Turbo Supervision

The history

From Sub-critical to super critical

Turbo SupervisionTurbo Supervision

GEN.GEN.

LPLP

IPIP

HPHP

The

turbine

http://www.pcez.com/~jmsc/spectrum_analysis.html

THENEED

FORFOR

• Safety• Cost of spares• Cost of repair• Procurement

difficulties• The criticality• The process • The thermal effects• The speed

TURBINEHYDROHYDRO

SLOW SPEED

MODERATE TEMPERATURE

QUICK TO START / STOP

MOSTLY SELF LUBRICATION

SUITABLE FOR GENERATOR/SYCHRONOUSCONDENSER OPERATIONS

ONCE ERRECTED REQUIRES LITTLE MAINTENANCE

NO THERMAL ENCOUNTERS

THERMALTHERMAL

HIGH SPEED

COMBINATION OF HIGH AND MODERATE TEMP. AND PRES.

LONG START UP TIME

FORCED LUBRICATION

SOFT ROTORS ( soft & rigid )

REQUIRES LOT OF CARE EVEN DURING SHUT DOWN

PERFORMANCE DEPENDS ON PURITY OF STEAM AND COOLANT, AND ALSO OF THE LUB

LOT OF MAINTENANCE WORRIES EVEN AFTER ERRECTION


$ SLOW SPEEDSLOW SPEED

$ $ MODERATE TEMPERATURE MODERATE TEMPERATURE

$ $ QUICK TO START / STOPQUICK TO START / STOP

$ $ MOSTLY SELF LUBRICATEDMOSTLY SELF LUBRICATED

$ $ SUITABLE FOR GENERATOR / SUITABLE FOR GENERATOR / SYCHRONOUS CONDENSER SYCHRONOUS CONDENSER OPERATIONSOPERATIONS $ $ ONCE ERRECTED REQUIRES ONCE ERRECTED REQUIRES LITTLE MAINTENANCELITTLE MAINTENANCE

DYNAMIC BALANCING

OF

TURBO MACHINES

HRD -TTI -TNEB

HIGH SPEEDHIGH SPEED

COMBINATION OF HIGH AND MODERATE TEMP. / PRES.COMBINATION OF HIGH AND MODERATE TEMP. / PRES.

LONG START UP TIME :: FORCED LUBRICATIONLONG START UP TIME :: FORCED LUBRICATION

SOFT ROTORS ( soft & rigid )SOFT ROTORS ( soft & rigid )

REQUIRES LOT OF CARE EVEN DURING SHUT DOWNREQUIRES LOT OF CARE EVEN DURING SHUT DOWN

PERFORMANCE DEPENDS ON PURITY OF STEAM ANDPERFORMANCE DEPENDS ON PURITY OF STEAM AND COOLANT, AND LUBCOOLANT, AND LUB

LOT OF MAINTENANCE WORRIES EVEN AFTER ERRECTIONLOT OF MAINTENANCE WORRIES EVEN AFTER ERRECTION

TURBINE TURBINE - GENERATOR- GENERATOR


TURBINE

T – G BLOCK DIAGRAMT – G BLOCK DIAGRAM

5405400 0 C, 135 C, 135 ATAATA

54054000C, 40 ATAC, 40 ATA

Vacuum, 100Vacuum, 10000CC

statorstator

RotorRotorHeavy shell,

Fixed impellers/blades,

Slow expansion

Hollow shaft,

moving blades / impellers,

quick expansion,

affected by flow and temperatureHPHP--MPMP--LPLP

EXPANSION EXPANSION ??

TURBINETURBINEBearings out-of-operational areaBearings out-of-operational area

hpf hpr

Mpr || lpf

lpr

HP IPLP

JB

JB AND AXIAL THRUST

JB

JB

300 bar /600°C for the HP turbine

The LP turbine consists of adouble flow with a horizontal split casing.The typical steam conditions are up to 7 bar and 350°C. The steam will expand to thecondenser at condenser pressures in therange of 30 - 100 mbar. Because of the high volumetric flow rates special care has to be attributed to the appropriate choice of LP turbine exhaust area and design.The development of optimal last stage blade families and long last stage blades is the key for reasonable exhaust areas, i.e. reduced exhaust losses.

GeneratorGeneratorBearing in operational areaBearing in operational area

• Moderate temp. and just hydrogen pressure, added with electromagnetic and bearing - shaft interaction forces

gdegnde

generator

Assembly of turbine rotorsAssembly of turbine rotors

• No !

• They are not done as below, because….

HP IP LP

~gEN


• Catenary : a bow : bearing level , slope

||

DatumDatum

HP IP LP G




• THEY ACHIEVE LEVEL PLAYING WHEN ON NORMAL RUN AS THE THERMAL GROWTH ARE ACCOMPLISHED

• When the machine achieves its normal speed and parameters

HP IP LP g

Turbo supervisionTurbo supervision

HP – to measureHP – to measure

• Thermal growth of

• stator ( casing or shell ) - overall

• rotor ( shaft ) - differntial

• Oscillation of rotor in the bearing w.r.t housing ( relative motion of the shaft in relation to the stator )

• Overall vibration of bearing measured on the housing

IP- to measureIP- to measure

• Thermal growth of

• stator

• rotor

• Oscillation of rotor in the bearing w.r.t housing ( relative motion of the shaft in relation to the stator )

• Overall vibration of bearing measured on the housing

LP- to measureLP- to measure

• Thermal growth of Rotor

• Oscillations of the rotor w.r.t stator

• Vibration of the bearing

• The LP cylinder sits literally on the spring foot condenser. Hence it will be forced to float. The casing bolts are not set to full tight

Generator – to measureGenerator – to measure

• Only vibration of the bearings

• The heavies part of the T-G , which is solidly placed on the basic structure.

• Hardly subjected to any thermal growth of its own

• In many cases bearings are located on its shell or body

Common Common datadata

• The axial shift – the axial movement of the shaft due to forces of electrical, mechanical, hydraulic and structural interaction when the machine attains full dynamic status and also in-service

• ( salt formation in HP area is diagnosed by slightly higher curtis pressure than normal, and increase in axial shift )

• Rotational speed ( RPM )

The salient parametersThe salient parameters

• Overall expansion ( casing )• Differential expansion ( rotor )• Axial shift ( rotor movement on the axis of the entire

machine )• Eccentricity ( oscillation of the rotor with respect to

stationary body, ie., shaft axis with respect to machine or neutral axis )

• Speed ( especially to cross over critical speeds of the Gen., LP, IP, AND HP VERY CAUTIOUSLY – the cross over should be gradual and quick , as stalling at or near about critical will create resonance forcing the operator to stop and restart the machine )

TURBO - GEN

• STARTUP : process, expansion, eccentricity, axial shift, vibration ( at critical N )

• OPERATION : process, vibration ( monitoring : continuous and periodical ) VIBRATION ANALYSIS : on alarm and on demand


Stator ExpansionStator Expansion

In Turbine Supervisory Instrumentation (TSI), case expansion is an important measurement.

Case expansion (or shell expansion) is the growth of the machine shell with increase of temperature during machine startup and on-line operations.



The LVDT is mounted to the foundation at the opposite end from where the turbine casing is attached.

The LVDT provides information on the change of position of the point measured relative to the foundation.


• This is primarily a startup parameter allowing the machine casing and rotor growth to increase at a proportional rate. Thermal growth

• As different growth rates can cause internal rubbing between rotating and stationary parts of the machine.


• Case expansion should be measured by a pair of LVDT as this provides information on the position of both of the sliding feet on the machine case.

• This allows for a comparison of readings preventing damage should one foot become obstructed or jammed.

• Case expansion measurements also allow determination of whether expected thermal growth differentials are being exceeded on the machine.

Stator ExpansionStator Expansion(overall expansion)(overall expansion)

• Normally referred to as overall expansion

HP STATOR

lvdt

Linear Variable Differential Transformer

LVDT

Features:• 2" Range• Stainless Steel Housing• NEMA 4X• 1-6 VDC Output• 2.5 mV/mil

LVDT

The growth watchThe growth watch

• The turbine will be rolled initially to 500 RPM to warm the cylinders when starting from cold condition

• Then the speed is maintained at about 1000 RPM so that more steam can enter the shell allow it to develop thermal expansion. At the same time rotor also achieves some growth

Moving onMoving on

• Until the turbine stator attains the required growth the speed will not be increased further

• This type baking the machine in steam is referred to as SOAKING

Rotor DifferentialRotor Differential

• Using LVDT

Shaft disc

lvdtlvdt

Bearing

housing

Eddy probe

• Proximity or Non-contact pickup

Eddy ProbeEddy Probe

Eddy Probe MountingEddy Probe Mountingtoto

measure rotor differentialmeasure rotor differentialEddy probe

Air gap

Eddy Probe to measureEddy Probe to measureEccentricityEccentricity

• To measure rotor oscillations or vibrations of the shaft with respect to bearing housing ie., w.r.t stationary part

• Referred generally as ECCENTRICITY

Eddy Probe mountingEddy Probe mounting to measure eccentricityeccentricity

Probe

Shaft center line

Machine axis

Bearing housing

Eddy Probe for SpeedEddy Probe for Speed

Teeth wheel

probe

Tacho genShaft end

Teeth pulses

Typical probe in serviceTypical probe in service

Dual probe setupDual probe setup for orbit analysis to examine alignmentfor orbit analysis to examine alignment

Typical probe set up Typical probe set up recommended by western recommended by western

manufcturersmanufcturers

Bearing Overall VibrationVelocity Pickup

Mounting of Vibration pickups• On all turbine bearings as per

recommendations

H

v

a

Recommended by TNEBRecommended by TNEB

• Pickup locations : _|_ _|_ , and , and || || to axis to axis

H

V

SHAFT

Bearing

BEARING HOUSE

A

The circuit block

Continuous monitoring

DCS ON DATA MODEDCS ON DATA MODE

DCS on TIME MODE

TREND CHARTHourly, Weekly, MonthlyHourly, Weekly, Monthly

Variation of one parameterone parameter w.r.t anotheranother

RESONANCERESONANCE

NATURAL FREQUENCYNATURAL FREQUENCY

CRITICAL SPEEDCRITICAL SPEED

RESONANCERESONANCE

RESONANCERESONANCE

While designing the machine every care is taken to keep the natural frequency of the rotating (dynamic) and stationary (static) parts including that of civil structures very well above or well below the operating speed

RESONANCERESONANCE

Critical speed any rotating part of T – G will be crossed on its way from zero to rated RPM

While crossing critical speed of any part of the machine that particular component will experience high vibration

RESONANCERESONANCE

• If resonance sets in due to any reason, then the only recourse is to trip the machine and restart it

• If the machine continuous to stay at critical speed for long, the rotor will be subjected to severe fatigue due to abnormally high continuously raising vibration, which may result in damage to the rotors and its component parts

It is a must that the critical speed should It is a must that the critical speed should

be crossed smoothly and quicklybe crossed smoothly and quickly

RESONANCERESONANCE

• The natural frequency of bearing housing and structures are not identified openly as is done for critical speed

• If due to any external excitation like grid disturbance or sudden abnormality in the process, the area whose natural frequency coinciding with the excitation frequency will start resonating

RESONANCERESONANCE

• Then the only way out is to reduce or increase load to change stress

• When that fails the one and only option available is to stop the machine and restart it from rest

• If the high vibration vanishes on restarting the cause is resonance

RESONANCERESONANCE

• Analysis will reveal only 1XRPM component which may be due to unbalance or eccentric alignment of rotor

• Only the gradual increase of the vibration to abnormal levels in spite of our effort is the clear indication of the presence of resonance

Identifying critical speedsIdentifying critical speeds

• From the front pedestal legends :

Typical ::

Gen 1200 rpm

LP – IP 1700 – 1900 rpm

HP 2350 rpm

Identifying Critical speedby the rise in vibration at specific speed

Critical speedsCritical speedswhen more than one occurswhen more than one occurs

Identifying critical speedIdentifying critical speed

by rise in vibration of individual rotors

SPEED

AM

PLIT

UD

E

1 2 3

( x 1000 rpm )

Gen. LP IP HP

What happens when the machine What happens when the machine crosses over critical ?crosses over critical ?

• The SHAFTSHAFT under goes some bending movements

• This is christened as MODE SHAPESMODE SHAPES

• At each critical the SHAFT AXIS or MACHINE AXIS undergoes several modes

• Care is taken at the time of design to accommodate this modes by adopting necessary bearing clearances, and spacing between the stator – rotor, not forgetting alignment

Mode ShapesMode Shapes

Instruments to monitor vibration

H

A

v

Portable Portable vibrationvibration monitormonitor

AnalyserAnalyser

Periodic MonitoringPeriodic Monitoring / Alarm investigationAlarm investigation

• Machine : UNIT IV T-GT-G Load : 214 MW214 MW

• d / v d / v H H V V AA

• Beag. 5 ( LPR) 25/4 18/3 59/1059/10

Continuous monitoring Continuous monitoring Y Y N

No alarm from DCS - yet scheduled vibration measurement revealed high Axial vibrationhigh Axial vibration indicating development of a fault requiring detailed investigation and diagnosis

Alarm from DCSAlarm from DCS

• On instrumentation front :• Verification of the data by portable meter• Checking the cable, connections, and

pickup fastening

• On condition monitoring side :• If confirmed that high vibration is

present do investigate to diagnose

the fault/defect

InvestigationInvestigation

• Signature analysis Signature analysis

• Phase analysisPhase analysis• Coast / run-down time signature (to study

critical speed behavior, compatibility to the last recorded plot )

• Stethescoping ( scanning the vibration on the bearing house and supports inclusive of the civil structure )

Vibration signaturesVibration signaturesFrequency Frequency vs vs amplitudeamplitude

In service Time waveIn service Time waveBode / coast down plotBode / coast down plot

Some cluesSome clues

PhasePhase ?- the direction of forces of individual parts

• Talks about bearing – journal misalignment, and coupling misalignment between any two consecutive stages

OIL WHIRLOIL WHIRL---- due to insufficient bearing clearancedue to insufficient bearing clearance

1XRPM 2XRPM

1 / 2 X RPM

shaft

bearing

Oil clearance

MISALIGNMENT – PHASE MEASUREMENTMISALIGNMENT – PHASE MEASUREMENT • HIGH AXIAL VIBRATION• UNEVEN PHASE DIFFERENCE BETWEEN TWO BEARINGS

ACROSS ANY COUPLING• 360 /12 = 30 8X30 = 240 2X30 = 60• 240 – 60 = 180 OFF-SET MISALIGNMENT• ie., the centre lines of A and B are not in line

• 900 for angular and in-between for other positions

8

8

8

82

2 22

a b

Misalignment - orbitMisalignment - orbit

Dual input FFT

Dual trace oscilloscope

rubbingrubbing

Casing DistortionCasing Distortion

• When casing was prevented from free expansion

• Uneven heating in casing

• Rotor – stator clearances are uneven

• Unit III at ETPS : DUE TO TIGHTENING OF CASING BOLT IN EXCESS –

• The HP rotor got bent near curtis – confirmed by shaft run out test at works

Casing DistortionCasing Distortion

• Confirmed by the vibration data obtained on casing of HP on all corners and sides

• Overall expansion could not detect it

• Added evidence that the speed could not be taken beyond 2790 RPM

• Observation of the casing noise pattern

Loose Discs on rotorLoose Discs on rotor

• Vibration changing abnormally even when machine is operating on constant load

• Abnormal noise occasionally

• No change in phase – no misalignment detected

in a day

Loose discs on rotorLoose discs on rotor

• During overhaul the LP rotor root gap between discs were found to be “ nil ” for one and in “ excess ” for other

• Rotor was taken to works and checked and found the run out with discs and without discs were found to be much beyond permitted levels

Loose discs on rotorLoose discs on rotor

• Analysis confirmation of rotor trueness difficiency and moving of discs while in service were based on the large variation of vibration levels over a day ( trend ) and the noise pattern indicating the changes in blade excitation due to shifting of discs

Interference in BearingInterference in BearingUnit I at ETPSUnit I at ETPS

• Machine could not be run up to rated speed of 3000 RPM on commissioning just after overhaul

• The critical speed of generator was down to about 950 RPM instead of the design speed of 1200 RPM

• Shows mass or stress changed the machine generator rotor resonance

• Since mass could not be changed drastically to create such a situation only stress need to be suspected

Interference in BearingInterference in BearingUnit I at ETPSUnit I at ETPS

• Discussion with the maintenance revealed the presence of interference in GDE bearing due to catenary restrictions

• Suggested the reduction interference to zero or better bringing some clearance in the bearing setup

• Interference reduced to possible extent and machine was on bars

• The critical speed was found to be as per design after bearing correction

Journal – Bearing misalignmentJournal – Bearing misalignmentUnit II at ETPSUnit II at ETPS

• Uneven bearing clearance

• Poor journal – bearing blue matching ( the contact area should be upwards of 90 % )

• Machine was stopped and checked

gnde

generator

24 / 4 ( 17 / 3 ) 67 / 7 . 5 ( 19 /

4. .3 )

Rotor thermal instabilityRotor thermal instabilityUnit V at TTPSUnit V at TTPS

• The machine vibration in turbine were normal, but the generator vibration varied with time just after synchroning from mere 12 / 3 to 77 / 12 ( as observed during one of the several trials at GNDE in H )

• m/s BHEL suggested balancing and after umpteen attempts started blaming the machine structure

• TNEB did its own study and concluded that the rotor is thermally unstable and is beyond any possible repair based on previous experience on rotors

Rotor thermal instabilityRotor thermal instabilityUnit V at TTPSUnit V at TTPS

• No structural abnormality could be detected after conducting extensive study of the pillars, bearing pedestal fastening and other possible areas including causes for suspected resonance

• The rotor was replaced thereafter and unfortunately that was placed in UNIT II at NCTPS and experience was the same as encountered at TTPS.

Machine could not be loaded Machine could not be loaded beyond certain levelbeyond certain level

• Unit III at ETPS could not be loaded beyond 65 MW after R & M

• Detailed vibration analysis, especially the phase analysis revealed that the catenary was deficient


• The IP rotor bearing was below its correct level, causing severe LP – IP coupling misalignment causing excessive loading of IP bearing revealed by high vibration in IP

• Due to this the machine could not be loaded beyond certain level, in this case it is 65 MW

• It was encountered every time the machine was subjected to load beyond 65 MW


• The same could occur if the journal bearing alignment and clearances are not adequate

• The effort of M/S BHEL to correct this by balancing was in vain, only resulted in further straining and causing damage to the machine

• Never attempt to repair a running Never attempt to repair a running machine if it is performing absolutelymachine if it is performing absolutely

• When attempted, please note down the existing alignment and clearance values at the time of opening the machine

• If no change in components and modifications are in schedule, set the old levels before closing

No overhaul, pleaseVote for condition based maintenanceVote for condition based maintenance

Please care for them, as Please care for them, as they are like childrenthey are like children

Thank you,Thank you,Wishing you all the Wishing you all the

very bestvery best

Documents

Turbo Supervision