I have a 750 Mw turbine. The Generator side HP bearing is reading 5 to 6 mil with most of it sub-sync.

This is a tilting pad bearing with prox probes.

I also took shaft rider data and it shows very little sub-sync. Just 1.5 mils at running speed (3600 RPM).

I am still thinking oil or steam whirl but why are the shaft rider readings so low?

Thoughts?

Thanks in advance.

New plant; shaft rider and cap data low or reasonable or much better anyway.

I would first do a full calibration check of the eddy current system to ensure integrity. The shaft rider is going with the shaft and unlike the prox probes or cap data isn't a relative to the same degree (apple to orange). In addition: viscosity of the oil and its correct ISO grade for the application as sometimes new plants can suffer getting oil from the wrong drum from 'new' operator.

I would believe the shaft rider data -- assuming you were using a fish tail, wooden dowel or piggybacked on an installed shaft rider -- not plugged into a remote panel. You could have a loose prox probe, a bracket resonance, or some other problem. I would also believe my fingers. You should easily be able to feel 5+ mils of subsynch when using a shaft stick. 1.5 mils to me feels very smooth and is hard for me to even detect without an analyzer.

Michael Titone

Posting a spectrum would help, and orbit would be even more helpful. What is the frequency of the subsynch vibration? Is it a single distinct frequency, or is it a bunch of hash at many frequencies? This may be a case of bad probes, either grounded in the conduit, or wet connectors. If it was whip or whirl I would expect to see similar characteristics at the shaft rider as at the probe.

Shaft riders don't always see the first resonance if it has a low frequency. The frequency or characteristics (as Steve enquired)of the spectra can help clear this up.

The proximity probes will read a lower fequency than the shaft rider will. Compare the low frequency characteristics of the shaft rider to the shaft relative spectra from the proximity probes.

A number of nuclear turbines (1500, 1800 rpm) show unusual low frequency vibration when swithched to proximity probes. The operators may assume that they have a false vibration, when in fact they just didn't know they had this vibration for years. You could have an instability or something similar to these lower speed turbines.

Figured it out.

Both are right. The prox probes are mounted on the bearing and shaft rider is on the bearing housing. The bearing is not set correctly in the housing. Have to wait for shutdown.

This message has been edited. Last edited by: Ron Stiemsma,