During transient measuremts on a steam turbine the rotor rubbed only during shutdown at its critacal speed. The direct unfiltered orbit shows a dwell time of 23ms (38% of one rad./s) this 23 ms was parrallel with a straigt line. Oh yeah, the 0,5X reversed component was 80microns...keyphasor dots where 180 dergrees from eachother.

The next start-up reveals no rub related vibs. We tripped again and suddenly the 0,5X component was like a cirkel (!) and shows also keyphasor dots 180 degrees cross each other. the direct unfiltered orbit amplitude at that time was 200microns.... This was shown on both the bearing positions!! The shaft centerlines are laying in the middel on both bearings.

Question:
The main question is : Why are these orbits looked different at the same speed (4100RPM)? the first orbit had straight dwell time(23ms), the second was like a smooth big cirkel??
(ps. the direct slow roll direct data was 14micron @ both bearings, no scratches where seen in TWF)

You are asking

quote:

Why are these orbits looked different at the same speed

. How about posting some data (graphics) for us to examine?

John from PA

I think the keyphasor dots are not exactly 180 degree apart . So the frequency is <0.5X. May be whirling.
Irshad