Robust repair fundamentals for 7FA (GE Energy, Atlanta) second-stage turbine nozzles was the subject of an invited vendor presentation before the 7F Users Group last May. It was one of 18 half-hour presentations by equipment and services providers arranged in three parallel sessions of six papers each.
That tells you many users attending the meeting did not hear the presentation by Matt Walton of Sulzer Turbo Services, La Porte, Tex. But most of those who did found it of significant value—at least judging from the number questions asked. What the questions by owner/operators also revealed was a general unfamiliarity with shop procedures for repairs to critical turbine parts.
The editors got with Walton to review the highlights of his presentation for the greater community of F-class gas-turbine (GT) users. He stressed the term “robust repairs” throughout the instructional session. It implies upgrading of second-stage nozzles where possible rather than just “repairing” (for example, simply welding a crack) and reinstalling the part.
Why focus on the second-stage nozzle? Walton said it was a unique component and one challenging to repair, given the material used, geometry, duty, etc (1). Coverage here encompasses the inspection process, as well as repair procedures to correct downstream deflection (DSD), distortion, cracking, and other issues.
Inspection incorporates in-shop and field dimensional checks, component disassembly, cleaning, and nondestructive examination (NDE).
In-shop dimensions for calculating nozzle pitch and throat area are illustrated in (2). Walton explained that nozzles control inter-stage flow and if the spacing between adjacent partitions is not correct, harmonics and wheelspace-temperature problems can surface.
In (2), blade pitch (W) and throat area (A) are measured in three places along the airfoil, which are identified as Ax, Ay, and Az. Throat area is influenced by nozzle height (H), and the distance from the tangent of one blade (T) to the adjacent nozzle.
The measurements identified in (3) are taken in the shop with the nozzle row removed from the turbine and reassembled on the donut fixture shown in (4). Note that the grooves in table match the nozzle hookfits. The donut fixture allows craftsmen to check diameters on a true basis, adjust/align the gas path and hookfits, and check seal engagement. The last is to verify that the seal segments for all nozzles match up and are performing as an integrated assembly. Note that the dimensions identified in red are critical.
Field clearance measurements enable a comparative check against shop measurements, Walton said; they must be taken by trained, experienced personnel. Knowing field clearances, Sulzer shop personnel can make repairs that address the idiosyncrasies of your engine rather than recreate an as-new assembly. This approach can improve performance, reduce rubbing after repair, prevent vibration issues, etc.
Measurements suggested to verify shop dimensions and determine axial and radial seal clearances and the axial blade clearance are identified in (5). Again, the color red stresses importance. Photo 5B offers evidence of a discourager seal rub. The more numbers you have, Walton said, the better view you have of what’s going on in your engine. Plus, work cannot be warranted without a thorough dimensional check.
Labyrinth seal rubs on the rotor associated with the second-stage nozzle are shown in (6). Photos like this one offers indications of operating conditions and provides the service engineer critical information about specific repair requirements. Ideally there should be no rubbing, which usually occurs because the casing goes out of round.
Disassembly of the nozzles is the next step. It requires removal of packing, packing hardware, joint seals, and core plugs and covers. Thermo wells and joint seals are replaced with new ones; usually, some cooling tubes and air guides can be reused. The core plugs in (7), which meter cooling air to the nozzle, typically are cleaned up and reinstalled.
Cleaning calls for stripping of coatings on gas-path surfaces of the nozzles as well as on the sidewalls and in the core cavity. Walton stressed that Sulzer prefers mechanical rather than chemical stripping for external coatings. The reason, he said, was that chemical stripping indiscriminately attacks aluminum content in the coating and base material. Non-uniform coating thicknesses guarantee at least some level of base-metal attack when using a chemical strip.
Walton admitted that unless technicians were highly skilled, the same could happen with mechanical stripping. Here’s how the process works at Sulzer: Technicians remove a conservative amount of material, the nozzles are placed in an atmospheric furnace and heated to highlight any (8), strippers remove any remaining coating with precision, and the process is repeated as necessary.
Prior to cleaning, a material sample is taken for metallurgical analysis of the base metal and both internal and external coatings. In most cases, the internal coating is acceptable for continued use. Chemical stripping may be applied in extreme cases. Once the coating, oxidation, and corrosion are removed, the part is sent for NDE.
The first NDE inspection includes ultrasonic (UT) and visible penetrant (PT) steps. The first is for thickness measurement, the second to identify cracks not visible with the naked eye. A photographic record of all 24 nozzles in the second stage is maintained for the life of the parts (9).
Goals for the repair process, Walton said, include the following:
- Be “right,” at a minimum.
- Facilitate installation.
- Reduce maintenance cost and extend repair intervals.
- Provide upgrades when appropriate.
- Custom-tailor repairs to suit unit- or customer-specific requirements.
Steps in the repair of second-stage nozzles include distortion correction, weld repair, stress relief, fixture checks, in-process inspections, recoating, heat treatments (diffusion, age), machining as needed, final assembly, and final inspection. Left-hand photo in (10) shows a nozzle as-received; one at right is being reassembled after repair.
Correction of downstream deflection (DSD) and distortion is a significant part of the repair effort for all second-stage nozzles. DSD happens because the nozzle (1) is loaded, (2) has a complex geometry, and (3) is arranged in a cantilever configuration. Load sources include a pressure differential of about 30 psi across the airfoil, a thermal gradient of about 250F, and forces caused by the nozzle’s conversion of pressure energy to velocity.
In effect, the load sources are trying to overcome the hookfit restraint and make the nozzle move downstream. Many powerplant personnel think of DSD as axial motion, but it’s not as simple as that: The component also is twisting.
These two points are described by the simplified diagrams in (11A) and (11B). The left-hand drawing describes the downstream deflection, the right-hand sketch the twisting. In the latter, notice the potential for twist to reduce or negate axial deflection in portions of the wall. Cracking that results from the nozzle trying to “get free” is shown in (12A). The craze cracking in the adjacent photo (12B) is indicative of creep caused by long-term use.
Work begins by installing the second-stage nozzles in a fixture and measuring the difference in axial deflection between the left and right sides (13). As noted above, twist contributes to deflection in all nozzles.
Eliminate the twist in a 7FA second-stage nozzle, however, and the deflection takes care of itself, Walton said. Because of metallurgy and geometry, there is no need to perform a DSD correction for this application if distortion is addressed correctly. Where corrective effort is required is obvious with the nozzles in the donut fixture. A sidewall mismatch is evident in (14).
Twist is eliminated in the donut fixture by use of bottle jacks, adjusting and realigning the gas path (15A). Hastelloy X braces are welded in the nozzle segments to anchor the inner and outer sidewalls (15B). The braces remain throughout the repair process to prevent distortion during welding and heat treatment.
When extreme twisting is in evidence, the nozzles may be welded to a flat plate to assure restraint (16). In such cases, the plate must have a similar coefficient of thermal expansion as the nozzle to avoid problems during heat treatment.
Weld repairs are made after routing out cracks and a full-solution annealing step. Inconel 625 filler wire is used on the GTD 222 vane material (essentially Inconel 939) primarily because of its favorable ductility. Stresses created during welding are relieved at solution temperature. Sulzer’s shop requirement for NDE following stress relief is no visual indication using red dye and no magnification. The entire process is repeated if one or more indications are found.
When a buildup in the thickness of airfoil walls is required because of extreme oxidation or material loss, Sulzer relies on a proprietary process the company calls parent-metal bonding. It avoids large amounts of surface welding and minimizes the potential for distortion, cracking, and adding stresses unnecessarily.
The material used is a mixture of parent metal and filler, which is formed into strips matching the shape of the airfoil (17). The material is machinable and weldable; thicknesses range from 20 to 120 thousandths. Bonding of the strips to the airfoils is accomplished in a furnace held at constant temperature.
After all repair work is complete, parts are cleaned with a media blast and white light is used to find any surface indications that may exist. Any found are blended. Next, the nozzles are coated and heat-treated. Then there’s another NDE step. Sulzer’s rule: NDE and fixture check each time you strike an arc or heat a part.
Finally the nozzles are ready for the final QC checks. They are reinstalled in the donut fixture to check diameters on a true basis and to adjust/align gas path and hook fits. Seal engagement also is verified to ensure installation will go smoothly. Lastly, the nozzle segments are checked in a slide-through simulator (18) to check hook geometry and seal engagement, measure throat area and pitches, and verify the elimination of DSD. ccj