Through-bolt failures, R4 blade locking failures among hot topics at 501G meeting

501G logoThe value of users groups to gas-turbine owners—the 501G organization, in particular—cannot be overstated. Small by industry standards because there are only 24 engines at a dozen sites in the US and one in Mexico, it is the rare G meeting that doesn’t have at least one representative from each location; attendance typically is in the mid-30s, or an average of 1.5 attendees for every machine in the fleet. No other user group comes close to that level of commitment from owner/operators.

The organization’s size, capable leadership (sidebar), and collaborative nature allow it to think and act as a unit to resolve issues experienced on the deck plates and with the OEM. Minimal turnover in the top positions at G facilities, at least compared to the industry at large, means most attendees know the other participants and are aware of their plants’ idiosyncrasies, minimizing the need for “reviews” and repetitive discussion. In effect, each of the semiannual meetings picks up where the previous one ended.

501G Steering Committee

Chairman: Steven Bates, plant manager, Wise County Power Co LLC, GDF Suez Energy Generation NA Inc
Scott Wiley, fleet maintenance manager, GDF Suez Energy Generation NA Inc
Greg McAuley, VP, Turbine Maintenance Group, Calpine Corp
Kevin Robinson, operations manager, Lakeland Electric
Dan Jorgensen, maintenance manager, Granite Ridge Energy LLC, Cargill Inc
John Wolff, technical support/compliance manager, PPL Ironwood, PPL Generation LLC
Mark Winne, plant manager, Millennium Power Plant, Millennium Power Partners LP

A closed session for G users at the Savannah conference, Feb 25, 2015, was held the day after Siemens Day. The OEM’s technical program updated owner/operators on progress in resolving turbine rotor bolt, compressor through bolt, and R4 blade locking hardware issues, and included status reports on activities concerning combustion systems, quality improvement, disc-cavity thermocouples, bellybands, and generator spring bars.

The user-only program began with an open-discussion review of Siemens Day to identify issues requiring clarification, follow-up with the OEM, etc. The bolt failure at one of the G plants and Siemens’ redesign of that component was the first topic of discussion. RCA (root cause analysis) results and the plan for installing these new bolts in the fleet obviously were on the minds of attendees.

Iron oxide found in the compressor section of at least one unit was another concern raised. What’s the cause? Might it be water washing, carryover from evap coolers, or what? You got the sense this would be a topic of ongoing interest until the gremlin was uncovered. Another concern: Large (12 in.) spiral-wound gaskets failed in service shortly after a gas-turbine outage. A user reported that all gaskets provided for the outage were manufactured in China. Suggestion was to specify US-manufactured gaskets.

Headaches associated with wear and tear of disc-cavity (DC) thermocouples (t/c) were in evidence. A user reported that one failed on his unit; wiring was fine, but the tip broke off because of wear. The guide tube also revealed wear. A Siemens technician was required to remove the tip section at the bottom of the tube. Another attendee said the t/c for DC3 lasted only a couple of months; guide-tube wear also was experienced at his plant.

General comments on the quality of parts (belly bands, gaskets, etc) and workmanship kept the discussion lively for more than just a few minutes.

Around the plants. Most independent users groups serving GT owner/operators organize their technical programs by sections of the engine—for example, compressor, combustion section, turbine, etc. The G users begin with an “annual report” from each plant and follow that half-day program with user presentations on emerging and significant plant-wide issues of importance to the fleet. Notes taken during the plant reports follow:

Plant 1, equipped with three 501Gs, each with about 50k equivalent base hours (EBH), said one unit suffered a R4 locking-hardware failure which forced it out of service for several weeks.          

Damage to the R2 interstage seal also was mentioned.

Plant 2, equipped with two 501Gs, each with about 20k EBH (one near 650 starts; the other, more than 700) reported three R4 locking-hardware incidents on one unit, two on the other.

Plant 3, equipped with one 501G at about 75k EBH/750 starts, reported no new issues but ongoing challenges operating at 50% of rated load. Some dynamics issues (low, medium, and high frequency) are expected at low loads and low ambient temperatures. Wintertime dynamics were said to be “all over the lot” with no consistency in frequency.

Plant 4, equipped with two 501Gs, both north of 70k EBH/1500 ES, reported a slight performance improvement from installing refurbished baskets with uniform liner gaps on both engines. Cracks in the exhaust ducts required weld repair. Key issues included these: Plant unable to achieve full-load operation on the original (design) firing curve, oil leakage at steamer No. 2 bearing, extensive weld repairs required to restore hookfits (unexpected) during most recent combustion inspection (CI).

Plant 5, equipped with three 501Gs having EBHs ranging from 21k to 25k and ESs from the mid-500s to more than 700, reported on the first HGP for one of the gas turbines. Material liberated from wear and tear of the R3 Z shrouds went downstream and damaged R4 blades beyond serviceable limits. Also found was significant cracking inside the inlet scroll where the strut bolts up to the wall; this was mentioned as an ongoing issue.

The steamer for this 1 × 1 combined cycle continues to experience binding in the HP turbine section during hot starts. Plant is installing heating blankets to reduce the temperature differential between the upper and lower casing halves to a maximum of 10 deg F. Additional work includes stiffening the turning-gear platform and recommissioning of the system. A couple of users in the room thought turning-gear maintenance would be a good presentation/discussion subject at a future meeting.

Plant 6, equipped with two 501Gs having 57k EBHs and about 1600 starts each, reviewed its experience cleaning the ammonia injection grids and NOx catalyst for both units. The presenter mentioned that the plant had changed out two combustors (7 and 11) on one engine because of flashback damage. Exhaust temperature was derated by 28 deg F on that unit because of the flashbacks. Earlier, a GT had suffered a forced outage because of flashback damage to baskets 9 and 10. The DC3 t/c failed on one GT two months after replacement in both the spring and fall outages.

Plant 7, equipped with two 501Gs, both slightly beyond 30k EBH and 500 ES, learned of a cooling-air flow issue in DC2 of both units during a modified HGP inspection on one engine. Problem was discovered when R2 vanes suffered premature failure because of overheating. The replacement new-style vanes require less cooling air flow. An RCA suggested an improper flowmeter setup during site commissioning: The setting gave indication of proper flow but actual flow was about 30% less than indicated.

Plant 8, a 1 × 1 facility, mostly had positive O&M results to share over the years, but the tables turned in 2014. More specifically:

      • Combustion hardware issues were identified following a switch to 12k parts. The most serious was TBC (thermal barrier coating) delamination from a transition piece attributed to a steam coupling failure. A borescope inspection revealed three baskets had signs of flashbacks, suggesting a modified CI was needed. Metal spatter also was in evidence. Inspection of the steam circuit revealed debris.

      • HRSG fouling forced a GT backpressure derate. This initiated a brief discussion on the causes of fouling. An attendee with expertise in this area said that ammonia slip is known to increase during low-load operation, contributing to greater deposition.

      • LP economizer tube leaks.

      • A crack of significant size was found near a weld in the 18-in.-diam P22 hot-reheat pipe at a turn downstream of the attemperator. The spool piece was replaced.

      • A turbine through-bolt failure was found at 77.5k EBH/2500+ starts. Unit tripped on high vibration, which began at the exhaust bearing and cascaded through the other bearings. A borescope inspection revealed a broken bolt; a major inspection ensued. An unbladed replacement rotor was borrowed from another plant to reduce downtime. The outage took 43 days, about a month less than estimated for a repair without benefit of a replacement.

Back in the OEM’s shop, ultrasonic testing confirmed the cracked bolt. NDE was said to be of little value for identifying a crack before it propagates to failure, which reportedly can occur within about four hours after crack initiation. Machining of the GT and generator couplings was necessary for proper fit-up; new coupling bolts were required as well.

Plant 9, equipped with two 501Gs at about 25k EBH (one having more than 1000 starts, the other 750), required controls changes and reported on a disconnect between Alpharetta and Orlando for this work. This has been an ongoing user criticism for at least six years that the editors know of.

Plant 10, another 1 × 1, reported no major GT issues since the 2014 meeting. The facility ran more the past winter that it had in recent years. A positive was the installation of a new hydrogen dryer skid which has improved H2 purity to a consistent 97%-98%. The plant representative said the old dryer had no scavenge capability; dump and add was the method used to keep up purity. No payback period was reported for the new dryer, but better generator performance was evident. For example, less windage loss contributed a few hundred kilowatts of additional output.

The speaker reported that rotor air cooler leaks have occurred; they were expected at some point given fleet experience. Other highlights: the plant’s alarm-reduction effort is nearing completion; elevated bearing temperatures experienced were thought possibly caused by an accumulation of varnish; erosion/corrosion (unusual wear pattern) of blade-path t/cs is under study.

Plant 11, equipped with two 501Gs in mid 70s EBH/about 1500 ES, noted steam-turbine trip issues following conversion to T3000 controls as well as processor failures. Failures of new-style DC t/cs were reported as well. Leaks were experienced in the IP rotor air cooler just before the meeting. Unit was refurbished onsite with an all Type-304 stainless steel system—including tubesheet and tubes. Failure mechanism was stress corrosion cracking. Water seeps in between the tube OD and tubesheet hole and attacks the weld.

Two plants with a common owner, total of three 501Gs, reported on HGP outages. Two of the three engines were operating with 8k hardware, now all three have 12k. At the two-unit plant, after 24k EBH only five R1 vanes had overheating indications showed only minor wear on R1 vane rails. One transition revealed TBC delamination; pilot nozzles were burned up and missing pieces at three locations.

Linear indications were identified in a total of 35 R3 turbine blades on the two units, but after chemical stripping those indications were found in the base coat. The OEM cleared the blades for 48k service. Factory belly bands were replaced with the U-clip style, new seals provide for borescope access. Exhaust system weld repairs were required in the strut area. A Hastelloy backing bar/weld solution has given early success—no re-cracking in the first year of service.

Open discussion. A question was raised regarding the correlation between low-load turndown (LLT) and thrust-bearing vibration. An attendee with knowledge of the subject said that as thrust clearances open up you can get damaging vibration as you reduce load below about 50% of rated output. He had personal experience of severe vibration at 42% load. Clearances in the high teens (mils) are not problematic if you do not implement LLT, he continued. But if you want to operate at such low loads, suggestion was to install a machined plate (not shims) to close up the clearance to about 8 mils. There would be no overheating concerns at that clearance, the expert said, and you can go down to 30% of rated load without vibration.

Planning. A user presentation on the benefits of good planning in terms of safety, cost, plant availability, and equipment reliability was a segue to meaningful discussion. For most in the room, this was not new material, but reviews of this type serve as great reminders for O&M personnel overburdened with “must do” activities. Here were some of the reminders presented:

      • Well-planned work reduces, and may even eliminate, expediting fees.

      • Planned work is conducive to organized and time-efficient access to tooling, parts, procedures, and craftsmen with the proper skillsets.

      • Employees know what is expected of them each day, giving them meaningful goals.

      • Reduces stress: Having a list of tasks relieves the employee of the responsibility for making decisions on what work to do.

      • Doing planned work and preventive maintenance reduces “firefighting” issues and changes the concept of how work gets done.

      • Planned work reduces downtime for critical equipment.

Summary of benefits: Reduce backlog of work orders to a manageable level, increase the amount of productive work completed each day, allow timely ordering of critical and long-lead-time items, reduce “firefighting” by increasing the number of PMs completed.

The cost of poor quality. If there was any doubt in your mind regarding the value of user groups—the small, tightknit 501G users in particular—consider the following results of a three-year study undertaken by one of the members. Unselfish sharing of information among the plants pointed to 105 “quality occurrences” experienced by the fleet. The combustion section was the source of 46% of these problems, the turbine section 42%.

The project leader calculated that the OEM cost its customers 187 outage days (47 days parts only, 32 days parts and service, and 26 service only). The dollar value associated with these outages was estimated conservatively at about $12 million. The speaker point out that this figure assumes the cost of a cover lift at $1 million, but that number could easily double depending on specific circumstances. Example: an unplanned cover lift is sure to cost more than $1 million and take more than 10 days.

Posted in 501 F&G Users Group |

Comments are closed.