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Condition monitoring of rotary machines

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Condition monitoring of rotary machines

  1. 1. UNDERSTANDING CONDITION MONITORING OF ROTARY MACHINES
  2. 2. AUTOMATED MONITORING CONCEPTS  Detect Detailed Machinery Problems  Unbalance, Misalignment, Looseness, Shaft Cracks, Oil Whirl, Phase, Rubs, Gear and Bearing Problems Looseness Problem!
  3. 3. TOTAL VIBRATION IS SUM OF ALL THE INDIVIDUAL VIBRATIONS
  4. 4. RELATIONSHIP BETWEEN FORCE AND VIBRATION  • Forces that cause vibration occur at a range of frequencies depending on the malfunctions present  • These act on a bearing or structure causing vibration  • However, the response is not uniform at all frequencies. It depends on the Mobility of the of the structure.  • Mobility varies with frequency. For example, it is high at resonances and low where damping is present
  5. 5. CONVERSION OF VIBRATION PARAMETERS METRIC UNITS  • Displacement, Velocity and acceleration are related by the frequency of motion  • Parameters in metric units  – D = Displacement in microns (mm/1000)  – V = Velocity in mm/sec  – A = Acceleration in g’s  – F = Frequency of vibration in cycles /minute (CPM)  • V = D x F / 19,100  • A = V x F / 93,650  • Therefore, F = V / D x 19,100
  6. 6. SELECTION OF MONITORING PARAMETERS  • Where the frequency content is likely to be low (less than 18,000 CPM) select displacement  – Large, low speed, pumps and motors with sleeve bearings  – Cooling tower fans and Fin fan cooler fans. Their gear boxes would require a higher frequency range  • For intermediate range frequencies ( say, 18,000 to 180,000 CPM) select Velocity  – Most process plant pumps running at 1500 to 3000 RPM  – Gear boxes of low speed pumps  • For higher frequencies (> 180,000 CPM = 3 KHz) select acceleration.  – Gear boxes  – Bearing housing vibration of major compressor trains including their drivers  • Larger machines would require monitoring more than one parameter to cover the entire frequency range of vibration components
  7. 7. Signal Processing Flow FFT Waveform Spectrum Transducer AmplitudeAmplitude Time FrequencyData Collector/Analyzer
  8. 8. Rotation Heavy Spot 1 revolution Time Amplitude 0 + - Time Waveform 3600 rpm = 3600 cycles per minute 60 Hz = 60 cycles per second 1 order = one times turning speed 360 degrees
  9. 9. 1000 rpm 1 revolution Time Amplitude 0 + - Time Waveform 4 blades = vibration occurs 4 times per revolution 4 x 1000 rpm = vibration occurs at 4000 cycles per minute = 4000 cpm
  10. 10. 12 tooth gear 1000 rpm 1 revolution Time Amplitude 0 + - Time Waveform 12 teeth are meshing every revolution of the gear 12 x 1000 rpm = vibration occurs at 12,000 cycles per minute = 2,000 cpm = 200 Hz
  11. 11. Time0 + - Time0 + Time0 + - -
  12. 12. Time0 - + Time Waveform contains all the different frequencies mixed together Complex Time Waveform
  13. 13. Time Waveform contains all the different frequencies mixed together Complex Time Waveform
  14. 14. We are now entering the Frequency Domain •FFT - Fast Fourier Transform •Separates individual frequencies •Detects how much vibration at each frequency
  15. 15. TIME WAVEFORM  AMPLITUDE VS TIME
  16. 16. Amplitude Amplitude Amplitude Time Amplitude Time Frequency
  17. 17. Time0 - Time0 + Time 0 + - - Frequency Frequency Frequency 1x 4x 12x
  18. 18. Predefined Spectrum Analysis Bands 20000 0.3 0.6 0.9 1.2 1.5 1.8 1xRPM - BALANCE 2xRPM - ALIGNMENT 3-5xRPM - LOOSENESS 5000 10000 15000 Frequency Hz 5-25xRPM 25-65xRPM ANTI-FRICTION BEARINGS & GEARMESH
  19. 19. TRANSDUCERS -
  20. 20.  Vibration transducers can be divided into three groups, based on the physical measurement that they make: acceleration, velocity and displacement.  measurements of rotor displacement within the clearances of fluid film bearings made with eddy current displacement transducers.
  21. 21.  output signal will include two components, the varying ac voltage (resulting from the mechanical vibration of the rotor), and the dc or average voltage (representing the average distance between the rotor and the probe).
  22. 22.  The most common parameter associated with the vibration signal is the overall (unfiltered or “broadband”) amplitude, which for displacement measurements is typically expressed as a peak-to- peak (pp) value, as shown on Figure
  23. 23. PHASE LAG/LEAD- Looking from left to right, it is easy to see that signal B reaches its maximum (point 1) level before signal A does (point 2). In other words, signal B leads signal A by 138 degrees
  24. 24. ABSOLUTE PHASE-
  25. 25. PHASE – PHYSICAL SIGNIFICANCE
  26. 26. STEADY STATE PLOTS- Vibration trend
  27. 27. VIBRATION VECTOR  • A vibration vector plotted in the transducer response plane  • 1x vector is 90 mic pp /220o  • Zero reference is at the transducer angular location  • Phase angle increases opposite to direction of rotation
  28. 28. BODE PLOT-
  29. 29. POLAR PLOT
  30. 30.  Polar plot is made up of a set of vectors at different speeds.  Vector arrow is omitted and the points are connected with a line  Zero degree is aligned with transducer location  Phase lag increases in direction opposite to rotation  1x uncompensated Polar Plot shows location of rotor high spot relative to transducer  This is true for 1x circular orbits and approximately true for 1x elliptical orbits
  31. 31. ORBIT PLOT-  • The orbit represents the path of the shaft centerline within the bearing clearance.  • Two orthogonal probes are required to observe the complete motion of the shaft within.  • The dynamic motion of the shaft can be observed in real time by feeding the output of the two orthogonal probes to the X and Y.  • If the Keyphasor output is fed to the Z axis, a phase reference mark can be created on the orbit itself
  32. 32. CONSTRUCTION OF AN ORBIT  • XY transducers observe the vibration of a rotor shaft  • A notch in the shaft (at a different axial location) is detected by the Keyphasor transducer.  • The vibration transducer signals produce two time base plots (middle) which combine into an orbit plot (right)
  33. 33. MEASUREMENT OF PEAK-TO-PEAK AMPLITUDE OF AN ORBIT  X transducer measurement axis is drawn together with perpendicular lines that are tangent to maximum and minimum points on the orbit
  34. 34. SHAFT ROTATION AND PRECESSION  • Precession is the locus of the centerline of the shaft around the geometric centerline  • Normally direction of precession will be same as direction of rotation  • During rubbing shaft may have reverse precession
  35. 35. DIRECTION OF PRECESSION IN ORBITS  In the orbit plot shaft moves from the blank towards the dot. In the plot on left the inside loop is forward precession  In the right orbit the shaft has reverse precession for a short time at the outside loop at bottom
  36. 36. EFFECT OF RADIAL LOAD ON ORBIT SHAPE  Orbits are from two different steam turbines with opposite rotation. Both machines are experiencing high radial loads  Red arrows indicate the approximate direction of the applied radial load.  Red arcs represent the probable orientation of the bearing wall
  37. 37. FULL SPECTRUM  • Half Spectrum is the spectrum of a WAVEFORM  • Full Spectrum is the spectrum of an ORBIT  • Derived from waveforms of two orthogonal probes  – These two waveforms provide phase information to determine direction of precession at each frequency
  38. 38.  First Waveform and its half spectrum  Second Waveform and its half spectrum  Combined orbit and its full spectrum
  39. 39.  Forward Precession  Spectrum on forward side of plot <-- Reverse Precession  Spectrum on reverse side of plot  Direction of rotation – CCW  <-- Forward Precession  Spectrum on forward side of plot  Direction of rotation – CW  <-- Reverse Precession  Spectrum on reverse side of plot  Direction of rotation - CW

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