GAS TURBINE PERFORMANCE

Transcription

1 GAS TURBINE PERFORMANCE Rainer Kurz Solar Turbines Inororated 9330 Skyark Court San Diego, CA Klaus Brun Southwest Researh Institute 6220 Culebra Road San Antonio, TX Dr. Rainer Kurz is the Manager of Systems Analysis at Solar Turbines Inororated, in San Diego, California. His organization is resonsible for analyzing omression requirements, rediting omressor and gas turbine erformane, for onduting aliation studies, and for field erformane testing. Dr. Kurz attended the Universitaet der Bundeswehr in Hamburg, Germany, where he reeived the degree of a Dr.-Ing. in He has authored numerous ubliations about turbomahinery related tois, is an ASME fellow and a member of the Turbomahinery Symosium Advisory Committee. Dr. Klaus Brun is the Diretor over the Mahinery Program in the Fluids and Mahinery Deartment at Southwest Researh Institute. His researh interests are in the areas of turbomahinery aerothermal fluid dynamis, roess system analysis, energy management, advaned thermo-dynami yles, instrumentation and measurement, and ombustion tehnology. He is widely exeriened in erformane redition, off-design funtion, degradation, unertainty diagnostis, and root-ause failure analysis of gas turbines, ombined-yle ower lants, entrifugal omressors, steam turbines, and ums. Dr. Brun is the inventor of the Single Wheel Radial Flow Gas Turbine, the Semi-Ative Plate Valve, the Planetary Gear Mounted Auxiliary Power Turbine, and the Comressor Seed-Pulsation Controller. He has authored over 60 aers on turbomahinery, given numerous invited tehnial letures and tutorials, and ublished a textbook on Gas Turbine Theory. Dr. Brun obtained his Ph. D. and Master s Degree at the University of Virginia. ABSTRACT The ower and effiieny harateristis of a gas turbine are the result of a omlex interation of different turbo mahines and a ombustion system. In this tutorial, we will address the basi harateristis of eah of the omonents in a gas turbine (omressor, gas generator turbine, ower turbine) and the imat of tyial ontrol limits and ontrol onets. The goal is to rovide exlanations for the oerational harateristis of tyial industrial gas turbines, emhasizing the interation between the gas turbine omonents. The onet of omonent mathing is exlained. Additionally, methods are introdued that allow the use of data for trending and omarison uroses. The imat of omonent degradation on individual omonent erformane, as well as overall engine erformane is disussed, together with strategies to redue the imat of degradation In artiular, the following tois will be disussed: The gas turbine as a system Thermodynamis and aerodynamis Comonent mathing Off-design behavior of gas turbines Low fuel gas ressure Aessory loads Single-shaft versus two-shaft engines Variable inlet and stator vanes Control temerature Transient behavior Thermo dynamial arameters of exhaust gases The tois resented should enhane the understanding of the riniles that are refleted in erformane mas for gas turbines, or, in other words, exlain the oerating riniles of a gas turbine in industrial aliations. The onets develoed will be used to derive basi riniles for suessful ondition monitoring and erformane testing of gas turbines. INTRODUCTION Gas turbines have been used for many aerosae and industrial aliations for many years (Figure 1). Gas turbines for industrial aliations onsist either of an air omressor driven by a gas generator turbine with a

2 Page 2 searate ower turbine (two-shaft engine, Figure 2) or of an air omressor and a turbine on one shaft, where the turbine rovides both ower for the air omressor and the load (single-shaft engine, Figure 2). The ower and effiieny harateristis of a gas turbine are therefore the result of a omlex interation of different turbo mahines and a ombustion system. Figure 2. Single-shaft (old end drive) and two-shaft (hot end drive) gas turbines Whatever ower is left is used as the mehanial outut of the engine. This thermodynami yle an be dislayed in an enthaly-entroy (h-s) diagram (Figure 3). The air is omressed in the engine omressor from state 1 to state 2. The heat added in the ombustor brings the yle from 2 to 3. The hot gas is then exanded. In a single-shaft turbine, the exansion is from 3 to 7, while in a two-shaft engine, the gas is exanded from 3 to 5 in the gas generator turbine and afterwards from 5 to 7 in the ower turbine. The differene between lines 1 to 2 and 3 to 7 desribes the work outut of the turbine, i.e., most of the work generated by the exansion 3 to 7 is used to rovide the work 1 to 2 to drive the omressor. Figure 1. Gas turbine aliations The tois resented should enhane the understanding of the riniles that are refleted in erformane mas for gas turbines, or, in other words, exlain the oerating riniles of a gas turbine in industrial aliations. THERMODYNAMICS OF THE GAS TURBINE CYCLE (BRAYTON CYCLE) The onversion of heat released by burning fuel into mehanial energy in a gas turbine is ahieved by first omressing air in an air omressor, then injeting and burning fuel at (ideally) onstant ressure, and then exanding the hot gas in turbine (Brayton yle, Figure 3). The turbine rovides the neessary ower to oerate the omressor. Figure 3. Enthaly-entroy diagram for the Brayton yle In a two-shaft engine, the distanes from 1 to 2 and from 3 to 5 must be aroximately equal, beause the omressor work has to be rovided by the gas generator turbine work outut. Lines 5 to 7 desribe the work outut of the ower turbine. For a erfet gas, enthaly and temerature are related by

3 h For the atual roess, the enthaly hange h for any ste an be related to a temerature rise T by a suitable hoie of a heat aaity for eah of the stes. We an thus desribe the entire roess, assuming that the mass flow is the same in the entire mahine, i.e., negleting the fuel mass flow and bleed flows, and further assuming that the resetive heat aaities, e, and a are suitable averages. e a ( T T ) + ( T T ) E W f e T ( T T ) P / W f 3 / W In the first equation, the first term is the work inut by the omressor, and seond term desribes is the work extrated by the turbine setion. The seond equation, relates the temerature inrease from burning the fuel in the ombustor with the energy ontained in the fuel. For two-shaft engines, where the gas generator turbine has to balane the ower requirements of the omressor, and the useful ower outut is generated by the ower turbine, we an re-arrange the equation above to find: e a ( T ( T 5 2 T ) e T ) P / W ( T 3 T ) This relationshi neglets mehanial losses(whih are in the order of 1%) and the differene between the gas flow into the omressor and into the turbine due to the addition of fuel mass flow. However, the resulting inauraies are small, and don t add to the understanding of the general riniles. The omressor and the turbine setions of the engine follow the thermodynami relationshis between ressure inrease and work inut, whih are for the omressor 5 Page 3 with the amount of ower yielded. The thermal effiieny is thus and the heat rate is P ηth W E 1 HR η th f f W f E P In this aer, T 3, TIT, and TRIT will be (loosely) referened as firing temeratures. The differenes whih lie simly in fat that temeratures ustream of the first turbine nozzle (TIT) are different from the temeratures downstream of the first nozzle (TRIT) due to the ooling of the nozzles, are not imortant for the understanding of the toi of this aer. Aendix A shows an examle for a tyial GT yle. A QUICK EXCURSION TO AERODYNAMICS Any gas turbine onsists of several turbo mahines. First, there is an air omressor, and after the ombustion has taken lae, there is a turbine setion. Deending on the design of the gas turbine, the turbine setion may onsist either of a gas generator turbine, whih oerates on the same shaft as the air omressor, and a ower turbine whih is on a searate shaft. The task of the omressor is to bring the inlet air from ambient ressure to an elevated ressure. To do this, ower is neessary, i.e., the omressor imarts mehanial ower into the air. The same relationshis that aly to the omressor an also be alied to the turbine, exet that the turbine extrats work from the flow. The transfer of energy is aomlished with rotating rows of blades, while the stationary rows allow the onversion of kineti flow energy (i.e., veloity) into ressure or vie versa. f P h W W ( T T ) 2 1 T1 W η 2 1 γ 1 γ 1 and the turbine P h W W ( T T ) W η T 3 7 t γ 1 γ In the two equations, ideal gas laws are assumed. The effiieny of a gas turbine is defined by omaring the amount of ower ontained in the fuel fed into the engine Figure 4. Veloities in a tyial omressor stage

4 The fundamental law desribing the onversion of mehanial energy into ressure in a turbo mahine is Euler s Law. Euler s law onnets thermodynami roerties (head) with aerodynami roerties (i.e., veloities u and, Figure 4): h u 2 u 2 u1u1 or, for axial flow mahines, where the rotational seed u is about the same for inlet and exit of a stage: h u(u 2 u1 ) This orrelation exresses the fat that the fore on the rotating blade in diretion of the rotation is roortional to the defletion of the flow in irumferential diretion, i.e., Fu W ( u1 u 2 ) and therefore, the ower introdued into the flow is the angular veloity ω times the torque generated by Fu: P ω r Fu ω r W (u1 u 2 ) W [u (u1 u 2 ) ] W h A rotating omressor blade assage (stage) imarts energy on the fluid (air) by inreasing the fluid's angular momentum (torque). Page 4 seed, the firing temerature, and the exhaust gas omosition (thus, the load, the fuel and the relative humidity). The ower turbine Mah number deends on its seed, the ower turbine inlet temerature, and the exhaust gas omosition. For a given geometry, the referene diameter will always be the same. Thus, we an define the Mahine Mah number also in terms of a seed, for examle the gas generator seed, and get the so alled orreted gas generator seed: N orr N T / Tref Even though Norr is not dimensionless, it is a onvenient way of writing the mahine Mah number of the omonent. In the following text, we will also use this simlified exression N/ T, whih is based on the above exlanations. In a modern gas turbine, the omressor front stages are transoni, whih means that the relative flow veloity into the rotor is higher than the seed of sound, while the flow veloity leaving the rotor is below the seed of sound (Figure 5). Turbine stages usually see subsoni inlet veloities, but the veloities within the blade hannels an be loally suersoni (Figure 6). Mah Number The aerodynami behavior of a turbine or omressor is signifiantly influened by the Mah number of the flow. The same turbine or omressor will show signifiant differenes in oerating range (flow range between stall and hoke), ressure ratio, and effiieny. The Mah number inreases with inreasing flow veloity, and dereasing Temerature T. It also deends on the gas omosition, whih determines the ratio of seifi heats γ and the gas onstant R. To haraterize the level the Mah number of a turbo mahine, the 'Mahine' Mah number Mn is frequently used. Mn does not refer to a gas veloity, but to the irumferential seed u of a omonent, for examle a blade ti at the diameter D: Mn u γrt 2πDN γrt Figure 5. Mah number distribution for tyial transoni omressor blades. The flow enters at suersoni seeds and is deelerated to subsoni seeds at the exit (Shodl, 1977) This oints to the fat that the Mah number of the omonent in question will inrease one the seed N is inreased. The onsequenes for the oeration of the gas turbine are: The engine omressor Mah number deends on its seed, the ambient temerature, and the relative humidity. The gas generator turbine Mah number deends on its

5 Page 5 Figure 8. Tyial omressor erformane ma with oerating lines for a two-shaft engine Figure 6. Veloity distribution in a turbine nozzle at different ressure ratios. As soon as the maximum loal flow veloity exeeds Mah 1 (at a ressure ratio of 1.5 in this examle), the inlet flow an no longer be inreased (Kurz, 1991). Comonent erformane mas show a signifiant sensitivity to hanges in Mah numbers. There is a strong deendeny of losses, enthaly rise or derease, and flow range for a given blade row on the harateristi Mah number. Figure 7 and Figure 8 with the omressor mas for tyial gas turbine omressors show in artiular the narrowing of the oerating range with an inrease in Mah number, whih in this ase is due to inreasing omressor seed N GP (Cohen et al., 1996). Figure 7. Tyial omressor erformane ma with oerating lines for a single-shaft engine For turbine nozzles, one of the effets onneted with the Mah number is the limit to the maximum flow that an ass through a nozzle. Beyond a ertain ressure ratio, the amount of atual flow Q that an ass through the nozzle an no longer be inreased by inreasing the ressure ratio. As demonstrated with Figure 6 whih shows the flow veloities in a turbine nozzle for inreasing ressure ratios, the veloity or Mah number levels in the nozzle beome higher and higher, until the seed of sound ( Mah 1)is reahed in the throat (for a ressure ratio of 1.5 in the examle).a further inrease of the ressure ratio yields higher veloities downstream of the throat, but the through flow (whih is roortional to the veloity at the inlet into the nozzle) an no longer be inreased. Beause eah gas turbine onsists of several aerodynami omonents, the Mah number of eah of these omonents would have to be ket onstant in order to ahieve a similar oerating ondition for the overall mahine. While the harateristi temerature for the engine omressor is the ambient temerature, the harateristi temerature for the gas generator turbine and the ower turbine is the firing temerature T 3 and the ower turbine inlet temerature T 5 resetively. Therefore, if two oerating oints (o1 and o2) yield the same mahine Mah numbers for the gas omressor and the gas generator turbine, and both oerating oints are at the resetive otimum ower turbine seed, then the thermal effiienies for both oerating oints will be the same- as long as seond order effets, suh as Reynolds number variations, effets of gas and learanes et., are not onsidered. The requirement to maintain the mahine Mah number for omressor and gas generator turbine an be exressed by N GPorr onstant (whih leads to idential Mah numbers for the omressor): N GP, o1 T 1, o1 N T GP, o2 1, o2

6 and, in order to maintain at the same time the same Mah number for the gas generator turbine, whih rotates at the same seed as the omressor, we require for the firing temerature: T T 3, o1 1, o1 T T 3, o2 1, o2 In this ase, the fat that the volumetri flow through the turbine setion is determined by the nozzle geometry also enfores (aroximately) idential head and flow oeffiients for omressor and turbine. Therefore, the engine heat rate will remain onstant, while the engine ower will by hanged roortional to the hange in inlet density. This aroah does not take effets like Reynolds number hanges, hanges in learanes with temerature, hanges in gas harateristis, or the effet of aessory loads into aount. This aroah also finds its limitations in mehanial and temerature limits of an atual engine that restrit atual seeds and firing temeratures (Kurz et al., 1999). Reynolds Number While the Mah number essentially aounts for the omressibility effets of the working gas, the Reynolds number desribes the relative imortane of frition effets. In industrial gas turbines, where neither the working temeratures, nor the working ressures hange as dramatially as in the oeration of airraft engine, the effets of hanges in the Reynolds number are tyially not very ronouned. A hange in the ambient temerature from 0 F to 100 F hanges the Reynolds number of the first omressor stage by about 40%. The tyial oerating Reynolds numbers of omressor blades and turbine blades are above the levels where the effet of hanging the Reynolds number is signifiant. Page 6 erformane of the gas turbine, beause some (but not all) of the work to omress the air is lost if air is used for ooling uroses. Figure 9. Tyial ooling arrangements: Convetion and imingement ooling (left) and film ooling (right). COMBUSTION The engine ombustor is the lae where fuel is injeted (through fuel injetors) into the air reviously omressed in the engine omressor. The released fuel energy auses the temerature to rise: The heat aaity ~ e ( T3 T2 ) E fw f / W ~ e in the equation above is a suitable average heat aaity. Modern ombustors onvert the energy stored in the fuel almost omletely into heat (Tyial ombustion effiienies for natural gas burning engines range above 99.9%). This is also evident from the fat that the results of inomlete ombustion, namely CO and unburned hydroarbons, are emitted only in the arts-er-million level. Blade Cooling The temerature in the hot setion of gas turbines requires the ooling of nozzles and blades (as well as ooling for the ombustor liner). Pressurized air from the engine omressor is brought to the blade and nozzle internals. In some designs, steam is used rather than air. There are a number of different ways to aomlish the ooling (Figure 9): The air is ushed through the inside of the blade with goal to remove as muh heat from the blade surfae as ossible. To this end, ribs are used to inrease the turbulene, and thus the heat transfer (onvetion ooling), and jets of air are blown though small holes to iminge on the blade inside (imingement ooling). Another design brings old air from the inside of the blade through small holes to the outer blade surfae, generating a thin layer of ooler air between the blade surfae and the hot gas (film ooling). The amount of air used imats the Figure 10. Axial temerature distribution in a

7 ombustor Only some art of the omressed air artiiates diretly in the ombustion, while the remaining air is later mixed into the gas stream for ooling uroses. The temerature rofile in a tyial ombustor is shown in Figure 10. The loal temeratures are highest in the flame zone. Cooling of the ombustor liner and subsequent addition of air to redue the gas temerature lead to an aetable ombustor exit temerature. The flow will also inur a ressure loss due to frition and mixing. Lean Premix Combustion (LPM) Systems To further redue the NOx emissions of gas turbines, Lean Premix ombustion systems were develoed. The general idea behind any Lean Premix ombustor urrently in servie is to generate a thoroughly mixed lean fuel and air mixture rior to entering the ombustor of the gas turbine (Greenwood, 2000). The lean mixture is resonsible for a low flame temerature, whih in turn yields lower rates of NOx rodution (Figure 11). Figure 11. Flame temerature as a funtion to fuelto-air ratio Beause the mixture is very lean, in fat fairly lose to the lean extintion limit, the fuel-to-air ratio has to be ket onstant within fairly narrow limits. This is also neessary due to another onstraint: The lower ombustion temeratures tend to lead to a higher amount of roduts related to inomlete ombustion, suh as CO and unburned hydroarbons (UHC). The neessity to ontrol the fuel-to-air ratio losely yields different art-load behavior when omaring gas turbines with onventional ombustors and LPM engines. 1 At ertain levels of art load, LPM engines usually bleed a ertain amount of air from the omressor exit diretly into the exhaust dut. Therefore, while the airflow for any two-shaft engines is Page 7 redued at art load, the redution in airflow is greater for a onventional ombustion engine than for a LPM engine. This sounds aradoxial beause the amount of air available at the ombustor in art-load oeration has to be less for a LPM engine (to maintain the fuel-to-air ratio) than for an engine with onventional ombustion. However, due to the bleeding of air in a LPM engine, the flow aaity of the turbine setion is artifiially inreased by the bleeding dut. The ombustor exit temerature at art load dros signifiantly for engines with onventional ombustion, while it stays high for LPM engines. One the bleed valve oens, the art-load effiieny of a LPM engine dros faster than for an engine with onventional ombustion. Sine the oening of the bleed valve is driven by emissions onsiderations, it is not diretly influened by the load. Regarding emissions, the dro in ombustor temerature in engines with onventional ombustion, leading to a leaner fuel-to-air ratio, automatially leads to NOx emissions that are lower at art load than at full load. In LPM engines, there is virtually no suh redution beause the requirement to limit CO and UHC emissions limits the (theoretially ossible) redution in fuel-to-air ratio. However, the NOx emissions levels of LPM engines are always lower than for engines with onventional ombustion. The imat of design onsiderations on NOx emissions needs to be onsidered: Fortunately, there is no evidene that ressure ratio influenes NOx rodution rate (on a m basis) in LPM systems. There might be some omromises neessary for engines with high firing temeratures regarding the ooling air usage. But this is a seondary effet at best, beause the ombustor exit temerature and the flame temerature are not diretly related. Some aeroderivative engines, whih tend to have high ressure ratios, have sae limits for the ombustion system, thus might be at a disadvantage. But this is not rimarily due to the high ressure ratio, but rather due to the seifi design of the engine. A limiting fator for lowering NOx emissions is often driven by the onset of ombustor osillations. Again, there is no evidene that the oerating windows that allow oeration without osillations are influened by oerating ressure or firing temerature. They rather seem to deend far more on the seifi engine design. 1 Regarding the requirements for Lean Premix engines, multi-sool engines show no fundamental differenes from single-sool engines. Figure 12. Interation between the gas turbine omonents

8 COMPONENT INTERACTION: DESIGN AND OFF- DESIGN BEHAVIOR When the omressor, the gas generator turbine, and the ower turbine (if aliable) are ombined in a gas turbine, the oeration of eah omonent exerienes ertain oerating onstraints that are aused by the interation between the omonents (Figure 12). For examle, the engine omressor will omress a ertain mass flow, whih in turn ditates the omressor disharge ressure neessary to fore the mass flow through the turbine setion. On the other hand, the gas generator turbine has to rodue suffiient ower to drive the generator. The firing temerature influenes both the ower that the turbines an rodue, but it also imats the disharge ressure neessary from the omressor. The omonents are designed to work together at their highest effiienies at a design oint, but the oeration of the omonents at any other than the design oint must also be onsidered. The onstraints and requirements are different for single-shaft and two-shaft engines; hene, they are treated searately. In the following setion, we will look into the interation between the engine omonents, beause it is this interation that generates the tyial behavior of gas turbines. Single-Shaft Engines A single-shaft engine onsists of an air omressor, a ombustor, and a turbine. The air omressor generates air at a high ressure, whih is fed into to the ombustor, where the fuel is burned. The ombustion roduts and exess air leave the ombustor at high ressure and high temerature. This gas is exanded in the gas generator turbine, whih rovides the ower to turn the air omressor. The exess ower is used to drive the load. Most single-shaft turbines are used to drive eletri generators at onstant seeds. We will not onsider the rare ase of single-shaft turbines driving mehanial loads at varying seeds. The oeration of the omonents requires the following omatibility onditions: Comressor seed Gas generator Turbine seed Mass flow through turbine Mass flow through omressor - Bleed flows + Fuel mass flow Comressor ower < Turbine ower Tyial omressor mas are shown in Figure 7 and Figure 8, resetively. The fat that the gas turbine oerates at onstant seed means any oerating oint of the engine omressor (for given ambient onditions) lies on a single seed line. Load inreases are initiated by inreasing the fuel flow, whih in turn inreases the firing temerature. Due to the fat that the first turbine nozzle is usually hoked, the omressor oerating oint moves to a higher ressure ratio to omensate for the Page 8 redued density (from the higher firing temerature). The ossible oerating oints of the omressor deending on the load running are also shown in the omressor mas (Figure 7 and Figure 8). In the ase of the single-shaft engine driving a generator, redution in outut ower results in only minute hanges in omressor mass flow as well as some redution in omressor ressure ratio. A single-shaft engine has no unique mathing temerature. Used as a generator drive, it will oerate at a single seed, and an be temerature toed at any ambient temerature as long as the load is large enough Two-Shaft Engines A two-shaft gas turbine (Figure 2) onsists of an air omressor, a ombustor, a gas generator turbine, and a ower turbine. 2 The air omressor generates air at a high ressure, whih is fed into the ombustor, where the fuel is burned. The ombustion roduts and exess air leave the ombustor at high ressure and high temerature. This gas is exanded in the gas generator turbine, whih has the sole task of roviding ower to turn the air omressor. After leaving the gas generator turbine, the gas still has a high ressure and a high temerature. It is now further exanded in the ower turbine. The ower turbine is onneted to the driven equiment. It must be noted at this oint, that the ower turbine (together with the driven equiment) an and will run at a seed that is indeendent of the seed of the gas generator ortion of the gas turbine (i.e., the air omressor and the gas generator turbine). The gas generator is ontrolled by the amount of fuel that is sulied to the ombustor. Its two oerating onstraints are the firing temerature and the maximum gas generator seed (on some engines, torque limits may also onstrain the oeration at low ambient temeratures). If the fuel flow is inreased, both firing temerature and gas generator seed inrease, until one of the two oerating limits is reahed. Variable stator vanes at the engine omressor are frequently used, however, not for the urose of ontrolling the airflow, but rather to otimize the gas roduer seed. In two-shaft engines, the airflow is ontrolled by the flow aaities of the gas generator turbine and ower turbine nozzles. Inreasing the seed and temerature of the gas generator rovides the ower turbine with gas at a higher energy (i.e., 2 Some engines are onfigured as multi-sool engines. In this ase, the gas generator has a low-ressure omressor driven by a lowressure turbine and a high-ressure omressor driven by a highressure turbine. For this onfiguration, the shaft onneting the LP omressor and turbine rotates inside the shaft onneting the HP omressor and turbine. In general, all the oerating harateristis desribed above also aly to these engines.

9 higher ressure, higher temerature and higher mass flow), whih allows the ower turbine to rodue more ower. If the ower sulied by the ower turbine is greater than the ower absorbed by the load, the ower turbine together with the driven omressor will aelerate until equilibrium is reahed. The oeration of the omonents requires the following omatibility onditions: Comressor seed Gas generator Turbine seed Mass flow through turbine Mass flow through omressor - Bleed flows + Fuel mass flow Comressor ower Gas generator turbine ower (- mehanial losses) The subsequent free ower turbine adds the requirement that the ressure after the gas generator turbine has to be high enough to fore the flow through the ower turbine. Tyial omressor and turbine mas are shown in Figure 8 and Figure 13, resetively. The gas generator for a two-shaft engine adats to different load requirements (and aordingly different fuel flow) by hanging both seed and firing temerature. Note that the omressor oerating oints are very different between a single-shaft and a two-shaft engine. Figure 13. Shemati turbine erformane ma for two turbines (gas generator and ower turbine) in series Two-shaft engines oerate with the gas generator turbine and the ower turbine in series. The ower turbine ressure ratio 5 / a is thus related to the omressor ressure ratio 2 / a by the identity: 5 a 2 a The ressure dro in the gas generator turbine 5 / 3 and the ressure inrease in the omressor 2 / a are related insofar as the gas generator turbine has to rovide enough ower to drive the omressor Page 9 The maximum ossible ressure ratio 5 / a is ontrolled by the flow aaity Q 5 of the ower turbine. In artiular if the ower turbine is hoked, it will ause the gas generator turbine to oerate at one fixed oint (Figure 13). In many ases, both the gas generator turbine first-stage nozzle and the Power turbine nozzle oerate at or near hoked flow onditions. In this ase, the atual flow Q 3 through the gas generator turbine nozzle is ratially onstant. The mass flow is then only deendent on the ombustor exit ressure 3, the firing temerature T 3, the gas omosition (whih determines γ, and thus the volume inrease during the exansion), and the geometry of the nozzle, whih determines the through flow area(in reality, it is determined by the ritial nozzle area, the learane area and the effetive bleed valve area). The above relationshi has the following onsequenes: 1. Inreasing the firing temerature (without hanging seed or geometry) will lead to a lower mass flow. 2. Inreasing the gas generator seed, thus inreasing 2 and 3, will allow for a larger mass flow. 3. Pressure ratio, seed, and firing temerature are all related, and annot be hanged indeendently of eah other. The turbine geometry determines both flow aaities Q 3 and Q 5, as well as the gas generator turbine effiieny. The omressor geometry and seed set the airflow. With variable IGV's the airflow an be altered without hanging the gas generator seed, thus also setting a new T 3 and a different omressor ressure ratio 2 / 1. The relationshi between 2 / 1 and T 3 remains, however, unhanged: The turbine flow aaities alone determine the gas generator math, not the IGV setting. Closing the IGV's will raise the seed of a temerature toed gas generator, but sine the temerature remains onstant, the airflow tends to remain unhanged (beause the flow through the gas generator turbine nozzle Q 3 remains onstant). If, however, η ggt inreases due to the hange in seed, T 3 has to dro, leading to an inrease in omressor mass flow: The gas generator ums more airflow with the IGV losed and the higher seed than with the IGV oen and the lower seed. The IGVs thus allow to trim the engine suh that the rated T 3 is always reahed at full orreted NGP. Therefore, at high ambient temeratures, when the gas generator would normally slow down, IGV's an be used to kee the seed at a higher level, thus avoiding effiieny enalties in the gas generator turbine. Reduing the net ower outut in a two-shaft engine involves a redution in omressor seed and hene in air flow, ressure ratio and temerature rise. From a omarison of the mas (Figure 14) we see that the omressor in a two-shaft engine oerates for most of the load oints lose to its best effiieny.

10 Page 10 Figure 14. Two-shaft gas turbine erformane ma Figure 15. Two-shaft gas turbine erformane ma

11 Page 11 Figure 16. Single-shaft gas turbine erformane ma Two-shaft engines have a ower turbine where the shaft is not mehanially ouled with the gas generator shaft. These omonents need to be 'mathed', suh that the overall erformane of the gas turbine is otimized for a defined oerating ambient temerature. The seed of the gas generator is therefore not ontrolled by the seed of the driven equiment (suh as in single-shaft generator set aliations). The gas generator seed only deends on the load alied to the engine. If the ower turbine outut has to be inreased, the fuel ontrol valve allows more fuel to enter the ombustor. This will lead to an inrease both in gas generator seed and in firing temerature, thus making more ower available at the ower turbine. The setting of the flow aaity of the ower turbine has obviously a great influene on the ossible oerating oints of the gas generator. For a high resistane of the ower turbine (i.e., low mass flow W for a given 5/1), the gas generator reahes its limiting firing temerature at lower ambient temeratures than with a low resistane of the ower turbine. By altering the exit flow angle of the first stage ower turbine nozzle the required ressure ratio for a ertain flow an be modified (i.e. the flow aaity). This effet is used to math the ower turbine with the gas generator for different ambient temeratures.. Due to mehanial onstraints, both the gas generator seed and the firing temerature have uer limits that annot be exeeded without damaging the engine or reduing its life. Deending on the ambient temerature the aessory load the engine geometry (in artiular the first ower turbine nozzle) The engine will reah one of the two limits first. At ambient temeratures below the math temerature, the engine will be oerating at its maximum gas generator seed, but below its maximum firing temerature (seed toing). At ambient temeratures above the math temerature, the engine will oerate at its maximum firing temerature, but not at its maximum gas generator seed (temerature toing). The math temerature is thus the ambient temerature at whih the engine reahes both limits at the same time (Figure 14 and Figure 15) Beause the first ower turbine nozzle determines the amount of ressure ratio needed by the ower turbine to allow a ertain gas flow it also determines the available ressure ratio for the gas generator turbine. If the ressure ratio available for the gas generator does not allow the balaning of the ower requirement of the engine omressor (see enthaly-entroy diagram), the gas generator will have to slow down, thus reduing the gas flow through the ower turbine. This will redue the ressure ratio neessary over the ower turbine, thus leaving more head for the gas generator to satisfy the omressor ower requirements. Some effets an ause the gas turbine to exhibit an altered math temerature:

12 Gas fuel with a low heating value or water injetion inreases the mass flow through the turbine relative to the omressor mass flow. The temerature toing will thus be shifted to higher ambient temeratures. Dual fuel engines that are mathed on gas, will to early on liquid fuel. This is aused by the hange in the thermodynami roerties of the ombustion rodut due to the different Carbon to Hydrogen ratio of the fuels. The mathing equations indiate, that a redution in omressor effiieny (due to fouling, inlet distortions) or turbine effiieny (inreased ti learane, exessive internal leaks, orrosion) will also ause early toing. Aessory loads also have the effet of leading to earlier toing. Single-Shaft versus Two-Shaft Engines The hoie of whether to use a single-shaft or two-shaft ower lant is largely determined by the harateristis of the driven load. If the load seed is onstant, as in the ase of an eletri generator, a single-shaft unit is often seified; an engine seifially designed for eletri ower generation would make use of a single-shaft onfiguration. An alternative, however, is the use of a two-shaft engine. If the load needs to be driven with varying seeds (omressors, ums), two-shaft engines are advantageous. The two tyes have different harateristis regarding the suly of exhaust heat to a ogeneration or ombined yle lant, rimarily due to the differenes in exhaust flow as load is redued; the essentially onstant air flow and omressor ower in a single-shaft unit results in a larger derease of exhaust temerature for a given redution in ower, whih might neessitate the burning of sulementary fuel in the waste heat boiler under oerating onditions where it would be unneessary with a two-shaft. In both ases, the exhaust temerature may be inreased by the use of variable inlet guide vanes. Cogeneration systems have been suessfully built using both single-shaft and two-shaft units. The torque harateristis are very different and the variation of torque with outut seed at a given ower may well determine the engine's suitability for ertain aliations. The omressor of a single-shaft engine is onstrained to turn at some multile of the load seed, fixed by the transmission gear ratio, so that a redution in load seed imlies a redution in omressor seed. This results in a redution in mass flow, hene of ower and torque. This tye of turbine is only of limited use for mehanial drive uroses. However,the twoshaft engine, having a free ower turbine, has a very favorable torque harateristi. For a onstant fuel flow, and onstant gas generator seed, the free ower turbine an rovide relatively onstant ower for a wide seed range. This is due to the fat that the omressor an suly an essentially onstant flow at a given omressor seed regardless of the free turbine seed. Also, at fixed gas generator oerating onditions, redution in outut seed results in an inrease in Page 12 torque. It is quite ossible to obtain a stall torque of twie the torque delivered at full seed. The atual range of seed over whih the torque onversion is effiient deends on the effiieny harateristi of the ower turbine. The tyial turbine effiieny harateristi shown in Figure 14 suggests that the effiieny enalty will not be greater than about five or six erent over a seed range from half to full seed. Load Any gas turbine will exeriene a redued effiieny at art load (Figure 17). The redution in effiieny with art load differs from design to design. In artiular, DLN engines show different art-load effiienies than their onventional ombustion ounterarts. The neessity to ontrol the fuel to air ratio losely yields different art load behavior when omaring gas turbines with DLN ombustors and with onventional ombustors. A tyial way of ontrolling these engines is by ontrolling the airflow into the ombustor, thus keeing the ombustor rimary zone temerature within narrow limits. The art load behavior of single-shaft and twoshaft, Standard Combustion and Dry Low Nox onets is fundamentally different. This is both due to the different aerodynami onfiguration, and the requirements of keeing the fuel to air ratio within a narrow window for DLN engines. Figure 17. Thermal effiieny of tyial industrial gas turbines as a funtion of load To oerate the engine at art load, the fuel flow is adjusted, until some ontrol arameter (for examle the flow through the driven omressor, or a ertain kw value for a generator) is satisfied. Other adjustments, suh as guide vane settings or byassing ombustion air may be neessary. For a single-shaft engine, whih has to oerate at onstant gas generator seed (to kee the generator frequeny onstant) this means that the firing temerature will be hanged with load. A governor will kee the seed onstant and will inrease the fuel flow with inreasing load, thus inreasing the firing temerature, until the ontrol limit is reahed. Due to the onstant seed, the airflow through the engine will not

13 vary greatly between full load and art load. This means, that the fuel to air ratio dros signifiantly at art load and the ombustor exit temerature dros signifiantly from full load to art load. Therefore, most single-shaft dry-low NOx engines use variable stator vanes on the engine omressor to vary the airflow, and thus kee the fuel to air ratio relatively onstant. For a given mass flow, any inrease in firing temerature would inrease the volume flow through the turbine setion. Therefore the ressure sulied from the omressor has to be inreased, beause the turbine nozzle is at or near hoked onditions. Sine the omressor also oerates at onstant seed, the result is a redution of mass flow until equilibrium is reahed. A tyial erformane ma for a single-shaft engine shows this inrease in omressor disharge ressure at inreased load. If the engine is equied with VIGVs to kee the fuel to air ratio in the ombustor onstant, a redution in load will require a losing of the VIGVs to redue the airflow. Closing the VIGVs also redues the ressure ratio of the omressor at onstant seed. For single-shaft engines, as desribed above, the art load effiienies of gas turbines with Lean Premix (LPM) ombustion and onventional ombustion are very similar. The need to bleed ombustion air for two-shaft engines tyially leads to lower art load effiieny for engines equied with LPM ombustors. For a two-shaft engine, both gas generator seed and firing temerature hange with load. An inrease in load at the ower turbine will ause the fuel flow to inrease. Beause the gas generator is not mehanially ouled with the ower turbine, it will aelerate, thus inreasing airflow, omressor disharge ressure, and mass flow. The inrease in gas generator seed means, that the omressor now oerates at a higher Mah number. At the same time the inreased fuel flow will also inrease the firing temerature. The relative inrease is governed by the fat that the ower turbine requires a ertain ressure ratio to allow a given amount of airflow ass. This fores equilibrium where the following requirements have to be met: 1. The omressor ower equals gas generator turbine ower. This determines the available ressure ustream of PT. 2. The available ressure ratio at the ower turbine is suffiient to allow the airflow to be fored through the ower turbine. Deending on the ambient temerature relative to the engine math temerature, the fuel flow into the engine will either be limited by reahing the maximum firing temerature or the maximum gas generator seed. The ambient temerature, where both ontrol limits are reahed at the same time is alled engine math temerature. Page 13 Variable stator vanes at the engine omressor are frequently used, however, not for the urose of ontrolling the airflow. This is due to the fat that in two-shaft engines, the airflow is ontrolled by the flow aaities of the gas generator turbine and ower turbine nozzles. To ontrol the fuel to air ratio in the ombustor, another ontrol feature has to be added for two-shaft engines with DryLowNox ombustors: Usually, a ertain amount of air is bled from the omressor exit diretly into the exhaust dut. This leads to the fat that while the airflow for two-shaft engines is redued at art load, the redution in airflow is larger for an engine with a standard ombustion system. Like in single-shaft engines, the ombustor exit temerature at art load dros signifiantly for engines with standard ombustion, while it stays relatively high for DLN engines. The dro in ombustor temerature in engines with standard ombustion, whih indiates the leaner fuel to air ratio, automatially leads to NOx emissions that are lower at art load than at full load. In DLN engines, there is virtually no suh redution, beause the requirement to limit CO and UHC emissions limits the (theoretially ossible) redution in fuel to air ratio. Power Turbine Seed For any oerating ondition of the gas generator, there is an otimum ower turbine seed at whih the ower turbine oerates at its highest effiieny, and thus rodues the highest amount of ower for a given gas generator oerating oint. Aerodynamially, this otimum oint is haraterized by a ertain ratio of atual flow Q5 over rotating seed N PT. The volumetri flow deends on the ambient temerature and the load. This exlains why the otimum ower turbine seed is a funtion of ambient temerature and load. If the ower turbine does not oerate at this the otimum ower turbine seed, the ower outut and the effiieny of the ower turbine will be lower (Figure 15). The imat of hanging the ower turbine seed is easily desribed by: N 2 N PT, ot N N PT, ot P P ot PT This equation an be derived from basi relationshis (Brun and Kurz, 2000) and is retty aurate for any arbitrary ower turbine. When using this relationshi, it must be onsidered that the otimum ower turbine seed (N t,ot ) deends on the gas generator load and the ambient temerature (Kurz and Brun, 2001). In general, the otimum ower turbine seed is redued for inreasing ambient temeratures and lower load (Figure 15). The heat rate beomes for a onstant gas generator oerating oint: PT 2

14 HR HR ot Off otimum seed of the ower turbine redues the effiieny and the ability to extrat head from the flow. Even if N GG (and the fuel flow) do not hange, the amount of ower that is rodued by the PT is redued. Also, beause of the unhanged fuel flow, the engine heat rate inreases and the exhaust temerature inreases aordingly. Theoretially, any engine would reah its maximum exhaust temerature at high ambient, full load and loked PT. Another interesting result of the above is the torque behavior of the ower turbine, onsidering that torque is ower divided by seed: τ τ ot P N 2 N ot P PT PT, ot The torque is thus a linear funtion of the seed, with the maximum torque at the lowest seed This exlains one of the great attrations of a free ower turbine: To rovide the neessary torque to start the driven equiment is usually not diffiult (omared to eletri motor drives or reiroating engines) beause the highest torque is already available at low seeds of the ower turbine. Influene of Emission Control Tehnologies All emission ontrol tehnologies that use lean-remix ombustion require a reise management of the fuel to air ratio in the rimary zone of the ombustor (i.e., where the initial ombustion takes lae) as well as the reise distribution of ombustor liner ooling and dilution flows. Deviations in these areas an lead to inreased NOx rodution, higher CO or UHC levels, or flame-out (Greenwood, 2000). Lean-remix ombustion ahieves redution in NOx emissions by lowering the flame temerature. The flame temerature is determined by the fuel to air ratio in the ombustion zone. A stoihiometri fuel to air ratio (suh as in onventional ombustors) leads to high flame temeratures, while a lean fuel to air ratio an lower the flame temerature signifiantly. However, a lean fuel to air mixture also means, that the ombustor is oerating lose to the lean flame-out limit. Any art load oeration will ause redution of fuel to air ratio, beause the redution in air flow is smaller than the redution in fuel flow. Several different aroahes to ontrol the fuel to air ratio are ossible to avoid flame-out at art load or transient situations, for examle bleeding air overboard Page 14 using variable inlet guide vanes managing the ratio between fuel burned in lean remix mode and in a diffusion flame to name a few. Obviously, all these aroahes an have an effet on the art load erformane harateristis of the gas turbine. Variable Inlet and Stator Vanes Many modern gas turbines use variable inlet guide vanes and variable stator vanes in the engine omressor. Adjustable vanes allow altering the stage harateristis of omressor stages (see exlanation on Euler Equations) beause they hange the head making aability of the stage by inreasing or reduing the re-swirl ontribution. This means that for a resribed ressure ratio they also alter the flow through the omressor. It is therefore ossible to hange the flow through the omressor without altering its seed. There are three imortant aliations: 1. During startu of the engine it is ossible to kee the omressor from oerating in surge 2. The airflow an be ontrolled to maintain a onstant fuel to air ratio in the ombustor for dry low NOx aliations on single-shaft mahines. 3. Two-shaft engines an be ket from droing in gas generator seed at ambient temeratures higher than the math temerature, i.e.; the gas generator turbine will ontinue to oerate at its highest effiieny. Aessory Loads Aessory loads are due to mehanially driven lube oil or hydrauli ums. While the aessory load an be treated fairly easily in a single-shaft engine - its ower requirement is subtrated from the gross engine outut - this is somewhat more omliated in a two-shaft mahine: In a two-shaft gas turbine, the aessory load is tyially taken from the gas generator. In order to satisfy the equilibrium onditions the gas generator will have to run hotter than without the load. This ould lead to more ower outut at onditions that are not temerature limited. When the firing temerature is limited (i.e., for ambient temeratures above the math oint), the ower outut will fall off more raidly than without the load. That means that an aessory load of 50 h may lead to ower losses at the ower turbine of 100 or more h at higher ambient temeratures. The heat rate will inrease due to aessory loads at all ambient temeratures. The net effet of aessory loads an also be desribed as a move of the math oint to lower ambient temeratures.

15 Control Temerature One of the two oerating limits of a gas turbine is the turbine rotor inlet temerature (TRIT or T3). Unfortunately, it is not ossible to measure this temerature diretly - a temerature robe would only last for a few hours at temeratures that high. Therefore, the inlet temerature into the ower turbine (T5 ) is measured instead. The ratio between T 3 and T 5 is determined during the fatory test, where T 5 is measured and T 3 is determined from a thermodynami energy balane. This energy balane requires the aurate determination of outut ower and air flow, and an therefore be erformed best during the fatory test. It must be noted, that both T 3 and T 5 are irumferentially and radially very non-uniformly distributed in the referene lanes (ie at the ombustor exit, at the rotor inlet, at the ower turbine inlet). Performane alulations use a thermodynami average temerature. This is not exatly the temerature one would measure as the average of a number of irumferentially distributed temerature robes. Rather than ontrolling T 3 the ontrol system limits engine oerations to the T 5 that orresonds to the rated T 3. However, the ratio between T 3 and T 5 is not always onstant, but varies with the ambient temerature: The ratio T 3 / T 5 is redued at higher ambient temeratures. Modern ontrol algorithms an take this into aount. Engines an also be ontrolled by their exhaust temerature (T7). For single-shaft engines, measuring T 7 or T 5 are equivalent hoies. For two-shaft engines, measuring T 7 instead of T 5 adds the omliation that the T 7 ontrol temerature additionally deends on the ower turbine seed, while the relationshi between T 3 and T 5 does not deend on the ower turbine seed. INFLUENCE OF AMBIENT CONDITIONS Ambient Temerature Changes in ambient temerature have an imat on fullload ower and heat rate, but also on art-load erformane and otimum ower turbine seed (Figure 14) Manufaturers tyially rovide erformane mas that desribe these relationshis for ISO onditions. These urves are the result of the interation between the various rotating omonents and the ontrol system. This is artiularly true for DLN engines. If the ambient temerature hanges, the engine is subjet to the following effets: Page 15 with inreasing temerature. 2. The ressure ratio of the omressor at onstant seed gets smaller with inreasing temerature. This an be determined from a Mollier diagram, showing that the higher the inlet temerature is, the more work (or head)is required to ahieve a ertain ressure rise. The inreased work has to be rovided by the gas generator turbine, and is thus lost for the ower turbine, as an be seen in the enthaly-entroy diagram. At the same time N Ggorr (ie the mahine Mah number) at onstant seed is redued at higher ambient temerature. As exlained reviously, the inlet Mah number of the engine omressor will inrease for a given seed, if the ambient temerature is redued. The gas generator Mah number will inrease for redued firing temerature at onstant gas generator seed. The Enthaly-Entroy Diagram (Figure 3) desribes the Brayton yle for a two-shaft gas turbine. Lines 1 to 2 and 3 to 4 must be aroximately equal, beause the omressor work has to be rovided by the gas generator turbine work outut. Line 4-5 desribes the work outut of the ower turbine. At higher ambient temeratures, the starting oint 1 moves to a higher temerature. Beause the head rodued by the omressor is roortional to the seed squared, it will not hange if the seed remains the same. However, the ressure ratio rodued, and thus the disharge ressure, will be lower than before. Looking at the ombustion roess 2 to 3, with a higher omressor disharge temerature and onsidering that the firing temerature T3 is limited, we see that less heat inut is ossible, ie., less fuel will be onsumed.the exansion roess has, due to the lower 2 3, less ressure ratio available or a larger art of the available exansion work is being used u in the gas generator turbine, leaving less work available for the ower turbine. On two-shaft engines, a redution in gas generator seed ours at high ambient temeratures. This is due to the fat that the equilibrium ondition between the ower requirement of the omressor (whih inreases at high ambient temeratures if the ressure ratio must be maintained) and the ower rodution by the gas generator turbine (whih is not diretly influened by the ambient temerature as long as omressor disharge ressure and firing temerature remain) will be satisfied at a lower seed. The lower seed often leads to a redution of turbine effiieny: The inlet volumetri flow into the gas generator turbine is determined by the first stage turbine nozzle, and the Q 3 /N GG ratio (i.e., the oerating oint of the gas generator turbine) therefore moves away from the otimum. Variable omressor guide vanes allow keeing the gas generator seed onstant at higher ambient temeratures, thus avoiding effiieny enalties. 1. The air density hanges. Inreased ambient temerature lowers the density of the inlet air, thus reduing the mass flow through the turbine, and therefore redues the ower outut (whih is roortional to the mass flow) even further. At onstant seed, where the volume flow remains aroximately onstant, the mass flow will In a single-shaft, onstant seed gas turbine one would inrease with dereasing temerature and will derease see a onstant head (beause the head stays roughly onstant

16 for a onstant omressor seed), and thus a redued ressure ratio. Beause the flow aaity of the turbine setion determines the ressure-flow-firing temerature relationshi, equilibrium will be found at a lower flow, and a lower ressure ratio, thus a redued ower outut. 1. The omressor disharge temerature at onstant seed inreases with inreasing temerature. Thus, the amount of heat that an be added to the gas at a given maximum firing temerature is redued. 2. The relevant Reynolds number hanges At full load, single-shaft engines will run a temerature toing at all ambient temeratures, while two-shaft engines will run either at temerature toing (at ambient temeratures higher than the math temerature) or at seed toing (at ambient temeratures lower than the math temerature). At seed toing, the engine will not reah its full firing temerature, while at temerature toing, the engine will not reah its maximum seed. The net effet of higher ambient temeratures is an inrease in heat rate and a redution in ower. The imat of ambient temerature is usually less ronouned for the heat rate than for the ower outut, beause hanges in the ambient Page 16 temerature imat less the omonent effiienies than the overall yle outut. Inlet and Exhaust Pressure Losses Any gas turbine needs an inlet and exhaust system to oerate. The inlet system onsists of one or several filtration systems, a silener, duting, and ossibly de-iing, fogging, evaorative ooling and other systems. The exhaust system may inlude a silener, duting, and waste heat reovery systems. All these system will ause ressure dros, i.e. the engine will atually see an inlet ressure that is lower than ambient ressure, and will exhaust against a ressure that is higher than the ambient ressure. These inevitable ressure losses in the inlet and exhaust system ause a redution in ower and yle effiieny of the engine. The redution in ower, omared to an engine at ISO onditions, an be desribed by simle orretion urves, whih are usually sulied by the manufaturer. The ones shown in Figure 21 desribe the ower redution for every inh (or millimeter) of water ressure loss. These urves an be easily aroximated by seond order olynomials. The imat on heat rate is easily alulated by taking the fuel flow from ISO onditions and dividing it by the redued ower. Figure 18. Corretion fators for inlet losses, exhaust losses, and site elevation

17 Page 17 Figure 19. Power and heat rate as a funtion of site elevation (tyial) Figure 20. Effet of low fuel gas ressure on different engine designs. PCD is the ressure in the ombustor. The imat is universal for any engine, exet for the Ambient Pressure result of some seondary effets suh as aessory loads. If the ambient ressure is known, the erformane orretion an be easily aomlished by: The imat of oerating the engine at lower ambient ressures (for examle, due to site elevation or simly due to hanging atmosheri onditions) is that of a redued air density (Figure 21 and Figure 19). The engine, thus, sees a lower mass flow (while the volumetri flow is unhanged). The hanged density only imats the ower outut, but not the effiieny of the engine. However, if the engine drives aessory equiment through the gas generator, this is no longer true, beause the ratio between gas generator work and required aessory ower (whih is indeendent of hanges in the ambient onditions) is affeted. δ ambient " ( in _ Hg) " Hg If only the site elevation is known, the ambient ressure a at normal onditions is: sealevel elevation( ft) a e

18 Fuel While the influene of the fuel omosition on erformane is rather omlex, fortunately the effet on erformane is rather small if the fuel is natural gas. Fuel gas with a large amount of inert omonents (suh as CO 2 or N 2 ) has a low Wobbe index, while substanes with a large amount of heavier hydroarbons have a high Wobbe index. Pieline quality natural gas has a Wobbe index of about In general, engines will rovide slightly more ower if the Wobbe Index WI LHV SG is redued. This is due to the fat that the amount of fuel mass flow inreases for a given amount of fuel energy when the Wobbe index is redued. This inreases the mass flow though the turbine setion, whih inreases the outut of the turbine. This effet is to some degree ounterated by the fat that the omressor ressure ratio inreases to ush the additional flow through the flow restrited turbine. In order to do this, the omressor will absorb somewhat more ower. The omressor will also oerate loser to its stall margin. The above is valid irresetive whether the engine is a two-shaft or single-shaft engine. The fuel gas ressure at skid edge has to be high enough to overome all ressure losses in the fuel system and the ombustor ressure, whih is roughly equal to the omressor disharge ressure 2. The omressor disharge ressure at full load hanges with the ambient temerature, and therefore, a fuel gas ressure that is too low for the engine to reah full load at low ambient temerature may be suffiient if the ambient temerature inreases. If the fuel suly ressure is not suffiient, single and two-shaft engines show distintly different behavior, namely: A two-shaft engine will run slower, suh that the ressure in the ombustor an be overome by the fuel ressure (Figure 20). If the driven equiment is a gas omressor (and the roess gas an be used as fuel gas), 'bootstraing' is often ossible: The fuel gas is sulied from the gas omressor disharge side. If the initial fuel ressure is suffiient to start the engine and to oerate the gas omressor, the gas omressor will inrease the fuel gas ressure. Thus the engine an rodue more ower whih in turn will allow the gas omressor to inrease the fuel ressure even more, until the fuel gas ressure neessary for full load is available. A single-shaft engine, whih has to run at onstant seed, will exeriene a severe redution in ossible firing temerature and signifiant loss in ower outut, unless it uses VIGVs. With VIGVs, the omressor exit ressure, and thus Page 18 the ombustor ressure an also be influened by the osition of the VIGVs, thus leading to less ower loss (Figure 20). Without VIGVs, the only way to redue PCD ressure is by moving the oerating oint of the omressor on its ma. This an be done by reduing the bak ressure from the turbine, whih requires a redution in volume flow. Sine the seed is fixed, only a redution in firing temerature -whih redues the volume flow through the gas generator if everything else remains unhanged- an ahieve this. A redued volume flow will redue the ressure dro required for the gas generator turbine. Industrial Gas Turbines allow oeration with a wide variety of gaseous and liquid fuels. To determine the suitability for oeration with a gas fuel system, various hysial arameters of the roosed fuel need to be determined: Heating value, dew oint, Joule-Thomson oeffiient, Wobbe index, and others (Elliott et al., 2004). However, fuel borne ontaminants an also ause engine degradation. Seial attention should be given to the roblem of determining the dew oint of the otential fuel gas at various ressure levels. In artiular, the treatment of heavier hydroarbons and water must be addressed. Sine any fuel gas system auses ressure dros in the fuel gas, the temerature redution due to the Joule-Thomson effet has to be onsidered and quantified (Kurz et al., 2004). Gas fuels for gas turbines are ombustible gases or mixtures of ombustible and inert gases with a variety of omositions overing a wide range of heating values and densities. The ombustible omonents an onsist of methane and other low moleular weight hydroarbons, hydrogen and arbon monoxide. The major inert omonents are nitrogen, arbon dioxide, and water vaor. It is generally aeted that this tye of fuel has to be omletely gaseous at the entry to the fuel gas system and at all oints downstream to the fuel nozzle (ASME, 1992). Gaseous fuels an vary from oor quality wellhead gas to high quality onsumer or ieline gas. In many systems, the gas omosition and quality may be subjet to variations (Newbound et al., 2003). Tyially, the major soures of ontaminants within these fuels are: Solids Water Heavy gases resent as liquids Oils tyial of omressor oils Hydrogen sulfide (H 2 S) Hydrogen (H 2 ) Carbon monoxide (CO) Carbon dioxide (CO 2 ) Siloxanes Other fators that will affet turbine or ombustion system life and erformane inlude lower heating value

19 (LHV), seifi gravity (SG), fuel temerature, and ambient temerature. Some of these issues may o-exist and be interrelated. For instane, water, heavy gases resent as liquids, and leakage of mahinery lubriating oils, may be a roblem for turbine oerators at the end of a distribution or branh line, or at a low oint in a fuel suly line. Water in the gas may ombine with other small moleules to rodue a hydrate a solid with an ie-like aearane. Hydrate rodution is influened, in turn, by gas omosition, gas temerature, gas ressure and ressure dros in the gas fuel system. Liquid water in the resene of H 2 S or CO 2 will form aids that an attak fuel suly lines and omonents. Free water an also ause turbine flameouts or oerating instability if ingested in the ombustor or fuel ontrol omonents. Heavy hydroarbon gases resent as liquids rovide many times the heating value er unit volume than they would as a gas. Sine turbine fuel systems meter the fuel based on the fuel being a gas, this reates a safety roblem, eseially during the engine start-u sequene when the suly line to the turbine still may be old. Hydroarbon liquids an ause: Turbine overfueling, whih an ause an exlosion or severe turbine damage). Fuel ontrol stability roblems, beause the system gain will vary as liquid slugs or drolets move through the ontrol system. Combustor hot streaks and subsequent engine hot setion damage. Overfueling the bottom setion of the ombustor when liquids gravitate towards the bottom of the manifold Internal injetor blokage over time, when traed liquids yrolyze in the hot gas assages. Liquid arryover is a known ause for raid degradation of the hot gas ath omonents in a turbine (Meher-Homji et al., 1998). The ondition of the ombustor omonents also has a strong influene and fuel nozzles that have aumulated ieline ontaminants that blok internal assageways will robably be more likely to miss desired erformane or emission targets. Thus, it follows that more maintenane attention may be neessary to assure that ombustion omonents are in remium ondition. This may require that fuel nozzles be inseted and leaned at more regular intervals or that imroved fuel filtration omonents be installed. Relative Humidity The imat on engine erformane would be better desribed by the water ontent of the air (say, in mole%) or in terms of the seifi humidity (kg H20 /kg dry air ). Figure 21 Page 19 illustrates, this, relating relative humidity for a range of temeratures with the seifi humidity. Figure 21. Grahi exlanation of seifi and relative humidity as a funtion of temerature The main roerties of onern that are affeted by humidity hanges are density, seifi heat, and enthaly. Beause the moleular weight of water (18 g/mol) is less than dry air (28 g/mol), density of ambient air atually dereases with inreasing humidity. When the density of the ambient air dereases the total mass flow will derease, whih then will derease thermal effiieny and outut ower. Performane of the ombustor and turbines as a funtion of humidity is dominated by the hanges in seifi heat and enthaly. Inreases in water ontent will derease temeratures during and after ombustion (the same reason water is injeted into the fuel to redue NO x levels). Sine the water onentration in the air for the same relative humidity inreases with inreasing temerature, the effets on engine erformane are negligible for low ambient temeratures and fairly small (in the range of 1 or 2%) even at high temeratures of 38 C (100 F). The water ontent hanges the thermodynami roerties of air (suh as density and heat aaity) and thus auses a variety of hanges in the engine. For single-shaft engines, inreasing humidity will derease temeratures at the omressor exit. Humidity also auses dereased flame temeratures at a given fuel air ratio. As a result T2, ombustor exit temerature, TRIT and T5 all derease with an inrease in humidity. Sine the seed is set in single-shaft engines, the ontrols system will inrease fuel flow in order to get T5 temerature u to the toing set oint. Desite the inrease in fuel flow, the total exhaust flow still dereases due to the derease in airflow. Outut ower inreases throughout the range of temeratures and humidity exeriened by the engines, whih shows that the inreased fuel energy inut has a greater influene on outut ower than does the dereased total flow. In two-shaft engines, we have to distinguish whether the engine runs at maximum seed (NGP toed), or at maximum firing temerature (T5 toed). Inreasing humidity will derease air density and mass flow when running NGP toed,

20 whih will derease outut ower. This is the general trend in outut ower notied in all two-shaft engines when running NGP toed. As reviously disussed, inreased humidity auses lower T2, Flame temerature, TRIT, and T5 temeratures. When running T5 toed, the trend in outut ower reverses due to the engine inreasing fuel flow to inrease temeratures, and results in inreased outut ower. So for two-shaft engines, outut ower will be seen to inrease when running T5 toed, and to derease when running NGP toed. WHAT DO TYPICAL MAPS SHOW? Beause the gas turbine erformane varies signifiantly from one design to the other, the roedure to determine the erformane of the engine for a seified oerating oint is to use the manufaturer s erformane mas. Today, these mas are usually embedded in software rograms that allow the alulation of erformane arameters of the engine. Tyial engine erformane mas are shown in Figure 16 for single-shaft engines and in Figure 14 and Figure 15 for two-shaft engines. In general, these mas an be used to determine the engine full load outut at a given ambient temerature, and a given ower turbine seed. They also show the fuel flow at any load, as well as exhaust flow and temerature. Additional mas allow orretion for inlet and exhaust losses as well as for the site elevation. For diagnosti uroses, the mas also allow to determine the exeted omressor disharge ressure, ontrol temerature (tyially ower turbine inlet temerature or exhaust temerature) and gas generator seed at any oerating oint. Disreanies between the exeted and the atual values may be indiative of engine roblems. In order to fully understand the information dislayed on engine erformane mas, we want to determine what the reason is for an engine to behave the way it does 3. It should be noted that, artiularly in the field, the measurement of ower outut, heat rate, exhaust flow and exhaust temerature are usually rather diffiult (Brun and Kurz, 2001). Understanding the oerating riniles of the engine is therefore a useful tool of interreting data. PERFORMANCE DEGRADATION Page 20 Any rime mover exhibits the effets of wear and tear over time. The roblem of rediting the effets of wear and tear on the erformane of any engine is still a matter of disussion. Beause the funtion of a gas turbine is the result of the fine-tuned ooeration of many different omonents, the gas turbine has to be treated as a system, rather than as isolated omonents (Kurz and Brun, 2001). Treating the gas turbine akage as a system reveals the effets of degradation on the math of the omonents as well as on the math with the driven equiment. The mehanisms that ause engine degradation are hanges in blade surfaes due to erosion or fouling, and the effet on the blade aerodynamis; hanges in seal geometries and learanes, and the effet on arasiti flows, hanges in the ombustion system (e.g. whih result in different attern fators) The funtion of a gas turbine is the result of the fine-tuned ooeration of many different omonents. Any of these arts an show wear and tear over the lifetime of the akage, and thus an adversely affet the oeration of the system. In artiular the aerodynami omonents, suh as the engine omressor, the turbines, the driven um, or omressor have to oerate in an environment that will invariably degrade their erformane. The understanding of the mehanisms that ause degradation as well as the effets that the degradation of ertain omonents an have on the overall system are a matter of interest. Several mehanisms ause the degradation of engines: Fouling is aused by the adherene of artiles to airfoils and annulus surfaes. The adherene is aused by oil or water mists. The result is a build-u of material that auses inreased surfae roughness and to some degree hanges the shae of the airfoil (if the material build u forms thiker layers of deosits). Many of the ontaminants are smaller than 2 µm. Fouling an normally eliminated by leaning. Hot orrosion is the loss of material from flow ath omonents aused by hemial reations between the omonent and ertain ontaminants, suh as salts, mineral aids or reative gases. The roduts of these hemial reations may adhere to the aero omonents as sale. High temerature oxidation, on the other hand, is the hemial reation between the omonents metal atoms and oxygen from the surrounding hot gaseous environment. The rotetion through an oxide sale will in turn be redued by any mehanial damage suh as raking or salling, for examle during thermal yles. 3 API 616 (1998) resribes another form of reresenting engine erformane than desribed above. The ma for single-shaft engines in API 616 (1998) is not artiularly useful for single-shaft engines driving generators, beause it shows the erformane as a funtion of gas generator seed. For these generator set aliations, however, the gas generator seed is always onstant. The API 616 mas used to reresent two-shaft engines do not allow a desrition of the engine erformane at varying ambient temeratures. Also, the ontrol temerature for two-shaft engines is usually not the exhaust temerature (as ostulated bin one of the API 616 urves), but the Erosion is the abrasive removal of material from the flow ower turbine inlet temerature. The most useful urve in API 616 is ath by hard artiles iminging on flow surfaes. These essentially a subset of the ower turbine urve in Figure 15.

21 artiles tyially have to be larger than 20 µm in diameter to ause erosion by imat. Erosion is robably more a roblem for airraft engine aliations, beause state of the art filtration systems used for industrial aliations will tyially eliminate the bulk of the larger artiles. Erosion an also beome a roblem for driven omressors or ums where the roess gas or fluid arries solid materials. Damage is often aused by large foreign objets striking the flow ath omonents. These objets may enter the engine with the inlet air, or the gas omressor with the gas stream, or are the result of broken off iees of the engine itself. Piees of ie breaking off the inlet, or arbon build u breaking off from fuel nozzles an also ause damage. Abrasion is aused when a rotating surfae rubs on a stationary surfae. Many engines use abradable surfaes, where a ertain amount of rubbing is allowed during the run-in of the engine, in order to establish roer learanes. The material removal will tyially inrease seal or ti gas. While some of these effets an be reversed by leaning or washing the engine, others require the adjustment, reair, or relaement of omonents. It should be noted, that the determination of the exat amount of erformane degradation in the field is rather diffiult. Test unertainties are tyially signifiant, eseially if akage instrumentation as oosed to a alibrated test faility is used. Even trending involves signifiant unertainties, beause in all ases the engine erformane has to be orreted from datum onditions to a referene ondition. Three major effets determine the erformane deterioration of the omressor: Inreased ti learanes Changes in airfoil geometry Changes in airfoil surfae quality While the first two effets tyially lead to nonreoverable degradation, the latter effet an at least be artially reversed by washing the omressor. The overall effet of degradation on an engine omressor yields added losses and lower aability of generating head. Tyially, a degraded omressor also will have a redued surge or stall margin (Sakovszki et al., 1999). This will not have any signifiant effet on the steady state oeration, as long as other effets that lower the stall margin (suh as water or steam injetion) are avoided (Brun et al., 2005). For a given seed of a degraded omressor, eah subsequent stage will see lower Mah numbers (beause of the higher temerature) and an inreased axial veloity omonent (beause ρ /RT, where is redued, T is inreased, thus the density gets redued). Page 21 The net effet will be that while in the new mahine, all stages were working at their otimum effiieny oint at design surge margins. The degradation will fore all stages after the first one to work at off-otimum surge margins and lower than design effiieny. This will not only lower the overall effiieny and the ressure ratio that an be ahieved, but also the oerating range. Calulations for a tyial axial omressor (Kurz and Brun, 2001) reveal that the ombined effets of airfoil fouling and inreased learanes lead to loss of ressure ratio, loss of effiieny and loss of range or stall margin. In artiular the inreased learanes ause hoke at lower flow. RECOVERABLE AND NON-RECOVERABLE DEGRADATION The distintion between reoverable and non-reoverable degradation is somewhat misleading. The majority of degradation is reoverable; however, the effort is very different deending on the tye of degradation. The reovery effort may be as small as water or detergent on-line washing, or detergent on-rank washing. The degradation reovery by any means of washing is usually referred to as reoverable degradation. However, a signifiant amount of degradation an be reovered by engine adjustments (suh as resetting variable geometry). Last but not least, various degrees of omonent relaement in overhaul an bring the system erformane bak to as-new onditions. PROTECTION AGAINST DEGRADATION While engine degradation annot entirely be avoided, ertain reautions an learly slow the effets down. These reautions inlude the areful seletion and maintenane of the air filtration equiment, and the areful treatment of fuel, steam, or water that are injeted into the ombustion roess. It also inludes obeying manufaturer s reommendations regarding shut-down and restarting roedures. For the driven equiment surge avoidane, roess gas free of solids and liquids, and oeration within the design limits need to be mentioned. With regards to steam injetion, it must be noted, that the requirements for ontaminant limits for a gas turbine are, due to the higher roess temeratures, more stringent than for a steam turbine. The site loation and environment onditions, whih ditate airborne ontaminants, their size, onentration, and omosition, need to be onsidered in the seletion of air filtration. Atmosheri onditions suh as humidity, smog, reiitation, mist, fog, dust, oil fumes, or industrial exhausts will rimarily effet the engine omressor. Fuel quality will imat the hot setion. The leanliness of the roess gas, entrained artiles or liquids, will affet the driven equiment erformane. Given all these variables, the rate of degradation is imossible to redit with reasonable auray.

22 Thorough on-rank washing an remove deosits from the engine omressor blades, and is an effetive means for reovering degradation of the engine omressor. The engine has to be shut down, and allowed to ool-down rior to alying detergent to the engine omressor while it rotates at slow seed. Online leaning, where detergent is srayed into the engine running at load an extend the eriods between onrank washing, but it annot relae it. If the omressor blades an be aessed with moderate effort, for examle, when the omressor asing is horizontally slit, handleaning of the blades an be very effetive. TRANSIENT BEHAVIOR All the above onsiderations were made with the assumtion that the engine oerates at a steady state onditions. We should briefly disuss the engine oeration during load transients, i.e., when load is added or removed. Figure 22 shows the engine limits (for a two-shaft engine) from start to the full load design oint: The engine initially is aelerated by a starter. At a ertain GP seed, fuel is injeted and light-off ours. The fuel flow is inreased until the first limit, maximum firing temerature, is enountered. The engine ontinues to aelerate, while the fuel flow is further inreased. Soon, the surge limit of the engine omressor limits the fuel flow. While the starter ontinues to aelerate the engine, a oint is reahed where the steady state oerating line an be reahed without violating surge or temerature limits: at this oint, the engine an oerate self-sustaining, i.e. the starter an disengage. The maximum aeleration (i.e. the maximum load addition) an now be ahieved by inreasing to the maximum ossible fuel flow. However, the maximum ossible fuel flow is limited by either the surge limit of the engine omressor or the maximum firing temerature. CONCLUSION Page 22 The revious ages have given some insight into the working riniles of a gas turbine and what the effet of these working riniles on the oerating harateristis of gas turbines is. Based on this foundation, it was exlained what the effets of hanges in ambient temerature, barometri ressure, inlet and exhaust losses, relative humidity, aessory loads, different fuel gases, or hanges in ower turbine seed are. The tois resented aim at enhaning the understanding of the oeration riniles of a gas turbine in industrial aliations NOMENCLATURE A E f h L M M n N P Q q R s T U V W w η τ γ ω throughflow area veloity vetor in stationary frame seifi heat lower heating value enthaly length Mah number Mahine Mah number rotational seed in rm stagnation ressure ower volumetri flow rate heat flow seifi gas onstant entroy temerature blade veloity veloity vetor mass flow rates veloity vetor in rotating frame differene effiieny torque seifi heat ratio rotational seed in rad/se 2?N/60 Figure 22. Start and aeleration ma for a gas turbine If the load suddenly dros, the maximum rate deeleration is limited by the flame out limits of the engine. ACRONYMS GG -Gas generator IGV -Inlet guide vane NGP -Gas Generator Seed PCD -Gas Generator Comressor disharge ressure PT -Power turbine TRIT -Turbine rotor inlet temerature VIGV -Variable inlet guide vane SUBSCRIPTS 1 at engine inlet 2 at engine omressor exit 3 at turbine inlet