TSI 20.10.08

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TURBOVISORY instruments

Anil Kapoor En r. O&M/C&I

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TURBOSUPERVISORYINSTRUMENTS

VIBRATION PICKUPS THERMAL EXPANSION PICKUPS

AXIAL SHIFT PICKUP ECCENTRICITY SPEED PICKUP BEARING TEMPERATURE

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NEED FOR TURBOVISORYINSTRUMENTS

Heavy mass rotating at high speed Small clearances between moving and

stationary blades Ensures turbine healthiness Gives idea of any problem in turbine

Helps in increasing turbine operatinglifetime by giving timely indications

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HPTIPT LPT

GENERATOR EXCITER

BRG.1 BRG.2 BRG.4BRG.3 BRG.5 BRG.6 BRG.7

TURBINE BEARINGSARRANGEMENT

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BEARING 1 (HPT FRONT)INSTRUMENTS

HALL PROBE FOR SPEEDMEASUREMENT

HPT ROTOR EXPANSION HP CASING EXPANSION THERMOCOUPLES FOR BEARING

TEMP.(4 NO.S) PEDESTAL VIBRATION PICKUP SHAFT VIBRATION PICKUP

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BEARING 2 (HPT REAR)INSTRUMENTS

AXIAL SHIFT PICKUP THERMOCOUPLES FOR BEARING


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BEARING 3 (IPT REAR)INSTRUMENTS

IP CASING EXPANSION IP ROTOR EXPANSION

ECCENTRICITY PICKUP THERMOCOUPLES FOR BEARING

TEMP.(2 NO.S)

PEDESTAL VIBRATION PICKUP SHAFT VIBRATION PICKUP

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BEARING 4 (LPT REAR)INSTRUMENTS

IP+LP SHAFT EXPANSION THERMOCOUPLES FOR BEARING


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BEARING 6 (GENERATORREAR) INSTRUMENTS

THERMOCOUPLES FOR BEARINGTEMP.(2 NO.S)

PEDESTAL VIBRATION PICKUP

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BEARING 7 (EXCITER)INSTRUMENTS

THERMOCOUPLES FOR BEARINGTEMP.(2 NO.S)

PEDESTAL VIBRATION PICKUP SHAFT VIBRATION PICKUP

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VIBRATIONMEASUREMENTS

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VIBRATION CAUSES

Unbalance forces Internal damages

Misalignment Poor lubrication Eccentricity Thermal stresses

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VIBRATION TYPES

PEDESTAL VIBRATION SEISMIC MASS TYPE

VELOCITY TYPE ACCELERATION TYPE

ABSOLUTE SHAFT VIBRATION

PROXIMITY TYPE(NON CONTACT) EDDY CURRENT TYPE PICKUP

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4 steps in vibrationmeasurement

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SENSOR TYPES(SEISMIC MASS)

Velocity sensors Velocity of vibration is the speed at which

the part is moving at any instant during thevibration cycle Measured in mm/s

Rms value of velocity gives vibration Seismic mass type sensors Velocity gives the kinetic energy

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Velocity sensors Works on PLUNGER COIL PRINCIPLE.

Two permanent magnets rigidly fixed to thecasing with coils arranged as seismic mass. The Relative motion between the coil and

magnet generates voltage proportional to

vibration velocity. Operates within range of 10hz to 150 hz

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CONSTRUCTION

Coil

Magnet

Magnet

Aluminum case

DamperTerminals

spring

Output is proportional to velocity, hence the name Velocity sensor

Output=B*l*V B & l are constant

Seismic sensor

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Accelerometer Piezoelectric accelerometer transducers Rate of change of vibration velocity at any instant

during vibration cycle Measured in mm/s 2 Acceleration gives the force Emf proportional to force

Internal amplifier for boosting signal Can measure Acceleration, velocity and

displacement

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ACCELEROMETER

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SHAFT VIBRATION

Proximity (Non contact) type Shaft vibration measures the radial motion of the

rotating shaft relative to the case.

This measurement gives the first indication of afault, such as unbalance, misalignment, crackedshaft, oil whirl or other dynamic instabilities

Total distance moved by the vibrating part in agiven direction

Measured in micrometers

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SENSOR TYPES

Proximity (Non contact) type Proximity probe senses the distance between the

probe tip and the observed surface. Proximeter generates a radio frequency signal,

which is radiated through the probe tip into theobserved surface. Eddy currents are generated

in the surface and loss of strength in the returnsignal is detected by the proximeter.

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OSCILLATOR

PROBE GAP

V O L T S

0 100

EXTENSION CABLEAND PROBE

OBSERVEDMATERIAL

PROXIMITOR

VOLTAGEDETECTOR

CONDUCTIVEMATERIAL

PROXIMITOR

Eddy Current

Radio FrequencySignal

PROXIMITY SENSOR

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NON CONTACT TYPE

PROXIMITY SENSOR

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Measuring Amplifier

45O 45O

ProximityPick-up

L RSHAFT

2 transducers spaced 90 deg to each other for a 2 dimensional view ofeach bearing The 315 and 45 points are used to avoid the half joints of the bearingsand to ensure that when bearings are removed the probes are removedalong with them.This moves the probes away from possible mechanical damage when theturbine is being worked on.

PROXIMITY SENSOR

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VIBRATIONSENSITIVITY DISPLACEMENT

VELOCITY

ACCELERATION

FREQUENCYCPM

60 600 6000 60000 600 000

10

1

0.3.1

.01

.001

WHEN TO USE DISPLACEMENT,VELOCITY & ACCELERATION

V

i b r a

t i o n a m p

l i t u d e

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SHAFT VIBRATION

For no variation in gap we get DC signalwhich indicates gap or average distancefrom transducer tip.

In case shaft is vibrating there will be bothAC & DC components and varying voltagewill give vibration signal

Sensitivity around 8V/mm

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VIBRATION ANALYSES

Each machine defect produces a uniqueset of vibration components that can beused for identification.

By identifying different frequencycomponents we can make out the defect inturbine.

Spectrum analysis, shaft centerline plots,orbit plots, waterfall plots are some of thetechniques used for analysis

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PEDESTALVIBRATIONPICKUP

SHAFTVIBRATIONPICKUP

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Thermal expansions

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THERMAL EXPANSIONMEASUREMENT

Differential expansion Overall casing expansion

Rotor expansion

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HPTIPT LPT

GENERATOR EXCITER

BRG.1 BRG.2 BRG.4BRG.3 BRG.5 BRG.6 BRG.7

ANCHOR POINT

HP ROTOR/CASINGEXPANSION

IP ROTOR /CASINGEXPANSION

IP+LP SHAFTEXPANSION

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DIFFERENTIAL/ROTOREXPANSION

Difference between the thermal growth of the rotorcompared to the thermal growth of the case.

Important during a turbine "cold" start-up. Since the pedestal are of fixed type, differential

expansion can not be measured directly. Hence casingexpansion & rotor expansion are measured separatelyand differential expansion between the two rotorexpansion is measured at HP front & IP rear brgs

Due to the mass of the case it will grow slower than the

rotor, so the operator must make sure the case hasexpanded enough to keep it from making contact with therotor.

If shaft expands more than casing it is positive expansionand if casing expands more than shaft than it is negativeexpansion

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THERMAL EXPANSION/AXIAL SHIFT PICKUP

PROXIMITY TYPE VARIABLE INDUCTANCE TYPE

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ROTOR EXPANSIONPICKUP

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d2 d1

L1 L2

Turbine Casing

I

-

MeasuringAttachment

Power Supply Unit

Large Relative Expansion, Shaft with double cone

L1 L2 Sensor Coilsd 1 d 2 Air gap

IP+LP SHAFT EXPANSION

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IP+LP SHAFTEXPANSION PICKUP

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CASING/OVERALLEXPANSION

Steam temperature varies greatly betweenstartup, operation, and shutdown.

Shell expansion is a measurement of how much

the turbine's case expands from its fixed pointoutward as it is heated. Continuous indication of shell thermal growth

allows the operator to manage the amount of

shell distortion as the load is increased ordecreased. LVDT Type pickup

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OUTPUT VOLTAGE = DIFFERENCE OF VOLTAGE BETWEEN 2SECONDARY COILS

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RANGES

HP Rotor Expansion: 0-25mm IP Rotor expansion:0-21mm

HP Casing Expansion:0-30mm IP Casing Expansion: 0-20mm IP+LP Shaft Expansion; (-)15-48mm

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SPEED PICKUP HALL PROBE

ECCENTRICITY PROXIMITY PICKUP

BEARING TEMPERATURE THERMOCOUPLES

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ECCENTRICITY

Eccentricity is measured as the deviation ofmass centre from the geometrical centre of thebearing journal.

Eccentricity is the measurement of Rotor Bow atrotor slow roll which may be caused by any or acombination of reasons

Fixed mechanical bow

Temporary thermal bow Gravity bow

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Eccentricity is measured while the turbine is on slow roll(1 to 240 RPM below the speed at which the rotorbecomes dynamic and rises in the bearing on the oilwedge) and requires special circuitry to detect the peak-to-peak motion of the shaft.

For eccentricity measurements it is recommended thatthe transducer be mounted vertically .

Slow roll and heating can help reduce the eccentricity tothe point where the turbine can safely be brought up tospeed without damage from excessive vibration or rotorto stator contact.

ECCENTRICITY

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Eccentricity

Eccentricity is normally measured P/P (Peak to Peak)The actual excursion from shaft centerline caused bybow would be one half that measurement or the 0/P(Zero to Peak) measurement

ECCENTRICITY

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Special consideration for using eddy current probes

RUNOUT Error signal generated by mechanical, electrical or

metallurgical irregularities of the shaftMECHANICAL RUNOUT

A smooth (64 micro-inch) area approximately 3 times thediameter of the probe tip must be provided for a viewingarea.ELECTRICAL RUNOUT

Eddy Probes are sensitive to the permeability andresistivity of the target material and the field of thetransducer extends into the surface area of the shaft byapproximately 15 mils (0.015"), care must be taken toavoid non homogeneous viewing area materials such asChrome.

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System Cable Length

Eddy Probe Systems are a "tuned" length,and several system lengths are available.

System length is measured from the probetip to the Oscillator/Demodulator, and ismeasured electrically which can be slightlydifferent than the physical length.

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The RF Field emitted fromthe probe tip of thetransducer isapproximately a 45 conicalshape. Clearance must beprovided on all sides ofthe Probe tip to preventinterference of the RFField.

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To prevent Ground Loops from creatingsystem noise, system common, groundand instrument wire shield must beconnected to ground at one location only.

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GAP VOLTAGE

Gapping thetransducer to thecenter of the linear

range is adequate

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TURBINE SPEEDMEASUREMENT

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BASIC ARRANGEMENT OF HALLPROBES AT BEARING 1

SENSOR

WIRE

BRACKET

PERMANENT MAGNETS

N-POLES-POLE

DISK

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SPEEDMEASUREMENT

Allows the operator to increase speed quicklythrough shaft critical frequencies

Hold the speed stable during heat soakplateaus

For electric generators, to accurately match thesynchronous frequency before connecting the

generator to the power grid. Used for tripping turbine in case of overspeed(2/3 logic)

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Bearing temperature

Measure of the how hot a bearing isoperating.

May be due to overloading, misalignment,improper lubricant pressure and/or flow.

Thermocouples used to measuretemperature. Each thermocouple has 3elements

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AXIAL SHIFT

Shifting of rotor due to unbalanced forces in the turbine Positive towards generator, negative towards front

pedestal. Very critical measurement as very small gap between

stationary and rotating blades Three probes mounted and used in 2/3 protection logic. Total range from (-)1.5mm to + 1.5mm Protection at (-)1.0mm & +1.0mm Proximity probes or relative inductance type probes can

be used for measurement

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PORTABLE VIBRATIONMEASUREMENT

Vibration measurement closeto machinery avoids errorintroduced due to cableproblems.

Used to check vibration ofmachinery located at remotelocation.

Easy to carry Evaluate machine condition

and bearing problemsassociated with pumps whosesignal is not available incontrol room

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PHASE MEASUREMENT

Phase measurement is required foraccurate balancing of any rotor.

It also provides an indication of shaftcracks, misalignment, mass loss (such asthrowing a blade), and other faults.

TSI 20.10.08

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