C10_ST(CC)

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10. Steam Turbines for Combined Cycle 10. Steam Turbines for Combined Cycle Power PlantsPower PlantsPower PlantsPower Plants

207D-17 Steam Turbine

Combined Cycle Power Plants 10. Steam Turbines for CCPPs 1 / 78

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Steam Turbines for Combined Cycle 21

Factors Affecting Cycle Performance 383

Type of Steam Turbines 222

Part Load Operation of Steam Turbines 735

LP Exhaust Hood 624


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Rankine Cycle

The thermodynamic cycle for the steam turbine is the Rankine cycle

Rankine cycle.

In the steam cycle, water is first pumped to high pressure.

It is then heated to the boiling temperature corresponding

to the pressure, boiled (heated from liquid to vapor), and then superheated (heated to a temperature above that of boiling).

boiling).

A multistage turbine expands the pressurized steam to lower pressure and the steam is then exhausted either to a condenser at vacuum conditions or into an intermediate

T a condenser at vacuum conditions or into an intermediate temperature steam distribution system that delivers the steam to the industrial or commercial application.

The condensate from the condenser or from the steam utilization system returns to the feedwater pump for continuation of the cycle.


s

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Combined Cycle

Fuel Combustor

G

5

6

Steam Turbine

2 3

G G

HR

SG

Compressor Turbine

Air

Exhaust gas1 4

7

9Condenser

8

Pump

[ A simple schematic diagram of a combined cycle system ]


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Power Output Ratio

T

Topping Cycle

1

pp g y(Brayton Cycle)

2

GT : ST = 2 : 1

Bottoming Cycle(Rankine Cycle)

GT : ST 2 : 1s


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Combined Cycle

Three PressureReheat Cycle

FuelFuel

GHeat Recovery

Steam Generator

Air

Gas Turbine

IP Steam

LP Cold Reheat Hot Reheat

Steam Main St

G

SteamSteamSteam

SteamHPIP/LP

Condenser

Steam TurbineCondensate Pump

WaterFuelAir


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Typical STs for CCPPsD 11 (GE)

1,800 psig/1,050F/1,050F

D-11 (GE)

SV/CVCRV

SV/CV


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D 11 (GE)

Typical STs for CCPPs

207FA 209FA

D-11 (GE)

207FA 209FA

Steam conditions, psig/F/F(psig/C/C)

1800/1050/1050(1800/566/566)

P t l d ti d Slidi ti ith f ll d i iPart load operating mode Sliding pressure operation with full arc admission

Rating, MW 180 283

Number of casings 2 2

Number of stages

HP 11 10

IP 7 8

LP (per flow) 5 5

rpm 3600 3000

30LSBs, inch

3033.540

33.542

207FA: means that two 7FA gas turbines are applied in a “two on one” configuration with a single


207FA: means that two 7FA gas turbines are applied in a two-on-one” configuration with a single steam turbine

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D 11 (GE)


1) Opposed flow HP/IP section

D-11 (GE)

Even temperature gradient from the center of the casing to the ends. Minimum thrust force. HP/IP section utilizes single shell.

2) Steam path design

HP/IP stages are designed by low reaction design theory. The HP section was designed to accommodate up to 45% additional throttle mass flow for

supplementary firing.

3) LP turbine


Same as the LP turbine used for Korean standard 500 MW class.

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GEA18 86 (J /2011)



GEA18786 (June/2011)

109D-14 ST



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KN S T bi (Si )


SIEMENS – up to 700 MWfor steam power plants

KN Steam Turbine (Siemens)

X-over pipe Double flow LP

p pand CCPP

Synchronous self-shifting clutch (not on picture)

Expansion joint

T iclutch (not on picture) Turning gear

C bi d HP/IP f


Combined HP/IP for full arc admission

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Steam Turbine

Steam turbines are one of the most versatile and oldest prime mover technologies still in use.

Power generation using steam turbines has being used for about 100 years due to higher efficiencies and lower costs.

The HRSGs are used to convert the heat energy contained in the The HRSGs are used to convert the heat energy contained in the gas turbine exhaust gas to steam.

This steam is sent to the steam turbine and converted to mechanical energy to turn the generatormechanical energy to turn the generator.


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Si l Sh f [GE]

Steam Turbine ArrangementSingle-Shaft [GE]


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Si l Sh f [Si ]

Steam Turbine ArrangementSingle-Shaft [Siemens]


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Si l Sh f

Steam Turbine Arrangement

Single-shaft with generator between gas turbine and steam turbine enables installation of a clutch between steam turbine and generator.

Single-Shaft

One problem of a Jaw clutch, which was used previously, is that it can only be engaged when the gas turbine is at rest. This means that in the event of a failed gas turbine start, the operator must wait until the gas turbine is stationary before engaging the jaw clutch to re-start.

Currently, SSS(Synchronous Self-Shifting) clutch has been employed popularly. The SSS clutch engages in that moment when the steam turbine speed tries to overrun the rigidly coupled gas turbine generator and disengages if the torque transmitted from the steam turbine to the generator becomes zero.

The clutch allows startup and operation of gas turbine without driving the steam turbine.

This results in a lower starting power and eliminates certain safety measures for the steam turbine, such as g ycooling steam or sealing steam.

The clutch also provides design opportunities for accommodating axial thermal expansion.

However, the clutch is an additional component with a potential impact on availability. Additionally, the generator located at the end of the line of shafting has advantages during generator overhaul.

Single-shaft units without a clutch definitely need auxiliary steam supply to cool the steam turbine during


Single shaft units without a clutch definitely need auxiliary steam supply to cool the steam turbine during startup. This is not necessary in units with a clutch.

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M l i Sh f [207FA GE]

Steam Turbine ArrangementMulti-Shaft [207FA – GE]


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Steam Turbine Arrangement

Single Shaft Multiple Shaft

Less generator required One large ST instead of 2 smaller STsComponents

Less generator requiredOne compact lube oil system

One large ST instead of 2 smaller STsLess auxiliaries (pumps etc) required

Civil Smaller plant area Higher flexibility in plant layout

Costs Lower specific cost of plant

Performance Same level in larger plants Steam turbine has higher efficiency because of larger steam volumeof larger steam volume

Operational Flexibility

Suitable for daily start and stop (DSS) operation Suitable for base load operation


Availability Higher (less complexity)

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Enhancement of Main Steam Conditions

Traditional Modern

EGT C 450 550 550 650EGT, C(F)

450-550 (842-1022)

550-650 (1022-1202)

Steam Temperature, C(F)

420-520 (788 968)

520-600 (968 1112)(F) (788-968) (968-1112)

Number of Steam Pressure Level 1 or 2 2 or 3

Steam Pressure, bar 30-100 100-170(psig) (420-1430) (1430-2460)

Reheat Steam Cycle no yes

Number of Gas Turbines per Steam Turbine 1 to 5 1 to 2

As the exhaust temperatures of gas turbines are over 600C, there is sufficient spare heat in the flue gases that can be used to reheat the steamgases that can be used to reheat the steam.

The output of the steam turbines are higher and HRSG heat rate is reduced by the employment of reheating cycle.


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Main Steam Conditions

복합발전용 증기터빈은 HRSG에서 생산된 증기를 모두 발전에 사용하는 경우 가스터빈 1기 출력의 약 50%생산 (복합발전용 증기터빈은 대형 화력발전용 증기터빈에 비해 소용량)

복합발전 주증기 조건은 HRSG에 대한 증기압력과 LSB 습분침식을 고려하여 결정

급수가열기는 사용하지 않거나 사용하더라도 작은 수 사용 (목적: 신속한 기동특성 확보 및 저압터빈에 필요한 증기유량 확보)

지속적인 성능 개량이 이루어지고 있는 복합발전용 가스터빈에 대응하여 HRSG와 증기터빈 성능도 지속적개량

복합발전용 LP터빈은 대형 화력발전용 LP터빈을 그대로 사용 (이유: 새로운 LP터빈을 개발하기 위해서는 3차원 블레이드 설계에 많은 시간 요구. 따라서 상대적으로 짧은 기간에 설계가 가능한 HP터빈과 IP터빈을개선하여 대처)

한국표준화력 500 MW LP터빈 최대출력은 125 MW

복합발전 증기온도 석탄화력발전 수준 도달

가스터빈 EGT가 600℃를 초과함에 따라 증기의 재열에 필요한 충분한 열량을 확보할 수 있게 되었으며, 이로 인해 3압 재열사이클(3-pressure, reheat cycle) 증기터빈이 일반화되고 있음

주증기 온도 상승 HRSG 증기배관 증기터빈 소재가격 상승


주증기 온도 상승 HRSG, 증기배관, 증기터빈 소재가격 상승

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Main Steam Conditions

재열사이클을 채택하면 상대적으로 높은 주증기 압력을 이용해야 하는데, 주증기 압력이 상승하면 증기터빈효율 저하 (주증기 압력 상승 체적유량 감소 짧은 블레이드 사용 효율 저하)

높은 주증기 압력을 이용하는 경우 2-on-1 configuration을 채택하여 증기터빈 효율저하 문제 해결

주증기 압력 증가 튜브 두께 증가 가격 상승 및 튜브 열적 유연성(thermal flexibility) 저하

복합발전 증기압력은 100~170 bar로서 석탄화력발전의 160~300 bar에 비해서 크게 낮음

비재열 증기터빈의 경우 주증기 압력 하한선은 증기터빈 출구 습분 및 LSB 습분침식에 의해 제한 받음 비재열 증기터빈의 경우 주증기 압력 하한선은 증기터빈 출구 습분 및 LSB 습분침식에 의해 제한 받음

복합발전에서는 흔히 서로 다른 압력을 가지는 증기를 생산하며, 이런 경우 다수의 증기 입구를 가지기 때문에 증기터빈 출구로 가면서 증기의 질량유량 증가

3압 증기터빈의 경우 증기터빈 출구 증기 질량유량은 증기터빈 입구 질량유량에 비해 30% 정도 증가 (대형화력발전 증기터빈 출구 질량유량은 입구 질량유량에 비하여 약 60% 정도로 감소)

이런 이유 때문에 복합발전용 LP터빈 LSB에서 증기터빈 전체 출력의 약 15% 생산 (일반 화력발전 LSB는증기터빈 전체 출력의 약 10% 생산)


복합발전용 증기터빈은 빈번한 기동/정지로 인하여 짧은 기동시간 요구

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Summary for Main Steam Conditions

Higher main steam temperatures require more expensive alloys in the HRSG, steam piping, and steam

The followings must be considered when the main steam parameters are increasedg p q p y p p g

turbine. The gain in output must, therefore, justify this additional investment.

Higher main steam pressures cause wall thickness to increase, which, in general, reduces thermal flexibility and increases cost. Once-through HRSGs will be installed more frequently to avoid the negative y g q y gimpact on thermal flexibility of the higher main steam pressures.

High main steam pressure in combination with reheat steam turbine reduces the main steam volume, which may result in a reduction in the efficiency of the high-pressure steam turbine due to shorter blades. y y g pHowever, this problem can be overcome by employment of 2 on 1 configuration.


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Type of Steam Turbines2


LP Exhaust Hood 4


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Reheat Turbines

High pressure steam

Power output

To condenserCold

reheatHot

reheat To condenser

HP LPIP

Reheat turbines are special condensing units applied to large, high pressure power plants.

To condenserreheat reheat To condenser

Reheat is becoming common for the large combined cycle power plants, where inlet pressures and temperatures are rising to those commonly found in traditional fossil power plants.

In the reheat turbine all the steam expanded through the high pressure turbine is removed from the unit and In the reheat turbine, all the steam expanded through the high pressure turbine is removed from the unit, and taken as cold-reheat to the reheater of a HRSG.

Usually, cold-reheat steam is reheated back to the original inlet steam temperature, and admitted back into the IP turbine as hot reheat steam at the same temperature as the inlet although at a lower pressure


the IP turbine as hot-reheat steam, at the same temperature as the inlet although at a lower pressure.

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Reheat Turbines

Reheat turbines maintain acceptable levels of wetness in the LP turbine, while the increased length of steam expansion line results in increased power and efficiency although it requires a significantincreased power and efficiency although it requires a significant increase in equipment cost.

Multi-cylinder configurations are suitable for reheat turbine, because of the large diameters of the piping takeoffs and inlets.g p p g

It requires reheat stop valve and intercept valve to control the unit during startup and transients.


[ GE D11 reheat steam turbine ]

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Condensing Steam Turbines

The primary type of turbine used for central power generation is the condensing turbine.

Condensing turbines exhaust steam directly to condensers that

High pressure steam

Condensing turbines exhaust steam directly to condensers that maintain vacuum condition, that is significantly below atmospheric pressure.

An arrays of tubes, cooled by cooling water, condensates the

Power output

steam into water and creates condenser vacuum.

As a result of condensation, the volume of the exhaust steam reduces by several order of magnitude, and since the process t k l i l d l t d t t itakes place in a closed vessel, a steady-state vacuum is created and maintained, which keeps the process and cycle going.

The term ‘condensing’ refers to the condensing system that is

Condenser

The term condensing refers to the condensing system that is attached to the unit and acts to condense the exhaust steam.

The condensing turbines have large exhaust areas since the steam is expanded to low pressures, extracting as much of the p p guseful energy as reasonably possible to being exhausted.

Under the vacuum pressure, specific volume of steam is very high, thus longer blades are required at the last stage before t i t th d C d i


steam goes into the condensers. Condensing steam turbine (65 MW)

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Condensing Steam Turbines

An exhaust hood or diffuser is provided downstream last stage blade to minimize exhaust loss by reduction of the steam velocity.

The steam exhausted into condenser has a quality of near 90% and this is the main cause of the water The steam exhausted into condenser has a quality of near 90%, and this is the main cause of the water droplet erosion of blades.

The large power plants for electricity generation are this type of turbine because of maximum power output d l t i l ti ffi iand electrical generation efficiency.

The power output of condensing turbines is sensitive to ambient conditions.

C d i t bi d f ll l fi d il d fi d t t bi ll l t t bi Condensing turbines are used for all coal-fired, oil- and gas-fired steam turbines, all nuclear steam turbines, and most of combined cycle plants.

The size range of the condensing turbines is from as small as 5 MW, up to 1500 MW units employed in power utility.

For combined cycle plants, the maximum size will be 200~250 MW.

Typical condensing pressure are 0.75 in.Hga for the coldest condensing systems, up to a maximum of 10 in.Hga for air-cooled condensing system.

As a small amount of air leaks into the system when it is below atmospheric pressure, a relatively small


pump removes non-condensable gases from the condenser.

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Condensing Steam TurbinesH B l f C d i T bi

Single PressureSupplementary Firing

Heat Balance for a Condensing Turbine

Supplementary FiringCondensing Turbine


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Back Pressure Steam Turbines

The term “back-pressure” refers to turbines that exhaust steam at atmospheric pressures and above.

High pressure steam

Power

At these pressure, the specific volume of the steam is relatively small, and consequently blade height and exhaust piping are also relatively small.

Power output

also relatively small.

Backpressure steam turbines do not have condenser, and this is the reason why the backpressure turbine is called as non-

Low pressure steam to process

condensing turbine.

Backpressure turbine exhausts its entire steam flow to the i d t i l tindustrial process steam.

Usually, the steam sent for process is not much above saturation temperaturesaturation temperature.

The specific application establishes the discharge pressure, and 50 150 and 250 psig are the most typical pressure levels Backpressure steam


and 50, 150, and 250 psig are the most typical pressure levels. Backpressure steam turbine (28 MW)

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District heating system most often use the lower pressures, and industrial processes use the higher pressurespressures.

Industrial processes often include further expansion for mechanical drives, using small steam turbines for driving heavy equipment that runs continuously for long periodsdriving heavy equipment that runs continuously for long periods.

Power generation capability reduces significantly when steam is used at appreciable pressure rather than being e panded to ac m in a condenserbeing expanded to vacuum in a condenser.

Discharging steam into a steam distribution system at 150 psig can sacrifice slightly more than half the th t ld b t d h th i l t t diti 750 i d 800 F t i l f llpower that could be generated when the inlet steam conditions are 750 psig and 800F, typical of small

steam turbine systems.

The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure.

These are commonly found at facilities where large amount of low pressure process steam is needed.


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C d i B k T bi

Back Pressure Steam TurbinesCondensing vs. Backpressure Turbine

Condensing steam turbine (65 MW)turbine (65 MW)


Backpressure steam turbine (28 MW)

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H B l f B k T bi


Single PressureSupplementary Firing

Heat Balance for a Backpressure Turbine

pp y gBackpressure Turbine


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Extraction Turbines

The extraction turbine has opening(s) in its casing for extraction of a portion of the steam at some intermediate pressure before condensingsome intermediate pressure before condensing the remaining steam.

The term extraction means that the removing steam from an intermediate stage of the turbinesteam from an intermediate stage of the turbine during the expansion of the steam through a turbine,

In most power plants some steam is extracted In most power plants, some steam is extracted from the turbine expansion process and supplied to heat exchangers for feedwater heating.

Thi t f t ti i i d This type of extraction varies in pressure and flow as a function of load.

This variation is acceptable and therefore no ff t i d t l t theffect is made to regulate the pressure.

This type of extraction called uncontrolled, or nonautomatic extraction.


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Extraction Turbines

The steam extraction pressure may or may not be automatically regulated.

High pressure steam

Turbines supplying steam for process applications typically must supply steam at a constant pressure.

Since the steam pressure varies with load unless

Power output

controlled, valve is included in the turbine steam expansion path to control the pressure.

The extraction turbine has opening(s) in its casing for extraction of a portion of the steam at some intermediate pressure before condensing the remaining steam. Condenser

Medium/low pressure steam to process

This valve restricts the flow to the downstream stages as required to maintain the pressure of the extraction stage.

This type of turbine is called a controlled or automatic extraction turbine.

If several controlled extraction points are required in any one turbine, multiple internal control valves are provided.

Regulated extraction permits more steam to flow through the turbine to generate additional electricity


g p g g yduring periods of low thermal demand by the CHP system.

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Extraction Turbines

Feedwater heating deaeration extraction from LP turbine is generally used for cycles where the gas turbine

[ Layout with possible extraction – D-11 (GE) ]

fuel has relatively high sulfur content.

Occasionally, a concern with high sulfur gas turbine fuels is acid condensation on low temperature heat transfer surfaces. In these cases, an LP turbine extraction may be used to heat feedwater above the acid de point prior to feed ater s ppl to the HRSG economi er


dew point prior to feedwater supply to the HRSG economizer.

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Extraction Turbines

[Packaged cogeneration unit with multiple inlet control valves and automatic extraction ]


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Admission Turbines

In large, often complex, industrial plants, additional steam may be admitted (flows into the casing and increases the flow in the steam path) to the steam turbine.

Often this happens when using multiple boilers at different pressure, because of their historical existence.


pp g p p ,These steam turbines are referred to as admission turbines.

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Admission Turbines

[ 207D 17 Steam Turbine (GE) ][ 207D-17 Steam Turbine (GE) ]


[ KN Steam Turbine (Siemens) ]

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LP Exhaust Hood 4


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1_Superheating

T3

For a better understanding of various steam cycle parameters, a thorough investigation of the ideal Carnot cycle is very helpful.

2

3

3

y y p

It is not mechanically practical to partially condense steam to a particular quality from state 4 to state 1 and then compress the wet steam from state 1 to state 2.

411

2

4

p

For these reasons, actual steam cycles are based on a modified version of the Carnot cycle called the Rankinecycle.

s

411

a b c

4cycle.

Cycle T-s Diagram Remarks

Vapor Carnot cycle 1-2-3-4 th = 1 – T4/T3 = 1 – TL/TH

Rankine cycle without • Nuclear power steam cycleRankine cycle without superheating 1-2-3-4 • Nuclear power steam cycle

• Cycle efficiency is lower than Carnot cycle

Rankine cycle with superheating 1-2-3-4

• Fossil power steam cycle• Cycle efficiency increase by application of


superheating y y y ppsuperheating

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1_Superheating

T Equivalent Carnot Cycle

Equivalent Cycle Hot Temperature T

3

Equivalent Cycle Hot Temperature

3

3

2 2

s

1 4

s

1 4

[Ideal Rankine Cycle for a Typical Nuclear Power] [Ideal Rankine Cycle for a Typical Fossil Power]

The higher the equivalent cycle hot temperature, the greater the cycle efficiency.

The average temperature where heat is supplied in the boiler can be increased by superheating the steam.


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1_Superheating

The overall efficiency is increased by superheating the steam. This is because the mean temperature where heat is added

T

4

pincreases, while the condenser temperature remains constant.

Increasing the steam temperature not only

energy

3 Increasing the steam temperature not only improves the cycle efficiency, but also reduces the moisture content at the turbine exhaust end and thus increases the turbine internal efficiency energy

added2

55

efficiency.

The turbine work out is also increased by superheating the steam without increasing the

s

energy lost

1 5boiler pressure.

When the superheating the steam is employed in the cycle the important thing is that thein the cycle, the important thing is that the quality of the steam at the turbine exhaust is higher than 90%.


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2_Condenser Pressure

Except for choked turbine exhaust conditions, the lower the turbine exhaust pressure, the higher the cycle efficiency.

60%1001 HR

HRKW

70%

Throttle Flow Rate

kWh

100KW = change of generator kW

output, %.HR = change of heat rate, %

100%90%

80%

at ra

te, B

tu/k

T3

3.5

4.5

100%

CondenserPressure(in Hga)Tu

rbin

e he

a

p2p4

2.5(in.Hga)T

2p4’

11

2

b

4

4


Turbine output, kWsa ba

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1952 F-40.0"LSB D-11 steam turbine for GE 207FA,

1800 psia / 1050F / 1050FW

195

1902 F-33.5"LSB

2 F-30.0"LSBOut

put,

MW 190

am T

urbi

ne 185

Ste

a

180

Condenser Pressure, in.Hga0.5 1.51.0 2.52.0 3.53.00.0

1754.0


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When the ambient temperature is low, gas turbine output and HRSG steam production are increased above plant rating point.

Condenser (e ha st) press re directl infl enced b ambient air or cooling ater temperat re Condenser (exhaust) pressure directly influenced by ambient air or cooling water temperature.

Condenser pressure is expected to be lowest at low ambient air / cooling water temperature, and exhaust annulus velocity will be the highest.

[ Combined Cycle Output Power Factor ](ISO ambient temperature base)


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[Exercise 10.1]복수기압력이 2.5 inHga로운전되는증기터빈출력이 700 MW, heat rate가 7826 Btu/kWh이다. 그런데throttle steam 조건을그대로유지시킨상태에서복수기압력을 4 5 inHga로운전하였더니열율이 7980throttle steam 조건을그대로유지시킨상태에서복수기압력을 4.5 inHga로운전하였더니열율이 7980 Btu/kWh가되었다. 이때나타나는출력변화율을계산하시오.

[Solution 1]증기터빈출력변화율은다음식으로구할수있다.

HRKW

HR = {(7980–7826)/7826}100 = 1.97%KW = 1 97/(1+1 97/100)= 1 93%

1001 HRKW

KW = –1.97/(1+1.97/100)= –1.93%

[Solution 2]HR = Q/W Q = 700,000 kW 7826 Btu/kWh = 5,478.2106 Btu/hrW′ = Q/HR′ = 686,491 kW KW = {(700,000–686,491)/700,000}100 = –1.93%


{( 00,000 686, 9 )/ 00,000} 00 93%

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3_Reheat

The steam from boiler flows to the HP turbine where it expands and is exhausted back to the boiler for reheating

T qRH

3 5

boiler for reheating.

The efficiency of the Rankine cycle can be improved by reheating on the right hand side of

qH

4

the T-s diagram.

An improvement in cycle efficiency from a single reheat is only 2-3% Although this is not

2

wP

wTA

B

single reheat is only 2-3%. Although this is not dramatic, it is a useful gain which can be obtained without major modification to the plant.

qL 41 6

wP

s


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3_Reheat

The effect on the cycle efficiency depends on the reheat pressure.

• In general, the optimum reheat pressure for maximum cycle efficiency is usually about 1/4 of the main boiler pressure.boiler pressure.

• High reheat pressure gives a high mean temperature of heat reception during reheating, but only a small extra heat input leading to small increase in cycle efficiency.

L h t i l t t t ith th i l Th f th i• Low reheat pressure gives an almost same mean temperature with the main cycle. Therefore, there is no significant improvement in cycle efficiency.

The pressure drop in the reheater and associated piping is important.

A relatively large pressure drop can significantly offset the benefit due to reheating.

When large diameter piping is used, the pressure drop is reduced, but initial cost may increase proportionately. As in the main steam pipe, pressure drop in the reheater and its piping must be appropriately balanced against the initial cost.

The pressure drop in the reheater and associated piping is about 7% to 10%.

This pressure drop results in a poorer power plant heat rate of 0.7% to 1.0%.

As a rule of thumb for the effect of pressure drop on heat rate any place in the steam path is 0.1% poorer


heat arte per 1% pressure drop in a fossil plant.

HIPT

3_Reheat

h D The trend for higher gas turbine firing and exhaust

temperatures has made reheat combined cycles common.A

A-B-C: NonreheatA-B: HP TurbineB D: Reheater

A structured, modular approach to the combined cycle steam turbine product line allows application of non-reheat steam cycles with advanced gas turbines as well.

B B-D: ReheaterD-E: IP and LP Turbine

y g

The reheat cycle benefits thermodynamic by adding heat to the steam cycle at a higher average temperature than to t e stea cyc e at a g e a e age te pe atu e t athe non-reheat cycle, and by reducing moisture loss in the low pressure section.

The drier low pressure section expansion reduces the

4%

8%E

The drier low pressure section expansion reduces the potential for last stage moisture erosion.

The gain from the reheat cycle is seen as greater steam turbine output for the same heat to the HRSG 8%

16%

12%C

turbine output for the same heat to the HRSG.

Reduced heat rejected to the condenser reduces the size of the cooling system and the amount of cooling flow required


srequired.

HIPT

3_Reheat

Boundary between reheat and non-reheat steam cycle is gas turbine exhaust gas temperature of approximately 1000F/538C. However, steam turbines for non-reheat cycles are also available when the


EGT exceeds 1000F/538C.

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4_Increasing Steam Pressure

9000

Main Steam conditions strongly influence the turbine performance.

, Btu

/kW

h 9000

At a given pressure the turbine performance can be improved by increasing the main steam temperature.

Throttle Pressure2400 psiaH

eat R

ate,

8000

steam temperature.

The higher the steam temperature, the better the turbine performance.

2400 psia

3500 psia6 Flows, 30” LSBThrottle steam 2400 or 3500 psia, 1000 F/ 1000F

Turb

ine

7000

However, there is a temperature limit beyond that turbine and boiler will become less reliable.

Nominal output 700 MW at 1.5 in.Hga.

Turbine Output, MW200 400 600 800

7000

An increase in steam pressure at turbine inlet will increase the cycle thermal efficiency.

The casing becomes quite thick as the steam pressure increases, and consequently steam turbines exhibit large thermal inertia. Therefore, steam turbine must be warmed up and cooled down slowly to minimize the differential expansion between the rotating blades and the stationary parts. Large steam turbine can take


over ten hours to warm up.

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4_Increasing Steam PressureE l i f R ki C lEvolution of Rankine Cycle

T Ultra SupercriticalSupercritical

1960

1940s

Early 20th century


s

HIPT

5_Regenerative Feedwater Heating

TTurbine4

34

BoilerG

5

1

2

51 5 condenser

23

5

s

1 51 5

a b c d

condenser

pump1

If the liquid heating could be eliminated from the boiler, the average temperature for heat addition would be increased greatly and equal to the maximum cycle temperature.

I th id l ti R ki l th t i l t d th t bi i d fl i th In the ideal regenerative Rankine cycle, the water circulates around the turbine casing and flows in the direction opposite to that of the steam flow in the turbine.

Because of the temperature difference, heat is transferred to the water from the steam. However, it can be id d th t thi i ibl h t t f th t i t h i t th t t f t i


considered that this is a reversible heat transfer process, that is, at each point the temperature of steam is only infinitesimally higher than the temperature of water.

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Reversible heat transfer and an infinite number of feedwater heaters would result in a cycle efficiency equal to the Carnot cycle efficiency.

The greater the number of feedwater heaters used, the higher the cycle efficiency. This is because if a large number of heaters is used, the process of feedwater heating is more reversible.

However, each additional heaters results in lower incremental heat rate improvement because of the decreasing benefit of approaching an ideal regenerative cycle.

Th i b fit f dditi l h t i li it d b f th di i i hi i t i l The economic benefit of additional heaters is limited because of the diminishing improvement in cycle efficiency, increasing capital costs, and turbine physical arrangement limitations.

The amount of steam flow into condenser can be reduced dramatically by the employment of regenerative y y p y gRankine cycle.

The LSB problems, such as water droplet erosion and longer active length, could be solved by the regenerative Rankine cycle which is made by steam extraction in many turbine stagesregenerative Rankine cycle, which is made by steam extraction in many turbine stages.

Regenerative Rankine cycle also diminish the influence of the LP turbine, which has worst performance.


HIPT


There are usually no extractions from the steam turbine, since feedwater heating is generally accomplished within the HRSG

복합발전에서 급수가열기는 사용하지 않거나 사용하더라도 작은 수를 사용

이는 신속한 기동특성 확보와 저압터빈에 필요한 증기유량을 확보하기 위함


HIPT

5_Regenerative Feedwater HeatingH B l f F il P PlHeat Balance of a Fossil Power Plant


HIPT

6_LSB

When configuring any steam turbine it is very When configuring any steam turbine, it is very important to choose the proper annulus area for the anticipated exhaust flow and condenser pressure.

Wh 9000

LSB strongly influence the turbine performance.

ate,

Btu

/kW

The length of the LSB is determined by the number of exhaust flows.

In general the longer LSB the lower the full

LSB = 26"

bine

Hea

t Ra

800030"33.5"

In general, the longer LSB, the lower the full-load heat rate.

However under the part-load operation

6 Flows, 2400 psia / 1000 F/ 1000FNominal output 700 MW at 1.5 in.Hga.

Turb

7000 However, under the part load operation,

turbines having longer LSB deteriorate more rapidly in performance. Turbine Output, MW

200 400 600 800


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6_LSBD 11 S T bi f GE 207FAD-11 Steam Turbine for GE 207FA

195The best steam turbine choice for any combined-cycle is strongly influenced by the site exhaust pressure which in turn

2 F-40.0"LSB D-11 steam turbine for GE 207FA, 1800 psia / 1050F / 1050F

W

195

190

It can be seen from this figure

exhaust pressure, which, in turn, is largely determined by the temperature of the cooling media.

2 F-33.5"LSB

2 F-30.0"LSBOut

put,

MW 190

It can be seen from this figure that the units with largest annulus area have the best performance at low back pressures am

Tur

bine

185

pressures

However, the units with smaller exhaust area have better performance at high back

Ste

a

180

performance at high back pressure because of lower turn-up loss.

Condenser Pressure, in.Hga0.5 1.51.0 2.52.0 3.53.00.0

1754.0


HIPT

7_Pressure Drop in Reheater System

The total reheater pressure drop includes

1

2

Turbine Heat

The total reheater pressure drop includes the pressure drop associated with the cold reheat piping from HP turbine exhaust to the reheater section of the boiler the reheater section of the boiler

ange

, % 0

Turbine Heat Rate

boiler, the reheater section of the boiler itself, and the hot reheat piping from the reheater to the IP turbine intercept valves.

Output

Cha -1

-2

A typical design value for total reheatersystem pressure drop is 10% of the HP turbine exhaust pressure.

R h t P D %

10 155-3

20

For a 1% decrease in reheater pressure drop, the heat rate and output improve approximately 0.1% and 0.3%, respectively Reheater Pressure Drop, %respectively.


HIPT

8_EGT vs. ST Output

95 90

When the EGT is lowered, both the thermodynamic quality of the steam

85

75

80

70

, MW

process and the HRSG efficiency decrease.

Th ff t i d ith th

65

55

60

50

40bine

Out

put

ffici

ency

, %

The effect is more pronounced with the single pressure than with a dual or triple-pressure reheat cycle because the energy utilization rate falls off faster.

55

45

40

30

20Stea

m T

urb

HR

SG

Ef

35

25400 450 500 550 600 647

10

0

S

Steam turbine output and HRSG efficiency versus gas

400 450 500 550 600 647GT Exhaust Gas Temperature, C


turbine exhaust temperature for a single-pressure cycle

HIPT

9_Makeup Flow (for Fossil Power)

The makeup is necessary to offset the steam losses in the cycle and losses in the boiler associated with boiler blowdown and steam soot blowing.

Typical amounts of the steam used for makeup are from 1% to 3% of the throttle flow.

Boiler blowdown is necessary to maintain proper boiler chemistry.

Consideration should also be given to process extractions that involve less than 100% return of condensate.

The makeup water is typically supplied to the condenser hot well increasing the total flow through the The makeup water is typically supplied to the condenser hot well, increasing the total flow through the heaters and pumps, and therefore must be heated in the feedwater cycle on the way to the boiler.

This additional flow results in higher feedwater heater thermal duties and therefore higher extraction flows, d hi h i tand higher pump power requirements.

This results in a negative effect on cycle performance.

The effect of makeup on net turbine heat rate is approximately 0.4% higher per percent makeup. The effect of makeup on output is approximately 0.2% lower per percent makeup.


These values are based on boiler blowdown at saturated conditions at the boiler drum pressure.

HIPT





LP Exhaust Hood 4


HIPT

Typical Exhaust Loss Curve

UEEP = ELEP + Exhaust Loss

The internal efficiency of a ow

50Annulus

Restriction Loss

steam turbine does not include the loss at the turbine exhaust end.The exhaust loss includes /lb

of d

ry fl

o

30

40 Gross Hood Loss

Total The exhaust loss includes (1) actual leaving loss, (2) gross hood loss, (3) annulus-restriction loss, (4) turn-up loss. st

Los

s, B

tu20

30Turn-up

Loss

Exhaust Loss

( ) tu up ossE

xhau

s

10Actual

Leaving Loss

0S i

200 400 600 800 1000 1200 1400 16000

Loss


Annulus Velocity, fps Sonic

HIPT

Exhaust Loss [3,600 rpm, GE]

50

46Bucket Pitch Last stage

Curve length diameter annulus areano. (inches) (inches) single flow (ft2)1 14 3 52 4 16 3

1 23452 3 4 5

flow

42

38

34

1 14.3 52.4 16.31 16.5 57.5 20.71 17 52 19.31 20 60 26.22 23 65.5 32.93 26 72 41 1

u/lb

of d

ry f 34

30

26

3 26 72 41.14 30 85 55.65 33.5 90.5 66.1

Van = Annulus velocity (fps)m = Condenser flow (lb/hr) = Saturated dry specific volume (ft3/lb)Aan Annulus area (ft2)

ust L

oss,

Bt 26

22

18(1) R d th h t l t th l l it bt i d f

Aan = Annulus area (ft2)Y = Percent moisture at ELEPELEP = Expansion line end point at actual

exhaust pressure (Btu/lb)UEEP = Used energy end point (Btu/lb)

1

Exh

au

14

10

(1) Read the exhaust loss at the annulus velocity obtained from the following expression:

Van = m(1-0.01Y) / 3600Aan

(2) The enthalpy of steam entering the condenser is the quantity obtained from the following expression:

200 400 600 800 1000 1200 1400

6

20

UEEP = ELEP + (Exhaust loss)(0.87)(1-0.01Y)(1-0.0065Y)

(3) This exhaust loss includes the loss in internal efficiency which occurs at light flows as obtained in tests.


Annulus Velocity, ft/s

HIPT

Total Exhaust Loss

W = Workh

hT1

h

HL = Hood LossLL = Leaving LossEL = Exhaust Loss

hS1

EEL = Effective ELUEEP = Used Energy End Point (or TEP)ELEP = Expansion Line End Point

Total Expansion LineStatic Expansion

LineW

SEP = Static End PointEL = Change in ELW = Change in Work

sB

EEL = Change in EELpc = Static Pressure at Turbine

Exhaust FlangeT t l P t L t Bl d E it

EELW=EEL

LLEL

hT2

EL

pTB = Total Pressure at Last Blade ExitpSB = Static Pressure at Last Blade ExitELEPSEPHL

EL


s

HIPT

Steam Turbine Exhaust Size SelectionE h L

Nozzle

Exhaust Loss

[ Velocity triangle in LSB ] C2W2

ULSBU1

2

3W3C3

LSB

Minimum LL

4

3C3 2

The exhaust area of the steam turbine

5

3

5

should be determined by the balance between exhaust loss and capital investment in steam turbine equipment.

4


HIPT

Exhaust LossT LTurn-up Loss

Turn-upRegion


Normal Rating Operation Low Load Operation

HIPT

Exhaust Loss2) T L2) Turn-up Loss

Water supply line

Water spray

Water running down casing walls

p yLSB

[ Eroded Trailing Edge of LSB near the Hub ] [ Recirculation in the Exhaust Hood ]

casing wallsRecirculating steam


[ Eroded Trailing Edge of LSB near the Hub ] [ Recirculation in the Exhaust Hood ]

HIPT

Trailing Edge Erosion

A crack emanating from a trailing edge gouge

Trailing edge erosion on the suction side

Crack in the trailing edge caused by erosion - PT


y

HIPT

Turn-up Region에서의 사고사례


HIPT

Turn-up Region에서의 사고사례


HIPT

Mass Flow at the Exhaust

Price = 00억원/152 개 (for LSB installed in one LP cylinder of 500 MW Korean standard fossil power plants), 2010 년 기준.

28 MW 출력 저하 (스팀터빈 기준 17%)


HIPT





LP Exhaust Hood 4


HIPT

Velocity Triangle at Various Loads

Steam turbines designed for combined cycle employ slidingcombined cycle employ sliding pressure operation.

Ahead of First Stage Nozzles

Main Steam

Stage Nozzles

After First Stage

Load[ Sliding Pressure Operation ]


[ Sliding Pressure Operation ]

HIPT

Sliding Pressure Operation

Sliding pressure operation, which is also called variable pressure operation, is a method of controlling throttle flow and thus power output by varying boiler pressure.

The important factor to be considered when evaluating sliding pressure operation relative to constant throttle pressure are:

• Reduced available energy

• Improved HP turbine efficiency

• Higher hot reheater temperatures at part loads

Hi h th ttl t t t t l d• Higher throttle temperatures at part loads

• Reduced power for BFPT and motors driven with variable speed hydraulic couplings

The combined effect of improved HP turbine efficiency and reduced available energy can be evaluated to The combined effect of improved HP turbine efficiency and reduced available energy can be evaluated to adopt sliding pressure operation.

The live steam pressure can be controlled by adjusting the boiler pressure, i.e. a sliding boiler pressure controlcontrol.

One purpose of this is to reduce the inlet pressure to the first stage with maintained temperature at part loads.


HIPT


This result in a more stable inlet volumetric flow, which is beneficial for the isentropic turbine efficiency. Otherwise, the turbine will be operated under the off-design condition.

Both feed pump power and throttling losses in the turbine control valves can be reduced during the part load operation because of lower pressure.

It requires a fast response of the boiler, therefore, a forced circulation boiler is beneficial.

Reduced pump power at low load operation with variable pressure can be a significant additional heat rate p p p p p gimprovement.

Sliding pressure operation decreases the potential for low cycle thermal fatigue in the turbine during load changes as compared to constant initial pressure operationchanges as compared to constant initial pressure operation.

In sliding operation, first stage exit temperature is almost constant over the load which reduces thermal stress. (see the Figure shown in next page)

However, with constant initial pressure, and partial arc admission over the load range, the heat rate at light load is better than the sliding pressure operation heat rate.


HIPT


100 The steam cycle is operated in sliding

pressure mode with fully open steam turbine valves down to approximately

Fixed Pressure Mode50

50% live-steam pressure.

The conventional steam plants are generally operated at a fixed pressuregenerally operated at a fixed pressure, that is steam pressure and temperature remain constant.

Load, %50 100

Items 결과 이유 효율변화

밸브 교축손실 모든 출력에서 VWO상태이기 때문에 교축손실 매우 작음

HP Turbine 효율 HP 1단에서 full arc 운전이며, 밸브에서 교축손실 작음

Rankine cycle 효율 부분출력에서 사이클 압력 저하로 available energy 감소

부분출력에서 펌프 동력 절감 (출력이 낮아질수록 펌프 동


급수펌프 동력 부분출력에서 펌프 동력 절감 (출력이 낮아질수록 펌프 동력 절감 크기 증대)

HIPT

질의 및 응답질의 및 응답

작성자: 이 병 은 (공학박사)작성일: 2014 03 03 (Ver 3)


작성일: 2014. 03. 03 (Ver.3)연락처: ebyeong @ naver.com

Mobile: 010-3122-2262저서: 실무 발전설비 열역학/증기터빈 열유체기술

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