R&t 2008 principles and practices of vibrational analysis - keefer

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R&t 2008 principles and practices of vibrational analysis - keefer

  1. 1. Vibration Analysis ServicesFES Systems Inc. 1
  2. 2. FES Systems Inc. 2
  3. 3. What is Vibration? Vibration is the movement of a body about its reference position. Vibration occurs because of an excitation force that causes motion.FES Systems Inc. 3
  4. 4. Vibration TermsFES Systems Inc. 4
  5. 5. Time Waveform Analysis q. h fre hig cy en overall vibration tim qu e q. fre fre low Individual vibration signals combine to form a complex time waveform showing overall vibration complex time waveformFES Systems Inc. 5
  6. 6. Overall Vibration The total vibration energy measured within a specific frequency range. – includes a combination of all vibration signals within measured frequency range – does not include vibration signals outside measured frequency range – produces a numerical valueFES Systems Inc. 6
  7. 7. Amplitude vs. Frequency – Vibration amplitude indicates the severity of the problem. – Vibration frequency indicates the source of the problem. 1X amplitude 3X 2X 4X frequencyFES Systems Inc. 7
  8. 8. Vibration- Measurable Characteristics 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). 0 90 180 270 360 Acceleration Velocity Displacement TimeFES Systems Inc. 8
  9. 9. Scale Factors – When comparing overall vibration signals, it is imperative that both signals be measured on the same frequency range and with the same scale factors. NOTE: RMS is .707 of peak.FES Systems Inc. 9
  10. 10. 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) G’s or in/sec2 or mm/sec2FES Systems Inc. 10
  11. 11. Lines of Resolution Individual 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. 7200 CPM 7200 CPM 3570 CPM 3570 CPM Amplitude Amplitude Frequency in CPM Frequency in CPMFES Systems Inc. 11
  12. 12. 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)FES Systems Inc. 12
  13. 13. Velocity Sensors • Often Measure Bearing Housings or Machinery Casing Vibration • Effective in Low to Mid Frequency Range (10 Hz to around 1,500 Hz) • Self Generating DevicesFES Systems Inc. 13
  14. 14. Displacement Probe/Eddy Probe • Measure Relative Distance Between Two (2) Surfaces • Accurate Low Frequency ResponseFES Systems Inc. 14
  15. 15. Multi-Parameter Monitoring Same Data in Velocity and Acceleration - Model 32L S/N AB10099P FES Model 32L S/N AB10099P #2 West -C3H Compressor Inboard Horizontal #2 W est -C3H Compressor Inboard H orizontal 0.40 5 Route Spectrum Route Spectrum 06-Feb-01 14:02:05 06-Feb-01 14:02:05 OVRALL= .6123 V-DG OVR ALL= .6123 V-DG 0.32 PK = .6091 4 PK = 8.19 LOAD = 100.0 LOAD = 100.0 RPM = 2990. RPM = 2990. PK Acceleration in G-s RPS = 49.83 RPS = 49.83 PK Velocity in In/Sec Reference Env/Prf-Std Reference Env/Prf-Std 0.24 3 0.16 2 0.08 1 0 0 0 40 80 120 160 200 240 0 40 80 120 160 200 240 Frequency in kCPM Frequency in kCPM On the same bearing, low freq. events (imbalance, Velocity misalignment, etc.) show Acceleration Spectrum best in the velocity spectrum; while high freq. Spectrum events (bearing faults, gearmesh) show best in the acceleration spectrumFES Systems Inc. 15
  16. 16. Sensor Relationships 100 Displacement (mils) 10 Acceleration (gs) Amplitude 1.0 (mils, in/sec, g’s) Velocity (in/sec) 0.1 Common Machinery 1 Operating Range 0.01 10 100 1,000 10,000 Frequency (Hz)FES Systems Inc. 16
  17. 17. Resonance typically 10% or greaterFES Systems Inc. 17
  18. 18. Detection vs. Analysis Detection Alarm limits are established for each measurement. When the measurement’s value exceeds its programmed alarm limits, the predictive maintenance software or data collector notifies the analyst of a problem. Analysis Once detected, analyzing exceptional measurements provides insight to the problem itself, and to its root cause.FES Systems Inc. 18
  19. 19. Important Frequency Peaks Rotational Speed or Even Multiples Always present but excessive amplitude or multiple harmonics can indicate a problem. Electric motors always have frequency peaks at shaft rotational speed and at line frequency i.e. 60 Hz. Two pole motors will always display a 2X line frequency peak.FES Systems Inc. 19
  20. 20. Important Frequency Peaks Gas Pulsation Frequencies Screw 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.FES Systems Inc. 20
  21. 21. FES Systems Inc. 21
  22. 22. 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 FactorsFES Systems Inc. 22
  23. 23. Selecting the Machinery Critical - If a failure or shutdown occurs, production is stopped, or machine performance creates an unsafe environment Essential Spared - If a failure or shutdown occurs, production is disrupted Non Essential Spared - If a failure or 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 productionFES Systems Inc. 23
  24. 24. Thinking Ahead Walk Through Machinery Data SheetsFES Systems Inc. 24
  25. 25. Measurement Planes radial – vertical – horizontal axialFES Systems Inc. 25
  26. 26. 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)FES Systems Inc. 26
  27. 27. 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.FES Systems Inc. 27
  28. 28. Sensor LocationFES Systems Inc. 28
  29. 29. Mounting MethodsFES Systems Inc. 29
  30. 30. Hand-held/Probe Mounting Rapid and convenient. Subject to many sources of error. Use only as a last resort.FES Systems Inc. 30
  31. 31. FES Systems Inc. 31
  32. 32. Types of Alarms Overall Vibration Limits Spectral Enveloping Spectral Bands Phase AlarmsFES Systems Inc. 32
  33. 33. FES Systems Inc. 33
  34. 34. Acceptable Vibration Levels Tables are published that show overall vibration levels as a function 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.FES Systems Inc. 34
  35. 35. ISO Guidelines ISO 2372 overall velocity vibration guidelinesFES Systems Inc. 35
  36. 36. velocity - in/sec (peak) Assessing Overall acceleration - Gs (peak) Vibration Severity Frequency - CPMFES Systems Inc. 36
  37. 37. Overall Vibration Trend Plot VIB - Alignment Fault ALIGNMENT -M1H MOTOR OUTBOARD BRG. - HORIZONTAL 0.24 Trend Display FAULT of 0.20 OVERALL VALUE PK Velocity in In/Sec 0.16 -- Baseline -- Value: .06350 0.12 Date: 11-AUG-95 0.08 ALERT WARNING 0.04 0 0 100 200 300 400 500 Days: 11-AUG-95 To 11-DEC-96FES Systems Inc. 37
  38. 38. Spectral Enveloping VIB - Balance Fault BALANCE -M2A MOTOR INBOARD AXIAL 0.40 Route Spectrum 0.35 14-MAR-96 12:10:26 PK Velocity in In/Sec 0.30 OVRALL= .3260 V-DG PK = .3257 0.25 LOAD = 100.0 RPM = 1777. 0.20 RPS = 29.62 alarm is triggered Reference Envelope 0.15 0.10 0.05 0 0 400 800 1200 1600 2000 Frequency in HzFES Systems Inc. 38
  39. 39. Phase Alarms A2 - Machine #6 (Various Setups) MACH#6 -PPH PEAK PHASE DATA 0 Correlation Display Phase vs Peak Data Period: 26-Dec-96 To 28-Dec-96 2.500 90 270 180 PeakFES Systems Inc. 39
  40. 40. Acceptable Vibration Levels Motor and compressors with sleeve bearings do notnot lend Motor and compressors with sleeve bearings do lend themselves well to to readings with accelerometers and for themselves well readings with accelerometers and for dependable information a device such as a proximity probe dependable information a device such as a proximity probe should be used to to measure vibration mils displacement. should be used measure vibration in in mils displacement. Probes should be oriented in two planes 90 ° apart and Probes should be oriented in two planes 90 bearingand displacement cannot exceed the shaft to ° apart clearance. displacement cannot exceed the shaft to bearing clearance.FES Systems Inc. 40
  41. 41. Acceptable Vibration Levels For Individual spectrum peaks limits are set by their perceived cause but some general limits are shown below: perceived For Individual spectrum peaks limits are set by their cause but some general limits are shown below: Compressors: Compressors: Rotational speed 1X, 2X, 2X, 3X 0.25 ips RMS RMS Rotational speed 1X, 3X 0.25 ips Gas Pulsation at compressor Gas Pulsation at compr. 0.27 ips RMS RMS 0.27 ips Bearing fault frequencies 0.15 ips RMS Bearing fault (2000-3000 Hz) Roller bearings frequencies 2.5 g’s 0.15 ips RMS Roller bearings (2000-3000 Hz) 2.5 g’sFES Systems Inc. 41
  42. 42. Acceptable Vibration Levels Motors: Rotational Speed 1X, 2X, 3X 0.25 ips RMS Line Frequency 1X, 2X Motors: 0.13 ips RMS Bearing fault frequencies 3X Rotational Speed 1X, 2X, 0.15 ips RMS 0.25 ips RMS Line Frequency 1X, 2X 0.13 ips RMS Bearing fault frequencies 0.15 ips RMSFES Systems Inc. 42
  43. 43. Readings-How Often? •At start up - Baseline • At start up - Baseline •Six months after start up unless aaproblem is is suspected. After • Six months after start up unless problem suspected. After that every 66month to one year after that unless aa deteriorating that every month to one year after that unless deteriorating trend isisobserved. trend observed. •At 25000 hours readings should be taken every three months • At 25000 hours readings should be taken every three months toto extendthe time before an internal inspection isis required. extend the time before an internal inspection required. •Anytime an unusual noise or vibration is noticed. • Anytime an unusual noise or vibration is noticed.FES Systems Inc. 43
  44. 44. Spectrum Analysis Techniques Collect Useful Information Analyze 500 HP/3570 RPM Motor Model 23LE C3 C4 M1 M2 C1 C2 CFES Systems Inc. 44
  45. 45. Spectrum Analysis Techniques Some compressors have a combination of sleeve bearings and ball thrust bearings that require different analysis techniques. FES Model GL Series Compressors Thrust and Sleeve Bearing Location Sleeve Bearing LocationFES Systems Inc. 45
  46. 46. Sleeve Bearing Wear Pattern 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 one pronounced 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 Source: Technical Associates Inc. clearances were set to specifications. Illustrated Vibration ChartFES Systems Inc. 46
  47. 47. FES Systems Inc. 47
  48. 48. Why Do Bearings Fail? Inadequate Lubrication - too much - too little - contaminated Excessive Load Caused by: - misalignment - imbalance - bent shaft - etc..... Improper Handling or Installation Spall On Outer Race AgeFES Systems Inc. 48
  49. 49. Typical Bearing Failure RateFES Systems Inc. 49
  50. 50. Bearing Defect Frequencies BPFO Ball Pass Frequency Outer Race BPFI Ball Pass Frequency Inner Race BSF Ball Spin Frequency FTF Cage Frequency or Fundamental Train FrequencyFES Systems Inc. 50
  51. 51. Bearing Failure Stages Stage 1 Stage 2 No apparent change on typical velocity spectrum Defect’s harmonic frequencies appear defect’s “fund.” defect’s “harmonic” frequency range frequency range Stage 3 Stage 4 Defect’s fundamental frequencies also appear Defect’s harmonic frequencies develop multiple and may exhibit sidebands sidebands (haystack), fundamental freqs. grow and also develop sidebandsFES Systems Inc. 51
  52. 52. Sidebands VIB - Alignment Fault ALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL 0.14 Route Spectrum 0.12 01-AUG-96 15:15:26 RMS Acceleration in G-s OVRALL= .0665 V-DG 0.10 RMS = .2506 LOAD = 100.0 0.08 RPM = 3606. RPS = 60.10 0.06 0.04 0.02 0 Freq: 2634.6 0 1000 2000 3000 4000 Ordr: 43.84 Frequency in Hz Spec: .02417 Dfrq: 120.19FES Systems Inc. 52
  53. 53. Harmonics VIB - Alignment Fault ALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL 0.06 Route Spectrum 01-AUG-96 15:15:26 0.05 PK Velocity in In/Sec OVRALL= .0665 V-DG PK = .0660 0.04 LOAD = 100.0 RPM = 3606. 0.03 RPS = 60.10 0.02 0.01 0 Freq: 57.69 0 1000 2000 3000 4000 Ordr: .960 Frequency in Hz Spec: .02572FES Systems Inc. 53
  54. 54. Waterfall Plot VIB - Alignment Fault ALIGNMENT -M2H MOTOR INBOARD BRG. - HORIZONTAL 0.05 Max Amp .0456 PK Velocity in In/Sec 0 11-DEC-96 21-NOV-96 28-OCT-96 30-SEP-96 01-AUG-96 05-SEP-96 15:15:26 RPM= 3550. 01-AUG-96 Freq: 57.69 0 1000 2000 3000 4000 Ordr: .975 Frequency in Hz Sp 1: .02589FES Systems Inc. 54
  55. 55. FES Systems Inc. 55
  56. 56. Stator problems generate high vibration at 2X line frequency (2FL). Stator eccentricity produces uneven stationary air gap between rotor and stator which produces very directional vibration. Differential Air Gap should not exceed 5% for induction 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 cause uneven, 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 ChartFES Systems Inc. 56
  57. 57. 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 2FL and running speed harmonic. Eccentric rotors generate 2FL surrounded by Pole Pass frequency sidebands (FP), as well as FP sidebands 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 ChartFES Systems Inc. 57
  58. 58. Broken or Cracked rotor bars or shorting rings; bad joints between rotor bars and shorting rings; or shorted rotor laminations will produce high 1X running speed vibration with pole pass frequency sidebands (FP). In addition, these problems generate FP sidebands around the second, third, fourth and fifth running speed harmonics. Source: Technical Associates Inc. Illustrated Vibration ChartFES Systems Inc. 58
  59. 59. 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 2FL sidebands); but little or no increase in amplitudes at 1X RBPF. Source: Technical Associates Inc. Illustrated Vibration ChartFES Systems Inc. 59
  60. 60. Phasing problems due to loose or broken connectors can cause excessive vibration at 2X Line Frequency (2FL) which will 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 is only sporadically making contact. Loose or broken connectors must be repaired to prevent catastrophic failure. Source: Technical Associates Inc. Illustrated Vibration ChartFES Systems Inc. 60
  61. 61. Dosk - RAM 700 HP Motor Test1 RAM TEST 1-M2L Mot. Inboard Horiz./2X Line Freq 0.40 Route Spectrum 10-Apr-01 08:20:35 OVRALL= .3045 V-DG 0.32 PK = .3028 LOAD = 100.0 RPM = 3579. RPS = 59.64 Reference Env/Prf-Std PK Velocity in In/Sec 0.24 0.16 0.08 0 Freq: 7200.0 0 8000 16000 24000 Ordr: 2.012 Frequency in CPM Spec: .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.FES Systems Inc. 61
  62. 62. Dosk - RAM 700 HP Motor Test1 RAM TEST 1-M2L Mot. Inboard Horiz./2X Line Freq 0.40 Route Spectrum 10-Apr-01 08:20:35 OVRALL= .3045 V-DG 0.32 PK = .2922 LOAD = 100.0 RPM = 3579. RPS = 59.64 2 x Line Freq. Reference Env/Prf-Std PK Velocity in In/Sec 0.24 0.16 7140 RPM 0.08 2x turning speed 0 Freq: 7200.0 6800 7000 7200 7400 7600 Ordr: 2.012 Frequency in CPM Spec: .283 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 The 2 x Line F number of lines of resolution to separate these two frequencies to a sufficientFES Systems Inc. 62
  63. 63. Vibration analysis can be used to determine rotor problems in motors. The rotor bar pass frequency has penetrated the narrow band alarm.FES Systems Inc. 63
  64. 64. Vibration Analysis ServicesFES Systems Inc. 64

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