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•Bharat Heavy Electricals Limited one of the top engineering companies in India ranks among the top 12 Power Equipment manufacturers in the world. Setup in mid fifties, BHEL has diversified its product base over the years and today every key sector of the economy is served by BHEL. •BHEL’s products cater to a wide spectrum of customers in various fields like power generation and transmission, oil exploration and production, transportation, steel & metals, fertilizer, petro-chemicals, refineries, cement plants, engineering industries, non conventional energy sources, defense etc. •More than 65% of installed capacity for electrical power in India is contributed by BHEL. With 14 manufacturing units and over 63,000 skilled manpower, BHEL has built up tremendous engineering strength. BHEL-Hyderabad manufactures almost all critical high technology products requires for Power Sector viz. Gas Turbine, Steam Turbine, Generator, Steam Generator, Heat Recovery Steam Generators, Pumps & Compressors, Heat Exchangers, Control equipment etc. IN the factory area of BHEL there are altogether broadly 8 workshops in which all the major work is done. They are: 1.ELECTRICAL MACHINES-Electrical equipment like generators, transformers are produced and tested here. 2.TC &GT-Turbines, compressors and gas turbines are produced here. This work area is further divided into sub sections for undertaking different operations like Heavy Machine shop, light BHEL-INTRODUCTION

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•Bharat Heavy Electricals Limited one of the top engineering companies in India ranks among the top 12 Power Equipment manufacturers in the world. Setup in mid fifties, BHEL has diversified its product base over the years and today every key sector of the economy is served by BHEL.

•BHEL’s products cater to a wide spectrum of customers in various fields like power generation and transmission, oil exploration and production, transportation, steel & metals, fertilizer, petro-chemicals, refineries, cement plants, engineering industries, non conventional energy sources, defense etc.

•More than 65% of installed capacity for electrical power in India is contributed by BHEL. With 14 manufacturing units and over 63,000 skilled manpower, BHEL has built up tremendous engineering strength.

BHEL-Hyderabad manufactures almost all critical high technology products requires for Power Sector viz. Gas Turbine, Steam Turbine, Generator, Steam Generator, Heat Recovery Steam Generators, Pumps & Compressors, Heat Exchangers, Control equipment etc. IN the factory area of BHEL there are altogether broadly 8 workshops in which all the major work is done. They are: 1.ELECTRICAL MACHINES-Electrical equipment like generators, transformers are produced and tested here.2.TC &GT-Turbines, compressors and gas turbines are produced here. This work area is further divided into sub sections for undertaking different operations like Heavy Machine shop, light machine shop, rotor shop, blade shop, testing and assembly area.


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Radial compressors are also produced and tested here. Several other operations like forging, welding (small scale) are done. GT components are also done and assembled separately. This project was done in this workshop. Basic workshop operations like tapping, drilling etc. are done on the vertical drilling machine. For different materials different coolants are used. Various specifications for processes are 1. Reaming-shell type or normal type reamers are the tools used.M-10,M-20…denote the type of thread. They have their own characteristics like pitch ,drill dia, etc.2. Drilling-According to requirement, the size of drill tool is selected. Radial drilling is used to drill holes sideways. 3. Tapping-this operation is used for threading. There are many types of threads like gas threads, NPT (National Pipe Taper), BSW, BSF etc. one example is shown below.

Thread size

Major dia. Pitch dia. Drill dia. TPI(TEETH PER INCH

1/4 6.35 5.53 4.72 20

1 25.4 22.36 21.33 8

BSW thread characteristics

•The use of taper pins in assembling two parts is that it does not allow their separation.

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There is a design section adjoining the work area where analysis of the process is done. Actual drawing sheets for various components are made here. 3.FOUNDRY - Input to this shop are details like drawing, temp. of boring, allowances etc. Here the moulding process takes place. Exhaust roots, bearing pedestal ,guide wheels are manufactured. 4. FORGING – the various welding operations are shielded metal arc welding, termite welding etc. Closely related to this workshop is the metallurgical testing lab where the composition of the metal is monitored. This is done by UV (ultraviolet) testing and chemical analysis. Mechanical testing lab is used for determining the properties of the metals. Here the various tests done on are UTM (Universal testing machine), various hardness tests like Brinnell, vicker etc.5. SURFACE TREATMENT WORKSHOP- In most of the cases heat treatment is done here.

Tempering ,nitriding and other processes are made here.

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Steam turbines-introduction• The steam turbine is a prime mover in which the potential energy of the steam is

transformed into kinetic energy, and later in turn transformed into mechanical energy of rotation of the turbine shaft

•Transformation of potential energy of the steam into mechanical energy of rotation of the shaft is brought about by different means.

• The steam passes through the main steam connection, the steam strainer and the emergency stop valve before the servo valves into the inlet part of the outer casing. After opening of the servo valves, the steam flows into the steam chamber, to pass through the jet groups into the expansion area of the turbine, by giving off its energy capacity and expanding up to the final pressure in the exhaust part.

•In a steam turbine the force exerted by the blades is due to the velocity of the steam. This is due to fact that the curved blades by changing the direction of the steam receive a force of impulse. The action of steam in this case is said to be dynamic in a steam turbine.

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TURBINEGeneral layout of a steam turbine plant

1.3 STEAM TURBINE POWER PLANT CYCLE Steam turbine power plants are based on the Rankine cycle investigated by a Scotch Engineer and Scientist William Rankine (1820 -1872). Rankine cycle for Steam turbine power plant with ideal turbines and pumps is shown below.

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THE IMPULSE PRINCIPLEIf steam at high pressure is allowed to expand through a stationary nozzle, the

result will be a drop in the steam pressure and an increase in steam velocity. The effect of this change in direction of the steam flow will be to produce an impulse force, on the blade causing it to move.

REACTION TURBINE PRINCIPLEA reaction turbine has rows of fixed blades alternating with rows of moving

blades. The steam expands first in the stationary or fixed blades where it gains some velocity as it drops in pressure. It then enters the moving blades where its direction of flow is changed thus producing an impulse force on the moving blades. In addition, however, the steam upon passing through the moving blades again expands and further drops in pressure giving a reaction force to the blades.

Special Aspects of Reaction Turbines 1. There is a difference in pressure across the moving blades. The steam will therefore

tend to leak around the periphery of the blades instead of passing through them. Blade clearances therefore must be kept to a minimum.

2. Also, due to pressure drop across the moving blades, an unbalanced thrust will be developed upon the rotor and some arrangement must be made to balance this.

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MAJOR PARTS OF STEAM TURBINE• The major parts of impulse-reaction turbines are as follows—

1.NOZZLE: The nozzle expands steam of comparatively low velocity and high static pressure within considerable increase in velocity

2. DIFFUSER: It is a mechanical device that is designed to control the characteristics of steam at the entrance to a thermodynamic open system. Diffusers are used to slow the steam's velocity and to enhance its mixing into the surrounding steam

3. BLADES OR BUCKETS: The blades or buckets form the rotor flow passage and serves to change the direction and hence the momentum of the steam received in the stationary nozzles. 4. GUIDEBLADES: Often a turbine is arranged with a series of rotor flow passages. Intervening between the blades comprising the rotor passages are rows of stationary guide blades.5.CASING:The turbine enclosure is called the casing. The nozzle and guide blades are fixed on the casing.6.ROTOR:The rotor is the rotating part in the turbine on which the moving blades fixed.7.BLADE CARRIER: The carrier carries fixed blades. It is rested on the casing.8.TURBINE BEARINGS AND JOURNAL BEARINGS: The main purpose of bearings is to keep the rotor in the exact position in the casing. It is also used to absorb the axial thrust.9.GOVERNORS:They are used to regulate the steam inflow into the turbine.

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10.EXHAUST HOOD: The exhaust hood is the portion of the casing which collects and delivers the exhaust steam to exhaust pipe or condenser.

11.THROTTLE OR STOP VALVES: The throttle and stop valves are located in the steam supply line to the turbine. The stop valve is hydraulically operated quick opening and shutting valves designed to be either fully opened or shut. .

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Machining of casing:Machining of casing:• STEAM CHAMBER: The steam chamber represents the connection

between the control valve bores in the outer casing and the nozzle groups of the control stage.

• The steam chamber contains the nozzle groups for the control stage(3),provision for stem flow(2),inner shaft gland(5).

•Movable L-rings seal the steam path between the steam chamber and theouter casing so as to permit thermal movement.•The detailed design is as shown below---

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• The turbine casing is divided into an admission and exhaust section. Back pressures as well as condensing turbines have admission sections of identical design. Depending on the inlet steam conditions, the admission sections of comparable size are designed with casings of different wall thickness. The admission section will be completed by an exhaust section of adequate size.

1. Outer casing2. Exhaust steam part sealing shells3. Support blocks9. Webs to accommodate the blade carriers11 Mounts for rear bearing6 Main steam line connection 12 Exhaust steam flange

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Initial machining operations:

•It is general practice to let the thickness of walls and flanges decrease from inlet- to exhaust-end. •Rough machining by a Gantry machine is done. Shoulder milling operation is done for machining circular surfaces. •On the inside, the outer casing has surrounding webs and contact cams to accommodate blade carriers, the inner case or the steam chamber. By means of these webs, the outer case is split into several sectors of differing pressure, out of which tappings can be obtained to suit requirements

•The casing of a condensation turbine essentially consists of an outer case withsteam inflow and the exhaust steam part bolted on via a ring flange .Flanges are projecting rims or edges used for guidance, strength and attachment to other components like the blade carrier.

•The overall length is adjusted by cylindrical or conical spacer rings. The outer diameter at the inlet and exhaust are measured.

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•By using drill tool for boring operation directly holes are drilled if required or using reaming operation the holes are machined according to specifications.•Now by using CNC operated machining( for small capacity turbines), blade groove roughing is done to get approximate shape of blade groove on blade slot machine.

•As shown, the drilling and boring operations as well as spot facing operations being done on the casing

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Operation characteristics of horizontal milling and boring machine type

Boring spindle diameter 200mm

Longitudinal travel (column on bed), x axis


Vertical travel, y axis 5000mm

Head stock cross travel, w axis


Boring spindle travel (z axis)


Max. reach on spindle 2650mm(w+z)

D.C spindle motor power

100 KW

Max. Spindle rotation speed


Max nominal torque 14101NM

Loading capacity 10^5kg

Corners chamfering


Feeds in all directions


Head stock section


Table size 4500*4500mm

The CNC (Computer numerical control) system is


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• G -CODES & M- CODES are used for writing the program. For different operations different codes are given.

• Some commands used are:

GO Rapid travels when machining is not done.

G01 Linear interpolation

G02 Circular interpolation

G04 Dwell time

G90 Absolute position

G91 Incremental position

G95 Feed per revolution



Programmable offset

G53 TO G56

Work offset


M05 Spindle shaft

M17 Program end

M04,M03 Table rotation

M30 End of main program

M01 Program start unconditional

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Overview of guide blade carrier:

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Blades manufactureBlades manufacture:• The different operations involved in the manufacture of blades are enumerated below::

1. RAW PIECE CUTTING: Using a power saw machine the lengthy bars are first cut into small 20cm pieces. The material used for blades is heat resistant alloyed steel having the composition X22CrMoV12.Here the use of chromium is to get a good finish, Mo is a heat resistant metal.

2. SIZE MILLING: According to the required specification, the depth and width of the w/p’s are set and machined on lathe using Angle cutter.

3. SIZE GRINDING: Horizontal grinding operation is done to set the w/p to required tolerance.

4. RHOMBOIDAL MILLING: The cubical w/p’s are now milled to obtain a rhomboidal shape whose edge angles are set.

5. RHOMBOIDAL GRINDING: According to the required tolerance grinding operation is done (on many w/p’s) to get accurate edges.

6. BLADE LENGTH CUTTING: Now, according to required length of blade the w/p’s are cut, leaving some tolerance limit.

7. ROOT MILLING: A T-root is now machined on to the base part of the w/p using a T-max cutter. Simultaneously three cutters are in operation on three sides of the w/p.

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Blade profile after rhomboidal milling

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8. WIDTH MILLING: For decreasing the side area for blade, width milling is used. Two stages are involved.-outlet WM & inlet WM. Outlet and inlet are determined according to the direction of steam flow.

9. BACK PROFILE USING COPYING MILLING: The back side of the w/p is now milled

according to dimensions of a standard blade using the cam arrangement. The cutter employed is convex cutter.

10. CHANNEL MILLING: The front portion of the blade is now machined using pre-determined values to obtain an aerofoil shape.

11. CHANNEL AND BACK PROFILE WIDENING: To Meet the exact stipulations this is done.

12. ROOT RADIUS: In order that the blade is assembled perfectly on the circular rotor, a slight indent is made on the T-Root as shown.

13. TAPER MILLING: For all the blades to get perfectly fitted beside one another it is now machined to get a taper, length-wise.

14. TAPER GRINDING: For smooth finishing the blades are now grinded.

15. POLISHING: To ensure meeting perfect specifications, polishing is done.

A-Wheel blade is made by cutting into small pieces a wheel, which has grooves. This acts like a fork and later the usual operations are done on the A-Wheel blades.

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Root radius operation enables blades to fit round circular rotor.

After polishing blades are fit onto rotor.

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Rotor machining:• Blades are assembled on the rotor and thus when steam makes impact it

rotates. It is connected to the generator shaft to convert rotational energy to electrical energy.

• The actual turbine shaft comprises of a monoblock forging.• A 4-check lathe is used for man grooves on the rotor.• The grooves have dimensions which allow exact fitting of blades.• Based on the steam inflow pressure the rotor is divided into primarily five

sections:1. Inlet section: It consists of A-Wheel which first receives

the high pressure steam and guides it onto the rotor.2. HP (High pressure) section: This section intercepts steam of high

pressure and hence the blades, grooves are small in size.3. IP (intermediate pressure) section: Due to the stationary stages, pressure

of steam is considerably reduced.4. LP (low pressure) section: to intercept steam of low pressure the profile

and the size of the blades are considerably altered. Now the blade root is of fork type.

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5. EXHAUST section: It consists of the pinion for rotating the generator shaft, journal bearing and other auxiliary equipment.

Scaling fins which are used for arresting the steam leakage ,and create more back pressure so that the steam enters the guide vanes in the right way, are embedded into the portion of the rotor between the grooves.

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ROTOR ASSEMBLY.•Actual assembly consists of individually inserting the blades into the grooves and locking their positions using a small metal piece between the base of blade and the rotor groove.

•This is done for all the stages except LP blades which have fork-root base.

•For the last blade in assembling a row of the blades, the base part remains

. solid (no root milling is done). It is marked and used for conducting some of the performance tests like rotor insensitivity test etc

•Shrouding operation is done on the projecting side of all the rows of moving blades. This is done so that steam does not escape out of the blades (rotor part).

Drum stage moving blades with shrouds.

•The moving blades have integral shrouding, with the moving blades of one row forming a closed, tightly fitting blade assembly at the shrouds.

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•The rotor is supported in two pressure lubricated journal bearings (4 and 12).•The axial position of the rotor is fixed by the thrust bearing collars (2).•Grooves for the electric speed measurement are provided in the right collar(3). The mechanical overspeed trip (1) and the trip cams for the axial positiontrips are arranged upstream of the thrust bearing (3).

1 Overspeed trip 9 HP nozzle group2 Thrust bearing 10 LP nozzle group3 Thrust bearing collar with grooves 11 Rear shaft gland region4 Front bearing support point 12 Rear point of bearing support5 Front shaft gland region 13 Gear for manual turning gear6 Balancing planes 14 Coupling7 Inner shaft gland region 15 Shaft nut8 Control Stage

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Assembly of casingAssembly of casing

•The turbine case is axially split; the top and bottom of the case are held together by reduced-shaft or solid shaft bolts. The outer faces of the casing parts possess webs to accommodate the sealing shells and cast-in channels with affiliated flange connections to remove leaking steam or to supply sealing

•As we know for small capacity turbines, the guide blades are embedded in the blade carriers whereas for the larger capacity ones, grooves are machined on the inner casing using CNC operated vertical boring machine.

STEAM CHAMBER:•The steam chamber is split axially. The two halves are bolted together by high-temperature bolts. Axial fixation in the outer casing is effected using a circumferential groove; the steam chamber is supported vertically by prepared supports on both sides. The transverse axial fixation is provided by an eccentric bolt installed in the outer casing bottom half.

•The blade carriers are axially split and are bolted together steam tight by partialarea screws. In the axial direction, the blade carriers are held in the outercasing by a slot-web connection.•Vertical support is provided via adjustment elements that are screwed into thepads of the blade carrier bottom. Radial mounting and guidance are ensuredby an axial bolt guide in the bottom of the blade carrier and the outer casing

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• A closed, tightly fitting blade assembly is obtained at the shrouds when the stationary blade carrier halves are bolted together during final assembly. The stationary blade carriers are suspended in the outer casing (5) so as to allow for thermal expansion. If required, a stage drain can be provided upstream of the last stationary blade stage to draw off any water that mayaccumulate on the blade vanes into the condenser.

•Now the rotor is placed onto the one half of casing.

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Final assembly operations

Task• The sealing shells serve the purpose of sealing between the outer casing and rotor

on the steam end. Besides sealing against internal excess pressures, the ingress of ambient air in the event of an internal partial vacuum must be reliably prevented.

Structure•The sealing shell is axially split, and the halves are bolted to one another. Onthe outer circumference of the sealing shell body, there is a surrounding slotwith which the shell is fixed radially and axially in the outer casing.


1 Sealing shell 6 Turbine rotor2 Caulking material3 Sealing tip4 Steam extraction (steam feed) D Steam from turbine casing5 Vapour chimney

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• The bearings for small turbines are often self-aligning spherical ball or roller-bearings or they may be ring lubricated sleeve bearings with bronze or Babbit lining.

• Thrust Bearings: • The main purposes of the thrust bearing are:

To keep the rotor in an exact position in the casing.

To absorb any axial thrust on the rotor.

1 Turbine Casing 8 Stud bolt2 Turbine support paw 3 Pedestal support4 Stud bolt

5 Bearing housing6 Adjusting element: Bearinghousing (vertical)

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GAS TURBINES• The gas turbine is a rotating internal combustion engine, which takes air

from the atmosphere and compresses it to a higher pressure in a axial compressor (compressor section) and the compressed air flows into combustion chamber where fuel is admitted and ignited with the help of a spark plug. The products of combustion are used as a working fluid for developing power in the turbine section of the Gas Turbine

•The flow diagram of a simple gas turbine is as shown. The gas undergoes isentropic compression before undergoing combustion, followed by expansion in the turbine stage.

•The thermodynamic cycle upon which a gas turbine works is called the Brayton Cycle.

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• Work input to the turbine is needed to start rotating the shaft. During start-ups 50-60% of energy is consumed by the compressor. The self-sustaining speed is taken as 40-45% of max. speed. The motor is cutoff from rotating the shaft.

• A Brayton Cycle is characterized by two very significant parameters: pressure ratio and firing temperature. The pressure ratio of the cycle is the pressure at point 2 divided by the pressure at point 1. The firing temperature point 3 on the fig., is the highest temperature reached in the cycle. The thermodynamic cycle efficiency and Power output of a Gas Turbine depends on these two parameters

• The efficiency of gas turbine is in the range of 40-46% which is low compared to a steam power plant. But, further refinement to the cycle is at the cost of simplicity and not feasible. The high temperature of the exhaust gases can be used to heat steam for steam turbine application.

1-2:isentropic compression in compressor.

2-3:isobaric heat addition.

3-4:isentropic expansion in turbine.


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• The advantages of gas turbine are:Efficient Compact Reliable Eco-Friendly…Rotary Internal Combustion Engine• Self contained• Light weight• High Power Density• Low installation time• Quick start• Black Start Capability

NOTES: 1)Though the Otto cycle is more efficient it has many inherent disadvantages like knocking, not practicable for high output applications.

2)As it is an IC engine it does not require any external inputs as in the case of steam turbine. Hence the installation is quite easy

3)Hence it occupies very less space. In steam turbines for generating the same 125 MW of power coupled system is used.

4)Since air fuel mixture is 30:1 the exhaust gases do not contain high concentration of pollutants.

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Ancillary base.

The major components of a Gas Turbine are the compressor, combustion system and turbine section. CDC stands for (compressor discharge casing). The various parts that compose each individual section is dealt herewith.

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COMPRESSOR SECTION • The axial-flow compressor section consists of the compressor rotor and the

enclosing casing, as also CDC.

• Within the compressor casing are the inlet guide vanes, 17 stages of rotor and stator blading, and the exit guide vanes.

• Each compressor stage consists of a rotating row of blades (aerofoil) that increases the velocity of the incoming air there by increasing its kinetic energy, followed by stationary row of blades that act as diffusers, converting the kinetic energy to pressure increase.

• The number of stages used for a particular gas turbine compressor depends upon the design pressure ratio for that turbine. Typical pressure ratio ranges from 6:1 to 14.9:1.

• The compression efficiency is 85% that of the ideal isentropic process but handling is easy. From stage to stage, the compressor ratio is n*o.85.(n approx 1.4)

NOTE:1) during startups ,50-60% of energy is consumed by compressor.

2)Some air is extracted from the intermediate stages to enable the rotor to rotate freely.

AT 95% of the full speed we close breed valve. Then compressor operates at full efficiency.

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•At the compressor inlet there is a row of stationary blades, called inlet guide vanes (IGV's) that direct the incoming air on to the first rotating stage in a smooth way. The flow angle of the IGV's can be changed to control the volume of air being drawn into the compressor. The variable inlet guide vanes (VIGV's) are used to ensure aerodynamically smooth operating compressor throughout a large operating range.

Lower casing

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• Movement of these guide vanes is accomplished by the inlet guide vane control ring that turns individual pinion gears attached to the end of each vane. The control ring is positioned by a hydraulic actuator and linkage arm assembly


The inlet casing is located at the forward end of the gas turbine. The inlet casing also supports the No.-1 bearing housing, a separate casting that contains the No.-1 bearing. The No.-1 bearing housing is supported in the inlet casing on machined surfaces on either side of the inner bellmouth of the lower half casing. To maintain axial and radial alignment with the compressor rotor shaft, the bearing housing is shimmed, doweled and bolted in place at assembly. The inner bellmouth is positioned to the outer bellmouth by eight airfoil-shaped radial struts that provide structural integrity for the inlet casing. The struts are cast into the bellmouth walls.





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•The compressor rotor is an assembly of 15 individual wheels, two stub shafts (each with an integral wheel) a speed ring, tie bolts, and the compressor rotor blades.•Unlike the steam turbine rotor here in GT the rotor is not a single block forge.

MACHINING: Each wheel and the wheel portion of each stub shaft has slots machined around its circumference. The rotor blades and spacers are inserted into these slots and are held in axial position by staking at each end of the slot. The blades are manufactured using the same process as in the case of a steam turbine described earlier except that here the base profile is not a T-Root but an inverted U.STAKING is the process of inserting a thin metal piece between the compressor groove and the sides of the blade base. •The blades are not of the same size for all the stages,but differ according to their position. Small size bladesare to be assembled at the front end of the rotor wherethe pressure is more and large size blades are to beassembled at the rear end of the rotor where pressure is minimum.


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ASSEMBLY:• The wheels and stub shafts are assembled to each other with

mating rabbets for concentricity control and are held together with tie bolts. Stub shaft is the protruding part of the connecting rod of wheels of the compressor. They are two in number.

• Selective positioning of the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynamically balanced to a fine limit

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CASING:•The stator (casing) area of the compressor section is composed of four major sections. They are inlet casing, forward compressor casing, aft compressor casing, compressor discharge casing.

•These sections, in conjunction with the turbine shell, form the primary structure of the gas turbine. They support the rotor at the bearing points and constitute the outer wall of the gas path annulus. The casing bore is maintained to close tolerances with respect to the rotor blade tips for maximum efficiency.

FORWARD CASING•The forward compressor casing contains the first four compressor stator stages. It also transfers the structural loads from the adjoining casing to the forward support which is bolted and doweled to this compressor casing’s forward flange. Note: The forward compressor casing is equipped with two large integrally cast trunnions which are used to lift the gas turbine when it is separated from its base.

Bleed air from the fourth rotor stage can be extracted through four ports which are located at the aft section of the compressor casing



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The compressor stator blades are airfoil shape and are mounted by similar dovetails into ring segments. The ring segments are inserted into circumferential grooves in the casing and are held in place with locking keys.

AFT CASINGThe aft compressor casing contains the 5 through 10 compressor stages. Extraction ports in the casing permit removal of 5 and 11th-stage compressor air. This air is used for cooling and sealing functions and is also for starting and shutdown pulsation control. BLADING DESCRIPTION

The stator blades of the last nine stages and two exit guide vanes have a square base dovetail that are inserted directly into circumferential groovesin the casing. Locking key holds them in place

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COMPRESSOR DISCHARGE CASING• The compressor discharge casing is the final portion of the compressor stages. It

is the longest single casting. It is situated at the midpoint—between the forward and aft supports and is, in effect, the keystone of the gas turbine structure.

• The functions of the compressor discharge casings are:1. to contain the final seven compressor stages,2. to form both the inner and outer walls of the compressor diffuser, and3. to join the compressor and turbine stators. 4. They also provide support for the No. 2 bearing, the forward end of the

combustion wrapper, and the inner support of the first-stage turbine nozzle.



As shown the grooves for fitting the blades are of square dovetail shape

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ASSEMBLY•The compressor discharge casing consists of two cylinders, one being a continuation of the compressor casings and the other an inner cylinder that surrounds thecompressor rotor. •The two cylinders are concentrically positioned by twelve radial struts.These struts flair out to meet the larger diameter of the turbine shell, and are the primary load bearing members in this portion of the gas turbine stator.•The supporting structure for the No. 2 bearing is contained within the inner cylinder.

A diffuser is formed by the tapered annulus between the outer cylinder and inner cylinder of the discharge casing .This diffuser converts some of the compressor exit velocity into added pressure. There are also stationary diffuser blades which give maximum pressure

CDC-upper portion:

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•The combustion system is a reverse flow type with 14 combustion chambers arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles, spark plug ignition system, flame detectors, and crossfire tubes. Hot gases, generated from burning fuel in the combustion chambers, are used to drive the turbine.

SPARK PLUGSCombustion is initiated by means of the discharge from two high-voltageretractable-electrode spark plugs installed in adjacent combustion chambers.

FUEL NOZZLESEach combustion chamber is equipped with a fuel nozzle that emits a metered amount of fuel into the combustion liner. Gaseous fuel is admitted directly into each chamber through metering holes located in the outer wall of the gas swirl tip.

CROSSFIRE TUBESAll fourteen combustion chambers are interconnected by means of crossfire tubes. These tubes allow flame from the fired chambers to propagate to the unfired chamber

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Arrangement of Combustion chambers.

Combustion flow diagram.

Spark plug

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• TURBINE SECTION•The three-stage turbine section is the area where energy, in the form ofhigh-temperature pressurized gas produced by the compressor and combustion sections, is converted to mechanical energy.

•The major components of Gas turbine are the turbine rotor, turbine casing exhaust frame, exhaust diffuser, nozzles and shrouds.

ROTOR•The turbine rotor consists of two wheel shafts; the first, second, and third-stage turbine wheels with buckets; and two turbine spacers.•The 3-stage turbine wheels are manufactured in the following way: The outer rim part consists of grooves for assembling the buckets. These rims are forged onto a supporting circular shaft by heat treatment, forging.

•These wheels are held together with through bolts.

• The turbine rotor must be cooled to maintain reasonable operating temperatures and, therefore, assure a longer turbine service life. Cooling is accomplished by a positive flow of cool air radially outward through a space between the turbine wheel with buckets and the stator, into the main gas stream. This area is called the wheelspace.

WHEELSPACE: There are three stages of wheelspaces according to their positions.1. FIRST STAGE: The first –stage forward wheelspace is cooled by compressor discharge

air. High-pressure packing is installed at the end of the compressor rotor between the rotor and the inner barrel of the compressor discharge casing.

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The second-stage aft wheel space is cooled by air from the internal extraction

system. The air enters the wheel space through slots in the forward face of the spacer


The third-stage forward wheel space is cooled by leakage from the second-stage aft

wheel space through the inter stage labyrinth.

The third-stage aft wheel space obtains its cooling air from the exhaust framecooling system. This air enters the wheel space at the rear of the third-stage turbine wheel and then flows into the gas path at the entrance to the exhaust diffuser.

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• NOZZLES• In the turbine section, there are three stages of stationary nozzles which direct the

high velocity flow of the expanded hot combustion gas against the turbine buckets causing rotor to rotate.

NOTE :These nozzles are not machined using the regular practices but by using sophisticated procedures like EDM (Electrode dynamic machine) ,profile projector etc.


The 18 cast nozzle segments, each with two partitions or airfoils, are contained by a horizontally-split retaining ring which is centerline supported to the turbine shell on lugs at the sides and guided by pins at the top and bottom vertical centerlines. This permits radial growth of the retaining ring, resulting from changes in temperature, while the ring remains centered in the shell

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• BUCKETS: The turbine bucket increase in size from the first to the third-stage. Because of the pressure reduction resulting from energy conversion in each stage, an increased annulus area is required to accommodate the gas flow; thus

necessitating increasing the size of the buckets.•Turbine buckets for each stage are attached to their wheels by straight, axial-entry,multiple-tang dovetails that fit into matching cutouts in the turbine wheel rims. Bucket vanes are connected to their dovetails by means of shanks. •These shanks locate the bucket-to-wheel attachment at a significant distance from contact with hot gases so that they are at low temperature.

•Air is introduced into each first-stage bucket through a plenum at the base of the bucket dovetail. It flows through cooling holes extending the length of the bucket and exits at the recessed bucket tip.

•The material used for these blades is high temperaturesteel. At the site they are mostly imported. Since they require highly precisioned measurements devices likeCo-ordinate Measuring Machine (CMM) is used.Machining is done further on highly sophisticatedequipment like EDM, electronic enabled CNC machines etc.

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CASING:• The turbine shell and the exhaust frame constitute the major portion of the gas

turbine stator structure. The turbine nozzles, shrouds, #2 bearing and turbine exhaust diffuser and internally supported from these components.

• The turbine shell controls the axial and radial positions of the shrouds and nozzles. It determines turbine clearances and the relative positions of the nozzles to the turbine buckets

SHROUDS•Unlike the compressor blading, the turbine bucket tips do not run directly against an integral machined surface of the casing but against annular curved segments called turbine shrouds. The shrouds’ primary function is to provide a cylindrical surface for minimizing bucket tip clearance leakage.

•The turbine shrouds’ secondary function is to provide a high thermal resistancebetween the hot gases and the comparatively cool shell. In doing this, shell cooling load is drastically reduced, the shell diameter is controlled, the shell roundness is maintained, and important turbine clearances are assured SHROUDS

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• The shroud segments are maintained in the circumferential position by radial pins from the shell. Joints between shroud segment are sealed by

interconnecting tongues and grooves




•Hot gases contained by the turbine shell are a source of heat flow into the shell. Tocontrol the shell diameter, it is important that the shell design reduces the heat flow into the shell and limits its temperature. Heat flow limitations incorporate insulation, cooling, and multi-layered structures. The external surface of the shell incorporates cooling air passages.


Structurally, the shell forward flange is bolted to flanges at the aft end of thecompressor discharge casing and combustion wrapper. The shell aft flange is bolted to the forward flange of the exhaust frame. Trunnions cast onto the sides of the shell are used with similar trunnions on the forward compressor casing to lift the gas turbine when it is separated from its base.

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Turbine Shell

Inner Barrel

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1. Pick up turbine case & shroud assembly and assemble on the height blocks with its AFT face upwards. Clean AFT face free of burrs & high spots

2. Pick up exhaust frame. Clean the FWD face and assemble the exhaust frame on to turbine case AFT face, aligning the horizontal parting plane of both frame and turbine case.

3. Assemble the Frame to turbine case loosely with the above screws. They are inserted into the holes drilled on the outer casing of turbine and exhaust.

4. Align the exhaust frame to turbine case by taking drop checks at 6 points.5. Tighten the vertical flange bolting and recheck the alignment.6. Using the quacken-bush drilling equipment and fixture drill and ream dia19H7

dowel pin holes.7. Assemble correctly the dowel pins into the holes. Match mark the pins and holes.8. Remove the pins and screws from vertical flanges and preserve. Dismantle the

exhaust frame from turbine case.9. Reverse the turbine case and assemble it on height block such that FWD face isup wards and repeat the operations. Thus the exhaust frame is aligned with the turbine casing.ALIGNMENT OF CDC WITH TURBINE:1. Bring the compressor discharge casing assy. Loosen the support ring bolts all

around & tighten to a snug condition.2. Assemble the C.D.C assy. on turbine casing. Rotate the casing to align the partingplanes closely

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3.Assemble the two casings loosely with bolts move the alignment pole within 0.05, maintaining horizontal joint alignment to turbine case.

4. 4.Bring the casing tube to horizontal position. Attach the FWD support fixture to FWD COMP. casing.

5. Position the FWD & AFT support blocks. Lower the tube and rest it on the supportblocks. Ensure the tube is supported properly on blocks. Level the tube and release the

crane.6. Bring the inlet casing including IGV assembly. Assemble the inlet casing to comp.casing with bolts as before.7. Assemble exhaust frame to turbine casing with sock screws, we should not tighten

the bolts. ASSEMBLY OF ROTATING PARTS INTO CASING:1. Before removing the top half of casing match mark all dowel pins and body bound

studs to matching holes in casing. 2. Remove the upper cover from variable I G V & control ring.3. Remove the vertical bolting from combined compressor casing upper half. Remove

thehorizontal joint bolting.4.Similarly unbolt the vertical and horizontal flange bolting of exhaust frame and remove

the upper half 5.These bolts and their order should be clearly demarcated. Now the rotating parts are

ready to be assembled into the corresponding stator parts.

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