Vibration Monitoring & Analysis
Vibration Monitoring
What is Vibration ?
It is motion of mechanical parts back and
forth from its position of rest /neutral
position.
Vibration Monitoring
What causes Vibration ?
Induced Force
&
Freedom for Movement
Vibration Monitoring
Harmful Effects of Excess vibration
• Increased load on BRGs: Reduced BRG Life
• Higher Forces on Mountings:
Foundation Loosening and Damage of
Support Structure
• Increased Stresses of M/c : Risk of fatigue
components
Vibration Monitoring
Harmful Effects of Excess vibration
• Decreased Equipment efficiency.
• Reduced Output Quality.
• Increased Maintenance Cost due to
more Component Failures and
Unplanned Operations
• Unsafe Operating Environment
Vibration Monitoring
Problem Identifications
•
•
•
•
•
Unbalance
Misalignment
Mechanical Looseness
Antifriction / Sleeve Bearing Defects
Gear Defects
Vibration Monitoring
Problem Identifications
•
•
•
•
•
Belt Defects
Impeller / Blade Defects
Bent Shaft
Electrical Problems
Resonance
Vibration Monitoring
Fundamental Realities
• All Machines vibrate.
• An increase in vibration level is a sign of
trouble & amplitude of Vibration depends on
the extent of defect in the machinery
components
• Each trouble will create vibration with different
characteristics
VIBRATION FUNDAMENTALS
90
Upper Limit
Neutral Position
180
TIME
Lower Limit
Period(T)
(1 complete cycle)
270
Characteristics of Vibration
• Vibration characteristics are
Displacement
Amplitude
Velocity
Acceleration
Frequency
Hz or CPM
Phase
Angle or clock face
Parameter Selection
• Frequency sensitivity
Displacement
<600CPM
Velocity
600-60,000CPM
Acceleration
>60,000CPM
Spike
Energy/SEE
Ultrasonic range
Frequency sensitivity
Vibration Monitoring
Displacement
Velocity
Acceleration
FFT
FAST FOURIER TRANSFORM.
• THE PROCESS OF TRANSFORMING TIME
DOMAIN SIGNAL TO FREQUENCY
DOMAIN.
• THE TIME DOMAIN SIGNAL MUST
FIRST BE SAMPLED AND
DIGITIZED.
FFT SPECTRUM ANALYSIS
A method of viewing the vibration signal in a way that is more useful for analysis
is to apply a Fast Fourier Transformation (FFT). In non-mathematical terms, this
means that the signal is broken down into specific amplitudes at various
component frequencies.
Indian Institute For Production Management
Time Domain - overall data is the sum of
all exciting and reacting forces
Imbalance
Rolling
Element
Bearing
Gearmesh
Coupling
chatter
Resultant Complex
Waveform
Time
Spectrum Analysis
Enables precise evaluation of
machinery condition and prediction
Fmax, LINES, AVERAGES.
• Fmax REPRESENTS THE MAXIMUM
FREQUENCY RANGE IN CPM OR HZ TO BE
SCANNED BY THE INSTRUMENT.
• Fmax SHOULD NOT BE SET TOO HIGH SO
THAT THE RESOLUTION AND ACCURACY
SUFFERS OR IT SHOULD NOT BE TOO LOW
SO THAT WE MISS SOME IMPORTANT HIGH
FREQUENCIES.
GUIDELINES FOR SETTING Fmax.
• FOR MACHINES HAVING ANTIFRICTION BEARINGS:- Fmax = 60 x
RPM
• FOR MACHINES HAVING SLEEVE
BEARINGS:- Fmax = 20 x RPM
• FOR GEAR BOXES:- Fmax = 3.25 x GMF
LINES OF RESOLUTION
• THE RESOLUTION IS THE NUMBER OF LINES
OR CELLS WHICH ARE USED TO CALCULATE
AND DISPLAY THE FREQUENCY SPECTRUM.
• THE BANDWIDTH CAN BE CALCULATED BY
DIVIDING Fmax BY THE LINES OF RESOLUTION.
• THE GREATER THE NUMBER OF LINES , THE
BETTER IS THE ACCURACY.
FREQUENCY
RESOLUTION
F max
Bandwidth
=
total lines of resolution
Amplitude
total lines of resolution
lines or bins or cells
of resolution
Frequency
Fmax
Spectrum Data Collection Time
• FFT Calculation Time = Time to calculate FFT
from Time Waveform [assuming no overlap
processing]
(60) ( #FFT Lines) (#Averages)
FFT Calculation Time =
Frequency Span
Where:
#FFT = Number of FFT Lines or Bins in Spectrum
# Averages = Number of Averages
Frequency Span measured in CPM
FFT SPECTRUM
OVERALL VIBRATION
Total summation of all the vibration,with no
regard to any particular frequency.
OVERALL VIBRATION
Overall vibration is the total vibration energy measured
within a frequency range. Measuring the “overall”
vibration of a machine or component, a rotor in relation
to a machine, or the structure of a machine, and
comparing the overall measurement to its normal value
(norm) indicates the current health of the machine. A
higher than normal overall vibration reading indicates
that “something” is causing the machine or component
to vibrate more.
Overall Vibration
Total summation of all the vibration,with no
regard to any particular frequency.
OA =
A1 2 + A2 2 + ………………………+An 2
NBF
OA=Overall level of Vibration Spectrum , Ai = Amplitude of each FFT line
n = No. of FFT Lines of resolution , NBF= Noise Bandwidth for Window chosen
NOTE: Don’t be concerned about the math,
the condition monitoring instrument
calculates the value. What’s important to
remember is when comparing overall
vibration signals, it is imperative that both
signals be measured on the same frequency
range and with the same scale factors.
What is Phase?
• The position of a vibrating part at a given
instant with reference to a fixed point or
another vibrating part.
• The part of a vibration cycle through which
one part or object has moved relative to
another part.
The unit of phase is degree where one
complete cycle of vibration is 360 degrees.
Phase is a measurement, not a processing
method. Phase measures the angular
difference between a known mark on a rotating
shaft and the shaft’s vibration signal. This
relationship provides valuable information on
vibration amplitude levels,shaft orbit, and shaft
position and is very useful for balancing and
analysis purposes.
Vibration Phase
Additional
Illustration on
Phase
PHASE AN ILLUSTRATION
30 Micron
10 degrees
32 Micron
10 degrees
Shaft centre line moves up and down in a planer fashion
PHASE AN ILLUSTRATION
30 Micron
10 degrees
32 Micron
190 degrees
Shaft center line moves up and down in a rocking fashion
MACHINE TRAIN MISALIGNMENT
TURBINE
AXIAL PHASE
(degrees)
G/B
0
10
12
8
5
12
10
6
HP COMP
15
22
20
16
18
24
22
20
LP COMP
198
210
208
200
Note: All phase readings corrected for pickup direction
215
220
218
210
Comparing Overall Levels
Across Mounting Interfaces
Phase application
A
B
C
A
B
C
5 Microns, 10 degrees
7 Microns, 12 degrees
25 Microns, 175 degrees
Bolt at C is loose
Vibration Analysis of
Common Problems
Vibration Analysis
Unbalance
• Amplitude proportional to the amount of
unbalance
• Vibration high normally in radial direction
(may be also in axial direction incase of
overhung and flexible rotors ).
• 1* RPM vibration is greater than 80%
(normally) of the overall reading.
Vibration Analysis
Unbalance
• Horizontal and vertical 1* RPM amplitude
should be nearly same, although it also
depends on system rigidity on the
particular direction.
• Other frequency peaks may be less than
5% of the 1*RPM amplitude
• Phase shift of 90 deg. When sensor
moves from horizontal to vertical.
UNBALANCE
• Operating conditions such as load, flow
condition and temperature effect
unbalance
– Balance under normal operating conditions
• Changes in track and pitch angle of fan
blades can result in “Aerodynamic
Unbalance”
Typical Spectrum For
Unbalance
MISALIGNMENT
• BIGGEST PROBLEM INITIALLY
• Operating temperature can affect
alignment
– Machines aligned cold can go out when
warm
• Bases or foundations can settle
• Grouting can shrink or deteriorate
• Increases energy demands
MISALIGNMENT
• Forces shared by driver and driven (not
localized)
• Level of misalignment severity is
determined by the machines ability to
withstand the misalignment
– If coupling is stronger than bearing the
bearing can fail with little damage to the
coupling
Three Types of Misalignment
• Combination (most common)
• Angular
• Parallel or Offset
General Characteristics Of
Misalignment
• Radial vibration is highly directional
• 1X, 2x, and 3x running speed
depending on type and extent of
misalignment
– Angular
1x rpm
axial
– Parallel
2x rpm
radial (H & V)
– Combination
1,2,3x rpm radial and
axial
Typical Spectrum for
Misalignment
Vibration Analysis
Misalignment
Angular Misalignment
• High axial vibration
( Greater than 50% of the radial vibration)
• 1* , 2*, 3* RPM normally high.
• 180 deg. Out of phase across the coupling
Angular Misalignment
• Produces predominant 1x rpm component
• Marked by 180 degree phase shift across the
coupling in the axial direction
Vibration Analysis
Misalignment
•
•
•
•
Off-Set Misalignment
High Axial vibration. Also shows high radial
vibrations.
1*, 2*, 3* RPM high. 2* often larger than 1*
In case of severe misalignment, much high
harmonics (4* - 8*) or even a whole series of
high frequency harmonics will be generated.
180 deg. Out of phase across coupling
Parallel Or Offset Misalignment
• Produces a predominant 2x rpm peak in the
spectrum
• Marked by 180 degree phase shift across the
coupling in the radial direction.
Typical Spectrum for
Misalignment
Axial Phase Showing
Misalignment
Other Types Of Misalignment
Vibration Analysis
Mechanical Looseness
Caused by structured looseness / weakness of
machine feet, base plate or foundation; also by
deteriorated grouting, loose base bolts and
distortion of the frame or base.
• Radial vibration high
• 2* RPM & 1* RPM dominant
• 180 deg. Phase differences between mating
surfaces which have looseness between
them.
Vibration Analysis
Mechanical Looseness
Caused by structured looseness / weakness of
machine feet, base plate or foundation; also by
deteriorated grouting, loose base bolts and
distortion of the frame or base.
• Radial vibration high
• 2* RPM & 1* RPM dominant
• 180 deg. Phase differences between mating
surfaces which have looseness between
them.
Looseness
• Looseness produces
2X RPM Freq.
Vibration Analysis
Mechanical Looseness
Caused by looseness in bearing housing bolts and
cracks in the frame structure.
• Radial vibration high
• 2* RPM normally dominant. 0.5*, 1* and 3* RPM
may also be present
• Substantial Phase difference between mating
surfaces which have looseness between them
LOOSENESS
• Not an exciting force
• Allows exciting frequencies already
present to exhibit much higher
amplitudes
• Loss or reduction in normal stiffness
• Caused by:
– loose mounting bolts
– deterioration of grouting
– cracked welds
Two Types Of Looseness
• Looseness of Rotating Components
– Loose Rotors
– Bearings Loose on the Shaft or in Housing
– Excessive Sleeve Bearing Clearances
• Looseness of Support System
–
–
–
–
–
Loose Mounting Bolts
Grouting Deterioration
Cracks
Poor Support
Frame Distortion
Looseness Of Rotating
System
• Rattling condition cause impacts due to
excessive clearance in a rolling element
or sleeve bearing
• Impacts cause multiple running speed
harmonics to appear in the spectra
• Identified by:
– multiple harmonics
– unstable phase
– highly directional radial vibration
Typical Spectrum for Looseness
of Rotating System
Looseness Of Support System
• FFT readings show 1x rpm, 2x rpm, and 3x
rpm components
• Structural looseness / weakness will cause
high 1xrpm peak in FFT
• Identified by
– Highly directional radial vibration
– Bouncing
– Taking comparative phase readings across
interfaces and look for amplitude variation
– Typically loose in vertical direction
Looseness Of Support System
Modern Trend
in
Vibration Technology
Condition Monitoring
System Integration
WALKAROUND
SURVEILLANCE
ON-LINE
ANALYSIS
CENTRALISED
PROTECTION
DISTRIBUTED
PROTECTION
PROTECTIO
N
PERIODIC
OFF- LINE
NETWORK
PdM TECHNOLOGIES
CONTINUOUS
ON - LINE
SOFTWARE
DCS
CMMS
Overall Data Acquistion
4-20mA
MONITOR
THE DCS
time waveform
DCS OUTPUT
Overall Data Trendsthis is what the DCS records
hi alarm
lo alarm
current value
changes over time
The limitation is that it does not adequately reflect changes
at higher frequencies which can increase by 100% but only add
1% to the overall energy level
Vibration Analysis
Protection Monitor
and / or
Data Collector
Vibration Spectrum
transducer
time waveform
Band Alarms, associate with each rotating element
hi alarm
lo alarm
frequency bands
Band Trending, the new way forward
hi alarm
lo alarm
changes over time
Trend and alarm the:
• Machine unbalance
• Alignment
• Gear mesh
• Bearings etc
Emonitor Odyssey:
spectrum band alarming though its
diagnostic tools feature for both On & Off line gives advanced
machinery analysis and reduces False Alarms
EMONITOR Odyssey: Frequency
Trends
Frequency Trend of Single Measurement
Band
DIAGNOSTICS -
the advantage of frequency band trending
• Root cause analysis is a complex machine
specific exercise considering all eventualities
• Expert systems are a one off diagnosis and do
not show a trend
• Frequency band trending is specific to root cause
analysis
• Band alarming also indicates vibration signals
that are outside the established norms
• Trending alignment, unbalance, gear meshing
and bearing condition condition is more specific
• A complex issue simplified without the need of
specialist customisation and regular updates
DCS Limitations - Summary
• We have shown that putting total belief in the DCS
vibration trend is highly risky
• Machinery failures still happen with on-line vibration
monitoring with 4-20mA data to the DCS. Most causes
are due to higher frequency signals swamped by the
overall levels.
• Advanced machinery protection through Frequency Band
Trending and Alarming - more specific than an Expert
system.
• The latest S/w based Analysers incorporates Narrow
Band Alarming. They offer machinery protection and
narrow band alarming.
• A lower cost solution is periodic manual Data Collection.
ESHAPE: Modal analysis using phase for
advanced diagnosis and better understanding of
system response
On line Vibration and other
monitors
•
•
•
•
•
•
Innovative, fully-digital design
Exceeds API 670 specification
Widely-used system
Fully field programmable
Low installation cost
ModBus protocol
TYPICAL APPLICATION
POWER PLANT INTEGRATION
DCS
OPERATIONS
DATA
LOGGER
FS
HP
LP
GEN
TURBINE
SUPERVISORY
ODYSSEY
SERVER
ENGINEERING
VIBRATION
ANALYSER
EX
STATOR END
WINDING
BFP
ID
BFP
FD
CWP
PA
AUXILIARIES
GATEWAY
TO CMMS
Plant Integration with LAN or WAN
ENGINEERING
DCS
VIBRATION
ANALYSER
ETHERNET
CONTROL ROOM No 1
FS
HP
LP
TG 1
GEN
CONTROL ROOM No 2
EX
FS
ODYSSEY
CLIENT SERVER
HP
LP
TG 2
GEN
EX
CONTROL ROOM No 3
FS
HP
LP
GEN
TG 3
EX
GATEWAY
TO CMMS
ANURAKSHAN
NETWORKING THE INFORATION - LAN
/ WAN e.g.
RIHAND
TALCHER
PLANT
OPERATION
VINDHYACHAL
S
NOIDA HQ
GATEWAY
TO CMMS
ANURAKSHAN
UNCHAHAR
CM CELL
KAYAMKULAM
Using PlantLink
Digital Picture of Plant
Vibration Trend Plot
Hyperlink to
equipment
Hierarchy
Automatic E-Mail notification on
Equipment Alarm Status
Click on Measurement
Label to link to plots or
other views.
Information however you want it !
X-Window Screen Captures
Scenario of Instruments &Sensors & Probes
• Velocity sensors are made in India
• Accelerometers range over 150 types
– standard
– Low frequency
– High temperature (Gas Turbines)
– Special application
• Eddy current probes - comprehensive range
• Others available for process measurement
Vibration Datacollectors
Many vendors
Select on ‘Fitness
for Purpose’
Intrinsic Safety
Dust & Moisture proof
Diagnostic Capability