Energy and Power Engineering, 2013, 5, 274-277
doi:10.4236/epe.2013.54B053 Published Online July 2013 (http://www.scirp.org/journal/epe)
Analysis of the Unit Performance Degradation for Top
Heater Out-of-service Operation Condition
Haisheng Yang, Shuping Chang, Ruitao Wu
Thermal Technology Depart, Hebei Electric Power Research Institute, Shijiazhuang, Hebei PRC
Received September, 2012
ABSTRACT
To analyze the unit performance degradation for top heater out-of-service operation condition, two calculation methods
were introduced, that was the model based calculation method and the simplified variable operation condition analysis
method. A 600MW sub-critical steam turbine unit with air cooled condenser was analyzed using the two introduced
methods. It is shown that calculation data got can reflect the redistribution effect on steam flows of the turbine extrac-
tions and turbine internals due to the top heater out-of-service, and similar and close calculation results were got from
the two methods, and both results were lower than result calculated using the commonly used equivalent enthalpy drop
method.
Keywords: Top Heater; Out-of-service; Performance Degradation; Analysis Method
1. Introduction
Presently the Equivalent Enthalpy Drop Method is com-
monly used in the performance quantitative analysis of
the steam turbine units. For the top heater out-of-service
operation condition for the steam turbine unit, the quan-
titative analysis equation in [1] is widely used to deter-
mine the effect on unit performance.
In Ref [2], the impact of top heater out-of-service con-
dition was analyzed under the consumption that the top
heater out-of-service operation was a small disturbance
to the normal operation conditions. In Ref [3], the impact
of top heater out-of-service condition was calculated by
the Variable Operation Condition Analysis Method and
Matrix Method (based on the small disturbance con-
sumption), and it was concluded that there was minor
difference between the two calculated results.
Theoretically, the Equivalent Enthalpy Drop Method is
based on the certain assumption, that is the parameters of
the live steam, reheat steam, the turbine end, and the ex-
tractions are kept constant, or the change of the turbine
expansion line is not considered. The mentioned assump-
tion is the precondition for establishing the concept and
equations of the Equivalent Enthalpy Drop Analysis
Method.
Under practical operation conditions, when the top
heater is out-of-service, there is a dramatic change of the
unit operation condition and turbine internal flow and
parameters. Thus, the Equivalent Enthalpy Drop Method
is not very applicable for analysis of this operation con-
dition change.
2. Two Calculation Methods for Analyzing
the Impact of Top Heater Out-of-Service
Condition
To analyze the unit performance degradation for top
heater out-of-service operation condition, two calculation
methods are introduced, that is the Model-based Calcula-
tion Method and the Simplified Variable Operation Con-
dition Analysis Method.
2.1. The Model-based Calculation Method for
Thermal System
During normal operation, the operation parameters and
the equipment performance are inter-related and coupled.
The model-based calculation method can reflect the cou-
pling relationship between the equipment performance
and the operation conditions, thus provide a clear picture
of the influence between the equipment and boundary
conditions, especially for operation conditions with large
scale change of operation parameters, equipment per-
formance and operation loading.
Model-based calculation software such as GateCycle,
can provide analysis for all various changes of the opera-
tion parameters and equipment performance degradation.
This method can consider the change factors and its re-
lated effect on the thermal system, thus providing an
complete variable Operation Condition Analysis for the
system.

H. S. YANG ET AL. 275
Advantages of the Model-based Calculation Method.
Some advantages of using the model based analysis or
calculation method are listed below:
a) Interaction of varying operating conditions can be
modeled: Models can allow wide variations in operation
parameters from the rated conditions.
b) Model-based calculation can compute impacts of
parameters for which no curves are available.
c) Model-based calculation can handle various equip-
ment and thermal systems, such as fossil-fired generation
unit, combined cycle generation unit, cogeneration unit,
and advanced gas turbine generation unit.
d) Model-based calculation can share information be-
tween different operation conditions, such as design and
off-design conditions.
The typical process of model-based performance anal-
ysis includes the following steps:
a) Model building: Build a model of the equipment or
system being monitored.
b) Model testing: Test the model versus manufacture
guarantee data or measured data over a wide range of
operation conditions. Correct the model where necessary.
c) Data input: Input the measured equipment operation
conditions into the model.
d) Run the model: obtain the expected equipment or
system performance model output.
e) Evaluate degradation: comparing the expected per-
formance from the model calculation to the measured
performance.
2.2. Simplified Variable Operation Condition
Analysis Method for Thermal System
The Simplified Variable Operation Condition Analysis
Method handles the steam turbine into stage groups
based on turbine extractions. The turbine extraction pa-
rameters under variable operation condition can be de-
termined if the steam turbine expansion process is deter-
mined. Thus the thermal performance of the system can
be calculated.
First, based on the operation data of the baseline op-
eration condition, the flow coefficient for turbine extrac-
tions can be calculated as:
/
w
Cp

 (1)
where, C is the flow coefficient; is the steam flow
upstream of the turbine extractions; p is the pressure up-
stream of the turbine extractions; v is the steam specific
volume upstream of the turbine extractions. If the extrac-
tion temperature change is relatively minor during the
operation condition change, the flow coefficient can be
simplified as the ratio of the steam flow to the pressure.
w
Secondly, the extraction parameters under variable
operation condition can be calculated based on the as-
sumed turbine expansion line (related to turbine effi-
ciency), the changed steam flow, and the determined
flow coefficient of the extractions. The thermal per-
formance of the system can be calculated.
Based on the new steam flow distribution data, the
turbine extraction parameters can be corrected until the
bias between the two iterations is within a set limit.
Finally, based on the exhaust steam flow result from
the previous step, and the exhaust loss curve provided by
the manufacture, the exhaust loss of the turbine last stage
and the turbine used energy of end point (hUEEP) can be
calculated, and the unit power output can be determined
by energy balance calculation.
The above mentioned calculation process is similar to
the correction process of the calculation example in-
cluded in the Appendix of the ASME PTC 6 standard.
The steam turbine expansion line can be determined
based on the baseline operation data. The turbine casing
efficiency under variable operation condition is set the
same as the baseline operation condition.
3. Calculation Example of Analyzing the
Impact of Top Heater Out-of-Service
Condition
For one example generation unit, the 660MW steam tur-
bine is a sub-critical, single-shaft, four-casing
four-exhaust condensing steam turbine with air-cooled
condensers. Steam turbine is designed with seven extrac-
tions, which supplies steam to three HP heaters, one
deaerator, and three LP heaters. The top HP heater heat-
ing steam is supplied from the HP turbine extraction, and
second HP heater heating steam is supplied from the HP
turbine exhaust, and the third HP heater heating steam is
supplied from the IP turbine extraction.
The main design data of the steam turbine is summa-
rized in Table 1.
Table 1. Main design data of the 600 MW steam turbine.
Item Unit Design Data
Generator Output MW 600
Main Steam Flow Rate t/h 1843.63
Hot Reheat Flow Rate t/h 1584.03
Main Steam Pressure MPa 16.70
Hot Reheat Pressure MPa 3.435
Main Steam/ Hot Reheat Steam Temperature ℃ 538/538
Heat Rate kJ/kWh8080.4
Final Feedwater Temperature ℃ 274.96
Makeup Water Rate % 0
Turbine Back Pressure kPa 16

H. S. YANG ET AL.
276
Table 2. Calculation result by the model-based c alculation method.
Item Unit
Baseline
Condition Top Heater Out-of-Service (Fixed Pressure Mode) Top Heater Out-of-Service (Sliding Pressure Mode)
Generator Output kW 599631 626266 624117
Total Heating Energy kJ/sec -1345921 -1414107 -1409912
Unit Heat Rate kJ/kW.h8080.56 8128.8 8132.4
Relative Change of the Generator Output % 4.4418 4.0835
Relative Change of the Unit Heat Rate % 0.5970 0.6415
Table 3. Calculation result by the simplified variable operation condition analysis method.
Item Unit
Baseline
Condition Top Heater Out-of-Service (Fixed Pressure Mode) Top Heater Out-of-Service (Sliding Pressure Mode)
Mail Steam Flow t/h 1843.628 1843.628 1837.440
Main Steam Pressure MPa 16.700 16.700 16.700
Main Steam Temperature ℃ 538.000 538.000 538.000
Extraction #1 Pressure MPa 5.977 6.412 6.391
Extraction #1 Temperature ℃ 382.163 391.656 391.495
Top HP Heater Outlet Temperature ℃ 274.956 249.862 249.673
Top HP Heater Drain Temperature ℃ 251.414 255.465 255.272
Extraction #1 Flow t/h 126.151 -0.013 0.002
Extraction #2 Pressure MPa 3.817 4.090 4.076
Extraction #2 Temperature ℃ 322.665 331.091 330.960
#2 HP Heater Outlet Temperature ℃ 245.814 249.865 249.672
#2 HP Heater Drain Temperature ℃ 217.253 220.679 220.515
Extraction #2 Flow t/h 126.894 139.889 139.214
HP Turbine Exhaust Pressure MPa 3.817 4.090 4.076
HP Turbine Exhaust Temperature ℃ 322.665 331.091 330.960
HP Turbine Exhaust Flow t/h 1584.031 1697.201 1691.671
HP Turbine Efficiency 0.884 0.885 0.883
Hot Reheat Steam Pressure MPa 3.435 3.680 3.668
Hot Reheat Steam Temperature ℃ 538.000 538.000 538.000
Extraction #3 Pressure MPa 2.032 2.174 2.167
Extraction #3 Temperature ℃ 456.393 456.050 456.065
#3 HP Heater Inlet Temperature ℃ 184.112 186.991 186.850
#3 HP Heater Outlet Temperature ℃ 211.653 215.079 214.915
Extraction #3 Flow t/h 74.763 82.591 82.219
#3 HP Heater Drain Flow t/h 327.808 222.466 221.436
Condensate Flow t/h 1427.319 1522.438 1517.687
IP Turbine Exhaust Pressure MPa 0.455 0.485 0.483
IP Turbine Exhaust Temperature ℃ 257.070 256.229 256.261
IP Turbine Efficiency 0.940 0.940 0.940
LP Turbine Exhaust Flow t/h 1220.466 1295.409 1291.676
LP Turbine Exhaust Pressure kPa 16.000 16.000 16.000
LP Turbine Exhaust Enthalpy (UEEP) kJ/kg 2444.659 2432.940 2433.490
HP Turbine Work kW 185685.64 181960.22 181340.88
IP Turbine Work kW 233858.72 250197.36 249400.39
LP Turbine Work kW 190266.08 205653.72 204881.90
Relative Change of HP Turbine Work ％ -2.0063 -2.3398
Relative Change of IP Turbine Work ％ 6.9865 6.6458
Relative Change of LP Turbine Work ％ 8.0874 7.6818
Generator Output kW 600104.72 627769.56 625607.70
Unit Heat Rate kJ/kWh 8080.402 8125.045 8128.772
Relative Change of Generator Output ％ 4.6100 4.2498
Relative Change of Unit Heat Rate ％ 0.5525 0.5986

H. S. YANG ET AL. 277
Table 4. Calculation result by the equivalent enthalpy Drop
method.
Item Symbols UnitResult
Increasing of the Equavalent Enthalpy
Drop △H kJ/kg66.97
Increasing of the Total Heat Absorbed △Q kJ/kg171.57
Relative Increasing of the Generator
Output △P ％ 5.639
Relative Decreasing of the Cycle
Efficiency (Heat Rate) δη % 0.838
For the top HP heater out-of-service operation condi-
tion, two calculation methods are adopted, and the calcu-
lation results are described below.
1) Calculation Result by the Model-based Calculation
Method
The calculation result by the Model-based Calculation
Method is summarized in Table 2.
2) Calculation Result by the Simplified Variable Op-
eration Condition An alysis Method
The calculation result by the Simplified Variable Op-
eration Condition Analysis Method is summarized in
Table 3.
3) Comparison and Analysis of the Calculation Results
It is shown that calculation results from the proposed
two methods are very close. Because in both methods,
the redistribution effect on steam flows of the turbine
extractions and turbine internals due to the top heater
out-of-service is considered, and there is no significant
difference between the two methods in essence.
For evaluating and crosscheck of the calculated results,
the calculated result using the commonly used Equivalent
Enthalpy Drop Method is also shown in Table 4.
It can be shown clearly that both results by the pro-
posed methods are lower than result calculated using the
commonly used equivalent enthalpy drop method.
In summary, the calculated results calculated by the
proposed methods have higher credibility than the result
calculated by the commonly used method. The com-
monly used Equivalent Enthalpy Drop Method is not
recommended for analyzing the significant operation
condition change situations, because the dramatic change
of the operation conditions under these situations can’t
meet the assumptions and preconditions of this method,
which is the basis for deducing the concept and equations
of this method.
4. Conclusions
To analyze the unit performance degradation for top
heater out-of-service operation condition, two calculation
methods are introduced, that is the model based calcula-
tion method and the simplified variable operation condi-
tion analysis method.
A 600 MW sub-critical steam turbine unit with air
cooled condenser is analyzed using the two introduced
methods. It is shown that calculation data got can reflect
the redistribution effect on steam flows of the turbine
extractions and turbine internals due to the top heater
out-of-service, and similar and close calculation results
are calculated from the two methods, and both results are
lower than result calculated using the commonly used
equivalent enthalpy drop method.
REFERENCES
[1] W. C. Lin, “Energy-saving Theory of Fossil Fired Power
Plant Thermal Systems,” Xi’an, China: Xi’an Jiaotong
University Press, 1994.
[2] S. L. Yan, “Research on the Calculation Model of the
Unit Performance Degradation for Top Heater
Out-of-service Operation Condition,” Turbine Technol-
ogy, Harbin China, Vol. 49, No. 6, 2007, pp. 420-422.
[3] C. F. Zhang, “Quantitative Study on the Limitation of
Small Disturbance Theory based on the Top Heater
Out-of-service Operation Condition,” Turbine Technol-
ogy, Harbin China, Vol. 50, No. 1, 2008, pp. 34-36.