we have 660MW ELECTROSILA MAKE TURBO GENERATOR, having generator gas volume of 126 cubicmeter & single oil flow seal system. The H2-side seal oil flow is 3 liters per minute (in both side TE & EE). We are facing the problem of falling of generator casing H2-purity @ 0.25% per day.

ARE THERE ANY CHANCES OF OXYGEN DIFFUSION FROM STATOR WATER SYSTEM (OXYGEN CONTENT IS 4000PPB) TO GENERATOR CASING HYDROGEN OTHER THAN THE AIR RELEASE FROM H2-SIDE SEAL OIL?

PLZ ADVICE FOR REMEDIAL MEASURE??

 

When did this problem start?

What have you done to try to resolve the problem?

What were the results of the efforts to try to resolve the problem?

When is the last time the generator seals were inspected and/or replaced?

Has(Have) the seal oil flow rate(s) increased recently? Significantly?

Seals wear, and as they wear they require more seal oil flow to maintain the differential pressure. More flow equals more entrained air which can be released into the generator casing.

We don't know how the seal drains are "scavenged". Sometimes it's as simple as increasing the scavenging rate (which also increases H2 consumption, but that's to be expected).

It's presumed you have confirmed the H2 purity analyzer is working properly.

About the only way I could conceive of cooling water causing a problem with H2 purity would be if there was excessive cooling resulting in condensation on the H2 side of the coolers. Have you seen any evidence of water/liquid in the generator casing drain?

Do you have any means of "drying" the H2? Some generators have H2 Dryers.

Are there moisture indicators in the H2 purity sensing lines? Do they indicate moisture in the gas?

That's about all I can think of. When posting a problem like this, it's most helpful if you provide as much information as possible about the problem, when it was first noticed, what you've done to try to resolve the problem, and the results of your efforts.

 

Further to CSA's comments,

1. have you checked the quality / cleanliness of your seal oil?

2. Are your oil purifiers doing their job properly?

3. Have you checked for the presence of water in your lube-oil tank?

4. Do you have a seal somewhere in the turbine end which is leaking steam close to a shaft bearing possibly pumping steam into the bearing?

I stated all the above as any water in the seal oil will tend to migrate to the hydrogen where the oil come in contact with the hydrogen, lowering your purity. In all the above I am presuming that the bearing lube oil and the seal oil are coming from the same lube oil tank.

 

for 660mw unit, the 3lpm (0.8gpm)seems low, does that rate correspond to unit design data?

I'm familiar with vacuum treated oil supply units. when untreated oil is used, I calculated purity degrade using 10% air absorption, so your total 6lpm would be about 0.8 cubic meters per day of air entering the generator. the purity in the seal casing area should be maintained at lest 85%, thus a scavange <i>(scavenge?)</i> rate 6 times air being released is expected. this should be with fresh 99% H2 entering main casing, but exiting from seal casing area (H2 detraining section)

as for the possibility of air from SCWS, the H2 pressure should be at least 1atm (15psi) above SCW pressure. If you have any leaks in windings/clips/tubes, then you should be seeing H2 in the SCW tank vent.

operating with SCW pressure higher than H2 would be a risk of water leak into gas volume and gas humidity will increase. If you have seen H2 in SCW vent, then you must avoid operation with low H2 pressure