I am hoping to get some help in determining the cause of excessive H2 loss in our generator. We have been going through about 3 to 4 bottles/day compared to a normal of about 1/2 to 1 bottle/day (or 5 psig per day compared to about 1 or less). This has occurred following a purge incident while the unit was offline but not immediately, as immediately after for about two weeks usage was normal and then suddenly a sharp increase. The background information is as follows and any ideas are appreciated:

GE 7FA simple cycle turbine.
seal oil flow, DP, and scavenging flow rates are normal, and in fact scavenging flow rates on the E-One DHCP have been reduced from total flow of 3000sccm to 2500sccm in an effort to slow H2 losses (no noticeable effect on either purity or H2 usage).

Purge incident was due to power loss from substation (not at our plant) trip caused by weather.

CO2 Purge valve line-ups have been closed since the incident due to a stuck solenoid valve. we have done this so that we don't lose H2 through the CO2 header and out the CO2 inlet line which is normally valved into the purge vent line via another solenoid.

As mentioned after purge for about two weeks H2 usage was normal and then we had a couple of normal runs (about 10 hours on separate days) over the course of a week. then we went on an extended run (for us) of 5 days and toward the end of that run is when we can determine that the usage increased.

Logic would point to a relationship between the purge incident and H2 increase even with the two weeks of normalcy, but what exactly could that have done to cause this? We are leaning toward seal damage but aren't seeing any other indicators of seal damage or possibly something having to do with the CO2 valving closed off. However it seems highly unlikely to be related to the CO2 valve as it is closed (double blocked) at the header and therefore isn't connected to any other piping. The only reason CO2 valving is suspect at all is because it is an abnormal condition, but in reality it seems almost impossible to be the cause.

Thanks folks and hopefully someone can help because I dread the thought of a very expensive and time-consuming seal replacement if that is not the cause.

 

I am not familiar with CTs, but think the seal oil is about the same as STs (Also not familiar with scavenging either, but never mind)

I would think there would be a generator bearing drain detraining tank for the bearing oil and air side seal oil. this should have separate vent (GBV?)from the main oil tank. I have measured the total venting flow out of the vent and then measure the % H2 with the air to determine and quantify a H2 leak across the seals and into the bearing cavity.

the method I used decades ago was a large trash bag and a stopwatch. then forced the bag content through purity meter for %

Most Generator bearing caps had a pipe plug where a gas sample could be pulled out to determine which end.

Given it was during an extended run, could it be a cracked weld in some piping?

 

We have F7E GT. We had same problem with excessive H2 loss at one of the GT. We found out after hard sniffings that core plug on collector end was bad (this is a plastic one and it almost behind the fans and hard to imagined). Two leads for exciter have gaskets and due to excessive heat and no maintenance over the time they developed the leak. We end up getting GE at site to change the core plug. After that this plug check was a PM item. Hope this will help.

 

Could you explain specifically where that cap is located. I'll be going out later today to sniff around the bearing caps and if I can concentrate on that area it would be a big help.

 

> Could you explain specifically where that cap is located.

You can not sniff around the cap unless you remove fan cover and fan (and expose copper leaves) and unit should be down. we sniffed all over the other possible places. But with H2 meter you can move your H2 analyzer (meter) to read the readings (PPM). Best place to move H2 meter just above the fan assembly starting from top of the casing and moving upwards as H2 is lighter your reading will show high PPM of H2 if leak persists there. And if move upward and not vertically but diagonally towards doors reading will decrease. If you talk to GE and mention core plug at collector end, they will tell you more in details.

Good Luck please post your findings

 

Eventually found the problem. For others who may have a similar problem.... Found a small leak on the generator liquid level switch sight glass. This was not the ultimate cause however. Also, this bullseye type sight glass was a local site addition to the system for visual indication of liquid in the generator so anyone looking into similar issues may not have that.

The ultimate cause was found to be a flange on the H2 inlet piping upstream of the vertical filter and H2 inlet manifold. This is outside the collector cab package and was discovered using the old fashioned isolation technique. We closed the valve closest to the H2 header and the leak went away (H2 pressure loss over 24 hour period dropped to almost nil). This indicated a leak pre-generator. We then did the same test over 12 hour periods by opening that valve and then closing the next valve in the system, then the next, then the next until the leak returned and isolated the leak to a 15ft section of pipe. We had snooped and air sniffed that section prior but did it again more in depth and discovered the leak on the back side of the flange up against a frame where it was difficult to detect using bubbles and outside where apparently wind diluted enough to evade the sniffer. Replaced flange seal and leak went away.

 

DFoster,

Thanks for the feedback--and the troubleshooting process you employed! Both are most appreciated.