Localized Demagnetization of Rotor Journals without removing from Casing ??

[faisalakhan]

Dear Experts;
I have seen and experienced that for demagnetization of Rotor journal areas, complete rotor need to be removed from Casing (Turbine / Compressor) and best practice is to hang it, using overhead crane, to avoid its contact with ground.
Is is also possible to do localized demagnetization of rotor sensitive parts (Journals / Thrust disc etc) if rotor is not removed from casing and we have only inspected the bearings?
I would like to know the suggestion / guidance from all experts in the field and any care if this is possible without any Reliability issue.

best regards,

 

[JJPellin]

We recently had a small steam turbine that had high magnetism in the shaft ends. We did not want to fully disassemble the turbine to address it. We attempted to demagnetize the shaft using a tape eraser, a bearing heater, a demagnetizer and a welding machine. We failed to get it within our specifications for residual magnetism. Since it was a small, general purpose turbine, we accepted it and installed it away. It has been running well for a couple of months. If it was a critical machine, we would have disassembled it and sent the rotor to a shop with the capability of demagnetizing the entire rotor.

We have a critical centrifugal compressor that is currently experiencing high shaft voltage which has resulted in the failure of one radial bearing. We have not had a chance to figure out what is magnetic. But, we believe that we are experiencing electrical discharge from some residual magnetism. We installed an upgraded grounding brush and have been able to limit the shaft voltage to about 70 mV which should be low enough to avoid any further damage. Before we installed the new grounding brush, we were reading shaft voltage in the range of 50 to 70 volts even with the OEM grounding brushes installed.

Johnny Pellin