Combustion Gas Turbines

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services.

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Engineering Encyclopedia Saudi Aramco DeskTop Standards

COMBUSTION GAS TURBINES

Engineering Encyclopedia Rotating Equipment

Combustion Gas Turbines

Saudi Aramco DeskTop Standards i

Section Page

INTRODUCTION............................................................................................................. 3

MAJOR COMPONENTS OF A GAS TURBINE .............................................................. 4 How a Gas Turbine Works......................................................................................... 4

Air Compressor..................................................................................................... 5 Combustor ............................................................................................................ 6 Power Turbine ...................................................................................................... 6

Auxiliary Equipment ................................................................................................... 6 Control Systems......................................................................................................... 7

GAS TURBINE TYPES AND CONFIGURATIONS ......................................................... 8 Gas Turbine Types .................................................................................................... 9

Heavy Duty ........................................................................................................... 9 Aero Derivative ................................................................................................... 11

Gas Turbine Configurations ..................................................................................... 13 Single Shaft ........................................................................................................ 13 Dual Shaft........................................................................................................... 14 Three Shaft Gas Turbine .................................................................................... 15

Available Models of Gas Turbines ........................................................................... 16 Fuels for Gas Turbines ............................................................................................ 16

GAS TURBINE CYCLES............................................................................................... 17 Efficiency Definitions................................................................................................ 17 Simple Cycle ............................................................................................................ 17 Other Cycles ............................................................................................................ 18

Regenerative Cycle ............................................................................................ 18 Exhaust Heat Recovery...................................................................................... 19 Combined Cycle ................................................................................................. 20 Supplementary Firing ......................................................................................... 21

PERFORMANCE CALCULATIONS.............................................................................. 22 Site Rating ............................................................................................................... 22 Exhaust Gas Composition ....................................................................................... 24 Gas Turbine Performance Curves ........................................................................... 24

WORK AID 1: GAS TURBINE ALTITUDE CORRECTION FACTOR FOR OUTPUT AND HEAT CONSUMPTION AND ALSO ALTITUDE VS ATMOSPHERIC PRESSURE ....................................................................................... 25

WORK AID 2: GENERAL ELECTRIC MODEL M5382C GAS TURBINE -- EFFECT OF COMPRESSOR INLET TEMPERATURE ON MAXIMUM OUTPUT, HEAT RATE, AND AIR FLOW...................................................................... 26

WORK AID 3: GENERAL ELECTRIC MODEL M5382(C) *38,000 HP GAS TURBINE ...................................................................................................................... 27



Saudi Aramco DeskTop Standards ii



GLOSSARY .................................................................................................................. 30

REFERENCES.............................................................................................................. 32

LIST OF FIGURES

Figure 1. How a Gas Turbine Works .............................................................................5

Figure 2. Gas Turbine Internals.....................................................................................8

Figure 2a. Gas Turbine Internals.................................................................................10

Figure 2b. Second Generation Aero Derivative Gas Turbine ......................................12

Figure 3. Single Shaft Gas Turbine .............................................................................13

Figure 4. Dual Shaft Gas Turbine................................................................................14

Figure 4a. Three Shaft Gas Turbine............................................................................15

Figure 5. Regenerative Cycle......................................................................................18

Figure 6. Exhaust Heat Recovery................................................................................19

Figure 7. Combined Cycle...........................................................................................20

Figure 8. Schematic Diagram - Supplementary Firing.................................................21

Figure 9. Altitude Correction Factor for Output and Heat Consumption and Altitude Versus Atmospheric Pressure ......................................................25

Figure 10. GE Model M5382C Gas Turbine – Effect of Compressor Inlet Temperature on Maximum Output, Heat Rate, and Air Flow.....................26

Figure 11. GE Model M5382C – 120ºF Load Limit 989 ºF Exhaust Temperature..............................................................................................27





Saudi Aramco DeskTop Standards 3

INTRODUCTION

Gas turbines are used to drive process equipment and electric generators. Because they require few utilities, they are suitable for installation in remote locations. Models are available with a wide range of horsepower. Gas turbines, unlike steam turbines, are not custom designed for each application. In Saudi Aramco, gas turbines are used to drive electric generators, process gas compressors, re-injection gas compressors, crude oil pipeline pumps and water injection pumps.




MAJOR COMPONENTS OF A GAS TURBINE

How a Gas Turbine Works

Refer to Figure 1. A gas turbine has three major components:

• Air compressor

• Combustor

• Power turbine

In Figure 1, the air compressor and the power turbine are mounted on the same shaft. The temperatures and pressures shown are typical values. However, there is a considerable range in these values, depending on air compressor and combustor design.

Air from the atmosphere enters the inlet of the air compressor. The air compressor is usually an axial bladed compressor. At the outlet of the air compressor, the pressure can be as high as 400 psig, depending on the compressor speed and the number of compressor stages and the temperature is approximately 800ºF. The air flows from the compressor to the combustor. In the combustor, fuel is added and combustion raises the temperature of the gas mixture to approximately 1800ºF – 2300ºF, depending on the type of gas turbine. The temperature rise increases the pressure of the air significantly, since the volume is fixed and this greatly increases the amount of energy available in the air.

The heated air fuel mixture flows to the power turbine. The power turbine is also an axial device, somewhat like a steam turbine. In the power turbine, the pressure is reduced from 100 psig to near atmospheric pressure. Work is extracted from the air fuel mixture as it flows through the power turbine.

Because the gas flowing through the turbine has been heated in the combustor, the energy available to the turbine is greater than the energy consumed by the air compressor. The net difference between these two energies is available as shaft work to drive a machine. Approximately 60% of the total power produced by the gas turbine is required to drive the air compressor.




Air Compressor

The air compressor is usually an axial compressor with 8 to 20 rows of blades. In small gas turbines (below 2000 bhp), the air compressor can be a centrifugal compressor. Compression ratios vary from 5 to 30. For present designs, a compression ratio of 20 - 30 is most common.

Note: Temperatures and Pressures are Typical

Figure 1. How a Gas Turbine Works




Combustor

The combustor burns fuel in the compressed air, increasing its volume and therefore its energy potential. Only a small part of the available oxygen is consumed in the combustor, because there is a limit on the temperature that can be reached. The higher the temperature, the higher the efficiency and power output, but nozzle and blade materials limit the practical temperature to about 2500°F.

Power Turbine

The power turbine is a hot gas expander. It is usually an axial flow turbine, with 2 to 6 rows of blades.

Figure 2 is a cutaway diagram of the components of a gas turbine.

Auxiliary Equipment

In addition to the major components, there are a number of auxiliary items in a gas turbine installation.

• A common lubrication system for all of the rotating components. The system will contain a reservoir, circulating pumps, coolers, and piping to the various bearings. For aeroderivative gas turbines, a separate lube oil reservoir is required since the oil used is different than the driven load lubricating oil (synthetic vs. mineral).

• Air filter. It is very important that the air to a gas turbine be clean. Therefore, a major component, particularly in desert environments, is the air filter, which removes airborne solid particles. The present requirements for Saudi Aramco inlet air systems are pulse air self cleaning systems. In these systems, a pulse of air is blown backward through one section, while the other sections are operating normally.




• Noise suppression. Gas turbines are inherently very noisy. Therefore, silencers are usually included to control the noise. There is usually a silencer on both the inlet and the exhaust. If low noise levels are important, the turbine is mounted inside an acoustic enclosure.

• Starting systems. An auxiliary starting device is needed to get the air compressor up to minimum speed before fuel can be introduced. The starting device may be an electric motor, a hydraulic motor, a diesel engine or a small turbine. A starting turbine can be driven by steam, compressed air, or natural gas.

Sometimes the starting turbine is a steam turbine that is also used during normal operation. This turbine is called a "helper" and is used to increase the power output of the installation.

Control Systems

There are two basic control systems. The first is the speed control during operation. If the driven machine is variable speed, this controller is a speed governor. If the turbine drives an electric power generator, the speed is fixed by its connection to the grid. Therefore, the primary controller determines the amount of load or the amount of power generated by the turbine.

The other control system is an automatic sequence controller. This controls the steps taken during startup and shutdown to ensure that all conditions for safe operation are satisfied before proceeding to the next step and to ensure equal thermal growth of components. During startup, this system increases the speed and the load gradually through a programmed sequence. The shutdown sequence decreases the load gradually.




GAS TURBINE TYPES AND CONFIGURATIONS

With Permission from Solar Turbines Inc., a Division of Caterpillar

Figure 2. Gas Turbine Internals




Gas Turbine Types

Heavy Duty

Heavy-duty gas turbines are designed to run approximately three years continuously under ideal conditions without a shutdown for maintenance. To achieve this goal, heavy duty turbines are conservatively designed. They operate with relatively low firing temperatures (approximately 1500º - 1800ºF). They are available in a wide range of sizes including very large models producing over 100,000 bhp. For power generation, gas compression, and water reinjection, Saudi Aramco uses heavy-duty gas turbines in the range of 20,000 - 25,000 site horsepower. A typical two-shaft heavy-duty gas turbine is shown in Figure 2a.




Figure 2a. Gas Turbine Internals




Aero Derivative

This type of gas turbine is similar to aircraft jet engines. It is lightweight and compact. For this reason, it is frequently used on offshore platforms. These machines are designed to operate with high firing temperatures (2100º - 2300ºF) to achieve high efficiency. Because of their lightweight design and relatively high firing temperatures, they have shorter run lengths between overhauls. Saudi Aramco uses aero derivative gas turbines for pipeline pump drives, gas injection compressors, and electrical power generation.

Aero derivative gas turbines are gaining in popularity because they are much lighter in weight, easier to transport, smaller in size, and higher in efficiency. In addition, their on-line factor (availability) has increased significantly with the second-generation design of aero derivative turbines. An example of a second-generation gas turbine that is used by Saudi Aramco (G.E. LM 2500) is shown in Figure 2b.




Figure 2b. Second Generation Aero Derivative Gas Turbine




Gas Turbine Configurations

Single Shaft

A single shaft gas turbine has the air compressor and the power turbine on the same shaft, running at the same speed (Figure 3). This type is best for constant speed applications. Therefore, it is the type commonly used to generate electric power. It is not usually used for mechanical drive (pump or compressor) applications since the starting power requirement is much greater than that required for a generator.

Figure 3. Single Shaft Gas Turbine




Dual Shaft

A dual shaft gas turbine has the air compressor and the high pressure turbine that drives it mounted on one shaft. See Figure 4. A second low pressure turbine, commonly called the power turbine, and the load are connected to a second shaft. Because there are two shafts, the compressor and the power turbine can operate at different speeds. This makes the turbine suitable for variable speed applications. It is used to drive process equipment such as pumps and compressors. Since the high pressure turbine is not connected directly to the load, the starting horsepower requirement is considerably less than that of a single shaft turbine.

Figure 4. Dual Shaft Gas Turbine




Three Shaft Gas Turbine

A three shaft gas turbine has a two shaft air compressor (low pressure and high pressure) and a one-shaft power turbine connected to the load. By operating the high pressure compressor at a higher speed than the low pressure compressor, the overall efficiency of the gas turbine is increased. Saudi Aramco uses the Pratt and Whitney model FT-4 gas generator to provide exhaust gas to a power turbine to drive pumps on the East-West Pipeline. Refer to Figure 4a.

Figure 4a. Three Shaft Gas Turbine




Available Models of Gas Turbines

A wide range of gas turbines is available to the industry with horse powers ranging from 700 to 200,000. Approximately 20 different manufacturers make gas turbines. See the GPSA Manual Figure 15-33 for a partial list of available models.

Saudi Aramco uses combustion gas turbines in the following services:

• Electric power generators • Pipeline pumps • Water injection pumps Process compressors

Fuels for Gas Turbines

Gas turbines can operate with a wide variety of fuels, both gases and liquids. The most common are:

• Natural gas • Mixed refinery gases, H2 and C1 to C5

• Kerosene • Diesel fuel Natural Gas Liquids (NGL)

It is also possible to burn heavier liquids, such as crude oil and heavy fuel if the fuel is properly treated.

The combustors must be designed for the actual fuel that is used.

Fuel pressure must be high enough to pass through a control valve and then enter the combustor. The combustor operates at the discharge pressure of the air compressor. For liquid fuels, the gas turbine installation can include a fuel pump. Gas fuels must be supplied at the required pressure or a fuel gas booster compressor must be used.




GAS TURBINE CYCLES

"Cycle" is a term used to describe thermodynamic processes of the gas turbine, as affected by the way gas turbines are connected to other components, particularly heat recovery devices. Simple cycles have few components but are low in efficiency. More complex cycles can improve the efficiency of a gas turbine installation.

Efficiency Definitions

Gas Turbine Efficiency is power produced divided by fuel consumed (lower heating value, LHV).

Cycle Efficiency is power plus useful heat produced divided by fuel consumed (LHV). Useful heat is heat that is recovered from the exhaust and used to make steam or to heat a process. If no heat is transferred to steam or to a process, the turbine efficiency is the same as that of the cycle efficiency.

Simple Cycle

The gas turbine in Figure 1 is a simple cycle machine. The hot gas from the expander is vented directly to atmosphere. Since this exhaust is quite hot, approximately 900ºF, a large amount of energy is lost to the atmosphere. A typical efficiency for a simple cycle gas turbine is approximately 25% to over 40%. Heavy duty gas turbines have efficiencies from 25% to 32%. Aero derivatives have efficiencies from 32% to over 40%.

At the present time, all gas turbines in Saudi Aramco installations are simple cycle. As the cost of fuels becomes higher, regenerative cycle and combined cycles will be used. These options are being considered in new projects (e.g.. Abqaiq Plants).




Other Cycles

Several cycle improvements can be made to improve turbine efficiency.

• Regenerative cycle • Exhaust heat recovery • Combined cycle, combining gas and steam turbines • Supplementary firing in a heat recovery boiler

Regenerative Cycle

The regenerative cycle is illustrated in Figure 5. Heat from the turbine exhaust preheats the air before it enters the combustor. Again, typical temperatures are shown and there are variations in actual machines. The exhaust gas heats the air from 400ºF to 800ºF. Since the air entering the combustor is preheated, less fuel is required to heat it the rest of the way to 1800ºF. This is the source of improvement for cycle efficiency. Since the regenerator cools the exhaust gas to about 500ºF, less heat is lost to the atmosphere.

Figure 5. Regenerative Cycle




Exhaust Heat Recovery

Figure 6 shows an example of exhaust heat recovery. The gas passes through a waste heat boiler, where the heat converts water to steam. The exhaust gas leaving the waste heat boiler will have a temperature of about 300ºF. Thus, significantly less energy is lost to the atmosphere. The heat from gas turbines usually generates steam as shown. However, it is also possible to use the hot gas for direct heating of processes. The generated steam can be used for the process, this is also called co-generation.

Figure 6. Exhaust Heat Recovery




Combined Cycle

The combined cycle operation is shown in Figure 7. Heat from the exhaust gases again generates steam. The exhaust gas from the heat recovery device, commonly called a heat recovery steam generator (HRSG), goes to the atmosphere at about 250ºF. The steam is generated at high pressure. It then drives a steam turbine to produce more power. The steam turbine is a condensing type. A pump returns the condensate to the waste heat recovery steam generator. This cycle is used to produce maximum power and when no process steam is desired.

Figure 7. Combined Cycle




Supplementary Firing

Supplementary firing can be added to waste heat recovery. Remember, that only a portion of the oxygen is consumed in the gas turbine combustor. The exhaust gas still contains about 16% oxygen. If additional fuel is added, the temperature of the exhaust gas rises considerably. This results in more steam production. It also results in higher cycle efficiency because the efficiency of the supplementary firing increment is 100%. See Figure 8 for a schematic diagram of supplementary firing.

Figure 8. Schematic Diagram - Supplementary Firing




PERFORMANCE CALCULATIONS

The main performance calculations for gas turbines are:

• Site rating: The maximum power available from a gas turbine at actual site conditions.

• Heat rate: The ratio of fuel consumed to power produced.

• Thermal efficiency.

• Exhaust gas composition.

Site Rating

The amount of power that a gas turbine can produce depends on air temperature and barometric pressure. As temperature rises, or as barometric pressure decreases, the air density decreases. With lower air density, the gas turbine will produce less power.

Manufacturers provide standard ratings for their gas turbines, based on conditions set by the International Standards Organization (ISO). The standard conditions are as follows:

• Ambient air temperature: 59ºF.

• Altitude: sea level.

• Ambient air pressure: 29.92 in Hg.

• Inlet and exhaust pressure losses: zero.

• Natural gas fuel with a specific heating value.

Site Rated Power is the maximum continuous power that the turbine can generate at actual conditions of the site. To calculate the Site Rated Power (refer to SAES-K-502), one starts with Standard Rated Power and makes corrections for site conditions.




To make these corrections, the first choice is to use curves that have been supplied by the manufacturer for each machine. If these are not available, the curves in the GPSA Handbook, Figures 15-29 to 15-32, can be used to make approximations.

Heat Rate is the amount of fuel required per unit of power. The units are Btu per horsepower-hour or Btu per kilowatt-hour. Btu's are the heat of combustion of the fuel, lower heating value (LHV).

The heat rate is affected by:

• Inlet and outlet pressure losses

• Ambient air temperature

• Percentage of rated load

Note that the heat rate is not affected by altitude.

Graphs are available to make these corrections. For pressure losses and ambient air temperature use manufacturers' curves, or the GPSA Manual Figure 15-30 to 15-32. For correction due to percentage load, the manufacturers curves are the only reasonable source.

Thermal Efficiency is the power delivered to the load divided by the heat of combustion of the fuel. Keep in mind the following conversion factors. At 100% efficiency:

One horsepower (hp) = 2544 Btu/hr

One kilowatt (kW) = 3414 Btu/hr

Therefore, thermal efficiency = hrhpBtu,RateHeat

2544−

= hrkWBtu,RateHeat

3414−




Exhaust Gas Composition

The principal components of the exhaust gases are nitrogen, oxygen, carbon dioxide and water. The amounts of carbon monoxide and unburned hydrocarbons are negligible because there is a large amount of excess oxygen in the combustor. The manufacturers' performance curves will usually give the oxygen in the exhaust gas as a function of percentage of full load. Alternatively, if the fuel rate is known, the oxygen content of the exhaust gas can be calculated by stoichiometry. To obtain the other components, carbon dioxide and water, a stoichiometric calculation using balanced chemical equations is necessary.

Gas Turbine Performance Curves

The information normally provided on a manufacturer's performance curve is as follows:

• Effect of altitude on maximum power output.

• Effect of inlet air temperature on maximum power output, heat rate and air flow rate.

• Effect of percentage load and speed on the heat rate and exhaust temperature.

Work Aids 1-5 are the manufacturer's curves for a General Electric Frame 5 turbine, dual shaft. They can be used in the exercises.




WORK AID 1: GAS TURBINE ALTITUDE CORRECTION FACTOR FOR OUTPUT AND HEAT CONSUMPTION AND ALSO ALTITUDE VS ATMOSPHERIC PRESSURE

With Permission from General Electric Company K.D. Knapp Sept. 21, 1970 Notes: 1. Altitude Pressure Calculated by Methods of NACA Report No. 218. 2. Heat Rate and Thermal Efficiency Unaffected by Altitude.

Figure 9. Altitude Correction Factor for Output and Heat Consumption and Altitude Versus Atmospheric Pressure




WORK AID 2: GENERAL ELECTRIC MODEL M5382C GAS TURBINE -- EFFECT OF COMPRESSOR INLET TEMPERATURE ON MAXIMUM OUTPUT, HEAT RATE, AND AIR FLOW

With Permission from General Electric Company V. Poua, Rev A Feb. 26, 1987

Notes: 1. Compressor Speed - 5100 rpm; 100% Speed 2. Load Turbine Design Speed - 4670 rpm

Figure 10. GE Model M5382C Gas Turbine – Effect of Compressor Inlet Temperature on Maximum Output, Heat Rate, and Air Flow




WORK AID 3: GENERAL ELECTRIC MODEL M5382(C) *38,000 HP GAS TURBINE


Figure 11. GE Model M5382C – 120ºF Load Limit 989 ºF Exhaust Temperature





With Permission from General Electric Company V. Poua, Rev A Dec. 2, 1986











GLOSSARY

Combined Cycle A cycle that includes a gas turbine to generate power, a waste heat boiler to recover heat from the gas turbine exhaust, and a steam turbine that consumes steam from the waste heat boiler and generates power.

Combustor The component of a gas turbine between the air compressor and the power expander. It is the place where fuel is burned in the compressed air.

Compressor The first component of a gas turbine, which compresses ambient air that is supplied to the combustor.

Dual Shaft Gas Turbine A gas turbine having two shafts. This permits the air compressor and the load turbine to run at different speeds. It also reduces the load on the starting device.

Expander The power turbine of a gas turbine. It generates power to the load from the compressed and heated air.

Governor A device that regulates the speed of a gas turbine.

Heat Rate A measure of fuel consumption in a gas turbine. It is the fuel fired divided by the power output, in Btu/hp-hr or Btu/kW-hr.

Helper Turbine Auxiliary turbine connected to a gas turbine usually driven by steam. The turbine is used for starting the gas turbine and may also run continuously to supplement power output.

Simple Cycle A configuration of a gas turbine in which the exhaust is vented to atmosphere.




Power Turbine An expansion turbine that converts the energy of a hot compressed gas to shaft power. Same as expander.

Regenerative Cycle A gas turbine cycle that includes a heat exchanger. The heat exchanger transfers heat from the exhaust gas to the compressed air before the combustor.

Sequence Controller An instrument that controls the startup or shutdown sequence of a gas turbine.

Single Shaft Gas Turbine

A gas turbine in which the air compressor, the power turbine, and the load are all connected to the same shaft and therefore run at the same speed.

Site Power Rating The power capability of a specific gas turbine at actual site conditions of air temperature and air pressure.

Stoichiometric Calculation

A calculation of the components of the exhaust gas using balanced chemical equations.

Starting Device Device that provides power to accelerate the compressor to sufficient speed so that the air flow through the compressor is sufficient to sustain combustion (commonly called “light-off”).

Supplementary Firing The combustion of extra fuel in the exhaust stream of a gas turbine to increase the thermal energy of the exhaust gas. Supplementary firing takes place in the exhaust duct or in the heat recovery steam generator (HRSG).

Thermal Efficiency For a gas turbine cycle, the sum of power output plus useful heat output divided by the fuel consumed.




REFERENCES

Supplementary Text

• Gas Processors Suppliers Association

• Engineering Data Book - Section 15

Saudi Aramco Standards

• SAES-K-502, Combustion Gas Turbines

Industry Standards

• API-616, Type H Industrial Combustion Gas Turbines for Refinery Service (Heavy duty)

• API-679, Type G Aeroderivative (Lightweight) Combustion Gas Turbines for Refinery Service

Documents

Combustion Gas Turbines