Gas Turbines

FUNDAMENTALS OF MACHINERY ENGINEERING

COMBUSTION GAS TURBINES

PARTICIPANT MODULE


PAGE

OBJECTIVES 1

INFORMATION SHEET

How a Gas Turbine Works 2 Major Components 2 Gas Turbine Types 4

Gas Turbines 4

Fuels for Gas Turbines 8 Gas Turbine Cycles 8 Performance Calculations 14 Gas Turbine Performance Curves 15 Auxiliary Equipment 16 Control Systems 16

EXERCISES SHEETS 17

WORK AIDS 20

GLOSSARY 26

REFERENCES 28

WRITTEN EVALUATION 29

TERMINAL OBJECTIVE After completing the module, you will be able to calculate the performance of various gas turbines in service at affiliate plants.

ENABLING OBJECTIVES

Identify the three major components of a combustion gas turbine.

Calculate how much of the oxygen in the incoming air is used up in the combustor.

Compare and contrast the four types and configurations of gas turbines.

Calculate the thermal efficiency of a typical gas turbine, exhausting to atmosphere without heat recovery.

Calculate the maximum continuous power rating, given a General Electric gas turbine and location conditions.


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.

HOW A GAS TURBINE WORKS

Figure 1 illustrates a gas turbine and its three major components:

Air compressor

Combustor

Power turbine In Figure 1, the air compressor and the power turbines are mounted on the same shaft. The temperatures and pressures shown re typical values; however, there is a considerable range in these values. 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 is about 100

psig and the temperature has risen to about 400 F. The air flows from the compressor to the combustor. In the combustor, fuel is added and combustion raises the temperature of the air

to approximately 1800 F. The temperature rises increases the volume of the air significantly, which greatly increases the amount of energy in the air. The heated air 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 as it flows through the power turbine. Because the air flowing through the turbine has been heated in the combustor, the energy available to the turbine is greater than energy consumed by the air compressor. The net difference between two energies is available as shaft work to drive a machine.

MAJOR COMPONENTS

Air Compressor The air compressor is usually an axial compressor with 8 to 20 rows of blades. In some cases, the air compressor can be a centrifugal compressor. Compression ratios vary from 5 to 18, though a compression ratio of 8 to 10 is most common.

HOW A GAS TURBINE WORKS

FIGURE 1

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 which can be reached. The higher the temperature, the higher the efficiency and power output, but nozzle and blade materials limits

the practical temperature to about 1800 F. Power Turbine The power turbine is a hot gas expander. It is usually an axial flow turbine, with 2 to 4 rows of blades. Figure 2 is a cutaway diagram of the components of a gas turbine.

GAS TURBINE INTERNALS

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

FIGURE 2

GAS TURBINE TYPES

Heavy Duty Heavy duty gas turbines are designed to run approximately three years continuously without a shutdown for maintenance. To achieve this goal, heavy duty turbines are conservatively designed. They operated with relatively low combustion temperatures. They are available in a wide range of sizes including very large models. Aircraft Derivative Another 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 temperatures to achieve high efficiency. As a result they have shorter run lengths between overhauls, approximately one year.

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 common used to generate electric power.

SINGLE SHAFT GAS TURBINE

FIGURE 3

Dual-Shaft A dual-shaft gas turbine has the air compressor and the power turbine that drives it mounted on one shaft. See Figure 4. A second 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 compressor.

DUAL SHAFT GAS TURBINE

FIGURE 4

AVAILABLE MODELS OF GAS TURBINES

A wide range of gas turbines is available to the industry with horsepowers ranging from 700 to 200,000. Approximately 20 different manufacturers make gas turbines. See GPSA Engineering Data Book Figure 15-32 for a partial list of available models. Oil and Gas companies use combustion gas turbines in the following services:

Electric power generators

Gas and process air compressors

Pipeline pumps

Water injection pumps

Offshore platforms

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 It is also possible to burn heavier liquids, such as crude oil ad heavy fuel. The combustors must be designed for the actual fuel which 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.

GAS TURBINE CYCLES

"Cycle" is a term used to describe 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 (low heating value). 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 process, the turbine efficiency is the same as the cycle efficiency. Simple Cycle The gas turbine in Figure 1 is a simple cycle. The hot gas from the expander is vented

directly to atmosphere. Since this exhaust is quite hot, approximately 900 F, a large amount of energy is lost to the atmosphere. A typical efficiency for a simple cycle gas turbine is 20 to 25%. Some of the gas turbines in affiliate installations are simple cycle. Other Cycles Several cycle improvements can be made to improve efficiency.

Regenerative cycle

Exhaust heat recovery

Combined cycle, combing gas and steam turbines

Supplementary firing

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 the actual machines. The exhaust gas heats the air from 400 F to 800 F. Since the air entering the combustor is preheated, less fuel is required to heat the rest of the

way to 1800 F. This is the source of improvement for cycle efficiency. Since the regenerator

cools the exhaust gas to about 500 F, less heat is lost to the atmosphere.

REGENERATIVE CYCLE

FIGURE 5

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 300 F. Thus significantly less energy is lost to the atmosphere. The heat from the gas turbines usually generates steam, as shown. However, it is also possible to use hot gas for direct heating of processes.

EXHAUST HEAT RECOVERY

FIGURE 6

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 goes to the atmosphere at

about 250 F. 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.

COMBINED CYCLE

FIGURE 7

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% of 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.

SCHEMATIC DIAGRAM - SUPPLEMENTARY FIRING

FIGURE 8

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 can 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: 59 F

Altitude: sea level.

Ambient air pressure: 29.92 in Hg.

Inlet and exhaust pressure losses: none. Site Rated Power is the maximum continuous power that turbine can generate at actual conditions of the site. To calculate the Site Rated Power, one starts with Standard Rated Power and makes corrections for the sit conditions. To make these corrections, the first choice is to use curves that have been supplied by the manufacture for each machine. If these are not available, the curves in the GPSA Engineering Data Book, Figures 15-28 to 15-31, 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 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.

Site Rating (Cont'd) Graphs are available to make these corrections. For pressure losses and ambient air temperature use manufacturer's curves, or GPSA Figures 15-28 to 15-31. 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 = 2544 .

Heat rate, Btu/hp-hr = 3414 . Heat rate, Btu/kW-hr

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 manufacturer's performance curves will usually give oxygen in the exhaust gas as a function of percentage of full load. Alternatively, if the fuel 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 1-6 are manufacturer's curves for a General Electric Frame 5 turbine, dual shaft. They can be used in the exercises.

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.

Air filter. It is very important that the air to gas turbine be clean. Therefore, a major component, particularly in desert environments, is the air filter, which removes airborne solid particles. The primary air filter is often an inertial device to remove large particles. If a significant number of small particles are present there will be a second stage containing a fabric filter medium.

A new type of filter called pulse clean is becoming popular. In this system a pulse of air is blown backwards 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 may be a silencer on both the inlet and the exhaust. If low noise levels are important, the turbine may also have a cocoon, or acoustic enclosure around the casing.

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

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 turbine 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. During startup, this system increases the speed and the load gradually through a programmed sequence. Shutdown sequence is normally controlled only on large turbines. It decreases the load gradually.

EXERCISE 1

Directions: Answer the following questions in the space provided. 1. A single shaft gas turbine drives a gas compressor. The bhp of the compressor is

21,000hp. The total output of the power turbine is 46,000 hp.

Question: What is the power consumption of the air compressor section of the gas turbine?

2. The exhaust gas from a combustion turbine typically has 16 to 17% oxygen.

Question: Why is the oxygen content so high?

3. a. What type of gas turbine is used to drive electric generators?

b. What type of gas turbine is used to drive pumps at variable speeds?

c. What type of gas turbine is used where small size and weight are needed?

d. What type of gas turbine is used for the longest possible running time

between shutdowns?

EXERCISE 2

Directions: Use the information below to perform the required calculations. A gas turbine has the following ISO ratings hp 38,000

Heat rate 8700 Btu/hp-hr The Inlet air filter takes a pressure drop of 6 inches water. There is no pressure drop on the exhaust side. The fuel is 100% methane (977 Btu/SCF, LHV). The manufacturer's curves are Figure 9, 10 and 11 (Work Aids 1, 2 and 3) Questions: 1. If the turbine is installed inland at 800 ft. elevation, where the daytime temperature is

110 F, what will be the maximum continuous power available?

hp 2. What will the fuel consumption be at maximum continuous power?

SCF/hr 3. What is the thermal efficiency of the turbine?

EXERCISE 3

Directions: Use the information below to perform the required calculations. A GE frame 5 combustion gas turbine drives a pipeline pump. The manufacturer's curves are Figure 9, 10, and 11 (Work Aids 1, 2, & 3). The required conditions are:

Turbine output hp 19,500 hp Turbine output speed 3,700 rpm Altitude 1,200 feet Ambient temperature 120 F Inlet pressure drop 4 inH2O Outlet pressure drop zero

Calculate the following:

1. Exhaust temperature F 2. Fuel rate Btu/hr

MODULE NO. PARTICIPANT WORK AID 1

FIGURE 9


FIGURE 10


FIGURE 11


FIGURE 12


FIGURE 13


FIGURE 14

MODULE NO. PARTICIPANT GLOSSARY

GLOSSARY

Cocoon An acoustical enclosure surrounding a gas turbine. It is

used to reduce noise emission.

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.

Dual-Shaft Gas Turbine A gas turbine having two shafts. This permits the air compressor and the load turbine to run at different speeds.

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

Governor A device that regulates the speed of the 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.

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.

Open 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 from the exchanger transfers heat from exhaust gas to the compressed air before the combustor.

MODULE NO. PARTICIPANT GLOSSARY

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.

Supplementary Firing The combustion of extra fuel in the exhaust stream of a gas turbine.

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

MODULE NO. PARTICIPANT REFERENCES

Supplementary Text and Software

Gas Processors Suppliers Association: Engineering Data Book - Section 15 Gas Turbine World Magazine: Annual Performance Specifications Edition. Address: Gas Turbine World Magazine, PO Box 447, Southport, CT 06490, USA.

Thermoflow GTPRO®, PC software for analysis of simple and combined cycle gas turbines. Address: Thermoflow Inc., 888 Worcester Street, Wellesley, MA 02181, USA.

Industry Standards

API 616 Gas Turbines for the Petroleum, Chemical and Gas Industry Services

API 11PGT (RP) Packaged Combustion Gas Turbines Engineering Practices (EMEPS - Global Practices)

GP 10-08-01 Combustion Gas Turbines GP 10-08-02 Packaged Gas Turbines

GP 10-08-04 Gas Turbine Drivers

EPT 07-T-01 Gas Turbines (Tutorials)

International Practices

IP10-8-1 Combustion Gas Turbines

IP 10-11-1 Sizing of Drivers and Transmissions for Compressors, Fans, and Pumps

Design Practices

Section XI-N Reciprocating Engines and Gas Turbines

Section VIII-M Combustion Calculations

MODULE NO. PARTICIPANT WRITTEN EVALUATION SHEET

WRITTEN EVALUATION SHEET

1. Name the three major components of a combustion gas turbine.

1. 2. 3.

2. How much of the oxygen in the incoming air is used up in the combustor?

Choose one.

a. About 90% b. About 50% c. Less than 50% 3. Which type of gas turbine is best for variable speed drive?

4. What is the thermal efficiency of a typical gas turbine, exhausting to atmosphere

without heat recovery? Choose one.

a. 10 to 15% b. 20 to 25% c. 60 to 70 % d. 80 to 85%

5. A General Electric gas turbine, model M5382 (c) will be installed at an inland location.

Ambient air temperature = 110 F Altitude above sea level = 1200 ft Pressure drop, inlet duct and air filter = 6 in H2O Pressure drop, exhaust duct and silencer = 4 in H2O

What is the maximum continuous power rating at these conditions? hp

Gas Turbines

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