Steam Turbine 5. Performance 1 / 93
HIoPE
5. Steam Turbine Performance
Nuclear Reheat
HP = 88-90%
LP = 90-91%
LP = 87%
LP = 85%
HP = 82%
Nuclear Non-Reheat
Fossil
Reheat
IP = 90-94%
Saturation
Line
h
s
Steam Turbine 5. Performance 2 / 93
HIoPE
Thermal Kit 65 4
Steam Turbine Section Efficiencies 15 2
Factors Affecting Cycle Performance 21 3
Heat Balance 2 1
Steam Turbine 5. Performance 3 / 93
HIoPE
Major Parameters Affecting Steam Turbine Performance
Steam Turbine Performance
Thermodynamic Efficiency
Working fluids
• Throttle pressure and temperature (enthalpy)
• Throttle flow (specific heat)
Steam Cycles
• Reheating (non-, single-, double)
• Regenerating
Exhaust pressure
• The amount of enthalpy drop
Others
• Air preheating
• Desuperheating
• Part load operation mode
Mechanical (Fluid Dynamic) Efficiency
Section efficiencies (in terms of energy conversion)
• Throttling losses occurred in valves
• Stage losses (profile, secondary flow, leakage loss)
Advanced airfoil shapes for nozzle and bucket
Advanced vortex blades
• Exhaust loss, and interface loss
Mechanical and electrical losses
• Bearing loss
• Generator loss
Power for auxiliary system in plant
• Fan power
• Power for a lube oil pump
Others
• LSB
• Makeup flow
• Pressure drop in boiler and extraction lines
Heat Balance
Steam Turbine 5. Performance 4 / 93
HIoPE
Turbine (Generator) Output [1/2]
Required generator output = plant net output + plant auxiliary loads.
Steam turbine output = generator output + generator electrical losses + turbine generator mechanical losses.
Historically, turbines have been designed to have 5% margins above required rated steam flows and
pressure to provide for manufacturing tolerances and variations in flow coefficients.
Therefore, the steam flow is 5% greater than that required for rated output with rated steam pressure (normal
pressure).
Under VWO-NP (VWO, normal pressure) condition, the turbine generator output is approximately 104% of
rated.
The pressure margin is included to operate safely and continuously at 105% of rated pressure (overpressure)
with VWO.
Under VWO-OP (VWO, overpressure) condition, the turbine generator output is approximately 109% of rated
and the main steam flow is 110% to 111% of rated.
Steam Turbine 5. Performance 5 / 93
HIoPE
Recent trends indicate that some manufacturers are not including all of the 5% steam flow margin. The
designer may want to include only a part of the steam flow margin with consideration of the full over-pressure
operation margins.
The designers should specify that the turbine be capable of operation at VWO-OP because operators
typically attempt to operate at those conditions.
The designer needs to design the steam generator and balance of plant equipment to support the VWO-OP
conditions if he has included them in the establishment of the steam turbine generator rating.
Designing for the VWO-OP condition is recommended even if not included in the rating definition, because
significant output increase can be achieved at little cost by being capable of operating at VWO-OP.
Turbine (Generator) Output [2/2]
Steam Turbine 5. Performance 6 / 93
HIoPE
Typical Power Plant Steam Flow
Gen
Stop V/V
Control V/V
HP IP LP
Condenser
Reheater
Reheat
Stop and
Intercept
V/V
Main Steam
Hot Reheat
Cold
Reheat
Crossover
Ventilation
V/V
Exciter
Steam
Generator
Front
Standard
Steam Turbine 5. Performance 7 / 93
HIoPE
Heat Balance
Fossil, 700 MW, 3500 psig/1000F/1000F
Steam Turbine 5. Performance 8 / 93
HIoPE
Fossil, 500 MW, 2400 psig/1000F/1000F
Heat Balance
Steam Turbine 5. Performance 9 / 93
HIoPE
Nuclear, 1000 MW
Heat Balance
Steam Turbine 5. Performance 10 / 93
HIoPE
Heat balances are provided by the turbine manufacturer.
Cycle performance is represented in the heat balance diagram which shows the steam/condensate flows,
pressures, temperatures, and enthalpies. These parameters are used to determine equipment design
conditions.
A complete heat balance provides enough information to balance the energy distribution.
Heat balance diagram also indicates ELEP and UEEP, generator losses and net generator output.
On the basis of this information, the engineer can perform an energy balance for the major equipment
associated with the turbine, feedwater, condensate, and heat rejection systems.
A number of heat balance computer programs are commercially available. However, it also can be performed
by hand calculation.
Hand calculation, which is time consuming caused by iteration, is instructive because it permits the engineer
to gain an understanding of the interrelationships of the various equipment.
Heat Balance
Steam Turbine 5. Performance 11 / 93
HIoPE
Establishing a Heat Balance
Reheater
Generator
Condenser
2.0 in.Hga
SSR
HTR1
5F
5 F
HTR2
5F
10 F
HTR3
2F
10 F
HTR4
2F
10 F
HTR5
(DA)
0 F
HTR6
0F
10 F
HTR7
-3F
10 F
10%P 3%P
3%P
3%P 6%P 6%P 6%P 6%P
6%P
BFP
BFP = 87%
Main Steam BFPT
HP IP LP
Make U
p
3%P
SPE
99
99
BFPT = 82.6%
51
2 1
4 5
3
6
4
20
21
86
52 52
86
20
21
54 51
2
6
5
54
Steam Turbine 5. Performance 12 / 93
HIoPE
Typical Assumptions Made in Establishing a Heat Balance
1. The boiler feed pump suction conditions will be the temperature and pressure of the deaerator. Boiler feed
pump discharge pressure is 125% of the turbine throttle pressure.
2. The boiler feed pump efficiencies will vary with load as follows:
Condition BFP Efficiency
VWO-OP, VWO-NP, and rated load 84%
75% of rated 83%
50% of rated 67%
25% of rated 40%
3. For a turbine cycle with a motor-driven boiler feed pump, the variable speed coupling efficiency will vary with
load as follows:
Condition Coupling Efficiency
VWO-OP 85%
VWO-NP, and rated load 82%
75% of rated 76%
50% of rated 73%
25% of rated 68%
The combined motor and transmission efficiency will vary with load as follows:
Condition Motor and Transmission Efficiency
VWO-OP, VWO-NP, and rated load 94%
75% of rated 93%
50% of rated 92%
25% of rated 89%
Steam Turbine 5. Performance 13 / 93
HIoPE
4. For a turbine with a turbine-driven boiler feed pump exhausting to the main condenser, the BFPT(Boiler Feed
Pump Turbine) will operate at an exhaust pressure 0.5 in.Hga greater than the exhaust pressure of the main
turbine. BFPT expansion efficiencies will vary with load as follows:
Condition BFPT Efficiency
VWO-OP, VWO-NP, and rated load 80%
75% of rated 78%
50% of rated 77%
25% of rated 77%
The pressure drop in the extraction line to the BFPT is 3% of the inlet pressure.
At low loads, the BFPT will require steam from a source of higher pressure than is available in the
crossover line. Below approximately 0.35 TFR(Throttle Flow Ratio), the BFPT takes steam as required
from the main steam line.
5. There is a pressure drop between the turbine stage and the extraction flange. This value is typically 3% of
stage pressure. A pressure drop also occurs from the turbine extraction flange to the heater. This value is
usually 3% or 5% of the extraction flange pressure. For extractions at a turbine exhaust or at the crossover
pipe, no pressure drop due to an extraction flange exists, only an extraction line pressure drop.
6. There is a pressure drop from the HP turbine exhaust to the intercept valves of the IP turbine because of hot
and cold reheat piping and the reheater. This value is normally taken to be 10% of the HP turbine exhaust
pressure.
7. The condensate leaving the condenser will be at saturation temperature corresponding to the turbine
exhaust pressure.
8. The condensate will be considered to be saturated liquid at the heater inlet and outlet temperatures.
9. Calculations will consider the feedwater downstream of the boiler feed pump as compressed liquid.
Typical Assumptions Made in Establishing a Heat Balance
Steam Turbine 5. Performance 14 / 93
HIoPE Approximate Flow Distribution for Typical Regenerative/Reheat Cycles
1. The reheater flow is approximately 90% of the throttle flow.
2. The BFPT extraction flow from an IP to LP turbine crossover is 4% to 6% of the throttle flow.
3. The turbine exhaust flow is 65% to 75% of the throttle flow, with the remaining flow being taken for heating
the feedwater and driving the BFPT.
4. Rules of thumb, when the temperature rise across a heater is known:
a. For the low pressure (LP) heaters and deaerator, the extraction flow is approximately 1% of throttle flow
for each 14F temperature rise.
b. For the high pressure (HP) heaters, the extraction flow is approximately 1% of throttle flow for each 10F
temperature rise.
5. If a heat balance is available for other than the desired load, ratio the extractions by the ratio of the throttle
flows for a first guess. However, these may differ up to 30% from the final calculations.
Steam Turbine 5. Performance 15 / 93
HIoPE
Thermal Kit 4
Steam Turbine Section Efficiencies 2
Factors Affecting Cycle Performance 3
Heat Balance 1
Steam Turbine 5. Performance 16 / 93
HIoPE
The Rankine cycle, the basic cycle used for electric power generation, mainly consists of four components:
steam generator, turbine, condenser, and pump.
The performance of a power plant is influenced not only by the steam turbines, but also by the choice of
steam turbine cycle.
Thermal efficiency of the cycle can be increased either by reducing the condenser pressure or by
increasing the turbine inlet pressure and temperature.
The principal cycle considerations are those of regenerative feedwater heating by turbine extraction steam
and of reheating.
The performance of the steam turbine is governed by the losses occurred in it.
Steam Turbine Performance
3
4
2 1
46
45
44
43
42
41
43
42
41
40
39
% %
1.15
1.25
120C
130C
300 bar/600C
250 bar/540C
Single
reheat
Double
reheat
1.9 in.Hga
0.88 in.Hga
Excess Air Discharge
Flue Gas
Temperature
Main Steam
Condition
Reheat Back
Pressure
Pla
nt N
et
Eff
icie
ncy B
ased o
n L
HV
Pla
nt N
et
Eff
icie
ncy B
ased o
n H
HV
USC
Steam Turbine 5. Performance 17 / 93
HIoPE
발전시스템에서의 열역학 제2법칙
TH
TL
QL
W
QH
에너지 변환
외부에 어떠한 영향도 남기지 않고 한 사이클 동안에 계가 열원으로부터 받은 열을 모두 일로 바꾸는 것은 불가능하다. (열효율이 100%인 열기관을 만들 수 없으며, 작동유체는 반드시 공급된 에너지의 일부를 주위와 열교환시켜야 한다).
모든 열기관은 고온의 열원에서 열을 흡수하여 역학적 일을 하고 저온의 열원으로 열을 방출한다.
발전시스템의 경우 증기발생기에서 고온의 열을 공급받고 복수기에서 열을 방출한다.
그러나 열기관 주변에 있는 자연상태의 온도보다 낮은 온도로 열을 버릴 수는 없다.
H
L
H
LH
H
thQ
Q
Q
Q
W
1
T
s
1
2
3
4 qout
wturbine qin
qin
qin
wpump
Steam Turbine 5. Performance 18 / 93
HIoPE
Stage Efficiency
Ava
ilab
le E
ne
rgy
Usefu
l E
ne
rgy
(Sta
ge W
ork
)
1
2s a
2
p1
p2
Stage Loss
h
s
b
Nozzle profile loss
Bucket profile loss
Secondary flow loss +
leakage loss
s
sthh
hh
21
21
Stage efficiency is defined as the ratio of mechanical work produced by the
stage to the thermal energy available.
Steam Turbine 5. Performance 19 / 93
HIoPE
545 psia
534 psia
180 psia
175 psia
92.7%
89.1%
91.2%
1378.2 h
1367.1 h
1364.6 h
1361.0 h
Pressure drop in the IP
exhaust hood, the cross-over
pipe, or the LP turbine inlet
Pressure drop in
intercept valve (2%)
h
s
IP Turbine Section Efficiencies
Nuclear Reheat
HP = 88-90%
LP = 90-91%
Nuclear Non-Reheat
IP = 90-94%
Saturation
Line
h
s
Fossil Reheat
Steam Turbine Section Efficiencies
Steam Turbine 5. Performance 20 / 93
HIoPE
In general, HP turbine efficiency includes the losses occurred in the stop valves, control valves, and HP
exhaust hood.
Blade profile loss increases with blade length. However, the amount of secondary loss is not changed,
although blade length increases. This is same for the leakage loss. Therefore, stage efficiency increases with
blade length(height).
For this reason, the efficiency of HP turbine is lower than that of IP turbine.
LP turbine has longer blades, but its efficiency is lower than IP turbine. This is because the last several
stages of LP turbine are operated in the wet steam region. Typically, every 1% of wetness gives a 1% loss in
isentropic efficiency.
The efficiency of nuclear HP turbine is lower than that of fossil HP turbine because of moisture loss. This fact
is same for LP turbines.
Nuclear LP turbine uses moisture removal buckets to reduce the moisture loss as well as water droplet
erosion.
The turbine section efficiencies may have different values because the losses occurred at the interface are
included or not.
Steam Turbine Section Efficiencies
Steam Turbine 5. Performance 21 / 93
HIoPE
Thermal Kit 4
Steam Turbine Section Efficiencies 2
Factors Affecting Cycle Performance 3
Heat Balance 1
Steam Turbine 5. Performance 22 / 93
HIoPE
Effects of Cycle Parameters
Base
A. Condenser Pressure, in.Hga 2.0 2.5 3.0 3.5 4.0
Relative net output (kW) 2,914 --- -4,073 -9,078 -14,091
Relative net heat rate (Btu/kWh) -31.3 --- 44.1 99.0 154.8
B. Pressure Drop in Boiler (including piping
to the turbine), % 15 17 19 21
Relative net output (kW) 346 --- -345 -732
Relative net heat rate (Btu/kWh) -4.0 --- 3.9 8.3
C. Pressure Drop in Reheater, % 5 7 9 11
Relative net output (kW) 1,403 --- -1,445 -2,935
Relative net heat rate (Btu/kWh) -17.8 --- 18.3 37.1
D. Pressure Drop in Crossover Pipe, % 0 3 5 7
Relative net output (kW) --- -1,824 -3,037 -4,318
Relative net heat rate (Btu/kWh) --- 19.7 33.2 46.8
E. Boiler Steam Temperature, F 950 1,000 1,050 1,100
Relative net output (kW) -35,474 --- 34,585 68,738
Relative net heat rate (Btu/kWh) 118.9 --- -106.0 -202.9
F. Use of Air Preheater Yes No
Relative net output (kW) --- 8,291
Relative net heat rate (Btu/kWh) --- -88.3
Data is produced using the fossil power plant, 726 MW, 3500 psig/1000F/1000F
Steam Turbine 5. Performance 23 / 93
HIoPE
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.
T
s
1
2
3
4
Equivalent
Carnot Cycle
Equivalent Cycle
Hot Temperature T
s
1
2
3
4
Equivalent Cycle
Hot Temperature
[Ideal Rankine Cycle for a Typical Nuclear Power] [Ideal Rankine Cycle for a Typical Fossil Power]
1. Superheating [1/3]
Steam Turbine 5. Performance 24 / 93
HIoPE
T
s
heat
added
heat
lost 1
2
3
3
4
4
The overall efficiency is increased by
superheating the steam. This is because the
mean temperature where heat is added
increases, while the condenser temperature
remains constant.
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.
The turbine work out is also increased by
superheating the steam without increasing the
boiler pressure.
When the superheating the steam is employed in
the cycle, the important thing is that the quality
of the steam at the turbine exhaust is higher than
90%.
1. Superheating [2/3]
Steam Turbine 5. Performance 25 / 93
HIoPE
T
s
Early 20th century
1940s
1960
Ultra Supercritical
Supercritical
Evolution of Rankine Cycle
1. Superheating [3/3]
Steam Turbine 5. Performance 26 / 93
HIoPE
2. Condenser Pressure [1/3]
2.5
3.5
4.5
100%
90%
80%
70%
60%
Throttle Flow Rate
Condenser
Pressure
(in.Hga)
Turbine output, kW
Tu
rbin
e h
ea
t ra
te, B
tu/k
Wh
1001
HR
HRKW
KW = change of generator kW
output, %.
HR = change of heat rate, %
T
s
2 p4
1 1
2
3
a b
4
p4
a
4
Except for choked turbine exhaust conditions, the lower the turbine exhaust pressure, the higher the
cycle efficiency.
Steam Turbine 5. Performance 27 / 93
HIoPE
Condenser Pressure, in.Hga
2 F-40.0"LSB
2 F-33.5"LSB
2 F-30.0"LSB
D-11 steam turbine for GE 207FA,
1800 psia / 1050F / 1050F
Ste
am
Tu
rbin
e O
utp
ut,
MW
0.5 1.5 1.0 2.5 2.0 3.5 3.0 0.0 175
195
190
185
180
4.0
2. Condenser Pressure [2/3]
Steam Turbine 5. Performance 28 / 93
HIoPE
[Exercise 5.1]
복수기 압력을 2.5 in.Hga로 운전했을 때 발전기 출력이 700 MW, heat rate가 7826 Btu/kWh이다. 주증기
조건을 그대로 유지시킨 상태에서 복수기 압력을 4.5 in.Hga로 운전하였더니 열율이 7980 Btu/kWh가 되었다. 이때 나타나는 출력변화율을 계산하시오.
[Solution 1]
증기터빈 출력변화율은 다음 식으로 구할 수 있다.
HR = {(7980–7826)/7826}100 = 1.97%
KW = –1.97/(1+1.97/100)= –1.93%
1001
HR
HRKW
[Solution 2]
HR = Q/W Q = 700,000 kW 7826 Btu/kWh = 5,478.2106 Btu/hr
W′ = Q/HR′ = 686,491 kW
W = {(700,000–686,491)/700,000}100 = –1.93%
2. Condenser Pressure [3/3]
Steam Turbine 5. Performance 29 / 93
HIoPE
The cycle maximum temperature is constant.
Increased boiler pressure has a higher mean
temperature of heat addition.
However, the temperature of heat rejection is
unchanged.
Usually, the amount of the cycle work is not
changed although boiler pressure is increased.
This is because the amount of the increased work
(top side) and the amount of the decreased work
(right hand side) caused by pressure increase is
almost same.
However, the amount of heat rejected is
decreased. Thus, the cycle efficiency increases
with boiler pressure.
The only one drawback is that the quality of the
exhaust flow become worse.
T
s
2
1
2
3
a b
4
b
4
3
decrease in qin
increase in qin
decrease in qout
c
3. Increasing Steam Pressure [1/5]
Steam Turbine 5. Performance 30 / 93
HIoPE
3. Increasing Steam Pressure [2/5]
Throttle Pressure
2400 psia
3500 psia 6 Flows, 30” LSB
Throttle steam 2400 or 3500 psia, 1000F/1000F
Nominal output 700 MW at 1.5 in.Hga
Turbine output, MW
Tu
rbin
e h
ea
t ra
te, B
tu/k
Wh
200 400 600 800
9000
8000
7000
Main steam conditions strongly influence
the turbine performance.
At a given maximum cycle temperature,
the turbine performance can be improved
by increasing the main steam pressure.
The higher the steam pressure, the better
the turbine performance.
However, there is a temperature limit
beyond that turbine and boiler will
become less reliable.
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.
Steam Turbine 5. Performance 31 / 93
HIoPE
T
s
1
2
3
4
USC
Subcritical
2
3
Tmax, subcritical
Tmax, USC
3
2
Critical
Point
3. Increasing Steam Pressure [3/5]
Steam Turbine 5. Performance 32 / 93
HIoPE
Normally, the manufacturing companies indicate the guaranteed and expected performance of steam
turbines.
In the guaranteed performance the steam turbine is specified to produce a certain number of kilowatts while
operating at rated steam conditions, 3.5 in.Hga exhaust pressure, 0% cycle make-up, and other cycle
feedwater heating conditions.
To assure that the steam turbine will pass the guaranteed throttle flow, the turbine is frequently designed for
a steam flow rate larger than the guaranteed value.
This new value is sometimes called the expected steam flow and is usually around 105% of the guaranteed
value. For this reason, the actual output of the turbine is expected to be larger than the guaranteed value.
The turbine is guaranteed to be safe for continuous operation with valve wide open.
Furthermore, the turbine is also capable of operating continuously with VWO and at the same time at 105%
of rated initial pressure.
Under these conditions the expected steam flow would become maximum (approximately 110% of the
guaranteed value) and thus the expected turbine output.
3. Increasing Steam Pressure [4/5]
Steam Turbine 5. Performance 33 / 93
HIoPE
분류 VWO MGR NR 75 50 30
Constant Pressure Operation Sliding Pressure Operation
출력 (kW) 550,000
(110%)
541,650
(108.3%)
500,000
(100%)
375,000
(75%)
250,000
(50%)
150,000
(30%)
유량 (lb/hr) 3,757,727
(112.7%)
3,684,046
(110.5%)
3,335,116
(100%)
2,389,835
(71.7%)
1,564,131
(46.9%)
980,271
(29.4%)
복수기 압력 (in.Hga) 1.5 1.5 1.5 1.5 1.5 1.5
주증기 온도 (F) 1000 1000 1000 1000 1000 1000
주증기 압력
(psia)
3514.7
(100%)
3514.7
(100%)
3514.7
(100%)
2860.2
(81.38%)
1870.2
(54.47%)
1152.6
(32.79%)
1st STA Bowl P.
(psia)
3409.3
(100%)
[97.00%]
3409.3
(100%)
[97.00%]
3409.3
(100%)
[97.00%]
2774.4
(81.39%)
[97.00%]
1814.7
(54.49%)
[97.03%]
1118.0
(32.79%)
[97.00%]
1st STA Shell P.
(psia)
2630.8
(113.9%)
2573.8
(111.5%)
2309.0
(100%)
1683.5
(72.9%)
1128.0
(48.9%)
723.9
(31.4%)
FWPT 동력 (kW) 18,755
(3.41%)
18,390
(3.40%)
16,611
(3.32%)
9,622
(2.57%)
4,125
(1.65%)
1,523
(1.02%)
[ Operating conditions for Korean standard 500 MW fossil power ]
3. Increasing Steam Pressure [5/5]
Steam Turbine 5. Performance 34 / 93
HIoPE
The steam from boiler flows to the HP turbine where it expands and is exhausted back to the boiler for
reheating.
The efficiency of the Rankine cycle can be improved by reheating on the right hand side of the T-s diagram.
An improvement in cycle efficiency from a 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.
4. Reheating [1/6]
T
s
qRH
qH
3
qL
2
5
4
4 1 6
wP
wT A
B Condenser
Pump
Steam
generator
Turbine
qRH
qH
G LP
qL
1 2
3
4
5
6
wP
wT
Reheater
HP
Steam Turbine 5. Performance 35 / 93
HIoPE
[Exercise 5.2]
재열압력의 크기에 따른 증기터빈 출력 및 사이클효율을 비교하시오.
4. Reheating [2/6]
Steam Turbine 5. Performance 36 / 93
HIoPE
h
s
4%
8%
16%
12% C
E
B
A
D
A-B-C: Nonreheat
A-B: HP Turbine
B-D: Reheater
D-E: IP and LP Turbine
4. Reheating [3/6]
Steam Turbine 5. Performance 37 / 93
HIoPE
1-nuclear reactor, 2-steam generator, 3-HP turbine, 4-moisture
separator, 5-reheater, 6-LP turbine, 7-generator, 8-condenser,
9-condensate pump, 10-LP FWH, 11-LP FWH, 12-BFP,
13-HP FWH, 14-main circulating pump
G
1
10
9
8
7
6
5 4
3
2
11 12 13
14
Schematic of Nuclear Power Plant
4. Reheating [4/6]
Steam Turbine 5. Performance 38 / 93
HIoPE
HP Turbine
Condensate
Pump
Main Steam
To
Feedwater
Heaters
Steam
Dryers LP Turbines
From
Main
Steam
Moisture Separator Reheater
To
Feedwater
Heaters
Condensers
Main
Electrical
Generator Generator
Exciter
To Main
Transformer
[ Steam Turbine (APR 1400) ]
이 그림에서 잘못 그려진 부분 ?
4. Reheating [5/6]
Steam Turbine 5. Performance 39 / 93
HIoPE
In an ideal Rankine cycle for saturated steam
with Moisture separator and reheater, steam
expands in the HP turbine to pressure p4 and is
reheated to superheated steam (T6<T3).
It is clear that the equivalent Carnot cycle
temperature is this case is lower than for the
initial cycle. Thus, such steam reheat does not
improve the thermal efficiency.
Practically, however, thermal efficiency is
improved by using the MSR because of much
less moisture loss in LP turbine caused by an
improved LP turbine exhaust quality.
T
s
1
2
3
7
4
5
6
7
4-5: Steam separator
5-6: Reheater
Ideal Saturated-Steam Rankine Cycle with MSR (Nuclear)
4. Reheating [6/6]
Steam Turbine 5. Performance 40 / 93
HIoPE
5. Regenerative Feedwater Heating [1/5]
T
s
1
2
3 4
5 1 5
a b c d
Condenser
Pump
Boiler
G
Turbine
1
2 3
4
5
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.
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
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.
Steam Turbine 5. Performance 41 / 93
HIoPE
At the end of the heating process the water enters the boiler at the saturation temperature.
Since the decrease of entropy in the steam expansion line is exactly equal to the increase of entropy in the
water heating process, the ideal regenerative Rankine cycle will have the same efficiency as the Carnot
cycle.
The boiler, in this case, would have no economizer, and the irreversibility during heat addition in the boiler
would decrease because of less temperature difference between the heating and heated fluids.
Unfortunately, however, this ideal process is practically impossible.
Instead, the turbine is furnished with the definite number of heaters to heat feedwater with extracted steam in
some stages.
This improves the cycle efficiency significantly, even though it remains lower than the Carnot cycle efficiency.
This cycle is called as a regenerative cycle.
The heat input in the boiler decreases as the final feedwater temperature increases and the heat rejected in
the condenser getting smaller as the feedwater is heated higher using the extracted steam.
5. Regenerative Feedwater Heating [2/5]
Steam Turbine 5. Performance 42 / 93
HIoPE
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.
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
Rankine 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 stages.
Regenerative Rankine cycle also diminish the influence of the LP turbine, which has worst performance.
5. Regenerative Feedwater Heating [3/5]
Steam Turbine 5. Performance 43 / 93
HIoPE
Cycle No. of Feedwater
Heaters HARP Heat Rate Benefit
Single Reheat
(4500 psi, 1100F/ 1100F)
7
8
8
9
No
No
Yes
Yes
Base Case
+0.2%
+0.6%
+0.7%
Double Reheat
(4500 psi, 1100F/ 1100F/1100F)
8
9
9
10
No
No
Yes
Yes
Base Case
+0.3%
+0.2%
+0.5%
Heat Rate Impact of Alternative Feedwater Heater Configurations
HARP means that steam extraction to a heater above reheat point.
If HARP is involved in the cycle, the percentage of reheat flow to main steam flow is 75 to 80% instead of 85
to 92% as with the earlier designs without HARP.
When HARP is included in the cycle, the cycle efficiency is improved because feedwater temperature
becomes higher.
5. Regenerative Feedwater Heating [4/5]
Steam Turbine 5. Performance 44 / 93
HIoPE
SSR: Steam Seal Receiver, SPE: Steam Packing Exhaust
HARP: Heater Above Reheat Point
Single Reheat Cycle with HARP
5. Regenerative Feedwater Heating [5/5]
Steam Turbine 5. Performance 45 / 93
HIoPE
6. LSB
LSB strongly influence the turbine
performance.
The length of the LSB is determined by the
number of exhaust flows.
In general, the longer LSB, the lower the full-
load heat rate.
However, under the part-load operation,
turbines having longer LSB deteriorate more
rapidly in performance.
LSB = 26
6 Flows, 2400 psia / 1000 F/ 1000F
Nominal output 700 MW at 1.5 in.Hga
Turbine output, MW
Tu
rbin
e h
ea
t ra
te, B
tu/k
Wh
200 400 600 800
9000
8000
7000
30
33.5
Steam Turbine 5. Performance 46 / 93
HIoPE
7. Pressure Drop in Reheater System
Reheater pressure drop, %
Output
Ch
an
ge
, %
10 15 5 -3
1
0
-1
-2
20
2
Turbine Heat
Rate
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
itself, and the hot reheat piping from the
reheater to the IP turbine intercept valves.
A typical design value for total reheater
system pressure drop is 10% of the HP
turbine exhaust pressure.
For a 1% decrease in reheater pressure
drop, the heat rate and output improve
approximately 0.1% and 0.3%,
respectively.
Steam Turbine 5. Performance 47 / 93
HIoPE
8. Pressure Drop in Extraction Line [1/2]
Reheater
Generator
Condenser
2.0 in.Hga
SSR
HTR1
5F
5 F
HTR2
5F
10 F
HTR3
2F
10 F
HTR4
2F
10 F
HTR5
(DA)
0 F
HTR6
0F
10 F
HTR7
-3F
10 F
10%P 3%P
3%P
3%P 6%P 6%P 6%P 6%P
6%P
BFP
BFP = 87%
Main Steam BFPT
HP IP LP
Make U
p
3%P
SPE
99
99
BFPT = 82.6%
51
2 1
4 5
3
6
4
20
21
86
52 52
86
20
21
54 51
2
6
5
54
Steam Turbine 5. Performance 48 / 93
HIoPE
The extraction line pressure drop occurs between the turbine stage and the reheater shell.
For extractions not at turbine section exhausts (HP exhaust and IP exhaust), 6% of the turbine stage
pressure is a typical design pressure drop.
Three percent is the drop across the extraction nozzle, and 3% is for the extraction piping and valves.
(Extraction nozzle pressures are typically 2% to 3% lower than the shell pressure. Heater operating
pressures are typically 3% to 5% lower than the nozzle pressure.)
For extractions at the turbine exhaust section, no extraction nozzle loss occurs and the total pressure drop is
3%.
The higher the extraction line pressure drop, the worse the cycle heat rate.
For a 2% increase in extraction line pressure drop for all the heaters (from 6% to 8%), the change in output
and heat rate would be approximately 0.09% poorer.
8. Pressure Drop in Extraction Line [2/2]
Steam Turbine 5. Performance 49 / 93
HIoPE
9. Makeup Flow
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
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,
and 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.
Steam Turbine 5. Performance 50 / 93
HIoPE
Air Heater
Gas Out
Air In
Furnace
Gas
Recirculating
Fan
Pulverizer Primary Air
Fan
Forced
Draft Fan
Induced
Draft Fan
Steam
Coil
Air
Preheater
10. Air Preheating [1/3]
Air Preheater Using Steam Coil
Steam Turbine 5. Performance 51 / 93
HIoPE
10. Air Preheating [2/3]
Steam Turbine 5. Performance 52 / 93
HIoPE
The combustion air is heated by flue gas leaving the boiler prior to entering the boiler in order to improve
boiler efficiency by lowering the flue gas exit temperature.
Preheating of the combustion air prior to air heater is used to keep the flue gas exit temperature above its
dew point temperature.
The water dew point occurs at approximately 120F, and the flue gas dew point varies with the quantity of
sulfur trioxide in the flue gas. The acid dew point occurs at a higher temperature than the water dew point.
If the flue gas temperature falls below the dew point temperature, sulfuric acid which can damage the air
heater and flue gas duct is formed.
LP extraction steam or hot water from the turbine cycle is often used as the preheating source. These
heating sources are readily available and minimize the impact on the turbine cycle because the
thermodynamic availability of the supply source is low.
The air preheater steam supply is often supplied from the deaerator extraction point which is normally the
IP/LP turbine crossover point.
If the air preheater has steam coils, crossover steam is used directly and condenses in the preheater.
If the air preheater use hot water, saturated water from the deaerator is supplied to the air preheater.
The condensate is either pumped back to the deaerator, returned to the condenser, or returned to an
intermediate LP feedwater heater point such as flash tank (고압 증기의 드레인을 모아 감압하여 저압의 증기, 즉 재증발 증기를 발생시키는 탱크).
10. Air Preheating [3/3]
Steam Turbine 5. Performance 53 / 93
HIoPE
11. Condensate Subcooling
Condensate subcooling is the cooling of the cycle condensate in the condenser hot well below the saturation
temperature corresponding to the turbine exhaust pressure.
Condenser are normally specified to provide condensate at the condenser saturation temperature (0F
subcooling).
When subcooling occurs, the duty on the first feedwater heat increases, causing the extraction flow to the
heater to increase.
This decreases the turbine output and increases the turbine heat rate.
.024
.023
.022
.021
.020
.019
.018
.017
.016
.015
.014
.013 30 40 50 60 70 80 90 100 110
Throttle Flow, %
He
at R
ate
In
cre
ase
, %
Correction for 5F
Condensate Subcooling
Steam Turbine 5. Performance 54 / 93
HIoPE
12. Spray Flows for Desuperheating [1/2]
One method used to control the main steam and reheat steam temperatures is desuperheating by the spray
water into steam.
The source of spray water is typically boiler feed pump discharge for main steam spray, and an interstage
bleed off the boiler feed pump for reheat spray.
Alternatively, the spray water is taken from after the final feedwater heater.
Both main steam and reheat steam spray flows have an adverse effect on the turbine heat rate when the
spray water is taken from the boiler feed pump discharge.
The reason for this, in the case of main steam spray, is that the spray flow evaporates in the boiler and
becomes part of main steam flow. However, it bypasses the HP feedwater heaters, thus makes the cycle
less regenerative (using only five feedwater heaters).
In the case of reheat spray, the effect on heat rate is worse because cycle becomes less regenerative and
reheat spray flow bypasses the HP turbine and expands only through the reheat turbine section; thus, for the
steam flow that is reheat spray, the cycle is nonreheat.
Steam Turbine 5. Performance 55 / 93
HIoPE
Throttle flow, %
Co
rre
ctio
n fo
r 1
%
de
su
pe
rhe
ating
flo
w, %
60 90 40
.4
.3
.5 Load Correction Reheat Steam Desuperheat
Heat Rate Correction Reheat Steam Desuperheat
Load Correction Main Steam Desuperheat
Heat Rate Correction Main Steam Desuperheat
50 70 80 100 110
.1
0
.2
.6
.7
12. Spray Flows for Desuperheating [2/2]
Steam Turbine 5. Performance 56 / 93
HIoPE
13. Removing Top Heaters [1/2]
Feedwater heaters may need to be removed from service due to tube leaks.
Removing the top heater(s) from service eliminates turbine extraction for these heaters and increases steam
flow through the remaining sections of the turbine.
For a given throttle flow, turbine output increases because of the increased steam flow and cycle heat input
increases because of the lower final boiler feedwater temperature.
The turbine and cycle heat rates are poorer when removing the top heaters from service.
Some power plants are designed for removal of the top feedwater heaters to increase net plant output.
In this case, the boiler has higher heating duty because the boiler produces the steam having same throttle
steam conditions with maximum continuous rating under the condition with the lower final feedwater
temperature.
The turbine would need to be designed to accommodate the higher HP turbine exhaust pressure, increased
shaft power requirements in the IP and LP turbines, increased electric power generation, and increased
steam flow in the LP turbine last stage.
If the turbine specification requires increased output with removal of top heaters, the manufacturer may have
to select a larger last stage blade than optimal.
For existing units, the steam loading limit on LSB may prohibit increased output.
The engineer or operator should check with the turbine manufacturer’s literature or contact the manufacturer
directly for limitations on operation with heater removed from service.
Steam Turbine 5. Performance 57 / 93
HIoPE
Parameter
(500 MW, 7 feedwater heaters)
Case
All heaters in service Heater 7 out of
service
HP turbine output, kW
IP and LP turbine output, kW
Generator and mechanical losses, kW
Net turbine output, kW
Net turbine heat rate, Btu/kWh
Final feedwater temperature, F
Turbine cycle heat input, MBtu/h
Turbine cycle heat rejection, MBtu/h
Steam loading on LSB, lb/h/ft2
151,400
379,583
8,707
522,316
8,001
482
4,179
2,373
14,233
142,823
416,512
9,206
550,129
8,136
413
4,476
2,574
15,459
[ Effect on turbine cycle performance with removal of top heater from service]
When the heater removed from service, the HP turbine output decreases because turbine expansion
is reduced as a result of higher exhaust pressure caused by the greater cold reheat flow.
However, the output of the IP and LP turbine increases significantly because of increased steam flow.
13. Removing Top Heaters [2/2]
Steam Turbine 5. Performance 58 / 93
HIoPE
A closed feedwater is a heater where the feedwater and the heating steam do not directly mix.
Open feedwater heaters (deaerators) directly mix the feedwater and the heating steam.
A closed feedwater heater may consist of three zones: the desuperheating zone, the condensing zone, and
the drain cooling zone.
All closed heaters have a condensing zone where the feedwater is heated by the condensation of the
heating steam.
Feedwater heaters that receive highly superheated steam require a desuperheating zone to reduce the
steam temperature to approximately 50F above saturation temperature before it enters the condensing
zone.
14. Feedwater Heater Design Parameters [1/6]
Steam Turbine 5. Performance 59 / 93
HIoPE
A desuperheating zone may not be required
for heaters that receive heating steam with
less than 100F superheat.
Usually, a drain cooler is also included in a
feedwater heater to recover the heat
contained in the drains before the drains leave
the heater.
The feedwater heater performance is
determined by DCA (drain cooler approach)
and TTD (terminal temperature difference).
The DCA is the difference between the
temperature of the drains leaving the heater
and the temperature of the feedwater entering
the heater.
The TTD is the difference between the
saturation temperature at the operating
pressure of the condensing zone and the
temperature of the feedwater leaving the
heater.
Travel Distance
Te
mp
era
ture
Feedwater Inlet Extraction
Steam Outlet
Feedwater Outlet
Extraction Steam Inlet
Extraction
(Ne
ga
tive
) T
TD
Desuperheating
Zone Condensing Zone
Drain
Cooling
Zone
TSAT
DC
A
[ Temperature profile for a closed feedwater heater ]
14. Feedwater Heater Design Parameters [2/6]
Steam Turbine 5. Performance 60 / 93
HIoPE
By decreasing the DCA of a heater, cycle efficiency is improved while the heater surface area is increased,
resulting in higher capital cost.
The practical minimum DCA for an internal drain cooler is 10F. But, the minimum practical limit is 5F for an
external drain cooler.
The heater may have a negative TTD when the temperature of the feedwater leaving the heater is higher
than the saturation temperature of the condensing zone because of the desuperheating zone.
If the desuperheating zone of the heater is removed, the feedwater temperature leaving the heater would be
less than the saturation temperature, resulting in a positive TTD.
The practical lower limit of TTD on a heater without a desuperheating zone is +2F.
The negative TTD limit for a heater with a desuperheating zone depends on the amount of superheat in the
extraction steam entering the heater.
The lower the TTD and DCA, the higher the cycle efficiency and the larger the heater surface area.
The more efficient cycle results in a lower heat rate and reduced fuel consumption, while the larger surface
area of a heater results in a higher capital cost.
14. Feedwater Heater Design Parameters [3/6]
Steam Turbine 5. Performance 61 / 93
HIoPE
[ Effect of TTD on net turbine heat rate – Heater 7 (500 MW)
cycle, HP heaters 6 and 7 ]
TTD, F
Ne
t tu
rbin
e h
ea
t ra
te
co
rre
ctio
n fa
cto
r
4 1 6
1.0000
0.9995
1.0005
5 3 2 0
0.9985
0.9980
0.9990
1.0010
TTD varied independently from
base for each heater
Heater 6
Heater 7
14. Feedwater Heater Design Parameters [4/6]
Steam Turbine 5. Performance 62 / 93
HIoPE
Ne
t tu
rbin
e h
ea
t ra
te
co
rre
ctio
n fa
cto
r
3 6 1
1.0000
0.9995
1.0005
2 4 5 7 8
0.9985
0.9980
0.9990
1.0010
1.0015
TTD, F
TTD for LP heaters 1,2,3, and 4
varied as a group from base
[ Effect of TTD on net turbine heat rate – Heater 7 (500 MW)
cycle, LP heaters 1, 2, 3, and 4 ]
14. Feedwater Heater Design Parameters [5/6]
Steam Turbine 5. Performance 63 / 93
HIoPE
Ne
t tu
rbin
e h
ea
t ra
te
co
rre
ctio
n fa
cto
r
11 14 9
1.00010
1.00005
1.00015
10 12 13 15 16
0.99995
0.99990
1.00000
1.00020
1.00025
DCA, F
DCA varied on closed heaters 7, 6,
4, 3, and 2 as a group. External
heater 1 drain cooler DCA
remained fixed.
Heater 5 is the deaerator.
[ Effect of DCA on net turbine heat rate – Heater 7 (500 MW)
cycle ]
14. Feedwater Heater Design Parameters [6/6]
Steam Turbine 5. Performance 64 / 93
HIoPE
How to best apply the capital funding available on a power plant project is a critical question for the plant
designer.
The cost basis of technological improvements must be known to make an economic evaluation in today’s
competitive marketplace.
One open literature investigated that the ranking of several technology improvement steps for better plant
efficiency. From least cost to highest cost per efficiency improvement, million US$ / % net LHV efficiency,
these were.
1) Reducing condenser back pressure, 4.6
2) Increasing to 8th extraction point feedwater heater, raising feedwater temperature, 5.7
3) Raising main steam temperature and reheat steam temperature, 12.3
4) Raising main steam temperature, 12.7
5) Using separate BFPT instead of main turbine driven pump, 14.2
6) Raising main steam pressure, 39.1
7) Changing from single to double reheat, 56.7
8) Using separate BFPT condenser, 60.7
Economic Efficiency Improvement
Steam Turbine 5. Performance 65 / 93
HIoPE
Data is provided based on a GE steam turbine having output of 412 MW and main
steam condition of 2,400 psig/1,000F/,1000F. The turbine is a reheat, tandem
compound, four-flow with 26LSB.
Thermal Kit 4
Steam Turbine Section Efficiencies 2
Factors Affecting Cycle Performance 3
Heat Balance 1
Steam Turbine 5. Performance 66 / 93
HIoPE
Turbine Thermal Kit
The turbine thermal kit is provided by the turbine manufacturer and consists of numerous characteristic
curves those are used to determine the steam turbine performance for various steam cycle conditions.
These curves are used to develop computer programs or to perform hand calculation of steam turbine
performance.
In addition, the turbine thermal kit includes correction curves that can be used to adjust actual turbine test
data to design or guaranteed turbine performance conditions.
These correction curves facilitate the comparison of actual performance to guaranteed performance.
The turbine manufacturer should supply a complete set of these curves to permit the adjustment of all cycle
parameters that may vary between guaranteed conditions and actual operating conditions.
These correction curves should be obtained and their use understood prior to conducting the performance
test.
In addition, turbine test procedures should be developed and agreement reached on their use prior to testing.
These procedures should illustrate methods of adjustment to reference conditions.
Steam Turbine 5. Performance 67 / 93
HIoPE
Characteristic curves Correction curves
• Extraction stage shell pressures versus flow to the
following stage
• Gland leakage and mechanical losses
• Expansion lines
• HP turbine internal efficiency
• HP turbine expansion line end points
• Reheat turbine internal efficiency
• Reheat turbine expansion line end points
• Correction to expansion line end points
• Exhaust loss curve
• Generator losses
• First-stage shell pressure versus throttle flow
• Throttle pressure correction
• Throttle temperature correction
• Reheat pressure drop correction
• Reheat temperature correction
• Exhaust pressure correction factors
This information can be used to estimate changes in unit performance at off-design conditions.
These estimations can be performed by hand. However, some calculations can be lengthy, and if several
conditions are being evaluated, a detailed computer model is typically used with this information to predict
the performance of the actual turbine purchased.
Turbine Thermal Kit
Steam Turbine 5. Performance 68 / 93
HIoPE
3. Expansion Lines
[ Expansion lines for HP turbine ]
Turbine expansion lines, drawn on Mollier diagram, are
lines depicting the thermal state of the steam that has
different thermal state as it expands through the turbine.
These lines are developed based on throttle, governing
stage, and reheat conditions to determine the steam
enthalpy at the various extraction points on the turbine.
These lines are used in conjunction with a heat balance
and the extraction stage shell pressure curves or constants
to establish the extraction pressure at which to read the
expansion line enthalpy for a given extraction point in the
turbine.
In a thermal kit, the expansion line for the HP turbine is
given only for the steam expansion downstream of the first
stage.
A pressure drop from turbine throttle conditions of
approximately 3% is usually indicated to describe pressure
losses between the main stop valve and the HP turbine
bowl conditions.
sT
Pressure Drop through
Control Valves
Exit from Governing
Stage
Parallel Expansion
Line
Partial Flow
Expansion Line
Design Flow
Expansion Line ELEP
ELEP
AE
AE
pT pB
p1
pX
pX
hT
hXS
TT
p1
hXS
HP Turbine
Steam Turbine 5. Performance 69 / 93
HIoPE
1
p1
Availa
ble
Ene
rgy
p6
p5
p4
p3
p2
p0
T0
h
s
2
3
4
5
6
3. Expansion Lines
Steam Turbine 5. Performance 70 / 93
HIoPE
The assumption of 3% is typical for turbine
operation in partial arc admission mode.
The steam entering the turbine then expands from
the HP turbine bowl conditions to the exhaust
conditions of the first stage.
Turbine heat balance developed on the basis of
this assumption are considered to be on a locus-
of-valve best points basis. This heat balances
describe heat rates assuming an infinite number of
small valves having a 3% pressure drop.
Generator output
He
at ra
te
40 0 20 60 80 100
Valve Loop Basis (True Curve)
Mean of Valve Loop Basis
Valve Point Basis (Locus-of-valve best points)
Actual turbine performance is shown on a valve loop basis heat rate curve. This curve reflects the steam
throttling effect as the steam passes through a partially closed steam admission.
The throttling pressure drop reduces the available energy of the steam as the throttled admission steam
expands across the control stage.
Depending on the steam turbine manufacturer, curves of heat rate effect due to control valve position are
provided in the thermal kit.
HP Turbine
3. Expansion Lines
Steam Turbine 5. Performance 71 / 93
HIoPE
An alternative method of representing turbine heat rate impact due to turbine valve losses at part load is by a
mean of valve loop method.
This method is an approximation of the heat rate impact illustrated on the valve loop basis curve and
represents a mean of the turbine heat rate and passes through the valve loop curve.
For units operating with constant throttle pressure in partial arc admission mode, the pressure ratio through
the control stage is not constant.
As a result of the variation in pressure ratio, the available energy across the stage and the control stage
efficiency vary with throttle steam flow and conditions.
Therefore, expansion lines at different flow conditions for the control stage are not parallel to one another.
However, HP turbine stages downstream of the control stage operate with essentially constant pressure ratio,
and their expansion efficiency is essentially constant.
Therefore, at lower steam flows, the expansion line of the HP turbine stage group downstream of the control
stage is typically described as a straight line that is drawn parallel to the VWO expansion line.
HP Turbine
3. Expansion Lines
Steam Turbine 5. Performance 72 / 93
HIoPE
[ Expansion lines for reheat turbines ]
The expansion line for the reheat turbine (IP and LP
turbines) typically includes a 2% pressure drop between
the reheat stop valve inlet and IP turbine bowl to account
for the pressure drop across the stop/intercept valves.
In addition, for combined HP/IP turbines, the steam
leakage from the HP turbine is mixed with the hot reheat
steam to determine the reheat bowl steam conditions.
The steam then expands through the IP turbine to LP
turbine.
The steam exiting the IP turbine is often conveyed to the
LP turbine through a crossover pipe.
A 2% allowance for crossover pipe pressure drop is
typically included by the turbine manufacturer to
determine the LP turbine inlet conditions.
sm
Pressure Drop Through
Intercept Valves, 2%
For Machines with No
Leakage Entry
hm = hIV
sm = sIV
Expansion Line AE
pB
pIV
pX= 1.5 in.Hga
hm TIV
hXS
hIV
sIV
pX= 1.0 in.Hga
ELEP1.5 in.Hga
ELEP1.0 in.Hga
ELEP 1.5 in.Hga to
1.0 in.Hga
Reheat Turbine
3. Expansion Lines
Steam Turbine 5. Performance 73 / 93
HIoPE
On a Mollier diagram, the turbine expansion line is drawn to a ELEP.
ELEP is plotted at the turbine back pressure used as the basis of the heat balance and represents a
complete expansion of the steam to the condenser pressure.
However, the steam leaving the LP turbine never actually reaches ELEP steam conditions because there is
exhaust loss occurring in the LP exhaust hood.
The actual exhaust condition, referred to as UEEP, is calculated as the sum of the ELEP and the exhaust
loss.
Since the stages upstream are unaffected by the exhaust loss, the expansion line describing the steam
condition in the IP and LP turbine stages is drawn to the LP turbine ELEP.
This permits determination of the steam condition for the reheat turbine extractions.
The expansion line for the IP turbine is essentially a straight line.
However, the expansion line for the LP turbine exhibits a curvature or varying slope. This variation in the
expansion line represents efficiency degradation caused by moisture loss.
Reheat Turbine
3. Expansion Lines
Steam Turbine 5. Performance 74 / 93
HIoPE
[ HP turbine internal efficiency, GE ]
4. HP Turbine Internal Efficiency
The HP turbine internal efficiency represents the overall
efficiency of it and is applied to the calculation of the
available energy from the turbine throttle conditions to
the HP turbine exhaust pressure.
A composite of the effect of throttle valve pressure drop,
first-stage efficiency, and HP turbine stage group
efficiency are represented by this curve.
Since the curve is drawn using the assumption of locus-
of-valve best point, the curve does not reflect the
throttling losses of partially open control valves for flows
above the throttling flow ratio of the first admission.
These throttling losses are small at high load because of
the relatively small portion of flow that is throttled
compared to the flow that is passing through the valves
those are fully open.
However, as load is decreased, a greater portion of
turbine flow becomes throttled, further impacting turbine
efficiency.
This curve is on a valve best point basis.
TFR = (throttle flow at any steam conditions)/ (VWO
throttle flow at same steam conditions)
Apply the efficiency from this curve to the available
energy from the turbine stop valves to the HP turbine
exhaust.
Break in curve is first admission point, throttling
control occurs at all lower throttle flow ratios.
For off-rated steam conditions use equivalent TFR.
ratedoff
ratedeq
p
p
flowDesign
flowratedOffTFR
)/(
)/(
HP
Tu
rbin
e In
tern
al E
ffic
ien
cy,
%
.3 .6 .1
70
65
75
.2 .4 .5 .7 .8
55
50
60
80
85
Throttle Flow Ratio (TFR)
.9 1.0
Steam Turbine 5. Performance 75 / 93
HIoPE
The throttle flow ratio at first admission point
with respect to the number of admission is
approximately as follows:
1 admission (full throttling) ------
2 admissions -----------------------
3 admissions -----------------------
4 admissions -----------------------
8 admissions -----------------------
1.0
0.85
0.60
0.35
0.30
[ Throttle flow ratio at
first admission point, GE ]
Turbines are provides with a limited numbers of
valves, some of those are operated in unison.
The actual number of valves and turbine admission
is a function of the mode of operation and the
manufacturer’s design practice.
Steam flow control at throttle flow ratios below the
first admission point is accomplished by throttling
process using valve(s).
In this mode, all flow to the turbine is throttled which
results in a decrease of efficiency.
Because of throttling losses at lower loads, the
throttle flow ratio at the first admission has a
significant impact on performance at loads below
this point.
4. HP Turbine Internal Efficiency
Steam Turbine 5. Performance 76 / 93
HIoPE
HP
Tu
rbin
e E
LE
P, B
tu/lb
2.0 3.5 1.0
1320
1310
1330
1.5 2.5 3.0 4.0 4.5
1290
1280
1300
Intercept Valve Pressure – 100 psia
5.0 5.5
This curve assumes a pressure drop of 10 percent from high pressure
turbine exhaust to intercept valve. These high pressure expansion line
end points cannot be used for other pressure drops.
6.0
5. HP Turbine ELEPs
This curve is limited to
applications with rated steam
conditions and further based on
the assumption of a 10%
reheater pressure drop from HP
exhaust to intercept valve.
The curve is provided as a quick
reference for the leaving
enthalpy of the steam exhausting
the HP turbine.
The HP turbine efficiency curve
discussed previously provides a
more versatile method for
determining expected HP turbine
exhaust conditions.
Steam Turbine 5. Performance 77 / 93
HIoPE
Intercept valve pressure, 100 psia
Re
he
at tu
rbin
e th
erm
al
effic
ien
cy,
%
2.0 0.0 1.0 3.0 4.0 5.0
Reheat Temp., F
94
93
92
91
90
89
88 6.0
1000
950
900
850
800
750
700
[ Reheat turbine internal efficiency, GE ]
Apply efficiency to available energy between intercept valve
pressure and bowl enthalpy (including packing leakage mixture if
any) and 1.5 in.Hga.
ELEP1.5 = hbowl – Eff. (AE)
See ASME paper 62WA209 for construction of expansion line
6. Reheat Turbine Internal Efficiency
The reheat turbine internal
efficiency curves are plotted
as a function of pressure
ahead of the intercept valve
because it relates to steam
flow.
The turbine efficiency is
applied to the available energy
between the intercept valve
pressure and mixed bowl
enthalpy and a reference LP
turbine exhaust pressure of
1.5 in.Hga.
Steam Turbine 5. Performance 78 / 93
HIoPE
7. Reheat Turbine ELEPs
The end points are plotted as a function of intercept
valve pressure and are based on a turbine exhaust
pressure of 1.5 in.Hga.
This curve conveniently provides the end point
conditions for rated reheat steam temperature;
however, it is not as flexible as the internal
efficiency curve because end points are valid only
for 1,000F reheat steam.
These ELEPs are for heat balance calculations in which
grand leakage steam is used in the feedwater heating cycle.
To obtain the enthalpy of the steam entering the condenser,
read the curve at 1.5 in.Hga and correct to the desired
exhaust pressure, and correct for exhaust loss.
Reh
ea
t T
urb
ine
EL
EP
s a
t 1
.5 in.H
ga
, B
tu/lb
3 6 1
1030
1020
1040
2 4 5 7 8
1000
990
1010
1050
1060
Intercept Valve Pressure, 100 psia
9
1070
Steam Turbine 5. Performance 79 / 93
HIoPE
8. Correction to ELEPs for Exhaust Pressure
1. When ELEP at 1.5 in.Hga is in the moisture region, use the following equation:
ELEP = ELEPY=0(0.87)(1-0.01Y)(1-0.0065Y), Y=percent moisture at ELEP at 1.5 in.Hga.
2. When ELEP at 1.5 in.Hga is in the superheat region, multiply the change in ELEP at 0 percent
moisture both by the factor 0.87 and the ratio of specific volume at ELEP under consideration to
the dry saturated specific volume at 1.5 in.Hga.
0.5
0.6
0.7
0.8 0.85
0.85
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.5
2.6
2.7
2.8
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8 4.9
5.0
-64
-62
-60
-58
-56
-54
-52
-50
-48
-46
-44
-42
-40
-38
-36
-34
-32
-30
-28
-26
-24
-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0
+2
+4
+6
+8
+10
+12
+14
+16
+18
+20
+22
+24
+26
+28
+30
+32
+34
+36
+38
+40
+42
+44
+46
+48
+50
+52
+54
+56
+58
+60
+62
+62
+64
+66
+68
+70
+72
+74
+76
E
LE
PY
=0 -
Cha
nge
in E
LE
P w
ith
o%
Mo
istu
re, B
tu/lb
Exhaust Pressure, in.Hga
Steam Turbine 5. Performance 80 / 93
HIoPE
9. Exhaust Loss Curve
UEEP = ELEP + Exhaust Loss per unit steam flow
The exhaust loss plus the ELEP
defines the UEEP, that is also
called as TEP (Turbine End Point).
The UEEP is the actual leaving
enthalpy of the LP turbine steam
and includes losses associated
with the hood loss, leaving loss,
and restriction loss or turn up loss.
It is clear that the work produced
by the turbine should be
determined by performing an
energy balance with the use of the
UEEP, not the LP turbine ELEP.
Exh
au
st L
oss, B
tu/lb o
f d
ry
flo
w
0
Annulus Velocity, fps Sonic
200 400 600 800 1000 1200 1400 1600 0
10
20
30
40
50 Annulus
Restriction
Loss
Gross Hood
Loss
Actual
Leaving
Loss
Turn-up
Loss
Total
Exhaust
Loss
Steam Turbine 5. Performance 81 / 93
HIoPE
Annulus Velocity, ft/s
Exhaust Loss, B
tu/lb o
f dry
flo
w
200 400 600 800 1000 1200 1400
6
50
46
42
38
34
30
26
22
18
14
10
2 0
Bucket Pitch Last stage
Curve length diameter annulus area
no. (inches) (inches) single flow (ft2)
1 14.3 52.4 16.3
1 16.5 57.5 20.7
1 17 52 19.3
1 20 60 26.2
2 23 65.5 32.9
3 26 72 41.1
4 30 85 55.6
5 33.5 90.5 66.1
(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:
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.
Van = Annulus velocity (fps)
m = Condenser flow (lb/hr)
= Saturated dry specific volume (ft3/lb)
Aan = Annulus area (ft2)
Y = Percent moisture at ELEP
ELEP = Expansion line end point at actual
exhaust pressure (Btu/lb)
UEEP = Used energy end point (Btu/lb)
1
1 2 3 4 5
2 3 4 5
9. Exhaust Loss Curve
Steam Turbine 5. Performance 82 / 93
HIoPE
511,000 kVA at 45 psig H2 pressure
Conductor cooled 3600 rpm
Note: Generator losses assume rated hydrogen pressure at all loads.
Generator loss at reduced hydrogen pressure (P) = Loss at rated hydrogen pressure –
11.2(Prated – P).
Use generator reactive capability curve to determine generator capability at reduced
hydrogen pressure.
Turbine generator mechanical losses are not included in the generator loss curve.
If hydrogen and stator liquid coolers are located in the condensate line, the loss
transferred to the coolers is 474 kW les than the generator loss at all loads.
Ge
nera
tor
Loss,
1,0
00
kW
100 250 0
6
5
7
50 150 200 300 350
3
2
4
Generator Output, 1,000 kVA
400 450 500 550
10. Generator Losses
The generator losses are a function of
generator kVA (not kW).
Therefore, if generator losses at a
power factor other than design is
required, the curve should be read at
desired output (in kilowatts) multiplied
by the rated power factor divided by
the desired power factor.
Generator losses do not include the
turbine generator fixed or mechanical
losses.
The mechanical losses should be
accounted for separately and do not
vary with unit load.
Steam Turbine 5. Performance 83 / 93
HIoPE
11. 1st Stage Shell Pressure vs. Throttle Flow
To estimate the first stage shell pressure at
other than rated initial temperature, the
following equation should be used to
determine an equivalent throttle flow before
entering the curve.
design
actualactualthcorrectedth mm
__
correctedthm _
actualthm _
actualdesign
corrected turbine throttle flow, lb/hr
actual turbine throttle flow, lb/hr
actual throttle steam specific volume, ft3/lb
design throttle steam specific volume, ft3/lb
Flow, lbs/hr x 1,000
Pre
ssu
re, p
sia
1000 2500 0
2000
1500
500 1500 2000 3000
500
0
1000
Steam Turbine 5. Performance 84 / 93
HIoPE
Correction Curves
There are five typical correction curves as follows:
• Initial pressure correction
• Initial temperature correction
• Reheat pressure drop correction
• Reheat temperature correction
• Exhaust pressure correction factors
The correction curves typically found in a thermal kit are intended to give approximate output and heat rate.
The correction curves are often provided for correction turbine test date to guaranteed conditions.
Steam Turbine 5. Performance 85 / 93
HIoPE
Decre
ase
, %
3
4
2
-5
6
5
1
Rated Load
½ Load ¼ Load
Incre
ase
, %
+5
4
3
5
1
0
2
6
Change in initial
pressure, %
Rated Load
½ Load ¼ Load
Change in Kilowatt Load, %
Change in Heat Rate, %
Poorer, % 2
-5
1
Rated Load ½ Load ¼ Load
Better, %
+5
2
1
Rated Load ½ Load
¼ Load
Change in initial
pressure, %
Method of Using Curves
These correction factors assume constant control
valve opening and are to be applied to heat rates
and kilowatt loads at rated steam conditions.
1. The heat rate at desired condition can be found
by multiplying the heat rate at rated conditions
by the following:
2. The kilowatt load at the desired conditions can
be found by multiplying the kilowatt load at rated
conditions by the following:
3. These correction factors are not guaranteed.
% change in gross heat rate
100 1 +
% change in kilowatt load
100 1 +
1. Initial Pressure Correction
Steam Turbine 5. Performance 86 / 93
HIoPE
The curve allows for the correction of turbine output and heat rate for changes in throttle pressure.
The curve is developed based on holding a constant control valve opening while varying the turbine throttle
pressure.
A similar curve is provided in the ASME PTC(Power Test Code) 6.1, Interim Alternative Test Procedure for
Steam Turbines.
The manufacturer may provide a curve specifically developed for the turbine.
Increased throttle pressure at constant valve opening increases the mass flow to the turbine, which
increases the output of the unit.
The increased throttle pressure improves the turbine cycle efficiency as a result of increased available
energy.
1. Initial Pressure Correction
Steam Turbine 5. Performance 87 / 93
HIoPE
2. Initial Temperature Correction
Increasing the throttle temperature results in an increased specific volume, a decreased mass flow, and
decreased output.
The increase in throttle temperature increases the available energy to the turbine, increases the turbine cycle
efficiency, and reduces heat rate.
Method of Using Curves
These correction factors assume constant control
valve opening and are to be applied to heat rates
and kilowatt loads at rated steam conditions.
1. The heat rate at desired condition can be found
by multiplying the heat rate at rated conditions
by the following:
2. The kilowatt load at the desired conditions can
be found by multiplying the kilowatt load at rated
conditions by the following:
3. These correction factors are not guaranteed.
% change in gross heat rate
100 1 +
% change in kilowatt load
100 1 +
Change in Heat Rate, %
Poorer, % 2
-5
1 Rated Load
¼ Load
Better, %
+5
2
1 Rated Load ¼ Load
Change in initial
temperature, F
Change in Kilowatt Load, %
Increase, % 2
-50
1
Rated Load
½ Load ¼ Load
Decrease, %
+50
2
1
Rated Load
½ Load ¼ Load Change in initial
temperature, F
Steam Turbine 5. Performance 88 / 93
HIoPE
3. Reheat Pressure Drop Correction
The decrease in reheater pressure drop
results in an increase in turbine output
and a decrease in turbine heat rate.
HP turbine output increases, and the
HP turbine expansion end point
becomes lower.
As a result, the reheater duty increases.
Change in Kilowatt Load, %
Increase, % 2
5
1
Rated Load ¼ & ½ Load
Decrease, %
15
2
1 Change in initial
temperature, F Rated Load ¼ & ½ Load
10
Change in Heat Rate, %
Poorer, %
2
5
1
All Loads
Better, %
15
2
1 All Loads
Reheater pressure
drop, %
10
Steam Turbine 5. Performance 89 / 93
HIoPE
4. Reheat Temperature Correction
The increased reheat temperature results
in increased turbine output and decreased
turbine heat rate.
Increased reheat temperature does not
impact the amount of main steam flow,
however, it increases the intercept valve
pressure and decreases the amount of
work produced in the HP turbine due to
greater HP turbine exhaust pressure.
However, the reduced HP turbine output is
offset by the greater available steam
energy supplied to the reheat turbine and
its increased turbine output.
Change in Heat Rate, %
Poorer, % 2
-50
1
Better, %
+50
2
1 Change in reheat
temperature, F
0
Rated Load ½ Load ¼ Load
Rated Load ½ Load ¼ Load
Increase, % 3
2
1
Rated Load ½ Load ¼ Load
Decrease, % 3
2
-50
1
Rated Load ½ Load ¼ Load
+50
Change in reheat
temperature, F
Change in Kilowatt Load, %
0
Steam Turbine 5. Performance 90 / 93
HIoPE
5. Exhaust Pressure Correction
A change in LP turbine exhaust pressure results in a
change in available energy. In addition, exhaust losses
and LP turbine exhaust flow are impacted.
Exhaust pressure correction provides an equation to
correct the turbine output.
This equation is based on constant heat input and
indicates that turbine heat rate is inversely proportional
to the turbine output.
For the case of decreasing exhaust pressure, the
condensate temperature of the first LP heater
decreases because it is typically the saturation
temperature corresponding to the turbine exhaust
pressure.
The colder condensate results in an increased
extraction flow to the LP heater, thus reduced both
exhaust flow and turbine output.
Values near curves are flows at 2400 psig, 1000F.
These correction factors assume constant control valve opening.
Apply corrections to heat rate and kW loads at 2.0 in.Hga. and
0.0 percent mu.
The percent change in kW load for various exhaust pressures is
equal to (minus percent increase in heat rate) 100/(100 +
percent increase in heat rate)
These factors give change in net turbine heat rate.
2 4
4
0
8
3 1 0
-8
-4
12
Incre
ase
in H
ea
t R
ate
, %
Exhaust Pressure, in.Hga
5
2,980,170 lb/h, VWO-NP
2,049,969 lb/h, 75%
2,846,828 lb/h, 100%
1,355,163 lb/h, 50%
697,935 lb/h, 25%
Steam Turbine 5. Performance 91 / 93
HIoPE
In addition, lower exhaust pressure typically results in higher exhaust loss because of increased exhaust
velocity due to the higher specific volume at the exhaust pressure.
However, these effects are offset by the increased available energy. Therefore, output and efficiency typically
improve with reduced exhaust pressure.
However, as back pressure continues to decrease, the turbine may become choked.
Below the choking pressure, the turbine no longer benefits from increased available energy.
The condensate continues to cool further and heater extraction increases.
Therefore, the output and efficiency can decrease.
The turbine used in the thermal kit becomes choked at approximately 2.0 in.Hga for the maximum flow
condition.
As turbine exhaust flow decreases, the turbine becomes choked at lower exhaust pressures.
The point at which the turbine becomes choked depends on turbine design, exhaust flow, and exhaust
pressure.
5. Exhaust Pressure Correction
Steam Turbine 5. Performance 92 / 93
HIoPE
0.5 1.0 1.5 2.0 1.76 in.Hga
0 2 12 16 19
18
16
14
12
10
8
6
4
2
0
Cooling Water Inlet Temperature
Po
we
r G
ain
, M
W
4F30"
4F32"
4F38"
6F32"
C
Choking
5. Exhaust Pressure Correction
Steam Turbine 5. Performance 93 / 93
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
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