Steam Turbine PPT

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STEAM TURBINE

Dr. K.C. Yadav, Head,Training & Development

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Learning Agenda Expansion of steam and work done Description of the nozzle angles (α), blade angles (β) and

surface roughness (µ) and their impact on turbineperformance

Velocity vector diagrams and estimation of turbine stageoutput and efficiency

Purpose, principle, classification, construction andfunctioning of steam turbine

Physical significance of turbo-supervisory parameters Performance of steam turbine

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Purpose of Steam Turbine Steam turbine is prime-mover for electric power generation,

which converts heat energy of steam to mechanical energyof Steam Turbine Rotor.

This mechanical energy is utilized to spin rotor (magnet) ofthe electricity generator to produce electric power.

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Steam Expansion Steam expands, whenever it is subjected either to lower

pressure or to a higher temperature. It is considered to be free expansion when the expanding

boundary is free from any resistance from the surrounding.Though the free expansion has no engineering application butit provides enough guidelines to the designers of steamturbines/engine to properly deal with steam operatingparameter to avoid any possibility of free expansion.

Expanding steam (thermodynamic System) does work onsurrounding irrespective of its being a solid, liquid or gasseparated by well defined boundary.

Expansion of steam in turbine is facilitated to do work onturbine blades mounted on the freely rotating shaft.

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Principle of Steam TurbineWhen steam is allowed to expand through anozzle, then its heat is converted to kinetic energyof steam itself, which in turn converts into kineticenergy (mechanical energy) of Turbine Rotorthrough the impact (impulse) or in an other way,when it expands through Turbine Rotor Bladeswithout any change in its velocity then its heat isconverted directly in to kinetic energy (mechanicalenergy) of Turbine Rotor through reaction ofsteam expansion against the blades.

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Types of Steam Turbine

Impulse Turbine (DR = 0) Reaction Turbine (DR = 1) Impulse - Reaction Turbine (DR > 0 & <1)

Pressure drop in Moving Blades________________________

Total Pressure drop =DR

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Impulse Turbine

Velocity compounded Pressure compounded Pressure - Velocity compounded

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Reaction TurbineExpanding steam has to be accommodated in movingblades without any change in velocity by suitablyincreasing the space in the blade down stream, which isvery difficult and hence no steam turbine is constructed tobe pure reaction turbine.

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Impulse - Reaction TurbineExpanding steam does work on surrounding bladesurface by virtue of its volume change and at the sametime incremental velocity of steam stream also doessignificant work on moving blades by impaction.


Turbine Blade


Vector Diagram


Multistage Turbine Blade Arrangement


3


Stationary Diaphragm


Components of Steam Turbine Foundation (TG & Pillar) Base plate / sole plate Bearing pedestal / pedestal plate Casing

Single / double (Inner or outer casing) / Triple casingBarrel type or axially spilt (bottom or top flange)Body liners and stationary diaphragm

RotorInbuilt (solid), key & shrunk fit and welded Moving diaphragm

Studs and nuts


Components of Steam Turbine HP, IP & LP turbine. Bearings. Shaft sealing . Stop & control valves. Turbine control system. Turbine monitoring system. Turbine oil system. Turbine turning gear


TG Foundation


*


IP Cylinder of a 500 MW Unit


Barrel Type HP Turbine


Hydraulic Turning GearThe function of the hydraulic turning gear isto rotate the shaft system at sufficient speedbefore start-up and after shutdown in orderto avoid irregular heating up or cooling downand also to avoid any distortion of theturbine rotors. The hydraulic turning gear issituated at the front end of the HP turbinefront bearing pedestal.


Hydraulic Turning Gear


Mechanical Barring Gear

The turbo- generator is equipped with amechanical barring gear, which enablesthe combined shaft system to berotated manually in the event of afailure of the normal hydraulic turninggear. It is located at IP-LP pedestal(Brg No-3).


Mechanical Barring Gear


Low Pressure TurbineOuter casing ,upper half

Outer shell, upper half Inner shell, upper half

Inner shell, lower halfOuter shell, lower half

Outer casing, lower half

STEAM FLOW


Fixed Points of a 250 MW TurbineCasing Expansion:

HP Turbine outer Casing expands towards front Pedestal.

IP Turbine Casing expands towards Generator side. LP Turbine outer casing expands towards both ends

from center.Rotor Expansion:

HP Rotor towards front Bearing. IP Rotor towards Generator side. LPT Rotor towards Generator.


Casing ExpansionThe bearing pedestals are anchored to the foundation bymeans of anchor bolts and are fixed in position. The HPand IP turbines rest with their lateral support horns on thebearing pedestals at the turbine centerline level. The HPand IP casings are connected with the bearing pedestalsby casing guides which establish the centerline alignmentof the turbine casings. The axial position of HP and IPcasings is fixed at the HP-IP pedestal. Hence, when thereis a temperature rise, the outer casings of the HP turbineexpand from their fixed points towards Front pedestal.Casing of IP Turbine expand from its fixed point towardsthe generator.


Casing ExpansionThe LP Turbine outer casing is held in placeaxially, at centre area of longitudinal girder bymeans of fitted keys. Free lateral expansion isallowed. Centering of LP outer casing is providedby guides which run in recesses in the foundationcross beam. Axial movement of the casings isunrestrained. LP Casing expands from its fixedpoint at front end, towards the generator at centrearea of longitudinal girder by means of fittedkeys. Free lateral expansion is allowed.


Rotor ExpansionThe thrust bearing is housed in the rear bearing pedestalof the HP turbine. The HP turbine rotor expands from thethrust bearing towards the front bearing pedestal of theHP turbine and the IP turbine rotor from the thrust bearingtowards the generator. The LP turbine rotor is displacedtowards the generator by the expansion of the shaftassembly, originating from the thrust bearing.


Turbo Supervisory Parameters Over all expansion Axial shift Differential expansion Eccentricity Vibration


Performance of Steam Turbine THR = [(Qms*Hms – Qfw*hfw) + Qrh*(Hhrh – Hcrh)]/P

P = PGen.Ter. – (Pexc + Pmin) ηta = 3600/THR = ηt*ηg*ηc

ηt = Wt/Hise

ηg = MW/Wt

ηc = Hise /[(Qms*Hms – Qfw*hfw) + Qrh*(Hhrh – Hcrh)] or ηc = [Qms*(Hms–Hcrh)+Qrh*(Hhrh – Hexh)–Sum(qb*Hb)] /

[(Qms*Hms–Qfw*hfw)+Qrh*(Hhrh–Hcrh)]


Enthalpy Drop Across the TurbineHPT

Qms*(Hms-H7) + (Qms-q7)*(H7-Hcrh) IPT

+ Qrh*(Hhrh-H5) + (Qrh-q5-qd)*(H5-H4) + (Qrh-q4-q5-qd)*(H4-H3) + (Qrh-q3-q4-q5-qd)*(H3-H2)

LPT

+ (Qrh-q2-q3-q4-q5-qd)*(H2-H1) + (Qrh-q1-q2-q3-q4-q5-qd)*(H1-Hexh)


Velocity Vector Diagram for Pure Impulse Turbine

β1α1 β2α2

β1α1

β2α2


Blade Performance of Pure Impulse Turbine

Wo = C2 Cos α2 (clockwise tangential component) Wi = C1 Cos α1 (anticlockwise tangential component) R2 < R1 & R2 = µ*R1

For smooth surface µ = 1 & R2 = R1

P = [Wi - (-Wo)]*u = [C2 Cos α2 + C1 Cos α1]*uC2 Cos α2 = R2 Cos β2 –u = R1 Cos β1 –uor C2 Cos α2 = C1 Cos α1 - u – u = C1 Cos α1 – 2uP = [C1 Cos α1 + C1 Cos α1 – 2u]*u = 2*u*[C1 Cos α1 – u]ηb = 2*P/C1**2 = 4*[(u/C1)*Cos α1 – (u/C1)**2]


Velocity Vector Diagram for Impulse-Reaction Turbine

β1α1 β2α2

β1 α1

β2α2


Work Done in Imp-Reaction Steam Turbine


Deduction of C2 & R1 in terms of R2 & C1


Degree of ReactionPressure drop in Moving Blades

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Total Pressure drop =DR =

Enthalpy drop in Moving BladesTotal Enthalpy drop

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Stage Efficiency


Internal Losses

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Steam Turbine PPT

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