Intense, interactive program covering major issues defines annual meeting

The V fleet in the Americas is not very large—fewer than 200 units by one unofficial count. It is characterized by several engine models and several owner/operators that are true “fans” of the machine—for want of a better term. By way of example, Conective Energy has a dozen V84.2s just at its Hay Road Power Complex in Delaware and Bethlehem Commerce Plant in Pennsylvania.

So when three-score users attend an annual meeting, the fleet is well represented. That’s how many came to Newport for the 2006 conference in mid June. The rewards for attendance: productive technical meeting, cloudless skies, and first-class treatment by host Siemens Power Generation Inc, Orlando.

The core of the meeting is two days of intense sessions flanked by a “fun” day to start and a tour on the fourth day. Newport is not an easy city for most participants to access, so a first day of golf, informal discussion, and product demonstrations provides arrival flexibility. This year’s tour was of Dominion Energy’s Manchester Street Generating Station.

Chairman Robert S Pasley, a gasturbine (GT) maintenance coordinator for the proactive cooperative power wholesaler Associated Electric Co-op Inc, Springfield, Mo, directed the user-only portion of the meeting on Day Two; formal presentations by Siemens were on Day Three.

The energy level of the group and the presenters seemed especially high—perhaps the result of a compressed two-day classroom format, fresh ocean air, attendees who knew each other well, or a combination of these. Questions from the floor were probing, answers comprehensive. And users were comfortable in asking challenging questions of the OEM’s experts and they equally comfortable in answering them. One big difference between a V meeting and other user-group annual conferences in the Americas is that about half of the OEM’s participants are from Europe—a welcome diversity of engineering talent.

Open discussion

Pasley worked all portions of the engine into the open forum on Day Two, which also included the user presentations profiled in the section immediately below. Mundane discussion of leaning silencers and filter performance in the shadow of a coalfired unit got things rolling. From there it was on to IGVs (inlet guide vanes) and the long delivery times associated with replacement motors and actuators. Plus, analysis of compressor blade failures attributed to flow instabilities caused by IGVs out of position.

Debate on the value of compressor-blade coatings and wet compression ensued, with cameras and windows suggested to identify droplet agglomeration and puddles in the filter house before they caused downstream erosion problems. The nitty gritty of opening the compressor and accessing parts for in-situ repairs or removal was a user-forum highlight.

Coverage of the combustion section included fuel-nozzle wear and how to deal with it, flame-scanner tube erosion, and the long lead times associated with replacement parts—particularly tiles—for the combustors proper. Back in the turbine section, users had interest in the performance of single-crystal blades and experience with extended inspection intervals.

User investigates vane issue

The typical GT user conference relies on one or more of the following session formats for providing valuable information to owner/operators: OEM presentations, open user-only discussions, vendor fair, and formal user presentations. The last are particularly well received. At the Newport meeting, John Macdonald’s presentation on RWE npower’s Didcot B compressor incidents early in 2006 probably would have won “best in class” had there been a vote.

Background notes: (1) Macdonald is an experienced GT engineer in the energy company’s central engineering offices and knows the subject well. (2) Didcot B consists of two 2 × 1 combined cycles. So-called Module 50 has two V94.3 engines (GT51 and GT52); Module 60, two V94.3A1 machines (GT61 and GT62). (3) RWE npower, one of the UK’s top three energy suppliers, produces more than 10% of the electricity used in England and Wales. Total generating capability exceeds 8000 MW.

Compressor issues dominate podium time at user-group meetings these days. Every frame OEM has had its challenges, virtually all of which are resolved over time and at significant cost. Compressor problems probably should not be surprising given (1) the pressure on OEMs to deliver machines with the highest power ratings and efficiencies possible at the lowest possible prices, and (2) the cycling operating mode of most large frames today, which was not envisioned by OEMs and users when contracts were signed.

Attendance at user-group meetings is important for learning about issues experienced by others in the fleet, and how to identify and correct potential problems before damage occurs. Awareness is your first line of defense against forced outages.

Issues with vane rings in Vx4.3x compressors are not new to V user discussions. In fact, a formal presentation by the OEM at the group’s 2005 meeting explained to owner/ operators the advantages of its design for compressor vane rings, fleet experience, analyses conducted to resolve outstanding issues, and revised service criteria.

Macdonald began his presentation, which was based in large measure on a report (“Compressor Vane Failure Investigation and Conclusions”) that he co-authored with colleague Philip Morgan, by reviewing experience with the older Module 50 GTs. Units 51 and 52 have vanes similar to, or identical to, those in the later Module 60 engines.

Excessive wear was found in Row 4 in April 2002. Fretting wear continued to the point where some airfoils moved far enough downstream to contact the following rotating disk. In January 2005, vane rings in R4, R8, R11, and R13 were replaced to the extent parts were available. Vanes in R8 and R11 were fillet-welded to the inner ring and are still in service (Fig 2).

Cracking of airfoils in R3 and R4 of Module 60 engines was found in April 2004 (Fig 3); fretting of vane hooks also was evident in several stages. A routine inspection last January found debris at the back of both Module 60 compressors. A subsequent borescope inspection discovered airfoil impact damage.

The two units were opened for closer inspection and the damage found was almost identical in both— except for R2 vane cracking in GT62 only. The crack in one vane was part way across an airfoil (Fig 4) which Macdonald said implied a slow growth rate.

Damage found in both machines included R4 vanes with cracks that originated in the hook area and resulted in liberated metal (Fig 5). Fragments hit several vanes and a R8 rotating blade at mid height. The impact on the R8 blades (one each in GT61 and GT62) caused high-cycle fatigue (HCF) failure (Fig 6).

Vanes damaged by liberated metal were changed out where replacement vanes were available; where not, the impact areas had to be dressed back further than recommended by the manufacturer (Fig 7).

A special repair procedure for R7 vanes was developed by RWE engineers to correct trailing edge impact damage (Fig 8). The vanes were welded up in a vane-ring fixture at the repair facility. A female profile form, produced from vane measurements, was used to dress back the repairs to match the desired profile. Macdonald said that the OEM did not offer such a repair—at least at that time—and it saved three months on the outage schedule.

RWE npower engineers conducted detailed analyses of R2 and R4 vanes. First the vanes were measured using CMM (coordinate measuring machines). Next, finite-element vibration models of R2 and R4 were analyzed. Engineers noted that vibration response is dependent on the hook-to-ring fit. To illustrate: For R4 vanes, if the hook is rigidly clamped, then the vane resonant frequency corresponds with the excitation from the R3 blades; if less rigidly clamped, the frequency corresponds with R2 blades.

To verify these findings, impact testing of the assembled vane rings was conducted using an accelerometer attached to individual vanes. Macdonald stressed that measuring the vibration response of each vane, tuning, and retesting was a time-intensive process. He suggested that the root cause of the failures might be traceable to over-peening of the vanes during manufacture, noting that peening is a highly variable process and one that impacts frequency response.

In sum, frequency measurements were made on both new OEM stage 2 and 4 vane ring assemblies as well as on service-run assemblies. The repaired stage 4 vane assembly for GT62 and the new stage 4 assembly for GT61 both were detuned to mitigate the risk of cracking in service. Also detuned were the stage 6 upper vane ring and stage 7 repaired vane ring assemblies for both machines.

Strain gauge tests were conducted on the operating engines to confirm that the vane staking and tuning work done by the generating company had achieved the goals intended. Such tests are needed to determine the response levels of blade-wake excitations. Two vanes each in R2, R4, and R7 were instrumented (strain gauge and thermocouple). Results: stress levels in R2 and R7 were not significant during the test runs. R4 stress levels measured during run-up of GT61 were just within the safe zone.

Aftermath. The repair effort for Module 60 was considerable. GT61 required blending of 59 blades and replacement of 127; 119 vanes were blended or weld-repaired, 84 replaced. More than 100 blades in GT62 were blended and another 100+ replaced; 74 vanes were blended or weld-repaired, 58 replaced.

The detuning exercise reduced stress levels in R4 vanes as planned, but probably at the expense of increased fretting wear. Recall that fretting is inherent in the design, which allows movement at the inner ring (flexible bearing). RWE has installed more borescope ports in its machines and is now conducting visual examinations every six months. Macdonald cautioned that while a good borescope technician can ID vane cracking, he can’t find horizontal cracks in the hook. The company’s engineers expect that reworking or replacement of vane rings will be required every 24,000 EOH unless a design change is introduced.

In wrapping up, Macdonald said most Vx4.3x users are finding vane cracking at around 50,000 EOH but some with a high number of starts have found cracks at as few as 15,000 EOH. He added that the 50-Hz V Users Group is working closely with Siemens and compiling statistics on fretting and cracking.

Other user presentations. There were several other excellent user presentations at the Newport meeting and one or more will be developed into short articles for a future issue. Paul Tegen, chief CT engineer for Cogentrix Inc, Charlotte, and the chairman of the 501F Users Group, discussed his company’s experience with V64.3A compressors as well as with burner fretting wear. Interestingly, neither Macdonald nor Tegen had any indication of impending vane or blade failures before they occurred.

One participant said he had been told by metallurgists that many vanes still look fine even when as much as 90% of their fatigue life has been used up. Don’t be fooled: If you put them back into service, serious problems can develop quickly. An analogy might be hypothermia. A person who falls into cold water initially appears normal. But once his body temperature drops below a threshold value, loss of consciousness occurs virtually instantaneously and he will drown.

Conectiv Energy’s Bill Robbins followed Tegen with a presentation on the challenges of modifying a spare rotor to accommodate the needs of a fleet of V engines. Then Jeff Gillis, senior staff engineer, ExxonMobil Chemical Co, Baytown, Tex, and cochair of the Frame 6 Users Group, addressed the group on post-hurricane operational problems caused by pipeline gunk.

Theory and evidence suggest that with the gas pipeline out of service during one of last year’s Gulf Coast hurricanes, flow reversed and a black gooey residue that had been accumulating for years worked itself loose. After a couple of months of operation following restart, the residue migrated to customer sites and plugged burners. All supply piping back to the source had to be cleaned.

Siemens Day

John Schuck, director of business excellence, was introduced by Chairman Pasley and got the day’s presentations rolling with a review of the OEM’s progress in satisfying customer needs as determined from surveys and interviews conducted by Six Sigma Program personnel.

“Two-way communications into the hearts of large organizations can be challenging,” Schuck began. “At times,” he continued, “it can be difficult for customers to get the specific information they require for decisionmaking, and for the OEM to respond to the unique needs of individual owner/operators.

“But Siemens is committed to listening carefully to its customers—the analysis of customer satisfaction surveys is one way—and is putting forth strong efforts to improve the transparency of its communications with them. The Customer Extranet Portal (CEP) and use of online ‘Net meetings’ and Webcasts have been especially valuable in this regard.”

The CEP (https://customers. pg.siemens.com) provides a window or portal directly into the OEM’s information network. Owners and operators can access a variety of unit-specific technical information— including technical bulletins, spareparts catalogs, and comparative operating statistics; plus, presentations from conferences and Net meetings.

At the time of the V Users annual meeting, the CEP contained more than 1500 unit-specific repair and outage reports and more than 900 technical bulletins. In the three years since inception, CEP membership has grown to more than 800 owner/operators— about half of them signing up in the last year. Member increase among V users in the first five months of 2006 alone grew by 36%.

Looking ahead, Schuck said the company’s plan is to expand content to include instruction manuals and outage planning documentation. Further, to implement advanced functionality that would enable online reporting of operating statistics, order tracking, and e-commerce.

Net meetings. Just two years ago, Siemens Power Generation reached out to the company’s telecom group to develop and implement online meeting software that would facilitate communications between the OEM and GT user groups it supports—on demand. The versatile software provided allows for effective open discussion forums as well as for formal technical presentations.

Net meetings are highly regarded for their ability to connect Siemens technical experts, wherever their location, to customers with an immediate need for specific information. Agenda for Net meetings range from communicating details on a particular topic to providing general updates on many.

Recent Net meetings have featured coverage of the new igniters designed for the 501G engine and a service bulletin for Westac (for Westinghouse air-cooled) generators. The “Siemens channel” also carries regularly scheduled open discussions on issues related to TXP and WDPF control systems (see “Web forum facilitates communication between users, OEM,” 501F User’s Group report, COMBINED CYCLE Journal, 2Q/2006, p 12).

Schuck said that the I&C forums, in particular, have demonstrated the value of collaborative online communication both by improving user understanding of control system design and by facilitating the integration customer feedback into software upgrades.

No need for anyone to miss a Net meeting: Presentations can be accessed 24/7 on the CEP.

Schuck’s presentation covered other subjects as well—such as the Plant Optimization Center, Field Service Support Center, spare parts, etc. Consult the Siemens Day profiles that accompany both the 501D5/D5A and W251 Users Group reports, presented elsewhere in this issue, for details.

Customer satisfaction survey results were presented by Harvey Grassian, director of market and customer analysis, who compiled the information with Jean Matkovich. Grassian opened with a review of the four-step customer-satisfaction business process: assess the voice of customers, analyze information, improve products and/or processes, and validate improvements with customers.

The six-sigma approach to gain and sustain customer satisfaction begins with surveys and interviews. Content analysis and the development of meaningful statistics is the next step, followed by identification of the source of the dissatisfaction: commercial, technical, service, or customer-driven. Actions to correct deficiencies include process/product improvements, customer programs, and/or R&D spending. Finally, resurvey.

Grassian reminded the V users to submit accurate operating statistics in timely fashion, explaining to attendees how important the information is to the OEM for helping owner/operators maximize reliability, availability, and efficiency while decreasing emissions. Just under half of the 2006 survey respondents have V84.3Ax machines, another 35% V84.2s. Dutycycle: Half of the engines are in standby service, nearly a quarter in peaking duty, and most of the remainder are base-load.

Overall satisfaction rating among participating users is virtually the same as it was when Grassian began compiling formal stats back in 2003. Translating numbers to words, one would say that customers are “somewhat satisfied overall.”

Regarding the all-important parts life, V84.2 owner/operators reported higher levels of satisfaction with all hot-gas-path (HGP) components since the 2004 survey; turbine blades received a “significant improvement” rating. Nearly the same scorecard from V84.3Ax owner/operators except that “significant improvement” ratings went to turbine blades as well as several other parts. Outage report content/timeliness and response to GT technical issues also improved significantly.

The entire spare-parts value chain received complimentary ratings, with “significant improvement” going to ease of ordering, status information, availability of HGP, and on-time delivery.

The top grade in level of customer satisfaction among all areas evaluated went to V84.2 availability/reliability which finished well ahead of second-place O&M plant supervision. Remainder of the top 10 key areas evaluated were: V84.2 starting reliability, V84.3Ax heat rate, V84.3Ax availability/reliability, GT service workmanship, outage schedule and safety performance, spareparts invoicing, outage implementation, and post-outage equipment performance.

The V fleet availability/reliability report was delivered by Rich Rogalin, senior program engineer in the fleet and component data management group. Key points from his presentation:

  • Both 50- and 60-Hz Vframe engines are highly reliable. Most units are above 99% (12-month rolling average).
  • Availability is improving for the two frames that dominate the Americas fleet, “significantly” for the V84.3A. The V84.2 is at 97% with the V84.3Ax close and gaining. Most maintenance is conducted during planned outages, which may be of widely varying duration.
  • Starting reliability is high, averaging 95%. However, there is a wide variation among individual units.
  • Average fleet service factor (V84.2 and V84.3Ax frames) is just south of 30% because of the large number of standby units.

A generator update was conducted by Tom Schuchart, manager of global generator service engineering. Schuchart updated attendees on several issues impacting both hydrogen- (THRI) and air- (TLRI) cooled units that had been announced previously in technical advisories and other communications. Here’s a summary:

  • Cracks observed on pole cross-over brazed joints in some generators (60 Hz, air- and hydrogen-cooled machines). Crack propagation is very slow. Root causes: low-cycle fatigue and/or braze substrate deposits. Correction includes installation of a redesigned crossover connector; repair process has been qualified in the shop and in the field (may take up to seven days to complete). Inspect under retaining ring to see if cracking has occurred. This can be done with the rotor in place using a technique developed by Siemens and should be included as a standard inspection procedure.
  • Gas-baffle improvements to prevent fracturing in service include lower-stress design, use of more flexible materials.
  • Some tooth top cracks have been reported in the fleet; however, they have caused no forced outages. Siemens is systematically developing an improved tooth-top/retaining- ring fit geometry for very high cyclic operation. Design-life goal is 10,000 start/stop cycles; while the entire Siemens fleet leader has accumulated more than 7000. The new design is being installed in new units. OEM recommends modifying operating units in accordance with planned-outage schedules. The rotor must be removed to implement this mod.
  • TLRI and THRI generators have two so-called J straps on the excitation ends of the machines. These are J-shaped rotor electrical lead connections between the radial leads and the first turns of the rotor winding. They originally were designed to IEC (International Electrotechnical Commission) standards; however, J straps on a very small percentage of the more than 600 generators so equipped suffered low-cycle fatigue failures after several years of operation. Cracking on all but one of the failed J straps was found on the top-turn connection. Note that the J strap is difficult to access for inspection, a procedure best left to experts. A design to accommodate high cyclic operation is being installed in new units and is available as a retrofit. Recommendation is that replacements be scheduled consistent with planned outages.
  • Root cause of rotor blower blade cracking has been identified as flow buffeting. Performance of a new blower design that incorporates tuned, steel T-root blades has been verified in shop tests and in field operation on more than two score retrofitted units. Periodic inspection of blades on the original blowers, which have threaded aluminum blades, is recommended with replacement in-kind when indications are found. A switch to the new blower is recommended.
  • Stator-frame axial vibration, if identified, is easily corrected by adding supports/ gussets.
  • Stator end-windings should be bump tested and inspected as part of a comprehensive maintenance program. Verified techniques for tuning the end-winding are available and are recommended to be implemented for cause.

A comprehensive GT service update was provided by Andreas Skrobotz, field support manager. Skrobotz covered compressors and combustion systems for Vx4.2 and Vx.3Ax machines in rapid-fire fashion, slowing only to field questions during nearly two non-stop hours at the front of the meeting room. Maintenance managers in attendance had to be impressed with the level of detail he provided in the four major areas addressed:

  • Flame tubes, findings and recommendations for F-ring oxidation.
  • Compressors, findings and recommendations for stator vanes.
  • Lifetime extension, early experience.
  • Ceramic heat shields, materials update and other topics.

Consider the stator-vane presentation, for example. Skrobotz reviewed findings reported during major outages— including some wear at the contact area between vane hook and inner ring, and airfoil cracks in the hook area. Note that there have been no reports of stator-vane airfoil tear-off initiated by cracking in compressor vane hooks.

Engineering investigations of fleet findings, including finite-element analysis of single- and double-sided staking, were explained in detail. Laboratory and field testing proved double-sided staking effective for significantly reducing wear.

However, because of cracks at the vane hooks on some frames (V64.3x and V94.3/3A1), permanent solutions are in development with a short-term new design featuring an increase in hook thickness and improved transition geometry from airfoil to hook scheduled for introduction this year. Plan is to validate the solution on 50-Hz machines. Future development plans call for making the first compressor stages more robust. The design of this long-term solution should be ready by the end of 2007.

Lifetime extension (LTE) is a topic of increasing interest as fleet’s age. Skrobotz discussed Siemens extensive knowledge of long-term operation: More than 50 of its machines have passed the 100,000-EOH (equivalent operating hours) mark and two are beyond 200,000 EOH.

The presentation explained the LTE process, quantified the benefits, and ran through a case history. Last explored in detail where component replacement was most likely in the compressor section (vanes and blades), the rotor (damping cones), combustion section (mixing chamber), and turbine (blades and vanes in some rows), and where components can be released for an additional 100,000 EOH or 3000 starts after certain NDE—such as specially developed ultrasonic testing of forged rotor parts.

Repair technology and experience were covered by Christian Baehr, manager of V-engine repairs. He began with a review of the refurbishment process, explaining the steps associated with incoming inspection, stripping, and fact-finding (nondestructive examination and definition of the level of repair necessary). Also, how repairs are made: blending, brazing, and welding. Final steps include recoating and the reopening of cooling passages.

The presentation—a workshop might be a better description of this session—was of particular value for those attendees responsible for due diligence and verification of repair procedures and work. To illustrate: An overview of applicable welding processes identified base metal and welding materials; the specific steps and care required for successful brazing were reviewed; matching of inspection findings and the optimum repair technology was covered.

Baehr closed wi th a review of current and planned repair technologies. For example, a vendor has been qualified for narrow gap brazing of R1 and R2 vanes on V84.2 engines and an improved coating for R4 blades and vanes increases the interval between recoating.

Long-term development includes special welding and brazing techniques targeting repair of single crystal parts with a single-crystal joint, while conventional repair technologies for these parts are already available. Attend the 2007 meeting for updates on technology development and experience.

A service update on auxiliaries was given by Volker Poloczek, manager of auxiliaries and system design for the service engineering group. An adjunct presentation by George Van Deventer, commercial marketing manager, addressed the issue of the long lead times associated with repair of Argus gas valves: A valve exchange program was offered as one solution for several of the Argus gas valves/actuators, and qualification of a fullservice domestic supplier to include both valve and actuator repair is underway.

Poloczek got down to the nitty gritty of engine servicing. Fuel valves, hydraulics, and other auxiliaries, often overlooked in the “big picture” of GT overhaul, can cause engine trips when not given the respect they deserve. He outlined a service plan for ball valves and low- and high-pressure hydraulics of particular value to maintenance managers.

In brief, it suggested functional testing of components at 25,000 hours (typical interval for first hot-gas-path inspection) and overhaul at 50,000 hours (first major); then the cycle repeats. Functional testing includes checking for valve leakage (internal and external), stroke time, limit-switch control, and step response. Valve overhaul includes disassembly and change-out of inner and outer seals, metal bellows, and packing gland.

Special mention was made of fueloil transfer valves which have been problematic at a couple of plants. Overtorquing of the valve by the actuator has caused shaft breakage in two cases. Incidents occurred during the purge sequence after an oil trip; valves remained open. Recommendat ion: Replace valves and actuators with ones of more robust design.

Alternative fuels. Poloczek returned to the podium late in the afternoon with a primer on alternative fuels for GTs.

Life-cycle support of the control system was discussed by Tom Franks, a regional marketing manager. Essentially the presentation offered the case for migrating to the SPPA-T3000 control system. For details, refer to “Upgrading controls to maximize performance, availability,” COMBINED CYCLE Journal, 2Q/2006, p 111.

Mods and upgrades (M&U) always get the audience’s attention. Marco Tappan had a particularly attentive group as he took the podium after an afternoon break consisting of coffee, cookies, and ice cream. Tappan’s new product development roadmap stretched out to 2009, covering such improvements as higher compressor mass flow, advanced coatings, IGV (inlet guide vane) upgrades, and many others. All were evaluated with regard to improvements in power output, efficiency, emissions reduction, reliability/availability, and operational flexibility; plus reduced cost of maintenance.

Plant and asset managers were particularly interested in what Tappan had to say because they are responsible for planning the outages that would permit the modifications necessary to implement the upgrades, and to begin the long process of lobbying upper management for the necessary funding.

Two topics of considerable interest given the desire on the part of many owners to squeeze every possible kilowatt from a particular frame and to extend turndown for greater operating flexibility:

  • Redesign of the first five compressor rows to increase GT mass flow and power by about 3% with no adverse impact on GT efficiency. Turbine exhaust temperature decreases because of the higher compressor discharge pressure and can be restored by slightly increasing GT firing temperature to optimize plant output.
  • Turndown to less than 50% of the machine’s nameplate rating is possible with an M&U offering applicable to nearly all units in service. Minimum compressor mass flow is reduced to 62% of design by reducing air flow through the IGVs. Operation in the pre-mix mode maintains a constant exhaust temperature to less than half load while holding emissions to permit levels.

In both cases, design calculations have been confirmed by more than three years of experience in 50-Hz commercial units. Scaled designs for 60-Hz application are available. ccj