2 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

The HRSG Users Group, well into its second decade of ser-vice to the industry, contin-ues to deliver fresh, vital information on the design, opera-tion, and maintenance of HRSGs and their associated steam systems. That certainly was evident to the nearly 350 attendees who participat-ed in the organizations 15th Annu-al Conference & Exhibition, held in Kansas City (Mo), March 26-28.

The HRSG Users Group hosts one of the largest and most dynamic meetings for owners and operators of gas-turbine-based combined-cycle and cogeneration plants. Several fac-tors account for this success, begin-ning with the meetings format: A series of Open Discussions dominate the agenda, enabling users to fully explore the specific issues theyre currently facing at their plants. The issues may address any aspect of the HRSG, as well as the entire, integrated steam cycle the HRSG servesfrom the makeup-water pre-treatment system to the last-stage steam-turbine blading.

A second factor is the type and number of people actively contributing to the meeting. In contrast to many user forums, all HRSG Users Group sessions are open to all participantsincluding manufacturers, EPC con-tractors, water-treatment suppliers, engineering consultants, insurance carriers, and so on. This infuses the room with the wide range of techni-cal knowledge and robust diversity of opinion needed to tackle the most vex-ing of technical problems.

Increasingly, the participants are diversified in geography, as well, reports Executive Director Rob Swanekamp. Registered members of the HRSG Users Group are now located in over 50 countries, further expanding the talent and resources the organization brings to bear.

Perhaps the most important fac-tor in the meetings successand the most difficult to emulateis the lengthy experience and considerable moderating skills of Chairman Bob Anderson. A 30-yr industry veteran, Anderson has chaired this confer-ence for most of its existence, each year further honing his ability to keep the discussions on-point, to put esoteric details into perspective, and to extract key information from sometimes-bashful attendees. Ander-son says with pride, Dont attend this meeting if you want to sit quietly in the back row and avoid having to learn anything. We work hard to make this a participatory event, for users and suppliers alike.

Heat transferAlthough the Open Discussions are wide-ranging in content, they are tackled in an orderly manner. Swanekamp presorts the scores of questions submitted by users into nine technical categories, which get addressed sequentially as the meet-ing progresses. Damage to super-heaters, reheaters, evaporators, and economizers tend to dominate HRSG user concerns, so the rightful leadoff category for the Open Discussions is heat-transfer equipment.

This year, a plant manager from Georgia displayed photos of fin-tube corrosion occurring in the latter stag-es of his HRSGs, and opened up an

hour-long discussion when he asked about the potential causes and cures of this so-called cold-end corrosion. Consensus was that it occurs across all OEM lines, that it can affect base-load as well as cycling plants, and that several different mechanisms can cause it.

Sulfuric acid is one potential cul-prit, explained an attendee. Plants firing liquid fueleven onceare more likely to experience this prob-lem, but gas-only facilities can be vul-nerable, too, because normal sulfur levels in some pipeline natural gas runs as high as 15 ppmand high-er during upsets. A combined-cycle plant in western Washington, search-ing for the cause of its cold-end cor-rosion, found that the sulfur content in its suppliers pipeline averaged between 20 and 30 ppm, and peaked at nearly 60 ppm last year.

Another potential culprit is an improper cold-layup procedure, said a former OEM engineer turned HRSG consultant. Making an anal-ogy to the condensation he gets on his cold garage floor when warm, moist air hits it in the springtime, the con-sultant explained that moisture in the ambient air will condense on the tubes of an HRSG in cold layup.

HRSGs tend to sit idle more fre-quently during the fall and springtraditionally the wet seasonsthan they do in winter and summer, increasing the potential for this phe-nomenon. Rainwater and humidity entering through an open stack or leaking roof-casing pipe penetrations exacerbate the problem.

But even units that are staying hot during daily cycling can experience cold-end corrosion, Chairman Ander-son pointed out, so users need to look at additional factors. For example, he said that HRSGs equipped with selec-tive catalytic reduction (SCR) systems will often get ammonia salt deposits

Discussion forums, formal presentations, exhibition provide guidance needed to improve plant performance, availability

AndersonSwanekamp

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4 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

on the tubes. Ammonia salts are capa-ble of absorbing moisture from the atmosphere, so they will become moist over time and cause corrosion.

A veteran engineer from a leading OEM submitted that fin-tube corro-sion on the back end is not likely to cause any significant steam-produc-tion problems. The more important symptom to watch for, he said, is increasing backpressure created by the rust flakes. That can degrade overall plant efficiency. Only a few cases were known to the group where cold-end corrosion was so extensive that it forced the replacement of an entire heat-transfer module. Most of the time, diligent users can track down the specific cause of their cold-end corrosion, take steps to mitigate the corrosion rate, and control back-pressure by regularly inspecting and cleaning the heat-transfer surfaces.

Appropriately, a formal presenta-tion delivered later in the conference covered one users experience with cleaning an extreme buildup of fin-tube corrosion products on HRSGs at a coal gasification site (Fig 1).

Water chemistryQuestions on water chemistry are always popular at HRSG Users Group meetings. This year one of the most heavily discussed was how to set up a recirculation system to be used during periods of wet layup. Few combined-cycle plants are built with such a system, perhaps because the owners intended the facilities to operate base-load. But most plants now find themselves cycling, so great-er attention must be paid to HRSG layup conditions.

While in wet layup, recircula-tion of the fluid can be beneficial for several reasons. As one attendee pointed out, it prevents stratifica-tion, promoting a uniform environ-ment and inhibiting the development of differential concentration cells. In addition, pitting damage can be mini-mized by eliminating the stagnant

regions in which pits tend to develop. Recirculation also is necessary each time chemical additions are made, to ensure proper mixing of the chemi-cals with the layup water.

Weve seen several [recirculation] systems that our clients have put in, offered one HRSG consultant. Generally, they are helpful. But a pitfall, he warned, is to forget that the evaporators have multiple paral-lel circuits, and that the drums have baffling. As a result, the recirculation system often fails to generate flow through all of the evaporator tubes, or throughout all of the drum inter-nals. Its easy to get flow through the economizers, he said, but you need to think through how youre going to get flow in the evaporators.

To accomplish this, one user explained how he installed a sys-tem with redundant recirc pumps and extensive piping. He uses one pump to recirculate flow in the low-pressure (LP) evaporator circuit, and the other to recirculate flow in the high-pressure (HP) circuit. The sys-tem also has multiple injection points where ammonia can be added to con-trol pH, and nitrogen can be injected to blanket the system.

One attendee cautioned that these recirculation systems are not designed to handle the high pres-sures of an operating HRSG, there-fore the system must be properly isolated from the unit before transi-tioning from layup into startup. To ensure that this isolation occurs, he recommended that a written proce-dure gets developed specifying iso-lation as a critical step. Its also a good idea to install spectacle flanges for these temporary connections, Chairman Anderson chimed in, so that operators can easily see that the recirc pumps are isolated, and that startup may then safely proceed.

Several water-treatment special-ists and savvy users spoke up during this layup discussion regarding the benefits of a relatively new technolo-gy: gas-transfer membranes to strip

out dissolved oxygen from the make-up water. Historically, forced-draft degasifiers, chemical agents, and steam deaerators have been used to remove dissolved gasses from boiler feedwater. But gas-transfer mem-branes increasingly are being adopt-ed since the recent introduction of industrial-grade devices.

As one supplier explained, these devices contain micro-porous, hydro-phobic membranes. Hydrophobic refers to the tendency of a substance to repel water or to be incapable of completely dissolving in water, so these membranes enable gas and water to be brought into direct con-tact without actually having the two mix. When the pressure of the gas in contact with the liquid is lowered, it creates a driving force to remove the dissolved gassesmostly oxygen and carbon dioxidefrom the water.

A water-treatment specialist at the conference reported that one of his clients uses the technology to drive dissolved-oxygen levels in the feedwater down to 10 ppb, with-out having to inject any of the oxy-gen-scavenging chemicals that have recently been identified as contribu-tors to flow-accelerated corrosion.

PipingThe third category in the meetings Open Discussionspipingmight seem elementary and stale. How-ever, todays steam-piping systems transport fluids at exceptionally high temperatures and pressures, thus they can generate significant O&M concerns, not to mention personnel safety hazards. Whats more, the use of advanced alloys for combined-cycle/cogen piping has generated new concerns that the industry is just now beginning to understand. A question from a user in the Rocky Mountain region instantly brought these issues to the surface, and triggered a vigor-ous dialogue.

The plant, commissioned only four years ago, experienced a failure in a section of P91 piping. During the subsequent failure analysis, met-allurgical testing revealed differ-ent hardness readings in the weld compared to the base metal, even though the graphs provided by a contractor for the post-weld heat treatment (PWHT) processes looked nearly perfect. The user asked of the assembled industry veterans: What could have caused the differ-ences in hardness?

A bit of background for new sub-scribers to the COMBINED CYCLE Journal: Designers increasingly are specifying 9% chrome-1% molybde-

1. Extreme example of plugging (left), attributed in large part to the syngas at an IGCC facility, was successfully cleaned by CO2 blasting in the first two rows of tubes (right). However, deposits in rows deeper than that proved very difficult to remove

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6 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

num steel (9Cr-1Mo) for critical sec-tions of the HRSG and steam plant. Referred to as Grade 91or more commonly P91 for piping and T91 for tubingthis high-temperature, high-strength alloy can dramatically reduce thermal stresses in superheat-ers, reheaters, and main-steam pip-ing, and thereby improve service life.

However, its superior mechanical properties require that a very specific microstructure be obtained during original steel production, and main-tained throughout its service life. Any action that alters the microstructure of the alloysuch as the hot bending, forging, or welding that regularly occurs during component fabrica-tion, plant construction, and outage repairscan seriously degrade the alloys properties (Fig 2).

As a result, quality-control steps typically include precise temperature records of all heat-treatment process-es like PWHT. One convenient way to verify the microstructure of Grade 91 is with hardness testing, hence the users concern about his different hardness readings.

Presenting at the 2007 conference was one of the leading authorities on this particular topica metal-lurgist who chairs the ASME Task Group currently revising how the ASME Code addresses Grade 91 and other creep-strength-enhanced fer-ritic steels. This was no coincidence, since the HRSG Users Group meet-ing tends to draw leading authorities from throughout the industry.

The metallurgist was one of sever-al participants who responded to the

users question, explaining first of all that its not uncommon after PWHT on Grade 91 for the weld metal to be slightly harder than the base metal. For example, if the base-metal hard-ness is around 200 or 210, the weld-metal hardness generally will come in around 230, he said. If this is the magnitude of difference the user was seeing, then he shouldnt be too con-cerned with the readings, the metal-lurgist advised.

However, he was troubled by the statement that the PWHT graphs looked nearly perfect. Generally, when the PWHT chart looks that good, the metallurgist warned, then you need to start asking questions. Because there are some people out there who are much more skilled at making good charts than they are at making good heat treatments. Though his comment generated some laughs, users understood his serious point about possibly fraudulent docu-mentation.

Much discussion ensued about the types of failures users are experienc-ing in P91 and T91 components, and the quality-control methods needed to properly handle the alloy. Weve seen a number of T91 failures in our laboratory, reported one participant, and overheating is one of the causes. He explained that duct burners can cause a transient spike in temper-ature that exceeds the materials critical temperature. This causes the microstructure to change, which weakens the material and sets it up for probable failure.

Another problem reported is the specification of Grade 91 in low-tem-perature areas of the HRSG. Appar-ently, some owners believe the old cliche that if some is good, more is better. However, components made of T91 need to be exposed to tempera-tures approaching 1000F, in order for the residual stresses from fabrication to relax. If used, for example, in the evaporator and economizer sections, T91 tubing wont be exposed to these temperatures and always will be subjected to higher-than-expected residual stresses.

Another failure mechanism dis-cussed in detail was dissimilar-metal weldsthe joining of Grade 91 to either Grade 11, Grade 22, or auste-nitic stainless steels. One US plant owner was forced to repair the welds between the main-steam piping made of P91 and the steam-turbine stop valve made of 1.25Cr low-alloy steelnot on one, but on three of its 500-MW F-class combined-cycle plants. The oldest plant, which had less than 5000 service hours, found a through-wall crack 135 deg around

the weld, and 24.5 in. long, while the steam line was in service.

Warned by this problem, the owner conducted liquid-penetrant testing on a second plant and found a crack that was 20 deg around the joint but fortunately had not yet pen-etrated the wall. The youngest of the three plants could not detect a crack with liquid-penetrant testing, but replaced the weld anyway because its joint design was the same as the other two plants.

Attendees were advised that when joining Grade 91 to other metals, they need to (a) pay strict attention to welding and PWHT procedures; (b) use cold-spring in the piping to make up for the different relaxation rates of the different metals, and (c) care-fully design the transition geometry.

Whenever there are dissimilar-metal joints, the ASME Task Group chairman explained, a weak zone is inevitably present, as a result of the decarburization that occurs at the interface between different chromium levels. Therefore, the transition geom-etry must place that weak zone in the lowest-stress region of the joint.

Experience with P91/T91 use in HRSGs and high-temperature HP steam systems was addressed at the CTOTF Spring Turbine Forum a few weeks after the HRSG Users Group meeting. Presentations by a well-known industry consultant and two users are summarized in the CTOTF report elsewhere in this issue (see sections on the Generic and GE Roundtables).

ControlsFor several years, HRSG users have reported difficulty in control-ling superheater and reheater out-let temperature while the GT is at low loads. This is especially trouble-some in HRSGs operating behind GE Energys 7FA gas turbines. In Kan-sas City, the topic received in-depth attention.

Once again, some brief background is in order: When a 7FA is changing load in the range between approxi-mately 40 and 100 MW, its exhaust temperature increases rapidly from about 1050F to the so called the iso-thermal limit, just above 1200F. This exhaust-temperature spike is necessary to maintain flame stability in the combustor.

Unfortunately, exhaust tempera-ture peaks at a time when steam flows in the HRSG are low. This makes it very difficult for the super-heater and reheater attemperators to maintain steam outlet temperatures within design limits, while at the

2. The microstructure of P91 com-ponents can be damaged by improper hot bending, forging, or welding, thus degrading the alloys mechani-cal properties. Catastrophic material failuresuch as this pipe rupturecan result

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8 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

same time avoiding desuperheater overspray. As a result, a mixture of saturated steam and water droplets can enter the downstream super-heater or reheater coils, potentially causing severe fatigue damage.

Aware of the problem, GE devel-oped a control-system modification called OpFlex, which some users are beginning to install. An early adopter asked other users at the con-ference about their experience with the modification, which touched off a long discussion on both OpFlex soft-ware and controls in general.

As further evidence of the quality of attendees at the meeting, the prod-uct manager for OpFlex was in the audience. He explained how users can get the most out of the offering, including the use of a turndown option that further assists in low-load operation.

Ductwork, dampers, stacksA question in the next categoryStructuresunderscored the groups increasing diversity in geography, and the value in assembling an inter-national audience. An OEM engi-neer from the Netherlands inquired about acid-dewpoint corrosionnot of the tubing, as discussed earlier in the meetingbut of the HRSG casing. Most HRSGs, particularly the horizontal-gas-path type most popular in North America, have a cold casing design. This engineerwho has more experience with the vertical-gas-path type popular in

Europewondered why chemical theory predicts sulfuric-acid dew-point corrosion will occur in cold-casing HRSGs, but in real life he had not seen such corrosion.

An HRSG specialist from the US promptly raised his hand. Yes, there is experience with this type of corro-sion damage, he reported. Ive seen it personally, and I know of at least one other site that experienced it. Several other reports added to his assertion that, yes, sulfuric-acid dew-point corrosion of a cold casing actu-ally has occurred.

An HRSG OEM engineer raised the if some is good, more is not necessarily better argument as it relates to this issue. Apparently, installing too much thermal insula-tion in a cold-casing system can cause the outer casing to operate below the acid dewpoint, resulting in severe casing corrosion.

After much discussion a consensus seemed to emerge that so long as the casing is kept relatively dry, then the problem will be relatively rare. The real bad actor, one attendee explained, is the free moisture, or a stream of moisture from a leak, because that allows the SO2 to go into the water, and then you get bad, acidic reactions. So if you keep the casing dry, you should be in pretty good shape.

ValvesTurbine bypass valves are critical to the operation of an advanced com-bined-cycle plant. Unfortunately,

improper selection and sizing of these valves has been a common problem in the industry, resulting in unscheduled unit trips and, in some cases, cata-strophic piping damage. Users report that turbine bypass valves have been replacedan expensive and time-consuming taskin as little as two years of service. Other common design problems with turbine bypass valves include inappropriate logic control, unreliable valve mechanics, andas was discussed in Kansas Cityweld failure on the downstream (letdown) side of the valves.

A user in Florida who was expe-riencing such weld failures quickly found colleagues with the same trou-ble. A lengthy discussion of the prob-lem suggested that the underlying causes are not yet fully understood, and that nobody has a panacea. As a result, the HRSG Users Group Steering Committee identified Main-taining and repairing severe-service valves as a topic to be explored at its Steam-Plant Workshop scheduled for later this year (Sidebar).

Rounding out the nine categories of Open Discussion were supplemen-tal firing, environmental systems, and balance-of-plant. In addition, there were four formal presentations at the 2007 conference: n Latest industry experience with

P91/T91, Jeff Henry, associate, Structural Integrity Associates.

n Evaluation of gas-side tube clean-ing options, Maggie Lelak, chemi-cal engineer, Duke Energy.

n Pros and cons of heavy duct-burner firing, William Byrd, operations manager, Entegra Power Group LLC, Gila River Station.

n Condenser performance/trouble-shooting, William Lutz, performance engineer, GE Energy.

Integrated exhibitionA companion exhibition, supported by 84 suppliers of combined-cycle equipment and services, was inte-grated with the conference sessions. Exhibitors were not restricted to the expo hall, but rather encouraged to participate in all of the technical ses-sions. This not only keeps suppliers attuned to user concerns, but affords them the opportunity to get involved in technical questions during the conference, then invite interested respondents to meet them at their booth, over coffee, or during one of the meal functions for follow-up dis-cussions.

The 2008 Conference & Exhibition will be held April 7-9, in Austin. Visit www.HRSGusers.org for details as they become available. ccj

Steam-plant workshop co-locates with PowerGenIn addition to its spring conference and trade show, the HRSG Users Group conducts a Steam-Plant Workshop in the fall. The annual meeting is characterized by a free-ranging discussion open to the broad array of issues that users request; the workshops provide in-depth seminars focused only on one or two topics selected by the steer-ing committee.

This years workshop will be co-located with PowerGen Interna-tional in New Orleans, December 11-13. This enables registrants also to participate in the largest power-generation expo in the world.

Theme for Day One of the 2007 workshop is Evolution of steam-plant design. Presentations include the following: n Developing specifications for

your next project.n Scaling up the once-through

steam generator.n Plant designs for a water-con-

strained world.n New ASME, NFPA, and European

design codes.n Using P91/T91 and other

advanced alloys.Theme for Day Two is Mid-life

maintenance for combined-cycle/cogen plants, including:n HRSG retubing.n Chemical cleaning of HRSGs.n Servicing transformers, switch-

gear, and generators.n Maintaining and repairing severe-

service valves.n Cooling-tower overhauls.

Workshop registration includes full access to the PowerGen Expo; group discounts are available to users via the Energy Provider Pro-gram. Visit www.HRSGusers.org, or call 918-831-9160.

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