7/31/2011

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

Basic Concept V

Vibration Transducers
 
          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

           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 gap and nothing else.
           Coil at probe tip is driven by oscillator at around 1.5 MHz
           If there is no conducting surface full voltage is returned
           Conducting surface near coil absorbs energy
           Therefore, voltage returned is reduced
           Proximitor output voltage is proportional to gap

 Eddy Current Proximity Probe System
Eddy Current Proximity Probe System Calibration

           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

          Only probe that can measures shaft position – both radial and axial
          Good signal response between DC to 90,000 CPM
          Flat phase response throughout operating range
          Simple calibration
          Rugged and reliable construction
          Suitable for installation in harsh environments
          Available in many configurations
          Multiple machinery applications for same transducer – vibration, position, phase, speed

 Proximity Probe DisAdvantages

          Sensitive to measured surface material properties like conductivity, magnetism and finish
        Scratch on shaft would be read as vibration
        Variation in shaft hardness would be read as vibration
          Shaft surface must be conductive
          Low response above 90,000 CPM
          External power source and electronics required
          Probe must be permanently mounted. Not suitable for hand-holding
          Machine must be designed to accept probes – difficult to install if space has not been provided

 Seismic Velocity Pick-Up IRD 544

           Coil of fine wire supported by low-stiffness springs
           Voltage generated is directly proportional to velocity 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

           All velocity pick ups have low natural frequency (300 to 600 CPM)
           Therefore, cannot measure low frequencies in the resonant range.
           Their useful frequency range is above - 10 Hz or 600 CPM

Advantages of Velocity Pick-Up


          Measures casing absolute motion
          It is a linear self generator with a high output
        IRD 544 pick up – 1080 mv 0-pk / in/sec= 42 mv / mm/sec
        Bently pick up – 500 mv 0-pk / in/sec =  19.7 mv / mm/sec
          High voltage Output
        Can be read directly on volt meter or oscilloscope
        Therefore, readout electronics is much simplified
        Since no electronics needed in signal path, signal is clean and undistorted. High signal to noise ratio
          Good frequency response from 600 to 90,000 CPM 
          Signal can be integrated to provide displacement
Easy external mounting, no special wiring required

 Disadvantages of Velocity Pick-Up

          Mechanically activated system. Therefore, limited in frequency response – 600 to 90,000 CPM
          Amplitude and phase errors below 1200 CPM
          Frequency response depends on mounting
          Large size. Difficult to mount if space is limited
          Potential for failure due to spring breakage.
          Limited temperature range – usually 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.



 






1 comment:

  1. Those are really good features there. Look like it is of high standard items too. Great.
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