Condition monitoring of rotary machines orbit polar spectrum system1

1. UNDERSTANDING CONDITION
MONITORING OF ROTARY MACHINES

2. AUTOMATED MONITORING CONCEPTS
 Detect Detailed Machinery
Problems
 Unbalance, Misalignment,
Looseness, Shaft Cracks, Oil
Whirl, Phase, Rubs, Gear and
Bearing Problems
Looseness
Problem!

3. TOTAL VIBRATION IS SUM OF ALL THE
INDIVIDUAL VIBRATIONS

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. 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. 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

8. Signal Processing Flow
FFT
Waveform
Spectrum
Transducer
AmplitudeAmplitude
Time
FrequencyData Collector/Analyzer

9. 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

10. 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

11. 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

12. Time0
+
-
Time0
+
Time0
+
-
-

13. Time0
-
+
Time Waveform contains all the different
frequencies mixed together
Complex Time Waveform

14. Time Waveform contains all the different
frequencies mixed together
Complex Time Waveform

15. We are now entering the Frequency Domain
•FFT - Fast Fourier Transform
•Separates individual frequencies
•Detects how much vibration at each
frequency

16. TIME WAVEFORM
 AMPLITUDE VS TIME

17. Amplitude
Amplitude
Amplitude
Time
Amplitude
Time
Frequency

18. Time0
-
Time0
+
Time
0
+
-
-
Frequency
Frequency
Frequency
1x
4x
12x

19. 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

20. TRANSDUCERS -

21.  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.

24.  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).

25.  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

26. 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

27. ABSOLUTE PHASE-

28. PHASE – PHYSICAL SIGNIFICANCE

29. STEADY STATE PLOTS- Vibration trend

32. 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

33. BODE PLOT-

34. POLAR PLOT

35.  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

36. 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

37. 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)

38. 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

39. 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

40. 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

41. 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

42. 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

43.  First Waveform and
its half spectrum
 Second Waveform
and its half spectrum
 Combined orbit and
its full spectrum

44.  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