(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.

h

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-

tions.

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

ACCs

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

H. S. YANG ET AL.

Copyright © 2013 SciRes. EPE

1476

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.

-10

-8

-6

-4

-2

0

2

4

6

8

051015 20 25 3035 40

Turbine Back P re s sure (kP a )

Unit Output Relative Change (

%

THA

90%THA

75%THA

50%THA

Figure 2. Performance curves of the unit output with turbine back pressure.

H. S. YANG ET AL. 1477

0

2

4

6

8

10

12

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

线性

(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

technology.

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 .

REFERENCES

[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.

Copyright © 2013 SciRes. EPE