r&t 2008 - principles and practices of vibrational analysis - keefer (1)

7/29/2019 R&T 2008 - Principles and Practices of Vibrational Analysis - Keefer (1)

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Vibration Analysis Services


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What is Vibration?

Vibration is the movement of a body about its reference position.

Vibration occurs because of an excitation force that causes motion.


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Vibration Terms



Time Waveform Analysis

complex time waveform

Individual vibration signalscombine to form a complextime waveform showing overallvibration

frequ

ency

lowfreq

.

high

freq

.

timeoverall vibration



Overall Vibrationz The total vibration

energy measuredwithin a specificfrequency range.

includes a combinationof all vibration signalswithin measuredfrequency range

does not includevibration signals outsidemeasured frequencyrange

produces a numericalvalue


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Amplitude vs. Frequency Vibration amplitude indicates the severity of the problem. Vibration frequency indicates the source of the problem.

2X3X

4X

frequency

amplitud

e

1X


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Vibration- Measurable Characteristics

0 90 180 270 360DisplacementVelocityAcceleration

Time

Velocity is the first derivative of displacement as a function of time, it is the rateof change in displacement (the speed of the vibration).

Acceleration is the second derivative of displacement, it is the rate of change of

velocity (the change in speed of the vibration).


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Scale FactorsWhen comparing overall vibration signals, it isimperative that both signals be measured on the

same frequency range and with the samescale factors. NOTE: RMS is .707 of peak.


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Measurements & Units

Displacement (Distance)mils or micrometer, mm

Velocity (Speed - Rate of change of displcmt)in/sec or mm/sec

Acceleration (Rate of change of velocity)Gs or in/sec2 or mm/sec2


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Lines of ResolutionIndividual Vertical Lines or Bins Located Adjacent to One AnotherAlong the Frequency Axis. Each Bin is used to Store Individual Amplitude

at a Specific Frequency Location.

Amplitude

Frequency in CPM

7200CPM

3570CPM

Amplitude

Frequency in CPM

7200CPM

3570CPM


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Accelerometers

Rugged Devices Operate in Wide Frequency

Range (Near 0 to above 40 kHz)

Good High Frequency Response

Some Models Suitable For High

Temperature

Require Additional Electronics

(may be built into the sensor housing)


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Velocity Sensors

Often Measure BearingHousings or Machinery Casing

Vibration

Effective in Low to Mid

Frequency Range (10 Hz to

around 1,500 Hz)

Self Generating Devices


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Displacement Probe/Eddy Probe

Measure RelativeDistance Between

Two (2) Surfaces

Accurate LowFrequency

Response


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Multi-Parameter Monitoring

Same Data in Velocity and Acceleration

VelocitySpectrum

AccelerationSpectrum

On the same bearing, lowfreq. events (imbalance,misalignment, etc.) showbest in the velocityspectrum; while high freq.events (bearing faults,gearmesh) show best in the

acceleration spectrum

FES Model 32L S/N AB10099P#2 West -C3H Compressor Inboard Horizontal

Route Spectrum06-Feb-01 14:02:05

OVRALL= .6123 V-DGPK = 8.19LOAD = 100.0RPM = 2990.RPS = 49.83

0 40 80 120 160 200 240

0

1

2

3

4

5

Frequency in kCPM

PKAccelerationinG-s

Reference En v/Prf-Std

- Model 32L S/N AB10099P#2 West -C3H Compressor Inboard Horizontal

Route Spectrum06-Feb-01 14:02:05

OVRALL= .6123 V-DGPK = .6091LOAD = 100.0RPM = 2990.RPS = 49.83

0 40 80 120 160 200 240

0

0.08

0.16

0.24

0.32

0.40

Frequency in kCPM

PKVe

loc

ity

inIn/Sec

Reference Env/Prf-Std


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10 100 1,000 10,000

Frequency (Hz)

10

1.0

0.1

1

0.01

100

Displacement (mils)Acceleration(g's)

Velocity (in/sec)

Common MachineryOperating Range

Amplitude(mils, in/sec, gs)

Sensor Relationships


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Resonance

typically 10% or greater


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Detection vs. Analysisz Detection

Alarm limits are established for each measurement.When the measurements value exceeds itsprogrammed alarm limits, the predictive maintenance

software or data collector notifies the analyst of aproblem.

z Analysis

Once detected, analyzing exceptional measurementsprovides insight to the problem itself, and to its rootcause.


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Important Frequency Peaks

Rotational Speed or Even MultiplesAlways present but excessive amplitude or multiple

harmonics can indicate a problem.

Electric motors always have frequency peaks at shaftrotational speed and at line frequency i.e. 60 Hz.

Two pole motors will always display a 2X line frequency

peak.


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Important Frequency Peaks

Gas Pulsation FrequenciesScrew compressors - gas pulsation frequency(cpm) occurs at[No. of lobes on male rotor] X [ rotational speed (RPM)]

Pumps or fans - fluid pulsation frequency(cpm) occurs at[No. of vanes, lobes or blades] X [ rotational speed (RPM)]

Recip. Compressors - gas pulsation frequency (cpm) occurs

at [No. of pistons] x [ rotational speed (RPM)]Harmonics or even multiples (2X and 3X) of gas pulsationfrequencies always present and are most noticeable on oilseparator vessels.


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Setting Up the Measurementz Physical Considerations

Selecting the Machinery Selecting Measurement Planes

Selecting Sensor Locations

Surface Preparation Sensor Mounting Techniques

z Database Considerations

Parameters (multi-parameters) Alarm Limits

Setting Fmax

Scale Factors


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Selecting the MachineryCritical - If a failure or shutdown occurs,

production is stopped, or machineperformance creates an unsafe environment

Essential Spared - If a failure orshutdown occurs, production is disrupted

Non Essential Spared - If a failureor shutdown occurs, production loss isinconvenienced, however, a spare unit canbe brought on-line, or a repair can bring theproduction unit back on-line withoutsignificant loss of production


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ThinkingAhead

z Walk Through

z Machinery Data

Sheets


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Measurement Planes

z radial

vertical

horizontal

z axial


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Sensor Location(qualifying and identifying)

Measurement POINT numbering

follows flow of power:

Motor Non-Driven End (NDE)Motor Driven End (DE)Compressor Driven End (DE)Compressor Non-Driven End (NDE)


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Sensor Location

The accelerometer must be located over the bearing of interest. Avoid air gaps

in housings whenever possible. Air gaps will skew vibration readings.


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Sensor Location


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Mounting Methods


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Hand-held/Probe Mounting

Rapid andconvenient.

Subject tomany sourcesof error.

Use only as alast resort.


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Types of Alarms

z

Overall Vibration Limitsz Spectral Enveloping

z

Spectral Bandsz Phase Alarms


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Acceptable Vibration Levels

Tables are published that show overall vibration levels as afunction of rotational speed or vibration frequency for the

purpose of determining whether vibration levels are acceptable.As a general rule for compressors operating at 3600 RPM an

overall vibration level of 0.3 ips RMS would be cause for concern

For piping and valves overall readings exceeding 1.0 ips RMS

would be cause for concern though actual stress values inducedby the vibration may be quite low and no corrective action

needed. Some engineering evaluation should be conducted todetermine this.


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ISO Guidelines

ISO 2372overall

velocityvibrationguidelines


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AssessingOverall

VibrationSeverity

ve

loc

ity-

in/sec

(pea

k)

Frequency - CPM

ac

ce

lera

tion-

Gs

(pea

k)


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Overall Vibration Trend PlotVIB - Alignment Fault

ALIGNMENT -M1H MOTOR OUTBOARD BRG. - HORIZONTAL

Trend Display

of

OVERALL VALUE

-- Baseline --Value: .06350

Date: 11-AUG-95

0 100 200 300 400 500

0

0.04

0.08

0.12

0.16

0.20

0.24

Days: 11-AUG-95 To 11-DEC-96

PKVeloc

ityinIn/Sec

WARNINGALERT

FAULT


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Spectral Enveloping

alarm is triggered

VIB - Balance Fault

BALANCE -M2A MOTOR INBOARD AXIAL

Route Spectrum14-MAR-96 12:10:26

OVRALL= .3260 V-DGPK = .3257

LOAD = 100.0RPM = 1777.

RPS = 29.62

0 400 800 1200 1600 2000

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Frequency in Hz

PKVeloc

ity

inIn/Sec

Reference Envelope


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Phase Alarms

A2 - Machine #6 (Various Setups)

MACH#6 -PPH PEAK PHASE DATACorrelationDisplay

Phasevs

Peak

Data Period:26-Dec-96

To28-Dec-96

0

180

90 270

2.500

Peak


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Motor and compressors with sleeve bearings do not lendthemselves well to readings with accelerometers and for

dependable information a device such as a proximity probe

should be used to measure vibration in mils displacement.Probes should be oriented in two planes 90 apart and

displacement cannot exceed the shaft to bearing clearance.


Motor and compressors with sleeve bearings do not lendthemselves well to readings with accelerometers and fordependable information a device such as a proximity probeshould be used to measure vibration in mils displacement.Probes should be oriented in two planes 90 apart anddisplacement cannot exceed the shaft to bearing clearance.


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For Individual spectrum peaks limits are set by their perceived

cause but some general limits are shown below:

Compressors:

Rotational speed 1X, 2X, 3X 0.25 ips RMSGas Pulsation at compr. 0.27 ips RMS

Bearing fault frequencies 0.15 ips RMS

Roller bearings (2000-3000 Hz) 2.5 gs


For Individual spectrum peaks limits are set by their perceivedcause but some general limits are shown below:

Compressors:

Rotational speed 1X, 2X, 3X 0.25 ips RMSGas Pulsation at compressor 0.27 ips RMS


Roller bearings (2000-3000 Hz) 2.5 gs


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Motors:

Rotational Speed 1X, 2X, 3X 0.25 ips RMS

Line Frequency 1X, 2X 0.13 ips RMS



Motors:Rotational Speed 1X, 2X, 3X 0.25 ips RMSLine Frequency 1X, 2X 0.13 ips RMS



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At start up - Baseline

Six months after start up unless a problem is suspected. After

that every 6 month to one year after that unless a deteriorating

trend is observed.

At 25000 hours readings should be taken every three months

to extend the time before an internal inspection is required.

Anytime an unusual noise or vibration isnoticed.

Readings-How Often?

At start up - Baseline

Six months after start up unless a problem is suspected. Afterthat every 6 month to one year after that unless a deterioratingtrend is observed.

At 25000 hours readings should be taken every three months

to extend the time before an internal inspection is required.

Anytime an unusual noise or vibration is noticed.


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Spectrum Analysis Techniquesz Collect Useful Information

z Analyze

500 HP/3570 RPMMotor Model 23LE

C1 C2

C3 C4

M1 M2

C


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Spectrum Analysis TechniquesSome compressors have a combination of sleevebearings and ball thrust bearings that require differentanalysis techniques.

FES Model GL Series Compressors

Thrust andSleeveBearingLocation

Sleeve Bearing Location


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Latter stages of journal bearing wear are normally evidencedby presence of whole series of running speed harmonics (up to10 or 20). Wiped journal bearings often will allow high vertical

amplitudes compared to horizontal, but may show only onepronounced peak at 1X RPM. Journal bearings with excessiveclearance may allow a minor unbalance and/or misalignment tocause high vibration which would be much lower if bearingclearances were set to specifications. Source: Technical Associates Inc.Illustrated Vibration Chart

Sleeve Bearing Wear Pattern


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Why Do Bearings Fail?Inadequate Lubrication- too much

- too little- contaminated

Excessive LoadCaused by:

- misalignment- imbalance- bent shaft- etc.....

Improper Handling orInstallation

AgeSpall On Outer Race


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Typical Bearing Failure Rate


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Bearing Defect FrequenciesBPFO

Ball Pass Frequency Outer RaceBPFIBall Pass Frequency Inner Race

BSFBall Spin Frequency

FTFCage Frequency orFundamental Train Frequency


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Bearing Failure StagesStage 1 Stage 2

Stage 3 Stage 4

No apparent change on typical velocity spectrum Defects harmonic frequencies appear

Defects fundamental frequencies also appearand may exhibit sidebands

Defects harmonic frequencies develop multiplesidebands (haystack), fundamental freqs. grow

and also develop sidebands

defects fund.frequency range

defects harmonicfrequency range


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SidebandsVIB - Alignment FaultALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL

Route Spectrum01-AUG-96 15:15:26

OVRALL= .0665 V-DG

RMS = .2506

LOAD = 100.0RPM = 3606.

RPS = 60.10

0 1000 2000 3000 4000

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Frequency in Hz

RMSAcce

lera

tion

inG-s

Freq:

Ordr:

Spec:

Dfrq:

2634.6

43.84

.02417

120.19


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HarmonicsVIB - Alignment FaultALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTALRoute Spectrum

01-AUG-96 15:15:26

OVRALL= .0665 V-DGPK = .0660

LOAD = 100.0

RPM = 3606.

RPS = 60.10

0 1000 2000 3000 4000

0

0.01

0.02

0.03

0.04

0.05

0.06

Frequency in Hz

PKVeloc

ity

inIn/Sec

Freq:

Ordr:

Spec:

57.69

.960

.02572


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Waterfall Plot

PKVelocityinIn/Sec

Frequency in Hz

VIB - Alignment Fault

ALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL

0 1000 2000 3000 4000

0

0.05Max Amp.0456

01-AUG-96

05-SEP-96

30-SEP-96

28-OCT-96

21-NOV-96

11-DEC-96

RPM= 3550.15:15:2601-AUG-96

Freq:

Ordr:Sp 1:

57.69

.975.02589


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Stator problems generate high vibration at 2X line frequency (2FL). Statoreccentricity produces uneven stationary air gap between rotor and statorwhich produces very directional vibration. Differential Air Gap should notexceed 5% for induction motors and 10% for synchronous motors. Soft footand warped bases can produce an eccentric stator. Loose iron is due to

stator support weakness or looseness. Shorted stator laminations can causeuneven, localized heating which can distort the stator itself. This producesthermally-induced vibration which can significantly grow with operating timecausing stator distortion and static air gap problems.

Source: Technical Associates Inc.

Illustrated Vibration Chart


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Eccentric Rotors produce a rotating variable air gap between the rotor andstator which induces pulsating vibration (normally between 2FL and closestrunning speed harmonic). Often requires "zoom" spectrum to separate 2FLand running speed harmonic. Eccentric rotors generate 2FL surrounded byPole Pass frequency sidebands (F

P), as well as F

Psidebands around running

speed. FP appears itself at low frequency (Pole Pass Frequency = SlipFrequency X #Poles). Common values of FP range from about 20 to 120 CPM(0.3 - 2.0 Hz). Soft foot or misalignment often induces a variable air gap dueto distortion (actually a mechanical problem; not electrical).

Source: Technical Associates Inc.



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Broken or Cracked rotor bars or shorting rings; bad joints betweenrotor bars and shorting rings; or shorted rotor laminations willproduce high 1X running speed vibration with pole pass frequency

sidebands (FP). In addition, these problems generate FP sidebandsaround the second, third, fourth and fifth running speed

harmonics.Source: Technical Associates Inc.



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Loose or open rotor bars are indicated by 2X line frequency (2FL) sidebandssurrounding Rotor Bar Pass Frequency (RBPF) and/or its harmonics (RBPF =Number of Bars X RPM). Often will cause high levels at 2X RBPF, with only asmall amplitude at 1X RBPF. Electrically induced arcing between loose rotorbars and end rings will often show high levels at 2X RBPF (with 2FLsidebands); but little or no increase in amplitudes at 1X RBPF.

Source: Technical Associates Inc.Illustrated Vibration Chart


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Phasing problems due to loose or broken connectors can cause excessivevibration at 2X Line Frequency (2FL) which will have sidebands around itspaced at 1/3 Line Frequency (1/3 FL). Levels at 2FL can exceed 1.0 in/sec if

left uncorrected. This is particularly a problem if the defective connector isonly sporadically making contact. Loose or broken connectors must berepaired to prevent catastrophic failure.

Source: Technical Associates Inc.Illustrated Vibration Chart

Dosk - RAM 700 HP Motor Test1


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RAM TEST 1-M2L Mot. Inboard Horiz./2X Line Freq

Route Spectrum

10-Apr-01 08:20:35

OVRALL= .3045 V-DG

PK = .3028

LOAD = 100.0

RPM = 3579.

RPS = 59.64

0 8000 16000 24000

0

0.08

0.16

0.24

0.32

0.40

Frequency in CPM

PKVe

loc

ity

inIn/S

ec


Freq:

Ordr:Spec:

7200.0

2.012.283

The 2x Line frequency on this motor is .283 in/sec.this indicates a stator eccentricity problem.The spectrum was taken at 6400 lines of resolution.

Dosk - RAM 700 HP Motor Test1

RAM TEST 1-M2L Mot. Inboard Horiz./2X Line Freq

Route Spectrum

10 A 01 08 20 35

0.40


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10-Apr-01 08:20:35

OVRALL= .3045 V-DGPK = .2922

LOAD = 100.0

RPM = 3579.

RPS = 59.64

6800 7000 7200 7400 7600

0

0.08

0.16

0.24

0.32

Frequency in CPM

PKVe

loc

ity

inIn/Sec


Freq:

Ordr:

Spec:

7200.0

2.012

.283

The 2 x Line F

The 2 x Line Frequency must be separated from 2 x turning speedto determine rotor or stator problems. The data collector must be setto a sufficient number of lines of resolution to separate these two frequencies

2 x Line Freq.

7140 RPM

2x turning speed


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Vibration analysis can be used to determine rotor problems in motors.The rotor bar pass frequency has penetrated the narrow band alarm.


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Vibration Analysis Services

r&t 2008 - principles and practices of vibrational analysis - keefer (1)

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