Steam Turbine Vibration Characteristics [PDF]

Zitiervorschau

October 2004

Steam Turbine Vibration Characteristics Mike McGuire

Works Assembly of 1130 MW HP Turbine

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LP Rotor

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Introduction z

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Avoidance of damaging vibration essential for long term plant reliability Comprehensive dynamic analysis tools used at design stage to assess: – Shaft vibration – Blading Integrity Assessment tools utilise modern analytical techniques which have been validated by well established feedback from service experience and test rigs Ensure that new designs are developed with confidence and without risk

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Shaft Vibration

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Shaft Vibration z

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Comprehensive shaft line analysis performed – rotor (inertia & stiffness) – bearing oil films (stiffness & damping) – pedestal/foundation (mass, stiffness & damping) – blading & seals (steam excitation & damping) – excitation (unbalance, concentricity errors, rotor asymmetry) Steady state response (synchronous) - unbalance Stability assessment (sub-synchronous) - oil/steam whirl Acceptance criteria based on detailed R&D programmes and well established practical experience Results in low levels of vibration which are continuously monitored

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Dynamic Modelling of Rotor Line

ROTORS BEARINGS PEDESTALS FOUNDATION

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Rotor and oil films most important elements

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Design guidelines ensure that same rotor line operates successfully on different foundations

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Bearing Oil Films 8 dynamic coefficients: ⎛ Fx ⎞ W ⎜⎜ ⎟⎟ = ⎝ Fy ⎠ C r

⎛ a xx ⎜ ⎜a ⎝ yx

a xy ⎞ ⎛ x ⎞ W ⎟ ⎜⎜ ⎟⎟ + a yy ⎟⎠ ⎝ y ⎠ Ω Cr

⎛ b xx ⎜ ⎜b ⎝ yx

b xy ⎞ ⎛ x& ⎞ ⎟ ⎜⎜ ⎟⎟ b yy ⎟⎠ ⎝ y& ⎠

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anisotropic coupling between horizontal and vertical vibration provide stiffness and substantial damping coefficients influenced by bearing type (fixed arc, tilting pad), geometry (length, diameter, clearance), steady load, speed, lubricant properties (viscosity) – derived from thermo-hydrodynamic analysis for both ALSTOM and third party bearings – can induce self excited vibration (oil whirl)

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Steady State Response z z

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Vibration response to unbalance Non dimensional response calculated for different modal unbalance cases, e.g. – Centre span excitation for first mode – Couple excitation for second mode Assessment made at ‘critical speeds’ and normal operating speed using established guidelines Each rotor high speed balanced in factory to long established criteria (ref: ISO 11342) In service vibration meets international standards (ref: ISO 7919-2 & ISO 10816-2)

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Shaft Line Stability z

Sub synchronous bearing instability phenomenon well known for many years (oil whirl/whip)

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Steam whirl phenomenon emerged from 1960’s onwards as unit outputs and steam conditions increased Significant experimental and theoretical work carried out for steam turbines in UK, USA and Europe during 1970’s (Benckert, Slocombe/C.E.G.B.) Analysis methods initially developed based on previously established theory for compressors (e.g. Wyssmann) Increasing use of modern aerodynamic analysis methods for assessment of steam excitation forces

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Factors Influencing Stability z

Rotor mass and stiffness - ‘critical speed’

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Oil film characteristics - bearing type

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Bearing support stiffness

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Steam Excitation - ‘steam whirl’ – Steam density - HP/Supercritical most susceptible – Tip seals and shaft glands > Thomas effect (Tip seals only) > Influence of inlet swirl (Tip seals & shaft glands)

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Steam Excitation: Thomas Effect Variations in blade force

Tip leakage Blading force Net destabilising force….

... system rotates causing Whirl

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Steam Excitation: Influence of Swirl

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Steam Excitation: Influence of Swirl P(A) > P(B)

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P(B)

Inlet Point

P out

B Steam Swirl

A Outlet Point

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Destabilising Force…

Outlet Point

... rotor moves to close clearance…

P(A) P in

A Inlet Point

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... system rotates causing Whirl 14

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Swirl Breaks

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Swirl Breaks High exit steam swirl (circumferential velocity) from fixed blades

Swirl Breaks reduce circumferential velocity thus reduce the destabilising force

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Tip Seal with Swirl Breaks

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Diaphragm Gland Segment Inlet Swirl Breaks zUsed

successfully on a UK 500MW turbine (1980’s) Tested efficiency : 75% (removes 75% of inlet swirl)

33% of tip seal effectiveness 10% of power loss

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Stability Analysis Process z

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Full shaft line stability analysis carried out to establish datum case in absence of steam excitation – shaft line stable to maximum overspeed (i.e. no oil whirl) Complete audit of steam excitation forces carried out – shaft/diaphragm glands & blade tip seals Influence of steam excitation on rotor stability assessed If lower limit stability criterion not satisfied at full load rotor geometry and/or bearing oil films improved. If necessary steam excitation forces reduced by use of swirl breaks – shaft/diaphragm glands and/or tip seals Full shaft line stability analysis repeated Conservative approach applied for third party retrofits

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Shaft Vibration - Summary z z z z z z z

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Comprehensive rotordynamics analysis assesses both steady state response and stability Acceptance criteria based on detailed R&D programmes and long established practical experience Complete audit of steam forces carried out If necessary, steam excitation forces reduced by use of swirl breaks Swirl break behaviour validated by comprehensive test programme Proven operational experience with standard sealing arrangements Low levels of vibration in service

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Torsional Vibration

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Torsional Vibration z z z z z

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Low Damping ( Damping Factor D ≈ 0.001) with high resonance amplification factors ( >100 ) - no influence from bearings or supports During normal resonance free operation very small vibration amplitudes (< 0.1 °) Torsional excitation due to the electrical system Short term transient disturbances (e.g. mal-synchronisation, line to line fault) can induce transient torques ~6-8 times normal full load torque Long term disturbances of electrical system frequency - negative sequence current - due to unbalanced phases or interactions with long transmission lines. Frequencies tuned to avoid first two harmonics of electrical system frequency (e.g. 50/60 & 100/120 Hz) Potential excitation of coupled blade/rotor modes Excitation magnitude dependent on generator mode shape

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Torsional Vibration 2 x Grid Frequency at Generator Rotor Exciter

Transformer

Required Conditions for Failure z

Excitation from the electrical Grid due to non-symmetrical electrical loading e.g. Steel Industry, Aluminium Production, ...

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Torsional natural Frequency close to 100/120 Hz R S T -

HighVoltage Bus

Lines

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Coupled Mode Shape Shaft / Blade

to the Consumer

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Torsional Vibration Summary z z

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Conservative design criteria adopted to avoid resonance at 1x & 2x electrical system frequency Coupled blade/rotor modes fully assessed

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Low interaction between blades and welded drum type LP rotors

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Accuracy of analysis confirmed by experimental validation

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Routine in situ validation unnecessary - if confirmation is required this can be achieved with confidence in factory spin pit

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Good Experience with new machines and LP-Retrofits (Zion 1 & 2, Indian Point 3, Maanshan 1 & 2, Kori 1-4, San Onofre . . . . . .) - no adverse experience

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