  (1)
The steam parameters of the LP turbine extractions
can be determined by similar method. First calculate the
extraction steam flow for the LP heaters, and calculate
the new corrected extraction pressure. The new extrac-
tion parameters can be found using the corrected extrac-
tion pressure and the same LP turbine expansion line.
This iteration process continues until the parameter dif-
ference of the two iterations is lower than certain error.
Based on the LP turb ine exhaust flow calculated in the
above Simplified Variable Operation Condition Analysis,
and the last stage exhaust loss curve provided by the tur-
bine manufacture, the LP turbine last stage exhaust loss
and the LP turbine used en ergy end point enth alpy UEEP
can be calculated, and the final generator output change
can be calculated using the heat balance method.
This Simplified Variable Operation Condition Analy-
sis Method considers both the LP turbine last stage ex-
haust loss change and the extraction steam flow change
of the LP heaters for turbine back pressure change condi-
This Simplified Variable Operation Condition Analy-
sis Method is used in [2] for analyzing the generator
output change with the turbine back pressure. The calcu-
lation result matches the real situation for one 600MW
sub-critical generation unit.
3. Calculation Example of Turbine Choked
Back Pressure for Steam Turbine with
For one example generation unit, the 660MW steam tur-
bine is a critical, single-shaft, two-casing two-exhaust
Figure 1. LP turbine expansion line under different LP tur-
bine back pressure operation conditions.
condensing steam turbine with ACCs. There are totally
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
The turbine choked back pressure result is shown in Ta-
ble 2 and Figure 3.
28 stages in the turbine, including 1 control stage and 9
pressure stages for the HP turbine, 6 pressure stages for
the IP turbine, and 2×6 pressure stages for the LP turbine.
The LP turbine last blade has a height of 939.8 mm. The
steam turbine is controlled by nozzles. The steam flow
into the four nozzle groups on the HP turbine is con-
trolled by the four mail control valves. The ACC system
is the mechanically drafted direct air-cooled system pro-
duced by the domestic manufacture.
Table 1. Main design data of the steam turbine.
Item Unit THA TRL
Unit Output kW 660.01660.00
Unit Heat Rate kJ/kWh 7778.318225.81
Main Steam Pressure MPa(a) 24.20 24.20
Hot Reheat Steam Pressur e MPa(a) 3.71 3.94
HP Turbine Exhaust Pr e ssure MPa(a) 4.12 4.38
Main Steam Temperature 566.00 566.00
Hot Reheat Steam Temperature 566.00 566.00
HP Turbine Exhaust Temperature 305.07 312.13
Main Steam Flow t/h 1878.422013.35
Reheat Steam Flow t/h 1600.901701.48
Turbine Back Pressure kPa 16 33
LP Turbine Exhaust Entha l py kJ/kg 2454.672550.0
LP Turbine Exhaust F l ow t/h 1254.781349.40
Makeup Water % 0 3
Final Feedwater Temperature 273 277.2
The main design data of the steam turbine is summa-
rized in Table 1.
3.1. The Calculation Result of the Steam
Turbines Choked Back Pressure
Based on the turbine last stage exhaust loss curve, the
turbine exhaust steam used energy end point enthalpy
and the exhaust loss and the final generator output are
calculated for various turbine back pressure operation
conditions. For each load condition, the turbine back
pressure corresponding to the maximum generator output
can be regarded as the turbine choked back pressure.
Using this method, the turbine choked back pressures
under different operation load conditions are calculated.
On Figure 2, the generator output change with the back
pressure is shown for various operation load conditions.
051015 20 25 3035 40
Turbine Back P re s sure (kP a )
Unit Output Relative Change (
Figure 2. Performance curves of the unit output with turbine back pressure.
H. S. YANG ET AL. 1477
40 5060 70 8090100110120
Unit L oad Rate (%)
T urbine Choked Back Pr essur e (kP a
Sub-Critica l 600MW Unit with Wet Cooling Techno logy
Super-Critical 600MW Unit with ACC
(Sb C iil 600MWUiih WCliThl)
Figure 3. Performance curves of the turbine choked back pressure with unit load.
Table 2. Calculation results of choked back pressure under
various operation loads.
Operation Load (%) Turbine Choked Back Pressure (kPa)
100 9.0
90 8.2
75 7.0
50 5.0
For comparison purpose, the choked back pressure for
600 MW four-casing four-exhaust sub-critical generation
unit with wet cooling technolog y is illustrated on Figure
3. It can be shown from the Table 2 and Figure 3 that
the turbine choked back pressure is in nearly linear rela-
tionship with the operation loads. For the choked back
pressure curves, there is a significant increasing of the
chocked pressure values for the units with ACCs com-
pared with the similar curve of the units with wet cooling
4. Conclusions
Based on the clear definition of the turbine choked back
pressure, the solution of the choked back pressure is
changed into a problem of how to determine the turbine
power output change as a function of the turbine back
pressure change.
The proposed simplified variable operation condition
analysis method is adopted for one sub-critical genera-
tion unit with ACCs. The performance curve of the
power output versus the turbine back pressure under
various operation loads is given, and curve of the turbine
choked back pressure under different loads is provided.
These curves provide valuable guidance for the economic
operatio n of t he power plant u ni t s .
[1] H. Lin, “Calculation and Correction of the Impact of Tur-
bine Back Pressure on Turbine Output,” Turbine Tech-
nology, Harbin China, Vol. 46 No. 1, 2004, pp. 18-21.
[2] H. S. Yang, “Simplified Calculation Method of Turbine
Back Pressure Correction Curves,” Turbine Technology.
Harbin China, Vol. 49, No. 1, 2007, pp. 32-34.
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