F-class hot-parts repair clinic helps users write better specs, identify optimal solutions

You’re probably tired of hearing about the loss of experienced managers, engineers, and technicians to retirement and the knowledge gap this has created on the deck plates. You might even want to scream, “Enough already, the turbine shaft is still turning and we’re making electricity.” True enough, but look around, who will you turn to when questions arise? What if less senior employees—and management—expect you to have the answers? This may be the right time to invest in skills/knowledge development. It’s the first step on the pathway to career advancement.

Participation in user-group meetings is a good way to get started. The all-volunteer steering committees generally excel at selecting subject-matter experts to speak on topics conducive to improving plant availability/reliability, reducing costs, improving safety, maintaining emissions within regulatory limits, etc. The presentations on hot-parts repairs for F-class gas turbines by Allied Power Group’s (APG) Technical Director Aaron Frost at the CTOTF™ spring meeting are a case in point.

It’s unusual for a small fleet to have more than one or two persons both knowledgeable about damage assessment and repair of turbine blades (buckets in GE speak) and vanes (nozzles) and familiar with repair shops and their practices. This is a business risk given the cost impact of having an inexperienced person approve unnecessary repairs or, worse, not making repairs needed to maintain reliability goals.

7FA+e first-stage nozzle repair. The CTOTF Leadership Committee invited Frost, who has more than two decades of experience in the metallurgy and repair of hot parts—including a few years with the OEM and another third-party services provider before joining APG in 2009. Two characteristics of a Frost presentation: Slides are jam-packed with information (imagine a CliffsNotes version of a handbook) and a rapid-fire delivery.

His presentation on 7FA+e first-stage nozzle repair was first on the agenda at the GE F-class Roundtable. Frost is big on history, to help users better understand why things are the way they are. His backgrounder on stage 1 nozzles showed how component design progressed from the 7221 (7FA.01) through the 7FA.05. For example, nozzles for the first three engine models in this series were made of FSX-414 material (cobalt), the last two of GTD-111 (nickel); first and third models had some areas of the airfoils coated, the second none.

The Dot 04 and Dot 05 have some characteristics of the 7FB (only about a dozen built)—specifically single nozzles (nozzles on the earlier engines are arranged with two airfoils per segment) and full thermal-barrier coating (TBC). Two other distinguishing characteristics of the 7FA.04 and 7FA.05 are that they have trapezoidal cooling holes; plus the GTD-111 material essentially is unweldable.

Thinking out loud, Frost guessed that the singlet design of first-stage nozzles for the Dot 04 and Dot 05, and the full coating of those airfoils, might be the OEM’s solution to the first-stage cracking experienced on the earlier models in the 7FA series. He pointed to several photos of extensive nozzle cracking included in one slide, calling it “intimidating.” Cracking is most severe in the uncoated areas, the metallurgist continued, with significant thermal fatigue damage often identified as early as the first repair cycle in airfoils for peakers. But such distress is not exclusive to simple-cycle machines, he added; it has been found on base-load engines as well.

The required weld repairs typically are extensive, requiring bars and strongbacks to control distortion to the degree possible (Fig 1). Component shrinkage should be expected.

APG Fig 1

Caveat emptor. One got the impression from listening to Frost that the changing economics of buying new parts versus repairs to existing ones is challenging the current industry structure and may limit choices for owner/operators in the future. Perhaps not, but the new economics certainly means users will have to bone up on their knowledge of repair processes, industry standards, repair-shop capabilities, inspection technologies, etc, to protect generation assets.

Today, he said, new nozzles may only be twice the cost of repair. This means due diligence on repair shops is critical. With repair pricing exceedingly competitive, you have to be sure you’re getting what you think you’re getting, Frost said. Make sure you’re comparing apples to apples when evaluating competitive bids.

Here are some numbers to ponder and help you understand what’s going on. Assume the estimated cost of new first-stage nozzles from the OEM in 2003 was X. Four years later that price had dropped by about 25%; by 2010 it was about 0.5X; now it’s approaching 0.4X. There have been significant cost reductions on the repair side as well. Consider the following timeline:

      • In 2003 it cost Y for weld only and coupon repair.

      • Four years later, the cost of repair had dropped by about 25%, similar to the reduction in the price of new parts; however, the scope had changed to weld/coupon/braze as required.

      • This year you’re likely to see bids in the neighborhood of 0.55Y, but for a reduced scope of brazing and minor weld repair.

Frost illustrated the impact of cost pressures by way of example. First repairs in 2008 by the OEM on nozzles with severe distress were characterized by minimal scope: Cracks were fixed using Renewalloy™, inspection was visual (no red dye or fluorescent penetrant), nozzles were stripped and recoated. Photos of the same parts received by APG five years later confirmed most cracks had been brazed, outer sidewall cracks were not repaired, surface craze cracking was extreme, overall nozzle fatigue health was poor. Frost’s assessment was that the as-received condition likely would have been no worse had repairs not been attempted in 2008.

The APG technical director presented several other case histories as well. One revealed that one third-party shop wanted an owner to scrap a set of first-stage nozzles after two full repairs (one by the OEM) and about 40,000 hours/1600 starts and 50 trips, based on incoming inspection results and condition review after an acid strip.

APG accepted the challenge, installing trailing-edge coupons and welding all cracks with modified Mar-M-918 (a/k/a Nozzaloy). Butt gaps were repaired using Haynes 214 weld wire. The airfoils—which came to be known as the “train wreck” nozzles—ran more than 16,000 additional hours and 750 starts before replacement in 2012. A fourth repair cycle was not economically attractive for this set of nozzles.

APG Fig 2Frost illustrated the importance of quality repairs and detailed photographic records in another example. As Fig 2 shows, Nozzaloy™ repairs, properly done, give excellent results. Cracks should not reappear during the next run; however, the inherent stress in the nozzles is relieved by a crack in another spot on the airfoil.

Frost’s thoughts on braze alloys followed. He said the chemistry of braze alloys is provable and provided some examples. The metallurgist added that aero braze alloys sometimes must be customized for land-based service: “Just because it can fly, doesn’t mean it’s good for powerplants.”

Several more topics were covered in the presentation, including a useful primer to help you better understand the value of fatigue data and what it means, how to calculate coating density using basic arithmetic, importance of bond coats, etc. All this material is available to users in the CTOTF Presentation Library. Some of the takeaways:

      • High-density coatings (less than 10% porosity) achieve maximum cyclic life. A tightly controlled coating process is required to achieve this level of quality. Keep in mind that the thicker the coating, the more likely it is to spall; high-density thin coatings are optimal.

      • Visual inspection has its shortcomings as a reliable indicator of repair quality.

      • Non-OEM casting quality is higher than the OEM’s in some cases.

      • Users should carefully evaluate repair alternatives and repair services providers; the OEM often is not the optimal choice.

      • Unrealistic specifications drive the wrong shop behavior.

      • Performance loss and tuning issues many times are blamed on repaired parts. More often than not, this has no basis in fact.

      • Braze is used for filling cracks where cost is the primary consideration.

      • Acid stripping of cobalt-base alloys without pre-strip heat treatment is ineffective.

      • Learn how to interpret parts numbers. All castings are not created equal; some suppliers are better than others and mixing blades of different origins can give you headaches. Likewise, parts number revisions by the OEM can put you at risk in the repair process; know your parts and keep meticulous records.

501F row 1 turbine blades. Frost’s second presentation of CTOTF’s spring meeting, before the Siemens Advanced Frame Roundtable, reviewed the design history, repair, and field experience with 501F row 1 turbine blades. He began the history lesson with the 20-yr-old 501FC engine (170 MW, 15:1 pressure ratio, 2375F firing temperature, 1050F exhaust) and carried through to the 232-MW 501F4 machine with a pressure ratio of 18.9:1, firing temperature of 2475F, and 1100F exhaust.

Frost focused primarily on the design of Gen 1 and Gen 2 platform cooling circuits, tip plates, a successful seal-pin slot modification, and welding materials for this group of gas turbines, covering the various alternatives offered by the OEM, an alternative OEM, and major third-party parts manufacturer. With all the parts and repair options available to 501F owner/operators it might be in your best interest to review Frost’s slides posted on a shelf in CTOTF’s Presentations Library.

Considerable wear and tear in the platform area generally is addressed by the repair shop. For an OEM set of blades, all burned and weakened base material at the platform edge must be removed before weld repairs. Photos in Frost’s slides illustrated the problem. After the platform is fully welded, the front face is remachined to assure proper seal-pin fit-up, APG’s platform cooling mod is installed and another mod implemented to prevent buildup of problematic foreign material behind the seal pin.

Suggested tip-cap welding and reconstruction procedures follow. Allied uses Haynes 230 material for its tip cap. First step in the process is to remove old tip caps and cooling holes, then pre-cap weld, tack weld the new tip cap, complete the tip-cap weld, and add the squeeler tip. Finally, the cooling holes are replaced, finishing the repair. Each piece then is checked by x-ray. Final step in the rehab process is the application of dense, vertically cracked TBC.

Want more information on repairs? A good place to start might be the special report, Six steps to successful repair of GT components by Hans van Esch, founder of Turbine End-User Services Inc (Houston). If that’s the kind of information you’re looking for, consider attending van Esch’s specialty training course, Metallurgical aspects of IGT component refurbishment, held two or three times annually.

Posted in Best Practices |

Comments are closed.