CTOTF™: Addressing user concerns from the GT air inlet out to the transmission line

Plant operations personnel expressed a healthy skepticism over prognostics. All of the presenters, in one way or another, addressed what perhaps is the greatest challenge: the disruption to traditional plant processes and organizations. Centralized M&D often is perceived, at least initially, by the plants as “big brother” and a nuisance because advisories generally are not well-vetted.

No one on the panel would argue that it is critical to pay as much, if not more, attention to communication within the organization as to the technology implementation. Yeager stressed at the top that “the human is an integral part of the control loop.” Even so, prognostic control can be run closed-loop, with the “system” simply alerting the human about what it is up to.

Makansi observed that prognostics advocates focus almost exclusively on the “catches,” alerts, and predictions which prevented failures. He suggested users measure and report non-catches as well, and develop metrics that capture value lost from poor predictions or poor communication relative to value gained from preventing failures and incidents.

Clearly, there is much value in these technologies. Entegra has cut in half the labor required for calibrations. All documentation for instrumentation is now managed with greater accuracy and consistency in the AMS digital system. In one instance, NV Energy received an alert that avoided close to $1-million in expenses associated with a potential attemperator failure (Fig 1). Whether they detect issues better or faster than properly trained and competent human operators remains a valid point of contention, but virtually everyone knows the real question is whether the new technologies can do as much, or more, for less money.

Tiger. One of the featured presentations at the Generators/HV Electrical/I&C Roundtable, “Techniques for Effective Monitoring and Diagnostics of Gas Turbines,” extended the diagnostics and predictive-analytics theme of the conference through the final day of the meeting. The presenter was Dr Charlie Nicol, diagnostic systems manager for Turbine Services Ltd, a wholly owned subsidiary of Chromalloy.

Nicol has been the chief architect for the Tiger turbine monitoring and diagnostic system since its birth more than 20 years ago. Background: Tiger began as a European funded project, involving a multinational consortium taking leading-edge research and applying it to the monitoring and diagnosis of GTs. The commercial product, developed by Intelligent Applications, was acquired by Sermatech and subsequently by Chromalloy.

Tiger is installed on 90 turbines today—including several models of gas-turbine frames and aeros, as well as steam turbines. The product generally has an excellent reputation among long-term users, but one has to wonder why there aren’t many more plants using this diagnostic system. Perhaps it was the prolonged period of development and too many participants in that process; it took 17 years to go from a prototype to 60 installations two years ago.

After listening to the presentations on the first day of the fall meeting, it may be that GE Intelligent Platforms (read SmartSignal) and others have passed Tiger by—at least in the US. Aggregating data from many like machines improves the predictive models and an aggressive OEM certainly has a leg up in this regard. Engines current Tiger users are operating include GE Frames 5 (two shaft), 6B, and 7EA; GE aeros LM2500 and LM6000; P&W FT4; 501F; GE steam turbine.

Nicol’s presentation began with the ABCs—the reasons for monitoring and diagnostics, the skill sets required by those interpreting and using the information collected from your engine, how an incident profile benefits the engine owner/operator (such as planning corrective action), etc. He then dissected the software and explained its structure and functionality, and how the company’s M&D center in Glasgow works.

Nicol noted that Tiger classifies diagnostics according to severity, turbine area, relevance to the main conclusion, and number of occurrences. It enables the user, he said, to easily replay the data, graph the information, access turbine manuals, link to user-specific notes, etc. Nicol then gave some examples of Tiger’s “finds,” such as sticking fuel and bleed valves, faulty controller cards, improperly operating speed ratio valve, faulty vibration sensor, etc. Digging into the details, he used bearings to illustrate how Tiger uses diagnostics (vibration data in this case) to detect bearing unbalance, misalignment, oil whip, oil whirl, rubs, and/or blade damage.

There were a few dozen more slides illustrating information displays in chart and table form, how to track startups and shutdowns and use aggregated data to guide future starts and stops, how different variables are displayed on-screen, etc. Find Nichol’s presentation, and others made at CTOTF meetings over the last 10 years, in the organization’s keyword-searchable library at forums.ctotf.org. The archive, which now contains more than 500 presentations on GT technology, operation, and maintenance, is available only to users. Sign up today if you are not already registered—no charge. Instructions are in the sidebar.

HEP concerns

The attemperator photo (Fig 1) was the perfect introduction to the Combined Cycle Roundtable, where this equipment is discussed. Ed Sundheim, CTOTF vice chair for program development and director of engineering for Essential Power LLC, made a short presentation summarizing common failures in “boom-era” high-energy piping (HEP) systems. It provided particularly valuable perspective for engineers new to the industry and those whose professional positions focus on the Brayton side of combined cycles.

It was not Sundheim’s intent to provide details, but rather to identify potential problem areas that bear watching. Background and current information on each of these topics can be accessed by using the search function in the top right corner of the CCJ ONline home page at www.ccj-online.com. He focused on three areas:

Attemperators, noting poor design, improper location in piping systems, leaking spray valves, thermal fatigue damage, etc.

P91 and material issues in pipe runs, valves, fittings, welds, etc, primarily related to deficient quality control during manufacture and installation.

Liberation of stellite hard-facing in main and reheat stop and non-return valves, and steam turbine stop and control valves as well.

Sundheim spoke to what he called a “daisy-chain” of HEP failures. His professional opinion, based on a 50-year association with powerplant equipment, is that many problems owner/operators face today can be traced to poor initial design, fabrication, and construction. He believes that some designers did not/may still not adequately understand high cycling stresses or the cures. Also, that some contractors did not/may still not fully appreciate the critical nature of P91 welding and heat-treatment requirements and the need for tight quality control. Finally, he questioned the origin of some Code documentation owner/operators have been given.

High cycling and part-load operation of combined-cycle units designed for base-load service have contributed to the issues noted. Sundheim mentioned the overuse of attemperators and the resulting failures caused by quenching, as well as nightly shutdowns that are conducive to condensation and slugs of water in steam systems and consequent quenching damage.

Stellite liberation is an issue Sundheim is following closely and he urged others to do the same. He said it has become a significant industry-wide problem with more than 50 cases already reported. Damage has been associated with the products of most all manufacturers of P91 stop and non-return valves for HEP systems; plus, the major steam-turbine OEMs have acknowledged problems with their valves. At the present time, Sundheim said, there’s no foolproof method for attaching stellite to P91—at least based on his research.

EPRI is leading a study on the problem but no concrete findings have been announced yet to the editors’ knowledge. In the meantime, at least one plant owner believes it’s time to consider alternatives to stellite hardfacing. It is taking a proactive approach in buying replacement valves from a supplier with little experience in the manufacture of steam valves for F-class combined cycles but with positive experience in the use of chromium-carbide-based proprietary material on wear faces. Stay tuned.

P91 valves. Next, Sundheim alerted attendees to a new problem associated with P91 steam valves: the incompatibility of Type 316 stainless steel seat rings with P91 bodies because of differential expansion issues. In wrapping up, he suggested that users consider starting an HEP health monitoring program, or upgrading the one they have, consistent with new industry findings. Here are his recommendations:

• Inspect all P91 welds and heat-affected zones for proper hardness.
• Establish a Code compliance program.
• Research your HEP Code documents.
• Establish strict standards for HEP repairs and monitor/inspect work carefully.
• Vet all HEP suppliers and services providers.
• Avoid field welding of P91 where possible.

Other recommendations:

• Regarding steam turbines, follow prescribed OEM guideline documents—such as GE’s Technical Information Letter 1629-R1, “Combined Stop and Control Valve Seat Stellite Liberation.”
• Monitor and inspect attemperators regularly.
• Monitor main steam and reheater drains for proper operation.

HRSG drains. Sundheim paused here to stress the importance of proper operation and maintenance of drain systems for preventing the formation of condensate and subsequent chill-shocking of valves and attemperators. In recent conversations with other combined-cycle operators he reported hearing this common theme: HRSG drain systems are maintenance hogs because of their (1) location at the bottom of the boiler where they are prone to attack by gas-path condensation /corrosion, (2) parsimonious allowance for thermal expansion, and (3) cycling beyond design assumptions.

“At our Newington plant,” he added, “we have gone through an extensive refit of the HRSG drain systems” and have done the following:

• Added insulation to prevent corrosion to the extent possible.
• Provided for thermal expansion.
• Installed redundant stop valves so that one valve throttles and the other only sees full-open/closed service. “By eliminating the fears of operators that they might cause a valve or line failure, plant personnel are now less reluctant to operate the drains and we have much better control over condensation/slugs,” Sundheim said.
• Planned to add drain pots in each line when the maintenance budget permits. The drain pots will be equipped with thermocouples so the control-room operator will know when the line is blowing liquid or wasting steam.

Cold-weather operational impacts

Personnel at plants in cold climes generally are adept at dealing with winter weather events and prepare accordingly. However, facilities to the south often are caught “napping.” The consequences can be severe. During the Regulatory and Compliance Roundtable, a user reported on lessons learned from cold-weather incidents. The information was compiled by NERC. The following case histories were excerpted from that presentation.

Case History 1. During a severe winter weather event, human performance errors caused generating units to trip, requiring rolling load sheds.

Lessons learned: To maximize human performance during adverse conditions consider doing the following:

• Train personnel on the plant’s plans for freeze protection and emergency cold weather operation to provide them better insight into the challenges presented by winter events and how to deal with them.
• Develop and practice procedures to manually receive critical operating data in case instruments and transmitters malfunction or freeze.
• Assign to the plant for the duration of extreme weather events additional personnel who are trained to perform critical jobs.

Case History 2. Very low temperatures forced a plant offline. The balancing authority and reliability coordinator had not been informed that the plant was not designed to operate during such extreme conditions.

Lessons learned. The generator owner/operator is responsible for the following:

• Know the ambient-temperature limitations of the plant and inform grid management of the low-temperature limit.
• Establish procedures that stress identification of possible freeze areas after maintenance or design changes and upgrade protection as necessary.
• Enable operation at low ambient temperatures by identifying areas of the plant susceptible to freezing and adding insulation, heat tracing, and wind breaks to maintain water temperature at 40F or above.
• Document and institutionalize knowledge and experiences from previous severe winter weather events and apply this learning to winterization procedures.

Case History 3. During an extreme winter weather event, a five-unit combined cycle had engines fail to start, or trip, numerous times because of frozen instrument transmitters and sensing lines. The loss of generation forced the balancing authority and reliability coordinator to shed load.

Lessons learned included the following:

• Appropriate consideration should be given to the potential effects of wind in removing heat from equipment.
• Bypass the cooling water system, if possible, during very cold weather to prevent freeze-up.
• Purge transmitter sensing lines inside the GT package prior to winter weather events to prevent the build-up of condensation which could then freeze.
• Identify unit-specific critical freeze-protection areas and the work required to decrease the likelihood of long-duration freeze impacts.
• Develop emergency operating procedures to allow unit operation in freezing weather when critical instrumentation is lost.
• Where feasible, monitor heat-tracing circuits to verify circuit integrity. Also, check amperage and voltage periodically to assure that circuits are operating at their design output.
• Heat tracing doesn’t last forever; develop a replacement schedule in accordance with the manufacturer’s recommendations.

GE F-class engines

The GE F-Class Roundtable was intense, running virtually non-stop from 8 am to 5 pm. Session Chair Pierre Boehler, a senior engineer for NRG Energy Inc, and Vice Chair Mike Hartsig, plant manager for Star West Generation LLC’s 2 × 1 Griffith Energy, organized 11 prepared presentations and several open discussion sessions that more than filled the day.

The morning was dedicated to presentations by the OEM and open discussion between owner/operators and the GE team. Key presentation topics were an update on the compressor section, 7F design evolution and hot-section changes, new naming convention, compressor-blade monitoring system, hot-section hardware, and the OEM’s designs to address new market requirements—such as fast start/fast ramp capability.

An open discussion on getting more life from existing hardware was good segue for repair expert Hans van Esch’s (TEServices™, Houston) afternoon tutorial, “Extending 7FA Parts Life Beyond the OEM’s Recommendation.”

Several of the GE presentations were valuable for their ability to put everyone in the room on the same page, no matter what their level of experience. For example, the compressor presentation offered details on the company’s enhanced packages for both flared and unflared units. Unless this information is available on the screen, users can get lost in the storm of nomenclature and tune out.

Example: One slide describing the components of each of the several enhanced packages for the flared compressor allowed attendees to see what elements were included in the Package 3 that they might have and to compare those elements to what others in the room had with a Package 4 or 5.

Fleet experience with each of the packages—for both flared and unflared compressors—also was made available. This information included numbers of units equipped with, as well as planning to install, each package—plus starts and hours leaders for each option. Such perspective contributes to a user’s ability to make good decisions.

Other compressor design updates presented by the OEM’s engineers included the following:

• R0 retention issue addressed in TILs 1796 and 1870.
• Aft stator twist.
• Shrouded S17 distress.

Significant portions of GE’s compressor presentation were dedicated to airfoil blending and fleet corrosion experience. The OEM discussed its blending process, history, analyses done to confirm blending is within airfoil design limits, performance impact, etc. Corrosion of compressor components, which is influenced by the local environment, effectiveness of inlet filters, maintenance of the inlet filter house, and water washing, among other variables, was of considerable interest to the group.

New naming convention. A PowerPoint you’ll probably want to review in the CTOTF Presentations Library is “GE HDGT Naming,” which describes the new naming convention for GE frames. You may recall the OEM’s first series of descriptors: PG7221FA, PG7231FA, etc. Then came a simplification: 7FA, 7FA+, etc. The next change was to the not-so-intuitive, but logical: 7FA.01, 7FA.02, etc. Now there’s the 7F 3-series, 7F 5-series, etc. The latest code comes in long and short versions for both unit and model designations. It is too complex to describe in words. A “Berlitz” type course is available in the form of a table included in the presentation. It is easy to follow but has too much information to remember. Consider printing the table and taping it to the refrigerator in the break room for easy access.

Life extension of hot-section parts launched the afternoon session of the GE F-Class Roundtable at a high energy level with the polished Hans van Esch at the front of the room. His presentation was based on the experiences of EPRI, Duke, and TEServices, the company he founded, regarding engineering and assessments of 7FA buckets, nozzles, and shroud blocks.

The metallurgical authority, who spent the early part of his career in component engineering and repairs at industry leaders such as Hickham Industries and Elbar (both now part of Sulzer Turbo Services) and Chromalloy, opened by saying the OEMs are conservative about life expectations of their industrial gas turbine (IGT) parts—especially for advanced-technology components.

End users have three options, he said: follow the OEM’s low-risk/high-cost approach, extend service intervals without proper engineering and assessment (low cost/high risk), or opt for a qualified third party to perform the required engineering and assessments, with some cost and risk involved.

Van Esch believes that with 20 years of experience, F-class technology has reached a level of maturity that allows turbine owners to shift from a risk-mitigation to an O&M cost-reduction strategy. An in-depth understanding of the design evolution and superalloy materials provides the engineering basis for safely extending useful hot-section economic life.

The operating hours remaining in IGT hot-section components, van Esch continued, can be determined by analysis based on data gathered while monitoring parts through design, manufacturing, operation, and repairs. Traceability and meticulous tracking of each part are critical to success, he said. Knowing a given part’s history, metallurgical evaluations, mechanical testing, and modeling can be used to reliably assess its true condition.

Van Esch breezed through a mountain of material, captured in five-dozen or so slides, in the hour allotted for his tutorial. Attendees who were not knowledgeable about component manufacturing processes, repair techniques, methods of nondestructive examination, and metallurgical assessments would have had difficulty keeping up with the “professor.”

Those participants and others wanting to learn more, to possibly take advantage of the opportunity for extending parts’ lives, are referred to the series of articles van Esch wrote for CCJ several years ago. You can access them using the search function on the www.ccj-online.com home page.

While there have been improvements in materials and processes since those articles were written, the technical principles are sound and provide a good backgrounder for the novice. To build a solid foundation in the various technologies important to life-extension decision-making, the two-and-a-half-day workshop conducted by van Esch twice annually comes highly recommended by many in the industry. Write hvanesch@teservices.us for a schedule and course outline.

A couple of takeaways from the presentation included the following:

• OEMs provide little, if any, information on processes used in the manufacture of hot parts. This suggests the need for a thorough engineering and metallurgical assessment of critical parts to guide life-extension decisions, if you opt for the third-party approach.
• First-stage buckets typically are the “weak link” in the hot gas path (HGP). Redesign of under-performing buckets often is possible and may include the addition of cooling holes to reduce hot spots on the airfoil. However, simply drilling a few more holes is not the answer; fine-tuning of air flows is critical, van Esch warned.
• The casting method, and its execution, are critical to bucket lifetime. It’s important to audit the casting house, the speaker said; some are better than others. Grain structures of the castings demand close metallurgical examination. They can vary widely among suppliers, with some significantly more susceptible to cracking than others.
• Regarding bucket-cap repairs, be sure to verify full-penetration welds.
• The operational impacts on component life can be significant. Temperature is particularly critical, van Esch said. Example: A firing temperature increase of 100 deg F can reduce component life by a factor of six.
• Detailed records of a unit’s operating history are critical to the analysis of remaining life. He reminded the attendees that an emergency start is equivalent to 20 normal stars, operation at peak load consumes six times the life as operation at the OEM-specified “full” load, etc.
• Contaminants in fuel, NOx water or steam if used, and air also can significantly impact parts life.
• Internal coatings can extend bucket life, but van Esch cautioned that not many companies are adept at stripping and/or applying internal coatings during the repair process.

A user sharing his experience on 7F parts inspection and repair was next to the podium. He offered commentary on disassembly inspection, parts inspection prior to and during repair, and final inspection of components. Check parts as soon as they come out of machine, the user suggested, to get an overall impression of machine condition. For the engine discussed in the presentation, tip burning of the first-stage buckets was in evidence (Fig 2), as was loss of the thermal barrier coating (TBC) from the DLN combustion liner and associated cracking of that component (Fig 3).