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Figure 1: Frame 7FA transition piece fixture fit and check[/caption]
Who would have thought that the insurance field could provide insight into the operations and maintenance of turbomachinery? Yet that is exactly what occurred at the 39th annual Combustion Turbine Operations Technical Forum (CTOTF) Spring 2014 meeting in Palm Beach Gardens, Florida.
“Top-notch maintenance management, excellent condition monitoring and accurate life assessment are vital for good insurance rates,” said Donald Schubert, Vice President for Global Engineering Practice at insurance broker, Marsh Inc., during his keynote on opening day.
He was followed by dozens of sessions and panels covering topics, such as rotor life extension, alternatives to OEM Long Term Service Agreements (LTSAs), cybersecurity, blade repair, how to deal with opposition to CT projects, environmental regulatory concerns, business markets, high voltage and electrical, Heat Recovery Steam Generators (HRSGs), instrumentation and control, and plant winterization.
As a young engineer, Ed Sundheim of Essential Power and Executive Vice Chair for CTOTF Program Administration recalled being able to call upon two full floors of engineering experts for support. “Now we are very thin on such resources yet the level of technology has risen exponentially,” said Sundheim. “Most people coming in new will not yet have gotten their hands dirty on turbomachinery and they have to deal with a very steep learning curve.”
CTOTF encompasses all areas that influence the CT, such as transformer failures, for example. “Transformers need to be protected and maintained well,” said Sundheim. “If one goes down, it can take the plant out for 18 months as that is the typical lead time on a new one.”
Insurance figures reinforced this sentiment. When one investigates the equipment itself, Schubert’s data indicate that heavy frame turbines make up 28% of claims followed by boilers at 18%, generators at 16%, steam turbines 13%, aeroderivatives 11%, transformers 10% and others at 4%.
Within these categories, though, there is plenty of change. In particular, Schubert noted that generator and transformer losses are on the rise while aeroderivative and boiler claims are dropping.
“We have seen many more large transformer losses globally, some of which were followed by fires which caused even more destruction,” he said. “The increased use of fast cycling in plants appears to be a major factor in these incidents.”
On the steam turbine side, the numbers showed a greater incidence of overspeed incidents, especially for large machines. This included nine non-catastrophic incidents in the U.S (one with a bowed shaft) and four catastrophic overspeed events in other countries.
Emergency lube oil system and control valve failures were behind some of these situations. Schubert recommended redundant components in these systems, changing from mechanical bolts to electrical shutdown systems and upping the operations and maintenance (O&M) game in these areas, as well in battery maintenance.
Further root causes, added Schubert, included cracked low-pressure blades. He highlighted superficial analyses of blade damage leading to catastrophic failures before the next maintenance interval. The moral is clear: be very careful about life extension actions and perform multiple tests to verify the actual extent of detected blade anomalies.
“How insurers react to providing coverage is based almost solely on losses paid for the plants they insure,” said Schubert. “Thus their reactions can be treated as a weather vane for how well O&M technology is performing and how well turbomachinery is being received by the banking community for new construction.”
It turns out that claims paid and deductible settings can offer a window into OEM best practices. Far from just crunching numbers, the insurance industry takes a close look at plant operations and puts maintenance under deep scrutiny. “The insurance field is a little conservative and so has some restrictions in place concerning newer turbomachinery technology as, in their eyes, it is not yet field proven,” said Schubert.
Figure 2: Critical flow measurement of Frame 7FA fuel nozzles[/caption]
But while catastrophic equipment damage is a major cause for claims, it is dwarfed by the dollar value requested due to unscheduled downtime. “The biggest hit in insurance claims is from business interruptions,” said Schubert.
Another chart demonstrated the critical nature of test, commissioning and initial startup periods. Most insurance exposure takes place at this time. But Schubert also mentioned that operational exposure is on the rise.
He ended with a rundown of factors that can lead to carriers demanding higher deductibles and longer periods when they are not liable for a business interruption, i.e., the plant is not covered for business losses incurred until a 60-day or 90-day period has passed. “Top-notch maintenance management, excellent condition monitoring and accurate life assessment are vital for good insurance rates,” said Schubert.
He told the assembled users to be careful of OEM engine modifications as these have been a source of more claims of late. He advised the audience to take great care during turbomachinery rerates. Steam turbine rerates, for example, have sometimes resulted in significant rubbing.
Spare parts availability, too, is a big insurance concern. Do you or the OEM have a spare rotor, or guarantees in place for parts availability? he asked. If your deducible period is 90 days, the insurance firm wants to know how well you are set up on parts delivery and repair so that you can take care of most problems before that period has expired. If you cannot demonstrate your capability for fast and effective resolution, expect rates and time periods to be extended.
“Carriers want to see successful hours of operation on a given model of turbomachinery before they will give good rates,” said Schubert. “For example, the latest Hclass and ‘flex’ product lines from various OEMs are not yet regarded as proven by some insurance carriers so you can expect higher rates.”
Cycling, fuel and air quality are additional areas of concern for some carriers. Small print in contracts may even spell out lack of liability if the insurance company finds the plant lax in its O&M efforts concerning fuel quality or air filtration.
Figure 3: V84.2 turbine blade internal detail[/caption]
Looking at the big picture, Schubert shared some good news. He expected 2015 to see lower deductibles, broader coverage and overall lower insurance rates. One element of this change is more firms entering or reentering the market for turbomachinery insurance. That raises competition and ultimately benefits the end user.
Following Schubert’s briefing, CTOTF supplied three more days of material addressing most of the points he mentioned about maintenance best practices, repair options and turbomachinery lifespan.
Aaron Frost, Technical Director, Allied Power Group, addressed blade repair strategies. He began by explaining some of the history of GE Frame 7 stage 2 buckets. Over the years, it has evolved from an equiaxed to directionally solidified (DS) material and from eight radial cooling holes to 10 holes combined with integral tip shroud cooling.
GE moved from a 2,020°F firing temp on the E-class, to 2,350°F for the 7F, accomplished mainly by drilling additional and more complex cooling holes to prevent hot spots. The company has taken the 7FA beyond 2,500°F but more recently retreated to a firing temperature a little below that level.
Today, the 7FA.04 has a rotor and turbine wheels that are largely unchanged from the original 7F, but the airfoils are dramatically more curved, said Frost. To gain more lifespan, the OEM has developed a more elaborate tip shroud. “When drilling complex holes, it is hard to center them correctly and difficult to exactly control the metal thickness,” said Frost.
The tip shrouds were made heavier a few years ago. But as this caused creep and other issues, the shrouds have been slimmed down. Due to the changes in these shrouds, and the associated airfoils and cooling holes, Frost said that 7FA.04 buckets are hard to repair if the tip shroud gets damaged. Casting manufacturing location can also impact O&M.
“The volume of repairs on 7F buckets appears to be driven by where they were cast, even more so than the operational profile,” said Frost. “Different casting facilities have slight differences in the DS casting process.”
Frost was complimentary about changes made by GE when the company went from nine to ten holes on the second stage buckets. While these new blades are not interchangeable with older ones, he said the OEM has increased the thickness of the bucket near the shroud, and has improved the grain structure in the casting process.
While OEM bulletins place lifespan of stage 2 buckets at two maintenance intervals, independent repair shops have extended that to 96,000 hours and 2,700 starts, and beyond, said Frost.
Looking at the big picture, Frost believes that on the OEM side of the ledger, design complexity is winning against robust designs which are more repairable. The philosophy behind this appears to be to extend intervals by needing fewer repairs over the lifecycle of the parts and this is portrayed as a significant end user saving.
However, Frost wondered what happens when simple foreign object damage takes out thin and highly cooled metal before a fraction of the design life is realized. Regardless of vendor efforts to extend their lifecycle, those buckets will need to be replaced.
“Future repair strategy will shift from a restoration philosophy to a preservation philosophy due to the fact that performance and output will only be mandated post repair by minimizing airfoil shape changes that inevitably occur during today’s repairs,” said Frost.
He also touted the benefits of Thermal Barrier Coatings (TBCs). On 7FA blades, for example, he said that TBCs help blades become even stronger than new blades as they keep the temperature of the metal lower. “Heat treating can be done on these buckets without having to strip and recoat the internal aluminide coating, and without damaging the aluminide,” said Frost.
The thorny world of gas turbine component life management was the topic for an afternoon combustion technology tutorial by Rich Curtis, Vice President of Engineering at Eta Technologies. He walked the attendees through the process of inspecting fuel nozzles, liners, transition pieces, buckets, shrouds and blades in 7EA and 7FA turbines.
But he began by encouraging listeners to develop the right mindset by understanding the tools and processes at their disposal. “What do you have to pay attention to in order to manage the repair shop and its processes to control your own parts life destiny?” asked Curtis.
He pointed out that one set of 7FA buckets is the equivalent cost of one Hinckley 70 luxury sailing yacht, and said the reality is that the OEMs do not want to repair components, they want to sell new parts. But they were forced to develop repairs in the late 70s due to the blossoming aftermarket component repair industry.
“OEM repairability limits are generally more conservative than third party repair shops,” said Curtis. “In general, if shop problems arise, it is almost always in the execution of the established repair process.”
A proper inspection, then, is needed to figure out whether a part can be repaired so that it performs to its design intent and runs safely for the next planned service interval. A wise strategy is to adopt an inspect and advise strategy (p. XX).
“Pre-repair inspection results also define the economic viability of proceeding with repair,” said Curtis. “Adopting OEM fleet-wide recommendations for component life often result in retirement of parts with significant additional service life.”
Curtis advised that instead of a set number of hours, starts or maintenance intervals, users should let the components tell you how long they will last, not an OEM document. He also pointed out that as complexity has risen on parts, repairability has dropped. For example, Eclass buckets were robust and more easily fixed compared to the F-class.
Of course, there are trade-offs in component design. Thin trailing edges to optimize flow, for example, can come at the expense of durability. Similarly, upping the firing temperature to boost output places more strain on the turbomachinery.
Inspection tools that can be used in a parts management strategy include visual inspection, penetrants, ultrasonic, eddy current, radiographic, dimensional, airflow and non-destructive testing (NDT). Curtis noted the major advances in NDT over the last 10 or 15 years.
Interestingly, no standards exist for what is repairable or serviceable, i.e., able to be run as-is without repair. This lies in sharp contrast to the aerospace industry.
Instead, availability of parts, economics and timing of maintenance intervals are often used as elements in the decisionmaking process. Further, there are no agreed-upon categories for repairs, such as minor, medium and major. “Repair shops can sometimes work this lack of definition to make everything major,” said Curtis.
Finally, he commented on aftermarket parts. While purchasing used parts can be a cost effective approach, these parts are available because another user deemed them scrap. “Not once in 35 years have I seen a set of used parts sold with real and accurate documentation,” said Curtis. “There is serious risk in using mixed sets so rigorous inspection of all parts prior to purchase is a must.”
The repair theme was continued by John Downing, owner of Turbine Controls and Excitation (TC&E) Group. He outlined several areas identified by GE and users concerning 7FA turbines.
“When those turbines run on the turning gear for extended periods, they can develop wear and damage in the third stage turbine wheel blade roots and dovetail,” he said. “The cause is extensive blade rock at 6 rpm.”
Koenig Engineering and GE have proposed a modification to the turning gear using a parallel motor which reduces the shaft speed to .1 rpm during lengthy on gear operation. However, Downing noted that this was not a simple or inexpensive fix.
Tuning of the 7FA DLN 2.6 was another issue covered. This combustor required periodic tuning to address emission levels and combustion dynamics. In some cases, the plant had to bring in trained field engineers, paying a premium. In this instance, TC&E created a custom logic package that enable plant managers to make the adjustments themselves.
The latter part of CTOTF was taken up by OEM equipment briefings, followed by a Q and A period. Alstom, for example, featured available uprates to its units as well as an LTSA alternative attempted at an Alstom user’s facility.
Stephane Jacquemond of Alstom focused on how to adapt Alstom units for fast start. This, he said, is becoming even more crucial as more renewables come onto the grid.
As a result, the company is offering a lifetime extension and power boost package. The goal is to improve startup times, ramp rates and turn down to help these units stay viable and add value. “Grid stabilization requires capacity reserve and flexible plants,” said Jacquemond.
The uprate is currently available for Alstom units such as its GT8 and GT11 . The company is working to add the GT24 to the list of those with fast start capabilities.
Jacquemond said that the basic steps involve prepping the system, an HRSG purge, run up (accelerate up to nominal speed), then loading the GT up to baseload at 5% per minute. Achieving fast start involved improvements at each step.
Alstom has developed a fast start preparation sequencer to speed the time required in readiness to start. Some preparations, for example, are moved to a standstill phase and performed by the sequencer before push button. This includes the fuel valve test, the protection test and others.
Sequencer logic optimization, said Jacquemond, has also reduced run up time by opening the fuel valves and variable inlet guide vanes faster. But that only shaved off another 30 seconds. So a boiler purge was instituted to remove combustible materials from the turbine and replace them with air.
Instead of performing this function at startup (consuming 10 to 15 minutes), engineers figured out how to do this after turndown and still maintain purge conditions during turbine standstill. This is known as a purge credit.
“To ensure tightness of the turbine with no gas reentry, minor modifications were made and implemented active monitoring,” said Jacquemond. “The purge credit reduces startup time as well as auxiliary power consumption at startup, and avoids condensation within the HRSG.”
Further benefits cited included increased MW during normal startup, lower fuel consumption during startup as it is faster to reach base load, and lower emissions per start. The downside is higher a maintenance cost per start.
“Going to fast start tends to put more stress on components and influences the lifecycle,” said Jacquemond. “Wear and tear is worse on cycling units.”
Alstom offers two fast start versions. The FS15 has a 15 minutes startup while the FS10 takes that down to 10 minutes, but requires more modifications and changes to the controls.
One user of this retrofit is the Millbank plant in Canada, which runs four Alstom GT11Ns installed in 1990. They each produce 87 MW. By adding FS10, the facility achieved a loading rate of 19 MW per minute.
The consortium approach is said to retain LTSA advantages, such as maintaining continuity of support services while keeping the service providers focused on common goals, providing a single source of accountability for services, and obtaining a synergy that drives innovation. Meanwhile the plant retains ownership of parts.
Further consortium advantages allow the plant to control the decision making process as opposed to following OEM dictates. In addition, this alternative to the LTSA can open doors to creative service and repair solutions, help address end-oflife concerns, is typically lower cost and provides a simple exit strategy.
One example of a consortium is that offered by Liburdi Turbine Services and PIC Group. These companies combine their resources and incorporate the services of other aftermarket providers as needed to service a plant using Alstom turbines.
“We can offer continuity of service, i.e., the same people are always coming back to the facility so information and familiarity is retained,” said Doug Nagy, Manager of Components Repair at Liburdi. “If field service people find a component that is not lasting well, they talk to us in the repair shop and have the part adjusted so that it works better.”
The plant in question ran two Alstom 11N2 gas turbines in a combined cycle configuration along with an ABB VAX steam turbine and ABB generators. It had reached about 60,000 equivalent operating hours (EOH), running in load-following mode with occasional shut-downs. It has never had an LTSA, preferring OEMexclusive transactional buying.
“They were experiencing premature failures, high costs and had components that were not lasting the complete lifecycle,” said Terry Hinson of PIC Group. “They also wanted to be able to procure some parts via other third party providers.”
The consortium supplies combustion, hot gas path, generator and steam inspections. In addition, it analyzed long-term parts needs and built a parts utilization and rotation plan based on available inventory. “Overall plant availability has increased from 92% to 96%,” said Hinson.
CTOTF is a user group that is all about keeping those turbines running. It achieves this by serving up a mix of repair shop briefings, user experiences, Q and A sessions, industry overviews, and a vendor fair, as well as closed-door meetings between the users and OEMs, such as Siemens, GE, Mitsubishi, Alstom, Rolls- Royce and Solar. The goal is education and information exchange.
But sitting back of that is concern about the future of the industry’s knowledge base in light of workforce attrition, retirements and personnel cutbacks. “We are the premier forum for the exchange of information and experiences related to the design, operations and maintenance of combustion turbine (CT) and combined cycle power plants, and to provide a collective voice for its members to express issues and concerns to the CT industry,” said Jack Borsch of Colectric Partners who is Chairman of the CTOTF. “We are working hard to get younger people to come to this group so we have the next generation of operators, maintenance personnel and turbomachinery experts.”
The CTOTF Fall 2014 meeting will take place near San Diego, California from the 7th to the 12th of September. For more information, visit www.ctotf.org.