Hi all,

I'm hoping someone here can help me with an issue we here at my plant have recently encountered. We operate 7FA turbines. While shutting down a unit the P2 pressure started to hunt from 350-450 PSI (typical P2 pressure is 400 PSI) while the unit was at ~10 MW's. Unit MW output started to hunt from 5-20 MW's, and the GCV's appeared to be cycling. After a few minutes of this, the unit eventually did shut down normally.

Is anyone able to help me at least begin to troubleshoot this issue? I would greatly appreciate it, and I thank you in advance.

 

madmaccc,

There's really too little information to say with any degree of certainty what might have happened.

If the unit has DLN-2.n combustors and I'm not mistaken I think there's a mode switch somewhere in this range--but I'm not 100% certain about that. That could have triggered an instability with the SRV which could also have a problem with a gas fuel supply pressure regulator that caused even more instability.

If your L.O. (which is also used as the hydraulic fluid) is in need of cleaning/conditioning and/or the last-chance filters at the valve actuator blocks are dirty and/or have ruptured that could cause servo-valve issues.

Depending on the age of the units and how often they cycle up and down it's possible the hydraulic valve actuators have developed worn areas where they are now "sticking" or ineffective or even passing/leaking oil past o-rings. (A LOT of F-class units use right-angle gas valves made by Woodward which require very regular maintenance and refurbishment.)

A knowledgeable person could use the LVDT calibration feature of the turbine control system (provided it's a Mark*) to do some checks on the LVDTs and the servo/actuator. I'm NEVER one to propose "calibrating" something without knowing if it even needs calibration (and--really--ONLY the LVDT feedback can ever be calibrated, not the servo or the valve, just the LVDT feedback). But there are ways to initiate tests using the LVDT calibration function to plot and analyze LVDT feedback and servo current in an attempt to try to isolate the problem.

If this has only happened once, and you're certain the gas fuel supply pressure was very stable when this happened, and that the L.O. is clean and the gas control valves and their hydraulic actuator are in good condition then I'd wait to see if it happens again. I might try using the LVDT calibration function--if I knew what I was doing--and if time permits (it's not a simple thing, especially the first time it's used by someone who's never used it before (actually, it is pretty simple, but it's also pretty intimidating--WAY MORE intimidating that just "calibrating" the valve...)). If this really caused some consternation in the plant and beyond, then you should probably be checking things like hydraulic oil filters, and replacing last chance filters at each of the control valves (they're not very expensive--certainly less than the cost of a new or refurbished servo valve, and MUCH faster to change out that a servo valve).

Since you have an F-class machine, you may also have some kind of data historian feature. If so, you could probably analyze some data from that run. Trip History data would be pretty useless for this, but real-time historical data (which might be captured on the HMI hard drive) might have some interesting information. Problem is--GE has a really, Really, REALLY bas habit of, one, changing the folders where that data is stored, and, two, of not properly enabling the feature, especially on multi-unit sites with multiple HMIs. So, we can't really tell you where to go and try to find any data--if you don't have and arent' familiar with a data historian which might be in use at the site. (And, even then, the data is probably pretty low-speed--once-per-second or ever slower).

We would really need to know a lot more about your site, and your experience, and what you're comfortable of and capable of doing, and your familiarity with the turbine control system--and really, most of the experience on Control.com is with Mark* turbine control systems, so if you have something else, we probably can't help too much with that. But the same things are probably going to be the root cause: dirty oil; dirty filters; worn actuators; LVDT issues; servo issues; gas fuel supply pressure regulator problems; even P2 pressure transmitter problems. Those aren't going to change with a different turbine control system, just the way one views and troubleshoots them.

Best of luck!