Dr. K.C. Yadav, Head, Training & Development
STEAM TURBINE
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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 turbine performance Velocity vector diagrams and estimation of turbine stage output and efficiency Purpose, principle, classification, construction and functioning 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 energy of Steam Turbine Rotor. This mechanical energy is utilized to spin rotor (magnet) of the 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 but it provides enough guidelines to the designers of steam turbines/engine to properly deal with steam operating parameter to avoid any possibility of free expansion. Expanding steam (thermodynamic System) does work on surrounding irrespective of its being a solid, liquid or gas separated by well defined boundary. Expansion of steam in turbine is facilitated to do work on turbine blades mounted on the freely rotating shaft. 4 of 99
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Principle of Steam Turbine When steam is allowed to expand through a nozzle, then its heat is converted to kinetic energy of steam itself, which in turn converts into kinetic energy (mechanical energy) of Turbine Rotor through the impact (impulse) or in an other way, when it expands through Turbine Rotor Blades without any change in its velocity then its heat is converted directly in to kinetic energy (mechanical energy) of Turbine Rotor through reaction of steam expansion against the blades. 5 of 99
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Types of Steam Turbine
Impulse Turbine (DR = 0) Reaction Turbine (DR = 1) Impulse - Reaction Turbine (DR > 0 & <1)
DR =
Pressure drop in Moving Blades ________________________ Total Pressure drop
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Impulse Turbine
Velocity compounded Pressure compounded Pressure - Velocity compounded
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Reaction Turbine Expanding steam has to be accommodated in moving blades without any change in velocity by suitably increasing the space in the blade down stream, which is very difficult and hence no steam turbine is constructed to be pure reaction turbine.
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Impulse - Reaction Turbine Expanding steam does work on surrounding blade surface by virtue of its volume change and at the same time incremental velocity of steam stream also does significant work on moving blades by impaction.
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Turbine Blade
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Vector Diagram
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Multistage Turbine Blade Arrangement
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Multistage Turbine Blade Arrangement
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3
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Stationary Diaphragm
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Components of Steam Turbine
Foundation (TG & Pillar) Base plate / sole plate Bearing pedestal / pedestal plate Casing Single / double (Inner or outer casing) / Triple casing Barrel type or axially spilt (bottom or top flange) Body liners and stationary diaphragm Rotor Inbuilt (solid), key & shrunk fit and welded Moving diaphragm Studs and nuts 21 of 99
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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
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TG Foundation
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IP Cylinder of a 500 MW Unit
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Barrel Type HP Turbine
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Hydraulic Turning Gear The function of the hydraulic turning gear is to rotate the shaft system at sufficient speed before start-up and after shutdown in order to avoid irregular heating up or cooling down and also to avoid any distortion of the turbine rotors. The hydraulic turning gear is situated at the front end of the HP turbine front bearing pedestal. 50 of 99
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Hydraulic Turning Gear
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Mechanical Barring Gear The turbo- generator is equipped with a mechanical barring gear, which enables the combined shaft system to be rotated manually in the event of a failure of the normal hydraulic turning gear. It is located at IP-LP pedestal (Brg No-3). 58 of 99
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Mechanical Barring Gear
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Low Pressure Turbine Outer shell, upper half
Inner shell, upper half
Outer casing ,upper half
STEAM FLOW
Outer shell, lower half
Inner shell, lower half
Outer casing, lower half 67 of 99
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Fixed Points of a 250 MW Turbine Casing 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. 72 of 99
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Casing Expansion The bearing pedestals are anchored to the foundation by means of anchor bolts and are fixed in position. The HP and IP turbines rest with their lateral support horns on the bearing pedestals at the turbine centerline level. The HP and IP casings are connected with the bearing pedestals by casing guides which establish the centerline alignment of the turbine casings. The axial position of HP and IP casings is fixed at the HP-IP pedestal. Hence, when there is a temperature rise, the outer casings of the HP turbine expand from their fixed points towards Front pedestal. Casing of IP Turbine expand from its fixed point towards the generator. 73 of 99
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Casing Expansion The LP Turbine outer casing is held in place axially, at centre area of longitudinal girder by means of fitted keys. Free lateral expansion is allowed. Centering of LP outer casing is provided by guides which run in recesses in the foundation cross beam. Axial movement of the casings is unrestrained. LP Casing expands from its fixed point at front end, towards the generator at centre area of longitudinal girder by means of fitted keys. Free lateral expansion is allowed. 74 of 99
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Rotor Expansion The thrust bearing is housed in the rear bearing pedestal of the HP turbine. The HP turbine rotor expands from the thrust bearing towards the front bearing pedestal of the HP turbine and the IP turbine rotor from the thrust bearing towards the generator. The LP turbine rotor is displaced towards the generator by the expansion of the shaft assembly, originating from the thrust bearing.
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Turbo Supervisory Parameters
Over all expansion Axial shift Differential expansion Eccentricity Vibration
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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)] 77 of 99
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Enthalpy Drop Across the Turbine HPT
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) 78 of 99
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α2
β1
α2
Velocity Vector Diagram for Pure Impulse Turbine
β2
α1
α1
β2
β1
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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]*u C2 Cos α2 = R2 Cos β2 –u = R1 Cos β1 –u or C2 Cos α2 = C1 Cos α1 - u – u = C1 Cos α1 – 2u P = [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] 80 of 99
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β2
α2
β1
Velocity Vector Diagram for ImpulseReaction Turbine
α1
α2
α1
β2
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Work Done in Imp-Reaction Steam Turbine
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Deduction of C2 & R1 in terms of R2 & C1
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Degree of Reaction ________________________
DR =
=
Total Pressure drop
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Enthalpy drop in Moving Blades ________________________
Pressure drop in Moving Blades
Total Enthalpy drop
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Stage Efficiency
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Internal Losses
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Thank you
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