STG L-0 turbine blade failure
STG L-0 turbine blade failure
(OP)
I have a 130 MW STG in a CCGT plant application that experienced high cycle fatigue failure of several of the L-0 buckets in less than 8700 hours of operation. OEM is stating that 4 separate events of low vacuum opertion caused this failure (a total of 21 minutes in duration), The last low vacuum event occured 2 weeks before the failure (over an 11 minute period of time). Profilometry inspection of the failure surface indicated that the crack growth during this 11 minute period of time was insignificant. We did a whole battery of metalurgical tests and examinations that still support HCF.
One item that we found was that there was more than 20 % variation in the area check (measured at 3 points from the radius) of the L-0 diaphragm and as much as a 12 % variation in the area check of the L-1 diaphragm. We are thinking that the poor area check control by the manufacturer could have created enough pulsations to excite the L-0 blades, considering that the blade design only has a 5.8% margin at the 1X natural frequency. It might be important to note that this is a 60 Hz application with a STG specification requirement for the STG to operate at 60 Hz +/-5% without limitation.
Has any other user out there been able to correlate Area Check variation in stationary diaphragms with resonance excitation of downstream blades? What levels of acceptable area variations are user seeing in STG diaphragms? What margin of safety off of the 1X and 2X natural frequency is typically being seen on new LP blade designs?
Thanks
One item that we found was that there was more than 20 % variation in the area check (measured at 3 points from the radius) of the L-0 diaphragm and as much as a 12 % variation in the area check of the L-1 diaphragm. We are thinking that the poor area check control by the manufacturer could have created enough pulsations to excite the L-0 blades, considering that the blade design only has a 5.8% margin at the 1X natural frequency. It might be important to note that this is a 60 Hz application with a STG specification requirement for the STG to operate at 60 Hz +/-5% without limitation.
Has any other user out there been able to correlate Area Check variation in stationary diaphragms with resonance excitation of downstream blades? What levels of acceptable area variations are user seeing in STG diaphragms? What margin of safety off of the 1X and 2X natural frequency is typically being seen on new LP blade designs?
Thanks





RE: STG L-0 turbine blade failure
RE: STG L-0 turbine blade failure
And, how did you get this vacuum excursion?
We must live at opposite ends of the earth. Where I live no one is troubled with low vacuum right about this time of year.
rmw
RE: STG L-0 turbine blade failure
Plant is located in the Amazon, so it is generally hot all year, plus this is an islanded grid, so there are a lot of issues with power system impacts that always hammer the plant.
RE: STG L-0 turbine blade failure
rmw
RE: STG L-0 turbine blade failure
You mention that crack growth during one event was insignificant. Almost any crack will have a major effect on the response of a bucket surface condition has a major effect on endurance limit.
Vacuum excursions are an easy target for the manufacturer and can cause L-0 failures but design and operation of condenser dumps and low load transients should also be investigated.
RE: STG L-0 turbine blade failure
i.e. did both L-0 stages fail? or just one?
usually the clearance between the diaphragm and the L-0 is pretty high... so there is a lot of steam mixing downstream of the diaphragm.
nevertheless... you could model the situation with CFD and see if there is a significant excitation due to the uneven area.
saludos.
a.
RE: STG L-0 turbine blade failure
Low vacuum running has also been attributed to causing failures in this case poor design of exhaust annulus can be an important factor.
Regarding design margins the blading characteristics should have been accurately established and ideally be clear of lower order engine orders over a speed range of +/-6% of design running speed (fixed speed turbine).
As always it can be difficult to establish the specific contributing mechanism of failure however detailed examination of the failed blading can give useful information in establishing the nature and mode of failure.
RE: STG L-0 turbine blade failure
You are actually quite lucky the entire unit wasn't trashed.
Cheers
RE: STG L-0 turbine blade failure
RE: STG L-0 turbine blade failure
I'd still try and press your EPC contractor to fit blade vibration monitoring to ensure the blades are operating in a safe zone (load+vacuum wise). This is quite different from having blades designed for operation at +-5% frequency. The former is a function of the strength in the blade/root design, the latter is a function of how the blade is 'tuned' to avoid certain natural frequencies.
I'd also make sure you're steam chemistry is faultless because the OEM metalurgists will try and prove SCC or poor steam chemistry played a part in the failure.
RE: STG L-0 turbine blade failure
by all means chemistry MUST be checked and DOCUMENTED...
but if the OEM tries to blame it on the chemistry...
then the NEW limitation on low load operation should not apply, right?
the OEM should have no objections to guarantee, provided the chemistry is within specification, that there are no problems at any load.
also... if chemistry was the problem... there should be indications elsewhere and not only in the L-0.
cheers
saludos.
a.
RE: STG L-0 turbine blade failure
RE: STG L-0 turbine blade failure