Fuel Nozzle End Cover Insert Life Extension
MD&A Fuel Nozzle Services Division can extend the service life of a Fuel Nozzle End Cover Insert. The life expectancy of an end cover increases from the OEM claim of 48,000 hrs. to over 125,000 hrs. with a new maximum of 7 insert replacements per cover.
A braze-cracking issue can occur with F-Class DLN 2.0 and 2.6 end cover designs. Turbines at the greatest risk are those burning unheated fuel.
Low-cycle fatigue is the primary cause of braze-joint fracture at the 4 possible braze locations on each insert location. Thermal stresses are a contributor because temperatures in the susceptible locations can cycle from 100°F to 700°F.
For fuel nozzles of OEM design, insert braze cracking creates the opportunity for gas to leak into neighboring circuits and contaminate burner-tip cooling air. This scenario can create emissions and tuning challenges, not to mention the possibility of increased flash-back potential. Flashbacks can result in nozzle damage which can lead to catastrophic failure.
These failing conditions are identified if insert replacements requirements are met and reported on at incoming inspection (pressure test). At this point, under these conditions, the industry standards are to perform a full insert replacement of each failed cover assembly.
Additionally, the currently understood industry standard for brazed insert replacements (as set forth and published by the OEM) is a maximum of 3 insert replacements per cover before the cover is deemed scrap. Each time an insert replacement occurs, .030” of material is removed from the bore to establish clean parent metal for the new braze joint.
Our team has performed extensive research and development found that the “3-and-out” strategy was only true for the original 7FA DLN 2.0 cover design and not the others.
Using the original 7FA gas turbine DLN 2.0 combustion end cover as a baseline for our assessment, which used the as-designed 5/8-11 threaded wire inserts in order to secure the outer fuel nozzles to the end cover. A minimum wall thickness, at pressure vessel requirements, had been calculated to be .078”.
It was confirmed that the 7FA DLN 2.0’s 3rd repair cycle, the minimum wall thickness is optimally .081”. Therefore the 3 insert maximum was met.
We modeled and destructively tested the 6FA DLN 2.0 end cover which had also been assessed by the OEM to only permit 3 insert replacements.
However, we had also discovered through destructive testing and 3-D modeling of the “6FA” 2.0 cover’s internal passages, that while maintaining minimum wall thicknesses insert replacements can double over the OEM recommendations of “no more than 3 replacements allowed”.
Solution & Life Extension:
MD&A Fuel Nozzle Services found that insert replacements up to 7 repair cycles, had a minimum wall thickness at .093”. The same is true for the 7FA DLN 2.6 since the 6FA DLN 2.0 and 7FA DLN 2.6 designs use ½-13 threaded wire inserts rather than the 5/8-11 of the 7FA 2.0 to secure the nozzles to the end cover.
Our team can extend the life of the end cover on the 6FA DLN 2.0 and 7FA DLN 2.6 past the currently understood industry standard by continuing the repair cycles until the minimum wall thickness is obtained. We have also incorporated additional machining of the cover to address the warping issue that is associated with the end covers throughout their lifetime, especially with those that are utilized in an unheated application.
Conclusions & Results:
With a new maximum of up to a final 7th insert replacements per cover, the life expectancy of an end cover increases from the OEM claim of 48,000 hrs. to over 125,000 hrs. At an average estimated rate of repairing 4 covers every 3 years, you would not need to replace a cover for 75 years!
MD&A is a worldwide leader in the repair and refurbishment of gas turbine fuel nozzles. Our advanced engineering expertise combined with leading-edge equipment delivers reliable, extended life for new or refurbished fuel nozzle products.
Call MD&A today for Fuel Nozzle End Cover Insert Life Extension at +1 (864) 721-4200 or use our Contact form.Tags:
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