Drop of hydrogen purity in GT generator 1

[npower0073]

On site we have a 324 GE generator hydrogen cooled. 8 months before GE made a MAGUC inspection when it removed the end shield and seal ring from collector end. During the last month we have experienced sudden drop of H2 purity while the unit starts up and mainly when it syncjronizes. From 98% we go to 93% during the period from 2700 rpm to synchronization. It is also interesting that during the last two outages we have the lube oil pumps stopped along with turning gear and the aux seal oil worked to keep the dif pressure. Is there any suggestion fo rthe sudden drop of H2 purity? From the time that the H2 pressure is higher than the oil we cant have entrapped air. We also dont have oil drainage to the seail oil enlargemnts drain pots , but bear in mind that due to a mistake last outage 8 months before there was a mistake and a lot of oil was inserted in the generator. If we dont ahve air or oil contamination then it should be something from the generator itself (e.g dust from fretting? that creates the contamination.

 

[byrdj]

You didn't mention transferr anyliser flow from vent (while below speed) to fan delta when at speed.

Do you stay in vent flow all the time? any way, what I would look at is did you verify proper flow and current setting, these change during roll up and will effect the "indicated" purity.

Now does your fan dP manometer also indicate a reduced purity?

 

[byrdj]

Also, does the unit have Vacumm treated seal oil or is it a "scavanging" design.

If scavanging (no vacuum treated seal oil) then verify that the scavinging vents at both ends are venting. I only saw a couple scavinging units and not sure exactly where they are but they should be very near the gas side seal drain. being on gear would allow a very low purity to build up near the seal, and when spinning mix it up and get it to the anylsier flow.

If vacuum unit, did you go on emergancy seal oil while on gear, that oil is not treated. did you operated with out vacuum?

You are aware that lube oil has entrained air, and if lube oil is used for seals, that air will come out in the gas side. some where I have the ratio CFair per gallon seal flow

Again I'm more familair with LSTG vacuum and not as much with those MSTG designs

 

[byrdj]

One last thing, was this a one time event and were you able to "feed and bleed" while online to get purity back up.

During the initial purging of air/co2/h2, do you include the h2 detrainig tank. Nearly every station I've been to does not and it is expected to have a purity drop. Even after I make them aware of purging the h2 detraining, they may perform the co2 purge, but not the h2 since the upper tank vents were not piped to the purge manifold vent, just blows under the wrapper.

 

[npower0073]

byrdj
Thanks for the answer. Well the seal oil is non vacuum sealed. We have scavenging rates that are adjusetd daily to GE recommendation. The fan dp transmitter also indicated diff pressure slightly increasing 98% purity with 9 inches Dif Pres, 93% purity with 11 inches Diff Pres.

 

[byrdj]

I don't know the published scavinging recomendations. I'll dig through some old files and see if I can comment any more.

Have you measured the actual seals flows? I could see if the gas side flow was excessive (due to a misassembled/worn seal assembly) then the scavinging rate needs to be incresed due to the extra air from the extra oil. Anyway the scavinging rate would need to be increased to "bleed and feed" to increase the purity. I know this takes a lot of wasted h2 do to.

Do you think the purge could have left a volume of air in the h2 detraining tank?

 

[npower0073]

We did twice scavenging, the values increased but then the values of purity dropped after unit start up.

 

[byrdj]

Summerizing (just to help me think)
1)Air could have been left in cavities during purge, then mixed during rolling
2)Air is entering with the seal oil, greater than scavinging. excessive seal oil flow
3)Instrumentation. I was thinking of the anylyser type where you have to adjust gas flow differently from still to running (and balance the current base on gas temp). But fan dP shows low purity



Sorry, I haven't had time to dig through old notes yet about scavanging formulars

 

[byrdj]

still looking for scavinging unit notes, but I did find and simplfy the equation for expected air entering via seal oil
A cubic feet per hour = seal oil gallons per minute X 0.8

But I had a though. Is it possible a station or instrument air line is still connected. that line might have been used for "air leakage" test. the line still connected but the isolation valve leaking. there should be NO solid connections to the air system, there needs to be a removable connection. I would look over the manifold, h2 detraining and instrument tubing real good, especially if you don't know the exact conections that have been used by others to put air into the unit. Maybe some one tried to clean a line with air during outage?

 

[npower0073]

After thorough check there is no air piping connected to he H2 measuring or distribution circuit. The interesting thing is that the geenrator after some days of operation shows decrease in purity. It is really with no explanation except from the fact that something is inside the generaotr and is mixed with the hydrogen thus decreases its purity.

 

[npower0073]

I think thtat it is also a higher seal oil flow issue meaning that if there is a faulty orifice in the lube oil circuit that permits higher seal oil flow to come to the geneartor seals and thus allow more air from the seals to to the generaotr then it can cause this issue.

 

[byrdj]

Your seal oil pressure must be greater than gas pressure for the seals to work. normally 6 to 12 psi. If the oil flow required to seal is restricted and thus oil pressure falls, the indicating gauges will only drop to the gas pressure, thus while 2 psi would work, it is best to have it enough it can be easily seen on a questionable calibrated guage.

The seal oil flow GPM is determined by the seal ring clearances. the ring clearance becomes the "orifice". Now with the seal oil pressure a constaint say 8psi higher than machine gas, the flow would increase if the rings were worn or the casings misassembled thus making the "orifice" bigger.

the oil enters between two rings (air and gas side). since the delta pressure will be the same 8psi for the gas side, the flow to the gas side will be the same no mater the casing pressure.

The flow for the air side will increase with gas pressure and is much greater than gas.

On turning gear the flows are expected to be less than 25% of when at rated speed. If they are greater, that is a good indication something is wrong

If your seal oil regulator can not provide the pressure to maintan the desired delta pressure is also an indication the flow is excessive.

I still haven't gone to the barn yet to look, but the scavinging rates of H2 need to be propertioanl to the oil flow rate on the gas side.

The recomended scavinging rates I found for a vacuum treated machine are extremely high. due to have to feed and bleed the entire casing volume, verses just the seal cavitites of the untreated oil desigh arrangement.

BTW what is your machine volume and what gas pressure, so if i every go get notes I could have some calulations done

 

[byrdj]

well, I had no details about the operation of a scavinging in my notes.
so I need to ask
Do you have a scavange vent for the two ends. what I did have implies the vents are off the top of the H2 detraining tank. that tank is divided to keep the two drain sides seperate so there should be two conections there, but i don't know if that tees to give you a single adjustment.

Have you verified both sides of the H2 detraining are being vented.

Now once the low purity gas gets to the casing and that volume's purity goes down, the bleed and feed to get purity back is a LOT of H2