2. Turbine Manual

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Power Plant

General Series Course

Volume 2

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Turbine manual Version (a) 15 September, 2010` Page 2 of 190

1. Introduction

This module is designed to provide a trainee power station operator with detailed information on the construction and operation of a generic type turbine.

NOTE: This module contains detailed information relating to a generic turbine and its ancillary equipment. Portions of this module may reflect the type of equipment at your location but should not be interpreted as being modelled on any particular plant.

Prior to commencing this module you may wish to obtain a copy of the module Power Plant Induction Course (coal fired boiler) which covers „Introduction to Power Generation‟ produced by TechComm Simulation. It contains a basic overview of how a thermal (coal fired) power generating plant is constructed and operates. It will assist you in gaining an overview prior to specialising on individual items of plant covered in this module.

This module comprises the second in a series of six modules that cover the following topics:

Boiler

Turbine (covered in this module) Generator

Electrical Controls

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2. Learning Outcomes

The trainee after completion of this module should have gained a detailed understanding of the component parts that go together to form an efficient steam turbine.

This course is constructed in such a fashion that the trainee and the trainer/mentor determine which parts of the course the trainee needs to complete. It is a self-guided course in which the trainee operates alone or in cooperation with other trainees. This course does not require attendance at formal training sessions but does require the trainee to venture into the plant and inspect equipment currently under study. The trainer/mentor will monitor trainee progress and provide guidance during the program.

3. Disclaimer

While every care will be taken to ensure the accuracy and adequacy of information, concepts, advice and instructions conveyed to participants in the Course, no responsibility or liability is accepted by either TechComm Simulation, the course leaders or their associates, for any errors or omissions which may arise through no fault of the parties, and which may be attributed to errors or omissions in the information, advice or instructions given to the parties by the Client or others. Nor is any responsibility or liability accepted for any consequent errors, omissions or acts of the participants or others

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4. Assessment: Evaluation, Recording and

Reporting

Assessment of trainee achievement of the learning outcomes is an essential part of the training process. Regular assessments during the training will enable trainee‟s progress to be monitored and any parts of the training where a trainee may be having difficulty to be identified and appropriate corrective action to be taken. Each module includes Trainee exercises that are to be completed at the end of each section and an open book final assignment to be completed at the end of the module.

The final assignment will assess if the trainee has progressed to a level suitable for sitting the closed book end of module test.

If a trainee does not satisfy any of the assessment criteria, the trainee will have to be reassessed, this may require further training.

Assessment will take into account that not only has the trainee studied this module but also closely examined the equipment at their location.

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5. History of the steam turbine

Early steam engines were of the reciprocating type where steam acted upon a piston contained within a cylinder. The piston operated through a connecting rod and onto a

crankshaft that was rotated to give the engines mechanical output.

In the early twentieth century electrical generators had reached a capacity of 5 megawatts and were driven by a reciprocating steam engines.

As electrical generator outputs increased an alternative form of prime mover needed to be developed as the reciprocating steam engine had reached its practical output limitations.

Although not a new idea at the time; the steam turbine had the ability to fill the requirement of larger outputs.

5.1 Early applications

The steam turbine did not have a smooth transition in taking over from reciprocating steam engine, as early designs had high noise levels along with difficult regulation and were prone to frequent breakdowns.

First applications of the steam turbine were in sawmills and woodcutting shops; with one actually being fitted to a steam locomotive.

5.2 Benefits of steam turbines

As steam turbines became more accepted; rapid development ensued. With the use of superheated steam, turbine performance and efficiency exceeded that of the reciprocating engine and the era of the steam turbine had commenced.

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6. Steam turbine operation

6.1 Introduction

A steam turbine can be considered as a rotary heat engine constructed of a number of cylinders (each cylinder comprises a cylinder casing that contains a rotor). Individual rotors are supported within their respective cylinder casing by journal bearings. The cylinder casing is the stationary component of the turbine while the rotating section of the turbine is referred to as the rotor.

The cylinder casing contains rows of stationary or fixed blades with rotating blades connected to the rotor. These rotating blades are installed between the fixed blades. The stationary blades are fitted into the cylinder casing in such a fashion as to direct or redirect the steam onto the next row of rotating blades. The cylinder rotors are coupled together and connected to the alternator rotor. Steam governor valves control the turbine output.

A condenser installed at the exhaust or low pressure end of the turbine receives and condenses the steam prior to it being pumped back to the boiler.

6.2 Principles of operation of a steam turbine

When high temperature steam passes through a steam turbine; heat energy contained within the steam is converted into kinetic energy (energy due to motion). The steam flowing from the high pressure to a lower pressure is then converted into rotating mechanical energy as the high velocity steam acts on a series of rows of blades mounted on the rotor.

In a typical condensing turbine high pressure; high temperature steam is allowed to expand progressively in stages through the various rows of blades until it is exhausted to the condenser.

As the steam progresses through the turbine the pressure reduces and the volume of the steam increases. To compensate for this volume increase the blade passages of the turbine take the shape of an expanding cone; with the

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largest diameter blades located at the low pressure end of the turbine.

The amount of heat that is converted into kinetic energy by the fixed blades (or nozzles) is dependant on the design shape of these blades.

6.3 Classification of turbines

Turbines are classified as to the: Type of flow (axial or radial)

Cylinder arrangement (number of cylinders; whether single, tandem or cross-compound in design)

Type of blading (impulse or reaction)

6.3.1 Type of flow

Turbine construction is either of the radial or axial flow design. With a radial flow turbine the steam flows outward from the centre of the casing through stages of blading. Figure 1 shows the principle of a radial flow turbine.

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The radial turbine is not normally the preferred choice for electricity generation and is usually only employed for small output applications.

Axial flow turbines have the steam flow through the turbine in a parallel direction to the turbine shaft. Figure 2 shows an axial flow turbine.

Figure 2: Axial flow turbine

The axial flow type of turbine is the most preferred for electricity generation as several cylinders can be easily coupled together to achieve a turbine with a greater output.

In some modern turbine designs the steam flows through part of the high pressure (HP) cylinder and then is reversed to flow in the opposite direction through the remainder of the HP cylinder. The benefits of this arrangement are:

outer casing joint flanges and bolts experience much lower steam conditions than with the one direction design

reduction or elimination of axial (parallel to shaft) thrust created within the cylinder

lower steam pressure that the outer casing shaft glands have to accommodate

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A simplified diagram of a reverse flow high pressure cylinder is shown in Figure 3.

Figure 3: Reverse flow turbine cylinder

6.3.2 Cylinder arrangement

Turbines can be arranged either single cylinder or multi-stage in design. The multi-stage can be either velocity, pressure or velocity-pressure compounded (more about this later).

Single cylinder construction

Single cylinder turbines have only one cylinder casing (although may be is multiple sections). Steam enters at the high pressure section of the turbine and passes through the turbine to the low pressure end of the turbine then exhausts to the condenser.

Figure 2 shows a single cylinder turbine with a high, intermediate and low pressure section contained within the one cylinder casing.

Cylinder exhaust High pressure steam inlet

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Tandem construction

Dictated by practical design and manufacturers considerations modern turbines are manufactured in multiple sections also called cylinders. Greater output and efficiency can be achieved by coupling a number of individual cylinders together in what is referred to as tandem (on one axis). A tandem two cylinder turbine with a single flow high pressure (HP) cylinder and a double flow low pressure (LP) cylinder is shown in Figure 4.

Figure 4: Tandem two cylinder turbine

You will notice that the turbine shown in Figure 4 has what is referred to as a double flow LP cylinder. The steam enters the centre of the double flow cylinder and then divides and flows to opposite ends of the cylinder where it exhausts to the condenser. This type of arrangement provides sufficient cross sectional area for the large volume of low pressure steam. If a single flow design was employed an excessively large diameter cylinder would be required. With the double flow design the length of the blades are significantly reduced thus simplifying the construction while reducing the centrifugal force on the rotor. In addition the double flow arrangement balances out axial thrust on the rotor.

In Figure 5 a tandem three cylinder turbine is shown. It has a double flow LP cylinder with an IP cylinder arranged so that the steam flow through it is in the opposite direction to the

HP Rotor LP Rotor

Exhaust steam to condenser Steam from

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HP cylinder. This design also greatly reduces the axial thrust on the rotor.

Figure 5: Tandem three cylinder turbine

Large modern turbines are required to deliver high output and are generally constructed of four cylinders with the

Ex h a u s t s te a m to c o n d e n s e r Ste a m fr o m b o il e r L P R o to r H P R o to r IP R o to r

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exhaust steam from the HP cylinder passing through a reheater before entering the IP cylinder. This arrangement is shown in Figure 6.

Figure 6: Four cylinder turbine with reverse flow HP cylinder and two double flow LP cylinders

Ex h a u s t s te a m t o c o n d e n s er Ste a m fr o m b o il e r L P 1 R o to r H P R o to r IP R o to r L P 2 R o to r Ste a m re h e a te r

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In some larger overseas installations that operate at 60 hertz (frequency) the use of cross-compounding is sometimes employed. Cross-compounding is where the HP and IP cylinders are mounted on one shaft driving one alternator while the LP cylinders are mounted on a separate shaft driving another alternator. This is done so as the LP cylinder with its large diameter blading can be operated at a greatly reduced speed thus reducing the centrifugal force. This arrangement is shown in Figure 7.

Figure 7: Tandem cross-compound turbine Exhaust steam to condenser

Steam from boiler Steam reheater LP 2 Rotor LP 1 Rotor Alternator No 2 1800 rpm 4 pole 60Hz HP Rotor IP Rotor Alternator No 1 3600 rpm 2 pole 60Hz

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The final turbine arrangement that is becoming increasingly popular is the “Tandem four cylinder turbine with reverse flow HP cylinder, double flow IP and twin double flow LP cylinders”. This arrangement is shown in Figure 8.

Figure 8: Tandem four cylinder turbine with reverse flow HP cylinder, double flow IP and LP cylinders

Ex h a u s t s te a m t o c o n d e n s e r Ste a m fr o m b o il e r L P 1 R o to r H P R o to r IP R o to r L P 2 R o to r Ste a m re h e a te r

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6.3.3 Trainee exercise:

Attempt the following Trainee exercises to gauge how you are progressing. Your answers can then be compared with the model answers at the end of this module.

3. What determines the amount of heat that is converted into kinetic energy within a turbine:

...

2. How are turbines classified:

a) ...

b) ...

c) ...

3. Why is the axial flow type turbine preferred for electricity generation:

...

4. What are the advantages of reverse flow turbine cylinders:

a) ...

...

b) ...

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c) ...

...

5. Draw the steam flow path through the tandem three cylinder turbine shown in Figure 9:

Figure 9: Tandem three cylinder turbine

6.4 Types of blading

The heat energy contained within the steam that passes through a turbine must be converted into mechanical energy. How this is achieved depends on the shape of the turbine blades. The two basic blade designs are:

impulse reaction

6.4.1 Impulse

Impulse blades work on the principle of high pressure steam striking or hitting against the moving blades. The principle of a simple impulse turbine is shown in Figure 10.

Impulse blades are usually symmetrical and have an entrance and exit angle of approximately 200_{. They are} generally installed in the higher pressure sections of the turbine where the specific volume of steam is low and

HP IP _LP

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requires much smaller flow areas than that at lower pressures. The impulse blades are short and have a constant cross section.

Figure 10: Principle of impulse turbine

In a single stage impulse turbine the steam is expanded to the required pressure in fixed diaphragm nozzles thus producing high velocity steam.

The expanded, accelerated steam is then directed onto the moving blades transferring its kinetic energy to the blades. The velocity of the steam (relative to the moving blades) as it leaves the blades should be zero; indicating that no further energy may be transferred to the moving blades.

The characteristic features of an impulse turbine are:

all the pressure drop of the steam occurs in the fixed nozzles

no pressure drop occurs over the moving blades, ie. there is no pressure difference between the two sides of a row of moving blades (with this feature there is little tendency for steam to leak past the moving blades) Rotation Nozzle Rotor Boiler Flame Steam Bearings

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Figure 11 shows a section of impulse type blading.

Figure 11: Section of an impulse turbine blade

A cross section of a single stage impulse turbine is illustrated in Figure 12. The drop in pressure across the nozzles and the velocity change across the moving blades are also shown in Figure 12. Force Steam IN Leading edge Steam OUT

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Figure 12: Cross section of an impulse blade stage Shaft B N V P Fixed Nozzles Steam flows Moving blades Motion Rotor Casing Live steam entering PC VL Exhaust steam leaving Section

P – pressure of steam entering turbine V – velocity of steam entering turbine N – nozzle (fixed blade)

B – blades (moving) PC – condenser pressure

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Velocity compounding

When the velocity energy produced by one set of fixed nozzles is unable to be efficiently converted into rotational motion by one set of moving blades then it is common to install a series of blades as shown in Figure 13. This arrangement is known as velocity compounding.

Figure 13: Velocity compounded impulse turbine Shaft VL B Moving B Fixed Rotor B Moving N V P Fixed Nozzles Steam flows Moving blades Motion Casing Live steam entering PC Exhaust steam leaving Motion Section Fixed blades

B – blades (moving and fixed) PC – Condenser pressure

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Figure 13 shows the arrangement of a velocity compounded impulse turbine giving a section of the blading corresponding to a graph of pressure and velocity as the steam flows through the turbine.

As the steam flows through the fixed nozzles its pressure drops as its velocity is increased. It then enters the first row of moving blades where the kinetic energy of the steam is transferred to the moving blades forcing them to rotate. The steam pressure remains the same but the velocity decreases as it travels across the blades. The steam then enters the intermediate fixed blades which are installed in the cylinder between each row of moving blades. These fixed blades have no pressure or velocity drop across them as they only change the steam direction towards the next row of moving blades. The process continues through the remaining sets of moving and fixed blades until the steam exhausts the turbine.

Pressure compounding

With pressure compounding the total steam pressure to exhaust pressure is broken into several pressure drops through a series of sets of nozzles and blades. Each set of one row of nozzles and one row of moving blades is referred to as a stage.

Figure 14 shows a two stage pressure compounded impulse turbine. The steam passes through the first set of nozzles where it looses pressure as it gains velocity. It then passes across the first row of moving blades where the steam velocity is reduced while imparting rotational force. The steam then enters the second row of fixed nozzles where it once again loses pressure as its velocity is increased. It then passes across the second row of moving blades where the steam velocity is reduced while imparting additional rotational force. The second row of nozzles (and any subsequent rows of nozzles) are installed on a diaphragm. This diaphragm minimises any steam leakage occurring around the nozzles due to the high pressure drop across the nozzles.

When designing a steam turbine the actual number of stages installed will depend on the total energy available and desired blade speed.

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Figure 14: Two stage pressure impulse turbine Shaft gland Fixed nozzle Shaft VL B Moving N Rotor B Moving N V P Fixed Nozzles Steam flows Moving blades Motion Casing Live steam entering PC Exhaust steam leaving Motion Section

B – blades (moving and fixed) PC – Condenser pressure

VL – velocity of steam leaving turbine

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Combination of pressure and velocity compounding

Most modern turbines have a combination of pressure and velocity compounding. This type of arrangement provides a smaller, shorter and cheaper turbine; but has a slight efficiency trade off. Turbines using this arrangement are often referred to as CURTIS turbines after the inventor. Individual pressure stages (each with two or more velocity stages) are sometimes called CURTIS stages.

6.4.2 Reaction

The principle of a pure reaction turbine is that all the energy contained within the steam is converted to mechanical energy by reaction of the jet of steam as it expands through the blades of the rotor. A simple reaction turbine is shown in Figure 15. The rotor is forced to rotate as the expanding steam exhausts the rotor arm nozzles.

Figure 15: Principle of reaction turbine

Rotor

Boiler

Flame Nozzle

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A section of reaction type blading is shown in Figure 16 while Figure 17 shows a turbine section with pressure and velocity relationship.

Figure 16: Section of reaction turbine blading

In practice it is impossible to achieve a pure reaction effect as the steam already has velocity when it reaches the moving blades. Therefore the steam on passing across the moving blades imparts some impulse to the blades due to its change in direction. The force developed by impulse compared with the force developed by reaction will depend on the blade speed/steam speed ratio.

In a reaction turbine the steam expands when passing across the fixed blades and incurs a pressure drop and an increase in velocity. When passing across the moving blades the steam incurs both a pressure drop and a decrease in velocity.

Steam IN Force Leading edge Steam OUT

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Figure 17: Turbine section showing pressure and velocity relationship. Section Shaft B N V P Fixed Nozzles Steam flows Moving blades Rotor Casing Live steam entering PC VL Exhaust steam leaving

P – pressure of steam entering turbine V – velocity of steam entering turbine N – nozzle (fixed)

B – blades (moving) PC – Condenser pressure

VL – velocity of steam leaving turbine

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1. What is the operating principle of an impulse turbine blade:

...

2. Impulse blades are usually installed in which section of a steam turbine:

...

3. Shown below in Figure 18 is an incomplete diagram of the pressure and velocity curves for a reaction turbine stage. Complete the diagram showing steam velocity:

Figure 18: Reaction turbine stage B N V P Steam flows PC VL Motion

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4. What are the characteristic features of an impulse turbine:

a) ...

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6.5 Turbine Nozzle Plates or Diaphragms

6.5.1 Nozzle Plate

Nozzle plates are installed as the first row of fixed blades or nozzles. A nozzle plate is constructed of three major components:

Nozzle segments

Centre ring(s) or diaphragm

Baffle strip gland (not required on double flow turbines)

A diagram of a nozzle plate is shown in Figure 19.

Nozzle segments

Nozzle segments are shaped and positioned in the nozzle plate to direct steam onto the rotating blades at the most effective angle to gain maximum efficiency from the steam.

Centre ring(s) or diaphragm

Centre rings support the nozzle segments and are located in groves machined into the cylinder casing. In most large turbines the nozzle plates are in two halves. The top half of the nozzle plate is installed into the top half of the turbine cylinder casing while the bottom half is installed in the bottom half of the turbine cylinder casing. This arrangement allows for easy dismantling should maintenance be required.

Baffle strip gland

These are installed to prevent steam from bypassing the rotating blades by passing around the outer tip of the rotating blades. A diagram of a double flow turbine nozzle plate showing a baffle strip is displayed in Figure 19.

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Diaphragms

The function of a diaphragm is to contain the nozzle segments and prevent pressure leakage along the rotor shaft to the next lower pressure stage within the cylinder. A diagram of a diaphragm is shown Figure 20.

A diaphragm is constructed of three major components: Nozzle segments

Centre ring(s) or diaphragm Baffle strips

Nozzle segments

Nozzle segments are shaped and positioned in the diaphragm so to direct or redirect the steam onto the rotating blades at the most effective angle to gain maximum efficiency from the steam.

Centre ring(s) or diaphragm

Centre rings support the nozzle segments and are located in groves machined into the cylinder casing. In most large turbines the diaphragms are in two halves. The top half of the diaphragm is installed into the top turbine cylinder casing while the bottom half is installed in the bottom half of the turbine cylinder casing. This arrangement allows for easy dismantling should maintenance be required.

Baffle strip gland

Baffle strip glands in this instance prevent steam pressure leakage along the rotor shaft to the next lower pressure stage within the cylinder. The baffle strip gland can be seen in Figure 20.

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6.5.2 Trainee exercise

Attempt the following trainee exercises to gauge how you are progressing. Your answers can then be compared with the model answers at the end of this module.

1. Why are nozzle plates manufactured in two halves:

...

2. What are the three major components of a turbine diaphragm:

a) ...

b) ...

c) ...

3. What part of a diaphragm is inserted into the machined groove of the turbine casing:

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6.6 Basic steam cycle

To gain an understanding of how a turbine functions we must first understand where a turbine fits into the basic steam cycle.

Lets us first start with the simplified diagram of a basic steam cycle shown in Figure 21.

We will start our journey at the bottom of the condenser which is known as the condenser hotwell. At this point the water is in liquid form and is termed condensate. The condensate is drawn from the condenser hotwell by the condensate extraction pump. It is then pumped through the non-contact low pressure (LP) heater/s. Travelling through the low pressure heater/s the condensate is heated. It then passes to the deaerator (DA) for further heating and oxygen removal.

The deaerator is a multi function device in that it acts as a contact type low pressure heater, oxygen remover and a storage vessel allowing for system fluctuations.

Once the condensate exits the DA it enters the feedwater pump. The feedwater pump boosts the pressure to that greater than boiler pressure and therefore forces what is now known as feedwater through the high pressure (HP) heater/s and into the boiler. The feedwater gains further heating in the HP heater/s but is still in a liquid form when it enters the boiler.

As the feedwater travels through the boiler it becomes high pressure, high temperature steam known as superheated steam. The superheated steam is now in a gaseous state. Superheated steam exiting the boiler is piped to the control valve/s (or throttle valve/s). The control valves regulate admission of steam to the turbine depending upon load. Once the superheated steam enters the turbine it expands and gives up heat causing the turbine rotor to rotate.

Once the superheated steam has exhausted its energy it exits the turbine and enters the condenser. The condenser has

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circulating water passing through tubes installed in the condenser. As the exhaust steam comes in contact with these circulating water tubes it is cooled and changes from a gaseous state back to a liquid. It then gravitating to the bottom of the condenser and collects in the condenser hotwell ready for pumping once again around the water/steam cycle.

For efficiency reasons bled steam (or extraction steam) is drawn off from the turbine at various stages. This bled steam containing heat is piped to the various low and high pressure heaters and is used to preheat the condensate/feedwater. Upon entering the LP or HP heaters the bled steam releases its heat energy preheating the condensate/feedwater. In giving up this heat it changes from gaseous to liquid form. This liquid form is known as drainate and passes to the condenser for reuniting with the condensate.

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Figure 21: Basic steam cycle Condensate Extraction Pump Feedwater Pump Condenser Generator Inlet Canals Outlet Turbine Stack Boiler Precipitator or fabric filter Fuel Air HP Heater Circulating Water Pump LP Heater Deaerator Control valve

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1. Starting at the condenser hotwell explain the passage of water and steam around the basic water/steam cycle:

... ... ... ... ... ... ... ... ... ... ... ... ...

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6.7 Turbine efficiency and wet steam

As with any machine it is important to operate in the most efficient manner. To achieve this with a steam turbine we must extract the maximum possible energy from the steam as it passes through the turbine.

The factors that affect turbine efficiency are: Steam inlet conditions

Steam exhaust conditions Type and stages of feed heating Turbine efficiency losses due to:

 Inaccuracy in blade profile or worn parts  Deposits on blades

 Clearances between fixed rows of blades and/or nozzles and the moving rows of blades or nozzles  Radiation of heat from the casing

 Bearing and gland friction

 Steam leakage at valve glands, turbine glands and joints

A number of the above factors are design features and are out of the control of operating staff. There are however a few that affect turbine efficiency that are under the control of

operating staff:

Deposits on blades Steam inlet conditions Steam exhaust conditions

6.7.1 Deposits on blades

If we have contaminants dissolved in our boiler water this will tend to carryover from the boiler with the steam and deposit on the turbine blading. The principle element that deposits on turbine blading is silica. This silica is brought into the boiler during filling or as make-up using contaminated water.

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To prevent silica deposits occurring on the turbine blading we must ensure that any water entering the boiler is of a pure nature.

Silica deposits affect the efficiency of the turbine blading and therefore all precautions must be taken to prevent their formation.

Silica deposits can be removed from the turbine blading by what is called washing. To achieve washing the inlet steam temperature to the turbine is reduced. In doing this the steam quickly becomes wet steam as it passes through the turbine. This wet steam has a tendency to wash the silica deposits from the blading. The down side to this is that impinging upon the turbine blades takes place causing erosion which gives us a permanent efficiency loss.

Another problem is that when silica is washed from turbine blades it goes back into solution with the condensate and is returned to the boiler. Once returned to the boiler it can only be removed by blowing down or it will once again redeposit itself onto the turbine blades.

6.7.2 Steam inlet conditions

As we have just mentioned if we have lower than design steam temperature and pressure at the turbine inlet then the steam tends to condense prior to exiting the turbine. If this occurs we once again have wet steam and this wet steam erodes our turbine blades. Particular attention must be made to ensure that turbine inlet steam conditions are maintained at correct design values.

6.7.3 Steam exhaust conditions

To gain the maximum energy transfer from the steam passing through the turbine it is common practice with modern

turbines to have the condenser under a vacuum. From studying the boiler manual you are aware that the boiling point of water increases as pressure increases. Conversely the condensing point of steam is lowered by lowering the pressure. A typical steam turbine exhaust temperature of 33 - 35o_{C is quite common in modern turbines that are}

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By operating the condenser under a vacuum the steam condenses at a lower temperature and therefore we are able to extract additional work from the steam. This gives us an efficiency improvement for the turbine.

Condenser vacuum is often called condenser back pressure and may be expresses as:

kPa absolute, or

kPa gauge (reading a minus pressure below atmospheric)

eg: 5kPa absolute = 96.7kPa gauge

where atmospheric pressure = 101.7kPa (or1 bar)

It is important to maintain condenser vacuum at design values to prevent the turbine exhaust steam condensing within the turbine and causing an efficiency loss along with blade erosion.

Most modern turbines are designed to operate with a small percentage of wetness factor to improve the energy extraction from the steam.

Wetness factor is the quantity of moisture contained within the steam expressed as a percentage. Normal wetness factor for a modern turbine is in the vicinity of 10-15% when

operating at low loads.

When operating a turbine with a slight wetness factor it leaves the final few rows of blades in the low pressure section of the turbine exposed to blade erosion. To minimise this erosion on the final few rows of blades they are installed with stallite tips on the leading edge. Stallite is an extremely hard material and resists the erosion process.

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6.7.4 Factor affecting condenser back pressure.

Back pressure in a condenser can be affected by a number of factors:

Loading on the turbine

Circulating water inlet temperature

Circulating water quantity passing through condenser Cleanliness of condenser tube surfaces

Air entrainment in the circulating water Air in the steam side of the condenser

Operating personnel have varying degrees of control of all of the above factors.

Loading on turbine

The load on any turbine is usually at the discretion of system control but as an operator you can ensure that steam inlet conditions are at their optimum for that prescribed load.

Circulating water inlet temperature

If lake, river or ocean water is used it is normally seasonally dictated and beyond the control of the operator. If cooling towers are employed ensure fans are operating correctly, correct distribution of circulating water throughout cooling tower and correct quantity of circulating water contained within the system.

Circulating water quantity passing through condenser Operators can ensure that trash racks are clean, no

backwash valves inadvertently left open, canal level is correct and circulating water screens are operating in a clean

condition. If cooling tower employed ensure correct quantity of circulating water contained within the system and bebris screens are clean.

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Cleanliness of condenser tube surfaces

Ensuring correct chemical dosing of circulating water to prevent algae growth, that condensers are back washed at regular intervals and/or condenser ball cleaning plant operating correctly.

Air entrainment in the circulating water

Ensuring a tight circulating water system by checking all valves are fully closed to prevent air being drawn into the system. Canal level is correct so as air is not entering the system through the suction of the pumps.

Air in the steam side of the condenser

Air leaks at valve glands, out of service plant not isolated correctly, valve gland sealing not in service, valves open on out of service plant. Air ejector equipment malfunctioning or not being operated correctly.

Further information about the above factors contained within this module and volume 6 covering External Plant.

1. Name three factors affecting turbine efficiency that operators have control over:

a) ...

b) ...

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2. What six factors influence condenser back pressure: a) ... b) ... c) ... d) ... e) ... f) ...

3. If a condenser was operating at a back pressure of 8.7kPa absolute what would this be displayed as gauge pressure:

...

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7. Components of a Turbine

We have up to now been talking about steam flow through a turbine, the effects the steam has on the turbine blades and how it forces them to rotate. It is now time to discuss the components that go together to construct a complete and functional turbine.

As mentioned earlier most modern turbines are constructed of multiple cylinder coupled together to achieve the desired output. We will focus on this type of turbine construction in our explanations. Smaller turbines are constructed using fewer cylinders but their construction philosophy is the same.

The construction of a modern turbine employs the following components:

Turbine cylinder(s) Turbine rotor Turbine glands Bearings

Lubricating oil system Turbine thrust

Governor Condenser

 Air extraction equipment  Circulating water system

Turbine couplings Turbine turning gear

Steam chest(s) (containing emergency and control valves) Drains

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7.1 Turbine cylinder(s)

The casings of turbine cylinders are of simple construction to minimise any distortion due to temperature changes. They are constructed in two halves (top and bottom) along a horizontal joint so that the cylinder is easily opened for inspection and maintenance. With the top cylinder casing removed the rotor can also be easily withdrawn without interfering with the alignment of the bearings.

Most turbines constructed today either have a double or partial double casing on the high pressure (HP) and intermediate pressure (IP) cylinders. This arrangement subjects the outer casing joint flanges, bolts and outer casing glands to lower steam condition. This also makes it possible for reverse flow within the cylinder and greatly reduces fabrication thickness as pressure within the cylinder is distributed across two casings instead of one. This reduced wall thickness also enables the cylinder to respond more rapidly to changes in steam temperature due to the reduced thermal mass.

A cutaway diagram of a HP cylinder is shown in Figure 22. The HP cylinder is a single flow cylinder with steam entering the inner casing, passing through the blading and then exhausting to the outer casing before passing to the reheater.

Figure 23 shows a double flow IP cylinder. Steam enters the centre of the cylinder where it divides into halves before passing through blading and exhausting at each end of the cylinder.

Low pressure (LP) cylinders are manufactured of either cast iron or fabricated steel and are shaped to allow smooth passage of steam as it leaves the last row of blades and enters the condenser that is usually situated directly below the LP cylinder(s).

Two double flow LP cylinders are shown in Figure 24 with a cutaway section on one of the cylinders. Steam enters each cylinder in the centre dividing into halves before passing through blading and exhausting at each end of that cylinder. The condenser (not shown) is installed directly below the two LP cylinders and receives the exhaust steam.

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In the HP, IP and LP cylinders casings are constructed, suitable spaces or belts to provide exit apertures for bled steam used in the LP and HP heaters.

7.1.1 Casing flanges

One method of joining the top and bottom halves of the cylinder casing is by using flanges with machined holes. Bolts or studs are insertion into these machined holes to hold the top and bottom halves together. To prevent leakage from the joint between the top flange and the bottom flange the joint faces are accurately machined. A typical bolted flange joint is shown in Figure 25.

Figure 25: Bolted cylinder joint

Bolted turbine flanges for a HP cylinder can be seen in Figure 22 while the IP cylinder and LP cylinders may be seen in Figure 23 and Figure 24 respectively.

The bolts or studs holding the flanges together must be tightened to precise values to effectively maintain their integrity once the cylinder is exposed to high temperatures. This is achieved by using a bolt or stud with a hole drilled through the centre. A carbon heating rod is inserted into

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these holes in the bolt or stud to heat the assembly during tensioning. This can be seen in Figure 25.

Another method of joining the top and bottom cylinder flanges is by clamps bolted radially around the outer of the cylinder. The outer faces of the flanges are made wedge-shaped so that the tighter the clamps are pulled the greater the pressure on the joint faces. This method of joining top and bottom casings is shown in Figure 26.

Figure 26: Clamped cylinder joints

With this method heating rods are insertion into the clamps during the tensioning process. The holes for these heating rods can also be seen in Figure 26.

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7.1.2 Flange warming

As the flanges on a cylinder are relatively thick with respect to the thickness of the casing there is a tendency for the flanges to lag behind when temperature changes occur. A cross section of a turbine cylinder showing the relationship between the casing and flange thickness is displayed in Figure 27.

Figure 27: Cross section of simple turbine cylinder

With casing flanges being much thicker than the casing itself they are slower to cool than the casing and are also slower to warm when the casing is heated. When rapid temperature changes occur the casing temperature changes much faster than the flange temperature thus subjecting the casing to abnormal and unwanted thermal stresses. These thermal stresses reduce the expected working life of the material.

The most critical time when the greatest thermal stress occurs is when the turbine is being returned to service and the steam to metal temperature differences are at their greatest.

To minimise the thermal stress occurring on the casings a system of flange warming is employed. The flange warming system supplies a regulated flow of steam through ducts or holes in the flanges and/or flange bolts/studs. Flange warming through flange ducts is shown in Figure 28. With this method warming steam passes through the flange and into the bolt/stud hole, it then passes along the bolt/stud outer shaft transferring heat to the casing and bolt/stud. It

Thicker casing flange Thinner casing Flange joint Turbine rotor Flange bolt/stud

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then passes through the flange to the next bolt/stud to continue the warming process.

Figure 28: Cross section side view of casing flanges

Another method of flange warming is shown in Figure 29. With this method a small hole is drilled at an angle through the centre of the bolt/stud to allow steam passage from one flange duct to the next. During assembly accurate alignment of the bolt/stud is required to ensure that the flange and bolt/stud holes line-up.

With both methods of flange warming we regulate the flow of steam through these ducts or holes to maintain design temperature differential limits between the casing and the casing flanges.

In reducing the temperature differential, the expansion differentials of the varying thickness of casing and flanges along with the rotor are kept to a minimum allowing turbine start and run-up time to be reduced. More about this when we discuss turbovisory equipment covered later in this module. Flange bolt/stud Casing flanges Flange joint Flange warming steam entering

flange Flange warming _{steam exiting}

flange From auxiliary steam To Condenser via turbine drains Holes drilled through flanges

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Figure 29: Cross section side view of casing flanges with drilled bolts/studs

1. Why are most modern turbine casings constructed in two halves: ... ... ... ... Flange bolt/stud Casing flanges Flange joint Flange warming steam entering

flange Flange warming _{steam exiting}

flange From auxiliary steam To Condenser via turbine drains Holes drilled

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2. What is the advantage of constructing a turbine cylinder with a double casing:

...

3. What are two methods of joining the top and bottom cylinder casings together:

a) ...

b) ...

4. What procedure is employed to ensure correct tensioning of turbine casing flange bolts or studs:

...

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5. Why are turbine casing flanges slower to heat than the casing itself: ... ... ... ...

6. How is the thermal stress of a turbine casing and casing flanges kept within limits during turbine run-up:

...

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7.2 Turbine rotor

As the name suggests the turbine rotor is the component of a turbine that rotates. Most modern turbines operate at either 1800rpm when driving a 60Hz 4 pole generator, 3000rpm when driving a 50Hz 2 pole generator or 3600rpm when driving a 60Hz 2 pole generator.

Special attention must be given to the construction of a turbine rotor due to the centrifugal force generated by the high speed operation.

Turbine rotors are constructed by the following methods: Forged steel drum rotor

Solid forged rotor Disc rotor

 Shrunk and/or keyed to the shaft  Welded construction

7.2.1 Forged steel drum rotor

Drum rotors as they are commonly referred to are a single steel forging for the high pressure steam inlet end rotor (drum) with another separate forging for the exhaust end disc. After machining the drum is shrunk onto the exhaust end disc forging and secured by bolts and driven dowels. Grooves are machined in the body of the drum to accommodate the blading. A diagram of a drum rotor construction can be seen in Figure 30

The drum type rotor has limitations in its application due to the excessive stresses encountered if manufactured in large sizes. For this reason its applications are limited to small machines or the high pressure cylinder of multiple cylinder machines.

The main advantage of this type of construction is that there is approximately the same mass of metal contained within the rotor as in the cylinder casing. With their mass being almost equal the same response to a change in temperature conditions occurs for both the rotor and the casing. By having similar response characteristics the internal working

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clearances can be kept to a minimum thus improving efficiency.

Figure 30: Forged steel drum rotor construction

7.2.2 Solid forged rotor

Solid forged rotors have wheels and a shaft machined from one single solid steel forging. This type of construction is extremely rigid and eliminates the problems of looses wheels that other types of construction can experience. Groves are machined into the wheel rims to accommodate the necessary blading. A diagram of a solid forged turbine rotor is shown in Figure 31.

Solid forged rotors of creep resistant alloy steel are predominately used in the HP and IP cylinders employing impulse type blading and the IP cylinder for reaction type blading. The modern trend is to bore a hole through the entire length of the shaft to permit inspection by video camera or other viewing method. This hole through the centre

Rotor blades _Driven dowels Exhaust end shaft and disc Shrink fit HP steam inlet end

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of the shaft also relieves stresses during the heat treatment process.

Gland rings are machined between the discs to align with the diaphragm glands. The outer faces of the first and last discs have machined slots which allow the attachment of balance weights

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7.2.3 Disc rotor

Shrunk and/or keyed to the shaft

Construction of the disc rotor type is made up using a central shaft with separately forged discs or wheels and the hubs of these wheels shrunk and keyed onto the central shaft. The outer rims of the wheels are suitably grooved to allow for fixing of the blades. The central shaft is usually stepped so that the wheels hubs can be easily threaded then pressed and shrunk or welded into their correct position. A shrink fit disc rotor is shown in Figure 32.

Suitable clearances are provided between the hubs to allow for expansion axially along the line of the shaft.

Figure 32: Shrink fit disc rotor

The disadvantage with this type of construction is that if the rotor is subjected to a rapid temperature rise in excess of

Rotor shaft Hole through shaft Wheel Blades Locking ring _Weights

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manufacturers recommendation the wheels being much smaller in mass than the shaft expand quicker and can become loose on the shaft.

Disc rotor balance is achieved by adjusting the position of the weights in a channel machined in the outer face of the first and last disc. When the rotor is balanced the weights are locked in position in the channel by grub screws.

Welded construction

Welded rotors are assembled from a number of discs and two shaft ends. The discs are joined together by welding at the circumference. Figure 33 shows this type of construction prior to welding while Figure 34 shows the rotor after being welded and the blading installed.

Figure 33: Rotor showing discs before welding Discs

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Figure 34: Welded rotor construction after assembly Blades

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7.3 Turbine blade fixing

Various root fixing shapes have been developed for turbine blading to suit both construction requirements and conditions under which turbines operate. The most popular types of blade root fixing available are:

groove straddle rivet

Groove construction

The groove type of root fixing fits into a machined grove around the circumference of the rotor wheel or disc. Some examples of typical groove type blade root designs are shown in Figure 35 while a rotor disc with a machined groove arrangement is shown in Figure 36.

Figure 35: Groove type root fixing Cut-off blade

section

Blade root

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Figure 36: Disc periphery for annular fir-tree root blades

Blade roots are installed through the closing blade window and then slid around the circumference of the disc into their desired position. The last blade root is installed in the closing blade opening and secured in position by dowel(s).

Straddle construction

Straddle construction is where the blade root fits over the machining on the outer periphery of the rotor wheel or disc. An example of straddle fir-tree blade root construction is shown in Figure 37. while the disc peripheral machining is shown in Figure 38. Closing blade window Dowel hole Rotor disc

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Figure 37: Two shoulder straddle fir-tree blade root

Figure 38: Disc periphery two shoulder fir-tree root anchor

Once again with this type of construction the blade roots are installed through the closing blade window slid around the circumference of the disc into position, then the last blade inserted is doweled in the closing blade window location.

Dowel hole

Closing blade window

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Rivet construction

Rivet construction is where the blade root either inserts into a groove or straddles the disc and all blades are doweled into position.

Peripheral blade fixing

On larger blading where the blade length is relatively long a system of lacing wire or shroud rings are installed to give the blading additional support and reduce vibration.

The lacing wire is installed a small distance from the outer ends of the blades while the shoud rings are fitted to tangs on the outer edges of the blades and secured by peening the tangs. A section of blading showing the installation of the lacing wire is shown in Figure 39 while a section of blading showing shroud ring installation is shown in Figure 40.

Figure 39: Blading supported with lacing wire

Reaction blading

Overlap of lacing wire at start and finish

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Figure 40: Shroud ring installation

Often gland sealing is installed around the outer circumference of the shroud ring to minimise pressure leakage around the outer tips of the blades. A shrouding single baffle ring gland can be seen in Figure 41. while a shrouding side baffle gland can be seen in

Shroud ring

Tang

Blades

Tang peened over

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Figure 41: Shrouding single baffle ring gland

Figure 42: Shrouding side baffle gland Casing

Gland

Casing

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7.4 Couplings

With multi-cylinder turbines it is necessary to have some method of connecting individual cylinder rotors. It is also a requirement to connect the turbine to the alternator rotor. To achieve these connections we use a device known as a coupling. These couplings must be capable of transmitting heavy loads and in some turbines are required to accommodate for axial expansion and contraction.

The types of couplings generally employed in power plants are:

Flexible coupling Solid shaft coupling

7.4.1 Flexible couplings

Where axial shaft movement is required a flexible coupling is employed and these are either:

Sliding claw (or tooth)

Flexible connection (between the two flanges)

With both of the above flexible couplings it is necessary to have a separate thrust bearing for each shaft to maintain the same relative position between rotor and cylinder casing.

Sliding claw (or tooth)

Sliding claw couplings consists of an inner gears or tooth coupling half. The inner half is shrunk onto its respective shaft and secured by keys or driven pins. The outer coupling half; machined in the reverse shape is installed onto the other shaft.

The gear or teeth coupling is positioned inside the outer coupling half where it is able to slide back and forth to allow for expansion or contraction. A diagram of a sliding claw coupling prior to the inner claw section being inserted into the outer half is shown in Figure 43 while a gear tooth coupling is shown in Figure 44

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Figure 43: Claw coupling

Figure 44: Gear tooth coupling

Flexible connection coupling

Flexible connections such as the bibby coupling are constructed in two halves. Each half is shrunk onto their respective shaft and secured with keys or driven pins. The halves are machined with groves parallel or nearly parallel to that of the alignment of the shaft. Flexible spring steel grids are inserted into these machined groves and held in place with an outer cover. This type of coupling is effective in allowing axial expansion and contraction along with the ability to tolerate minor misalignment. A bibby coupling is shown in Figure 45. Inner claw Outer half of coupling Shaft

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Figure 45: Bibby coupling

The flexible couplings just mentioned are by no means the only flexible couplings available but they are the preferred choice for high load applications.

Solid shaft coupling

When shaft movement is not required it is usual to install a solid type coupling. Two flanges are installed onto their respective shafts and then the two flanges are bolted together to form a solid joint as shown in Figure 46.

Often teeth are machined on the outer rim of these couplings and used as a point for barring the turbine shaft. (more about barring the turbine later). Figure 47 shows a solid shaft coupling with a barring gear fitted.

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2. Turbine Manual

Power Plant

General Series Course

Volume 2

Table of contents

1. Introduction

2. Learning Outcomes

3. Disclaimer

4. Assessment: Evaluation, Recording and

Reporting

5. History of the steam turbine

5.1 Early applications

5.2 Benefits of steam turbines

6. Steam turbine operation

6.1 Introduction

6.2 Principles of operation of a steam turbine

6.3 Classification of turbines

6.4 Types of blading

6.5 Turbine Nozzle Plates or Diaphragms

6.6 Basic steam cycle

6.7 Turbine efficiency and wet steam

7. Components of a Turbine

7.1 Turbine cylinder(s)

7.2 Turbine rotor

7.3 Turbine blade fixing

7.4 Couplings