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Tip-rotor vibration

By Greg Adams, Arizona

Power plants use proximiters to monitor rotor vibration, many of which weigh hundreds of tons. As the shaft moves around in its bearing clearances, it will move closer to and farther away from the proximity probe. The proximiter converts gap between the end of the probe and the shaft into a DC offset, or gap voltage. Any shaft vibration becomes a sine wave output. The amplitude of the sine wave is proportional to the shaft movement in mils, peak-to-peak. During an overhaul, an I&C tech planned to set the gaps between the probes on a generator by gap voltage alone. This should work. But setting them incorrectly would cost several man-hours plus additional units down during a peak load season. I insisted on "wobbulating" the probes and doing O-scope checks.

Wobbulation replicates a vibrating shaft by means of a beveled rotating disk. Using our 196C ScopeMeter, it became obvious that the amplitude of the sine wave was much greater than the expected 200mV per mil: It was 600mV/mil. This would triple the apparent vibration amplitude and cause a vibration trip at 1/3rd of the actual. In addition to embarrassing down time due to the trip and phantom vibration chase, we'd have to pull the bearings apart again to reposition the probes costing several man-hours plus more down time. But there was no denying it: The 196C told the story in living color. It turns out that the I&C tech had replaced the cable between the proximity probe and its amplifier with a much shorter cable not realizing the consequences. The reduction in cable resistance caused the increase in signal output. But the cost avoidance more than paid for the 196C.