Ontelaunee details experience with GT uprate, exhaust cylinder fix

The 501F Users Group’s 2018 conference and exhibition is only a few months away—Feb 25 – Mar 2, at the Hyatt Regency Grand Cypress, Orlando, Fla. It is your only chance in the coming year to visit with the major service providers and parts suppliers for the 501F fleet. Exclusive closed sessions for owner/operators with Siemens, Mitsubishi, PSM, and GE are a highlight of the program.

Add to that package the following:

      • User presentations on recent experiences (both good and bad).

      • Direct participation in roundtables dedicated to the compressor, turbine, combustion system, rotor, generator, inlet and exhaust sections, auxiliaries, safety, and hot-gas section.

      • The opportunity to select from more than three-dozen vendor presentations on O&M topics selected and vetted by the steering committee.

      • The opportunity to spend quality time with nearly 100 vendors in the exhibition hall.

No budget, no time is no excuse for not attending this meeting. Participation will save significant staff time and O&M costs in the coming year because you’ll be better prepared to anticipate problems already experienced in the fleet, know whom to call about them, and solutions to implement.

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Following up on key topics from the 2017 meeting a couple of weeks ago, the editors caught up with Adam Sensenig, who made two presentations at last February’s conference in Reno, Nev. During a visit to Dynegy’s Ontelaunee Energy Facility in Reading, Pa, Sensenig, Ontelaunee’s plant engineer, provided additional details on the recent uprate of the facility’s two gas turbines and a “fix” for cracking of strut shields in the two-piece exhaust cylinder, a 501F fleet-wide issue.

As background, Ontelaunee is a 2 x 1 combined cycle which went commercial in 2002, and features 501FD2 machines. In recent years, the plant has been operating primarily at base load, with an average capacity factor of 87% for 2016. In 2014, the plant contracted with PSM for a long-term service agreement (LTSA) and the company’s GTOP 6 upgrade package.

GTOP (Gas Turbine Optimization Package) is PSM’s non-OEM performance-enhancement offering for the 501F market. The uprate increases mass flow to 501FD3 levels.  By signing with PSM, the plant avoided the exhaust cylinder and R4 blade-ring replacements characteristic of the OEM’s uprates.

The plant was first out of the gate with GTOP for the 501F. About 18 months transpired between project kickoff and completion.  Overall, Ontelaunee gained 7% in net plant output with a 1.7% improvement in net plant heat rate. It was able to maintain a large portion of the increase in output by way of an aggressive online water-wash program.

Five upgrade elements. Sensenig breaks down the uprate modifications into three areas: new blade-path components that take advantage of prevailing metallurgical, coating, and design improvements; modified inlet-guide-vane (IGV) actuators to boost air flow by extending stroke length from minus 2 to minus 6 deg; and by adding auto-tune capability.

Overall goals were to increase the air/mass flow through the turbine, reduce the amount of air required for blade path cooling, and achieve a higher total exhaust temperature.

Regarding the blades, the R16 compressor blades feature a different airfoil shape, and were a “drop-in” replacement. The R1 blades, vanes, and seals were modified to decrease cooling-air flow, as were the R2 blades and vanes. The last-stage R4 GT blades are about a ¼-in. taller to accommodate the higher mass flow and additionally reduce exhaust swirl. R3 blades and R3 and R4 vanes remain the same. Of these, Sensenig credits opening up of the IGVs and the taller R4 blades as having the biggest impact on the results.

The IGV actuator mods could be accomplished by replacing the actuator or modifying existing ones. Ontelaunee had one spare actuator which was modified for one unit and the plant purchased a new actuator for the other GT.

Auto-tuning, through the combustion dynamics monitoring system (CDMS) and PSM’s Autotune Version 2, assures flame stability—and emissions stability—under all operating conditions. One consequential saving with the auto-tune, says Sensenig, is that the plant no longer has to call someone out to adjust the controls for seasonal ambient conditions.

Attention to BOP. The upgrade did require the plant’s (and its third-party engineers) careful attention to balance-of-plant (BOP) impacts. PSM conducted the plant assessment up to through generator output but only guaranteed simple-cycle GT performance. Some BOP impact examples:

      • The four-way joint where the turbine casing and the combustor shell meet has significant fleet-level issues. The uprate leads to higher shell pressure, and the impacts, such as greater potential for leakage, has to be monitored.

      • More air flow through the turbine, of course, means more air flow through the HRSG; plant personnel need to keep up with HRSG maintenance, and be cognizant of the higher HRSG backpressure.

      • The HRSG high-pressure steam drum safety valve had to be resized and replaced.

      • The SCR’s ammonia vaporizer is running at near capacity at maximum output; while ammonia consumption per megawatt dropped, the absolute level of ammonia feed increased because unit throughput increased.

      • Water-treatment chemical consumption increased because of higher demineralizer demand and cooling-tower load.

In general, Sensenig notes, “We’re anxious to see what the parts will look like after 24 months of operation.”

Because Ontelaunee was Rev 0 for the PSM GTOP, a substantial effort was required to qualify the R4 blade design, which added three to four days to the outage. Blade monitoring had to be conducted under a variety of operating modes, including startups, shutdowns, speed sweeps, IGV sweeps, and inlet fogging and steam power augmentation.

Other issues Sensenig describes as run-of-the-mill for outages and significant equipment modifications. The plant experienced a trip during over-speed/under-speed testing because of the trip-limit settings. Capability to modify the controls within the TXP DCS “was limited” and the plant had to “clean up” unused counters and other items to free up processing space, but Sensenig notes this should not be an issue with the newer control systems.

Exhaust-cylinder fix solves “most” problems. Ontelaunee was not first for the exhaust-cylinder fix, performed by Texas-based Braunflex LLC. Important to note is that this fix doesn’t solve all the problems experienced fleet-wide with the support struts, but for a fraction of the cost of a new exhaust cylinder, it solves the most important ones.  

At the 501F Users Group meeting, Sensenig reported that, upon inspecting one modified unit after 5000 operating hours and 10 starts, only minor stress-relief type cracking on the inner load plates had been observed. At the time of the CCJ visit, both GTs had been modified. With 25 starts and 11,000 hours on the first modified unit, they both are “looking good.” No additional severe cracking has been observed.

The two-piece cylinder design issues stem from differential thermal expansion between the inner diffuser piece and the outer case. The issues range from common strut shield cracking to complete liberation of the load plates. Other repair options offered included new flanged load collars and replacing load plates with a new material, but neither adequately addresses the fundamental thermal expansion issue.

The modification at Ontelaunee is essentially a Hastelloy X collar overlay onto the original strut shield. The collar allows for controlled growth while still supporting the outer diffuser. Said another way, the outer diffuser can expand independently of the inner diffuser and outer casing.

Get the slides! Readers interested in knowing more are urged to visit the 501F Users Group website at http://501f.users-groups.com. The slides include diagrams with rich detail of blade comparisons, 3-D graphics of the modified components, before and after performance graphs and tables, and much more.

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