Electostatic Charges in Gas Turbine Compressor

If someone did a rotor repair and did not properly degauss the rotor, then that might result stray currents.

Are you sure the pitting marks were ESD and not bearing cavitation or overload or fatigue?

Where is the machine? Is the air very humid? Are you doing inlet cooling? Compressor water wash? Often static charge on a rotor comes from water droplets in the gas stream.

Pitting marks are looking bcos of leakage current probably bcos of ESD. About the machine:-GE Fame 9e gas trurbine [exciter]==5[gen ]4===3[ Turbine ]2===[Compressor]1===Aceessory gear Box(AGB) As the plant is in coastal area ,so air is humid especially during rainy season. No Inlet cooling. Compressor water washing offline id done in 2-3 months. During the combustion inspection of the Turbine we do not open any bearings(1-5) indicated above,but as the AGB vibration was on higher side we inspected the AGB and pitting marks were observed in half bearings of the main shaft of the accesory gear box

Assuming it is electrostatic charge in the turbine stage, I feel it is just a bit unlikely due to the temperature of the rotor, which even at the compressor stage is quite high. If i remember, compressor outlet air temperature is about 600⁰C or so.

In case the culprit is from the turbine side the first attack should have been on the its bearings, both the journals and active pad. Even the other thrust bearings too would not have been unaffected and the results at that load should have been quite significant. (What is the oil debris report of the turbine drain?)

Similar should have been the case had it been from the generator side.

What is the prime mover (starting mechanism)? if it is motor, has it something to do with it? Else as SS said it has to be cavitation/starvation of the specific bearings. Is it on shaft#1? which side is more affected Turbine side or upstream side? How are the gears? with these current flowing, there should have been significant pitting on the gear flanks too.

Check the history, make a detailed observation, the answer will be obvious. Lastly keep us informed.

About the machine:-GE Fame 9e gas trurbine [exciter]==[5 gen 4]===[3 Turbine 2 ]===[Compressor 1]===Aceessory gear Box(AGB) Compressor outlet air temp after 17 stage is 350-360 Deg C and inlet is ambient temperature. Also the flow of air is from bearing no 1 to brg no 2 side i.e from AGB to turbine side During the combustion inspection of the Turbine we do not open any bearings(1-5) indicated above,but as the AGB vibration was on higher side we inspected the AGB and pitting marks were observed in half bearings of the main shaft of AGB.

I am aware of the configuration and the flow. The point I made is if it is a problem from GT side of generator side, the affected would have been the LGBs,Turbine and Generator bearings as well, and you would have been forced to trip due to turbine vibration/ load line vibration.

The temperature I mentioned is due to natural demag, at higher temperature in the compressor zone, though it is quite below curie point, still there will be a gradual weakening.

But the affected looks to be only AGB - and that too shaft #1 - what do you mean by half bearings?

I meant whether it has got something to do with the starting mechanism? - Is it motor started or Diesel engine started ? In this line, first earthing will be through the AGB, after the starting means.

Also there is a good possiblity of some residual magnetism left out in the Gears of AGB, this may happen after improper demag subsequent to MPI or any other mechanism to induce the residual.

Still I doubt the ESD theory, what about the thrust faces of the bearing? Normally I will go with lube failure and contamination theory.

Any repair/replacement activity done on the machine? What is the current running hours of this bearing?

It was a combustion inspection ,so all the bearings were not opened ,only the accessory gear box was inspected ,bcos of high vibration in the Accessory gear box. On inspection it was observed that the bearings of AGB were having pitting. About the starting means:--- This machine is started with 1 MW cranking motor( electric driven) after firing and 60 % of running rpm it is cut off. Accessory gear box is between the Compressor and the cranking motor. Thrust faces of bearing also have same type of pitting marks. No repair /replacement was done from last major overhauling approx 18 -24000 operating hours. What could be the other cause ,if pitting is not due to leakage current. Please elaborate. Regards BSJhala

Sounds like a lubrication failure.

Why Lubrication failure?????