501F and 501G Users Groups: Synergy between groups benefits all participants

The 501F and 501G User Groups co-located for the third consecutive year last February. The part­nership works well and is a win/win/win for the users, equipment and services providers, and Sie­mens Energy, Orlando, the OEM for both engines. Technical user-only sessions are combined or separate depending on the subject matter; members of both groups come together for the vendor fair and all meals and social events.

The 2010 meetings for both groups are scheduled for the Disney Yacht & Beach Resort, February 21-25. Caren and Lisa Genovese will be coordinating the conference as in the past. Refer questions to carengeno­vese@charter.net.

Owner/operators of 501F (501G) engines interested in par­ticipating in the 2010 meeting who are not registered members of the 501F (501G) Users Group are urged to submit their profes­sional profiles as soon as possible via the membership drop-down menu at http://501F.Users-Groups.com/Membership/User­Candidate.shtml or http://501G.Users-Groups.com/Membership/UserCandidate.shtml. Only reg­istered members are invited to attend the annual conferences.

In a perfect world, the steering committees, through their plant-level contacts would personally invite each employee at every generating facility equipped with 501Fs and 501Gs to join these organizations. But committee members are volunteers all with real day jobs— like yourself. There is just not enough time for the one-on-one approach.

501F steering committee

Chairman: Paul Tegen, chief CT/I&C engi­neer, Cogentrix Energy Inc.
Vice Chairman: Russ Snyder, plant man­ager, Cleco Generation Services LLC.
Gary Giddings, power block supervisor, Progress Energy Florida.
Ivan Kush, interim director of outage ser­vices (Siemens fleet), Calpine Corp.
Martha Leskinen, senior engineer, SRP.
Ray Martens, plant manager, Iberdrola Renewables Inc.
Paul Terry, maintenance engineer, PPG Industries Inc.

501G steering committee

Chairman: Steve Bates, plant manager, Hot Spring Power Co LLC.
Vice Chairman: Mark Winne, plant man­ager, Millennium Power Plant.
Timothy L Bachand, PE, manager of engi­neering (production), Lakeland Electric.
Ken Daycock, plant manager, AES Iron­wood LLC.
Ralph Leidy, plant manager, Granite Ridge Energy LLC.
Scott Wiley, manager of fleet maintenance, GDF Suez Energy Generation NA Inc.
Bill Wimperis, director of project manage­ment, Constellation Energy Inc.

So, consider this a personal invitation from 501F Chair Paul Tegen and 501G Chair Steve Bates to join these productive user groups and share your ideas on improving plant performance and safety, reducing operating costs and emissions, etc. Membership (there’s no fee) has tre­mendous value even if you can’t get to the meetings in person.

The websites for both organi­zations are repositories for con­ference presentations and corre­spondence among members that can help you solve problems. Plus, there’s access to an e-mail sys­tem that puts you in direct con­tact with other members who can answer your questions, loan you a much-needed part, etc. Sign up today and make your job easier.

Vendor profiles. Likewise, companies wanting to participate in upcoming meetings as a spon­sor or exhibitor must submit a vendor profile. This is done at http://501F.Users-Groups.com/Membership/AffiliateCandidate.shtml. Only companies approved by the steering committee receive invitations to exhibit.

The 2009 501F/501G meeting in Glendale, Ariz, ran a full four days. Sessions went from bell to bell; plus, there was a group dinner Monday (Day One) evening, ven­dor fair Tuesday evening (sidebar, p 105), and group dinner Wednes­day evening (sidebar, p 106).

The G group got rolling first with a robust Monday morn­ing program of user presentations and discussion covering redesign of valves for the rotor air cooler (RAC), retubing of a kettle boil­er, R1 blades and vanes, genera­tor rewind, installation of a new rotor and compressor, and genera­tor vibration. F owner/operators eased into the day, their first for­mal group session starting after lunch. F users looking to broaden their horizons were welcomed at the G session.

Siemens Energy, Orlando, saw an opportunity in the relaxed Monday morning and offered a special 100-min 501F mods and upgrades breakout session for the F group, expanding its normal coverage of the subject and opening up room on the Siemens Day schedule for other topics. There were “real world” results on mods and upgrades to report on this year: Much of the development work the company has been doing over the last several years is out of the laboratory and off company test rigs and now in beta trials or full commer­cial operation.

Among the topics covered—ther­mal performance upgrade and low-load CO control—were implemented last spring at Iberdrola Renewables Inc’s Klamath Cogeneration Plant, managed by Ray Martens, a member of the 501F steering committee. That experience is profiled in special fea­ture beginning on p 110 which also includes details on the plant’s conver­sion from WDPF to Siemens’ relative­ly new SPPA-T3000 control system.

Monday afternoon was busy, with the F and G groups coming togeth­er for an opening plenary that included a report by the TXP Focus Group to the full membership. Recall that issues with this system prompted the steering committee to support an auxiliary TXP group under the leadership of Mike Magnan, PPL Generation LLC. Ivan Kush of Calpine Corp assumed respon­sibility for this effort when Magnan transferred to a renewables position after the 2008 meeting. Members have worked closely with Siemens since 2004 via regular web forums.

After the plenary, attendees had a choice between controls and genera­tor breakout sessions. Dividing the audience into multiple subject areas has two big benefits: Attendees get to select the topic of greatest interest to them and the more intimate setting improves the group discussion.

Tuesday was Siemens Day for the G users. Presentations by the OEM focused on compressor hook-fit wear, R1 ring segments, combustor baskets, R4 deflection, blade-ring oxidation, an update on the rotor-bolt failure issue, bellyband seals, exhaust-cylinder repairs, RAC leak­age, torque converter, and several other topics. G users who could not make the meeting can access the Sie­mens presentations on the company’s Customer Extranet Portal (CEP). If you are not registered for access to that information resource, contact your Siemens representative today.

The F owner/operators spent Tuesday immersed in user presen­tations and group discussion on a broad range of topics, including: fleet safety issues, a compressor forced-outage case history, major inspection, FD3 upgrade (this topic is covered in detail in the Klamath article ref­erenced earlier), turbine enclosure platforms, exhaust diffuser failure, and torque converter failure.

Plants that do not send at least one representative to every 501F annu­al conference miss out on the les­sons learned by others and best prac­tices adopted by the industry. This increases your chances of overlooking a critical O&M finding and/or making the same mistake someone else has already paid for. No reason for that. There isn’t a user group in the GT sec­tor that doesn’t provide a minimum 10:1 return on the attendance invest­ment. You always come back to the plant with an idea or two that will cut costs by more than $10,000.

Wednesday the group programs were the reverse of those the day before, with the F attendees par­ticipating in Siemens Day and the Gs hosting user presentations and open discussion. The OEM’s track featured presentations on hot topics associat­ed with the compressor, combustion system, turbine section, generator, and exhaust end and casings; plus, a NERC (National Electric Reliability Council) advisory on the potential for lean blowout, cold-end bearing, repair quality, and outage support planning. G discussions focused on a R4 turbine-blade failure, general quality issues, unit updates, and lessons learned during the previous fall’s outages.

On Thursday, the final day of the meeting, the G users were invited to the morning F session, which offered topics of interest to members of both groups—for example, combustion dynamics, gas-only pilot nozzle issues, inlet-screen failure, and inlet and exhaust sections. The steering commit­tees for both groups also solicited feed­back from attendees both on the value of the Siemens presentations and the user-only sessions and what might be done to improve the conference.

Thursday afternoon featured two breakout sessions: one on steam tur­bines, the other on operational issues associated with the use of liquid fuel. Each of these sessions, which ran until 4 pm had a user-only compo­nent and an accompanying Siemens presentation.

Having a steamer session as part of the meeting makes perfect sense because most 501F and G engines are incorporated into combined-cycle plants and there is no independent steam-turbine user group to serve owner/operators. The 7F Users Group also covers steam turbines; plus, it has had a half-day workshop on heat-recovery steam generators (HRSGs) included in its value proposition for several years (p 2).

The liquid-fuel session was impor­tant because some users are convert­ing gas-only systems to dual fuel. Most activity in this area is related to the concerns public power genera­tors have regarding the possible need to burn oil to meet customer expecta­tions in the event of a gas emergency (loss of a pipeline, for example). There also is the possibility that payments for firm power may be tied to having both gas and oil capability.

To learn more about what’s involved in a gas-to-gas/oil conver­sion, access www.combinedcyclejour­nal.com/archives.html, click 2Q/2009, click “Termocandelaria” on the cover. See also in the same issue, the article on the Arvah B Hopkins repowering.

User presentations

Case history No. 1. A 501FC installed in mid 2000 that operated 16 hr/day suffered a performance loss of more than 10 MW. An off-line detergent wash recovered the lost megawatts. Over the next month, both a perfor­mance loss and decrease in compres­sor discharge pressure were noted.

Unit was shut down, forced-cooled, and detergent washed. Inlet guide vanes and R1 compressor blades were hand cleaned to remove a black sub­stance. Unit was returned to service; lost megawatts again were regained. Plant personnel assumed there was a bad oil leak at the No. 1 journal but were not able to prove this. Inlet fil­ters and evap media were near end of life and thinking was they might be a contributing factor. While looking for a bearing oil leak, a tear in a second-row compressor blade was found.

The OEM’s engineering team reviewed a photo of the tear and thought there was a high probability that it was caused by impact dam­age. A borescope inspection down­stream of R2 was recommended. Another R2 airfoil was found missing a corner of its blade tip. That failure was believed to have been caused by high cycle fatigue and the liberated piece considered the cause of the tear in the first blade. A forced outage ensued and 177 compressor blades were replaced.

Case history No. 2. Same turbine described in the preceding case his­tory tripped about four months later when the fire suppression system was initiated. Probe that initiated the trip is located at the left-side exhaust lou­ver. A plant inspection team entered the compartment and found an insu­lation blanket smoldering directly under the exhaust louver and fire-sensor probe. The cause was thought to be a loose union in the oil vent line that allowed an insulation blanket to become soaked with oil and ignite.

Oil vent line was tightened, insu­lation blanket replaced, and the FM-200 bottles refilled. Engine was restarted and taken to base load while checking for potential fire haz­ards. Everything looked good. An hour later there was another unit trip initiated by the same fire protec­tion probe as the first time.

A search for the gremlin was initi­ated immediately. Eureka! The root cause was an exhaust leak at the for­ward flange of the exhaust manifold. Crack was welded closed, exhaust manifold reinsulated, the FM-200 system refilled again, and the unit started. An infrared temperature gun was used to closely monitor the tem­perature of the fire detection probes until the unit reached thermal equi­librium at base load.

Lessons learned included the fol­lowing:

  • Make every effort to determine the root cause of forced outages and abnormal operating conditions.
  • Locate fire detection probes where they provide the protection needed and are least likely to cause acci­dental activation of the suppres­sion system. Install thermocou­ples adjacent to the fire detection probes above the exhaust manifold and program the DCS to alarm if the temperature exceeds the pre­set limit.
  • Be sure you have easy access to spares, a backup charge of sup­pressant (if a water mist system is not installed), and anything else necessary to reactivate the protec­tion system in timely fashion.

There was some follow-on discus­sion regarding potential issues asso­ciated with a breach in the exhaust manifold beyond unwanted activa­tion of the fire suppression system. For example, exhaust gas bypass­ing downstream catalyst could put the plant out of compliance on NOx and/or CO in areas with particularly stringent air emissions limits. A pos­sible safety risk: The presence of gas from a failed start or flowing gas for a system check.

Case history No. 3. This is the powerplant equivalent of a Sher­lock Holmes mystery that can be traced to an OEM Technical Advisory (2004-017) that the user never saw. Editorial comment: This happens. E-mails can be deleted accidentally, hardcopy mail can be misplaced, etc. A good reason every plant should be represented at the user group meet­ing serving its model of gas turbine is that the OEM always reviews TAs and other alerts issued during the past year. It’s the easiest way to find out if you have missed something.

Here’s how this mystery began, progressed, and was eventually solved:

  • Fall 2004. First scheduled outage after COD. Borescope inspection revealed no indications on com­pressor blades.
  • Spring 2005. Casing was removed. No relevant indications were found during a visual inspection of rows 1-16.
  • Fall 2005. Unit was in service for two years at this point. One blade had impact damage to its lead­ing edge; liberated material left a notch measuring about three quarters of an inch by a third of an inch. Inspection of the inlet came up empty and the damage was attributed to FOD (foreign object damage), source undetermined.
  • Spring 2006. Borescope inspection identified nicks in one R2 blade (leading edge), one R3 blade (tip of trailing edge), two R4 blades (both trailing edge), and one R5 blade (leading edge). Once again, inspec­tion of the inlet house revealed no obvious source and plant staff assumed that the “new” damage noted might actually have been caused by the material liberated previously.
  • Fall 2006. Minor impacts were noted on the leading edges of sev­eral R7 and R11 blades. No cause identified.
  • Spring 2007. NDE (nondestructive examination) evaluation revealed no new indications. Might the gremlin have exited the engine?
  • Fall 2007. No such luck. A new ding was found on the leading edge of one blade. Plant personnel returned to the inlet house and methodically combed it from the clean side of the filters to the trash screen and on the downstream side of the trash screen as well.

Perhaps the air filters were loose or pulling away from the wall during air-puff cleaning, allowing insects and debris to enter. Some insect debris was found; loose fil­ters were tightened; a few alumi­num nuts that held the filters in place had been stripped. All joints were thoroughly inspected and vacuumed. Once again, the source of the stealth foreign material was not found. The pesky gremlin had to be laughing at this point.

  • February 2008. A quick inspec­tion at the inlet revealed a ding that did not exist the previous fall. The unit was released for service after blending.
  • Spring 2008. Borescope inspec­tion revealed several indications throughout the compressor. A cover lift and significant repairs were nec­essary. The entire inlet structure was inspected once more, this time hand-over-hand and from top to bottom. Nothing. But then a second look at the inlet screen revealed a couple of places where small pieces seemed to have gone missing.

Tests confirm viability of compressor-diaphragm hook-fit solution

 Mitsubishi Heavy Industries, Orlando, confirms the suc­cessful completion of field tests of redesigned replacement compressor diaphragms for the W501FD. This solution was developed to address hook-fit wear in W501FD engines which sometimes required removal of the compressor cylinders to repair the hook-fit grooves without the need to modify or replace com­pressor cylinders. An inspection of the redesigned diaphragms and hook-fit grooves was performed in May 2009 with no signs of wear after 10,500 equivalent base-load hours and 73 equivalent starts.

During the 2008 501F User Group meeting in Orlando, Mitsubishi Power Systems announced that a design enhancement for W501FD2 compres­sor diaphragms had been developed and was being installed in a custom­er’s W501FD2 gas turbine (GT).

The upgraded diaphragm design is based on replacing the fabricated dia­phragm assemblies (tennon welded airfoils) with assembled diaphragms in which the airfoils are mechanically attached and banded to improve mechanical damping.

This assembly technique was first applied in 2001 and has accumulated almost 2 million hours of successful operation in Mitsubishi F- and G-class GTs. The thickness of the airfoils in the diaphragms was increased relative to the original airfoils to improve the rigidity of the assemblies—with only a minor penalty in compressor effi­ciency (approximately 0.1% for three stages).

Design validation tests began in February 2008. Strain gages, accel­erometers, and pressure transmitters were installed to monitor the response of vanes during startup, loading, and steady-state operation (Fig A). Results indicated that the diaphragms per­formed as well or better than similar designs applied to Mitsubishi’s fleet of large frame GTs. Initial indications were that the design would resolve the hook-fit wear issue.

The compressor diaphragms and hook-fit groove were inspected last May for signs of wear near the hori­zontal joint. The results: No visible wear with the Mitsubishi upgraded diaphragm design (Fig B). Also, the geometry modification of the hook-fit grooves to address this issue is not required.

By comparison, the original diaphragms would likely have worn the cylinder to the point that hook-fit repairs would have been required based on the operating hours accumulated at the time this borescope inspection was performed.

Mitsubishi has continued the design and development of replacement diaphragms for the W501FD compressors and now offers assembled-type diaphragms for rows one through six.

Sherlock’s magnifying glass car­ried the day. It confirmed that piec­es of metal had liberated from the screen at the edges of the structure. Close examination showed that the metal had failed at weld joints and other attachment points. Solution: Welded trash screens were replaced with woven screens. There are sever­al 501s at this site—a couple of FC+ engines and several FD2s. The ones with the woven screens had operated for years without any indication of metal liberation.

Social events critical to effective networking

You can’t meet someone half a room away during a discussion session. You might meet someone waiting for coffee during a break—if it’s a slow line. More than likely, however, the best opportunity for extending your network is at the vendor fair (p 105) or another social function.

The 501F and 501G users always have had first-class dinners on Mon­day and Wednesday nights courtesy of Mitsubishi Power Systems Ameri­cas and Siemens Energy. This year the user groups added a golf tourna­ment for one more networking oppor­tunity.

It was a tough day at The Legends at Arrowhead in Glendale: cloudy, windy, and even a hail storm. But somehow crummy weather has a way of bringing people closer together—particularly when they huddle trying to avoid hail stones. And there’s nothing like a good luncheon, complete with gifts, to stimulate the comradery. The winning foursome: Russ Snyder, Far­rell O’Malley, Dean Motl, and Brad Keating.

Mitsubishi’s Monday dinner at the Skye Restaurant in Peoria was special: Comical Chef Scott Tompkins shared some cooking techniques, there was a wine-tasting, and live entertainment. Some of the guests got into the swing of things and joined the band on stage. Word from an unbiased source is that Cleco’s Mike Bishoff might be ready for his own label.

Siemens’ Wednesday dinner at the Saddle Ranch Chop House was in sharp contrast to the Monday din­ner. Called a “hoot” by many who attended, it featured a western style dinner and included western and rock music, line dancing, hand-rolled cigars, bull (mechanical) riding, horse lasso, tomahawk throwing, and other fun things.

Vendor presentation

At user-group meetings for Siemens 501 and V engines, D-class through G-class, the one constant is in the area of generators. Difficult to recall a presentation in the last few years that was not made by Tom Schucha­rt or Jim Lau of Siemens or Howard Moudy of National Electric Coil (NEC). All three were in Glendale.

Moudy spoke at the 501F/501G user-only generator breakout ses­sion on Monday afternoon, bringing attendees up to date on spark erosion of stator bars and rotor pole-to-pole crossovers.

He said that NEC has been involved intimately in the inves­tigation of several failures attrib­uted to spark erosion, finding that the primary contributor to failure is loose bars in the stator core slots. Loose bars are conducive to vibra­tion, which causes sparking. Vibrat­ing coils make and break contact with the core, initiating the spark that causes erosion.

The type of semi-conductive side packing used in the stator core slots can be part of the problem, Moudy continued. NEC has found in its investigations several alternatives to side ripple springs, which it prefers for assuring long-term tight fit-up of stator bars in their slots.

He described two such systems: One uses flat semi-conductive side packing, the other a combination of semi-con­ductive tape and RTV silicone. Both were said to hold bars tight in the rotor slots at least initially. But over time the materials could shrink, enabling the bars to loosen and vibrate. This would not happen to a side packing system using ripple springs.

Moudy was on a roll, determined to convince the group that spark ero­sion should occur far less frequently than it does—if ever. Maintaining sta­tor bars tight in their slots is the fun­damental requirement for preventing this phenomenon, he emphasized. But gaps sometimes do occur despite best efforts. To minimize the damaging effects of high current levels in these instances, it is important to have a minimum value of surface resistivity on the bar semi-conductive coating.

Failures attributed to spark erosion have occurred in genera­tors made by more than one OEM, Moudy added, wrapping up. Com­mon denominators include large, air-cooled machines rated 18 kV or higher. However, it also has been observed on similarly designed air-cooled generators rated 13.8 kV.

Concerns with rotor connec­tors are not new, but they continue, he said switching subjects. While causes may vary among designs and OEMs, concerns can be addressed. Moudy offered several case studies illustrating the point and noted that NEC has corrected pole-to-pole cross­over issues both in the shop and in the plant—even with the rotor still in the stator.

The company’s experience with pole-to-pole crossover issues indicates that start/stop cycles are a major fac­tor in their occurrence. A statistical data base covering a range of genera­tor designs, Moudy said, allows NEC to help customers better predict when repairs probably will be required.

He concluded his prepared remarks by inviting users to learn more about spark erosion and rotor connectors, and other generator issues as well, by visiting the NEC library at www.national-electric-coil.com.

The 501G Users Group once again had an enviable turnout for its annual meeting, aver­aging more than two attend­ees for each of the 12 plants in opera­tion and the one under construction.

As mentioned at the beginning of this report, profiles of user experi­ences and group discussion dominate the 501G User Group’s sessions on all but Siemens Day.

Case history No. 1. Conversion of a 501G control system from TXP to SPPA-T3000 began with the rea­sons for making the change. Those stated typically were the same as the ones given for conversions from TPP to T3K for other Siemens frames: legacy control-related problems, TXP obsolescence, high cost of replace­ment parts, NERC CIP (critical infra­structure protection) compliance, and employee preference.

Project highlights and lessons learned were similar to those for the WDPF conversion to T3K for the 501FD2 engines at Klamath Cogen­eration plant (p 110), including:

  • Proper planning and preparation are critical to success.
  • Schedule too aggressive.
  • Significantly improved trouble­shooting capabilities.
  • No DCS-related trips during com­missioning.
  • Performance improvements.

Managing exhaust-system repairs

There are some subjects you can almost guarantee will be covered at a user group meeting year after year. For large frames—such as the 501G, 501F, 7EA, 7FA—exhaust-system wear and tear is one of those. Cracking of components, broken struts, material distress, and leaks at joints are almost sure to occur in virtu­ally any system handling a 1000 lb/sec or more of turbine exhaust gas at 1000F or higher and moving at near mach speed.

All OEMs face exhaust-system issues—especially given that many of the engines they designed for base-load combined-cycle service now start daily. However, users generally agree that the design of exhaust-system components could be better. They don’t believe they should have to deal with cracking—often in the same locations—every outage or almost every outage.

Many users prefer specialty con­tractors over the OEM for exhaust sys­tem work—usually because they can make comparable repairs at a lower price. But rigorous due diligence is required to assure the firm you select is capable of high-quality repairs so unit availability is not compromised.

User group meetings are a good place to learn about and/or meet capable aftermarket equipment and services providers. The care­ful screening given to presenters, in particular, pre-qualifies candidate vendors for your business. The 501G Users Group invited Industrial Air Flow Dynamics Inc, Glastonbury, Ct, to review its exhaust-system repair capabilities with owner/operators at the 2009 conference.

IAFD Project Engineer Ryan Sachetti came prepared to speak about both the 501G and 501F machines (Fig A). He began by identi­fying the problems IAFD typically sees on G engines—including ovalization of the exhaust manifold (Fig B); broken or bent retaining fingers (Fig C); bind­ing of the floating ring on the exhaust manifold (Fig D); and damage to the round-to-square transition that allows wicking of insulation installed to pro­tect the fabric expansion joint.

Typical F-engine concerns include burning-up of the exhaust-manifold gasket, forgetting to remove the ship­ping tabs, and the presence of an extended flow shield.

Even though the Siemens G fleet has fewer than two dozen engines in operation, he continued, there are four different exhaust-manifold con­figurations. The first generation has a large inner exhaust diameter of nearly 16.5 ft; the second has a steep-angle expansion joint flange; the third has a tall vertical flange and thicker mate­rial; and the fourth generation has the expansion-joint flange free-floating while bound and bolted through gas­ket material.

The first-generation exhaust mani­fold (refer back to Fig 2, in particular, has difficulty supporting its own weight under demanding GT service conditions and tends to settle into an oval shape. This opens up space at the joint between the exhaust manifold and the round-to-square transition, thereby allowing exhaust gas to bypass. The second-, third-, and fourth-generation manifolds are of smaller diameter and less likely to assume a new shape, but they too have ovalized.

Note that the floating liner ring sits on the exhaust manifold and acts as a mechanical heat barrier to protect the expansion joint (refer back to Fig C). It often breaks, cracks, and/or cocks to one side when the manifold ovalizes, Sachetti said pointing to Fig D, which allows hot gas direct access to the fabric expansion joint.

The F-frame expansion-joint area has had fewer modifications, Sachetti told the editors. However, some problems continue to resurface. Perhaps the most common cause of fabric expansion-joint failures is the failure to remove shipping clips (Fig E). Any clips not removed act as a wedge between the manifold gasket and flange, opening a pathway for hot gas to contact the fabric.

IAFD has computer-aided design tools that allow it to 3D-model failures and determine their root causes. Then engineers can use these tools to simulate repairs and replace damaged sections of the floating liner and ring with ones modified to accommodate the new shape of the exhaust manifold.

The last is particularly important to any owner/operator investing in a conversion project such as this: “In the end, what do I get; what can I expect for my money?” This user compared data for the year immedi­ately before the upgrade to that for the first six months after it. Starting reliability improved by 20 percent age points from 67% to 87%; equiva­lent base hours (EBH) of operation between trips increased from 393 to 515; EBH between runbacks nearly doubled from 127 to 224.

Case history No. 2. Problems caused by poorly performing valves in combined-cycle plants cannot be overstated: boiler tube failures and cracks in main steam piping caused by leaking spray valves on attem­perators, condenser damage caused by leaking bypass valves, water loss from leaking boiler drains, etc. So it should come as no surprise that some users are experiencing problems with kettle-boiler bypass and flow control valves.

Binding of kettle-boiler valves can be linked to issues with one or more of the following parts: seat energizer rings, shafts, bearings/bushing, valve body, packing-gland follower and possibly the valve positioner. First problem the user encountered—this was back in mid 2007—was valve sticking on a back-seat. Manufacturer said it was the seat energizer ring. New parts were ordered, the valves pulled, and parts replaced/repaired. Sticking prob­lems continued.

It soon became clear that if the problem was going to be solved, the plant would have to find the solu­tion. Factors considered included: (1) bushings and shaft galling; (2) shaft straightness, bushing and packing bores; (3) concentricity of bores; (4) clearance between shaft and bushing and the interference on the bushing to the valve body; and (5) clearance on the packing follower to shaft,

The engineering effort pointed to the need for changes to materials and dimensions. Examples: Bush­ings were switched to Stellite® and an interference fit of 0.003 in. was added to the valve body; valve shaft was changed to Type-410 stainless steel and the shaft-to-bushing clear­ance was opened up by five thou­sandths.

The refitted valve was put in a test stand, heated to 800F, pressurized to 250 psig, and stroked 1500 times. No lock-up was experienced. Testing complete, the valve was disassem­bled and all components inspected. No issues—such as galling—were identified.

All valves for one of the plant’s three units were modified with the new internals in fall 2008. Between then and the meeting four months later there were no generation incident reports related to the RAC valves.

Case history No. 3. Kettle boilers for two of the plant’s three units were experiencing a high frequency of tube leaks, which were detected through the air-side drain system. A typical repair consisted of plugging the leak­ing tubes and performing a hydro to verify both plug integrity and that all leaking tubes had been identified. This strategy obviously works for just so long.

The fact that the kettle boilers for one unit had no leaks was not lost on plant personnel. Close inspection and review of drawings indicated the boil­ers for two units had been installed incorrectly: They were set on the foundation and mounting hardware was tightened and did not allow for thermal growth. The tube bundles and shells wanted to grow but could not expand and the tubes deformed.

To learn more about kettle boil­ers and some of the issues plant personnel encounter, access www.combinedcyclejournal.com/archives.html, click 2Q/2008, click 501F Users Group on the cover. Work done by Scott McLellan and his colleagues at Arizona Public Service Co’s West Phoenix Generating Station on rotor air coolers begins on the first page of that report. ccj