Field Balancing of Critical Rotating Machines

1
BALANCING OF CRITICAL
ROTATING MACHINERY
PRAVEEN MANOCHA
MACHINERY DIAGNOSTICS ENGINEER
manochap66@gmail.com
Phone: (44) 7423-659794

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Introduction
 I have started the presentation with
fundamentals of vibration and tools (plots)
used in machinery diagnostics.
 Differentiate unbalance from other causes of
high vibrations with case studies.
 Theory behind balancing with my experiences
in balancing of rotating machinery with case
studies.

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FUNDAMENTALS OF
VIBRATION
 Mechanical vibration is the dynamic motion of
machine components.
 Vibration measurement is the measurement
of this mechanical vibration relative to a
known reference, viz. machine pedestal or
ground.
 The four transducer systems to measure
dynamic motion machine components are:
Proximity Transducers, Velocity Transducers
Accelerometer & Velomitor Transducers

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FUNDAMENTALS OF
VIBRATION
 What we measure
 Vibration & Position
 Rotor Speed
 Bearing Temperature
 Process Condition and its effect on
machines

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FUNDAMENTALS OF
VIBRATION
 Rotor Vibration Frequency ranges – 1/4 X to
3X shaft rotative speed (Machine rotative
speed varies – 1200 to 3600 RPM)
 These Machines Generate Rotor related
Vibration Signal ranging from 10 - 200Hz
(600-12000RPM).
 Gear-Box and Rolling Element Bearings
generate higher frequency components that
requires high frequency measuring
transducers (Accelerometers).

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FUNDAMENTALS OF
VIBRATION-INVESTIGATION
 Know the machine that is investigated –
Impulse/Reaction Turbine, Steam admission
(partial/full arc), bolted /welded Gas Turbine design,
Bearing type, Coupling type, etc.
 Every machinery malfunction has an exciting force or
forcing component – Find the excitation force (root-
cause analysis – RCA).
 Know the history of the machine to be analyzed –
previous failures.
 Request for machine build-up information – alignment
figures, bearing clearances etc.

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FUNDAMENTALS OF
VIBRATION-INVESTIGATION
 Every machine has its own signature, use both
vibration and process data to determine it.
 Believe in what the vibration data tells you.
 4 Ws – essential tool for root-cause analysis
 Validate your findings/diagnostics.
 Visual inspection of the machine – essential tool to
look for abnormalities. Look-out for a changes that
might influence performance – oil leaks, structural
change, heat etc.
 Continuously analyze the performance of the machine.

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DATA PLOTS
 Timebase Plot
 Orbit
 Average Shaft Centerline
 Polar
 Bode
 APHT
 Half and Full Spectrum
 Trend

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DATA PLOTS
 Machinery Diagnostics plots can be divided
into two categories:
1. Steady-state data analysis – when
the machine is on load and there is no
change in speed. Vibration data is
collected with-respect-to time.
2. Transient data analysis – Vibration
data captured on the machine during Run-
Up, Run-Down and Over-Speed test.
Vibration data is collected with-respect-to
speed and Time.

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DATA PLOTS
 Collection/display of vibration data is also
categorized into Synchronous and
Asynchronous format. Most vibration
diagnostic equipment collects/display data
plots in both formats.
1. Synchronous plots are collected/
displayed with reference to the machine
speed (keyphasor).
2. Asynchronous plots are collected/
displayed without speed reference.

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DATA PLOTS- INFORMATION AVAILABLE
FROM STEADY-STATE PLOTS
 Machine behavior on steady state with Amplitude &
Phase information in Trend format.
 Orbit plots - direction of shaft precession, bearing
loading (pre-load), Rub condition, oil instability etc.
 Spectrum plots – major vibration vector & frequency
is used to determine machine malfunction.
 Waterfall plots – vibration spectra is displayed over a
period to determine periodic machine malfunction.
 Acceptance region (Polar) Plots – change in machine
condition caused by machine malfunction

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DATA PLOTS- INFORMATION
AVAILABLE FROM TRANSIENT PLOTS
 Slow Roll Speed / Slow Roll Run-out Vector
 Amplitude, Phase, and Frequency of Res.
 Synchronous Amplification Factor
 High / Heavy Spot Relationship
 Structural and Split Resonance
 Rotor Mode Shape / Deflection Shape
 Preload Identification
 Frequency Relationships

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UNBALANCE
 Static unbalance
 Dynamic unbalance
 Rotor bow
 Coupling problems
 Run-out
 Loose Part or Debris

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UNBALANCE
 Unbalance is “offset” between geometrical
centerline and mass centerline of a Rotor
System.
 Unbalance can also be described as vector
resultant of unequal distribution of cast
material and off-center machining of bore.
 Primary symptom of Unbalance is an
increased 1X vibration levels on a rotor
system.
 Vibration = Force / Dynamic Stiffness

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UNBALANCE
Reasons for mechanical unbalance on a
running machine
 Loss of mass from rotor system (gradual or
sudden)
 Thermal bow – caused by a “rubbing”
between stationary and rotating components
of rotor system.
 In both cases change in “slow roll” vectors
(run-out) and 1X vibration levels through
rotor’s balance resonance is recorded.

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STATIC UNBALANCE
 Static unbalance the balance condition
where the principal inertial axis is offset from
the rotor rotational axis by a parallel
distance ‘r ’.
 This locates the Center of Gravity (CG) away
from the axis of the rotation resulting in an
unbalance force.

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STATIC UNBALANCE
 Static unbalance can be corrected in either
of two methods.
1. In the first method, a correction weight can
be located directly 180° opposite the
unbalance, (CG) and at the appropriate
radius from the rotor centerline to equal the
unbalance.
2. In the second method, the weight is divided
and distributed to each end of the rotor. The
location of the weights must remain in line
with the weight location of the first method,
i.e. 180 ° opposite the CG of the rotor.

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COUPLE UNBALANCE
 Couple unbalance is a condition where the mass
centerline intersects the rotor rotational axis and this
intersection is also the CG.
 A rotor with couple unbalance will have no static
unbalance (the CG lies on the rotor rotational axis),
but, if rotated, the opposing forces produce vibration
in the bearings.
 Couple unbalance must be addressed by making
weight corrections in the same axial plane of the
unbalance at each end of the rotor, i.e. 180°apart.

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DYNAMIC UNBALANCE
 Dynamic Unbalance is the balance condition where
the mass centerline (principal inertial axis) does not
intersect either the rotor rotational axis or the CG of
the rotor.
 This condition is a combination of static and couple
unbalance wherein the static component lies in an
axial plane different from one of the couple
unbalance forces. This type of unbalance is common.
 Like couple unbalance, dynamic unbalance must also
be addressed by making weight corrections in a
minimum of two planes.

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UNBALANCE
Other malfunctions that appear like
unbalance:
 Run-out – mechanical or electrical
 Rotor bow (thermal)
 Coupling problems
 Shaft-crack
 Loose components
 Rubs
 Misalignment

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CASE 1-Unbalance caused by blade loss on a LP-rotorCASE 1-Unbalance caused by blade loss on a LP-rotor
• While operating at constant load, two (2) step changes in shaft displacement level were
displayed across the LP Turbine in the trend plot with a very directional vector movement
displayed on the Polar plot.
• Subsequent to this step change in levels, shaft displacement levels across the LP-turbine
displayed significant change with load.
• Unit was operated on constant load prior run-down for inspection.
Two Step changes in vibrationTwo Step changes in vibration

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•Significant increase
in shaft displacement
levels recorded
through LP rotor’s 1st
balance resonance
during unit’s Run-
Down after blade loss.
•Change in “slow roll”
vectors also observed
during the Run-Down.
•Steady-state and
transient vibration
behavior of the LP-
Turbine suggested a
blade loss close to the
center of the LP
Turbine.
•Inspection of theInspection of the
LP-Turbine revealedLP-Turbine revealed
loss of two bladesloss of two blades
on LP rotor Stage 1on LP rotor Stage 1
(Generator side).(Generator side).
RUN-DOWN PRIOR BLADE LOSSRUN-DOWN PRIOR BLADE LOSS RUN-DOWN AFTER BLADE LOSSRUN-DOWN AFTER BLADE LOSS
CASE 1-Unbalance caused by blade loss on a LP-rotorCASE 1-Unbalance caused by blade loss on a LP-rotor

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•The steam Turbine generator is a single
bearing per shaft design where the HP/IP
and LP-Turbine Rotors are each supported
on a single bearing.
•The Steam turbine operates with the HP-
turbine bypass during the run-up, i.e. the IP-
turbine drives the unit train to its operating
speed.
•Significant increase in shaft displacement
levels was recorded across the HP-Turbine
at operating speed resulting in vibration
protection trip.
•.Significant increase in 1X Vibration levels
was also seen during the unit’s Run-Down
with increased “slow roll vectors” suggesting
thermally bowed HP rotor caused by a
severe “rub condition”.
CASE 2-Unbalance caused by severe rotor rubbingCASE 2-Unbalance caused by severe rotor rubbing

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•Investigation of process parameters during
run-up revealed fluctuating ICVs of the IP-
turbine during the run-up as the unit
reached its operating speed.
•This vibration behavior had been observed
in the past and was corrected by re-tuning
the IP-turbine’s ICV controls.
•Unit was Run-Up after approximately 6
hours of barring with slightly higher 1X
vibration levels across the HP-turbine at
speed suggesting that the thermal bow
caused by the “rub condition” had not
completely cleared.
•The shaft displacement levels reduced on
load as the unit soaked and the thermal
bend cleared.
EXCITATION FORCE – Fluctuating ICVs and
thermal distortion on Pedestal 2 caused severe
rub on the HP rotor resulting in a thermal
induced bow.
CASE 2-Unbalance caused by severe rotor rubbingCASE 2-Unbalance caused by severe rotor rubbing

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•On the Steam-turbine train, pedestal
no 2 (between HP/IP turbine) is a
sliding pedestal with self aligning
spherical bearing that is free to move
and align with unit’s load change.
•This pedestal requires regular
greasing for movement.
• During unit’s load change, if the
pedestal gets “stuck” resulting in
distortion of the spherical bearing.
•The HP-Turbine rotor moves to a
position within bearing clearance that
is more susceptible to Fluid induced
instabilities and light rubs on the
steam labyrinth resulting in a thermal
bow and increase in over-all vibration
levels.
CASE 3 – Change in balance response due toCASE 3 – Change in balance response due to
distortion of Sliding Pedestaldistortion of Sliding Pedestal

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• Note the increase in 1X vibration
amplitude and change in phase.
•Change in 1X amplitude and phase
can be caused by
•Rub
•Rotor moving to a different
position within bearing
clearance that is normally seen
on this bearing as change in
wedge pressure and bearing
temperature.
•The Unit had to run-down and
subsequently run to resolve this issue
with regular greasing of the sliding
pedestal.
EXCITATION FORCE – Thermal distortion of Pedestal 2 causes the HP/IP
rotors to move to a different position within bearing clearance.
CASE 3 – Change in balance response due toCASE 3 – Change in balance response due to
distortion of Sliding Pedestaldistortion of Sliding Pedestal

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CASE 4 - MISALIGNMENT CAUSED BY SWASH(GAP)CASE 4 - MISALIGNMENT CAUSED BY SWASH(GAP)
AND CONCENTRICITY ERROR on a 520MW AlstomAND CONCENTRICITY ERROR on a 520MW Alstom
STGSTG
•The Exciter rotor is coupled with the
Generator rotor through a stub-shaft.
•The Generator/ Exciter coupling was
assembled with a significant gap
(swash) error and concentricity error
between the Exciter rotor and
intermediate stub-shaft.
•Although the Exciter rotor was high
speed balanced, excessive vibration
levels were witnessed during run-up
through the 1st
balance resonance and
the unit was manually tripped.
EXCITATION FORCE – significant swash (gap) error on coupling and concentricity
error

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CASE 14 - MISALIGNMENT CAUSED BYCASE 14 - MISALIGNMENT CAUSED BY
SWASH(GAP) AND CONCENTRICITY ERRORSWASH(GAP) AND CONCENTRICITY ERROR
on a 520MW Alstom STGon a 520MW Alstom STG
•The Exciter rotor was rebuild with a
reduced swash (gap) error and lower
concentricity error between exciter
rotor and stub-shaft at RWE workshop
since the equipment supplier could not
achieve the desired concentricity
figures.
•The Exciter rotor was re-balanced
prior to assembly on site.
• Excessive vibration levels were
witnessed during return-to service were
further reduced with in-situ balancing.
error

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BASIC BALANCING
 Unbalance and rotor responses
 Balancing Methodology
 Selecting the calibration weight
 Weight Splitting
 Influence Vector

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BASIC BALANCING
 Balancing is the process of adding forces to
the rotor to “offset” the current unbalance
distribution forces and other 1X vibration
forces in the rotor system.
Balancing is Categorized into
 Single plane balancing
 Multiple plane balancing

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BASIC BALANCING
-PROCESS
 Evaluate machine condition
1. Collecting transient data
2. Recognizing resonances
3. Consider rotor mode shapes
 Understanding machine response
1. Linearity in machine response
2. Repeatability in machine response
3. Repeatability in “slow roll” vectors

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BASIC BALANCING
-PROCESS
 Decision making
1. Compensating for Run-Out (slow roll)
2. Applying 10% rule for Trial weight
3. Verify the response of trial weight and if
possible do a repeat run.
4. Apply Cal 1 and verify the response of Cal 1
before attempting further balancing and if
response is non-linear abort balancing.

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BALANCING
Unbalance results in increase in 1X vibration
levels. Other machinery malfunctions that
can increase 1X vibration levels
1. Thermal Bow
2. Rubs
3. Run-out
4. Shaft crack
5. Broken blades/ vanes
6. Internal / external misalignment.

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BASIC BALANCING –
INFLUENCE VECTORS
 If the rotor system responds linear to the installed
Cal 1/ Cal 2 weights (balancing), Influence vector
can be derived for the rotor system and can be used
for future balancing.
 Influence vectors determines how a balance weight
can influence the vibration response of a machine.
 However, influence vectors should be used with
following caution:
1. Same operating speed.
2. Same load
3. Same process conditions.

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BALANCING – WEIGHT
SPLITTING
When a balance solution requires the
balance weight to be positioned
1. Between balance holes
2. Holes that are full
3. Require more weight that will fit in one hole
Weight splitting can be carried out that
combines the effect of two weights, mounted
on available holes, that will have the same
effect of the balancing solution.

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• Arnot Power station has single bearingArnot Power station has single bearing
per shaft design where the HP/IP andper shaft design where the HP/IP and
LP-Turbine Rotors are each supportedLP-Turbine Rotors are each supported
on a single bearing.on a single bearing.
• Bearing 2 is self-aligning sphericalBearing 2 is self-aligning spherical
bearing, 2 supports the HP/IP rotorsbearing, 2 supports the HP/IP rotors
• Massive two-dual stage LP Turbine rotorMassive two-dual stage LP Turbine rotor
(68 ton) is supported on the LP front(68 ton) is supported on the LP front
bearing (3) and self aligning Generatorbearing (3) and self aligning Generator
front bearing (5b).front bearing (5b).
• Substantial difference in vibration levelsSubstantial difference in vibration levels
is recorded through LP-Turbine 1is recorded through LP-Turbine 1stst
balance resonance during Run-Up andbalance resonance during Run-Up and
Run-Down especially after run-downRun-Down especially after run-down
down from full load.down from full load.
• The station requested to balance theThe station requested to balance the
LP-Turbine in order to reduce vibrationLP-Turbine in order to reduce vibration
levels through its 1levels through its 1stst
balance resonance.balance resonance.
Preliminary Root Cause:Preliminary Root Cause:
1.1. Alignment /Concentricity errorsAlignment /Concentricity errors
between LP-turbine/Generator.between LP-turbine/Generator.
2.2. Thermal condition of theThermal condition of the
Generator.Generator.
3.3. Thermal distortion of pedestal 2Thermal distortion of pedestal 2
restricting “freedom ofrestricting “freedom of
movement /sliding” causingmovement /sliding” causing
misalignment of rotors.misalignment of rotors.
CASE 3- Balancing of LP-Turbine’s 1st
balance resonance

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1X Vibration Vectors through
LP-Turbine 1st
Balance
Resonance during 1st
Run-Up on
29th
Dec 2010
TRIAL WEIGHT - 1X Vibration
Vectors through LP-Turbine 1st
Balance Resonance during 1st
Run-
Up on 29th
Dec 2010
CAL1 - 1X Vibration Vectors
through LP-Turbine 1st
Balance
Resonance during 1st
Run-Up on
29th
Dec 2010
Balancing weight Trial Run
Plane 2 - 280grams@Slot7 (285’)
LOCATION 1X Phase 1X Phase 1X Phase
BRG 3 – Y-Probe 206 194 140 217 102 204
BRG 5a – Y-Probe 999 854 733 275 541 277
BRG 5b – Y-Probe 254 197 168 261 116 268
balance resonance
• Adding trial weight in the centre of LP Turbine reduced vibration levels through the 1Adding trial weight in the centre of LP Turbine reduced vibration levels through the 1stst
balance resonance of LP Turbine.balance resonance of LP Turbine.
• With past experience, TSS has observed that adding additional weight on Plane 2 andWith past experience, TSS has observed that adding additional weight on Plane 2 and
3 does not give the desired results3 does not give the desired results
• Additional balance weight was added on plane 4 that reduced vibration levels throughAdditional balance weight was added on plane 4 that reduced vibration levels through
the 1st balance resonance of LP Turbine to acceptable levels.the 1st balance resonance of LP Turbine to acceptable levels.
Note: There is no physical bearing on 5a, vibration probes are installed to measureNote: There is no physical bearing on 5a, vibration probes are installed to measure
the rotor deflection shape during transients and used as guidelines tothe rotor deflection shape during transients and used as guidelines to
determine internal rubs.determine internal rubs.

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Balance weight added on plane 2, 3 & 4Balance weight added on plane 2, 3 & 4
reduced vibration levels on across the LP-reduced vibration levels on across the LP-
turbine during transient events through theturbine during transient events through the
11stst
balance resonance to acceptable levels.balance resonance to acceptable levels.
A significant improvement in vibrationA significant improvement in vibration
levels was also observed on Exciter rearlevels was also observed on Exciter rear
bearing (7) at steady state, levels reducedbearing (7) at steady state, levels reduced
from 180 um to 40 um pk-pk. Severalfrom 180 um to 40 um pk-pk. Several
unsuccessful balancing attempts wereunsuccessful balancing attempts were
carried out in the past when the balancecarried out in the past when the balance
weight was added to slip-rings, refer toweight was added to slip-rings, refer to
Case 4.Case 4.
balance resonance

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•Arnot Power station, Unit 2 had a capacityArnot Power station, Unit 2 had a capacity
increase with a new Toshiba Generator,increase with a new Toshiba Generator,
commissioned in March 2005 and Turbinecommissioned in March 2005 and Turbine
upgrade in May 2008.upgrade in May 2008.
•Since the Turbine upgrade, a substantialSince the Turbine upgrade, a substantial
increase in vibration levels had beenincrease in vibration levels had been
recorded through LP-Turbine’s 1recorded through LP-Turbine’s 1stst
balancebalance
resonance during transient event.resonance during transient event.
•A significant difference was observedA significant difference was observed
during “COLD” and “HOT” transient events.during “COLD” and “HOT” transient events.
•Also, an increase in Vibration on bearing 6Also, an increase in Vibration on bearing 6
& 7 was observed after the Turbine& 7 was observed after the Turbine
upgrade when load was increased aboveupgrade when load was increased above
350 MW.350 MW.
•Station requested to trim balance theStation requested to trim balance the
Exciter to reduce vibration levels at speed.Exciter to reduce vibration levels at speed.
Preliminary Root Cause:Preliminary Root Cause:
1.1. Thermal condition of theThermal condition of the
Generator.Generator.
2.2. Bearing no. 7 (pedestal) notBearing no. 7 (pedestal) not
sufficiently loaded.sufficiently loaded.
CASE 4- Balancing of an Exciter on a 400MW STG

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LOCATION 1X AMP & PHASE AT 3000
RPM
Unexcited on 10 Sep 08.
1X AMP & PHASE AT 3000
RPM
Unexcited on 14 Nov 08.
1X AMP & PHASE AT 3000
RPM
at approx. 40 MW Load
Balance
weight
225 grams @135 degree 225 grams @135 degree
Probe - 6X 60 @ 296 48 @ 277 72 @ 306
Probe -6Y 36 @ 182 26 @ 45 23 @ 266
Probe - 7X 155 @ 299 46 @ 312 108 @ 350
Probe - 7Y 108 @ 189 28 @ 57 27 @ 306
Bearing 7 Orbit plot before the unit’s run-
down on 9th
Nov before adding the balance
weight – suggested bearing is not loaded
On 16 Nov after adding the balance weight
orbit suggested a slightly loaded bearing at
lower loads.
These levels however, deteriorated with time.
The spread of the balance weight was ~ 90
degrees and the OEM (Toshiba) did not allow
the use of heavy metal weights or increase
pre-load on bearing 7

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The in vibration level
started to increase
once the unit’s speed
reached 3000 RPM
from 45 to 89 μm Pk-
Pk [1X]
Balance weight added on the Stub-shaft reducedBalance weight added on the Stub-shaft reduced
vibration levels on bearing 7 as the unit reached itsvibration levels on bearing 7 as the unit reached its
rated speed. However, an increase in vibrationrated speed. However, an increase in vibration
levels was observed at the unit’s rated speed.levels was observed at the unit’s rated speed.
The improvement in balancing could not beThe improvement in balancing could not be
achieved as increasing the weights on theachieved as increasing the weights on the
balancing plane of Stub-shaft increased the spreadbalancing plane of Stub-shaft increased the spread
and the OEM did not allow to use heavy metaland the OEM did not allow to use heavy metal
weights on the Stub shaft.weights on the Stub shaft.

43
CASE 5 – Balancing of Power
Turbine Load Shaft and Generator at
RWE Npower, Ellesmere Port
Pedestal Velocity levels of ~11 mm/s
rms on Wheel gear outboard bearing
at operating speed
Unit’ return-to-service
in November 2011
after an inspection
outage where the
alignment across the
Gas-turbine Alternator
was checked and
corrected revealed
Shaft displacement levels of
~200 um pk-pk on high speed
load shaft at operating speed

44
CASE 5 – Balancing of Gas Turbine
Output Load Shaft
Transient Polar plotTransient Polar plot
indicating 1X vectorindicating 1X vector
movement of loadmovement of load
shaft was veryshaft was very
repeatable during run-repeatable during run-
up and run-down. Noup and run-down. No
significant change insignificant change in
the vibration behaviorthe vibration behavior
was observed after thewas observed after the
Flender gearbox wasFlender gearbox was
refurbishedrefurbished
Run-up - Blue; Run-down -Run-up - Blue; Run-down - RedRed

45
Output Load Shaft
Balance weightsBalance weights
added on the loadadded on the load
shaft coupling (closeshaft coupling (close
to Pinion Gear DE)to Pinion Gear DE)
reduced vibrationreduced vibration
levels at operatinglevels at operating
speed to acceptablespeed to acceptable
levels.levels.
Run-up - Blue; Run-down -Run-up - Blue; Run-down - RedRed

46
Output Load Shaft
A significant improvement
in shaft vibration levels was
observed on the load shaft
at Pinion Gear DE bearing
after installing a balance
weight.
No significant deterioration
in vibration levels was
observed on load (thermal)
after in-situ balancing

CASE 5 – Balancing of the Alternator
47
High velocity levels
recorded on the
Wheel gear O/B
bearing with levels
increasing while
the Unit operated
on load. This
bearing also
supports the
Alternator shaft and
probably caused by
the Alternator
rotor’s thermal
behavior on load

48
CASE 5 – Balancing of the Alternator
• Significant improvement in
pedestal vibrations was observed
across the Alternator bearings
after installing a balance weight.
• Three (3) planes were used to
balance the Alternator with
balance weights installed on the
inboard and outboard balance
planes of the Alternator and
overhung Exciter
• No significant deterioration in
vibration levels was observed on
load (thermal) after in-situ
balancing

49
CASE 6 – Balancing of Solar’s Mars 100
Gas Turbine on a Conoco Vikings Platform
Shaft vibration levels on theShaft vibration levels on the
power turbine exhaustpower turbine exhaust
/coupling end increased/coupling end increased
with speed and tripped thewith speed and tripped the
machine.machine.
In-situ balancing reducedIn-situ balancing reduced
these levels from ~2,4 milsthese levels from ~2,4 mils
pk-pk to ~0,34 mils pk-pk.pk-pk to ~0,34 mils pk-pk.

CASE 7 – Balancing of SPEY Gas
Turbine on a Type 23 Frigate – HMS
Argyll
50
Pedestal vibration on the
Power Turbine Aft bearing
increased to 9mm/s rms at
operating speed.
In-situ balancing reduced the
levels from 9mm/s rms to
~0,5mm/s rms to comply with
maximum allowable vibration
per marine standard DEF
STAN 02-305, Issue 3.

51
•LP Gland box housing the labyrinth seals is installed on the LP Turbine casing.LP Gland box housing the labyrinth seals is installed on the LP Turbine casing.
•Restriction was observed on the LP Turbine’ s shaft-centerline “rise” during run-up.Restriction was observed on the LP Turbine’ s shaft-centerline “rise” during run-up.
•During investigations, it was found that the severe rubbing was caused byDuring investigations, it was found that the severe rubbing was caused by
restriction in expansion of the LP gland box, which is connected to the LP outerrestriction in expansion of the LP gland box, which is connected to the LP outer
casing.casing.
EXCITATION FORCE – Severe rubbing on the labyrinth seals during run-up
caused by restriction in expansion of the LP gland box fixed to the LP turbine
outer casing.
CASE 10 – Rubbing on LP Turbine Labyrinth
Seals on a 200 MW Parsons Turbine during
return to service

52
•The Exciter rotor is coupled with the
Generator rotor through a stub-shaft.
•The Generator/ Exciter coupling was
assembled with a significant gap
(swash) error and concentricity error
between the Exciter rotor and
intermediate stub-shaft.
•Although the Exciter rotor was high
speed balanced, excessive vibration
levels were witnessed during run-up
through the 1st
balance resonance and
the unit was manually tripped.
error

53
•The Exciter rotor was rebuild with a
reduced swash (gap) error and lower
concentricity error between exciter
rotor and stub-shaft at RWE workshop
since the equipment supplier could not
achieve the desired concentricity
figures.
•The Exciter rotor was re-balanced
prior to assembly on site.
• Excessive vibration levels were
witnessed during return-to service were
further reduced with in-situ balancing.
error

54
ROTOR BOW
 What is rotor bow
 Causes of rotor bow
 Diagnosing rotor bow
 Removing rotor bow

55
ROTOR BOW
Rotor bow is a condition that results in bent shaft
centerline.
Rotor bow can be categorized in two
 Mechanical Rotor bow can be caused by improper
handling, transportation and storage of rotor system.
 Thermal Rotor bow can be caused by uneven heating
/ cooling of rotor system and “rubbing” between
stationary and rotating components of rotor system.

56
ROTOR BOW - Diagnosis
Mechanical bow – Permanent bow
 All characteristics of mechanical unbalance are present in
vibration data.
 Vibration data will be repeatable for consecutive runs.
 “Slow roll” vectors will be high and orbit at “Slow roll” &
speed will be circular.
Thermal bow
 All characteristics of mechanical unbalance are present in
vibration data.
 Vibration data will not be repeatable for consecutive runs.
 “Slow roll” vectors will be different for start-up and shut-
down.

57
ROTOR BOW
Rotor can have temporary or permanent bow depending
on the stress on the rotor system
 Temporary bow can occur due to uneven heating
of rotor surface or anisotropic thermal material
properties. If a “hot spot” develops on one side of the
rotor, it will expand and become longer than the other
side. If the rotor system is unconstrained, resulting
deflection will not be permanent. Temporary bow can
be cured by barring the machine for longer duration
and also “heat soak” the machine at low speed for
longer duration.

58
ROTOR BOW
 Permanent bow results when the rotor system has
been deformed to a condition that is not self
reversing without special intervention. It usually
happens when stresses on the rotor system has
exceeded the yield strength of material.

59
CASE 18 – HP Rotor thermal bow caused
by rubbing on 400MW Alstom HP-Turbine
•This Unit the HP/IP Turbine had a severe “rub
condition” on the glands caused by fluctuating
ICVs.
•Significant increase in Vibration levels through
HP resonance was observed during the unit’s
Run-Down after severe “rub condition”.
•Change in “slow roll” vectors were also
observed during the Run-Down after the unit
had a severe “rub condition”.
•Unit was Run-Up after approximately 6 hours
of barring and had higher vibration levels
(direct & 1X) at speed suggesting that the
thermal bow caused by the “rub condition” had
not cleared.
EXCITATION FORCE – Fluctuating ICVs and
thermal distortion on Pedestal 2 moved the
HP/IP rotors into a severe “rub condition” at
speed.

60
•This Unit the HP Turbine on
Pedestal 1 had a “rub condition”
resulting in an increase in 1X
vibration amplitude and change in
phase.
•Once the HP-Turbine rotor had a
“rub condition” and developed a
thermal bow it moved to a position
within bearing clearance that is more
susceptible to Fluid induced
instabilities. Increase in over-all
vibration levels.
EXCITATION FORCE – Thermal distortion of Pedestal 2 causes the HP/IP
rotors to move closer to baffle / gland clearance resulting in a “rub condition”
at speed.
CASE 19 – Thermal bow on HP rotor
caused by rub due to pedestal distortion

61
GENERATOR THERMAL
 What is a thermal effect/defect on a
generator
 How can you identify this effect

62
GENERATOR THERMAL
 Generator rotor thermal sensitivity is a phenomenon
which may occur on the generator rotor causing the
rotor vibration to change as the field current is
increased.
 The thermal sensitivity can be caused by an uneven
temperature distribution circumferentially around the
rotor, or by axial forces which are not distributed
uniformly in the circumferential direction.
 The primary driver of this second cause is the large
difference in coefficient of thermal expansion
between the copper coils and the steel alloy rotor
forging and components.

63
GENERATOR THERMAL
 If the rotor winding is not balanced both electrically
and mechanically in the circumferential direction, the
generator rotor will be unevenly loaded which can
cause the rotor to bow and cause the vibration to
change.
 In most cases, a thermally sensitive rotor will not
prevent a generator from running, but may limit the
operation at high field currents or VAR loads due to
excessive rotor vibration.

64
CAUSES GENERATOR
THERMAL
 Shorted turns.
 Blocked ventilation or Unsymmetrical cooling
 Insulation variation
 Wedge fit
 Retaining Ring/Centering Ring Assembly Movement
 Tight slots
 Heat Sensitive Rotor Forging
 Distance Block Fitting

65
GENERATOR THERMAL
 A thermally sensitive rotor is characterized by a once-
per-revolution frequency response signature due to a
change in the rotor balance arising from the rotor
bow. If the total vibration of the field stays within
acceptable limits, the field is not considered
“thermally sensitive.”
 Since vibration is characterized by amplitude and
phase angle. If the vibration vector stays within the
acceptable vibration levels, the vibration is not
considered to be a problem.
 Similarly if the phase angle changes also more than
the allowable limit the generator rotor is considered
“thermally sensitive.”
 The change in vibration and phase angle within the
polar plot from the starting operating point to the end
operating point is called the thermal vector.

66
GENERATOR THERMAL
There are two types of thermal sensitivity -
Repeatable (or reversible) & irreversible. Both types
vary with field current
 Reversible type follows the field current as it is
increased and decreased. For example, if the
vibration on a field increases from 1 mil to 3 mils as
field current is increased and then decreases in the
same manner as the field current decreases, then
the thermal sensitivity is considered to be reversible.
In these case, the field can be compromised
balanced so that the thermal vector passes through
zero and the maximum vibration remains within
acceptable limits.

67
GENERATOR THERMAL
 Irreversible - if the vibration increases as the field
current is increased but does not respond to a
decrease in field current, then this type of thermal
sensitivity is referred to as irreversible or “slip-stick”.
If this situation occurs, the generator frequently must
be taken off-line and brought down to turning gear
speed to unlock the forces that induced the rotor
bow.

68
•This Generator rotor had a thermal condition that was confirmed by the “heat run” test
performed in the 300 Ton balancing facility at Rotek works.
•When the excitation current was passed through the Generator rotor coils, the rotor
showed a thermal behavior that is associated with the bending mode of the rotor.
•Significant increase in Vibration levels were observed when the rotor was Run-Down.
CASE 22 – 400MW GENERATOR ROTOR “HEAT RUN” TEST
BEFORE REPAIRS
POLAR PLOT FOR VECTOR MOVEMENT ACROSS GENERATOR RUN-DOWN PLOTS AFTER HEAT RUN TESTPOLAR PLOT FOR VECTOR MOVEMENT ACROSS GENERATOR RUN-DOWN PLOTS AFTER HEAT RUN TEST

69
•This Generator rotor’s thermal condition was acceptable after repairs done on the
Generator. “Heat run” test performed in the 300 Ton balancing facility at Rotek works
indicated small vector movements.
•When the excitation current was passed through the Generator rotor coils, the rotor
showed a slight vector movement.
•Significant decrease in Vibration levels were observed after Generator repairs.
CASE 22 – 400 MW GENERATOR ROTOR “HEAT RUN” TEST AFTER
REPAIRS
POLAR PLOT FOR VECTOR MOVEMENT ACROSS GENERATOR RUN-DOWN PLOTS AFTER HEAT RUN TESTPOLAR PLOT FOR VECTOR MOVEMENT ACROSS GENERATOR RUN-DOWN PLOTS AFTER HEAT RUN TEST

70
CASE 23 – 400MW GENERATOR
ROTOR THERMAL DURING LOADING
•This Generator rotor showed a thermal
behavior on load when a significant
vector movement was seen on
Generator outboard as the unit was
loaded from unexcited to 350 MW.
•Significant increase in Vibration levels
was observed and the phase across the
generator moved from out-of-phase to in-
phase. (Change of mode shape)

71
CASE 24 – 400MW GENERATOR ROTOR THERMAL CONDITION AT 1200
RPM DURING RUN-DOWN
•This Generator rotor’s thermal behaviour was confirmed by the 1X vectorThis Generator rotor’s thermal behaviour was confirmed by the 1X vector
movement when the unit was kept at 1200 Rpm (just above its balance resonance)movement when the unit was kept at 1200 Rpm (just above its balance resonance)
during run-down.during run-down.
•This allowed the generator rotor to relax (straighten) at 1200 Rpm resulting inThis allowed the generator rotor to relax (straighten) at 1200 Rpm resulting in
lower vibration levels through LP 1lower vibration levels through LP 1stst
balance resonance.balance resonance.
Both the LP Turbine’s and
Generator’s 1st
balance resonance
were close to each other at ~1100
rpm.
During run-down the generator
rotor’s 1st
balance resonance
excited the LP rotor’s 1st
balance
resonance resulting in substantially
high vibration levels through LP
rotor’s its 1st
balance resonance
resulting in severe rubbing.

72
CASE 25 – Offset balance of a generator
thermal on a 40 MW Gas-Turbine
Generator
A significant generator
rotor thermal was
seen as the unit was
loaded, especially on
BB11 (Generator I/B
bearing)
Thermal offset
balancing using 3
balance planes on the
Generator, decreased
the levels to
acceptable

73
CASE 26 – 12MW Alternator Rotor Instability
While the unit operated
at lower load and the LC
heaters were brought
into service, significant
variations in shaft
displacement levels was
recorded across the
Alternator bearings.
LC heater
operations
Orbit plot of Alternator DE and NDE bearings indicated significant instability
that is normally seen on rotors with fluid induced instability with the presence
of near ½ X vibration component.

74
Waterfall plot of Alternator DE and NDE bearings indicated presence of near
½ X vibration component.

75
The instability in orbit was not observed prior the LC heaters were
brought in service and after the LC heaters tripped. However, change
in orbit shape of the Alternator DE bearing was noted between LC
heaters operations (prior they were brought in service and after they
tripped)

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