HIoPE
h
HP = 88-90%
IP = 90-94%
Fossil Reheat
LP = 90-91%
HP = 82%
LP = 87%
Saturation Line
LP = 85%
Nuclear Reheat Nuclear Non-Reheat
s
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Heat Balance
Steam Turbine Performance
Thermodynamic Efficiency
Mechanical (Fluid Dynamic) Efficiency
Working fluids
Section efficiencies (in terms of energy conversion)
• Throttle pressure and temperature (enthalpy)
• Throttling losses occurred in valves
• Throttle flow (specific heat)
• Stage losses (profile, secondary flow, leakage loss)
Steam Cycles • Reheating (non-, single-, double) • Regenerating
Exhaust pressure • The amount of enthalpy drop
Others • Air preheating • Desuperheating • Part load operation mode
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
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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, pressure). the steam flow is 5% greater than that required for rated output with rated steam pressure (normal
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.
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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.
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Main Steam Steam Generator
Crossover
Stop V/V Control V/V
Front Standard Cold Reheat
HP
IP
LP
Ventilation V/V
Reheater
Hot Reheat
Reheat Stop and Intercept V/V
Condenser
Gen
Exciter
HIoPE
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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.
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10%P
3%P
Reheater Main Steam
BFPT BFPT = 82.6%
3%P 99
51
86
99 2
1
IP
HP
Generator
LP
3 5
4
6
Condenser 2.0 in.Hga
6
3%P
86
5 20
3%P
6%P
6%P
21
6%P
2
6%P
20
SSR
6%P
4
51 52 52
HTR7
HTR6
-3F 10 F
0F 10 F
BFP
BFP = 87%
HTR5 (DA) 0F
HTR4
HTR3
HTR2
HTR1
2F 10 F
2F 10 F
5F 10 F
5F 5F
54
SPE
54
p U e k a M
21
HIoPE
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 VWO-OP, VWO-NP, and rated load 75% of rated 50% of rated 25% of rated
3.
BFP Efficiency 84% 83% 67% 40%
For a turbine cycle with a motor-driven boiler feed pump, the variable speed coupling efficiency will vary with load as follows: Condition VWO-OP VWO-NP, and rated load 75% of rated 50% of rated 25% of rated
Coupling Efficiency 85% 82% 76% 73% 68%
The combined motor and transmission efficiency will vary with load as follows: Condition VWO-OP, VWO-NP, and rated load 75% of rated 50% of rated 25% of rated
Motor and Transmission Efficiency 94% 93% 92% 89%
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4.
5.
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. 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.
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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 10 F 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.
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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. % 46
3
4
2
1
V H L n 45 o d e s a B44 y c n ie 43 ic ff E t e N42 t n a l P 41
% V H H43 n o d e s a42 B y c n ie ic ff 41 E t e N t n40 a l P
39
0.88 in.Hga
Double reheat 1.9 in.Hga 300 bar/600C Single reheat
USC
1.15 1.25
Excess Air
120C 130C
Discharge Flue Gas Temperature
250 bar/540C
Main Steam Condition
Reheat
Back Pressure
HIoPE
T
3 qin qin
wturbine
qin
2
wpump
1 th
W QH
QH QL QH
1
외부에 어떠한 영향도 남기지 않고 한 사이클 동안에 계가 .( 100%인 만들 수 없으며, 와 한다).
모든
경우 그러나 열기관 주변에 있는
열을
s
QL QH
고온의
받은 열을 모두 일로 바꾸는 것은 반드시 공급된 일부를 주위
역학적 일을 하고 저온의
고온의 열을
4
qout
열을
열을 낮은 온도로 열을 버릴 수는 없다.
.
.
HIoPE
Stage efficiency is defined as the ratio of mechanical work produced by the stage to the thermal energy available.
h
p1
1
st y rg e n E l u f e s U
y rg e n E e l b ila a v A
2s
b
a
2
h1 h2 h1 h2 s
Nozzle profile loss Bucket profile loss Secondary flow loss + leakage loss
p2
Stage Loss
) rk o W e g ta (S
s
HIoPE
IP Turbine Section Efficiencies h
h
545 psia 534 psia Pressure drop in intercept valve (2%)
IP = 90-94% HP = 88-90%
92.7%
89.1% 180 psia
91.2% LP = 90-91%
175 psia 1378.2 h
Saturation Line 1367.1 h Fossil Reheat Nuclear Reheat Nuclear Non-Reheat
s
1364.6 h
Pressure drop in the IP exhaust hood, the cross-over pipe, or the LP turbine inlet
1361.0 h
s
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.
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Base A. Condenser Pressure, in.Hga
2.0
2.5
3.5
4.0
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
Relative net heat rate (Btu/kWh)
-4.0
---
3.9
C. Pressure Drop in Reheater, %
5
7
9
1,403
---
-1,445
Relative net heat rate (Btu/kWh)
-17.8
---
18.3
0
-732 8.3 11
Relative net output (kW) D. Pressure Drop in Crossover Pipe, %
-2,935 37.1
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 Relative net output (kW) Relative net heat rate (Btu/kWh) F. Use of Air Preheater
3.0
Relative net output (kW)
950
1,000
1,050
1,100
-35,474
---
34,585
68,738
118.9
---
-106.0
-202.9
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
HIoPE
T
Equivalent Carnot Cycle
Equivalent Cycle Hot Temperature
T
Equivalent Cycle Hot Temperature
3
3
2
2 1
4
1 s
[Ideal Rankine Cycle for a Typical Nuclear Power]
4 s
[Ideal Rankine Cycle for a Typical Fossil Power]
The higher the equivalent cycle hot temperature, the greater the cycle efficiency.
The average temperature where heat is supplied in the boiler can be increased by superheating the steam.
HIoPE
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.
T
3 3
Increasing the steam temperature not only improves the content cycle efficiency, but also reduces the moisture 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%.
heat added 2 4 1
heat lost
4 s
HIoPE
T
Ultra Supercritical Supercritical 1960
1940s
Early 20th century
s
HIoPE
Except for choked turbine exhaust conditions, the lower the turbine exhaust pressure, the higher the cycle efficiency. KW
HR
1
HR
60%
100
Throttle Flow Rate h W k / u t B , e t ra t a e h e in b r u T
KW = change of generator kW
output, %. HR = change of heat rate, %
T
3
p4
2 2 1
70% 80% 90% 100% 4.5
Condenser Pressure 3.5 (in.Hga) 2.5
p4
1
4 4
Turbine output, kW a a
b
s
HIoPE
195 2 F-40.0 LSB "
W 190 M ,t u p t u O e 185 n i rb u T m a te 180 S
175 0.0
D-11 steam turbine for GE 207FA, 1800 psia / 1050F / 1050F
2 F-33.5 LSB "
2 F-30.0 LSB "
0.5
1.0
1.5
2.0
2.5
Condenser Pressure, in.Hga
3.0
3.5
4.0
HIoPE
[Exercise 5.1] 복수기 압력을 2.5 in.Hga로 조건을 그대로 었다. 이때
때 발전기 출력이 700 MW, heat rate가 7826 Btu/kWh이다. 주증기 복수기 압력을 4.5 in.Hga로 열율이 7980 Btu/kWh가 되
.
[Solution 1] 다음 식으로 구할 수 있다. KW HR HR 1
100
HR = {(7980–7826)/7826}100 = 1.97% KW = –1.97/(1+1.97/100)= –1.93%
[Solution 2]
HR = Q/W Q = 700,000 kW 7826 Btu/kWh = 5,478.2106 Btu/hr W′ = Q/HR′ = 686,491 kW W = {(700,000–686,491)/700,000}100 = –1.93%
HIoPE
The cycle maximum temperature is constant.
Increased boiler pressure has a higher mean temperature of heat addition.
T 3
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.
3
c
increase in qin 2
decrease in qin
2 1
4
4 decrease in qout a
b
b
s
HIoPE
Main steam conditions strongly influence the turbine performance. 9000
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.
h W k / u t B , te ra t 8000 a e h e n i b r u T
7000
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 200
400
600
800
Turbine output, MW
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.
HIoPE
T
3
Tmax, USC
USC 3
3 Tmax, subcritical
Subcritical Critical Point
2 2 2 1
4
s
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.
HIoPE
[ Operating conditions for Korean standard 500 MW fossil power ] VWO
MGR
NR
75
50
30
분류
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
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%)
(psia)
HIoPE
qRH
T
5
3
3 Turbine
Steam generator
qH
qH
4
qRH
HP
Reheater
wT
5 6
B
Condenser
2 2 Pump
wP qL
4
G wT
A
1
LP
4
6
wP
1 qL
s
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.
HIoPE
[Exercise 5.2] 크기에 따른
출력 및
.
HIoPE
h
D A A-B-C: Nonreheat A-B: HP Turbine B-D: Reheater D-E: IP and LP Turbine
B
4%
E 8% 12%
C 16%
s
HIoPE
2
4
5 7
3
6
1
8
14
13
12
11
10 9
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
HIoPE
Moisture Separator Reheater To Feedwater Heaters
Generator Exciter
Main Electrical Generator
From Main Steam Steam Dryers LP Turbines HP Turbine
Main Steam
To Main Transformer
[ Steam Turbine (APR 1400) ]
Condensate Pump Condensers
이 잘못 그려진 부분 ?
To Feedwater Heaters
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
T
3
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.
4 2 1
6
5
4-5: Steam separator 5-6: Reheater 7
7
s
HIoPE
T
4
3
Turbine
4
G Boiler 5
2
2
3 1
1
5
5
Pump a
b
c
d
1
Condenser
s
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.
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.
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.
HIoPE
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.
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%
HIoPE
HARP: Heater Above Reheat Point
SSR: Steam Seal Receiver, SPE: Steam Packing Exhaust
HIoPE
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 fullload heat rate.
However, under the part-load operation, turbines having longer LSB deteriorate more rapidly in performance.
9000
h W k / u t B , e t ra t 8000 a e h e n i b r u T 7000
LSB = 26 30 33.5
6 Flows, 2400 psia / 1000 F/ 1000F Nominal output 700 MW at 1.5 in.Hga
200
400
600
Turbine output, MW
800
HIoPE
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.
2 1
Turbine Heat Rate
% 0 , e g n a -1 h C
Output
-2 -3
5
10
15
Reheater pressure drop, %
20
HIoPE
10%P
3%P
Reheater Main Steam
BFPT BFPT = 82.6%
3%P 99
51
86
99 2
1
IP
HP
Generator
LP
3 5
4
6
Condenser 2.0 in.Hga
6
3%P
86
5 20
3%P
6%P
6%P
21
6%P
2
6%P
20
SSR
6%P
4
51 52 52
HTR7
HTR6
-3F 10 F
0F 10 F
BFP
BFP = 87%
HTR5 (DA) 0F
HTR4
HTR3
HTR2
HTR1
2F 10 F
2F 10 F
5F 10 F
5F 5F
54
SPE
54
p U e k a M
21
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.
HIoPE
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.
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Air Heater
Furnace Air In
Gas Recirculating Fan
Pulverizer
Primary Air Fan
Air Preheater
Steam Coil
Forced Draft Fan
Gas Out
Induced Draft Fan
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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 모아 저압의 증기, 즉 재증발 증기를 탱크).
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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 e s a .021 re .020 c .019 n I .018 e t a .017 R.016 t a .015 e H.014 .013 30
Correction for 5 F Condensate Subcooling
40
50
60
70
80
90 100 110
Throttle Flow, %
HIoPE
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 andfrom reheat flowsdischarge. have an adverse effect on the turbine heat rate when the spray watersteam is taken thesteam boiler spray feed pump
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.
HIoPE
.7
.6 % 1 r o f n o it c re r o C
% , .5 w lo f g tin a .4 e h r .3 e p u s .2 e d .1
0
Load Correction Reheat Steam Desuperheat Heat Rate Correction Reheat Steam Desuperheat Load Correction Main Steam Desuperheat Heat Rate Correction Main Steam Desuperheat
40
50
60
70
80
Throttle flow, %
90
100
110
HIoPE
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.
HIoPE
[ Effect on turbine cycle performance with removal of top heater from service] 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
151,400 379,583 8,707
142,823 416,512 9,206
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
522,316 8,001 482 4,179 2,373 14,233
550,129 8,136 413 4,476 2,574 15,459
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.
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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.
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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.
Drain Cooling Zone
e r tu ra e p m e T
Condensing Zone TSAT
Desuperheating Zone
Extraction
D T T
A C D
Feedwater Outlet Extraction Steam Inlet Feedwater Inlet Extraction Steam Outlet
Travel Distance [ Temperature profile for a closed feedwater heater ]
) e v ti a g e N (
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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 10 F. 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.
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1.0010
e t a r t a e h e in b r tu t e N
1.0005
r o t 1.0000 c a f n 0.9995 o i t c e rr 0.9990 o c
Heater 6
Heater 7 TTD varied independently from base for each heater
0.9985 0.9980 6
5
4
3
2
1
0
TTD, F [ Effect of TTD on net turbine heat rate – Heater 7 (500 MW) cycle, HP heaters 6 and 7 ]
HIoPE
1.0015
e t ra t a e h e in b r u t t e N
1.0010
r 1.0005 to c fa 1.0000 n o i t 0.9995 c e rr o c 0.9990 TTD for LP heaters 1,2,3, and 4 varied as a group from base
0.9985 0.9980
1
2
3
4
5
6
7
TTD, F [ Effect of TTD on net turbine heat rate – Heater 7 (500 MW) cycle, LP heaters 1, 2, 3, and 4 ]
8
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1.00025
e t ra t a e h e in b r u t t e N
1.00020
r 1.00015 to c fa1.00010 n o i t 1.00005 c e rr o c1.00000
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.
0.99995 0.99990
9
10
11
12
13
14
15
DCA, F [ Effect of DCA on net turbine heat rate – Heater 7 (500 MW) cycle ]
16
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 8 th 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
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Data is provided based on a GE steam turbine having output of 412 MW and main steam condition of 2,400 psig/1,000 F/,1000F. The turbine is a reheat, tandem compound, four-flow with 26LSB.
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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.
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Characteristic curves
Correction curves
• Extraction stage shell pressures versus flow to the
following stage • Gland leakage and mechanical losses • Expansion lines
• Throttle pressure correction
• HP turbine internal efficiency
• Throttle temperature correction
• HP turbine expansion line end points
• Reheat pressure drop correction
• Reheat turbine internal efficiency • Reheat turbine expansion line end points
• Reheat temperature correction • Exhaust pressure correction factors
• Correction to expansion line end points • Exhaust loss curve • Generator losses • First-stage shell pressure versus throttle flow
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.
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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.
sT
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.
Pressure Drop through Control Valves
hT
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 are used in conjunction withcurves a heator balance and thelines extraction stage shell pressure constants to establish the extraction pressure at which to read the expansion line enthalpy for a given extraction point in the turbine.
pT pB
Exit from Governing Stage
TT p1 p1
AE
Parallel Expansion Line Partial Flow Expansion Line
AE ELEP
Design Flow Expansion Line
pX
ELEP hXS
pX
hXS
[ Expansion lines for HP turbine ]
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p0
h
p1 T0
1
p2
2 y g r e n E e l b ila a v A
p3 p4
3 4
p5
5
p6
6
s
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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.
Valve Loop Basis (True Curve)
Turbine heat balance developed on the basis of this assumption are considered to be on a locusof-valve best points basis. This heat balances describe heat rates assuming an infinite number of small valves having a 3% pressure drop.
e t a r t a e H
Mean of Valve Loop Basis
Valve Point Basis (Locus-of-valve best points) 0
20
40
60
80
100
Generator output
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.
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.
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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.
sm sIV hm
hIV
pIV
pB
Pressure Drop Through Intercept Valves, 2% TIV
For Machines with No Leakage Entry
In addition, for combined HP/IP turbines, the steam leakage from the HP turbine is mixed with the hot reheat
hm = hIV sm = sIV
steam to determine the reheat bowl steam conditions. Expansion Line
AE
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.
pX= 1.5 in.Hga pX= 1.0 in.Hga
ELEP1.5 in.Hga hXS
A 2% allowance for crossover pipe pressure drop is typically included by the turbine manufacturer to determine the LP turbine inlet conditions.
ELEP1.0 in.Hga ELEP 1.5 in.Hga to
1.0 in.Hga
[ Expansion lines for reheat turbines ]
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.
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85
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 thebest curve is drawn usingdoes the assumption of locusof-valve point, the curve 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.
% , 80 y c n ie75 c if f lE70 a rn te65 In e60 in b r u T55 P H 50 .1
.2
.3
.4
.5
.6
.7
.8
.9
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.
However, as load is decreased, a greater portion of turbine flow becomes throttled, further impacting turbine efficiency.
1.0
Throttle Flow Ratio (TFR)
For off-rated steam conditions use equivalent TFR.
TFReq
Off rated flow Design flow
( p / ) rated ( p / ) off rated
[ HP turbine internal efficiency, GE ]
HIoPE
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.
The throttle flow ratio at first admission point with respect to the number of admission is approximately as follows: 1 admission (full throttling) ------
1.0
2 admissions -----------------------
0.85
3 admissions -----------------------
0.60
4 admissions -----------------------
0.35
8 admissions -----------------------
0.30
[ Throttle flow ratio at first admission point, GE ]
HIoPE
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.
1330 l/b1320 tu B , P1310 E L E e in1300 b r u T P1290 H
1280 1.0
1.5
2.0 3.0 4.0 3.5 4.5 2.5 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
HIoPE
94
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.
l a m r e h t e n i b r u t t a e h e R
93
Reheat Temp., F 1000 950 900 850
% 92 ,y c n 91 e i ifc f e 90
800 750 700
89 88 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Intercept valve pressure, 100 psia 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 [ Reheat turbine internal efficiency, GE ]
HIoPE
1070
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 conditionsitfor rated however, is not asreheat flexiblesteam as thetemperature; internal efficiency curve because end points are valid only for 1,000F reheat steam.
b /l tu 1060 B , a g 1050 .H n i 5 . 1040 1 t a s P 1030 E L E 1020 e n i rb 1010 u T t a e h 1000 e R 990
1
2
3
4
5
6
7
8
9
Intercept Valve Pressure, 100 psia 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.
HIoPE
0.85
-32 1.5 0 +62 -64 b /l 2.5 u t -62 +32 4.0 +64 -30 +2 B , -28 +4 +34 4.1 +66 -60 re 0.5 0.9 4.2 u t -58 -261.6 +6 2.6 +36 4.3 +68 is o -56 -24 +8 2.7 +38 4.4 +70 M 4.5 +72 -54 -22 1.7 +10 % 2.8 +40 1.0 o 4.6 -52 +42 +74 -20 +12 h 4.7 1.8 3.0 it -50 +14 +44 -18 w 4.8 +76 3.1 P 0.6 4.9 +16 +46 1.1 -16 -48 E 5.0 1.9 L 3.2 E -46 -14 +18 +48 in 3.3 +50 2.0 +20 -44 -12 e 1.2 g 3.4 +52 -42 -10 n +22 2.1 a 0.7 h -8 +24 3.5 +54 -40 C 2.2 +26 1.3 -6 -38 3.6 +56 0 = Y
P E L E
-36 0.8 -34 0.85
-4 1.4 -2 0 1.5
2.3 +28 +30 2.4 2.5
3.7
+58 3.8
+60 3.9 +62 4.0
Exhaust Pressure, in.Hga 1. When ELEP at 1.5 in.H ga is in the moisture re gion, use the foll owing 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 th e 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.
HIoPE
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.
Annulus Restriction Loss
50
y r d f o b l/ u t B , s s o L t s u a h x E
40
Gross Hood Loss
30 Turn-up Loss
w lfo
Total Exhaust Loss
20 Actual Leaving Loss
10
0
0
200
400
600
800
1000
Annulus Velocity, fps
1200 1400 Sonic
1600
HIoPE
50
3 4 5
2
Curve no. 1 1 1 1 2 3 4 5
46 42 38
w o fl 34 ry d f 30 o u t/lb 26 B , s s 22 o L t 18 s u a h x 14 E
Bucket length (inches) 14.3 16.5 17 20 23 26 30 33.5
Pitch diameter (inches) 52.4 57.5 52 60 65.5 72 85 90.5
23 4 5
1
Van = Annulus velocity (fps) m = Condenser flow (lb/hr)
= Saturated dry specific volume (ft 3/lb)
Aan = Annulus area (ft 2) Y = Percent moisture at ELEP
1
ELEP = Expansion line end point at actual exhaust pressure (Btu/lb) UEEP = Used energy end point (Btu/lb) (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:
10
UEEP = ELEP + (Exhaust loss)(0.87)(1-0.01 Y)(1-0.0065Y)
6 2
Last stage annulus area single flow (ft 2) 16.3 20.7 19.3 26.2 32.9 41.1 55.6 66.1
(3) This exhaust loss includes the loss in internal effi ciency which occurs at light flows as obtained in tests.
0
200
400
600
800
1000
Annulus Velocity, ft/s
1200
1400
HIoPE
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.
7
0 0 0 , 1 , s s o L r o t a r e n e G
6 5
W k 4
3 2
0
50
100
150
200 250
300
350
400
450
500
Generator Output, 1,000 kVA
Generator losses do not include the turbine generator fixed or mechanical losses.
511,000 kVA at 45 psig H2 pressure Conductor cooled 3600 rpm
The mechanical losses should be accounted for separately and do not vary with unit load.
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.
Note:
550
HIoPE
2000
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.
th _ corrected m th _ corrected m th _ actual m
th _ actual m
actual
1500 a i s p , e r 1000 u s s e r P 500
0
0
design
corrected turbine throttle flow, lb/hr
actual turbine throttle flow, lb/hr
actual actual throttle steam specific volume, ft 3/lb design design throttle steam specific volume, ft 3/lb
500
1000
1500
2000
Flow, lbs/hr x 1,000
2500
3000
HIoPE
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.
HIoPE
Change in Kilowatt Load, %
Method of Using Curves
6
¼ Load These correction factors assume constant control ½ Load valve opening and are to be applied to heat rates Rated Load and kilowatt loads at rated steam conditions.
5 4
% , e s 3 a e r 2 c In
1. The heat rate at desired condition can be found by multiplying the heat rate at rated conditions by the following: 1 + % change in gross heat rate 100 2. The kilowatt load at the desired conditions can
1 -5
0 1
Rated Load ½ Load ¼ Load
Change in initial D pressure, %
be found by multiplying the kilowatt load at rated conditions by the following: 1 + % change in kilowatt load 100 3. These correction factors are not guaranteed.
+5
2 e c re 3 a s e , 4 % 5 6
Change in Heat Rate, % 2
¼ Load ½ Load Rated Load
Poorer, %
1 +5 -5
Change in initial pressure, %
1 2
Better, %
Rated Load ½ Load ¼ Load
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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.
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Change in Kilowatt Load, % 2
½ Load ¼ Load Rated Load -50
Method of Using Curves
Increase, %
1 +50
Change in initial temperature, F
Rated Load ¼ Load ½ Load
1 2
Decrease, %
Change in Heat Rate, % 2
Rated Load
Poorer, %
1
¼ Load -5
+5
Change in initial temperature, F
1
¼ Load Rated Load
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: 1 + % change in gross heat rate 100 2. The kilowatt load at the desired conditions can be found by multiplying the kilowatt load at rated conditions by the following: 1 + % change in kilowatt load 100 3. These correction factors are not guaranteed.
2
Better, %
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.
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Change in Kilowatt Load, % 2
Rated Load ¼ & ½ Load
The decrease in reheater pressure drop results in an increase in turbine output and a decrease in turbine heat rate.
Increase, %
1 10 5
Change in initial temperature, F
15
1
¼ & ½ Load Rated Load
2
Decrease, %
HP turbine output increases, and the HP turbine expansion end point becomes lower.
Change in Heat Rate, % Poorer, %
2 1
As a result, the reheater duty increases.
All Loads
5
All Loads
10
15
1
Reheater pressure drop, %
2
Better, %
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Change in Kilowatt Load, %
Increase, %
The increased reheat temperature results in increased turbine output and decreased turbine heat rate.
Rated Load ½ Load ¼ Load
3 2 1
-50
0 +50
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.
Change in reheat temperature, F
1
Rated Load ½ Load ¼ Load
2 3
Decrease, %
Change in Heat Rate, % 2
However, the reduced HP turbine output is offset by the greater available steam energy supplied to the reheat turbine and its increased turbine output.
Rated Load ½ Load ¼ Load
Poorer, %
1 0
-50
Change in reheat temperature, F
1 2
Better, %
+50
Rated Load ½ Load ¼ Load
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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.
12 697,935 lb/h, 25%
8 % , te a R4 t a e H n i 0 e s
1,355,163 lb/h, 50% 2,846,828 lb/h, 100% 2,049,969 lb/h, 75%
2,980,170 lb/h, VWO-NP
a re c n I -4
-8 0
1
4 2 3 Exhaust Pressure, in.Hga
5
Values near curves are flows at 2400 psig, 1000 F. 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.
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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.
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18 6F32
16
"
14 W12 M , in 10 a G r 8 e w o 6 P 4
4F38 Choking
"
4F32
"
4F30
2
"
0 0.5 0
1.0 2
12
1.5
1.76
16
19
Cooling Water Inlet Temperature
2.0 in.Hga C
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작성자: 이 병 은 (공학박사) 작성일: 2015.02.11 (Ver.5) 연락처:
[email protected] Mobile: 010-3122-2262 저서: 실무 발전설비 열역학/증기터빈 열유체기술
