Almost every one of the 250 attendees of the HRSG User's Group meeting got their turn as a speaker, thanks to the group's high-energy, interactive forum. Topics included: streamlined assessment of HRSG performance, options for HRSG lay-up, and maintenance concerns with SCR systems
Do you know you can keep your heat-recovery steam generator hot during a weekend shutdown, even in the frozen Canadian tundra, with a trip to the neighborhood camping store? Do you know that the crank purge, required for safety reasons prior to gas-turbine startup, could actually be hazardous? Do you know when partial penetration is preferable to full penetration?
If you're not involved in the design, operation, or maintenance of combined-cycle plants, you may not care about such things. But if you are involved, then these and scores of other HRSG-related questions affect your business. The 250plus attendees at this year's HRSG User's Group meeting--which was co-located with the Electric Power 2001 Conference in Baltimore, Md--clearly understood that these questions affect their business; that's why they vigorously participated in the event, and stayed till its very last session.
The nonprofit HRSG User's Group, now in its ninth year and enjoying record attendance, is led by Chairman Robert Anderson, Florida Power Corp, St. Petersburg, Fla. Anderson makes the meetings stand out from the crowded field of industry conferences by expertly moderating the discussions, hustling up and down the aisles with a cordless microphone, and engaging the entire audience in a three-day, nonstop, question-and-answer session.
The audience, also in contrast to many industry conferences, is not comprised of marketing reps looking to push a new product or academics trying to publish their research. The audience--who Anderson said "really run the meeting, not me"--is dominated by maintenance supervisors, powerplant managers, and practicing engineers. The result is an in-depth, candid training program on operating and maintaining HRSGs.
Consider the discussion of full-penetration vs partial-penetration welds. An audience member explained that endurance curves based on actual test data show that a full-penetration weld has 10 times the life of some partial-penetration welds. As a result, full-penetration welds are increasingly being applied to tube-to-header joints in HRSGs--particularly those expecting to operate in cycling mode.
But while full-pen welds are better from a theoretical standpoint, another attendee interjected, they are more difficult to perform correctly. If not properly performed, full-pen welds offer no endurance advantage, and may even be weaker.
"That's right," a plant manager agreed. "And they're more expensive. So there's a balance that the HRSG user has to strike. For example, you might use full-pen welds on bottom headers, where strength is critical; then use the less-expensive, easier-to-fabricate partial-pen welds on top headers."
Another detailed discussion concerned the "crank purging" required by the National Fire Protection Assn (NFPA) prior to gas-turbine startup. Though purging has proved to be an effective way of clearing accumulated fuel gas prior to combustor ignition, some users are concerned that the crank purge is, in the longterm, damaging to an HRSG. This is because condensate can form in the superheater from the cooling effect of the purge, and thereby cause significant thermal stress to tubes and headers.
An audience member, who sits on the NFPA committee studying the issue, was able to offer up-to-date insight. A new code is being published, he explained, that includes revised "enforceable language." He encouraged HRSG users to obtain the new code-NFPA 85: Boiler and Combustion Systems Hazards Code, 2001 Edition--which was scheduled to publish in April for $35. Continued study of the issue and refinement of the code is under way, with the involvement of several members of the HRSG User's Group.
The topic of stack dampers was bandied about, and several lessons offered up by veteran HRSG users. Stack dampers often are installed at combined-cycle plants to retain heat during short shutdowns and thereby minimize thermal cycling of the HRSG.
But one attendee cautioned that dampers have been known to inadvertently close while the plant is operating, as a result of interlock failures. On a larger and more serious scale, this is akin to shoving a potato up your car's exhaust pipe while the engine is running.
One suggestion, from the left side of the room, was to counter-weight the damper so that it would fail in the open position. Another suggestion, from the right side of the room, was to avoid the butterfly-type damper, and to instead use louver-type or clamshell dampers, which reportedly are less susceptible to inadvertent closing.
The lowest-cost suggestion came from a Canadian plant manager. Instead of a stack damper to retain heat during a weekend shutdown, he simply draped a large piece of canvas across the gas-turbine air inlet. "This was in February," he added, "but it still kept the HRSG relatively hot during a repair on the economizer."
Sprinkled in between the three days of question-and-answer sessions were a handful of short, formal presentations on specific issues facing HRSG users. For instance, Scott Wambeke, Deltak LLC, Minneapolis, Minn, presented "Streamlined Assessment of HRSG Performance." Wambeke explained how plant operators can quickly get a feel for "actual" vs "as-designed" HRSG performance, by monitoring three key temperatures: approach, pinch, and stack. Instrument accuracy is critical, he cautioned, so users must be aware of measurement bias, flue-gas temperature distribution, effects of radiation on gas-side thermocouples, pressure/temperature compensation of flow meters, and so on (POWER, January/February 2001, p 56).
Another formal presentation concerned HRSG chemistry. As Cal Boris, Applied Specialties Inc, Avon Lake, Ohio, explained, cycling has become the normal mode of operation for many combined-cycle units, both in the US and overseas. As a result, protecting the HRSG during frequent startups, shutdowns, and short-term layups has presented some unique water treatment problems. Rather than trying to sell a certain chemical or a "one-size-fits-all" solution, Boris presented the pros and cons of numerous chemistry options, and emphasized that the best solution is site-and system-specific.
A third presentation came from Houston Outing, Cormetech Inc, Durham, NC, on the subject of selective catalytic reduction (SCR) systems. As increased numbers of combined-cycle plants accumulate increased numbers of operating hours, field experience with SCRs has grown. Lessons from the field, Outing reported, point to the need for:
• Ongoing testing of catalyst samples.
• Prevention of temperature excursions--the primary means of catalyst deterioration.
• Use of water-only or phosphate-free detergents during gas-turbine compressor cleaning to prevent catalyst poisoning.
• Protection of the catalyst when welding in the HRSG, to prevent poisoning by welding fumes or slag.
The final presentation--there are only four over a three-day period because the meeting's key feature is open-forum discussion--came from Robert Krowech, HRST, Wayzata, Minn. One of the founding members of the HRSG User's Group, Krowech looked back on nine years of meetings, to summarize lessons learned and chart the progress of the HRSG industry.
Many of the problems that dominated early meetings--such as stress corrosion cracking and feedwater heater failures--have diminished as users and manufacturers found solutions. Continuing problems, Krowech noted, include water chemistry, failures of expansion joints and liners, and air emissions. Looking ahead, Krowech believes that HRSG users and manufacturers will work to:
• Improve repairability, though better access lanes, more access doors, use of skyclimber ports, etc.
• Increase fuel efficiency, by designing stack temperatures less than 250F and minimizing desuperheat at base-load.
• Enhance cycling capability--finding ways to minimize thermal shock of economizers and fatigue of "thick sections" of the HRSG.
To track these future developments, plan on attending next year's HRSG User's Group meeting, March 19-21, 2002, in St. Louis, Mo.
PHOTOS (COLOR): Unique "talk-show" style format, led by Bob Anderson, Florida Power Corp (top, left), allows virtually every HRSG user to be a speaker. Formal presentations were made by (clockwise from Anderson) Scott Wambeke, Deltak; Houston Outing, Cormetech Inc; Cal Boris, Applied Specialties Inc; and Bob Krowech, HRST (not pictured)
PHOTOS (COLOR)
By Robert Swanekamp, PE, Editor-in-Chief
