What plant owners should know about today’s materials before specifying pressure parts

Perhaps the most compelling takeaway from the HRSG Forum with Bob Anderson was “How innovations in steelmaking (sidebar) are changing the way pressure-part materials behave.” As attendees listened intently to Jeff Henry, one of the industry’s top materials experts, now affiliated with Applus+ RTD, a global leader in nondestructive testing, the details unfolded into a wakeup call. Henry also is chairman of the ASME Boiler & Pressure Vessel Code’s Section II (materials) committee.

Henry turned back the clock to 2015 when a Fluor representative alerted ASME to problems with some carbon-steel fittings and forgings at a work site. Unexpected cracking appeared during normal handling, at or below room temperature (Fig 1). All materials met ASME/ASTM specifications, but subsequent analysis showed poor toughness with brittle trans-granular fracture and nil lateral expansion. Re-normalizing did not restore toughness.

The following year, XGEN engineering’s inspectors at a nuclear site had troubling nondestructive examination (NDE) issues with stainless-steel and nickel-based materials. UT was showing false reflectors (indicating cracks). Inspectors were unable to transmit sound to critical areas.

Testing showed highly anisotropic mechanical properties (varying by measurement direction). Analysis also indicated accelerated growth rates for stress cracking in cast and banded structures in some well-known alloys.

Stated Henry, “Creep-strength-enhanced ferritic (CSEF) steels, such as Grade 91, are examples of what we consider advanced alloys for boiler and pressure-vessel applications.” He then added, “Implications for advanced alloys could be the most significant, particularly for those at elevated temperatures and under frequent cycling. These materials have more complex metallurgy, so small changes in their composition, heat treatment, or processing may produce significant adverse changes in properties.”

So what are we seeing and what are the implications?

We know there are potential changes in material behavior, but all of this is not yet clearly understood. Again in the past, OEMs filled part of this knowledge area with dedicated laboratories and veteran personnel, but they can no longer fill this role.

Material suppliers (steelmakers) are concerned with delivery and production costs.“Also,” Henry added, “most suppliers will charge a high premium to produce materials with more restrictive requirements for individual purchasers.”

Recent work at EPRI has focused on the effect of residual elements on both long-term creep rupture strength and ductility (Fig 2).

One finding might be even more concerning: “Significant differences in both strength and ductility have been found in different heats of Grade 91, all of which had been manufactured in the same manner and operated under exactly the same conditions.”

Henry then turned to compositional issues specific to Grade 91, noting ASTM’s requirements adopted in the mid-1980s. Details for more restrictive requirements (residuals) are now incorporated in a new Section I code case that was approved in 2016. And in February 2017, the responsible ASME groups voted to reduce allowable stress values for the existing Grade 91 steel (Fig 3). Other studies and potential actions are ongoing.

One message was clear: Don’t panic, but do be aware!

Although there were many questions and focused discussions during Henry’s presentation, there were many “still looking” statements. The primary conclusion: “Effects of fabrication processes on advanced alloys are not yet clearly understood,” but we all need to stay informed. We need to observe.

Henry summarized the implications: “There is no question that the concerns raised for performance of Grade 91 apply to all CSEF steels, and to other advanced alloys.” All codes will need to consider these “unintended consequences.”

In a more pointed remark, he noted that “Action has been taken on Grade 91 not because it is the only advanced alloy adversely affected by the changes, but because it is the only alloy that has received the attention necessary to define some of the required actions.”

He concluded with a look ahead, and the need to do the following:

      1. 1. Better understand these changes.

      2. 2. Implement the necessary Code changes.

      3. 3. Better define what constitutes critical service.

Closely related topics of personnel safety and plant downtime also were discussed.

Lively dialogue followed on the status, outlook, and use of 91, 91 Type 2, and 92. Topics included ASTM material and product processes, general upgrading of specifications including OEM’s lists of supplementary requirements (based on experience), and the troublesome challenge of management’s fallback: “If it’s not in the Code, we don’t need it.”

Time will tell.

Steelmaking evolves, impacts product quality

Traditional steelmaking reduces iron ore in a blast furnace to get high-carbon molten iron. This is desulfurized and charged into a furnace; carbon content is reduced and alloying elements are added. Residuals like sulfur, phosphorus, copper, and tin (even some dirt) exist, but at tolerable levels that are not worth the high cost of removal. At the known and allowable levels, these residuals should not change material behavior. Simple enough.

Now add some more-modern activities. Quantities of scrap metal are charged into an electric furnace, saving time and money. But the potentially pesky residuals are now determined more and more by the scrap content. According to Jeff Henry of Applus+ RTD, “The number of residual elements—such as copper, tin, chromium, nickel, niobium, and titanium—have increased with the greater use of scrap.”

But even residuals are a complex topic. In carbon steel, chromium is residual. In Grade 22, it is alloying.

Now more progress. Years ago, hefty ingots were sent out from the mills as the main starting point for steel parts. The industry knew that as these large ingots slowly cooled, their cast structure became a mix of grain structures with pronounced macro and micro segregation. Breaking down the ingots into sheets improved product homogeneity. Knowingly, the process chain added several mechanical steps with intermediate heat treatments. This produced a relatively consistent and uniform structure and composition.

But today there’s more continuous casting, again saving time and money. Molten metal is solidified into intermediate shapes more closely aligned with the final product. On the positive side, the continuously cast products cool more quickly, eliminating much of the macro segregation and structural variety. But the effects of micro segregation and finer-scale structural inconsistency remain.

Because the product is now close to final size, the amounts of mechanical working and heat treating are reduced, and some remnants of the original cast heterogeneous structure can move freely into the final part. And don’t forget the pesky residuals.

More on materials

There were two additional presentations focusing on materials—one by Jean-Francois Galopin of CMI Energy, who used WebEx technology to deliver his message and answer questions from Brussels, the other by Kent Coleman of EPRI.

Galopin discussed higher efficiencies and the resulting higher exhaust gas temperatures entering the HRSGs. “We are already operating,” he noted, “at the upper limits of Grade 91 materials.”

CMI’s recent involvement includes Bouchain in France and Hamitabat in Turkey, both high-temperature plants now in operation. He is also involved in advanced ultra-supercritical installations.

His discussion covered material selection and design, weldability, steam oxide resistance, allowable stress, and Code approvals (both ASME and EN). For the highest temperatures, he discussed Super 304H, good for resistance to stress corrosion cracking and stress relaxation cracking. Galopin also reviewed experience with headers mode of Grades 91 and 92. To show variety and characteristics he discussed Super 304H (fine grain structure), TP347H (course grain structure), and high-cost Incoloy 617.

Specific to maximum allowable stress/creep and impact on design, he presented the data for component thickness and cycle fatigue performance (Fig 4).

Other items included header mockups for research, welding procedures, and metallurgical aspects for dissimilar welds, and alternative weld locations. In one summary Galopin stated, “Welding procedures and metallurgical aspects of dissimilar welds exist, but long-term thermal cyclic behavior of dissimilar welds is not well established.”

He concluded with finite element analysis and a focus on cyclic behavior.

Questions included the pros and cons of horizontal and vertical HRSG design, cold metal working and shot peening, and stress-relief applications.

EPRI’s Coleman discussed issues with CSEF piping, including Grade 91. He first noted several design and manufacturing flaws that have led to premature damage in Grade 91 piping systems including:

      • Dissimilar metal welds.

      • Improper materials.

      • Fabricated fittings and wyes.

      • Fabrication/design errors.

      • Soft materials.

      • Longitudinal seam risks.

      • Thermal fatigue at sky vents and drip pots.

      • Lower alloy filler metal/base metal with and without heat treatment for attachments and lining lugs.

      • Use of high nickel and manganese fillers for improved toughness.

Many of these practices, he stated, “are not prohibited by Code.” Similar to the discussions with Henry, therein could lie part of the problem.

For specifics, Coleman stated that “dissimilar metal weld failures can be quite dangerous and can occur between Grade 91 and austenitic alloys as well as lower-alloy ferritic materials.” Main areas of concern include flow elements, thermowells, RT plugs, bypasses around stop valves (warmup lines), and material transitions (Fig 5).

Coleman also highlighted longitudinal welds, stating that current reinforcement rules are unacceptable for materials in the creep range where HAZ damage is likely.

He also questioned various cross and attachment welds and then offered some specific suggestions to minimize the problems:

      • Require lifting and alignment lugs to be nominally the same composition as the base metal to which they are attached.

      • Follow PWHT requirements.

      • Protect in a dry environment until PWHT is performed.

      • Ensure PWHT on lifting lugs before they are used or stressed.

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Drones: Consumable inspection tools with financial benefits

Scott Wambeke, principal HRSG engineer, offered a detailed review of Xcel Energy’s new indoor unmanned aerial system (UAS) tools, a/k/a drones, at the HRSG Forum with Bob Anderson (Fig 1). Although the company has used drones outside to survey transmission systems, pipelines, and other difficult-to-traverse systems, Wambeke’s team concentrates on what’s inside the plant—more specifically what’s inside the boiler. He was at the meeting to explain the experiences, successes, and lessons learned as his crew launched and improved these new inspection tools.

The Xcel team has come a long way in two years of  flying, beginning in large coal-fired units looking at tube-wall slag and burners. Skill and precision have progressed rapidly, and the team is finding comfort and new applications inside the more restrictive HRSGs.

Wambeke clearly labeled drones as consumable inspection tools. In corporate terms the cost is very reasonable. And the savings in time, equipment and labor can be enormous. Scaffolding alone, in a large boiler, can cost up to $100,000 to deploy.

“Our primary purpose is to have a safe, compliant, and efficient internal UAS program that saves both time and money,” he explained. Wambeke’s overview stressed how these systems offer immediate access to information so engineers and maintenance personnel can quickly determine needs and assess what conditions they might be facing. It’s a decision-making tool on whether or not they need to expend further time and effort, and where to focus if they do. 

And it’s nearly spontaneous. When called, the UAS team can be up and running quickly (in as little as 30 minutes).

“We’ve had our challenges,” said Wambeke, “and we know that personnel safety is a primary concern.” The drones can, for example, be pulled by draft into tube walls, or lose power and just drop. They can also veer toward objects and people, and can move horizontally at up to 50 mph. But Xcel adds a “handler” who is equipped to “swat them down.” Think pesky bats.

For the pilot, stress levels can also become high. “Sometimes it’s like driving a car in a snow storm,” stated Wambeke.

But the immediate and recorded inspection results offer great benefits.

Xcel began with a DJI Phantom 3 Pro, modified with carbon-fiber blades, blade guards, and eventually LED lighting. At times, a simple sheet of plywood served well as a launch site.

Most camera systems are reasonably priced, as are memory cards. Wambeke offered a warning on the latter: “Have multiple cards,” he cautioned. “The last thing you want is to lose a memory card in an ash pit or bunker.” So the team replaces cards at each landing. “It’s OK to lose the drone,” he said, “but you don’t want to lose the data.”

There have been other challenges and a few crashes with various causes including HRSG liner bolt snags, draft-induced collision with catalysts, and the occasional yaw malfunction (software-induced). But tabletop repairs can be fairly simple.

Other restrictions become obvious through experience. Examples include:

      • Flight time is 10 to 12 minutes per battery.

      • Minimum space requirement is normally 6 to 12 ft.

      • High dust and ash loading can make flight manipulation a challenge (think snowstorm).

      • Any draft above 5 mph makes flight difficult in tight areas.

      • Standard lighting can be limiting; Xcel has modified and continues to improve theirs (Fig 2).

Challenges. Commercially available drones are designed for outdoor use, and most are equipped with GPS. This positioning system does not work inside a steel boiler. So Xcel is working with others on a possible artificial GPS. They are also working on new object-avoidance software.

Wambeke offered a list of other challenges, including these:

    1. 1. Inspector/operator training and stress can be limiting factors, especially with crew members having other responsibilities and with long intervals between flights.

    2. 2. Safety consciousness around people is critical, and personal protection is required.

    3. 3. Communication between pilot and spotter requires training and coordination.

    4. 4. Drifting in elevation and yaw are nearly unavoidable (at present).

    5. 5. Maintaining a precise position is difficult, especially with ambient drafts.

    6. 6. Compass interference can prevent takeoff; an angle iron near the compass seems to help.

Next generation. Wambeke gave some specific ideas for the next fleet of craft. These included:

      • Continuous lighting improvements.

      • New versions of wall-roller attachments.

      • Upgraded blade guards and unit cages (Fig 3).

      • And eventually, very highly refined, custom-built systems.

Xcel collaborates with universities on some of these developments and shares information with other utilities and users.

Although the possibilities seem endless, Wambeke stressed that drones are still an inspection tool—not (yet) a service and repair tool. But again, the possibilities seem endless.

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Mark your calendar: HRSG Forum with Bob Anderson, Houston, Mar 5 – 7, 2018

Every buying decision—business and personal—should be supported by an objective evaluation of the alternatives. Meeting participation included. Tight budgets, staffing constraints, and the pressure on you to continually improve plant performance make conference selection particularly important. Most station personnel in responsible positions can get authorization to attend only one meeting a year these days. Obviously, you want to participate in the conference offering greatest professional value.

Heat-recovery steam generators may be the most undervalued assets in combined-cycle plants. Simply put, the HRSG is the big hunk of metal that sits between the “money machines”—gas and steam turbines—to make steam. Certainly not out of sight, but typically out of mind. You regularly hear about long-term service agreements (and other such arrangements) for gas turbines, steam turbines, and electric generators, what about the HRSG? Tubes fail, drum-to-downcomer welds crack, flow-accelerated and under-deposit corrosion consume metal, heat-transfer surfaces foul, etc, but that’s for the plant to deal with—based on the editors’ observations.

It’s important to expand your level of expertise on HRSGs as plants are forced to follow increasing levels of renewables generation. More than just a few combined cycles now are starting multiple times daily and doing so more rapidly. You have a pretty good idea about the cost of operating and maintaining rotating equipment in such a stressful environment because of the formulas the LTSA provider uses to schedule outages, but what about the HRSG? What should you be doing vis-à-vis monitoring, evaluation of pressure parts, regular maintenance, water chemistry, etc, to ensure a HRSG failure does not take out your combined cycle suddenly and put a dent in earnings?

With 2018 planning and budgeting already underway at most generating companies, perhaps the articles in this issue of CCJ ONsite, developed from material presented at the first HRSG Forum with Bob Anderson earlier this year (February 28-March 2, Charlotte), will help in your evaluation of conferences that you might attend next year. Consulting Editor Steve Stultz, who spent more than three decades at boilermaker Babcock & Wilcox Co, covered Anderson’s inaugural meeting, which featured participation by some of the leading HRSG experts, and wrote the articles below:

Development of the 2018 program for the HRSG Forum with Bob Anderson is ongoing. An unreleased draft of the preliminary program reflects a diversity of subject matter, with presenters divided among owner/operators, consultants, and products/services providers. In addition to formal presentations by some of the industry’s leading experts, open-floor discussions are sprinkled throughout the program to dig deeper into topics of interest to attendees. Access to vendors/sponsors will be during breaks, receptions, and meals throughout the meeting.

Anderson released the names of these three 2018 presenters and their topics:

      • Barry Dooley, Structural Integrity Associates Inc, “Film-Forming Products for HRSG Cycle Chemistry.” Dooley recently chaired the “First International Conference on Film-Forming Amines and Products” and will report on bleeding-edge technology developments worldwide. To whet your appetite, read a recent CCJ article on the subject authored by EPRI’s leading experts on water chemistry. 

      • Robert Hassing, NEM, “Assessing the Impact of Modifying the Gas Turbine on the Combined-Cycle/HRSG Plant Before Making Changes to the GT.”

      • Scott Wambeke, Excel Energy, “3-D Printing to Create True-to-Scale Mockups of HRSG Components for Planning Repair Activities.”

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Presentations by owner/operators get high marks from HRSG Forum attendees

Four user presentations followed Scott Wambeke’s detailed review of Xcel Energy’s experience with unmanned aerial systems (drones) at the HRSG Forum with Bob Anderson. That material is summarized below.

Converting from simple to combined cycle. Yogesh Patel, Tampa Electric (TECO), discussed the recently commissioned Polk Unit 2 Expansion. The project began in 2012 with four existing 7FA simple-cycle gas turbines. The result would be a 4 × 1 combined cycle with new diverter dampers, auxiliary firing for an additional 30 MW per HRSG, and one 500-MW steam turbine. Cooling towers were added to support the new auxiliary equipment.

Patel clearly outlined TECO’s planning and assessment factors for all major project participants. The common theme was what he called a “blended team arrangement,” including hands-on participation by the owner/operator.

Also of interest, HRSG supplier bid criteria included all standard items, then added adaptability to cycling service, and ease of inspection and maintenance. Cycling design elements were therefore critical, as was full-system accessibility. For example, inspection ports, doors, and platforms would be installed in both the front and rear of the boilers.

Wide maintenance bays (23 versus 18 in.) were specified. Lower headers also required good access and room was specified under all modules for blowdown and drain inspection. As Chairman Anderson clearly acknowledged, “The industry is changing. We will have more cycling and inspections; we need more room.”

Discussion also took place (in general terms) on the cost impact of the added features. Methods of vendor quality control also were noted.

For resource selection, weaving a thread with Jeff Henry’s discussion on steelmaking, raw materials originating in certain countries were not allowed. Patel noted that the entire supplier list was scrutinized and evaluated. Weld qualification tests were mandated for P91 tube-to-header welds.

He then reviewed the construction management team setup, organized in part to strengthen owner/operator personnel for future involvement in field engineering and construction.

Tube cleaning. Jacob Pursley, Southern Power, gave a new perspective on HRSG tube cleaning by discussing both CO2 and an alternative, GE PressureWave Plus™ (developed by Bang & Clean Technologies AG of Switzerland).

Scaffolding is not required; after setup, pressure waves penetrate the tube bundles. With the unit offline, a lance is placed between the modules and a bag is inflated with a combustible mixture that is then remotely ignited. The pressure waves and tube vibrations remove the deposits. This is performed at various locations.

Pursley explained the process, site history with CO2 cleaning, and the decision to use this technology.

The HRSGs at Southern’s Plant Rowan F-class 2 × 1 Unit 4 were cleaned by ice blasting in 2015. Corrosion at the time was visible with excessive bridging of rust between the fins. Tubes are in-line. Backpressure was approaching the GT trip point of 24 in. H2O (original design backpressure was 15 in.). Three tons of debris were removed from each unit. Backpressure decreased by 3 in. for Train A and 2.5 in. for Train B.

But soon after the outage, Train-B backpressure increased to 28 in. Potential reasons were found in areas that had no personnel access (18 in. between modules). Pursley did point out that “previous ice blasting showed good results on the surface but various concerns were raised in the ability to access hard-to-reach areas, as well as depth of penetration.” He added, “We did not use a spreading or deep-clean method previously.”

Also, during an 86-day outage in 2015, Unit 4 had no dehumidification or layup, and there was rain “almost every week.”

For PressureWave Plus, cleaning locations were set in five horizontal and nine vertical spots. There would be at least four bangs at each location in the Train B HRSG. The primary target was the middle of Module 4, between HP Economizer 2 and the IP superheater section. Roof-access sky climber ports were installed for the lance rigging. The results: 14.5 tons of debris removed; back pressure decreased by 8 in.

Both units then were cleaned in fall 2016. The results: Train-A debris 24.8 tons and backpressure decrease of 9 in.; Train-B debris another 3.3 tons, backpressure decrease another 2 in.

Pursley then gave comparative results (both cleaning methods) for personnel required, backpressures, stack temperatures, and tons of debris. There were several questions—including the specific type of fouling, potential impact on seals and baffles, potential impact on catalysts, and theoretical depth of cleaning. Experience with this technology is growing and under review. An EPRI program or paper is being considered.

Penetrations and seals. In another site-specific example, the OG&E/Dekomte presentation focused on the 500-MW McClain Power Plant commissioned in 2001. Thermal and visual HRSG inspections in 2014 and 2015 showed several bellows and penetration-seal concerns.

McClain’s Benn Privett explained the inspection findings, ranging from sediment and debris at the bottom of an expansion joint to severely leaking penetration seals (essentially overheating the surrounding casing). There are 103 seals per HRSG at this plant. After initial inspection, Dekomte performed a complete thermal survey.

Side-wall penetration technology was reviewed first, noting various OEM installation types and retrofit options. Discussion then centered on converting OEM bellows to fabric (to increase flexibility), mechanical seals to fabric, and labyrinths to fabric.

Specific and detailed examples followed, supporting the conclusion that “each site and application requires a tailored solution.” These details also showed that “monitoring and annual inspection are critical,” to address small issues early rather than major problems later.

The McClain replacement project was 80% complete at the time of the meeting.

Dekomte’s Jake Waterhouse then discussed pumpable fiber insulation, defined as a mix of short fibers dispersed in high temperature binders which, upon drying, produces a strong insulation structure with low thermal conductivity. It can be pumped or troweled into place for use in insulation, sealing and duct repairs while the unit is operating.

PI dashboard. Madeline Dean, Exelon Generation, discussed the PI Dashboard, “Using PI to mitigate thermal transients.” PI (plant information) performance equations, with real-time sensors, offer thermal transient calculations for:

      • Condensate detection.

      • Attemperator leakage.

      • Valve instability.

Condensate: The system identifies times when undrained condensate travels across selected thermocouples, identifying large and sudden temperature drops by looking at slope and standard deviation. This helps focus corrective actions on the specific portion of the HRSG.

Attemperator: These calculations identify when water is exiting the attemperator with the valve closed, allowing detection of valve degradation.

Valve instability: Thermal transient calculations will count the number of times a valve changes from open to closed over a specific time interval, providing a specific maintenance alert.

“The Transpara LLC interface,” she added, “is the visual end product that can be used on computer, phone or tablet.” She also stressed the value of having historical data in an easily searchable and adaptable format. Other applications discussed were system chemistry and catalyst monitoring for pressure drop and ammonia use.

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Invited vendors address top user concerns at HRSG Forum

Top concerns of HRSG owner/operators haven’t changed much in the last several years. Emissions control, drain issues, attemperator problems, etc, remain etched in the minds of users. Some of the key takeaways from the 2017 meeting follow.

HRSG drain systems improperly designed and/or operated continue to torment personnel at some plants. Glen Wilson, Nooter/Eriksen, offered a timely project-based look at installation of an EPRI/Flexim-developed ultrasonic steam/water detection system for control of superheater/reheater drain valves.

The overall objective: Automatically detect and remove condensate from superheaters and reheaters without excessive release of steam, thereby preventing damage to coils and other steam-path equipment. This would reduce damage from tube-metal failures, stretching and bowing of tubes, and a host of related issues.

 “Simply put,” noted Wilson, “the industry has vast experience using ultrasonic meters to measure flow. The application to detect condensate uses the same sensing method, now calibrated to distinguish condensate from steam.” 

Next came a discussion on condensate removal. This is a severe service involving large pressure drops and flashing liquid; flow capacities vary with operations.

Continued throttling reduces valve life, and discussion turned to the master/martyr drain-valve arrangement in which the master valve is opened first and closed last. This maintains reliable shutoff tightness of the master valve by using the sacrificial martyr valve.  In this case, the master metal-seated ball valve opens and closes in two seconds with an unobstructed flow path.

The martyr valve, for throttling, can adjust to fairly low flows. As Wilson explained, “The martyr valve will open to a preset position based on current system pressure and then modulate based on presence of condensate at the ultrasonic sensor. Modulation rate may be lower prior to the purge and faster during the purge and after the CT is fired.”

The all-inclusive goal is to operate the master valve as little as possible.

Installation and calibration should be by experienced technicians the group was told. Although more expensive than the commonly used thermocouples, “ultrasonic sensing element signals can be directly correlated to the presence of liquid or vapor, with fast response to changes in state. Thermocouples respond more slowly and cannot distinguish water from steam at a saturated condition,” Wilson noted.

Many questions followed, some stressing again the importance of proper installation. Chairman Anderson added that a downward piping slope is necessary, as water must be able to flow toward the sensor. Therefore, it is best to specify this equipment before construction. Retrofits can be challenging with high and low spots in the drain pipe, but there are now retrofits in full-time service.

Catalyst technology. Brian Helner, Cormetech Inc, presented on Meteor™, a multi-pollutant catalyst technology patented by Siemens. This is a homogeneously extruded honeycomb catalyst in one layer with both oxidation and SCR functionality. It has been “optimized and fully developed into commercial production by Cormetech,” stated Helner.

A principal benefit is reduced pressure drop, along with accessibility (less space requirement). 

He then presented a case study, Ennis Power Co in Texas, a G-class combined cycle. Results after one year of operation showed “capability to operate at lower loads while maintaining CO emissions compliance,” plus faster CO compliance during startup. Field test data verified 99% CO oxidation at the 36% GT load point (previous was 55% to 60%), low ammonia slip, and NOx within the target value. Inspection showed excellent catalyst condition with clean and open cells.

Further installations are scheduled. One general point noted was possible permitting obstacles with substitute technologies.

Online monitoring. Through its acquisition of Alstom’s power and grid businesses, GE can now supply and service HRSGs. (GE also has acquired the Doosan’s HRSG business.) With GE’s digital background, the presentation by Pascal Decoussemaeker concentrated on turning data into actionable intelligence to improve plant performance. His overall theme was asset performance management (APM).

Typical HRSG failure modes and locations were discussed (Fig 1).

Decoussemaeker then compared traditional single-sensor monitoring with real-time, multi-sensor analysis systems with dynamic alert band, to more easily detect early stages of damage.

Next, he concentrated on the monitoring of some of the main life-limiting failure modes for HRSGs—such as fatigue, creep, and flooding events. The last, he explained, include desuperheater overspray and condensate flooding of the lower headers or manifolds during startup. However, it is not sufficient to just monitor. GE uses these insights to optimize the outage scope in an application called the Outage Advisor (Fig 2).

“Operation optimization,” he explained, “uses actual versus expected values for thermal performance.”  This is traditionally applied in monitoring systems to proactively manage plant performance. However, advances in IT also allow using artificial intelligence in completely new ways. In one application, a newly developed signal called “maximum likely demand in the next 30 minutes” was created to optimize the increase of duct-firing activity to free up gas-turbine capacity for rapid response, supporting the ancillary reserve market. This led to an annual fuel saving of $1 million.

“This example shows,” Decoussemaeker said, “how the exchange between plant operators and IT developments can lead to differentiators in a changing market environment.”

Various questions and discussions followed on how to retrofit life-monitoring systems, factory versus site installation methods for instrumentation, adjustments for cycling operation, and incorporation of site-specific data such as measured material properties.

Attemperators. One of the headliners on the Forum program was a panel discussion on HRSG attemperator concerns. Panel members were:

      • Tom Freeman, GE Power.

      • Joe Schroeder, JES Consulting.

      • Ory Selzer, IMI-CCI.

      • Justin Goodwin, Emerson Fisher.

Chairman Anderson moderated the panel after introducing the topic and panelists. Ensuing discussions centered on the following topics:

      • Tube and pipe failures, steam-pipe distortions, and weld cracking.

      • Performance problems, including high steam temperatures and overspray.

      • Attemperator hardware damage.

      • System controls.

Superheater and reheater tube bowing was voted a common problem by the panelists. Anderson began by asking how many people with this problem operated GE 7FA machines. Most indicated “yes.” Discussions then centered on reheater and superheater piping length, master/martyr valve combinations, and turndowns.

The panel was asked about attemperator design features (developments) to address fast starts. Selzer opened the discussion with a summary of inter-stage and terminal units in different thermal and operating environments. Goodwin continued, explaining that current operating ranges are not consistent with original attemperator designs. This led to discussion of two-stage setups.

One participant discussed a site’s move to Inconel spray nozzles; benefits were noted. Others indicated this might be a good form of “cheap insurance.” The option of reheater bypass for attemperation also was reviewed.

The gas-turbine perspective. Anderson set the stage with a summary of common experience: GT exhaust temperatures can go very high, and the attemperator cannot spray enough water. So, operators turn down the set point to prevent outlet-temperature overshoot. This increases the probability of overspray.

Freeman discussed the original GT design for nightly turndowns, and the differences inherent in cycling operation. “The world has changed,” he said. “Maintenance people want to protect the hardware; dispatch people want to make money and worry about parts life later.”  He then added, “The money-making piece plays heavily.”

Freeman examined three topics. He began with the market perspective, noting that when most existing powerplants were commissioned no one conceived of the world in which they function today. He described this as a global market and mission shift. 

Next, he extended this shift to the CT operational profile. Most existing plants were designed with part-load paths that were merely transients on the way to baseload, where units were expected to run for most of their lives.

Often, part-load curves were designed for what was then the new generation of low emissions combustors. But before long the North American market would see dramatic operational shifts. Turndown was becoming a significant factor. And sites were beginning to feel new pressures on HRSG operation and maintenance.

Freeman continued: “GE is rethinking the gas turbine load path [GT load versus exhaust temperature]. The way the load path was configured,” he continued, “was to hold temperature rise as a constant as long as possible. Some GE units are 100 deg F hotter on exhaust than counterpart GTs.”

A few years back, he said, his gas turbine team was largely unaware of HRSG constraints. “When information began to emerge, it became clear that there were two primary regions of interest: startup and turndown. The startup regime is generally related to independent GT/plant controls and often can be solved with a simple feed forward loop. In GE, that is called OpFlex Advanced Attemperation.

“The more intriguing issue is turndown,” he noted. “If you look closely at the operation, the concern often is not at low turndown but rather in the high-energy portion of the turndown, as the gas turbine approaches the isotherm while air flow (inlet gas velocity/IGV) remains fairly high.”

He then relayed some more history.  “A few years ago, GE was experimenting with a much broader load path range. The low end of the range could be called cold load path. Essentially, it becomes a trade-off on giving up air flow more quickly than T-fire (uprating the base firing temperature). Old timers would call it a simple-cycle load path. Interest grew because the modern DLN systems were now fairly robust and could provide greater flexibility.

The HRSG high energy corner could be bypassed. Attemperation valves that were running to 100% stroke could be held down around 40%. Thus, GE began to include the optimized load path in the later implementations of the advanced gas path (AGP)” (Fig 3).

He cautioned, however, about locking into any one path.

Freeman summarized the industry situation this way: GE has some methods to mitigate near-term operational boundaries. Yet, he sees the industry as needing to make uprates to the HRSG subsystems. Looking at industry direction, Freeman predicted increased exhaust conditions and the need to make an improved integrated system decision focused not on status quo, but on where the industry will likely go.

And his summary message to the HRSG community: “Scale up what you are doing. Don’t solve for today; solve for tomorrow. If you do an upgrade without understanding where the market is going, then you might end up paying twice.”

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Same old cycle chemistry issues persist

Barry Dooley, a senior associate at Structural Integrity Associates Inc and executive secretary of IAPWS, provided participants in the HRSG Forum with Bob Anderson a global view of optimum cycle chemistry for HRSGs and combined-cycle plants. He consolidated information from aligned HRSG sessions in Australasia, Europe, Russia, and Canada. Dooley and Anderson share the chairmanship duties for these conferences. 

A well-known and highly-respected presenter and cycle-chemistry expert, Dooley repeated that flow accelerated corrosion (FAC) is a paramount concern, and proper chemistry must either be designed into or incorporated into each system installation. He also noted, with a sense of frustration, that under-deposit corrosion and FAC are still occurring at the same rate as 20 years ago. In the US alone, three incidences of FAC-induced failure were reported in the past year.

But he stressed that this can be prevented with proper cycle chemistry.

Failure repetition was the underlying theme, and Dooley referenced a summary table of data from 185 plants showing repeat problem situations. He then presented the repeat cycle-chemistry events specific to combined-cycle plants and HRSGs.

Repeat causes of corrosion and corrosion products in HRSGs include, among others:

      • Inadequate online alarmed instrumentation.

      • Drum carryover.

      • Not challenging the (site) status quo.

      • Lack of shutdown protection.

      • Boiler waterwall/evaporator deposition.

      • No action plans for repeat situations.

Carryover also must be addressed because it leads to steam-turbine failures in the phase transition zone. “Steam-turbine issues in combined-cycle plants are increasing,” he noted.

Dooley also stated emphatically that “air-cooled condenser chemistry absolutely controls the plant chemistry.”

For FAC control, he noted the need to identify both single- and two-phase activity, and to distinguish between the two. To emphasize the seriousness and what we do know, Dooley stated that single- and two-phase FAC are still occurring worldwide, and locations have remained constant for more than 15 years. He then gave details and background, along with relevant IAPWS rules for avoidance.

Dooley then pointed out typical FAC locations in horizontal- and vertical-gas-path systems. His discussion featured some common ways to identify both location and type.

Then another rule of thumb: Two-phase is controlled by pH; single-phase is controlled by the oxidizing power of the liquid. Proper levels of each were discussed. He also noted that amines and film-forming products must be used with extreme care.

Under-deposit corrosion (UDC) was also explained and reviewed, with specific histories, causes, examples, and treatments.

Relevant IAPWS Technical Guidance Documents (TGDs) also were listed and reviewed.

EPRI’s Mike Caravaggio, senior program manager for major component reliability, followed Dooley with “Achieving Cycle Chemistry Excellence in HRSGs.” This included current industry statistics, examples of chemistry-related damage to both HRSGs and turbines (for example, FAC and iron transport, hydrogen), and a preferred instrumentation suite. Caravaggio concentrated on what he labeled “practical steps” to achieving proper and obtainable chemistry control.

Topics included the most common tube failure areas (LP economizer and HP evaporator), turbulence and FAC risk, UDC and hydrogen damage, and corrosion fatigue (now increasing with aging of the fleet).

Iron transport and oxygen levels also received high attention.

Caravaggio then presented the case study of a 700-MW plant that has shown dramatic improvement reducing iron transport by using an ammonia and neutralizing amine blend.

Concluding thoughts included understanding the damage mechanisms and chemistry control options, performance monitoring, and the critical importance of acting on the measured results. All too often, it became clear, such action requirements are overlooked (or simply ignored).

Water/steam sampling. Manual Sigrist, Swan Systeme AG, discussed specific tools to monitor cycle chemistry, emphasizing refurbishments of existing plant sampling equipment. Systems in legacy powerplants typically have high O&M costs, low monitoring reliability, and can increase risk of collateral damage to major components. Space for new instrumentation can be limited.

For refurbishments, he emphasized the “project concept phase” to define options and obtain commercial justification. Personnel safety should always be included as a valid economic factor.

He then reviewed examples of both refurbishments and new installations, in both cases putting strong emphasis on operator training.

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