By the time Chairman Ben Meissner opened the forum portion of the 21^st annual conference of the 7F Users Group Tuesday morning at the Westin La Cantera, many of the 250 owner/operators in attendance had already participated in a day of special events. Monday featured an eclectic program that included a tour of Pratt & Whitney Power Systems’ San Antonio shop, a workshop on heat recovery steam generators conducted by engineers from HRST Inc, a golf tournament, and a special session on 7F rotor dynamics, vibration analysis, and troubleshooting developed by GE Energy. A reception and dinner completed the day.

The golf tournament, which began at 8 a.m. on one of the host hotel’s two onsite courses, was dominated by players from the 108 participating equipment and services providers. The morning-long HRSG Spotlight Session focused on fatigue cracking and was attended by about three dozen users. Amy Sieben, PE, and Scott Wambeke, PE, divided up the subject matter, with Sieben covering fatigue and other failure mechanisms in superheaters/reheaters and panelized economizers and Wambeke handling return-bend economizers and steam-drum nozzle cracks.

The approximately 100 users who visited the PWPS shop found a bright, modern repair facility with the latest inspection and repair tools and efficient production lines (photos). The facility is a one-stop shop—strip to ship—for inspection, refurbishment, and repair of F-class parts. General Manager Gerald D Hill and his team of knowledgeable tour guides told the gas-turbine owners and operators that, in addition to restoration repairs, PWPS pursues design improvements with the goal of providing refurbished parts that are “better than the original” where possible. The collaborative process of component improvement involves (1) identification of issues based on field experience, (2) re-engineering and modeling of improved parts, (3) laboratory validation and finally (4) field validation.

Vane repair line extends from one side of the shop to the other

New tip is welded on a 7FA+e first-stage bucket

Obvious from the visit was PWPS’s commitment to a culture of continual improvement. If you haven’t toured this facility in the last couple of years, you might consider returning for an update. A great deal has been accomplished since then. In fact, new equipment was in commissioning during the 7F visit while other machines were being installed in ever-diminishing space.

The users-only sessions on Tuesday covered the 7F compressor, safety practices and lessons learned, controls, and auxiliaries. Vendor presenters included PSM, ExxonMobil Corp, National Electric Coil, and Advanced Turbine Support LLC. Wednesday’s user-only lineup includes a combustion and turbine session, generator session, and open discussion before presentations by Environment One Corp, Praxair Surface Technologies Inc, PWPS, and Turbine Technology Services Corp, among others. Vendor fairs, including dinner, complete the Tuesday and Wednesday programs.

Thursday is GE Day. Presentations and discussion cover the entire engine from the air inlet house through the generator, capped off with a reception and GE product fair. Friday’s program, which ends at noon, features parallel sessions on D-11 and A-10 steam turbine maintenance and GE controls and diagnostics.

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The half-day “HRSG Spotlight” session that HRST Inc, Eden Prairie, Minn, has conducted in conjunction with the 7F meeting for the last several years is designed for plant personnel who want a refresher on heat-recovery steam generators and an update on industry concerns with large triple-pressure units.

Focus of the 2012 workshop was fatigue cracking, with Amy Sieben, PE, presenting on failures in superheater/reheaters and panelized economizers, and Scott Wambeke, PE, addressing drum-nozzle cracking and fatigue in return-bend economizers. Sieben opened the session by saying that fatigue cracking is one of five mechanisms that account for more 90% of all pressure-part failures suffered by HRSGs. The others are flow-accelerated corrosion (FAC), corrosion fatigue, chemical attack/under-deposit corrosion, and dew-point corrosion.

Fatigue cracking occurs, she said, when material is repeatedly stressed beyond its yield point. Low-cycle fatigue is the term used to describe fatigue failures that occur in fewer than 1000 cycles. Sieben introduced an important term into the lexicon of many attendees when she stressed the importance of managing the HRSG’s “fatigue bank account.” Fatigue cracking can initiate on the inside or outside surfaces of pressure parts, the boiler designer continued, noting the three components of fatigue: pressure, temperature, and external piping stress.

Fatigue cracking in superheaters (reheaters). Tube-to-tube temperature differences cause cracking in superheater (reheater) panels for two primary reasons:

• Condensate blockage and poor drain design. Inability to remove condensate in timely fashion during/following a unit purge often is traced to undersize, ganged, or closed drains.

• Water introduced through interstage desuperheaters, which are located between the primary and secondary superheaters and reheaters. Typical causes include leaking spray-water supply valves, hunting, poor piping arrangements, overspray, and a primary/secondary superheater (or reheater) surface arrangement that is incompatible with a given turbine’s performance at startup or low load.

Note that attemperators sometimes are installed downstream of the final superheater or reheater surface in lieu of, or in conjunction with, an interstage desuperheater. Two concerns shared by owner/operators regarding the use of “downstream” attemperators: (1) Additional cost and (2) the risk of steam turbine damage in the event of a failure. A couple of slides illustrated for first timers the two basic types of desuperheaters used in HRSGs: (1) Probe style with single or multi-nozzle axial injection and single or multi-nozzle radial injection. Reported advantages of the latter are that spray nozzles are not in the steam path and steam/water mixing generally is more efficient than with the probe style attemperator. For more on the subject, access “Avoid desuperheater problems with quality equipment, proper installation, tight process control” by HRST’s Scott Wambeke.

Sieben then expanded her coverage of the two bullet points above. She began with a few photos and drawings illustrating how humping of lower headers equipped only with center drains can allow condensate to block tubes at the ends of superheater and reheater panels and cause buckling of those tubes. Having multiple drain locations is one way to solve this problem. Discussion of the dos and don’ts of drain system design came next. Here are the important take-aways:

• Purge condensate from lower headers before every start. Automatic valves are needed to do this effectively.

• Drain condensate as it forms during the gas-turbine purge cycle.

• Proper sizing of drains is critical. Keep in mind that drains too large or too small can be problematic.

• Locate blowdown tanks below header drain locations.

• Avoid combining drains (Fig 1). Be especially careful not to interconnect drains operating at different pressures: The higher pressure drain can block condensate flow from the lower-pressure line. More on drains at “Incorporate lessons learned into specifications for new units” and “Learn the basics of HRSG inspection.”

1. Poorly designed drain arrangement has manually operated under-size drain lines and multiple drains ganged together

Sieben next noted that a proper drain must allow for a full range of motion between the penetration seal and the access hole in the HRSG casing. Differential expansion between ganged panels amplifies drain lateral displacement, she said, illustrating the point with Fig 2. Drains that collide with the floor liner or casing often suffer stress-induced cracking (Fig 3).

2. Differential expansion amplifies drain lateral displacement

3. Drains that collide with the floor liner or casing often suffer stress-induced cracking

Ineffective draining of cold reheat lines, and occasionally main steam piping, also is conducive to damage. Sieben spoke about water hammer resulting from a slug of condensate being pushed through steam piping during startup. Typically, she said, pipe supports are bent, or thrown out of position; piping may be damaged as well. Tube damage is a possibility, too, if a slug of water reaches the HRSG. Sieben offered a checklist on how to avoid water hammer:

• Drain steam piping before every gas-turbine start. Best practice: Automate valves and install condensate detection for added protection.

• Confirm piping slope and the ability of the drain system to clear condensate from the entire line.

• Prevent the possibility of water accumulation upstream of valves—especially the steam-turbine bypass.

• Tightly control bypass/letdown valve attemperation.

• Check for leaking bypass attemperator spray water.

Before addressing in detail the desuperheater problems identified earlier, Sieben reminded attendees of recent (2007 and 2009) changes to the ASME Boiler & Pressure Vessel Code regarding attemperators. First, drain pots downstream of desuperheaters must be able to detect water automatically and to drain it without operator intervention. Second, superheater and reheater drains must be able to detect and drain condensate both under pressure and at atmospheric pressure.

Leakage by block and control valves usually can be prevented, Sieben added, sometimes by simply specifying class V shutoff or better. Confirm leak tightness by finding no drop in steam temperature across the desuperheater. Attemperator hunting, most common at low load, causes chronic cycling with the possibility of fatigue damage in the probe, liner, and/or piping. Hunting increases the likelihood of finding water in the superheater.

Attemperator overspray can damage superheaters and reheaters, and, in the case of desuperheaters downstream of those heat-transfer surfaces, may cause catastrophic damage to the steam turbine. More on overspray at “2011 Outage Handbook – HRSG Clinic.” Overspray usually is attributed to one or more of the following conditions:

• Poor atomization of spray water because of probe/nozzle damage or partial plugging.

• Improper piping design—in particular an insufficient straight run of pipe upstream and/or downstream of the attemperator.

• An arrangement of superheater and/or reheater surface that allows overspray to occur at some operating points (typically startup or low load) because all the water cannot be evaporated.

Sieben spent several minutes explaining the last point by way of diagrams with actual gas and steam temperatures and spray-water flow rates for varying loads both with and without supplementary firing, and for different ratios of superheater and reheater primary and secondary surface. One example presented: A superheater for an F-class HRSG designed with 60% primary surface area and 40% secondary surface area requires no spray water when operating at base load without duct burners in service, but needs 43,000 lb/hr of spray water at min load without supplemental firing. For a superheater having 70% of its surface area in the secondary bundle, 55,000 lb/hr of spray water would be required at min load without duct burners.

Correcting for overspray can be extremely challenging and expensive, Sieben continued. Options she offered included these:

• Bypass a portion of the HP saturated steam flowing from the drum to the primary superheater thereby cooling steam exiting the secondary superheater. Same logic can be applied to the reheat circuit, with some of the cold reheat steam being withdrawn ahead of the heat-transfer surface to cool hot reheat. This option is rarely practical because of the expense involved and because reducing steam flow to superheater/reheater panels can increase metal temperatures above recommended limits.

• If too much surface is installed, remove fins and/or gas baffles, or use tube shields, to reduce heat transfer.

• Add a final attemperator or an additional interstage desuperheater.

• Minimize or eliminate the need for spray water on startup by installing an air attemperation system. More at “Air attemperation protects HRSGs against damage at low loads.”

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Immediately following the announcement of the 7F User Group’s new website, Chairman Ben Meissner polled the more than 250 attendees in the Westin La Cantera’s ballroom to learn more about them. About half of the audience, by show of hands, said this was their first 7F meeting. Well over half the group had at least two years of experience operating and maintaining the GE engine, with 40% saying they had participated in at least one major inspection. Units in peaking, cycling, and base-load service were represented about equally. The starts leader in the room was over the 3000 mark; six engines were said to have recorded more than 100,000 operating hours. More than half the group had GE long-term or parts services agreements; about 30% self-performed maintenance.

The first user to present said he was concerned by the migration of compressor rotor-blade spacers discovered on two 7241s during their second HGPs. The units had about 1800 starts each but were transitioning from a starts-based regimen to base-load service. The units’ first majors would probably are two years off. This owner/operator was not sure what should be done to address the migration issue, if anything, and asked his colleagues in the audience to share relevant experience.

Not to worry. His investigation revealed no vibration problems resulting from spacer migration. In fact, calls to several users before making the trip to San Antonio did not uncover any vibration issues in the fleet that could be linked to spacer migration. The OEM’s response, he said, was “don’t worry.” However, GE expressed some concern that if the migration had occurred in rows 7 and 8, where the spacers align, a combined migration of the two rows could happen in the future.

Four attendees said spacer migration had occurred on their machines but there had been no negative effects. One of these users said he used to work for the OEM and had seen many instances of migration but no problems resulting from that movement. Yet another owner/operator had experienced migration on Frame 5s and 7EAs and thought it might be the result of poor craftsmanship—specifically, poor staking.

Someone else said that if just one or two spacers migrate it’s one thing, but if several in a small section of the rotor move, then you could throw the unit out of balance. He reported seeing this on peaking machines and attributed the migration to “thermal ratcheting.” A few others confirmed this based on their experiences with 7B-EAs. The OEM reportedly did field restaking for one user to correct for movement of one spacer.

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SRO, standing room only, is no surprise when PSM presents at a user group meeting. Chris Johnston, a senior R&D manager, moved quickly through understandable technical detail in bringing users up to date on PSM’s 7FA compressor solutions and field experience with state-of-the-art parts for F-class engines.

This year’s 7F program accommodated three vendor presentations in each of two 45-min time slots between the user-only sessions and the vendor fair. The format was the same both Tuesday and Wednesday.

Johnston divided the compressor section of his presentation into four parts: R0, S0-S4, S13-16, and S17/EGV (exit guide vanes). PSM’s compressor solutions have been operating in the fleet for four years with parts installed in more than 30 units, he said, adding that fleet-leader sets have been validated by way of on-going in-situ, destructive, and dimensional inspections. The company’s redesigned R0 blade, which has a different airfoil shape than the OEM’s, is meeting expectations in units that fog and online water wash. Its R0 retention plug replaces the biscuit familiar to many users (Fig 1).

Success with the flared R0 airfoil prompted development of an unflared R0, the R&D manager continued. The new offering also operates without restriction and is now running in four or five engines. The development approach used PSM’s successful methodology: field assessment, problem identification, solution implementation, and validation.

1. PSM’s R0 retention plug

2. Corrosion resistant carrier ring segment for S0-S4 is designed for ease of installation and removal

S0-S4 field issues, including high cycle fatigue (HCF) and corrosion concerns, have been addressed with a redesigned S3 airfoil and corrosion resistant carrier ring (Fig 2). Johnston said PSM believes that the OEM’s S3 vane has the lowest design margin in this section of the compressor and required redesign to mitigate HCF failures initiating at its leading edge. OEM vanes in S5-S16 have a history of hook fretting and other hook-fit issues. PSM’s S5-S12 vanes remain stator singlets, redesigned to better accommodate the circular case. For rows S13-S16, the company’s “hook ring” packed design provides increased damping.

The second part of Johnston’s presentation concerned the 7FA GTOP3 upgrade. GTOP is the acronym for Gas Turbine Optimization Program. This enhancement incorporates PSM’s low-pressure-drop combustion system and redesigned first- and second-stage HGP buckets and nozzles. The upgraded components have been operating since 2005 and have accumulated nearly 100,000 service hours. They are interchangeable with OEM parts.

The higher firing temperature allowed by the redesigned standard-life (24,000 hours/900 starts) first- and second-stage turbine buckets and nozzles can increase power output of a simple-cycle engine by 5% and improve simple-cycle heat rate by 1.3%, while still achieving less than 9 ppm NOx and accommodating turn down to 50% of the full-load rating. Extended-life (32,000 hours/1200 starts) airfoils are compatible with a 2% increase in simple-cycle power and a 1% improvement in heat rate.

Johnston noted the 22-MW gain attributed to GTOP3 for the company’s first combined-cycle install at a 2 x 1 plant. PSM provided field services, the gas-turbine TFAs, and tuning services. Third-party performance testing conducted before and after the outage validated a 5.2% increase in power for one GT and 5.1% on the other. Efficiency gains were 1.6% and 1.5%, respectively.

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When members of the borescope inspection team from Advanced Turbine Support LLC talk, virtually everyone with responsibility for gas-turbine assets listens. Reason: Rod Shidler and company generally don’t tell you what you want to know, they tell you what you have to know to keep running. Field Service Manager Mike Hoogsteden’s presentation to a room packed with users on Tuesday discussed the importance of technical information letters (TILs) 1509-R3, 1638, and 1795 in reducing operational risks.

Much of what Hoogsteden had to say in San Antonio he and colleague Dustin Irlbeck reported to the industry last December by webinar. CCJ ONscreen’s Scott Schwieger coordinated and produced that event, which was sponsored by Dresser-Rand LETT. A recent CCJ article, 7FA TIL Updates, summarized the webinar’s content. Regarding TIL 1509-R3, recent findings ATS shared with users (HEADS UP for 7FA users: S1 tip liberations) suggest that TIL 1509-R4 may not be far off. 

For Tuesday’s presentation, Hoogsteden added a section on TIL 1562 to stress the importance of monitoring the condition of compressor shims and the corrective actions necessary to mitigate the risks of migrating shims on both E- and F-class machines (photo). There are 20 possible shim locations in the first five stages (0-4) of 7F units and they are spaced approximately 60 deg apart from each other.

Most people like stats and Hoogsteden gave the owner/operators some numbers to ponder with what he called the ATS Scorecard. Keep in mind when reviewing these data that there are 821 engines in the 7F fleet, according to Ben Meissner, chairman, 7F Steering Committee, and that the fleet leaders are at about 126,000 factored fired hours and about 3500 factored fired starts.

• TIL 1509. ATS has completed more than 2000 in-situ inspections since 2001, identifying more than 200 cracked rotor blades, over 70 S0 cracked vanes (six in unflared compressors), and three S1 cracked stator vanes.

• TIL 1638. ATS has completed nearly 1100 in-situ inspections of R0 rotor blades (P-cut, standard, and enhanced) over the last five years, identifying 55 R0 blades with cracks adjacent to the P-cut relief, 55 R0 blades with cracks in the suction-side mid-span dovetail fillet, two R0 blades with cracking on the suction-side mid-span dovetail sloped face, two R0 blades with cracks in the pressure-side dovetail fillet (both in the leading and trailing edges), and two cracked R1 rotor blades.

All of the above indications have been validated; there were no false positives.

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If the gas-turbine user group community were to vote for a Generator Champion, National Electric Coil’s Howard Moudy would probably win hands down. Well known, he attends most user meetings and speaks passionately on the maintenance and overhaul of these vital rotating machines. Moudy’s presentation on Tuesday was on “Considerations in Purchasing Quality Stator Windings and Rewinds.”

His goal was to provide much needed perspective on what steps owner/operators should take to assure that their project expectations will be met. The specification is critical, he stressed, and challenged attendees to think about the possible improvements they could make by not replacing in-kind. All capable suppliers have the engineering ability to upgrade an existing coil, Moudy said. For example, thin insulation material available today allows you to increase the amount of copper in an old machine, reducing I2R losses; thinner strands can reduce eddy current losses.

His slides walked the users through the coil manufacturing process, covering types of ground insulation, winding fit-up inspections, etc. Tests described included blackout, corona camera examination, hi pot, voltage endurance, and thermal cycling. Moudy’s presentation, minus sensitive slides, will be posted to the 7F Users Group’s new website at www.7fusers.org within a few weeks.

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