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# Vibration analysis of process plant machinery best

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### Vibration analysis of process plant machinery best

1. 1. A Brief Introduction to Vibration Analysis of Process Plant Machinery (I) Basic Concepts I Machinery Vibration is Complex Vibration of a machine is not usually simple • Many frequencies from many malfunctions • Total vibration is sum of all the individual vibrations • Unfiltered overall amplitude indicates overall condition • Displacement amplitude is not a direct indicator of vibration severity unless combined with frequency • Velocity combines the function of displacement and frequency • Unfiltered velocity measurement provides best overall indication of vibration severity Characteristics of Vibration • • • • Vibration is the back and forth motion of a machine part One cycle of motion consists of Movement of weight from neutral position to upper limit Upper limit back through neutral position to lower limit Lower limit to neutral position The movement of the weight plotted against time is a sine wave Simple Spring- Mass system • Movement plotted against time Free and Forced Vibration When a mechanical system is subjected to a sudden impulse, it will vibrate at its natural frequency. Eventually, if the system is stable, the vibration will die out Forced vibration can occur at any frequency, and the response amplitude for a certain force will be constant
2. 2. • • • • Relationship between Force and Vibration Forces that cause vibration occur at a range of frequencies depending on the malfunctions present These act on a bearing or structure causing vibration However, the response is not uniform at all frequencies. It depends on the Mobility of the of the structure. Mobility varies with frequency. For example, it is high at resonances and low where damping is present Various Amplitudes of a Sine Wave • • • • A = Zero to Peak or maximum amplitude – used to measure velocity and acceleration 2A = Peak to Peak = Used to measure total displacement of a shaft with respect to available bearing clearance RMS = Root Mean Squared amplitude - A measure of energy - used to measure velocity and acceleration – mainly used in Europe Average value is not used in vibration measurements Characteristics of Vibration (2) • Time required to complete one cycle is the PERIOD of vibration If period is 1 sec then the number of cycles per minute (CPM) is 60 Frequency is the number of cycles per unit time – CPM or C/S (Hz)
3. 3. • Peak to peak displacement is the total distance traveled from one extreme limit to the other extreme limit • Velocity is zero at top and bottom because weight has come to a stop. It is maximum at neutral position • Acceleration is maximum at top an bottom where weight has come to a stop and must accelerate to pick up velocity Root Mean Squared Amplitude • • RMS amplitude will be equal to 0.707 times the Peak amplitude if, and only if, the signal is a sine wave (single frequency) If the signal is not a sine wave, then the RMS value using this simple calculation will not be correct Displacement, Velocity & Acceleration • • • • • • • • Displacement describes the position of an object Velocity describes how rapidly the object is changing position with time Acceleration describes how fast the velocity changes with time If Displacement d = x = A sin (wt) , then Velocity = rate of change of displacement v = dx / dt = Aw cos wt = Aw sin (wt + 90o) Acceleration = rate of change of velocity a = dv /dt = - Aw2 sin wt = Aw2 sin (wt + 180o)
4. 4. 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”
5. 5. • • 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 90o; that is, it reaches its maximum ¼ cycle or 90obefore displacement maximum Acceleration leads displacement by 180o. Acceleration leads velocity by 90o Small yellow circles show this relationship clearly
6. 6. 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
7. 7. – – • – – 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
8. 8. 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
9. 9. 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
10. 10. – – • – – – – – – 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)
11. 11. 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
12. 12. 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
13. 13. 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
14. 14. • • • 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
15. 15. – • – 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
16. 16. • • • • • • • 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 (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
17. 17. 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
18. 18. • – – • • • • • 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
19. 19. 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
20. 20. • • • • • • – • – 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 120oC 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. 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
21. 21. 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 1/3rd of resonance Resonance frequency, however, depends on mounting
22. 22. 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
23. 23. • 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 200oC to a high temperature of > 600oC 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 120oC for ICP Acceleroms
24. 24. • • • 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 3. Vibration Vector 4. 5. • A vibration vector plotted in the transducer response plane 6. • 1x vector is 90 mic pp /220o 7. • Zero reference is at the transducer angular location 8. • Phase angle increases opposite to direction of rotation 9. 10. 11. 12. Polar Plot 13.
25. 25. 14. 15. • Polar plot is made up of a set of vectors at different speeds. 16. 17. • Vector arrow is omitted and the points are connected with a line 18. 19. • Zero degree is aligned with transducer location 20. 21. • Phase lag increases in direction opposite to rotation 22. 23. • 1x uncompensated Polar Plot shows location of rotor high spot relative to transducer 24. 25. • This is true for 1x circular orbits and approximately true for 1x elliptical orbits 26. 27. 28. 29. Read more » 30. Posted by Fajar at 12:35 PM 1 comments 31. Email ThisBlogThis!Share to TwitterShare to Facebook 32. 8/06/2011 33. Shaft Orbit Plots (II) 34. Not- 1X Compensation of an Orbit 35. 36. • At Left orbit is the uncompensated orbit 37. • At right is the same orbit with the 1X component removed 38. • The remaining vibration is primarily 1/2X from a rub 39.
26. 26. 40. 41. 42. Measurement of peak-to-peak amplitude of an Orbit 43. 44. X transducer measurement axis is drawn together with perpendicular lines that are tangent to maximum and minimum points on the orbit 45. 46. 47. Direction of Precession in Orbits 48. 49. • In the orbit plot shaft moves from the blank towards the dot. In the plot on left the inside loop is forward precession 50. • In the right orbit the shaft has reverse precession for a short time at the outside loop at bottom
27. 27. 51. 52. 53. 54. Effect of Radial Load on Orbit Shape 55. 56. • Orbits are from two different steam turbines with opposite rotation. Both machines are experiencing high radial loads 57. • Red arrows indicate the approximate direction of the applied radial load. 58. • Red arcs represent the probable orientation of the bearing wall 59. 60. 61. 62. Deflection Shape of Rotor Shaft 63. 64. • When keyphasor dots of simultaneous orbits at various bearings along the length of the rotor are joined an estimate of the three dimensional deflection shape of the rotor shaft can be obtained 65. * This is a rigidly coupled rotor system 66. 67. 68. Posted by Fajar at 1:45 PM 1 comments 69. Email ThisBlogThis!Share to TwitterShare to Facebook
28. 28. 70. 8/05/2011 71. Shaft Orbit Plots (I) 72. The Orbit 73. • The orbit represents the path of the shaft centerline within the bearing clearance. 74. • Two orthogonal probes are required to observe the complete motion of the shaft within. 75. • The dynamic motion of the shaft can be observed in real time by feeding the output of the two orthogonal probes to the X and Y of a dual channel oscilloscope 76. • If the Keyphasor output is fed to the Z axis, a phase reference mark can be created on the orbit itself 77. • The orbit, with the Keyphasor mark, is probably the most powerful plot for machinery diagnosis 78. 79. Precession 80. 81. Once a gyroscope starts to spin, it will resist changes in the orientation of its spin axis. For example, a spinning top resists toppling over, thus keeping its spin axis vertical. If atorque, or twisting force, is applied to the spin axis, the axis will not turn in the direction of the torque, but will instead move in a direction perpendicular to it. This motion is called precession. The wobbling motion of a spinning top is a simple example of precession. The torque that causes the wobbling is the weight of the top acting about its tapering point. The modern gyroscope was developed in the first half of the 19th cent. by the 82. 83. 84. Construction of an Orbit 85. 86. • XY transducers observe the vibration of a rotor shaft 87. • A notch in the shaft (at a different axial location) is detected by the Keyphasor transducer. 88. • The vibration transducer signals produce two time base plots (middle) which combine into an orbit plot (right) 89. 90. 91. 92. Probe Orientation and the Orbit Plot
29. 29. 93. 94. 95. • On the left side, when the probes are mounted at 0o and 90oR, the orbit plot and oscilloscope display show the same view. 96. 97. 98. • On the right, when the probes are mounted at 45oL and 45oR, the orbit plots are automatically rotated 99. 100. 101. • The oscilloscope, however, must be physically rotated 45oCCW to display the correct orbit orientation 102. 103. 104. 105. 106. 107. 108. 109. 110.
30. 30. 111. Examples of 1X and Subsynchronous Orbits 112. 113. • Orbit at left shows subsynchronous fluid-induced instability. Note the multiple keyphasor dots because the frequency is not a fraction of the running speed 114. • The orbit at right is predominantly 1X. The keyphasor dots appear in a small cluster indicating dominant 1X behavior 115. 116. 117. 118. Slow Roll Vector Compensation of 1X Filtered Orbit 119. 120. • Slow roll vector compensation can considerably change the amplitude and phase of the orbit 121. 122. • Slow roll vectors of X= 1.2 mil pp /324oand Y= 1.4 mil pp /231o 123. 124. 125. 126. 127. 128. 129. 130. Slow roll Waveform Compensation of a Turbine Orbit 131. Note how compensation makes the orbit (right) much clearer
31. 31. 132. 133. 134. 135. 136. Posted by Fajar at 12:49 PM 0 comments Email ThisBlogThis!Share to TwitterShare to Facebook 137. 8/04/2011 138. Full Spectrum Plots 139. Full Spectrum 140. 141. • Half Spectrum is the spectrum of a WAVEFORM 142. • Full Spectrum is the spectrum of an ORBIT 143. • Derived from waveforms of two orthogonal probes 144. – These two waveforms provide phase information to determine direction of precession at each frequency 145. – For phase accuracy they must be sampled at same time 146. • Calculated by performing a FFT on each waveform 147. • These FFT’s are subjected to another transform 148. – Data converted to two new spectra – one for each direction of precession – Forward or Reverse 149. – Two spectra are combined into a single plot 150. Forward to the right, Reverse to the left 151. 152. Calculation of Full Spectrum Plot
32. 32. 153. 154. 155. 156. 157. 158. 159. First Waveform and its half spectrum 160. 161. 162. Second Waveform and its half spectrum 163. 164. Combined orbit and its full spectrum
33. 33. 165. 166. Circular Orbits and Their Full Spectra 167. 168. Forward Precession 169. Spectrum on forward side of plot 170. 171. <-- Reverse Precession 172. Spectrum on reverse side of plot 173. Direction of rotation – CCW 174. 175. <-- Forward Precession 176. Spectrum on forward side of plot 177. Direction of rotation – CW 178. 179. <-- Reverse Precession 180. Spectrum on reverse side of plot 181. Direction of rotation - CW 182. 183. 184. 185. Full Spectrum of Elliptical Orbit
34. 34. 186. 187. Orbit is generated by two counter rotating vectors 188. 189. Forward spectrum length is twice the length of forward rotating vector 190. 191. Reverse spectrum length is twice the length of reverse rotating vector 192. 193. Major axis of ellipse = a +b 194. Minor axis of ellipse = a - b 195. 196. Original orbit cannot be reconstructed from full spectrum because there is no phase information. 197. 198. 3 possible orbits are shown 199. 200. 201. Circular & Elliptical 1x Orbits
35. 35. 202. 203. • Direction of precession is indicated by dominant line of “Forward” and “Reverse” components. 204. 205. • Flatness of ellipse is determined by the relative size of forward and reverse components 206. 207. • When orbit is circular there is only one spectrum line 208. 209. • When orbit is a line the spectrum components are equal. 210. 211. • Therefore, the smaller the difference between components, the more elliptical the orbit. 212. 213. Orbit and Spectrum of a ½x Rub 214. 215. • Orbit and spectrum of a steam turbine with a ½ x rub 216. • Full spectrum clarifies the complex orbit which is a sum of ½ x, 1x and their harmonics. 217. • From the ratio of forward ad reverse components 218. • 1x is the largest, forward and mildly elliptical 219. • ½ x and 2x orbits are nearly line orbits 220. • Small component of 3/2 x is third harmonic of ½ x fundamental
36. 36. 221. 222. 223. Half and Full Spectrum Display of a ½ x Rub 224. 225. 226. 227. Differentiating ½ x Rub and Fluid Instability from Full Spectrum Plots 228. • Half and full spectrum display of a ½ x rub (red data) and fluid induced instability (blue data) 229. • Note similarity in appearance of the two half spectrum plots 230. • The full spectrum plots clearly show the difference in the subsynchronous vibration 231. – The ½ x rub orbit is extremely elliptical – small difference between forward and reverse components 232. – The fluid induced instability orbit is forward and nearly circular – large difference between forward and reverse 1x and ½ x components. 233. • The unfiltered orbits are at the bottom 234. Full Spectrum Cascade Plot of Machine Start Up
37. 37. 235. 236. • Horizontal axis represents precession frequency 237. 238. • Rotor speed is to the left and amplitude scale is on the right 239. 240. • Order lines drawn diagonally from the origin show vibration frequencies that are proportional to running speed 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. • • • • • • • Display of spectra plots taken at different speeds during start up Base of each spectrum is the rotor speed at which the sample was taken Diagonal lines are “Order” lines. Usually 1x, 2x and ½ x are plotted Resonances and critical speed can be seen on 1x diagonal line Sudden appearance of ½ x indicates rub which can produce harmonics. Phase relationships cannot be seen on cascade plot. Many harmonics at low speed usually due to scratches on shaft
38. 38. 252. 253. 254. Horizontal ellipse shows rub second balance resonance (critical) 255. 256. Vertical ellipse shows ½ x rub frequency is almost equal to first critical. Slight shift to right is due to stiffening of rotor system from rub contact. 257. 258. 259. 260. 261. 262. Full Spectrum Waterfall Plot 263. 264. • Displays spectra with respect to time 265. 266. • Used for correlating response to operating parameters 267. 268. • Time on left and Running Speed on right. Amplitude scale is at extreme right 269. 270. • Plot of compressor shows subsynchronous instability whenever suction pressure is high (red). 1x component is not shown on plot. 271.
39. 39. 272. • Full spectrum shows subsynchronous vibration is predominantly forward. 273. 274. 275. 276. 277. Waterfall of Motor with Electrical Noise Problem 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. • High vibration at mains frequency (60 Hz) during start up (red). 1x is low. • Vibration reduces when normal speed and current are reached (green) • When motor is shut down (blue) 60 Hz component disappears suddenly. • 1x component reduces gradually with speed. 291. Summary 292. 293. • Conventional spectrum is constructed from the output waveform of a single transducer 294. • Full Spectrum is constructed from the output of a pair of transducers at right angles. 295. – Displays frequency and direction of precession 296. – Forward precession frequencies are shown on right side 297. – Reverse Precession frequencies are shown on left side 298. • Full spectrum is the spectrum of an orbit 299. – Ratio of forward and reverse orbits gives information about ellipticity and direction of precession 300. – However, there is no information about orientation of orbit
40. 40. 301. • spectra 302. Cascade and Waterfall plots can be be constructed either from half or full 303. 304. 305. Posted by Fajar at 10:54 AM 3 comments Email ThisBlogThis!Share to TwitterShare to Facebook 306. 8/03/2011 307. Half Spectrum Plots 308. Spectrum Plot-1 309. • Machines can vibrate at many different frequencies simultaneously 1x, 2x, 3x, vane passing etc. 310. • Timebase and orbit have frequency information but only a couple of harmonics can be identified – impossible to identify nonsynchronous frequencies 311. • Using an analog tunable analyzer the amplitude and phase at each individual frequency can be identified but only one at a time. 312. – All frequencies cannot be seen simultaneously. 313. – Trend changes in individual frequencies cannot be followed 314. – Each frequency sweep may take one minute during which short duration transient events may be missed 315. • A Spectrum Plot by a FFT Analyzer shows all frequencies instantaneously. 316. 317. Spectrum Plot-2 318. • Spectrum plot is the basic display of a Spectrum Analyzer. It the most important plot for diagnosis 319. • Spectrum plot displays the entire frequency content of complex vibration signals in a convenient form. 320. – It has frequency on X-axis and amplitude on Y-axis 321. – It is constructed from sampled timebase waveform of a single transducer – displacement, velocity or acceleration 322. • Fast Fourier Transform (FFT) calculates the spectrum from the sample record which contains a specific number of waveform samples 323. • Spectrum plots can be used to identify harmonics of running frequency, rolling element bearing defect frequencies, gear mesh frequencies, sidebands 324. Periodic motion with more than one frequency 325.
41. 41. Above waveform broken up into a sum of harmonically related sine waves 326. 327. 328. Illustration of how the previous signal can be described in terms of a frequency spectrum. 329. Left - Description in time domain 330. Right - Description in frequency domain 331. 332. 333. 334. 335. Spectrum Frequency as a Function of Pulse Shape 336. 337. Construction of Half Spectrum Plot - 1 338. • Raw timebase signal (red) is periodic but complex. 339. • Fourier transform is equivalent to applying of a series of digital filters 340. • Filtered frequency components are shown as sine waves (blue) 341. • Phase for each signal can be measured with respect to trigger signal 342. • We can see components’ amplitude, frequency and phase
42. 42. 343. 344. Construction of Half Spectrum Plot - 2 345. • If we rotate the plot so that the time axis disappears we see a two dimensional spectrum plot of amplitude v/s frequency 346. • Component signals now appear as series of vertical lines. 347. • Each line represents a single frequency 348. • Unfortunately, the phase of the components is now hidden. 349. • It is not possible to see phase relationships in spectrum plot. 350. 351. 352. 353. These plots show why it is impossible to guess the frequency content from the waveform. 354. Vertical lines in top plot show one revolution 355. It is clear that 2x and higher frequencies are present 356. But 3x and 6x could not be predicted from the waveform. 357. A Fourier spectrum shows all the frequencies present
43. 43. 358. 359. 360. Linear and Logarithmic Scaling 361. • Amplitude scaling can be Linear or Logarithmic 362. • Logarithmic scaling is useful for comparing signals with very large and very small amplitudes. 363. – Will display all signals and the noise floor also 364. • However, when applied to rotating machinery work 365. – Log scale makes it difficult to quickly discriminate between significant and insignificant components. 366. • Linear scaling shows only the most significant components. 367. – Weak, insignificant and low-level noise components are eliminated or greatly reduced in scale 368. • Most of our work is done with linear scaling 369. 370. 371. 372. 373. 374. 375. 376. 377. 378. 379. 380. 381. Illustration of Linear and Log scales • Log scale greatly amplifies low level signals • It is impossible to read 1% signals in linear scale • It is very easy to read 0.1% signals on the log scale Limitations of Spectrum Plots • FFT assumes vibration signal is constant and repeats forever. • Assumption OK for constant speed machines . – inaccurate if m/c speed or vibration changes suddenly. • FFT calculates spectrum from sample record
44. 44. 382. – Which has specific number of digital waveform samples 383. – FFT algorithm extends sample length by repeatedly wrapping the signal on itself 384. – Unless number of cycles of signal exactly matches length of sample there will be discontinuity at the junction 385. – This introduces noise or leakage into the spectrum 386. • This problem is reduced by “windowing” 387. – Forces signal smoothly to zero at end points 388. – Hanning window best compromise for machinery work 389. 390. Effect of Windowing 391. 392. 393. 394. 395. 396. 397. • Figure shows a timebase plot with a mixture of 1/2x and 1x frequencies. Two examples of half spectrum plots are shown below • • Without window function the “lines” are not sharp and widen at the bottom This “leakage” is due to discontinuity at sample record ending 398. • When “Hanning” window is applied to the sample record 1/2x spectral line is narrower and higher 399. • Noise floor at base is almost gone.
45. 45. Half Spectrum Plots Spectrum Plot-1 • Machines can vibrate at many different frequencies simultaneously 1x, 2x, 3x, vane passing etc. • Timebase and orbit have frequency information but only a couple of harmonics can be identified – impossible to identify nonsynchronous frequencies • Using an analog tunable analyzer the amplitude and phase at each individual frequency can be identified but only one at a time. – All frequencies cannot be seen simultaneously. – Trend changes in individual frequencies cannot be followed – Each frequency sweep may take one minute during which short duration transient events may be missed • A Spectrum Plot by a FFT Analyzer shows all frequencies instantaneously. Spectrum Plot-2 • Spectrum plot is the basic display of a Spectrum Analyzer. It the most important plot for diagnosis • Spectrum plot displays the entire frequency content of complex vibration signals in a convenient form. – It has frequency on X-axis and amplitude on Y-axis – It is constructed from sampled timebase waveform of a single transducer – displacement, velocity or acceleration • Fast Fourier Transform (FFT) calculates the spectrum from the sample record which contains a specific number of waveform samples • Spectrum plots can be used to identify harmonics of running frequency, rolling element bearing defect frequencies, gear mesh frequencies, sidebands Periodic motion with more than one frequency Above waveform broken up into a sum of harmonically related sine waves
46. 46. Illustration of how the previous signal can be described in terms of a frequency spectrum. Left - Description in time domain Right - Description in frequency domain Spectrum Frequency as a Function of Pulse Shape • • • • • Construction of Half Spectrum Plot - 1 Raw timebase signal (red) is periodic but complex. Fourier transform is equivalent to applying of a series of digital filters Filtered frequency components are shown as sine waves (blue) Phase for each signal can be measured with respect to trigger signal We can see components’ amplitude, frequency and phase
47. 47. • • • • • Construction of Half Spectrum Plot - 2 If we rotate the plot so that the time axis disappears we see a two dimensional spectrum plot of amplitude v/s frequency Component signals now appear as series of vertical lines. Each line represents a single frequency Unfortunately, the phase of the components is now hidden. It is not possible to see phase relationships in spectrum plot. These plots show why it is impossible to guess the frequency content from the waveform. Vertical lines in top plot show one revolution It is clear that 2x and higher frequencies are present But 3x and 6x could not be predicted from the waveform. A Fourier spectrum shows all the frequencies present Linear and Logarithmic Scaling
48. 48. • • • Amplitude scaling can be Linear or Logarithmic Logarithmic scaling is useful for comparing signals with very large and very small amplitudes. Will display all signals and the noise floor also However, when applied to rotating machinery work Log scale makes it difficult to quickly discriminate between significant and insignificant components. Linear scaling shows only the most significant components. Weak, insignificant and low-level noise components are eliminated or greatly reduced in scale Most of our work is done with linear scaling • • • Illustration of Linear and Log scales Log scale greatly amplifies low level signals It is impossible to read 1% signals in linear scale It is very easy to read 0.1% signals on the log scale – • – • – • • – • – – – – • – – Limitations of Spectrum Plots FFT assumes vibration signal is constant and repeats forever. Assumption OK for constant speed machines . inaccurate if m/c speed or vibration changes suddenly. FFT calculates spectrum from sample record Which has specific number of digital waveform samples FFT algorithm extends sample length by repeatedly wrapping the signal on itself Unless number of cycles of signal exactly matches length of sample there will be discontinuity at the junction This introduces noise or leakage into the spectrum This problem is reduced by “windowing” Forces signal smoothly to zero at end points Hanning window best compromise for machinery work Effect of Windowing
49. 49. • Figure shows a timebase plot with a mixture of 1/2x and 1x frequencies. Two examples of half spectrum plots are shown below • • Without window function the “lines” are not sharp and widen at the bottom This “leakage” is due to discontinuity at sample record ending • When “Hanning” window is applied to the sample record 1/2x spectral line is narrower and higher Full Spectrum Plots Full Spectrum • • • – – • • – – Half Spectrum is the spectrum of a WAVEFORM Full Spectrum is the spectrum of an ORBIT Derived from waveforms of two orthogonal probes These two waveforms provide phase information to determine direction of precession at each frequency For phase accuracy they must be sampled at same time Calculated by performing a FFT on each waveform These FFT’s are subjected to another transform Data converted to two new spectra – one for each direction of precession – Forward or Reverse Two spectra are combined into a single plot Forward to the right, Reverse to the left Calculation of Full Spectrum Plot
50. 50. First Waveform and its half spectrum Second Waveform and its half spectrum Combined orbit and its full spectrum
51. 51. Circular Orbits and Their Full Spectra Forward Precession Spectrum on forward side of plot <-- Reverse Precession Spectrum on reverse side of plot Direction of rotation – CCW <-- Forward Precession Spectrum on forward side of plot Direction of rotation – CW <-- Reverse Precession Spectrum on reverse side of plot Direction of rotation - CW Full Spectrum of Elliptical Orbit
52. 52. Orbit is generated by two counter rotating vectors Forward spectrum length is twice the length of forward rotating vector Reverse spectrum length is twice the length of reverse rotating vector Major axis of ellipse = a +b Minor axis of ellipse = a - b Original orbit cannot be reconstructed from full spectrum because there is no phase information. 3 possible orbits are shown Circular & Elliptical 1x Orbits
53. 53. • Direction of precession is indicated by dominant line of “Forward” and “Reverse” components. • Flatness of ellipse is determined by the relative size of forward and reverse components • When orbit is circular there is only one spectrum line • When orbit is a line the spectrum components are equal. • Therefore, the smaller the difference between components, the more elliptical the orbit. Orbit and Spectrum of a ½x Rub • • • • • • Orbit and spectrum of a steam turbine with a ½ x rub Full spectrum clarifies the complex orbit which is a sum of ½ x, 1x and their harmonics. From the ratio of forward ad reverse components 1x is the largest, forward and mildly elliptical ½ x and 2x orbits are nearly line orbits Small component of 3/2 x is third harmonic of ½ x fundamental
54. 54. Half and Full Spectrum Display of a ½ x Rub • • • – – • Differentiating ½ x Rub and Fluid Instability from Full Spectrum Plots Half and full spectrum display of a ½ x rub (red data) and fluid induced instability (blue data) Note similarity in appearance of the two half spectrum plots The full spectrum plots clearly show the difference in the subsynchronous vibration The ½ x rub orbit is extremely elliptical – small difference between forward and reverse components The fluid induced instability orbit is forward and nearly circular – large difference between forward and reverse 1x and ½ x components. The unfiltered orbits are at the bottom Full Spectrum Cascade Plot of Machine Start Up
55. 55. • Horizontal axis represents precession frequency • Rotor speed is to the left and amplitude scale is on the right • Order lines drawn diagonally from the origin show vibration frequencies that are proportional to running speed • • • • • • • Display of spectra plots taken at different speeds during start up Base of each spectrum is the rotor speed at which the sample was taken Diagonal lines are “Order” lines. Usually 1x, 2x and ½ x are plotted Resonances and critical speed can be seen on 1x diagonal line Sudden appearance of ½ x indicates rub which can produce harmonics. Phase relationships cannot be seen on cascade plot. Many harmonics at low speed usually due to scratches on shaft
56. 56. Horizontal ellipse shows rub second balance resonance (critical) Vertical ellipse shows ½ x rub frequency is almost equal to first critical. Slight shift to right is due to stiffening of rotor system from rub contact. Full Spectrum Waterfall Plot • Displays spectra with respect to time • Used for correlating response to operating parameters • Time on left and Running Speed on right. Amplitude scale is at extreme right • • Plot of compressor shows subsynchronous instability whenever suction pressure is high (red). 1x component is not shown on plot. Full spectrum shows subsynchronous vibration is predominantly forward. Waterfall of Motor with Electrical Noise Problem
57. 57. • High vibration at mains frequency (60 Hz) during start up (red). 1x is low. • Vibration reduces when normal speed and current are reached (green) • When motor is shut down (blue) 60 Hz component disappears suddenly. • 1x component reduces gradually with speed. Summary • • – – – • – – • Conventional spectrum is constructed from the output waveform of a single transducer Full Spectrum is constructed from the output of a pair of transducers at right angles. Displays frequency and direction of precession Forward precession frequencies are shown on right side Reverse Precession frequencies are shown on left side Full spectrum is the spectrum of an orbit Ratio of forward and reverse orbits gives information about ellipticity and direction of precession However, there is no information about orientation of orbit Cascade and Waterfall plots can be be constructed either from half or full spectra Vibration Vector • A vibration vector plotted in the transducer response plane • 1x vector is 90 mic pp /220o • Zero reference is at the transducer angular location • Phase angle increases opposite to direction of rotation
58. 58. Polar Plot • Polar plot is made up of a set of vectors at different speeds. • Vector arrow is omitted and the points are connected with a line • Zero degree is aligned with transducer location • Phase lag increases in direction opposite to rotation • 1x uncompensated Polar Plot shows location of rotor high spot relative to transducer
59. 59. • This is true for 1x circular orbits and approximately true for 1x elliptical orbits Read more » Posted by Fajar at 12:35 PM 1 comments Email ThisBlogThis!Share to TwitterShare to Facebook 8/06/2011 Shaft Orbit Plots (II) Not- 1X Compensation of an Orbit • At Left orbit is the uncompensated orbit • At right is the same orbit with the 1X component removed • The remaining vibration is primarily 1/2X from a rub Measurement of peak-to-peak amplitude of an Orbit X transducer measurement axis is drawn together with perpendicular lines that are tangent to maximum and minimum points on the orbit
60. 60. Direction of Precession in Orbits • In the orbit plot shaft moves from the blank towards the dot. In the plot on left the inside loop is forward precession • In the right orbit the shaft has reverse precession for a short time at the outside loop at bottom Effect of Radial Load on Orbit Shape • • Orbits are from two different steam turbines with opposite rotation. Both machines are experiencing high radial loads Red arrows indicate the approximate direction of the applied radial load.
61. 61. • Red arcs represent the probable orientation of the bearing wall Deflection Shape of Rotor Shaft • When keyphasor dots of simultaneous orbits at various bearings along the length of the rotor are joined an estimate of the three dimensional deflection shape of the rotor shaft can be obtained * This is a rigidly coupled rotor system Posted by Fajar at 1:45 PM 1 comments Email ThisBlogThis!Share to TwitterShare to Facebook 8/05/2011 Shaft Orbit Plots (I) The Orbit • The orbit represents the path of the shaft centerline within the bearing clearance. • Two orthogonal probes are required to observe the complete motion of the shaft within.
62. 62. • The dynamic motion of the shaft can be observed in real time by feeding the output of the two orthogonal probes to the X and Y of a dual channel oscilloscope • If the Keyphasor output is fed to the Z axis, a phase reference mark can be created on the orbit itself • The orbit, with the Keyphasor mark, is probably the most powerful plot for machinery diagnosis Precession Once a gyroscope starts to spin, it will resist changes in the orientation of its spin axis. For example, a spinning top resists toppling over, thus keeping its spin axis vertical. If atorque, or twisting force, is applied to the spin axis, the axis will not turn in the direction of the torque, but will instead move in a direction perpendicular to it. This motion is called precession. The wobbling motion of a spinning top is a simple example of precession. The torque that causes the wobbling is the weight of the top acting about its tapering point. The modern gyroscope was developed in the first half of the 19th cent. by the Construction of an Orbit • XY transducers observe the vibration of a rotor shaft • A notch in the shaft (at a different axial location) is detected by the Keyphasor transducer. • The vibration transducer signals produce two time base plots (middle) which combine into an orbit plot (right)
63. 63. Probe Orientation and the Orbit Plot • On the left side, when the probes are mounted at 0o and 90oR, the orbit plot and oscilloscope display show the same view. • On the right, when the probes are mounted at 45oL and 45oR, the orbit plots are automatically rotated • The oscilloscope, however, must be physically rotated 45oCCW to display the correct orbit orientation
64. 64. Examples of 1X and Subsynchronous Orbits • Orbit at left shows subsynchronous fluid-induced instability. Note the multiple keyphasor dots because the frequency is not a fraction of the running speed • The orbit at right is predominantly 1X. The keyphasor dots appear in a small cluster indicating dominant 1X behavior Slow Roll Vector Compensation of 1X Filtered Orbit
65. 65. • • Slow roll vector compensation can considerably change the amplitude and phase of the orbit Slow roll vectors of X= 1.2 mil pp /324oand Y= 1.4 mil pp /231o Slow roll Waveform Compensation of a Turbine Orbit Note how compensation makes the orbit (right) much clearer
66. 66. Posted by Fajar at 12:49 PM 0 comments Email ThisBlogThis!Share to TwitterShare to Facebook 8/04/2011 Full Spectrum Plots Full Spectrum • Half Spectrum is the spectrum of a WAVEFORM • Full Spectrum is the spectrum of an ORBIT • Derived from waveforms of two orthogonal probes – These two waveforms provide phase information to determine direction of precession at each frequency – For phase accuracy they must be sampled at same time • Calculated by performing a FFT on each waveform • These FFT’s are subjected to another transform – Data converted to two new spectra – one for each direction of precession – Forward or Reverse – Two spectra are combined into a single plot Forward to the right, Reverse to the left Calculation of Full Spectrum Plot
67. 67. First Waveform and its half spectrum Second Waveform and its half spectrum Combined orbit and its full spectrum
68. 68. Circular Orbits and Their Full Spectra Forward Precession Spectrum on forward side of plot <-- Reverse Precession Spectrum on reverse side of plot Direction of rotation – CCW <-- Forward Precession Spectrum on forward side of plot Direction of rotation – CW
69. 69. <-- Reverse Precession Spectrum on reverse side of plot Direction of rotation - CW Full Spectrum of Elliptical Orbit Orbit is generated by two counter rotating vectors Forward spectrum length is twice the length of forward rotating vector Reverse spectrum length is twice the length of reverse rotating vector Major axis of ellipse = a +b Minor axis of ellipse = a - b Original orbit cannot be reconstructed from full spectrum because there is no phase information.
70. 70. 3 possible orbits are shown Circular & Elliptical 1x Orbits • Direction of precession is indicated by dominant line of “Forward” and “Reverse” components. • Flatness of ellipse is determined by the relative size of forward and reverse components • When orbit is circular there is only one spectrum line • When orbit is a line the spectrum components are equal. • Therefore, the smaller the difference between components, the more elliptical the orbit.
71. 71. Orbit and Spectrum of a ½x Rub • Orbit and spectrum of a steam turbine with a ½ x rub • Full spectrum clarifies the complex orbit which is a sum of ½ x, 1x and their harmonics. • From the ratio of forward ad reverse components • 1x is the largest, forward and mildly elliptical • ½ x and 2x orbits are nearly line orbits • Small component of 3/2 x is third harmonic of ½ x fundamental Half and Full Spectrum Display of a ½ x Rub
72. 72. Differentiating ½ x Rub and Fluid Instability from Full Spectrum Plots • Half and full spectrum display of a ½ x rub (red data) and fluid induced instability (blue data) • Note similarity in appearance of the two half spectrum plots • The full spectrum plots clearly show the difference in the subsynchronous vibration – The ½ x rub orbit is extremely elliptical – small difference between forward and reverse components – The fluid induced instability orbit is forward and nearly circular – large difference between forward and reverse 1x and ½ x components. • The unfiltered orbits are at the bottom Full Spectrum Cascade Plot of Machine Start Up • Horizontal axis represents precession frequency • Rotor speed is to the left and amplitude scale is on the right • Order lines drawn diagonally from the origin show vibration frequencies that are proportional to running speed
73. 73. • Display of spectra plots taken at different speeds during start up • Base of each spectrum is the rotor speed at which the sample was taken • Diagonal lines are “Order” lines. Usually 1x, 2x and ½ x are plotted • Resonances and critical speed can be seen on 1x diagonal line • Sudden appearance of ½ x indicates rub which can produce harmonics. • Phase relationships cannot be seen on cascade plot. • Many harmonics at low speed usually due to scratches on shaft Horizontal ellipse shows rub second balance resonance (critical) Vertical ellipse shows ½ x rub frequency is almost equal to first critical. Slight shift to right is due to stiffening of rotor system from rub contact.
74. 74. Full Spectrum Waterfall Plot • Displays spectra with respect to time • Used for correlating response to operating parameters • Time on left and Running Speed on right. Amplitude scale is at extreme right • • Plot of compressor shows subsynchronous instability whenever suction pressure is high (red). 1x component is not shown on plot. Full spectrum shows subsynchronous vibration is predominantly forward.
75. 75. Waterfall of Motor with Electrical Noise Problem • High vibration at mains frequency (60 Hz) during start up (red). 1x is low. • Vibration reduces when normal speed and current are reached (green) • When motor is shut down (blue) 60 Hz component disappears suddenly. • 1x component reduces gradually with speed. Summary • Conventional spectrum is constructed from the output waveform of a single transducer • Full Spectrum is constructed from the output of a pair of transducers at right angles. – Displays frequency and direction of precession – Forward precession frequencies are shown on right side – Reverse Precession frequencies are shown on left side
76. 76. • Full spectrum is the spectrum of an orbit – Ratio of forward and reverse orbits gives information about ellipticity and direction of precession – However, there is no information about orientation of orbit • Cascade and Waterfall plots can be be constructed either from half or full spectra Posted by Fajar at 10:54 AM 3 comments Email ThisBlogThis!Share to TwitterShare to Facebook 8/03/2011 Half Spectrum Plots Spectrum Plot-1 • Machines can vibrate at many different frequencies simultaneously 1x, 2x, 3x, vane passing etc. Timebase and orbit have frequency information but only a couple of harmonics can be identified – impossible to identify nonsynchronous frequencies • • Using an analog tunable analyzer the amplitude and phase at each individual frequency can be identified but only one at a time. – All frequencies cannot be seen simultaneously. – Trend changes in individual frequencies cannot be followed – Each frequency sweep may take one minute during which short duration transient events may be missed • A Spectrum Plot by a FFT Analyzer shows all frequencies instantaneously. Spectrum Plot-2 • Spectrum plot is the basic display of a Spectrum Analyzer. It the most important plot for diagnosis • Spectrum plot displays the entire frequency content of complex vibration signals in a convenient form. – It has frequency on X-axis and amplitude on Y-axis – It is constructed from sampled timebase waveform of a single transducer – displacement, velocity or acceleration
77. 77. • Fast Fourier Transform (FFT) calculates the spectrum from the sample record which contains a specific number of waveform samples • Spectrum plots can be used to identify harmonics of running frequency, rolling element bearing defect frequencies, gear mesh frequencies, sidebands Periodic motion with more than one frequency Above waveform broken up into a sum of harmonically related sine waves Illustration of how the previous signal can be described in terms of a frequency spectrum. Left - Description in time domain Right - Description in frequency domain Spectrum Frequency as a Function of Pulse Shape
78. 78. Construction of Half Spectrum Plot - 1 • Raw timebase signal (red) is periodic but complex. • Fourier transform is equivalent to applying of a series of digital filters • Filtered frequency components are shown as sine waves (blue) • Phase for each signal can be measured with respect to trigger signal • We can see components’ amplitude, frequency and phase Construction of Half Spectrum Plot - 2 • • If we rotate the plot so that the time axis disappears we see a two dimensional spectrum plot of amplitude v/s frequency Component signals now appear as series of vertical lines.
79. 79. • Each line represents a single frequency • Unfortunately, the phase of the components is now hidden. • It is not possible to see phase relationships in spectrum plot. These plots show why it is impossible to guess the frequency content from the waveform. Vertical lines in top plot show one revolution It is clear that 2x and higher frequencies are present But 3x and 6x could not be predicted from the waveform. A Fourier spectrum shows all the frequencies present • Linear and Logarithmic Scaling Amplitude scaling can be Linear or Logarithmic • Logarithmic scaling is useful for comparing signals with very large and very small amplitudes. – Will display all signals and the noise floor also
80. 80. • – • – • However, when applied to rotating machinery work Log scale makes it difficult to quickly discriminate between significant and insignificant components. Linear scaling shows only the most significant components. Weak, insignificant and low-level noise components are eliminated or greatly reduced in scale Most of our work is done with linear scaling Illustration of Linear and Log scales • Log scale greatly amplifies low level signals • It is impossible to read 1% signals in linear scale • It is very easy to read 0.1% signals on the log scale Limitations of Spectrum Plots • FFT assumes vibration signal is constant and repeats forever. • Assumption OK for constant speed machines . – inaccurate if m/c speed or vibration changes suddenly. • FFT calculates spectrum from sample record – Which has specific number of digital waveform samples
81. 81. – FFT algorithm extends sample length by repeatedly wrapping the signal on itself – Unless number of cycles of signal exactly matches length of sample there will be discontinuity at the junction – This introduces noise or leakage into the spectrum • This problem is reduced by “windowing” – Forces signal smoothly to zero at end points – Hanning window best compromise for machinery work Effect of Windowing • Figure shows a timebase plot with a mixture of 1/2x and 1x frequencies. Two examples of half spectrum plots are shown below • • Without window function the “lines” are not sharp and widen at the bottom This “leakage” is due to discontinuity at sample record ending • When “Hanning” window is applied to the sample record 1/2x spectral line is narrower and higher
82. 82. Shaft Orbit Plots (I) The Orbit • The orbit represents the path of the shaft centerline within the bearing clearance. • Two orthogonal probes are required to observe the complete motion of the shaft within. • The dynamic motion of the shaft can be observed in real time by feeding the output of the two orthogonal probes to the X and Y of a dual channel oscilloscope • If the Keyphasor output is fed to the Z axis, a phase reference mark can be created on the orbit itself • The orbit, with the Keyphasor mark, is probably the most powerful plot for machinery diagnosis Precession Once a gyroscope starts to spin, it will resist changes in the orientation of its spin axis. For example, a spinning top resists toppling over, thus keeping its spin axis vertical. If atorque, or twisting force, is applied to the spin axis, the axis will not turn in the direction of the torque, but will instead move in a direction perpendicular to it. This motion is called precession. The wobbling motion of a spinning top is a simple example of precession. The torque that causes the wobbling is the weight of the top acting about its tapering point. The modern gyroscope was developed in the first half of the 19th cent. by the Construction of an Orbit • XY transducers observe the vibration of a rotor shaft • A notch in the shaft (at a different axial location) is detected by the Keyphasor transducer. • The vibration transducer signals produce two time base plots (middle) which combine into an orbit plot (right)
83. 83. Probe Orientation and the Orbit Plot • On the left side, when the probes are mounted at 0o and 90oR, the orbit plot and oscilloscope display show the same view. • On the right, when the probes are mounted at 45oL and 45oR, the orbit plots are automatically rotated • The oscilloscope, however, must be physically rotated 45oCCW to display the correct orbit orientation
84. 84. Examples of 1X and Subsynchronous Orbits • Orbit at left shows subsynchronous fluid-induced instability. Note the multiple keyphasor dots because the frequency is not a fraction of the running speed • The orbit at right is predominantly 1X. The keyphasor dots appear in a small cluster indicating dominant 1X behavior Slow Roll Vector Compensation of 1X Filtered Orbit
85. 85. • • Slow roll vector compensation can considerably change the amplitude and phase of the orbit Slow roll vectors of X= 1.2 mil pp /324oand Y= 1.4 mil pp /231o Slow roll Waveform Compensation of a Turbine Orbit Note how compensation makes the orbit (right) much clearer
86. 86. Not- 1X Compensation of an Orbit • • • At Left orbit is the uncompensated orbit At right is the same orbit with the 1X component removed The remaining vibration is primarily 1/2X from a rub Measurement of peak-to-peak amplitude of an Orbit X transducer measurement axis is drawn together with perpendicular lines that are tangent to maximum and minimum points on the orbit
87. 87. Direction of Precession in Orbits • • In the orbit plot shaft moves from the blank towards the dot. In the plot on left the inside loop is forward precession In the right orbit the shaft has reverse precession for a short time at the outside loop at bottom Effect of Radial Load on Orbit Shape • • • Orbits are from two different steam turbines with opposite rotation. Both machines are experiencing high radial loads Red arrows indicate the approximate direction of the applied radial load. Red arcs represent the probable orientation of the bearing wall
88. 88. Deflection Shape of Rotor Shaft • When keyphasor dots of simultaneous orbits at various bearings along the length of the rotor are joined an estimate of the three dimensional deflection shape of the rotor shaft can be obtained * This is a rigidly coupled rotor system Bode and Polar Plot Vibration Vector • • • • A vibration vector plotted in the transducer response plane 1x vector is 90 mic pp /220o Zero reference is at the transducer angular location Phase angle increases opposite to direction of rotation Polar Plot
89. 89. • Polar plot is made up of a set of vectors at different speeds. • Vector arrow is omitted and the points are connected with a line • Zero degree is aligned with transducer location • Phase lag increases in direction opposite to rotation • 1x uncompensated Polar Plot shows location of rotor high spot relative to transducer • This is true for 1x circular orbits and approximately true for 1x elliptical orbits Bode Plot and Polar Plot Show the Same Detail • • Bode’ Plot displays the same “vibration vector data” as the Polar Plot Vibration amplitude and phase are plotted separately on two plots with speed on the horizontal axis.
90. 90. Effect of Slow Roll Compensation • • • Slow roll compensation removes slow roll runout from filtered vibration What remains is mainly the dynamic response Compensated vector has zero amplitude at the compensation speed Detecting Resonance with Bode & Polar Plots • In a Bode plot balance resonance is indicated by peak amplitude and sharp, significant change of phase at the frequency of the peak. On Polar plot rotor modes will produce large, curving loops.Small system resonances are more easily visible as distinctive small loops