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

Basic Concept VI

Piezoelectric Accelerometers

 

•          Piezoelectric crystal is sandwiched between a seismic mass and outer case.

•          Preload screw ensures full contact between crystal & mass

•           When mounted on a vibrating surface seismic mass imposes a force equal to mass x acceleration

•           Charge output of piezo crystal is proportional to applied force

•           Since mass is constant, output charge is proportional to acceleration

Piezoelectric Accelerometers

Converting Charge to Voltage

•          The output of accelerometers is charge. Usually expressed as picocoulomb / g (pc/g)

•          Electronic charge amplifier is required to convert charge signal to voltage signal

–        Impedance of accelerometer is high. Cannot be connected directly to low impedance instruments

–        Charge amplifier has high input impedance and low output impedance so that long cables can be used.

•          Charge amplifier can be external or internal

–        In bigger accelerometers amplifier can be located inside

–        In small, high frequency units amplifier is located outside

–        Also located outside in high temperature accelerometers

Accelerometers Mounting

•          Mounted resonance of accelerometer drops with reduction in mounting stiffness.

–        This causes a reduction in the upper frequency range

•          Ideal mounting is by threaded stud on flat surface

–        Maximum stiffness, highest mounted resonance

–        Resonant frequency 32 KHz. Usable range 10 KHz.

•          Magnet mounting simpler but lower response

–        Resonant frequency drops to 7 Khz. Usable range 2 KHz

•          Handheld probe convenient but very low frequency response

–        Due to low stiffness of hand resonant frequency < 2 KHz

–        Frequency response < 1 KHz

 Accelerometers Resonance & Frequency Response

•          Frequency response depends on resonance frequency

•           Higher the resonance frequency, higher the useful range

•           Maximum useable frequency range is 1/3^rd of resonance

•           Resonance frequency, however, depends on mounting

Frequency Response - Screw Mount

•          Screw mount has the highest resonance and, therefore the highest frequency response

•           This film of silicon grease improves contact.

•           Make sure bottom of accelerometer contacts measured surface

 Frequency Response - Magnet Mount

•          Weight of magnet determines the mounted resonance 

•           Smaller the magnet higher the frequency response

 Use the smallest magnet that holds the accelerometer without slipping. Use a machined surface for the best grip

Frequency Response Hand Held

•          Poor high frequency response - < 1 KHz

•           Response may change with hand pressure

•           Repeatability is poor when high frequencies are present

•           Hand holding accelerometers should be avoided except for low frequency work

Filtering Necessary for Accelerometers

•          Any high frequency vibration in the resonant range will be highly amplified. 

–        Amplification can be up to 30 dB or almost 1,000 times

–        Filtered amplitudes will be highly distorted

•          Resonant frequency highly depends on mounting

–        By previous example – 32 KHz for screw mount. Only 2 KHz for handholding

•          Therefore, resonance range should be filtered out

–        For screw mount low pass filter should be set at 10 KHz

–        For hand holding filter should be set at 1 KHz.

–        Analyst must know frequency response of accelerometer used for different mounting conditions.

Filtering can be done in FFT Analyzer by setting maximum frequency correctly.

 Advantages of Accelerometers

•          Measures casing or structural absolute motion

•          Rugged and reliable construction

•          Easy to install on machinery, structures, pipelines

•          Small size, easiest to install in cramped locations

•          Good signal response from 600 to 600,000 CPM

•          Low frequency units can measure down to 6 CPM

•          High freq units can reach 30 KHz (1,800,000 CPM)

•          Operates below mounted resonance frequency

•          Flat phase response throughout operating range

•          Internal electronics can be used to convert acceleration to velocity – Bently Velometer

•          Units available from a cryogenic temperature of minus 200^oC to a high temperature of > 600^oC

 Disadvantages of Accelerometers

•          Sensitive to mounting and surface conditions

•          Unable to measure shaft vibration or position

•          Not self generating – Need external power source

•          Transducer cable sensitive to noise, motion and electrical interference

•          Low signal response below 600 CPM (10 Hz)

•          Temperature limitation of 120^oC for ICP Acceleroms

•          Double integration to displacement suffers from low frequency noise – should be avoided

•          Signal filtration required depending on mounting

•          Difficult calibration check

Machine With Both Shaft and Bearing Housing Vibration Monitoring

 Refferensi Book

1. Machinery Malfunction Diagnosis and Correction – Robert C Eisenmann – Prentice Hall
2. Fundamentals of Rotating Machinery Diagnostics – Donald E. Bently – Bently Pressurized Bearing Press

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