basic vibration

BASIC VIBRATION

BYTPC MAINTENANCE

OSI TEAM

OVERVIEWIntroduction to Condition monitoringWhat causes vibration in machinery?Vibration terminology

Basic vibration theory Unit use in vibration analysis

Vibration transducer and signal processing

Different transducers used in vibration monitoring Basic signal processing from transducer output to analyzer

OVERVIEW

Fault diagnosis of: Unbalance Misalignment Looseness Anti-friction bearing

TPC Case HistoryTPC Case History Highlight different problems successfully detected in Highlight different problems successfully detected in

TPC I & IITPC I & II 1P-33 Motor bearing defect1P-33 Motor bearing defect 2B-302 High impacting2B-302 High impacting 2R-102B High enveloping vibration2R-102B High enveloping vibration 3C-102A Improved vibration after precision 3C-102A Improved vibration after precision

alignmentalignment 5B-125 High vibration due to bearing looseness5B-125 High vibration due to bearing looseness

OVERVIEW

Used of Vibration Simulator Simulate different fault and look at the related

spectrum

Introduction to Condition Monitoring

Maintenance Practices

Reactive Based• Run to failure• Unpredictable machine condition• High cost

Time Based• Machine are disassembled & overhaul on fixed schedule• More prominent practices in the 80’s• High cost

Condition Based• Repair only machine with known fault•Technology based monitoring such as

•Vibration analysis•Oil analysis•Thermography•Motor current analysis•Ultrasonic

Proactive Based• Root cause failure analysis• Used technology as in condition based maintenance


TPC Vibration Monitoring Program

TPC I TPC II

SPEL INo. Critical Equipments Monitor: 42

SPPNo. Critical Equipments Monitor: 64

SPEL IINo. Critical Equipments Monitor: 23

CPNo. Critical Equipments Monitor: 54


Vibration provides the most information about plant machinery non-intrusivelyVibration is a superior foundation for a Condition Based approach to maintenanceOf course, many other valid measurement can be made, such as:current, voltage, temperature, flow, torque, act

A mature Condition Based Maintenance program will include them all as needed

What causes vibration?

Vibration is caused by an excitation force such as: Driving torque acting on the machine Reaction from the load on the machine Additional stresses caused by a problem

unbalance misalignment loose part fluctuating fluid pressure bearing defect

To analyse a problem, the analyst tries to relate the vibration to the excitation force

BASIC VIBRATION

What is Vibration?How is it described?How is it measured?

What is Vibration?The movement of a body about a reference position.

+

A bearing moving back and forth on its pedestal A shaft in a journal moving

about the center of rotation

Describing Vibration(Terminology)

Amplitude How big is the vibration?

Waveform How the vibration change over time?

Frequency How rapidly does the vibration

change?

Phase What is the delay between event?

The Basic Vibration Parameter

Displacement Distance of an object from its reference

point (micron, mils)

Velocity The rate of change of displacement with

time (mm/sec, in/sec)

Acceleration The rate of change of velocity with time

(mm/sec², g’s)

Different ways of describing the amplitude of waveform

RMS = 0.707 times the peak valueAvg = 0.637 times the peak valuePk-Pk = 2 times the peak valueCaution! This only applies to the "simple" vibration shown here, not for complex waveforms

Average Root MeanSquare

Peak toPeak

Zero toPeak

Period and FrequencyThe period of a waveform is the time taken for the waveform to repeat itself

This waveform has a period of 0.04 sec.This waveform repeat itself every 0.04 sec

Period and FrequencyThe Calculation for Frequency is:Frequency = 1/TT = Time For this example T =0.04 sec = time to complete one revolution.Frequency = 1/0.04 = 25 Hertz or 1500 cpm

Period and FrequencyFrequency is the rate at which a simple waveform repeats itself

Peak at 25 Hz

Period and Frequency

Oh! Just one more thing: Sometimes frequency is expressed in

orders, which are multiples of shaft speed

For example, if the shaft speed is 1500 rpm, a frequency of 25 Hz is 1order

Then 50 Hz is the second order and so on

Simple Vibrations and Frequency

The simplest “simple” vibration( like our example) has just one frequency These vibration are very easy to handle

Typical machine vibration waveform are not simple, often a mix of many frequencies Not so easy, but we’ll see how to do it later

Measuring Machine Condition

Any parameterMeasurable without stopping machine,

given measure of operating status

Transducer (sensor)

Electrical signal

Signal processing

Problem detection

Automated Diagnostics Manual Diagnostic

Vibration TransducersVibration transducers are devices for converting vibration into equivalent electrical signals for analysisThe most common transducers include: Displacement Transducer

measures shaft displacement relative to bearing

Velocity Transducers measures casing velocity vibration

Accelerometer measures casing acceleration vibration

Application of Vibration Transducers

Displacement transducers: Typically used for shaft relative movement at

low frequencies

Velocity transducers commonly used for low to intermediate

frequency, where velocity believed to give best guide to vibration severity

Accelerometers: best for high frequency such as bearing

impacting, high speed gear & blading problems

transducers of choice for industrial application

Displacement TransducersMeasures relative displacement between probe tip and rotating shaft or targetUseful on machine with high case to rotor weight ratio (e.g. Steam turbine)May be already installed as OEM equipmentUsually permanently installedLimited frequency range due to run out typical 0 to 1000 Hz

Requires special power supply, signal conditioner & cables

Displacement Transducers

-9V DC

-24V DC

Driver

CLShaft

Probe Tip Near Shaft

Probe Tip Far Away From Shaft

Bias or DCGap Voltage

DisplacementProbe

Velocity TransducersSeismic transducer works well where there is significant casing vibrationOlder style of transducers; developed in 1940Gives velocity signal directlySelf-generating, no power requiredLimited frequency range (10Hz to1000Hz)Tend to be relatively large and heavyCalibration may shift due to wear and temperature( due to damping)

Velocity TransducersTransducer Connector

Transducer Case

Spring

Transducer Coil

Permanent Magnet

Damping Fluid

Accelerometer TransducersThe transducer of the choice in industry todayVery wide frequency range possible from 0 to 20 kHz( different transducers) typically from 2 to 15kHz

Extremely rugged, no moving partsRelatively small and lightweightEasy mount for permanent or intermittent useRequires constant current power supply for built-in amplifierSignal output in acceleration

Accelerometer TransducersAmplifier

InertialMass

PiezoelectricCrystal

Insulator

ConductivePlate

Insulator

Preload Bolt

Signal ProcessingWe have already looked at measuring the level of a simple waveform (single frequency)A complex signal can be broken down into “simple” components, using the process of frequency analysisThe level of each component can be measured using the simple approachMoreover, the frequencies of the components in a machine vibration signal can be used to identify many different faultsFault severity is linked to the amplitude of the components associated with that fault

Signal Processing

Displacement waveform from 40 megawatt steam turbine. Notice the complexity of the waveform - its NOT simple.

Signal ProcessingA

mpl

itude

Time

We begin with a vibration waveform, represented by the transducer signal...

Signal Processing

Am

plitu

de

Time

Am

plitu

de

Time

Analyzer

... we feed the signal into the analyzer and the process begins...

Signal Processing

Am

plitu

de

Frequency

Am

plitu

de

Time

Am

plitu

de

Time

Time Frequency

The waveform picked up by the transducer is a complex mixture of waveform

Complex waveform is broken down into simple waveform components

Each waveform component has a separate frequency ...

Spectrum, or signature, shows the amplitudes of all the frequency components in the signal

Signal ProcessingA

mpl

itude

Time

Am

plitu

de

Frequency

SpectrumWaveform

The spectrum and the waveform are just twoways of looking at the same information

In general, both the waveform and spectrumshould be used together to maximize information

Signal Processing

Looseness

Misalignment

Imbalance

Oil Whirl

The spectrum of the displacement from 40 MW steam turbine is much easier to understand. It is easy to see the relative severity of oil whirl, unbalance, misalignment and looseness.

DIAGNOSTICS

Identifying the CharacteristicVibration Patterns of Common

FaultsUnbalanceMisalignmentLoosenessAnti- Friction Bearing

UnbalanceSimple unbalance occurs when the center of mass of a rotating object differs from the center of rotation. (static unbalance)

Center of

Mass

Center of

Rotation

UnbalanceWhen rotation axis is not coincident with a principal of inertia of the rotation body- often called dynamic unbalance

Axis of Rotation

Principal Axis of Inertia

Unbalance

Causes of unbalance include: Improper assembly of parts Material buildup on blades or

impellers Wear on rotating elements Broken or missing rotor parts Improper or no balance specs on

rotor

Characteristics of Unbalance

High amplitude radial peaks at 1x shaft RPMVery low axial vibration level at 1x shaft RPMHarmonics of shaft RPM are very low in amplitude1x RPM pattern visible in the waveformIf harmonic of shaft RPM have significant amplitude, other faults should be suspected

Characteristics of Unbalance

1x shaft speed

Very little harmonics

Misalignment

Misalignment occurs when the center lines of two shafts are offset or meet at an angleDifferent categories of this defect include: offset misalignment angular misalignment bearing misalignment

We’ll look at offset/angular misalignment

Misalignment

Angular Misalignment

Offset Misalignment

Characteristic of misalignment

High axial vibration at 1,2,3 x shaft RPMHigh radial vibration at 1,2,3 x shaft RPMHigher orders of shaft RPM (>4x) are generally low in amplitudeA repeatable pattern is visible in the time domain with little or no visible impactingA phase shift of about 180 deg seen across the coupling or between bearing


Shaft centerlines meet at an angleOften strong component at 1X RPMMay cause dominant vibration in any planeTherefore important to take 2 radial readings per bearing plus one axial reading per shaft

Angular Misalignment


Shaft centerlines parallel but offsetStrong radial component at 2 X Run SpeedOther shaft orders also appearAlmost always combined with angular misalignment

Offset Misalignment

Misalignment

1x shaft speed

2x shaft speed

3C-102A Before Alignment

Misalignment3C-102A After Alignment

Vibration at 1x & 2x shaftspeed reduced

LOOSENESS

There are two main types of looseness:

Structural Looseness Faulty or Eroded Base Mounts Cracked or Split Casings Improperly Torque Bearing Caps Bearing Supports

Rotating Element Looseness Impellers Fan Blades Bearings Couplings

Characteristics of Looseness

Spectra show large number of ordersVibration is often directional: horizontal & vertical amplitudes may differ significantlySub-synchronous peaks sometimes occur"Half-harmonics" and "one-third harmonics", etc., may occur in some casesWaveform often shows erratic impacting and no clear pattern as with misalignment

LOOSENESSOB-153A Bearing Looseness before repair

Vibration at 1x shaft speedand its harmonics

LOOSENESSOB-153A Bearing Looseness after repair

Vibration at higher harmonic reduced after repair done on the bearing

ANTI-FRICTION BEARING

Causes of premature bearing failures include:

Improper techniques for mounting bearing on shaft and in housing during machine rebuildExcessive or inadequate lubrication

More bearings fail from too much lubrication than from inadequate lubrication.

Incorrect application of bearings shortens life E.g. high axial load on non-thrust type bearing

Excessive vibration causes increased stress Two-fold stress increase may reduce life ten-fold

Incorrect bearing fits may cause damage

Bearing Defect Characteristics

Rolling element bearing defects show characteristic vibration frequency patterns Harmonics of non-synchronous peaks visible

Multiples of bearing frequencies Sidebands may become apparent

shaft frequency for inner race fault cage frequency for rolling element fault

Spectrum floor level may show broadband rise Acceleration waveform shows impacting

Calculating Defect Frequencies Require physical dimensions and shaft speed

pitch and ball/roller diameters, number of balls/rollers Or need manufacturers bearing data..


Frequency Type Description

FTF Fundamental Train Frequency

Rate at which cage orretainer rotates

BSF Ball Spin Frequency Rate at which ball fault contacts either race

BPFO Ball Pass Frequency Outer Race

Rate at which balls contact outer race fault

BPFI Ball Pass Frequency Inner Race

Rate at which balls contact inner race fault


BearingType

No. of Balls or Rollers

FTF BSF BPFO BPFI

TOR 23318 15 .412 2.665 6.177 8.823TOR 23322 14 .407 2.527 5.703 8.297TOR 23324 15 .411 2.619 6.160 8.839TOR 23326 14 .407 2.516 5.700 8.299TOR 23328 14 .409 2.574 5.726 8.274TOR 23330 15 .410 2.601 6.149 8.850TOR 23332 15 .412 2.652 6.174 8.826TOR 23334 16 .416 2.795 6.652 9.348TOR 23336 16 .416 2.791 6.651 9.349TOR 23338 15 .415 2.786 6.232 8.768TOR 23340 16 .416 2.815 6.661 9.228TOR 23344 16 .415 2.777 6.644 9.356TOR 23348 16 .415 2.773 6.641 9.259TOR 23352 16 .415 2.770 6.638 9.362TOR 23356 16 .415 2.765 6.637 9.363TOR 238-1060 54 .476 10.494 25.720 28.280TOR 23926 31 .459 5.978 14.220 16.780TOR 23928 32 .461 6.379 14.760 17.240

Multiply Shaft Speed By Factor to Get Bearing Frequency

CASE HISTORIES

Process member inform that the motor was noisy and vibration measurement was carried on the motor on 08-01-01.Since the equipment was not in OSI monitoring schedule at that time, 2P-33 vibration measurement was also taken for comparison.Vibration spectrum taken on Motor DE bearing show peak at outer race frequency and its harmonic with sidebands of cage frequency.Peaks from bearing frequencies had disappear after bearing was replaced.

1P-33 Motor bearing defect

CASE HISTORIES1P-33 Motor Bearing Defect (before repair)

Peak at 3rd & 4th harmonic of BPFOMotor DE bearing

Velocity measurement taken on 1P-33 Motor bearing DE

CASE HISTORIES

Peak at 3rd & 4th harmonic of BPFOMotor DE bearing disappear afterreplacing bearing

1P-33 Motor Bearing Defect (after repair)Velocity measurement taken on 1P-33 Motor bearing DE

CASE HISTORIES1P-33 Motor bearing defect

Enveloping vibration show peak at BPOR (outer race) frequency and its harmonic

Sideband of cage frequency

Enveloping vibration measurement taken on 1P-33 Motor bearing DE


Enveloping vibration show peak at running speed and its harmonic. No peak at bearing frequencies

Enveloping vibration measurement taken on 2P-33 Motor bearing DE


Outer race defect only on one sideDiscoloration of inner race most probably

due lubrication breakdown and load

Defect on two balls and discoloration of ball most probably due lubrication breakdown

Close up view of the defect area

CASE HISTORIES

High rotor pass frequency(4x) and enveloping vibration at driven rotor pulley side.Five sense check had found broken drain nozzle of the air chamber at pulley side.The high vibration was due to rotor rub as a result of unbalance air pressure at pulley side.Vibration level went back to normal after replacement of broken drain nozzle & before deteriorating into unplanned break-down maintenance.

2B-302 High Impacting

CASE HISTORIES2B-302 High Impacting

Data taken on 6/10/00 with amplitude of 9.61 mm/sec

Data taken on 17/10/00 with amplitude of 6.35mm/sec after replaced broken air chamber nozzle.

Velocity vibration taken on Point 5AV (Driven rotor pulley side)


Enveloping vibration taken on Point 5HE (Driven rotor pulley side)

Data taken on 6/10/00 with amplitude of 4.94 g’s

Data taken on 17/10/00 with amplitude of 0.92 g’s after replaced broken air chamber nozzle.


Vibration spectrum taken at Point 5AV before repair

Vibration spectrum taken at Point 5AV after repair

Peak at 4x rotor pass frequency at 187.5 Hz

Peak at 4x rotor pass frequency at 187.5 Hz


Waveform data taken on 17/10/00 doesn’t show clear sign of impacting and amplitude reduced to 20 g’s.

Waveform data taken on 05/10/00 show high impacting up to 50 g’s.

CASE HISTORIES

Monthly monitoring show an increased in the enveloping vibration on the Mechanical Seal since September 00.The equipment was under closely monitoring for about one month to see the rate of increased.Mechanical group and Process member was informed about the increased and site check found that the needle valve was throttled to inadequate flow rate.The valve was adjusted and the oil was topped up.The vibration on the Mechanical Seal was back to normal after the adjustment.

2R-102B High Enveloping

CASE HISTORIES2R-102B High Enveloping

Data taken on 02/08/00 with amplitude of 0.007

Data taken on 14/09/00 show increased in value from 0.007g’s to 0.0308g’s (340%)

Data taken on 11/10/00 with amplitude of 0.0488g’s

Data taken on 20/10/00 with amplitude of 0.009 after topping oil and adjusting needle valve.

CASE HISTORIES2R-102B High EnvelopingEnveloping vibration taken on Reactor mechanical seal bearing before adjusting needle valve

Spectrum shows high noise level without any particular peak indicated that the bearing was running in very rough condition.

Spectrum shows very low noise floor level after adjusting needle valve and topping of oil

Enveloping vibration taken on Reactor mechanical seal bearing after adjusting needle valve

CASE HISTORIES

Vibration on motor had increased after servicing in August 1999.The increased was due to peak at 1x running speed and its lower harmonics.Misalignment was suspected and detail analysis was carried out to confirm the misalignment problem.Precision alignment was carried by Electrical group.Vibration on the motor reduced after the alignment.

3C-102A Improved vibration level after precision alignment

CASE HISTORIES3C-102A Improved vibration level after precision alignment

Vibration started to increased in 27-08-1999 and maintain at that level

Vibration level reduced after precision alignment carried out in August 2000

CASE HISTORIES3C-102A Improved vibration level after precision alignment

Vibration level reduced after precision alignment carried out in August 2000

Vibration level increased in August 1999 due peak 1X running speed and its lower harmonic

CASE HISTORIES

Monthly monitoring done on May 2000 showed an increased in vibration level on the blower at drive rotor pulley side.Quarterly oil sample in June also showed an increased in wear particles from 142 to 2780. Spectral showed peak at rotor pass frequency and its harmonicsThe blower was overhauled in mid June and found the drive rotor shaft was loosed & worn out at inboard bearing position. The shaft was rebuilt and installed back.Vibration level went back to normal after the repair work.

5B-125 High vibration due to looseness

CASE HISTORIES5B-125 High vibration due to looseness

Vibration level increased due to peak at rotor pass frequency on 7-06-2000

Vibration measurement taken on drive rotor pulley(Point 5HV)


Vibration show higher peak at rotor pass frequency and its harmonic. The noise floor was also higher.

Vibration show lower peak at rotor pass frequency and its harmonic. The noise floor was also lower.

Vibration measurement taken on drive rotor pulley side before repair

Vibration measurement taken on drive rotor pulley side after repair


Inner race of bearing and shaft was found to be loose fit not thigh fit Pitting found on the shaft where

the bearing sit

basic vibration

Documents

critical equipments

adjusting

rotor pass

153a bearing

precision

vibration

shaft speed

shaft rpm