Steady increase in DC gap voltage of one of two radial vibration probes


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Two perpendicular coplanar (BentlyNevada) proximity probes are installed at steam inlet end of a steam turbine. one probe is 45 deg left from vertical and 2nd is at 45 deg right from vertical. (Right and left are when looking along the axis of turbine in the direction of steam flow)

Dc gap voltages of one of these two probes (one at 45 deg left) have steadily increased from -8.9 volts (April 2011) to -5.7 volts (August 2011). No appreciable change has been observed in DC gap voltages of 2nd probe (one at 45 deg right) at the same end.

The amplitude of unfiltered vibration signal at steam inlet end increases with inlet steam flow but its variation pattern is same from April to August