Reliability Of
Rotating Equipment By Vibration Monitoring
|
To give an overall idea to the
Maintenance Personnel about the Predictive Maintenance Programme / Reliability
& Condition Monitoring Concepts, Principles and Techniques.
This Manual is
intended to explain in general about different types of condition monitoring
techniques, analysis and diagnostics on any type of rotating equipment.
1 Introduction
1.1 Predictive maintenance concepts and
objectives
1.2 Vibration and its causes
2 VIBRATION MEASUREMENT TYPES
2.1 Vibration Displacement
2.2 Vibration Velocity
2.3 Vibration Acceleration: (Figure-1)
2.4 Vibration Frequency
2.5 Vibration Phase: (Figure-2)
2.6 Acoustic emission and Shock pulse
3 transducer types
3.1 Contact type:(Figure-3)
3.1.1 ELECTRO
MAGNETIC/VELOCITY PROBES:
3.1.2 PIEZO-ELECTRIC/ACCELEROMETER
PROBES:
3.2 Non-contact type
3.2.1 EDDY
CURRENT/PROXIMITY PROBES
3.3 Transducer selection
4 Monitoring TYPES
4.1 On-line monitoring:
Figure-6*
4.2 Off-line or periodic monitoring
5 vibration analysis and machinery diagnostics
5.1 Types of Vibration Plots
5.1.1 TREND PLOT :
(Figure-11)
5.1.2 SPECTRUM PLOT:
(Figure-12&13)
5.1.3 TIME WAVEFORM PLOT:
(Figure-14)
5.1.4 WATERFALL PLOT:
(Figure-15)
5.1.5 CASCADE PLOT:(Figure-16)
5.1.6 ORBIT
PLOT:(Figure-17)
5.1.7 BODE PLOT:(Figure-18)
5.1.8 POLAR
PLOT:(Figure-19)
5.1.9 CENTER LINE PLOT
5.2 Typical problems and Diagnostics
6 Vibration standards
1.1 Predictive maintenance concepts and objectives
In recent years industry has
started to realise the financial benefits and engineering improvements created
by utilising Predictive Maintenance techniques.These techniques can give
advanced warning of any impending mechanical or any other machinery/structural
related problems allowing them to be rectified in a planned manner thus
improving the machinery reliability. Problems are diagnosed
in advance or at an early stage itself to avoid any unexpected machine breakdown thus avoiding plant downtime.
Various techniques like Vibration
monitoring,Process monitoring,Lube oil analysis are used in Predictive
maintenance programs out of which Vibration Monitoring is considered to be a
widely used and proven technique.
The objective of machinery monitoring
is to increase the availability and reliability of machinery and decrease the
maintenance costs by:
·
Preventing catastrophic failures
·
Detecting machinery distress to allow corrective action before major
damage occurs
·
Reducing spareparts requirements
·
Improving operation and maintenance by providing better knowledge of
machinery rotor and bearing behaviour
·
Prolonging the periods between machinery major overhauls by providing
better insight into the condition of machinery in operation
·
Providing information for correction of basic design deficiencies
·
Reducing the downtime by detecting the onset of a problem,enabling
remedial repairs to be made during scheduled outages
·
Predicting maintenance requirements,allowing advanced scheduling and
planning of maintenance work
1.2 Vibration and its causes
Vibration is the motion of a
machine or part of a machine to and from its position of rest.
The cause of vibration must be a
force which is varying in either its direction or its amount.It is the force
which causes vibration and the resulting characteristics will be determined by
the manner in which the forces are generated.That is why each cause of
vibration has its own specific characteristics.
With few exceptions, mechanical
problems in a machine cause vibration.Some of the common problems which produce
vibration are:
·
Unbalance of rotating parts
·
Misalignment of couplings
and bearings
·
Worn or damaged bearings
·
Bent shafts
·
Looseness
·
Rubbing
·
Machine/structural
resonance
·
Worn,eccentric or damaged
gears
·
Aerodynamic/Hydraulic forces
2 VIBRATION MEASUREMENT TYPES
The displacement,velocity and
acceleration are the three characteristics of vibration by which the vibration
amplitude is quantified (Refer figure 1).In terms of the operation of a
machine, the vibration amplitude is the indicator used to determine the
condition of the machine.Types of measurement differs from machine to machine
mostly depending upon the machine type and bearing type.Vibration monitoring
and machinery diagnostics gives good results only when right type of measurement
is chosen for the machines. Common types of measurements done with rotating machinery are,
·
Relative shaft vibration -- using non-contact type pickups
·
Absolute casing vibration -- using contact type pickups
2.1 Vibration Displacement:
The total distance
travelled by the vibrating body from one extreme limit of travel to the other
extreme limit of travel is referred to as the “peak to peak displacement” and
mostly measured in “microns” or in “mils” (peak,peak-to-peak or rms).
This type of measurement is
chosen when a machine has a low rotor- to-casing mass ratio i.e the rotating
parts are much lighter than the stationary parts(casings, bearing housings
etc.).There is much less energy(vibration)to measure on the casing and it is
more important to measure the actual movement of the shaft within its bearings.
A good example of a machine with low rotor-to-casing mass ratio is a barrel
type compressor.
2.2 Vibration Velocity:
The velocity at which the
vibrating body covers the displacement is referred to as “vibration velocity”
and mostly measured in “mm/sec” or in “inch/sec” (peak or rms).
Velocity is the most widely used
parameter in vibration measurement.This is due to equal weighting of velocity
amplitude through a wide frequency range, thus allowing to identify a wide
range of problems.The International Standards Organisation (ISO), in its work
to establish internationally acceptable units for measurement of machinery
vibration, decided to adopt “vibration velocity”as the standard unit of
measurement. Vibration velocity is mostly measured on machines with low casing-to-rotor mass ratio
like pumps,blowers,fans,reciprocating compressors etc..
2.3 Vibration Acceleration: (Figure-1)
Acceleration is normally referred
to as time rate of change of velocity and measured in “G’s” (peak) where one
“G” is the acceleration produced by the force of gravity (1G’s=9.8m/sec2).
Figure:1
Acceleration leads velocity by 90
degrees in time, and leads displacement by 180 degrees in time.
Acceleration amplitudes are weighted
towards the higher frequency ranges and as a result, it can give a better
indication of incipient anti-friction bearing problems,gear tooth meshing
problems and high speed machinery problems.
2.4 Vibration Frequency:
Frequency is usually expressed in
Hertz(Hz) or cycles per minute(CPM).Frequency is also often quoted in multiples
of rotating speed of the machine train, this is because most machinery
vibrations occur in direct multiples or sub-multiples of rotating speed.It also
provides an easy means to express the frequency of vibration, it being easier
to refer to frequency as once times rpm(1X),two times rpm(2X) and so on rather
than having to express all vibration in cpm or Hertz.Frequency is probably the
most important characteristic of vibration as it indicates the probable source
of vibration.
Machines can simultaneously
generate a number of frequencies relative to the various forces which are
acting and reacting in the machine and it is necessary at times to know what
the dominant frequency of a particular machine is, if we are measuring overall
levels.
2.5 Vibration Phase: (Figure-2)
Accurate diagnosis of machinery
problems requires a complete set of machine vibration information.That
information comes from the three primary parts of the data that we can obtain
from a vibration signal, like direct amplitude, frequency and when, there are
two signals, phase (Refer figure 2). Relative phase is the timing relationship
between two vibration signals, while absolute phase compares a vibration signal
to a once-per-turn reference pulse.The keyphasor is a once-per-turn voltage
pulse provided by a proximity type transducer and the information is called as
phase. The keyphasor signal is used by monitoring,diagnostic and management
systems to generate filtered vibration amplitude, phase lag, speed and a variety of other information.Without phase
information,overall machine
condition and machine faults would often be very difficult to diagnose.
Phase information can be used to
detect subtle changes to the machinery that otherwise go unnoticed. Listed
below are a few of the many phase angle applications:
·
Direction of vibration
·
Shaft crack detection
·
Rub detection
·
Shaft balancing
·
Shaft / structural
resonance detection
·
Shaft mode shape
Trending
amplitude and phase information can provide early shaft crack detection and
alert the operators to other possible machine problems.Rub conditions can be
detected with the use of phase presented in steady state polar plots.Machinery
specialists use phase extensively in their analysis to confirm suspected
machine problems.
Figure:2
2.6 Acoustic emission and Shock pulse
Acoustic emissions and shock
pulse methods(SPM) of monitoring anti-friction bearings are basically similar
techniques, in that both use crystal or piezoelectric type transducers that
measure stressed bearing activity in a frequency range between 35 KHz to 50
KHz.The transducer mounted natural frequency is also in this frequency range
and typically has low damping.This gives an effective amplification of the
signal and defects can be detected earlier.
The term acoutic emission is
applied to the so-called shock pulse which is emitted by a material under
stress.The pulse is generated in the material and into the adjacent
structure.In a ball or roller bearing this pulse is generated when, for
example, a crack occurs in an inner or outer race.Each time a ball or roller
crosses the crack ,it is stressed, and a pulse is generated in the ball or the
race and transmitted through the race into the mounting structure.
The pulse has a very high energy
content for a very short time.The frequency of this signal is well above the
normal bearing vibration range.It is in the band of frequencies centered about
40,000 Hz.The pulses occur at a rate directly related to the number of balls
and rotational speed of the bearing.
The acoustic emissions are in a
frequency range distincly different from sounds produced as a result of
vibration of the bearing or its mounting structure.These latter vibrations
occur at much lower frequencies and over a wide range dependent on the speed of
rotation,resonances of the structure etc,.. Vibration frequencies range from
about 20 Hz to 15 Khz for shaft speeds of up to 100,000 rpm.It should also be
noted that acoustic emission occurs long before vibration is measurable.If
vibration is in evidence,the bearing has long since started to fail.
3 transducer types
To measure vibration, a
transducer or vibration pickup is needed to convert the mechanical motion into
an equivalent electrical signal. There are two major classification of
transducer types:
·
Contact type
·
Non-contact type
3.1 Contact type:(Figure-3)
This class of transducers are used
for casing vibration measurements mostly velocity and acceleration (Refer
figure 3). These type of probes can be mounted as follows:
·
Stud mounting
·
Magnetic mounting
·
Hand held probe
The method of mounting of
the transducer will affect the useful frequency range of the transducer. There
are certain advantages and disadvantages in these type of probes.
Figure:3
3.1.1 ELECTRO MAGNETIC/VELOCITY PROBES:
The velocity type transducer is a
moving coil or magnetic seismic device.When a conductor is moved through a
magnetic field or if a magnetic field is moved past a conductor a voltage will
be induced.Either the magnet or the coil of the pickup are mounted on the
machine bearing housing, the moving element is held suspended by low stiffness
springs.When the bearing housing vibrates the fixed section of the transducer
moves relative to the floating section and hence a voltage is generated in the
coil which is directly proportional to the velocity of the vibration.
The voltage output of a velocity
transducer is normally expressed in mv/mm/sec.This is also referred to as the
sensitivity of the vibration pickup.Velocity probes has got the limitations
like,
·
Limited frequency range i.e
10-1000Hz
·
Direction oriented i.e
horizontal or vertical
3.1.2 PIEZO-ELECTRIC/ACCELEROMETER PROBES:
An accelerometer uses a
piezo-electric crystal that produces an electrical charge directly proportional
to strain (and hence force) when loaded either in tension,compression,shear.
When the accelerometer is
subjected to vibration the mass exerts a varying force on the piezo-electric
crystal which produces a charge directly proportional to the vibration
acceleration.The charge is then converted into a voltage signal.The conversion
is done internally inside the probe by an in-built charge amplifier.
Their output voltage is
proportional to vibration amplitude and the square of the frequency. The normal
sensitivity range is in the range of 100mv/g.
Accelerometers are widely used as
compared to electromagnetic type because of its own advantages like,
·
Better frequency response
characteristics (dc-0 Hz to typically 20 KHz)
·
No mounting orientation
restrictions
·
Extended temperature ranges
·
Compact and light weight
·
No moving parts and hence
rare failures
·
Available at very low
frequency to high temperature ranges for different applications
3.2 Non-contact type
This type of probes are widely
used for relative shaft displacement measurement on critical compressors.
Non-contact type pickup is a transducer used for vibration and position
measurement that doesn’t require contact with the target whose vibration or
position being measured.This type of probe works on eddy-current principle.
3.2.1 EDDY CURRENT/PROXIMITY PROBES:
Shaft vibration,radial and axial
shaft movement, shaft and housing expansion can all be measured using
non-contact type displacement pickups (Refer
figure 4). Eddy current is referred as the electrical current which is
generated (and dissipated) in a conductive material when such material
intercepts the electromagnetic field of a proximity probe.The proximity/eddy
current probe is powered by the instrument/monitor.The probe is driven by a
high frequency carrier signal and whenever the target moves , the carrier
signal is altered which is sensed by the de-modulator and a proportional
voltage output is generated corresponding to the peak-to-peak vibration
displacement of the target(shaft).The probe measures both dynamic motion and
average position of the shaft with a good frequency response. The probe gap is
the physical distance between the face of a proximity probe tip and the
observed surface. The distance can be expressed in terms of
displacement(mils,micrometers) or in term of voltage(volts).
The normal working frequency
range is dc 0-10000Hz with a displacement measuring range of 2mm. Nominal
sensitivity is in the range of 8mv/micron(200mv/mil)
Figure-4
3.3 Transducer selection
Transducer selection for a
particular machine depends on the machine type,bearing type and the type of
measurement required.
Shaft observing eddy-current
proximity probes directly measure the relative displacement between the probe
and the conductive surface it observes,generally a rotating or reciprocating
machinery shaft.These transducers are extremely sensitive and accurate in
measuring displacements in the order of micrometers.These probes directly
observe the relative shaft motion inside the bearing clearances of fluid film
bearings.These bearings are employed in the majority of large,high speed
turbomachinery and proximity probes are permanently mounted on these machine
types for continuous on-line monitoring.
Casing observing transducers
include both velocity transducers and accelerometers.As the name implies, they
are both used to measure casing motion and are mounted temporarily or
permanently on the machine or bearing housing.These type of probes cannot be
used directly to observe rotor motion within the bearing clearances.Therefore,
their usefulness as the primary or only measurement on machines incorporating
fluid-film bearings is normally not appropriate.The proper application of these
transducers requires detailed knowledge of their limitations and the effects of
signal conditioning such as integration and filtering.
4 MOniTORING TYPES
Two types of vibration monitoring
techniques are widely used worldwide.They are
·
On-line monitoring
(Continuous/Permanent)
·
Off-line monitoring
(Periodic/Temporary)
The
scope, extent and type of monitoring, modified by operating experience,
judgement and economic analysis should be based on whether it is:
·
A critical unspared machine
·
A critical or possible
failure prone machines
·
A critical spared machine
in light service
·
A non-critical unspared
machine
·
A non-critical spared
machine
4.1 On-line monitoring:
Continuous collection of
vibration data by means of permanent monitoring systems is called as on-line
monitoring (Refer figure 5 to 9 for such
typical systems).Dedicated on-line systems with diagnostic capabilities
makes it possible to make timely decisions regarding machinery health that can
be used to better schedule of maintenance,delay shutdowns,thus eliminating any
unscheduled outages as well as preventing catastropic failures.Various systems
with different capabilities are available in the market. Most of the process
critical machineries like compressors, gear boxes, high pressure steam
turbines, expander turbines, large electric motors, boiler feed water pumps
are monitored by permanently installed
non-contact type proximity probes. In
certain hazardous area locations
with limited access,accelerometers are also permanently mounted on pumps for
on-line continuous vibration monitoring.
A typical on-line monitoring
system consists of either a non-contact proximity or accelerometer type probes connected to the
Data management software work station through rack mounted signal transmitters
and communication processors. Work stations with Data management software is
usually located in control room or in engineer’s office whereas rest of the
hardware is field mounted.The latest systems are capable of collecting data in
steady-state and also in startup/shutdown(transient) operating conditions.
This type of monitoring is most
suitable for critical/super critical machineries since continuous vibration
monitoring of those machines are inevitable due to their process importance.
Any increasing vibration trend can be noticed immediately and further action
can be taken to avoid any unexpected plant shutdown and production losses.
Maintenance activities can be planned in advance to avoid any prolonged
shutdown due to non-availability of spares,manpower etc.,if any.
Another advantage of on-line
monitoring is its capability to correlate vibration data with process
parameters. Process data like pressure, flow, temperature can always be related
with vibration data under two different operating conditions especially for
process critical compressors for better performance analysis and reliability
studies.
The major parameters monitored by
this on-line monitoring includes,
·
Radial relative shaft
vibration
·
Axial thrust position
(Average position, or change in position, of a rotor in the axial direction
with respect to some fixed reference)
·
Speed / Phase angle
·
Torque
Figure-5
Figure-6
Figure-7
Figure-8
Figure-9
4.2 Off-line or periodic monitoring
Vibration data collection of
machineries at certain time intervals using portable data collection systems is
called as Off-line or Periodic vibration monitoring (Refer figure 10). Most of the non-critical / critical machineries
like utility pumps, blowers, fans, small motors etc., fall under this category
of monitoring.
A typical off-line or periodic
monitoring system consists of a data collector with contact type accelerometers
used in conjunction with a data collection software loaded into a PC. Most of
the data collected with this type of monitoring are by casing vibration
measurement in three different directions (horizontal / vertical / axial) at
bearing locations.Measurement points are downloaded from the PC software in to
the datacollector as per monthly / biweekly schedules for data collection. Once
the route data collection is completed, data is uploaded in to the PC software
for further analysis and diagnostics.
Figure-10
5 vibration analysis and machinery diagnostics
There are many ways that
vibration data can be obtained and displayed for the purpose of detecting and
identifying specific problems in rotating machinery.Some of the common
techniques include:
·
Amplitude vs frequency
(RPM) - Spectrum
·
Amplitude vs time - Waveform
·
Amplitude vs frequency vs
time - Waterfall
·
Amplitude/phase vs rpm - Bode plot
·
Orbits
·
Shaft center line plots
Vibration
signals measured on machineries are mostly complex.This signal consists of a
number of signals of different frequencies and amplitudes.From this signal it
would be difficult to abstract the different amplitudes and frequencies of
vibration which exist within signal.The signal can be processed electronically
into what is known as a vibration spectrum, fast fourier (FFT)spectrum or a
frequency spectrum.The vibration spectrum gives a breakdown of the different
vibration frequencies and their amplitudes which exist within the original
vibration signal.
The
identification of the vibration signal’s frequencies(source) and the
amplitudes(severity) for a machine holds the key to the use of vibration to
assess the machine’s condition.
When
interpreting vibration data we have to identify two things:
·
The levels of vibration i.e
vibration severity and whether they are good,acceptable or unacceptable.
·
The source of vibration
Various
standards organisations like ISO/API have produced guidelines on the assessment
of machinery vibration levels. But it is to be noted that complete assessment
is done only after studying the individual characteristics of the machine.
5.1 Types of Vibration Plots
5.1.1 TREND PLOT : (Figure-11)
A presentation in cartesian
format with the measured variable (like vibration amplitude) on the Y axis and
time on the X axis.
Figure-11
5.1.2 SPECTRUM PLOT: (Figure-12&13)
Presentation of the amplitude of
a signal as a function of frequency. An XY plot where the X axis represents
vibration frequency and the Y axis represents vibration amplitude.
Figure-13
5.1.3 TIME WAVEFORM PLOT: (Figure-14)
A presentation of the
instantaneous amplitude of a signal as a function of time. A vibration waveform
can be observed on an oscilloscope in the time domain.
5.1.4 WATERFALL PLOT: (Figure-15)
A graph in cartesian format
displaying a series of frequency spectra versus time. This type of plots are
useful in comparison of spectrums collected at different time intervals mostly
with same rotative speed.
Figure-14
Figure-15
5.1.5 CASCADE PLOT:(Figure-16)
A graph in cartesian format
displaying a series of frequency spectra versus shaft rorative speed. This XYY’
presentation shows amplitude (Y) versus frequency (X) incremented along a
second (sometimes vertically offset) Y axis , rpm. This data format is used to
evaluate the change in vibration frequency characteristics during machine
transient conditions like startups/coast down.
Figure-16
5.1.6 ORBIT PLOT:(Figure-17)
The dynamic path of the shaft
centerline displacement motion as it vibrates during shaft rotation. The orbit
can be observed on an oscilloscope connected to XY proximity probes.Sometimes
called precessional motion or orbital motion.
Figure-17
5.1.7 BODE PLOT:(Figure-18)
The Bode plot is two separate
cartesian plots of amplitude versus rpm and phase lag versus rpm. Rotative
machine response has usually been measured through the use of the Bode plot, a
technique used to plot rotative speed amplitude (1X) of a given measurement
against rpm, along with the phase lag angle of that amplitude vector against
rpm. This plot most useful in showing the rpm of variuos
resonances of the machine.
5.1.8 POLAR PLOT:(Figure-19)
The Polar plot is a combination
of three variables (amplitude, phase, rpm) into a single plot in a circular
format. Polar plot is much descriptive and useful to the machinery engineer in
displaying other parameters of machine response.
Figuare-18
Figure-19
5.1.9 CENTER LINE PLOT
The Shaft center line plot is a
plot in circular format with two variables i.e vibration amplitude and rotative
speed. It gives a very useful machinery information of radial shaft movement
inside the bearing clearances at different running
speeds.
5.2 Typical problems and Diagnostics
The following table illustrates
the typical or most common problems,their symptoms and diagnostics.
CAUSE /
PROBLEMS
|
FREQUENCY
|
AMPLITUDE
|
REMARKS
|
Unbalance
|
1
X RPM
|
Proportional
to unbalance. Largest in radial direction
|
Most
common cause of vibration.
|
Misalignment
Couplings
or bearings
and
bent shaft
|
1
X RPM usual
2
X, 3 X RPM
sometimes
|
Large
in axial
Direction.
50% or
More
of radial vibration
|
Best
found by appearance of large axial vibration.If sleeve bearing machine and no
coupling misalignment then balance the rotor.
|
Bad
bearings
Anti-friction
type
|
Very
high. several times RPM
|
Unsteady
|
Largest
high frequency vibration with associated noise. Special techniques like SPM /
SEE / Enveloping can be adopted.
|
Bad
gears or Gear noise
|
Very
high. Gear teeth times RPM
|
Low
|
Usually
accompanied by high temperature and noise.
|
Mechanical
looseness
|
2
X RPM and hormonics
|
|
Usually
accompanied by unbalance and /or misalignment.
|
Bad
drive belts
|
1
X, 2 X, 3X RPM of belts
|
Erratic
or pulsing
|
Strobe
light can be used to freeze faulty belts.
|
Electrical
|
1
X RPM or 1 or 2X synchronous frequency
|
Disappears
when power is switched off
|
If
vibration amplitude drops down instantly when power is turned off, the cause
is electrical.
|
Aerodynamic
Hydraulic
forces
|
1
X RPM or number of blades on fan or impeller X RPM
|
Moderate
& steady
|
Rare
as a cause of trouble except in cases of resonance.
|
Reciprocating
forces
|
1,
2 & higher orders X RPM
|
High
& steady
|
Inherent
in reciprocating machines. Can only be reduced by design changes or
isolation.
|
6 Vibration standards
Various international
organisations have developed standards for different type of rotating
equipment. Mostly these standards are considered as guidelines only in
detemining the machine condition since individual machineries have their own
characteristics which will be used in arriving at the final conclusion of
machine condition detemination.
The most common vibration standards
used nowadays are:
·
International Standards
Organisation (ISO)
·
American Petroleum
Institude (API)
·
National Electrical
Manufacturer’s Association (NEMA)
CLASS
3: Large machines on rigid CLASS 4: Large machines on flexible
Foundations (steam turbines, foundations (large
turbo-generators, large centrifugal compressors) fans, blowers)
API 610 : Centrifugal pumps for general refinery service
·
Pumps with sleeve bearings
RPM Shaft vibration
·
Pumps with anti-friction bearings
API 611 : General purpose steam turbines
API 612 : Special purpose steam turbines
API 613
: Special purpose gears
API 617 : Centrifugal
compressors
API 672 : Centrifugal plant and
instrument air compressors
API RP541 : Electric motors
RPM Casing vibration
720 to 1499 <2 .5="" font="" mils="" pp="">2>
1500 to 2999 <2 .0="" font="" mils="" pp="">2>
>3000 <1 .0="" font="" mils="" pp="">1>
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