1. Komandur Sunder Raj
Power & Energy Systems Services
Last Stage Performance
Considerations in LP
Turbines of Power Plants –
A Case Study

2. Factors Affecting Last Stage
Performance of LP Turbines
Case Study – Relationships Between
Last Stage, Unit Rating, End Loading
& Operating Range of Condenser
Last Stage Exhaust Loss Curves
Recommendations for Optimum
Performance
Objectives

3. Last Stage Blades (LSB) Historically
Specified & Designed by Matching
Turbine to Condenser Cooling System
for Specified Unit Rating
Last Stage End Loadings Vary from 8000
lb/hr-sq.ft to 14000 lb/hr-sq.ft
Corresponding Unit Loadings 1.1 to 2.1
Mwe/sq.ft
Background

4. Several Plants Upgrading/Replacing LP
Turbines
Considerations Include Efficiency,
Reliability, SCC, Power Uprates, Aging,
Inspection, Maintenance, etc.
New LP Rotors, Blading, Steam Paths,
Longer LSB, Redesigned Exhaust Hoods
Background

5. Condensers Typically Have Not Been
Upgraded to Match Turbine Retrofits,
Modifications
Potential Mismatch Between Upgraded
LP Turbine and Condenser/Cooling
System
Background

6. For LSB Sizes 38” to 52”, Pitch
Diameters 10’ to 13’
Blade Velocities 1000-1300 fps for 1800
RPM
Last Stage Performance Function of
Load & Pressure Ratio
Last Stage Output Reaches Maximum at
Critical Pressure Ratio (Choked Flow)
General

7. Exhaust Loss Curve Composite of:
Leaving Loss (Kinetic Energy)
Pressure Loss in Exhaust Hood
Turn-Up Loss Occurring at Low Annulus
Velocities
Choke Conditions for Diffusing Hoods
Occur at Axial Mach Nos. 0.87-0.90
(Annulus Velocity of ~1058 fps)
General

8. Figure 1 – Exhaust Loss Curves
0
5
10
15
20
25
30
35
40
45
50
55
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Annulus Velocity, Fps
DryExhaustLoss,Btu/lb
43" 44"
52"
40"
44"
47"
45"
Appoximate
Choke Point

9. Figure 2 – Annulus Velocity for Different
End Loadings, Exhaust Pressures
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
Exhaust Pressure, in.HgA
LastStageAnnulusVelocity,fps
Condenser Operating Range
52 "LSB
40 "LSB
43 "LSB
47 "LSB

10. Case Study
1100 Mwe Power Plant (TC6F-43”LSB)
Condenser Operating Range 2.0 - 4.0
in.HgA (35.0 F to 80.0 F Circulating Water
Inlet Temperature)
Design Pressure 2.3 in.HgA at 52.0 F
Once-Through Cooling
End Loading ~ 11,000 lb/hr-sq.ft
General

11. Table 1 – Last Stage Performance for 43”LSB
(1) (2) (3) (4) = (3)/(2) (5) (6) (7) (8) = (5) - (7) (9) (10)
2.0 4.85 0.98 0.203 1044.2 1.70763 953.0 91.2 967.9 8,093,233
2.5 4.89 1.23 0.251 1044.5 1.70726 964.7 79.8 977.3 8,154,128
3.0 4.92 1.47 0.299 1044.7 1.70702 974.4 70.3 985.2 8,205,506
3.5 4.95 1.72 0.347 1044.9 1.70675 982.8 62.2 991.9 8,250,172
4.0 4.98 1.96 0.394 1045.1 1.70643 990.0 55.0 997.9 8,289,834
(11)
(12) = (10) x
(11)
(13) See
Note 1
(14) (15)
(16) = {(5) -
(15)}/(8)
(17) See
Note 2
(18)
(19) =
{(17)/(18)} x
100
(20) =
(10)/742.8
294.206 2.381E+09 890 21.1 982.5 67.70% 146.4 1128.851 12.97% 10,896
239.937 1.956E+09 732 12.5 986.0 73.26% 139.7 1121.771 12.46% 10,978
203.160 1.667E+09 623 8.5 991.1 76.20% 128.8 1110.617 11.60% 11,047
176.526 1.456E+09 545 7.1 997.0 77.13% 115.9 1097.568 10.56% 11,107
156.312 1.296E+09 485 7.2 1003.0 76.47% 102.3 1083.781 9.44% 11,160
NOTES:
1. Annulus area per last stage is 123.8 sq.ft. For six exhaust ends, total last stage annulus area is 742.8 sq.ft.
Annulus Velocity, fps = Last Stage Volumetric flow/(742.8 x 3600)
2. Last Stage Output, Mw = Last Stage Flow x (Last Stage Bowl Enthalpy - Used Energy End Point)/3412141.63
End
Loading,
lb/hr-sq.ft.
Last Stage
Annulus
Velocity Van,
fps
Dry
Exhaust
Loss EL,
btu/lb
Used
Energy
End
Point
UEEP,
btu/lb
Last Stage
UEEP
Efficiency,
%
Generator
Output,
Mwe
Last Stage
Specific
Volume v,
cft/lb
Last
Stage
Exhaust
Flow W,
lb/hr
Cond.
Press.,
in.HgA
Expansion
Line End
Point
ELEP,
btu/lb
Last Stage
Press.
Ratio,
PSh/PB
Last Stage
Output, %
of Gen.
Ouput
Last Stage
Bowl Press.
PB, psia
Last Stage
Bowl
Entropy
SB, btu/lb
Last Stage
Available
Energy AE,
btu/lb
Isentropic
Enthalpy
HISEN,
btu/lb
Last Stage
Shell Press.
PSh, psia
Last
Stage
Bowl
Enthalpy
HB,
btu/lb
Last Stage
Volumetric
Flow Wv,
cft/hr
Last Stage
Output,
Mw

12. Table 2 – Last Stage Performance for 52”LSB
(1) (2) (3) (4) = (3)/(2) (5) (6) (7) (8) = (5) - (7) (9) (10)
2.0 4.85 0.98 0.203 1044.1 1.70753 953.0 91.2 967.9 8,093,667
2.5 4.89 1.23 0.251 1044.4 1.70716 964.6 79.8 977.3 8,153,834
3.0 4.92 1.47 0.299 1044.7 1.70695 974.4 70.3 985.2 8,204,912
3.5 4.95 1.72 0.347 1044.9 1.70670 982.7 62.1 991.9 8,249,484
4.0 4.98 1.96 0.394 1045.1 1.70643 990.0 55.0 997.9 8,289,181
(11)
(12) = (10) x
(11)
(13) See
Note 1
(14) (15)
(16) = {(5) -
(15)}/(8)
(17) See
Note 2
(18)
(19) =
{(17)/(18)} x
100
(20) =
(10)/1055
294.190 2.381E+09 627 10.0 974.7 76.10% 164.6 1146.978 14.35% 7,672
239.924 1.956E+09 515 10.5 984.6 75.00% 143.0 1125.284 12.71% 7,729
203.151 1.667E+09 439 12.0 993.6 72.68% 122.8 1104.932 11.12% 7,777
176.522 1.456E+09 383 13.7 1001.6 69.67% 104.7 1086.645 9.63% 7,819
156.310 1.296E+09 341 15.2 1008.7 66.18% 88.5 1070.27 8.27% 7,857
NOTES:
1. Annulus area per last stage is 175.8333 sq.ft. For six exhaust ends, total last stage annulus area is 1055 sq.ft.
Annulus Velocity, fps = Last Stage Volumetric flow/(1055 x 3600)
2. Last Stage Output, Mw = Last Stage Flow x (Last Stage Bowl Enthalpy - Used Energy End Point)/3412141.63
End
Loading,
lb/hr-sq.ft.
Last Stage
Annulus
Velocity Van,
fps
Dry
Exhaust
Loss EL,
btu/lb
Used
Energy
End
Point
UEEP,
btu/lb
Last Stage
UEEP
Efficiency,
%
Generator
Output,
Mwe
Last Stage
Specific
Volume v,
cft/lb
Last
Stage
Exhaust
Flow W,
lb/hr
Cond.
Press.,
in.HgA
Expansion
Line End
Point
ELEP,
btu/lb
Last Stage
Press.
Ratio,
PSh/PB
Last Stage
Output, %
of Gen.
Ouput
Last Stage
Bowl Press.
PB, psia
Last Stage
Bowl
Entropy
SB, btu/lb
Last Stage
Available
Energy AE,
btu/lb
Isentropic
Enthalpy
HISEN,
btu/lb
Last Stage
Shell Press.
PSh, psia
Last
Stage
Bowl
Enthalpy
HB,
btu/lb
Last Stage
Volumetric
Flow Wv,
cft/hr
Last Stage
Output,
Mw

13. Table 3 – Impact of 15% Power Uprate on
Last Stage Performance for 43”LSB
(1) (2) (3) (4) = (3)/(2) (5) (6) (7) (8) = (5) - (7) (9) (10)
2.76 6.01 1.36 0.226 1039.5 1.67891 953.8 85.7 968.1 9,576,736
2.85 6.02 1.40 0.233 1039.5 1.67871 955.5 84.1 969.5 9,588,814
3.25 6.07 1.60 0.263 1039.5 1.67783 962.0 77.5 974.7 9,637,186
3.95 6.14 1.94 0.316 1039.4 1.67649 971.9 67.5 982.6 9,711,370
5.00 6.24 2.46 0.394 1039.3 1.67488 984.1 55.2 992.4 9,804,425
(11)
(12) = (10) x
(11)
(13) See
Note 1
(14) (15)
(16) = {(5) -
(15)}/(8)
(17) See
Note 2
(18)
(19) =
{(17)/(18)} x
100
(20) =
(10)/742.8
215.802 2.067E+09 773 14.5 978.0 71.79% 172.7 1268.413 13.62% 12,893
209.450 2.008E+09 751 13.4 978.6 72.49% 171.3 1266.769 13.52% 12,909
185.840 1.791E+09 670 9.9 981.5 74.81% 163.7 1258.675 13.01% 12,974
155.439 1.510E+09 565 7.3 987.7 76.60% 147.2 1241.382 11.86% 13,074
125.309 1.229E+09 459 7.6 997.7 75.32% 119.6 1212.802 9.86% 13,199
NOTES:
1. Annulus area per last stage is 123.8 sq.ft. For six exhaust ends, total last stage annulus area is 742.8 sq.ft.
Annulus Velocity, fps = Last Stage Volumetric flow/(742.8 x 3600)
2. Last Stage Output, Mw = Last Stage Flow x (Last Stage Bowl Enthalpy - Used Energy End Point)/3412141.63
End
Loading,
lb/hr-sq.ft.
Last Stage
Annulus
Velocity Van,
fps
Dry
Exhaust
Loss EL,
btu/lb
Used
Energy
End
Point
UEEP,
btu/lb
Last Stage
UEEP
Efficiency,
%
Generator
Output,
Mwe
Last Stage
Specific
Volume v,
cft/lb
Last
Stage
Exhaust
Flow W,
lb/hr
Cond.
Press.,
in.HgA
Expansion
Line End
Point
ELEP,
btu/lb
Last Stage
Press.
Ratio,
PSh/PB
Last Stage
Output, %
of Gen.
Ouput
Last Stage
Bowl Press.
PB, psia
Last Stage
Bowl
Entropy
SB, btu/lb
Last Stage
Available
Energy AE,
btu/lb
Isentropic
Enthalpy
HISEN,
btu/lb
Last Stage
Shell Press.
PSh, psia
Last
Stage
Bowl
Enthalpy
HB,
btu/lb
Last Stage
Volumetric
Flow Wv,
cft/hr
Last Stage
Output,
Mw

14. Table 4 – Impact of 15% Power Uprate on
Last Stage Performance for 52”LSB
(1) (2) (3) (4) = (3)/(2) (5) (6) (7) (8) = (5) - (7) (9) (10)
2.74 6.01 1.35 0.224 1039.5 1.67891 953.5 86.1 967.8 9,574,576
2.84 6.02 1.40 0.232 1039.5 1.67873 955.2 84.3 969.3 9,587,724
3.26 6.07 1.60 0.264 1039.5 1.67782 962.1 77.4 974.7 9,637,599
3.98 6.14 1.95 0.318 1039.4 1.67646 972.2 67.1 982.9 9,713,204
5.05 6.24 2.48 0.398 1039.3 1.67483 984.6 54.7 992.8 9,807,062
(11)
(12) = (10) x
(11)
(13) See
Note 1
(14) (15)
(16) = {(5) -
(15)}/(8)
(17) See
Note 2
(18)
(19) =
{(17)/(18)} x
100
(20) =
(10)/1055
217.219 2.080E+09 548 10.1 974.7 75.29% 181.8 1277.477 14.23% 9,075
210.288 2.016E+09 531 10.3 976.3 75.04% 177.8 1273.083 13.96% 9,088
185.559 1.788E+09 471 11.2 982.5 73.63% 161.0 1256.005 12.82% 9,135
154.451 1.500E+09 395 13.3 992.1 70.42% 134.6 1229.071 10.95% 9,207
124.251 1.219E+09 321 16.0 1003.9 64.63% 101.7 1195.266 8.51% 9,296
NOTES:
1. Annulus area per last stage is 175.8333 sq.ft. For six exhaust ends, total last stage annulus area is 1055 sq.ft.
Annulus Velocity, fps = Last Stage Volumetric flow/(1055 x 3600)
2. Last Stage Output, Mw = Last Stage Flow x (Last Stage Bowl Enthalpy - Used Energy End Point)/3412141.63
End
Loading,
lb/hr-sq.ft.
Last Stage
Annulus
Velocity Van,
fps
Dry
Exhaust
Loss EL,
btu/lb
Used
Energy
End
Point
UEEP,
btu/lb
Last Stage
UEEP
Efficiency,
%
Generator
Output,
Mwe
Last Stage
Specific
Volume v,
cft/lb
Last
Stage
Exhaust
Flow W,
lb/hr
Cond.
Press.,
in.HgA
Expansion
Line End
Point
ELEP,
btu/lb
Last Stage
Press.
Ratio,
PSh/PB
Last Stage
Output, %
of Gen.
Ouput
Last Stage
Bowl Press.
PB, psia
Last Stage
Bowl
Entropy
SB, btu/lb
Last Stage
Available
Energy AE,
btu/lb
Isentropic
Enthalpy
HISEN,
btu/lb
Last Stage
Shell Press.
PSh, psia
Last
Stage
Bowl
Enthalpy
HB,
btu/lb
Last Stage
Volumetric
Flow Wv,
cft/hr
Last Stage
Output,
Mw

15. Figure 3 – Performance of 43”LSB &
52”LSB Before and After Power Uprate
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
Exhaust Pressure, in.HgA
LastStageAnnulusVelocity,fps
Original Range for 52 "LSB
Power Uprate Range for 43 "LSB
Original Range for 43 "LSB
Power Uprate Range for 52 "LSB
Original Operating Range
Power Uprate Operating Range

16. For Optimum Last Stage Performance,
Evaluate Performance Over Entire Operating
Range
Consider Mods if Evaluation Reveals Mismatch
or Less than Optimum Performance
For Power Uprates, LP Turbine Mods with
Different LSB Offerings, Evaluate Performance
Over Entire Operating Range
Consider Condenser/Cooling System Mods to
Properly Match LSB
Recommendations