6. gas turbine performance - engsoft power lab - … · 2015-05-18 · 950.24%. steam turbine -10...

Combined Cycle Power Plants 6. Gas Turbine Performance 1 / 101

HIoPE

6. Gas Turbine Performance

Condenser

(heat out)

T

s

Topping cycle

Bottoming cycle

Combustion

(heat In)

Stack

(heat

out)


HIoPE

The gas turbine is a standardized machine, and can be used under widely different ambient conditions.

Manufacturers quote gas turbine performances at ISO ambient conditions of 15C(59F), 1.013 bar (14.7

psia), and 60% relative humidity.

Gas turbine performance is mainly governed by pressure ratio, turbine inlet temperature, and efficiency of

each parts.

The performance of gas turbine is affected by its inlet and exit conditions. The most important items are

pressure and temperature. Ambient weather conditions are the most obvious changes.

Since the gas turbine is an air-breathing machine, its performance is changed by anything that affects the

density and/or mass flow rate of air intake to the compressor.

A smaller weight of air requires a smaller weight of fuel to mix with, and the mixture then produces less

power when burned.

Most peak power enhancement opportunities exists in the topping cycle.

In general, however, performance enhancements to the gas turbines will carry with them an increase in

bottoming cycle performance due to an associated increase in gas turbine exhaust energy.

Duct firing within HRSG is an exceptional performance enhancement occurred in the bottoming cycle.

Generals


HIoPE

Factors to be considered individually

Is there a need for peak power production with

premium paid for the resulting power?

Does peak power demand occur on hot days

(summer peaking) only?

Is there a need to compensate the power

reduction continuously during summer period?

Is frequency support required?

Solutions for power augmentation

Supplementary firing in HRSG

Steam / water injection

GT peak load firing

GT inlet air cooling

Factors Affecting GT Performance


HIoPE

Options for power enhancements Typical performance impact

Output Heat Rate

Base configuration Base Base

Evaporative cooling GT inlet air (85% effective cooler) +5.2 % -

Chill GT inlet air to 45F +10.7 % +1.6 %

GT peak load operation +5.2 % 1.0 %

GT steam injection (5% of GT airflow) +3.4 % +4.2 %

GT water injection (2.9% of GT airflow) +5.9 % +4.8 %

HRSG supplementary firing +28 % +9 %

Note: 1. Site conditions = 90F, 30% RH(Relative Humidity)

2. Fuel = NG

3. 3-pressure, reheat steam cycle

4. At sites where large power enhancement is possible, the owner must verify that the

added power is within the capabilities of the generator and transformer

Output = m h •

Various Options for Power Enhancement


HIoPE

Typical Gas Turbine Sensors/Effectors Source: GE


HIoPE

1. Ambient Temperature [1/7]

Because gas turbines are constant-

volume-flow engines, they are very

sensitive to changing ambient

temperature and pressure.

The output and thermal efficiency of the

gas turbines decrease as air

temperature increases. This is because

an air density decreases as the ambient

air temperature increases, thus the

mass flow rate of air decreases because

industrial gas turbines running at

constant speed are constant volume

flow machines.

The thermal efficiency decreases as the

air temperature increases. This is

because compressor driving power

increases as the air temperature

increase. In addition, heat transfer

efficiency of the blade cooling system

decreases as the air temperature

increases. Thus, more cooling air is

needed as the air temperature increases.

Perc

ent

desig

n

130

120

110

100

90

80

70 0 20 40 60 80 100 120 F

-18 -7 4 16 27 38 49 C

Compressor inlet temperature


HIoPE

The specific power consumed by the compressor increases

proportional to the inlet air temperature (in K) without a

corresponding increase in the turbine output.

The exhaust gas temperature increases as the inlet air

temperature increases because the turbine pressure ratio is

reduced, although the gas turbine inlet temperature remains

constant. This is the main reason for that the gas turbine

output and efficiency decrease while the ambient air

temperature increases.

However, the effect on the performance of the combined cycle

is somewhat less because a higher exhaust gas temperature

improves the performance of the steam cycle.

1

1

1,

CPR

TCw

C

op

C


T

s

1

2

3

4

1

2

3

4


HIoPE

Rela

tive e

ffic

iency,

%

Gas turbine

100

0

Air temperature, C

105

95

10 20 30 40 -10

Based on constant

condenser pressure

Steam turbine

Combined cycle

An increase in the inlet air temperature has a

slightly positive effect on the efficiency of the

combined cycle plant, while other ambient

conditions as well as condenser pressure

remain constant.

Because the increased gas turbine exhaust gas

temperature improves the efficiency of the

steam process, it more than compensates for

the reduced efficiency of the gas turbine unit.

According to the open literature, with each one-

degree temperature increase above 30°C, power

output of the gas turbines drops by 0.50%–

1.02% while efficiency drops by approximately

0.24%. Steam turbine power output and

efficiency are not significantly changed by

changing air temperature, while net CCGT power

output drops by 0.3%–0.6% and net efficiency

drops by approximately 0.01% per degree above

30°C.

Effect on Combined Cycle Efficiency

Source: Kehlhofer et al., 2009



HIoPE

Com

bin

ed c

ycle

effic

iency,

%

Ambient air temperature, K

Effect on Combined Cycle Efficiency



HIoPE

Net efficiency of a combined cycle power plant as a function of river water temperature.



HIoPE

Rela

tive p

ow

er

outp

ut, %

Gas turbine

100

0

Air temperature, C

120

90

10 20 30 40 -10

Based on constant

condenser pressure

110

The power output of the combined cycle

decreases as the inlet air temperature

increases.

In a combined cycle plant, gas turbines

contribute approximately two-thirds of the

power production, while the steam turbine

contributes the remaining one-third.

The combined cycle power output curve is

dominated by the gas turbine output curve, and

it is expected that changes in air temperature

will have more significant impact on plant

power output than changes in water

temperature.

The power output of the combined cycle is

affected differently from the efficiency because

change in mass flow of inlet air and exhaust

gases are more dominant than the exhaust gas

temperature.

Effect of Ambient Temperature on Combined Cycle Output




HIoPE

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

plant rating point.

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.



HIoPE

Gas turbine performance is quoted at an air pressure of

1.013 bar – ISO conditions, which corresponds to the

average pressure prevailing at sea level.

A different site elevation and daily weather variations

result in a different pressure.

The air density reduces as the site elevation increases.

Therefore, airflow and output decrease as the site

elevation increases.

However, the air pressure has no effect on the

efficiency if the ambient temperature is constant, even

though the output decreases as the pressure

decreases. This is because the backpressure of the

gas turbine is correspondingly lower at a lower ambient

pressure. This is also because both the thermal energy

supplied as well as airflow vary in proportion to the air

pressure.

Altitude x 103 feet

2 0 4 6 8 0.5

0.6

0.7

0.8

0.9

1.0

Correction

factor

Atmospheric

pressure

11.0

12.0

13.0

14.0

15.0

Corr

ectio

n fa

cto

r

Atm

osp

heric p

ressu

re, p

sia

2. Ambient Air Pressure

GT Model CC Configu-

ration

Ambient

Temp.,C Site

Site

Elevation, m

CC Thermal

Effcy., %

CC Net

Power, MW

GT Net

Power, MW

ST Net

Power, MW

PG7221FA 2-on-1 28.1

(82.5F)

Las Vegas 664 53.4 437.0 285.3 151.7

Miami Sea side 53.2 475.5 312.9 162.6


HIoPE

400 0 800 1200 1600

Combined

cycle output

Elevation above sea level, m

75

85

100 R

ela

tive p

ow

er

outp

ut, %

95

90

80

70 2000 2400

75

85

100

95

90

80

70

Rela

tive a

ir p

ressure

, %

Air pressure

2. Ambient Air Pressure

Effect of Air Pressure on Combined Cycle Output


HIoPE

Humid air is less dense than dry air.

In the past, this effect was thought to be too small to

be considered.

However, as the size of gas turbine increases, this

effect become important.

Steam or water injection for NOx control makes this

effect more significance.

3. Humidity

Specific humidity (kg water vapor/kg dry air)

ISO specific humidity

0.0064

60% RH

1.008

0.996

Co

rre

ctio

n f

acto

r

0.01 0 0.02 0.03

1.006

1.004

1.002

1

0.998

0.994

1.010


HIoPE

4. Inlet & Exhaust Pressure Drop [1/6]

Inlet filter

Evaporative cooler or chiller

Anti-icing system

Silencer (The large frontal areas of the

compressors reduce the inlet velocities, thus

reducing air noise)


HIoPE

Hot-end drive Cold-end drive

• In the hot-end drive configuration, the output shaft

extends out the rear of the turbine.

• The designer is faced with many constraints, such

as output shaft length, high EGT, exhaust duct

turbulence, pressure drop, and maintenance

accessibility.

• Insufficient attention to any of these details, in the

design process, often results in power loss,

vibration, shaft or coupling failures, and increased

down-time for maintenance.

• This configuration is difficult to service as the

assembly must be fitted through the exhaust duct.

• In the cold-end drive configuration, the output shaft

extends out the front of the compressor.

• The single disadvantage is that the compressor

inlet must be configured to accommodate output

shaft.

• The inlet duct must be turbulent free and provide

uniform, vortex free, flow over the all operating

range.

• Inlet turbulence may induce surge in the

compressor resulting in complete destruction of the

unit.

MS7001E, GE MS7001F, GE



HIoPE

A similar cost evaluation determines the optimum point that

dirty air filters, which have higher pressure losses, should

be changed out.

Inlet Pressure Drop

Inlet pressure drop is a function of the inlet air system design and cleanliness of the inlet air filters.

Lower inlet air pressure losses can be achieved by designing for lower inlet air velocities through the filter,

silencer, and duct.

The improved operating performance associated with a lower inlet air velocity design must be evaluated

against the associated higher capital cost.


Inlet pressure prop, in.H2O

Corr

ectio

n fa

cto

r

0 1 2 3 4 5 6 7 8 9 10 0.96

0.97

0.98

0.99

1.00

1.01

1.02


HIoPE

Self-cleaning filters were developed in the 1970s and now account for 90% of the new systems.

It combines high-efficiency filters, which can collect even small particles smaller than 1 m, and low

maintenance.

The design pressure drop of a new and clean filter bank is normally on the order of 2 mbar. The approach

velocity of air upstream of the filter surface is approximately 3 m/s.

When pressure drop builds up to a predetermined level (3~4 in.H2O = 7.5~10 mbar), the filter is cleaned by a

brief back-pulse of air, either extracted from the gas turbine compressor, or derived from an auxiliary source.

A filter compartment includes many filter elements (about 1,200 for 7FA), only a few of which are cleaned at

any given time; so the airflow to the gas turbine is not disturbed by the cleaning process.

A single cleaning cycle is usually completed in 20~30 minutes.

The filter elements are replaced when they begin to show signs of deterioration caused by heat and

ultraviolet rays, or when the cleaning is not effective any more.

It is impossible to keep the compressor completely clean and usually fouling occurs in the compressor. The

fouling that results causes losses in output and efficiency.

Two types of compressor cleaning can be used to help recover those losses, on-line washing and off-line

washing.

Self-Cleaning Filters



HIoPE

Exhaust pressure drop, in.H2O

Co

rre

ctio

n f

acto

r 1

1.020

1.015

1.010

1.005

0.995

0.990

0.985

0.980

2 3 4 5 6 7 8 9 10 11 12 13 14 0 15

1.025

1.000

0.975

Higher exhaust pressure loss is primarily a function of

the exhaust system design.

For a simple cycle applications, the exhaust system

typically consists of an exhaust duct, silencers, and a

stack.

Exhaust pressure losses of 4.0 to 5.0 in.H2O are

typical for simple cycle gas turbines.

For combined cycle or cogeneration applications, the

exhaust gases pass through an HRSG with the

associated additional.

Exhaust pressure losses of 10 to 17 in.H2O are typical

for combined cycle and cogeneration applications

depending on the complexity of the cycle arrangement,

exhaust emission control, or noise-abatement.

Exhaust Pressure Drop



HIoPE

4.0 in.H2O (10 mbar) Inlet pressure drop produces:

1.42% Power output loss

0.45% Heat rate increase

1.1C Exhaust temperature increase

4.0 in.H2O (10 mbar) Exhaust pressure drop produces:

0.42% Power output loss

0.42% Heat rate increase

1.1C Exhaust temperature increase

Inserting air filter, silencer, evaporative coolers or chillers into the inlet or heat recovery devices in the

exhaust causes pressure losses in the system.

The effects of these pressure losses are unique to each gas turbine models. This is because the amount of

pressure drop at the exit of compressor is pressure drop at the inlet times pressure ratio.

Hot-end drive has not been used since the cold-end drive type gas turbines have developed.

HRSG flue gas draft losses: approximately 25 mbar, 35 mbar if catalysts are required.

Hot-end drive (“E” technology)

MS7001EA 기준



HIoPE

Fuel affects combined cycle performance in a variety of ways.

Output of the gas turbine can be defined as the product of mass flow, specific heat, and temperature

differential across the turbine. Here, specific heat (Cp) means that the heat energy in the combustion

products.

The mass flow in this equation is the sum of compressor air flow and fuel flow.

Natural gas (methane) produces nearly 2% higher output than does distillate oil. This is because of the

higher specific heat in the combustion products of natural gas, resulting from the higher water vapor content

produced by the higher hydrogen/carbon ratio of methane. This effect is noted even though the mass flow of

natural gas is lower than that of distillate oil. Here the effects of specific heat were greater than and in

opposition to the effects of mass flow rate.

Model Fuel ISO base rati

ng, kW

Heat rate,

Btu/kWh

Exhaust flow,

kg/hr x10-3

EGT,

℃

Pressure

ratio

PG7251FB

N.G. 184,400 9,245 1613 623 18.4

D.O. 177,700 9,975 1677 569 18.7

5. Fuel [1/7]

4343 TTcmhhmW pTTT


HIoPE

C + O2 = CO2 + 33.9 MJ/kg

H2 + 1/2O2 = H2O(water) + 143.0 MJ/kg (HHV)

H2 + 1/2O2 = H2O(vapor) + 120.6 MJ/kg (LHV)

S + O2 = SO2 + 9.28 MJ/kg

5. Fuel [2/7]


HIoPE

The Composition of Natural Gases

The composition on a molar basis of natural gases is as follows:

The average heat content of natural gas is 1,030 Btu/ft3 on an HHV basis and 930 Btu/ft3 on

an LHV basis – about a 10% difference.

Composition, mol% A B C D E F

Methane

Ethane

Propane

Isobutane

Normal butane

Isopentane

Normal pentane

Hexane

Nitrogen

Carbon dioxide

Hydrogen sulphide

Heating value, Btu/ft3

95.0

1.9

0.5

0.5

0.1

0.1

0.1

0.1

1.5

0.2

0.0

?

94.3

2.1

0.4

0.0

0.2

0.0

0.0

0.0

0.0

0.0

2.8

1010

72.3

5.9

2.7

0.2

0.3

0.0

0.2

0.0

0.0

17.8

0.1

934

88.9

6.3

1.8

0.1

0.2

0.0

0.0

0.0

0.0

2.2

0.1

1071

75.4

6.4

3.6

0.6

1.0

0.2

0.1

0.0

0.0

12.0

0.1

1044

85.6

7.8

1.4

0.1

0.0

0.1

0.0

0.0

0.0

4.7

0.2

1051

5. Fuel [3/7]


HIoPE

Plant output and efficiency can be reduced when the fuels containing higher sulfur content are used. This is

because higher stack gas temperature is required to prevent condensation of corrosive sulfuric acid.

Plant output and efficiency can be reduced when the ash bearing fuels (crude oil, residual oil, blends, or

heavy distillate) are used because of fouling occurred in gas turbine and HRSG.

Heavy fuels normally cannot be ignited for gas turbine startup; therefore a startup and shutdown fuel, usually

light distillate, is needed with its own storage, forwarding system, and fuel changeover equipment.

The LHV of the fuel is important because it defines the mass flow of fuel supplied to the gas turbine.

The lower the LHV, the higher the mass flow of fuel required to provide a certain chemical heat input,

normally resulting in a higher power output and efficiency. However, there is no clear relationship between

fuel lower heating value and output.

This is why low BTU gases can result in high power outputs if they are supplied at the pressure required by

the gas turbine.

This effect is noted even though the mass flow of methane is lower than the mass flow of distillate fuel.

Here the effects of specific heat were greater than that of mass flow.

5. Fuel [4/7]


HIoPE

Degradation in CCPP after 8,000 hours of operation

Clean fuel Heavy or crude oil

Plant output, % 0.8~1.5 4.0~5.5

Plant efficiency, % 0.5~0.8 1.5~1.9

Ash deposition

on turbine

vanes

5. Fuel [5/7]


HIoPE

Hot corrosion of blades Burned turbine blades

In the past, corrosion is one of the major causes of gas turbine failures.

Corrosion problems have been eliminated by the use of advanced materials and coating.

Whenever heavy fuels are used, particularly those containing vanadium or sodium, it is necessary to use

additives or treat the fuel to prevent high-temperature corrosion.

The additives commonly used are based on magnesium, chromium, or silicon.

5. Fuel [6/7]


HIoPE

As the amount of inert gas is increased, the decrease in

LHV will provide an increase in output.

This is the major impact of IGCC type fuels that have

large amounts of inert gas in the fuel.

This mass flow addition, which is not compressed by the

gas turbine’s compressor, increase the turbine output.

Effects of Fuel Heat Value on Output

5. Fuel [7/7]


HIoPE

One way of improving the cycle efficiency is to

raise the apparent LHV (LHV + sensible heat) of

the fuel by preheating it with hot water from the

IP economizer of the HRSG.

Heated fuel gas gives higher turbine efficiency

because of the reduced fuel flow required to

raise the total gas temperature to firing

temperature.

Fuel heating will result in slightly lower gas

turbine output (almost negligible) because of the

incremental volume flow decrease.

The reduction in combined cycle output is

typically greater than simple cycle output

because energy that would otherwise be used to

make steam.

Actual combined cycle output and efficiency

changes are dependent on fuel temperature rise

and cycle design.

For combined cycle applications, fuel

temperatures on the order of 150 to 230°C

(300~450°F) are generally economically optimal.

6. Fuel Heating [1/2]

G

Fuel gas

ST

Condenser

G

Air

Stack gas

HRSG


HIoPE

Provided the fuel constituents are acceptable, fuel temperatures can potentially be increased up to

approximately 370°C(700°F) before carbon deposits begin to form on heat transfer surfaces.

Typical F-class three-pressure reheat systems use water from the intermediate pressure economizer to

heat the fuel to approximately 185°C (365°F). Under this conditions, efficiency gains of approximately 0.3

points can be expected for units with no stack temperature limitations.

Another factor is the gas supply pressure, depended on the combustor design and the gas turbine pressure

ratio.

If the gas turbine pressure ratio is high, a gas compressor may be required to increase fuel pressure. In this

case, the temperature of the fuel is increased in proportion to the pressure ratio and the benefit of gas

preheater will be reduced. In this case, the efficiency improvement is too slight to justify the additional

investment in the water/gas heat exchanger, HRSG surface, and piping.

It is important to ensure that the fuel does not enter the steam system because maximum steam

temperatures are typically above the auto ignition temperature for gas fuels.

For a system utilizing a direct water-to-fuel heat exchangers, the water pressure is maintained above the

fuel pressure so that any leakage takes place in the fuel system.

Additional system design and operation requirements ensure that the fuel does not enter the steam system

during periods when the water system is not pressurized.

6. Fuel Heating [2/2]


HIoPE

Compressor Inlet Temperature

Options for power

enhancements

Performance impact

Output Heat

rate

Base configuration Base Base

Evaporative cooling GT inlet air

(85% effective cooler) +5.2 % -

Chill GT inlet air to 45F +10.7 % +1.6 %

GT peak load operation +5.2 % 1.0 %

GT steam injection (5% of GT

airflow) +3.4 % +4.2 %

GT water injection (2.9% of GT

airflow) +5.9 % +4.8 %

HRSG supplementary firing +28 % +9 %

Note: 1. Site conditions = 90F, 30% RH(Relative Humidity)

2. Fuel = NG

3. 3-pressure, reheat steam cycle

7. Steam Injection [1/6]

Effects of Steam Injection on Output and Heat Rate [MS7001EA]


HIoPE


Source: Kehlhofer et al., 2009.

Rela

tive p

ow

er

outp

ut, %

Water or steam/fuel ratio,

114

Rela

tive

effic

iency,

%

112

110

108

106

104

102

100

98

96

94 0.5 1.0 1.5

Hot water injection (150C)

0.0

The steam is extracted from the steam turbine.

Cold water (15C) from the makeup water line.

Hot water (150C) at the outlet of the economizer.

Power output increases with

water/steam injection because

of increased mass flow rate.

Water injection has a greater

effect than steam because

steam turbine output is

decreased with steam injection.

Efficiency of the combined cycle

plant is decreased in both cases;

however, less so by steam than

by water, because steam brings

more internal energy to the

combustor.

Hotter water less reduce the

efficiency than cold one.

[ Effect of water and steam injection on relative combined-cycle power

output and efficiency versus water or steam/fuel ratio (with TIT = constant) ]


HIoPE

Diluent injection is accomplished by admitting water or steam in the cap area or head-end of the

combustion liner to reduce the peak flame temperature.

Actually, this has been used for NOx control to meet environmental regulation.

The mass flow passing through the gas turbine increase with the amount of water or steam injection.

Increased mass flow produces higher power output.

Generally, the amount of water is limited to the amount required to meet the NOx abatement in order to

minimize operating cost and impact on inspection intervals.

When steam is injected for power augmentation, it can be introduced into the compressor discharge casing

of the gas turbine as well as combustor.

Normally, gas turbines are designed to allow up to 5% of the compressor airflow for steam injection.

Steam must contain 50F(28C) superheat and be at pressures comparable to fuel gas pressures (at least

40 bar above the compressor discharge).

The way steam is injected must be done very carefully so as to avoid compressor surge.

Gas turbine output and heat rate increase 3.4% and 4.2% respectively, by the steam injection of 5% of the

compressor airflow.

Water or steam injection for emission control or power augmentation can impact parts lives and

maintenance intervals.



HIoPE

Wet control 3% steam injection

TF = 2020F(1104C)

Load ratio = 1.10

3% steam injection

TF = 1994F(1090C)

Load ratio = 1.08

0% steam injection

TF = 2020F(1104C)

Load ratio = 1.0

EG

T, F

Compressor discharge pressure, psig

Dry control

The wet control maintains

constant TF

[ Steam injection for 25 ppm NOx ]

EGT Control Curve – MS7001EA GER-3620K



HIoPE

The control system on most base load

applications reduces firing temperature as

water or steam is injected. This is known as

dry control curve operation.

The dry control curve operation counters

the effect of higher heat transfer on the gas

side, and results in no net impact on bucket

life.

This is the standard configuration for all gas

turbines, both with and without water or

steam injection.

Dry control

On some installations, however, the control system is

designed to maintain firing temperature constant with

water or steam injection level. This is known as wet

control curve operation.

The wet control curve operation results in additional unit

output, but decreases parts life.

Units controlled in this way are generally in peaking

applications where annual operating hours are low or

where operators have determined that reduced parts

lives are justified by the power advantage.

Wet control

An additional factor associated with water or steam injection relates to the higher aerodynamic loading on

the turbine components that results for the injected water increasing cycle pressure ratio.

This additional loading can increase the downstream deflection rate of the second- and third-stage nozzles,

which would reduce repair interval for those components.



HIoPE

Steam/water injection increases metal temperature of hot-gas-path components in the case

of constant firing temperature operation.

• Water affects gas transport properties:

k – thermal conductivity

Cp – specific heat

– viscosity

• This increases heat transfer coefficient, which increases metal temperature and

decreases bucket life

Example (MS7001EA 1st stage bucket):

• 3% steam injection (25 ppm NOx)

• h = +4% (heat transfer coefficient)

• Tmetal = +15F (8C)

• Life = – 33%

GER-3620K



HIoPE

In some gas turbine applications, it may be desirable to

extract air from the compressor.

In general, up to 5% of the compressor airflow can be

extracted from the compressor discharge casing without

modification to casings or on-base piping.

Air extraction between 6% and 20% may be possible,

depending on the machine and combustor, with some

modification to the casings, piping and controls.

Air extractions above 20% will require extensive

modification to the turbine casing and unit configuration.

As a rule of thumb, every 1% in air extraction results in a

2% loss in power. Ambient temperature %

Effect

on o

utp

ut

C

F 120 0 20 40 60 80 100

0 -7 4 16 27 38 49

100

80

60

40

20

0

-20

-40

-60

% E

ffect

on h

eat

rate

20%

15%

10%

5%

5%

10%

15%

20%

8. Air Extraction

Effects of Air Extraction on Output and Heat Rate


HIoPE

9. Inlet Air Cooling [1/29]

Roughly, 1C temperature decrease corresponds to a combined cycle power increase of about +0.4 to

0.5% and overall efficiency remains more or less same.

Compressor

Fuel gas

pre-heater

Turbine

Fuel oil

Fuel gas

compressor

Fuel oil

treatment

Fuel gas

Water

Evaporative

cooler

or

chiller

Wet compression

(Overspray)

Compressor

washing

Fuel additives

Water

(NOx reduction,

power augmentation)

Cooling air

cooler

Air

Air filter

Drain

Water

Steam

Water

Inlet Fogger

(spray cooler)


HIoPE

상대습도(relative humidity): 공기중에 있는 수증기의 양과 그 때의 온도에서 공기중에 최대로 포함할 수 있는 수증기의 양을 백분율로 표현한 값.

건구온도(dry bulb temperature): 일반 온도계 측정한 온도.

습구온도(wet bulb temperature): 온도계 아래 부분 동그란 구면을 거즈로 감싸고 거즈의 한쪽 끝을 물이 담긴 그릇에 넣어 그릇에서 빨아올린 물이 끊임없이 온도계의 구면에서 증발하도록 한 상태에서 측정한 온도. 물이 증발하면서 기화열을 빼앗아 가기 때문에 건구온도보다 더 낮은 온도를 나타냄.

공기중의 습도가 낮으면 물이 더 많이 증발할 수 있어서 열을 더 많이 빼앗아 가기 때문에 건구와 습구온도 차이가 더 커짐.

일반적으로 건구온도와 습구온도 차이에 의해서 습도를 계산.

습도 계산표가 있어서 건구온도와 습구온도를 알면 그 때의 습도를 찾을 수 있음. 대부분 습도 계산표를 이용하여 습도 확인.

[ 습구온도계 ]

건구온도 vs. 습구온도



HIoPE

For applications where significant power demand and highest electricity prices occur during the hot summer,

a gas turbine air inlet cooling system is a useful option for increasing power output.

Inlet air cooling increases output because the mass flow rate of air passing through the compressor

increases as air temperature decreases.

A decrease in the inlet dry-bulb temperature by 10F(5.6C) will normally result in around 2.7% power

increase of a combined cycle using heavy-duty gas turbines.

The output of the simple-cycle gas turbines is also increased by the same amount.

There are three basic systems currently available for inlet air cooling.

• The first and perhaps the most widely used system is evaporative cooler. Evaporative coolers use the

high efficiency evaporative media for the evaporation of water to decrease the gas turbine inlet air

temperature.

• The second one is a fogger system, also called as spray cooler. This is classified as evaporative cooling.

• The third system employs various ways to chill the inlet air. In this system, the coolant (usually chilled

water) flows through a heat exchanger located in the inlet duct to remove heat from the inlet air.

Evaporative cooling is limited by the wet-bulb temperature.

Chilling, however, can cool the inlet air to temperature that are lower than the wet-bulb temperature, thus

providing additional output, although chilling is much more expensive.

Depending on the combustion and control system, evaporative cooling may reduce NOx emissions; however,

this is very little because of current dry low NOx technology.



HIoPE

High efficiency evaporative media

Evaporative Cooler (Wetted Honeycomb Evaporative Coolers)



HIoPE


Conventional evaporative coolers use a wetted honeycomb type medium to maximize evaporative surface

area and the cooling effectiveness.

The medium for gas turbines is typically 12 inches thick and covers the entire cross-section of the filter

house or the inlet air duct.

The pressure drop caused by evaporative media and droplet eliminator is 1 in.H2O.



HIoPE

The plant output and efficiency decrease due to this pressure drop of about 1.5 to 3 mbar..

The reduction in gas turbine and combined cycle output is 0.35% and 0.3%, respectively.

A controller is provided to prevent operation of the evaporative cooler system below 60F(15.6C).

Icing could form if the system is allowed to operate below this temperature.

The whole system must be deactivated and drained to avoid damage to the water tank and piping if the

ambient temperature is expected to fall below freezing.

Evaporative cooling is a cost-effective method to recover capacity during periods of high temperature and

low or moderate relative humidity.

Evaporative cooling works on the principle of reducing the temperature of an air stream through water

evaporation.

The process of converting the water into a vapor state requires energy.

This energy is drawn from the air stream. The result is cooler, denser air.

There are limitations that must be considered for each site condition. The key design parameters are the wet

and dry bulb temperature and the allowable load limits for the generator and the transformer.

At sites where large reductions in the compressor inlet temperature are possible, the owner must verify that

the added power is within the capabilities of the generator and transformer.




HIoPE

Theoretically, the lowest temperature that can be achieved by adding water to the air is equal to the ambient

wet-bulb temperature.

Practically, however, this level of cooling is difficult to achieve.

The actual temperature drop realized is a function of both the equipment design and atmospheric conditions.

Other factors being constant, the effectiveness of an evaporative cooling system depends on the surface

area of water exposed to the air stream and the residence time.

T means air temperature. Subscripts 1 and 2 refer to inlet and exit of the cooler, respectively. Subscripts DB

and WB refer to ‘dry bulb’ and ‘wet bulb’, respectively.

Temperature drop of the compressor inlet air is proportional to the difference between wet and dry bulb

temperature. If the effectiveness is 85%, the temperature drop is

The effectiveness of evaporative cooler is typically 85% and of foggers somewhat higher at 90 to 95%.

WBDB

DBDB

TT

TTessEffectivenCooler

,2,1

,2,1

Evaporative Cooler - Theory

WBDB TTdropeTemperatur ,2,185.0



HIoPE

[Solution]

The corresponding wet-bulb

temperature is 70F.

T = 0.85(100-70) = 25F (14C)

[Example 9.1]

Ambient temperature is 100F (37.8C)

and relative humidity is 20%. Calculate

the temperature drop through the cooler.

The effectiveness of the evaporation

system is 85%.

Psychrometric Chart [건습계 차트]

Water

Evaporated

Degrees Cooled

Dry Bulb Temperature



HIoPE

Roughly, 1C temperature decrease corresponds to a

combined cycle power increase of about 0.4 to 0.5% and

overall efficiency remains more or less same.

The exact increase in power available from a particular gas

turbine as a result of evaporative cooling depends on the

machine model and site altitude, as well as on the ambient

temperature and humidity.

However, the chart given in the figure can be used to get the

power increase from evaporative cooling.

As would be anticipated, power increase is greatest in hot, dry

weather.

Evaporative cooling is limited to ambient temperatures (15C)

and above (compressor inlet temperature >7.2C) because of

the potential for icing the compressor.

An evaporative cooling does only make sense at locations

with humidity below 70 to 80%.

Evaporative Cooler



HIoPE

Gas turbines have been used foggers, also called as spray coolers, since mid-1980s.

These systems atomize the supply of water into billions of tiny droplets.

The droplets require a certain amount of residence time in the air stream to evaporate. The size of droplet

plays an important role in determining the surface area of water exposed to the airstream and, therefore, to

the speed of evaporation.

The water droplets should be atomized to less than 20 m in foggers.

Fogger

Demineralized water is used

to reduce compressor fouling

or nozzle plugging. However,

it necessitates the use of a

high grade stainless steel for

all wetted parts.



HIoPE

Fogger



HIoPE

Fogger - EPRI Spray Nozzle Array

A typical spray-impingement

fog nozzle

Nozzle fog spray pattern



HIoPE

Two methods are used for water atomization.

The first relies on compressor air in the nozzles to atomize the water.

The second uses a high pressure pump to force the water through a small orifice.

Air-atomized nozzles require less water pressure.

However, they result in low power output due to the air extraction from the gas turbine. An air-atomized

system using compressor discharge air would reduce the power output 1.3% (EPRI, TR-104612).

The typical air-to-water mass ratio is 0.6 (volume ratio is 500).

Some high-pressure pumps use swirlers to break the water into small droplets.

Other force the water on an impingement pin to generate the same effect.

A typical high-pressure pumped fog system has an operating pressure of between 1000 and 3000 psi (6.8

and 20.4 MPa).

In contrast to evaporative coolers, fogging systems have a negligible pressure drop and are ideal for

retrofitting.

Fogger



HIoPE

Careful application of these systems is

essential, because condensation or

carryover of water can be causes of severe

compressor fouling and performance

degradation.

These systems generally are followed by

moisture separators or coalescing pads to

reduce the possibility of moisture carryover.

More spray flow was removed (~70%) by the

Large Droplet Eliminator than was originally

anticipated (~58%) in the EPRI test (TR-

108057).

Normally, water droplets are agglomerated

by turbulent fluctuations and become large

droplets.

Analysis of the drain water gives some

beneficial air scrubbing effects when the

spray cooler is operating.

Fogger - Large Droplet Eliminator

A large droplet eliminator (LDE) is installed in the in the inlet

housing downstream of the spray nozzle array to remove

large water droplets from the air stream. The modules are

manufactured using polypropylene with sine curve shaped

vanes.



HIoPE

dp =

4 m

10 m

20 m

40 m




HIoPE


Droplet distribution

upstream of LDE

Droplet distribution

downstream of LDE

Single nozzle

droplet distribution



HIoPE

The power increase from evaporative cooling is about 3.5% for every 10F (5.6C) of cooling.

Evaporative cooling is limited by the difference between the dry bulb and wet bulb temperatures.

If sufficient water can be introduced into the air such that the air becomes fully saturated, the air

temperature will be reduced to the wet bulb temperature.

The amount of cooling is limited by the potential for icing as the air flow speeds up in the bellmouth and the

static air temperature drops.

The icing limit is engine dependent but typically varies from 40F to 50F.

Fogger - Summary of ERPI Test Results



HIoPE

Evaporative cooler Fogger (spray cooler)

Advantages

• Water quality requirements are less

severe than fogger system.

• Simple and reliable.

• More operating experience.

• Gas turbine inlet pressure drop is lower

than that of evaporative cooler and

provides increased output.

• Higher effectiveness.

• Potential for lower uprate costs and faster

installation time due to reduced duct

modifications compared to evaporative

cooler.

Disadvantages

• Uprates frequently require substantial

duct modifications.

• Higher gas turbine inlet pressure drop

than fogger system degrades output and

efficiency when not in use.

• Lower cooling effectiveness.

• Requires demineralized water.

• Higher parasitic load than evaporative

cooler for high-pressure pumped systems.

• Lower power increase for air-atomized

systems.

• Controls are more complex.

Evaporative Cooling



HIoPE

There are two types of inlet chilling systems, direct chillers and thermal storage.

Liquefied natural gas (LNG) systems use the cooling generated by the vaporization of liquefied gas in the

fuel supply.

Thermal storage systems use off-peak power to store thermal energy in the form of ice.

During peak power periods, the ice is used to perform inlet chilling.

Direct chilling systems use mechanical or absorption chillers.

All these options can be installed in new plants or retrofitted in older plants.

The chilling achieved by using cooling coils depends on the design of the equipment and ambient

conditions.

Unlike evaporative coolers, cooling coils are capable of lowering the temperature below the wet-bulb

temperature.

The capacity of the inlet chilling device, the compressor’s acceptable temperature and humidity limits, and

the effectiveness of the coils limit actual reduction in temperature.

Chiller



HIoPE

Figure illustrates a typical cooling process

from an ambient dry-bulb temperature of

100F(37.8C) and 20% relative humidity.

The initial cooling process follows a line of

constant specific humidity.

As the air approaches saturation,

condensation starts to occur.

Additional cooling results in further

condensation.

Mist eliminator should be installed

downstream of coils to prevent condensed

water from entering the gas turbine.

The air can be cooled below the ambient

wet-bulb temperature.

However, the compressor inlet temperature

should be higher than 45F(7.2C) with a

relative humidity of 95%.

Icing will form at lower temperature,

resulting in possible equipment damage.

Chiller

Water

Evaporated

Degrees Cooled

Dry Bulb Temperature

Evaporative

Cooling

Process

Specific

Humidity



HIoPE

Chiller



HIoPE

Chiller

F-Class gas turbine inlet filter house, showing

installation of chiller coils. Coil manifolds are the

vertical pipes along side the filter house. This filter

house is “passively balanced” with a third “reverse

return manifold”.

Filter houses for chilling applications are much

larger than standard models. A larger “face area”

keeps pressure drop across the coils low. This filter

house also has a symmetrical transition duct that

improves the airflow across the coils.

The fogger systems react to the ambient weather conditions, being limited to the spread between DB and

WB. However, chiller systems break through the WB and dew-point barriers that would limit fogger systems.

The power output enhancement associated with chiller systems can be nearly twice that of the fogger

systems. In addition, temperature of the inlet air can be as constant as possible using chiller systems.



HIoPE

Case Inlet air

cooling

GT output,

kW (each)

Total duct

burner fuel

input,

MMBtu/hr

(LHV)

ST output,

kW (gross)

Auxiliary

power, kW

Net plant

output, kW

Each GT fuel

input,

MMBtu/hr

(LHV)

Net plant

heat rate,

Btu/kWh

(LHV)

1 None 147207 0.000 169942 12173 452183 1440.373 6371

2 Fogger 159850 0.000 174642 12489 481853 1531.400 6356

3 Chiller 173833 0.000 178055 19799 505922 1632.324 6453

4 None 147185 654.636 252408 16575 530203 1440.300 6668

5 Fogger 159831 635.466 254283 16839 557106 1531.337 6638

6 Chiller 173813 619.586 255761 24071 579316 1632.256 6705

7 Chiller 164800 0.000 175366 15918 489048 1565.722 6403

• Model: STAG207FA

• Simulation software: GTPro & GTMaster

• Ambient conditions: 95F (35C), 40% RH

• Effectiveness of fogging system: 95%

• The chiller cools the inlet air temperature down to 50F (10C)

Summary for Performance Simulation Results

Source: T.C. Tillman, PowerGen International 2003



HIoPE

Case Inlet air

cooling

Duct

firing

Net plant

output, kW

Incremental output,

kW

Reference

cost, M$

Incremental

cost (to Case 1),

M$

Unit cost,

$/kW

1 None No 452183 212.4 470

2 Fogger No 481853 29670 6.6% 215.1 2.7 446

3 Chiller No 505922 53739 11.9% 225.3 12.9 445

4 None Yes 530203 78020 17.3% 229.5 17.1 433

5 Fogger Yes 557106 104923 23.2% 231.5 19.0 416

6 Chiller Yes 579316 127133 28.1% 239.9 27.5 414

• All cost figures are provided by Thermoflow’s PEACE costing module.

• This software uses the plant configuration as provided by GTPro.

Summary for Capital Cost Simulation Results

Source: T.C. Tillman, PowerGen International 2003



HIoPE

Comparison of Fogger and Inlet Chiller

Inlet Chiller

Fogger

No Augmentation GT: PG7221FA

Site: Las Vegas (Dry Weather Condition)

Peak Load Operation (Simple Cycle)

TR-1005039 (EPRI)


ration

Ambient

Temp.,C Site

Site

Elevation, m

CC Thermal

Effcy., %

CC Net

Power, MW

GT Net

Power, MW

ST Net

Power, MW

PG7221FA 2-on-1 28.1

(82.5F)

Las Vegas 664 53.4 437.0 285.3 151.7

Miami Sea side 53.2 475.5 312.9 162.6



HIoPE


Inlet Chiller

Fogger

No Augmentation

GT: PG7221FA

Site: Miami (Humid Weather Condition)

Peak Load Operation (Simple Cycle)

TR-1005039 (EPRI)


ration

Ambient

Temp.,C Site

Site

Elevation, m

CC Thermal

Effcy., %

CC Net

Power, MW

GT Net

Power, MW

ST Net

Power, MW

PG7221FA 2-on-1 28.1

(82.5F)

Las Vegas 664 53.4 437.0 285.3 151.7

Miami Sea side 53.2 475.5 312.9 162.6



HIoPE


Combined Cycle Output

Site: Las Vegas (Dry Weather Condition)

Inlet Chiller

Fogger

No Augmentation


ration

Ambient

Temp.,C Site

Site

Elevation, m

CC Thermal

Effcy., %

CC Net

Power, MW

GT Net

Power, MW

ST Net

Power, MW

PG7221FA 2-on-1 28.1

(82.5F)

Las Vegas 664 53.4 437.0 285.3 151.7

Miami Sea side 53.2 475.5 312.9 162.6

TR-1005039 (EPRI)



HIoPE

Comparison of Fogger and Inlet Chiller TR-1005039 (EPRI)

Combined Cycle Output

Site: Miami (Humid Weather Condition)

Inlet Chiller

Fogger

No Augmentation


ration

Ambient

Temp.,C Site

Site

Elevation, m

CC Thermal

Effcy., %

CC Net

Power, MW

GT Net

Power, MW

ST Net

Power, MW

PG7221FA 2-on-1 28.1

(82.5F)

Las Vegas 664 53.4 437.0 285.3 151.7

Miami Sea side 53.2 475.5 312.9 162.6



HIoPE



HIoPE

Compressor Turbine

Wet compression (Overspray )

Water

Evaporative

cooler

or

chiller

Wet compression

(Overspray)

Compressor

washing

Cooling air

cooler

Air

Air filter

Drain

Water

Steam

Water

Inlet Fogger

(spray cooler)

• Wet compression is defined as the excess spray beyond

that which is required to completely saturate the air.

• As an extension of the fogger system, water droplets are

allowed to enter the compressor and evaporation takes

place within the compressor.

• Droplets are evaporated inside the compressor to give

evaporative intercooling effect.

• Wet compression is also called as high fogging, over

spray, over-fogging system, and are usually in operation

together with a fogging or evaporation cooling system.

10. Wet Compression [1/11]


HIoPE

LM6000 Sprint

Wet compression (Overspray)

• The power increase resulting from overspray is about

5% for every 1% overspray (overspray water mass is

expressed as a percentage of inlet air mass).

• The amount of overspray will depend on ambient

conditions.


Source: GER-3620K


HIoPE

When water droplets enter the compressor, the process is

called as wet compression or overspray.

There are two methods of overspray. One is spraying

more water droplets into the inlet air stream than can be

evaporative with given ambient conditions. The other is

installing a separate fogger system (two-stage

evaporative cooler, also called as two-zone system)

downstream of the acoustic silencers to spray water

directly into the compressor inlet.

The airstream carries unevaporated fog droplets into the

compressor section. Then, they are evaporated in the

compressor because higher temperatures in the

compressor increase moisture-holding capacity of air.

When the water droplets evaporate, the compressed air

becomes cooler and denser.

This increases total mass flow of air through the gas

turbine and reduces the compressor work. Therefore, gas

turbine output increases.

As the compressor discharge temperature decreases

when the overspray system is operated, more fuel is

required to achieve a given TIT. In this case, the increase

of power output is greater than the increase of fuel

consumed resulting in a net decrease in overall heat rate.

Source: TR-108057 (EPRI)



HIoPE

There is one possible drawback to wet compression; if water droplets are too large, there is potential for

liquid-impaction erosion of compressor blades.

For this reason, spray droplet diameter should be less than 20 m.

EPRI has recommended that droplets on the order of 10 microns in diameter or smaller are desirable to

evaporate quickly, follow streamlines, minimize wall wetting and minimize erosion.

Evaporation of the water droplets inside the compressor provides continuous cooling of the air thus leading

to a reduction in the compressor work and compressor discharge temperature for a given pressure ratio.

The maximum desirable ratio of water-to-air flow is limited by compressor surge or stall and combustor

efficiency. A coating of liquid on the airfoil surfaces will change the blade path geometry and the related

position of the surge line.

The spray of untreated water will results in fouling on the airfoil surfaces. This, in turn, leads to a change in

airfoil geometry and the position of the surge line.

It has been estimated that an overspray limit of 0.5% of the air flow appears reasonable without reducing the

surge margin. The effect of this amount of injection on a W501AB is estimated to reduce the compressor

power, increase the turbine output, and results in a net output increase of 4.8%, based on 100F dry bulb

and 80F saturated air inlet conditions. (from TR-104612, EPRI)

The wet compression system should be operated at or above 50F because of the formation of ice.



HIoPE

Experimental investigation has shown that overspray into an axial compressor can reduce compressor work,

reduce the compressor discharge temperature, increase output power, and reduce NOx emissions.

The benefits of overspray are greatest when complete evaporation is achieved as soon as possible in the

compressor.

The cooling air temperatures often limit the firing temperature to achieve the desired hot section parts life.

The spraying of water adds mass flow and increase the specific heat, so the actual compression power

reduction is somewhat offset by these effects.

Issues of concern are the potential for erosion for the compressor blade and the reduced stall margin.

The use of very fine droplets that are less than 3 microns is diameter has been shown to reduce erosion to a

negligible rate.

The maximum amount of water sprayed to saturate the air throughout the compressor is nearly 10% of the

air flow. However, it is affected by many factors, such as ambient conditions, compressor surge margin,

compressor blade incidence angle, and choking condition of the 1st stage turbine nozzle.

The estimated heat rate changes for simple cycles are small and increase slightly for combined cycle.

The increase of power with fogging or evaporative cooling depends on ambient conditions. However, the wet

compression power increase is nearly independent of ambient humidity and temperature.




HIoPE

1 5

1

0

-1

-2

-3

Stage

Change in r

oto

r in

cid

ence a

ngle

, deg.

-4 3 9 7 13 11 17 15

2

1% inlet water spray



Effect of water overspray on rotor incidence angle (2 micron droplets)



HIoPE

Change in s

tato

r in

cid

ence a

ngle

, deg.

1 5

2

0

-2

-4

-6

Stage

-8 3 9 7 13 11 17 15

4




10

8

6

Effect of water ovesrpray on stator incidence angle (2 micron droplets)



HIoPE

Power output

Delta power

Spray water, %

Pow

er

outp

ut, M

W

De

lta

pow

er, %

30

29

28

27

26

25

24

23

22

21

20 0 0.5 1.0 1.5 2.0 2.5 3.0

120

118

116

114

112

110

108

106

104

102

100

GT: Alstom/ABB 9D gas turbine




HIoPE

Base load

75% load

50% load

Spray water, %

NO

x,

g/G

J

240

220

200

180

160

140

120

100

80

60 0 0.5 1.0 1.5 2.0 2.5 3.0

GT: Alstom/ABB 9D gas turbine




HIoPE

Comparison of Prediction and Measurement of Inlet Air Cooling

Base Load Power

50 55 60 65 70 75 80

Base + NOx WI 62.2 MW Evaporative cooling

6.3 MW (10.2%)

Overspray

1.8 MW (2.9%)

Max WI

2.9 MW (4.7%)

Pe

ak g

en

era

tor

ratin

g

Peak Load Power

Peak + NOx WI 67.7 MW Evaporative cooling

6.5 MW (10.2%)

Max WI

3.1 MW (4.6%)

Overspray

2.0 MW (3%)

(A) Predicted

[97F(36C),

34% RH]

GT Model: MS7001E

Evaporative cooling

3.2 MW

(B) Predicted

[97F(36C),

34% RH]

Overspray

1.25 MW (C) Predicted

[New and

clean compressor,

Ambient conditions

are not given]

Evaporative cooling

3.5 MW

Overspray

0.75 MW (D) Measured

[Ambient

conditions are not

given]

Power Output, MW




HIoPE

[ Frame 7FA R0 compressor blade leading edge liquid droplet erosion ]

Water droplets will cause leading edge erosion on the first few stages of the compressor.

This erosion, if sufficiently developed, may lead to blade failure.

Additionally, the rounded leading edge surface lowers the compressor efficiency and unit performance.

Utilization of inlet fogging or evaporative cooling may also introduce water carry-over or water ingestion into

the compressor, resulting in water droplet erosion.

Although the design intent of evaporative coolers and inlet foggers should be to fully vaporize all cooling

water prior to its ingestion into the compressor, evidence suggests that, on systems that were not properly

commissioned, the water may not be fully vaporized.



HIoPE

11. Supercharging [1/3]

Ducted fan-motor set (plan view)


HIoPE

External motor drive (Plan View)



HIoPE

TR-1005039 (EPRI)

Supercharging rounds out the technologies based on inlet air conditioning such that the inlet temperature,

humidity and pressure may be controlled.

An electric motor-driven fan (or blower) strategically located in the GT inlet air flow path increases the inlet

pressure at the axial compressor scroll.

A temperature rise accompanies the pressure rise, so after-cooling is desirable to capture the maximum

benefit of the pressure rise.

Evaporative cooling is also beneficial in reducing the fan power, but this is considered a separate technology.

The motor, fan and after-cooler ensemble constitute the supercharger.

During the late 1960s, Westinghouse offered their W301 gas turbine with supercharging.

The W301 was offered in a fully fired combined cycle configuration, so the supercharger blower was also

used for operating the furnace and steam turbine when the CT was not operating.

The Westinghouse performance data was modeled with SCAAD (Strategic Capacity Analysis and Design)

software developed for EPRI.



HIoPE

Some gas turbine models can be operated at a higher firing temperature than their base rating.

This is called peak firing. During the peak firing operation, both simple-cycle and combined-cycle output will

increase.

Peak firing is available to get 3~10% higher output than the output at base load.

Normally, thermal efficiency of the plant is increased during peak firing of gas turbine because of higher

firing temperatures.

This mode of operation results in a shorter inspection interval and increased maintenance.

Despite this penalty, operating at elevated peak firing temperatures for short periods is cost-effective way

for power gain without any additional peripheral equipment.

“Peaking” at 110% rating will increase maintenance costs by a factor of 3 relative to base-load operation at

rated capacity, for any given period.

For an MS7001EA turbine, each hour of operation at peak load firing temperature (+100F/56C) is the

same, from a bucket parts life standpoint, as six hours of operation at base load.

12. Peak Firing [1/2]


HIoPE

Maintenance Factor

E-class

F-class E-class

peak rating

life factor 6x

0 50 100 150

Firing temperature, F

Ma

inte

na

nce

fa

cto

r

100

10

6

0

12. Peak Firing [2/2]


HIoPE

13. Part Load Operation

Load, % 30 40 50 60 70 80 90 100

65

60

75

70

85

80

95

90

100


HIoPE

It can be used to increase steam turbine capacity by

as much as 100%.

This will increase plant capacity by about 33%.

Cogeneration of power and process steam is usually

the incentive for HRSG supplementary firing.

14. Supplementary Firing in HRSG

There is a small performance penalty when operating unfired compared to operating a unit designed without

supplementary firing, and the magnitude of this performance penalty is directly proportional to the amount of

supplementary firing built into the combined-cycle plant.

The performance penalty is due to two factors: unfired operation results in lower steam flows and pressures

and, thus, lower steam turbine efficiency; also, the pumps, auxiliary equipment and generator are sized for

higher loads.

Operating unfired results in comparatively higher parasitic loads compared to a unit designed solely for

unfired operation.

Normally, thermal

efficiency of the plant is

decreased during HRSG

supplementary firing.


HIoPE

The end pressure of steam expansion in the turbine is determined by the steam saturation

temperature depending on the cooling water temperature and heat transfer conditions on the

condenser tubes.

15. Cooling Water Temperature [1/2]

T

s

2

p4’

1

1

2

3

a b

4

p4

a

4


HIoPE

15. Cooling Water Temperature [2/2]

Effect of Condenser Pressure on Steam Turbine Output

110

105

100

95

90

85

80

20 40 60 80 100 120 140 160 180

Condenser pressure, mbar

Ste

am

tu

rbin

e o

utp

ut,

MW

Single pressure

Double pressure

Triple pressure reheat



HIoPE

16. Compressor Washing [1/11]

Performance Degradation

All guaranteed performance numbers are valid for a new and clean engine. New and clean conditions are

specified on a project-by-project basis, but typically are defined as performance during first 20 to 100 hours

of fired operation.

All engines have performance degradation, those are easily recoverable, or non-recoverable.

A typical recoverable loss is usually associated with compressor fouling which can be partially removed by

water washing or by mechanical cleaning after opening the unit. The removal of fouling deposits from the air

path components can restore partially the aerodynamic performance of the machine.

Non-recoverable loss is due to increased turbine and compressor tip clearances and changes in surface

finish and airfoil contour, mainly caused by corrosion and erosion, and sometimes by FOD.

These non-recoverable losses are main cause of the reduction in component efficiencies, and it may be

recovered only by replacement or repair of the affected parts at recommended inspection intervals.

OEMs typically recommend periodic maintenance inspections and overhauls which are scheduled on the

basis of hours of fired operation, type of fuel used, and the number of starting the machine.

On the basis of natural gas firing, it has generally being recommended that the inspection intervals are 8,000

hours of operation for combustors, 24,000 hours of operation for hot gas parts, and 48,000 hours of

operation for a major overhaul.

Typically, performance degradation during the first 24,000 hours of operation (the normally recommended

interval for a hot gas path inspection) is 2% to 6% from the guaranteed performance. This assumes

degraded parts are not replaced. If replaced, the expected performance degradation is 1% to 1.5%.


HIoPE

Compressor Fouling

The most significant performance degradation occurred in gas turbines is

caused by compressor fouling.

Compressor fouling takes place when particulate matter in the inlet air is

deposited and adhered to the bell mouth or compressor blades. The fouling

physically changes the shape of the compressor blades, reducing

aerodynamic performance.

Fouling may also block cooling air flow ports and passages, resulting in

thermal damage to the components.

A large quantities of air pass through the compressor, combustor, and turbine

section.

Although inlet air is filtered, some contaminants pass through the compressor

and turbine. Typical contaminants are submicron dirt particles entering the

compressor, oil vapors, smoke, and sea salt.

There is a temperature and humidity region showing higher compressor

fouling rate. Too little humidity gives too little water for sticking the particles

whilst too high levels might result in the effect of on-line wash.

Compressor fouling can be severer with inlet foggers.

Fouled gas turbine air inlet bell

mouth and blading, engine

operated in an industrial

environment


Front stages are usually fouled worst. This is because the particles adhere to the compressor blade mainly

by bell mouth condensation, or by oil leak from #1 bearing.

Rear stage fouling gives a smaller impact on performance; but due to higher temperatures, deposits can

become baked and difficult to remove. This baking effect is more severe on the machines with high

pressure ratio compressor.


HIoPE

The compressor efficiency is governed by the smoothness of

stator and rotor blade surfaces and their shapes. These

surfaces can be roughened by erosion, but more frequently by

fouling.

The shapes of compressor blades are also changed from its

design shape by fouling.

An axial compressor is a machine where the aerodynamic

efficiency of each stage depends on that of the previous stages.

Thus, when fouling occurs in the inlet guide vanes and the first

few stages, there may be a dramatic decrease in compressor

efficiency.

The effects of compressor fouling are decrease of airflow,

pressure ratio, and compressor efficiency, resulting in a

decrease in power output and thermal efficiency.

Airflow is reduced by 5% due to the compressor fouling, which

will reduce output by 13% and increase heat rate by 5.5%.

The output of a gas turbine can be reduced as much as 20% by

fouling .

Fouling Effects in terms of Performance

Deterioration of GT performance due to compressor blade fouling

Hea

t ra

te

incre

ase

, %

Ou

tpu

t

de

cre

ase, %

Pressure ratio decrease, %

-1 -2 -3 -4 -5 -6 -7 -8

-2

-4

-6

-8

-10

-12

-14

8

6

4

2

0

Fouling

Fouling

5% loss of

airflow

It has been estimated that the fouling is responsible for 70 to 85% of all gas turbine performance losses

accumulated during operation.

In extreme cases, fouling can also result in surge problems, as it tends to reduce the surge margin.



HIoPE

Severe fouling, dried carbonaceous type

of deposits

Severe carbonaceous oily type of

deposits on 1st stage vanes. Fouling

mainly caused by oil leaks in the bearing

system

Pre

ssu

re r

atio

Fouled

Mass flow

Design

point

Heavy stage

loading

Light stage

loading

Surge line

Compressor Fouling



HIoPE

Disposition for Compressor Fouling

Fouling is best controlled by a combination of two methods. The first one is to employ a high quality air

filtration system. Recently, fouling problems have reduced dramatically by the employment of HEPA (High

Efficiency Particulate Air) filters, which can remove 99.95-99.97% of 0.3 µm particles.

However, as fouling will inevitably occur, compressor washing should be used to control its impact.

In general, water washing is employed to remove the fouling. Under extreme fouling conditions, however,

hand washing of the IGVs may have to be conducted. During overhauls, hand cleaning of the full axial

compressor is most effective.

Previously, abrasive cleaning using crushed nutshells had been employed for compressor cleaning.

However, abrasive cleaning is not recommended anymore because it can damage compressor blade

coatings and compressor blade surface finish. Moreover, abrasive cleaning compounds are also a potential

cause of plugged cooling passages.


Before water

washing

After water

washing

Silica deposits resulting from untreated water

used for evaporative cooling of the compressor

inlet air are difficult to remove.

The various impurities can be washed by

condensed water at bell mouth. The droplets

and wet blade and end-walls become acidic

due to the scrubbed pollutants, such as CO2,

SO2, NOx, HCl, and Cl2. Therefore, corrosion

risk is elevated and protective coatings are

required.


HIoPE

On/off-line washing system diagram (GE)

Water washing piping system (GE)

On-line water washing (GE 7EA)



HIoPE

Off-line wash (Siemens) On-line wash (Siemens)


Off-line wash nozzles

On-line wash nozzles


HIoPE

There are two water wash systems performed on

gas turbines: on-line and off-line (crank wash).

On-line washing with the machine running at full

speed and loaded.

On-line is not as effective as off-line but on-line

washing is performed with the unit in full

operation, and outages or shutdown periods are

not required.

On-line washing is now very popular as a means

to control fouling by avoiding the problem from

developing.

The primary purpose of on-line washing is to

extend the operating period between off-line

washes by minimizing the compressor fouling,

thereby to provide peak availability.

On-line Washing


Pow

er

outp

ut, %

of

new

cle

an

machin

e

Operating hours 1000

100

97

94

91

0 8 16 24 32 88

On-line cleaning is straightforward but because the temperature increases through the compressor, the

cleaning solution evaporates and cleaning is limited to the first compressor rows. Fortunately, fouling of the

first stage guide vane is the primary cause of reduced air mass flow through the compressor.

On-line washing has become increasingly important with base load combined cycle plants and CHP plants.

On-line cleaning has a risk of getting dirt particles into the secondary air system.


HIoPE

Off-line washing is better suited to large, modern gas turbines because it is more effective. Normally, off-line

washing removes particulate matter and sticky coating on blades. On-line washing removes particulate

matter.

However, it requires shutting down and cooling the engine.

Off-line washing with the machine on crank.

Off-line involves injecting the cleaning solution into the compressor while it is turning at cranking speed (=

850 rpm for 7FA+e).

The downtime for a crank wash depends mainly on the time it takes for cooling the engine. In order to avoid

thermal shock, wheel spacer temperatures must be below 200F.

Larger heavy-duty engines can take 8 to 10 hours to cool whereas on light aeroderivative engines only 1.5

to 3 hours may be needed due to its low metal mass.

Optimal compressor washing can normally be achieved by adopting a combined program of regular and

routine on-line washing (every few days or weekly), plus periodic offline washing during planned outages.

Typical on-line cleaning cycles are in the order of 10 to 20 minutes.

Off-line Washing



HIoPE

Off-line Washing On-line Washing

• Objective: To clean a dirty compressor

• Objective: To keep a clean compressor cleaner for

longer. To extend operating period between off-line

cleaning, thus enhancing availability.

• Virtually full power recovery (approaches new and

clean values)

• About 1% power can be recovered per wash, with a

frequent on-line cleaning program

• Reaches all compressor stages • Primarily cleans the IGVs (no effect after water

evaporates)

• Involves shutdown and cool-down period (8 to 10

hours) • No shutdown

• Lost revenue during shutdown • No lost revenue

• Optimum time for cleaning may not be convenient,

especially with base load plants • Optimum cleaning frequency is site specific

• Effluent water for disposal • No effluent water for disposal

• Maintains safe margin to surge line

• Reduce risk of blade corrosion



HIoPE

Benefits of Compressor Washing

• Restoration of lost performance

• Maintaining higher efficiency

• Compliance with environmental regulations

• Increased availability and reliability

• Longer life expectancy

• Reduced O&M costs

• Reduction of gas turbine fuel consumption



HIoPE

For a MS7001E unit, a rub of 20 mils on

the labyrinth seal teeth equates to at

least 1.0% loss in unit performance.

To increase unit performance and to

reduce the rate of performance

degradation due to wear on labyrinth seal

teeth, a wire brush seal has been

employed.

Since the wire brush seal is flexible and

will bend (not wear) on contact with the

compressor aft shaft, a closer clearance

can be allowed for the initial installation.

Since the wire brush seal will “bounce back” to its original configuration after a “rub,” there will be

substantially less performance degradation over time than for the original labyrinth seal.

Performance improvement is typically about 1% output and 0.5% heat rate.

High Pressure Packing Brush Seal GER-3571H

17. Seal Rubbing [1/3]


HIoPE

Stage 2 Nozzle Inner Diaphragm Brush Seal GER-3571H

There is a large gap between the

stage 2 nozzle inner diaphragm and

the stage 1-2 wheel spacer to prevent

any contact due to rotor vibration,

thermal transients or nozzle deflection.

A brush seal has also been employed

to improve the inner stage packing

seal.

This seal is available for all single

shaft designs and provides a

performance improvement of

approximately 1% output and 0.5%

heat rate.



HIoPE

Stage 2 and 3 Shroud Honeycomb Seal

To avoid bucket tip rub, the clearance between

the bucket tip and stationary shroud blocks

have always been about 100 mils.

This large clearance allows a significant

amount of hot gas to flow over the bucket tip,

resulting in significant performance loss.

The honeycomb material is softer than the

shroud and bucket material, which makes it

sacrificial in nature for this application.

The bucket tip shroud labyrinth seals are

designed to cut a groove into the honeycomb

material. The clearance between the bucket tip

and the honeycomb shroud seals provide a

performance improvement up to 0.6% in both

output and heat rate.

GER-3571H



HIoPE

질의 및 응답

작성자: 이 병 은 (공학박사) 작성일: 2015.02.11 (Ver.5) 연락처: [email protected]

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

6. gas turbine performance - engsoft power lab - … · 2015-05-18 · 950.24%. steam turbine -10...

Documents