A Brief Introduction to Vibration Analysis of Process Plant Machinery (V)

Basic Concept V

Vibration Transducers

 

•          Transducer is a device that converts one form of energy into another.

•          Microphone  -  sound (mechanical) to electrical energy

•          Speaker - electrical to mechanical energy

•          Thermometer - thermal to electrical energy

•          Vibration is mechanical energy

•          It must be converted to electrical signal so that it can easily be measured and analyzed.

•          Commonly used Vibration Transducers

•          Noncontact Displacement Transducer

•          Seismic Velocity Transducer

•          Piezoelectric Accelerometer

•          Transducers should be selected depending on the parameter to be measured.

 Proximity Displacement Probes

•          Proximity probes measure the displacement of shaft relative to the bearing housing

•           They observe the static position and vibration of shaft

•           By mounting two probes at right angles the actual dynamic motion (orbit) of the shaft can be observed

Non Contact Displacement Probes

 (Eddy Current Proximity Probe)

•          Measures the distance (or “lift off”) of a conducting surface from the tip of the probe

•           Measures gap and nothing else.

•           Coil at probe tip is driven by oscillator at around 1.5 MHz

•           If there is no conducting surface full voltage is returned

•           Conducting surface near coil absorbs energy

•           Therefore, voltage returned is reduced

•           Proximitor output voltage is proportional to gap

 Eddy Current Proximity Probe System

Eddy Current Proximity Probe System Calibration

•          Eddy current “lift off” output is parabolic – not linear

•           Proximitor has a nonlinear amplifier to make the output linear over a certain voltage range

•           For a 24 Volt system the output is linear from 2.0 to 18.0 volts

 Proximity Probe Advantages

•          Measures shaft dynamic motion

•          Only probe that can measures shaft position – both radial and axial

•          Good signal response between DC to 90,000 CPM

•          Flat phase response throughout operating range

•          Simple calibration

•          Rugged and reliable construction

•          Suitable for installation in harsh environments

•          Available in many configurations

•          Multiple machinery applications for same transducer – vibration, position, phase, speed

 Proximity Probe DisAdvantages

•          Sensitive to measured surface material properties like conductivity, magnetism and finish

–        Scratch on shaft would be read as vibration

–        Variation in shaft hardness would be read as vibration

•          Shaft surface must be conductive

•          Low response above 90,000 CPM

•          External power source and electronics required

•          Probe must be permanently mounted. Not suitable for hand-holding

•          Machine must be designed to accept probes – difficult to install if space has not been provided

 Seismic Velocity Pick-Up IRD 544

•          Permanent magnet is attached to the case. Provides strong magnetic field around suspended coil

•           Coil of fine wire supported by low-stiffness springs

•           Voltage generated is directly proportional to velocity of vibration

•          When pick up is attached to vibrating part magnet follows motion of vibration

•           The coil, supported by low stiffness springs, remains stationary in space

•           So relative motion between coil and magnet is relative motion of vibrating part with respect to space

•           Faster the motion higher the voltage

 Velocity Pick-Up - Suspenped Magnet Type

•          Coil fixed to body, magnet floating on very soft springs

•           All velocity pick ups have low natural frequency (300 to 600 CPM)

•           Therefore, cannot measure low frequencies in the resonant range.

•           Their useful frequency range is above - 10 Hz or 600 CPM

 

Advantages of Velocity Pick-Up

•          Measures casing absolute motion

•          It is a linear self generator with a high output

–        IRD 544 pick up – 1080 mv 0-pk / in/sec= 42 mv / mm/sec

–        Bently pick up – 500 mv 0-pk / in/sec =  19.7 mv / mm/sec

•          High voltage Output

–        Can be read directly on volt meter or oscilloscope

–        Therefore, readout electronics is much simplified

–        Since no electronics needed in signal path, signal is clean and undistorted. High signal to noise ratio

•          Good frequency response from 600 to 90,000 CPM 

•          Signal can be integrated to provide displacement

Easy external mounting, no special wiring required

 Disadvantages of Velocity Pick-Up

•          Mechanically activated system. Therefore, limited in frequency response – 600 to 90,000 CPM

•          Amplitude and phase errors below 1200 CPM

•          Frequency response depends on mounting

•          Large size. Difficult to mount if space is limited

•          Potential for failure due to spring breakage.

•          Limited temperature range – usually 120^oC

–        High temperature coils available for use in gas turbines but they are expensive 

•          High cost compared to accelerometers

–        Accelerometer cost dropping velocity pick up increasing

Note - Velocity transducers have largely been replaced by accelerometers in most applications.

 

A Brief Introduction to Vibration Analysis of Process Plant Machinery (IV)

Basic Concepts IV

Basic Rotor and Stator System

•          Forces generated in the rotor are transmitted through the bearings and supports to the foundation

•           Displacement probe is mounted on the bearing housing which itself is vibrating. Shaft vibration measured by such a probe is, therefore, relative to the bearing housing

•           Bearing housing vibration measured by accelerometer or velocity probe is an absolute measurement

Type of Rotor Vibration

•          Lateral motion involves displacement from its central position or flexural deformation. Rotation is about an axis intersecting and normal to the axis of rotation

•          Axial Motion occurs parallel to the rotor’s axis of rotation

•          Torsional Motion involves rotation of rotor’s transverse sections relative to one another about its axis of rotation

•          Vibrations that occur at frequency of rotation of rotor are called synchronous vibrations.

•          Vibrations at other frequencies are nonsynchronous vibrations

 The Relationship Between Forced and Vibration

•          Forces generated within the machine have may different frequencies

•           The mobility of the bearings and supports are also frequency dependent. Mobility = Vibration / Force

•           Resultant Vibration = Force x Mobility 

 Alternative Measurements on Journal Bearings

•          Relative shaft displacement has limited frequency range but has high amplitude at low frequencies – running speed, subsynchronous and low harmonic components

•           Accelerometer has high signal at high frequencies – rotor to stator interaction frequencies – blade passing, vane passing

 Types of Machine Vibration

•          Casing Absolute is measured relative to space by Seismic transducer mounted on casing

•           Shaft relative is measured by  displacement transducer mounted on casing

•           Shaft Absolute is the sum of Casing Absolute and Shaft Relative.

Shaft Versus Housing Vibration

Shaft Versus Housing Vibration
(Selecting the Right Parameter) 

•          Shaft vibration relative to bearing housing

–        Machines with high stator to rotor weight ratio ( For example in syngas comp the ratio may exceed 20)

–        Machines with hydrodynamic sleeve bearings

–        Almost all high speed compressor trains

•          Bearing housing vibration

–        Machines with rolling element bearings have no shaft motion relative to bearing housing.

–        Rolling Element bearings have zero clearance

–        Shaft vibration is directly transmitted to bearing housing

•          Shaft absolute displacement

–        Machines with lightweight casings or soft supports that have significant casing vibration

 Bearing Housing Vibration
 

•          Shaft-relative vibration provides

–        Machinery protection

–        Low frequency (up to 120,000 CPM) information for analysis

•          Many rotor- stator interactions generate high frequency vibrations that are transferred to the bearing housing

–        Vane passing frequency in compressors

–        Blade passing frequency in turbines

–        These frequencies provide useful information on the condition and cleanliness of blades and vanes

•          These vibrations are best measured on the bearing housing using high-frequency accelerometers.

–        Periodic measurements with a data collector.

 Shaft Rotation and Precession

  

•          Precession is the locus of the centerline of the shaft around the geometric centerline

•           Normally direction of precession will be same as direction of rotation

•           During rubbing shaft may have reverse precession

 

 IRD Severity Chart

  

•          Values are for filtered readings only – not overall

•           Velocity is expressed in peak units (not RMS units)

•           Severity lines are in velocity

•           Displacement severity can be found only with reference to frequency.

•           In metric units

•           Very rough > 16 mm/sec

•           Rough                     > 8 mm/sec

•           Slightly rough > 4 mm/sec

•           Fair              - 2 – 4 mm/sec  

•           Good           - 1 – 2 mm/sec

 

A Brief Introduction to Vibration Analysis of Process Plant Machinery (III)

Basic Concepts III

Forced Vibration

•          Exciting Force = Stiffness Force + Damping Force + Inertial Force

•          Stiffness

–        Stiffness is the spring like quality of mechanical elements to deform under load

–        A certain force of Kgs produces a certain deflection of mm

–        Shaft, bearing, casing, foundation all have stiffness 

•          Viscous Damping

–        Encountered by solid bodies moving through a viscous fluid

–        Force is proportional to the velocity of the moving object

–        Consider the difference between stirring water versus stirring molasses

•          Inertial Forces

–        Inertia is the property of a body to resist acceleration

–        Mainly weight 

Physical Concept of Vibration Forces

•          Stiffness determines the deflection of a rotor by centrifugal forces of unbalance

–        Determined by the strength of the shaft

•          Damping force is proportional to velocity of the moving body and viscosity of the fluid

–        Damping is provided by lube oil

•          Inertial forces are similar to those caused by an earthquake when acceleration can be very high.

–        Acceleration is related to the weight of the rotor

–        It can cause distortion of structures

Physical Concept of Vibration Parameters

•          Displacement

–        Displacement is independent of frequency

–        Displacement is related to clearances in machine

–        If displacement exceeds available clearances, rubbing occurs.

•          Velocity

–        Velocity is proportional to frequency

–        Velocity is related to wear

–        In machines higher the velocity, higher the wear

•          Acceleration

–        Proportional to square of frequency

–        Acceleration is related to force

–        Excessive acceleration at the starting block can strain an athlete’s leg muscle

–        Acceleration is important for structural strength

Stiffness Influence

•          Stiffness is measured by the force in Kgs required to produce a deflection of one mm.

•          Stiffness of a shaft is

–        Directly proportional Diameter⁴ and Modulus of Elasticity

–        Directly proportional to Modulus of Elasticity

–        Inversely proportional to Length³

•          Typical Stiffness values in pounds / inch

–        Oil film bearings – 300,000 to 2,000,000

–        Rolling element bearings – 1,000,000 to 4,000,000

–        Bearing Housing, horizontal – 300,000 to 4,000,000

–        Bearing housing, vertical – 400,000 to 6,000,000

–        Shaft 1’ to 4” diameter – 100,000 to 4,000,000

–        Shaft 6” to 15” diameter – 400,000 to 20,000,000

Damping Influence

•          Damping dissipates energy

•          Rotor instability can be related to lack of damping

•          System Damping controls the amplitude of vibration at critical speed.

•           With low damping there is poor dissipation of energy and amplitude is high

Amplification factor Q through resonance is an indicator of damping

Relationship between Displacement, Velocity and Acceleration (For British Units)

Acceleration Varies as the Square of Frequency

 •          Acceleration is negligible at low frequencies.

•           It predominates the high frequency spectrum

•           Measure displacement at low frequency, velocity at medium frequencies and acceleration at high frequencies

A Brief Introduction to Vibration Analysis of Process Plant Machinery (II)

Basic Concept II

Concept of Phase

•         Weight “C” and “D” are in “in step”

•          These weights are vibrating in phase

•         Weight “X” is at the upper limit and “Y” is at neutral position moving to lower limit

      

•          These two weights are vibrating 90 deg “out of phase”

       

         •         Weight “A” is at upper limit and weight
                   “B” is at lower limit
         •          These weights are vibrating 180 deg
                    “out-of-phase”

 
Displacement, Velocity and Acceleration Phase Relationship

 

   •         Velocity leads displacement by 90^o; that is, it

              reaches its maximum ¼ cycle or 90^o before 

              displacement maximum

   •         Acceleration leads displacement by 180^o.

   •         Acceleration leads velocity by 90^o

   •         Small yellow circles show this relationship clearly

  

Units of Vibration Parameters

•         Displacement

–       Metric            - Micron        = 1/1000 of mm

–       English           - Mil                = 1/1000 of Inch

•         Velocity

–       Metric            - mm / sec   

–       English           - inch / sec

•         Acceleration

–       Metric                        - meter / sec²

–       English           - g  = 9.81 m/sec2 =

  

English Metric Unit Conversion

•         Displacement

                1 Mil = 25.4 Micron

•         Velocity

                1 inch/sec = 25.4 mm/sec

•         Acceleration

                Preferable to measure both in g’s because g is directly related to force

Conversion of Vibration Parameters Metric Units

•         Displacement, Velocity and acceleration are related by the frequency of motion

•         Parameters in metric units

–       D = Displacement in microns (mm/1000)

–       V = Velocity in mm/sec

–       A = Acceleration in g’s

–       F = Frequency of vibration in cycles /minute (CPM)

•         V = D x F / 19,100

•         A = V x F / 93,650

•         Therefore,  F = V / D x 19,100

Conversion of Vibration Parameters English Units

•         Displacement, Velocity and acceleration are related by the frequency of motion

•         Parameters in English units

–       D = Displacement in mils (inch / 1000)

–       V = Velocity in inch/sec

–       A = Acceleration in g’s

–       F = Frequency of vibration in cycles /minute (CPM)

•         V = D x F / 19,100 – same as for metric units

•         A = V x F / 3,690 – metric value / 25.4

Relative Amplitude of Parameters

•          V = D x F / 19,100 in metric units

–        This means that velocity in mm/sec will be equal to displacement in microns at a frequency of 19100 CPM.

–        At frequencies higher than 19,100 CPM velocity will be higher than displacement

•          A = V x F / 93,650

–        This means that acceleration in g’s will be equal to velocity in mm/sec at a frequency of 93,650 CPM.

–        At frequencies higher than 93,650 CPM acceleration will be higher than velocity

Selection of Monitoring Parameters

•          Where the frequency content is likely to be low (less than 18,000 CPM) select displacement

–        Large, low speed, pumps and motors with sleeve bearings

–        Cooling tower fans and Fin fan cooler fans. Their gear boxes would require a higher frequency range

•          For intermediate range frequencies ( say, 18,000 to 180,000 CPM) select Velocity

–        Most process plant pumps running at 1500 to 3000 RPM

–        Gear boxes of low speed pumps

•          For higher frequencies (> 180,000 CPM = 3 KHz) select acceleration.

–        Gear boxes

–        Bearing housing vibration of major compressor trains including their drivers

•          Larger machines would require monitoring more than one parameter to cover the entire frequency range of vibration components

•          For example, in large compressor and turbines

–        The relative shaft displacement is measured by permanently installed eddy current displacement probes.

–        This would cover the frequency range of running speed, low order harmonics and subharmonic components

–        To capture higher stator to rotor interactive frequencies such as vane passing, blade passing and their harmonics, it is necessary to monitor the bearing housing acceleration

•          Monitoring one parameter for trending is acceptable

•          However, for detailed analysis, it may be necessary to measure more than one parameter  

  
Example in Selecting Units of Measurement

•       Amplitude measurement units should be selected based upon the frequencies of interest

•       Following 3 plots illustrate how measurement unit affects the data displayed. Each of the plots contain 3 separate component frequencies of 60 Hz, 300 Hz and 950 Hz.

Displacement

This data was taken using displacement. Note how the lower frequency at 60 Hz is accentuated

 Velocity

The same data is now displayed using velocity. Note how the 300Hz component is more apparent

 Acceleration

The same data is now displayed using acceleration.  Note how the large lower frequency component is diminished and the higher frequency component accentuated