W251 Users Group: Cost-effective repairs, life extension dominate discussions on ageing fleet

Hats off to Federico Kitzberger, chairman of the W251 Users Group, for developing from his office in Argentina, a top-notch meeting in Canada that included a robust technical program and valuable shop tours. The conference was held at the Sheraton Hamilton Hotel, Hamilton, Ont, May 16-19. Recall that Kitzberger took over the reins of the small group from Procter & Gamble’s Chris Moshner after the 251 meeting in December 2004; no conference was held in 2005.

Kitzberger is the power generation manager for Capex SA, which owns Central Termica Agua del Cajon, a combined-cycle plant named for the city where it is located. The 657-MW (net, base-load) facility has five model B11-12A W251s among its generation resources.

The program focused on the needs of owner/operators that characterize this fleet—those with high-hours engines experiencing increasing maintenance costs in a competitive market that is adversely impacting revenues. User-only roundtable discussions and formal presentations by Siemens Power Generation Inc and third-party parts and services providers addressed the following specific topics relevant to that goal:

  • Durability and repair issues with principal turbine and compressor parts.
  • Operating results with non-OEM parts.
  • Rotor issues.
  • Life extension.
  • Second major overhauls (at a nominal 100,000 hours).

The two-dozen user attendees represented 35% of the world’s population of W251B10-12 machines, which is typical for this group’s meetings. They came from five countries. Still, Kitzberger seemed somewhat disappointed by last-minute cancellations, having spent the better part of a year gaining a consensus from a large majority of users on a meeting time and location.

Technical tours were a highlight of the conference. Users visited the Siemens factory in Hamilton and gained insights on the fabrication of forged turbine discs, rotor assembly, machining of turbine-disc serrations, fabrication of transition panels, and robot laser welding of compressor diaphragms.

A visit to Liburdi Turbine Services Inc’s facilities in nearby Dundas gave users a better perspective on component repair using the company’s proprietary LPM (Liburdi Powder Metallurgy) process, diffusion coating of blade air-cooling passages, x-ray inspections, compressor diaphragm repair, and borescope inspections of “turbulation ribs” in cooled blades.

W251 Users to meet at CTOTF’s Spring Turbine Forum

Joseph E Mitchell, facility manager for Newark Bay Cogeneration, which is managed by Wood Group Power Operations Inc, will chair the W251 Users Group in 2007; Federico Kitzberger, the power generation manager for Argentina’s Capex SA, will serve as vice chair.

The user organization announced that it will conduct its 2007 conference in conjunction with the CTOTF’s Spring Turbine Forum in Orlando, April 15-19. For more information, visit www.ctotf. org or e-mail joe.mitchell@woodgroup. com or meetings@ctotf.com.

The group invited Marty Magby, an experienced technical advisor with Turbine Generator Maintenance Inc, Cape Coral, Fla, to participate in user discussions related to maintenance issues. Magby, who was with Siemens before moving to TGM, told the editors of the COMBINED CYCLE Journal that the W251 has a light rotor and is prone to vibration. Exacting alignment is important, he said. The exhaust casing can be a source of vibration, Magby added, particularly if it has been lifted frequently and mating parts are worn.

A tutorial on W251 vibration was presented on the last afternoon of the meeting by Robert C Eisenmann Sr, PE, of Sulzer Hickham Inc, La Porte, Tex, and is summarized later in this report. To dig deeper on W251 overhaul, read Magby’s article, “Planning and executing your 100,000-hr overhaul,” on p OH-10 of the 2007 Outage Handbook bound into the center of this issue.

The following is a collection of notes from user discussion sessions. Summaries of the prepared presentations appear after this section.

Experience with non-OEM suppliers. Interest in and use of non- OEM new parts, repairs, and services continues to increase because users generally think that the higher prices for OEM parts and services are not always coupled to sufficient added value.

General experience of the group regarding parts and services from non-OEM suppliers ranged from acceptable to good. However, users noted that a comprehensive program of quality control/quality assurance (QA/QC) and vendor verification are necessary to ensure success, and this demands knowledge of the manufacturing/ repair processes involved. Assistance from independent consultants was suggested.

One user offered that new hotgas- path (HGP) parts from a thirdparty supplier have been in service for more than five years without problems. Non-OEM repairs of both OEM and non-OEM HGP parts from a half-dozen suppliers have been operating satisfactorily for more than six years.

Compressor parts. Compressor issues were reported by five B10-12 users. The group discussed how it might determine the root causes for each of the problems and disseminate the information among owner/operators. Reverse engineering of parts also was addressed.

Combustor basket repair by non-OEM vendors. Users suggested a thorough review of the spring seal configuration, dimensions, material, heat treatment, and coating before opting for a third-party supplier. A higher frequency of spring seal damage has been associated with non-OEM repairs than with OEM repairs.

Transition pieces and seals. Transitions continue to be an issue at some plants but design improvements introduced by Siemens have increased significantly the level of customer satisfaction. Principal lifelimiting factors are oxidation/wallthickness reduction and mouth and seal-rail deformation. Users discussed at what point repairs were impractical and replacement economically justified.

R1 vanes. Issues with row 1 vanes reported by several users include burning of the leading edge, trailingedge hole cracks, and suction-side bulging. One reason for the problems may be poor sealing at the transition exit, which causes excessive thermal gradients. Coating formulations, their application, and QA/QC were among the discussion points. Pros and cons of coupon versus braze repairs was another topic of interest. Flow testing after repairs was considered by the group as an imperative.

R2/R3 vanes. Downstream deflection correction adds significantly to repair cost. Siemens reported use of a different alloy for R2, one less prone to deflection. Proper heat treatment restores properties of vane alloys; fixture fit is critical.

Rotating blades. Discussions focused on life-limiting factors, repair limits, and alloy restoration. Lifelimiting factor for W251 rotating blades appears to be thinning of the airfoil at the cooling holes. Rejuvenation heat treatments are well developed and are able to restore material properties; however, there was not complete agreement among different repair vendors regarding how many times this can be done. More on the subject of blade repair can be found in the summary of the Liburdi presentation in the next section.

Third-party vendor presentations

Chairman Kitzberger invited several third-party vendors to participate in the W251 Users Group meeting because of the special products and services they provide to this fleet. The lineup and subjects addressed:

  • ETS Power Group, Stuart, Fla. Manufacturing capability for new combustion system and turbine parts.
  • Liburdi Turbine Services Inc, Dundas, Ont. Overall capabilities with emphasis on the company’s proprietary powder metallurgy repair process (LPM) and life analysis, rejuvenation processes, and repairs beyond “book” limits.
  • Life Prediction Technologies Inc, Gloucester, Ont. Case study of lifecycle management of W101 discs.
  • Stork H&E Turbo Blading Inc, Ithaca, NY. Manufacturing capability for compressor parts.
  • Sulzer Hickham Inc, La Porte, Tex. Comparison of model, field, and shop vibration behaviors of a W251 rotor.
  • Trinity Turbine Technologies LLC, Iowa Colony, Tex. Coupon repairs for R1 vanes and repairs with deflection correction for R2 vanes.

Combustion system, HGP parts

ETS is a perennial presenter at 251 meetings, having supported owner/ operators of the engine for nearly a decade. In a sense, the company is a “boutique OEM.” It offers virtually all replacement parts for the 251: compressor blades and vanes, combustor liners and transition pieces (TPs), turbine blades and vanes, and consumables. Plus it does repairs.

VPs Mark Dender and Bruce Beeman participated in the meeting. Dender offered some background on the firm which he described as an independent engineering company of about three dozen employees capable of producing HGP parts for the most popular mature frames. All engineering is done in-house, manufacturing is outsourced. Warranties are said to be equivalent to those offered by the OEMs.

Beeman began his presentation with a review of the company’s turbine-parts supply chain, customer base, manufacturing QA, and its various services—now including control system troubleshooting, repair, and calibration; DLN (dry low NOx) tuning; and technical field assistance and turnkey labor.

Greatest interest among 251 owner/operators seemed to be with the company’s “durability fixes.” Beeman talked about the application of new GT technology to older engines and the benefits it brings users. One enhanced part he discussed was the TP with enhanced cooling and improved sealing. Problem with the original TPs—at least in some cases—was distortion in the exit-mouth area that allowed cold air to enter the unit and adversely impact the lives of R1 vanes (Fig 1).

ETS’s fix was a brush seal designed to create a positive seal between adjacent TPs, even in the event the transitions distort and a gap opens up. The seal is an add-on to existing transitions, no design changes are necessary. The company also offers an improved cooling-duct mod as part of this upgrade, to reduce mouth distortion.

Another of ETS’s enhanced parts is a more robust first-stage vane than that provided with the engine. One feature is a thicker wall on the suction side of the airfoil. The replacement design is already beyond 16,000 hours in one machine—and headed for 24,000—with no repairs to date. Follow-on work in this area includes enhancement of vane cooling via preferential cooling or additional cooling.

Finally, Beeman explained ETS’s new Total Turbine Maintenance (TTM) Internetbased system for managing a fleet of GTs. This valueadded offering is a systematic method for reducing the cost of turbine operations. It includes parts management, turbine operations management (maintenance history, planning, and scheduling; lifecycle cost evaluation; performance monitoring, etc), budget planning and control; and studies for evaluating upgrades.

Life analysis of turbine blades

The presentation prepared by Liburdi’s Doug Nagy, business unit manager, and Paul Schumacher, metallurgist, essentially was a short, focused workshop on life analysis of Model B12 R1 and R2 turbine blades. It began by answering three questions:

  • What is a life analysis? Answer: A metallurgical investigation, typically destructive, of a component at the end of a know service interval. Life analysis determines the damage mode, repair needs, and remaining life of the component assuming the same operating regime.
  • How do you conduct a life analysis? First, assume that one blade is representative of the set or row. Next, mechanically and metallurgically test the blade for (1) surface and coating condition, (2) casting/forging quality, (3) microstructure and alloy phases, (4) metal temperatures, (5) mechanical properties, and (6) dimensional fidelity and distortions. Finally, evaluate the data collected within the context of component history and available repair technologies.
  • Why do a life analysis? Knowing the metallurgical condition of a used component, the following are possible: (1) Determination of engine operating conditions; (2) evaluation of coating performance; (3) sufficient information to recommend the optimal service interval and make suggestions on upgrades for future service; (4) accurately define the scope of work for repairs; and (6) make an informed prediction of total life.

Questions asked and answered, Nagy, who made the presentation, gave an example of an actual 251 blade life analysis. Blade history included these highlights: 65,425 hours in base-load service on natural gas; 241 starts, including 89 trips; three repair cycles. Expected “normal” replacement life for R1 blades is 72,000 hours, 100,000 for R2.

Nagy and Schumacher prepared several slides to illustrate the following:

  • Metallurgical condition of the sample R1 blade surface and of its root section (Fig 2).
  • Oxidation and impact cracks at the blade tip (Fig 3).
  • Metallurgical condition of the cooling holes (Fig 4).

The interactive presentation was extremely valuable because it provided perspective and background for informed decision-making by users when they are required to choose between repair or replace for their HGP components. It also offered a sample of the type of mechanical/ metallurgical condition report they should expect from a qualified repair vendor. To illustrate, here’s the diagnosis for the R1 blade sampled from this set:

  • Microstructure. Minor degradation of non-standard structure.
  • Tip of airfoil. Severe oxidation and impact damage.
  • Surface condition. Bond coat exhausted in upper-airfoil and platform areas.
  • Wall thickness. Under minimum thickness at trailing-edge cooling hole.
  • Roots. Acceptable alloy depletion.
  • Cooling holes. Five mils of oxidation/ alloy depletion.

Bottom line. What this report means is that the sample blade is not repairable because of less-thanminimum wall thickness at the trailing-edge cooling holes. However, the good news is that other blades in the set should be repairable where no “under-minimum” wall-thickness condition exists. The coating system protected the base metal, thereby enabling stripping of the coating, recoating, and heat treatment to rejuvenate the microstructure. Expected additional service life following repairs: 24,000 hours.

A similar presentation was made on the sample R2 blade. That diagnosis:

  • Microstructure. Ageing of abnormal structure; significant loss of creep strength.
  • Tip of airfoil. Oxidation and impact damage (Fig 5).
  • Surface condition. Bond coat in good condition.
  • Wall thickness. Acceptable.
  • Roots. Acceptable alloy depletion
  • Cooling holes. Five mils of oxidation/ alloy depletion.

Therefore, this part is repairable and the balance of the set should be. Note that inner-hole condition is critical. After stripping and recoating, proper heat treatment should ensure another 24,000 hours of service.

Suggestions, possible improvements. Continued use of TBC (thermal barrier coating) on R1 blades recommended, and suggested for R2 blades as well. Periodic rejuvenation of the blade alloy would improve strength and reliability. Coating of the cooling channels with an aluminide is suggested after the existing oxide and depleted layer are removed. Note that the stripping process must be carefully controlled to avoid excessive wall thinning.

Dig deeper into repair of HGP components by reading “Advanced repair techniques prolong hot-section component life,” p OH-72, in the 2007 Outage Handbook supplement inserted in the middle of this issue. That article references still other knowledge resources.

Rotor dynamics 101

Robert C Eisenmann Sr, PE, principal mechanical engineer for Sulzer Hickham, is one of vanishing breed— an engineer competent in both the theory and practical application of large powerplant rotating equipment. His 90-slide tutorial on the vibration behavior of the 251 engine expanded the perspective of many attendees. It would be impossible to do justice to Eisenmann’s presentation in the limited space available here. A brief summary of major points will have to do for now, saving the details for a feature article in an upcoming issue.

The content of the tutorial was particularly valuable to anyone responsible for a legacy machine and preparing for a major overhaul that involves rotor refurbishment. By the time a GT has reached the 100,000- hr mark—normally the first time the rotor is lifted out of the casing—the machine is different physically than the engine installed a dozen or more years ago. For example, foundations can settle, bearings can wear, rotating parts are dinged or damaged, high-temperature parts can creep. Any of these occurrences, and/or others, influences the dynamic behavior of the GT.

In the practical world of powerplant operation, changes in vibration signature that don’t trip the unit often are ignored. But after you pull the rotor and refurbish it, install upgrades, rebabbit and reset bearings, and make other adjustments, you want the machine to go together and run like it did when new. How can you be sure? What should the vibration signature look like?

Eisenmann originally had planned to talk about field vibration analysis but his practical experience dictated a “back to basics” approach. Reason, he said, was “you often go to a site and all that’s there are two velocity pickups—one is broken and the other isn’t reading properly.” He thought that powerplant operators could benefit from a better understanding of the importance of vibration information. For more background on the subject, read “Machinery-health monitoring helps plants achieve financial goals,” p OH-40, 2007 Outage Handbook supplement inserted in the middle of this issue.

Eisenmann stressed the value of having an accurate analytical model for evaluating the impact of new parts and modifications before the machine is reassembled and started—particularly when blade weight or bearings change. This allows any necessary corrections to be made prior to restart, with the potential for considerable saving in time and money.

Being prac t i cal by nature, he knows it takes a considerable amount of effort to develop an accurate model—beginning with actual physical measurements of the unit to thousandths of an inch—and that can’t be justified for a typical repair job. But if you’re considering investing hundreds of thousands, or millions, of dollars in upgrades, a model usually makes sense.

The model can be tweaked based on data obtained from a full-speed vacuum bunker test, which is recommended before any refurbished rotor is returned to the plant. Last thing you want is to install a rotor and then have to return it to the shop. If rotor dynamic behavior passes bunker tests and reassembly work is done properly, field data should confirm a well-balanced machine. Finally, the model should be fine-tuned as necessary to mirror field results.

To prepare for the meeting, Eisenmann invested about a week’s worth of personal time to develop an analytical model of the 251AA because Sulzer Hickham had much of the detailed information required for the effort on file. The basic machine configuration is shown in Fig 6.

After developing the model, Eisenmann put it to work and developed slides illustrating the vibration response of both shaft and housing at the damped first and second critical speeds (about 1200 and 2400 rpm, respectively). Next, he applied forcing functions to show the response of the rotor to the addition of weight at the shaft midspan and elsewhere.

The behavior of the rotor at the first, second, and third critical speeds (last is the running speed at 4850 rpm) then was projected on the screen. Eisenmann noted that if there’s a problem with the marriage coupling, you never get through the first critical; problems in the turbine section influence rotor behavior at the second critical. Poor alignment is reflected in high vibration readings at the running speed.

Bunker behavior. Following a low-speed balance that includes a run-out check, the rotor is installed in the full-speed vacuum bunker where it is driven from the turbine end. The sequence of activities: Highspeed balance without turbine blades installed, evaluate critical-speed response, install all turbine blades, conduct three high-speed runs and average data.

High-speed measurements include shaft displacements using proximity probes. Expected findings would be less than 1.5 mils at critical speeds, less than 0.5 mils at the maximum continuous operating speed.

Trinity Turbine’s Mark Dion, a mechanical engineer expert in the repair of rotating equipment, and Joseph Hale, PE, a metallurgist specializing in the analysis and repair of HGP parts, spoke to coupon repair and life extension of vane segments for R1 vanes in the W251B11-12 and deflection correction of R2 vane segments.

Dion said that Trinity has made these types of repairs, and many others related to them, for several different engine models, not just the W251. There are 28 individual R1 vanes in the W251. He explained that coupon repairs make financial sense when the leading edges of vanes are thinned (less than 90 mils thick) by normal wear and tear (vane at right in Fig 7) or simply “blown out” (Fig 7, left).

Trinity’s coupon material was said to be similar to that of the OEM’s offering, thicker (130 mils vs 60 to 100), and the repair less than one-fifth the price of a new vane. Dion explained that the repair process is well-proven and relatively simple. Here are the main steps: (1) Cast the coupon, (2) remove the leading edge of the damaged vane up to the bridge (Fig 8), (3) weld in the coupon, (4) do final machining, (5) drill cooling holes and recoat, (6) install vane inserts—they meter the flow of cooling air, and (7) flow test the finished piece. Fig 9 summarizes the work done: Damaged vane is at lower right, coupon is welded in place at lower left, finished vane is at top.

R2 deflection correction. There are two vanes per segment in the second row. Over time, the material creeps downstream—so-called vane deflection. Movement can be significant said Dion. For example, the second row being machined in Fig 10 was corrected for 200 mils of deflection. Here’s how this was accomplished: (1) Measure movement (the distance from the leading-edge hook to the interstage seal fit), (2) weld the leading-edge hook fit, (3) solution anneal, (4) mount in fixture and machine, and (5) inspect.

Note that the machining step involves skimming of the face, machining the outside diameter, and re-establishing the proper leadingedge hook-fit width and the trailingedge fit. Repaired assembly (Fig 11) conforms to OEM clearance requirements at significantly less cost than a new component.

Siemens Day

The 251 users got a change in venue on the second day of the meeting: After presentations by the Siemens team at the hotel, attendees were given the opportunity to visit the company’s nearby manufacturing plant. Highlights included a first-hand look at the fabrication of forged turbine discs, rotor assembly, machining of turbinedisk serrations, fabrication of transition panels, and robot laser welding of compressor diaphragms. The tour group also was taken through the Repair Development Center, which was recently added to enhance the Metallurgical Services Facility in the Hamilton plant.

Understanding terms key to good communication

Important to good (and accurate) communication is an understanding of technical and business terms related to gas-turbine operation. Duty-cycle and other definitions used by Siemens include the following:

Base load. Greater than 75% service factor (approximately 6500 hr/yr) and operation for more than 125 hr/start. Service factor is percentage of time over a given period—typically one year—that a unit is connected electrically to the grid.

Intermediate duty. Greater than 30% service factor (approximately 2500 hr/yr) and operation for more than 12.5 hr/start—and less than base-load duty.

Peaking. Greater than 5% service factor (approximately 400 hr/yr) and operation for more than 4 hr/start— and less than intermediate duty.

Standby. Less than peaking.

Start attempt is the action to bring a unit from shutdown to the in-service state. Specifically, it is the entering of the engine’s starting sequence whether or not the operator intends to synchronize the unit to the grid.

Successful start occurs when a unit achieves synchronization.

Start failure is an unsuccessful start attempt—that is, the engine does not reach synchronization because of an operational or equipment failure. It does not refer to the intentional abort of the starting sequence for such things as testing, troubleshooting, or economic reasons.

Fired abort is a start attempt that aborts, or is aborted, after ignition has occurred but before synchronization is achieved.

Unfired abort is a start attempt that ends before combustor ignition. Availability and reliability terms, as defined by ANSI/IEEE Standard 762 are:

Availability is the percentage of time that a plant is capable of providing service, whether or not it actually is in service and regardless of the capacity level that can be provided.

Forced-outage factor is the proportion of time a unit is in a forcedoutage condition over a given time period. Reliability (not defined by ANSI/IEEE 762) is the percentage of time a unit is not in a forced-outage condition over a given time period.

Starting reliability is the percentage of attempted starts that successfully synchronize with the grid.

The formal program opened with a review of progress in satisfying customer needs as determined from surveys and interviews conducted by the OEM’s Six Sigma Program personnel. Annual surveys are the users’ report card on OEM performance and important to ensure continual improvement in product offerings and customer service. Chairman Kitzberger thought the following Siemens communications initiatives were particularly valuable:

  • Net meetings—online, real-time Internet conferences that enable the OEM to interface directly with the user community regarding fleet updates.
  • Field Service Support Center in Orlando has an engineering help desk that provides onsite outage field engineers rapid response to technical questions. In 2005, 90% of the tickets were responded to in less than four hours; goal is three hours. Half of the tickets were for GT/generator outage support. Field personnel reported that the solution suggested through the help desk was implemented 95% of the time; only in 1% of the cases was the solution not used.
  • Customer Extranet Portal (CEP). “Greatly improved compared to previous meetings,” noted Kitzberger. Nearly 60% more W251 owners and operators used the CEP in 2005 than in previous years. Reasons included enhanced accessibility, more technical documents (service and product improvement bulletins, technical advisories, customer service letters), parts catalogs, contact information for Siemens personnel, outage reports, and the addition of GT shop repair reports.

Improved content of shop repair reports also has been well received by users. Recent enhancements include greater standardization, more inspection data detail, elimination of acronyms, inclusion of product mods implemented in repairs, better executive summary. Plus, reports are completed in less time than previously.

The customer satisfaction survey, conducted by Harvey Grassian and Jean Matkovich, revealed that two-thirds of respondents were operating their units base-load (sidebar defines duty cycles). Also, that most respondents owned and/or operated the latest engines in the fleet (B11-B12A) and were the most satisfied segment of the group. Overall satisfaction index has shown continual and significant improvement since it was first measured in 2002.

Customers expressed particular satisfaction in the significant partslife improvement achieved for transition pieces, turbine vanes and blades, and combustor baskets. Users also said that the OEM’s response to unit-specific issues and its timeliness in the delivery of solutions to address fleet issues during outages had improved significantly since the previous survey two years ago.

The quality of repairs to compressor components, baskets, transitions, vanes, and blades improved significantly as well. Customer experiences in the ordering of spare parts have improved. Parts are more available and easier to order and track than in the past; deliveries are more punctual and accurate. A substantial improvement in price competitiveness and a big jump in unit availability/ reliability were highly regarded by the user community. However, owner /operator s believe there is considerable room for improvement in the pricing of aftermarket services.

Looking ahead, users are most focused on extending both unit life and parts replacement intervals in addition to reducing the cost of repairing critical parts. As you would expect, the goals of higher availability, lower heat rate, improved starting reliability, and lower emissions get many votes on survey forms.

Reliability, availability. Jeffrey Kain, manager of mature frame product support, reported that currently there are 49 operating units in the B10-B12A fleet, which dates back to 1984. The leading engine had recorded more than 160,000 hours of service at the time of the conference; several more had passed 100,000 hours.

Kain was challenged in developing meaningful fleet-wide reliability and availability metrics for the meeting because data were available for only 20% of the units. The goal is to have more than 50% of the machines supplying O&M information regularly; participation had been at that level as late as February 2005. It is likely that the declining number of experienced O&M personnel—particularly at the smaller facilities— was the primary reason for the poor response: Too many tasks for too few hands.

However, the value of statistically significant operating data cannot be underestimated for a fleet with such high average operating hours. It is particularly important to track component performance to guide implementation of a lifetime extension program that maintains availability, reliability, and safety while holding O&M costs within reason. The W251 Users Group members attending the 2006 meeting were well aware of the value of fleet operating data and promised to redouble efforts to contact peers and regain their support.

With 10 of 49 units reporting, Kain calculated a 12-month average reliability for the Econopac of 98.4%; leader was at 100%. Average availability for the fleet was 91.7%; leader was at 100%. Starting reliability averaged 91% with the fleet leader again at 100%.

The component repair presentation,

compiled by Rennie Urias- Castillo and Gord Howie, emphasized refurbishment of turbine blades and vanes to extend parts lifetimes. Targeted replacement of R1 blades is 48,000 EOH (equivalent operating hours) or 1600 ES (equivalent starts); R2 and R3, 96,000 EOH or 3200 ES. Inspection and repair to achieve these goals is recommended at 24,000 EOH or 800 ES for R1, 48,000 EOH or 1600 ES for R2 and R3.

Fleet experience indicates that achieving 80,000 EOH on R1 blades is possible with multiple repair cycles. Service life is constrained by repair limits to airfoils and the platform area. Life-limiting damage mechanisms typically are platform cracks, heavy tip damage, and cracks in the trailing edge of the airfoil.

Fleet leader already has operated for more than 80,000 EOH on R2 blades and achieving 96,000 is not constrained by current repair limits. Similar performance also has been experienced by R3 blades.

Regarding vanes, replacement of R1 is targeted at 48,000 EOH or 1600 ES provided inspection and repair at every 16,000 EOH or 600 starts are conducive to that goal. Field experience is that an occasional set of R1 vanes lasts beyond 48,000 EOH. Coating changes in the late 1990s dramatically reduced the number of premature repair and change-out incidents experienced prior to that time.

Inspection and refurbishment of R2 vanes is recommended every 24,000 EOH or 800 ES with the expectation that they will not require replacement until the 72,000 EOH or 2400 ES milestone. Several sets of R2s have passed 48,000 EOH. Four sets have been retired after achieving between 70,000 and 84,000 EOH; one set went beyond 2400 ES successfully. Urias-Castillo and Howie noted that weld repairs become difficult after long service.

Design lifetime of R3 vanes is 96,000 EOH or 3200 ES with inspection and refurbishment suggested at 48,000/1600.

Other topics covered during the session: (1) New repair techniques for transition inner seals to enable refurbishment of parts previously scrapped, and (2) the importance of maintaining proper tip clearances on all three turbine stages—particularly the first—to minimize rubs and maximize power output.

Mods and upgrades always is a popular session on the Siemens Day program. Dan Stankiewicz and Robert Mozzoni discussed these features of the B10-12A product portfolio:

  • Disk-cavity reliability modification, includes more effective sealing solutions (replacement of labyrinth seals in the exhaust face of the interstage seal housing with brush seals and rope seals between the R2 and R3 interstage seal housings and the vane segments) and use of a bypass valve and orifice scheme to better control the flow of cooling air into the disk cavity than was possible with the original fixed orifices. Latter helps to prevent high temperatures in the disk cavity and allows the engine to run reliably and less likely to auto-unload.
  • DLN (dry, low NOx) combustion. Benefits include a reduction in NOx emissions to less than 22 ppmvd (corrected to 15% O2) when burning natural gas and 42 ppm on distillate oil.
  • Fuel conversion. This upgrade converts an existing single-fuelcapable turbine to dual fuel. Also offers the potential for switching between liquid fuel (distillate or crude oil) and natural gas online and for mixed-fuel operation.
  • Electrical ly actuated throt – tle valves. Potential benefits of replacing pneumatically actuated valves include improved accuracy and control and easier calibration.
  • Electrohydraulic IGV actuator. Replacing pneumatic inlet-guidevane actuators with EH actuators is relatively simple, requiring no changes to the inlet cylinder and IGV unison ring. Benefits include improved control, accuracy, and repeatability as well as tighter control of part-load exhaust temperatures.
  • System monitoring and control package measures/calculates operating conditions in real time at all loads and IGV positions for comparison against baseline data. Information provided helps operators to maximize overall plant efficiency and guides decisions on work required during the next outage. Calculated metrics include heat rate, compressor mass flow and efficiency, turbine efficiency, exhaust pressure drop, etc.

Additional mods and upgrades covered in less detail included steam power augmentation, starting package replacement, emissions control using steam or water injection, island control, and redundant instrumentation.

Lifetime extension piqued everyone’s interest because of its potential for deferring investment in new generation equipment. Michael Liao, manager of GT service engineering and projects for mature frames, began his presentation by explaining the ageing mechanisms that limit the life of turbine parts: wet corrosion that occurs during both normal and turning-gear operation (Fig 12, A), creep in metals operating at high temperatures under load (B), mechanical stress from both high- and low-cycle fatigue (C), contaminants in the ambient environment (D), erosion from the byproducts of combustion, and hightemperature oxidation and corrosion.

Photos of the various damage mechanisms were particularly valuable to users because they provided a frame of reference for visual inspection of their units.

Liao urged owners to conduct a life assessment of their engines at 96,000 EBH or 3200 ES when plans call for operation to continue. A program for lifetime extension (LTE) is customized for each engine based on (1) information gathered from the OEM’s pre-investigation checklist, which is critical to determining the feasibility of LTE; (2) analysis of component history; (3) results of the most recent HGP and combustor inspections; and (4) mods and upgrades implemented.

The findings from various inspections and tests—including both nondestructive and destructive examination, visual and dimensional inspections, and materials analyses— identify which of the major components can be requalified for extension of service.

To dig deeper, read “Planning and executing your 100,000-hr overhaul,” p OH-10, 2007 Outage Handbook supplement inserted in the middle of this issue. This article is based in large part on experience gained at a plant with a W251 GT.

Transition-piece redesign. Last place on the W251 customer satisfaction survey seems reserved for transition pieces. However, temporary fixes that satisfactorily addressed pressing issues and a recent redesign bumped the score dramatically from 2004 to 2006. As the new TP, available on limited release, gains commercial experience scores are expected to continue their upward trend. Bill Van Nieuwenhuizen reviewed the issues with the original TP:

  • Exit mouth deformation which caused excessive blade-path temperature spread and NOx emissions. Forced outages resulted.
  • Excessive panel temperatures— primarily associated with DLN applications which have combustion- basket exit cones to reduce CO emissions. Generally did not cause forced outages.
  • Cracking at the ends of stiffening ribs. Generally did not cause forced outages.

Design of the new TP was discussed in detail at the previous W251 Users Group meeting, in Orlando, December 2004. To review, access “Transition-piece repair/retrofit, blade/vane repair, compressor washing major topics at annual meeting” from the 2Q/2005 COMBINED CYCLE Journal at www.psimedia. info/ccjarchives.htm.

Comprehensive, heavily instrumented tests in an operating engine verified peak temperatures that were 500-600 deg F lower than measured for the previous design—as engineers predicted. Dynamic strain measurements showed no indications of resonance and the exit mouth remained dimensionally stable.

Retrofit of the new TPs requires a total outage duration of 12 days with one 12-hr shift/day—including any field modifications necessary. ccj