Degradation Effects on Comined Cycle Power Plant Performance

A. I. Zwebek P. Pilidis Department of Power Engineering and Propulsion, School of Engineering, Cranfield University, Bedford MK43 0AL, UK

Degradation Effects on Combined Cycle Power Plant Performance— Part III: Gas and Steam Turbine Component Degradation Effects This paper presents an investigation of the degradation effects that gas and steam turbine cycles components have on combined cycle (CCGT) power plant performance. Gas turbine component degradation effects were assessed with TurboMatch, the Cranfield Gas Turbine simulation code. A new code was developed to assess bottoming cycle performance man ce dete deterio riorati ration. on. The two codes wer weree the then n join joined ed to sim simula ulate te the com combin bined ed cycl cyclee performance deterioration as a whole unit. Areas examined were gas turbine compressor and turbine degradation, degradation, HRSG degradation, degradation, steam turbine degrada degradation, tion, condenser degradation, and increased gas turbine back pressure due to HRSG degradation. The procedure, assumptions made, and the results obtained are presented and discussed. The parameters that appear to have the greatest influence on degradation are the effects on the gas generator. ͓DOI: 10.1115/1.1639007͔

Introduction The rapid improvement of gas turbine technology in the 1990s drove combined cycle thermal efficiency to nearly 60% with natural gas as a fuel ͑Briesch and Bannister ͓1͔͒ . It will probably go even higher in the future. This high plant efficiency along with low emissions and competitive capital and running costs made the combined cycle gas turbine ͑CCGT͒ plant a very popular prime mover for electricity generation. This interest increase in the CCGT plants led to the users of such plants to become more concerned about the plant’s behavior after aft er run running ning for lon long g time times. s. As a resu result, lt, simulation simulation codes are developed to predict the behavior of such power plants and their subsystems on a thermo-fluid dynamic basis ͑Erbes and Gay ͓2͔, Roy-Aikins ͓3͔, and Thermoflow ͓4͔͒ . This is the third in a series of three technical papers looking at the degr degradat adation ion eff effects ects that dif differ ferent ent com compon ponents ents of com combin bined ed cycle have on the plant’s performance. The first paper ͑Zwebek and Pilidis ͓5͔͒ presented the effects that gas turbine components degradation have on gas turbine and hence on the overall CCGT plant, the second paper ͑Zwebek and Pilidis ͓6͔͒ discuss discussed ed the steam ͑bottoming͒ cycle component degradation effects have on CCGT plant. The conclusion of the two papers mentioned above is summarized herein. In the first paper, ͓5͔, it was concluded concluded that the GT turbine turbine deg degrada radation tion has the utmost effect on gas turbine as well as on steam turbine cycles performances perfor mances compared to GT compressor. compressor. Also, it was shown that the GT exh exhaust aust temperatu temperature re has a pred predomi ominant nant effect effect on steam cycle efficiency over the GT exhaust mass flow. Because the CCGT plant is more dependent on the gas turbine, and as it was expected, the CCGT plant performance was more sensitive to change chan ge in gas turb turbine ine cycle conditions conditions than to the changes changes in steam turbine cycle conditions. The conclusion from the second paper, ͓6͔, was that, within the HRSG HRS G uni unit, t, the evap evapora orator tor degr degradat adation ion is the utm utmost ost eff effecti ecting ng fault on steam turbine cycle performance compared to superheater and economizer. economizer. Also concluded that, the steam turbine isentropic Contributed by the International Gas Turbine Institute ͑IGTI͒ of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS for publication in the ASME JOURNAL OF NGINEERING ERING FOR GAS TURBINES AND POWER. Paper presented at the InternaENGINE tional Gas Turbine and Aeroengine Congress and Exhibition, Amsterdam, The Netherlands, June 3–6, 2002; Paper No. 2002-GT-30513. Manuscript received by IGTI, Dec. 2001, final revision, Mar. 2002. Associate Editor: E. Benvenuti.

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efficiency as a performance parameter efficiency parameter has the uppermost effect on steam turbine cycle power and efficiency. Finally, the effects of HRSG and condenser degradations on steam cycle and hence on CCGT CCG T plan plants ts perf perform ormance ance is ver very y low compared compared to the steam turbine unit compon components ents degrada degradation. tion. Gas ÕSteam Turbine Performance Deterioration. Even under normal engine operating conditions, with good inlet filtration systems, and using a clean fuel, the gas turbine engine flow path components will become fouled, eroded, corroded, and covered with wit h rus rustt sca scale, le, ͑Diakunchak ͓7͔, Lakshm Lakshminarasi inarasimha mha et al. ͓8͔, Tabakoff ͓9͔, and Tabakoff Tabakoff et al. ͓10͔, and others͒. The results will then be an engine performance deterioration. Since the gas turbine, bin e, in this cas case, e, is conn connecte ected d to ano another ther plant ͑steam cycle͒ which is entirely dependent on it, then the concern due to performance deterioration will increase. This is due to the fact that any failure or malfunctioning within the gas turbine will be magnified as it would be affecting the two ͑CCGT͒ plants at the same time. This paper explores different different compon component ent degradation effects on a simple combined cycle ͑CCGT͒ plant of Fig. 1. The plant under consideration is composed of a single-shaft industrial gas turbine coupled with a single-pressure HRSG steam ͑bottoming͒ cycle. The ͑design point͒ specifications of both gas and steam turbine plants used with this unfired cycle were chosen in such a way that they represent an existing typical real cycle, as follows: Gas Turbine Specifications inlet mass flowϭ408.6 kg/sec compressor compre ssor pressu pressure re ratioϭ15.2 turbine entry temperat temperature ureϭ1697.80 K exhaust mass flowϭ419.4 kg/sec exhaust temperatureϭ871.24 K powerϭ165.93 MW thermal efficien efficiency cyϭ35.57% Steam Turbine Specifications live steam pressureϭ65.4 bar live steam temperatureϭ537.8°C steam mass flowϭ67 kg/sec steam turbine isentropic effic.ϭ89.48% superheater superh eater surface areaϭ8424.8 m2 evaporator surface areaϭ29315.6 m2 economizer econom izer surfac surfacee areaϭ38004.1 m2 condenser surface areaϭ3942.9 m2

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Tab able le 1

Repr Re pres esen enta tati tion on of co comp mpon onen entt de degr grad adat atio ion n

Fault

Compressor fouling Compressor erosion Turbine fouling Turbine erosion FOD Gas turbine back pressure Econ Ec onom omiz izer er de degr grad adat atio ion n Evap Ev apor orat ator or de degr grad adat atio ion n Supe Su perh rheat eater er de degr grad adat atio ion n Cond Co nden ense serr de degr grad adat atio ion n Ste team am tu turb rbiine fou oullin ing g Ste team am tu turb rbiine er eros osiion

Fig. 1 Fig. plant

Sch che ema mati tic c di diag agra ram m of a sin ingl gle e pr pres ess sur ure e CCG CGT T po powe werr

HRSG efficienc efficiency yϭ81.11% steam turbine plant power outputϭ76.4541 MW steam turbine plant efficiencyϭ33.97% The effects of the gas turbine degradation on steam cycle, and hence on the CCG hence CCGT T plan plants ts per perfor forman mance ce as who whole le was investiinvestigated. The faults investigated were the following: i. compres compressor sor isentropic isentropic effic. degradat degradation, ion, ii. turbine isentropic isentropic efficiency efficiency degradation, degradation, iii. compressor and turbine fouling, iv.. compre iv compressor ssor and turbine erosion, erosion, v. econom economizer izer degradation degradation,, vi. evaporat evaporator or degradation, vii. superhe superheater ater degradat degradation, ion, viii. steam turbine fouling, ix. steam turbine erosion, erosion, x. ST isentropic isentropic efficiency efficiency degradation, degradation, xi. condens condenser er degradation, xii. combina combination tion of all faults mentioned mentioned above, and xiii. gas turbine back pressure increase due to heat exchanger ͑HRSG͒ surfaces fouling. The terms fouling and erosion are used in the context of other work, ͑Diakunchak ͓7͔ and Lakshminarasimha et al. ͓8͔͒ . In the case of gas turbine unit, because the combustion system is not likely to be a direct cause of gas turbine performance performance deterioration deterioration ͑Diakunchak ͓7͔͒ it was assumed not to degrade for the following reasons: i.

Combustion Combus tion chamber chamber faults faults that aff affect ect GT ove overall rall perforperformance are rare in comparison to those faults that may occur in the compressor and turbine. ii. Any mal malfun functio ctioning ning in the com combus bustio tion n cham chamber ber wou would ld mean increased emissions, emissions, which is not allowed by environmental laws in many places.

FOD

Represented By

Range

Drop in ⌫ Drop in ␩ C Drop in ⌫ Drop in ␩ C Drop in ⌫ Drop in ␩ T Rise in ⌫ Drop in ␩ T Drop in ␩ C and ␩ T GT Back pressure rise Drop Dr op in U Drop Dr op in U Drop Dr op in U Drop Dr op in U Dro rop p in ⌫ Drop in ␩ T Rise in ⌫ Drop in ␩ T Drop in ␩ T

0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑ϩ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑ϩ3.0%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑ϩ5.0%͒ 0.0– ͑Ϫ2.5%͒ 0.0– ͑Ϫ5.0%͒

component due to fouling along with a decrease in the component’s isentropic efficiency due to surface roughness, for example. Erosion:: Compressor erosion is represented by a lower inlet Erosion mass flow capacity and a reduction in compressor isentropic efficiency. On the other hand, GT and ST turbines erosion is represented by an increased flow capacity plus a reduction in the turbine isentro isentropic pic efficien efficiency cy ͑Lakshm Lakshminarasi inarasimha mha et al. ͓8͔͒ . Thesee two phenomena Thes phenomena are rep represe resented nted by chan changing ging the socalled nondimensional mass flow ͑Eq. ͑1͒ of the component maps ͑Table 1͒. ˙ ͱT W i PA

is increased or reduced

(1 )

Component Efficiency Degradation: Degradation: This is modeled by reducing the component isentropic efficiency of the appropr appropriate iate map and keeping all other parameters at their design point ͑DP͒ levels. In this case, it was assumed that the component isentropic efficiency might decrease from its DP value due to any reason, such as blade tip rubs or FOD. Heat Exchan Exchanger ger Degra Degradation dation:: The degradation of either of the heat exch exchange angers rs ͑econom economizer, izer, evapora evaporator, tor, superh superheater eater,, and condenser͒ was simulated by assuming a percent reduction in the original DP value of the overall heat transfer coefficient of the heat exchanger in concern. Gas Turbine Back Pressure: Pressure : The increased back pressure at the gas turbine exhaust is represented as an increase in the GT exhaustt outlet pressure. exhaus The above-mentioned faults are applied to different components of the plant in different values. Table 1 summarizes these faults and their ranges at which they were applied to each component. Therefore, throughout this study, it was assumed that there was no component washing or any type of maintenance carried out on the gas and steam turbine plants until the deterioration deterioration reached 5% from the original design point performance.

Fault Representation

Combined Cycle Degradation Simulation

In order to investigate the effects of faults mentioned in previous sectio the Gas Gas/st /steam eam tur turbine bine plants performan performan

Before starting any degradation simulations it was necessary to establish a datum working line ͑design point performance͒ of both establish





Table able 2 dation

Comp Compon onen entt isen isentr trop opic ic effic efficie ienc ncy y vari variat atio ion n with with degr degraa-

Physical Fault Compressor fouling Compressor erosion Compressor corrosion Turbine fouling Turbine erosion Turbine corrosion Foreign object damage Thermal distortion Blade rubbing

Nondimensional Mass Flow Change ͑A͒

⌫c↓ ⌫c↓ ⌫c↓ ⌫ T ↓ ⌫ T ↑ ⌫ T ↓ ⌫ C / T T ↓ ⌫ T ↑↓ ⌫ C ↓ & ⌫ T ↑

Isentropic Efficiency Change ͑B͒ ␩ c ↓ ␩ c ↓ ␩ c ↓

␩ T ↓ ␩ T ↓ ␩ T ↓

␩ C / T ↓ / T ␩ C / T ↓ / T ␩ C / T ↓ / T

Table able 3 GT back back pres press sure ure dist distri ribu buti tion on alon along g with ith othe otherr com components ponents degradation degradation GT Back Pressure

Ratio A:B

ϳ1:0.5 ϳ1:0.5 ϳ1:0.5 ϳ1:0.5 ϳ1:0.5 ϳ1:0.5 ϳ1:2.0 ϳ1:2.0 ϳ1:2.0

Tabakoff et al. ͓10͔, Diakunchak ͓7͔, and Lakshminarasimha Lakshminarasimha et al. ͓8͔, and others͒, the applied degradation magnitude to each component, when simulating gas turbines deterioration performance, in most cases is either arbitrary or based on some published experime per imental ntal res results ults.. Ther Therefo efore, re, in pres present ent stu study dy the valu values es men men-tioned by Diakun Diakunchak chak ͓7͔ and Escher ͓11͔ were taken as a guideliness fro line from m whi which ch the imp implant lanted ed faul faults ts wer weree esti estimat mated. ed. Table Table 2 ͑Zwebek and Pilidis ͓5͔͒ shows a summary of how component isentropic efficiency changes vary with degradation. These values were applied in all calculations to the appropriate components. Based on what is mentioned above, in the case of steam turbine plant, it was also assumed that every 1.0% deterioration in mass flow capacity ͑fouling or erosion͒ would result in a deterioration of ͑0.50%͒ in steam turbine isentropic efficiency. Unfortunately, not much literature was found on the subject of CCGT plant degradation, or on modeling of this problem, including the effect of GT back pressure rise. Therefore to simulate the effect of back pressure on gas turbine performance, due to HRSG degradation some assumptions have been made. An increase in back pressure by Ϸ0.0025 atm results in a reductio reduction n in gas turbine power pow er by Ϸ0.3 0.3%. %. Typi ypical cal back pressure pressure rang ranges es fro from m 0.0 0.025 25 to 0.037 atm above the design value. Because of the inherent problems which accompanies the increase of back pressure, e.g., high

GT Fouling

GT Erosion

ϩ1.0%

Ϫ1.0%

ϩ1.0%

ϩ1.5%

Ϫ2.0%

ϩ2.0%

ϩ2.0%

Ϫ3.0%

ϩ3.0%

ϩ2.5%

Ϫ4.0%

ϩ4.0%

ϩ3.0%

Ϫ5.0%

ϩ5.0%

torque on the shaft, coupling forces on thrust bearing, and vibration, it was assumed that maximum it can go up to 0.03 atm over the DP value. It is worth reminding the reader here that the values in Fig. 3 and in the following successive figures are also including the gas turbine back pressure rise due to HRSG degradation. This is accomplished by implanting a value of GT exhaust back pressure rise with a corresponding GT degradation ͑fouling/erosion͒ value as shown in Table 3.

Gas Turbine Degradation Simulation Results Due to its working nature and depending on the place where it is installed, installed, it was assumed assumed that the gas turb turbine ine might foul or erode. Therefore, the simulation strategy of the gas turbine was divided div ided into two dif differe ferent nt cate categor gories. ies. The firs firstt str strateg ategy y was to assume that the gas turbine will foul up ͑Ϫ5.0%͒ from its origina originall DP performance. On the other hand, the second strategy assumes an erosion erosion in gas turb turbine ine gas pat path h com compon ponents ents up to ͑ϩ5.0%͒ from fro m thei theirr DP performan performance. ce. In par paralle allell wit with h each of the cases mentioned mentio ned above, an amount increase increase in gas turbine back pressure due to degraded HRSG was assumed as shown in Table 3. As Fig. 2 shows, a back pressure increase of ͑3.0%͒ resulte resulted d in a re reduc ductio tion n in gas tur turbi bine ne th therm ermal al ef effic ficien iency cy an and d po powe werr by ͑Ϫ2.0%͒ approximately. While the exhaust mass flow was almost constan con stant, t, the exha exhaust ust tem tempera perature ture incr increase eased d by abo about ut ͑0.75%͒ from its original DP value. Figure 3 summarizes the main performance parameters of gas turbine and how they vary with degradation. As it can be seen, it





Fig. Fig. 3 GT per perform forman anc ce para param mete eters varia ariattion ion with with gas gas turbi urbine ne cycle ycle comp omponen onentt degradation

seems that the effect of either fouling or erosion is tending to have a similar curvature trend, but the magnitude is different. As this figure shows, the maximum degradation consequence was encountered with gas turbine power deterioration due to GT component erosion. This was around ͑Ϫ15.2%͒ from the original DP value. The corresponding plant thermal efficiency drop was about ͑Ϫ11.5%͒. The combination of decreased compressor mass flow with an increased turbine flow capacity, due to erosion by 5%, led to a higher ͑about 2.5%͒ reduction in the plant’s overall efficiency in comparison to the case where both components are experiencing fouling. This is due to the fact that the decreased pressure ratio through the turbine due to erosion resulted in a lower power output of the turbine, and hence a reduced overall power output of the engine which then reflected on the engine’s overall efficiency. This shows that the erosion effect of gas turbine gas path components on the gas turbine performance is higher than the fouling effect.

Steam Cycle Degradation Simulation Results As it was explained above, since the trend of the GT degradation due to either fouling or erosion is the same, and they only differ in the magnitude, and due to the limited space ͑page numbers allowed for this paper͒ it was decided to discuss only the fault fau lt whi which ch gave the high higher er imp impact act on GT performan performance ce when simulat sim ulating ing the bott bottomi oming ng cycl cycle. e. Ther Therefor efore, e, as alre already ady sho showed wed,, since the GT erosion effects were predominant over the effect of fouling, it was decided to use its values when simulating steam cycle perform performance ance degrada degradation. tion. The most important steam turbine cycle performance deterioration sim simulat ulation ion res results ults are repr represe esented nted grap graphica hically lly in Fig Figs. s. 4 through 8. It is worth reminding the reader here that the values in

8.3%͒ was at its highest value with superheater degradation by 5%. Although the expectation was to see the highest change in mass flow variation with GT degradation, degradation, the results came up with different values. In reality, this increase in mass flow was not due to degraded superheater. superheater. In fact as Zwebek and Pilidis ͓6͔ showed, the effect of degraded superheater alone on steam mass flow ͑with out gas tur turbine bine deg degrada radation tion͒ is almo almost st neg negligi ligible ble ͑ϩ0.51%͒. Therefore, Therefo re, as Eq. ͑1͒ shows, the inlet conditions at the ST inlet are controlled control led by the so-called nondimensional nondimensional mass flow ͑Eq. ͑1͒ above͒. Now by comparing the superheater degradation effects in Figs. 4, 7, and 8 with GT degradation effects it will be observed that while steam live pressure is almost constant ͑Fig. 7͒, there was an increase in steam mass flow. Now to fulfil the conditions of Eq. ͑1͒, then the live steam temperature must increase. This is the result obtained ͑as Fig. 8 shows ͒. The same discussion is almost applicable to all other conditions. It is well known from the very basics of steam turbine cycle theory that the steam turbine power is a function of steam mass flow and its enthalpy. Now by comparing Fig. 5 with Fig. 4 it would be observed that the steam turbine power is more or less following the mass flow behavior. The effects effects of degr degrade aded d topp topping ing as wel welll as bott bottomi oming ng cycl cyclee components on steam turbine power plant are illustrated in Fig. 5. As this figure shows, the largest displacement displacement of ST turbine power from its original DP value was encountered with superheater as welll as con wel condens denser er degr degradat adations ions.. Agai Again n as stat stated ed abo above, ve, this increase in ST power is merely due to increased gas turbine exhaust temperature due to GT degradation which led to increase steam mass flow and hence to increase the ST power. On the other hand, the lowest effect on ST power resulted from GT degradation degradation along with ST turbine isentropic isentropic efficiency degradation; this was around ϩ4.2%.





Fig. Fig. 4 ST stea steam m mass ass flow flow varia ariattion ion wit with gas gas and and stea team cyc cycles les com compone ponent nt degdegradation

sponding to the maximum GT efficiency; i.e., as GT exhaust temperature goes up, the CCGT efficiency goes up. Since gas turbine efficiency is already at its maximum, and still by increasing GT exhaust temperature ͑due to any reason͒ the CCGT will increase. This would then implicitly indicate that this increase is gained by increased steam turbine plant’s power due to the increased steam turbine inlet conditions as explain explained ed above. This exactly coincides with the results obtained in the current study ͑see Fig. 5 ͒. The next important performance parameter to discuss here is the steam turbine plant ͑Rankine͒ efficiency variation with degra-

dation, which is illustrated in Fig. 6. The thermal efficiency definition of steam turbine ͑bottoming͒ plant is given by ␩ R ϭ

W SC Q HRSG

.

(2)

This equation shows that the steam turbine ͑bottoming͒ cycle efficiency is a function of steam turbine net power output and the heat transferred in the HRSG ( Q HRSG), which is representing the heat input to the steam cycle. Now by looking at Fig. 9 it will be





Fig. Fig. 6 tion

Rank Rankin ine e effic efficie ienc ncy y vari variat atio ion n with with gas gas and and stea steam m cycl cycles es comp compon onen entt degr degrad adaa-

seen that all types of degradations resulted into an increase in the HRSG efficiency, efficiency, i.e., increased Q HRSG . As Fig. 5 sh show ows, s, although ST power increased with all types of degradation as well, yet the increase in the Q HRSG ͑relative to DP value͒ in some cases was higher higher than the incr increas easee in ST pow power er.. This led to the ST efficiency to fall with such cases. This can be clearly seen in the casee of ST tur cas turbine bine isentropic isentropic degradatio degradation. n. In this case, the increase in ST power was around 4.2% with 5% degradation. Although ͑as Fig. 9 shows͒ there was an increase in HRSG efficiency by about 4.2% ͑for the same magnitude of degradation͒ as

well, nevertheless, the increase in Q HRSG was predominant and hence resulted in decreasing the ST turbine efficiency by approximately 3.3%. As Figs. 5 and 6 show, the ST turbine isentropic efficiency has a predominant effect over all other types of ST cycle degradations. This is in agreement with the conclusion established by Zwebek and Pilidis ͓6͔. The degradation effects of gas and steam plants on live steam pressure and temperature are expressed on Figs. 7 and 8, respectively.. As Fig. 7 shows, the blockage of the steam turbine inlet due tively







Fig. Fig. 8 Live Live stea team tem temper peratur ature e vari varia ation tion wit with gas gas and and steam team cyc cycles les compo ompon nent ent degradation

to fouling by 5.0% resulted in about 11.6% increase in live steam pressure pressu re at the ST turbine inlet. The combina combination tion of all other types of degradation with steam turbine fouling reduced the inlet pressure to about 11.0%. On the other hand, the degradation of all components along with steam turbine erosion by 5.0% resulted in only 0.3% ͑approximately͒ reduction in live steam pressure. In the case of live steam temperature, temperature, as Fig. 8 shows shows,, the effect of different components degradation on live steam temperature is mostly controlled by increased gas turbine exhaust temperature. As alre already ady sho shown wn abov abovee ͑see Fig. 3͒ the effect of gas turbine turbine

components degradation was to increase the GT exhaust temperature. This, then by itself, led the live steam temperature temperature to increas increasee ͑see explanation above͒.

Combined Cycle Degradation Results The degradation results have been explained in part by addressing the two cycles separately. Figure 10 shows the whole CCGT plan pl ant’ t’ss po powe werr an and d ho how w it va vari ries es wi with th GT an and d ST pl plan ants ts degradation.





Fig. Fig. 10 CCGT CCGT powe powerr vari variat atio ion n with with gas gas and and stea steam m cycl cycles es comp compon onen entt degr degrad adat atio ion n

As this figu figure re shows, although although there was an incr increase ease in ST power ͑see Fig. 5͒, the decrease that was caused by GT power ͑see Fig. 3͒ was predominant. This actually is a straightforward result since GT power counts for the two thirds of the total amount of CCGT power. Figure 11 is a reproduction of Fig. 9 showing the CCGT CCG T effic efficienc iency y is actu actually ally following following the beh behavio aviorr of CCG CCGT T power. As the two previous figures showed, the GT turbine degradation degradation alone was having the least effect on CCGT plant power and efficiency. On the other hand, when the ST component effects were include incl uded d in the deg degrada radation tion,, the out outcom comee dete deterior rioratio ation n res results ults started to increase. As these two figures show, the largest degra-

dation effect was due to the ST turbine isentropic efficiency degradation along with GT degradation. This was about Ϫ10.7% and Ϫ4.3% deterioration in CCGT power and efficiency respectively with Ϫ5.0% degradat degradation. ion. Figures 9 and 12 illustrate the degradation effects of both plants on HRSG efficiency ( ␩ HRSG) and stack tempera temperature, ture, respectively. respectively. The stack temperature is mainly a measure of the amount of gas turbine exhaust heat utilization by the bottoming cycle. Also, by definition, definiti on, HRSG efficiency is a function of stack temperature temperature and HRSG exhaust inlet temperature for a given ambient temperature ͑Eq. ͑3͒





Fig. Fig. 12 Stac Stack k temp temper erat atur ure e vari variat atio ion n with with gas gas and and stea steam m cycl cycles es comp compon onen entt degdegradation

␩ HRSGϭ

T GinϪ T Stack T GinϪ T amb

.

(3)

This equation shows that, for a given HRSG inlet exhaust and ambien amb ientt tem tempera perature tures, s, the HRS HRSG G effic efficienc iency y incr increase easess wit with h decreased stack temperature ( T Stack ) and vice versa. While, as Zwebek and Pilidis ͓5͔ showed, the effects of GT component degradation resulted in decreasing T Stack , the effects effects of ST component degradation Zwebek and Pilidis ͓6͔ cam camee up ͑somewhat͒ wit with h opp opposi osite te res results ults.. Now Now,, as Fig Fig.. 12 sho shows, ws, although in this case both plants were degraded, all types of degradation led to decreasing T Stack and henc hencee incr increas easing ing the ␩ HRSG ͑see Fig. 11͒. This leads us to a conclusion that the effects of GT degradation degrada tion on ␩ HRSG are predominant over the effect ST component degrada degradation. tion.

Conclusions The results obtained showed that the erosion of gas turbine gas path components components has a predominant effect on its perform performance ance over

HRSG Q ST T ⌫

ϭ ϭ ϭ ϭ ϭ

␩ ϭ

U

ϭ

heat recovery steam generator heat transfer steam turbine temperature nondimensional nondime nsional mass flow efficiency heat transfer coefficient

Subscripts C CC GT ST i Gin SC T Stack

ϭ ϭ ϭ ϭ ϭ ϭ ϭ ϭ ϭ

GT compressor combined combine d cycle gas turbine steam turbine inlet HRSG inlet steam cycle turbine HRSG exit





͓10͔ Tabakoff, W., Lakshminarasimha, A. N., and Pasin, M., 1990, ‘‘Simulation of Compressor Performance Deterioration due to Erosion,’’ ASME J. Eng. Gas Turbines Power, 112 112.. ͓11͔ Escher, P. C., 1995, ‘‘Pythia: An Object-Oriented Gas Path Analysis Computer

Program for General Applications,’’ Ph.D. thesis, School of Mechanical Engineering, Cranfield University, UK. ‘‘Parametric Analysis of Comb Combined ined Gas-St Gas-Steam eam Cycles Cycles,’ ,’’’ ͓12͔ Cerri, G., 1987, ‘‘Parametric ASME J. Eng. Gas Turbines Power, 109 109..

Degradation Effects on Comined Cycle Power Plant Performance

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