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the service magazine of the PRÜFTECHNIK Group

In this issue:

PRÜFTECHNIK for turbomachinery

Strong shaft vibrations in aturbo-generator set

Evaluating shaft vibrations inturbomachinery

Determining turbine alignmenttargets

Alignment of a steam turbine

Measuring and evaluating shaftmovement: part 1 – part 8

News

Power plant turbo-generator setsshould run 24/7 – not only to keeppower flowing but also to improve themachinery’s life expectancy, which isshortened every time machines arestarted up and shut down. While it isunfortunate when a turbo-generator setneeds to be shut down for maintenanceor due to excessive vibration, it is entire-ly futile if no faults are subsequentlyfound.

PRÜFTECHNIK was contracted to con-duct a vibration analysis on a 55-MWturbo-generator set. This job came aboutdue to a steady rise in shaft vibrations atthe Drive-end (DE) bearing of the gener-ator. The cause was suspected to bevibrations from the turbo gear unit sincethe gearbox contained corrected toothedge damage. Would a new gear setneed to be ordered? Dr. Becker handledthis assignment himself since he is fa-miliar with turbo gear units from earlierjobs. This was also a good opportunityto test the new VIBXPERT® and OMNI-TREND® functions.

The different machine analyses werecombined. On the one hand, measure-ments were taken directly at typicalhousing measuring points using piezo-electric accelerometers. In addition,shaft vibrations were measured using aBently shaft vibration protection systeminstalled in the unit. Unfortunately,there were no vibration sensors mount-

ed on the gearbox. The gear conditionwas evaluated using accelerometers.

The first set of analyses showed thatthe gear vibrations were by no meansunusual and that no gear mesh faultsoccurred under load change. On theother hand, directional forces betweenthe generator and gearbox were found,as well as elliptical orbits on both gener-ator sides. In fact, on the DE side, theamplitudes were impermissibly high at123 µm. Had the machinery alignment

changed? This was checked in a series ofspecific examinations several days laterduring a scheduled shutdown. The visu-al inspection showed unusual contactpatterns in the double engagement gearcoupling and slight tarnishing on thegenerator Non-drive-end (NDE) bearing.The alignment had changed only slight-ly, and the small vertical deviation thatwas found was corrected. Was it possi-ble that the new gear coupling installedduring the last shutdown was the cause?

Fig. 1: Unusual contact patterns in the double engagement gear coupling.

Condition Monitoring Service

Shaft vibrations in a turbo-generatorDr. Edwin Becker

The new version of OMNITREND®

now lets you evaluate and fully analyzeshaft vibrations in power plant turbo-generator sets, industrial turbo-genera-tor sets, gas turbine sets, compressorsystems and other machinery in a stan-dards-compliant manner. Mobile mea-surement and diagnosis of shaft vibra-tions can be readily accomplished withthe 2-channel VIBXPERT® data collectorand signal analyzer, which is connected

to protection monitoring systems or totemporarily mounted displacement sen-sors. This issue is all about turboma-chinery and how new Condition Moni-toring approaches make your condition-based maintenance program even moreeffective through the analysis of shaftvibrations.

We hope you find this issue interestingand enjoyable – and will be glad toanswer any questions you may have.

PRÜFTECHNIK News

PRÜFTECHNIK for turbomachinery

No. 13 – Focus: Turbomachinery

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PRÜFTECHNIK investigated this whenthe turbo-generator set was recommis-sioned. They performed additional com-parative measurements, which couldalso be used to check run-out.

Several sensors, measurement resultsand the build-up of pressure in hydrody-namic bearings are shown in Figs. 2 – 6.The shaft vibrations measured at thegenerator NDE bearing were conspicu-ous. The rotating shaft was virtually‘glued’ to the upper bearing shell. Theassociated time waveforms in Fig. 4show that the harmonic motion of therotor was ‘disturbed’.

Time waveforms for the generator DEbearing are shown in Fig. 6. They pro-vide information on time-based shaftvibrations before and after correction ofthe alignment.

By chance, the turbo-generator sethad to be shut down again due to acontroller problem. “I quickly activatedthe recording function in VIBXPERT®,”reported Dr. Becker. During ramp down,it became apparent that the generatorwas operating at a overcritical level.This explained why the elliptical orbitswere so pronounced on both generatorsides. This type of rotor flexibility needsto be taken into account in the align-ment targets.

Only – why did the system run betterbefore? Does the current gear couplinglack the necessary crowning to balanceout the forces between the coupled ma-chines? Indeed – the coupling did nothave crowned outer toothing and wasthus unable to correctly align the dis-placements and eccentricities of the cou-pled machines. A crowned coupling ofthe same design was ordered and in-stalled. Measurements taken six monthslater showed that the new crownedtooth coupling eliminated the disturbingvibrations and the drive ran smoothly.

Fig. 2: Accelerometer on gearbox. Fig. 3: Measurement signals taken from theBently rack and recorded with VIBXPERT®.

Fig. 4: The time waveform of shaft vibration on the generator NDE bearingin +45° and -45° directions, with the associated frequency spectra.

Fig. 7: Alignment that takes rotor flexibilityinto account.

Fig. 5: Pressure buildup in a hydrodynamicjournal bearing.

Fig. 6: Time waveformsof shaft vibration fromthe generator DE bear-ing, before and afteralignment.

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Condition Monitoring Background Information

Evaluating shaft vibrations in turbomachineryMisel Tanasijevic

Turbomachines are highly complexsystems used in power plants. They havea relatively high machine value, exhibita certain risk potential and their failurecan result in high secondary costs. Forthis reason, turbo-generator sets are of-ten equipped with a vibration protectionmonitoring system to obtain timely in-formation on when to shut down themachine. When the threshold parame-ters are set up correctly, these protectionsystems detect slow changes in proper-ties such as imbalance, in dynamic mar-ginal conditions (bearings, screw con-nections) and in operating conditions/processes. Diagnostic vibration monitor-ing determines dependencies on operat-ing parameters, transient operatingstates, orbits, frequencies and orderanalyses, and uses correlation methodsto identify changes early on.

Standardized guidelines and/ornorms from the European and Americansector serve as decision-making criteria.Their main difference lies in the evalua-tion variables applied. While API stan-dards use peak-to-peak values as param-eters for amplitude evaluation, the Eu-ropean norms like to employ smax values.The applicable standards and guidelinesfor evaluating shaft vibrations areshown in Fig. 1. The standards assessconditions by assigning them to one offour zones:

Zone A: goodZone B: allowableZone C: just permissibleZone D: not permissible

Fig. 2: Example of general evaluation criteria.

These evaluations only apply to con-tinuous operation and not to transientconditions. Condition Monitoring differ-entiates between machine protection,vibration monitoring and vibration diag-nosis. While the protection and monitor-ing methods are well described in thestandards listed here, comprehensive

specifications for diagnosis methods donot yet exist. Initial steps have beentaken in VDI 3839, Sheet 3, but thesehave not yet been finalized.

Figures 3 – 5 show several nomo-grams from the DIN-ISO standards. Ifyou know the rpm of the machine, it isrelatively easy to determine how highthe shaft vibration amplitudes are per-mitted to be.

The standards, which are availablefrom the German Beuth-Verlag, also pro-vide information and make recommen-dations on setting warning and switch-off criteria.

Vib

ratio

ns

rpm

rpm-range

Fig. 1: Some applicable standards for shaftvibration in turbomachinery.

rpm x1000 [min-1]

Fig. 3: Recommended limit values for relativeshaft vibrations (stationary steam turbines andgenerators above 50 MW).

Fig. 4: Recommended alarm thresholds forshaft vibration in coupled industrial machines.

Fig. 5: Recommended alarm thresholds forshaft vibration in gas turbine sets.

rpm x1000 [min-1]

rpm [min-1]

S p-p

rela

tive

[µm

]Re

lativ

e sh

aft

vibr

atio

n di

spla

ce-

men

t, p

eak-

to-p

eak

in µ

m

S p-p

rela

tive

[µm

]S p-

p re

lativ

e [µ

m]

DIN/ISO 7919-2

DIN/ISO 7919-3

DIN/ISO 7919-4

Dadditionaldamageoccurs

Crestrictedoperation

Bcontinuousoperationwithoutrestrictions

Arecently putinto opera-tion

VDI 2059

DIN/ISO 7919-1DIN/ISO 7919-2DIN/ISO 7919-3DIN/ISO 7919-4

API 541API 546API 611API 612API 617

VDI 2059

DIN/ISO 7919-1DIN/ISO 7919-2DIN/ISO 7919-3DIN/ISO 7919-4

Machines withrotating masses

Shaft vibrations

Measurementduring

acceptance

Manufactureracceptance

Operationmonitoring

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PreviewThe next issue will focus on generatorsystems:

– Wear monitoring incombined heat and power plants

– Acceptance measurementson a ship generator

– Excessively high vibrations in a gasengine

– Applicable vibration standards forgenerators

To be able to correctly align a turbinesystem, its thermal growth characteris-tics must be known and taken into con-sideration. This is the only way to en-sure low-vibration operation and a longcomponent service life. In critical tur-bine systems, online continuous moni-toring of the alignment condition maybe wise. This measurement detects foun-dation and system movements to enableappropriate intervention early on.

PERMALIGN® measuresalignment changes

PERMALIGN® was developed byPRÜFTECHNIK specifically for these ap-plications and can be used with com-pressors, steam and gas turbines, andwater cooling systems. There is also aspecial model for the chemical, oil andgas industry with explosion protection(Ex ib IIc T4 Zone 1).

Four-machine trainwith increased vibrations

In a four-machine train consisting of asteam turbine, two compressors (HP, LP)and an expander (Fig. 2), repeat occur-rence of bearing damage and raisedvibration levels led to the conclusionthat misalignment existed due to a fail-ure to take thermal growth into account.This was a task for PERMALIGN® andthe PRÜFTECHNIK Machinery ServiceTeam.

With PERMALIGN®, relative machinedisplacements can be recorded duringoperation in four degrees of freedomusing a roof prism. Axial displacementsand thermal growth can be measured

Alignment Application

Determining the alignment targets of a turbineDirk Günther

directly with triple prisms – at a distanceof up to 10 meters with micrometerresolution.

The service callPERMALIGN® was installed using sta-

ble brackets with the system running(Fig. 3). Then the machinery wasramped down and switched off. Themeasurement data recorded over thisperiod was ideal for determining theoptimal alignment targets, according towhich the machine train was then

aligned. During the subsequent machin-ery startup, PERMALIGN® sensors re-mained installed to compare the dis-placements with the expected values.

The resultBy using alignment targets that take

thermal growth into account, the vibra-tion values at operating temperaturewere considerably reduced. The opera-tor now has an optimally aligned ma-chine train, which will have a positiveeffect on component service life.

Fig. 2: Measurement of a four-machine train.

Fig. 3: PERMALIGN® systems mounted on themachine train for measuring positional chang-es in the vertical and horizontal directions.

Fig. 1: PERMALIGN® roof prismwith mounting bracket.

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Alignment Application

Alignment of a steam turbineBernardo Quintana

Steam turbines are important ma-chines for power generation. More thanhalf of electricity produced worldwide isgenerated using steam turbines. Theyexist in a wide range of sizes. Typicalpower outputs for small industrial appli-cations are 2 – 3 MW, while large coal-burning or nuclear power plants haveturbines that generate up to 1 GW.Steam turbine efficiency is closely relat-ed to the air gap between the turbinecomponents and the rotor. This is whythe installation position of the rotorrelative to the internal housing is ex-tremely important. It should be checkedclosely, not only when first installed butalso when the machine is overhauled.

The CENTRALIGN® Ultra measure-ment system developed by PRÜFTECH-NIK has been in use to align turbineshells to rotors for many years aroundthe world.

Faster than piano wireor dummy shafts

Compared to conventional methodslike piano wire and dummy shafts, thismethod saves a lot of time and deliversunequaled accuracy with its laser-opti-cal measurement procedure.

With CENTRALIGN® Ultra, internalturbine elements like bearing shells, tur-bine casings, guide vanes andinternal shells can be alignedwith a high degree of precision.The use of a laser extends theworking range to 40 m withoutsag, and the sensor measureswith a resolution of 1 µm. Anadditional control sensor perma-nently monitors the measure-ment setup for changes andcompensates laser drift.

The service callPRÜFTECHNIK was contract-

ed to align half-shells to a rotorduring routine maintenance of aturbine. After the casing wasopened, the rotor was removedand transported to a partnerwho repaired or replaced theindividual blades and then bal-anced the entire rotor. In paral-lel, the turbine elements and lab-yrinth seals were renewed on the

turbine system. Finally, the CENTRA-LIGN® Ultra system and Machinery Ser-vice got to work.

The laser and the control sensor weremounted and adjusted outside of theturbine bearings – as shown in Fig. 1.The laser beam acts like a reference linefor measuring each individual turbineelement – just like a piano wire would.

Each half-shell was measured with thepatented sensor measurement equip-ment from PRÜFTECHNIK and the datawas transmitted remotely to the ROTA-LIGN® Ultra computer. After all measur-ing locations were recorded, two bear-ings or oil rings were selected inROTALIGN® Ultra as a reference and allhorizontal and vertical deviations rela-

Fig. 1: CENTRALIGN® Ultra measurement system setup.

ROTALIGN® Ultra computer withwireless signal transmission

Controlsensor

Measuring sensor withprecision holder

Laser

Fig. 2: The measurement values of the individual half-shells are remotely transmitted to the computer.

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tive to this reference were measured.The correction values for the bearings ofthe half-shells were computed fromthese deviations, enabling the installersto implement these promptly.

Fig. 6: Vertical (top) and horizontal (bottom) measurement results – as displayed by the ALIGN-MENT CENTER PC software.

Fig. 5: The aligned steam turbine with the control sensor in the foreground.

Fig. 4: Mounting of a new baffle.

Fig. 7: Finally, the overhauled rotor can be installed back in the turbine casing.

CENTRALIGN® Ultraadvantages

• Precise laser equipment with a resolu-tion of 1 µm

• No lengthy installations and adjust-ments of piano wire and dummyshafts

• The laser beam is not subject to theforce of gravity and does not sag

• Individual segments can be workedon immediately after the measure-ment data is taken since they are fullyaccessible

• Measurement data are immediatelyavailable in electronic form and tar-get values for rotor sag can be takeninto account using the ALIGNMENTCENTER software

Fig. 3: The precision laser mounted outside ofthe turbine bearings.

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+0S = >

S

S = <

S = 0

–+

0

0

–

1

0

0°

AMPLITUDE VS. TIME

AMPLITUDE VS. AMPLITUDEVERTICAL TIMEBASE WAVEFORM

HORIZONTAL TIMEBASE WAVEFORM

45°

T11

2

3

4

5

6

7

8

9T2

90° 135° 180° 225° 270° 315° 360°

-1

1

0

0.5

0.5

0° 45°

T1

12

3

4 5 6

7

89

T2

90° 135° 180° 225° 270° 315° 360°

-1

2

0°

180°

19

5- 0.5 + 0.5 90°270°

Y

X

7

3

+1

-1

24

6

HO

RIZ

ON

TAL

INP

UT

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RTI

CA

L IN

PU

T

T I M E B A S E O R B I T

Condition Monitoring Basics

Measuring and evaluating shaft movementNon-contact displacement sensors (part 1)Dr. Edwin Becker

Shaft movements take the form of(slow) displacements and/or (rapid) vi-brations of the turning rotor shafts inaxial and radial measurement direc-tions. Vibration displacements can beprecisely measured with non-contactdisplacement sensors. When two dis-placement sensors are mounted offsetby 90° and the signals are measuredsimultaneously beginning at 0 Hz, datais acquired on the static shaft position(gap) and the dynamic shaft movement(orbit) within the bearing plane. Bycomparing these values with the bearingclearance, information can be obtainedon formation of the lubrication gap,especially when starting up, and on thethickness and position of the narrowestlubrication gap. Longer measurementsalso make it possible to evaluate vectorcircles or displacement paths.

Eddy current sensors are among themost sensitive shaft vibration sensors.The sensor must, however, have a suffi-ciently large ‘visible area’ (at least200%). When the shaft begins moving(or vibrating) in the µm range, an elec-trical signal is produced. Both the DCand AC components are evaluated as afunction of the distance.

Measuring and evaluating mechanical shaft movement (part 2)

Mechanical and electrical runoutIf the measurement trace of the dis-

placement sensors contains irregulari-ties, notches, scoring, scratches, drilledholes or surface flaws, mechanicalrunout results. Runout measurementsbelong to the acceptance criteria fornew machine components and are de-scribed in detail in the API standards,for example. If runout is larger than10% of the permissible shaft vibrations,the measurement trace must undergocertain surface treatments. Eddy currentsensors are additionally affected by elec-trically induced runout due to micro-structure differences, residual stress andresidual magnetism. For this reason,shaft traces are often demagnetized. In

Measuring principleof eddy current sensors

The working principle behind eddycurrent sensors – see Fig. 1 – is based onthe fact that the coil in the sensor headgenerates an alternating magnetic fieldwhose field lines emerge from the sen-sor plane, pass through the object andthen close again. The measurement field(alternating magnetic field) generateseddy currents in the electrically conduc-tive object, leading to a loss in joules.These eddy current losses in the objectincrease as the distance to the objectdecreases. On the input side of thesensor coil, the eddy current losses arereflected in a change in the complexinput impedance, which is measuredand evaluated. An output signal propor-tional to the distance is formed, such as0 ... 10 V or 4 ... 20 mA.

On turbo-generator sets, the eddy cur-rent sensors are usually mounted whenthe sets are manufactured. They arepermanently integrated in the bearingand machine housing and the measuringsignals can be collected via separateoutputs of the protective systems. Eddycurrent measuring equipment is rela-tively expensive and is of limited use

Fig. 1: The runout acts independently of the measuring direction.

contrast, electrical runout is not an issuewith optical and induc-tive sensors.

The runout can beidentified through mea-surements during slowrotation, and can bestored and used for com-pensation if necessary.Fig. 1 illustrates runoutfor a geometric egg-shaped shaft. The posi-tion of the sensors is ir-relevant here.

Fig. 1: Measuring principle of an eddy currentsensor and definition of direction of movement.

ShaftHousingOil

when employed in a mobile or tempo-rary manner. Inductive displacementsensors and, increasingly, optical sen-sors are lower in cost and more flexible.Any nonlinearities, such as in inductivedisplacement sensors, are evened outeither by PRÜFTECHNIK equipmentthrough interpolations or by material-specific characteristic curves that arestored in the setup.

Inductive displacement sensors can beused to measure machines that are notequipped with displacement sensors atthe factory.

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Measuring and evaluating shaft movement (part 3)

Analysis of shaft displacements and bending linesRotating shafts are subject to varying

degrees of play. During standstill, hori-zontal shafts rest in the lower bearinghousing due to the force of gravity.When the oil supply is switched on inrotors with journal bearings, the shaftfloats up slightly. Depending on the posi-tion of the oil supply, a certain lateralchange in position may result.

When the rotor shaft is set into mo-tion, the shaft wanders upward in thebearing shell in a manner dependent onthe direction of rotation (Fig. 1) and


Shaft vibration analysesAmplitude spectra – time domain analyses – phase spectra

Vibration excitation sources can beidentified on the basis of frequency andorder spectra. After all, every rotatingshaft produces rotational excitationsthat lead to shaft vibrations with moreor less large amplitudes. First, however,it should be checked whether the timewaveforms of the shaft vibration arestable and harmonic. Then the ampli-tude spectra can be examined for furtherexcitations or even for natural frequen-cies. This method will also identify atyp-ical excitations. If several multiples are

reaches a more central operating posi-tion. If the shaft is accelerated evenfurther, it rises still higher due to itsinertia. The resulting pressure pointmust not lie in the area of the lubricat-ing oil supply. At an infinite rotationalspeed, the shaft would concentrate inthe center. This displacement is alsoexhibited during ramp down, a behaviorthat can be measured via the DC value.Irregularities in the displacement dia-gram point to rubbing, bearing damage,impermissible bearing clearance, incor-

discovered in the amplitude spectra, dy-namic time domain analyses becomenecessary. Thus, ‘tarnishing’ can lead tosuperimposed additional movements ofthe shaft. Also, phase dependencies canbe more easily identified by time-syn-chronous measurements.

Shaft vibration analysis should takeinto account that the measurements tak-en by displacement sensors are direc-tionally dependent. If the sensors aremounted at -45° and +45° instead of 0°and 90°, significantly lower amplitudes

result at sensor A and higher amplitudesat sensor B with the same orbit. This isillustrated in Fig. 1. The procedure re-quires at least a 2-channel vibrationanalyzer because it prevents vibrationalchanges from appearing to occur only atone sensor because the orbit is turning.

Continued at the bottom of page 9

rect alignment or sudden load changes.By also evaluating shaft displacement

in the other rotor planes, it is possible toplot the static and kinetic bending linesof the rotor shaft in the housing. Fig. 3shows the static and dynamic bendinglines of a shaft with three bearings.These types of bending lines can be usedto predict how shafts will be displacedafter the addition of load.

It can also be determined whetherforces, additional bending moments andadditional bearing loads act on the shaftduring operation.

Fig. 2: The displacement of the rotor shaftdepends on the direction of rotation and theshaft then moves along its orbit.

Fig. 3: The static (blue) bending line and two dynamicbending lines of a shaft with three bearings.

Fig. 1: The rotor shaft ‘wanders’ in the bea-ring shell in an rpm-dependent manner.

n = 0 n = slow n = fast n = ∞

ShaftLubricant

Bearing shell

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When a rotating shaft is displaced, itvibrates around the shaft centerpoint.The traces of the shaft around the shaftcenterpoint are known as orbits. Orbitanalysis is used to plot both the natural(unfiltered) and order-filtered shaft or-bits in the bearing plane.

Sudden changes in the amplitudesand form of the orbits are an earlyindicator of disturbing influences. Therotational orbits are a special featurethat also provides information on thecurrent balancing and alignment condi-tion of the shaft.


Unfiltered and filtered orbitsThe form of the orbit is influenced by

multiple factors. The most important ofthese are the rotational speed andthe mode of operation. When anisotropic machine is operated at asubcritical level and without con-straints, the orbits are circular. In con-trast, elliptical orbits indicate the pres-ence of forces due to alignment error oruneven bearing stiffness in two direc-tions.

If the orbit starts turning, contact aspecialist.

Fig. 1: The installation location of the vibrationaccelerometer influences the amplitude height.

Continued from page 8

Mounting of shaft vibration sensors at positions 0° and 90° and -45° and +45°

SensorA

SensorB

SensorA

Sens

or

B

SP–P

SP–P

SMAX

SMAX

SP–P

SP–P

Glossary of termsDid you know?

Shaft movements are exhibited as shaftdisplacements and shaft vibrations, acting ra-dially and/or axially.

The units of shaft movement are mm, µm,mils, inch. 1 mm = 0.001 m = 0.04 inch.1 inch= 25.4 mm. 1 µm = 0.04 mils.1 mils = 25.4 µm.

Journal bearings have considerably largerbearing clearance than roller bearings. Practi-tioners work with the assumption that radialbearing clearance is 2‰ of the shaft diameter.Journal bearings can be differentiated intoradial and axial bearings. Oval clearance bear-ings, multi-surface plain bearings and tiltingpad bearings are special designs.

Orbits describe the kinetic trace of the shaftcenter point. The gap is the distance betweenthe shaft vibration sensor and the rotatingshaft, and it changes with shaft displacement.

Oil whirl generates shaft vibrations at the0.38th to 0.49th order and results in a highprobability of metal-to-metal contact. Unfil-tered orbits jump significantly.

Oil whip is a type of oil whirl with exactly halfthe rotational frequency. The machine runswith double the bending critical speed andexcites half the rotational speed. The oil whirlfrequency remains constant as the rotationalspeed increases.

Anisotropy results from varying degrees ofstiffness in the bearing, shaft and/or housingand causes directionally dependent vibrations.It must be taken into account in shaft vibra-tion analyses.

Phase angle is the rotational movement fromthe point where the trigger pulse is output tothe maximum vibration amplitude.

Critical speed: The rotational speed at whichthe system becomes resonant.

Balancing: The procedure of improving themass distribution of a rotor so that the rotordoes not generate period forces on its bear-ings during rotation and the orbits remainsmall.

Bode plot: A graph that displays the transferfunction of a dynamic system. This type ofanalysis, which is also known as frequencyresponse, stems from the field of electricalengineering from the 1930s and is namedafter Hendrik Wade Bode.

Nyquist plot: A graph in polar coordinatesthat contains multiple vectors (magnitude andphase). The vector tips are connected by a lineand the parameterization is specified for eachone.

Waterfall diagram: A graph in which theresults are plotted against the rotationalspeed, independent of the measuring time.

Corbits are waterfall diagrams of orbits inwhich the rotor speed is plotted on the Z-axis.

Imbalance(thermal,

magnetic, mass-related)

Forces from- deposits- asymmetry

Alignment

Forces from- constraints- asymmetry- increased play

Oil whipOil filminstability(steam turbine)

Transverse crack Fatigue, stress

Too smalllocal clearance

Pressure to oneside

Rubbing(labyrinth seals)

Reduction inradial play

Fault Cause Frequencyspectrum Orbit

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Shaft vibrations in flexible rotorsRotors that are operated above the

bending critical speed are referred to asflexible rotors. Depending on the ma-chine setup, speed, balance conditionand rotor-dynamic properties, differentkinetic shaft traces and/or different or-bits occur at the individual shaft cross-sections.

When the behavior of a shaft withthree separate discs is depicted in adiagram, areas with multiple criticalspeeds and different vibration modescan be discerned.

Fig. 1: Several journal bearing designsa) Standard circular journal bearing shellb) Oval clearance bearingc) Journal bearing with offset oval clearanced) Multi-surface plain bearing with 4 sliding surfacese) Tilting pad bearing with 5 tilting pads


Orbits of rigid rotors in flexible bearingsUnfiltered orbits permit certain con-

clusions to be drawn on the type ofjournal bearing and the lubrication inuse. Journal bearings must absorb theload, deliver the necessary stiffness anddamping, and control the rotor position.The most commonly used journal bear-ings are divided cylindrical radial bear-ings. Oil is supplied through holes,grooves or pockets. Some journal bear-ings may only be operated in one rota-tional direction or unipolar bearings.

If there is a risk of vibration, themachine manufacturer takes steps toreduce this risk, usually by changing thebearing type (bearings are characterized

by their gap geometry, diameter-to-width ratio and load angle). Initially, heinstalls oval clearance or double-wedgebearings. If this is insufficient, the nextstep is to employ bearings with three orfour sliding surfaces or even radial tilt-ing pad bearings, which are the mostexpensive. Fig. 1 shows different bear-ing designs, which each exhibit specificcharacteristics in unfiltered orbits.

All bearing designs have in commonthat they are flexible, a fact that needsto be taken into account when analyzingshaft movements and shaft orbits. Theydampen vibrations and influence thedirectional vibration behavior.

A special vibration-related feature ofhigh-speed journal bearings is the self-induced vibration of the rotating shaftvia the oil film. The laminar lubricationgap flow becomes unstable when ex-posed to these self-excited vibrations,leading to ring vortices and even toturbulence. In extreme cases this caneven cause journal bearings to transportdrive energy into bending vibrations,resulting in whirl and oil whip. In thiscase, measures must be implementedthat increase outer damping, the naturalbending frequency of the rotor or thebearing topography.

Fig. 1: Bending lines of a rotor with threeseparate discs in the critical speed range.

Fig. 2: The frequency spectra with broadbandexcitations in the respective natural frequencyranges.

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nkrit

OW


Run-up & coast-down analyses of flexibly mounted rotorsThe vibration behavior of high-speed

turbomachinery can be very complex.Rotors and bearings often exhibit vibra-tions in different directions. In horizon-tal machinery, the horizontal stiffness isusually less than the vertical stiffness,which leads to anisotropic behavior withdifferent natural frequencies. In particu-lar, run-up and coast-down analyses

provide the specialist with a tool toobtain more information on discrete ma-chine excitations and natural frequencyexcitations. By analyzing shaft and cas-ing vibrations during run-up and coast-down, it is possible to differentiate be-tween housing or structural natural vi-brations and natural vibrations relatedto the rotor system. There are variousmethods of plotting the measurementresults:

a) Bode plotBode plots are the simplest analytical

method. The phase angle and usuallythe speed-filtered component of the vi-bration signal are plotted against thespeed. A turning point in the phaseangle together with an amplitude maxi-mum is a reliable indicator of resonance.

b) Nyquist plotIn the Nyquist plot, the magnitude

and phase are displayed in a singlediagram, making it easier to identifyresonances and couple-critical speeds.

c) Waterfall diagramFor the waterfall diagram, frequencies

and order analyses are measured atdifferent speeds and superimposed as afunction of speed.

d) Campbell diagramIn a Campbell diagram, natural fre-

quencies are plotted against the speedon the abscissa and compared to typicalexcitation frequencies.

e) SpectrogramA spectrogram is a time-variable, col-

or-coded display of the frequency distri-bution based on a short-term FFT. Soundspectrograms are a special form andwere formerly called sonograms.

f) Order spectrogramOrder spectrograms are a new

PRÜFTECHNIK capability. During run-up or when under load, resampled spec-tra are continuously recorded and su-perimposed on each other on a time-dependent basis.

g) Corbit (cascade orbit)Orbits can change spatially during

run-up analyses. Please see the VDI

3839 for a description of the availableevaluation methods.

h) Shaft movement diagramBoth shaft displacements and super-

imposed shaft vibrations can be depict-ed as spatial shaft movements or bend-ing lines in multiple dimensions (page 8,Fig. 3).

Other display methodsModal and operating deflection shape

(ODS) analyses, classification methods,shaker excitation and sweeps are further

display methods that make the vibrationbehavior of flexible rotors with flexiblebearings during run-up and coast-downeasier to understand.

The vibration technology departmentof the VDI is currently working on a newstandard in this field.

Fig.1: For natural frequency analyses, sensorsshould preferrably be mounted in a radial-horizontal and radial-vertical direction.

Fig. 3: Orbits change in their form, amplitude and rotational direction whenpassing through the critical speed.

Fig. 2: Imbalanced rotor with flexible bearings.

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PRÜFTECHNIKCondition Monitoring GmbH85737 Ismaning, GermanyTel: 089 99616-0Fax: 089 99616-341eMail: [email protected]

PRÜFTECHNIKAlignment Systems GmbH85737 Ismaning, GermanyTel: 089 99616-0Fax: 089 99616-100eMail: [email protected]

www.pruftechnik.com

Information on all trade fairs, semi-nars and other important events of thePRÜFTECHNIK Group can be found onour website at www.pruftechnik.com

Online monitoring of wearCounting and size classification of

wear particles in lubricating oil circuitsis a growing field in Condition Monitor-ing and supplements vibration-basedmethods. The WEARSCANNER® fromPRÜFTECHNIK is a compact sensor sys-tem that detects electrically conductiveparticles in the oil flowing through it,counts these and classifies them by sizeas per ISO 16232. Changes over time inthe size distribution allow users to drawconclusions on the wear condition anddamage development of the compo-nents.

One black line is all it takesThe laser-optical trigger and rotation-

al speed sensor (VIB 6.631) recognizesmarkings on the shaft. A black linepositioned at right angles to the direc-tion of rotation is usually sufficient as atrigger mark – even on highly reflectivesurfaces. After being adjusted to themarking, the non-contact sensor deliv-ers speed values and trigger pulses froma distance of up to two meters. Thismakes it easier to install and safelyoperate the sensor.

Measuring shaftmovement

For the measurementof shaft movement,PRÜFTECHNIK has aspecial displacementsensor and variousconnection adapters:1. Inductive displace-ment sensor (VIB 6.640)• Direct connection to VIBXPERT®

• Large working range (3 – 15 mm)• Linearization of the characteristic

line in the device• Works with a considerably smaller

detection area than an eddy currentsensor

• Includes a universal holder for axialand radial installation

• A suitable substitute for dial gauges2. Connection adapter for existing

protection monitoring systems• For measurement at buffered signal

outputs and at the keyphaser output(VIB 5.433, VIB 5.332)

• With overvoltage protection in com-pliance with interface conditions forVIBXPERT® EX (VIB 5.433-X,VIB 5.332-X)

3. Connection adapter for existingpowered displacement sensors(VIB 5.341)

• For example, the IN 085 fromSCHENCK.

In the lead – six times overVIBXPERT® II is not only valued by

customers. The international press, too,has singled out the fast vibration analyz-er with numerous awards: six respectedtrade publications see VIBXPERT® II asbeing outstanding in terms of innova-tion, power, flexibility and usefulness.More at www.vibxpert.com

A license to measureYou can become a certified vibration

expert according to ISO 18436 in thecategory I, II or III.

Manufacturers and operators of ma-chinery are seeking certified and skilledtechnicians in the field of vibration anal-ysis and diagnosis. PRÜFTECHNIK hastherefore expanded its seminar programand now offers a three-part ISO CATtraining that includes a standards-com-pliant qualification program with certifi-cation.

These three series of seminars buildon each other and take four days each.Participants who pass the final examina-tion become qualified certified vibrationexperts in the corresponding ISO cate-gory, either I, II or III.

PRÜFTECHNIK is represented in thetechnical committees that implementthe requirements of the above-men-tioned ISO standard in the German-speaking regions.

The WEARSCANNER® particle distributioncounter is a valuable Condition Monitoringtool not only in wind power plants.

prÜftechnik for turbomachinery - company | … · prÜftechnik for turbomachinery strong shaft...

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