During our T&I inspection, the turbine shell (casing) was removed for some job (not related to 1st stage nozzle). We found the 1st stage nozzles (S1N) at 9 & 6 clock positions have their trailing edge melted to some degree with TBC removed also from trailing edge. the leading edge is intact. S1N at other location like 12 and 3 clock are in good condition.
As per GE engineering, this could be due blocked cooling holes in the nozzles. Also they said the unit can run up to the next outage in this condition.
What makes this assumption valid (blocked holes) is that the compressor and the turbine stages were dusty compared to sister units. But I bore-scoped the nozzles from inside (since they are hollow from inside) and didn't find the cooling holes to be blocked. Further I blew some air to resemble the cooling process, and the air came out through the holes which suggests they are not blocked.
During normal operation, S1N are cooled from CPD directly, while S2N are cooled from stage 13 extraction, and S3N are cooled from stage 9 extraction.
Is there any possible reason other than blocked cooling holes to be investigated?
Although this not a mechanical forum, Can I ask a few questions to try and understand more.
1) How many hours does the machine have?
2) Do have pre-shutdown data, particularly Wheelspace Temps.
3) What fuel do run on? Do you use Liquid Fuel?
Thank you for your reply
1) Advanced Gas Path (AGP) carried out at 32K running hours. Now the unit running hours is 52K hours.
2) I trended the wheelspace for three different units. All of them (including the unit that has this melting issue) are around 800 F at the first stage forward wheelspace (the space that is between stage 1 nozzle and stage 1 bucket. The first stage forward wheel space is the nearest space to the melted S1N.
For the first stage aft wheelspace (space between stage 1 bucket and stage 2 nozzle) all the 3 units shows a temperature of 715 F approximately.
3) Our gas turbines mainly burn natural gas. You can say we run on liquid fuel less than 10 hours yearly.
Sorry for any confusion. All the wheel space data were taken at approximately 145 MW Load. The rated Load for our machines is 170 MW.
I don't usually recommend other websites, but if you own, operate or maintain a GE-design F-class heavy duty gas turbine you can apply to join the F-class Users Group, where they have many forums about various aspects of F-class turbines and a lot of people share a lot of information related specifically to those machines. Beware; some of it can be a little dodgey (as can any freely available advice or recommendations), but for the most part it's a pretty good source for all things mechanically related, especially hot gas path hardware (of which the first stage turbine nozzles are a part).
What were the spreads for the associated combustors (you would need a swirl angle calculator for that)? But, if the unit were being operated with high spreads for any length of time, it would stand to reason that there may be some follow-on damage, depending on the magnitude of the spreads.
Were you using OEM first-stage nozzles?
I believe some units (not sure about F-class machines) that use eccentric pins as part of the "adjustment" of cooling air flows. Are you sure the pins--if used on your machine--were properly installed and had not shifted during operation?
Hope this helps!