7/31/2011

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

Basic Concepts IV

Basic Rotor and Stator System
          Forces generated in the rotor are transmitted through the bearings and supports to the foundation
           Displacement probe is mounted on the bearing housing which itself is vibrating. Shaft vibration measured by such a probe is, therefore, relative to the bearing housing
           Bearing housing vibration measured by accelerometer or velocity probe is an absolute measurement


Type of Rotor Vibration

          Lateral motion involves displacement from its central position or flexural deformation. Rotation is about an axis intersecting and normal to the axis of rotation
          Axial Motion occurs parallel to the rotor’s axis of rotation
          Torsional Motion involves rotation of rotor’s transverse sections relative to one another about its axis of rotation
          Vibrations that occur at frequency of rotation of rotor are called synchronous vibrations.
          Vibrations at other frequencies are nonsynchronous vibrations







 The Relationship Between Forced and Vibration
          Forces generated within the machine have may different frequencies
           The mobility of the bearings and supports are also frequency dependent. Mobility = Vibration / Force
           Resultant Vibration = Force x Mobility 


 Alternative Measurements on Journal Bearings

          Relative shaft displacement has limited frequency range but has high amplitude at low frequencies – running speed, subsynchronous and low harmonic components
           Accelerometer has high signal at high frequencies – rotor to stator interaction frequencies – blade passing, vane passing


 Types of Machine Vibration




           Shaft relative is measured by  displacement transducer mounted on casing
           Shaft Absolute is the sum of Casing Absolute and Shaft Relative.







Shaft Versus Housing Vibration
Shaft Versus Housing Vibration
(Selecting the Right Parameter) 

          Shaft vibration relative to bearing housing
        Machines with high stator to rotor weight ratio ( For example in syngas comp the ratio may exceed 20)
        Machines with hydrodynamic sleeve bearings
        Almost all high speed compressor trains
          Bearing housing vibration
        Machines with rolling element bearings have no shaft motion relative to bearing housing.
        Rolling Element bearings have zero clearance
        Shaft vibration is directly transmitted to bearing housing
          Shaft absolute displacement
        Machines with lightweight casings or soft supports that have significant casing vibration

 Bearing Housing Vibration
 
          Shaft-relative vibration provides
        Machinery protection
        Low frequency (up to 120,000 CPM) information for analysis
          Many rotor- stator interactions generate high frequency vibrations that are transferred to the bearing housing
        Vane passing frequency in compressors
        Blade passing frequency in turbines
        These frequencies provide useful information on the condition and cleanliness of blades and vanes
          These vibrations are best measured on the bearing housing using high-frequency accelerometers.
        Periodic measurements with a data collector.

 Shaft Rotation and Precession
  
          Precession is the locus of the centerline of the shaft around the geometric centerline
           Normally direction of precession will be same as direction of rotation
           During rubbing shaft may have reverse precession

 IRD Severity Chart
  
          Values are for filtered readings only – not overall
           Velocity is expressed in peak units (not RMS units)
           Severity lines are in velocity
           Displacement severity can be found only with reference to frequency.
           In metric units
           Very rough > 16 mm/sec
           Rough                     > 8 mm/sec
           Slightly rough > 4 mm/sec
           Fair              - 2 – 4 mm/sec  
           Good           - 1 – 2 mm/sec

 

6 comments:

  1. So that is how it works. It does not look so complicated as I thought it is. I find it fascinating.
    babbit bearings

    ReplyDelete
  2. I appreciate all of the information that you have shared. Thank you for the hard work!

    - The Vibration Analysis

    ReplyDelete
  3. I appreciate for all the information you've offered here. Thanks very much!

    ReplyDelete
  4. Your work is very good and I appreciate you and hopping for some more informative posts. Thank you for sharing great information to us. we are waiting for your new post so please carry on.


    Are you looking for the professional Vibration Data Collector Service in Florida at the Lowest Cost? Don't worry Ludeca INC is always with you for help.

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  5. IRD Mechanalysis pioneered much of the early work in this field, its great seeing one of their original vibration charts !

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  6. i. What is the use of this chart and what is its limitation?
    ii. Which vibration parameter is more suitable for deciding vibration severity?
    iii. Determine the vibration severity for a bearing vibration amplitude of 0.80
    mils at speeds of 1200 and 1800 rpm.
    iv. What is your general observation from the chart?
    v. How this chart can be used for condition monitoring?
    Can someone answer me following questions on this graph.

    ReplyDelete