DESUPERHEATERS
Affordable, timely replacement requires precise planning, skilled labor
Owners of F-class heat-recovery steam generators (HRSGs) installed during the bubble years (2000 through 2004) that have not yet replaced one or more attemperators should consider themselves lucky, but not necessarily “in the clear.” Insidious damage mechanisms may be at work, despite best inspection efforts. Plant managers must at least consider that possibility and plan accordingly.
Desuperheaters—attemperators if you prefer—have been a particularly hot topic at user group meet
ings over the last 18 months or so. The latest presentation, prepared by Reg and Bob Morse of Bremco Inc at the 2014 meeting of the Combined Cycle Users Group (CCUG), walked attendees through a challenging retrofit project at a utility F-class combined-cycle plant. It may be the most compelling of the recent presentations, given the level of practical detail and depth of experience it provides the user community.
Earlier in the CCUG conference, Scott Wambeke, PE, Structural Integrity Associates Inc’s HRSG product manager, reviewed the causes of desuperheater issues. He has more than a decade of experience helping owner/operators identify and correct problems caused by poor design, installation, operation, and maintenance. Access more detail on Wambeke’s work in this area by using the search function on the CCJ menu bar above.
At the 2014 meeting of the 7F Users Group, last May, Lester Stanley, PE, of HRST Inc shared his extensive desuperheater problem-solving experience during the company’s respected half-day HRSG Spotlight Session. A month earlier, at CTOTF’s spring conference, the Combined Cycle Roundtable featured presentations on desuperheaters by Bill Lovejoy, PE, NAES Corp’s chief engineer, and Bremco’s VP/GM Bill Kitterman (see sidebar following this article). Lovejoy conducted a tutorial on the science and engineering of attemperation, while Kitterman focused on planning for desuperheater replacement. Both presentations can be accessed (by users only) in CTOTF’s Presentations Library.
Instructive, too, is the work done by Amy Sieben, PE, over the years on attemperation using both water and air. Sieben, who recently opened her own shop (ALS Consulting LLC), presented at the 2013 CCUG meeting on prioritization of HRSG corrective actions required to maintain continuity of service while budgeting realistically. You can access her engineering work on desuperheaters via CCJ search.
The desuperheater retrofit case history, presented at CCUG 2014 by Bremco Project Manager Bob Morse, revealed a complex job challenged by schedule, budget, equipment access, changing scope, weather, etc. Most experienced plant personnel might respond, “So, what else is new?” and they would be correct—for the most part. Except, this project involved four HRSGs, each having five HP superheater modules, three reheater tube panels, and five attemperators. Specifically, there are two desuperheaters in the high-pressure (HP) steam system (Fig 1), one in the reheat system (Fig 2), and one each in the HP and hot reheat (HRH) bypass systems (Fig 3).
The HRH desuperheaters were considered of a poor design by the plant owner and all four were replaced during the outage. One of the problems was that the original liners were short—that is, they began downstream of their respective spray-water valves. The replacement liners extend upstream of the spray-water valves. Material and liner thickness also were considered unacceptable and among the underlying causes of cracking and material liberation (Fig 4). The original thin 800-series Inconel liners, which had been repaired several times during the plant’s first decade of service, were replaced with more robust liners of P22.
All of the HP bypass desuperheaters were evaluated as fit for service, none requiring replacement. By contrast, all of the vertical HP2 attemperators, located between the fourth and fifth sections of the superheater, were in poor condition and replaced. Interestingly, the HP1 and reheat attemperators on two of the HRSGs were able to continue in operation, while the same desuperheaters on the other two boilers had to be replaced. Orders were placed for two HP1 and two reheat attemperators; they will be retrofitted during an upcoming outage as a preventive measure.
HRH bypass attemperators. Morse described several of the challenges presented by retrofit of the HRH bypass desuperheaters to provide attendees perspective for projects they may be considering. Points he made included the following:
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Temporary support steel was required to support piping and the attemperators during replacement (Fig 5).
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Spring cans and guides were pinned and blocked to prevent equipment damage.
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The new attemperator was longer than the original and an existing hanger had to be moved.
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One of the circumferential welds could not be radiographed because of its location and ultrasonic testing was substituted.
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Existing spray-water piping had to be modified and an isolation valve installed to control leak-by of spray water during shutdowns (Fig 6).
HP2 attemperator replacement required plenty of know-how, Morse told attendees, given its vertical orientation, P91 material, exposure to weather, etc. The effort was similar to that described previously in CCJ to enable the retrofit of a vertical reheat desuperheater at Ontelaunee Energy Center. Specific challenges for this phase of the project included the following:
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Support of P91 attemperator piping without welding to it in the field (Fig 7). Lifting lugs were installed during fabrication to simplify onsite work, which would have required pre- and post-weld heat treatment.
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Providing weather protection capable of accommodating attemperator installation (Fig 8).
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Minimum wall thickness of the 90-deg elbow shown in the photo was thicker than the new attemperator pipe, causing joint fit-up issues and the need for special machining techniques.
The reheat attemperator is installed under the HRSG in a manner that suggests no one on the design team seriously considered it might have to be removed (Fig 9). Note that half of the 180-deg return bend shown had to be removed, as did spray-water piping and hangers. Existing piping was out of square/plumb and a support/trolley system had to be installed to slide out the old attemperator and install the new one (Figs 10, 11). The section of elbow removed is reinstalled in Fig 12 and wrapped for heat treat in Fig 13.
The HP1 attemperator was tucked under a catwalk and cable trays (Fig 14) and the new spray-water-nozzle piping ring (halo) would be located in the same spot as catwalk supports. Solution was to have the attemperator fabricator add 40 in. of pipe to the upstream end of the unit to eliminate the interference. Temporary support steel was required to slide the desuperheater out from under the catwalk and transfer it to the crane (Fig 15). Fig 16 shows the new attemperator being rigged in.
Best practices taken away from the project included these:
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Replacement in-kind may not be the optimal retrofit approach; consider modifying replacement components before fabrication to simplify field installation.
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Check, double-check, triple-check for interferences.
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Verify pipe sizes and minimum walls to enable the development of proper welding and heat-treat procedures.
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Develop workscopes early to be sure connecting piping is properly supported/pinned when work is done.
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Customer and contractor should work closely from pre-outage planning through project completion to assure that any engineering required for added support steel, rigging plans, etc, are developed for work is started.
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Develop a master schedule that incorporates all parties involved in the project. CCJ
CTOTF roundtable discusses desuperheater issues faced by combined-cycle operators
Presentations on attemperator (desuperheaters) issues have been made at many user-group meetings over the last 10 years or so. The problems identified generally can be traced to hardware and/or controls ill-suited for the service, poor steam piping design, poor installation, and/or operating procedures not consistent with long service life.
Cycling plants designed for base-load service is at the root cause of many failures—particularly where spray nozzles are located directly in the steam path. They are thermal-cycled to destruction because each start quenches hardware, typically at 1050F or higher, with water at condensate or feedewater temperature—a delta T of several hundred degrees.
Pedestrian spray-water valves often leak, causing downstream damage to tubing—and sometimes headers—in HP and reheater panels. Quality valves are a must; don’t cut corners. Beware the designer trying to save money on steam piping, hoping spray-water evaporation will take less time than the physics says it will. Also beware the EPC contractor that orders an attemperator without a protective liner for the steam piping immediately downstream of the spray nozzles; or the contractor that doesn’t install the liner correctly—or at all.
There have been several attemperator failures of late on F-class units built during the ordering frenzy of 1998-2003. More are likely given the reasons stated above. This fact was not lost on CTOTF™ Leadership Committee—in particular Combined Cycle Roundtable Chair Roger Schnabel, plant manager of CAMS New Mexico LLC’s Hobbs Generating Station, and Vice Chair Rick Shackelford, plant manager of Green Country Energy LLC, an 800-MW combined cycle operated by NAES Corp.
Their session at the user group’s 39th Annual Spring Conference, last April, at PGA National Resort, dedicated half of the available presentation/discussion time to attemperators. The expert speakers were William Lovejoy, PE, NAES’ chief engineer, who presented a primer on the “Fundamentals of Desuperheating,” and Bill Kitterman, VP/GM of Bremco Inc, who focused on the following aspects of attemperator replacement, among others: project planning, contractor selection, project definition, and code requirements.
. . .But solutions are available to correct them
Lovejoy’s was a classic engineering-school type of presentation; it could have been titled, “The Thermodynamics of Desuperheating.” His stated goal was to explain the basics of attemperation and share real-world experience to raise the level of awareness on desuperheater failures.
He began by acknowledging that there are no easy rules or formulas regulating the process. However, Lovejoy said, “A number of different rough guidelines must be examined. The amount of straight pipe with minimal turbulence, location of the measuring element, and style of spray nozzle are critical. There is very little margin for error. Even in a well-engineered system, there is limited capability to operate outside design conditions.” The reality, he continued, is that engineers frequently do not understand the implications of failing to follow recommended practice.
Desuperheating is accomplished by mixing water in steam, Lovejoy told attendees. Water passes through a control valve and is sprayed into the steam; temperature of the fluid is measured downstream and the control system regulates control-valve position based on the measured temperature. His explanation of the evaporation process, heat-transfer equations, the importance of droplet size for achieving desired attemperation without damaging piping and panels, practical turndown limits, etc, was a valuable introduction for the hardware portion of the presentation to follow.
Lovejoy provided commentary on a range desuperheaters, beginning with the most simple type—essentially a pipe with holes, no nozzles. He said there are lots of issues with this type of desuperheater, but acknowledged it can work, but only in a very narrow range of flow conditions and near-perfect control-valve sizing.
The NAES chief engineer stressed that problems start small and a good inspection and maintenance plan is required to prevent them from graduating to waterhammer, quenched pipes, tube leaks, cracked lines, high-energy-piping (HEP) stress, etc. Several photographs illustrated these conditions.
Basic maintenance, he said, includes regular borescope inspection, nozzle replacement, liner and drain inspections, removal and inspection of spraywater valves, control-loop review. More advanced diagnostic techniques include thermocouple arrays and infrared scanning to assure the temperature of the pipe is constant around its circumference.
Most problems, Lovejoy went on, can be traced to an inadequate length of straight pipe downstream of the attemperator and/or failed spray nozzles. Once water droplets agglomerate by collision with pipe walls and/or turbulence, they drop out of the steam path and never evaporate. This water must be removed by drains that sometimes don’t exist. Poorly performing desuperheaters are relatively easy to detect using the thermocouple arrays and infrared cameras mentioned above: The top of a horizontal pipe will be at the superheated steam temperature and the bottom, or outside elbow on bends, will be at the saturation temperature.
Quenching (of pipe walls) is a condition that can occur as a plant ages, particularly when a desuperheater has little design margin. Lovejoy explained that the mass flow of hot gas entering a heat-recovery steam generator (HRSG) can drop by as much as 10% within five years of commissioning. This reduces drum pressure and can increase steam velocity, despite a lower overall steam mass flow. Higher velocity reduces the amount of time available to evaporate moisture before the first elbow or obstruction. Also, depending on HRSG design, lower gas-side mass flow can increase HP steam temperature, thereby increasing the amount of desuperheating capacity required.
Another common situation, Lovejoy said, is turndown beyond initial design. He pointed to plants designed for base-load service but required to operate at 50% capacity or less. Quenching is a possibility in such situations because of lower steam pressure/higher velocity, or control valves throttling on seat leaving inadequate pressure drop at the desuperheater for droplet atomization.
The speaker added, “For plants with marginally functional desuperheaters there are very few options; improving control-valve sizing, changing the desuperheater or nozzles to improve atomization, preheating the liquid to reduce the time required to begin evaporation, and relocation of thermowells all are options, but the real fix is more time, and more time means more pipe.
“The short-term symptom is a failure to control to desired temperature and/or water in the piping system. The long-term result is failure of the piping to reach design life. Pipe failures can be from quenching, low-cycle fatigue at stress risers (welds), or creep damage. If the underlying issues with desuperheater design/operating condition are not corrected, repaired piping.
Begin by buying quality equipment, hiring the right contractor, adopting best practices Bremco’s Kitterman had a few introductory words on scope development for attemperator repair and replacement projects before diving into the importance of a comprehensive “walkdown” to guide planning and scheduling. Critical to project success, he said, is having the contractor physically walk you through the job to identify sticking points early so “surprises” can be avoided. Written explanations are inadequate, Kitterman stressed. Goals for a walkdown should include the following, at a minimum:
Identify structural, electrical, and instrumentation interferences and determine how to deal with them.
Decide how hangers will be pinned/blocked and where temporary supports will be needed.
Obtain hanger details—such as method of attachment (welded or bolted), materials, weld procedures, heat-treatment and NDE requirements, etc.
Examine spray water piping to identify interferences, assure proper valve layout, condition, etc.
Survey laydown area available for crane and materials.
Kitterman next offered attendees his thoughts on key elements of the bid review process—including assessment of contractor experience, schedule, and the importance of a face-to-face interview. Much of what he had to say is summarized in How to select the right contractors to support your next outage.
Job planning is perhaps the contractor’s most important job other than actually doing the physical work. However, job success hinges on proper planning. Here are some of the key activities that should be on your planning “to-do” list for a typical attemperator project:
Code requirements, ASME and NBIC.
Welding processes and procedures, including pre/post-weld heat treatment and inspection methods.
Removal of existing attemperator: How will cutting be done (hand or machine) and where (establish cut line)?
Personnel: Crew size, supervision, trades, shifts, etc.
Schedule development: Tasks required, subcontractor selection and coordination, identification of other outage work being done in parallel and its impacts on schedule and work area. Kitterman urged attendees to plan early for a desuperheater replacement. If piping mods are included in the project, planning should start about a year in advance, he said. It takes about 16 weeks just to get parts for a straight replacement.
Safety training.
Budget development.
Rigging plans.
Project hazard analysis.
Kitterman put up on the screen about a dozen photos of specific tasks undertaken as part of most attemperator replacements. Several of these were similar to the images compiled in the CCJ’s report on an interstage attemperator replacement done at Ontelaunee Energy Center a couple of years ago.
The “Code Review and Paperwork” segment of Kitterman’s presentation contained a valuable checklist of documents that Bremco recommends be included in any project quality-control documentation package. The R-1 package should follow a cover sheet, table of contents, and state letter, if applicable, and have the following information:
R-1 form.
Code work information checklist.
Job traveler.
Weld maps.
Weld procedure specifications and procedure qualification records.
Code welder list, welder continuity log, and welder performance qualification test.
Receiving inspection reports and material test reports
Data reports and drawing.
NDT/NDE reports, technical qualifications, post-weld heat-treat charts, and technician qualifications.
Applicable pictures and miscellaneous information.
