Reliability Of Rotating Equipment By Vibration Monitoring

Reliability Of Rotating Equipment By Vibration Monitoring

Purpose:

To give an overall idea to the Maintenance Personnel about the Predictive Maintenance Programme / Reliability & Condition Monitoring Concepts, Principles and Techniques.

Scope:

 This Manual is intended to explain in general about different types of condition monitoring techniques, analysis and diagnostics on any type of rotating equipment.

Table of Contents

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       Introduction

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-12

                                                            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="">

1500 to 2999               <2 .0="" font="" mils="" pp="">

            >3000                          <1 .0="" font="" mils="" pp="">