MD&A Completes Significant Combined-Cycle Inspection and Overhaul
Hermiston Generating Plant
For Hermiston Generating Plant in Oregon, Mechanical Dynamics & Analysis (MD&A) completed a complex, multiple-component combined-cycle major inspection and overhaul project that successfully brought both turbines and both generators back to full and reliable operations.
The 474 MW combined-cycle plant was depending on the complete and efficient renewal of its Unit 2 system.
This major project demonstrates the full capabilities and product knowledge of MD&A personnel, and the organization’s commitment as a premier, fully integrated non-OEM supplier to the turbine/generator community.
It began with a full major inspection of a late 1990s vintage 170 MW GE® 7FA gas turbine and included the 7FH2 generator robotic inspection work. The GE® A-10 steam turbine and 7A6 generator would also be fully inspected and refurbished by MD&A in the same outage.
7FA Gas Turbine
MD&A mobilized and completed the disassembly process utilizing a two-shift operation. Detailed visual inspections began, coupled with detailed NDE, followed by the recommended refurbishments.
In the turbine section, first-, second-, and third-stage buckets were replaced due to rubbing wear and thermal barrier coating (TBC) loss.
First-, second-, and third-stage nozzles were also replaced due to foreign object damage, evidence of cracking, and coating loss. Inspections and similar indications revealed the need to also replace the shrouds.
In the combustion section, liners and transition pieces were replaced with refurbished sets due to TBC loss. New inner crossfire tubes and retainers were installed due to wear and outer crossfire tube packing was replaced at reinstallation.
Although no abnormal visible wear was found, forward combustion cans and fuel nozzles were replaced with customer-provided refurbished sets. Flow sleeves also showed no wear, but the flow sleeve piston rings were replaced.
Liner caps were replaced with refurbished, and transition piece bullhorn brackets were found worn and replaced with new.
For the compressor section inlet guide vanes, MD&A replaced gears, rack, inner and outer bushings, and spacers. Inlet guide vane blades themselves will need to be replaced at the next major inspection.
Rotating and stationary blades showed no damage. R-0 inlet compressor blades were replaced with a refurbished set and shims were added to stages 14, 15, and 16. The casing and rotor showed no need for immediate action, but the discharge casing retention bars were replaced.
The inactive thrust bearing showed heavy scoring and the T-1 & T-2 bearings revealed pitting and scoring, which were subsequently replaced with refurbished bearings. The active thrust bearing was cleared for service.
For the generator, the initial scope of work was visual inspection, robotic wedge map analysis, electromagnetic core imperfection detection (ELCID), and a full battery of electrical testing.
The borescope inspection showed substantial widespread greasing and several areas that had loose hardware. The field was recommended to be removed for a more comprehensive stator investigation. After field removal, a core wedge map was performed that showed approximately 90% of the wedge system was loose and/or hollow, not meeting MD&A criteria. A full stator re-wedge, replacement of greasing blocking/ties, and axial support tightening was recommended and performed.
MD&A also provided and installed an improved the wedge design.
Prior to re-wedging, a significant amount of time was expended cleaning the core. All slots were cleaned including dovetails.
New filler material and top ripple springs were installed during the re-wedge. A modification was made to the end wedges to improve mechanical strength. The original flux probe was installed without issue, and a final ELCID was performed with acceptable values.
Based on modal bump testing, MD&A recommended that the entire collector and turbine ends have series blocking installed to reduce resonant frequency response. Saturated felt and ties were added to dampen the response.
New axial support hardware was installed, replacing the loose axial supports and hardware found during initial inspections. Locking epoxy was applied on all hardware to ensure no complications during operation.
H2 seals were replaced with new, and field collector rings were ground.
Successful electrical testing was performed at the completion of all work performed.
A-10 Steam Turbine
MD&A performed a major inspection and overhaul of the 81MW GE® A-10 steam turbine, generator stator, and field installed in 1996.
The main steam valves were also removed by MD&A and sent to MD&A’s St. Louis Repairs Facility for inspection and repairs.
MD&A specialists performed a complete steam path structural audit of the A10 steam turbine.
Although many minor diaphragm indications could be repaired by MD&A onsite, ILP diaphragms 9, 10, 11, and 13 were shipped to MD&A’s repair facility for major repairs. Stages 10 and 11 would also have inserts installed on the steam seal face due to dishing.
HP and ILP rotors remained coupled and were removed for sand blasting and NDE. Minor bucket repairs were performed on site to correct impact damage and moderate solid particle erosion.
On reassembly, MD&A performed a Topless Laser Alignment®, and MD&A On-Site Seal Services fit and installed new diaphragm and gland steam packing.
The 7A6 air-cooled generator was disassembled and the field removed. The generator field was shipped to MD&A’s St. Louis Facility for a full rewind. In addition, MD&A’s Generator Division mobilized onsite to perform a full stator rewind.
The combined HP stop and control valve was disassembled and the cores were shipped to MD&A’s St. Louis Repair Facility for inspection and repairs. The Steam Turbine Repairs Division also received two reheat stop valve and two intercept valve cores for inspection and repair.
Concurrent with the major steam turbine inspection, an elevated workspace onsite was constructed to support the generator division for stator disassembly and reassembly work. A baseline ELCID was performed to determine integrity of the current stator core iron. No shorted laminations were noted.
The wedge system was removed, then the flex probe was carefully set aside for reassembly.
With wedges and series loop connections removed, bar removal began. Inner axial supports were left in place and prepared for the reassembly. Connection pieces were cleaned for reuse.
The stator was thoroughly cleaned to remove any contaminants from the wedge/bar removal process. A post wedge/bar removal ELCID indicated no core iron damage during wedge and bar removal.
Each core slot was cleaned, and a detailed inspection of any abnormalities was conducted. The core compression flange and all exposed areas where the end windings sit were painted with an epoxy paint for a uniform color on the compression flange.
After a thorough cleaning, the rewind began.
Bar boxes were moved to the scaffolding deck with an innovative safety-conscious method of disassembling the scaffolding roof and flying the boxes to the deck with a crane following completion of a detailed lift plan.
Each of the six circuit rings were acceptance tested, and the outside binding bands were installed.
A tapered gauge from the bar manufacturer was used to ensure concentricity was achieved on the four binding bands. Concentricity of each band is a vital step that will properly align each bar and subsequentially the end winding basket once the rewind is completed.
Two top and two bottom bars were installed to ensure alignment. Bars were fit into a shoe on the collector end and carefully transferred through the bore to the turbine end. All 72 bottom bars were installed, blocked, and tied. All 72 top bars were then installed, blocked, and tied, along with 12 new resistance temperature detectors (RTD’s).
After all bottom bars were installed, a Hipot test was performed using a high potential voltage to ensure no bar armor insulation damage. Another Hipot was performed on all top & bottom bars at the completion of top bar installation.
Wedges were then installed, and filler was adjusted at each wedge for proper radial compressive force. Axial locking pins were installed, followed by a final ELCID and brazing. The existing circuit ring copper connection pieces were re-used and brazed to respective top and bottom phase connections.
Upon completion of all rewind activities, final electrical testing consisted of winding copper resistance, insulation resistance, and a final Hipot of each respective phase. Each phase produced satisfactory resistance values.
The stator rewind activities progressed as expected throughout this project. The consistent bar shapes and robust bar design aided in completing the project without incident.
The 7A6 generator field was sent to MD&A in St. Louis for testing, disassembly, coil removal, cleaning, further testing, re-assembly, and high-speed balance.
During initial electrical testing of the heavily dished collector rings, collector studs, and bore copper, the collector studs failed high potential testing. This resulted in the replacement of the collector rings which included removing the old collector rings, manufacturing new collector rings, new collector ring insulation, and reinsulating of the collector studs.
Coils were removed and sent off-site for cleaning. They were then returned and checked by MD&A.
After reinstallation, each coil received AC Hi-Pot and turn-to-turn testing.
Also, during the rewind process, the blocking was upgraded to the MD&A standard block and tie design. Turn insulation was coated, requiring a rotor bake cycle.
Electrical testing, high-speed balance, acceptance testing, and shipping followed.
Post-Outage GT Performance Analysis
MD&A issued a report that calculates the relative change in output and heat rate after the gas turbine outage. Operating data from pre and post-outage was used for the analysis.
MD&A provided startup and balance support of the unit.
Our team provided full recommendations of what to look for or replace at the next outage.
This combined-cycle inspection and overhaul clearly reveal MD&A’s total service capability and non-OEM commitment with deep and focused product knowledge and experience. The precision of all MD&A divisions in support of a common goal merged into a powerful, comprehensive response, returning the facility to full and long-term operations.
With this project, MD&A has added to its impressive project resume as a holistic provider of services to all makes and models of turbines and generators, returning them to and above their original standards.
MD&A is a one-stop shop! Call MD&A about your next outage today at (518) 399-3616Tags: steam turbine, combined cycle, gas turbine, Generator, Combined-Cycle Inspection, Combined-Cycle Overhaul
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