What will happen when the carbon brushes provided in the steam turbine for draining the electrostatic charges fails? What will be the effect on the Steam turbine?

What will the pattern of vibration of HP, LP Gen bearings?
What will be the indication?

 

I've seen the braided copper rotor grounding brushes fail without any obvious sign of a problem with a 240 megawatt steam turbine, but that doesn't mean we were getting away with it. The generator rotors' static charge will simply find another path to ground, which will probably be through the first non-insulted bearing.

We have a different 240 megawatt generator that experienced a catastrophic failure of its driven end end-turn windings, during which copper plasma was sprayed onto the rotor. There was so much excitation current flowing from the rotor to ground that it burned off all four of the ground brush straps, wiping out the bearing of the combined generator/LP turbine and causing pitting damage to the generators' D.E. journal.

I think it would be safe to assume that a machine running without grounding brushes will experience the same sort of arcing/pitting damage but at a much slower rate without causing any noticeable problems until the metal loss is severe enough to cause overheating and even catastrophic bearing damage due to oil wedge loss. There will simply be less bearing to support the load.

The moral of the story is: check your grounding brushes weekly, as you (hopefully) do with your exciter brushes. If you're curious, you can measure the effectiveness of your grounding brushes by making up a braided copper brush which you would mount on a stick with a wire for connection to a high impedance volt meter referenced to ground. It should be pretty close to zero.

 

What will the effect and trend of vibrations of all the bearings?

 

This is a pretty open-ended question, and there's a lot we don't know about the machine at your site.

I think the effects would depend on the condition of the bearings before the brushes failed and would be more likely to be prominent on the uninsulated bearing(s).

If the bearings were in good condition and the failure was noted fairly quickly after occurring, it might be that there would be little or no effect.

What effects were observed immediately after the brush failures? Did all the brushes fail at once? How long was it before the brush failures were noticed, or is that information even known?

Have you seen a steady increase in vibration levels since the failure was discovered or believed to have happened?

A good vibration analysis consultant can take data and pretty much pinpoint the type of phenomenon which is causing the vibration. Having said that, good vibration analysts are hard to find.

This seems to be one of those questions that comes up because the vibrations suddenly increased and the only possible cause that turned up in the investigation was that the grounding brushes hadn't been properly monitored and replaced in a timely fashion. It's <b>so</b> hard these days to convince people that rounds, walking 'round the unit to observe conditions and in the process take notes and pass along observations, is really important. Everyone thinks the control system will monitor everything and protect the unit and the operators can sleep or eat or watch television or exercise (that's right--some control rooms have treadmills and dumbbells (the metal weight-lifting variety)--and not have to get up and walk outside and look for oil leaks or air leaks or water leaks before they become bigger problems.

 

why electrostatic charge may build in generator rotor coupled with gas turbine having wet compression?