Dear CSA,

Recently, during shutdown (GE 7FA machine with hydrogen cooled generator GE make) we have measured the seal oil flow from float trap

Please find the below email communication:

Seal oil flow measurement on the hydrogen side of GT#1 was conducted. Data was taken with the following conditions.

1. GT#1 was at complete stop with axillary lube oil pump in service. (turning gear off)

2. Five (5) minute duration was used after stable flow was observed.

3. Total seal oil flow as read locally on the flow meter was 5 USgallon/min.

4. Flow was taken using the float trap by simultaneously manipulating V/V’s 123 and 106.

5. Seal oil differential pressure at 7.1 psi.

Total six (6) liters of oil was collected during the 5 minute measurement. Approximately 1.25 liters/min of seal oil flow was there with the above stated conditions were existing. It can also be concluded that seal oil flow at air side will be approximately 18 liters/min.

What would be the ideal flow rates at hydrogen side & air side? are the flow rates are different while M/C on turning gear & M/C on base load?

What would be the ideal time to measure the flow rates (on turning gear off, turning gear on or on base load)

regards,
Parag

 

parag,

You ask some interesting questions, but don't provide any context (like, why you are interested in exact seal oil flow-rates).

To my mind, and I have no data to support this, but I believe more seal oil would be required when the shaft was turning if only because the shaft surface it was being sprayed against was moving so it would seem to require more oil to maintain the seal in the space between the seal oil rings and the shaft. How much more is a good question and I have no feel for that.

I've never physically measured seal oil flow rates, and I've always just considered the flow rate indicator to be a relative measurement. In other words, one watches the indicator to establish a baseline and then if the indication increases or decreases and there is a change in H2 purity as measured at either end of the generator that would be an indication of a potential problem. But, to use the indicator as any kind of exact measurement of actual flow-rate, I don't think they're that accurate (though I have been wrong about things like this before). Again, I just use it to establish a baseline flow-rate after a maintenance outage or during initial start-up, and then monitor the flow-rate to detect any change (increase or decrease).

I don't have a 7F Seal Oil P&ID so I don't know which valves were being throttled or if the flow through the Seal Oil Float Trap was completely isolated or not so it's hard to tell. It may also be that you were only measuring the flow from one Bearing Drain Enlargement Tank; without the diagram it's difficult to tell.

Are you trying to troubleshoot some H2 purity problem? Has there been a noticeable increase in seal oil flow rate recently?

I've seen the flow-rate indicators be blocked with dirt and debris after a flush and also after a maintenance outage and when the obstruction cleared (for whatever reason) the indication increased greatly. However, there was no appreciable change in H2 purity, nor was there any great change in H2 consumption and no hydrogen was detected leaking from the shaft seals.

I've been under the impression (again, no data to support this fact) that the hydrogen side seal oil flow-rate would be less than the air side flow-rate, because of the higher pressure inside the generator casing. But, as for how much less, I would have no idea. My recollection of the seal oil rings is that they are flat where they sit on the shaft and there is no difference in gap on the hydrogen- or air side, so the only difference is the pressure differential between the generator casing and the ambient pressure (about 2 bar). I don't have any feel for how that will affect the flow-rates to the two sides. I've never given it much thought until now, actually.

Sorry; I wish I could be more help, but that's about all I can add. It would interesting to know how GE responds. My main association with hydrogen-cooled generators was to try to keep the mechanical TAs from flooding the generator casing and hydrogen purity cabinet with oil during initial charging, trying to accurately calibrate the hydrogen purity meters, and trying to establish stable scavenging flow-rates during initial operation. I do recall one site that had a huge seal oil flow-rate on initial operation and it was subsequently found that the generator manufacturer (Asian) had installed the upper and lower seal oil rings in the lower and upper end cavities, respectively. That only caused a several day delay and a huge clean-up, not to mention a lot of problems with the hydrogen purity monitors.