the reason of seal oil ring corrosion? 1

[chemical321]

Hi.
we have a corrosion problem about the seal oil ring in the generator. this corrosion have took place on the hydrogen section( please see the attached picture). what kinds of corrosion may take place in this section? is it scc? what do you think about the reason of this corrosion? please help me about this problem.
thanks.

 

[byrdj]

could it be arcing?
is this seal ring on CE?
if so, it should be insulated from generator frame, can you measure the operating quaility of insulation
do you have a grounding brush on the TE generator shaft and do you verify it has shaft ground

Is the seal oil vacuum treated or do you scavange. however if it was off gassing water, I would expect some pattern not the randomnism like arcing

are the radial clearances equal for the air and gas side ring. I would have expected air having less clearance and thus more suceptable to arcing

 

[btrueblood]

" this corrosion have took place on the hydrogen section"

Um...so you are using hydrogen gas as a fuel? What alloy is the ring made from, and are you sure it does not suffer H2 embrittlement at the temperature and pressure you are using it at. That's really a shot in the dark, but: I would be asking these questions in the Materials Engineers>Corrosion Eng'r. forum, if it were me.

 

[waross]

Hydrogen gas for cooling rather than air inside the generator case, btb.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[byrdj]

I was thinking
1) with this oil being a thin boundary to contain several atms of hydrogen,
2) and if it is a scavanging seal oil system with air off gassing from the oil into the hydrogen,
3) Then, have improper insulation and grounding so there is discharge arcing through that thin film of petrolium oil....

BOOOM !

was there a groove "EDM" machined into the journal from this

 

[chemical321]

Mr byrdj thanks for attention.
(My English language is not good and I apologize for this)

Is there any test (for example NDT tests) to find out what's the real reason for this seal ring failure? Can you suggest some technique for this purpose?
Did you have any experience like this?

If there is arching in the generator, is it possible to see the impact of arching in the other instrument in the generator?
I will say to you later where the place of the ring (CE or TE) is.

Thanks for attention.

 

[chemical321]

Mr byrdj
why did you distinguish this problem as a arching effect? do arch have a especial sign?
thanks.

 

[byrdj]

mainly, a generator seal oil ring is very much suspetible to eletrical damage and is a common problem.

the pic look like craters "explosions" from a significant arc instead of the "frosting" for a less severe problem.

was the ring ID less for the side showing the damage?

which end of the generator?

How was the adjacent journal bearing?

 

[byrdj]

sorry, I only saw the last post and replied and didnot read the one above.

as for detection of electrical discharge damage to bearings/seals or gears on a turbine generator rotor, there is no instrument that detects. measuring the effectiveness of the grounding brush and if possible the quaility of the insulated bearings / seals on the collector end of the generator provide indications damages are possible

 

[rmw]

If this is arcing and there is a problem with the grounding brush, there could also be problems at one or both bearings. How many of the bearings of this generator are insulated?

Regarding the damage, is it isolated to the zone where the photo was taken or is it all around the ring?

If you had told me that this was a photo of a brass pump impeller wear ring, I would have made the call that this is cavitation. How pure is your hydrogen?

rmw

 

[byrdj]

I though about cavitation. however never heard of it on a hydrogen seal. thinking, if oil had a lot of water, it could flash when exiting the air side.

just need more info

 

[rmw]

ByrdJ,

You have already asked which end of the generator, so we will just have to wait until the OP responds and hope that he/she does.

If it is a steam turbine and this is the turbine end....

There is a definite strong straignt line through affected zone in the photo although the individual pits are just that, pits. The line down the center is rather pronounced.

I wonder if this generator gets shut down in freezing weather often???? Hydrogen seal oil is hydroscropic and if they aren't getting the water out, that gives possibilities.

rmw

 

[chemical321]

Hi.
Especially thanks from mr. rmw and mr.byrdj.
The seal oil ring is at the CE section.
I think this a pitting accident because at the adjacent of the pitting we can see tracks ( physical effect of something responsible for pitting)

If it is pitting why this is black? It must be the same color of ring.

Please if it is possible write your comment in a simple manner..
I am a chemical engineer at water treatment section of power plant and I am not master in electrical engineering and turbine and generator technology.
thanks.

 

[chemical321]

purity of H2 is about 98%.

 

[chemical321]

ring ID is equal for both.
state of adjacent journal bearing is good.

 

[byrdj]

whether or not what has occured is arcing damage, cavitation, corrosion or even porisity must be determined with consideration to all the info you can aquire. If an definite decission is not possible, then use info to indicate most likely.

given my back ground is related to the mechanical aspects of assembling generators, my information provided will be biased towards such and I will provide discription of the mechanism that would cause arcing.

the frame of the generator is grounded through out its lenght.
the rotor of the generator WILL devolp an electrical potential during opearation. to prevent this potential from becoming a current, the turbine end of the rotor is grounded with a contacting brush. the collector end components with the posibilty of current arc over are provided with an insulator. this would be the oil defectors, the bearing and the seals.

If the insulation becomes damaged or contaminated with low resitance oil, the potential on the shaft will drive a loop current. given the thin film of lube oil (either at the bearing or seals) provides a very small seperation, arcing will go through this film.

some methods of insulation do not allow the quality to be verified onced assembled. this seems to be the normal now. some manufactures provided double insulation so that the section between the two insulators can be measured to verify both insulators are working.

for the single insulator (or if one of the double type) fails, the effectiveness of the insulation is verified by measuring the potential at the collector end of the rotor. this needs to be a relative reading since the normal potential varies between units. I have normally seen 50 to 250 VAC

the turbine end of the rotor should be grounded, this is verified by also measuring shaft voltage and it is expected to be zero. IF this ground fails, and the turbine end voltage increases, the collector end potential will increase, but to a greater potential and can lead to failure of the insulation, thus devolping a current loop, with arcing across the oil film

as for the ring ID dimensions, as for being equal for the air side and the gas side ring, the differance expected is only 0.002" so were they they within 0.001" of each other (example the air side diametrical clearance is 0.006", with the gas side diametrical clearaance of 0.008")

thus to consider arcing as a possible cause, the quality of the seal insulation must have been poor. given no damage to the adjancent bearing, the bearing insulation was still effective

 

[byrdj]

 

[rmw]

Chemical321,

I take this seal ring, both by its color in the photo and application to be a brass ring. Would that be right?

Black pitting on copper bearing parts to me strongly indicates the presence of sulphur.

Is there any way to connect the presence of sulphur in any form, especially H2S to this generator?

If you are a ChemE, you should be able to track down the sulphur/copper reactions. If not, I can track down some links.

rmw

 

[chemical321]

i searched but i couldn't find reaction between brass and sulfur.
is it possible for you (mr. rmw ) to find it?

thanks.

 

[rmw]

Here is a good start:

thread338-122893

Here is another one:

http://www.olinbrass.com/companies/fineweld/Literature/Documents/MIC in Heat Exchanger Tubing.pdf

Google H2S and brass and corrosion all together.

rmw