Study of Manufacturing of Turbine Blades
A Summer Internship Project Report

Abstract
Steam Turbine is a prime mover which converts the energy associated with the steam into mechanical energy which results in the rotation of the turbine rotor. It is nearly an ideal working device among all heat engines and prime movers and widely used in Power plants and in all industries to generate power because of its greater thermal efficiency and higher power-to-weight ratio. Presently, 80% of all electricity is generated by using steam turbines alone. In this project report, I have mainly concentrated on the turbine-rotor-blades. Because, the rotor is the heart of the turbine and it affects (i.e. blading of rotor) the efficiency of the steam turbine. In this manufacturing of blades is widely focused.
Key Words: Steam Turbine, Blades, Rotor

BY
k.sai sailender
Summer Internship Program 2015

ACKNOWLEDGEMENT

I would like to express my deepest appreciation to all those who provided me the possibility to complete this report. A special gratitude I give to Prof. V.Srinivas, Head of the Department of Mechanical Engineering, GIT-GITAM, Visakhapatnam. Furthermore I would also like to acknowledge with much appreciation the crucial role of the staff of BHEL Hyderabad, who gave the permission to use all required equipment and the necessary materials to complete the task “ASSEMBLY OF STEAM TURBINES”
Last but not least, many thanks go to the guide of the project, Mr.VIVEKANADA MANDAL , Sr. Engineer, Bay-03, Blade shop, has invested his full effort in guiding me in achieving the goal. I have to appreciate the guidance given by other BHEL Employees as well as the Apprentices especially in our project report that has improved our practical experience, thanks to their comment and advices.

k.sai sailender(1210812132) 6th semester, B.Tech. Department of Mechanical Engineering
GITAM University, Visakhapatnam.

CONTENTS 1. INTRODUCTION TO BHEL
1.1. ORGANISATION PROFILE
1.2. BHEL HYDERABAD – AN OVERVIEW
1.2.1 DIFFERENT PRODUCTS AND PROCESSING AREAS IN BHEL HYDERABAD 1.3. TURBINES, COMPRESSORS AND GAS TURBINES 201 SHOP 2. STEAM TURBINES

2.1. HISTORY
2.2. INTRODUCTION
2.3. WORKING PRINCIPLE
2.4. TYPES OF STEAM TURBINES
2.5. CYCLES USED IN STEAM TURBINES (RANKINE CYCLE)
2.6. THE PARTS OF STEAM TURBINE
2.7. INSPECTION OF STEAM TURBINES
2.8. ADVANTAGES
2.9. DISADVANTAGES 3. GAS TURBINES 3.1. GAS TURBINE 3.2. GAS TURBINE DESCRIPTION 3.2.1. COMPRESSOR SECTION 3.2.2. COMBUSTION SECTION 3.2.3. TURBINE SECTION 3.3. APPLICATIONS OF GAS TURBINES 4. TURBINE BLADES 4.1. INTRODUCTION 4.2. BLADE MATERIAL 4.3. CLASSIFICATION OF BLADES 4.4. CONSTRUCTIONAL FEATURES OF BLADES 4.4.1. Camber Line 4.4.2. Bitangent Line 4.4.3. Setting Angle 4.4.4. Pitch 4.5. HIGH PRESSURE BLADES 4.5.1. PROFILE 4.5.1.1. CLASSIFICATION OF PROFILE 4.5.1.1(a) Impulse profile 4.5.1.1(b) Reaction profile 4.5.2. ROOT 4.5.3. SHROUD

4.6. LP BLADES 4.6.1. LP BLADE PROFILE 4.6.2. LP BLADE ROOTS 4.6.3. LP BLADE SHROUD 5. MANUFACTURING OF STEAM TURBINE BLADES 5.1. INTRODUCTION 5.2. VARIOUS TOOLS USED FOR MANUFACTURING OF TURBINE BLADES 5.2.1. Band saw machine 5.2.2. Duplex milling machine 5.2.3. Segmental grinding machine 5.2.4. Horizontal milling machine 5.2.5. Four spindle milling machine 5.2.6. Vertical milling machine 5.2.7. Surface grinding machine 5.2.8. Face lathe machine 5.2.9. Copy milling machine 5.3. STEPS INVOLVED IN MACHINING OF STEAM TURBINE BLADE 5.3.1. PROCUREMENT OF MATERIAL 5.3.2. CUTTING TO BAR SIZE 5.3.3. SIZE MILLING 5.3.4. SIZE GRINDING 5.3.5. RHOMBOIDAL MILLING 5.3.6. RHOMBOIDAL GRINDING 5.3.7. BLADE LENGTH CUTTING 5.3.8. ROOT MILLING 5.3.9. ROOT RADIUS MILLING 5.3.10. WIDTH MILLING 5.3.11. CONVEX MILLING 5.3.12. CONCAVE MILLING 5.3.13. TAPER MILLING 5.3.14. TAPER GRINDING CONCLUSION

1. INTRODUCTON TO BHEL

1.1 ORGANISATION PROFILE

Bharat Heavy Electricals Limited is the largest engineering and manufacturing enterprise in India in the energy related / infrastructure sector today and ranks among the top twelve manufacturers of power equipment in the world.
Today BHEL has 14 Manufacturing Divisions, 4 Power Sector regional Centers, 8 Service Centers 18 regional offices and large number of Project Sites spread all over India and abroad enables the Company to promptly serve its customers and provide them with suitable products, systems and services efficiently and at competitive prices.
BHEL caters to core sectors of the Indian Economy viz, power Generation and Transmission, Industry, transportation, Renewable energy, Defense etc.
BHEL has already attained ISO 9000 and all the major units/divisions of BHEL have been upgraded to the latest ISO-9001: 2000 version quality standard certification for quality management. All the major units/divisions of BHEL have been awarded ISO-14001 certification for environmental management systems and OHSAS-18001 certification for occupational health and safety management systems.

Vision, Mission & Values
Vision
A global engineering enterprise providing solutions for better tomorrow.

Mission
Providing sustainable business solutions in the field of Energy, Industry & Infrastructure.

Values
Governance: We are stewards of our shareholders and we take that responsibility very seriously. We are accountable and responsible for delivering superior results that make a difference in the lives of the people we touch.
Respect: We value the unique contribution of each individual. We believe in respect for human dignity and we respect the need to preserve the environment around us.
Excellence: We are committed to deliver and demonstrate excellence in whatever we do.
Loyalty: We are loyal to our customers, to our company and to each other.
Integrity: We work with highest ethical standards and demonstrate a behavior that is honest, decent and fair. We are dedicated to the highest levels of personal and institutional integrity.
Commitment: We set high performance standards for ourselves as individuals and our teams. We honor our commitments in a timely manner.
Innovation: We consistently support development of newer technologies products, improved process, better services and management practices.
Teamwork: We work together as a team to provide best solutions and services to our customers. Through quality relationships with all stakeholders we deliver value to our customers.

Main manufacturing facilities * Centralized Stamping Unit & Fabrication Plant (CSU & FP), Jagdishpur * Insulator Plant (IP), Jagdishpur * Electronics Division (EDN), Bangalore * Industrial Systems Group (ISG), Bangalore * Electro-Porcelains Division (EPD), Bangalore * Heavy Electrical Plant (HEP), Bhopal * Industrial Valves Plant (IVP), Goindwal * Heavy Electrical Equipment Plant (HEEP), Ranipur (Haridwar) * Central Foundry Forge Plant (CFFP), Ranipur (Haridwar) * Heavy Power Equipment Plant (HPEP), Hyderabad * Transformer Plant (TP), Jhansi * Boiler Auxiliaries Plant (BAP), Ranipet * Component Fabrication Plant (CFP), Rudrapur * High Pressure Boiler Plant (HPBP), Tiruchirappalli * Seamless Steel Tube Plant (SSTP), Tiruchirappalli * Power Plant Piping Unit (PPPU), Thirumayam * Heavy Plates & Vessels Plant (HPVP), Visakhapatnam
The company is also setting up a new Greenfield Power Equipment Fabrication Plant (GPEFP) at Bhandara, Maharashtra, the foundation stone for which was laid on 14 May 2013. Further, BHEL is planning to enter solar manufacturing in a big scale, as it has announced its plans for a 600 MW Solar Module Factory.

1.2.BHEL Hyderabad- An Overview
Heavy Power Equipment Plant (HPEP)
The Hyderabad Unit of BHEL is located at Ramachandrapuram which is around 30KM from the historic city of Charminar. Foundation Stone of the Plant was laid in 1959 and the production commenced in the year 1965. The Unit was set up mainly to manufacture 60MW and 110MW Steam Turbo Generator sets for State Electricity Boards and also 12 MW TG Sets. From this small beginning, the Ramachandrapuram Unit has been growing steadily in different phases of development and today it caters to a wide spectrum of business in Power, Industry, Transmission, Oil and Gas. It now boasts the largest number of products under a single roof as compared to any of the other BHEL Units.
Other Facilities in Hyderabad
PE & SD - Project Engineering and System Division
Corporate R&D

1.2.1. DIFFERENT PRODUCTS AND PROCESSING AREAS IN BHEL HYDERABAD
SHOP PRODUCT / PROCESS
201 STEAM TURBINES,GAS TURBINES & CENTRIFUGAL COMPRESSORS
202 TURBO GENERATORS / MOTORS ETC
203 SWITCH GEARS
204 FERROUS FOUNDRY
205 NON-FERROUS FOUNDRY
206 HEAT EXCHANGERS
207 TOOL ROOM
208 HEAT TREATMENT
209 PATTERN SHOP
210 SPARES MANUFACTURING, OIL FIELD EQUIPTS
251 COAL PULVERIZERS
270 CENTRIFUGAL PUMPS

1.3 TURBINES, COMPRESSORS AND GAS TURBINES 201 SHOP
It is the biggest shop floor with plinth area of 35,500 sq.-m. The various products manufactured in this shop floor are
1) Steam turbines: a) Industrial set 2.5 MW and above up to 60 MW
b) Utility sets 60 MW and above up to 120 MW
2) Gas turbines: a) Frame - 1 (5 MW)
b) Frame - 3 (10 MW)
c) Frame - 5B (25 MW)
d) Frame - 6B (37 MW)

e) Frame - 9E (123 MW)
f) Frame - 6FA (70 MW)
e) Frame - 9FA (240 MW)
3) Centrifugal Compressors:
a) Horizontal split type b) Vertical split type

INTRODUCTION

Steam turbine is a rotating machine which- CONVERTS HEAT ENERGY OF STEAM TOMECHANICAL ENERGY.

In India, steam turbines of different capacities, varying from 15 MW to 500 MW, are employed in the field of thermal power generation. The design, material, auxiliary systems etc. vary widely from each other depending on the capacity and manufacturer of the sets.

RANKINE CYCLE

WORKING PRINCIPLES
When steam is allowed to expand through a narrow orifice, ft assumes kinetic energy at the expense of its enthalpy (heat energy). This kinetic energy of steam is changed to mechanical (rotational) energy through the impact (impulse) 6r reaction of steam against the blades. It should be realized that the blade of the turbine obtains no motive force from the static pressure of the steam or from any impact of the steam jet.

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The blades are designed in such a way, that steam will glide on and off the blade without any tendency to strike it.

As the steam moves over the blades, its direction is continuously changing and centrifugal pressure exerted as the result is normal to the blade surface at all points. The total motive force acting on the blade is thus the resultant of all the centrifugal forces plus the change of momentum. (Fig. 1.3). This causes the rotational motion of the blades.

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TURBINE TYPES

Basically there are two broad classifications of steam turbines:

i) Impulse: In Impulse turbine(Fig.1.4),the steam is expanded (i.e. pressure is reduced) in fixed nozzles. The high-velocity steam issuing from the nozzles does work on the moving blades which causes the shaft to rotate, The essential feature of an impulse turbine is that all the pressure drops occur in the nozzles only, and there is no pressure drop over the moving blades.

ii) Impulse-reaction : In this type, pressure is reduced in both fixed and moving blades. Both fixed and moving blades act like nozzles and are of same shape. Work is done by the impulse affect due to the reversal of direction of the high velocity steam plus a reaction effect due to the expansion of steam through the moving blades. This turbine is commonly called a reaction turbine and is shown below in (Fig.1.5).
COMPOUNDING

Several problems crop up if the energy of steam is converted in one step, i.e. in a single row of nozzle-blade combination. With all heat drop taking place in one row of nozzles (or single row of nozzles and blades in case of reaction turbine) the steam velocity becomes very high and even supersonic (velocity of steam is proportional to square root of heat drop in nozzle; V=44.8/K(H1-H2) m/s. K=constant, H, Enthalpy at nozzle inlet ; H 2 Enthalpy at nozzle outlet. The rotational speed of the turbine also becomes very high and impracticable.

So, in order to convert the energy of steam within practical speed range, it is necessary to convert R in several steps and thus reducing the velocity of steam and rotor speed to practical levels. This is termed compounding.

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Following are the various types of compounding

Velocity Compounded Impulse Turbine (Fig. 1.6)

Like simple impulse turbine this has also only one set of nozzles and entire steam pressure drop takes place there. The kinetic energy of high velocity steam issuing from nozzles is utilised in a number of moving row of blades with fixed blades in between them (instead of a single row of moving blades in simple impulse turbine). The role of the fixed guide blades is just to change the direction of steam jet and guide it to next row of moving blades. This type of turbine is also called Curtis turbine.

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Pressure Compounded Impulse Turbine (Fig.1.7)

This is basically a number of simple impulse turbines in series on the same shaft - the exhaust of one steam turbine entering the nozzle of the next turbine. The total pressure drop of the steam does not take place in the first nozzle ring, but is divided equally between all of them. Steam is passed through the first nozzle ring in which it is only partially expanded. It then passes over the first moving blades wheel where most of its velocity is absorbed, From this ring it exhausts into the next nozzle ring and is again partially expanded. The velocity obtained from the second nozzle ring is absorbed by the next wheel of moving blades. This process is repeated in the remaining rings until the whole of the pressure has been absorbed. This type of turbine is also called Rateau turbine after its inventor.

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Pressure-Velocity Compounded Impulse Turbine (Fig. 1.8)

Pressure-Velocity Compounding is a combination of both the previous methods and has the advantage of allowing a bigger pressure drop in each stage and so less stages are necessary. Hence, for a given pressure drop the turbine will be shorter. But the diameter of the turbine is increased at each stage to allow for the increasing volume of steam. This type was once very popular. But it is rarely, used as efficiency is quite low.

Multistage Reaction Turbine

(Fig. 1.9) shows such a multisage reaction turbine consisting of a number of rows of moving blades attached to the rotor and an equal number of rows of fixed blades attached to the casing. Each stage txilises a portion of energy of steam. Theoretically this may be called pressure compounded turbine as the pressure of steam drops gradually over the succeeding stages.

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FIG. 1.8 COMPOUDING FOR PRESSURE FIG.1.9 REACTION TURBINE PRESSURE AND
AND VELOCITY VELOCITY CURVES
The fixed blades compare to the nozzle used in the impulse turbine. Steam is admitted over the whole circumference, and in passing through the first row of fixed blades, undergoes a small drop in pressure and its velocity is increased. It then enters the first row of moving blades and, as in the impulse turbine, suffers a change in direction and hence momentum giving an impulse on the blades. During the steam passage through the moving blades, k undergoes a further small drop in pressure resulting in an increase in velocity which gives rise to a impulse reaction turbine differs from the pure impulse turbine. Thus the gross propelling force in the impulse-reaction turbine, (or the 'reaction' turbine as it is commonly called), is the vector sum of the impulse and the reaction forces. (Fig. 1.9) also shows how the blade heights increase as the specific volume of the steam increases with reduction in pressure. The pressure falls gradually as the steam passes through the groups of blades. 7

There is a pressure drop across each row of blades both fixed and moving. This is of considerable practical , especially at the high pressure end of the turbine where the pressure drops are greatest. Because this difference of pressure tends to force some steam through the clearance spaces between the moving blades and the casing and between the fixed blades and the rotor.These clearances have to be carefully controkd by using axial and 1 or radial seals at the blade tips, otherwise the leakage would be so large that the turbine would be inefficient. The pressure drop across the moving blades gives rise to a large axial thrust on rotor, towards the low pressure end of the turbine, and special balance pistons / thrust bearings have to be fitted to counteract it. The dummy (balance) piston diameter is so calculated that the steam pressure acting upon it in the opposite direction to the steam flow, balances out the force on the rotor blades in the direction of steam flow. Preferably the dimensions are so arranged to keep a small thrust towards the inlet end of the turbine. To maintain this condition at all loads in some designs, a balance pipe is usually connected from the casing, on the outer side of the balance piston, to some tap off point down the cylinder. This pipe-maintains the steam pressure on the out board side of the dummy piston to correspond with pressure at the stage down the turbine cylinder to which the balance pipe is connected. Under steady load conditions the steam leakage through the dummy piston "labrinth packings flows, from out board side of dummy piston, through the balance pipe and does working the lower stages of turbine. This arrangement is shown in (Fig.1.10).

8

The steam velocities in this type of turbines are moderate. The velocity of steam for maximum blade efficiency being roughly equal to the blade velocity . The leaving loss is normally about the same as for the multistage impulse turbine. The impulse-reaction turbine was developed by the late Sir. Charles A. Parson and widely used in power stations. It is sometimes called Parson's turbine.

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CLASSIFICATION OF STEAM TURBINES

Steam turbines may be classified into different categories depending on their construction, the progress by which heat drop is achieved, the initial and final conditions of steam used and their industrial usage etc.

1. According to the Direction of Steam Flow

a) Axial turbines : in which the steam flows in a direction parallel to the axis of the turbine.

b) Radial turbines: in which the steam flows in a direction perpendicular to the axis of the turbine.

According to the Number of Cylinders

a) Single - cylinder turbines
b) Double - cylinder turbines
c) Three - cylinder turbines and
d) Four - cylinder turbines etc.

According to the Method of Governing

a) Turbines with throttle governing in which fresh steam enters through one or more (depending on the power developed) simultaneously operated throttle valves.

b) Turbines with nozzle governing which fresh steam enters through two or more consecutively opening regulators.

c) Turbines with by-pass governing in which steam besides being fed to the first stage is also directly led to one, tow or even three intermediate stages of turbine.
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According to the Principle of Action of Steam a) Impulse Turbine b) Reaction Turbine

According to the Heat Balance Arrangements
a) Condensing turbines with regeneration; in these turbines steam ad a pressure less than atmospheric is directed to a condenser; besides, steam is also extracted from intermediate stages for feed water heating, the number of such extractions usually varies from 2-3 to as much as 8-9. Small capacity turbines of earlier designs often do not have regenerative feed heating.
b) Condensing turbines with one or two intermediate stage extractions at specific pressures for in industrial and heating purposes.
c) Back pressure turbines : the exhaust steam from these turbines is utilized for industrial or heating purposes.

d) Topping turbines: these turbines are also of the back pressure type with the difference that the exhaust steam from these turbines is further utilized in medium and low-pressure condensing turbines. These turbines, in general, operate at high initial conditions of steam pressure and temperature, and are mostly used during extension of power station capacities, with a view to obtain better efficiencies. By extension of power stations capacities here is meant additional installation of high pressure boiler (critical and super critical pressures) and topping turbines as additional units, delivering steam to the already existing medium-pressure turbines from the exhaust of topping turbines.

e) Back pressure turbines with steam extraction from intermediate stages at specific pressures ;turbines of this type are meant for supplying the consumer with steam of various pressure and temperature conditions.

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f) Low-pressure (exhaust-pressure)turbine sin which the (exhaust-steam) from reciprocating steam engines, power hammers, presses etc. is utilised for power generation purposes.

g) Mixed pressure turbines with two or three pressure stages, with supply of exhaust steam to its intermediate stages.

According to the Steam Conditions at Inlet to Turbines

a) Low-pressure turbines, using steam at pressure of 1.2 to 2 ata.
b) Medium-pressure turbines, using steam at pressure of up to 40 ata.
c) High-pressure turbines, utilising steam at pressures above 40 ata. and below 170 ata.
d) Turbines of very high pressures, utilizing steam at pressures of 170 ata and higher and temperatures of 550oc and higher.
e) Turbines of supercritical pressures, using steam at pressures of 225 ata and above.

According to Shaft Arrangements

i) Tandem compounded turbines - Here all the cylinders are arranged so as to drive a single shaft. ii) Cross compounded turbines - Here cylinders are arranged to drive two or more shafts with separate generators with every shaft.

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ASSEMBLY SEQUENCE FOR A TURBINE

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Turbine Diagram

TURBINE COMPONENTS * CASING * ROTOR * BLADES * SEALING SYSTEM * STOP & CONTROL VALVES * COUPLINGS & BEARINGS * BARRING GEAR

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Turbine Casings HP Turbine Casing * outer casing: a barrel-type without axial or radial flange. * Barrel-type casing suitable for quick startup and loading. * The inner casing--- cylindrical , axially split. * The inner casing is attached in the horizontal and vertical planes in the barrel casing so that it can freely expand radially in all directions and axially from a fixed point (HP-inlet side).

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IP Turbine Casing * The casing of the IP turbine is split horizontally and is of double-shell construction. * Both are axially split and A double flow inner casing is supported in the outer casing and carries the guide blades. * Provides opposed double flow in the two blade sections and compensates axial thrust. * Steam after reheating enters the inner casing from Top & Bottom.

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Rotors HP Rotor: * The HP rotor is machined from a single Cr-Mo-V steel forging with integral discs. * In all the moving wheels, balancing holes are machined to reduce the pressure difference across them, which results in reduction of axial thrust. * First stage has integral shrouds while other rows have shroudings, rivetted to the blades are periphery.

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IP Rotor * The IP rotor has seven discs integrally forged with rotor while last four discs are shrunk fit. * The shaft is made of high creep resisting Cr-Mo-V steel forging while the shrunk fit disc are machined from high strength nickel steel forgings. * Except the last two wheels, all other wheels have shrouding riveted at the tip of the blades. To adjust the frequency of the moving blades, lashing wires have been provided in some stages.

LP Rotor * The LP rotor consists of shrunk fit discs a shaft. * The shaft is a forging of Cr-Mo-V steel while the discs are of high strength nickel steel forgings. * Blades are secured to the respective discs by riveted fork root fastening. In all the stages lashing wires are providing to adjust the frequency of blades. In the last two rows satellite strips are provided at the leading edges of the blades to protect them against wet steam erosion.

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There are two types of turbine rotor used in large turbines which have impulse type blading :

a) The built up rotor also called Disc Rotor consisting of a forged steel shaft on which separate forged steel discs are shrunk and keyed. (Fig. 3.20).

b) The integral rotor in which the wheels and shaft are formed from one solid forging. (Fig. 3.21). The built up rotor is made up of a number of separately forged discs or wheels and the hubs of these wheels are shrunk and keyed on to the central shaft. The outer rims of the wheels have suitable grooves machined to allow for fixing the blades. The shaft is sometimes stepped so that the wheel hubs can be threaded along to their correct positions. Suitable clearances are left between the hubs to allow for expansion axially along the line of the shaft. Integral rotors as said before have discs and shaft machined from one solid forging, the whole rotor being one complete' icce of metal.
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This results in a rigid construction and troubles due to lobse wheels of the shrunk on type are eliminated. Grooves are machined in the wheel rims to take the necessary blading. These are also called solid forged rotors.

INTEGRAL ROTOR

The built-up rotor tends to be the cheaper of the two since the discs and shaft are relatively easy to forge and inspect for flaws; also, the machining of these components can be carried out concurrently. On the other hand, integral rotors are expensive and difficult to forge and there is a high incidence of rejects; there is also a large amount of machinery time and waste material involved.In spite of the expenses involved, the advantages of integral rotors are such that they are invariably used for the high pressure rotors on high temperature plant; on reheat machines in particular they are often used for intermediate pressure and low pressure rotors as well. This is because of the difficulty of ensuring that the shrunk-on discs on intermediate and low pressure rotors cannot become loose, particularly at the high temperature end during start up when the shafts may be relatively cool and the discs are hot. Another source of trouble under conditions of high temperature and stress is the phenomenon of creep which could also cause the shrink-fft to disappear after a large number of running hours.

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With regard to low pressure rotors, the main problem is one of centrifugal stress, the last stage being the most heavily stressed part of the turbine. The last row wheels on the standard 500 MW turbine are the largest cap able of operating at 1 000 rev 1 min; the blades are 900 mm in length and are mounted on the disc so as to have a mean diameter of 2.5 m, the overall diameter is therefore 3.45 m. On large turbines using 50 per cent reaction, four types of rotor are used:
a) The hollow drum rotor which promotes even temperature distribution because it is designed with the same thickness of material as the casing.
(Fig. 3.22) illustrates the construction of the hollow drum type rotor.
b) The solid drum rotor suitable for cylinders where there are lower temperatures but large diameters, as in intermediate pressure cylinders without reheat.
c) The built up rotor previously described.
d) Welded Rotors which are built up. From a number of discs and two shaft ends. These are joined together by welding at the circumferences and because, there are no central holes in the discs the whole structure has considerable strength. Small holes are drilled in the discs to allow steam to enter inside the rotor body to give uniform heating when coming on load. Grooves are machined in the discs to carry the blades and (Fig. 3.23) shows this type of rotor construction.

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BLADES * most costly element of turbine * blades fixed in stationary part are called guide blades/nozzles and those fitted in moving part are called rotating/working blades. * blades have three main parts Aerofoil: working part Root Shrouds * shroud are used to prevent steam leakage & to guide steam to next set of moving blades.

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Impulse Type Fixed Blading (Fig. 3.26).
The fixed blading in an impluse turbine takes the form of nozzles mounted in diaphragms. The diaphragm is made in two halves, one half being fixed to the upper half of the cylinder casings by means of keys so that when expansion occurs fouling of the shaft seals is avoided. Special carrier rinfs are generally used to support the diaphragms in H.P. cylinders. Because of the steam pressure difference on each side of the diaphragm, seals are provided at the bore where the shaft passes through the diaphragm, to prevent steam leakage along the shaft.

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FIG. 3.26 IMPULSE TYPE FIXED BLADING Moving and Stationary Blades

The HP turbine blading consists of several drum stages. All stages are reaction stages with 50 per cent. The stationary and moving blades of the front stages (Fig. 3.28) are provided with T-roots which also determine the distance between the blades. Their cover plates are machined intergral with the blades and provide a continuous shroud after insertion.
The moving stationary blades are inserted into appropriately shaped grooves closed casing (1) and are bottom caulked with caulking material (9). The insertion slot in the shaft (8) i a locking blade which is fixed either by taper pins or grub screws. Special end blades which lock with t horizontal joint are used at the horizontal joints of the inner casing. Graub screws which are inserted from t joint into the material secure the stationary blades in the grooves.

28 Guide Blades Fig.

Gap Sealing

Sealing strips (3,7) are caulked into the inner casing (1) and the shaft (8) to reduce leakage and losses at the blade tips (4,5). Cylindrically machined surfaces on the blade shrouds are opposite the sealing strips. These surfaces have stepped diameters in order to increase the turbulence of the steam and thus the sealing effect Should an operation disturbance cause the sealing strips to come into contact with opposite surfaces they are rubbed away without any considerable amount of heat being generated.

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METHODS OF FITTING BLADES
Root Fixings

Many types of root fixing shapes exist for turbine blading to suit both the conditions under which the blade must operate and the preference of the particular designer concerned. In general there are the types which either fit in their appropriate groove or straddle ft, whilst other designs are fixed by rivets through the blade root. Some examples of these blade root fixings are shown in (Fig. 3.33).

In the case of reaction type turbines the H.P. blading is built up in packets of up to ten blades and held in the rotor groove with serrated looking strips as shown earlier in (Figure 3.27). A gap is left in the rotor groove to allow the last serrated locking strip to be inserted. This gap is then closed with a plate fixed by screws to the rotor body. In these turbines, the last L.P. rotor blades are subjected to a centrifugal force of about 250 Tons / blade.
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Here fir tree fixing has proved most effective. This is also shown at (Fig. 3.31). For some impulse type blading a gap or "gate" is left in the rotor groove and the blades are fed in and located by the particular shape of serrations used for the blade root and groove. The last blade, which has a plain root with no serrations, fits over the "gate" and is secured by riveting. In other methods of fitting a soft metal distance piece or wedge is driven in the groove to locate the last blade. Where the blades are all attached to the rotor wheels by riveting or are of the side entry type any blade can be last one and each must be separately secured by riveting or with locking plates.

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BEARINGS * General bearing * Thrust bearing Bearings are usually forced lubricated and have provision for admission of jacking oil. Front Bearing Pedestal * The Front Bearing Pedestal is located at the turbine side end of the turbine generator unit. * Its function is to support the turbine casing and bear the turbine rotor. * It houses the following components a. Journal bearing b. Hydraulic turning gear c. Main oil pump with hydraulic speed transducer d. Electric speed transducer e. Over speed trip f. Shaft vibration pick-up g. Bearing pedestal vibration pick-up

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MATERIAL FOR CASING AND DIAPHRAGMS

H.P. & I.P. turbine casings and diaphragms are normally Cr, Mo, V creep resistance steel castings. L.P. casings and diaphragms where the temperature never exceeds 230'C (e.g. L.P. cylinder on non-reheat machines) are sometimes made of cast iron. On large reheat turbines, however the temperature of steam entering the L..P. cylinder may be more than 230'C and because of this and the large over all dimensions of L.P. cylinders, these are usually fabricated from carbon steel castings or M.S. Plates, This construction also provides greater protection in the event of blading failures and speeds manufactures.

STEAM VALVES

A turbine is equipped with one or more emergency stop valves, in order to cut off the steam supply shut down and to provide prompt interruption of the steam flow in emergency. In addition governing valves are used to provide accurate control of steam flow entering the turbine. Reheat turbines require additional emergency and interceptor valves . in the return path from the reheater and dual pressure turbines require two @ of emergency and governing valves. (Fig.3.14) shows some basic schematic designs of valves in modem use.
a) Shows a “ double--beat" valve having two seatings, the object being to balance the forces due to steam pressure. It is suitable for most pressures, but not for high temperatures as differential expansion between the valve and cage would cause one or other sealing to owrapm.
b) Shows another double-beat valve of the hollow type in which the steam from one sealing is led through the centre of the valve. The thinner walls promote even heating and lesser differential expansion.
c) Shows a modern spherical valve used for controlling high temperature steam.

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d) Shows a similar valve fitted with an internal pilot valve which, by opening first, equalises the pressures and provides initial fine control.
e) Shows a cylindrical valve in which steam pressure is prevented from acting on the back of the valve by fine annular clearance.
f) Shows a flap valve, used for reheat emergency valves, where the steam pressures are moderate and the specific volumes (and hence the valve diameters) are large.
g) Shows a governing valve of the *mushroom' type, with a profiled skirt to give a more linear area 1 lift relationship. Other types of valves, such as piston and grid valves are used in pass-out turbines. The diameters of valves opening are generally calculated to dive maximum steam velocity of about 60 m / sec for. emergency valves, and about 120 m 1 sec for governing valves.

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36

SHAFT TURNING (BARRING) GEAR

Turning gear is provided to rotate turbine shafts slowly during the pre-run up operation and after shut down to prevent uneven heating or cooling of the shafts. The uneven heating or cooling would lead to bending and misalignment of shafts with possible fouling of stationary and moving parts. Use of turning gear during starting eliminates the necessity of admitting suddenly a large flow of steam to rotate the turbine from the rest. The turning gear speed is chosen to ensure satisfactory lubrication of the bearing and, at the same time, provide some circulation of air within the casing (particularly at the low pressure end) after shut down. The speed of turning gear varies considerably from one design to another. For example while BHEL 1 LMW 210 MW turbine is rotated by the turning gear at the speed of 3.4 rpm, in 500 MW KWU turbines, the T/ G rotates the turbineshaft at 270 R.P.M. / or 240 R.P.M. depending on whether the condenser is under vacuum or not. The turbine must remain on turning gear units metal temperature has dropped below 150"c with normal cooling, this will take approximately 72 hours. Before putting the turbine on turning gear a few conditions like-adequate bearing oil pressure, jacking oil pump running etc. must be satisfied.

JACKING OIL PUMPS (JOPS) are positive displacement pumps that provide high pressure (1 20 bar for KWU turbines) supply of oil under strategic journals @~turbo generator and the oil lifts the shaft slightly, This ensures that there is no metal contact between a journal and the bearing. This greatly reduces the static friction and bearing wear, also the starting torque headed by the turning gear drive. The JOP can be stopped after the lubricating oil film is established between the shaft and bearings. On early turbogenerators, turning of TG was done by hand withthe help of long barfitted with rachetworm and pinion mechanism. (Fig. 3.34).
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This gave the name "barring gear" or "barring" to this operation. Now, the driving force is provided by either electric motors or hydraulic pressure. Hand barring gear is used, nowadays, in emergency, when T.G. motor is non-operational, and for maintenance purposes, to rotate the turbine shaft manually. An auxiliary source of power from U.P.S. (Uninterrupted Power Supply) or Diesel Generating set is also provided in some cases for reliability of T /G operation. Fig.3.35 shows the functional arrangement of a turning gear. In BHEL 1 21 0 MW LMW turbine, the T./ G is mounted on LP rotor rear coupling. It consists of a worm,, worm wheel spur gear and pinion, spiral shaft and sliding shaft with lever. The system comes into operation when the shaft comes to stand still. When T / G is engaged, the turbine shaft rotates at 3.4 R. P.M. In KWU turbines, the turning gear is hydraulic. It is engaged when shaft speed comes down to 545 R.P.M The T 1 G rotates the shaft at 120 RPM or 80 R.P.M. depending on whether the condenser is under vacuum or not. The T / G assembly is located in the front bearing pedestal of LP cylinder and consists of two rows of moving blades mounted on coupling flange of I.P. rotor, an inlet nozzle box with stationary nozzles and .,guideblades (Fig. 3.36). The TG shaft system is rotated by the double row wheel which is driven by prerssurised oil supplied by auxiliary oil pump. After passing through the blading the oil drains to the bearing pedestal and combines with the bearing lube oil returning to the iube oil tank. In addition, the system is equipped with facility for manual barring in the even of failure of hydraulic turning.

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39

Balancing

When assembled complete with blades the rotor is balanced both statistically and dynamically. In the case of built-up rotors, each bladed disc is balanced individually before assembly. The aim of balancing is to reduce vibration to a tolerable level, usually accepted to be about 35μm at the bearing pedestal of a 3000 rev / min. machine. A stationary shaft supported between bearings has a natural frequency of vibration depending upon its diameter and the distance between the bearings. If its speed of rotation corresponds to its natural vibration frequency, the residual out – of-balance forces can build up to a dangerous extent. This speed is known as the Critical Speed. Sometimes it is above the running speed and sometimes it is below. If above, the shaft is said to be stiff, if below, the shaft is said to be flexible and the critical speed should be passed as quickly as possible when running the turbine up to speed.The critical speed should not be within 20 per cent of the running speed.

Rotors are generally classified into two types –

* Rigid Rotor- A rigid rotor is a rotor whose critical speed is its operating speed. * Flexible Rotor- A flexible rotor is a rotor whose critical speed is below its operating speed.

Balancing is done in two different process -

* By removal of weights * By addition of weights
The balancing is done after determining the point where the uniform mass distribution is not present and it is determined by two different process-

* Low speed Balancing(when the speed of rotating is below 400RPM) * High Speed Balancing(When the rotating speed is up to operating speed)

For a rigid Rotor only Low speed balancing is done as it has constant vibration through out its operating speed.
For a Flexible Rotor both the low speed and the high speed balancing is done because low speed balancing is done to balance the non-uniformity of mass and high speed balancing is done to stop the vibration.

Fig shows Position of cg of shaft with increasing of rpm

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Balancing Tunnel Scheme-

DC MOTOR
(400 KW *2)
GEAR BOX
(1:4)
JOG MOTOR
INNITIATE ROTATION
BLOWER TO COOL THE DC MOTOR
CARDON SHAFT
ROTOR FOR BALANCING IS CONNECTED

Servo 46 lubbricant is used for lubrication

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