HRSGs: Tapping global know-how to expand local options

Posted on July 4, 2018 by Team CCJ

A special report by Consulting Editor Steve Stultz based on information shared at the HRSG Forum with Bob Anderson

The 2018 HRSG Forum with Bob Anderson, held in Houston, March 5-7, like its predecessor in Charlotte, captured the topics of immediate interest to HRSG experts and combined-cycle system owner/operators. There were spirited discussions before, during, and after the formal technical agenda. Participants networked freely within this consolidation of equipment details, operating trials and errors, and forward-looking ideas. More than 40 exhibitors were present for specific discussions.

The report that follows is divided into these sections to facilitate access to information of greatest importance to you:

Many of these topics are of worldwide interest and are being discussed at the Australasian HRSG Users Group (AHUG), the European HRSG Forum, the Canadian HRSG Forum, and other such regional meetings. Information transfer to the US meeting is by way of Bob Anderson and Dr Barry Dooley of Structural Integrity Associates Inc, who actively participate in those conferences as well.

To illustrate, a full-day workshop at the 2017 AHUG meeting was dedicated to materials issues in HRSGs, the same concerns addressed in Houston. Topics included P91 damage mechanisms and case studies (CSEF steels), failure mechanisms, life-prediction and remaining-life assessment methods, weld repairs, and oxide growth and exfoliation of HRSG materials. Open discussions at the 2018 HRSG Forum, led by Anderson and other members of the steering committee, focused on these issues as well.

The third annual HRSG Forum with Bob Anderson will be held at the Hilton Orlando, July 22-24, 2019, where you’ll learn from the world’s leading experts on HRSG design, operation, maintenance, inspection, water treatment, layup, etc. Program development is in the formative stage. Visit the organization’s website in the early fall for important details.

Inspections by drones, benefits of 3D printing

Innovation spawns innovation. One technology advancement presented at last year’s inaugural HRSG Forum with Bob Anderson was inspection by unmanned aerial systems (UAS), a/k/a drones. Xcel Energy’s Scott Wambeke explained how these inspection tools were flying within the congested spaces of HRSGs, launching new dimensions in this cost-effective and time-saving technology.

Pilot skills had been established outdoors along vast transmission lines, then within the relatively open spaces of large coal-fired utility boilers, verifying the values of these recorded flights. Now, spaces were tight but beneficial results were expanding.

Returning this year, Wambeke explained the escalating benefits of 3D printing, an innovation developed while bringing UAS skills and benefits to Xcel’s full power-generation fleet. So what began as plastic drone parts has expanded into project planning and execution.

Working with a local university, Wambeke’s team has mastered specific drone-system improvements, driven in part by equipment damage during the all-too-familiar impact events. One fast-growing need has been replacement and more durable blade guards. Another has been customized attachment systems for prototype lighting.

The licensed outside flight crews at Xcel use some elaborate systems and equipment, and travel long distances in their investigations and inspections of power lines, rights of way, and similar tasks. The inside teams at Xcel began with less expensive, off-the-shelf commercial drones, often making their own repairs. “Every time we fly in these tight areas,” explains Wambeke, “we know that there’s a 30% chance of drone damage.” Their drones are still considered consumable inspection tools.

Then the team purchased its first 3D printer. Repairs were made more quickly and innovation opportunities began. “One critical advantage is being able to make improvements as you go,” said Wambeke. “You can print a component, try it, improve it, and quickly try it again.”

Last year’s presentation concluded with a forward look at drone system advancements, and expanded applications. Both developments are moving quickly.

Navigating to projects: An operating HRSG was experiencing repeated failures in preheater 1, row 2. Access clearance to the failures at the lower header was tight at 1.6 in. The boiler/HRSG group was investigating a new printer and decided to try 3D printing. Engineers would look at 1:20-scale components.

When printing was done, project planners were able to see (and show to management) the processes for cutting header piping and drains free, then gaining access for repairs. The models also would serve as planning tools for NDE and provide tube-to-header joint geometry familiarization to the welding crews. In sum, the simple 3D printing process became proof of concept for what was deemed a five-day outage.  

Said Wambeke, “This became an excellent tool for both planning and the actual repair. The project was finished ahead of schedule in four days.”

Wambeke concluded with a review of 3D printing hardware, use of dissolving media, and the joining of parts by friction welding. He also noted that 3D scanners can replicate geometries to help reverse-engineer old parts when drawings are not available.

Questions and discussions were varied. Small blimps also have been considered for inspection flights, but achieving lift is difficult without supplemental fans, and internal drafts can make stability difficult. Work on drones and options continues. One additional note: Printer plastic is priced well below printer ink.

Longer attemperators speed up spray evaporation to mitigate pipe cracking

Paul Lofton, Tampa Electric, and Ory Selzer, IMI-CCI, discussed HRSG attemperator replacements at two TECO Bayside power blocks installed during the “bubble” (2003 and 2004).

This presentation discussed systems for:

    • HP (main steam) to cold reheat (CRH) bypass.

    • HP (main steam) interstage.

    • IP (reheat steam) interstage.

During a walkdown, a crack was discovered in the circumferential weld at the HP-to-CRH pressure control valve. This indication was discovered completely by accident while escorting a vendor across the top of the plant. The weld was cut out and replaced.

An HP interstage attemperator then showed girth-weld indications during linear phased-array UT inspection of the P22 pipe. One indication, 8 ft down from the spray nozzle, was 0.625 in. deep, originating from the inside wall and progressing toward the outside, approximately one third of the way around the pipe. All systems on both units were then inspected.

Plant layout fortunately allowed room for longer attemperator steam pipes. Seven new attemperators were purchased, twice the length of the old ones for improved dwell time to evaporate spray-water droplets. The resulting 16 to 18 ft total length gives more time to prevent quench damage. Four radial spray nozzles replaced the single nozzle of the original design; the existing 180-deg elbows were reused.

All modifications were designed to provide added strength and life in cycling duty. The common theme: conventional equipment is rarely designed for cycling. System improvements included longer pipe legs, improved drain systems, radial nozzles, Stellite 6 hard-faced seats, and new actuators.

Many questions followed regarding thermocouple locations relative to welds. There were also extended questions and discussions about cycling impact on original designs, steam versus water atomization, modification costs and methods, OEM design variations, and overall design and repair options. Dig deeper with a recent CCGT case study: Pipe Repair Oddyssey. 

FAC remains the No. 1 problem for HRSGs

Structural Integrity’s Dooley provided an in-depth presentation on “the No. 1 problem” for HRSGs: flow-accelerated corrosion (FAC).  He reiterated the chemistry influences on FAC, typical locations, appearances, and common mistakes in attempts to identify and control. “The problem is global, and it’s not just the power industry,” Dooley stressed.

Background discussion included temperature ranges and the solubility of magnetite, timing of first appearance in both vertical and horizontal gas path units, and turbulent flows at bends and complex geometries. Air-cooled condensers also were addressed.

“It is very important to identify what type of FAC you have, because the two types (single- and two-phase) are controlled by different aspects of the cycle chemistry,” he stressed. Many examples of appearance, location, and damage level were shown and characterized in detail.

Treatment options (and overall system chemistries) were discussed along with relevant IAPWS guidance documents. Component materials were also discussed, as were film-forming substances (FFS) that might help in protection. These would follow in a later presentation.

Recent developments in film-forming substances

EPRI’s Stephen Shulder addressed the increasingly dynamic topic of FFS for corrosion resistance and layup protection.  This topic now has its own international conference, most recently in Prague, organized by IAPWS.

Shulder offered background. Amine/filming chemistry is being used or considered for many reasons, including:

    • Better layup protection during outages of varying duration. Film-forming substances can place a hydrophobic barrier between metal surfaces and liquid. Limits on capital expenditures or personnel could restrict the full use of dehumidified air systems, a viable alternative.

    • Non-optimized corrosion-product transport with recommended feedwater treatment programs. Cycling operation means more startups, and excessive use of ammonia can lead to vapors within the plant and short polisher runs.

    • Two-phase FAC damage. Neutralizing amines may provide better dissociation and distribution than ammonia, and higher localized at-temperature pH.

The EPRI chemist discussed filming amines and filming products, amine chemical structure, and application benefits and risks. He then turned to case studies, including potential impact on and benefits to air-cooled condensers.

Considerable time was spent on the details of FFS research, followed by storage, feed considerations, and application schedules. His key two-part message:

1. Any use of film-forming substances should be in combination with, not in lieu of, best cycle chemistry by international standards.

2. You can’t control what you can’t identify and measure.

Shulder ended with a list of questions to ask any FFS supplier.

Dooley added that “There is a whole series of failures that can occur with these substances,” and reiterated that a definitive consensus document is now available at no charge through IAPWS, namely Technical Guidance Document “Application of Film-Forming Amines in Fossil, Combined Cycle, and Biomass Power Plants” (TGD 8-16).

Beware residual elements in advanced alloys

One shot across the bow at last year’s inaugural HRSG Forum was a report by Jeff Henry, Applus+ RTD.  The topic: advanced alloys. The message: We now have residual elements in the steel to consider, and they’re changing the material chemistry on a microscopic level.

Returning in 2018, Henry offered an update on the evolving issues with creep-strength-enhanced ferritic (CSEF) steels. His theme remained the continuously changing knowledge base, and the need for related ASME Code updates.

One initial Code focus is a reduction by ASME in the allowable stress for P91. According to Henry, who chairs the ASME Section II Committee, “reductions are now approved by ASME and will be in the 2019 edition of the Boiler and Pressure Vessel Code. Modified chemistry for Grade 91 Type II, currently a code case, could be released at the same time.”

However, he explained, the Code does not adequately address heat-treatment specifics for these steels, even after 20 years of experience and data. “Heat treatment remains an important topic, and many questions are the same now as they were 20 years ago.”

As he explained in 2017, CSEF steels are relatively complex materials and can vary according to their production process, “depending on heat-specific chemical composition and processing histories.” This is where the interesting and disturbing topic of recycled materials appeared last year.

For clarity, Henry stated: “The degree of technical control required during all phases of implementation (design, production, manufacturing, and erection) is substantially greater than it is with traditional (that is, more tolerant) powerplant alloys, such as Grades 11 and 22.”

“Improper heat treatment,” Henry stated, “has been the single greatest cause of unnecessary failures, repairs, and replacements” involving these steels—particularly Grade 91.

Such improper practices have cost the industry hundreds of millions of dollars, and will continue to do so as “poorly heat-treated components that have not been properly inspected continue to fail prematurely.” 

This is not because of overly complex procedures or processes. “The requirements are not abnormally difficult to understand or implement,” Henry explained. “Rather, there has been too little effort made to understand how the basic metallurgy (microstructure) of this class of steels dictates successful heat-treatment practice.”

The industry is now left with many questions, including these:

    • How many times can these materials be heat treated?

    • Can poorly heat-treated materials be salvaged?

Henry followed this with a detailed review of typical heat-treatment processes, and various complicating issues. As one example, the Code specifies minimum hold times for post-weld heat treatment, but not does not specify maximums. For temperature, he explained, the Code sets an upper limit, above which a material should be rejected. But the intention was never to hold material at this temperature. “Code needs to review this,” he stated.

Many questions and discussions followed (as they did last year) including precise temperatures, cooling durations, weld inspections, impacts of wall thickness, adjacent materials, weld materials, and the benefits and risks of re-austenitizing.

“Austenitizing can reset the life clock,” explained Henry, “but may not duplicate original material properties.” EPRI’s John Siefert took this further. “If creep damage is present, you cannot restore the material to virgin conditions.”

Henry ended with a case history of salvaging improperly heat-treated Grade 91 large-diameter (28 in.) elbows.

More questions followed on weld repairs, inspections and acceptances, and the global state of (or lack of) knowledge. The questions and discussions made clear the obvious concerns about these material properties, their complexities, and their quality issues.

And a key question: How do we track all of the actions that have taken place on all the pieces and parts that we have? The awakening answer: Even looking back at the original materials, there is a lot that we just don’t know. The best advice is to keep good records of everything you can.

HRSG lifetime assessments

“HRSGs are millennials,” suggested Sargent & Lundy’s Danial Azukas, meaning that most of those operating today were built in the 1990s and early 2000s. They were base-load designs for 30-year operation (300 cold, 1500 warm starts, and 6000 hot starts).

Then gas prices fluctuated, both up and down, and green power entered. As discussed throughout the HRSG Forum, cycling and unanticipated owner/operator challenges began.  

Azukas discussed various cycling impacts through a case history, launching a theme of cycling and remaining life assessments that would appear throughout the organized meeting and the frequent open discussions.

One assessment element discussed was benchmarking with data and evaluations not tied directly to the subject plant. The North American Electric Reliability Corp maintains availability database systems that can add value to such reviews, highlighting outage events and other influencing data. Peer plant comparisons of outage factors, availability and capacity can also help in the look forward.

Assessments would become a common topic throughout the conference.

Tom Sambor, EPRI’s technical leader for Program 88 HRSG and BOP, discussed life assessment of boilers and pressure parts, particularly the fleet of HRSGs now reaching the soft design life.

Reviewing relevant background, he explained that in Western Europe fitness for service was “an integrated approach to operation and life management.” Re-assessment for continued operation was common, he explained, due in part to long-term use of more complex materials.

This is not the history in the US and Canada. He explained that in the 1980s and 1990s, service fitness became an urgent need in North America because of header ligament cracking, dissimilar metal weld failures, and some catastrophic material events.

EPRI began pioneering with its RP2253 project. 

Sambor explained that EPRI now advocates a seven-point life management approach:

1. Understand how design, operation, fabrication, and metallurgy affect component performance.

2. Appreciate the historical issues (failures, statistical analysis).

3. Develop optimum specifications.

4. Set improved quality guidelines for design, manufacture, and operation.

5. Determine when, where, and how to look for damage.

6. Develop component-specific methods for repair/replacement that exceed minimum ASME Code rules.

7. Employ technology transfer of information to codes, standards, and the global community.

His presentation concentrated on the search for damage.

Another bit of history: The Central Electricity Generating Board (CEBG) in the UK performed foundational work in power-generation fitness for service in the 1960s. “Thousands of welds were identified to have fabrication flaws,” explained Sambor. “Rather than simply repair or replace, the CEGB chose to evaluate.”  Assessments looked at “safe” versus “potentially unsafe.”

In the US, the wake-up catastrophic failure was in 1986, at the coal-fired DTE Monroe. The failure occurred in Unit 1. But Units 2 and 3 were not shut down. Both were needed to meet demand, and continued in reduced operation. This ongoing generation during assessment proved successful.

Sambor’s discussion centered on safe operation based on inspection results, and subsequent inspection planning. “In some cases,” according to National Boiler Inspection Code (Part 2), “a visual inspection of the pressure-retaining item will suffice. However, more comprehensive condition-assessment methods may be needed—including an engineering evaluation performed by a competent technical source.”

Detailed examples also are given in American Petroleum Institute’s API 579, Section 1.1.2. As Sambor explained, “By its own definition API 579 does not independently determine if a component is fit for service; it helps determine.” API 579 was released in 2000 and immediately received widespread acceptance both inside and outside of the refining industry.

Sambor explained further background and continuing activities, highlighting those by API and developments at ASME, specifically the Boiler and Pressure Vessel Code Main Committee. ASME and API now work together on the issues.

Then he presented specific examples of poorly performed fitness programs. Several cautions were discussed, including lack of communication and inherent limitations of some overly specialized consultants.

Sambor next focused on fitness-for-service challenges specific to HRSGs, including the following:

    • For operations: Cycling, and significant duct firing.

    • For materials: Extensive use of CSEF steels.

    • For maintenance: Plants are forced to “do more with less.”

    • Other: Gas-turbine upgrades can have unintended consequences on HRSGs (and other downstream equipment).

Sambor then discussed these specific component challenges:

1. Large branch connections. This has become a significant industry problem, particularly with Grade 91.

2. Tube-to-header connections. This has led to numerous drain evaluations for condensate removal.

3. Circumferential cracking at drum-nozzle toe welds is increasingly common.

Many related issues were discussed throughout the 2018 Forum, including valve bodies and attemperator-induced quench cracking. 

Sambor ended with EPRI’s overall fitness philosophy, stating that proper methodologies must be: 

    • Based on engineering rigor.

    • Relevant and no more complicated than necessary.

    • Benchmarked against well-pedigreed test cases from the industry.

    • Clear for key scenarios such as identified flaw, assumed flaw, creep (time-dependent), etc.

    • Based on well-maintained and relevant databases, and active sharing of both present and past information.

“Most of all,” he said, “safety must be paramount! Safety drives fitness for service.”

Lengthy discussions followed including hands-on training of inspectors (AIs) and increased interaction with insurance carriers.

Later in the program, EPRI’s John Siefert discussed technology-transfer achievements (and open information exchange) as an approach to equipment life management, specifically regarding CSEF steels. Looking back to 2006, he stated that “after 10 years of research and more than $15 million invested by collaborative industry projects, there is now sufficient insight to provide an integrated strategy for life management of CSEF steels. This includes more than 125 individual reports and the world’s most comprehensive understanding of these materials in powerplant components.”

He then repeated that “variability is a reality in 9Cr martensitic CSEF steels.” The inconsistency is in deformation and damage resistance. “This reality,” he stated, “increases the complexity of an integrated life-management strategy.”

Siefert outlined the resistance specifics and discussed the need for more stringent control of both up-front (heat-to-heat) composition and various material heat-treatment practices.

“One current emphasis,” he said, “is to merge two items of EPRI’s seven-point life management approach:

    • Understand how design, operation, fabrication and metallurgy affect performance, and

    • Transfer this information to industry codes and standards, and the global electricity-supply community.”

The presentation offered details on programs by EPRI, ASME, and NBIC, and included a list of publicly-available EPRI reports.

Questions included applicable codes for repair of older materials.

Monitor bypass-valve erosion to assure proper sealing

Bob Anderson discussed HP-bypass pressure control valves, stating that although these are considered severe duty, they can last 10 to 12 years before seat/plug repair, if properly controlled and operated. The primary concern is erosion caused by water and/or steam passing through the valve (HP to CRH). This is not a problem with all units, nor is it a problem specific to any valve OEM.

He began with a single-shaft unit example in somewhat unique operating modes including open cycle, then followed with a 2 × 1 plant in typical operating modes.

In the first example, the HP-bypass pressure control valve began leaking through after one or two runs.  Although correctly installed, the OEM modified the valve to improve erosion resistance, but rapid erosion continued. The issue was identified as part of open-cycle service when the gas turbine is at low load and the steam turbine is offline. This allows condensate to pool along the bottom of the main-steam pipe, shrinking it relative to the top of the pipe. The normal 1% slope downward becomes reverse slope.

In a second case study (2 × 1 in typical modes), leaking occurred after a period of years, and although modified by the OEM, recurred. Arrangement and start modes were reviewed. During a lag cold start, the bypass valve opens too early when main-steam temperature is still at saturation (Tsat). Delay in opening the PCV until 35-deg-F superheat is achieved should eliminate erosion damage. A bigger warming drain may be required (before the header isolation valve) to avoid excessive startup time.

The continuing evolution of controls and automation

Emerson’s Jim Nyenhuis provided an update on automation and controls for combined cycles. He posed this question: “How can automation help, based on the changing world of plant operations?”

The presentation began with a look at classic closed-loop controls, and an interesting point: Controls look in real time. So the key is to also understand system dynamics, including the elements of time. In one example, Nyenhuis looked at unstable final steam temperatures caused by cycling, and PID-based load control. The question: What can refined control do for the long-term life of the components?

“If we can develop very refined models,” he said, “this allows us to look forward and make predictions, then anticipate these in our controls strategy.” The in-depth presentation appealed to those knowledgeable in the systems and tuning processes, but also reinforced the movement toward more forward-looking operations (the cycling world).

Discussions highlighted the complexities, including these:

    • Predictive modeling should account for historical empirical data, weather, time of day, differing humidity.

    • Other units on and running.

    • Anticipating where the load will be in 20 to 30 minutes.

    • Other operational challenges, including justifying the cost of a control-system change to management.

Protect against high SH/RH metal temperatures

HRST Inc’s Bryan Craig explained the primary factors affecting HRSG superheater and reheater tube metal temperatures, and typical areas of concern.

“For some HRSGs,” he explained, “the highest tube-metal temperatures could be in Module 1, during low-load unfired operation. In HRSGs with duct burners,” he continued, “maximums will be in Module 2 with burners firing.” And why is this important? “A temperature increase of 15 to 20 deg F can reduce equipment life by half,” he stated.

Long flames and gas-temperature variations downstream of burners were discussed, and rule-of-thumb observations were given including duct-burner cameras.

Failing or missing gas baffles are also a major contributor, allowing high temperature exhaust gas to come in contact with downstream tubes designed for lower temperatures. Craig presented photos of numerous gas-baffle failures.

Gas-turbine upgrades can also push HRSG tube-metal temperatures above their design limit. Gas temperatures increase, but steam temperatures can remain constant, controlled by the attemperators.

Internal oxide growth also was discussed. During comments, Dooley mentioned that a workshop on materials and overheating was held at the Australasian HRSG Users Group, and was on the agenda for the European HRSG Forum, held recently in Spain. “We are discussing overheating in HRSGs as a serious problem, and looking at doing some proactive work, particularly with a large global database on oxides in various materials.” This will become a topic for the HRSG Forum.

Miscellaneous topics

Drum level instrumentation. Jim Kolbus, Clark-Reliance Corp, discussed recent developments in drum level instrumentation, and a focus on ASME Code requirements.  

“The only direct-reading device is a gage glass,” he explained. “Indirect reading is a remote device.” He then reviewed the options, pressure limits of each, and Code-permitted variations.

The discussion included minimum Section 1 water-gage requirement—including placement details, isolation and drain valves, permitted globe-type valves, and chain operation mechanisms for personnel and plant safety.

Common Code violations and concerns discussed included:

    • Isolated inoperable water gages.

    • Missing water gage glass.

    • Missing illumination from water gages.

    • Inadequate display of remote indicators in the control room.

Kolbus stressed “proper maintenance and routine inspection of these critical instruments,” such as blowdown of small-diameter sensing lines to remove sediment.

Tube-to-header weld evaluations.  Matt Dowling of Applied Technical Services followed with a case study on HP/RH-tube-stub-to-header welds at the Ontelaunee Energy Facility. The F-class 2 × 1 combined cycle has accumulated more than 80,000 operating hours since COD in 2002.

History includes 73 failures, all in the upper-header bent tube rows. Analysis showed that damage initiated at the outer edge of the heat-affected zone, approximately 0/125 in. from the toe of the weld on the tube side.

Dowling covered the details of OD, ID, and mid-wall inspections, some difficult because of geometry. To date, 27 repairs have been completed, and further testing continues. Root cause is not yet identified.

Forum participants offered ideas and suggestions, and interesting insights going forward. This is a bubble unit; subsequent designs were modified.

Exhaust casings. Jake Waterhouse, Dekomte, presented exhaust-casing insulation inspection and repair examples for liner upgrades, insulation repacking, hot-spot elimination, apertures, and openings.

He began with cold-casing duct insulation issues (insulation loss leading to hot spots). Operational impacts highlighted the multiple negative effects of two-shifting and cycling including temperature transient fatigue and stress, movement, and dew-point condensation (acid and water).

Both standard and high-turbulence liner arrangements were described including the potential benefits of scallops. Duct insulation options included traditional, staggered, and stacked.

Pumpable repairs were described, including techniques and thermographic modeling during application. High-turbulence exhaust-diffuser repairs followed, including an example repair during operation. Fabric expansion-joint technology was described in detail for penetration seals.

Effective monitoring during plant operation was emphasized throughout the presentation and discussions.

Steam-cooled desuperheaters. Attemperators that cool with saturated steam were described, primarily interstage and final desuperheaters designed to reduce thermal shock damage. The design discussed by Karel van Wijk, Advanced Valve Solutions, has been in operation for 10 years.

Thermal-shock calculations reviewed estimate that by reducing temperature differential by about 125 deg F, the number of expected startups without damage increases enormously. Sample calculations and predictions were reviewed.

Desuperheater positions, designs, and materials were examined in detail.

Capacitance sensing. Qussai Metashdeh, Tech4Imaging LLC, discussed non-invasive imaging and monitoring technologies. He concentrated on electric-capacitance volume tomography (ECVT), specifically for two-phase flow applications in HRSGs.

Also discussed was adaptability to various locations, concentrating on evaporator steam quality for ramping purposes.

ECVT is also applicable for three-phase flow (water, air, oil) and work is being supported by the DOE’s National Energy Technical Laboratory.

Questions included solids monitoring (fluidized beds) and testing with hydrocarbons.

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