Development of New High Efficiency Steam Turbine · Mitsubishi Heavy Industries, Ltd. Technical Review Vol.40 No.4 (Aug. 2003) 1 Development of New High Efficiency Steam Turbine …

Mitsubishi Heavy Industries, Ltd.Technical Review Vol.40 No.4 (Aug. 2003)

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Development of NewHigh Efficiency Steam Turbine

Mitsubishi Heavy Industries, Ltd. (MHI) has developed new high performance turbine blades (reaction blades,impulse blades, and LP end blades) and high performance seals, as new technologies that remarkably improve theperformance of steam turbines. Moreover, MHI has developed a new high efficiency steam turbine, adopting not onlythese new technologies but also other latest technologies to improve performance and operability. The first unit ofnew high efficiency steam turbine has started the operation at the “T-point” combined cycle verification power plantin Takasago Machinery Works of MHI, and performance and reliability were verified by special measurements. Thisarticle describes the characteristics of the new technologies and the structures that were applied to the newly devel-oped steam turbine and the results of the verification.

1. Introduction1. Introduction1. Introduction1. Introduction1. IntroductionSteady efforts are actively being pursued throughout

the world to improve the efficiency of thermal powerplants in response to growing movements to conserveenergy resources and achieve greater environmental pro-tection typically represented by the control of CO2

emissions. One of these efforts is an attempt aimed atimproving thermal cycles by increasing the steam tem-perature and pressures handled in the steam turbines.For this purpose, MHI has also realized a high efficiencyand large capacity commercial steam turbine that oper-ates at a steam temperature of 600°C class (1) and hasdeveloped materials capable of withstanding tempera-tures as high as 630°C (2). In addition, MHI has alsopositively strived to increase the efficiency of steam tur-bines by improving blade performance, reducing leaklosses and so forth. In particular, in order to improvethe blade performance, MHI has developed fully three-dimensional design blades to increase their efficiency,applying the latest computational fluid dynamics (CFD)to the maximum extent possible (3), and has adopted suchblades for use in actual turbines. On the other hand, inorder to cope with growing levels of turbine capacity, MHIhas striven to elongate last stage rotating blades, andhas led the industry in commercial use of 40 inch steelblades for 3 600 rpm machines and 48 inch steel bladesfor 3 000 rpm machines (4).

In order to improve turbine efficiency, MHI has nowdeveloped new high performance reaction blades, impulseblades, and low-pressure (LP) end blades with the in-tent of achieving further enhancements in efficiency,utilizing the state of the art technologies of three-dimen-sional multi-stage viscous flow analysis and unsteadyflow analysis (5). MHI has also developed new high per-

formance seal (Leaf seal) capable of significantly reduc-ing steam leakage through the gland seals and blade tip/base clearance. Moreover, MHI has developed a new highefficiency steam turbine that not only adopts these newtechnologies but also other latest technologies to achievehigh performance and easy operability. The performanceand reliability of this new type steam turbine was veri-fied by replacing the existing turbine at the “T-point”combined cycle verification power plant at TakasagoMachinery Works of MHI.

2. Outline of new high efficiency steam turbine2. Outline of new high efficiency steam turbine2. Outline of new high efficiency steam turbine2. Outline of new high efficiency steam turbine2. Outline of new high efficiency steam turbineThe new high performance blades and seals are ap-

plicable to all sorts of steam turbines ranging fromsmall sized machines to large scale units for use in com-bined cycle power plants and conventional thermal powerplants. Fig. 1Fig. 1Fig. 1Fig. 1Fig. 1 shows the first unit of new high efficiencysteam turbine that has been developed and manufacturedfor the T-point verification power plant applying all thenew technologies mentioned above.

This turbine is a single-casing reheat type turbinewith a rated capacity of 105 MW. The upstream bladesof the high-pressure (HP) section are designed for im-pulse stages, while the downstream blades of the HPsection and all the blades of the intermediate-pressure(IP) section are designed for reaction stages. The newlydeveloped high performance blades are applied for boththe impulse and reaction stages, with the intent of im-proving the performance of both stages. The blades ofthe LP section are composed of 36 inch LP end blades foruse in 3 600 rpm operation. These blades are designedby the profile and flow path designs that have been de-veloped based on the latest CFD technology. In addition,leaf seals are applied to the sealing section between theHP exhaust and the gland in order to reduce any steam

EIICHIRO WATANABE YOSHINORI TANAKA

TAKASHI NAKANO HIROHARU OHYAMA

KEIZO TANAKA TOSHIHIRO MIYAWAKI

MASANORI TSUTSUMI TANEHIRO SHINOHARA

2


leakage. The same type of seals are also applied to theHP and IP rotating blades for sealing the tip clearancein order to improve the blade performance.

In addition to these key technologies, various othertechnologies have been applied to contribute to improv-ing the performance and operability. For example, thenozzle box for the main steam inlet is designed as a scrolltype passage with the aim of homogenizing the steamflow in the circumferential direction and reducing pres-sure losses, while the shapes of the flow paths at HPexhaust and IP inlet parts have been designed utilizingthe CFD so as to reduce pressure losses. The ActiveClearance Control or ACC technology applied to the seal-ing area between the HP and IP sections improves thesealing performance using movable sealing segments.This design results in larger clearance when the turbineis starting or stopping, while a tighter clearance resultswhen the turbine operates under loaded conditions. Awelded rotor with large bore structure is employed inorder to reduce thermal stresses produced during start-up of the turbine (hetero-material welded rotor) (6). As aresult, the operability of the turbine is improved for quickstart.

3. Characteristics of new technologies3. Characteristics of new technologies3. Characteristics of new technologies3. Characteristics of new technologies3. Characteristics of new technologies3.1 New high performance impulse blades3.1 New high performance impulse blades3.1 New high performance impulse blades3.1 New high performance impulse blades3.1 New high performance impulse bladesA newly designed profile that reduces unsteady losses

has been applied for the rotating blades of impulse stageadopted as the upstream stages of the HP section of thenew high efficiency steam turbine for the T-point. Themechanism of unsteady losses that are produced due tothe interaction between the rotating blade and the sta-tionary blade has been clarified through the effective use

of unsteady flow analysis (Fig. 2Fig. 2Fig. 2Fig. 2Fig. 2). Based on this knowl-edge, a new profile was developed to reduce unsteadylosses through optimum design. Results of tests at theair turbine facility also confirmed that the performanceof the blade was remarkably improved by the new pro-file.

The structural design of the impulse stages wasmodified (called high performance diaphragm struc-ture). Conventional wide nozzles are separated intoprofile sections and support columns. This method isbased on the idea that the strength of diaphragm isreinforced by support columns located upstream of thestationary blades and the width of stationary bladescan be reduced to increase the aspect ratio. Improve-ment of stationary blade performance is also achievedby applying a three-dimensional design. The shape ofthe support columns and the profile of stationary bladesare optimized through the use of three-dimensional flowanalysis.

Fig. 1 New high efficiency steam turbine for the T-point (single-casing reheat type turbine)The notes indicate the position where the new technologies are applied.

New high performance impulse blades and leaf seals

New high performance reaction blades and leaf seals

Welded rotor capable of coping with high temperature operation and quick start

Scroll type main steam inlet and paths for reducing pressure losses in IP inlet and HP outlet

High performance axial exhaust hood

Hollow stationary blades and suction slits for moisture removal

New high performance LP end blades

Leaf seals

ACC seals

Fig. 2 Example of unsteady flow analysis

Stationary blades Rotating blades


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3.2 New high performance reaction blades3.2 New high performance reaction blades3.2 New high performance reaction blades3.2 New high performance reaction blades3.2 New high performance reaction bladesMHI has always made steady efforts to improve the

accuracy of the predictions of efficiency and internal tur-bine flows. Three-dimensional multi-stage flow analysistaking viscosity into consideration was used for this pur-pose and the results of this analysis were confirmed atair turbine verification test. Recently, in addition to con-ventional steady state analysis, MHI has established anunsteady flow analysis method capable of predicting loss-producing mechanisms even more precisely, as shown inFig. 3Fig. 3Fig. 3Fig. 3Fig. 3. As a result, MHI has developed new high perfor-mance reaction blades using these methods.

The degree of reaction and the three-dimensionalstacking of profile are further optimized, compared withthe flow patterns of conventional blades. As a result,production of the secondary flow vortexes are controlledand the vortex zones are shifted toward the inside andoutside endwall of each blade, so that losses due to thesecondary flows in the rotating blades are reduced. Inaddition, a new profile to reduce unsteady losses pro-duced by the interaction between the rotating blades andstationary blades is applied to the middle zone of theblade height where profile losses are dominant.

Unsteady flow analysis has also quantitatively clari-fied that vortexes produced in the vicinities of the insideand outside endwall of each blade due to leakage fromand inflow into the spaces between each rotating blade

and stationary blade interact with secondary flow vor-texes and increase the secondary f low losses .Accordingly, MHI has attempted to optimize the shapeof the flow paths including those between the inside cir-cumferences of the stationary blades and the rotor disksand those around the rotating blade shrouds. Fig. 4Fig. 4Fig. 4Fig. 4Fig. 4shows an analysis model of the flow fields including allof these flow paths.

As described above, new high performance reactionblades have been developed through the latest analysistechnologies and the detailed verification of the inter-nal flows, and applied to the downstream blades of theHP section and to all the blades of the IP section of thenew type steam turbine at the T-point.

3.3 New high performance LP end blades3.3 New high performance LP end blades3.3 New high performance LP end blades3.3 New high performance LP end blades3.3 New high performance LP end bladesThe performance of the LP end blades largely influ-

ences the performance of the turbine as a whole.Consequently, MHI has striven to realize its variousimprovements taking the progress of the flow analysistechnologies. The 3 600 rpm 36 inch blades shown inFig. 5Fig. 5Fig. 5Fig. 5Fig. 5, which have been developed this time as new highperformance LP end blades based on the latest unsteadymulti-stage viscous flow analysis, demonstrate furtheradvanced performance. This latest unsteady flow analy-sis can be used for evaluating the behavior of the wakesof stationary blades inside the downstream rotatingblades in multi-stage flow field, the behavior of wakes ofrotating blades inside the downstream stationary blades,the interaction between wakes and secondary flows, andthe interaction between shock waves and boundary lay-ers or secondary flows. The improvement in theperformance has been realized by taking these effectsinto consideration, in addition to the optimization of

Fig. 4 Example of computational mesh grids for analyzing entire stream path (Downstream of HP section)

ShroudsStationary blades

Stationary blades

Rotatingblades

Rotatingblades

Fig. 5 New high performance LP end blades (3 600 rpm - 36 inches)

Casing

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Fig. 3 Example of improvement of prediction accuracy using flow analysis

(a) Measurement of loss distribution using fast response aerodynamic probes

(b) Example of unsteady viscous analysis: Result of prediction analysis of (a)


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stage loading, loading distribution along the blade heightand the blade profile for high Mach number flow, whichhave been implemented so far. Since these effects inunsteady flow field are evaluated at every moment, thedesign was made to achieve maximum efficiency by time-averaging.

The advance of the flow analysis technology has re-markably increased the prediction accuracy of excitingforces acting on a blade and enabled to improve vibra-tory strength of the blades largely. This increases thedegrees of freedom related to the flow path design andstacking of the blade profiles, enabling MHI to thoroughlyreview the design to improve performance. As a result,the base diameters and heights of the upstream rotat-ing blades as well as the lengths of the last stage bladecould be increased, and the flow fields of the LP endblades as a whole including the blade endwall contourshas been further optimized. Further, the skewed profileinclined in the axial direction, in addition to the conven-tional bowed profile, was adopted for the stacking of thestationary blades.

On the other hand, it has also become possible to pre-dict drain trajectories precisely using the flow analysisdescribed above, so that the positioning of the drain slitscan be optimized in order to reduce losses caused by drain(moisture losses) peculiar to the LP end blades.

3.4 Seal technology3.4 Seal technology3.4 Seal technology3.4 Seal technology3.4 Seal technologyIn developing a new high efficiency steam turbine,

MHI has striven to develop and apply new sealing andclearance-controlling technologies that contribute toimproving turbine performance, as well as the technolo-gies to improve the performance of blades. The newsealing technologies also make it possible to reduce theaxial span needed to seal each section of a turbine, com-pared with conventional labyrinth seals, by improvingsealing performance. This also allows the design of theshaft system and blades to have some margin.

3.4.1 Leaf seals3.4.1 Leaf seals3.4.1 Leaf seals3.4.1 Leaf seals3.4.1 Leaf sealsThe structure of leaf seal is shown in Fig. 6Fig. 6Fig. 6Fig. 6Fig. 6, and the

mechanism of its operation is shown in Fig. 7Fig. 7Fig. 7Fig. 7Fig. 7. The leafseal is a seal based on a new design concept that differsfrom non-contact type seals represented by conventionallabyrinth seals and from contact type seals such as brush

seals whose application has been expanding recently.This seal consists of a number of thin metal plate (leaf)inclined in the circumferential direction so that the tipof the seal is kept in a non-contact state with negligiblysmall clearance when the rotor is rotating. This is doneby a lifting force produced due to a hydrodynamic pres-sure effect acting between the tip of the leaf and the rotor.The result is that both the seal and rotor are preventedfrom wear and the durability of the seal is increased whenthe turbine is running, different from contact type sealssuch as brush seals. In addition, since the seal itself isin the shape of plate with axial width, it has a higherrigidity in the direction of the pressure difference andthe sealing function can be kept up to a higher differen-tial pressure compared with brush seals.

Fig. 8Fig. 8Fig. 8Fig. 8Fig. 8 shows the results of the verification test of sealperformance and lifting characteristics (test to verify theelectricity discontinuity between the seal and rotor) car-ried out in the shop. The results confirm that the flowrate through the seal can be reduced to about one-thirdthat of conventional labyrinth seals. The result of theelectricity discontinuity test also indicates that the lift-ing-up of the seal is performs well even if the seal iseccentrically positioned against the rotor. The leaf sealhas already installed and tested at the existing steamturbine of the T-point power plant and soundness wasconfirmed during inspections carried out one year afterthe verification operation was begun. The further im-proved seals are applied to the new type steam turbinefor the T-point.

3.4.2 ACC (Active Clearance Control)3.4.2 ACC (Active Clearance Control)3.4.2 ACC (Active Clearance Control)3.4.2 ACC (Active Clearance Control)3.4.2 ACC (Active Clearance Control)The structure of the ACC is shown in Fig. 9Fig. 9Fig. 9Fig. 9Fig. 9. The ACC

seal is a seal with a structure in which segments of laby-rinth seal rings are made to be movable in the radialdirection. When the turbine is starting, stopping or dur-ing turning operation after stopping, the segments areraised by a spring force to keep the clearance betweenthe rotor and the seal fins large. On the other hand,when the turbine load is increasing, the seal segmentsare shifted until the proper clearance is maintained ra-dially toward the center by utilizing the pressureFig. 6 Structure of leaf seal

Direction of rotation

Fig. 7 Mechanism of leaf seal operationThe tip of the leaf is lifted-up by a balance of the pushing force due to pre-pressure of the setting, lifting force due to hydrodynamic pressure generated during rotation of the rotor, and lifting force due to the differential pressure of the seal.

Pushing force due to pre-pressure of the setting Leaf

Lifting force due to sealdifferential pressure

Lifting force due to hydrodynamic pressure

Direction of rotation

Rotor surface


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difference for sealing. Then, the narrowest clearance iskept during the loaded operation.

The verification test of the ACC seal in the actualmachine has already been carried out at the existingsteam turbine of the T-point power plant. The move-ment of the ACC seal segments was measured using gapsensors. As a result, it was confirmed that the clear-ance enlarged and narrowed together with the start andstop of the turbine, thereby ensuring that clearance con-trol is performed precisely. Inspections carried out oneyear after the operation of the turbine confirmed thatthe seal fins on the ACC seal segments remained sound,while traces of wear were found in the seal fins of theconventional segments installed adjacent to the ACC

seal. Based on these results, these seals have beenadopted to nine turbines including a 700 MW unit andhave started actual operations.

In the new type steam turbine developed for the T-point, four stages of the ACC seal are installed in theline at the HP dummy ring. In this case, wear of theseal fins due to deformation of the casing during start-ing and stopping of the turbine is avoided by controllingthe pressure distribution and spring force of each seg-ment. During the rated load operation, the clearance iskept small in order to reduce any leak losses and to im-prove the performance of the turbine.

4. Verification tests on the actual turbine4. Verification tests on the actual turbine4. Verification tests on the actual turbine4. Verification tests on the actual turbine4. Verification tests on the actual turbineThe new high efficiency steam turbine provided with

various new advanced technologies and installed at theT-point verification power plant replacing the existingturbine began operations in May 2003. In this turbine,pressure and temperature sensors have been installedat the necessary points inside the turbine casing in or-der to confirm the improvement in the performance ofeach component, that achieved through the applicationof new technologies, as well as the improvement in theperformance of the turbine as a whole. The measure-ment of the internal flows in the IP blades, LP blades,and the exhaust hood is also carried out using Pitot-tra-versing devices. In addition, confirmation of theperformance of the seals was carried out by measuringthe differential pressure through each seal and the gapbetween the rotating parts and stationary parts (Fig. 10Fig. 10Fig. 10Fig. 10Fig. 10).

The test results show that improvements in the per-formance of entire turbine were as expected. The resultsalso confirmed that the performances of the new high

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Fig. 8 Results of verification tests of leaf seal performance and lifting characteristics of the leaf

At time of startingTime (sec) Time (sec)

At time of stopping

: Electricity continuity : Rotating speed

: Electricity continuity : Rotating speed

Differential pressure (kg/cm2) Concept for electricity continuity measurement

Flo

w r

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h)E

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ricity

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mea

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: Labyrinth (Clearance: 0.5 mm, 4 stages): Leaf seal (Predicted): Measurements in test

Amount of leakage

Measurement of potential difference

Leaf

Test rotor

Slip ring

Rot

atin

g sp

eed

(rpm

)

P1

P2

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Fig. 9 Construction of ACCSeal ring moves in the radial direction as a result of the difference in force between the spring force and the force due to the differences in pressure between the inner and outer circumference of each seal ring.

HP side

Cle

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Pushing force


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performance blades applied to the HP section, the IPsection and the LP end blades as well as the advancedcomponents such as the leaf seals, ACC seals, and pas-sage for reducing inlet and outlet losses were achievedas planned.

5. Conclusion5. Conclusion5. Conclusion5. Conclusion5. ConclusionMHI has successfully developed a new high efficiency

steam turbine by integrating a range of techniques to re-alize the latest advanced technologies. Results ofverification tests carried out with the new steam turbinefor the T-point power plant confirmed that improvementsin performance could be achieved as intended, and veri-fied the reliability of the turbine and its each component.The new technologies adopted in this new type steam tur-bine can contribute to the improvement of the efficienciesof a wide range of turbines from small and medium sizedturbines to large scale turbines. Hence, it is planned thatthese technologies will be successively applied to bothnewly installed turbines and existing units.

MHI will continue to develop advanced technologiesaimed at achieving ever higher levels of efficiency inpower plants and higher performance in steam turbinesthrough the application of these technologies.

ReferencesReferencesReferencesReferencesReferences(1) Tanaka et al., Feature and Operating Experience of 1000 MW

Class Steam Turbine with Highest Efficiency in the World,Mitsubishi Juko Giho Vol.39 No.3 (2002) p.132

(2) Yoshikuni KADOYA et al., "Alloy Design and Production ofan Advanced 12Cr Steel Rotor Applicable to Elevated SteamTemperatures", Fifth International Conference on CLEANSTEEL (1997)

(3) Sugitani et al., Development of Advanced High PerformanceBlades and Their Operating Experience, Mitsubishi HeavyIndustries Technical Review Vol.31 No.3 (1994) p.125

(4) Hiroharu OHYAMA et al., Development of High Efficiencyand Reliability Low Pressure End Blade, The 8th National

Fig. 10 Special measurement points in the new steam turbine for the T-point

Clearance Clearance

Clearance

Performance of diffuser

Performance of leaf seal

Pressure distribution at HP inlet

Pressure distribution at IP inlet


Amount of removed drain


Performance of new HP reaction blades

Performance of new impulse blades

Performance of ACC


Performance of new IP reaction blades

Performance and blade vibration of 36 inch LP end blades

Performance of exhaust hood

: Pressure and temperature measurements

: Measurements by Pitot-traversing device

Symposium on Power and Energy Systems (SPES 2002), To-kyo, (2002.6), p.141

(5) V.S.P.Chaluvadi et al., Blade Row Interaction in a High Pres-sure Steam Turbine, Transaction of the ASME, Journal ofTurbomachinery Vol.125 No.1 (2003) p.14

(6) Shige et al., Development of Large-Capacity Highly EfficientWelded Rotor for Steam Turbines, Mitsubishi Heavy Indus-tries Technical Review Vol.38 No.1 (2001) p.6

POWER SYSTEMS HEADQUARTERS

TAKASAGO RESEARCH & DEVELOPMENT CENTER,TECHNICAL HEADQUARTERS

NAGASAKI RESEARCH & DEVELOPMENT CENTER,TECHNICAL HEADQUARTERS

TAKASAGO MACHINERY WORKS

Eiichiro Watanabe Yoshinori Tanaka Takashi Nakano

Hiroharu Ohyama

Keizo Tanaka

ToshihiroMiyawaki

MasanoriTsutsumi

TanehiroShinohara

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Development of New High Efficiency Steam Turbine · Mitsubishi Heavy Industries, Ltd. Technical Review Vol.40 No.4 (Aug. 2003) 1 Development of New High Efficiency Steam Turbine …