ACCUG 2023, June 20-22

The Air-Cooled Condenser Users Group’s 2023 meeting will be held at Dominion Energy’s Innsbrook Technical Center in Glen Allen, Va, June 20-21. Thursday, June 22, is reserved for an optional tour of the utility’s Greensville County Power Station’s ACC.

Visit https://acc-usersgroup.org for meeting details, hotel suggestions, registration, etc. Questions? Contact sheila.vashi@sv-events.net.

The Air-Cooled Condenser Users Group (ACCUG) held its 13th Annual Conference, in person, at the Hilton Stamford Hotel and Executive Meeting Center in Stamford, Ct, Sept 13-15, 2022.

A few weeks earlier, ACCUG conducted its first no-cost virtual training session on some items covered at the conference (Sidebar 1).

ACCUG is the electric power industry’s only user group dedicated to discussing and resolving operation and maintenance (O&M) issues with air-cooled condensers. Annual conference topics include corrosion and chemistry, design and performance, O&M, and publishing of ACCUG Guidance Documents.

More than 50 attendees discussed these topics at the 2022 meeting, followed by a tour of the induced-draft ACC now in operation at nearby Competitive Power Ventures’ (CPV) 805-MW Towantic Energy Center, commissioned in 2018 (Sidebar 2). Andy Howell, EPRI’s technical executive for Boiler and Turbine Steam and Cycle Chemistry, chaired the event.

Below are conference highlights as a guide for the 2022 presentation materials now available here.

1. ACCUG’s first virtual training session addresses air in-leakage, corrosion, and more

The Air-Cooled Condenser Users Group held its inaugural no-cost virtual training session last July (2022)—a two-hour online event focused on air in-leakage, corrosion, and ACC performance enhancements.

Moderators/presenters were Riad Dandan (Dominion Energy), Rishi Velkar (NV Energy), Barry Dooley (Structural Integrity), and Andy Howell (EPRI).

Co-host Howell began by explaining that content was based on input, questions, concerns, and ideas raised recently through discussion forums at https://acc-usersgroup.org.

The international assembly numbered 150 participants.

CCJ participated, finding value in the technical content, Q&A, and ideas to share with its readers. The takeaways, outlined below, offer insights, verifications, new approaches to old problems, and troubleshooting experiences that can apply to nearly all powerplants.

Air in-leakage. Most air-cooled condensers operating today are large units made of thousands of finned-tube bundles acting as the heat-transfer surface to condense turbine exhaust steam (forced convection). Condensing creates a required internal vacuum, but also raises the threats of air in-leakage and dissolved oxygen in the condensate.

All ACCs, whether forced- or induced-draft, are elevated structures to allow required air flow up and over the tubes.

Large units can contain more than 20,000 tubes—that’s 40,000 tube welds with in-leakage potential. In-leakage opportunities also exist throughout ACC system components and connections.

Checking for leaks is complex, painstaking, and should be comprehensive.  As one presenter clearly encouraged: “Don’t forget ground level.”

Here are a few examples of leakage potential, beyond the tubes themselves:

• Welds.
• Valves.
• Expansion joints.
• Bolts.
• Any corrosion areas.
• Rupture discs.
• Piping under insulation.
• Previously repaired leak areas.

Testing complications are many, and include factors such as:

• Size and volume of the structure.
• Height and testing access.
• Weather and winds.
• ACC fan operation during testing.
• Cost of tracer gas and restrictions on alternatives such as SF6.

Traditional detection methods and their normal frequencies and challenges were covered. Other alternatives, such as infrared (IR) scanners/cameras and acoustic leak detection, received mention.

Discussions quickly turned to the effect of in-leakage on dissolved oxygen within the system, and O2 levels considered acceptable for normal powerplant operation. Barry Dooley offered that less than 10 ppb at the condensate pump is considered normal when deaeration is part of the system, but this depends upon unit design, location, and ambient conditions.

“I have seen up to 600 ppb, which is very unusual,” he explained. “Normally, levels of 20 to 40 ppb are considered high.” Dooley added, “High oxygen levels can occur during cold start, but should soon settle unless there is leakage.”

Howell then offered a rule of thumb: “I would not advise doing a helium test unless there is a reason, such as high oxygen/carbon dioxide or abnormally high air ejection. If there is little indication of a leak, any present will be small and difficult to locate.”

Other system chemistry discussions followed, although online time was limited.

Participants were then encouraged to use the forum at https://acc-usersgroup.org for further questions and to share thoughts with other users.

Chemistry and corrosion. Dooley presented 10 slides on common chemistry concerns, most raised by participants during their registrations. Topics included common damage conditions, and the relatively new topic of film-forming substances for metal protection.

Flow-accelerated corrosion and detection methods dominated the discussions.

Performance. “Performance enhancements are always important, but become even more so in warmer weather,” explained Howell. “No ACC is big enough at 100F ambient to achieve ideal backpressure for the power cycle.”

Ideal operation is difficult to achieve, but many basic search strategies for improvement include these:

• Open doors.
• Gaps in the tubes.
• Bent fins.
• Recirculation of warmer air from perimeter cells.
• Need for tube fin cleaning.

Other improvement strategies could include:

• Spray misting.
• Adding a small condenser and wet cooling tower.
• Deluge cooling (flooding a tube row).
• Fan uprates.
• Wind screens.

Questions and concerns. The final section, led by NV Energy’s Velkar, reviewed other questions submitted by attendees. Principal topics were:

• The effects of ambient wind speed and direction.
• Wind-induced blade cracking concerns.
• Variable-speed fan experience.
• General ACC cleaning methods.
• Common gearbox issues.

CCJ’s Scott Schwieger, who coordinated and managed this two-hour event, offered a list of selected fundamental resources for the ACC community:

• https://acc-usersgroup.org
• https://filmformingsubstances.com
• https://iapws.org

Ongoing forum, future events. Online discussion forums are available at https://acc-usersgroup.org/forums/ and include announcements from the group’s steering committee for upcoming events.

Technical discussions are available under these categories:

• Design.
• Fans and gearboxes.
• Inspection.
• Operation and maintenance.
• Performance issues.
• Steam cycle, chemistry, and corrosion.

Future online virtual discussion sessions will be announced in CCJ and at https://acc-usersgroup.org.

Reliability

Rishi Velkar, a plant supervisor at NV Energy, gives specific examples for various reliability programs in ACC reliability improvement. Most who have visited an ACC will appreciate the elevator installed in 2016. The only follow-up costs have been annual certification and testing, and one overspeed governor replacement.

Elevators help with inspections, maintenance, carrying of supplies, walkdowns, and in one major case, a contractor life-threatening health rescue, and now are installed on all NV Energy ACCs.

Velkar discusses a master control console for fan motors, and repairs to a fan cubicle, all listed as low-cost derate avoidance. He also outlines a $1 million estimate for specific vibration, and oil pressure and temperature, monitoring equipment and upgrades. Velkar shows new cranes and concrete landing structures for gearbox replacements and drawings for a rupture-disc exchange.

He ends with a list of upcoming projects (fan motors and blades, motorized vacuum pumps, and an auxiliary boiler to hold vacuum and improve startup).

Warsaw waste-to-energy

In Poland and most of Europe, greenfield powerplants with ACCs have strict design targets being met by a new waste-to-energy unit presented by MVM EGI (Hungary). Details are provided in the PowerPoint, ACC of largest waste-to-energy plant under construction in Poland.

MVM EGI is a global cooling system provider based in Budapest; MVM Group is the largest power utility in Central and Eastern Europe.

Because waste-to-energy plants usually are located in or near cities, common benefits of this new unit include:

• Low-noise operation.
• Small visual impact.
• Small footprint.
• Zero water discharge.
• No plume.

The new unit in Warsaw (Fig 1) meets these objectives for cost-effective dry cooling and features ease of operation and maintenance. At 25 MW, the project will be the largest of its type in Poland.

The forced-draft ACC is located on the roof of the steam-turbine building (Fig 2), and wind characteristics and air flow are altered by the boiler building and high elevation. György Budik, commercial director, reviews the CFD analysis and wind-load calculations, as well as foundation and structural steel requirements.

He also explains the design challenges for expansion joints and other critical components, as well as special measures taken to avoid freezing. Limited space for the exhaust steam duct called for unique solutions for expansion joints and load-bearing parts of the duct (Fig 3).

Because the plant is used for district heating in winter, evaluation of anti-freezing measures is discussed in detail.

Corrosion and chemistry

In ACC corrosion and cycle chemistry, Howell presents information provided by Barry Dooley of Structural Integrity. He launches into flow-accelerated corrosion (FAC) and its consequences, then moves to inspections using the Dooley-Howell Corrosion Index method (DHACI), followed by an update on film-forming substances.

He stresses that corrosion and chemistry issues are common worldwide, regardless of unit size or location.

DHACI has become an accepted global method for accurately measuring and monitoring corrosion in critical parts of the ACC (tube entries, cross-member supports, lower ducts, etc). The index is discussed in detail in ACC.01: Guidelines for internal inspection of air-cooled condensers, available at https://acc-usersgroup.org/reports/. It reliably indicates high iron concentrations within the cycle. Consequences can include the following:

• HRSG deposits requiring expensive chemical cleaning.
• HRSG tube failures caused by overheating, under-deposit corrosion, and hydrogen damage.
• Steam turbine deposits.

Corrosion/FAC in the ACC therefore reveals the need for iron removal processes, condensate polishing, and/or filters—also discussed in the conference.

Howell clearly states two important points:

1. As explained in earlier conferences, the relationship between total iron and pH is consistent worldwide, and corrosion leads to elevated iron levels throughout the system. With the accuracy of the Dooley/Howell index, monitoring the ACC can essentially help control unit cycle chemistry.
2. Evidence is global and consistent, drawn from specific operating experience in the US, Australia, Canada, Chile, China, Cote d’Ivoire, Dubai, India, Ireland, Mexico, Qatar, Abu Dhabi, South Africa, Trinidad, and the UK.

Howell moves to a brief discussion on how ACC two-phase FAC appears arrested with film-forming substances (FFS). For a review on this expanding topic, see https://www.ccj-online.com/protection-of-metal-surfaces-a-wakeup-call-on-film-forming-substances/.

He ends with a quick review of selected Technical Guidance Documents published by the International Association for the Properties of Water and Steam (www.iapws.org) with particular relevance to plants with ACCs.

Flow-accelerated condensate corrosion (FACC)

Howell then turns to his own presentation and expands with Steam cycle chemistry items in ACCs, highlighting idiosyncrasies of steam-side corrosion based on microscopic investigations that reveal some variations from typical two-phase corrosion.

Those who want the details should look here and dive into the intergranular faceted surfaces and cross-section microstructures that show what Howell calls “flow-accelerated condensate corrosion (FACC).” Microscopic observations indicate the differences.

He explains further distinguishing conditions such as operating temperatures below those optimal for single- or two-phase FAC, and an observation that under consistent conditions, metal loss may actually stop if relative stability is reached in the corrosion process.

This leads to a focused discussion on film-forming chemicals and equilibrium.

Condensate polishing

A discussion on Condensate polishing in ACCs reminds that “if properly designed, installed, maintained, and operated, polishing ensures that condensate directed towards the boiler/HRSG will be high quality and have minimal contamination.”

Although required for once-through boilers and nuclear steam generators, polishers are an option for most combined-cycle HRSGs and that decision is based on many factors. Both deep-bed and powdered-resin types are reviewed.

For air-cooled plants, the most common sources of steam-cycle contamination are air in-leakage (AIL, the primary source), makeup-water system failures, contamination of chemical treatments, poor practices in regeneration of deep-bed polisher resin, and decomposition of resins at elevated temperatures.

This presentation also discusses polishing for once-through steam generators with ACCs and hybrid-cooled (wet/dry) systems.

The conclusion: “Condensate polishing is problematic in use with ACCs and multiple factors should be evaluated to determine whether polishers are useful for a specific plant and, if installed, how to select, design, and operate polishers for optimum performance under ACC conditions.”

Air pockets

Evapco Dry Cooling discusses the causes, effects, and prevention of Air pockets in ACC tubes. Primary causes are backflow, vacuum system performance, excessive saturated oxygen, and excessive air ingress. Common results are reduced ACC performance and freezing risk.

In A-frame units, air pockets are visible in cold sections of the bundles (Fig 4). The presentation walks through the process where, during backflow, steam condenses but air can be left behind in pockets.

Capacity differences in the tubes are generally from air velocity or temperature differences at the face of the tube bundle. Air pockets can form in the higher capacity tubes with lower relative air temperature or higher relative air velocity.

Vacuum system performance is also an issue because vacuum is necessary and “no ACC is 100% leak tight.”

In addressing excessive saturated oxygen, the presentation covers the various deaerator types and expected O2 levels.

Air-ingress discussions include examples of leaks seen near valves and turbine shaft seals. Detailed case studies on prevention and proper ACC design close out the presentation.

Helium and options

In Alternatives to helium for AIL detection in ACCs, Howell first presents the benefits of helium tracer gas for leak detection:

• Almost always used for AIL into the vacuum created by powerplant condensers.
• Can be challenging for ACCs because of structure size, additional potential leak locations, and the effects of air currents (tracer must be applied close to point of in-leakage).

Therefore, time requirements can be extensive. Primary alternatives are infrared cameras and acoustic measurement. Coverage of the pros and cons of these alternatives is excellent.

Basics for infrared are:

• Availability and convenience.
• Easy temperature adjustments.
• Relatively rapid process.
• Good screening tool for follow-up helium testing of specific areas.
• ACC remains in operation.

Howell then reviews equipment and processes and offers some interesting tips for infrared surveys, such as recording only on the outside of fan modules and easier detection in cold weather.

For acoustics (Fig 5):

• Both audio and imaging-based results now available.
• Improvements made in background noise cancellation.
• Good screening tool for follow-up testing.
• Best when unit is offline supporting vacuum.

He again presents typical equipment and experiences.

Howell ends with an ERDCO Armor-Flo® device for continuous monitoring of air exhaust flow rate, recorded on the plant DCS (Fig 6).

Gearbox

In Gearbox durability, Sumitomo Drive Technologies walks users through gearbox startup torques, sealing, and lubrication.

Kris Herijgers (Hansen Industrial Gearboxes, Belgium) presents data for both forced- and induced-draft units. He focuses on the market-driven challenge of frequent stops and starts.

Startup can lead to three times nominal torque (or higher), with an additional sinusoidal transient torque. Gear contact stresses must be kept below the endurance limit for both conditions.

With today’s cycling challenges, “Traditional American Gear Manufacturers Assn (AGMA) calculations are not sufficient,” the gear expert says. Sophisticated calculation tools are thus required that consider starting method and peak torque including the transient torque, as well as the sum of all start cycles.

He then provides details of sealing and lubrication to reduce maintenance costs.

Windscreens

In Wind effects on air-cooled condensers, Galebreaker Industrial reviews wind-screen applications to counteract wind-induced loss of thermal performance, mechanical stresses, and tube fouling.

The fundamental placement of windscreens includes:

• Perimeter screens to reduce blade stress.
• Cruciform screens for improved thermal performance.

Galebreaker covers crosswinds and performance impacts, and the dynamic impacts on blade loading and vibration. CFD analysis and example solutions follow.

Application to an innovative sloped-structure ACC is also discussed.

Fully automatic cleaning

To view a fully automatic ACC cleaning and cooling system with low water use, see Fully-automated cleaning robots by JNW Cleaning Solutions GmbH, Germany.

Managing Director Arndt Krebs discusses how his company has been cleaning ACCs in Europe and South Africa since 1995, and in the US with partner Conco since 2001. Fully automatic systems have been in place since 2012.

Most semi-automatic systems operating today require two hands-on operators. The fully automatic system is centrally controlled, often from the control room (Fig 7).

Designed for and when in continuous operation, the fully automated system also becomes an ACC cooling device with these benefits:

• The cooling performance of the ACC remains high.
• Increases in backpressure are reduced significantly.
• Turbine efficiency remains high.
• Turbine throttling can be avoided.

Krebs offers plant evidence, and discusses the benefits of cooled equipment versus higher cleaning-water consumption, along with JMW’s HP-pump innovations to save water. He concludes with conversion of semi-automatic to fully-automatic systems.

Cleaning induced-draft ACCs

AX Systems presents specifics of Cleaning induced-draft ACCs noting that these V-frame/W-style designs present some unique cleaning obstacles and all cleaning equipment must avoid collisions with obstacles in narrow walkways (Fig 8).

2. CPV shares experience with Towantic’s induced-draft ACC

Competitive Power Ventures’ Towantic’s air-cooled condenser is a V-frame/W-style Enexio induced-draft system composed of six streets, each with five fans (Fig A). There are 18 condenser (C) sections and 12 dephlegmator (D) sections, with steam isolation on streets 1, 5, and 6.

Recognized basic advantages of induced-draft systems are reduced total ACC height (less visual impact), smaller footprint of columns, and lower initial cost over traditional forced-draft units.

CPV’s Nick Levandoski takes this further, both pros and cons:

Operationally, there is less vibration stress due to elimination of the fan bridge for possible longer life of gearboxes and fans.

He lists more operational advantages:

• Reduced auxiliary power is possible because of a lower pressure drop on the air side.
• Less hot air recirculation.
• Less sensitive to wind effects.
• Higher flexibility of sectionalizing (part-load operation) if required.
• Uniformity of air flow is improved over the face of the ACC heat exchanger tubing.

A few negatives discussed during his presentation include access from the walkway to the elevated motors and gearbox, and cleaning space limitations.

The main steam duct at Towantic is of  all-welded construction for leak avoidance (Figs B and C). Length is more than 300 ft and the largest section diameter is 23 ft. Seven steam ducts then run across the top.

The steam header conveys steam to the condensing cells. Condenser tube bundles are installed below the fans. Condensing cells 1, 3, and 5 are parallel-flow tube bundles. The second and fourth cells in each row are combination modules which contain parallel condensing and counterflow D bundles. The D bundles are connected to the condenser air extraction system for removal of air and non-condensable gasses.

Expansion joints are varied:

• Single hinged.
• Tied universal.
• Dogbone.
• Single (2).

The Alex-system tube bundles are single-row, single-pass heat exchangers composed of relatively flat cross-section tubes (8 × 0.75 in.) with aluminum fins. All condensing bundles are welded together to the steam header. At the lower ends, all bundles are connected together via integrated condensate collection/steam crossover headers.

Each motor, fan brake, and Siemens/Flender right-angle gearbox is located 30 ft above the walkway (Fig D), and for that reason extra precautions are taken in preventive maintenance.

The 30, seven-blade Cofinco fans are 36 ft. across.

The as-built finned-tube cleaning system is automatic vertical and manual horizontal, with a cleaning width of 4.7 ft through 22 nozzles at 1450 psig. Flow rate is 32 gpm. It is of the no-access rolling ladder design by AX Systems.

Some issues encountered. One of biggest issues to date: A main steam-duct leak in the last expansion joint before it goes into the riser for the streets. This duct location is 23 ft diam. The expansion joint was replaced in spring 2021.

Another ongoing issue is air pockets in the D cells. Although not yet resolved, it is believed to originate upstream of the ACC. Investigation continues.

A few best practices. Gearbox seals have been an ongoing issue. Therefore, the plant began, on a semi-annual basis, to supply a filter press (water and particulate, 6 micron) to all gearboxes and run for 1.5 to 2 hours each. This, combined with oil samples prior to filter pressing, provides a good indication of oil condition for each gearbox. It also keeps the gearbox seals from failing prematurely and allows the plant to see if anything is breaking down inside.

Also, the plant installed three vibration probes on each ACC motor and three on each gearbox, running cables to a remote location. These allow the site to take vibration readings monthly on running ACC equipment, providing useful vibration data on a regular basis. The site can better understand condition of the equipment and look for alignment, coupling, and motor or gearbox bearing issues.

At CPV’s Valley Energy Center (also induced-draft), knowing the challenges and time associated with disassembly and reassembly of the fan blades and hub assembly to change out a seal, the Maintenance Dept worked to understand the failure mechanism and brought in Corrosion Products & Equipment (CPE) to collaborate on a solution. A flange-mount split seal was chosen. After a year of operation, the Inpro seal solution remains leak-free and reliable. This program will be implemented at Towantic.

For other best-practice achievements at Towantic, see https://www.ccj-online.com/?s=Towantic.

Noteworthy operating points. Condenser vacuum is pulled initially by operating both liquid-ring and maintained by collapsing of steam within the finned-tube bundles. The liquid-ring pumps remove the air and non-condensable gases from the ACC during normal operation. An air valve is provided to protect against pump cavitation.

Freeze protection is required at the site. During sub-freezing operation, ice can accumulate in the upper part of the D bundles. To remove the ice, the D fans are stopped periodically to warm the tubes. This warming function becomes active when ambient temperature is below 35.6F after a time delay of two hours. Typical D warming cycle duration is five minutes. Active warming cycle from street to street is separated by 30 minutes.

 

Stellenbosch research

The South African utility Eskom operates massive air-cooled condensers, and Stellenbosch University is heavily involved in both operational issues and research for improved efficiency.

In Axial-fan performance and noise modeling, members of the Dept of Mechanical and Mechatronic Engineering present detailed accounts of predicting both performance and noise in large-diameter axial-flow fans.

Those looking for a refresher on fan aerodynamic noise mechanisms can find it here with details on rotational and non-rotational noise and the dominant noise mechanisms. This includes fan numerical modeling and comparisons with experimental results.

Stellenbosch has developed the M-fan as a large-diameter (24 ft) axial-flow fan with rectangular blades, and cambers of 3.5% at the blade root and 0.8% at the tip. The design was tested in the ISO 5801 Type A fan test facility equipped to measure flow rate, pressure rise, shaft power and running speed.

Also defined are visualized numerical model results at the design flow rate.

Numerical fan noise predictions follow, breaking down the noise spectrum into components to achieve better understanding of individual noise mechanisms and how they can potentially be reduced.

Details emerge on:

• Turbulence ingestion noise.
• Tip vortex formation noise.
• Trailing edge noise.
• Total fan noise.
• Fan noise scaling and measurement.

Deep-dive details are provided. Some conclusions:

• Fan performance can be modeled accurately in a 3-D CFD virtual testing rig.
• By combining trailing-edge noise strip theory and turbulence ingestion noise models, the total fan noise spectrum can be predicted numerically with good accuracy compared to experimental measurements.
• The standard fan test facility can be used to perform comparative measurements to determine the noise characteristics of scaled fans.

The facility is now being refined to reduce self-noise during testing.

In another presentation, R&D at U of Stellenbosch, Prof Hanno Reuter defines the university’s involvement in South African ACCs, activities that have spanned five decades. Detlev Kroger initiated ACC research at Stellenbosch in the 1970s and South Africa’s large ACC designs and features are greatly influenced by this research.

Stellenbosch facilities outlined include a variety of experimental facilities with typically 50 to 70 post-graduate students per year.

Reuter includes selected current research and results from various university professors and PhD candidates, along with direct-contact details.

Published documents

Two guideline documents are currently available at https://acc-usersgroup.org/reports/:

• ACC.01: Guidelines for internal inspection of air-cooled condensers (2018 update).
• ACC.02: Guidelines for finned tube cleaning in air-cooled condensers (2021 update).

Also available are the following:

1. Flow-accelerated corrosion in seam generating plants by Barry Dooley and Derek Lester, PowerPlant Chemistry 2018, 20(4).
2. Corrosion in air-cooled condensers: Understanding and mitigating the mechanisms by Setsweke Phala et al, Eskom, Johannesburg, South Africa.

In-progress ACC Guidelines were discussed at the conference:

• ACC.03: Guidelines for air in-leakage in air-cooled condensers.
• ACC.04: Guidelines for wind mitigation in air-cooled condensers.

Outlines for upcoming ACC.03 and ACC.04 are available at ACC guideline documents, 2022 Annual Conference.

Question period

A 45-min discussion period at the end of the meeting focused on submitted questions dealing with wind impact on efficiency and blades, variable fan speed control, finned-tube cleaning and water collection, gearbox replacement frequency, known structural resonance issues, use of electric motors, safety protocols for blade liberation, and rupture-disc replacement strategies.

The Australasian Boiler and HRSG Users Group (ABHUG) held its 2022 conference last November, in Brisbane, Australia. Participants joined from Australia, Germany, New Zealand, Singapore, UK, and US. There were 24 technical presentations and a workshop on film-forming substances. Selected highlights follow.

ABHUG 2023

The Australasian Boiler and HRSG Users Group will conduct its 2023 meeting in Brisbane in November. Dates, venue, and other details will be available at www.ccj-online.com as they become available.

We are not alone

A unique element of all associated HRSG conferences (ABHUG, European HRSG, and the US HRSG Forum) is hearing and discussing the latest trends in thermal-transient and cycle-chemistry issues facing all HRSG owners and operators worldwide.

Conference Co-Chairman Anderson (see Steering Committee box) presented a summary of thermal-transient survey assessment results conducted worldwide. This appraisal now includes 64 combined-cycle plants surveyed from 2009 through 2022, tracking 31 key operational and equipment issues that are within owner/operator control.

One very important concern is that only six of those plants has a formal boiler-tube-failure root-cause program in place. A proper program includes tube-sample removal to define failure mechanisms, complete determination of root cause, and upper management agreement with the time and expense involved to remove the failure sample. Industry-wide use of such a program could easily improve.

That said, some surveyed areas are showing progress. For example, routine attemperator hardware inspection has increased, although it remains low at only 12% of the plants. Also improving are stable operation of attemperators and some areas of plant control system monitoring.

The most important takeaway, explained Anderson, is that leaking attemperator spray-water valves continue to cause more steam pipe and HPSH/RH tube failures than any other issue surveyed. That’s too bad because it’s a relatively easy thing to detect and repair. The major culprit is a plant’s use of Master control valve/Martyr block valve logic. Just reversing this logic—and of course repairing already damaged valves—will prevent or delay repeated leak-by.

Also, during the meeting, Co-Chairman Dooley offered an update on cycle-chemistry control where he reviewed a list of repeat cycle-chemistry situations found in more than 260 plants worldwide. Such common situations, which can include corrosion-product transport, air in-leakage, low levels of alarmed instrumentation, and lack of proper shutdown protection, lead to plant damage.

These updates from a global perspective, becoming hallmarks of the associated HRSG events, tell owner/operators they are not alone in their plant challenges. The interactive conferences are ideal for sharing details and discussing them with other plant users, equipment and service providers, and industry consultants.

Anderson and Dooley continue to collect and analyze these statistics and trends.

Darling Downs

The Darling Downs Power Station in Dalby, Queensland, is a 3 × 1 coal-seam-gas-fired 630-MW combined cycle with GE 9E gas turbines, a two-stage GE 270-MW steam turbine, and air-cooled condenser—all commissioned in 2010. Origin Energy’s Ashwin Shinde discussed the past four years of movement into flexible operations, specifically plant alterations and changes to both operations and maintenance.

The three horizontal-flow, dual-pressure HRSGs (Fig 1) were supplied by NEM, Netherlands. Water chemistry is ammonia-based AVT(O).

Darling Downs was originally designed for baseload operation. By 2018, it had to begin adjusting to flexible operation (Fig 2). Average annual starts have moved from 47 in early years to 566 since 2019. Annual total-plant running hours in baseload approached 20,000. In flexible operation, the annual average reduced significantly to 11,365 (Fig 3). Startups and downtime therefore became major operating and equipment concerns.

This led to some physical plant changes.

The main steam control valves were modified to avoid stop-valve stem erosion. HP and LP stop-valve actuation was added in 2020 to retain heat in the HRSGs following shutdowns. Also in 2020, the chemistry control room was upgraded to more accurately monitor cycle chemistry.

The next year, HP-bypass warming-valve actuation was added along with thermocouples to closely monitor main-steam pipework temperatures.

Controls and procedures also began to change.

Sky-vent valve operation was modified to improve pressure and temperature control, and to prevent cold steam from entering hot pipework. Startup drum control was used to avoid oscillations and swelling. Superheater drain and drip-leg valve opening times and durations were changed to improve condensate evacuation. Attemperator operating philosophy was modified to control steam-to-turbine temperature matching, and ramp rates were reduced and more carefully controlled.

Minimum warmup load was reduced from 20 to 10 MW to improve HRSG warmup control. For the steam turbine, the original (OEM) temperature matching program was replaced with a lower-inlet-pressure control setpoint and higher inlet-steam-temperature roll permissive.

A control-loop design change was needed to improve duct-burner response to the automatic generation-control setpoint.

The main-to-auxiliary steam supply setpoint was reduced to allow for continuous operation of the auxiliary boiler and maintain vacuum during shutdown.

Maintenance schedule changes also were implemented, and a detailed risk-based inspection (RBI) program was launched—including cycle chemistry reviews.

Shinde then reviewed component failures that have occurred during the past four years of flexible operation:

• HRSG2 LP economizer drain leak.
• HRSG1 LP economizer upper header leak.
• HRSG2 inlet-duct expansion-joint failure (Fig 4).
• HRSG1 and 2 LP economizer differential-pressure-gauge impulse line sheared.

Diamantina

APA Group’s Diamantina Power Station is a 230-MW combined-cycle site on a remote, isolated grid serving the mining industry, in Mount Isa, Queensland. The plant is operated in two 2 × 1 blocks commissioned in 2014 and has not yet achieved maximum output.

HRL Technology Group, represented by Sam Clayton, worked with APA to maximize plant output and improve flexibility, initially developing overall thermodynamic models for a range of operating scenarios.

One key issue has been high HRSG furnace exit temperature and steam-turbine degradation. Supplemental firing has not been used, and the two power blocks have operated in isolation. The two steam turbines, each rated 40 MW, have only reached 36 MW.

Improvement objectives included:

• Maximize steam-turbine output through supplemental firing of HRSGs.
• Maintain steam-turbine output for varying gas-turbine loads by modulating duct firing.
• Implement steam ranges in combination with duct firing to achieve full capacity for both STs when only three of the four GT/HRSGs are operating.

To do this, effective control of duct-firing rate and gas temperature is required for safe and effective operation, and to not overheat the final superheater tubes. Thus, advanced temperature measurements were required.

HRL joined with EUtech Scientific Engineering to integrate online monitoring of HRSG exit gas temperatures (Fig 5). The system at Diamantina is installed across a plane upstream of the first superheater.

This setup allows:

• Optical measuring; thermal radiation of CO/CO₂ is used to measure temperature.
• Temperature measurement from 750F to 3630F.
• Single-point and 2D/3D measurements using multiple sensors.
• Digital output for integration with plant DCS.
• Easy installation; robust service life.

Using six ports and sensors for each HRSG gives output with either one or two burners in operation.

A few benefits:

• Allows operation near maximum temperature limits without exceeding maximum allowable gas temperatures (protecting superheater).
• Allows maximum steam-turbine and plant load.
• Allows operation of steam turbines at steady load with operating flexibility achievable through gas-turbine and steam-flow load modulation.
• Allows control of modulated supplemental firing.

Clayton’s summary: “Integration of online monitoring along with a new control strategy has enabled full modulating control of the supplemental firing system. These improvements have led to enhanced flexibility to maximize output and meet network swings in electricity demand.”

Next steps at Diamantina are these:

1. Allow transfer of steam from one block to the other for full plant load with one gas turbine/HRSG out of service.
2. Operate in a more flexible mode to allow commissioning of 88-MW solar-farm capacity at Mount Isa.

Clayton also described advantages of using this same equipment to optimize combustion at coal-fired plants.

Sharing ideas for Kwinana

NewGen Power Kwinana Pty Ltd operates a 1 × 1, 320-MW combined cycle at a naval base in Western Australia. Commercial operation began in 2008 (Fig 6).

Steam-turbine exhaust steam is condensed in a seawater-cooled, titanium-tubed condenser. Boiler blowdown is recycled to the water treatment plant. Demineralized makeup water is produced via ion exchange using both raw potable and recycled blowdown water.

Water chemistry for Kwinana is AVT(O). Additional dosing of low-ppm amine-based film-forming substance at the feedwater tank for shutdown protection began in March 2022.

Two drains in close proximity within the HP evaporator system have been experiencing repeated failures, specifically the HP intermittent blowoff (IBO) and HP evaporator drains (Fig 7). Failure causes are hydrogen damage, erosion, and caustic gouging.

Some key details:

Caustic gouging was identified as the root cause of failures in the carbon-steel pipes. However:

• Blowdown line is in the lower crawl space, away from the hot gas path.
• Pinhole failures occur at the top of the horizontal pipe. Deposits/corrosion are more likely along the bottom.
• No evidence of overheating or localized hot spots was observed in the bulk microstructure or from hardness testing.

The presenter, Veronica Yeo, shared specifics of the repeated failures and asked for input on a permanent solution.

An HRSG engineer/inspector present identified this as a common design error and informed the group why these failures occurred and how to prevent them. Horizontal evaporator drain pipes inside the casing are exposed to gas temperatures above saturation, so the water boils away leaving deposits that result in corrosion and failure along the top of the drain pipes.

One possible solution discussed is to route the drains downward through the bottom of the casing, so the piping remains flooded. Another is to upgrade the carbon steel material to T11 or T22. The current plan at site is to replace all drain lines with P11. Also, monitoring the temperature on the top surface of the horizontal piping will confirm the mechanism.

This was an excellent example of conference-participant input and group discussion.

Film-forming journey

AGL Energy Ltd, Australia’s largest electricity generator, began a “film-forming journey” in 2019 at Torrens Island B. The four 200-MW gas-fired, natural-circulation drum boilers are constantly cycled and face possible “mothballing.” Torrens Island A is already mothballed.

The purpose is offline protection for short- and long-term standby.

AGL’s starting point was (and remains) Section 8 of IAPWS Technical Guidance Document 8-16 (2019), Application of film-forming substances in fossil, combined-cycle and biomass powerplants. Get your copy of this TGD gratis at www.iapws.org.

AGL’s Brad Soutar explained the overall dosing program at Torrens Island:

Station A (closed)

• Dosing varied for 3-6 months prior to mothballing.
• Dose rates calculated at 1 ppm in feedwater.

Station B1 (mothballed; closure by 2024)

• Dosed for 3 months prior to mothballing.
• Dose rates calculated at 1-2 ppm in feedwater.

Stations B2, 3, and 4 (available; recent decision to retain)

• B2 setup installed.
• B3 and 4 installations in FY23.

Product used is Nalco Powerfilm™ 10000 (filming corrosion inhibitor). Initial inspections showed adequate protection of the feedwater system (Fig 8) and negligible impact on water chemistry. There has also been a reduction in corrosion-product transport during startup. Upcoming plans include more inspections to monitor effectiveness, and possibly increased dosage rates and frequencies.

AGL’s Liddell power station houses four 500-MW forced-circulation drum boilers firing black coal. Units suffer significant challenges with tube failures, and frequent chemical cleaning. Dosing began in 2020.

The objective is to reduce boiler oxide growth by decreasing iron and copper corrosion-product transport. (Lindell has copper-based condensers and LP heaters.) Powerfilm 10000 is dosed after the condensate polishers with polishers in service. Dose rate is 1 ppm based on boiler full load.

Soutar discussed a few outcomes:

• No condenser resin fouling has been detected. There has been improvement in feedwater iron transport, but feedwater copper transport results are inconclusive. Boiler-tube oxide thickness has stabilized, and future testing will include oxide density.
• Future work also includes an increased dose rate to 2 ppm.
• Two other plants, Loy Yang A and Bayswater, are in the early stages of dosing and examination. Soutar stressed the need to begin with a formal, detailed review process, including accurate baseline data. He stated: “Success is seen at the end, but stems from the start.”

Other specific case studies

• Kogan Creek, 1 × 750 MW, coal: Reheater tubes, and film-forming substances (FFS; CS Energy).
• Loy Yang B, 2 × 580 MW, coal; Cycle chemistry challenges for flexible dispatch (LYB Operations & Maintenance Pty Ltd)
• Mount Piper, 2 × 700 MW, coal: Unit layup scenarios including film forming substances (Energy Australia).
• Pelican Point, 2 × 1 combined cycle, 485 MW: Operating history and preservation (Engie).

Aging of P91 steel

Charles Thomas, Quest Integrity (New Zealand), noted the historical improvement in creep strength of P91 over P9 steel by adding very small amounts of vanadium, niobium, and nitrogen but also noted a “disappointingly high number of unanticipated failures.” He attributed this largely to improper fabrication heat treatment (normalizing and tempering). He added that “We now see particular difficulties with Type IV cracking, and irreversible loss of creep strength in the weld heat-affected zone.”

Thomas explained how aging has decreased the material creep-rupture properties, making an important point that the effect is often not considered in remaining-life assessments.

He then proposed a Larson-Miller parametric equation to manage time/temperature creep data, suggesting a “time-dependent C constant” and asking, “What if the Larson Miller Constant is not a constant at all?” The result would be a correlation between C, time, and temperature.

Work is ongoing, including an extended database, and will be presented at ABHUG 2023.

Aging high-pressure headers

HRST’s Brian Craig stressed the importance of careful assessment for aging high-pressure headers. He began with a key point for all: We are seeing an increase in failures because of the accumulation of cycles and operating hours on 20-year-old equipment. Specific to HP superheater and reheater headers, he added the factor of risk to plant personnel.

“Header geometries include tube hole penetrations, branch connections, hanger lug welds, and end cap welds, all of which can create failure risk areas,” Craig explained. “Some OEM header end caps are a serious concern, because of their weld joint design.”

“In general,” he said, “failure risk is elevated in units that are more than 15 years old or have more than 1000 starts.”

Craig walked everyone through specific examples recently seen by HRST and stressed that “some have been analyzed for root cause, and some have not.”

He offered an example where the end cap was correctly manufactured according to the ASME Code (and acceptable to AS1228/AS1210) but failed after more than 2000 cycles. The root cause was a lack of fusion that cannot be avoided in this design. This led to a stress concentration at the gap root where a fatigue crack was able to initiate and propagate to failure (Fig 9).

Other end plate failures occurred in the same HRSG, and some of these liberated, posing a significant risk to personnel.

While the HRSG presented had seen a significant number of cycles, many HRSGs in Australasia (and globally) will approach these cycling levels soon and those with this particular design should inspect for this type of cracking.

Craig also discussed improper test locations and alternatives.

HP bypass-to-CRH connections

Chris Jones, Quest Integrity Aus Pty Ltd, discussed the integrity of HP bypass to cold-reheat pipe connections and described the best NDE methods for inspecting failures.

For an HRSG, the high-pressure bypass pipework can allow the gas turbine to remain on while the HRSG is pressurized (allowing main steam to bypass the turbine). But the associated pressure control valve can be subject to demanding operating conditions and erosion damage.

The failure mechanisms discussed included creep, fatigue, creep and/or stress relaxation cracking, fatigue cracks, water-hammer events, and resonance.

Jones’ recommendations for inspecting areas upstream of the pressure control valve and/or isolator valve included fluorescent magnetic particle, ultrasonic, replication, and hardness testing.

KinetiClean

Jeff Bause, president and CEO of Groome Industrial Service Group (US), was in Brisbane to introduce KinetiClean™, a new shockwave system for cleaning HRSG finned tubes.

KinetiClean uses a detonation-cord curtain (Fig 10) and automated high-pressure, high-volume air jet system to clean the tubes, followed by in-house debris removal and disposal services.

Benefits include reduced backpressure and increased heat rate, along with increased HRSG efficiency and flexibility. Bause presented specific case studies to support the use of this new cleaning system. He also explained Groome’s capabilities for SCR/CO catalyst cleaning, repacking and replacement; AIG retrofitting and cleaning; CO acid/water washing; ACC and cooling tower cleaning; and permanent sampling grid installation.

Other specific presentations included:

• Expansion joints and penetration seals (Dekomte).
• Ultrasonic flowmeter to monitor for attemperator spray leakage (Anderson and Duke Energy).
• Steam-turbine nozzle plates (HRL).
• Boiler-drum operational limit analysis (ALS).
• Pulverized fuel mill failure (Energy Australia).

Cycle chemistry and film-forming substances

ABHUG 2022 featured a Workshop on film-forming substances. The latest international activities were reviewed regarding the effectiveness, applications, and risks associated with these potentially game-changing additions to HRSG and boiler cycle chemistry.

According to Barry Dooley, several takeaway messages came through including:

• FFS can be effective in protecting water/steam-touched surfaces against FAC and other forms of corrosion, therefore reducing corrosion-product transport.
• FFS should only be used after all existing cycle chemistry program shortfalls are eliminated. If a station’s water chemistry is not known and optimized, experience shows that failures and damage can occur with both amine and non-amine FFS applications.
• Hydrophobicity is not a valid indication that FFS is protecting pressure-part surfaces.
• Owners take a substantial risk when feeding any product (including FFS) into the plant if the product’s constituents and potential interactions are not known.

David Addison, Thermal Chemistry, repeated the critical need for due diligence for any application of FFS in a combined cycle or other type of plant.

Dooley and others also provided and discussed IAPWDS, AUSAPWS and NZAPWS updates, including relevant Technical Guidance Documents issued and offered free of charge by the association (www.iapws.org).

Acknowledgements

Exhibitors in Brisbane were Duff & Macintosh/Sentry, Flotech Controls, hrl:, Mettler Toledo, Swan Analytical Instruments, and Talcyon.

Event sponsors were: IAPWS, hrl:, Swan Analytical Instruments, and Ecolab.

Steering committee

ABHUG is chaired by Barry Dooley, Structural Integrity Associates (UK), and Bob Anderson, Competitive Power Resources (US). Steering committee members in addition to Dooley and Anderson are the following:
David Addision, Thermal Chemistry, New Zealand*
Matthew Sands, CleanCo, Queensland**
Russell Coade, HRL Technology Group, Victoria*
Michael Drew, Australian Nuclear Science & Technology Organisation (ANSTO), NSW*
Armand du Randt, Genesis Energy, New Zealand*
Stuart Mann, AGL, Victoria**
Keith Newman, Synergy, Western Australia**
Charles Thomas, Quest Integrity, New Zealand**
—————————————————————————————————-
* Consultant
** Energy provider

European Technology Development Ltd (ETD) organized and conducted, virtually, the second High-Temperature Plant Materials, Inspection, Monitoring, and Assessment Conference (MIMA-2) last October (2022). Sponsors were AGTec GmbH, Austria, and the Institute of Materials, Minerals and Mining (IOM3), London.

Dr Ahmed Shibli, director, ETD Consulting, Leatherhead, England, and his staff organized and managed the event. Selected notes from the presentations follow. Conference specifics can be accessed here.

MarBN steel

As with the first MIMA conference in 2020, Session 1 explored ongoing research with MarBN steel, a 9Cr martensitic with additions of boron (B) and nitrogen (N). Development and testing of this steel, intended for high-temperature applications, is moving quickly in both Europe and Japan, and attracting strong attention worldwide.

The goal is to increase material high-temperature strength, life, and integrity for future ultra-supercritical powerplants, and apply these benefits to all high-temperature, high-stress systems. Combined-cycle HRSG plants fall into this category.

Research continues to focus on MarBN’s extremely sensitive heat-treatment processes during production. Click here for details.

A good MarBN status report was given in MIMA-1 (2020) by ETD’s David Allen, who listed temperature capability of P92 as about 20 deg C better than P91 and “expected temperature capability of MarBN to be at least 25 deg C better than P92.” He ended with this summary:

1. Today we can replace P91 with P92.
2. Tomorrow we could use MarBN for even greater security [material integrity and safety].

But the microalloying production process is extremely sensitive.

MIMA-2 presentations were quite specific. Steve Roberts, Goodwin Steel Castings (UK), looked at the UK version of MarBN steel (known as IBN-1) for wrought pipe manufacture noting that optimized heat treatment is required for best control of B, N, and creep performance. He stated that creep rupture testing has demonstrated a creep strength that is 35% to 50% greater than P92, resulting in a potential 25 to 30 deg C increase in operating temperature, and/or more fatigue-tolerant and thinner components.

Related UK-government co-sponsored programs are Implant (multi-partner project coordinated by ETD, Innovate UK project number 105769) and Impulse (Innovate UK project number 102468), both specific to next-generation MarBN steel for fossil plant use.

Commercialization programs are ongoing through various other government and industry co-funded programs such as Impact in the UK and Howeflex in Germany.

The German-funded Howeflex, a consortium of two turbine manufacturers (Siemens Energy and GE Power, Germany), a forge master, and two research organizations, is dealing with the upscaling of trial melt results of MarBN-family alloys for large rotor forgings with diameters of up to 1200 mm and typical weights representative of IP rotor forgings. Their presentation: Project Howeflex MarBN rotor qualification (9Cr-3Co-3W-B-N) for load flexible application (Figs 1 and 2). The results obtained so far indicate “promising properties” regarding the achievable forging quality and short- and long-term material behavior. Results show that:

• Large rotor forgings can be manufactured successfully with available steel-making technology at experienced forge masters.
• Mechanical properties have been determined including static strength, toughness, fatigue and creep properties, and complex creep-fatigue interactions.
• The design of rotors for flexible loading in combined-cycle applications, as well as for temperatures up to 1200F, is now possible.

An important distinction, raised during discussions, is that high-temperature research in Japan has reached and exceeded a working environment of 650C; some others target 620C.

The following presentations also focused, at least in part, on MarBN steel development:

• Role of inclusions on degradation in creep life and rupture ductility of ferritic powerplant steels (National Institute for Materials Science, Japan).
• Multi-component alloying element effects on solidification segregation in cast IMN-1 based CSEF steels (Univ of Birmingham, UK).
• Normalizing temperature selection for creep performance of advanced high-temperature alloy IBN-1 (Loughborough Univ, UK).
• Experience of P93 manifold welding under real fabrication conditions (Siemens Energy, The Netherlands).
• Development of Inconel alloy 740H and its application to supercritical CO2 powerplants (Special Metals Corp, UK).

Plant life assessment

A key benefit of MIMA conferences is learning specifics about life-assessment efforts for existing plants.

The Central Research Institute of the Electric Power Industry, Japan, began Session 2 with Prediction of long-term creep life based on short-term data of used Grade 91 steels. The focus was on how to deal with this experience limitation.

Masatsugu Yaguchi discussed life assessment using both standard and ultra-miniature samples. Both are effective, he said, but these issues remain:

1. We must assess long-term creep life using short-term data.
2. Test specimens taken from components are limited (destructive testing).

Those who want details of creep tests, along with equations, can request this presentation from ETD Consulting.

India’s largest energy conglomerate, National Thermal Power Corp Ltd (NTPC), looked at residual life assessment of critical piping systems. NTPC’s overall goals:

• Improve efficiencies of existing plants (71,500 MW installed).
• Operate the low-cost plants beyond their design lives.
• Ensure safe operation of plants nearing design life.

Focus is on piping systems operating in the creep range, where failures can be life-threatening and replacements have long lead times. NTPC has developed a hybrid set of code requirements using both Indian Boiler Regulations (IBR) and ASME B31.1. The company’s Bhaskara Santosh, Kumar Pudipeddi, and Vineet Kumar elaborated on the methods, as well as constraints and issues faced while finding a “prudent approach.”

Petroliam Nasional Berhad (Petronas) was also there to discuss the latest creep remaining-life prediction work on fired heaters in Malaysia. The premise: Most steam boilers operate within creep threshold temperatures and creep damage can only be assessed during shutdown through destructive methods. Therefore, an analytic digital solution is useful based on operating data and minimal retrofitting requirements.

Petronas is developing such a system in compliance with API 579, a program named F1RST™. This now will be validated using in-situ surface replication and hardness testing results obtained during shutdowns.

Italy’s INAIL (Workers Compensation Authority) walked through eight years of research and the drafting of a new standard for assessment of martensitic steels, by the Italian Thermotechnical Committee. Work is based on both P91 and P92 research.

Inspection and maintenance

With a combined-cycle focus, David Tuey of RWE Generation (UK) discussed Risk-based inspection and integrity management of HRSG headers and manifolds. “Modern HRSG designs can feature many hundreds of collector headers and manifolds located on the steam/water circuits and it would not be practical to carry out widespread inspection of all locations during the operational life of a normal site,” he noted.

RWE and member organizations of the Generator Safety Integrity Program in the UK are documenting a process to define best practices in applying risk-based inspection to HRSG headers. The document provides specific information on targeted inspection locations, appropriateness of specific inspection and NDT techniques, inferred condition on a hierarchical assessment of risk, sample sizes, consideration of wider fleet experience, and optimized record keeping.

One specific example used is Fig 3, noting that many large fabricated branches have proven to be a significant creep/fatigue risk. Use of forged components is a possible solution, based upon manufacturing expertise and material quality. Research on such alternatives is ongoing.

It is important to note that the host ETD Consulting has completed a detailed study which evaluates various risk-based management (RBM) practices, and makes recommendations for best practices. Also, ETD’s RBM procedure RiskFit, especially prepared for powerplants, consists of four risk-assessment levels, each of which can be carried out independently. Details are available through enquiries@etd-consulting.com. This was discussed in a presentation entitled Evaluation of various risk-based maintenance procedures and recommendations for best practices by ETD’s Feroza Akther.

And for “the un-inspectable?” John Trelawny of Uniper Technologies (UK) attracted attention with Inspecting the un-inspectable: A new inspection technique on complex geometries (Fig 4). His assertion: Visualization and characterization of flaws are possible with 3D Full Matrix Capture inspection.

His focus: socket welds within HRSG drains built to ASME standards. One plant in 2019, he said, experienced a “violent weld failure, considered to be one of the most significant steam/water leaks in the UK.” The current practice of surface inspection, he explained, is less than ideal.

The Uniper CCGT fleet contains vast quantities of socket welds (Fig 5) within the HRSG drains, vents, and interconnecting small-bore piping. One quarter of the steam leaks documented in the previous 2 years at one of the Uniper CCGT units have been from failed socket welds. The inspection management team was asked if a technique could be developed to locate these sub-surface flaws. This presentation detailed the work and outcomes of that development effort.

The chosen method uses Full Matrix Capture (Fig 6) and the resultant total focusing method. With this, the area of interest is pixelated, and each pixel has its own data (full matrix data). More on this phased-array technique is available within the MIMA-1 review. Access more by clicking here.

Snake or calamari? Paulo Debenest, hibot, Japan, explained that for 20 years, he and others have been working with robotic solutions to make inspection missions safer, faster, and more reliable. Note that hibot is a startup operation with roots in the Tokyo Institute of Technology.

Debenest’s presentation: Robotics applied to inspection of infrastructure: Examples of internal and external pipe inspection. “These are not human-like robots. They are smart tools” (Fig 7). “They might be ugly, but they are all very functional,” he said.

The snake-like unit shown in Fig 8 is a float-arm concept driven by air. And where the head goes (Fig 9), the rest will follow, able to overcome obstacles in crowded environments, he explained. Another, labeled the squid and specifically for boilers, is driven by water (Fig 10).

This presentation focused on pipe inspections in boilers, heat exchangers, and pipelines. Specific to HRSGs this applies to pipe inspection inside (boiler and cooling water pipes) and outside (pipe racks).

Post-presentation comments were many, including “We are glad to see this is going commercial.” Commercial use has begun in Japan and is moving quickly to a new base in Europe. At least four units will be in operation within Europe in 2023.

ETD’s Ahmed Shibli had the most pointed summary comment: “Absolutely brilliant, and useful!”

Other presentations in this section were:

• Digital transformation (DX) of boiler maintenance (Best Materia C, Japan).
• Risk-based maintenance for steam turbines and generators (GE, Switzerland).
• Inspection of pipelines with varying cross sections using a combined multidisciplinary and robotized solution (Engie Laborelec, Belgium).
• Possible microstructural resistant factors in P91 steel to the magnetic domain wall motion of electromagnetic inspection method (Nippon Steel, Japan).
• High-temperature corrosion data and mechanism of T122, Super 304H and HR3C in a 1000-MW ultra-supercritical powerplant (Xi’an Jiaotong Univ, China).
• Combined AC/DC potential drop for on and offline NDT for creep-life monitoring of pressure vessels (University College London, Matelect Ltd, and ETD).

Cracking and failure

Real-time damage monitoring software was presented by SRG Global, Australia. The impetus is damage caused by cycling. RTDMS allows plant personnel to link transient operation and associated operator actions to potential damage. The software accounts for different damage types simultaneously and in real time, which includes the primary creep rupture life, high-cycle fatigue damage, low-cycle fatigue damage and coupled creep fatigue damage. This gives an increased level of accuracy and confidence in predicting asset life.

Aron Abolis explained that “RTDMS uses a highly agile and optimized code that allows for online stress and damage monitoring with set alarm points when ramp rates exceed critical limits, where coupling between creep and fatigue occurs, or where fatigue exceeds a critical limit.”

ETD’s CrackFit, a software for crack assessment in pressure vessels and turbines, has been discussed previously in CCJ, attributed to ETD Consulting and others as part of the European Commission project on high-temperature defect assessment. This tool is designed to help engineers perform crack stability checks and defect/crack assessment of pressure vessels, piping, and turbine components operating at both low and high temperatures.

Crack initiation and growth for a host of components (pressure vessels, turbine rotors, plate and laboratory specimens) and commonly occurring crack geometries (embedded cracks, surface emerging cracks, various crack front shapes, etc) are incorporated into this software. CrackFit is also unique in that it contains a substantial crack growth database representing various combined-cycle/HRSG operation modes. Stuart Holdsworth of EMPA (Swiss Federal Laboratories for Material Science and Technology, Switzerland) joined this year’s presentation with direct plant experience.

ETD also presented Developing reliability framework for powerplants: Integrated RCM IV generation implementation.

Plant flexibility issues

EMPA’s Stuart Holdsworth then focused on turbines to discuss prediction of high-temperature component integrity subjected to flexible operation. He discussed testing history leading now to smaller specimen sizes and ultrasonic total-focusing method techniques.

The impact of cycling was also the basis for Cavitation during creep-fatigue loading by Rolf Sandström, KTH Royal Institute of Technology, Sweden.

Creep rupture models have become reliable, and cavitation is recognized as playing a central role during creep fatigue. But there is a need for more adjustable parameters when applied to cycle loading.

This presentation offered details on transferring the cavitation models to cyclic loading.

Film formers Anodamine and VPI Power Limited (UK) discussed Use of filming chemistry to improve corrosion protection of flexible operated powerplants. The filming chemical Anodamine has been used to mitigate flow-accelerated corrosion (FAC) and other internal corrosion threats. This presentation focused on plant experience at VPI plants originally designed for baseload operation.

Dosing at one European 2 × 1 combined cycle began in February 2019. The 2021 inspections discussed were tailored to further check the efficacy of Anodamine following positive initial results from the 2019 and 2020 interim and steam turbine outages. Overall, the 2021 inspection results indicate that Anodamine is providing good levels of protection against FAC (both single- and two-phase) and that a benefit to corrosion fatigue and offload corrosion mitigation are also likely.

Internal direct and remote visual inspections have shown build-up of protective Anodamine film in areas susceptible to both single-phase (that is, condensate, feedwater, and the economizer circuits) and two-phase (that is, evaporators, steam drums, deaerator, blowdown/drains and steam turbine exhaust circuits) FAC. Further, repeat ultrasonic thickness inspection of critical areas (that is, HP and LP evaporator risers) shows a general reduction in thinning rates compared to pre-2019 levels (Fig 11).

VPI’s Adrian Bailey explained “As a result of the good performance observed, it is judged that credit can now start to be taken via a reduction (gradual reduction of sample size rather than complete removal) of inspection scope for FAC damage at future outages.”

His conclusions:

1. Four consecutive outage inspections show progressive improvement in surface passivation/cleanliness; off-load corrosion has been mitigated.
2. Less iron transport to boilers during two-shifting; reduced boiler blowdown rates.
3. Cleaner boiler tube surfaces; better heat transfer.
4. Clearing out of historical oxide deposits; no acid cleans needed.
5. Ongoing FAC mitigated at HP risers, according to remote inspection and NDT; huge cost saver in not replacing HP boiler risers.
6. Both operations and management recognize the benefits of Anodamine.

Access more on film-forming substance development and experience here.

Plenary

With a look toward ongoing and future operation of combined-cycle HRSG plants, Ian Perrin, Triaxis Power Consulting (US), discussed HRSG design challenges: Material and mechanical integrity. HRSGs are being paired with next-generation gas turbines that are larger and required to operate at higher pressures and temperatures. This is accompanied by increasing demands for rapid start and flexible operation, as well as baseload operation at high temperature and pressure.

“This puts significant demands on material selection, which must consider fabrication practicality alongside durability,” he said.

Perrin outlined the key challenges:

• Grade 92 replacing Grade 91.
• Large-bore piping branches.
• Long-term tube life.
• Stainless-steel tubes and headers.
• Dissimilar metal welds.

He focused on high-temperature components.

Examples were provided to illustrate challenges and to identify where detailed analysis can help optimize designs.

Also discussed were some of the challenges related to Codes and Standards, which are struggling to keep pace with design demands and material developments.

The needs for future creep-strength-enhanced ferritic materials and integrity assessment methods were also covered.

Large bore piping branches and tees are susceptible to creep failures, in part because of the proximity of welds and thickness transitions, and forged replacement components are not always what you expect, and could be difficult to inspect. Simply meeting the ASME Boiler & Pressure Vessel Code, or others like it, does not mean acceptable material integrity, particularly with some new advanced materials.

Perrin also covered stainless-steel tubes and headers, temperatures and thermal fatigue, and dissimilar metal welds.

A few summary comments and caveats:

• High-temperature materials in modern powerplants create challenges.
• Meeting the Code does not mean acceptable material integrity.
• Codes assume materials are tough, ductile, and damage tolerant. Creep-strength-enhanced ferritic steels may not be!
• High temperatures (creep) and cycling (fatigue) are often not considered at the design stage.

ETD’s David Allen addressed the Role of materials design data in life assessment of high-temperature plants, breaking life assessments into two basic categories:

1. Specific problems where high-risk items may be damaged, faulty, substandard, or subject to excessive stress, temperature, or corrosion, and,
2. Generic concerns where all items of a specific set (for example, Grade 91 pipes and headers) may be approaching end of life.

In the second, you may need to assess total life rather than simply remaining life, he explained.

Both begin with root cause analysis.

Some interesting cautions (among many listed):

• When a high-temperature plant is designed, all Codes and Standards specify procedures which take account of materials performance data and enable the plant to be designed for a specified design life. This commonly assumes steady operation and hence ignores plant cycling issues, but is otherwise intended to be conservative.
• More recently, cost pressures and climate-change concerns have led to the development of advanced ultra-supercritical (USC) plants, and have promoted less conservative selection of materials and operating temperatures (for example, Grade 91 at above 580C, Grade 92 at 600-610C or higher). Generic wear-out risks could thence come to the fore as a USC plant ages.
• End users should take the lead on this. Manufacturers will not.
• Don’t just rely on what the alloy developers did in the distant past. Do the work again, update the data, and strive toward more precise and credible conclusions.
• Drawing any valid generic inferences from data on ex-service materials is fraught with difficulties. Testing ex-service items is only useful when solely the specific sampled heat is to be assessed.
• If you conclude that generic “wear-out” may be imminent, keep in mind that replacement of high-temperature pipework and boiler header systems will take time.
• Finally, don’t start by being complacent, and then flip into panic mode when something goes wrong.

The editors recommend obtaining a copy of this presentation from ETD.

Also in the plenary session:

1. Life evaluation and research subjects for safe service of 9Cr steels used in USC powerplants (Univ of Science and Technology, Beijing).
2. Activities to advance residual life-evaluation techniques for highly aged powerplant boiler materials in Japan (Tohoku Univ, Japan).

Attend MIMA-3 this fall

ETD announces the third international MIMA Conference, an in-person event, “Sustainable Power Generation: Materials, Inspection, Monitoring, and Assessment,” will be held Oct 17-19, 2023 in London. Get the details at https://www.etd-consulting.com/conferences/.

HRSG Forum debuts under the Power Users umbrella, June 12-15, in the Renaissance Atlanta Waverly Hotel & Convention Center. Bob Anderson, who has moderated the lion’s share of power-industry meetings focused on the information needs of HRSG owner/operators for the last 25 years, will be at the front of the room once again. Although the pandemic kept Anderson off the live stage for the last three years, he continued to serve the user community, broadcasting worldwide via the web on Channel CCJ.

The upcoming meeting and vendor fair will be packed end-to-end with information of incomparable value to users, consultants, and services providers. All three segments of the industry qualify for participation in all sessions. The long-awaited event begins on Monday, June 12, with two special workshops; a traditional conference program—one reminiscent of past HRSG meetings with Bob Anderson—airs Tuesday and Wednesday. EPRI Day is Thursday, focusing on the research organization’s comprehensive work in the fields of HRSGs and high-energy piping.

Here’s an overview of the four-day conference:

DAY ONE SPECIAL WORKSHOPS

The morning workshop focuses on water, specifically the importance of film-forming substances (FFS) in the modern world of powerplant operations. Barry Dooley of Structural Integrity (UK), the workshop moderator, will make the introductory presentation to bring attendees up to speed with a backgrounder on FFS, relatively new technology for powerplants in North America. Dooley, a member of CCJ’s Editorial Advisory Board, has been sharing his FFS experiences with the periodical’s subscribers for five years.

Several speakers—users and chemical suppliers, follow Dooley digging into the details of powerplant experience both here and in other countries. To learn more about the program, click the link.

The afternoon workshop, moderated by Jeff Henry of Applied Thermal Coatings, respected worldwide for his knowledge of boilers, materials, welding, and the ASME Code, will speak to the following:

• Tools for supporting the safe, efficient operation of aging high-energy piping.
• Creep damage experienced by operation of elevated temperatures.
• Structure of welds and damage in welds at elevated temperatures.
• Characterizing indications found in welds and their size and orientation.
• Understanding repair objectives.
• Proper excavation of damage.
• What the industry-wide loss of expertise means for plant owners and operators.

Before you pack your bags for the HRSG Forum meeting in Atlanta, be sure to do your homework. Absent a textbook, thumb through back copies of CCJ to jog your memory. The more you know, the better organized you are, more value you’ll extract from the meeting. There aren’t many opportunities to access directly the knowledge held by Anderson, Dooley, and Henry, as well as other experts on the program—without later receiving an invoice.

DAY TWO and THREE HIGHLIGHTS

• HRSG steam-vent silencer safety inspections.
• Innovative tube repair technology.
• Ultrasonic detection of spray-water leakage.
• HRSG safety/relief valve maintenance.
• HRSG damage monitoring system.
• Update and stats on HRSG cycle-chemistry control and FAC.
• NDE and inspections for the aging HRSG fleet.
• Attemperator inspections and repairs
• HRSG tube-failure analysis shared by the Qurayyah combined-cycle.
• Replacing HP evaporators.
• Wireless monitoring system for high-energy piping.

DAY FOUR

Learn from the details shared by EPRI from its HRSG and piping program—including the following:

• Industry challenges, in particular the loss of expertise and what this means to plant owners and operators.
• Recent activities with high-temperature components.
• Safety alert: State of knowledge and screening methodology for header endcaps.
• Mitigating damage related to attemperators/desuperheaters.
• Recent activities with low-temperature components.