Hello everyone,

I have a question regarding the gap voltage in a setup we're using to measure radial vibration on a load gearbox shaft for a Fame 6B gas turbine driving a generator. During our recent maintenance activities, we set the gap voltage to -10V and locked it tightly. However, after performing some ratchetting activities, we observed that the gap voltage changed to -12.7V, which was the largest deviation among all sensors.

I’m curious about what could be causing this change in gap voltage. Could it be related to mechanical issues during the ratchetting process, or might there be other factors at play? We have also tested the Mechanical and Electrical Runout for the High Speed Shaft(HSS) and Low Speed Sha

Additionally, I’d like to know if such a change in gap voltage could pose any issues for accurately measuring radial vibration during equipment operation.

Details that might help:
Equipment: GE Frame 6B Gas Turbine
Driver: Alstom Generator
Load Gear Box: Flender
The issue occurs at multiple radial vibration sensors at the Load Gear Box.

Thanks in advance

 

@Zulfaiz,

1) What was the as-found gap voltage of the proximity sensors before they were adjusted to -10V?

2) Was any work done on the load gearbox during the maintenance activities? If so, what was done?

3) Have you considered removing the proximity probes (sensors) and replacing them with new ones--or at least one, possibly two, of the suspect sensors? And/or replacing the proximitor for one or two of the suspect probes?

4) Have you considered using a wobulator to test the suspect probes?

5) Many proximity probes are inadvertently damaged by adjusting them such that the probe tip contacted the shaft. Have you removed the proximity probes to ensure the tip are not damaged?

6) Are you certain all intermediate wiring terminations between the monitor and the load gearbox proximitors are tight and are not loose in the crimp (if crimp connectors were used)? (I'm not talking about "gorilla tight"--just that the terminal board screws are firmly holding the wire/crimp terminal in place.)

 

Bently probes come with "incremental scale factors" and tolerances. Below the particular probe I was testing had an ISF of .2 Volt/mil. Using a dial micrometer, just barely kiss the face of the probe at 0, and back the gap out 10 at a time taking voltage readings along the way. Here you can see my percent scale deviation was within tolerance all the way out until 90. You would need a wobbulator to do the dynamic test, but just a dial micrometer to do the static test.

The problem with BN probes, cables, and prox boxes is that you can accidentally blend probes, cables, and prox boxes that don't have the same ISF. For example, in a static test incorrect cables for the probe would present itself as a non linear result.

-10 to -12 shouldn't affect accuracy. Why did it change? Don't know. Maybe it was set with lift oil off and now lift oil is on. Could be anything.