Today is...
Thursday, July 19, 2018
Welcome to, the global online
community of automation professionals.
Featured Video...
Featured Video
A demonstration of EtherCAT control of linear motors using the CTC EtherCAT master.
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
GE 9fa 3gts All Generator Casing Purity Gradually Coming Down
my site ge 9fa 3gts all generator casing purity gradually coming down,

My site GE 9fa 3gts all generator casing purity gradually coming down, both seal oil float trap kept in service, also it has been observed float trap oil level is fluctuating with foaming.

Hence my seal oil flow is showing little more 21usgpm compare to commissioning period.

Please anybody here to give me solution why my generator casing purity gradually coming down.97% to 91%

my email

1 out of 1 members thought this post was helpful...


Have you changed the scavenging flow-rates at all since the seal oil flow-rates increased?

Presuming the hydrogen purity hasn't changed, the ONLY way for air to get into a generator that is at approximately 2 barg is through liberation from the seal oil flowing into the seal areas. If the seal oil flow-rate increases, the amount of air liberated from the seal oil will also increase--hence the need to increase scavenging flow-rates.

The purity will not start to increase immediately after increasing the scavenging flow-rate, but it will eventually begin to increase. (Of course, hydrogen consumption will increase as scavenging increases.)

Dirty oil can cause accelerated wear of the seal rings, which can cause increased seal oil flow rates. Frequent starting and stopping can also cause generator pressure fluctuations which can lead to varying seal oil flow-rates and increased air liberation into the seal cavities and eventually into the generator casing.

ANY changes made by ANY operator (inside operator or outside operator) to any aspect of hydrogen-cooled generator functions should ALWAYS be logged to the operations logs books. Very often it's eventually discovered when problems like this arise that someone has been making changes without logging them--and often without understanding the knock-on effects of the changes they are making, not to mention not understanding the basic system operating fundamentals.

Again--the ONLY way for air to get into a generator casing that is pressurized to some pressure greater than ambient pressure is through liberation from seal oil. The tendency is for hydrogen--at a pressure higher than atmospheric pressure--to leak OUT of the seals, NOT for air to leak INTO the casing. And because seal oil is Lube Oil--which is exposed to air everywhere else in the system (especially in the L.O. reservoir), and the air which is entrained in the Lube zoo which is used as Seal Oil is liberated into the seal cavities as Seal Oil flows around the shaft. So, increased seal oil flows result in increased air liberation which, if not "countered" by increased scavenging flow-rates will eventually cause hydrogen purity to decrease.

Hope this helps!

1 out of 1 members thought this post was helpful...


I want to add that the normal sensing location for hydrogen purity is the seal drain enlargment tanks at each of the generator (turbine end and collector end), because that's where air is liberated from the seal oil. Eventually, the hydrogen/air in the seal drain enlargement tank gets circulated back to the generator casing, but because the purity is usually lowest in the seal drain enlargement tanks that's where the normal sensing is set to monitor hydrogen purity.

If the casing purity is decreasing for all three GTGs (Gas Turbine-Generators), then it could be that the hydrogen purity is not as good as it was previously. Does the site buy hydrogen from a vendor/supplier, or is the hydrogen produced on site? If it's produced on site, or the vendor/supplier has recently changed, it's possible the purity of the hydrogen being used to maintain pressure (due to losses through the scavenging system) is not as high as it should be. Again, I only mention this because you say the purity is decreasing for all three generators--so I'm trying to think of things that should be common to all three generators, and hydrogen supply/source would be one of those things.

Also, has the Lube Oil (not Lube zoo!) been changed recently? Or has new oil been recently added to the reservoir(s) to maintain L.O. level? Could it be that the new or added L.O. is not exactly the same as the previous L.O. (not all Lube Oil is the same as all other L.O.)? Could it be that the added L.O. has in some way negatively affected the oil quality of the entire tank, which has resulted in the increased foaming you report in the float trap? If you regularly have your L.O. tested, can you compare results from the last couple of tests (presuming the most recent test is fairly recent--since the purity/foaming/increased seal oil flow-rate(s) issue has become apparent)?

Dear respected sir,

Thank you very much for your valuable reply. I would like to ask some other points which could help you to give me a good solution. why my site all 3(three) ge9fa gas turbine generators casing,TE&CE purity gradually coming down (97% to 91%) with high scavenging since may 2017.

hence, we have 2 (two) h2 gen sets from proton energy. One of the h2 generator always is running to maintain gas turbine generator casing pressure 4.25 kg/cm sq

My observation:

1. My h2 dryer at present dew point is showing -30deg c. as my knowledge h2 dryer alumina/desiccant did not replace since commissioning.

2. After commissioning only 1 seal oil float trap was in service for 2 years without any oil fluctuation and foaming in float trap. Now both float traps are in-service with oil fluctuation and foaming in float trap.

3. We have kept hi scavenging h2 flow rate in rota meter >5500 sccm/min to increase h2 purity. it has been observed h2 purity is increasing but again decreasing while rotameter flow rate adjusted to normal flow rate at 3500 sccm/min.

4. Genset no 3 seal oil flow is fluctuating observed 16 usgpm to 14 us gpm. genset no- 1 & 2 seal oil flow is showing 21 usgpm.

5. All three gen set both h2 analyzer TE & CE calibrated by source h2 the purity was shown 99.9%

6. To resolve the following problem should i proceed to purge h2 by co2 from generator and fill again new h2.

7. We made plan to replace h2 dryer alumina/desiccant

Could you please give me suggestion how I can solve my site h2 purity problem.

1 out of 1 members thought this post was helpful...

What hydrogen purity analyzers are the unit using? GE, for many decades, used a wheat-stone bridge type of hydrogen purity analyzer, before switching to a digital analyzer. They have used, at various times, eOne analyzers, Druck analyzers, Yokogawa analyzers, and Panametrics analyzers (those are the only ones I'm aware of; there may have been others).

I would suggest you look at the analyzer manufacturer's manuals (unless it's the old, archaic, GE analyzers) and determine what might cause a problem with the measurement--such as humidity, or oil vapors (I have personally seen some poorly-executed hydrogen charging procedures (haste makes waste!) which caused L.O. to get into the sensing line piping and cause very erroneous readings. That was EXPENSIVE and time-consuming to "clean up"--it required replacing most of the sensing line tubing inside the H2 Purity cabinet, as well as the tubing from the generator.)

There used to be desiccant "fuses" in the sensing lines of the H2 Purity cabinet. They could be dried and re-used by putting them in an oven at 250-300 deg F for a few hours (allow them to cool before re-installing them in the cabinet).

Is the hydrogen continuously circulating through an electric dryer? Is the dryer energized at all times?

Again, when I think about three generators all experiencing the same problem at the same time, I have to think about something that's common to all of them--such as the hydrogen supply system, in this case.

When you say "hydrogen source" gas was used to test the purity analyzers, did you use gas from the hydrogen generators, or from a test gas cylinder of pure hydrogen?

Last thing to know about scavenging flow-rates--they will never be exactly the same for two, or three, machines. AND, they will generally need to be increased over time, until the hydrogen seal rings/springs eventually get replaced--which will decrease the seal oil flow rates and then the scavenging flow-rates can be decreased to conserve hydrogen. The seal rings are made of a soft brass, and they do eventually wear.

Scavenging flow-rates are not static (constant) over the life of a generator, or it's hydrogen seal rings. That's why it's ALWAYS recommended that ANY time ANY one makes an adjustment to a scavenging flow-rate, that it be logged for reference. (A lot of operators will walk by a H2 Purity cabinet and make a small adjustment to try to increase, or decrease, purity--and if this happens over several weeks/months, it's not too long before someone notices the flows have REALLY changed and wonder what has happened.

Again, scavenging flow-rates have to change with seal oil flow-rate changes. And seal oil flow-rates usually increase over time, until the seal rings/springs are replaced and then seal oil flow-rates--and subsequently scavenging flow-rates--will be lower with new rings/springs. A lot of people think the scavenging flow-rates are "set it and forget it"--but that's not true. Especially for units which experience frequent starts/stops.

Dear sir,

Again, thanks, a lot for your valuable reply.

At my plant all 3 gts unit continuously running, we are not doing frequently unit start stop.

We have TE & CE eone analyzer. Also we have dual tower LACTO DRYER which is continuous in-service in 3 mode, cooling/absorbing/heating. As per requirements every 8 hour interval it is automatic changeover 1 to others. in commission time it has been observed dryer dew point was -65 deg c but now it is showing -30 deg c,

We have tested h2 purity from our h2 generator output source by TE & CE eone h2 analyzer, it was showing 99.9%. still we did not check out side portable cylinder.

1 out of 1 members thought this post was helpful...


Have you checked the generator casing "belly" drains to see if there's moisture of any kind in the generator casing? It could be that the generator casing liquid level detector(s) aren't working or stuck, and there's no indication because the switches aren't working for some reason.

I would also check the low-points of the tubing from the TE and CE and casing sensing lines, including in the H2 purity cabinet. There could be moisture collecting there.

What do the hydrogen purity analyzer instruction books say can cause erroneous readings?