Initial inspection revealed numerous deficiencies that needed to be remedied. Outage consisted of High Pressure/Intermediate Pressure (HP/IP) turbine section major inspection, HP control valves #3 and #4 inspection, and the left-side combined reheat valve, as well as the Ventilator valve and Blowdown valve. Low pressure A bearings #3 and #4 were also inspected and a start-up trim balance was performed.

Further, recommendations were made for maintenance and repair tasks in the future. Significant work was performed as follows:

HPIP Rotor Repairs

MD&A’s Steampath division performed on-site inspection and repairs of the HPIP rotor. Several issues were noted and resolved, as well as more significant deficiencies, particularly in the IP steampath. Repairs included the replacement of the IP stage 7 buckets, blending of the stage 1 leading edge cover and the stage 2 leading edge airfoils. Strap lapping was performed on various diameters, and low speed balancing of the rotor was performed.

Upon inspection, the original stage 7 buckets were found to be in need of replacement. MD&A replaced the stage 7 buckets with new hardware provided by the customer. The buckets and covers were assembled onto the rotor and the tenons peened. They were assembled in sequence per the customer-provided moment weight diagram. The HPIP rotor was installed in the lathe, the covers were final machined, and the tenon rivet heads were NDE inspected and accepted.

Repairs were required on stage 1 and 2 due to foreign object damage. For stage 1, MD&A’s technicians blended and polished the leading edge, cover and airfoils, thus removing any raised or rolled over material. The airfoils for stage 2 were straightened, benched back, blended and polished to remove stress risers and any raised or loose material. Rotor was low-speed balanced at the completion of the repairs.

Minor circumferential scoring was found on the inboard oil deflector diameter of the turbine end shaft end. MD&A strap lapped both oil deflector diameters to remove any raised material.

Inner Shell

A detailed visual inspection of the removed HP inner shell upper half and in-place lower half was performed by MD&A’s Steampath division. Components were closely reviewed for any signs of corrosion, erosion, foreign object damage (FOD), or other damage. As a result of the team’s findings, on-site repairs were performed, which included thermowell repairs, general stoning, blending, and polishing, and replacement of inlet snout seal rings.

Inspection revealed that the tip of the thermowells on the shell’s ID had severe foreign object damage. MD&A repaired three thermowells on the upper half HP inner shell. Two outboard thermowells were removed, replaced, and re-welded. The third middle thermowell was no longer utilized by the plant and a plug was produced, installed, and welded.

Stoning, blending, and polishing was conducted on several components where needed. On both right and left axial locating fits, minor galling and raised material was found. Stoning was performed to remedy this. The upper half left side bolt hole spot face had arcing damage due to destructive nut removal, which was blended and polished. Minor galling and raised material was found on the faces of the transverse gib key post, which was stoned. Stoning was also performed on the N2 Packing Casing axial locating fits as well as HP2- HP6 diagram grooves to remedy rough surfaces and minor galling.

Inlet snout bore seal rings were found loose on both the left and right side of the upper half and required replacement. After the client had the snout bores machined and honed, MD&A’s technicians installed new inlet seal rings.

HPIP Diaphragm Repairs

A detailed inspection was also conducted on the HPIP diaphragms. The results showed mostly dings, bends and FOD on the partition trailing edge and sidewalls, as well as minor rub and/or bent teeth on the root radial on several stages. Many of the axial steam seal faces demonstrated minor galling and raised material, which were also noted on the HJ faces of several stages

MD&A’s Steampath division performed partition trailing edge straightening, blending, and polishing on stages 3 through 7, and 10 through 12. They also performed minor partition weld repairs to diaphragm halves on stages 7 and 9. Significant foreign object damage was noted on stages 2 and 8. On stage 2, major partition weld repairs were made on both the upper and lower diaphragm halves. Due to time constraints, full repair of stage 8 was postponed until the next outage. There was enough time to conduct immediate repairs, and some preparation for the next outage. Both diaphragm halves were benched back, and corners were weld repaired. Area and contour measurements were also taken and recorded as benchmarks for future reference.

Repairs also included stoning of surfaces as well as straightening and sharpening of seals. MD&A Outage Service division replaced tip seals packing segments on several diaphragm stages.

Alignment was also performed on internal diaphragms and packing casings by an MD&A Turbine Laser Alignment expert.

Also, during the inspection, several bolts and screws we found broken, and damaged threads were noted. Bolts were replaced, and the threaded holes were re-tapped.

L-0 Bucket Visual Inspections

MD&A conducted through the hood visual inspections on the L-0 buckets of the DFLP turbine sections, looking for typical issues. These include signs of mechanical damage, foreign object damage, visible cracks, erosion, corrosion, deposits, missing or “bound-up” midspan dampening sleeves, missing/damaged side entry covers, and missing/migrated wheel dovetail pins.

The findings revealed that most components were in good working order, with one issue that required short-term attention, and several minor issues that should be simply monitored. For instance, the L-0 buckets exhaust sides / trailing edges revealed minor deposits, while the inlet sides / leading edges revealed minor water erosion of the bucket’s nose.  Minor erosion was noted on the leading side bathtub tenons. The LP inner casing above showed minor water erosion / grooving.

One more significant deficiency was noted: atmospheric relief diaphragms rubber disks were found cracked. It was recommended that these be replaced during the current outage.

The visual inspection of the L-0 buckets did not reveal any other issues or findings worth noting or monitoring.

Control Valves

Control valves CV-3 and CV-4 were removed, inspected and serviced. For #3, the bypass seat in the main disc was worn flat, preventing proper seating, and replaced with a new one. The bypass disc for this section was also worn and replaced. For #4, the bypass disc and seat were lapped to restore proper contact. Both bypass discs were also machined to achieve bypass lift per manufacturer’s design specifications. Main seat on both #3 and #4 were lapped to restore 100% contact.

A hydraulic fluid leak was discovered on the CV-4 actuator, leading to replacement. The coupling rod was also replaced, and MD&A machined the rod shoulder to achieve proper clearances

Combined Reheat Valve, Left Side

The combined reheat valve, left side was disassembled, cleaned, inspected and had work performed on major components. The anti-rotation pin hole on the stop valve stem was found oversized. To remedy this, the valve stem and nut were replaced. Significant lapping was required on the reheat stop valve seat and PSH gasket fit to restore 100% contact.

Auxiliary Valves

The Blowdown valve and Ventilator valve were inspected during the outage, and both required repairs. Inspection of the Blowdown valve revealed that the stem and seat were unevenly worn. Both were replaced. Common wear parts on the Blowdown valve and the actuator were noted as worn and replaced. The piston rod bushing inner wall on the Ventilator valve was breaking apart and replaced with a new one.

Conclusion

Detailed above are just many of the significant and minor repairs that were completed based on a thorough inspection by MD&A’s experts. The overall size and success of this particular undertaking cannot be underestimated. In addition to inspection, and repair, MD&A provided single-point project management, which allowed for efficient and coordinated scheduling of all resources involved in the project.

MD&A is a full-service, OEM-alternative, and is respected and utilized worldwide for projects ranging in size and complexity. We are the service provider of choice for power generation asset owners based on their extensive experience, responsiveness, professionalism and dedication to getting the job done both quickly and properly. Above all, MD&A is proud to be a dependable one-stop solution for virtually any power-generation inspection, maintenance and repair need – within the domestic US, and at Global locations worldwide.

Call MD&A today with any steam turbine issue at +1 (518) 399-3616 or use our Contact form.

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MD&A welded and repaired the HPIP outer shell, HP & IP inner shells, and nozzle box on a 677MW large steam unit. Inspections revealed axial distortion and mechanical damage of the critical positional fits on the outer shell, inner shells, and nozzle box caused by galling during an overhaul disassembly, and out-of-roundness of the HP and IP inner shells due to operational thermal stresses.

With over 40 years of experience, MD&A has the technical expertise in the power industry to repair your turbine. Our philosophy involves intelligent, technically sound, and proven repair options.

The HPIP outer shell upper half galling damage on the axial positional fits to the inner shells was weld repaired on-site. The repair involved weld preparation, NDE, preheating, welding, heat treatment, final machining, runout checks, dimensional checks, and final NDE and hardness checks.

Due to the welding and machining operations performed on the outer shell to repair the damage to the axial fit face surfaces, this slightly changed the axial position of the sealing face and reduced the overall width of the tongue for the tongue and groove fit in the outer shell. In order to restore the correct axial position of both the HP and IP inner shells within the outer shell, and to ensure the tongue and groove fits had the correct designed clearances, the entire 360° seal faces and clearance faces of the HP and IP inner shell fits were weld restored and machined.

Weld repairs and a full stress relief re-rounding operation of the HP and IP inner shells was performed by MD&A’s experts in St. Louis Turbine-Generator Repair shop. The HP and IP inner shells were re-rounded to within 0.036” of original design diameters after the stress relief re-rounding operation was completed.

Also, due to the amount of as-found horizontal joint gaps and diameter out of roundness with all studs tensioned, it was decided to perform horizontal joint welding and heat treatment of both the HP and IP inner shells. The repair process involved weld preparation, NDE, preheating, welding, heat treatment, final machining, and checks.

The upper half nozzle box damage was weld repaired, the snout ring bores were honed, and new snout rings were installed. The raised material and galling damage on the axial fits of the nozzle box were ground out and the fits were prepped for welding. The areas were preheated, welded, and locally post-weld heat treated. A GTAW welding process was used for this operation. Once the work was completed, a final NDE inspection was performed on the welded and dressed areas and no indications were noted.

Because of our vast knowledge and experience with different types of OEMs, MD&A experts are capable of performing any welding repairs. We are your full-service turbine-generator service provider. No matter what the problem, MD&A’s experts have the solution!

Have MD&A look at your casings/shells today, call our Turbine-Generator Repair Facility at (314) 880-3000, or use our Contact form.

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