison made the selectionfor the variable planetary gear. Thisdrive consists of a planetary gear anda hydrodynamic torque converter inthe ”power-split” branch. It has aregulating range of 70 - 105 % andhigh efficiency. The drive allows thecompressor to operate within itsrequired speed range. A torsionalvibration analysis is presented alongwith a brief description of the design.Manufacturing installation and asuccessfully commissioning carriedout in two days is presented.
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Synopsis
Recent developments in hydro-dynamic-variable planetary geartechnology using the principle of”Power-Splitting” have allowed itsuse as an effective drive forcompressors. A standard inductionmotor used as the driver. Aneconomic solution is the variableplanetary gear drive with a highefficiency and reliability.
Biography
Ulf Sjoemar is Manager of theMechanical Group and responsiblefor rotating machinery at Scanraffrefinery in Lysekil Sweden. Scanraff ispartly owned by Preem PetroleumAB, which is a major Swedish oilcompany. He was allocated asMachinery Specialist for theextension of Preem Refinery inGothenburg Sweden 1995 to 1997.Mr. Sjoemar was graduated from theChalmers University of Technology inGothenburg 1973 with Master ofScience in Mechanical Engineeringand has over twenty years experienceof rotating machinery.
Georg Wahl is the General Manager ofEngineering in the Variable SpeedDrive Group at Voith Turbo PowerTransmission in Crailsheim, Germany.He has 32 years of experience indevelopment, manufacturing, testingand field service of hydrodynamicvariable speed drives. Mr. Wahl isresponsible for design and develop-ment of variable speed drives withtorque converter, hydraulic coupling,including gears, also for high speedapplication. He graduated fromFachhochschule Augsburg Germanyin 1961 with a Dipl.- Ing. (FH).
Retrofit of steam turbine with fixed speed inductionmotor and variable planetary gear
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Introduction
In May 1995, the Preem refinery(former OK refinery) in GothenburgSweden decided to replace a steamturbine driving a compressor (Figure1) by a new driving system. Originallythe compressor was installed in 1965and revamped in 1991.
The compressor is installed in agasoline producing reformer unit inthe refinery. Figure 2 shows the circuitdiagram.
The compressor is circulating gas(mainly hydrogen) through heatexchangers, heaters, reactors,coolers and separators in a closedloop. The reformer gas circuit is fedby naphtha after the compressor. Thenaphtha is then heated, transformedto high octane gasoline by a catalyticreaction in the reactors, cooled andseparated as a outgoing productbefore the compressor.
This process uses only onecompressor. There is no sparecompressor so a stop of the com-
pressor will shut down the wholereformer unit and also downstreamprocess units with a loss ofproduction and a great economicalimpact.This means that the compressorsystem should have a 100%availability between the mainturnarounds, which occur every fouryears.
Plant requirements
Justification of the project
The project was justified based on apay back on invested money belowtwo years:
� The turbine is restricted in power.With the new driving system thereis a possibility to keep a highcompressor flow and a high ratiobetween circulating gas andnaphtha feed for most gascompositions giving a slower cokebuild up in the catalyst. Thismeans that the number of unit
shut downs for regeneration of thecatalyst can be reduced fromtwice per year to one per year fora period of one week. The higherpower of the new driving systemwill also give the possibility for afaster regeneration, when thecompressor is used to circulate anitrogen/air gas mixture forreduction of coke.
� Reduced energy cost with aelectric motor drive. The cost forelectricity in Sweden is rather lowfor big consumers (about 3.25cent/kWh). As the electrical supplyis very reliable there is no need fora steam turbine for availabilityreason.
� As the production of high pressuresteam by the boilers is reducedthe national environmental fee forNOx emission is also reduced.
� The refinery steam balance isimproved.
Fig. 1: Old installation
Fig. 2: Schematic process description
Heater
Reactor
Separator
HeatSweet naphatha
Fuel gas, LPGreformate for gasoline blending
Excess
Air
Vol
Wat
Circulatingcompressor
4
� The refinery circulating coolingwater system, which is highlyloaded during summer time givingcooling problem, is unloaded by 9to 12 MW as the turbinecondenser is not needed. This willalso make it possible to add thenew gas oil desulphination unit tothe existing cooling water systemwithout modifications.
� The maintenance cost for the newcompressor electric motor drivingsystem will be lower than for theold (30 years) turbine (withauxiliaries) driving system.
� The old high maintenance deman-ding lube oil system can beexchanged with new integratedlube oil system of the variableplanetary gear
To be able to fulfil the differentoperating cases ranging from lighthigh hydrogen content gas to almostpure nitrogen gas the compressorhave to be speed controlled.
Selection of equipment
Two different electric motor drivensystem were evaluated. One with afrequency controlled induction motorwith a gearbox and one with aconstant speed two pole inductionmotor driving through a variableplanetary gear.
The evaluation can be summarised asfollows:
� The frequency controlled motorhad a slightly better efficiencythan the variable planetary gear.
� The frequency controlled motorhad a lower noise emission thanthe variable planetary gear. The
� A new lube oil system wasrequired with the frequencycontrolled motor. The variableplanetary gear has an integratedlube oil system.
� The frequency controlled motorhas a risk for producing torsionalshaft vibration and harmonics inthe electric system, despitepredictive calculations.
� A spare rotor assembly wasbought for the variable planetarygear as this was a long lead item.
When above factors were consideredand transformed to an economicalvalue the total cost (investment/operation/maintenance) for thevariable planetary gear was lower byapp. 9% when compared with thefrequency controlled motor. As thecommissioning and start up periodfor the new driving system was veryshort there was no time to solveunforeseen problems like shafttorsion vibrations.
Based of this the final selection wasmade in favour of the variableplanetary gear. Figure 3.The retrofit project was handled byDresser Rand, Le Havre France as themain contractor.
Variable Speed Drive
Analysis of the system data
The contractor responsible for theretrofit, analysed the required pres-sure and flow data of the process(Figure 4) and converted it to powerversus speed data of the compressor(Figure 5).
Fig. 3: Variable planetary gear
Fig. 4: Pressure vs. FlowM = Main shaftTC = Hydrodynamic torque converterPF = Planetary gear-fixedPR = Planetary gear-revolving
Rated caseLight caseRegeneration N2
Dis
ch p
ress
ure
(bar
)
Flow (m3/h)
2000
40
35
30
25
20
15
106000 10000
Fig. 5: Power vs. Speed
Rated caseLight caseRegeneration N2
Pow
er (k
W)
Speed (rpm)
6000
5000
4000
3000
2000
1000
08000 10000
planetary gear with the high efficiencyof app. 98%. Only app. 25% of thepower is branched from the mainshaft through a variable speed torqueconverter to the planet carrier. Thetorque converter has a efficiency upto 89%. The total efficiency of thevariable planetary gear can be up to95,5%.Figure 8 shows the function of aplanetary gear when the branchedpower is superimposed through theplanet carrier. The ring gear (annulus)is running with constant speed of theinduction motor. If the speed of the
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Three cases had to be fulfilled:� Rated case, higher content of
nitrogen (N2), normal powerdemand
� Light gas case, higher content ofnitrogen (N2), lower powerdemand
� Regeneration with nitrogen (N2)with low air content, highestpower demand
The final design data for thecompressor are
Rated power: 4400 kWOutput speed: 10450 rpm/9952 rpmSpeedcontrol range: down to 6600 rpmOperatingmode: 100 % continuous
and for the Motor
Rated Power 5000 kWInput speed: 2987 rpm
Variable planetary gear design
For this power and speed data, adrive was selected consisting of aconstant speed induction motor anda variable planetary gear. The variableplanetary gear include a variableplanetary gear and a hydrodynamictorque converter operating to the”power-split” principle (Figure 6).
Principle of operation
The main shaft is connected via aconnection coupling to the inductionmotor shaft. The sun wheel of theplanetary gear is connected to thecompressor shaft via a connectioncoupling. As shown in Figure 7.Approximately 75% of the power istransmitted directly from the mainmotor to the compressor via the
Fig. 6: Principle of operation
In the torque convertera part of the power issplit off. This part issubject to control.
The main motordrives withconstant speed.
This speed of the branched-off part is adapted in thestationary planetary gear.The main part of the poweris enters directly into therevolving gear.
The variable outputspeed guaranteesan exact adaptationof the speed of theworking machine.
In the revolvingplanetary gear bothpower strings areunited.
Fig. 7: Planetary gear; principle of power split
Pow
er [%
]
0
-20
20
40
60
60 70 80 90 100
P1 P1
P2
P2
P3
P3
= P1 + P2
50
80
100
Speed [%]
P1 = Power annulus gearP2 = Power planet carrierP3 = Power sun gear - Output
Sun wheelconnectedto outputshaft
Fig. 8: Planetary gear
Annulusconnectedto motor
Planet
Outputspeed
Carrierspeed
Motorspeed
Planet carrierconnected totorque converter
Torsional analysis
There are no torsional vibrationexcitations from the induction motorand not from the variable planetarygear drive during operation.
Only for switching on and failures inthe electric supply system torsionalvibration excitations can occur.According to the engineeringstandard VDI 3840 [5], for aninduction motor the followingexcitation cases have to beconsidered:
� Switching on� Momentary re-switching� Prolonged re-switching� Three pole terminal short circuit� Two pole terminal short circuit
The excitation cases are shown inFigure 11.
The torsional calculation for the wholedrive train was made by thecontractor Dresser Le Havre France,for switching on the motor, two poleterminal short circuit and three poleterminal short circuit. As an example,the calculation for the three poleterminal short circuit is shown inFigure 12.
The upper diagram shows theexcitation torque in the air gap of themotor. The other diagrams show theresponding torque in the shafts. Thistorque results in stresses in the shaftsand has to be compared with theallowable strength.As a result of the torsional calculationthe stiffness of the spacer in thediaphragm coupling between themotor and the variable planetary gearwas defined.
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planet carrier is zero the sun wheel isrunning with a higher speedaccording to the gear ratio which isthe difference of the diameters. If theplanet carrier is turned/driven,additional speed is applied to the sunwheel. The planet carrier can beturned/driven in both directions withvarious speeds. This gives theplanetary gear a continuouslychanging gear ratio and the sunwheel a variable speed.
Operating range:
The variable speed torque converterconsists of a pump wheel a turbinewheel and guide vanes which can beopened and closed. Fig. 9 shows thecharacteristic speed/torque curves atthe torque converter turbine wheelwith various guide vane positions.This operating characteristic of atorque converter was used to developthe operating envelope for thecompressor as shown on Figure 10.
Encompassed within this operatingenvelope are all operating cases ofthe compressor.
Fig. 9: Torque converter characteristic
Fig. 10: Operating characteristic of a torque converter
Fig. 11: Excitation response at differentcases
Dis
ch p
ress
ure
(bar
)
Speed (rpm)
-3000
15000
10000
5000
0-2000 -1000 0 1000 2000 3000 4000
Rated caseLight caseRegeneration N2Speed range of drive
Pow
er (k
W)
Speed (rpm)
6000
5000
4000
3000
2000
1000
08000 10000
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Manufacturing
Figure 13 shows the schedule fromdate of order through delivery. Thedelivery time was 10 month.Some of the special features of thisorder are as follows:
� Input motor speed of 3000 RPM� Shaft driven oil pumps� Oil supply for motor and com-
Figure 14 shows the variableplanetary gear drive on a skid whichwas designed as a common supportalso for the main motor.The skid is designed for lowresonance frequencies. Thelubrication system is an integral partof the variable planetary gear withshaft driven pumps. Lubrication isprovided to the compressor and themain machine. The oil tank is flangedon the bottom of the variableplanetary gear.
The test run was carried out asfunctional test with part load at themanufacturing facility.Here the oil flow and pressures wereset, temperatures and vibrationmeasured.No string test with the compressorwas carried out.
Fig. 12: Torque response at three pole terminal short circuit
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Fig. 13: Manufacturing schedule for the drive
Fig. 14: The drive is ready for shipment
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Installation
Figure 15 shows the location selectedfor the new drive train which isalongside the existing installation. Anew platform was constructed onpillars to support the drive train.
As the compressor shut down periodwas only three weeks, preparationswere made to move the compressorto a new foundation close to theoriginal position but on a lowerelevation.
The main equipment were deliveredwithin ten month and installed on thenew foundation while the compressorwas still in operation. Allprocess/utility piping were installedusing a compressor dummy.During the shut down in May 1996 thecompressor was moved to the newfoundation and all the piping werefinally connected.
Start up of the drive train
During start up, the compressorpower consumption is reduced asshown in figure 16. It also shows theoperating envelope for thecompressor and the characteristictorque curves of the torque converterat various guide vane position.
During start up of the induction motorthe compressor is accelerated toapprox. 6500 rpm. After that it can beoperated in the operating range. Due to the reduced power and speedof the compressor the inductionmotor is able to start at low load.Figure 17 shows the torque curve ofthe motor during start-up. As can beseen from the diagram, the motortorque during start-up is approx. 18% of the rated torque at full speed.This reduced starting torque allows areduced voltage start-up of the motor.
The commissioning and start up ofthe drive train were successfullycarried out in two days in thebeginning of June 1996.
Experience
As January 1999 the compressor hasbeen in continuos operation for morethan 22000 hours. The variableplanetary gear has provided thenecessary and accurate speedcontrol with no problems. Vibrationshave been below the specified limits.Bearing temperatures have beenwithin the normal operating range.The only maintenance performedsince June 1996 is the replacement ofthe lube oil filter inserts.
Conclusion
This retrofit has been in continuousoperation since June 1996 and hasfulfilled the requirements establishedby the customer:
� Increased gas flow� Reduced regeneration period� Reduced environmental fee� Reduced maintenance cost� Meet required schedule� No torsional excitation� Operation within the required
speed and power range� Simple reliable and efficient
solution
It have been able to confirm that thejustification of the project was rightand there have been no majorproblem.
Fig. 15: Foundation works
Fig. 16: Start-up torque
Operating rangeStart-up torque
Torq
ue (N
m)
Speed (rpm)
0
5000
4000
3000
2000
1000
04000 8000 12000
Fig. 5: Power vs. Speed
Torq
ue (N
m)
Speed (rpm)
0
15000
10000
5000
01000 2000 3000
Rated torqueStart-up torque
Bibliography
1. Peikert, G. H., ”Variable SpeedFluid Couplings driving CentrifugalCompressors and other Centri-fugal Machinery”,Voith Transmissions, Inc., HoustonTexas, USA, Reprint from: 13thTurbomachinery Symposium,Houston Texas, USA (1984).
2. Wahl, G. and Suhari, L., ”Improve-ment of Efficiency of Hydro-dynamic Variable-Speed CouplingMulti-Stage Variable-Speed Drive- A New Development -”, VoithTurbo GmbH & Co. KG, Crails-heim, Germany, Reprint from:Kraftwerkskomponenten (1986).
3. Henschel, F.-K. and Rappold, W.,”Conversion of a 19,000 HPpropylene compresser from steamturbine to electric motor withgeared variable speed turbocoupling”, 16th TurbomachinerySymposium (1987).
4. Fechner, G. and Wahl, G.,”VORECON Multi-Stage VariableSpeed Drive - Operating Experi-ence with a New Drive System”,Antriebstechnisches Kolloquium‘89, IME-Leitfaden, Institut fürMaschinenelemente und Maschi-nengestaltung an der RWTHAachen, Verlag TÜV Rheinland(1989).
6. Wahl, G., ”Hydrodynamic Multi-Stage Variable-Speed Drive -Stage of development and typicalapplications for high-capacity fluidflow machines”, 3rd WorldCongress on Gearing and PowerTransmission, Paris, France(February 1992).
7. Peikert, G. H. and Harnisch, B.,”Use of Modern Multi-StageVariable Speed Drives for the Driveof Gas Compressors in PTT’s GasSeparating Plant No. 3 and ParallelGas Pipeline - Thailand”, OIL GAS- European Magazine (4/1996).
8. Colboc, D., ”Le point de vue ducompressoriste - Expérience deDresser Rand France”, Pétrole etTechniques, N° 403 (août-septembre 1996).
9. Peikert, G. H., ”HydrodynamicPower Transmission in the Oil andGas Industry” Voith Turbo GmbH &Co. KG, Crailsheim, Germany(1996).
