Custom repair techniques match complexity of original design

It’s not a secret that many Row 1 (R1) vanes serving in 501F gas turbines are deemed scrap (or near scrap) after the second repair cycle. Perhaps less well known is the variety of repair techniques and modifications available to salvage more operating time from these expensive components.

According to Matt Lau and Jason Field of ACT Independent Turbo Services Inc, Houston and La Porte, Tex, the company’s objective is not just to repair the vanes but also to understand failures. With this information it can adapt and improve parts, when possible. CCJ editors visited ACT to dig deeper into recent presentations on hot-parts repairs made by senior staff at user-group meetings, to prepare for the 501F Users Group and CTOTF™ conferences early in 2016. Both of those meetings regularly cover 501F component repairs.

Distortion and airfoil thinness. The interview began with ACT personnel explaining the fundamentals of evaluating component condition. Lau stated that with any cooled industrial-gas-turbine (IGT) component, metallurgical condition, wall thickness, and crack evaluation are the critical starting points for making sound repair recommendations.

For 501F R1 vanes, it sometimes is observed that repairs are performed without restoring the vanes to adequate thickness first. Priority one is thickness restoration on these thin airfoils.

Leading-edge airfoil. Lau and Field say a critical evaluation during an incoming inspection is the condition of the airfoil. If the leading edges have experienced severe damage (Fig 1), further inspection of the core cavities is warranted. The leading-edge core cavity has two thin strips, often thought of as guide rails for the core insert (Fig 2).  

ACT Figs 1, 2

ACT believes these guide strips are very important for pressurizing the leading edge with critical cooling air and directing cooling air to the leading-edge cooling holes. Company experts say there is a direct correlation with failed or missing core rails and burn-through of leading edges on repaired vanes.

For R1 vanes in fair condition, ACT incorporates replacement of core rails to assure proper core insert fit-up and correct cooling. For vanes with severe damage, the entire leading edge of the airfoil is replaced with one having rails cast into the component. New cooling holes then are CNC EDM-machined post repair in accordance with the cooling configuration program for that specific vane type.

Trailing-edge airfoil. Company experts further explain that trailing-edge airfoil damage can be approached in a manner similar to that for the leading edge by correlating trailing-edge damage (Fig 3) to the associated cooling flow. In the case of the trailing edge, ACT says that issues with the inner-wall covers (sometimes called the “bathtub,” Fig 4) are associated with severe trailing-edge airfoil damage.

ACT Figs 3, 4, 5

Oftentimes the bathtubs are overlooked during the repair process; tubs either are either shortened during removal or thinned by oxidation. If replaced, sometimes subpar tubs are installed using inadequate weld processes.

ACT recommends replacement and installation of new tubs at every repair visit (Fig 5). New tubs should be slightly thicker than nominal, of correct form-geometry, and “double welded” to assure good weld penetration. Lau explains that the upgraded bathtub replacement, with improved weldment, is “cheap insurance” when compared to the alternative of losing trailing-edge cooling efficiency and a tub.

Field explains that the bathtub can be thought of as a reservoir for cooling air with a specific metered volume. When there are issues with the tub, they will affect other areas of the vane cooling. Tub position and height also are critical. If too tall or too short, the amount of cooling air in the reservoir may increase or decrease. Tub deviation is detected via sonic-nozzle flow testing. Additionally, if the tub is positioned incorrectly, it can interfere with the inter-vane joint gaps.

Dimensional checks. When asked about vane installation, Lau and Field explain repairs are ineffective if the vanes do not fit properly in the machine. They say the 501F R1 vanes are relatively thin, move easily, and the result is usually severe post-operation distortion.

The best way to approach distortion correction is to first mock-up the machine with a solid, tight-tolerance fixture that simulates the R1 blade ring. The second portion of the approach is to have “master segments” that are within ±1 mil of nominal specification. The tighter the tolerances of master segments, the better the end product.

With the correct tooling, the next approach is to clearly understand the datum planes, both radial and axial.

After correcting the distortion and performing weld repairs, joint gaps and the torque-block lug must be addressed for proper installation and fit-up. Field explains that the torque-block dimensions are critical, but often are overlooked by repair shops where fit-up and component relationships are not fully understood.

The evidence: W501F vanes often sent to ACT for repair do not have blade rings. One reason may be is that repair shops not associated with outage groups rarely get to assemble the vanes to see how they really work. Lau further explains that uniformity is critical to avoiding issues onsite. Example: Torque-block lugs must be held to a very tight tolerance (+0.002/-0.000 in.). Lugs must be machined, not hand-blended.

Another critical area is the joint seals and seal grooves (Figs 6 and 7). Joint seal grooves should be addressed after all other repairs and distortions are corrected. In cases where severe distortion correction was performed, all seal grooves should be welded solid and re-machined. All seal grooves should be test fit with actual seals and in an accurate fixture using master segments.

ACT Figs 6, 7

ACT’s goal is to make the joints so uniform that it does not matter where they go in the assembly. The result will be the same dimensional tolerances within all segments. Caution: If the grooves are done prior to correction, compounding errors will occur, causing incorrect engagement and sidewall misalignment.

Seal engagement, while maintaining correct sidewall alignment, is tough even for the most contentious repair shop—but it is critical. Lau says this is much easier to achieve if all of the distortion is addressed during repairs. ACT cuts the typical allowable tolerance in half to assure uniformity.

Cooling holes and configuration. Only when all of the above vane repairs are completed can a repair shop address the cooling holes. Field says it is ACT procedure to perform 100% weld repairs near and around any cooling holes to remove all cracks—even it means the cooling hole will be lost. Many times cooling holes need to be remade to assure the proper diameter for flow consistency.

Field states that cooling-air pressure and the flow of cooling air is critical to vane life. Re-machining the holes using CNC EDM processes re-establishes the correct cooling holes, position, and vector.

Lau says it is challenging for repair shops to program the often complex 3D geometry of the cooling holes and locations. Shops also have to identify and understand the multiple cooling-hole configurations and patterns. Not all OEM and aftermarket vanes have the same hole configurations, and a mistake there may cause issues with flow and cooling that can limit component life or result in failure.

Coatings. Lau says that he has always believed the 501F R1 vane required a robust coating upgrade when compared to typical specifications. With today’s advances in coating technology, there is no reason that F-class vanes should not get the coating they require for optimal life. For the 501F R1 vanes, Lau and Field prefer a high-aluminum-content CoNirAlY bond coat with an 8- to 14-mil-thick thermal barrier top coat. Robotically applied coatings with high bond strength are crucial for the demanding intervals that users are trying to achieve.

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