turbines - nikola milikic

8/9/2019 Turbines - Nikola Milikic

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Kragujevac, 2013.

UNIVERSITY OF KRAGUJEVAC

FACULTY OF ENGINEERING IN KRAGUJEVAC

Master level of study

SEMINARY WORK

- Turbines -

Student: Professor:

Nikola Milikić 390/2013 Sandra Stefanović


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Content:

1.

Introduction 2

2. Definition of turbine 3

3. Turbine development history 3

4.

Functioning principle 5

5. Constructional classification 6

5.1 Single-shaft gas turbine 6

5.2 Two-shaft gas turbine with a power turbine 6

5.3 Three-shaft gas turbine with a power turbine 7

6. Materials and fabrication 8

7. Failures to gas turbine vanes and blades 9

7.1 Mechanical failures 10

7.2 Negative thermal influence 12

7.3 Negative chemical influence 13

8. Advantages and disadvantages of turbines 13

9. Literature 15


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1.

Introduction

A term turbine encourages from the Greek word "τύρβη" ("turbulance"), and it

represents a rotary mechanical device that extracts energy from a fluid flow and converts it

into useful work. A turbine is a turbomachine with at least one moving part called a rotor

assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so

that they move and impart rotational energy to the rotor. Early turbine examples are

windmills and water wheels.

There is several types of turbine. They can be:

steam turbines,

gas turbines,

wind turbines,

water turbines, …

In this work a gas turbines are mostly considered, because they are most present in

practice.

World energy demand is likely to increase over the next 20 years, and it is well

ascertained that fossil fuels will still be the dominant source for power generation all over the

world. In this scenario, gas turbine (GT) engines will still represent a key technology, either

in stand-alone applications or combined with other power generation equipment [1].

The challenges in GT technology today deal with several issues, such as increased

on/off design efficiency, reduced performance degradation over time, and decreased pollutant

emissions levels. Major research efforts and investments are being bestowed for the

development of new, advanced GT technologies with superior performance, thus helping inthe fulfillment of the Kyoto Protocol objectives for greenhouse gases reduction, as well as of

many other transnational policies on sustainability and reduced environmental impact of

energy technologies. In fig. 1 is represented an scheme of turbine [1, 2].

Fig. 1. Principle of work for impulse and reactional turbine

http://en.wikipedia.org/wiki/Turbulancehttp://en.wikipedia.org/wiki/Turbulancehttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Work_%28physics%29http://en.wikipedia.org/wiki/Turbomachineryhttp://en.wikipedia.org/wiki/Turbine_bladehttp://en.wikipedia.org/wiki/Windmillhttp://en.wikipedia.org/wiki/Waterwheelhttp://en.wikipedia.org/wiki/Waterwheelhttp://en.wikipedia.org/wiki/Windmillhttp://en.wikipedia.org/wiki/Turbine_bladehttp://en.wikipedia.org/wiki/Turbomachineryhttp://en.wikipedia.org/wiki/Work_%28physics%29http://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Turbulance


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This seminary work presents in short notes definition, brief development history, the

functioning principle, some of classification, materials, fabrications, and most common

failures of gas turbines.

2.

Definition of turbine

A gas turbine (fig. 2), also called a combustion turbine, is a type of internal combustion

engine. It has an upstream rotating compressor coupled to a downstream turbine, and a

combustion chamber in-between.

F ig. 2. Turbine

Energy is added to the gas stream in the combustor, where fuel is mixed with air and

ignited. In the high pressure environment of the combustor, combustion of the fuel increases

the temperature. The products of the combustion are forced into the turbine section. There,

the high velocity and volume of the gas flow is directed through a nozzle over the turbine's

blades, spinning the turbine which powers the compressor and, for some turbines, drives their

mechanical output. The energy given up to the turbine comes from the reduction in the

temperature and pressure of the exhaust gas.

Energy can be extracted in the form of shaft power, compressed air or thrust or any

combination of these.

3.

Turbine development history

The history of the gas turbine goes back to 1791, when John Barber took out a patent

for „A Method for Rising Inflammable Air for the Purposes of Producing Motion and

Facilitating Metallurgical Operations”. Scheme of the first invented gas turbine is shown in

fig. 3.

http://en.wikipedia.org/wiki/Internal_combustion_enginehttp://en.wikipedia.org/wiki/Internal_combustion_enginehttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Combustion_chamberhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Combustorhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Earth%27s_atmospherehttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Nozzlehttp://en.wikipedia.org/wiki/Nozzlehttp://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Earth%27s_atmospherehttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Combustorhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Combustion_chamberhttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Internal_combustion_enginehttp://en.wikipedia.org/wiki/Internal_combustion_engine


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Fig. 3. Scheme of first gas turbine (Eddie Taylor’s paper collection)

Many endeavors have been made since then particularly in the early 1900's to build an

operational gas turbine. In 1903, a Norwegian, Aegidius Elling, built the first successful gas

turbine using a rotary/dynamic compressor and turbines, and is credited with building the first

gas turbine that produced excess power of about 8kW (11 hp). By 1904 Elling had improved

his design, achieving exhaust gas temperatures of 773 K (500 degrees Celsius), up from 673

K (400 degrees Celsius), producing about 33kW (44 hp). The engine operated at about 20

000 rpm. Much of his later work was carried out (from 1924 to 1927) at Kongsberg, in

Norway. Elling’s gas turbine was very similar to Frank Whittle’s jet engine, which was

patented in 1930 in England [2].

Fig. 4. Scheme of Frank Whittle’s jet engine

Whittle’s design also consisted of a centrifugal compressor and an axial turbine and the

engine was subsequently tested in April 1937. Meanwhile, in 1936, Hans von Ohain and Max

Hahn, in Germany, developed and patented their own design. Unlike Frank Whittle’s design,

von Ohain’s engine employed a centrifugal compressor and turbine placed very closetogether, back to back. The work by both Whittle and Ohain effectively started the gas

turbine industry. Today, gas turbines are used widely in various industries to produce

mechanical power and are employed to drive various loads such as generators, pumps,

process compressors, or a propeller. The gas turbine began as a relatively simple engine and

evolved into a complex but reliable and high efficiency prime mover. The performance and

satisfactory operation of gas turbines are of paramount importance to the profitability of

industries, varying from civil and military aviation to power generation, and also oil and gasexploration and production.

In the quest to perfect the gas turbine, compressor pressure ratios have increased from

about 4:1 to over 40:1 together with high operating temperatures (about 1800 K), resulting inthermal efficiencies exceeding 40%.


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4.

Functioning principle

For a turbine to produce power, it must have a higher inlet pressure than that at the exit.

A compressor is normally used to provide this increase in pressure into the turbine. If the

compressor discharge flow through the turbine is expanded, the turbine power output will be

less than the power absorbed by the compressor because of losses in the compressor andturbine. Under these conditions, the whole engine will cease to rotate. If energy is added into

the compressor discharge air, corresponding to the losses in the compressor and turbine, then

the system will run but will not produce any net power output. To produce net power from

the gas turbine, additional energy needs to be supplied into the compressor discharge air. The

energy supplied to the compressor discharge air is normally achieved by burning fuel in the

compressor discharge air and this is accomplished in a combustion chamber or combustor,

which is located or positioned between the compressor and turbine [2] (fig. 5).

Fig. 5. Schematic layout of a single-shaft gas turbine

Clearly, the power output from a gas turbine depends on the efficiency of thecompressor, turbine and the combustor. The higher the efficiency of these components, the

better will be the performance of the gas turbine, resulting in increased power output and

thermal efficiency. The gas turbine has developed over 50 years into a high efficiency prime

mover, and compressor and turbine efficiencies (polytropic) above 90% can be achieved

today.

From the above discussion, a gas turbine must therefore have at least the following

components:

1.

compressor

2.

combustor

3.

turbine.

A gas turbine comprising these components is often referred to as a simple cycle gas

turbine. Gas turbines can include other components, such as intercoolers to reduce the

compression power absorbed, re-heaters to increase the turbine power output and heat

exchangers to reduce the heat input. These types of gas turbines are referred to as complex

cycles. Although such complex cycles were developed in the early days of the gas turbine,

today, simple cycle gas turbines dominate, and this is due to the high levels of performance

achieved by engine components such the compressor, turbine and combustor. However, there

is a renewed interest in complex cycle designs as a means of improving the performance of

the gas turbine further.


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5.

Constructional classification

Various arrangements of the gas turbine components have evolved over the years.

Some are better suited for certain applications such as power generation (constant speed

operation of the load, i.e. the generator) and other layouts are more suited to mechanical drive

applications where the gas turbine is used to drive a process compressor or a pump (where thespeed of the driven equipment can vary with load). In this seminary work, it will be presented

these various arrangements, highlighting their advantages and disadvantages.

5.1 Gasna turbina sa jednim vratilom

Gasna turbina sa jednim vratilom sadrži kompresor, komoru za sagorevanje i turbinu

kao što je prikazano na slici 6. Kompresor uvlači vazduh i poveava njegov pritisak. Taj

kompresovani vazduh se odna šalje u komoru za sagorevanje, u kojoj se visoke temperature

postižu sagorevanjem goriva. Gasovi koji imaju visoku temperaturu i visok pritisak se onda

šire u turbini i tako se proizvodi snaga. Deo izlazne turbinske snage se šalje u kompresor da bi mu obezbedio potrebnu energiju za rad.

Sli ka 6. Gasna turbina sa jednim vratilom

Preostala proizvedena snaga iz turbine se koristi za pokretanje mašine kao što je

generator. Gasna turbina sa jednim vratilom se najviše koriste za fiksne brzine rotacije.Turbine sa jednim vratilom imaju prednost što su zaštićene od preopterećenja u slučaju da se

potražnja za snagom na izlazu poveća.

5.2 Turbina sa dva vratila i posebnom turbinom

Proces ekspanzije prikazan na slici 4., može da se razdovji na dve posebne turbine. Prva

se koristi da napaja kompresor potrebnom energijom a druga se koristi da proizvede snagu na

izlazu. Turbina koja obezbeđuje izlaznu snagu je mehanički nezavisna. Preostale komponente

u sisteu se nazivaju gasni generator. Slika 7 pokazuje šemu turbine se dva vratila koja je

verovatno i najprimenjivanija šema za izradu gasnih turbina.


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Sli ka 7. Šema turbine sa dva vratila i posebnom turbinom

The function of the gas generator is to produce high pressure and high temperature

gases for the power turbine. Two-shaft gas turbines operating with a power turbine are often

used to drive loads where there is a significant variation in the speed with power demand

(mechanical drive applications such as gas compression). Examples are pipeline compressorsand pumps. The process conditions may be such that the load runs at low speed but absorbs

or demands a large amount of power. In such a situation, the power turbine can run at the

speed of the load and the gas generator can run at its maximum speed. If a single shaft gas

turbine were employed to provide the power requirements for such applications, the whole

engine would be constrained to run at the speed of the load thus resulting in poor engine

performance due to the low operating speed condition. Two-shaft gas turbines are also

employed in industrial power generation with the power turbine designed to operate at a fixed

speed determined by the generator [2].

Fi g. 8. Two-shaft gas turbine with a power turbine (General Electric T700)

Unlike a single-shaft engine, the gas generator speed will vary with electrical load.

The main advantage is smaller starting power requirements, as the gas generator only needs o

be turned during starting, and better off design performance. The disadvantage is that the

shedding of the electrical load can result in over-speeding of the power turbine.

5.3 Three-shaft gas turbine with a power turbine

The gas generator (GG) can be divided further to produce a two-shaft or a twin spool

gas generator. When this is done, the high-pressure GG turbine drives the high-pressure GG

compressor, and the low pressure GG turbine drives the low pressure GG compressor.

However, there is no mechanical linkage between the high pressure and low pressure shafts

in the gas generator. Fig. 9 shows a schematic layout of a gas turbine with three shafts incombination with a power turbine.


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F ig. 9. Schematic layout of a three-shaft gas turbine with a power turbine

Fig. 10. Three-shaft gas turbine with a power turbine (SGT500)

The power turbine is still mechanically independent from the gas generator. Such three-

shaft arrangements, as with a two-shaft gas turbine with its own power turbine, are widely

used in mechanical drive applications. Much higher-pressure ratios and thermal efficiencies

may be achieved with such a layout without having to resort to variable geometry

compressors as would be required by two-shaft gas engines when designed to operate at high

compressor pressure ratios.

Three-shaft gas turbines also have the added advantage of lower starting powers

because only the high-pressure compressor and turbine in the gas generator need to be turned

during starting. Engines that use such a configuration are often derived from aircraft gasturbines and are referred to as aero-derivatives [2].

6

Materials and fabrication

Advancements made in the field of materials have contributed in a major way in

building gas turbine engines with higher power ratings and efficiency levels. Improvements

in design of the gas turbine engines over the years have importantly been due to development

of materials with enhanced performance levels. Gas turbines have been widely utilized in

aircraft engines as well as for land based applications importantly for power generation.

Advancements in gas turbine materials have always played a prime role – higher thecapability of the materials to withstand elevated temperature service, more the engine


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efficiency; materials with high elevated temperature strength to weight ratio help in weight

reduction. A wide spectrum of high performance materials - special steels, titanium alloys

and superalloys - is used for construction of gas turbines. Manufacture of these materials

often involves advanced processing techniques. Other material groups like ceramics,

composites and inter-metallics have been the focus of intense research and development; aim

is to exploit the superior features of these materials for improving the performance of gasturbine engines. The materials developed at the first instance for gas turbine engine

applications had high temperature tensile strength as the prime requirement. This requirement

quickly changed as operating temperatures rose. Stress rupture life and then creep properties

became important. In the subsequent years of development, low cycle fatigue (LCF) life

became another important parameter. Many of the components in the aero engines are

subjected to fatigue- and /or creep-loading, and the choice of material is then based on the

capability of the material to withstand such loads. Coating technology has become an integral

part of manufacture of gas turbine engine components operating at high temperatures, as this

is the only way a combination of high level of mechanical properties and excellent resistance

to oxidation / hot corrosion resistance could be achieved. The review brings out a detailed

analysis of the advanced materials and processes that have come to stay in the production ofvarious components in gas turbine engines [3].

While there are thousands of components that go into a gas turbine engine, the

emphasis here has been on the main components, which are critical to the performance of the

engine:

1) Compressor parts for aircraft engines – Titanium alloys [3] (Ti-6Al-4V, Ti-8Al-1Mo-

1V, Ti-6Al-5Zr-0.5Mo-0.25Si, Ti-6Al-2.8Sn-4Zr-0.4Mo-0.4Si)

2) Compressor building materials for land based gas turbines – Special steels and

superalloys + (Ti, Al, V, Mo, Zr, Mb, Si, Sn) [3]

7. Failures of gas turbine blades and vanes

The process of gas turbine operation is associated with various failures to structural

components of gas turbines, in particular blades. Condition of the blades is of crucial

importance to reliability and lifetime of the entire turbine, and the ‘parent’ subassembly

where it is installed. This is why the blades are subject to scrupulous checks, both during the

manufacture and at the stage of assembly, when any deviations from the specification are

detected and eliminated. Analysis of the literature show that only a small portion of

damages/failures to turbine vanes and blades are caused by material defects, structural

Damageability of Gas Turbine Blades – Evaluation of Exhaust Gas Temperature in Front of

the Turbine Using a Non-Linear Observer 437 and/or engineering process attributabledefects; most damages/failures are service-attributable causes of failures to aircraft turbine

engines during service durability of turbine vanes and blades is a sum of a number of factors,

where material quality is the matter of crucial importance [4].


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F ig. 10. Causes of failures to aircraft turbine engines during service

With respect to materials, durability can be defined as time of item operation whenalloy properties developed during the manufacturing process remain steady unchanged.

Stability of the properties (the assumed service time) is defined at the design stage by

selection of the desired characteristics (as compared to the expected loads and with account

taken of the fact that the properties are subject to changes with time). High and stable

strength properties of superalloys offer suitable microstructures that are resistant to any

deterioration during the service. These structural features have been assumed a durability

criterion. During the service, gas turbine components may be subject to failures resulting

from the following processes [2, 4, 5]:

1. Creeping,

2. Overheating and melting,

3. Low-cycle and high-cycle fatigue due to thermal and thermomechanical factors,

4. Corrosion and fatigue cracking

5. Chemical and intercrystalline corrosion,

6. Erosion

7. Other factors of less importance.

7.1 Mechanical failures

Mechanical failures to gas turbine vanes and blades most often are attributable to what

follows [7, 8]:

a) Mechanical failures a deformations due to foreign matter affecting the blade (fig. 11)


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F ig. 11. Deformations of turbine blades in the form of dents caused by foreign matter [2]: a) – onthe leading edge, b) – on the trailing edge, c) – on the on suction faces of blades

b)

Foreign-matter-attributable surface scratches (fig. 12) and erosive wear (fig. 13)

Fig. 12. Scratches on protective coatingconducive to corrosion on the leading edge of a

turbine blade

Fig. 13. Erosive wear of leading edges of rotorblades [5]

c) Fatigue

Fig. 14. Failures to gas turbine rotor blades caused by [2]: – fatigue cracking of leading edge, b) – fatigue fracture located at the blade’s locking piece


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7.2 Negative thermal influence

Thermal influence on turbine geometry is also very important because high working

temperatures may cause deformations and damages of working parts of turbine. That

damages are mostly refer on material creeping and with respect to that the classification of

damages it could be made [5, 6, 7]:

a) plastic deformations of vanes due to creeping of material

Fig. 15. Damaged vane due to creeping

b)

deformations of material due to overheating

F ig. 16. Overheating of vane material a) partial melting, b) breakage of leading edge and c) tearing ofvane [4]

c)

melting of turbine material

Fig. 17. Permanent deformations of impeller caused by exhaust gases and high temperatures: a)impeller burn through and b) vane melting


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7.3 Negative chemical influence

a) hightemperature corrosion

F ig. 18. Damages of turbine impeller in coast environment, caused by exhaust gases [4, 6]: a) on vane surface and b) on the supporting edge of the blades

b) precipitation of carbonhydrates microparticles

F ig. 19. Condition of injector in combustion chamber: a) clean and b) contaminated with the rest ofthe carbon combustion

8. Advantages and disadvantages of gas turbines

Advantages of gas turbine compared to other power machines are:

Very high strength / weight;

Lower overall dimensions with respect to the other machines of the same driving

force;

Movement in only one direction, with much less vibration;

Fewer moving parts;

Lower operating pressure;

Higher operating speed;

Lower prices and lower consumption of lubricating oil.


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Disadvantages of gas turbine are:

A high price;

Lower coefficient of efficiency, especially at lower modes and idle;

A long time required for starting;

Weaker and slower response to a request for changing the operating mode.


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9. Literature

1) A. M. Y. Razak: Industrial gas turbines - Performance and operability, Wood head

Publishing Limited, Abington Hall, Abington, Cambridge CB216AH, England, 2007.

2) Ernesto Benini: Advances in gas turbine technology; Janeza Trdine 9, 51000 Rijeka,

Croatia, 2011.

3) www.wikipedia.org

4)

http://www.saacke.co.rs/applications/gta-additional-combustion-systems.html

5) A. M. Y. Razak: Industrial gas turbines - Performance and operability, Wood head

Publishing Limited, Abington Hall, Abington, Cambridge CB216AH, England, 2007.

6)

Nageswara Rao Muktinutalapati: Materials for gas turbines – An overview, VITUniversity, India, 2011.

7) Józef Błachnio and Wojciech Izydor Pawlak: Damageability of gas turbine blades – Evaluation of exhaust gas temperature in front of the turbine using a non-linear

observer , Air Force Institute of Technology (Institute Techniczny Wojsk Lotniczxych-

ITWL), Poland, 2011.

8) Meherwan P. Boyce: Gas turbine engineering handbook – Second edition, Institute ofdiesel and gas turbine engineers, U.K., American society of mechanical engineers,

Butterworth – Heinman, Houston, Texas, United States of America, 2002.

9)

Bojan Teofilović: Proračun ciklusa gasne turbine, grafički rad, Univerzitet u NovomSadu, Fakultet tehničkih nauka, Novi Sad, 2009. godine.

10)

http://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-

Elektrotehnicki_fakultet
http://www.wikipedia.org/http://www.wikipedia.org/http://www.saacke.co.rs/applications/gta-additional-combustion-systems.htmlhttp://www.saacke.co.rs/applications/gta-additional-combustion-systems.htmlhttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://bk.docsity.com/sr-dokumenti/Gasne_turbine-Opsta_energetika-Slajdovi-Elektrotehnicki_fakultethttp://www.saacke.co.rs/applications/gta-additional-combustion-systems.htmlhttp://www.wikipedia.org/

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