Evaluating Installation of Vibration Monitoring Equipment for Compressors

7/25/2019 Evaluating Installation of Vibration Monitoring Equipment for Compressors

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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcos employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Mechanical For additional information on this subject, contactFile Reference: MEX-212.06 PEDD Coordinator on 874-6556

Engineering Encyclopedia

Saudi Aramco DeskTop Standards

EVALUATING INSTALLATION OF VIBRATIONMONITORING EQUIPMENT FOR COMPRESSORS


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Engineering Encyclopedia Compressors

Evaluating Installation of VibrationMonitoring Equipment for Compressors

Saudi Aramco DeskTop Standards i

Section Page

INFORMATION............................................................................................................... 3

INTRODUCTION............................................................................................................. 3

VIBRATION MONITORING EQUIPMENT ...................................................................... 5

Vibration Monitoring.............................................................................................. 5

Basic Vibration........................................................................................... 5

Transducers for Vibration Variables......................................................... 11

Seismic Probes........................................................................................ 24

Requirements for Positive-Displacement Compressors........................... 28

Temperature Monitoring ..................................................................................... 28

Temperature-Monitoring Probes.............................................................. 28

MAJOR CONCERNS OF CONDITION MONITORING, MALFUNCTIONDIAGNOSIS, AND PREDICTIVE MAINTENANCE ....................................................... 36

Dynamic Compressors ....................................................................................... 40

Vibration................................................................................................... 40

Axial Position ........................................................................................... 41

Bearing Temperatures ............................................................................. 43

Seal Fluid Flow ........................................................................................ 44

Seal Fluid Leakage .................................................................................. 45

Balance Line Differential.......................................................................... 45

Performance ............................................................................................ 45

Oil Analysis.............................................................................................. 47

Positive-Displacement Compressors.................................................................. 48

Vibration................................................................................................... 48

Rod Drop ................................................................................................. 49

Packing .................................................................................................... 50Bearing Temperatures ............................................................................. 50

Cooling Jacket Temperature.................................................................... 50

Performance ............................................................................................ 51

GLOSSARY .................................................................................................................. 53


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Saudi Aramco DeskTop Standards ii

LIST OF FIGURES

Figure 1. Basic Relationship of Measured Parameters with a SimpleHarmonic Motion............................................................................................. 8

Figure 2. Formation of a Complex Harmonic Signal....................................................... 9

Figure 3. Views from the Time and Frequency Domain ............................................... 10

Figure 4. Range and Limitations on Machinery Vibration Analysis Systems andTransducers .................................................................................................. 13

Figure 5. Eddy Current Proximity Probe....................................................................... 16

Figure 6. Noncontact Eddy Current Probe Orientation................................................. 20

Figure 7. API 670 Axial-Position Probe Installation for a Shaft with an Integral Thrust Collar...................................................................................... 22

Figure 8. API 670 Standard Axial-Position Probe Installation Arrangement................. 23

Figure 9. Velocity Transducer ...................................................................................... 25

Figure 10. Piezoelectric Accelerometer........................................................................ 26

Figure 11 Oil Drain Line Thermocouple Installation ..................................................... 35

Figure 12. Axial Position Limits .................................................................................... 43

Figure 13. Performance Degradation ........................................................................... 46

LIST OF TABLES

Table1. Advantages, Disadvantages, and Useful Ranges of Transducer Types.......... 12

Table 2. Potential Causes of Defects ........................................................................... 39


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Saudi Aramco DeskTop Standards 3

INFORMATION

INTRODUCTION

A vibration, axial position and bearing temperature monitoringsystem consist of the following equipment:

Probes

Accelerometers

Temperature Sensors

Signal Conditioning Devices

Interconnecting Cables

Power Supplies Monitors

Communication Devices

As defined by Saudi Aramco Engineering Standard SAES-J-604, Vibration, Axial Position and Bearing TemperatureMonitoring System will be referred to as the VibrationMonitoring System.

Vibration and axial position information is acquired bytransducers and proximity probes positioned at optimal locationson a compressor. Transducers convert mechanical responsesto electric signals that are conditioned and processed byelectronic instruments.

Bearing and compressor temperature information is acquired bytemperature detectors that are positioned at the compressorbearings and/or gas flow paths.

The vibration monitoring system provides the informationnecessary to monitor compressor condition, to verify perform-ance, and to diagnose faults. Vibration monitoring systemsprovide the electrical signals to the Rotating MachineryProtection System (RMPS) and the condition monitoringsystem. The RMPS automatically sends shutdown commandsto the rotating equipment train if compressor vibration, axialposition, or monitored temperature exceeds a specified limit.

The condition monitoring system is a computer-based datacollection system that communicates directly to the vibrationmonitoring system. The condition monitoring system will alsoaccept process data from communication links to the DistributedControl System (DCS) or directly from process instruments. The


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condition monitoring system collects, stores, processes, anddisplays and prints the compressor operating data in a variety offormats.

The condition monitoring system data will typically be used forhistorical trending, machinery diagnostics and predictivemaintenance purposes but not for shutdown protection.

This module describes both the types of vibration monitoringsystem equipment for dynamic and positive displacementcompressors and the installation arrangements used at Saudi

Aramco installations.


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VIBRATION MONITORING EQUIPMENT

This section of the module describes the following processes

and the equipment that are used for condition monitoring:

Vibration Monitoring

Temperature Monitoring

Vibration Monitoring

Vibration monitoring is a monitoring method and process.Vibration monitoring measures the condition of the machinefrom the initial vibration signature after installation and then atperiodic intervals throughout the machines life. This monitoringmethod and process enables an accurate accrual or trend ofinformation by which equipment may be diagnosed before anyproblems occur.

Because vibration is the most sensitive and accurate of theindicators that are used for monitoring machinery condition,vibration sensors are typically used to prevent unscheduleddowntime and/or equipment failure. Saudi Aramco requiresautomatic vibration shutdown at pre-set vibration levels on allcritical equipment. Vibration sensors and monitoring equipment

can identify a machinery defect earlier than other types ofsensors can, and they can also be used to pinpoint the specificsource or machinery component that is defective; therefore,vibration analysis is frequently used in predictive-maintenanceprograms to provide the basic guidance for performance ofmaintenance and overhauls.

Basic Vibration

Vibration is the back-and-forth motion across a point ofequilibrium. Rotating equipment vibration is usually periodic,

i.e., it is related in some manner to the action of the rotatingelement. At times, there are non-periodic vibrations in rotatingequipment, but such vibrations are normally from externalsources. The vibration motion is described by the variables offrequency, displacement, velocity, and acceleration.

The terms and expressions that are used in this discussion ofvibration monitoring are presented in the text that follows.Vibration is defined as the oscillation of an object about its


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position of rest. When the mass of an object is set in motion, itwill move back and forth between some upper and lower limits.This movement of the mass through all of its positions and backto the point where it is ready to repeat the motion is defined as

one cycle of vibration. The time it takes to complete this cycle isthe period of vibration.

Frequency is the number of cycles in a given period. Frequencyis stated in cycles per minute (cpm) or cycles per second (cps)and is also referred to as hertz (Hz); however, frequency morefrequently is expressed in multiples of rotative speed of themachine because of the tendency of machine vibrationfrequencies to occur at direct multiples or sub-multiples of therotative speed of the machine. Frequency of vibration isexpressed in terms such as one times rpm, two times rpm, or

48% of rpm, rather than expressing all vibrations in cycles-per-minute or hertz. Frequency is one of the basic characteristicsthat is used to measure and describe vibration. The force thatcauses the vibration is the first event that occurs in time. Theresponses to these forces are the other basic characteristics ormovements, such as displacement, velocity, and acceleration.The magnitude of each of these characteristics describes theseverity of vibration.

The magnitude of severity is described by the amplitude of themovement. Amplitude of vibration on most machinery withhydrodynamic bearings is expressed in peak-to-peak mils.

Vibration probes that are mounted near bearings or oncompressor casings can sense the maximum excursion(amplitude) of the shaft or the high frequency casing vibrations.

A normal operating machine will generally have a stableamplitude reading of an acceptable low level that is less than1.0 mil (25 microns). Any change in this amplitude readingindicates a change of the machine condition. Increases ordecreases in amplitude should be considered justification forfurther investigation of the particular machine condition.

Phase, or phase angle, is another characteristic of vibration that

is important to diagnose and correct machinery problems.Phase angle is used to compare the motion of a vibrating part toa fixed reference or to compare two parts of a machine structurethat vibrate at the same frequency. Phase angle can be definedas the angular difference at a given instant between two partswith respect to a complete vibration cycle. Phase angle isusually expressed in degrees. The phase angle measurementis a means of describing the location of the rotor at a particularinstant in time. Phase angle is also valuable in determining the


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In simple harmonic motion, vibration occurs at a singlefrequency, with acceleration being proportional to displacementand occurring in a direction opposite to displacement. Simpleharmonic motion can be represented by a sine wave and can be

illustrated as the linear vertical motion of a weight that issuspended or supported on a coiled spring. The displacementof the weight below and above its point of rest and the return tothe point of rest, as a function of time, is the frequency variable.The change in the amount of displacement as a function of timeis the velocity variable. During a single cycle, this velocityconstantly changes from a value of zero at the peakdisplacement above and below the rest or equilibrium point to amaximum velocity value as the weight passes through theequilibrium point at zero displacement. The rate of change in thevelocity is the acceleration variable. The acceleration variable is

a negative value as the velocity slows down and thedisplacement approaches maximum.

The phase relationships between the variables for vibrationmeasurement (displacement, velocity, and acceleration) areshown on a simple sine wave in Figure 1.

Figure 1. Basic Relationship of Measured Parameters with aSimple Harmonic Motion


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Typical vibration signatures are not as simple as a single sinewave. Most machinery vibration consists of complex harmonicsignals. A complex harmonic signal can be described as manysine waves mixed together. Figure 2 shows a basic example of

a complex harmonic signal that consists of two pure sine waves.The upper sine wave is four times the frequency and one-fourththe amplitude of the lower sine wave. The resulting complexharmonic signal results when the two sine waves are mixedtogether.

Figure 2. Formation of a Complex Harmonic Signal

The vibration signals shown in Figures 1 and 2 are shown as

amplitude verses time, which is also known as the timedomain. Amplitude is on the vertical axis, and time is on thehorizontal axis. If a vibration transducer is connected to anoscilloscope, the oscilloscope display is in the time domain.

Another method to view vibration signals is to plot the amplitudeverses the frequency, which is called the frequency domain.Figure 3 shows the same two sine waves previously shown inFigure 2, but as a three-dimensional plot illustrating the viewsfrom the time and frequency domain.


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TIME FREQUENCY

AMPLITUDE

AMPLITUDE

AMPLITUDE

TIME

FREQUENCY

Figure 3. Views from the Time and Frequency Domain

The French mathematician, Jean Babtiste Fourier, discoveredthat all complex harmonic signals can be broken down into a

series of simple sine waves by means of the application of amathematical method. The mathematical method can be usedto break down periodic signals into discrete waves (sine waves,square waves, and triangular waves) as long as the wavesrepeat themselves. An FFT spectrum analyzer takes a complexwaveform from a vibration transducer, calculates the discretewaves that form that signal using Fouriers mathematicalmethod, and displays the individual waves in the frequencydomain. Using digital technology, the process has been madefast, leading to the term fast Fourier transformation or FFT.

Besides sine waves, which are pure tones, there are randomvibrations. Random vibrations look similar to a complexvibration signal except that the vibrations do not repeat regularlyor on a cycle. It is difficult to assign a frequency to randomvibrations. Random vibrations can occur in gas compressorswhen the moving gas encounters stationary objects in the gasstream and creates vortices and turbulence. Friction can alsocause random vibrations.


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In rotating equipment, mechanical sources, such as rotorunbalance, misalignment, critical speeds, gearing, andlooseness in parts, are only partially responsible for anyvibration. Process-type sources also contribute to vibration,

such as the high velocity of the process gases being handledand the turbulence in the gas.

Transducers forVibrationVariables

There are two general applications for vibration sensors that areused on rotating equipment. Both applications are used bySaudi Aramco.

One application is used to detect the actual vibrations of therotating shaft within a hydrodynamic radial bearing and toprovide a signal to the appropriate monitoring equipment. Saudi

Aramco uses a noncontacting proximity sensor for the detectionpart of the vibration system in this type of application.

The second application is used to detect the effects of therotating element vibrations on the static equipment casingand/or bearing housings. The seismic sensor is used in thisapplication and is directly mounted on the surface of the body tobe monitored. When anti-friction bearings are used in a

machine, the seismic sensor gives a good indication of rotormotion because anti-friction bearings have essentially zeroclearance and the dynamic force of rotor vibration is directlytransmitted to the bearing bracket through the bearings.

Vibration information is acquired through the use of transducersthat are strategically located in various positions on thecompressor or the associated equipment. The vibrationtransducers convert the mechanical motion of the equipment toan electrical signal that is sent to a monitoring/control unit.Table1 describes the advantages, the disadvantages, and the

useful ranges of the transducer types. The selection andpositioning of the proper transducers are discussed later invarious parts of this module.


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Table1. Advantages, Disadvantages, and Useful Ranges of Transducer Types

TransducerType

UsefulFrequency

Range

Measure-ment

Advantages Disadvantages

Radial ShaftVibration

Transducer

0-1 kHz Displacement SensorObserves Shaft

Directly

Senses surfaceimperfections

Conductive partsonly

Mounting difficulty

Frequency limits

Velocity pickup 1-10 kHz Velocity Self-generating

Good indicatorof machinecondition

Hand-held

Moving parts

Large size

Senses EMFs

Frequency limits

Accelerometer With accelerationoutput = 10 - 100 kHz

With velocity output =2.5 - 100 kHz

Acceleration Highfrequencies

Rugged

Small size

Hand-held

Temperature limits

Figure 4 shows the range and the limitations on machineryvibration analysis systems and transducers. The accelerationline shows that the signal strength (vibration amplitude) is low atlow frequencies. The displacement line shows thatdisplacement probes have a low signal strength at highfrequencies but that their frequency response is flat atfrequencies where signal strength is good. The velocity sensorline indicates that the signal strength is good throughout a rangeof frequencies, but that frequency response rolls off at high orlow frequencies.


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Figure 4. Range and Limitations on Machinery Vibration Analysis Systems andTransducers

Displacement Probes- Displacement is generally the bestparameter to use for very low frequency measurements (i.e.,less that 600 cpm) in which velocity and acceleration amplitudesare extremely low. Displacement is traditionally used formachinery balancing at speeds up to 10,000 or 20,000 rpm, andit should also be used where stress levels or clearances are the

important criteria. Displacement probes are available for avariety of applications and are sometimes referred to astransducers. Saudi Aramco uses noncontacting proximitysystems for displacement probes.The noncontacting proximity systems, as used by Saudi

Aramco, have the following basic applications that are related tothe proximity probe installations: radial to the rotating shaft,axial to the rotating shaft, shaft speed, and phase reference.Regardless of the application, the same types of proximity


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Oil mist or process-fluid vapors may distort the light in theprobe-to-shaft gap and cause noise and errors due to thevariations in gap transmittance.

Due to the erratic responses, the light proximity probe is onlyused as a phase reference transducer by Saudi Aramco.

The inductance proximity probe consists of a ferromagnetic coreinside a coil of wire. A high frequency alternating current issupplied to the coil, which establishes an alternating magneticfield at the tip of the probe. The proximity of a metallic surfacenear the probe tip varies the strength of the magnetic field andthereby changes the probe inductance, which modulates theamplitude of the high frequency alternating current.

It is not necessary that the rotating element under theinductance probe tip be made of a magnetic material, but theinductance probe tip must be conductive and magneticallypermeable. The probe will not sense non-conducting materials;therefore, if the conducting material has a non-conductingcoating applied to it, the probe will only respond to theunderlying metal. Any defects or eccentricity of the underlyingsurface will cause noise and erratic false readings even thoughthe actual finished shaft surface is running true. Because theprobe calibration curves are relatively non-linear and becausethey vary with different materials, the inductance proximityprobe is not satisfactory for use on Saudi Aramco rotatingequipment.

The capacitance probe is basically only one plate of a capacitor.The rotating element forms the other plate, and the air in thegap is the dielectric material. The variable capacitance of theprobe is generally placed in the feedback loop of an operationalamplifier with a high frequency ac excitation signal. Variationsin the probe-to-shaft gap size vary the capacitance of this circuitelement, and this variance in capacitance changes theexcitation signal. The readout circuitry transforms this signal toa dc voltage that is proportional to the instantaneous gap.

The capacitance system offers the greatest accuracy, linearity,and freedom from drift and temperature effects of all theproximity systems; however, the capacitance system is notapplicable for many industrial uses because the type of materialin the probe-to-shaft gap affects the output signal. Differentgases or water vapor that pass through the probe tip gap will


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change the dielectric characteristics and affect the output signalor will short-circuit the output completely. When the rotatingshaft is coated with dielectric materials, such as plasma-sprayedceramics, the probe senses only the metallic substrate.

The eddy current probe consists of a small coil, usually a flatpancake shape, at the tip of the probe. A high-frequency ac(in the frequency range for radio transmission) is applied to thiscoil from an oscillator circuit. The proximity probe sets up amagnetic field in the gap between the end of the probe and therotating shaft. In turn, the magnetic flux induces eddy current inthe portion of the shaft that is exposed to this flux. Loss ofenergy in the returning signal is detected through use of theproximitor. Relative distance or displacement is measuredbetween the probe tip and the surface by sensing the change in

the gap. The eddy current probe is useful for gaps from about10 to 70 mils, which is the approximate linear range of the eddycurrent probe. The sensitivity of most eddy current probes is200 mV/1 mil. The demodulator circuit in the proximitorconverts the amplitude-modulated ac to a varying dc signal(along a scale of 0 to -24V).

The eddy current type of noncontact proximity probe is shown inFigure 5.

PROXIMITYPROBE

SHAFT

Figure 5. Eddy Current Proximity Probe


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The eddy current system is not affected by different gases in theprobe tip gap. The output signal provides an indication (in mV)of the varying gap between the sensor and the observed shaftsurface.

The impedance of the probe to proximitor system is a criticalitem as the proximitors are tuned to a matching impedance inthe connecting wire cable. Impedance matching prevents errorsin measurement. Tuning is controlled through the use of onlycertain equivalent electrical lengths of cable that match therequired impedance. During field installation, this cable lengthmust never be cut to make an attractive installation. The excesscable should be rolled and neatly installed. If the cable length ischanged, the system will require recalibration. If the system isever replaced, it should be with a cable of the same impedance

or equivalent electrical length.

Proximity Probe Installations- The noncontact proximitysystems as used by Saudi Aramco have the following proximityprobe installation positions: radial to the rotating shaft, axial tothe rotating shaft, rotative speed, and phase relationship.

To analyze the surface of a rotating shaft, a noncontactproximity probe is usually permanently mounted in a bearinghousing. Noncontact proximity probes can also be clamped tothe bearing housing, in which case the mounted resonance of

the fitting must be taken into consideration. The probe must becalibrated for the specific shaft material, and the material mustbe electrically conductive in order to enable the proximity probeto properly set up a magnetic field and thereby sense any gaps.

The proximity probe senses shaft surface defects, such asscratches, dents, thermal growth, and variations in conductivityand permeability. The proximity probe also senses electricaland mechanical runout but has difficulty distinguishing vibrationfrom runout. Electrical runout can be described as an electricalsignal from a proximity probe due to the effect of irregular shaft

conductivity and magnetic permeability in the shaft material.Mechanical runout can be described as the measurements ofshaft surface imperfections. Shaft surface imperfections arealways present. A proximity probe cannot readily distinguishshaft runout (mechanical runout) from vibration. A slow rollmay be performed, however, to allow the electronic circuit tomemorize all of the shaft imperfections, which include therunout, and subtract the imperfections from the signal that theproximity probe reports at running speed. Slow roll is low rpm


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that occurs during the compressor startup or coastdown. Adigital vector filter (used to obtain the Bode plot) must be zeronulled so the runout will not be a factor during the slow roll.The acceptable shaft vibration limit, excluding electrical runout,

can be determined by the following equation:

Allowable shaft vibration in mils peak-to-peak =rpm

000,12

Saudi Aramco Standard 31-SAMSS-001 specifies that the totalmechanical and electrical runout in the shaft-sensing area mustbe less than 0.25 mils.

The measurement of radial vibration is accomplished by

monitoring the dc output of a displacement probe that isassociated with the radial vibration at the bearings. Undernormal operation and with no internal or external pre-loads onthe shaft, the shaft of most machine designs will ride on the oilpressure dam; however, as soon as the machine receives someexternal or internal type pre-load (steady-state force), the radialposition of the shaft in the journal bearing can be anywhere.The radial position measurement can be an excellent indicatorof bearing wear and heavy pre-load conditions, such asmisalignment.

Radial displacement should be closely monitored duringcompressor startup or coastdown. During a compressorstartup, the shaft would be expected to rise from the bottom ofthe bearing to some place toward the horizontal centerline of thebearing. This movement is fundamentally due to the oil flowingunder the shaft, which causes the shaft to rise in the bearing. Itis generally believed that the oil film is about one mil inthickness.

Because of the ability of the radial position to change undervarying conditions of machinery load and alignment, the

proximity probe transducer system must have a sufficiently longlinear range to allow for the large radial position changes. Along linear range is required in large machines in which largebearing clearances are normally present.For packaged, internally geared, centrifugal compressors, Saudi

Aramco practice recommends that whenever vendor design andshaft sizes permit, two proximity probes should be installed onthe pinion shaft at 90 degrees apart. As a minimum for eachpinion rotor shaft bearing, a single, radial, proximity-type, shaft


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vibration probe must be provided. If possible, the probes shouldbe installed in the same relative orientation to the vertical.Probe orientation may be restricted by physical equipmentconfiguration.

For gear-driven compressors, two vibration probes shouldalways be installed adjacent to the bull gear shaft bearing on thecoupling side if the bearing is a sleeve bearing. The use of twovibration probes that are mounted adjacent to the bull gear isnot required whenever roller or ball bearings are used by thevendor. If bull gear bearings are anti-friction types, proximityprobes should not be installed. A gear casing accelerometersystem that reads out acceleration and velocity values shouldonly be provided if it is found to be practical after review with themachinery vendor. The monitors for the vibration systems are

to be part of the instrument control that, preferably, will bemounted on the air compressor base plate.

For a radial vibration transducer, Saudi Aramco requires thattwo noncontact proximity probes be mounted to or in each radialhydrodynamic bearing. Unless the rotating equipmentconstruction prevents access to the bearings, there should bestrict adherence to this requirement. As shown in Figure 7, thetwo probes should be installed with as close to 90 degrees ofradial separation as is feasible. The probes must be in thesame radial plane to the shaft in order that a true representation

of the shaft movement can be monitored. Also, the probes mustbe installed in order that each probe is offset by 45 degreesfrom the top dead center of the bearing. The probes should beidentified as X and Y and not as horizontal and vertical, andthey should be oriented to rotation as shown in Figure 6. Theposition of the X and Y probes is defined by Saudi Aramcoconvention. The probes are positioned by standing outboardfacing the compressor driver; the left-hand probe is vertical andthe right-hand probe is horizontal.


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(along the x axis of the X/Y plot) from a pulse mark to the firstpositive peak in the waveform.

Axial displacement measurements are typically used to monitor

the condition of thrust bearings in rotating machinery. Axiallymounted noncontact proximity probes are used to detect theaxial movement of the rotating element during operation. Allrotating elements have some axial movement in response toexternal forces, such as forces that are imposed throughcouplings from other equipment in the train or from the couplingitself, and in response to internal forces in the rotatingequipment, such as changes in process conditions and thermalchanges. All hydrodynamic machines have sufficient axialclearance that allows relatively large gaps to be set fordisplacement alarm and trip setpoints. The typical setpoint for

alarm is 5 mils into the surface (wear) of the bearing babbitt.The typical setpoint of machine trip is 10 mils into the surface(wear) of the bearing babbitt. At least two axial thrust positionprobes should be mounted to provide axial thrust positionprotection. Under the normal operating conditions of acentrifugal compressor, thrust position can vary with the load ofthe machine; therefore, a variation in thrust positionmeasurements under differential loads and conditions of amachine are not uncommon. The thrust position measurementmay also be important in the determination of surge or incipientsurge conditions.

The axial shaft movements are normally constrained withinallowable limits by the design of the equipment. Axial shaftmovement constraints are commonly thrust bearings or thrustshoulders, both of which interact between the rotating andstationary parts of the equipment.

During normal operation, rotating equipment will have a thrustload in one direction. The direction of the thrust load dependson the direction of gas flow through the compressor, theexternal loading, and the design of the balance drum. The

design of the thrust bearing compensates for any residual axialthrust force. The rotating element must be protected fromexcessive axial movement that is caused by normal thrustbearing wear, balance drum seal wear, or thrust bearing failurethat would then permit compressor rotor wear and catastrophicfailure.Two axially mounted noncontact proximity probes are installedto sense changes that occur in the axial position of the shaft inany direction. The movement will be restricted to allowable


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values for the particular machine by alarm and shutdownfunctions.

In accordance with 34-SAMSS-625, Saudi Aramco uses axial-

position probe arrangements specified in API Standard 670.There are two probe installation arrangements: an arrangementfor a shaft that is equipped with an integral thrust collar and anarrangement for a shaft without an integral thrust collar. Figure7 shows the axial-position probe installation for a thrust bearingwith an integral thrust collar. One probe is mounted to measurethe integral thrust collar, and the other probe is mounted tomeasure the end of the shaft.

INTEGRAL THRUSTCOLLAR

PROBE VIEWINGINTEGRAL THRUST COLLAR

PROBE VIEWINGEND OF ROTOR

Figure 7. API 670 Axial-Position Probe Installation for a Shaft with an IntegralThrust Collar


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Figure 8 shows the axial position probe installation for a shaftwithout an integral thrust collar. This configuration is referred toby the API Standard 670 as the standard axial position

arrangement. Both axial position probes are mounted tomeasure the end of the shaft. Noncontact proximity probesmust never be installed to observe a non-integral thrust collar.The arrangement prevents incidental compressor shutdown oralarm in the event that a non-integral thrust collar comes looseand allows the shaft to move axially.

Figure 8. API 670 Standard Axial-Position Probe Installation Arrangement

In accordance with the requirements specified in API Standard670, the axial-position monitoring system must use dual votinglogic. In a dual voting logic system, the measurements

processed from the outputs of each transducer must equal orexceed the setpoint to activate the danger alarm or shutdown(two out of two).

Axial vibration is not normally continuously monitored oncentrifugal equipment, but it has proven valuable in diagnosingsome particular machinery malfunctioning conditions. If axialvibration is monitored or used for diagnosis of a particularmachine, the monitored surface must be relatively smooth (16


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rms finish) and perpendicular to the centerline of the rotor.Monitoring a smooth perpendicular surface will minimize anyeffect of mechanical runout on the dynamic output of the probe,which provides accurate axial vibration readings. Axial vibration

measurements can be read from the same proximity probe thatis used for axial thrust position measurements. Probe-mountinglocations must ensure minimum effect of thermal expansion ofthe rotor and minimize the effect of springiness of the thrustbearing assembly in the accuracy of the reading.

Seismic Probes

Seismic (mass-spring) transducers use the response of a mass-spring system to measure vibration. The seismic transducerconsists of a mass that is suspended from the transducer case

through the use of a spring of specific stiffness. The motion ofthe mass within the case may be damped by a viscous fluid, aspring, or an electric current. When the transducer case iscontact-mounted to the moving part, the transducer may beused to measure velocity or acceleration, depending on thefrequency range of interest.

Velocity transducers are no longer being used by SaudiAramco. Velocity measurements (usually required for allstructural vibrations with the exception of high frequency gearmesh vibrations) are obtained through the use of an

accelerometer with signal integration to velocity. This type oftransducer configuration is sometimes called a piezoelectricvelocity transducer.

Saudi Aramco establishes some recommended practices forseismic transducers, and these practices are partially based onwhether a machine is horizontal or vertical. All vibrationreadings should be taken as close as possible to the top

bearing, perpendicular to the shaft, in four positions, 45to eachother, with one seismic transducer in line with the processpiping connected to the compressor. The acceptable reading

level is 0.18 inches/sec peak rms.

The alarm level is set at one and a half times the acceptancelevel. The shutdown level is set at two times the acceptancelevel.


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measurement (referenced to inertial space). For this reason, avelocity transducer is also called a seismic transducer.

The velocity transducer has an internal natural frequency

(referred to as mounted resonance) of about 8 Hz (those sizesthat are used for machine monitoring). This natural frequency issimply the resonance of the single degree of freedom of theinternal mass suspended on springs. The response atresonance is highly damped because of the internal fluid. Thistransducer produces a linear output only above this resonantfrequency.

Accelerometers- The most common acceleration transducer isthe piezoelectric accelerometer, as shown in Figure 10. Thepiezoelectric accelerometer consists of piezoelectric disks that

are made of a quartz crystal (or an industrial ceramic calledbarium titanate) with a mass bolted on top and a spring thatcompresses the quartz. A piezoelectric material generates anelectric charge (voltage) output when the material iscompressed.

OUTPUT

TERMINALS

PIEZOELECTRIC

DISKS (QUARTZ)MASS

BASE

Figure 10. Piezoelectric Accelerometer


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In operation, the accelerometer base is contact-mounted to thevibrating object, and the mass wants to stay stationary in space.With stationary mass and the base moving with the vibration,

the piezoelectric disks get compressed and relaxed. In the mosttypically used compression-type models, the seismic mass andthe base alternately exert compression in the piezoelectricdiscs. The piezoelectric disks generate a charge (voltage)output going positive and negative as the disks are alternatelycompressed tighter and relaxed. The charge output follows themotion of the surface in the direction of the accelerometerssensitive axis. The immediate millivolt output of this transduceris proportional to the acceleration of the vibrating subject; if theacceleration level is high, the force transmitted from the shaft toits supporting radial bearing is high. This force is the cause of

excessive wear and premature failure in a radial bearing.

The measurement processed from an accelerometers outputsignal is seismic (absolute motion relative to inertial space).Unlike the velocity pickup, it is practically unaffected by externalelectrical or magnetic fields. Accelerometers are as sensitive toground loops as are other pickups. Ground loops can be easilyeliminated by providing ground isolating washers at theaccelerometer base.

As specified in API Standard 670, the accelerometer channel

accuracy for measuring casing vibration must be within 5percent of 100 millivolts per g (mV/g) over a minimum range of0.1 g to 75 g, peak, and over the frequency range of 10 Hz to 10kHz. The electrical impedance of the cable linking theaccelerometer to the signal conditioner and to the channel plug-in module is matched to the electrical impedance of theaccelerometer case to avoid problems from electronic noise andto minimize error in measurement.

The accelerometer has a very high mounted resonance,typically 25,000 kHz, because the accelerometer has no moving

parts. The response is linear for the first third of theaccelerometers range and it is used below its mountedresonance. The range is 5 to about 10,000 kHz, depending onits size. Small accelerometers have low sensitivities but higheroperating frequencies. Some small accelerometers are usefulabove 50,000 kHz. Large accelerometers have highsensitivities but lower high-frequency limits (800 to 1000 kHz).


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Requirements forPositive-Displacement

Compressors

Because of design, reciprocating compressors are subject tovibration. Reciprocating masses, reversing loads, and pulsatinggas streams all contribute to a normal vibration level on thecompressor; however, if the normal vibration level is exceeded,it indicates that something abnormal is happening, and thesituation should be investigated. Some reciprocatingcompressors use a spring or magnet-type vibration switch thatis mounted to the frame. API 618, Reciprocating Compressorsfor General Refinery Services, states that the use of ball-and-

seat or magnetic-type vibration switches are unacceptable;therefore, Saudi Aramco standard 31-SAMSS-003 requires thatreciprocating compressors for air or process gas service musthave one piezo-velocity transducer located horizontally on thecrank case perpendicular to the crank axis.

Temperature Monitoring

Temperature monitoring systems are primarily used to monitor

bearing conditions on compressors and compressor drivers.These temperature detectors are sometimes used as protectiveinstrumentation to actuate a compressor shutdown and toprevent compressor damage. Temperature monitoring systemsare also used to predict failure or wear of compressorcomponents, such as interstage valves and piston rings onreciprocating compressors.

Temperature-Monitoring Probes

A Resistance Temperature Detector (RTD) is a device thatsenses temperature through a measurement of the change inresistance of a material. All metals produce a positive changein resistance for a positive change in temperature. RTDs areavailable in many forms; however, they usually appear insheathed form. An RTD probe is an assembly that consists of aresistance deterrent, a sheath, a lead wire, and a terminationconnection. The sheath, which is a closed-end probe thatimmobilizes the element, protects the element against moisture


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and the measured environment. The sheath also providesprotection and stability to the transition lead wires from thefragile element wires. Some RTD probes can be combined withthermowells for additional protection. In this type of application,

the thermowell will also isolate the system gas from the RTD.

When the nominal value of the RTD resistance is large, systemerror is minimized. To obtain a high RTD resistance, a metalwire with high electrical resistance must be chosen. Platinumhas the highest resistivity of the selected metals that arecommonly used for RTD construction.

RTDs can be constructed of several different types of metal.Tungsten has a relatively high resistivity, but it is reserved forvery high temperature applications because it is extremely

brittle. Tungsten would also suffer in an oxidizing environmentbecause of the high reaction rates. Copper is occasionally usedas an RTD element. Coppers low resistivity forces the elementto be longer than a platinum element, but its linearity and verylow cost make it an economical alternative. Copper RTDs have

an upper temperature limit of 120C (248F).

The most common RTDs are made of either platinum, nickel, ornickel alloys. The economical nickel derivative wires are usedover a limited temperature range. Nickel wire output is non-linear and tends to drift with time. For the best measurement

integrity, platinum is the metal of choice. Platinum is used asthe primary element in all high-accuracy resistancethermometers. Platinum is especially suited for a wide range ofdegrees as it can withstand high temperatures while maintainingexcellent stability. Platinum shows limited susceptibility tocontamination, which can affect temperature readings. Saudi

Aramco practice recommends platinum RTDs, three-wire, and

calibrated to 100 ohm at 0C (32F).

Although the RTD is an accurate temperature measurementdevice, some errors may develop. The RTD is a passive

resistance element, and a current must be applied to the RTD todevelop an output signal. This current generates heat, whichbecomes objectionable when it is sufficient to significantlychange the temperature to be measured. This self-heatingeffect causes minor errors. To prevent self-heating, a limitedamount of power is used to produce the output signal tominimize the error.


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Another error that may affect the accuracy of the temperaturemeasurement may be caused by the lead wire. The copperlead wire for connection of the RTD to the transducer, although

a satisfactory trade-off between cost and resistance, representsa resistance in series with the RTD, and thus is a source ofinaccuracy. For long transmission distances, ambienttemperature effects can cause appreciable errors. These errorscan be compensated for by using three-or four-terminal RTDdesigns.

Lack of standardization among manufacturers concerning therelationships between resistance and temperature may causean accuracy problem. Errors can occur when RTDs of severalmanufacturers are used in a single system, or when the element

of one manufacturer is replaced with the element of anothermanufacturer. These errors can be avoided by not mixing RTDswith different temperature versus resistance curves.

Inaccuracy of an RTD may also result from slow dynamicresponse. Slow response may be caused by the RTDconstruction - the RTD sensing element consists of anencapsulated wire that is cut to a length that provides a

predetermined resistance at 0C. The temperature-sensitiveportion of the probe, which depends on the length of the sensingelement, is from 0.5 to 2.5 in. The RTD is thus considered to be

an area sensitive device, and it has a significantly slowerdynamic response than point sensitive devices likethermocouples. Because RTDs are sheathed or may beinstalled in thermowells, the sheathing or thermowell representa much larger contribution to the slowing of the dynamicresponse; therefore, the slow dynamic response of the RTD isof little significance.

Thermocouples are another reliable source for temperaturemeasurement. The thermocouple (T/C) consists of twodissimilar metal or alloy wires that are joined together at one

end, the so-called measuring (or hot) junction. The free endsof the two wires are connected to the measuring instrument toform a closed path in which current can flow. The point at whichthe T/C wires connect to the measuring instrument isdesignated as the reference (or cold) junction.Thermocouples function very differently from RTDs butgenerally appear in the same configuration. Thermocouples areusually sheathed and are possibly used in conjunction with athermowell. Thermocouple-type instruments have a range of -


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280 to +2750C (-440 to +5000F) and an accuracy of 0.1C

(0.2F)

Application of heat to the measuring junction causes a small

electromotive force (EMF or voltage) to be generated at thereference junction. When a readout device is employed, itconverts the EMF that is produced by the temperaturedifference between the measuring and the reference junctionsto display the temperature of the measuring junction. When the

reference temperature is known (usually 0C), and when themeasuring junction is exposed to an unknown temperature, theEMF that is developed will vary directly with changes in theunknown temperature.

The noble metal T/C, Types B, R, and S, are all platinum or

platinum-rhodium T/C and share many of the samecharacteristics. Platinum wire T/C should only be used inside anon-metallic sheath, such as high-purity alumina, due to metallicvapor diffusion at high temperatures that can readily change theplatinum wire calibration. The only other acceptable sheathwould be one made from platinum, which would be ratherexpensive.

The platinum-based T/C is the most stable of all the commonT/C. Type S is so stable that it is specified as the standard fortemperature calibration. Type R is similar to type S; the onlydifference is that the rhodium makes up 10% instead of 13% ofthe wire.

The Type B T/C is the only common thermocouple that exhibitsa double-valued ambiguity. Due to the double-valued curve,

Type B is not used below 50C (122F). Because the output is

nearly zero from 0C (32F) to 42C (107.6F), Type B has theunique advantage that the reference junction temperature is

almost immaterial, as long as it is between 0C (32F) and 40C

(104F). However, the measuring junction temperature can be

in excess of 1700C (3092F).

The Type E T/C element is made from nickel-chromium metal.Saudi Aramco practice recommends chromel-constantanthermocouples manufactured in accordance with ANSI MC 96.1.Type E is ideally suited for low temperature measurementsbecause of their low thermal conductivity and high corrosionresistance. The Type E is useful for detecting smalltemperature changes.

Saudi Aramco practice recommends ISA Type J or Type E,


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unless an existing monitoring system requires a different type.Because of the larger Type E temperature range and higherEMF output, the Type E chromel (nickel-chrome/constantan vs.copper-nickel) thermocouple is specified for thrust and journal

bearing temperature sensors instead of the Type J. The Type Ealso has better resistance to corrosion caused by H2S.

The Type K T/C is similar to the Type E with the exception thatthe one element is made from nickel instead of constantan.

Iron is used as an element in a Type J T/C. Iron is aninexpensive metal and is rarely manufactured in pure form,which contributes to the poor accuracy and responsecharacteristics. Although the impurities in the iron are high, theType J T/C is popular because of its low price. The Type J T/Chas a more restrictive temperature limitation than most T/C. At

760C (1400F), an abrupt magnetic transformation occurs thatcan cause decalibration even when the T/C is returned to lowertemperatures.

Typically, the measuring instrument is located away from thepoint at which the temperature is measured, therefore, a wiringextension is needed. Because the temperature sensing resistorfor maintaining a constant reference junction EMF can be mostconveniently located in the temperature reading instrument as apart of its circuit, it is necessary to locate the reference junctionitself in the temperature reading instrument. Therefore, the

thermoelectric circuit must be extended from the measuringjunction, at the point where the temperature measurement isdesired, to the reference junction in the instrument. This is donethrough the use of extension wires.

Extension wires theoretically extend the T/C to the referencejunction in the instrument. This wire is generally furnished in theform of a matched pair of conductors. The simplest procedureis to use the same types of wire that the T/C itself is made of.However, in installations with noble-metal T/C where severalhundred feet of extension wire must be used, or where

numerous T/C are employed, such a procedure may becometoo expensive. In such cases alternative, lower-cost materialswith similar characteristics at lower temperatures are available.

Thermocouples, much like RTD, suffer from errors in theirmeasurement. Static electrical noise may be introduced intoT/C circuits by adjacent wires carrying ac power or rapidlyvarying (pulsating) dc. These noises can be minimized oravoided by shielding each pair of extension wires and grounding


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In addition, shutdown concerns are as follows: Low-lowcompressor suction temperature shutdown inputs arerecommended for cases in which process upsets may cause

operating temperatures to drop below design limitations of acompressor or cause undesirable phase changes of processgases. High-high temperature shutdown inputs and alarms arerecommended for compressor lube-oil temperature; compressordischarge temperature, or compressor bearing temperatureswhere high temperature excursions or process upsets maycause a compressor malfunction, internal damage or unsafeoperating conditions.

Embedded- An embedded temperature monitoring probe istypically an RTD or a thermocouple. Saudi Aramco does not

permit the use of spring-loaded, bayonet-type, temperaturesensors that contact the outer shell of the bearing metal.Experience has shown that a consistently good contact forreliable and accurate readings is not obtained. In addition,through-drilling and puddling of the babbitt is not permitted. Thethermocouple is inserted through a drilled hole in the bearingretainer, and its tip is made to firmly contact the backing metalbut not in contact with the babbitt. This installation methodprovides the most reliable results and can detect a temperaturechange more quickly than if the thermocouples were measuringthe temperature of the oil stream. Measurement of the backing

metal could be significant in the case of a sudden rapid rise inbearing temperature, which might lead to severe bearing orcompressor damage before the compressor could be shutdown.

Saudi Aramco practices require the installation arrangement ofradial bearing temperature sensors for sleeve-type and tilting-pad type journal bearings, and the installation arrangement ofaxial bearing temperature sensors, are to be installed inaccordance with API 670 unless otherwise stated in theequipment specifications. Bearing, casing, and oil throw-off

temperature monitoring equipment must comply with SAES-J-400. Saudi Aramco practice recommends that bearingtemperature sensors be installed to facilitate replacement duringcompressor operation.

When embedded elements are used for bearing temperaturemeasurement, extra elements must be installed in the bearingoil throw-off lines. API Standard 670 requires that twotemperature sensors be mounted on both the active and


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inactive sides of a hydrodynamic thrust bearing. The sensorsare to be located in the lower half of the bearing at 120 degreesapart. Radial bearing temperature sensors, which arereplaceable and embedded in the shoe, are to be in accordance

with FORM ISS 8020-415-ENG and FORM ISS 8020-416-ENG.

Oil Drain - Oil drain probes consist mainly of thermocouple-typetemperature detectors that are installed in the oil drain line, asshown in Figure 11. The thermocouple is installed in athermowell with the tip of the thermocouple in contact with thebottom of the thermowell. Oil drain temperature is monitored toidentify potential operational problems that may cause failure ofa bearing. When used with an embedded temperature sensor,

the temperature of the bearing metal is not to exceed 220F

(alarm level) with an oil inlet temperature of 140F, while the oil

drain return temperature should not exceed 180F. Thecompressor bearing shutdown temperature is 240F.

Figure 11 Oil Drain Line Thermocouple Installation


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MAJOR CONCERNS OF CONDITION MONITORING, MALFUNCTIONDIAGNOSIS, AND PREDICTIVE MAINTENANCE

Data collected from a condition monitoring system are typicallyused for historical trending, machinery diagnostics, andpredictive maintenance purposes. Historical trending,machinery diagnostics, and predictive maintenance are termsthat describe the process whereby some parameter ismeasured in a non-intrusive manner and trended over time.The measured parameter(s) have a direct relationship to theperformance and/or health of the equipment, or at least someaspect of the equipment health.

Trending and predictive maintenance are two important aspects

of a viable maintenance program. Trending is defined as amethod of establishing a baseline and monitoring parameterchange over time. Predictive maintenance is defined as anapproach to and a methodology for maintenance.

Trending establishes a baseline of the initial operationalparameters of a piece of equipment and monitors change fromthat value. A baseline can be a graphical presentation or simplya compilation of data. This baseline is then compared to datathat are taken during the service life of a piece of equipment toidentify changes in the operational efficiency and health of the

equipment. The changes can then be used to determine thecause of the differences or the future implications. Theoperational parameters measured for trending analysis may besophisticated (such as vibration analysis) or simple (such as thepressure drop across an oil filter).

The predictive approach to maintenance is, as mentioned,based on the material condition of the compressor and on theprediction of time to failure. Scheduling of maintenance basedon prediction of equipment failure can be less expensive thanthe preventive or run-to-failure approaches. As a methodology

for maintenance, predictive maintenance uses projected data ortrends from condition monitoring techniques to determine thetrouble free service life of equipment. These techniques monitordeterioration of processing conditions and specific events thatprecede the development of equipment faults or failures.


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Assessments of mechanical condition based on trending andpredictive maintenance will allow a facility to maintainequipment, which results in lower maintenance costs. Lowermaintenance costs are realized through limitation of

unnecessary replacement of equipment or equipmentcomponents. The accumulation of pertinent data that are usedto develop trend analysis will assist in the identification ofproblems in the operation of the equipment, which results infewer unexpected equipment failures.

The more typical vibration problems that are identified bymalfunction diagnosis and the percentage of time that they areencountered are as follows: imbalance 40%, misalignment30%, and resonance at critical speeds 20%. These three itemscollectively account for 90% of all vibration-related problems.

Bearings account for most of the remaining 10%. These causescomprise the majority of the causes of excessive vibrations.

Mass imbalance is a condition in which, due to an unbalancemass, the rotation center of gravity is not coincident with theshafts geometric center. In malfunction diagnosis, unbalance is

suspected when vibration frequency is 1rotational speed, withan amplitude of displacement proportional to the amount ofimbalance.

Perfect balance is a zero quantity and cannot be measured.Balance measures the centrifugal force on the rotor due to a

heavy spot. The heavy spot is an imaginary area that is appliedto a rigid rotor. Several factors contribute to the development ofthe heavy spot: voids, mass nuances, and other defects. Thisimbalance can be remedied through isolation of the spot and

through counterbalance of the weight with an equal weight 180opposite.

Coupling misalignments are frequently diagnosed as the causeof vibration. Coupling misalignment is the condition in which theshafts of the driver and the compressor are not on the samecenterline. Misalignment is usually referred to as being parallel

or offset. In parallel misalignment, the axes of the shafts of thedriver and of the compressor are parallel but not on the sameaxis. In offset misalignment, the axes of the driver and of thecompressor are at an angle to each other. Although flexiblecouplings are employed to remedy misalignment, someinstances arise in which the alignment is so far from perfect thatit causes excessive vibrations.

Malfunction diagnosis would recognize misalignment vibration in


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the frequency domain as a series of harmonics of the runningspeed. For instance, if the rotational speed was 1800 rpm(rotational frequency of 30 Hz), then the maximum amplitude ofacceleration would occur at rotational amplitudes such as 30,

60, 90 Hz, etc. Offset misalignment is frequently diagnosed at avibration frequency of 1the rotational amplitude and parallelmisalignment is frequently diagnosed at a vibration frequency of

2the rotational amplitude. The harmonics occur because ofthe strain that is induced in the shaft. The harmonics are thedisplaced portion of the distorted sine wave that did not reach itsfull excursion in amplitude. The misplaced energy from the sinewave will be displaced to another frequency level, typicallyhigher, and will most likely be indicated on an accelerometer.

Resonance is a condition in which the frequency of driving force

that is applied to a structural part or a rotating part is close tothe parts natural frequency of vibration. Amplitudes of resonantvibration are amplified. The source of the driving force is mostlikely residual imbalance in a rotor system or broadbandturbulence that is due to fluid motion. As a rotor turns, thecentrifugal force of the unbalance can be transmitted from therotor to the structure of the machine as a vibratory force. If thisforce encounters a structural part that is tuned to the rotationalfrequency by virtue of its mass and stiffness, then that part willbe excited into resonance. When the ratio of input forcefrequency over the natural frequency is equal to one, resonance

will occur.

In rotors, the speed at which resonant vibration occurs is calledcritical speeds. In accordance with the formula for centrifugalforce, the vibratory force should increase as the square of thespeed, which is true in the low-speed range. When approachingcritical speed, the vibration increases much more than expectedby the centrifugal force formula. The vibration peaks at thecritical speed and then smoothes out. Rotors run smootherabove the first critical speed than below it.

Table 2 provides a quick overview of mechanical defects thatcause problematic vibration and the typical symptoms of thesedefects.


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Table 2. Potential Causes of Defects

Defect Frequency of MainVibration Hz =

rpm/60

Direction Remarks

Unbalance 1 x RPM Radial A common cause.

Misalignmentor Bent Shaft

Usually 1x rpm

Often 2 x rpm

Sometimes 3 and 4 rpm

Radial andAxial

A common cause.

Imbalance 1Shaft critical speed Primarilyradial

Vibrations excited when passing throughcritical shaft speed are maintained at higher

shaft speeds.

Damaged

Bearings

Impact rate - Vibration at

high frequency ( 2 to 60kHz

Radial and

Axial

Uneven vibration levels.

LooseJournalBearing

Subharmonics of shaftrpm, exactly 1/2 or 1/3

rpm

PrimarilyRadial

A loose journal bearing may only develop atoperating speed and temperature.

Oil Whirl Slightly less than halfspeed, 48 to 53%

PrimarilyRadial

Applicable to high speed machines usingsleeve-type bearings.

Damaged orWorn Gears

Tooth-meshingfrequencies (shaft rpm x

number of teeth) andharmonics

Radial andAxial

Side-bands around tooth-meshingfrequencies indicate modulation at frequency

corresponding to side-band spacings.Normally only detectable with very narrow-

band analysis and cepstrum analysis.

Mechanicallooseness

2 x rpm Also sub- and inter-harmonics, as for loosejournal bearings.

Faulty beltdrive

1 - 4 x rpm of belt Radial The precise problem can usually be visuallyidentified through use of a stroboscope.

Unbalancereciprocatingforces and

couples

1x rpm and/or multiplesfor higher-order

unbalance

Primarilyradial

IncreasedTurbulence

Blade and Vane passingfrequencies and

harmonics

Radial andAxial

An increase in frequency indicates increasedgas turbulence.

Electricallyinduced

vibrations

1 x rpm or 1 or 2 timessynchronous frequency

Radial andAxial

Should disappear when power turned off.

Gasdisturbances

(stall)

.3 to .4 times thesynchronous frequency

Radial Frequency caused by diffuser stall.


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The remainder of this section will discuss the major concerns ofcondition monitoring, malfunction diagnosis, and predictivemaintenance as they apply to dynamic compressors andpositive displacement compressors.

Dynamic Compressors

Condition monitoring, malfunction diagnosis, and predictivemaintenance for dynamic compressors are concerned withvibration, axial position, bearing temperatures, seal fluid flow,seal fluid leakage, balance line pressure differential,performance, and oil analysis. These factors can be theindicators of a problem or a potential failure during dynamic

compressor operation.

Vibration

The identification of abnormal operating characteristics is mostreadily apparent through the use of vibration analysis. Althoughcentrifugal compressors run smoother than reciprocatingcompressors, they operate at much higher speeds.Consequently, if a problem does arise that causes abnormalpatterns of vibration to occur, the effects can lead to a fastercatastrophic failure.

Centrifugal compressors run considerably smoother thanreciprocating compressors due to the extra time that is taken tobalance the units in preparation for the higher operating speedsand due to the absence of pulsation forces present inreciprocating compressors. Centrifugal compressors must beextensively monitored due to the higher speeds and thecatastrophic compressor damage that may occur in event offailure. Because centrifugal compressors are typically regardedas critical machines, the expense incurred by incorporatingthem into a condition monitoring system can be easilyrationalized.

Centrifugal compressors frequently operate above their firstcritical speed and sometimes between the second and thirdcriticals. At these speeds, the rotors can display flexible modesof deflection. The deflections that occur at these speeds couldcause rotor components to come in contact with stationarycomponents, resulting in compressor damage. The bearingsand their lubrication system are important at these higherspeeds to provide damping and to limit radial vibration. Some


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vibration problems on compressors can be corrected through achange in the oil viscosity. Abrupt loss of damping in fluid filmbearings has led to catastrophic failures.

In centrifugal compressors that use hydrodynamic bearings,radial noncontact proximity probes are used to measure radialshaft vibration. However, if analysis of vibration within the highrange of vibration frequencies is desired, the frequencylimitations of the proximity probe about 2000 Hz will bedetrimental. In such cases, an accelerometer (with an outputsignal proportional to acceleration) can be used to measurevibration of the bearing housing.

Centrifugal and axial compressors can experience two causesof vibration that are unique: surging and choking. Surge occurs

when the demand that is required by the process exceeds themaximum energy (head) that the dynamic compressor canproduce. The energy that is required by the process is directlyproportional to the system resistance and is inverselyproportional to gas density. Gas then flows in the reversedirection through the compressor, which creates a turbulentcondition that results in vibration characteristics that are non-periodic and broadband. If surging is allowed to continue, it willcause extensive damage.

Choking is the opposite of surging and occurs when the energythat is required by the process is low. When the process energy

is low, the gas velocity through the compressor increases.When the velocity in the diffuser section approaches Mach 1, aturbulent or circulating flow between the compressor blades willoccur, which has the effect of blocking the flow of gas. Thevibration level increases because of the turbulent gas flowconditions in the compressor. The vibration is also non-periodicand broadband, just like surging. A check of the dischargepressure will determine whether the problem is surging orchoking. The solution to this problem is to reduce the gas flowthrough the compressor.

Axial Position

Axial position measurement indicates the amount of axial thrustthat is experienced by the compressor rotor. The primarypurposes of a monitoring system channel for axial position arethe following:

To ensure against axial movement of the rotor that cancause the rotating elements to make contact with thestationary components and cause extensive damage.


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To ensure against degradation of the axial thrust bearingthat would lead to bearing failures. Such failures can becatastrophic.

Thrust bearings are required to be loaded in the active thrustdirection with a minimum thrust bearing loading of 75 psi. Theactive thrust direction is the direction of a rotor axial thrust loadwhen the compressor is operating under normal conditions.Zero thrust position is the center of the cold float zone of thecompressors rotor. Figure 14 shows an example of a cold floatzone, of displacement values for tolerable thrust movement outof the cold float zone (in either the active or inactive direction),and of alarm (alert) and shutdown (danger) limits for axialposition.

The limits that are shown on Figure 12 are an example forconventional tilting-pad thrust bearings that are found in mostturbo machinery and in which babbitt thickness is in the range of25 to 40 mils. These values do not apply to most tapered-landor thin-babbitt bearings, such as those bearings that are foundin gearboxes. Bearings in this class must be considered on acase-by-case basis in determining rational thrust limits.Equipment manufacturers limits may be considered but, inmany cases, will be too conservative and lead to difficulty insetting up the monitoring system.


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Figure 12. Axial Position Limits

BearingTemperatures

When the bearing temperature of a sleeve bearing exceeds themelting point of its lowest-melting-point metal, which is usually

lead or tin, large areas of the bearing lining will be cleanlyremoved from the steel back. An increase in bearing padtemperature is a clear indication of an increase in bearing load.Increased bearing load will increase the lubricant film friction,which results in an increase in lubricant and bearing padtemperature. A scenario such as this demonstrates theimportance of bearing temperature monitoring.

Although the melting temperature of babbitt material is


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approximately 300F, and although the typical operating oil

temperature is 120 to 140F, some areas of the bearing surfacetemperature may vary greatly. The bearing metal temperature

must not exceed 220F (alarm setpoint) at the loaded area

based on an oil inlet temperature of 140F (31-SAMSS-001).

In accordance with 31-SAMSS-001, journal bearings must beprovided with two temperature sensors or one duplex-typesensor that is mounted at the loaded area. Replaceableembedded sensors that can be changed without changing thebearing are required. All wiring must terminate in a location thatis accessible without machine dismantling. Tilting pad-typethrust bearings must have two temperature sensors or oneduplex type sensor in each of the active and inactive sides.Bearing temperature monitoring must comply with 34-SAMSS-625.

Seal Fluid Flow

In the case of oil film seals, seal oil is injected between twocollars or bushings (sleeves) that run at close clearance to theshaft. For oil film type seals, seal oil is supplied as a controlleddifferential pressure of 5 psid above the internal gas pressure.The seal oil flow towards the gas stream of the system throughthe gas-side bushing. The oil and system gas mix anddischarge through the contaminated oil drain line to be

reclaimed or discarded. An oil film seal bushing will have a sealleakage of 10 to 20 gallons per day; larger compressors willhave even more flow. The oil that does not flow across the gas-side bushing returns to the oil reservoir for reuse.

In addition, the oil provides the cooling and lubrication. The sealoil flow toward the gas stream is a direct function of thepressure difference between oil and gas. The static pressurefrom an overhead tank, combined with the system gas streampressure, provide the necessary flow pressure.

Mechanical seal leakage is approximately 5 to 10 gallons perday flow at an oil-to-gas differential pressure of 35 to 50 psid.


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Seal FluidLeakage

Seal oil leakage can result in significant operating costs during

the life of the equipment. Such costs can result from both thereplenishment and the disposal of discarded oil. An extremecase is where compressors are located offshore, andcontaminated oil is shipped onshore for disposal.

Seal oil leakage rates should be thoroughly evaluated beforeorder placement and then enforced on the test stand prior toequipment acceptance. Stated shop test leakage rates mayvary slightly from guaranteed field leakage rates due to differentseal clearance and differential pressure on the test stand. Theshop test and field leakage rates may differ due to theadjustment of seal clearances for the shop test conditions. Oil

replacement costs and the cost of disposing of contaminated oilcan be significant on an offshore platform. The life of that oilcan be extended through use of a reclamation system. Thetype of system will depend upon the volume of oil and thecontaminants that are present.

Balance LineDifferential

Because a labyrinth seal is utilized, a certain amount of leakageoccurs across the balance piston. This leakage is normallyrouted back to the compressor suction, which creates a

differential pressure across the balance piston. Because thebalance piston seal must seal the full compressor pressure rise,integrity of the seal is crucial to efficient performance and thrustbearing load. A damaged seal results in higher leakage rates,higher horsepower consumption, and greater thrust loads. Abalance line differential pressure increase is an indication thatthe balance drum labyrinth clearance has increased.

Performance

Compressor performance degradation is expected over time.However, degradation beyond the expected amountunnecessarily reduces efficiency and performance. Identification

of these abnormal operations requires accurate trend analysis.Through the use of a properly established baseline and the plotof percentage change of a parameter, the initial and currentperformance curves can be compared.

The initial curve may be predicted by the manufacturer, but thecurve should be adjusted in accordance with established fielddata. Inlet conditions, discharge conditions, gas analysis, inletvolume flow, and speed must be taken into consideration and


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compensated for when identifying the magnitude of theperformance curve shift. The actual operating range willdetermine the urgency of any maintenance shutdown.

The performance curve generally shifts downward and towardreduced flow because of less head capability, as shown inFigure 13, due to polymer buildup, dirt, corrosion, and increasedinternal recirculation from seal wear. Additionally, the processsystem may be fouling, which causes a greater restriction for agiven flow, and results in more required head to overcome thesystem resistance.

The efficiency is reduced due to the increased frictional lossesand/or increased internal recirculation, which shifts the curvesdown. This increased resistance also effectively reduces thecapacity of the compressor, which shifts the curves to the left.

The shape of the curve will also change.

Figure 13. Performance Degradation


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Oil Analysis

Oil analysis is performed in acceptance with Saudi AramcoLubrication Manual. The actual conduct of oil analysis is not

performed by Saudi Aramco Engineers. The following sectionbriefly describes basic analysis that can be performed withoutanalytical chemistry equipment.

Several tests are available that help to identify potential trendsor imminent problems. Oil samples should be taken on aroutine basis as specified by a preventive/predictivemaintenance schedule. Daily sampling should be performed forvisual inspection of the oil. Oil sampling should also beperformed

Evaluating Installation of Vibration Monitoring Equipment for Compressors

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