The experiences shared at the Australasian HRSG Users Group (AHUG) annual meeting have been chronicled for the last decade in CCJ. Beginning here, and going forward, you’ll also benefit from the lessons learned/best practices of fired-boiler owner/operators facing technical and operational issues closely related to those in HRSGs.
The first annual meeting of the Australasian Boiler & HRSG Users Group (ABHUG), chaired by Barry Dooley of Structural Integrity Associates Inc, attracted 75 participants from Australia, Japan, New Zealand, Thailand, UK, and US to the Brisbane Convention & Exhibition Centre, Oct 30-Nov 1, 2019. About half of the participants were users.
The agenda included 21 technical presentations and a welding workshop that brought together multiple experts to present on the latest standards, welding of service-exposed materials, ligament cracking in superheater headers, cold repair of Grade 91 material, etc.
The steering committee voted to expand the attendee base because many of the failure/damage mechanisms occurring in HRSGs are similar to those found in conventional boilers, and that other equipment in combined-cycle plants have many of the same issues as those in fossil stations. Dooley told the editors that ABHUG will continue to concentrate on HRSG aspects but will add the technical areas common to fossil and HRSG plants—such as tube failures, steam turbines, high-energy piping, valves, etc.
The chairman characterized the inaugural ABHUG meeting as a highly interactive forum for the presentation of new information and technology related to HRSGs and boilers, case studies of plant issues and solutions, and open discussions among users, equipment suppliers, and industry consultants.
ABHUG is supported by the International Association for the Properties of Water and Steam together with the local national committees of IAPWS in Australia and New Zealand. It is held in association with the European HRSG Forum (EHF) and the US-based HRSG Forum with Bob Anderson.
Two gold sponsors—Bang&Clean Technologies AG and HRL Technology Group—and seven exhibitors—ALS, Duff & Macintosh and Sentry, Flowtech Controls, Quest Integrity, Mettler Toledo, Optimum Control, and Swan Analytical—provided financial support (see end notes).
- Pitting corrosion
Tarong. The 1400-MW Tarong Power Station in Queensland was commissioned in the 1980s with four subcritical coal-fired boilers, each rated at 350 MW. The primary reheater is located in the back pass, composed of two sections with horizontal tube elements—126 total elements across each boiler’s width. Design steam conditions at the headers are 935F/765 psig (610 psig operating).
In 2012, owner/operator Stanwell Corp announced plans to shut down two units for two years because of reduced demand and low wholesale electricity prices. When the cost of natural gas rose in 2014, Stanwell returned the units to service.
Following a Unit 3 major outage in 2018, four reheater tubes across different elements experienced primary failures. Two months later, another two tubes across different elements experienced similar failures (Fig 1).
An EPRI roadmap was used to identify, evaluate, and solve the boiler-tube failure issue—EPRI Technical Report 3002010388, “Boiler and heat recovery steam generator tube failures: theory and practice.”
Stanwell adopted the process described in the EPRI document to determine the following:
- Primary failure location.
- Primary failure mechanism.
- Primary failure root cause.
- Factors contributing to the primary failure mechanism.
Both visual and tube-removal analyses revealed the key factors and mechanism:
- Corrosion only in the bottom half of the tube.
- Partial or complete through-wall pitting corrosion on welds and randomly on tube parent material.
- Pit sites open or filled with reddish or reddish-brown corrosion products.
In this case, all pit sites were located in the center sections of horizontal tubes (Fig 2). There was no external erosion, apart from steam erosion, attributed to the primary failures. Stanwell’s conclusion: Pitting corrosion attributed to inadequate layup was the primary failure mechanism.
Investigations included metallography, water sources and system chemistry, shutdown procedures, reheat spray operations, and pressure excursions.
Stanwell’s G Wang stated the primary failure root cause as “condensate formed during shutdown. The accumulation of condensate at the bottom of the tubes with oxygen being subsequently introduced during the shutdown provides a stagnant oxygenated-water environment for pitting initiation and development.”
Wang also reviewed relevant contributing factors:
- History of shutdowns, durations, and long-term preservation practices.
- Chemical excursions (condenser leaks, for example).
- Corrosion at welds caused by weld/tube galvanic attack.
- Sagging at center of horizontal tube elements.
- Pressure excursions.
- Lack of protection during washdown after tube failure events.
Stanwell’s long-term strategy includes repair/replace option reviews, enhanced inspection methods, and a direct action for shutdown of “drying the tubes before condensate formation and maintaining RH below 35%.”
Wang then listed specific future inspection methods:
Welds. Cobra PAUT/ToFD (ultrasonic phased array and time-of-flight diffraction) ultrasonic.
Tube parent material. EMAT and FMC/TFM (electromagnetic-acoustic transducer and full-matrix capture/total focusing method).
During the discussion period, Chairman Dooley stressed the benefits of “sharing issues across combined-cycle and conventional fossil plants. Corrosion can be initiated during inadequate layup. This and other presentations illustrate just how severe pitting damage can be for both combined-cycle and conventional units.”
Kogan Creek Power Station, owned by CS Energy Australia, is a 750-MW supercritical coal-fired unit in Queensland, commissioned in 2007. In 2018, a tube leak occurred in the horizontal reheater section of the boiler, causing significant secondary damage and an 11-day forced outage. Fifteen tubes were replaced (Fig 3). The primary failure was a pinhole leak that developed near the middle of the horizontal run, at the bottom of the tube. The failure mechanism was identified as pitting corrosion.
Galvanic corrosion caused by moisture and oxygen led to rapid pitting. CS Energy’s Luke Smith explained that “the base of the pit, low in oxygen compared to the wet metal surface, became anodic. This occurred during shutdown when steam was allowed to condense (not during operation). In this particular case, a low level of sulfate also was present.”
In 2019, the entire reheater was replaced (three banks of 196 elements), extending an outage from 56 to 77 days.
The long-term prevention objective is to replace dry steam with dry air (dry storage). Smith explained that “once the reheater has had about five changes of volume, the HP bypass is opened to allow air to flow to the superheaters. The flow of dry air will be maintained for the duration of the outage if possible. We aim for an RH below 30%.”
This was followed by participant discussion on use of dehumidified air (DHA) for systems including reheaters and steam turbines. Dooley again offered input: “Unfortunately,” he said, “the application of DHA is usually added after the damage has occurred, instead of proactively beforehand.”
Common tube-failure approach. Pitting issues from improper shutdown protection can affect both the HRSG and the steam turbine, as discussed in the recently released report “Trends in HRSG Reliability: A 10-Year Review,” by Dooley and Bob Anderson of Florida-based Competitive Power Resources (and chairman of the HRSG Forum with Bob Anderson). The Combined Cycle Journal’s summary of this valuable reference work appeared in CCJ No. 61, p 44.
In that report, the authors discuss the importance of root-cause analysis and HRSG tube-failure (HTF) programs. They caution that a tube failure often is assumed to be a bad weld, but warn that if tube removal and analysis are not performed the problems likely will continue.
This review continues: “In many cases the actual root cause may be due to a cycle chemistry deficiency, design feature, or operating practice that has repeatedly inflicted corrosion, corrosion fatigue, or thermal-mechanical fatigue damage in the failed tube and its neighbors.”
It is indeed complex. “The only way to ensure that the corrective actions are taken and will prevent a tube failure from recurring is to remove the initial failure site, have the actual failure mechanism identified via a metallurgical laboratory analysis, then determine the root cause of the failure.”
The tube failure discussions at ABHUG gave examples of effective root-cause analysis.
- Boiler-tube failure mitigation
Stewart Mann, responsible for asset integrity at AGL Energy Ltd, discussed his company’s corporate-wide approach to mitigation of boiler-tube failures at both gas- and coal-fired units. The AGL tube-failure reduction program records each failure mechanism and root cause, the specifics of each repair, and all planned actions.
The approach-to-mitigation discussion raised many logical but often bypassed steps needed for a complete analysis program. First, the AGL program is based on the model in EPRI Report 1013098, “Integrated boiler tube failure reduction/cycle chemistry improvement program (2006).” Second, it is produced as a fleet-wide AGL standard. Third, it is now integrated with AGL’s cycle-chemistry standard.”
The goal is to apply risk-based systems to minimize repeat failures, reduce failures from new mechanisms, and decrease the number of failures at new locations for known mechanisms. As Mann stated, “Unless we have recently changed how we are operating, credible inspection and analysis should enable us to confirm whether a potential mechanism is an issue at our plants.”
Data gathering includes failure mode and mechanism, root cause, extent of damage to the tube with the primary failure, tubing in the vicinity of the primary failure (secondary damage), and the surface areas along the leaking tube.
“AGL is careful to preserve removed tubes for metallurgical examination, and to always consider possible removal of additional samples,” added Mann. Also, every repair has strict QA provisions for:
- Welder certification.
- Inspector certification.
- Welding procedures.
- Welding materials.
- Selection of tube materials.
“All of this,” he continued, “requires good collaboration among trading, operations, and engineering.”
Each ALG site now has a dedicated boiler-tube failure-reduction program team that includes:
- Site boiler engineer (lead).
- In-service inspector.
- Principal engineers from the company’s technical services group.
- Operations input.
- Maintenance input.
- Complete failure and inspection history reviews.
- Development of inspection strategy and action plans.
“All reports are finalized, distributed, and accessible to everyone,” he said.
Mann offered an interesting concluding challenge: “Play devil’s advocate, and be prepared to question any prevailing norms.”
- Update on IAPWS activities
International updates were provided on cycle chemistry, instrumentation, and flow-accelerated corrosion (FAC), plus a review of the recent IAPWS Technical Guidance Documents (TGD) in those areas—including the following:
- Applications of Film-Forming Substances (TGD11-19).
- Air In-leakage in Steam-Water Cycles (TGD9-18).
- Chemistry Management in Generator Cooling Water during Operation and Shutdown (TGD10-19).
Find these and other TGDs at www.iapws.org.
- Update on thermal transients
International updates on HRSG thermal transients associated with attemperators, condensate return and superheater/reheater drain management, and bypass operation were well received. Get details in the special report referenced earlier, “Trends in HRSG reliability, a 10-year review.”
- Hex chrome
A practical presentation focused on hexavalent chromium contamination of high-chrome materials in gas turbines, HRSGs, steam turbines (casing bolts), and steam piping was made by David Addison, principal, Thermal Chemistry Ltd, a frequent contributor to CCJ. He covered how and where hex chrome forms, the health risks it poses, PPE requirements, best work practices, proper disposal, etc.
Recall that chromium, a common alloy element in high-temperature/high-pressure steels used in powerplants, has multiple oxidation states. To illustrate: Chromium III is essential for human health; Chromium VI (hexavalent chromium) is extremely toxic. Chromium VI is also easily managed with standard industrial hygiene and personal protection strategies applied, combined with neutralization where needed.
Risks associated with welding of high-chromium materials are well understood in the industry, as are possible hazards during some chemical cleaning procedures.
Recently, however, hexavalent chromium has been identified on gas-turbine hot-gas-path components (Fig 4), steam-turbine hot external components (bolts), and on the external surfaces of hot HRSG/boiler piping.
For gas and steam turbines, there’s a link to calcium-containing anti-seize pastes often used on hot components.
The yellowish appearance can be misinterpreted as sulfur deposits from the fuel. “Bright yellow HRSG gas-side deposits should be considered a major warning sign, and treated with significant caution,” noted Addison.
Although understood and manageable, this alert involved one particular slide labeled “unconfirmed risk areas” and offered the following details:
Upper and lower crawl spaces with:
- High-chromium pipework.
- High-chromium liner places.
- Oxygen atmospheres.
- High temperatures.
- Insulation containing calcium oxide.
- Potential for rain water ingress that allows for calcium leaching.
- Superheater/evaporator upper and lower crawl spaces.
- Gas-turbine exhaust ductwork—mainly on the insulation side of plates.
- Digital twin
A new inspection tool for developing a digital twin of pressure vessels and other plant components using state-of-the-art imaging and image-capture technology was described. Chairman Dooley said an interesting application of the technology might be its use in the upper ducts of an air-cooled condenser (ACC) to view, without entry into the upper ducts (streets), the tube entrances and any associated FAC. He should know: Dooley is a member of the ACC Users Group steering committee and respected for his knowledge of ACC damage mechanisms. Visit www.acc-usersgroup.org.
- OGE index
A question/answer period included impromptu discussion of the oxidation limits for steels used in superheaters and reheaters. Dooley announced that a new index on oxide growth and exfoliation (OGE) is in preparation. It will discuss the formation of steam-side oxide, how the characteristics of oxide exfoliation vary from one material to another, and the various types of damage caused by different exfoliated oxides. OGE had piqued the interest of attendees at the 2019 meeting of the HRSG Forum with Bob Anderson as noted in the report for that event in the last issue (CCJ No. 62, p 42).
- Tube cleaning
Experiences with pressure-wave cleaning of fireside/gas-side surfaces in fossil boilers/HRSGs were shared at the same forum perhaps for the first time. Pressure-wave cleaning of HRSGs was a “hot” topic at the 2019 meeting of the HRSG Forum with Bob Anderson as mentioned in the report for that event in the last issue (CCJ No. 62, p 42).
ABHUG returns to the Brisbane Convention & Exhibition Centre next year, in early December. Follow the organization’s website for announcements.
Sponsor, exhibitor briefs
The sponsors and exhibitors active in ABHUG may be unfamiliar to readers in the Western Hemisphere. What follows are summaries of their activities:
ALS is one of Australia’s leading providers of asset reliability and integrity services geared to help power producers maximize production, extend asset life, and assure top operational performance.
Bang&Clean Technologies AG specializes in cleaning boilers and HRSGs by way of pressure waves created by closed and controlled gas explosions. The Swiss company’s patented system reportedly has been used in more than 20,000 cleanings since 2001.
Duff & Macintosh has specialized in the area of sample conditioning for the last 50 years. It is the exclusive agent in Australia Pacific for Sentry Equipment Corp.
Flotech Controls provides valve solutions, specializing in severe-service isolation and control applications.
HRL Technology Group focuses on laboratory testing, asset integrity, materials engineering, power and combustion performance engineering, and process and energy efficiency engineering.
Quest Integrity is active in the development and delivery of asset integrity and reliability management services.
Mettler Toledo Process Analytics specializes in inline analytical process solutions. Its Thornton unit provides state-of-the-art technology in conductivity, sodium, silica, chloride/sulfate, pH, ORP, ozone, dissolved oxygen, and TOC measurement.
Optimum Control Co represents a range of valve and actuator manufacturers and has a facility in Sydney to repair those components.
Swan Analytical Instruments offers a range of analyzers for pure, ultra-pure, and cooling-water applications—including pH, conductivity ORP, dissolved oxygen, silica, sodium, phosphate, chlorine, chlorine dioxide, bromine, iodine, ozone, and turbidity. CCJ