Combined Cycle Overhaul with Our One-Stop-Shop

Mechanical Dynamics & Analysis (MD&A) recently completed an overhaul at a combined-cycle plant consisting of gas and steam turbine majors, a steam turbine generator minor inspection, and a gas turbine generator major inspection. The customer’s combined scope was the largest single project implemented at the site since the combined-cycle plant was commissioned.

MD&A sent all three rotors—the steam turbine, gas turbine, and gas generator—to its one-stop-shop, which was a huge selling point for the customer. MD&A was able to leverage the full capabilities of its state-of-the-art 250,000 square-foot turbine-generator repair facility, centrally located in St. Louis, MO to inspect and repair the major components all under one roof.

To ensure the success of this large project, coordination between all MD&A divisions was critical, in addition to constant communication with the customer. MD&A assigned a dedicated project manager responsible for all on-site services, including subcontractors, parts supply, and shop services.

MD&A’s scope included a 7EA gas turbine major with a gas turbine rotor inspection and assessment in its St. Louis repair facility. Upon incoming inspection, the aft coupling patch ring migrated, leading to galling on the aft flange coupling no. 12 bolt hole, galling in forward-flange coupling no. 10 bolt hole, wear groove and scratches on no. 3 journal, and circumferential lines and scratches on no. 2 journal. MD&A experts removed the damaged patch ring, manufactured and installed a new ring, and machined the ring fit.

Incoming runouts and low-speed balance checks were performed on the gas turbine rotor with no runout issues found.

The MD&A team observed the coating peeling from S1 buckets. They measured dovetail wear and bucket rock and found them to be well within the MD&A acceptable range; therefore, dovetail coating was not recommended. Bucket rock is often caused by cycling power plants that spend a considerable amount of time on a slow roll or turning gear, and the result was wear between the wheel and the buckets, which often abrade the fir tree surfaces, causing a loose fit. MD&A performed an S1-S3 bucket replacement during the repair portion and confirmed final assembly with a low-speed balance. Its fuel nozzle services division also performed a 7EA dual-fuel dry-low NOx System 1.0 (DLN1) primary and secondary refurbishment.

The steam turbine major included removal of the rotor and shipping it to its steampath division for L-0 and L-1 bucket replacements. During the incoming inspection, MD&A experts found heavy damage to the Stage 2 buckets: They, too, needed to be replaced, which the end customer agreed to. The MD&A team removed the buckets and performed a rotor-blast cleaning to include the affected wheels. It reverse-engineered and manufactured the Stage 2 buckets and covers and also installed L-0 buckets and reamed for new pins, machined to fit the wheel, and installed L-1 buckets. Lastly, the final NDE and a low-speed balance were performed.

In addition, the scope included a full rewind on the 7A6 gas turbine generator field. After the field was removed from the stator, it was shipped to MD&A’s repair facility’s generator clean room. Upon incoming inspection, the team found Nomex® tab-style end blocking with broken and torn tabs, damage to the original creepage blocking, and cracked and deformed main lead copper leaves. The field was rewound with new creepage blocking, the main lead was replaced, and the company’s experts incorporated an end-winding blocking modification to optimize the field’s future reliability.

After rewind, the field was balanced in MD&A modern high-speed balance facility. The high-speed balance bunker can excite the field while at speed to ensure no shorted turns when operating at load.

On-site, MD&A’s generator specialist performed a test and inspection on the gas turbine’s generator stator, which included a visual inspection, end winding inspection, borescope inspection, and bore crawl through and inspection. The electrical testing consisted of winding insulation “megger” testing, polarization index (PI), winding resistance, DC leakage testing, and RTD testing. The specialist also performed a wedge-tap test to determine tight and loose wedges to produce a wedge tightness map in addition to a thorough cleaning. A minor inspection was also performed on the steam turbine’s generator.

Lastly, MD&A’s alignment division performed a topless laser alignment on the steam turbine that eliminates the reassembling of the turbine for “tops-on” measurements, saving hundreds of man-hours and labor per turbine section. Its experts performed a traditional tops-on/tops-off internal alignment on the gas turbine

Along with having access to MD&A’s turbine-generator repair facility, the customer also benefited from choosing MD&A to execute the outage by having a single point of contact for multiple services across the entire combined-cycle platform.

Single point project management allowed for coordinated scheduling of concurrent work on-site and in-shop, leading to an optimized execution that was achieved on schedule. The project manager had clear and responsive communication with the elimination of multiple vendor interfaces and provided daily divisional job activity reports directly to the end customer. With the elimination of multiple vendors, there is clear warranty responsibility, which gives peace of mind to the customer. Lastly, the customer was able to derive significant value from having one vendor supply services, parts, and repairs due to the inherent efficiencies of this approach.

MD&A is a full-service combined-cycle provider and OEM-alternative—services, repairs, and parts for your gas or steam turbine or generator. For every job, large or small, the speed and effectiveness of its response team are matched only by the depth and breadth of its engineering expertise. Call MD&A today for your combined-cycle outage at (518) 399-3616.