principles and practices of vibrational analysis
DESCRIPTION
Principles and Practices of Vibrational AnalysisTRANSCRIPT
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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
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Time Waveform Analysis
complex time waveform
Individual vibration signals
combine to form a complex
time waveform showing overall
vibration
frequ
ency
low
freq.
high
freq
.
timeoverall vibration
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Overall Vibration The total vibration
energy measuredwithin a specific
frequency range.
includes a combinationof all vibration signals
within measured
frequency range
does not includevibration signals outside
measured frequency
range
produces a numerical
value
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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 rate
of 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 is
imperative 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 ormicrometer, mm
Velocity (Speed - Rate of change of displcmt)in/sec ormm/sec
Acceleration (Rate of change of velocity)Gs orin/sec2 ormm/sec2
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Lines of ResolutionIndividual Vertical Lines or Bins Located Adjacent to One Another
Along 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 andAcceleration
VelocitySpectrum
AccelerationSpectrum
On the same bearing, low
freq. events (imbalance,
misalignment, etc.) show
best in the velocity
spectrum; 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 Spectrum 06-Feb-01 14:02:05
OVRALL= .6123 V-DG PK = 8.19 LOAD = 100.0 RP M = 2 990. RP S = 49.83
0 40 80 120 160 200 240
0
1
2
3
4
5
Frequency in kCPM
PK
AccelerationinG-s
Reference Env/Prf-Std
- Model 32L S/N AB10099P#2 West -C3H Compressor Inboard Horizontal
Route Spectrum 06-Feb-01 14:02:05
OVRALL= .6123 V-DG PK = .6091 LOAD = 100.0 RPM = 2990. RPS = 49.83
0 40 80 120 160 200 240
0
0.08
0.16
0.24
0.32
0.40
Frequency i n kCPM
PK
VelocityinIn/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. Analysis Detection
Alarm limits are established for each measurement.When the measurements value exceeds its
programmed alarm limits, the predictive maintenance
software or data collector notifies the analyst of aproblem.
Analysis
Once detected, analyzing exceptional measurements
provides insight to the problem itself, and to its root
cause.
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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 pulsation
frequencies always present and are most noticeable on oil
separator vessels.
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Setting Up the Measurement Physical Considerations
Selecting the Machinery Selecting Measurement Planes
Selecting Sensor Locations
Surface Preparation Sensor Mounting Techniques
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 is
inconvenienced, however, a spare unit can
be brought on-line, or a repair can bring the
production unit back on-line without
significant loss of production
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ThinkingAhead
Walk Through
Machinery Data
Sheets
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Measurement Planes
radial
vertical
horizontal
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 and
convenient.
Subject to
many sources
of error.
Use only as a
last resort.
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Types of Alarms
Overall Vibration Limits Spectral Enveloping
Spectral Bands 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 induced
by the vibration may be quite low and no corrective action
needed. Some engineering evaluation should be conducted to
determine this.
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ISO Guidelines
ISO 2372overall
velocityvibration
guidelines
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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
PK
Veloc
ityinIn/Sec
WARNINGALERT
FAULT
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Spectral Enveloping
alarm is triggered
VIB - Balance Fault
BALANCE -M2A MOTOR INBOARD AXIAL
Route Spectrum
14-MAR-96 12:10:26
OVRALL= .3260 V-DG
PK = .3257
LOAD = 100.0
RPM = 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
PK
Veloc
ity
inIn/Sec
Reference Envelope
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Phase Alarms
A2 - Machine #6 (Various Setups)
MACH#6 -PPH PEAK PHASE DATACorrelation
Display
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.
Acceptable Vibration Levels
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.
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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
Acceptable Vibration Levels
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 RMS
Gas Pulsation at compressor 0.27 ips RMS
Bearing fault frequencies 0.15 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
Bearing fault frequencies 0.15 ips RMS
Acceptable Vibration Levels
Motors:Rotational Speed 1X, 2X, 3X 0.25 ips RMSLine Frequency 1X, 2X 0.13 ips RMS
Bearing faul t frequencies 0.15 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. 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 is noticed.
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Spectrum Analysis Techniques Collect Useful Information
Analyze
500 HP/3570 RPM
Motor 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 and
Sleeve
Bearing
Location
Sleeve Bearing Location
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Latter stages of journal bearing wear are normally evidenced
by presence of whole series of running speed harmonics (up to
10 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 excessive
clearance may allow a minor unbalance and/or misalignment to
cause high vibration which would be much lower if bearing
clearances 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 li tt le- contaminated
Excessive Load
Caused 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 or
Fundamental 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 appear
and may exhibit sidebandsDefects harmonic frequencies develop multiple
sidebands (haystack), fundamental freqs. grow
and also develop sidebands
defects fund.
frequency range
defects harmonic
frequency range
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SidebandsVIB - Alignment FaultALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL
Route Spectrum
01-AUG-96 15:15:26
OVRALL= .0665 V-DG
RMS = .2506
LOAD = 100.0
RPM = 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
PK
Veloc
ity
inIn/Sec
Freq:
Ordr:
Spec:
57.69
.960
.02572
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Waterfall Plot
PK
VelocityinIn/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.
15:15:26
01-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 stator
which produces very directional vibration. Differential Air Gap should not
exceed 5% for induct ion motors and 10% for synchronous motors. Soft foot
and 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 produces
thermally-induced vibration which can significantly grow with operating time
causing 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 and
stator which induces pulsating vibration (normally between 2FL and closest
running speed harmonic). Often requires "zoom" spectrum to separate 2FLand running speed harmonic. Eccentric rotors generate 2FL surrounded by
Pole Pass frequency sidebands (FP), as well as F
Psidebands around running
speed. FP appears itself at low frequency (Pole Pass Frequency = Slip
Frequency 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 due
to distortion (actually a mechanical problem; not electrical).Source: Technical Associates Inc.
Illustrated Vibration Chart
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Broken or Cracked rotor bars or shorting rings; bad joints between
rotor bars and short ing rings; or shorted rotor laminations will
produce 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.
Illustrated Vibration Chart
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Loose or open rotor bars are indicated by 2X line frequency (2FL) sidebands
surrounding 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 a
small amplitude at 1X RBPF. Electrically induced arcing between loose rotor
bars and end rings will often show high levels at 2X RBPF (with 2FLsidebands); but lit tle 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 excessive
vibration at 2X Line Frequency (2FL) which wil l have sidebands around it
spaced 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 be
repaired 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
PK
Ve
loc
ity
inIn/S
ec
Reference Env/Prf-Std
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
PK
Ve
loc
ity
inIn/Sec
Reference Env/Prf-Std
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 speed
to determine rotor or stator problems. The data collector must be set
to a suff icient 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