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Vibration monitoring and its features for corelation

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A process and methodology behind vibration analysis for machine monitoring

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Vibration monitoring and its features for corelation

  1. 1. Fundamentals of Machinery Condition Monitoring, Vibration Analysis Boben Anto C
  2. 2. Introduction to Machinery Condition Monitoring Boben Anto C
  3. 3. Introduction to Condition Monitoring Boben Anto C
  4. 4. Maintenance Philosophy For many years maintenance was simply based on keeping the plant running. There was no real planning or thought involved; it was simply the case that if a machine failed it was repaired or a spare was used. The more later day philosophy on maintenance is based upon:  Optimisation of Production  Optimisation on Plant Availability  No Compromises to Safety Boben Anto C
  5. 5. Maintenance Strategies There are four main strategies regarding the execution of maintenance activities:  Opportunity Maintenance  Planned Preventative Maintenance  Breakdown Maintenance  Condition Based Maintenance  Reliability based Maintenance Boben Anto C
  6. 6. Opportunity Maintenance Adopts the philosophy ‘Fix It When The Opportunity Arises‟. Opportunity Maintenance is carried out, for example, when:  There is a plant shutdown  Other equipment is down for maintenance  A seasonal or weather window is available Opportunity Maintenance Advantages Disadvantages 1. Maintenance does not cause additional plant downtime. 1. Machine condition may worsen before the opportunity arises - causing secondary damage. Boben Anto C
  7. 7. Planned Preventative Maintenance Adopts the philosophy ‘Fix It Before it Fails‟. Planned Preventative Maintenance is carried out with a high level of planning at specific time intervals.  Based upon generic machinery reliability data „bath tub‟ effect  Time or running hours based e.g. after 6 months or 10,000 hours Preventative Maintenance Advantages Disadvantages 1. Maintenance is planned and carried out a convenient time. 1. Machines repaired when they may not have a problem. 2. Theoretically fewer catastrophic failures. 2. Repairs can cause more harm than good. 3. Control over storage of spare parts. 3. Unscheduled breakdowns still occur. 4. Not tailored to individual machines. Boben Anto C
  8. 8. Breakdown Maintenance Adopts the philosophy ‘Fix It When It Breaks‟.  No element of planning  No forewarning of failure Breakdown Maintenance Advantages Disadvantages 1. Machines are not over maintained. 1. Unexpected machine downtime - potential loss of production. 2. No costs associated with preventative or condition based maintenance. 2. Potential for secondary damage and catastrophic failure - leading to higher repair costs. 3. Lack of any form of control or planning Boben Anto C
  9. 9. Condition Based Maintenance Adopts the philosophy ‘If It’s Not Broken Don’t Fix It‟. Based upon measured parameters which are sensitive to the development of machinery faults  Maintenance related to machine condition  Predictive - forewarning of failure, diagnosis of faults and root causes Condition Based Maintenance Advantages Disadvantages 1. Unexpected downtime is reduced. 1. Expense of instrumentation and software, training personnel and use of specialist contractors. 2. Optimised spares. 2. Does not identify all machine faults e.g. seal leaks. 3. Causes of failure can be diagnosed. 4. Identifies many common machine faults. 5. Maintenance can be deferred and performed when opportunity arises. 6. Tailored to individual machines. Boben Anto C
  10. 10. Section 1.2: Condition Monitoring Strategy Boben Anto C
  11. 11. Condition Monitoring Strategy A condition monitoring strategy is developed to focus condition monitoring activities on business and safety critical machinery:  A Criticality Assessment is undertaken, based upon the likelihood of failure and consequences of failure, to define the equipment for inclusion within the condition monitoring programme.  Fault Matrices are utilised to identify detectable machine faults (for each type of critical machinery) and the parameters that can be monitored which are sensitive the development of these faults.  The Condition Monitoring Strategy is then defined based upon the critical machine list and failure modes which can be realistically identified through condition monitoring. Boben Anto C
  12. 12. Sample Fault Matrix – Motor Driven Pump (Motor) Boben Anto C
  13. 13. Sample Fault Matrix – Motor Driven Pump (Pump) The fault matrix is used to define the measurements for inclusion in the condition monitoring database. Boben Anto C
  14. 14. Condition Monitoring Techniques  Vibration Analysis  Lube Oil Analysis  Thermography  Electrical Motor Phase Current Analysis  Rogowski Coil Analysis  Performance Analysis Condition monitoring techniques include: Boben Anto C
  15. 15. Condition Monitoring vs. Machinery Protection Condition Monitoring Trending of „Fault Sensitive‟ machine parameters on a periodic basis, to provide information regarding current and forecasted machine condition. Allows the onset of fault conditions to be identified so that maintenance activities can be planned. This can prevent unscheduled machinery shutdowns. Machinery Protection Acquisition of vibration and temperature values from online systems to initiate machinery shutdown once a pre-set value has been exceeded. This avoids catastrophic failure. Boben Anto C
  16. 16. Vibration Theory & Measurement Transducers Boben Anto C
  17. 17. Section 2.1: Vibration Theory Boben Anto C
  18. 18. What is Vibration?  The number of times a complete cycle takes place per second is called the Frequency (measured in hertz (Hz)).  The motion can be of a single frequency, as with a tuning fork, or a number of frequencies such as the motion of a piston in an internal combustion engine or a gearbox. Vibration is defined as an oscillating motion about an equilibrium point e.g. a mass on a spring. Boben Anto C
  19. 19. Acceleration, Velocity and Displacement  Acceleration – is a vector quantity (i.e. it has magnitude and direction) which defines the rate of change of velocity. It is measured in units of m/s2 or more commonly g, where 1 g = 9.81 m/s2. An acceleration signal can be integrated to give a velocity signal.  Velocity – is a vector quantity which defines the speed of motion in a particular direction i.e. the rate of change of displacement. It is normally measured in units of mm/s (it can be found measured in Imperial units of „ips‟ (millionths of an inch per second where 1 ips = 25.4 mm/s). A velocity signal can be integrated to give a displacement signal.  Displacement – is a vector quantity which defines the change of position from a rest position. It is measured in units of mm or mm (microns). It can be found measured in Imperial units of „mils‟ where 1 mil = 25.4 microns. Vibration is described in one of three terms: Boben Anto C
  20. 20. Simple Harmonic Motion Where vibration is of a single frequency the motion is sinusoidal and repeats in an identical pattern over time. This is known as simple harmonic motion.  Vibration is described in terms of amplitude (the level or magnitude of the motion) and frequency (the number of repetitions of one full cycle per second).  When viewed in the time domain, the time waveform exhibits a pure sine wave.  When viewed in the frequency domain simple harmonic motion exhibits a single peak. Boben Anto C
  21. 21. Simple Harmonic Motion – Mass on a Spring Example The oscillating movement of a mass on a spring exhibits Simple Harmonic Motion : Boben Anto C
  22. 22. Quantifying The Amplitude of Vibration The amplitude of vibration describes it severity and can be quantified in several ways: peak-to-peak level, peak level, average level and RMS level. Boben Anto C
  23. 23. Quantifying The Amplitude of Vibration The Peak-to-Peak value is the maximum excursion (positive to negative motion) of the time waveform. Useful for measuring direct shaft displacement within a bearing housing and is often applied to acceleration envelope measurements. The Peak value (also termed true peak and zero-to-peak) is the absolute value from zero to the maximum point in the time waveform . Useful for measuring short duration shocks. The RMS (Root Mean Square) value is directly related to the energy content of the time waveform and is an indication of the destructive properties of the vibration. Used predominantly for machine casing (bearing cap) measurements. The Average value is of limited practical use in describing vibration. The following is true for purely harmonic motion (single frequency): Peak-to-Peak = 2 x Peak Peak =1.414 x RMS RMS = 0.707 x Peak (Average = 0.9 x RMS = 0.637 x Peak) Boben Anto C
  24. 24. Non-Sinusoidal (Complex) Waveforms The mathematical relationships between peak-to peak, peak, average and RMS levels are inaccurate once the time waveform is no longer sinusoidal. Boben Anto C
  25. 25. Time vs Frequency Domain Once more than one frequency is present within a time waveform it can become very difficult to analyse. A vibration data collector (Fast Fourier Transform analyser) will convert the time waveform into individual frequency components for ease of analysis. Boben Anto C
  26. 26. Phase Angle Measurement Phase measurements are used to assess the relationship between two vibration signals. It is commonly used in machine balancing and advanced diagnosis of machinery fitted with proximity probes. Phase information is taken in the time domain (carrying out a FFT of a time waveform looses the phase information). Phase Angle is the timing relationship between two signals of identical frequencies. Phase is normally measured in degrees and can be either relative or absolute:  Relative Phase requires two vibration signals of the same frequency.  Absolute Phase requires a vibration signal and a synchronous reference pulse e.g. Keyphasor or optical tachometer. Boben Anto C
  27. 27. Relative Phase Relative Phase is the timing relationship between two signals measured in degrees. It is measured from a point on one signal to the nearest corresponding point on another signal. Rules to follow when measuring relative phase: 1. Two vibration signals are required 2. Same frequency 3. Same units (e.g. mm/s, mm) 4. Either signal is chosen as the reference signal 5. Relative phase is measured as either lead or lag from 0 to 180 degrees Boben Anto C
  28. 28. Absolute Phase Absolute Phase is the number of degrees of a vibration cycle following the triggering of a once-per revolution pulse (e.g. keyphasor, optical pickup, strobe light or magnetic pickup). Rules to follow when measuring absolute phase: 1. Two signals are required (one vibration signal and one reference signal) 2. Measured from reference signal thus always a phase lag angle from 0 to 360 degrees 3. 0o location is the point at which the reference signal triggers 4.Vibration signal to be filtered to a single frequency which is a integer multiple of reference signal Boben Anto C
  29. 29. Phase Relationships of Acceleration/Velocity/Displacement It is very important to measure in consistent units when measuring phase. When a signal is integrated from acceleration to velocity or from velocity to displacement its phase angle changes. It is also essential that vibration transducer orientation is taken into account.  Velocity leads displacement by a phase angle of 90o.  Acceleration leads velocity by a phase angle of 90o.  Acceleration leads displacement by a phase angle of 180o. Period T = 360o Boben Anto C
  30. 30. Uses of Phase Angle Measurements Examples of how phase angle measurements can be used:  Shaft Balancing – vibration measurements are related to the phase reference to calculate the placement of balance weights  Shaft Crack Detection  Shaft/structural resonance detection  Shaft mode shapes  Direction of shaft precession  Confirming force or couple imbalance  Confirming misalignment Boben Anto C
  31. 31. Shaft Orbits When two XY vibration signals are added together the resultant signal shows a two dimensional picture of the vibration motion. This is known as an orbit.  If the two vibration signals are from casing mounted transducers (e.g. accelerometers) the orbit reveals the casing motion.  If the two vibration signals are from proximity probes the orbit reveals the actual shaft motion within the bearing clearance. Boben Anto C
  32. 32. Shaft Orbit Shape Shaft orbits must be taken from XY vibration transducers mounted orthogonally (90o apart). They do not need to be true vertical and true horizontal but must be 90o apart in the radial plane.  Figure 1 shows a typical orbit. As machines are typically more stiff vertically than horizontally the orbit is elliptical in shape.  Figure 2 shows a circular orbit. A circular orbit is normally indicative of an imbalance condition.  Figure 3 shows a figure of eight orbit. This shape of orbit is characteristic of misalignment. 1. Typical Orbit 2. Orbit Indicating Imbalance 3. Orbit Indicating Misalignment Boben Anto C
  33. 33. Resonance When a tuning fork is struck it rings at a single frequency. This frequency is known as its resonant frequency and is the same every time the fork is struck. The natural frequency of any system is a function of its stiffness and mass. Mass Stiffness 2 1   nFFrequencyNatural  Resonance occurs when the forcing frequency is equal to the natural frequency.  At resonance very little excitation is required to produce a large response. The same excitation above or below resonance will produce a greatly reduced response.  Increasing or decreasing the system‟s stiffness or mass will change the resonant frequency. [Note: the above equation is true for an undamped single degree of freedom system]Boben Anto C
  34. 34. Why Machines Vibrate? All machines have the properties of mass and stiffness and therefore possess the ability to vibrate. Perfect engineering would produce machines with no vibration, however, in reality all machines are built to tolerances and as such any rotating system will have an element of unbalance. This unbalance force will produce vibration when the machine rotates. Boben Anto C
  35. 35. Rigid vs Flexible Rotors 1st Critical 2nd Critical 3rd Critical Rigid Support and Flexible Rotor  A machine is said to have a Rigid Rotor if the rotating elements‟ natural frequency is above the running speed of the unit.  If a machine runs at a speed above the rotating elements‟ natural frequency it is said to have a Flexible Rotor. On run up and shutdown the machine will pass through resonance (a critical speed).  A Critical Speed is a natural frequency of the rotating element and its support system including bearings and lubricating film. Boben Anto C
  36. 36. Critical Speeds  It is important to know a machine‟s critical speeds so that they do not coincide with normal operating speeds.  The response of a rotor at critical speed will give an indication of the system‟s damping and hence the condition of the journal bearings.  The synchronous amplification factor between operating speed vibration and resonance should be determined during commissioning to calculate machine protection system alarms. Critical speeds of rotating machinery are speeds which correspond with the systems resonant frequencies. Boben Anto C
  37. 37. Synchronous Amplification Factor When a unit is running at a speed above resonance (shaft critical speed) the maximum allowable vibration should be governed by the level the vibration will reach when it passes through resonance upon shutdown. The simplest calculation of SAF is the Peak Ratio Method where the level of vibration at resonance is divided by the steady state running speed vibration. This is often used to calculate machinery trip levels in order that shutdown occurs at such a level of vibration to avoid damage on rundown. Example: If the level of running vibration is 100 microns (pk-pk) and the bearing oil clearance is 200 microns. The unit will only safely run down if the SAF is significantly less than 2. Boben Anto C
  38. 38. Section 2.2: Vibration Transducers Boben Anto C
  39. 39. Vibration Transducers  Accelerometer – measures acceleration from machine casing (the signal can be integrated to measure velocity or displacement).  Velocity Transducer – measures velocity from machine casing.  Proximity Probe (Eddy Current Probe) – non-contact transducer which measures shaft relative vibration. There are three main types of vibration transducer: Boben Anto C
  40. 40. Transducer Sensitivities  Accelerometer sensitivities are defined in units of mV/m/s2 (millivolts per metre per second squared) or more commonly mV/g (where g is gravity).  Velocity Transducer sensitivities are defined in units of mV/ips (millivolts per inches per second) or mV/mm/s (millivolts per millimetre per second).  Proximity Probe (Eddy Current Probe) sensitivities are defined in units of mV/mil (millivolts per millionth of an inch (1x10-6 inches)) or mV/mm (millivolts per micron or micrometre (1x10-6 metres). The sensitivity of a transducer describes its electrical output per unit of mechanical input. The higher the electrical output per unit input, the more sensitive the transducer. Sensitivities can be described in metric or imperial units. The nominal sensitivity of a transducer is normally displayed on its casing. For vibration transducers sensitivities are normally given in mV/EU (millivolts per engineering unit). Boben Anto C
  41. 41. Sensitivity Conversion Factors The following are the most common sensitivity conversion factors for Imperial to metric units. When setting up a condition monitoring database it is good practice to convert all sensitivities into metric units for ease of analysis: Sensitivity Conversion Divide by to Obtain mV/m/s2 9.81 mV/g mV/ips 25.4 mV/mm/s mV/mil 25.4 mV/mm Unit Conversion: 1 g = 9.81 m/s2 (meters per second squared) 1 ips (inches per second) = 25.4 mm/s (millimeters per second) 1 mil (millionth of an inch) = 25.4 mm (micrometers or microns) Boben Anto C
  42. 42. Transducer Frequency Response  Accelerometer – useable range 1 to 20,000 Hz (dependent upon mounting arrangement), specialist accelerometers exist for very low and very high frequency applications.  Velocity Transducer – useable range 10 to 1,500 Hz for electromechanical type and 1 to 2,000 Hz for piezoelectric type.  Proximity Probe (Eddy Current Probe) – stated useable range 0 to 10,000 Hz, typically used in the frequency range 0 to 2,000 Hz as higher frequencies can be influenced by shaft surface imperfections. Boben Anto C
  43. 43. Accelerometer An accelerometer converts acceleration into an electrical output. Typical Sensitivities are 25 mV/g, 50 mV/g and most commonly 100 mV/g. Very sensitive accelerometers for low frequency, low amplitude applications can be as high as 1000 mV/g (1000 V/g). Boben Anto C
  44. 44. Accelerometer Mounting An accelerometer‟s frequency response is highly dependent upon how it is mounted to the machine surface.  Stud mounted measurements provide the best frequency response and repeatability for machine condition monitoring. This is critical for rolling element bearing and gearbox vibration analysis.  Magnetic mounted measurements have reduced frequency response.  Hand Held measurements provide the poorest repeatability and frequency response (should only be used when no other alternative e.g. very high machine casing temperature). Boben Anto C
  45. 45. Velocity Transducer A velocity transducer measures the rate of change of displacement and is traditionally an electromechanical device. Boben Anto C
  46. 46. Proximity Probe (Eddy Current Probe) A proximity probe is a non-contact electromagnetic sensor which converts displacement (distance) to voltage. The DC component of the signal measures the average distance from the shaft whereas the AC component measures the dynamic fluctuation in displacement i.e. the vibration. Typical Sensitivities are 3.94 mV/mm (100 mV/mil) and most commonly 7.87 mV/mm (200 mV/mil). [Note: 1 mil = 25.4 mm]. Boben Anto C
  47. 47. Proximity Probe - Mounting Pairs of radial probes are orientated 90o apart and referred to as „X‟ and „Y‟. Processing these two signals together produces a shaft orbit. Boben Anto C
  48. 48. Proximity Probe – Gap Voltage The DC component of proximity probe is known as its gap voltage and accurately measures the distance of the probe tip from the shaft. A probe has a typical range of 0 to -18 Vdc (~ 2.3 mm). The probes response is linear across a large proportion of this range.  Proximity probe gap voltages are normally set up at – 9 Vdc.  As a rough guide if the gap voltage is between -6 Vdc and – 12Vdc it is well within its linear operating range. If the gap voltage is not between -3 Vdc to -15 Vdc it is potentially outside its linear range.  If a gap voltage is close to zero it is short circuited or to close to the shaft. If a gap voltage is -18 Vdc it is open circuit or pointing into space. Note: Gap voltages by convention are always negative. Thus the more „positive‟ the gap voltage the closer you are to the shaft. Boben Anto C
  49. 49. Proximity Probe – Run out  On high speed machines (>2,500 RPM) run out is measured as part of machine commissioning during slow roll tests (typically 300 to 600 RPM).  API standards set limits for acceptable levels run out.  As a guide 6 mm (microns) or 10% of the overall vibration signal is acceptable. Shaft surface imperfections (e.g. scratches, dents, irregular conductivity or permeability) are indistinguishable from vibration to a proximity probe. This additional „signal‟ is known as run out. Boben Anto C
  50. 50. Proximity Probe – Used as a Keyphasor Shaft Probe -18V or -24V Proximitor Out Shaft Probe -18V or -24V Proximitor Out Keyway Projection (20) (15) (10) (5) 0 -Volts (20) (15) (10) (5) 0 -Volts A Keyphasor provides a once-per-revolution pulse used as a reference to measure absolute phase. [Note: Keyphasor is a Bently Nevada trade name] Boben Anto C
  51. 51. Proximity Probe – Used as a Speed Reference A Keyphasor provides a once per revolution pulse which will give an indication of machine running speed. To use a proximity probe for precise speed control requires a pulse multiple times per revolution. For precision speed control a toothed wheel (also known as a Phonic Wheel) is targeted by a proximity probe. The signal is processed to give a highly accurate measurement of shaft speed which is updated multiple times per revolution. Boben Anto C
  52. 52. Vibration Transducer Comparison - Advantages Advantages Accelerometer Velocity Transducer Proximity Probe 1. Surface Mounted. 2. Small, Portable and Robust. 3. Large Dynamic Frequency Range. 4. Relatively Inexpensive. 5. Signal can be integrated to measure velocity or displacement. 1. Surface mounted and portable. 2. Self-generating no complex signal conditioning. 1. Direct measurement of shaft of shaft motion/position within journal bearing. 2. Very sensitive to low frequencies down to DC. Boben Anto C
  53. 53. Vibration Transducer Comparison - Disadvantages Disadvantages Accelerometer Velocity Transducer Proximity Probe 1. Requires amplifier electronics. 1. Bulky. 2. Limited Frequency Range (<1.5 kHz). 3. Moving parts potentially wear over time. 1. Limited frequency range (0 to 10 kHz). Practical range 0 to 2 kHz. 2. Permanently mounted (not portable) often difficult to replace. 3. Conditioning electronics required and interface panel must be housed in non-hazardous area. Boben Anto C
  54. 54. Vibration Transducer Comparison - Applications Applications Accelerometer Velocity Transducer Proximity Probe 1. Machines with rolling element bearings. 2. Gearbox Fault Diagnosis. 3. Heavy rigid rotors with light casing/foundations. 4. Highly utilised with portable handheld data collectors. 1. Portable transducer for measurement of low speed machines. 1. Machines with journal bearings. 2. Machines with lightweight high speed rotors in heavy casing/foundations. 3. Measurement of radial shaft vibration and axial shaft position. 4. Keyphasor (phase reference device). 5. Speed reference. Boben Anto C
  55. 55. ISO & API Standards Boben Anto C
  56. 56. Alarm Setting Guidelines American Petroleum Institute (API) and International Standards Organisation (ISO) have produced guidelines as to acceptable levels of vibration based upon generic machine types. These guidelines are used in acceptance testing, e.g. during commissioning, to ascertain if a new machine is fit for purpose. They are also useful for reference purposes but it should be noted that alarm settings for condition monitoring purposes should be set on a machine by machine basis taking into account historical data. The rate at which vibration levels and characteristics deteriorate over time is as important as the magnitude of vibration i.e. a high level of vibration that remains stable over time may be less cause for concern than a lower level of vibration which shows a deteriorating trend or changing characteristics. In routine condition monitoring we are looking for deterioration in vibrations levels not just absolute values. Boben Anto C
  57. 57. ISO-10816-1 Vibration Standard ISO-10816-1 – Mechanical Vibration – Evaluation of Machinery Vibration by Measurements on Non-Rotating Parts is the most commonly referenced standard in routine condition monitoring for the evaluation of overall vibration levels taken on machine casings.  The standard provides general guidelines for the severity of overall casing (i.e. bearing cap) vibration levels based upon Machine Classes I, II, III and IV. These classes are define by power rating and stiffness of the mounting arrangement (i.e. rigid or relatively soft mounts).  The severity of vibration is classified into Evaluation Zones A, B, C and D to quantify if the level of vibration is acceptable for long term operation of the machine. Boben Anto C
  58. 58. Extract from ISO-10816-1 – Machine Classifications The machine classifications are as follows: Class I: Individual parts of engines and machines, integrally connected to the complete machine in its normal operating condition. (Production electrical motors up to 15 kW are typical examples of machines in this category). Class II: Medium sized machines (typically electrical motors with 15 kW to 75 kW output) without special foundations, rigidly mounted engines or machines (up to 300 kW) on special foundations. Class III: Large prime-movers and other large machines with rotating masses mounted on rigid and heavy foundations which are relatively stiff in the direction of vibration measurements. Class IV: Large prime-movers and other large machines with rotating masses mounted on foundations which are relatively soft in the direction of vibration measurements (for example, turbo generator sets and gas turbines with outputs greater than 10 MW). [Note: soft or special foundations refer to anti-vibration mounts] Boben Anto C
  59. 59. Extract from ISO-10816-1 - Evaluation Zones The following typical evaluation zones are defined to permit a qualitative assessment of the vibration on a given machine and to provide guidelines on possible actions: Zone A: The vibration of newly commissioned machines would fall within this zone. Zone B: Machines with vibration within this zone are normally considered acceptable for unrestricted long-term operation. Zone C: Machines with vibration within this zone are normally considered unsatisfactory for long-term continuous operation. Generally, the machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial action. Zone D: Vibration values within this zone are normally considered to be of sufficient severity to cause damage to the machine. Boben Anto C
  60. 60. Extract from ISO-10816-1 – Typical Zone Boundary Limits ISO-10816-1 - Typical Zone Boundary Limits Vibration Velocity mm/s (RMS) Class I Class II Class III Class IV 0.28 A A A A 0.45 0.71 1.12 B 1.8 B 2.8 C B 4.5 C B 7.1 D C 11.2 D C 18 D28 D 45 Boben Anto C
  61. 61. API Standards The American Petroleum Institute (API) has produced numerous standards to satisfy the specific needs of the petroleum, chemical and gas industries. These standards closely specify the detailed design, inspection and testing of generic machine types, for example:  API 610 – Centrifugal Pumps  API 611 – General Purpose Steam Turbines  API 612 – Special Purpose Steam Turbines  API 613 – Special Purpose Gear Units  API 616 – Gas Turbines  API 617 – Axial and Centrifugal and Expander Compressors  API 618 – Reciprocating Compressors  API 619 – Rotary-Type Positive Displacement Compressors  API 670 - Vibration, Axial Position and Bearing Temperature Monitoring Systems  API 674 – Positive Displacement Pumps – Reciprocating  API 676 – Positive Displacement Pumps – Rotary  API 677 – General Purpose Gear Units  API 681 – Liquid Ring Pumps Boben Anto C
  62. 62. API Standards – Guidelines for Vibration API 670 „Vibration, Axial Position and Bearing Temperature Monitoring Systems‟ specifies requirements for the supply, installation and calibration of radial shaft vibration and axial-position transducers and bearing temperature sensors for online machinery protection systems. The various machine specific standards give guidance as to acceptable levels of vibration. The following equation is common to several of the API standards and defines the maximum allowable level of relative shaft vibration (i.e. vibration measured using a proximity probe): N A 000,12 4.25  Where: A = amplitude of unfiltered vibration, in micrometers true peak-to-peak N = maximum continues speed, in resolutions per minute e.g. Gearbox vibration shall not exceed 50 micrometers (peak-to-peak) or that defined by the above equation, whichever is less [API 613]. e.g. Centrifugal Compressor vibration shall not exceed 25 micrometers (peak-to- peak) or that defined by the above equation, whichever is less [API 617]. Boben Anto C
  63. 63. Machinery Fault Diagnosis Boben Anto C
  64. 64. Section 4.1: Imbalance Boben Anto C
  65. 65. Imbalance  Imbalance is one of the most common causes of excessive vibration in rotating machinery.  It is always characterised as radial vibration at the 1X running speed (rotational speed) of the shaft.  Dependent upon the relative support stiffness, radial vibration may be more prominent in the horizontal or vertical axis.  It is often misdiagnosed as many faults exhibit 1X running speed characteristics - other vibration symptoms should be investigated before balancing is attempted. Imbalance (also known as unbalance) occurs when there is a deviation between the geometric centre of a rotor and its centre of mass. Or put more simply – when there is a heavy spot on the shaft. Boben Anto C
  66. 66. Static (Force) Imbalance  Is characterised by a dominant 1X running speed component.  Is measured in the radial direction and is in-phase.  Is corrected by one balance weight in one plane at Rotor centre of gravity (single plane balance). Boben Anto C
  67. 67.  Is again characterised by a dominant 1X running speed component.  Will be 180o out of phase on same shaft and can exhibit both high axial and radial vibration.  Is corrected by applying balance weights in more than one plane (multi-plane balance). Couple Imbalance Boben Anto C
  68. 68. Dynamic Imbalance A rotor with static imbalance can be diagnosed when the machine is not running. This is carried out by placing the rotor in frictionless bearings. If the rotor has a heavy spot it will rotate within the bearings until the heavy spot is at the bottom. Conversely a rotor with pure coupled imbalanced will not rotate when placed in frictionless bearings and will only manifest itself when the machine is running. In practice a rotor is likely to have a combination of static and couple imbalance which is collectively referred to as Dynamic Imbalance. Boben Anto C
  69. 69.  Is again characterised by a dominant 1X running speed component.  Will show a high axial 1X running speed component.  Axial vibration readings tend to be in-phase whereas radial readings may be unsteady. Overhung Rotor Imbalance Boben Anto C
  70. 70.  Characterised by a dominant 1X running speed radial vibration.  Will show highest vibration at the motor non-drive end irrespective of source. Vertical Rotor Imbalance Note: Vertically mounted pumps will often show large 1X running speed vibration at the motor non-drive end for a number of faults (e.g. pump bush wear, flow turbulence). Try to isolate the problem by uncoupling the motor/pump. Carry out measurements on motor whilst uncoupled. If 1X vibration is still relatively high the motor is at fault if not it is the pump. Boben Anto C
  71. 71. Fan and Overhung Imbalance  Fan imbalance is characterised by a dominant 1X running speed component in the radial direction.  Overhung fan imbalance is characterised by a dominant 1X running speed component in the axial direction.  Ensure the fan blades are clean and show no signs of damage. Often the cause of imbalance can be a build up of deposits on the fan blades.  Fan balancing can often be carried out in situ. The impeller can normally be balanced by applying a single weight. Imbalance can be very common in fans but should not be mistaken for belt drive problems which can also reveal 1X running speed vibration. Boben Anto C
  72. 72. Imbalance Severity Manufacturer‟s and API Standards may impose acceptable levels of imbalance for specific machine types. The following is a rough guideline to the severity of imbalance relating to the 1X running speed vibration component of vibration (for machines running between 1800 and 3600 RPM). Very high speed machines will have lower tolerance levels as the forces generated by imbalance, increase with machine speed. Severity of Imbalance Guidelines for Machines Running at 1800 to 3600 RPM 1X Vibration Level Diagnosis Repair Priority VdB (US) re 10E-8 m/s (rms) Equivalent mm/s (rms) <108 0 to 2.5 Slight Imbalance No Recommendation 108 to 114 2.5 to 5.0 Moderate Imbalance Desirable 114 to 124 5.0 to 15.8 Serious Imbalance Important >124 >15.8 Extreme Imbalance Mandatory Boben Anto C
  73. 73. Effects of Imbalance  All machines inherently have some form of residual imbalance.  Some slight imbalance will have little effect on the operating lifespan of a machine or its components.  An unacceptable level of imbalance can severely reduce the lifespan of bearings and seals.  A high level of imbalance can have catastrophic effects for large machinery with flexible rotors (running above shaft critical speeds). Boben Anto C
  74. 74. Misalignment Boben Anto C
  75. 75. Misalignment  Misalignment can be parallel (offset) or angular which will be diagnosed by whether the vibration characteristics are dominant in the radial or axial planes respectively. It can, however, be a combination of both.  Misalignment introduces a static preload force into the coupled shafts.  It is typically characterised by 1X, 2X and 3X running speed vibration components.  Misalignment characteristics may also indicate coupling problems.  Misalignment will be effected by thermal and dynamic growth and may manifest itself more prominently once the machine reaches its steady state operating condition. Misalignment occurs when the axis's of coupled machine components are not collinear. Boben Anto C
  76. 76. Parallel (Offset) Misalignment  Parallel misalignment is characterised by dominant 1X and 2X and to a lesser extent 3X running speed components of vibration in the radial direction.  Approaches 180o out of phase across the coupling in the radial direction. Boben Anto C
  77. 77. Angular Misalignment Boben Anto C
  78. 78. Cocked Bearing A cocked bearing is a form of misalignment which can generate high axial vibration, characterised by high 1X, 2X and 3X running speed characteristics.  Will show 180o phase shifts (in the axial plane) top/bottom and/or left/right on the same bearing housing.  Realignment or balancing will not cure the problem - the bearing will need to be removed and reinstalled correctly. Boben Anto C
  79. 79. Mechanical Looseness and Rotor Rub Boben Anto C
  80. 80. Mechanical Looseness Mechanical looseness is a common cause of high vibration and is by far one of the easiest problems to check.  Mechanical looseness should ALWAYS be checked before more intrusive maintenance activities are considered.  Mechanical looseness often reveals high 1X vibration.  Mechanical looseness can exhibit high 1X, 2X and 3X times running speed components of vibration which can be misinterpreted as misalignment.  A raised noise floor or ½ x multiples of running speed harmonics of vibration can sometimes be associated with looseness.  The Technical Associates of Charlotte define mechanical looseness in three categories; A: Structural, B: Fasteners and C: Component Fits. Boben Anto C
  81. 81. Mechanical Looseness – Type A: Structural “Type A is caused by Structural looseness/weakness of machine feet, base plate or foundation; also by deteriorated grouting, loose hold-down bolts at the base; and distortion of the frame or base (i.e., soft foot). Phase analysis may reveal approximately 90° - 180° phase difference between vertical measurements on bolt, machine foot, base plate, or base itself.” - Technical Associates of Charlotte, Inc. Boben Anto C
  82. 82. Mechanical Looseness – Type B: Fasteners “Type B is generally caused by loose pillowblock bolts, cracks in frame structure or in bearing pedestal.” - Technical Associates of Charlotte, Inc. Boben Anto C
  83. 83. Mechanical Looseness – Type C: Component Fits “Type C is normally generated by improper fit between component parts. Causes a truncation of time waveform and a raised noise floor in the spectrum. Type C is often caused by a bearing liner loose in its cap, a bearing loose turning on its shaft, excessive clearance in either a sleeve or rolling element bearing, or a loose impeller on a shaft, etc. Type C Phase is often unstable and may vary widely from one measurement to next, particularly if rotor shifts position on shaft from one startup to next. Mechanical Looseness is often highly directional and may cause noticeably different readings comparing levels at 30° increments in radial direction all the way around one bearing housing. Also, note that looseness will often cause subharmonic multiples at exactly 1/2 or 1/3 RPM (.5X, 1.5X, 2.5X, etc.). .” - Technical Associates of Charlotte, Inc. Boben Anto C
  84. 84. Rotor Rub “Rotor Rub produces similar spectra to Mechanical Looseness when rotating parts contact stationary components. Rub may be either partial or throughout the entire shaft revolution. Usually generates a series of frequencies, often exciting one or more resonances. Often excites integer fraction subharmonics of running speed (1/2, 1/3, 1/4, 1/5,...1/n), depending on location of rotor natural frequencies. Rotor rub can excite many high frequencies (similar to wide-band noise when chalk is drug along a blackboard). It can be very serious and of short duration if caused by shaft contacting bearing babbitt. A full annular rub throughout an entire shaft revolution can induce "reverse precession" with the rotor whirling at critical speed in a direction opposite shaft rotation (inherently unstable which can lead to catastrophic failure). - Technical Associates of Charlotte, Inc. Boben Anto C
  85. 85. Rolling Element Bearings Boben Anto C
  86. 86. Rolling Element (Frictionless) Bearing Faults Rolling element bearing faults are characterised by discrete (non-synchronous) frequency vibration components which are dependent upon the bearing construction. Prism4 includes a database of the most common bearing tags and can compute these frequencies through it‟s Frequency Analysis Module (FAM). Boben Anto C
  87. 87. Deep Groove Ball Bearing Components Seal Rolling elements Inner ring Outer ring Cage Seal Boben Anto C
  88. 88. Rolling Element Bearing Stages of Failure The very early stages of bearing faults are detected at very high frequencies using spike energy, shock pulse or HFD (High Frequency Detection) techniques. Boben Anto C
  89. 89. Rolling Element Bearing Stages of Failure The development of bearing faults can be tracked using acceleration enveloping techniques. Final stages of bearing faults will become evident in the vibration velocity spectra. Boben Anto C
  90. 90. Acceleration Enveloping – How it Works?  All low frequency vibration components which are attributed to mechanical faults such as imbalance, misalignment, mechanical looseness etc. are filtered out.  Bearing fault frequencies are non-synchronous components of vibration i.e. they are not exactly 1X, 2X etc. running speed multiples. They may however be very close to a multiple of running speed (e.g. 4.9X running speed) and thus difficult to separate out in an unfiltered signal. Enveloping inherently provides this filtering.  Very sensitive to the onset of bearing faults. By the time a fault is visible in a vibration spectra, the bearing is already significantly worn. The principles behind acceleration enveloping: Boben Anto C
  91. 91. Acceleration Enveloping – How it Works? Time waveform illustrating dominant 1X running speed vibration with frequent impacts (the transient superimposed on the waveform is like the ringing of a bell): Boben Anto C
  92. 92. Firstly the low frequency components of vibration are removed using a high pass filter: Acceleration Enveloping – How it Works? Boben Anto C
  93. 93. The resulting filtered signal contains only the high frequencies with the lower frequencies removed: Acceleration Enveloping – How it Works? Boben Anto C
  94. 94. The time waveform would now only show the bearing transient impacts (note the 1X running speed waveform has been filtered out): Acceleration Enveloping – How it Works? Boben Anto C
  95. 95. The signal is now demodulated so that the high frequencies are flipped over into the baseband of the frequency scale: Acceleration Enveloping – How it Works? Boben Anto C
  96. 96. The negative components of the signal waveform are flipped over to the positive portion of the signal: Acceleration Enveloping – How it Works? Boben Anto C
  97. 97. A low pass filter is now applied to remove any unwanted signals from other sources of modulation: Acceleration Enveloping – How it Works? Boben Anto C
  98. 98. The high frequency components are now removed. This is the enveloped signal: Acceleration Enveloping – How it Works? Boben Anto C
  99. 99. Acceleration Enveloping - Analysis The analysis on the remaining spectra is based upon trending of the frequency peaks against the noise floor: Boben Anto C
  100. 100. Acceleration Enveloping - Considerations Acceleration enveloping has become the favoured technique for assessing the onset and deterioration of rolling element bearing faults. It is both highly sensitive and easy to trend, however, great care should be taken in interpreting the results as:  The measurement location/technique needs to be highly repeatable. It is advised to stud mount the accelerometer. Swapping between stud and magnetic mounted or handheld measurements will produce highly spurious results/trends.  The measured characteristics are analysed as deterioration relative to the signature of a new bearing. Starting measurements well into a bearings lifespan provides a less reliable reference point.  Acceleration enveloping can give an indication of insufficient lubrication which could be misinterpreted as poor bearing condition. Boben Anto C
  101. 101. Acceleration Enveloping - Guidelines  There are basic guidelines to unacceptable levels of acceleration enveloping (gE); based upon machine speed and shaft diameter.  These should be used with caution as it is the relationship between the discrete bearing fault frequencies and the noise floor (often termed „carpet level‟) which provides the best indication of bearing condition.  This relationship will differ dependent on the stage of bearing wear which is why it is always important to gather baseline data when a new bearing is fitted. Boben Anto C
  102. 102. Acceleration Enveloping Guidance Levels: Boben Anto C
  103. 103. Section 4.5: Journal Bearings Boben Anto C
  104. 104. Journal Bearing Faults Journal Bearings also known as Sleeve, Plain, Fluid Film or White Metal Bearings show very different fault characteristics to rolling element bearings. The most common Journal bearing faults are:  Wear and Clearance Problems Due to improved tolerances in Journal Bearing design the following faults are nowadays less common but can still cause catastrophic effects:  Oil Whirl Instability  Oil Whip Instability NOTES: 1.Wear, Clearance and Oil Whirl problems can be detected in steady state vibration spectra, whereas Oil Whip is more likely to occur during machine start-up, which requires more advanced transient analysis. 2. Acceleration Envelope and HFD/Spike Energy/Shock Pulse measurement techniques are NOT applicable to Journal Bearings. Boben Anto C
  105. 105. Journal Bearing Wear and Clearance Problems Journal bearing wear and clearance problems show very similar symptoms to mechanical looseness and are identified by strong running speed harmonics: Wiped journal bearings will often show high vertical vibration compared with the horizontal measurement. Boben Anto C
  106. 106. Oil Whirl Instability Oil whirl can be characterised by vibration frequencies just below, but never equal to, half times running speed: Changes in lube oil viscosity, lube oil pressure and external preloads can all effect oil whirl. Boben Anto C
  107. 107. Oil Whip Instability “Oil Whip may occur if machine operated at or above 2X rotor critical frequency. When rotor brought up to twice critical speed, whirl will be very close to rotor critical and may cause excessive vibration that oil film may no longer be capable of supporting. Whirl speed will actually "lock onto" rotor critical and this peak will not pass through it even if machine is brought to higher and higher speeds. Produces a lateral forward processional subharmonic vibration at rotor critical frequency. Inherently unstable which can lead to catastrophic failure.” – Technical Associates of Charlotte, Inc. Boben Anto C
  108. 108. Journal Bearings Journal bearings are often fitted to large machinery with online protection systems. Alert and trip setting will be set for vibration, axial displacement and bearing temperatures. When a journal bearing wipes both the vibration and temperature will increase instantaneously, most likely tripping the machine. Boben Anto C
  109. 109. Gear Analysis Boben Anto C
  110. 110. Gear Faults When analysing gears (e.g. helical, spur, worm, bevel, epicyclic) gear mesh frequencies can be calculated from:  Input & Output Shaft Speed  Number of Teeth on Pinion & Wheel (Note: For a two stage gearbox the shaft speed and teeth of the intermediate gears would also be required). When collecting vibration data on gearboxes, where possible, time waveform data should be captured along with the FFT Spectra, in at least one axis. Theoretically Acceleration Envelope techniques can be applied to gear analysis, they should be utilised with caution however, as the transmission path between the meshing gears and vibration measurement location is often indirect (especially on large gearboxes). Boben Anto C
  111. 111. Gear Mesh Frequency Gear mesh frequency is defined as the number of teeth on a gear multiplied by its shaft rotating frequency: Gear mesh frequency = (Low Speed Shaft RPM / 60) x number of teeth on wheel Or (High Speed Shaft RPM / 60) x number of teeth on pinion [NOTE: The RPM has been divided by 60 to convert it into Hertz] When analysing gearboxes it is essential that data is captured using a suitable frequency range to capture up to 3.25 x gear mesh frequency in order to assess gear misalignment issues. We would normally capture a vibration velocity measurement with a frequency range up to 5 kHz to capture gear natural frequencies and an acceleration reading up to 20 kHz to capture harmonics of gear mesh frequency. Boben Anto C
  112. 112. Gear Mesh Characteristics A typical gearbox vibration spectrum will show low speed and high speed shaft running speed components accompanied by low amplitude gear mesh frequency with shaft running speed sidebands.  The highest level of vibration will be either radial or axial dependent upon the type of gear e.g. spur or helical gear.  The time waveform should show evenly spaced impulses of similar amplitude for a healthy gearbox. A pulse is produced as each tooth meshes.  The time waveform is often easier to analyse than the vibration spectra in the diagnosis of gear faults. Boben Anto C
  113. 113. Gear Tooth Wear When gear teeth start to wear the sidebands of gear mesh frequency become more pronounced – the amplitude and number of sidebands will increase. Gear natural frequencies will also be excited.  The gear mesh frequency sidebands will correspond to the gear with the wear e.g. if the sidebands are equal to the high speed shaft running speed it will be the pinion gear teeth which exhibit wear.  The gear natural frequencies are lower than gear mesh frequency and will also exhibit sidebands relating to the bad gear. Boben Anto C
  114. 114. Gear Tooth Load Gear mesh frequencies can be very sensitive to load. High gear mesh frequencies do not necessarily indicate a problem provided sideband frequencies remain at low amplitudes and gear natural frequencies are not excited.  In order to trend gear mesh activity, vibration measurements should be recorded with the machine operating at the same load each survey (wherever possible).  Machine load should be recorded each survey as a manual entry reading. Boben Anto C
  115. 115. Gear Eccentricity and Backlash Relatively high sidebands around gear mesh frequency can indicate eccentricity, backlash or non-parallel shafts. The bad gear will be indicated by the spacing of the sideband frequencies.  Eccentricity will normally show a high 1X running speed component of vibration.  Improper backlash often excites gear mesh harmonics and gear natural frequencies.  Gear mesh frequency amplitudes will often reduce with increasing load if backlash is the problem. Boben Anto C
  116. 116. Gear Tooth Misalignment Gear tooth misalignment is diagnosed in a similar manner to angular or parallel misalignment except it is 1X, 2X and 3X gear mesh frequency which reveals the symptoms.  In order to assess for gear tooth misalignment vibration measurements must be taken with a frequency range > 3.25 x gear mesh frequency.  Gear tooth misalignment will cause uneven tooth wear. NOTE: A loose fit journal bearing can also exhibit high 1X, 2X and 3X times gear mesh frequency vibration. Boben Anto C
  117. 117. Gear Cracked or Broken Tooth A cracked or broken tooth is best diagnosed in the time waveform which will show a large impulse every time the problem tooth tries to mesh with the teeth on the mating gear.  The frequency spectra will reveal gear natural frequencies.  The time waveform will reveal high amplitude 1X running speed component of the problem gear. These spikes will reveal themselves in the time waveform up to 10 to 20 times higher than in the frequency spectrum.  In the example opposite (gear with 12 teeth) the time waveform reveals a very large impulse for the cracked/broken tooth. Boben Anto C
  118. 118. Gear Hunting Tooth Hunting tooth problems occur due to faults on both the gear and pinion created during manufacture or improper handling. A hunting tooth problem can often be overlooked as it is revealed at very low frequencies, often less than 10 Hz.  A gearbox with a hunting tooth problem may emit a „growling‟ sound.  The effect is at its worst when the faulty gear and pinion teeth try to mesh at the same time. This may only occur once every 10 to 20 revolutions.  The number of teeth on a gear are often a prime number to avoid hunting tooth problems i.e. two imperfect teeth will not repeatedly mesh. Boben Anto C
  119. 119. Hydraulic and Aerodynamic Faults Boben Anto C
  120. 120. Hydraulic and Aerodynamic Faults Hydraulic and Aerodynamic forces in pumps, fans and compressors will produce vibration at Blade Pass or Vane Pass frequency. Blade pass frequency can be calculated by multiplying the number of blades or vanes by the shaft rotational speed: Blade Pass Frequency = Number of Blades x (RPM / 60) Hz Vane Pass Frequency = Number of Vanes x (RPM / 60) Hz Blade pass frequency is an inherent characteristic of the machine which will vary with process conditions and does not normally cause a problem. Boben Anto C
  121. 121. Blade Vane Faults Large blade pass frequencies are generated if the gap between the rotating vanes and stationary diffusers is not equal all the way round i.e. it is eccentric.  High blade pass frequencies, with 1X running speed sidebands, can be generated if an impeller wear ring seizes on the shaft or if welds which fasten impeller vanes fail.  High blade pass frequencies can also be generated by flow disturbance or the eccentric positioning of the pump or fan rotor within its housing. NOTE: Blade pass frequency fluctuates significantly with process condition so a deteriorating trend must be established before intrusive maintenance is considered. Closely monitor for increases in the blade pass frequency sidebands. Boben Anto C
  122. 122. Flow Turbulence Flow turbulence is caused by variations in the pressure or velocity of air passing through a fan or blower.  Flow turbulence will normally exhibit sub-synchronous (below 1X running speed) random noise in the vibration spectra.  Excessive turbulence can also exhibit broadband high frequency vibration i.e. an increase in noise floor above the blade pass frequency. Boben Anto C
  123. 123. Cavitation Cavitation is normally caused by insufficient suction pressure (starvation) or inlet flow and normally generates random high frequency broadband vibration which appears as a raised noise floor above the blade pass frequency.  Cavitation can be audible – sounding like gravel is passing through the pump.  Cavitation can cause erosion of pump internals and impellers if left uncorrected.  Cavitation may vary from one survey to the next but can often be overcome by increasing the pump suction pressure. Boben Anto C
  124. 124. Electrical Faults Boben Anto C
  125. 125. Electrical Faults Mechanical faults such as imbalance, misalignment and bearing problems are typically more common in electric motors than electrical faults. Electrical fault frequencies may be present in a vibration spectra at low levels, which could be a characteristic of the machine and not likely to cause long term detrimental effects. One of the simplest ways to identify whether a vibration component is mechanical or electrical in origin is to shutdown the power to the machine whilst recording real time vibration. If the fault frequency immediately disappears when the power is switched of the problem is electrical but if the faults frequency reduces with running speed the fault is mechanical in origin. Boben Anto C
  126. 126. AC Induction Motors 3-Phase AC Induction Motors are the most common motors utilised in industrial applications due to their relatively high efficiencies.  A motor with a 60 Hz line frequency and 2 stator poles will run at a speed of 3600 RPM (or 3000 RPM with a 50 Hz line frequency). A motor with a 60 Hz line frequency and 4 stator poles will run at a speed of 1800 RPM (or 1500 RPM with a 50 Hz line frequency).  In an induction motor the motor speed is always slightly less than synchronous speed. The difference between the actual speed and the synchronous speed is known as Slip. The difference between the running speed frequency and synchronous speed frequency is known as the Slip Frequency.  The greater the slip, the greater the induced current in the rotor bars and the greater the output torque. This is why the actual speed of an induction motor will vary slightly with load. Boben Anto C
  127. 127. AC Induction Motors – Stator Eccentricity Stator problems generate high 2X line frequency components of vibration i.e. 100 Hz or 120 Hz dependent upon whether the line frequency is 50 Hz or 60 Hz respectively. Stator eccentricity produces a uneven stationary air gap between the rotor and stator which produces very directional vibration.  Differential air gap should not exceed 5% for induction motors and 10% for synchronous motors.  Soft foot and warped bases can produce eccentric stators.  Shorted stator windings can produce thermally-induced vibration which can significantly increase with operating time causing stator distortion and air gap problems. NOTE: Electric motors will have a low level of 2X line frequency vibration as a normal characteristic. Boben Anto C
  128. 128. AC Induction Motors – Eccentric Rotor An eccentric rotor will produce a variable air gap between the rotor and stator producing pulsating vibration. Eccentric rotors produce 2X line frequency components with pole passing frequency sidebands.  Pole passing frequency = slip frequency X numbers of poles.  Not to be confused with soft foot or misalignment which can produce variable air gaps due to distortion (mechanical problem not electrical).  Zoom analysis may be required to separate 2X line frequency from 2X running speed harmonics.  On high voltage motors; Motor Phase Current Analysis can be used to assess rotor eccentricity. Boben Anto C
  129. 129. AC Induction Motors – Rotor Bow Uneven heating of a rotor due to unbalanced rotor bar current distribution can cause a rotor to warp or bow. Rotor bow can be misdiagnosed as mechanical imbalance as it has similar 1X running speed characteristics.  Rotor bow can be distinguished from imbalance as it will worsen when the motor is hot and the symptoms will subside when the motor cools down.  If the local heating effect is very severe it can cause the offending rotor bar to melt and lodge within the air gap. Boben Anto C
  130. 130. AC Induction Motors – Broken or Cracked Rotor Bars Broken/Cracked rotor bars or shorting rings, bad joints between shorting rings and rotor bars or shorted rotor laminations will produce high levels of 1X running speed vibration harmonics with pole pass frequency sidebands.  High resolution measurements are required typically using a 3200 line FFT spectra.  Running speed harmonics may be notable to 5X running speed and above.  On high voltage motors; Motor Phase Current Analysis is often employed to assess for broken rotor bars. Boben Anto C
  131. 131. AC Induction Motors – Loose Rotor Bars Loose rotor bars will exhibit a peak at rotor bar passing frequency (the number of rotor bars times the motor RPM) with 2X line frequency sidebands.  A relatively high frequency measurement is required to detect loose rotor bars as the rotor passing frequency is often over 2000 Hz.  Even if the number of rotor bars is unknown a high frequency vibration component exhibiting 2X line frequency sidebands is most likely caused by loose rotors. Boben Anto C
  132. 132. AC Induction Motors – Phasing Problems Phasing problems due to loose or broken connectors can result is excessive 2X line frequency vibration with 1/3 line frequency sidebands.  Very high 2X line frequency vibration levels in excess of 25 mm/s can result if the problem is left uncorrected.  The problem is accentuated if the defective connector makes intermittent contact.  The loose or broken connector must be repaired to avoid catastrophic failure. Boben Anto C
  133. 133. Belt Drive Faults Boben Anto C
  134. 134. Belt Drive Faults Belt drives are an inexpensive means of power transmission. They can however be prone to a number of faults including:  Belt Wear  Misaligned Sheaves (Pulleys)  Eccentric Sheaves (Pulleys)  Belt Resonance Boben Anto C
  135. 135. Belt Drive Equations The following equations are useful in determining operating speeds and fault frequencies for belt drives: DiameterSheaveDriven DiameterSheaveDrivingRPMDriving RPMDriven __ ___ _   LengthBelt DiameterSheaveRPMSheavePI FrequencyBelt _ __ _   Where PI = 3.1416 TeethBeltofNumberFrequencyBeltFrequencyBeltgTi ______min  SheaveonTeethofNumberRPMSheaveFrequencyBeltgTi _______min  or Boben Anto C
  136. 136. Worn, Loose or Mismatched Belts Belt frequencies are below both the driving and driven units running speeds. A worn, loose or mismatched belt will exhibit up to 3X or 4X harmonics of belt frequency.  A high amplitude of 2X belt frequency is normally present.  Amplitudes are unsteady and will sometimes fluctuate between the driving and driven unit running speed.  Timing belts will exhibit high amplitudes of timing belt frequency. Boben Anto C
  137. 137. Sheave (Pulley) Misalignment Misaligned sheaves will produce high vibration at 1X RPM predominantly in the axial direction. Often with pulley misalignment, the highest axial vibration on the motor will be at fan RPM, or vice versa.  A high amplitude of 1X belt frequency will be present in the axial direction.  Harmonics of belt frequency may sometimes be present in the axial direction. Boben Anto C
  138. 138. Eccentric Sheaves (Pulleys) Eccentric sheaves will generate high 1X running speed vibration, especially in the axis parallel to the direction of the belts.  This condition is very common and can be misdiagnosed for imbalance.  The 1X running speed of an eccentric sheave will be evident on both the driving and driven unit bearings.  Pulley eccentricity can be confirmed by phase analysis which should reveal horizontal and vertical phase differences close to either 0o or 180o. Boben Anto C
  139. 139. Belt Resonance Belt resonance can reach high amplitudes if the belt natural frequency coincides with either the driving or driven unit running speed.  This condition can be checked by tensioning and then releasing the belt whilst taking vibration readings on the pulleys or bearings.  The natural frequency of the belt can be altered, by either changing the belt length or tension, to correct this problem. Boben Anto C
  140. 140. Section 4.10: Machinery Fault Diagnosis Guidelines Boben Anto C
  141. 141. Machinery Fault Diagnosis Guidelines When carrying out machinery fault diagnosis there is often more than one type of fault that can be associated with the vibration characteristics measured. The following guidelines should be taken into account before making recommendations involving intrusive maintenance:  Vibration levels change with load and process conditions. If a step increase in 1X running speed vibration is evident look for evidence of changes in process conditions. Process parameters should be recorded wherever possible (as manual entry readings) for comparison with historical data.  Pump 1X running speed and blade pass frequency vibration can often fluctuate spuriously due to altering process conditions.  If vibration levels show a marked increase or deteriorating trend; survey more often. By surveying weekly instead of monthly the rate of deterioration can be more easily established. If the vibration levels appear stable revert to monthly monitoring.  Always check for mechanical looseness if 1X, 2X or 3X vibration is present as it is one of the most inexpensive and easiest maintenance actions to carry out. Boben Anto C
  142. 142. Machinery Fault Diagnosis Guidelines Continued  High acceleration envelope levels on rolling element bearings can be associated with lack of lubrication. To check this, grease the bearings and then repeat vibration measurements. The acceleration envelope readings should notably reduce. Repeat the vibration measurements after the machine has been allowed to run for a further 24 hours. If the acceleration envelope readings remain stable, lack of lubrication is confirmed. If the acceleration envelope readings start to increase closely monitor for bearing fault frequencies.  Ensure that the vibration spectra reveals good quality data. A ski-slope in the vibration spectra is an indication of a poorly taken measurement and should be repeated: Boben Anto C
  143. 143. Machinery Fault Diagnosis Guidelines Continued  Ensure measurements are always taken at the same locations. Stud and mark- up the measurement locations wherever possible to ensure the best possible repeatability of measurements.  If a stud should fall off a measurement location immediately replace it. Acceleration envelope measurements are highly dependent upon a repeatable mounting arrangement. If the measurement is changed from stud to magnetic mounting the envelope levels will be inconsistent and the baseline/trend will be lost.  Look for changes in vibration characteristics i.e. frequency component changes as well as changes in overall vibration levels. A fault may not manifest itself as an increase in overall vibration levels. Boben Anto C
  144. 144. Section 6: Supplementary Condition Monitoring Techniques Boben Anto C
  145. 145. Supplementary Condition Monitoring Techniques Vibration analysis is a very powerful technique in the diagnosis of many common rotating machinery problems. Additional condition monitoring techniques can be employed in support of vibration analysis and to provide supplementary information regarding both machinery and plant condition. Such techniques include:  Lube Oil Analysis  Thermography  Watch Keeping  Motor Phase Current Analysis  Rogowski Coil Analysis  Performance Monitoring Boben Anto C
  146. 146. Lube Oil Analysis Lube oil analysis serves two main purposes:  To assess oil quality to ensure the oil is fit for further use i.e. meets the lubricating requirements for the machine  To assess machine condition by examining the oil for signs of mechanical wear Boben Anto C
  147. 147. Lube Oil Sampling Guidelines Care should be taken when collecting lube oil samples to ensure the oil sample is representative of the oil circulating in the machine and to ensure that the sample is not contaminated. As a guide:  Oil samples should only be taken from running machines. Cold samples taken from non-running machines will allow any particles contained within the oil to separate and settle. Similarly, when taking samples from piping systems ensure the sample is taken from a location where oil is circulating e.g. a bend where flow is more turbulent as opposed to straight pipe where flow is laminar.  The sample must be taken from the same location each survey. Note: placing a sampling tube too far into oil reservoir is likely to collect the debris at the bottom of the tank which must be avoided.  Samples must be taken upstream of any filters.  Always use fresh tubing and bottles for each sample and ensure they are properly stored to avoid contamination.  Reference oil samples should be taken whenever a new batch of oil is utilised. Boben Anto C
  148. 148. Lube Oil Analysis – Laboratory Analysis Lube oil analysis is carried out on a periodic basis to compare the chemical and elemental properties of a used oil to a baseline of unused oil. Over time a trend is built up to determine the rate of deterioration of oil quality or abnormalities which could be indicative of mechanical wear. Standard tests include:  Viscosity – too low a viscosity reduces oil film strength, weakening its ability to prevent metal-to-metal contact.  Spectrographic Analysis – measures the concentration (normally in parts per million (ppm)) of elements (e.g. lead, copper, sodium etc.) entrained in the oil to determine wear metals, contaminants and additives.  Total Acid Number (TAN) – is used to measure the acidic content of the oil.  Total Base Number (TBN) – indicates the ability of an oil to neutralise acid. TBN is an important test for diesel engines. A low TBN can indicate overdue oil changes and overheating. Boben Anto C
  149. 149. Lube Oil Analysis – Laboratory Analysis Continued  Water Content – is a standard test used to assess the percentage of water within the oil sample. Water content should not normally exceed 0.1%.  PQ Index (Particle Quotient) – gives an indication of ferrous wear debris in the oil sample. The PQ Index can be easily trended over time and is normally carried out as part of a standard analysis. A high PQ Index can indicate that wear is present but ferrographic analysis is required to identify the type of wear.  Ferrographic Analysis (also known as Wear Debris Analysis) – is a relatively expensive test in comparison to standard analysis. It is used to assess the size, shape and number of wear particles suspended in the oil sample. The size and shape of the wear particles can be associated to specific wear modes indicative of mechanical faults. Boben Anto C
  150. 150. Lube Oil Analysis – Example Laboratory Report Part 1 Boben Anto C
  151. 151. Lube Oil Analysis – Example Laboratory Report Part 2 Boben Anto C
  152. 152. Lube Oil Analysis – Example Laboratory Report Part 3 Boben Anto C
  153. 153. Thermography Thermography is a highly utilised technique in the condition monitoring of electrical switch gear but can also be used as an indicator of mechanical wear. It works on the principle that temperature changes occur as the condition of components alter e.g. electrical arcing and bearing wear. Thermography measures infrared radiation emitted from different materials to allow the remote (non-contact) measurement of temperature and temperature differences. Special viewing panels are often fitted to electrical switch gear enclosures to allow thermographic survey; as opening the enclosure would let the heat escape. Boben Anto C
  154. 154. Thermography - Applications Boben Anto C
  155. 155. Watch Keeping Watch keeping is normally carried on a daily basis to record operating and process parameters around the plant. This is an ideal opportunity to walk around any machinery and use your senses to look and listen for any abnormalities in machine condition, for example:  Seal Leaks - vibration analysis cannot diagnose seal leaks however a very quick visual inspection can.  Bearing wear and lack of lubrication – if a machines‟ bearings are worn or inadequately lubricated they can emit audible noise. This can be confirmed by vibration analysis.  Cavitation sounds like gravel is passing through the pump and will emit an audible noise. This can be confirmed by vibration analysis.  Low pump discharge pressure or high motor current readings can indicate a pump is not running efficiently. Use Your Senses! Boben Anto C
  156. 156. Motor Phase Current Analysis Motor Phase Current analysis is used for rotor fault diagnosis of AC induction motors to detect broken rotor bars and air gap eccentricity. The technique is based upon frequency analysis of the phase current supplying the motor.  Motor Phase Current Analysis is generally applicable to large AC induction motors.  Measurements can be carried out using portable equipment by connecting a current clamp to the low voltage side of the power transformer. Boben Anto C
  157. 157. Rogowski Coil Analysis Rogowski coil analysis is used to assess the condition of stator winding insulation in high voltage electrical machines such as power generators. Assessment of condition is based upon the examination of high frequency signals caused by partial discharge activity measured using Rogowski coils.  Rogowski coil analysis is generally undertaken on larger, high voltage machines such as electrical generators and motors of 6.6 kV or greater.  Rogowski coils can be fitted to machines during manufacture on the client‟s request.  Specialist data collection equipment is required to interface to the fitted instrumentation to carry out diagnostics. Boben Anto C
  158. 158. Performance Monitoring Performance monitoring can give an indication of machine condition and running efficiency based upon calculations of measured operating and process parameters. Performance indicators can be utilised to:  Ensure the efficient running of power turbines and compressors; where reduced efficiency can result is substantial financial loss.  Indicate when a gas turbine should be water washed.  Indicate deterioration in machine condition based upon a reducing trend in performance. Boben Anto C
  159. 159. Performance Indicator Example – MOL Pump Calculating the ratio of the energy out of a system to the energy in, will give and indication of the system‟s efficiency. In this example the differential pressure multiplied by the flow rate of a pump was divided by the motor current. Over time it was seen that the pump performance indicator showed a deteriorating trend. The pump was overhauled and found to have badly worn impellers. After overhaul the performance indicator improved. MOL Pump 0.00 2.50 5.00 7.50 10.00 19/03/1999 19/05/1999 19/07/1999 19/09/1999 19/11/1999 19/01/2000 19/03/2000 19/05/2000 19/07/2000 19/09/2000 19/11/2000 19/01/2001 19/03/2001 19/05/2001 19/07/2001 19/09/2001 19/11/2001 19/01/2002 19/03/2002 19/05/2002 19/07/2002 19/09/2002 19/11/2002 Date PerformanceIndicator I QP IndicatorePerformancPump  __ Boben Anto C
  160. 160. Boben Anto C bobenntpc@yahoo.co.uk

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