Turbine Eccentricity

A rotor which has been sitting idle during overhaul or has been inadvertently stopped during coastdown for an extended period will develop a bow or bend.

This condition must be corrected by turning gear operation and, possibly, with auxiliary heating prior to high speed operation to prevent internal clearance rubbing.

Eccentricity is the measurement of Rotor Bow at rotor slow roll which may be caused by any or a combination of Fixed mechanical bow, Temporary thermal bow or Gravity bow


In extreme cases of thermal/gravity bow, caused by a sudden trip of the unit and failure of the turning gear to engage, the rotor may be positioned and stopped 180o out of phase (bow up) to allow gravity to work entirely on the bow and substantially shorten the time required to reduce the bow.

Eccentricity is measured while the turbine is on slow roll (1 to 240 RPM below the speed at which the rotor becomes dynamic and rises in the bearing on the oil wedge) and requires special circuitry to detect the peak- to-peak motion of the shaft.



Assuming uniform stiffness and weight, the rotor mid- span eccentricity may be expressed as the ratio of the transducer span from the bearing over the transducer measured eccentricity to 1/2 the bearing span over the midspan eccentricity or calculated using the following formula, (Tecc x ½Bspan)/Tspan = MSecc.
Where Tecc = Transducer measured eccentricity
Bspan= Bearing Span
Tspan= Transducer span from bearing
MSecc= Midspan eccentricity



Eccentricity is normally measured P/P (Peak to Peak) to agree with previously established conventions. The actual excursion from shaft centerline caused by bow would be one half that measurement or the 0/P (Zero to Peak) measurement.

Theory above can make us confused but lets check on Toshiba's turbine that have excellent vibration reading at 38 µm, good reading at 75µm and at 125µm the shaft need to balancing.

Shaft eccentricity at turning gear speed 6 RPM should no more than 110% from normal value or 55µm (100% = 50µm; 110% = 55µm), (usually reading 14µm). At 400RPM reading eccentricity is 8µm, and the eccentricity reading at 650RPM is not use anymore.