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Vibration Analysis – Level 2 for assessing machine potential failure modes
Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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ANALYSIS TECHNIQUES • • • • • • • • • • • • • •
Broad Band Vibration Analysis Bearing Condition Analysis Frequency Analysis (FFT) Time Synchronous Averaging Analysis Time Waveform Analysis Multispectrum Envelope Analysis Constant Percentage Bandwidth Analysis Cepstrum Shaft Orbit Tracking Analysis Vector Analysis (Amp. & Phase) Startup / Coastdown Analysis Impact Test
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Broad Band Vibration Analysis Also know as Overall vibration measurement Typically...
Velocity measurement mm/s, RMS from 2 Hz to 1000 Hz
ISO 10816-3
Displacement measurement um, RMS from 2 Hz to 1000 Hz Can be overall acceleration measurement eg. Gear box monitoring
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Vibration Monitoring v mm/s 0.45 0.40
Effect of Machine speed variation on Vibration measurement
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.05 0.10
v m m /s
0.15
4 .0
0.20
3 .8
0.25
3 .6
0.30
3 .4
0.35
3 .2 0.40
3 .0 0.45
2 .8 1500
2000
2500
3000
3500
Time Signal
4000
4500
5000
2 .6
5500
6000
6500
7000 t ms
2 .4 2 .2 2 .0 1 .8 1 .6 1 .4 1 .2
rms
1 .0 0 .8 0 .6 0 .4 0 .2
0 7 /0 2 /2 0 0 1 4 :5 9 :0 0 PM
0 7 /0 2 /2 0 0 1 4 : 5 9 : 1 0 PM
0 7 /0 2 /2 0 0 1 4 : 5 9 :2 0 PM
0 7 /0 2 /2 0 0 1 4 :5 9 :3 0 PM
0 7 / 0 2 /2 0 0 1 4 :5 9 :4 0 PM d a te
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Overall Vibration Level
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Bandpass Measurement
Peak RMS (0.707xPeak) Avg (0.637xPeak) Peak to Peak
Freq. = 1/Time
Freq. = Hz = rev. per second
Always ask.... Are you measuring RMS or Peak , etc ?? What is the frequency range ?? How much averaging?
Machine Freq are function of RPM ie. rev. per minute
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Frequency Analysis
Machine Vibrations
Time Signal
Time, s = Frequency, Hz Time = 1 / Frequency
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Frequency Analysis How to make a frequency analysis? FFT - Fast Fourier Transform is merely an efficient means of calculating a DFT (Discrete Fourier Transform). Basically, it transform a time signal into a frequency spectrum.
Time
F
F (Hz)
T Time = 1 / Frequency
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Frequency Analysis
How to make a frequency analysis? Frequency analysis can be made using frequency selective devices called filters
dB
dB
B
0
B = Bandwidth
f1
fc
f2
f
An ideal filter will only signals to pass within its bandwidth
-3
f1
fc
f2
f
Practical filter have roll-off, express as half-power (-3dB) For good filters the two will be very similar.
In FFT analysis, the bandwidth = Frequency span / no. lines Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Types of Bandwidth Constant Percentage Bandwidth (CPB)
Vibration Amplitude
0.1 1
2
3
4
Constant Bandwidth (FFT)
5
7
8
y
frequency
9
b
a x
6
a=b=c
10 kHz
c z
x, y, z are constant % of their center frequency
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Frequency Analysis
Types of filters: f
High-Pass filters
- As the name imply, a high pass filter allows high frequencies to pass. (lower frequency limit)
Low-Pass filters
- Allow low frequencies to pass through (upper limit)
Bandpass filters
- Allows only frequencies within the band f
Anti-aliasing filters
- Low pass filter at half the sampling frequencies
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FFT (DFT) - Pitfalls Discrete Fourier Transform (DFT) - Pitfalls FFT - Fast Fourier Transform is an efficient means of calculating a DFT (Discrete Fourier Transform). Basically, it transform a time signal into a frequency spectrum.
1. 1. Aliasing Aliasing-- high highfrequencies frequenciesappearing appearingas aslow lowfrequencies frequencies 2. 2. Leakage Leakage-- Memory Memorycontents contentsforced forcedto tobe beperiodic. periodic. Can Cangive givediscontinuities discontinuitieswhen whenends endsjoined joined
3. 3. Picket Picketfence fenceeffect effect––Actual Actualspectrum spectrumsampled sampledat atdiscrete discrete frequencies. frequencies.Peaks Peaksmay maybe bemissing missing
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FFT pitfalls - Aliasing Effect
Sampling rate too slow High frequency analysis results in false low frequency signal Solution: Use Anti-aliasing filter Typically a 1K (1024 point) transform, 512 frequency components are calculated and 400 lines displayed. Similarly a 2K transform 800 lines are displayed.
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FFT pitfalls - Leakage +ve 1st Sample
-ve
2nd Sample
…..give discontinuities when ends joined
+ve
-ve Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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FFT pitfalls - Picket Fence Effect
Actual Spectrum
Measured Spectrum
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Frequency Analysis
Basic law of frequency analysis
BT > 1 Bandwidth
Analysis Time
T Time
min. analysis time must allow the measured freq. to complete it’s cycle / period
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T
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Fungsi window
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FFT Spectrum 400 lines FFT
1X
2x
3X
1 kHz
IF Freq. Span is 1 KHz then resolution = 1000 / 400 lines = 2.5 Hz 2.5 Hz 5 Hz 7.5Hz
(eg. 2 - IF Span is 40Khz then resolution= 100Hz)
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Measurement time
• Harmonic signals can be measured in short time • Random and Pulsed signals need longer time • For FFT spectra C Theoretical Theoretical CC==33for Signals forHarmonic Harmonic Signals = 1 pr. average. C = 30 For Random Signals C = 30 For Random Signals
B B ** TT == C C
BB==Highest Highestresolution resolutionofofAnalysis Analysis TT==The Shortest measurement The Shortest measurementtime time CC==Constant. Constant.
In InPractice Practice
CC==55for forHarmonic HarmonicSignals Signals CC==100 For Random 100 For RandomSignals Signals
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FFT - Fast Fourier Transformation
Sample 1
Sample 2
Filtering
Filtering
Window Function
Window Function
Detectors
Detectors
FFT
+
FFT
Raw Machine Time Signal
/n = Avg
FFT Spectrum 1
FFT Spectrum
FFT Spectrum 2
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Time Synchronous Averaging Analysis Tacho
Sample 1
Sample 2
Raw Machine Time Signal Sample triggered by tacho (measured wrt speed)
Filtering
Filtering
+ Window Function Detectors
FFT Averaged Time Signal
Spectrum
Non synchronous signal will be averaged out. Reduced vibration effect– sole from nearby machine Copyright 2005PT. Putranata Adi Mandiri agent Prüftechnik AG, Germany
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FFT - How to select Freq. Ranges, lines, Averages
Shaft Rotating Speed Journal Bearings instability
Blades 2x
Rolling Element Bearings
Gear 3x
1 KHz
3KHz
40KHz
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Monitoring Techniques Types of Bearings
Journal JournalBearings Bearings
••Stationary StationarySignals Signals ••Relative RelativeLow LowFrequency Frequency ••Displacement Displacementtransducer transducer
Use Proximity probes Rolling RollingElement ElementBearings Bearings ••Modulated ModulatedRandom RandomNoise Noise ••Pulsating Pulsatingsignals signals ••High HighFrequency Frequency ••Accelerometers Accelerometers
Use Accelerometers Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Informasi penting tentang mesin
Amplitudo vibrasi
frekuensi
Apa saja yang mungkin menyebabkan vibrasi ? Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Analisa Amplitudo, Frekuensi dan Fase - 1 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
1. Unbalance
Sebanding dgn ketidak balance, dominan pd radial (2x aksial)
1 x rpm
Single reference mark
Kondisi sering ditemui
A
f 1x
Ve = 15
Vf = 15
Pengukuran getaran : Ae = 8
Af = 8
Va = 4 Vb = 3
Vc = 4 He = 15
Aa = 3
Ab = 4
Hf = 15
Ac = 5 Ad = 5
Ha = 4
Hb = 5 Hc = 3 Hd = 2
Vd = 4
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Analisa Amplitudo, Frekuensi dan Fase - 2 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
2. Misalignment kopling atau poros bengkok
Dominan pd aksial, 50% atau lebih dari arah radial
Sering 1 x & 2 x Single rpm. Kadang 3 x double rpm triple
KETERANGAN
GAMBAR SPECTRUM
Ditandai timbulnya vibrasi A aksial. Gunakan alat laseralignment. Apabila mesin baru dipasang terjadi vibrasi, maka kemungkinan besar karena misalignment.
Ve = 3
f 1x
2x
Vf = 4
Pengukuran getaran : Ae = 4
Af = 5
Va = 4 Vb = 10
Vc = 10 He = 4
Aa = 7
Ab = 15
Hf = 3
Ac = 15 Ad = 7
Ha = 5
Hb = 10 Hc = 10 Hd = 5
Vd = 4
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Analisa Amplitudo, Frekuensi dan Fase - 3 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
3. Anti friction bearing buruk
Tidak stabil, ukur percepatan, gunakan acceleration probe
Sangat tinggi, beberapa kali Rpm, 1x, 2x, 3x, 4x … 10x
Tdk tentu, Berubahrubah
A Vibrasi akan timbul apabila bearing sdh parah. Gunakan vibrotip / shockpulse u deteksi awal
Ve = 5
GAMBAR SPECTRUM
f 1x
2x
3x
Vf = 3
Pengukuran getaran : Ae = 4
Af = 2
Va = 2 Vb = 4
Vc = 5-10 He = 4
Aa = 4
Ab = 3
Hf = 4
Ac = 10-15 Ad = 5
Ha = 3
Hb = 3 Hc = 5-10 Hd = 4
Vd = 3
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4x
Frekuensi bearing karakteristik
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Dimensi bearing
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Kalkulasi frekuensi dari elemen bearing
n ⎛ Kerusakan di outer race = ⋅ fr ⋅ ⎜1 − 2 ⎝ n ⎛ Kerusakan di inner race = ⋅ fr ⋅ ⎜1 + 2 ⎝
BD ⎞ ⋅ cos β ⎟ ( Hz ) PD ⎠ BD ⎞ ⋅ cos β ⎟ ( Hz ) PD ⎠
2 ⎛ ⎛ BD ⎞ PD ⎞ Kerusakan pada elemen berputar = ⋅ fr ⋅ ⎜1 − ⎜ ⋅ cos β ⎟ ⎟ ( Hz ) ⎜ ⎟ BD PD ⎠ ⎝ ⎝ ⎠ fr ⎛ BD ⎞ Kerusakan pada cage = ⋅ ⎜1 − ⋅ cos β ⎟ ( Hz ) 2 ⎝ PD ⎠ dimana: BD &PD : lihat gambar
fr : Frekuensi rotasi dari inner race n : jumlah elemen berputar β : sudut kontak Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Why shock pulses for rolling bearing noise ? c m
fnat = x
(
Machine vibration
1 m
∼
1 l
,
1 d
,
1 a
c = stiffness
) m = Mass
Material crack plastical / elastical deformation
Shock pulse range rolling bearing
Natural frequencies rolling bearing pieces fnat,O
fnat,B
fnat, Ι 2 1
Example
l 1
d
l = n ⋅m f ≈ x ⋅ 1/1m fnat ≈ x ⋅ 30 Hz
2
d = n ⋅ 1 mm f ≈ x ⋅ 1/1 000 m fnat ≈ x ⋅ 30 000 Hz
a
a f fnat
1 000
10 000
36 000
100 000
= n ⋅ μm ≈ x ⋅ 1 / 1 00 000 m ≈ x ⋅ 3 00 000 Hz
flog / Hz
ultra sound emission Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany 31 velocity acceleration shock pulses
Overall values for Bearing condition Acceleration - Crest Factor Spike Energy Value BCU - Value Kurtosis Factor gSE - Value SEE - Value
Shock Pulse Measurement Normalising with… • Shaft speed (rpm) • Shaft Diameter (Bearing Size)
? ? ? Time
Time Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Normalising of shock pulse signals dBsv
dBsv 90
35
90 ideal measurement
dBn
measurement location with signal damping
P dBn
25 15
dBm
C
dBm
10 dBc dBc
dBi
dBia 0
0
-9
-9 dB sv = absolute shock pulse value
dBn = normalised shock pulse value
dBi = initial value →
dBia = adjusted inital value
→ signal damping of real measurement location → influencing factors like load condition Copyright 2005PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany 33 → determined through RPM and lubrication and bearing type Basic value of the normalised shock pulse values diameter of the bearing
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Fungsi envelope
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Signature Rolling Bearing Defects No rolling track defect:
Rolling track defect:
Time signal:
Time signal: a in m/s2
a in m/s2
Envelope
Enveiope
t in s
t in s
Ta
Envelope spectrum:
Envelope spectrum:
a in m/s2
a in m/s2
f in Hz
fRPOF
2•fRPOF 3 •fRPO F 4 •fRPOF
f in
Hz
• fRPOF=
1 TRPOF
Defect frequency
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Envelope Spectrum bearing Location :PT. Caltex\Water Plant\Fresh Water Pump\Centrifugal Pump\Coupling Side\rolling bearing >120
am /s² 2.0
#
X
Y
1.8
0
0.63
0.96
1.6
1
25.00
0.21
2
50.00
0.14
Alarm
3
176.88
0.10
W arn
4
151.88
0.10
0.6
5
126.88
0.08
0.4
6
4.38
0.08
7
20.63
0.07
8
29.38
0.07
1000 fHz 9
15.00
0.06
1.4 1.2 1.0 0.8
0.2 0.0 0
100
200
300
400
500
600
700
800
900
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Analisa Amplitudo, Frekuensi dan Fase - 4 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
4. Sleeve, metal, Jurnal bearing (friction bearing) / eksentrik
Tidak besar, aksial lebih tinggi
1 x rpm, seolaholah seperti unbalance
Single
pd rodagigi vibrasi segaris dengan pusat kontak. pd motor/gen vibrasi hilang bila mesin dimatikan. pd pompa/blower kemungkinan unbalance
A
Ve = 4
f 1x
Vf = 4
Pengukuran getaran : Ae = 4
Af = 5
Va = 4 Vb = 7
Vc = 3 He = 4
Aa = 7
Ab = 15
Hf = 3
Ac = 4 Ad = 4
Ha = 3
Hb = 8 Hc = 5 Hd = 3
Vd = 5
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Analisa Amplitudo, Frekuensi dan Fase - 5 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
5. Rodagigi buruk atau bersuara
Rendah, ukur kecepatan & percepatan, gunakan acceleration
Sangat tinggi Jumlah gigi x rpm
Tdk tentu
Awal rusak bersuara, semakin lama keras. Vibrasi biasanya dalam toleransi.
A
Ve = 7
f 2x
1x
3x
tooth
Vf = 3
Pengukuran getaran : Ae = 8
Af = 5
Va = 4 Vb = 3
Vc = 7 He = 6
Aa = 3
Ab = 4
Hf = 4
Ac = 8 Ad = 9
Ha = 3
Hb = 2 Hc = 7 Hd = 7
Vd = 7
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4x
Analisa Amplitudo, Frekuensi dan Fase - 6 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
6. Gear mesh buruk atau bersuara (pada saat start / stop)
Rendah, ukur kecepatan & percepatan, gunakan accel.
Sangat tinggi Jumlah gigi x rpm
Tdk tentu
Sering terjadi pada saat pemasangan
A
f 2x
1x
3x
tooth
Ve = 7
Vf = 3
Pengukuran getaran : Ae = 8
Af = 5
Va = 4 Vb = 3
Vc = 7 He = 6
Aa = 3
Ab = 4
Hf = 4
Ac = 8 Ad = 9
Ha = 3
Hb = 2 Hc = 7 Hd = 7
Vd = 7
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4x
Gear frequencies for Parallel Offset Gear
• Pinion speed, rpm.......................................(Rp) • Number of teeth on gear.............................(Ng)
Data needed from the gear
• Number of teeth on the pinion...................(Np)
• Gear speed, rpm.........................................(Rg) • Gear rotational frequency, Hz...................(frg)
• Mesh frequency, H................................(fm) • Tooth repeat frequency, Hz......................(ftr)
Info calculated from the data
• Pinion rotational frequency, Hz.................(frp)
• Assembly phase passage frequency, Hz.....(fa) Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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• Pumps are found in nearly every industry in a wide array of sizes and capacities. Larger pumps, such as boiler feed pumps and reactor recirculation/coolant pumps, are often permanently monitored, though many smaller units are not. Regardless, the following parameters are necessary to effectively evaluate process-related phenomena: • Speed • Suction pressure and temperature • Discharge pressure and temperature • Flow • Bearing metal and oil drain temperatures • Driver power Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Air Compressor
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Air Compressor
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Centrifugal Compressor
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Centrifugal Compressor
•
• • • • • • •
The compressor is one of the petrochemical industry's most durable and dependable machines. In general, there is a more limited set of variables to be monitored in compressors than in gas and steam turbines, which helps when you are analyzing and troubleshooting. However, rotational speeds tend to be much higher. The following process parameters are considered key items: Suction pressure and temperature Discharge pressure and temperature Product (gas) flow rate Gas analysis (mole weight) Compressor speed Driver power Bearing metal and oil drain temperatures
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Generator
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Generators
• Generators are generally well-behaved dynamically, due to their less complicated construction, compared to gas and steam turbines. Unbalance, thermal bows, and seal rubs comprise the majority of problems seen. The process variable list reflects this: • Output (kW or MW) • Reactive loading (vars) • Power factor • Coolant gas temperature and pressure • Winding temperatures • Field current • Bearing metal and oil drain temperatures Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Gas Turbine
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gas turbines
•
It is easy to see the interaction of process and vibration characteristics by studying industrial and aeroderivative gas turbines, because they are really three machines in one. They are a compressor that pressurizes ambient air, a combustor that introduces fuel and burns the air/fuel mixture, and an expansion (or power) turbine through which the hot, high pressure combustion gases expand, driving the compressor and any other connected machinery. Gas turbines are subject to wide performance and vibration variations when ambient air, fuel, or load values change. For example, high inlet air temperature reduces gas turbine performance, requiring higher fuel consumption for a specific power level. Conversely, low air temperature causes the power to increase. If humidity is high, ice can form on the inlet filters, inlet ducting, and inlet casing of the compressor. Large accumulations of ice reduce and distort the airflow, which may cause compressor stall and surge.
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Steam turbines
•
• • • • • • •
Steam turbines are used in almost every industry for driving compressors, generators, pumps, and other equipment. Sizes vary from small, single stage units of less than 100 hp to large power generation units capable of over 1,000 MW in a single machine train. However, despite these size variations, steam conditions generally provide significant insight into any rotor response changes, such as rubs and shaft bow. Process variables that should be monitored on each driver include: Steam supply and exhaust conditions - temperature, pressure, flow, quality Extraction conditions (if applicable) Condenser vacuum Bearing metal and oil drain temperatures Gross generation (kW) or shaft speed and torque Reheat steam conditions (if applicable) Kvars (generator drive applications)
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Phase Analysis
• • • • • • •
Trending for Acceptance Regions Shaft crack detection Rub detection Shaft balancing Shaft/structural resonance detection Shaft mode shape Location of a fluid-induced instability Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
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Trending
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Shaft Crack
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Rubs
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Shaft Structure
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Shaft balancingShaft mode shape
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Location of fluid-induced instability
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Rotational & Mesh Gear Frequencies
Gear & Pinion rotational frequencies : f rg =
Rg 60
( Hz ),
f rp =
Rp 60
( Hz )
Mesh frequency : f m = f rp × N p = f rg × N g ( Hz )
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Assembly phase passage gear frequency (1)
Ng = 15 Gear tooth 1-10-4-13-7 2-11-5-14-8 3-12-6-15-9
Q
Np = 9 Pinion Tooth 1-7-4 2-8-5 3-9-6
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Assembly phase passage gear frequency (2)
Ng = 15 Gear tooth 1-10-4-13-7 2-11-5-14-8 3-12-6-15-9
Q
Np = 9 Pinion Tooth 1-7-4 2-8-5 3-9-6
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Assembly phase passage gear frequency (3)
Assembly phase passage frequency : fm ( Hz ) fa = Na N a = Product of common prime factors example : Fg = 1,3,5,15 Fp = 1,3,3,9 N a = 1× 3 = 3
Ng = 15 Gear tooth 1-10-4-13-7 2-11-5-14-8 3-12-6-15-9
Q
Np = 9 Pinion Tooth 1-7-4 2-8-5 3-9-6
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87
Tooth repeat gear frequencies
Tooth repeat frequency : fm × Na ( Hz ) f tr = Ng × N p or for a true hunting tooth combination ( when N a = 1) : f tr =
f rg Np
( Hz )
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Summary gear frequencies for parallel offset gear To obtain
f tr
f rg
f rp
N a /N g
1/M g
1
N p /N a N p
N a /N p
1
Mg
N g /N a N g
fa
fm
multiply
f rp f rg
fm
by
N a /(N g × 1/N g 1/N p Np )
1/N a
1
N a = Number of assembly phases, N p = Number of teeth on pinion N g = Number of teeth on gear, M g = Ratio gear, f tr = Tooth repeat freq (Hz) f rg = Gear rotational freq (Hz), f rp = Pinion rotational freq (Hz) f a = Assembly phase passage freq (Hz), f m = Mesh frequency (Hz) Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
89
Gear frequencies for Planetary Gear
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90
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91
Application mill drive - cement industry
[bar] [m/s²] [bar]
4x [°C]
Machine speed Alarm status
[m/s²]
[m/s²]
[m/s²] [m/s²]
PCS
External expert
Data backup LAN / WAN
Internal expert
Interne t Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
92
Gear frequencies for Planetary Gear Planetary Gear : ⎛ Ts x Tr ⎞ Tooth Mesh Freq = ⎜ ⎟ x Ns = Tr x Nc (Hz) ⎝ Ts + Tr ⎠ Tr ⎛ np x Tr ⎞ x Nc (Hz) x Ns np x Defect on Sun = ⎜ = ⎟ Ts ⎝ Ts + Tr ⎠ Tr Defect on Planet = 2 x Nc x (Hz) Tp Defect on Ring = np x Nc (Hz) where : Ns = speed of sun gear (output), Nc = speed of carrier (input) Ts = number of teeth on sun, Tr = number of teeth on ring np = number of planets Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
93
Comparison of Sinusoidal and Impact Gear Tooth Contact
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Analisa Amplitudo, Frekuensi dan Fase - 7 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
7. Mechanical looseness (Housing bearing aus)
Tinggi pada aksial
2 x rpm
2 referensi Sering agak kacau bersamaan dgn unbalance / misalignment
Ve = 3
GAMBAR SPECTRUM A
f 2x
Vf = 3
Pengukuran getaran : Ae = 4
Af = 4
Va = 4 Vb = 12
Vc = 5 He = 4
Aa = 3
Ab = 15
Hf = 2
Ac = 5 Ad = 3
Ha = 3
Hb = 12 Hc = 5 Hd = 4
Vd = 5
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Analisa Amplitudo, Frekuensi dan Fase - 8 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
8. Mechanical Looseness (Pondasi kendor – dudukan lemah/karatan – baut kendor)
Tinggi pada vertikal
Kurang dari 1 x rpm
Tdk tentu
Kencangkan baut Untuk memastikan
A
f # poles = (2 x 3000) / 1480 = 4 -> Ns = 120 x 50 / 4 = 1500 RPM -> SF = 1500 – 1480 = 20 RPM = 0.33 Hz -> Fp = 4 x 20 RPM = 80 RPM = 1.33 Hz -> RBPF = 40 x 1480 RPM = 59200 RPM = 986.67 Hz Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
103
Analisa vibrasi pada motor listrik – 1 / 4
- Stator eccentricity, loose iron, shorted laminations : Amplitude
1x
2FL
FL = Line Frequency (3000 CPM, for 50 Hz Line Freq.)
2x Frequency
Amplitude
- Uneven air gap (variable air gap) / Eccentric rotor : 1x
2FL
FL = Line Frequency (3000 CPM, for 50 Hz Line Freq.) Fp Sidebands around FL
Fp Frequency • Pole pass frequency (Fp) = SF x (# poles) • Slip frequency (SF) = Ns – actual speed • Magnetic field speed, RPM (Ns) = 120 x FL / (# poles)
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Analisa vibrasi pada motor listrik – 2 / 4
- Rotor problems 1 (broken/cracked rotor bars / shorting rings, shorted rotor laminations) : Amplitude
1x
3x 2x
* Fp Sidebands around 1x for broken rotor bars * Fp Sidebands around 1x, 2x, 3x, …. for cracked rotor bars
Frequency
- Rotor problems 2 (loose/broken rotor bars) : Amplitude
RBPF 1x
2FL Sidebands around RBPF or its harmonic freq. 2x
RBPF = Rotor Bar Pass Frequency = # Bars x RPM
Frequency
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105
Analisa vibrasi pada motor listrik – 3 / 4
- Phasing problems (motor beroperasi hanya 2 dari 3 phasa, disebabkan oleh loose / broken connectors) : 2FL Amplitude
1/3 FL Sidebands around 2FL
Frequency
Loose stator coils pada synchronous motors : Amplitude
CPF 1x RPM Sidebands around CPF = Coil Pass Freq. 1x 2x CPF = # stator coils x RPM
Frequency
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106
Analisa vibrasi pada motor listrik – 4 / 4
- DC motor problems 1 (broken field winding, bad SCR and loose connection) : Amplitude
6FL = SCR Firing Freq. or its harmonic freq.
1x 2x
Frequency
- DC motor problems 2 (loose/blown fuses, shorted control card) : Amplitude
FL
Amplitude tinggi pada 1x hingga 5x Line Freq. 2FL 3FL
4FL
5FL
Frequency
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Rekommendasi untuk analisa vibrasi motor listrik
Untuk mendeteksi uneven airgap, eccentric rotor : -> 3 titik “resolusi tinggi” (diambil 1x setahun) * HOH : high resolution, motor outboard horizontal * HIH : high resolution, motor inboard horizontal * HOA (or HIA) : high resolution, motor outboard (or inboard) axial -> Fmax = 200 Hz, 1600 lines -> Resolusi = 0.125 Hz
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Rekommendasi untuk analisa vibrasi motor listrik
Untuk mendeteksi munculnya rotor bar pass frequency atau stator slot pass frequency : -> 2 titik “extended range” (diambil 1x setahun) * EOH : extended range, motor outboard horizontal * EIH : extended range, motor inboard horizontal -> Fmax = 5000 Hz, 3200 lines, jika tidak diketahui jumlah rotor atau stator slot, sebenarnya cukup s/d frekuensi : (2x rotor / stator slot pass freq. + 400 Hz) -> Jika ingin mengambil data ini 1x sebulan, cukup dengan 400 – 800 lines untuk menghemat memori Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
109
Analisa Amplitudo, Frekuensi dan Fase - 12 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
12. Gaya aerodinamik / hidrolik
Tinggi pada vertikal atau horizontal
1 x rpm atau jumlah sudu atau fan atau impeler x rpm
Tdk tentu
Lebih terasa bila beban tidak stabil.
A
f 1x
Ve = 14
Jml x
Vf = 13
Pengukuran getaran : Ae = 7
Af = 7
Va = 1 Vb = 2
Vc = 4 He = 13
Aa = 1
Ab = 3
Hf = 14
Ac = 5 Ad = 3
Ha = 2
Hb = 2 Hc = 3 Hd = 4
Vd = 4
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Analisa Amplitudo, Frekuensi dan Fase - 13 PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
13. Gaya reciprocating
Dominan aksial
1 x,2 x rpm atau lebih
Single, double, triple
Pada mesin reciprocating bisa ganti desain/isolasi
A
Ve = 2
f 1x
2x
Vf = 2
Pengukuran getaran : Ae = 3
Af = 3
Va = 7 Vb = 8
Vc = 3 He = 4
Aa = 6
Ab = 7
Hf = 2
Ac = 4 Ad = 4
Ha = 8
Hb = 7 Hc = 2 Hd = 4
Vd = 3
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Ringkasan Analisa Amplitudo, Frekuensi dan Fase PENYEBAB
AMPLITUDO
FREKUENSI
FASE
KETERANGAN
GAMBAR SPECTRUM
1. Unbalance
Sebanding dgn ketidak balance, dominan pd radial (2x aksial)
1 x rpm
Single reference mark
Kondisi sering ditemui
A
Dominan pd aksial, 50% atau lebih dari arah radial
Sering 1 x & 2 x Single rpm. Kadang 3 x double rpm triple
Ditandai timbulnya vibrasi A aksial. Gunakan alat laseralignment. Apabila mesin baru dipasang terjadi vibrasi, maka kemungkinan besar karena misalignment.
Tidak stabil, ukur acceleration untuk freq. tinggi
Sangat tinggi, beberapa kali Rpm, 1x, 2x, 3x, 4x … 10x
Tdk tentu, Berubahrubah
Vibrasi akan timbul apabila bearing sdh parah. Gunakan enveloping & shockpulse
A
4. Sleeve, metal, Jurnal bearing (friction bearing)
Tidak besar, aksial lebih tinggi
1 x rpm, seolaholah seperti unbalance
Single
pd rodagigi vibrasi segaris dengan pusat kontak. pd motor/gen vibrasi hilang bila mesin dimatikan. pd pompa/blower kemungkinan unbalance
A
5. Rodagigi buruk atau bersuara
Rendah, ukur kecepatan & percepatan, gunakan accel.
Awal rusak bersuara, semakin lama keras. Vibrasi biasanya dalam toleransi.
A
2. Misalignment kopling atau poros bengkok
3. Anti friction bearing buruk
f 1x
Sangat tinggi Jumlah gigi x rpm
Tdk tentu
f 1x
2x
1x
2x
f 3x
4x
f 1x
f 1x
2x
3x
tooth Copyright 2005- PT. Putranata Adi Mandiri – sole agent Prüftechnik AG, Germany
112
4x
Analisa Amplitudo, Frekuensi dan Fase - 2 PENYEBAB
AMPLITUDO
FREKUENSI
6. Gear mesh buruk atau bersuara pada saat start/stop
Rendah, ukur kecepatan & percepatan, gunakan accel.
Sangat tinggi Jumlah gigi x rpm
7. Mechanical looseness (Housing bearing aus)
Tinggi pada aksial
2 x rpm
8. Mechanical Looseness (Pondasi kendor – dudukan lemah/karatan – baut kendor)
Tinggi pada vertikal
9. Mechanical looseness (Pondasi melengkung)
Tinggi pada vertikal, horizontal & aksial
2 x rpm
10. Drive belt buruk
Tdk tentu/berpulsa
1,2,3 atau 4 x rpm belt
Kurang dari 1 x rpm
FASE
Tdk tentu
KETERANGAN
Sering terjadi pada saat pemasangan
GAMBAR SPECTRUM A
f 1x
2 referensi Sering agak kacau bersamaan dgn unbalance / misalignment
A
Tdk tentu
A
Kencangkan baut Untuk memastikan
2x
3x
4x
f 2x
f
