Summary

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Introduction*

Result*

Turbine Cylinders*

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Casings*

Requirements *

Bands in stead of Flanges & Bolts *

Single Flow H.P. Turbine*

Barrel Design No Horizontal Flanges*

Single flow H.P.Turbine*

Double Flow H.P.Glands avoided Balanced Thrust*

Double Flow L. P. Turbine*

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Steam Turbine DesignImpulse Turbine- Velocity CompoundingReaction Turbine- Pressure CompoundingCompounded Turbine First Stage Velocity and later stages reaction.Most large Turbines have an Impulse stage where the pressure drops greatly and after velocity compounding, the remaining stages are reaction.

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Comparison-In Impulse type,the pressure drops and velocity rises greatly.

In reaction type,the drop & rise is gradual.*

*True Impulse Turbine And

Reaction Turbine

Difference between Impulse and Reaction Turbine: An impulse turbine, has nozzles and moving & fixed blades in series.No pressure drop in the bladesReaction turbine has fixed blades and moving blades.In impulse turbine pressure falls in nozzle only, while in reaction turbine pressure drops over fixed & moving blades.In impulse turbine velocity of steam increases greatly in nozzle, while velocity increases gradually over the fixed blades in the reaction turbine.*

4. Compounding is done in impulse turbines to increase efficiency while no compounding is necessary in reaction turbine.5. In impulse turbine pressure drop per stage is more than in reaction turbine.6. Less number of stages required in impulse turbine compared to the reaction turbine. 7. Power developed in impulse turbine is less than in a reaction turbine. 8. Efficiency of impulse turbine is lower than reaction turbine. 9. Impulse turbine requires less space than reaction turbine. 10. Blade manufacturing of impulse turbine simpler than reaction turbine blades.

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Turbine Type SelectionSmaller sizes are Impulse type as they are cheaper, although less efficient.Medium and large size turbines must be reaction type with initial velocity stages.As a large pressure drop occurs in the Nozzle, the casing of Impulse turbine is thinner and hence cheaper.Warming of Impulse Turbine is faster.*

Layout Options Transverse or Longitudinal ?*

More space is required in Longitudinal L/OWidth of Boiler is much less than length of Turbine.So space is wasted on Boiler side.But EOT crane is cheaper as the span is less.Transverse layout saves space and the energy flow is in a straight line.As width of Turbine hall is more, the EOT crane becomes costlier.Space has to be provided for Feed heaters and maintenance purposes.*

Michell Thrust BearingFluid-film thrust bearings were invented by Australian engineer George Michell (pronounced Mitchell) who patented his invention in 1905. Michell bearings contain a number of sector-shaped pads, arranged in a circle around the shaft, and which are free to pivot. These create wedge-shaped regions of oil inside the bearing between the pads and a rotating disk, which support the applied thrust and eliminate metal-on-metal contact.Michell's invention was notably applied to the thrust block in ships. The small size (one-tenth the size of old bearing designs), low friction and long life of Michell's invention made possible the development of more powerful engines and propellers. They were used extensively in ships built during World War I, and have become the standard bearing used on turbine shafts in ships and power plants worldwide.*

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Thrust Pads*

Double Casing (Shrink Rings in lower fig.)*

Barrel Design*

Double Flow I.P. Turbine *

Double Flow L.P. Turbine *

Blade Materials

Blade material must have following properties,Corrosion resistance (especially in the wet LP stage)Tensile strength (to resist centrifugal and bending stresses)Ductility (to accommodate stress peaks and stress concentrations)Impact strength (to resist water slugs)Material damping (to reduce vibration stresses)Creep resistance12% Cr stainless steels are a widely used material. Their weakness is at very high temperatures (> 480C). A typical high temperature steel is 12% Cr alloyed with molybdenum and vanadium (to 650C).*

Interstage Sealing*

Blade Erosion

Water droplets in the last stages of a turbine can cause erosion at the leading edge of moving blades, and cracks can form. Leading edges can be protected by surface hardening or by welding a shield of hard material such as tungsten chromium tool steel or satellite (an alloy of cobalt and chromium). Shields will probably need to be replaced once during the lifetime of the turbine.*

Turbine Casings

A turbine casing (cylinder) is a high pressure vessel with its weight supported at each end on the horizontal centreline. It is designed to withstand hoop stresses in the transverse plane andto be stiff in the longitudinal direction to maintain accurate clearances between the stationary and rotating parts.Casings are split along the horizontal centreline to allow internal access and insertion of the rotor as a complete assembly.High pressures necessitate very thick flanges and bolting. The temperature of these changes more slowly than the rest of the casing during start-up so a flange warming system is used.HP and IP casings are cast. LP casings can contain some fabrication. Casings are tested to 150% of highest working pressure.*

Double Casing*

High Pressure CasingsCross-sections of a single-flow HP casing are shown in Fig. HP casings are usually of a double shell design. The space between the shells is filled with steam at exhaust conditions. Then each casing can be designed for smaller temperature and pressure differentials. Some exhaust steam leaks past a baffle to fill the space between the shells. The rotor is protected from high pressure steam at the inlet by a deflector ring. Steam leaking past the gland at the HP end is piped to exhaust connections, so there is only a gentle flow between the casings.Triple casings are used in some machines to further reduce temperature and pressure differentials.*

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Couplings

Shaft couplings are needed between the various stages of a turbine/generator set. Ideally, couplings should: Transmit torque Allow angular misalignment Transmit axial thrustEnsure axial location or allow relative axial movement Provide torsional resilienceFlexible or semi-flexible couplings can provide this but they are impracticable on large turbines because of the high torque to be transmitted.*

Rigid couplings are used in large turbines so that the joined shafts can behave as one continuous rotor. They are either integral with the shaft forging or shrunk on to the shaft . In the latter case, high pressure oil can be injected into annular grooves to ensure correct seating during assembly, or to aid removal.Couplings are designed to withstand a three-phase fault or out-of-phase synchronising without damage (4-5 times full load torque).*

Rigid Forged Coupling*

Rotor Alignment

Excessive misalignment of a multi-bearing shaft line can affect the vibration behaviour. It causes bending moments at couplings which act like a rotating out-of-balance.It can cause bearing unloading which alters shaft vibration behaviour.A long shaft bends naturally under its own weight to form a catenary and revolve around a curved centreline. The shape of the catenary depends on the masses and stiffnesses of the rotors.The aim of alignment is to ensure insignificant bending moments and shear at the couplings.Bearing heights are adjusted so that coupling faces are square to each other, with centrelines coincident and with the same slope where the faces meet. This is done by slightly separating the coupling and turning the rotor to different positions. Bearings are adjusted to get uniform gap and concentricity (measured with a dial gauge).*

Shaft Catenary*

Journal Bearings

Bearings on the shaft line of a large turbine/generator set are invariably white-metalledjournal bearings because of their:high load capacity reliabilityabsence of wear through use of hydrodynamically generated films of lubricating oil .(no metal-to-metal contact)*

Construction of Journal BearingsTurbine bearings have diameters up to 550 mm, with length/diameter (L/D) ratios of 0.5 to 0.7. Generator bearings have L/D ratios of 0.6 to 1.0 because of the weight of the generator rotor. They are split in halves for assembly of the rotor, with bolts and local dowels. White metal is either cast into a mild steel liner or cast into the bearing body. The bearing body is spherically seated into the pedestal for angular alignment. Shims are available for vertical and horizontal alignment.*

Exploded view of a Journal Bearing*

White metal (or babbit) is usually composed of 80 to 90 % tin to which is added about 3 to 8% copper and 4 to 14% antimony. These alloys have very little tendency to cause wear to their steel journals because of their ability to embed dirt. They are easily bonded, cast and shaped, and can have good load-carrying and fatigue properties.The bores of journal bearings are usually elliptical to provide the geometry for hydrodynamic lubrication.*

A circular bore is machined with shims in the horizontal split. The shims are removed in assembly to give typical diametrical clearance/diameter ratios of 0.001 vertically and 0.00015 horizontally. Oil is fed into the bearing via lead-in ports at two diametrically opposite points on the horizontal centreline. This is to cool and lubricate the bearings and comes from the main turbine lubricating-oil pump.Each bearing also has a high pressure jacking oil supply at the bottom. This lifts the shaft when starting from rest, until speed is high enough for hydrodynamic lubrication to start-up.

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Pedestals

Pedestals support the turbine rotor via the journal bearings in a fixed axial relationship with each cylinder so that gland clearances are maintained. They are usually fabricated in steel and stiffened by ribs and gussets plates.In the LP area, pedestals are normally bolted and dowelled to the foundations. At the HP end, provision is made for the cylinders to expand by way of sliding points at the top or bottom of the pedestals*

*T U R B I N EDESIGN ASPECTSHigh temperature.Creep.Thermal Stress.High pressure-Hoops stress.(for cylinders)High Speed (3000 RPM)High centrifugal stress proportional to (r*w*w)Unbalance-Vibrations -Close clearances between fixed & rotating parts.

*Turbine Rotor RequirementsHigh temperature rotors (HP & IP) need a combination of-Creep strengthRupture strengthDuctility.Chromium, Molybdenum, and Vanadium (Cr-Mo-V) provide these properties to steel.They provide a ferritic material for best creep properties.

*CreepCreep is plastic deformation (strain) at high temperature at a stress even below the yield point.Permissible creep strain is 0.2% during 10 lakh hours of operation.(About 12-1/2 years with annual one month outage for overhaul.1%Cr-1%Mo-0.25%V give best results for creep strength.

*L.P. RotorsHere the temperature is low but centrifugal stress is highest.So, L.P. rotors need high tensile strength & High toughness.3.5%Ni,Cr,Mo,V mono-block rotors are generally used.In 200/210 Russian design-the discs on LP Rotor are shrunk fit.Hence sudden change in diff.expn.from 600 to 3000 RPM.

*Rotor sizesMono block rotors for 900 MW 3000 RPM.LP Rotor for 660 MW- 100 MT forged weight.This is made from ingot of twice this size.Vacuum degassing eliminates Hydrogen embrittlement cracking.Ultrasonic test- No internal cavity greater than 5mm diameter is permissible.

*Boroscopic InspectionAll rotors have a central axial bore for boroscopic inspection.The central hole doubles the centrifugal stress .With superior inspection techniques, no bore is drilled.This greatly reduces the stress.For built-up rotors,(only L.P.)-good welding techniques are developed now.The conflicting needs are Tensile strength and ductility.

*Fracture Appearance Transition temperature.(FATT)FATT is the Temperature at which small cavities may enlarge under stress.FATT is kept as low as possible.LP rotors operate at near ambient temperatures.With stringent NDT and fracture mechanics study,FATT is kept well below the ambient temperature.3.5%Ni,Cr.Mo.V. steel has low FATT.To get high fracture toughness, proper composition control and heat treatment is required.This ensures trouble free operation of LP Rotor.

*Rotor Testing-Thermal (On Load) StabilityThere are three types of instabilities-Permanent-Due to asymmetrical expansion coefficients across the rotor diameter.Close metallurgical control of forging process is essential.2.Temporary-Due to residual stresses in rotor.Stress relieving of rotor before & after machining. For this the rotor is kept rotating in a special furnace.3. Transient-Differences in conductivity & emissivity.

*Transient InstabilityTo demonstrate this, one side of a rotor was mirror finished, and other side was kept as usual.Vibrations were noticed in service although the rotor was balanced.Generally ferritic materials remove this problem.While cleaning a rotor by sand blasting, care must be taken to ensure uniform cleaning on all sides of rotor.

An unstable rotor develops a bow and consequent vibrations develop.

*Over-speed Testing20% proof O/S. after manufacture.It proves the rotor balance at operational speed.

The rotor gets proof stress tested as much higher centrifugal stress is imposed at 20% O/S.

Stress is proportional to Square of rotational speed.

*Rotor BalancingStatic Balancing- It proves even distribution of mass.This is tested by rolling the rotor on knife edge supports.Dynamic Balancing.-It take care of unbalanced couples.The rotor is rotated (400 rpm) and vibrations are noted.Balance weights are added till vibrations become negligible.For full speed balancing, vacuum chambers are used.

*Unbalance coupleSuch a rotor will show static balance but is dynamically unbalanced. It will vibrate in service. Provision is made on rotors to fix balance weights.As site conditions differ from test bed conditions, trim balancing may be needed at site.

*CRITICAL SPEEDNatural Frequency of a rotor is directly proportional to dia.and inversely proportional to length.When a critical speed is below the operating speed, it must be quickly passed while rolling the turbine.Otherwise there will be resonance and severe vibrations.This is a FLEXIBLE rotor.Rotor shafts are joined by couplings. There may be 6-7 couplings. Coupled shafts are treated as ONE Shaft.Each rotor is supported on two bearings.These are not simple supports.Hydro dynamics of bearing oil film changes with temperature & oil condition.All this may significantly alter the critical speed calculated by theory.

*Rotor Fast Fracture Risk AssessmentAdvanced forging techniques try to give defect free rotor.But due to the huge size of the forging, some small defects (cavities/cracks) may remain.These defects may grow in service, (due to centrifugal stress) and cause fracture.Permissible size of defect is determined by calculations & experience.To assess the failure possibility 100% volumetric NDT is done. and it is ensured that defects are below the permissible size.

*5. The rotor is subjected centrifugal & thermal stress cycling.6. Growth rate of defect is observed. It should be within permissible band.

Mal-operations during start-up may cause higher growth rate and failure.Hence the need to strictly follow the established operations procedure.(Thermal soaking of Turbine is important)HP & IP Rotors operate in creep range and any defect may grow even under steady loading.

*COUPLINGSFlexibleSemi FlexibleRigid.Flexible-Claw or gear-tooth couplings, Bibby coupling can-Absorb small angular & axial misalignment.Double flexible couplings can absorb eccentricity.Bibby couplings satisfactory for medium loads.It has torsional resilience.Useful for jerky loads like coal mills.Torsional stiffness increses with load.These couplings require lubrication.

*Semi-flexible and Rigid couplingsSemi-Flexible:-Bellow type.Allow angular bending only. Absorb some axial movement. They are stiff in torsion.No lubrication.Coupling between LP Rotor & Generator. Rigid Couplings:- These are integral with the shaft.Shrunk on coupling flanges may be used on Turbines & Generator-rotor. These can be taken off to remove the rotor discs or the rotor retaining rings.(End bells).

*Torque transmission by couplingsThe torque is transmitted by the friction at the flanges and shear load on the bolts.Coupling bolt tolerance is about 0.02mm.For this reaming of holes is necessary which is a lengthy procedure.Normally ALL bolts are not fitted by reaming.Only sufficient no. of bolts are fittedto withstand system fault torques.Remaining bolts are loose fit. A close by 3-ph. Fault or mal-synchronising torque may be 4-5 times full-load torque.So-Synchronise very carefully.

*Coupling BoltsCoupling Bolts have cylindrical heads with an integral hexagon for tightening.

They are recessed into the coupling flanges to reduce windage.

Coupling guards are provided to reduce windage heating of adjacent bearing pedastal and formation of oil mist.

*TURBINE CASINGSCasing is a pressure vessel supported at both ends on a horizontal centerline.Hoops stress in transverse plane.Very stiff in longitudinal direction to keep accurate clearances between rotating and fixed parts.All casings are split for internal access. For placing diaphragms carrying fixed blades and the rotor in position.So, there are horizontal flanges and bolts.

*Flanges are massive (compared to casing thickness) as bolt holes are present and huge force has to be applied for tightness of the flange.So they tend to expand differently relative to the casing.For this , flange heating is provided.Gland housings further complicate the casing.HP & IP casings are castings. Circular cross section,flanges,bolting steam entry etc. are kept symmetrical to reduce thermal asymmetry & distorsion.

*LP casing may be fabricated. Or a combination of casting & fabrication.Hydarulic test is conducted at 150% of highest working pressure.HP casings are usually double casings.This reduces the shell thickness.Each casing has to be designed for lesser thermal & pressure stress.Space between casings is filled with exhaust steam from HP cylinder.Faster warming possible due to thinner shells.Thinner shells are easier to cast and have less defects.

*Tripple CasingInner casing has flanges.Outer casing is barrel type.It has no flanges,so design is simpler.In earlier machines, the control & stop valve-chest was integral with the casing.This complicated the design and valve maintenance was delayed as cooling was very slow.Now valves may be mounted in the pipe line,slighly away from the casing.Casing becomes simpler and valve maintenance is easier.

*IP & LP CasingTemperature is same as HP casing but the pressure is much less.So the casing is thinner.In double flow IP, no glands are needed at high pressure end.L.P. casing is usually double flow-double casing.Inner casing contains the diaphragms & extraction points.Outer casing directs exhaust steam to condensor and provides support for the inner casing.

*CASING MATERIALSH.P. and I.P. Casing:- High temperature, creep resistant material.2.25% Cr.-1Mo-for temperatures up-to 538 deg.C.0.5%Cr-0.5Mo-0.25V for temperatures up-to 565.

L.P. casing:- Generally fabricated from Carbon Steel.Inner casing may be spheroidal Cast Iron.Any large defect is ground out and repaired by welding.

*Casing gland & Diaphragm glands.To prevent leakage from the gap between fixed & moving parts, glands are provided.Glands are labyrinth or see-through type.They provide a torturous path for the leaking steam which greatly reduces its pressure and hence the leakage.Glands prevent steam leakage in HP & IP casing.L.P. casing glands prevent air entry.Here sealing steam must always be provided.HP & IP casing gland sealing steam is needed only during start-up.

*BearingsThrust bearing. It is an axial bearing.

*Thrust Bearing Monitor

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*SHAFT CATENARY

Chart1

6

3.5

1.7

0.15

1

5

15

25

Bearing No.

Lift in mm

Turbo-Generator Shaft Catenary

Sheet1

32 M.

61.4 M

198 MT/Hour

2016 MT/Hour

Sheet2

Type of FiringOpposedFrontCornerCornerOil Opposed

M.C.R. Load660500500350660

No.of Furnace Chambers21222

Heat Release upto Nose-Projected area Kw/M2551.9544.7502433753

Heat Release upto Nose-Volumetric Kw/M3211196.3228207342Turbine Shaft Catenary

Plan Area MW/M25.755.0558XY

Residence Time Seconds2.42.52.292.28****061

Dist.from top burner to Nose12.65M13.4M9.75M11.12M***53.52

Platen Pitch0.762M0.616M0.6090.663**101.73

SIZE COMPARISON150.154

FUELOILCOAL2015

HEIGHT123056

WIDTH11,235157

DEPTH11.1437258

Sheet2

0

0

0

0

0

0

0

0

Bearing No.

Lift in mm

Turbo-Generator Shaft Catenary

Sheet3