Mechanical Dynamics & Analysis (MD&A) has completed a quadruple Major inspection and repair of two steam turbine systems and two gas turbine generators, using the precision skills and state-of-the-art capabilities of its dedicated manufacturing, service, repair and parts facilities.
Coordination and comprehensive planning and scheduling were the pivotal keys to success of this complex work that included rings-off generator fields, diaphragms, steam turbine rotors, valves and actuators, and a variety of related items including parts supply.
Consolidated scheduling, communication, accurate reporting and corporate-wide commitment flawlessly united to ensure customer and MD&A-wide confidence in this landmark achievement.
Multiple Divisions
The focal point was the one-stop shop, MD&A’s Turbine Generator Repair Facility in St. Louis, MO, and encompassed various divisions including Generator, Steam Path, Machining Services, as well as the Ohio-based Bearings, Seals and Hydraulics Division.
The precision-based and long-established service and manufacturing capabilities of MD&A joined forces to seamlessly complete each detail of this multifaceted and time-dependent outage.
Scope
Overall scope of work included inspection and repair of 9A4 and 7FH2 generator fields, two HP rotors, HP and IP diaphragms, spill strip supply and installation, two sets of valves and two sets of actuators, and critical parts supply.
Generator
Upon arrival in St. Louis, MD&A completed rings-off testing, inspection and permanent blocking replacement of two steam turbine generator fields. Electrical testing showed unsatisfactory results and mechanical inspection indicated noticeable dishing of the collector rings. These would be honed and polished. Existing amortisseurs showed heat damage and axial cracking, requiring replacement by MD&A.
Existing blocking was replaced with the MD&A strap and tie design. These unique axial blocks have spring forces acting in opposite directions to add interference to prevent any unwanted axial or radial movement of the blocking after installation.
After repairs, the fields were moved to our high-speed balance facility in St. Louis where heat runs with electrical testing were successfully completed. Fields were then moved to machining services for collector ring work scope, including hone and polish. The fields were then final inspected by the Generator Division before returning to site.
Detailed recommendations were prepared for ongoing operation. In addition, MD&A recommended a full rewind at the next rings-off opportunity.
The two gas turbine generator fields also received complete inspection and repair. Mechanical testing indicated noticeable dishing in the collector rings. Slot spring misalignment was found.
Blocking would be replaced with the MD&A strap and tie design. Inspection showed that vent holes for the coils were partially blocked, which can result in reduced hydrogen gas cooling flow, leading to thermal instabilities with the rotor leading to excessive rotor vibration and forced outages.
Our experts performed a unique, patented 7FH2 generator field repair process (top hat modification) that eliminated the axial migration of slot leaf springs that restricted the air flow. Collector rings were honed and polished to correct the dishing.
The following components received non-destructive examination: snap rings, retaining rings, and field wedges. Because these gas-turbine units are hydrogen-cooled, pressure decay testing was performed.
After machining services work was completed, fields were released to the high-speed balance facility, then to the Generator Division for final inspection and shipping preparation.
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Steam Turbine Rotors
MD&A conducted detailed inspection and repair of the steam turbine rotors in St. Louis. Upon arrival the rotors were visually, dimensionally and non-destructively inspected. Repairs were conducted and detailed recommendations were made for long-term operation.
Dimensional, run out and non-destructive evaluation (NDE) inspections (including metallurgical evaluations) yielded minor findings that were addressed. Minor indications were identified, reviewed and dispositioned. Low speed balance and a full quality inspection was completed prior to return to service.
Minor repairs completed consisted of:
- Rabbet fit restoration via peening and machining
- Minor bucket repairs were completed by MD&A’s expertly trained bucket technicians
- Deburring and cleaning to remove any stress riser concerns, and
- Low speed balance
All detailed data inspections, repairs and as left conditions, including future repair recommendations were included in final reports to support future planning.
Valves
Example valve repairs, following complete inspection, included the following:
- Main Stop Valve
- Installed new stem & anti-rotation key
- Lapped the pressure seal head backseat to achieve a 100% blue contact check
- Installed a new stem/disc nut anti-rotation pin and staked, torqued and blue checked to the disc
- Machined the pressure seal head gasket face to correct runout and true it to the bushing bore
- Ground & polished the main disc seating surface to remove runout and surface imperfections
- Control Valve
- NDE revealed indications in the nitride around the disc OD, a new disc was purchased and installed
- New disc nut anti-rotation pin for the was installed and staked into the new disc
- Turbine End Reheat Stop Valve
- Honed pressure seal head bushing tight spots to restore design clearance to the stem
- Lap the pressure seal head backseat to achieve a 100% blue contact check
- Performed a weld buildup on the pressure seal head casing fit and machined to restore clearance to the steam chest on-site
- Machined the pressure seal head gasket face to correct runout and true it to the bushing bore
- Ground & polished the main disc seating surface to remove runout and surface imperfections
- Installed a new stem anti-rotation pin
- Installed a new disc cap bolt
- Generator End Reheat Stop Valve
- Honed the disc bore to restore design clearance to the stem
- Honed pressure seal head bushing tight spots to restore design clearance to the stem
- Lap the pressure seal head backseat to achieve a 100% blue contact check
- Machined the pressure seal head gasket face to correct runout and true it to the bushing bore
- Ground & polished the main disc seating surface to remove runout and surface imperfections
- Installed a new stem anti-rotation pin
- Intercept Valve #1
- Honed the disc bore to restore design clearance to the stem
- Restored the split ring ID to stem OD design clearance and verified
- Ground the split ring face in the surface grinder to restore the design stem lift
- Stoned the seal ring face to restore the axial clearance upon valve reassembly
- Ground & polished the main disc seating surface to remove runout and surface imperfections
- Intercept Valve #2
- Honed the disc bore to restore design clearance to the stem
- Restored the split ring ID to stem OD design clearance and verified
- Ground the split ring face in the surface grinder to restore the design stem lift
- Stoned the seal ring face to restore the axial clearance upon valve reassembly
- Ground & polished the main disc seating surface to remove runout and surface imperfections
- Installed a new seal ring bolt
At MD&A’s Bearings, Seals and Hydraulics in Ohio, two sets of steam turbine valve actuators (Rexroth and MOOG) were completely refurbished. The refurbishment included disassembly, cleaning, and inspection of all components. All soft parts and sealing parts were replaced, and in some instances some critical components were replaced, and this includes the following: large disc springs, piston shafts, servos, solenoids, LVDT components, packing glands, bushings, piston rings. Additionally, some components were restored using chrome and grind restoration methods to reestablish critical dimensions.
After repairs were complete the actuators were re-assembled, fluid flushed and tested at 110% operating pressure with the same fluid used at site. This ensures that the actuators function properly and ensures that the actuators are leak free during operation.
This testing greatly reduces the need for field troubleshooting and adjustment on site during start-up.
Explore the Benefits of MD&A
These Steam Turbine-Generator Majors and repairs led to their life extension, continued operation and longevity of the plant in the future.
Mechanical Dynamics & Analysis enjoys a firmly-established global reputation as a reliable and trustworthy non-OEM service firm with deep roots in all major OEM products and developments. MD&A’s professionals are up to date on all industry issues and offer these benefits:
- Comprehensive service and precise coordination.
- Multi-talented MD&A personnel in multi-faceted long-established facilities.
- One-stop gas turbine, steam turbine and generator point of contact for estimating, scheduling and project execution.
- Long-term operating assurance and operating guidelines for the owner/operator.
- All backed by MD&A’s world-class corporate commitment.
No job is too complex for MD&A. To discuss and schedule your next turbine generator service and repair project, contact the experienced professionals at MD&A. Call us at (518) 399-3616 today or use our contact form.
