Ust

1

Mitsubishi Reheat Boiler

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1. General In the “UST”, improvement of fuel consumption will be planned to apply reheat cycle in conventional marine steam turbine plant. In case of applying “Reheat cycle”, the Reheater will be applied for main boiler additionally. And exhaust steam of High Pressure Turbine (HP Turbine) is led to Intermediate Pressure Turbine (IP Turbine) through the Reheater for reheating steam up to Boiler Superheater outlet steam temperature. (Refer to Fig.1) 2. Basic Design Concept of Reheat Boiler Generally, main boiler for marine propulsion system is required following items;

High reliability and redundancy Compact and light weight considering engine

room arrangement Easy maintenance and operation due to

simple system Also, there is case of no steam flow or extremely low steam flow conditions in frequent load changes or astern / loading / stand-by of the ship which is the major difference points with on-shore

plant. Consequently, the Reheater tubes must be avoided tube failure by excessive tube metal temperatures. Even if tube failure occurs as worst case, continuous operation of the ship is desired. As for marine reheat boiler, it would appear that there are following three kind of type mainly.

Dual Furnace Type Gas By-pass Type Individual Type

Dual Furnace Type has main furnace and reheat furnace in one boiler. These furnaces are divided by “Divided Water Wall” or “Divided Water Tube”. Gas By-pass Type is divided two ~ three flow paths of combustion gas. One of path is installed Superheater and Reheater. The other paths are installed Superheater only. Also, outlet damper is arranged in outlet of each path. Individual Type is that boiler for Reheater is completely separated from Main Boiler. And exhaust steam of HP turbine is led to this separated Reheat Boiler for reheating. Each type of reheat boiler has both advantages and disadvantages (Refer to attached table). However, separated Reheater in the Dual Furnace type is selected for “UST” considering protection and reliability of the Reheater. In this type of boiler, Reheater and reheat furnace are located flow path outlet of combustion gas which is low gas temperature. Then, continuous operation can be done with Reheat Burner stop only when no steam flow or extremely low steam flow conditions.

HP

LP

R/G

M/C

AST

BOILER

TURBINE

HP IP

LP

R/G

M/C

AST

BOILER

TURBINE

REHEATE

Fig.2 Overview of Reheat Boiler

Reheater

Economizer

Reheat Burner

Fig.1 Comparison of Plant Configuration

2



Table 1 Comparison between conventional and reheat boiler

Conventional Boiler Reheat Boiler

Construction

Design Pressure 7.65 MPa 12.0 MPa

Material of Drum LR Grade 490 Carbon Steel Plate ASTM A299 Gr.A Material of Header LR Grade 410 Carbon Steel Pipe LR Grade 460 Carbon Steel Pipe Material of Tube LR Grade 360 Carbon Steel Tube LR Grade 410 Carbon Steel Tube

3. Features of Reheat Boiler 1) Basic Construction of Boiler

Basic Construction of Boiler and Economizer are applied MHI MB type main boiler which is used for conventional steam turbine plant. Therefore, its pressure parts by themselves can offer the adequate rigidity and yet absorbed the thermal expansion without difficulty. Reheat furnace which has reheat burner at lower position, is added at flow path outlet of combustion gas. Both main furnace and reheat furnace are of completely water cooled welded wall construction which assures complete sealing against combustion gas leakage. Also, high strength carbon steel plate is applied for steam drum and water drum to comply with high pressure application.

2) Construction of Superheater

For improvement of plant efficiency in UST, high pressure and temperature for main steam is applied compare with conventional steam turbine plant.

Accordingly, increasing of heating surface area for the Superheater and improvement of thermal resistance are required. For increasing of heating surface area, secondary Superheater is added behind of primary Superheater in parallel. This construction is still remained futures of MB type main boiler, drainable and easy maintenance construction. As for improvement of thermal resistance of Superheater tubes, secondary Superheater which is higher steam temperature compare with that of primary Superheater, is not exposed from furnace radiation directory to locate downstream of flow path.

Furthermore, combustion gas temperature will be down due to heat absorption of primary Superheater. Therefore, metal temperature of Superheater tubes can be kept similar with that of conventional main boiler even if steam temperature is higher than conventional. This kind of construction for Superheater, called Twin Superheater, has been manufactured in large capacity main boilers and proven technology.

SecondarySuperheater

PrimarySuperheater

SuperheaterHeader

Fig.4 Construction of Superheater

Fig.3 Basic construction of Reheat Boiler

Similar construction as conventional

non-reheat boiler

3



Reheat Boiler Superheater Metal Temp Calculation (Non RH, Oil Firing)

250

300

350

400

450

500

550

600

650

Superheater Tube

Ste

am /

Met

al T

empe

ratu

re (℃

)

Metal Temp.

Steam Temp.

1 pass 2 pass 3 pass 4 pass 5 pass 6 pass 7 pass 8 pass 9 pass

Primary Superheater Secondary Superheater

For material of Superheater Tubes, Chrome-Molybdenum alloy steel, ANSI A213-T22 equivalent, is applied for relatively low metal temperature region as same as conventional main boiler and Stainless steel, ANSI A213-TP347H equivalent, which has already proven material for On/Off-shore application is applied for high metal temperature region. This stainless steel material, contained much Chrome, excels at anti high temperature corrosion by Vanadium etc. Therefore, it can prevent corrosion from progressing by bad fuel.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

540 560 580 600 620 640 660

Metal Temperature (℃)

Cor

rosi

on R

ate

(mm

/yr)

18Cr Stainless Steel

2.25Cr-1Mo Alloy Steel

Estimated Max Metal Temp.

Composition of Test Soot : 80%V2O5 - 20%Na2SO4

The material of primary Superheater Header is applied as same as conventional main boiler and those of secondary Superheater header is applied high strength characteristic material, ANSI A335 Code case 2199, in high temperature. In addition, connection method between Superheater header and Stainless steel tubes is applied as same as method for On-shore boiler shown in Fig.6.

Allowable Stress Curve

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400 500 600 700

Metal Temperature (℃)A

llow

able

Str

ess

(N/m

m2 )

LR Gr.440 Steel Pipe

ASME SA335 Cade Case 2199

CST: 515℃

UST: 560℃

3) Reliability of Reheater

Reheater is designed to achieve specified steam temperature by using Reheat Burner which is located bottom of Reheat furnace. However, as mentioned former, there is case of no steam flow or extremely low flow condition in frequent load changes or astern / loading / stand-by of the ship which is the major difference points with On-shore plant. In this situation, there is possibility to burn Reheater tubes by excessive tube metal temperature due to reduction of steam cooling effect. To solve above problems, Reheater and Reheat furnace are located flow path outlet of combustion


Furnace Radiation

Max. Metal Temp forConventional Boiler


Fig.5 Conceptual image of furnace radiation

Fig.6 Metal Temp of Superheater Tubes

Fig.7 Characteristics of 18Cr Stainless Steel

Fig.8 Allowable Stress Curves

Fig.9 Detail of Superheater Header

Furnace Radiation

2.25Cr-Mo-W alloy steel header

18Cr Stainlesssteel tube

4



gas which is low gas temperature. Then, Reheater Tubes are not exposed high temperature combustion gas so that Reheat Burner is stop. Therefore, Reheater is thoroughly protected due to low temperature combustion gas from main furnace.

10MPa x 565℃

Main Burner

Reheat BNR

Furnace

2.0MPa x 560℃

2.2MPa x 360℃

420℃

６6０℃

160℃

Reheater

11MPa x 138℃

Superheater

10MPa x 565℃

Main Burner

Reheat BNR

Furnace

2.0MPa x 560℃

2.2MPa x 360℃

420℃

６6０℃

160℃

Reheater

11MPa x 138℃

Superheater

10MPa x 565℃

Main Burner

STOP

Furnace

2.0MPa x 450℃

2.2MPa x 335℃

365℃

530℃

Reheater

11MPa x 138℃

Superheater

155℃

10MPa x 565℃

Main Burner

STOP

Furnace

2.0MPa x 450℃

2.2MPa x 335℃

365℃

530℃

Reheater

11MPa x 138℃

Superheater

155℃

In addition, combustion gas at Reheater inlet during Reheat Burner operation will be high. However, construction of Reheater casing is applied inner insulation method which is proven construction of On-shore Heat Recovery System. Then, these constructions are protected against thermal expansion etc.

4) Environmental Issue

Fig.12 shows exhaust gas emission for each propulsion plant during normal operation and using boil off gas as fuel. Content of carbon oxide for steam turbine plant is lower than other plant because of source of exhaust gas coming from main boiler. And fuel consumption for UST plant will be reduced compare with conventional steam turbine plant due to high plant efficiency. Therefore, further reduction can be expected. Fig.13 shows exhaust gas emission during port operating condition. During port condition, steam turbine plants have dual fuel burning modes. However, a diesel electric plant is restricted to diesel mode only, because gas mode cannot be used at low load condition. Therefore the NOx and SOx emissions from steam turbine plant will be lower than other plants.

Under Normal Sea Going (Gas mode)

0

50

100

150

200

250

300

CST UST DFE GTCC GTSC

CO2

NOx

Fig.10 Normal Operating Condition

Fig.11 Reheat Burner Stop Condition

Fig.12 Construction of Reheater

Fig.13 Emission during normal operation

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4. Boiler Control System For reheat boiler, Automatic Boiler Control System has also been developed suitable control for additional Reheater and Reheat Burner. Basic function of conventional boiler control system is as follows. - Automatic Combustion Control (ACC) - Feed Water Control (FWC) - Steam Temperature Control (STC) - Steam Dump Control (SDC) - Burner Management System (BMS)

In case of Reheat Boiler Control, following functions are required additionally. And above additional functions are confirmed by using UST plant simulator. (Fig.16)

1) ACC for Reheat Burner Basically, total fuel quantity and combustion air quantity is controlled according to steam demand. And fuel distribution to main burner and reheat burner is shared with constant rate at steady operating condition.

2) Reheat Steam Temperature Control The reheated steam temperature is naturally varied according to boiler load. And rapid temperature variation or excessive high temperature is compensated the fuel quantity of reheat burner by detecting reheat steam temperature. Also, the superheated steam temperature is controlled to avoid excessive temperature difference between superheated steam temperature and reheated steam temperature at low load.

3) Reheater Protection System As mentioned former, there is case of no steam flow or extremely low flow condition in frequent load changes or astern / loading / stand-by of the ship. In this situation, there is possibility to burn Reheater tubes by excessive tube metal temperature. Therefore, automatic reheat burner operation at low load etc. is performed by this system.

300.0

350.0

400.0

450.0

500.0

550.0

600.0

0 10 20 30 40 50 60 70 80 90 100

Turbine load (%)

Steam Temp (℃)

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

Temp Difference (℃)

Oil Firing (Non Bleed Condition)

SH Temp (RHB off)

RH Temp (RHB off)

Non Control SH - RH

Allowable Temp Diff

Control SH - RH

5. Conclusion Generally, propulsion plant for LNGC requires reliability and safety concern. MHI Reheat Boiler designed considering these requirements mainly. Even if new technology is applied in this boiler, we carried out several confirmation not only numerical analysis but also scale model test, firing test etc. As a result of our investigation, we are confident about the design of Reheat Boiler and believe that our customers will be satisfied for applying UST plant for new LNGC.

0

100

200

300

400

500

600

700

800

CST UST DFE*1 GTCC

NOxSOxCO2

Fig.14 Emission during port operation

Fig.15 Local operation panel for Main Boiler

Fig.16 Example of Simulator Output

Fig.17 Characteristics of Steam Temperature

1

Mitsubishi Reheat Turbine


1. General Steam turbine plant has so many records as marine propulsion system and it is sufficiently attractive even at present because of maintenance - free and low NOx Emission for the environment except for a weak point that fuel efficiency in the plant is relatively low. In recent years, diesel propulsion such as DFE/DRL become conspicuous in LNG market due to a sudden rise of crude oil and their high engine performance, which surpasses steam propulsion by far. It is obvious that a supreme proposition to achieve the restoration of the steam turbine plant is to make the plant efficiency improved remarkably. In the latest reheat turbine plant called “UST Plant” which has been proposed by MHI (See Fig.1), fuel efficiency in the plant is improved by about 15% as compared with that in the conventional steam turbine plant (CST) without the present convenience and it can rival DFE/DRL plant in a total cost including initial, running and maintenance one. Main points of difference between UST and CST are as follows. 1) Application of reheat plant (Addition of Reheater

and Intermediate turbine) 2) Application of higher pressure and higher

temperature steam 3) Introduction of the latest technologies to improve

turbine internal efficiency

Fig.1 UST Plant (MHI)

Success of the UST plant certainly depends on main propulsion reheat-steam turbine (UST turbine) for the plant (See Fig.2). The UST turbine shall consist of one (1) high

pressure turbine, one (1) intermediate pressure turbine and one (1) low pressure turbine with an astern turbine incorporated in the low pressure turbine casing, and shall be arranged for driving a propeller through a reduction gear and shafting.

Fig.2 Components of the UST Turbine 2. Basic Specification of UST Turbine The basic specification is shown in Table 1. Main frame of the UST turbine is called MR types (Mitsubishi marine propulsion Reheat turbine), and they are classified into “MR36-II (26MW)”, “MR40-II (30MW)”, “MR45-II (33MW)” and “MR50-II (37MW)” in accordance with the shaft output power and revolution specified (See Table 2). For over 37MW reheat turbine, MHI is now diligently in developing and also the next generation UST for huge LNG carrier (over 200Km3) with twin-screw system will be sooner released to you.

Table 1 Basic Specification of UST Turbine (25MW base)

CST UST

Type MS36-2 MR36-II

Main Steam 5.68MPa x 520℃ 9.8MPaG x 555℃

Reheat Steam - 2.0MPaG x 555℃

Exhaust Steam 722mmHgv. x 27℃ 728mmHgv. x 24℃

Shaft Generator N/A Applied

Thrust 25MW x 78rpm 23.5MW x 76rpm

Turb

ine

Out

put

Electric - 1.5MW

Total 25MW x 78rpm 25MW x 76rpm

No. of Extraction 3 3

Feed Water Heater 3-stage F.W. system 2-stage F.W. system

・ CST: Conventional Steam Turbine (Non-Reheat) ・ UST: Ultra Steam Turbine (MHI Reheat)

HP/T IP/T

LP/T AST/T

M/C

R/G

HP/T Exh. to Reheater

Main Steam (AHD)

Reheat Steam

Main Steam (AST)

Main Steam : abt. 9.8MPa x 555℃Reheat Steam : abt. 2.0MPa x 555℃

Generator Turbine

Main Condenser

Gland Condenser

LP F.W.Heater

Drain Tank

Air H

Economizer

Feed Water Pump

Dearator

HP/T LP/T IP/T

Aux. Steam Line

Reh

eate

r

Main Boile

2



Table 2 Major Frame of MHI UST Turbine Frame Base Power HP/IP Frame LP Frame

MR36-II Abt. 36,000ps

(26MW) HR-22 LR-18


(30MW) HR-22 LR-18


(33MW) HR-26 LR-20


(37MW) HR-28 LR-23


(44MW) Now

developing Now

developing

The main steam and reheat steam condition at turbine inlet is designed as below.

Main Steam: 9.8MPaG x 555℃ Reheat Steam: 2.0MPaG x 555℃

In a case that the reheat plant is equipped with the shaft generator held in the intermediate shaft, the main turbine has to develop a total output power including the electric power generation. In general, the electric power expected at the guaranteed point in the plant is about 1.2 – 1.3MW. Therefore, the plant can gain much higher efficiency by developing the electric power with using rather the main turbine than the alternator turbine (Specified output power: 3.5MW), which is of lower performance at the partial load. However, it should be decided as to whether the shaft generator is adopted in consideration of an effect-to-cost including an initial cost, installation and a complexity of PMS (power management system) etc. Meanwhile the plant is a typical SAH (Steam Air Heater) system composed of two (2)-stage feed water heating system with three (3)-stage extraction and no high pressure heater is installed. It is because a simplicity and easy maintenance in the plant is regarded as important. Theoretically GAH (Gas Air Heater) system with multi-stage high pressure heater and multi-stage extraction has a little higher efficiency (about 2 – 3% in terms of FOC) as compared with normal SAH system; however, GAH system is extremely complex and strictly inferior to SAH system in operativeness and maintenance.

Our advanced SAH system adopting the shaft generator, dual economizer and so on has equal efficiency of the plant to or higher than GAH with easy operation and maintenance-free. 3. Introduction of UST Turbine Major points of difference from CST are as follows. 1) Intermediate pressure turbine integrated with high

pressure turbine for the reheat plant 2) Turbine rotor and single cylindrical turbine casing

for higher temperature use (560℃) with the latest materials and construction proven by land use turbines

3) Introduce of the latest technology to improve the turbine performance (3-D Rateau Nozzle, Unsteady Loss Reduced Blade and ISB etc.)

4) Application of the latest curtis stage with much higher efficiency, 3-diemnsional stationary blade for reaction stages and optimized construction of exhaust steam guide etc

5) 2-stage with 2-line Astern turbine for high pressure and high temperature use

MHI UST turbine can improve the performance of about 22 – 23% by comparison with CST due to the above-mentioned items to get higher turbine internal efficiency, the application of the reheat cycle and the adoption of main steam with higher pressure and temperature. Cross sections of the UST turbine (HR-22 and LR-18) are shown in Fig.3. The UST turbine is based on some small-medium sided land use reheat turbines proven.

Fig.3-1 Cross Section of UST Turbine (HR-22)

AHD Nozzle Valve (Bar-Lift Type) Main Steam Inlet

HP/T Part IP/T Part

Thermal Shield Integrated with Multi-Stage Diaphragm Ring

Dummy Ring integrated with Nozzle Block

HP/T Exhaust

IP/T ExhaustReheated Steam Inlet

3



Fig.3-2 Cross Section of UST turbine (LR-18)

In consideration of a spare-saving, regular maintenance and overhauling each docking, it is needed to reduce a number of parts composing the UST turbine and to make it simplified for some unique requirements to marine use. 4. Major Parts in UST Turbine Major parts composing the UST turbine are as follows. a) High-Intermediate Pressure Turbine (HR-22)

Main and reheat steam flow into the turbine in the middle of the turbine and each exhaust steam goes out from the end of the turbine. HR-22 turbine has basically followed similar size and specification of land use turbine. The detailed structural analysis in steady/unsteady condition has been performed in accordance with UST plant condition.

b) Low Pressure Turbine (LR-18) In LR-18 turbine, there is no big constructional change from the existing low pressure turbine except for the turbine casing and rotor material.

c) Astern turbine

Astern turbine element is incorporated in the low pressure turbine and arranged on the forward end of the turbine as well as CST turbine. The blade and steam chamber material are selected as the high temperature use in accordance with the UST plant condition.

d) Ahead stop valve and maneuvering valve There is no big constructional change from the existing system except for valve body and bolt material. The detailed structural analysis in steady/unsteady condition has been performed in accordance with the UST plant condition.

The comparison of turbine components between CST and UST is shown in Table 4.

Table 4 Comparison of Turbine Components between

CST and UST CST UST

Unit Type MS36-2 MR36-II

Power x rpm 25MW x 78rpm 25MW x 76rpm

HP/IPT Type H-22 HR-22

LP/T Type L-18 LR-18

HP 1-Curtis 2 lines

+ 7 Rateau 1-Curtis 2 lines

+ 5 Rateau

IP - 6 Rateau

LP 4 Rateau + 4

Reaction 6 Rateau + 4

Reaction

Stag

e

AST 2-Curtis with 2 lines 2-Curtis with 2 lines

In order to improve the turbine efficiency, HR-22 type of high-Intermediate pressure turbine is composed of 12 stages (1 Curtis with 2 lines and 11 Rateau Stages) in total and LR-18 type of low pressure turbine is composed of 10 stages (6 Rateau Stages and 4 Reaction Stages) in total. UST Astern turbine is of 2 Curtis with 2 lines as well as CST, however; it is refined in accordance with the UST steam condition.

5. Major Materials in UST Turbine MHI have so many experiences and records of reheat turbine applying the steam condition with high pressure and high temperature in commercial land use power plants and for marine use also, our know-how of the latest reheat turbine technologies and special materials of course is applicable. Major materials (Casing, valve body, turbine blades, diaphragm etc.) have suitably referred to those applied to our land use reheat turbines. It is understood that the reheat turbines for the land use cannot be applied to the UST turbine as it is

To Main Condenser

Flow Guide & Bearing Cone

LP/T Exhaust

Astern Turbine

LP/T Inlet

Rateau Stages Reaction Stages

4



because there are some differences in size and/or output power between the UST turbine and land use ones. MHI UST turbine is just a marine use reheat one for state-of-the-art that takes the latest technology of our land use reheat turbines while succeeding peculiar structural, functional concept for. The major materials applied for the UST turbine are shown in Table 5 with comparing to those for CST turbine.

Table 5-1 Materials for Casing/Body

Part CST UST

AHD Stop Valve

AHD Maneuving Valve

AST Maneuving Valve

HR(HP) Turbine Casing

Steam Chest at AST Turbine

1.25Cr.0.5Mo. Cast Steel

or 2.5Cr.1Mo. Cast Steel

MJC-12 12Cr.Cast Steel

Table 5-2 Materials for Bolt Part CST UST

Main Steam Line -

AHD Stop Valve

AHD Maneuving Valve

AST Maneuving Valve

M8B M8B

HR(HP) Turbine Casing 12Cr.Mo. MTB10A

Table 5-3 Moving Blades/Nozzle & Diaphragm/Rotor Materials for UST Turbine

Part CST UST

HR(HP) Turbine Moving Blades

12Cr.Mo. S 13Cr.Mo. S


HR(HP) Turbine Nozzle & Diaphragm


SA 182 F91 12Cr.Mo. S

HR(HP) Turbine Rotor Ni.Cr.Mo.V. S Cr.Mo.V. S

LR(LP) Turbine Rotor Cr.Mo. S 2.5Ni.Cr. Mo.V. S

For the UST turbine casing, High-Cr. Casting Steel (MJC-12 casing steel), which was developed for land use reheat turbines as a special material used under high pressure and high temperature condition, is applied as specified in Table 5-1. Consequently it has been succeeded to make strength of the casing high enough to endure the UST steam condition. And also MTB10A, which has almost similar coefficient of linear expansion to that the material of the casing has, is applied as the material of the casing bolt (See Table 5-2). In the UST turbine, it has been confirmed by the detailed turbine condition analysis using 3-demensional FEM (Steady/Unsteady) that no fatal steam leakage from flange at casing/valve bodies occurs at all considerable operating cases such as turbine starting, running-in, steady powering and long-term operation for creep strength and the turbine condition at all the cases is sufficiently satisfactory for the criteria of MHI turbine design. Meanwhile M8B, which has a special quality of high creep strength in high temperature condition, is applied as the material of holding bolts at AHD stop valve and AHD/AST maneuvering valve cover giving because these valves are always under high temperature condition. For turbine blades, the existing materials are well applicable in the UST steam condition in consideration of application records of land and/or marine use (See Table 5-3). For nozzle and diaphragm being in high temperature steam, SA 182 F91 (Super 9Cr.) developed for power plant is applied. The materials of HR turbine rotor and LR turbine rotor are properly selected from application records of land use turbines based on metal temperature of each rotor in working condition. In addition, HR turbine is equipped with the rotor cooling system in the structure (See below for further details) and it can be expected to prevent a drop in the creep strength caused by the long-term operation and to make a lifetime of the rotor elongated even more.

5



6. Remarkable Features of UST Turbine The most significant feature in the UST turbine is that the remarkable improvement of turbine performance can be achieved with sufficient safety at all considerable operation modes while the existing high-reliability, maintenance-free and easy-operation are being preserved as they are. Major features including the mentioned above are enumerated as follows.

Turbine performance About 25% Improvement of Steam Consumption Rate by Comparison to CST

Reliability Proven Design Based on Land Use Reheat Turbine

Safety Higher Creep Strength, No Steam Leakage from flange parts

Steadiability and Livability Low-Vibration, Low-Noise

Maintenance No Additional Consumable Part, Extremely Low Maintenance Cost, Easy-Overhauling

Lifetime Robust Design with Sufficient Safety Factors, Support of Long Lifetime due to Regular Maintenance every dock

The improvement of turbine performance is expected as the mentioned above, and just over 12% improvement of fuel oil consumption rate (FOC) specified as the plant performance is estimated due to the steam condition for the UST plant, the application of reheat cycle and the improvement of turbine internal efficiency etc. Based on the latest technologies for improvement of the turbine internal efficiency, which have been already proven by MHI land use reheat turbines and they have given satisfactory results as expected, the UST turbine has been tuned up and optimized as marine use reheat turbine and the turbine performance proposed at present has been methodically and empirically validated. And also, steady state analysis, simulation of starting-up/program operation and examination of

long-term creep strength were performed for a safety aspect and in addition to them, unsteady state analysis for the UST turbine has been completed to grasp an ever-changing turbine condition with the plant dynamic simulator developed for the UST turbine plant. Due to extracting any unforeseen troubles beforehand, taking the correct measures for and getting rid of them, all possible measures to ensure the success of the UST turbine have been taken.

7. The Latest Technologies to Improve Turbine Internal Efficiency As mentioned above, just over 12% improvement of FOC can be achieved only by applying the UST turbine and boiler to a plant.

Steam condition for the UST turbine about 2% FOC gain

Application of Reheat Cycle about 8% FOC gain

Improvements of Turbine Internal Efficiency about over 2% FOC gain

To achieve the further FOC gain in the plant, it is strictly imperative to improve the internal efficiency in the UST turbine itself. The following items are considered and applied for the UST turbine to optimize its performance. a) Optimization of Base Diameter at the HR/T Rotor b) Optimization of Number of Turbine Stages and

Steam Pressure of Each Extraction c) Application of ISB with Multi-Seal Fin d) Optimization of Nozzle Angle at HP Stages e) Development of the Latest Curtis (Control) Stage

with much higher efficiency for the UST steam condition

f) Application of 3-D Rateau (Bow) Nozzle g) Application of 3-D Stationary Blades at LR

Reaction Stages h) Application of Unsteady Loss Reduced Blade i) Optimization of Flow Guide for LR/T Exhaust

It is expected that each item will contribute to the turbine efficiency as shown in Fig.4.

6



Fig.4 Improvement of Turbine Efficiency (UST)

1) ISB with Multi-Seal Fin A tenon-shrouding type of blade (CST) can be equipped with only two (2) radial fins a stage at the tip of the blade. Meanwhile in the UST turbine, ISB (Integral Shroud Blade: See Fig.5) is applied and it can allow to be equipped with three (3) or more radial fins (Multi-Seal Fin) in accordance with a width of the blade. Due to the multi-seal fin, steam leakage in axial direction at the tip of blade can be decreased at each stage and so that turbine efficiency is improved.

Fig.5-1 ISB (Integral Shroud Blade)

Fig.5-2 ISB with Multi-Seal Fin

2) 3-D Rateau (Bow) Nozzle (See Fig.6). Due to bowing nozzle in its height direction, main steam flow is accelerated and pressure on rear side of nozzle (especially base side) will rise up by restraining of growth of boundary layer on nozzle surface and blade force of adjacent nozzles Consequently a differential pressure between base and tip side on front and rear surface of nozzle will be decreased and it is expected that a growth of the secondary flow (Cross flow), which is of the low-energy and causes a drop in turbine efficiency, can be restrained. In stages with low aspect nozzle, the above improvement is not sufficiently anticipated. Therefore, 3-D Rateau (Bow) nozzle has been applied for Rateau stages on IP turbine and LP turbine only.

Fig.6 3-D Rateau (Bow) Nozzle

3) 3-D Stationary Blade at LP Turbine

At some reaction stage(s), 3-D stationary blade (Refer to Fig.7), which improves blade profile performance and gauging distribution of the stationary blade, is applied and it results in decrease of the secondary flow.

Fig.7 3-D Stationary Blade (LP Turbine Reaction Stage)

2-Dimensional Stationary Blade (CST)

Big-pitched 3-Dimensional Stationary Blade (UST) ISB (UST)

Decrease of Steam Leakage

Tenon Blade (CST)

Steam Flow

0

0.5

1

1.5

2

2.5

Impr

ovem

ent o

f Tur

bine

Eff

icie

ncy

(%)

Optimization of Flow Guide at LR/T Exh.

Unsteady Loss Reduced Blade

3-Dimensional Stationary Brade

3-Dimensional Rateau (Bow) Nozzle

Development of UST Turbine Curtis Stage

Optimization of HR Turbine Nozzle (HP/T)

ISB with Multi-Seal Fin

Optimization of No. of Stages & Bleed Press

Optimaization of Dia. of HR/T Rotor

IP Turbine 1st Stage

7



4) Optimization of Exhaust Steam Chest (Flow guide & Bearing Cone) at LP Turbine Due to the optimization of exhaust steam chest at LP turbine, exhaust loss is decreased and exhaust pressure recovery (⊿Cp) of 0.1 or more is expected as compared with CST (See Fig.8) .

Fig.8 Exhaust Steam Chest at LP Turbine

8. High-Intermediate Pressure Turbine Casing (HR Turbine/HR-22) 1) Casing Material for the HR Turbine

As mentioned the above, High-Cr. casting steel (The UST turbine: 12Cr. casing steel) is applied for the HR turbine casing. In general, a material is in a creep region when used in high temperature condition (400~450℃) and its strength extremely lowers in the region as compared with that in a normal temperature. The high-Cr. casing steel is no exception to the above; however, it still keeps much higher strength even in high temperature. The comparison of allowable design stress between a conventional material (WC9: 2.5Cr. casing steel) and MJC-12 is shown in Fig.9. Allowable stress of MJC-12 under the UST steam condition (560℃) is about twice as much as WC9 and its stress level is almost equal to that of WC9 in temperature of about 480℃. In the UST turbine, this MJC-12 is uniformly applied for all valves and casing which exposed to higher temperature.

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400 500 600TEMPERATURE (℃)

Allo

wab

le S

tres

s (kg

/mm

2)

WC9 (2Cr. Casting Steel)

MJC-12 (12Cr. Casting Steel)

Fig.9 Comparison of Allowable Design Stress

between WC9 (2.5Cr. Casing Steel) and MJC-12 2) Casing Construction of the HR Turbine

In the construction, the inside temperature of HP turbine nozzle box is about 460℃. Meanwhile it is expected that the inside temperature of IP turbine inlet will be extremely high because high temperature reheat steam (about 555℃) directly flows into IP turbine. In this case, temperature gradient in axial direction at the part concerned of the HR turbine casing caused by the big temperature difference is much steep and it leads to unbalanced tightening force at neighboring casing bolts, reducing of surface pressure on casing flange of the HR turbine. It causes a dangerous steam leakage from the flange. The unexpected steam leakage must not be caused under any condition. So in addition that high-Cr. material as mentioned above is applied for the UST turbine (HR turbine) casing and the UST turbine is equipped with “Thermal Shield System for Cooling of Turbine Casing” to make the temperature gradient smooth and to keep the material in high creep strength (See Fig.10). At inlet part of IP turbine, a screen ring called “Thermal Shield” is arranged between IP turbine casing and high temperature reheat steam (Refer to Fig.11) and low temperature steam (HP Turbine exhaust steam) is led into a back space of the screen. The above-mentioned cooling system for turbine casing not to expose turbine casing inside directly to high temperature is introduced. In consequent it can be stated that no steam leakage or fatal damage of the flange occurs in the UST turbine.

Control of steam velocity increase at diffuser hub

LP Turbine Exhaust (CST) LP Turbine Exhaust (UST)

Rectification of exhaust flow due to the modification of the exhaust skirt

Bearing Cone 30° (CST)

Bearing Cone 12°/35° (UST)

8



Fig.10 Cross Section of HR Turbine

Fig.11 Thermal Shield System for HR/T Casing

The estimated temperature distribution at turbine casing flange at each case is shown in Fig.12. In a way of being equipped with the thermal shield, turbine casing well drops in temperature and the temperature distribution at the flange and bolt is almost even.

0

100

200

300

400

500

600

IP Exh

.IP

6IP

5IP

4IP

3IP

2IP

1

HP/IP D

ummyHP1

HP2HP3

HP4HP5

HP6

HP Exh.

Axial Position

The

mpe

ratu

re in

side

Cas

ing

( ℃)

Temperature Distribution at Flange (No Cooling System for Casing)Temperature Distribution at Flange (Cooling System for Casing)Temperature Distribution at Bolt (Cooling System for Casing)

Fig.12 Temperature Distribution at Casing Flange

and Bolt (HR Turbine) 3) Detailed Examination of Casing/Bolt Condition at normal operation (MCR), Long Term Operation and Unsteady Operation (Starting-up/Program-in)

In order to confirm the soundness of the UST turbine, 3-D FEM analysis using casing/bolt combination model (See Fig.13 and 14) at the following operations to be estimated has been performed.

Fig.13 3D-FEM Model (Casing/Bolt Combination)

Fig.14 Temperature Distribution at MCR Powering

HR Turbine Casing (HP

Turbine Side)

Thermal Shield

Dummy Ring

HR Turbine Casing (IP Turbine Side)

HP/T Exhaust

Shield SteamReheat Steam Inlet

AHD Maneuvering Valve

HP/T Exhaust

Shield Steam

IP Turbine Exhaust

Main Steam Inlet

Thermal Shield

HR Turbine Rotor (HP/IP Turbine Rotor)

HR Turbine Casing (IP/T Inlet)

IP turbine Inlet (Reheat Steam: 555℃)

Shield Steam (abt. 420℃)

Inside of HP Turbine Nozzle Box (abt. 460℃)

AHD Maneuvering Valve

Dummy Ring integrated with Nozzle Block

HP Turbine Exhaust

IP Turbine Exhaust

Thermal Shield Integrated with Multi-Stage Diaphragm Ring

9



a) Steady State Analysis at Normal Operation (See Fig.15)

b) Low-Cycle Fatigue Strength Analysis at normal operation (104 cycles)

c) Creep Strength Analysis at Long Term Operation (2×104 hours) (See Fig.16 and 17)

d) Unsteady (Starting-up/Running-in) State Analysis

Fig.15 Distribution of Surface Pressure at Casing Flange

Fig.16 Transition of Surface Pressure due to Creep

Strength Analysis (See Fig.14 for Mark A～D)

Fig.17 Creep Strength Analysis

(2 x 104 hours/Mises Stress)

Operation characteristic of the UST plant at starting-up and running-in is based on the output of the UST plant dynamic simulator, which has been developed in parallel and can simulate all operation modes including considerable emergency cases (Fig.18 and 19). Using with main and reheat steam condition at each time-step come from the simulator, the UST turbine performance calculation has been carried out and the calculation results have been used as input data for the unsteady state analysis.

Fig.18 UST Plant Dynamic Simulator

0

10000

20000

30000

0 20 40 60 80 100 120Passage Time (min)

Tur

bine

Out

put P

ower

(kW

)

300

400

500

600

Mai

n/R

ehea

t Ste

am T

emp.

(℃)

Turbine Output Power (kW)

Main Steam Temp. (℃)

Reheat Steam Temp. (℃)

Fig.19 Steam Condition at Program-In

(CASE: 40 min./Harbor Full to Navigation Full)

There is quite small difference of coefficient of linear expansion between MJC-12 (Casing material) and MTB10A (Casing bolt material), so that uneven thermal expansion between them never occur. In also point of casing and bolt strength, those can be well kept in sufficiently safe level because casing bolts and flange do not excessively rise in temperature due to the thermal shield system.

0.01 0.1 1 10 100 1000 10000 100000

Passage Time（H）

1 2 3 4 5 6

0

Surf

ace

Pres

sure（

kgf/m

m2 ) A

BCD

Criteria for Surface Pressure at Casing FlangeHP/T SIDE : 1.0kgf/mm2 or more

IP/T SIDE : 0.5kgf/mm2 or more

HL 2 or more 1 0

Surface Pressure (kgf/mm2)

HP SIDEIP SIDE

A B C D

Bolt (MTB10A): Initial Tightening 32kgf/mm2

10



And also as a result of creep strength analysis, it has been confirmed that bolt tightening force does not extremely go down and specified surface pressure at casing flange is properly maintained.

9. High-Intermediate Pressure Turbine Rotor (HR Turbine/HR-22) 1) Vibration Characteristic of Turbine Rotor

As the mentioned above, the intermediate turbine is incorporated into the HR turbine and a bearing span of the HR turbine extends by about 1.2m from that in CST (See Fig.20). In accordance with the extension of the HR turbine rotor, detailed rotor characteristic analysis (Rotor Dynamics) has been performed to confirm the rotor critical speed, stability, response and balancing characteristic. With reference to the result, characteristics of the rotor and bearing have been settled. The Tilting type bearing is adopted in the UST turbine to improve the vibration characteristic.

Fig.20 HR Turbine Rotor (HR-22) 2) Cooling System for Turbine Rotor

In order to prevent high temperature creep strength of the material from decreasing and to give a sufficiently robust strength to discs of the rotor, the cooling system is adopted. The steam after the HP turbine curtis stage is led to 1st stage disc at IP turbine through the dummy ring and cool the rotor not to expose the rotor to the high temperature reheat steam directly (See Fig.21).

Fig.21 Turbine Rotor Cooling System (HR Turbine)

10. Conclusion We are much confident of achieving the absolute success of the UST plant and offering it to you as the outstanding propulsion plant. The UST turbine is the most significant key to the success and its development has been fruitfully completed due to numberless examinations for it. We are eagerly looking forward to taking the last step to make the UST plant embodied and succeeded.

IP Turbine Inlet (555℃)

IP Turbine Side HP Turbine Side

Cooling Steam for Turbine Rotor (Dummy Leakage Steam: about 460℃)

Dummy Ring between HP Turbine and IP Turbine

Turbine Thrust

Dummy Part between HP/T and IP/T

HP/T Stages

Speed Control Stage(Advanced Curtis Stage with

Higher Performance for UST)

IP/T Stages

IP/T Steam Flow

HP/T Steam Flow

Ust

Documents

prior written

mitsubishi

international

big constructional

detailed structural

unfair competition

heating surface

creep strength