Introduction_To_Gas_Turbines.pdf

Engineering Encyclopedia Saudi Aramco DeskTop Standards

INTRODUCTION TO GAS TURBINES

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use o f Saudi Aramco’s employees. employees . Any material contained in this document which is not already in the public p ublic domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Mechanical

For additional information on this subject, contact

Engineering Encyclopedia

Gas Turbines Introduction to Gas Turbines

Section

Page

INFORMATION INFORMATION ...................................................... ................................................................................. ...................................................... .............................. ... 3 INTRODUCTION................................... INTRODUCTION.............................................................. ...................................................... ............................................... .................... 3 TYPES OF GAS TURBINES TURBINES AND AND THEIR OPERATING OPERATING CYCLES ................................. ................................. 7 HEAVY-DUTY HEAVY-DUTY (TYPE (TYPE H – INDUSTRIAL) INDUSTRIAL) .................................................. ............................................................... ............. 7 AIRCRAFT-DERIVA AIRCRAFT-DERIVATIVE TIVE (TYPE (TYPE G) ................................................... ....................................................................... .................... 9 HYBRID .................................................. ............................................................................ .................................................... .................................... .......... 12 GAS TURBINES: MAJOR CYCLES AND COMPONENTS........................ COMPONENTS........................................... ................... 13 CYCLE TYPES ................................................... ............................................................................. .................................................. ........................ 13 Closed Cycle Cycle .................................................... ............................................................................... ............................................. .................. 13 Open Cycle .................................................. ............................................................................ ................................................. ....................... 14 Regenerative Regenerative Cycle ................................................. .......................................................................... ...................................... ............. 14 Combined Cycle.................................................................. Cycle........................................................................................... ........................... 15 MAJOR COMPONENTS COMPONENTS .................................................. ............................................................................ ..................................... ........... 16 COMPRESSOR COMPRESSOR TYPES ................................................ ......................................................................... ....................................... .............. 19 Centrifugal Centrifugal Compressor Compressor ................................................. .......................................................................... ............................... ...... 19 Axial Compressor....................................... Compressor................................................................ .................................................. ........................... 22 COMBUSTOR COMBUSTOR TYPES ................................................. ........................................................................... ......................................... ............... 24 Can-Type Can-Type Combustor .................................................. ........................................................................... .................................. ......... 25 Annular Type Combustor Combustor .................................................. ........................................................................... ............................ ... 26 Can-Annular Can-Annular Type ................................................. .......................................................................... ........................................ ............... 29 SHAFT TYPES.................................. TYPES............................................................ .................................................... .......................................... ................ 31 Single-Shaft.................... Single-Shaft.............................................. .................................................... .................................................... ............................ 31 Split-Shaft....................... Split-Shaft................................................. .................................................... .................................................... ............................ 32 Multiple-Shaft Multiple-Shaft (Multiple (Multiple Spool) Spool) .................................................. ...................................................................... .................... 34 GLOSSARY .................................................... .............................................................................. .................................................... .................................... .......... 36



List of Figures

Figure 1. Similarities between Gas Turbine and Reciprocating Engine Cycle ................................................. ........................................................................... .................................................... ..................................... ........... 3 Figure 2. Pressure and Temperature Levels Occurring in a Typical Gas Turbine ................................................... .............................................................................. ....................................................... ............................... ... 5 Figure 3. Typical Horsepower Horsepower Levels Levels for a 7,000 HP Gas Turbine ................................ ................................ 6 Figure 4. Heavy-Duty Heavy-Duty Gas Turbine (MS5002)....................... (MS5002) ................................................. ......................................... ............... 7 Figure 5. Aircraft-Derivative Aircraft-Derivative Gas Turbine (GE LM2500)............................. LM2500).................................................. ..................... 9 Figure 6. Closed Cycle Turbine.................................................................. Turbine..................................................................................... ................... 13 Figure 7. Simple-Cycle Simple-Cycle Gas Turbine .................................................. ........................................................................... ........................... 14 Figure 8. Basic Regenerative Regenerative Cycle Cycle Gas Turbine Turbine ................................................. ........................................................ ....... 15 Figure 9. Schematic of Typical Typical Combined Combined Cycle Cycle....................... ............................................... ................................... ........... 16 Figure 10. 10. Single-Shaft Single-Shaft Gas Turbine Turbine Assemblies................... Assemblies............................................. ...................................... ............ 17 Figure 11. Typical Typical Split-Shaft Split-Shaft Gas Turbine Assemblies Assemblies ................................................ ................................................ 18 Figure 12. Single Suction (above) and Double Suction (below) Centrifugal Compressor Compressor Configurations............................................ Configurations..................................................................... ................................ ....... 21 Figure 13. Drum-Type Drum-Type Compressor Compressor Rotor .................................................. .................................................................... .................. 23 Figure 14. Disc-Type Disc-Type Compressor Compressor Rotor....................................... Rotor................................................................. ............................... ..... 23 Figure 15. 15. Elements of a Can-Type Can-Type Combustion Combustion Chamber.............................. Chamber.......................................... ............ 25 Figure 16. Single Can-Type Can-Type Combustion Combustion Chamber Chamber Arrangemen Arrangementt ................................ ................................ 26 Figure 17. Annular-Type Annular-Type Combustion Combustion Chamber Chamber Arrangemen Arrangementt ..................................... ..................................... 28 Figure 18. Typical Can-Annular Can-Annular Arrangement Arrangement .................................................. .............................................................. ............ 29 Figure 19. Single Shaft Gas Turbine (Allison 501-K17) for Electrical Generator Application......................................................... Application................................................................................... ............................ .. 32 Figure 20. Split-Shaft Split-Shaft Gas Turbine (General (General Electric LM-2500) LM-2500) .................................... .................................... 33 Figure 21. 21. Typical Twin-Spo Twin-Spool ol Gas Turbine Turbine (Pratt (Pratt and Whitney Whitney CG-4)........................ 34



INFORMATION INTRODUCTION Gas turbine operation is based on a thermodynamic cycle called the Brayton Cycle, and is known as a simple gas turbine cycle. In a gas turbine, compression and expansion occur continuously rather than in the intermittent manner of a reciprocating internal combustion engine. Gas turbine power is available continuously, whereas reciprocating engine power take-off is available only during the expansion stroke. A gas gas turb turbine, ine, like any other other heat heat engin engine, e, is is a device device for convert converting ing part of a fuel's chemical energy into useful available mechanical power. The energy transfer occurs in a manner similar in many ways to the system used by a four-cycle reciprocating internal combustion engine system. Both gas turbines and reciprocating engines are internal combustion engines; however, the gas turbine is a dynamic internal combustion engine, whereas the reciprocating engine is a positive displacement internal combustion engine. Figure 1 illustrates the similarities between the two cycles.

Figure 1. Similarities between Gas Turbine and Reciprocating Engine Cycle



As indic indicate ated d by this illustra illustratio tion, n, air air is is drawn drawn into the compressor , usually through an air intake filter system, to remove any harmful solid particles and vapors from the air stream. This air is then compressed to the proper value for the particular design application by the gas turbine compressor. The hot, compressed air is then discharged to the combustion chambers, or combustor s, s, where it mixes with injected fuel. In the combustion chambers, the fuel burns and adds energy to the air. A spark spark plug or flame flame igniter igniter in one one or more more chambe chambers rs initi initially ally starts combustion. Once established, combustion is sustained by a continuous flow of air and fuel, and the spark plug or igniter can be deenergized. The combustion process raises the gas to a flame zone temperature of 3500 F to 4000 F . This temperature is immediately reduced to usable values by the mixing of secondary air that enters the combustion chamber through holes placed in the combustion liners. The hot high-pressure gas mixture is then ducted to the turbine section, where it expands to exhaust pressure. In the expansion process, energy is removed from the gas to drive the compressor, the auxiliaries, the generator, and the external load equipment. °

°

The spent gas is allowed to flow to the exhaust stack system. Because there is still heat energy in this gas, the heat can be put to use in a variety of ways, such as air or water heating, process drying, or as hot-air-feed supply to a separately fired boiler. Any of these recovery methods helps to increase the overall thermal efficiency of the turbine cycle. Figure 2 illustrates pressure and temperature levels that occur in a typical heavy-duty gas turbine, and Figure 3 illustrates some typical horsepower levels that occur in a typical simple-cycle, single-shaft, 7000 horsepower gas turbine.



DUAL FUEL NOZZLE COMBUSTION CASING NO. 1 BEARING

NO. 2 BEARING EXHAUST DIFFUSER TURNING VANES

TRANSITION PIECE

VARIABLE INLET GUIDE VANE COMPRESSOR CASE

AIR INLET

COMPRESSOR

COMBUSTION TURBINE

EXHAUST

3000

180

160 2000 140

G I S P , E R U S S E R P

120

1000

100

80

F . G E D , E R U T A R E P M E T

60

40

20

0

E R U S S E R P

500

E U R T A E R P M T E 0

Figure 2. Pressure and Temperature Levels Occurring in a Typical Gas Turbine



EXHAUST 13340 HP

AIR IN

COMBUSTOR FUEL 33500 HP

ACCESSORIES 160 HP

COMPRESSOR 13000 HP

TURBINE 20340 HP

USEFUL OUTPUT 7000HP

Figure 3. Typical Horsepower Levels for a 7,000 HP Gas Turbine

In a reciprocating engine, as the piston moves downward on the intake stroke, an air/fuel mixture is drawn into the cylinder. When the piston reaches the bottom of the intake stroke, the intake valve closes to form a closed chamber. As the piston moves upward during the compression stroke, the air/fuel mixture is compressed. When the piston reaches the top of the compression stroke, the compressed air/fuel mixture is ignited. The combustion of the reciprocating engine is an intermittent process that occurs at a constant volume. The combustion of the air/fuel mixture forces the piston downward to rotate the crankshaft, which extracts energy from the combustion products. As the piston piston moves upward again, the exhaust valve opens, opens, and the waste gases are exhausted from the engine. This Module presents an overview of gas turbines, and it introduces the Mechanical Engineer to the fundamentals and operation of several gas turbine types and configurations.



TYPES OF GAS TURBINES AND THEIR OPERATING OPERATING CYCLES There are two major types of gas turbines: heavy-duty (Type H industrial) and aircraft-derivative (Type G - aircraft-derivative). The two types of gas turbines reflect the original intended use of the turbines. Type H stands for heavy (stationary) industrial use, and Type G stands for light engines in flight use. Additionally, gas turbines that use a Type G for gas generation and a Type H for power generation are sometimes classified as hybrid type gas turbines. Saudi Aramco engineering standard SAES-K-502 is the governing document for the design of gas turbines used at Saudi Aramco Aramco faciliti facilities. es.

Heavy-Duty (Type H – Industrial) Figure 4 illustrates a typical heavy-duty type gas turbine. Their robust design and construction characterize heavy-duty gas turbines. Heavy-duty gas turbines are specifically designed for stationary ground applications. The design philosophy for a heavyduty gas turbine is similar to the design philosophy for steam turbines used for large central stations; therefore heavy-duty gas turbines are ideal for base load power generating operating conditions. Heavy-duty gas turbine units are designed with safety factors and stresses that are comparable to the safety factors and stresses for steam turbine design. These design considerations allow heavy-duty gas turbines to run continuously for a long period without requiring major maintenance.

Figure 4. Heavy-Duty Gas Turbine (MS5002)



A heav heavy-du y-duty ty gas gas turbi turbine ne is is const construct ructed ed diffe differen rently tly than an aircraftderivative gas turbine. The construction can be used to identify a heavy-duty gas turbine. Heavy-duty gas turbines always use hydrodynamic bearings because the gas turbine was designed for land use. The heavy-duty gas turbine casings are normally split on the horizontal centerline. Multiple-shaft, heavy-duty turbines typically have variable load turbine nozzles. Heavy-duty gas turbines are presently available in a range of sizes from a variety of manufacturers. Unit output ranges are typically from around 4 MW to more that 225 MW in simple cycle operation. Heavy-duty industrial gas turbines have an advantage of relatively high simple cycle efficiencies (28 to 38 percent) with high exhaust temperatures. Because high gas turbine exhaust temperature is ideal for combined cycle operations, heavy-duty gas turbines are ideal for this configuration, and they allow overall combined cycle efficiencies in excess of 55 percent, based on the lower heating value of the fuel. Recent developments in combustion systems for heavy-duty gas turbines have also reduced the NO X emissions at the turbine exhaust to as little as 25 ppm. Heavy-duty gas turbines also operate differently than aircraftderivative gas turbines in that heavy-duty gas turbines generally have low compressor pressure ratios (approximately 15:1) and relatively low combustion temperatures (approximately 1500 F to to 1800 F); newer designs of heavy-duty gas turbines, however, are increasing this temperature. Heavy-duty gas turbines have the capability to burn a variety of fuels, and they are available in a wide range of sizes that include very large models that are capable of producing over 200,000 bhp. °

°

Advanta Advantages ges and disadva disadvanta ntages ges of Heav Heavy-du y-duty ty gas gas turbi turbines nes,, when when compared to aircraft-derivative gas turbines include: Advantages: •

Longer operation without shutdown for maintenance

•

Lower combustion temperatures

•

Ability to burn a larger variety of fuels

•

Higher horsepower capabilities

•

Greater tolerance to upsets



•

Disadvantages:

•

Lower efficiency

•

Increased amount of time to overhaul (maintainability)

•

Cannot be transported to maintenance centers as easily as aircraft-derivative turbines

•

Large footprints

•

High specific weight

•

Longer start sequences

Aircraft-Derivative (Type G) Figure 5 shows a typical aircraft-derivative type gas turbine. As the name implies, aircraft-derivative gas turbine designs are based on aircraft propulsion gas turbines that are modified to produce shaft power instead of thrust. Aircraft-derivative gas turbines are lightweight and compact, which frequently makes them the gas turbine of choice for use on offshore platforms and in remote areas. Another reason that aircraft-derivative gas turbines are frequently chosen for offshore platforms is their suitability for quick change-out and replacement with spare engines. 1st stage compressor Combustion blades mid-span chamber dampers 16th stage compressor blades

HP nozzle guide vanes 1st and 2nd stage

1st and 2nd stage HP turbine blades

1st of 6 stages of power turbine blades

Figure 5. Aircraft-Derivative Gas Turbine (GE LM2500)

Aircraf Aircraft-de t-deriva rivative tive gas turbine turbine constru constructio ction n is differe different nt than than the construction of heavy-duty gas turbines. Aircraft-derivative gas turbines always use anti-friction bearings because of the small tolerances required for flight maneuver environments (for instance, if a plane banks or flies inverted). The compressor turbine may have two or three concentric shafts that divide the compressor into



a low pressure section, intermediate pressure section followed by a high pressure section. Each section is driven by a turbine through one shaft. In this configuration, each compressor-shaftturbine combination is called a spool. Aircraft-derivative gas turbines generally have high compressor pressure ratios (as high as 30:1) and relatively high combustion temperatures (as high as 2500 F); thus, they are designed to operate with high efficiency and high horsepower. However these high temperatures also result in reduced reliability. °

Aircraft Aircraft-der -derivat ivative ive machine machines s combin combine e high high tempera temperature ture technol technology ogy and high pressure ratios with advanced metallurgy to achieve these high simple cycle efficiencies. Simple cycle efficiencies of up to 43 percent have been demonstrated in some newer designs. Aircraft Aircraft-de -deriva rivative tive gas turbine turbine designs designs maintain maintain as much much commonality with the flight engine as possible for economy. The result is a more effective approach to on-site maintenance and preventive and corrective actions. Such actions include partial disassembly of the engine and replacement of components, including blades, vanes, and bearings. Aircraf Aircraft-d t-deriv erivativ ative e gas gas turbi turbines nes evolve evolved d from from aircr aircraft aft engine engines s in which reduced unit size and weight are extremely critical. Rotor speeds (between 3,000 and 20,000 rpm) and casing pressures (20 to 30 atmospheres) for aircraft-derivative engines appear high when compared to other types of gas turbines; however, the choice of the materials used in aircraft engines offers high strength capabilities, and the resulting stress margins are equivalent to those of other types of gas turbines. For example, the commonly used aircraft engine casing material, cast Inconel 718, has a yield strength of 104 ksi at 1200 F (649 C), while cast iron that is commonly used in other types of gas turbine casings has a yield strength of 40 ksi at 650 F. °

°

°

The aircraft-derivative design uses low weight rotors. An example is the GE LM 5000 high-pressure rotor that only weighs 1,230 lbs (558 kg). Consequently, this rotor design uses anti-friction bearings. Anti-friction bearings do not require large lubricating oil reservoirs, coolers, pumps, or a pre- and post-lubricating cycle. Anti-fri Anti-frictio ction n bearin bearings gs are are rugg rugged, ed, and they have have demons demonstra trated ted good characteristics in industrial service to where most bearings are expected to provide reliable service for over 100,000 hours.



In practice, bearing replacement is prudent when the bearings are exposed during major repairs. Generally, the bearing replacement should occur at 50,000 hours for gas generators and 100,000 hours for power power turbines. Anti-friction bearings are also required to meet the application duty. The aircraft-derivative high-efficiency design makes it a good choice for simple cycle power generation. The same is true for cyclic applications such as peaking power, which, to some extent, parallels what an aircraft engine would see in flight operation. With start times in the one to three minute range, aircraft-derivative turbines are ideal for emergency power applications. The inherent low rotor inertias and the variety of air or gas pneumatic and hydraulic starting options available for aircraft-derivative gas turbines have simplified the black start capability of these machines. A black start is the ability to bring a “cold iron” machine on-line when a source of outside electrical power is unavailable. Aircraft Aircraft-de -deriva rivative tive gas turbine turbines, s, howe however, ver, require require more frequen frequentt maintenance in view of their higher compression ratios and firing temperatures. For example, hot gas path inspections are typically required after 12,000 hours of operation, and major overhauls are typically performed after 25,000 to 30,000 hours of operation. Typically, complete hot sections of aircraft-derivative gas turbines or the gas turbines themselves are pulled out and sent to a vendor's service facility for overhaul, and a spare section or turbine is installed in its place to increase availability. Saudi Aramco MSSD shops in Dhahran presently have the capability to perform maintenance on the hot sections of Pratt & Whitney, Allison, Solar, and GE gas turbines. Aircraft Aircraft-de -deriva rivative tive gas turbine turbines s are are also also employe employed d in the STIG STIG (steam injected gas turbine) cycle. The STIG cycle allows for significant amounts of steam injection at various points in the gas turbine for enhanced power output (up to 25 percent more than simple cycle output) and increased efficiency; however, at the higher steam injection rates, carbon monoxide (CO) emissions tend to increase. In simple cycle operation, aircraft-derivative gas turbines have relatively low exhaust temperatures. The low exhaust temperature limits their application in an unfired combined cycle power plant. Natural gas and distillate oil are the most frequently utilized fuels. Gaseous fuels with heating values as low as 300 Btu/lb are suitable for aircraft-derivative gas turbine designs. Fuels such as



gaseous propane and butane are equally suitable for aircraftderivative gas turbine designs. •

Aircraft-derivative gas turbines have the following advantages when compared to heavy-duty gas turbines:

•

Higher efficiency

•

Quick overhaul and replacement capability

•

Lighter and more compact

•

Smaller footprints

•

Faster start sequence

However, aircraft-derivative gas turbines have the following disadvantage when compared to the heavy-duty gas turbines: •

Shorter operation between maintenance overhaul periods

•

Less tolerance to upsets

Hybrid Two-shaft gas turbines are sometimes considered to be a third type of gas turbine, which is referred to as the hybrid type. These gas turbines are referred to as hybrids because they display characteristics of both the heavy-duty and the aircraft-derivative type gas turbines. On the hybrid type of gas turbine, one shaft is normally equipped with a compressor and and the compressor compressor turbine, which is commonly called the high-pressure turbine or the gas generator. This section of the gas turbine typically has characteristics that are similar to aircraft-derivative type gas turbines. The second shaft contains a power turbine (PT), which is also commonly called the low-pressure turbine, and the load is usually connected to this shaft. Typically, this section of the gas turbine has characteristics that are similar to heavy-duty gas turbines. The split-shaft arrangement allows the compressor and the load to operate at different speeds.



GAS TURBINES: MAJOR CYCLES AND COMPONENTS Heavy-duty, aircraft-derivative, and hybrid type gas turbines all incorporate major key components such as compressors, combustors and turbines. Construction of these components is obviously dependent on the design configuration. Gas turbines are also used in a variety of thermodynamic cycles, which are selected based on need and application requirements.

Cycle Types The basic types of gas turbine cycles include the open cycle, the closed cycle, a regenerative cycle, and a combined cycle. Closed Cycle

The closed cycle is not used at Saudi Aramco facilities. In general, this cycle allows for using high pressures, hence high gas density throughout the cycle, which in turn would result in a reduced machine size for a given output, and allows power output to be controlled by changing the system pressure level. Figure 6 shows a schematic description of a closed cycle. As may be seen in the figure, non-combustion heated gas is cycled between the engine compressor and turbine components. Also, the closed cycle requires a separate external heating cycle, which is considered as the key disadvantage of this cycle. Besides the size advantage this cycle provides, the closed cycle also avoids hot section turbine blades and nozzle erosion detrimental effects caused by combustion byproducts.

Figure 6. Closed Cycle Turbine



Open Cycle

In an open-cycle turbine, atmospheric air is used once in the air's passage through the turbine. Open-cycle gas turbines are generally referred to as simple-cycle gas turbines. Figure 7 shows a schematic of a simple-cycle gas turbine. Air is compressed by the compressor, and it is exhausted to the combustion chamber. The compressed air is mixed with fuel and is ignited in the combustion chamber. The hot exhaust gases from the combustion process are then directed to the turbine section. The energy of the hot exhaust gases is used to drive the turbine before the gases are exhausted from the gas turbine unit.

Figure 7. Simple-Cycle Gas Turbine Regenerative Cycle

A regen regenerat erative ive cycle cycle gas gas turbi turbine ne recycle recycles s the the hot hot exhau exhaust st gase gases s through a regenerator to preheat incoming, compressed, combustion air before the air enters the combustion chamber. The addition of a regenerator increases the efficiency of a simple opencycle gas turbine. Figure 8 shows a basic regenerative cycle gas turbine. Air is compressed by the compressor and exhausted to the combustion chamber through the regenerator. The regenerator is a heat exchanger that heats the compressed air through the use of the turbine exhaust gases. The heated compressed air is then mixed with fuel and ignited in the combustion chamber. The hot exhaust gases from the combustion process are directed to the turbine section. The energy of the hot exhaust gases is used to drive the turbine before the gases are exhausted from the turbine section back to the regenerator and finally to the atmosphere.



Figure 8. Basic Regenerative Cycle Gas Turbine Combined Cycle

A combin combined ed cycle, cycle, as show shown n in Figure Figure 9, 9, is a thermo thermodyn dynamic amic system that combines two or more independent power cycles. Each power cycle uses a different working fluid. Combination of the independent power cycles can result in higher efficiency than would be achieved by the independent operation of the individual cycles. To achieve this higher efficiency the individual cycles must exchange energy so that the ratio of the source to sink temperature of the combined cycle is greater than the ratio of the source to sink temperature of any of the individual cycles. The gas turbine cycle and the steam power system cycle, which are shown in Figure 9, are two independent cycles that can compliment each other to form an efficient combined cycle. The gas turbine cycle has a high source temperature and exhausts at a temperature that can be the energy source for the steam cycle. Heat from the gas turbine (GT) exhaust is used to generate steam in a heat recovery device, which is commonly called a heat recovery steam generator (HRSG). In the example shown in Figure 9, the exhaust from the GT enters the HRSG at about 900 F and exhausts to the atmosphere at about 250 F. The HRSG is normally a high-pressure steam generator with three sections: the superheater, the evaporator, and the economizer. The highpressure steam drives a steam turbine (ST) that drives a generator to produce more power. The steam turbine is normally a condensing type turbine that exhausts to a condenser. The °

°



condenser uses cooling water (CW) to condense the turbine exhaust steam. A pump returns the condensate water to the HRSG.

Figure 9. Schematic of Typical Combined Cycle

Major Components The major components of a gas turbine consist of three or four assemblies depending on the shaft arrangement. These assemblies are the following: •

Compressor assembly

•

Combustion assembly

•

•

High-pressure turbine assembly or gas generator assembly Power turbine assembly



Figure 10 shows the assemblies of a single-shaft gas turbine. Single-shaft gas turbines typically consist of an air inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section.

AIR INLET SECTION

COMPRESSOR SECTION

TURBINE SECTION

COMBIUSTION SECTION

Figure 10. Single-Shaft Gas Turbine Assemblies

EXHAUST SECTION



Figure 11 shows the assemblies on a typical split-shaft gas turbine. Split-shaft and multiple-shaft gas turbines use the compressor, the combustion, the gas generator, and the power turbine assemblies. 1st stage compressor Combustion blades mid-span chamber dampers 16th stage compressor blades

HP nozzle guide vanes 1st and 2nd stage

1st and 2nd stage HP turbine blades

1st of 6 stages of power turbine blades

Compressor Assembly

Combustion Assembly

Gas Generator Assembly

Power Turbine Assembly

Figure 11. Typical Split-Shaft Gas Turbine Assemblies

Each assembly can be used to classify the gas turbine by the compressor and combustion assembly type and by the shaft type.



COMPRESSOR TYPES To accomplish the first part of the Brayton Cycle—compressing the air and raising its pressure—gas turbines may utilize either a multistage axial compressor, a centrifugal compressor, or a combination of a multistage axial compressor followed by a single centrifugal stage. The difference between the two types of compressors lies in the direction of the air flow through the compressor. In principle, both axial and centrifugal compressors take suction from the atmosphere, increase the velocity and pressure of the air, and discharge to the combustion chamber. A centrifugal compressor draws in air axially at the center (or eye) of the impeller, turns it in the radial direction and accelerates it by a centrifuging action. On the other hand, an axial compressor compresses air while maintaining the original air flow direction, which is parallel to the axis of the rotor. Centrifugal Compressor

The air flow leaves the centrifugal compressor impeller with much higher kinetic energy (velocity) and thermal energy (enthalpy, pressure and temperature) than the entering flow. A radial diffuser then receives the high velocity flow and converts much of that kinetic energy into pressure by gradually increasing the flow area and thus decelerating the flow. The high pressure, high temperature flow exiting the diffuser is then ready for the next step of the Brayton Cycle, namely combustion. A centrifugal centrifugal compressor compressor is typically located located between between the the accessory section and the combustion section of the gas turbine, and consists of an impeller, a diffuser, and a compressor manifold. Figure 12 shows two typical centrifugal compressor configurations for gas turbines. The impeller may be either single suction or dual suction. The differences between the single suction and the dual suction are: the size of the impeller (in the axial and radial directions), the ducting arrangement, and the amount of air flow that can be passed for a given impeller diameter. The single suction impeller allows for convenient ducting directly to the impeller eye. Turning vanes are installed in the outlet elbows to reduce air pressure losses during the change in flow direction from radial (as air leaves the impeller) to axial (in order to enter the combustor). The dual suction impeller uses a more complicated ducting arrangement to channel the air to reach the inlet of the



second (inside) impeller. Single suction impellers are slightly more efficient due to lower losses in their simpler ducting. However, single suction impellers must have larger diameter to pass sufficient air flow, which increases the overall diameter of the engine and subsequently increases impeller stresses. Dual suction impellers are typically smaller in diameter for a specified air flow, and they may rotate at higher speeds to allow even higher air flow. Most centrifugal compressor gas turbines use dual suction compressors to reduce turbine diameter. Therefore air must enter the gas turbine at almost right angles to the turbine axis. A plenum chamber (not (not shown) shown) is therefore therefore required required to allow the air to surround the compressor uniformly at positive pressure before entering the compressor. It is established that multistage axial compressors are better suited for very high air flow rates than centrifugal compressors. This is so due to several factors involving efficiency, weight, relative simplicity of design and construction …etc. Therefore, since the horsepower produced by a gas turbine is a function of air flow rate, multistage axial compressors are commonly found in high output gas turbine engines, while centrifugal impeller are normally employed in low power applications such as helicopter engines (and micro-turbines) (~ less than 2000 hp).



Figure 12. Single Suction (above) and Double Suction (below) Centrifugal Compressor Configurations



Axial Compressor

The two main elements of an axial flow compressor are the rotor blades and the stator vanes. The rotor blades are attached to a disk that, with the other disks, constitutes the rotating assembly. When the disk rotates in the proper direction, the accurately shaped blades move tangentially and force the air to axially flow rearward, much like an aircraft propeller or an electric home air fan. By this action, the rotor blades impart energy into the flow, mostly as kinetic energy (velocity increase) and also as thermal energy (rise in pressure and temperature). Behind each rotor blade row is a stator vane row that converts much of the flow kinetic energy (i.e., reduce velocity) into pressure. The stator blades also direct the flow so it would properly enter the next rotor blades for further compression. Each consecutive pair of rotor and stator blades constitutes a compressor stage. The number of stages required is determined by the amount of air and total pressure rise required by the gas turbine. As the number of stages increases, the compression ratio (absolute exit pressure divided by absolute inlet pressure) also increases. Most modern axial compressor gas turbines typically have eight to sixteen stages, depending on the power requirements, with compression ratios in the range of 20 to 30. As for the the construction construction of axial compressor compressor rotors, rotors, there there are two main types: the drum-type and the disk-type. The drum-type rotor consists of rings that are flanged to fit one against the other. Figure 13 shows a typical drum-type rotor. The entire assembly is held together by through bolts or by welding. The blades and stators decrease in length from front to rear. The drum-type construction is typically used for relatively slow speed compressors in which centrifugal forces affecting the component’s structure are low.



Figure 13. Drum-Type Compressor Rotor

The disk-type rotor consists of a series of titanium alloy, low alloy steel, or stainless steel discs. As the compressor pressure ratio continues to increase, along with the corresponding temperature, other higher temperature nickel based materials are being considered for the latter stages of compression. Figure 14 shows a typical disc-type rotor. The disk-type rotor is typically used in high-speed gas turbines.

Figure 14. Disc-Type Compressor Rotor



COMBUSTOR TYPES The combustion section is the area in which the fuel is injected in the air discharged by the compressor and in which high temperature gas is produced. Several important considerations go into the design of gas turbine combustion systems. Except in some special applications, the room available for the burner (combustor) is relatively small and the temperature distribution of the gases leaving the burner must be as uniform as possible (to provide good performance and to prevent localized overheating problems). Also, combustion must be continuous and stable, which can be a difficult requirement since the air velocity entering the burner is often on the order of 100 - 200 ft/sec (68 - 136 mile/hr or 30 - 60 meters/sec). If the combustion process is not efficient, carbon particles can form and erode turbine blades decreasing their useful life and reducing engine efficiency. Other problems concerning the pressure, temperature, metal fatigue, and stresses are also important. There are three types of combustion chambers: the can-type, the annular-type, and the can-annular type. The can-type chamber is used on gas turbines with centrifugal compressors and some older, heavy industrial-type gas turbines. The annular and canannular type chambers are typically used on gas turbines with axial flow compressors.



Can-Type Combustor

The can-type combustion system consists of individual liners and cases mounted around the axis of the gas turbine. Figure 15 shows the elements of a typical can-type combustion system. The combustion chamber is connected to the compressor section at the combustion chamber inlet duct. Each chamber contains a fuel nozzle. The can-type arrangement makes removing a chamber easy; however, the arrangement is bulky and makes for a structurally weak turbine. The combustion chamber housing is typically welded to a ring that directs the gases into the turbine nozzle . Each of the casings is linked to the other casings by a short tube (not shown) containing a flame tube, which in turn joins adjacent combustion chamber liners (not shown). Linking the combustors with a flame tube ensures that combustion occurs in all the burners simultaneously.

Figure 15. Elements of a Can-Type Combustion Chamber



A single can-type combustion combustion chamber chamber is sometimes sometimes used used on industrial gas turbines or in very small gas turbines. Figure 16 shows the typical arrangement of a single can-type combustion chamber.

Figure 16. Single Can-Type Combustion Chamber Arrangement

Annular Type Combustor

Annular-type Annular-type combustion combustion chambers chambers are typically found on axialflow gas turbines. The annular-type combustion chamber is the most popular type of combustion system in use. The construction consists of a housing and liner similar to the can-type. Figure 17 shows a typical arrangement of an annular-type combustion chamber. The difference between the can-type and annular-type combustion chamber is the design of the liner. On large gas turbines, the inner liner consists of an undivided circular shroud that extends all the way around the outside of the turbine shaft housing. A large one-piece outer combustor case covers the liner and is attached at the turbine section and compressor



section by mounting flanges. Typically, the dome of the liner admits primary combustion air through the use of small slots and primary air holes in the liner. The slots and holes in the dome of the liner impart a swirling motion to the air, which causes better fuel atomization and fuel/air mixing. There are also holes in the dome for the fuel nozzles to extend into the combustion area. A fuel manifold connects to multiple air spray fuel injector nozzles that are located circumferentially around the dome. The position of the fuel nozzles provides for even combustion geometry around the entire combustion chamber. In the case of the double-annular chamber, two rows of fuel nozzles are used. The inner and outer liners form the combustion space. The outer liner keeps the flame from contacting the combustor case. The inner liner prevents the flame from contacting the turbine shaft housing. Large holes and slots are located along the liners. The holes and slots (dilution air holes) admit some cooling air into the combustion space to help cool the hot gases to a safe level, to center the flame, and to admit the balance of air for combustion in a secondary combustion zone downstream of the nozzles. The hot combustion gases are cooled to prevent the hot flow path structures from overheating damage. The combustion gases leave the combustion chamber through the turbine nozzle guide vanes. The advantage of the annular-type combustion chamber is a high system efficiency that minimizes bulk and is effective in limited spaces. The disadvantage of the annular-type combustion chamber is the difficulty in performing maintenance. On some gas turbines, the liners are constructed as a single piece and cannot be removed without turbine disassembly. Gas turbines that use a one-piece combustor dome must be disassembled to remove the dome. Modular assemblies, however, can allow changes to complete combustor assemblies.



Figure 17. Annular-Type Combustion Chamber Arrangement



Can-Annular Type

The can-annular type of combustion chamber combines some of the features of both the can and annular burners. Figure 18 shows a typical can-annular arrangement. This combustion system is the reverse flow type with the combustion chambers arranged around the periphery of the compressor discharge casing. Combustion chambers are numbered counterclockwise when viewed looking toward the compressor section (AFT), starting from the top of the machine. The can annular system also includes the fuel nozzles, a spark plug ignition system, flame detectors, and crossfire tubes. Fuel nozzles atomize the fuel for efficient combustion. The spark plug ignition system provides the ignition source for the combustor. Typically, two spark plugs are used for the ignition source, but not every combustor will have them. Crossfire tubes are used to distribute the combustion chamber ignition to all combustors instantaneously. The flame detectors detect the presence of flame in the combustor and are used to immediately trip the gas turbine on a loss of flame detection.

Figure 18. Typical Can-Annular Arrangement



High pressure air from the compressor discharge is directed around the transition pieces and into the annular spaces that surround each of the combustion chamber liners. The combustion wrapper forms a plenum through which the compressor discharge air flow is directed to the combustion chambers. The combustion air enters the combustion zone through metering holes for fuel combustion and through openings in the combustion liner for cooling. The secondary purpose of the combustion wrapper is to act as a support for the combustion chamber assemblies. In turn, the combustion wrapper is supported by the compressor discharge casing and the turbine shell. Fuel is supplied to each combustion chamber through a nozzle assembly designed to disperse and mix the fuel with the proper amount of combustion air. Advanta Advantages ges of usin using g the the can-a can-annul nnular ar arran arrangeme gement nt are are the the following: •

•

•

•

•

•

Arrangement permits the entire turbine to be factory assembled, tested, and shipped without interim disassembly. Improved control of the turbine inlet temperature profile. This improved control provides for longer turbine life without reducing cooling airflow requirements. Smaller parts can be handled more easily during maintenance. Smaller transition pieces are less susceptible to damage from dynamic forces generated in the combustor. Smaller combustors generate less NO x because of better mixing and shorter residence time. Due to their smaller size, can-type combustors can be fully tested in the laboratory before being installed in the field.



Shaft Types Gas turbines may use one of three different shaft types: the single-shaft type, the split-shaft type, and the multiple-shaft or multiple spool type. It is often difficult to determine the type of gas turbine shafting simply by performing an external inspection of the turbine unit. Each shaft type is discussed individually below. Single-Shaft

In a single-shaft gas turbine, as shown in Figure 19, all stages of the air compressor and the power turbine are mounted on a common shaft. The common shaft is coupled to the driven load unit. The single-shaft gas turbine has limited speed flexibility; its speed is typically dictated by the driven equipment, and is best used for constant speed applications such as electric power generators. Single-shaft gas turbines are not typically used by variable speed machines such as pumps and compressors. Also, single shaft turbines are usually used for electrical generator drives, because excessively large starting devices would be required if the gas turbine load were mechanical, such as a compressor or a pump. However, when electric generators are used the unit can be unloaded during start up, hence minimizing the starting device torque requirements. Upon reaching a selfsustaining engine speed, the single-shaft gas turbine can then be loaded. Maintenance of single-shaft gas turbines is relatively difficult because the gas turbine must be completely disassembled.



COMBUSTOR

AIR COMPRESSOR

LOAD POWER TURBINE COMMON

Figure 19. Single Shaft Gas Gas Turbine (Allison 501-K17) for Electrical Generator Application

Split-Shaft

A split split-sh -shaft aft gas turbine turbine contain contains s two two or or more more shafts. shafts. Split-s Split-shaft haft gas turbines that have two shafts, as shown in Figure 20, normally have the compressor and its driving turbine, which is commonly called the high-pressure (HP) turbine or the gas generator, mounted on one shaft. The power turbine (PT), which is also commonly called the low-pressure turbine, is mounted on the second shaft. The load is connected to the second shaft. The splitshaft arrangement allows the compressor and the load to operate at different speeds. Because pumps and compressors are much more difficult to unload for startup, split-shaft gas turbines are normally used to drive pumps and compressors and are also suitable for variable speed applications.



During startup of a split-shaft gas turbine, the starting device needs to only accelerate the compressor and gas generator. The PT and the driven load are disconnected from the starting device. The hot gas leaves the gas generator and enters the PT to drive the load by the energy extracted by the PT. The gas leaves the PT through the gas turbine exhaust. The gas generator of a split-shaft gas turbine typically has a speed range of 50 to 105 percent of rated speed, while the PT speed range for pump and compressor drives is in a small range of 85 to 105 percent to optimize the compressor efficiency. SEPARATE INDUSTRIAL TYPE POWER TURBINE (NOT SHOWN BELOW)

COMBUSTOR COUPLING

LOAD SHAFT SHAFT

COMPRESSOR

HIGHPRESSURE TURBINE

LOWPRESSURE TURBINE

Figure 20. Split-Shaft Gas Turbine (General Electric LM-2500)



Multiple-Shaft (Multiple Spool)

Twin spool gas turbines use two separate compressors and two separate turbine rotors. Figure 21 shows a typical twin spool gas turbine. The two separate compressor/rotor units are referred to as the low-pressure (LP) compressor and turbine rotor and the high-pressure (HP) compressor and turbine rotor. The LP shaft runs through the hollow shaft that connects the HP turbine to the HP compressor. The starter (not shown on the cross-sectional view) drives the HP assembly during a startup. The power turbine driving the equipment (not shown on the cross-sectional view) functions the same as in the split-shaft gas turbine design. A larger volume of air can be be handled by a twin spool gas turbine as compared to a single or split-shaft gas turbine; however, the turbine has more moving parts and is more complex than other gas turbine configurations.

Figure 21. Typical Twin-Spool Gas Turbine (Pratt and Whitney CG-4)



The twin-spool gas turbine shown in Figure 21 functions as a gas generator. A separate industrial design, low-speed power turbine (not shown) is directly coupled to the driven equipment. The twin-spool gas turbine arrangement is used to drive the main pumps on the Saudi Aramco East-West crude pipeline. General Starting Sequence Summary

The starting device rotates the common shaft, the compressor, the power turbine, and the driven unit until a speed is reached at which the compressor builds up pressure, combustion is initiated, and the gas turbine becomes self-sustaining. As the compressor rotates, it takes suction from the air intake duct and compresses it. Part of the air is used in the combustors for combustion. The remainder of the air is used to dilute the hot gases and to increase the mass flow through the turbine, and to cool the combustor, the power turbine rotor, the blades, and the nozzles. The hot gas from the combustor passes through the power turbine. The power turbine extracts the energy from the hot gases and drives the compressor and driven load. The gases leave the power turbine through the gas turbine exhaust. When self-sustaining speed is reached, the driven devices are loaded.



GLOSSARY aircraft-derivative gas turbine combined cycle

An airc aircraft raft jet engine engine that that is modif modified ied for ground ground applica application tions s to produce shaft power instead of thrust. A cycle cycle that includ includes es a gas turbine turbine to gene generate rate power, power, a waste waste heat boiler to recover heat from the gas turbine exhaust, and a steam turbine that consumes steam from the waste heat boiler and that generates power.

combustor

The component of a gas turbine in which the fuel and air are mixed and ignited at a constant pressure to produce a hot gas.

compressor

The first major component of a gas turbine. The compressor increases the pressure and temperature of the ambient air.

turbine / expander

The turbine/expander of a gas turbine. The expander extracts power from the compressed and combusted air/fuel mixture. Gas generator HP turbines drive the engine compressor, while power turbines drive the load equipment.

heavy-duty gas turbine

A type type of of gas gas turbin turbine e that that is is spec specific ifically ally designe designed d for for groun ground d applications, using a design philosophy that is similar to that of the steam-turbine industry. Casings are split on the horizontal centerline, with onsite maintenance planned after long periods of operation.

open cycle (simple cycle)

A config configurat uration ion of a gas turbine turbine in which which the the exhaust exhaust is vent vented ed to atmosphere. An open cycle is also called a simple cycle.

regenerative cycle

A gas gas turbin turbine e cycle cycle that include includes s a heat heat exchan exchanger ger.. The The heat heat exchanger transfers heat from the exhaust gas to the engine compressed air before entering the combustor.

gas turbin turbine e in which which the the air compress compressor, or, the power power turb turbine, ine, single-shaft gas turbine A gas and the load are all connected to the same shaft and therefore run at the same speed. split-shaft gas turbine

A gas gas turb turbine ine with two separa separate te shaf shafts. ts. This This type type of gas gas turbine permits the air compressor and the power turbine to run at different speeds.

Introduction_To_Gas_Turbines.pdf

Recommend Documents