ECCENTRICITY & SHAFT VIBRATION

Eccentricity monitoring can be subdivided into shaftvibration and bent shaft monitoring.Bent shafts normally result when the turbine is stationary and thermal arching or bowing of the shaft occurs or the shaft sags under its own weight.The Turbine is rotated slowly (barring) toprevent this happening or to straighten the shaft after ithas occurred.The accurate monitoring of the shaft,both when at fullspeed and when on barring is therefore vitally important but often requires two techniques to monitor themeffectively.At shaft speeds above 300rpm,conventionaldetection circuits are used but below this speed,analogue meters and recorder traces fluctuate at the frequencybeing measured.Recorder traces become a blur of ink as the pen ‘bands’ at the frequency of the barring speed.Detection circuits can be employed that hold the peakvalue of eccentricity so that a continuous line trace is obtained.In order to monitor gradually decreasingeccentricities,the peak hold function is discharged by The graphs illustrate that misalignment effects are readily ignored through utilising the peak to peak measurements of eccentricity.When higher frequency components are present (e.g hammer marks) the RMS value is more representative.Sensonics have developed a dual path eccentricity moduleto specifically eliminate the effects of marks and dents on the shaft.The module utilises a speed signal derived froma second probe to actively tune the low pass elliptical filter response of the eccentricity unit to remove the highfrequency components of the eccentricity waveform.Thisprovides accurate peak measurements,particularly at low barring speeds and is effective through the full speedrange.To ensure that all the shaft vibration data is captured,probes mounted in the X and Y axis are normally used. Probes are invariably mounted at the 0º and 90º points or at the 315º and 45º points.The eddy current probe measures displacement in theplane of its own axis only.Displacement vibration perpendicular to the probe axis is not measured. 1 %per rev using a tacho signal.One of the difficulties encountered when using shaftdisplacement transducers be they the eddy current probe or the older inductive probes,is the problem of “runout”. Runout is the error signal generated by mechanical, electrical or metallurgical irregularities of the shaft surface.These error signals are generally of a low magnitude in comparison to the vibration signal and are often at a much higher frequency. The 315º and 45º points are used to avoid the half joints of the bearings and to ensure that when bearings are removed the probes are removed along with them.This moves the probes away from possible mechanical damage when the turbine is being worked on.
Eddy current probes monitoring vibration at 45º and 315º Points Measuring eccentricity with runout using RMSor peak to peak detection Measuring lower frequency signals using RMS orpeak detection Typical probe mounting configuration

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