Hi to all!
A visual inspection reveals that all the tilting paths of the axial thrust bearing of a steam turbine show evidence of electrical current passage. Material is removed from the tilting paths surface, apparently as a result of the electrical current passage. During normal operation the turbine axial rotor position prox probes tell us the rotor is gradually moving in the axial direction and is approaching the axial displacement limit. Steam turbine is directly coupled to axial flow compressor, coupling is electrically conducting, speed 13000 rpm, power 4 MW.
It looks obvious electrical discharge is the cause of electrical current. My question is: Has anyone come across a case where magnetic fields in a steam turbine or compressor generate an electrical current that likes to eat tilting paths?
Regards,
Arie Mol

I have not come accross that.

Generators of course usually have insulated bearings, but I'm not sure about turbines.

Just as a clarification in terminology - I think you are referring to the "tilting pads"

excusez-moi: yes tilting pads!

Is it magnetic fields? Water droplets can result in static electric discharges through the bearings. Some process compressors can create static electricity too.

Electrostatic charge can be generated in steam turbine due to continuous friction with steam which may find its path through the tilting pad bearings. But how sure are you that it is due to electric current passage ? Pl. attach any photo of damaged bearing. Although I have not seen any such problem in non electrical machines , it can be rectified by insulating one bearing or providing earthing brush on shaft.

Dear Arie,

About 25 years ago some problems of electrical pitting were observed in steam turbine in power plants (not in thrust pads but elsewhere, especially in journal bearing or bearing housing).

After checking the healthiness of generator bearing insulation & other things, designers recommended insulating the MOP from the rest of the turbine. The problem of pitting got solved, though some sets had occasional failure of MOP bearing. But inspecting/replacing MOP bearing is lot easier.

Hope this provides some clue.

Arie, see attached, which is a case of a steam turbine, gear box, compressor machine train. I see one of these cases about once a year. Similar to the description provided by Bill Foiles, it was once described to me that steam moving through a turbine was much like a cloud moving across the surface of the earth. Lighting gets formed!

John from PA

Electrostatic_Discharge_on_a_Compressor_Train.pdf (674 KB, 44 downloads)

Thanks guys for the responses.
It looks like a grounding brush, provided this is correctly riding on turbine shaft eleviates the problem.
And electrical discharge being a root cause makes sense to me.
However my question referred to this: I have come to know that some sources claim that the origine of electrical current is magnetic not static. And de-magnetising the shaft is the answer.
I wonder how would this magnetic origine work?
Or:
Has anyone come across a case where magnetic fields in a steam turbine or compressor generate an electrical current that likes to eat tilting pads?

BTW: Ganesh, what is MOP referring to?

Regards,
Arie Mol
NL

Dear Arie,

MOP refers to Main Oil Pump. This is a shaft driven pump and is at one end of the turbine.

It is true, that grounding brush is important and is usally provided in turbine-generator sets. I am not aware whether designers envisage brushes for turbo-blower/turbo-compressor sets.

True, there are two possible types of current flow: static & magnetic.

Before attempting de-magnetizing, do confirm really whether the rotor/shaft got magnetized or not. If required contact your OEM (Original Equipment Manufacturer)

I thought that turbine rotor/shaft/babbit materials are made of non-magnetic alloys!

This message has been edited. Last edited by: ganesh, v.j,

quote:

I thought that turbine rotor/shaft/babbit materials are made of non-magnetic alloys!

Something like AISI 4140 alloy steel, a common material in turbomachinery, non-magnetic? Don't think so...

John from PA

Arie,

Is the damage on the whole face of the pad or is it just towards the leading edges of the pads?

Also, does it look frosted/pitted?

Those are more common to static discharge, as the charge builds up then the leading edge of the pad creates a path or a close enough area to over come any di-electric resistance for the current to flow..

Also a shaft voltage measurement can be taken and from the trace you should be able to tell between static or electo-magnetic currents.


Jeremy
jgroover@gaussbusters.com

Thanks Jeremy for your reply.
Went to visit your website and found interesting information!
Arie Mol