Dr Ahmed Shibli, managing director, European Technology Development Ltd (ETD), recently contacted Consulting Editor Steve Stultz to invite CCJ’s participation in the upcoming High-Temperature Defect Assessment (HIDA) conference, April 20-22. The virtual meeting will assess the behavior of high-temperature plant components containing defects and operating under steady and/or cyclic load conditions. This is considered by many experts to be an area of urgent need and interest to the global power-generation community.

Presentations and discussions will incorporate the thinking of experts from the UK, Belgium, Germany, Sweden, Switzerland, Poland, Italy, US, South Africa, Australia, and Japan, among others.

The program is divided into the following three segments:

• • Inspection, damage, and cracking under creep, fatigue, and oxidation conditions.
  • Defects/cracks and life assessment.
  • Martensitic steels—cracking, life assessment, and modeling.

Register today at www.edt-consulting.com.

Shibli and the UK-based consultancy he leads has had a long-term collaboration with CCJ on technologies of importance to powerplant owner/operators. To review some of ETD’s work, access www.etd-consulting.com and/or conduct a simple keyword search at www.ccj-online.com.

The first HIDA conference, a European Commission and industry-support research initiative, was held in Paris in 1998, back when all industry meetings were in-person events.

Diaphragms are a hot topic at most conferences where owner/operators gather to discuss issues with their steam turbines, such as the Steam Turbine and Combined Cycle User Groups operating under the Power Users Group umbrella. To get the most from these meetings, it’s important to know the basics of steam-turbine design, the differences among machines offered by the leading manufacturers, and typical challenges faced by O&M personnel.

The intent of this article, written by Moe Fournier and Bryan Grant of Advanced Turbine Support LLC, is to provide users a backgrounder on the different types of steam-turbine diaphragms and their associated repair challenges. Recall that the purpose of the stationary blades in a diaphragm—a/k/a nozzles—is to redirect steam from the exit of one rotating stage of blades into the entrance of the next rotating stage. The design intent of the nozzle is to optimize both the angle of steam flow and its velocity into the downstream stage of rotating blades to maximize energy conversion.

Diaphragms are a two-piece assembly consisting of upper and lower halves that are installed in the upper and lower halves of the steam-turbine casing, respectively (Fig 1).

Recall that impulse turbines (Fig 2) typically have far fewer stages than an equivalent reaction turbine. Thus, the energy transfer across each stage of an impulse turbine is much greater than it is in a reaction turbine.

Almost all of the stage-to-stage pressure drop in an impulse turbine occurs across the stationary blades, virtually none across the rotating blades. This means stresses on an impulse blade holder (diaphragm) are significantly higher than they are for a reaction blade holder—dictating that impulse stationary-blade holders be axially larger and more robust than reaction ones. Most are of welded construction to tie all parts together rigidly.

Because they are stationary components, diaphragms often do not receive the same level of attention as rotating blades/buckets. However, improper maintenance and/or repair of diaphragms can have a negative impact on steam-turbine thermodynamic performance. Plus, the mechanical failure of a diaphragm can cause significant turbine damage. Damaged diaphragms also can act as a stimulus on downstream buckets leading to their premature failure and consequential damage to the turbine.

Diaphragm designs

Spacer-band construction (Fig 3), the most common style of impulse diaphragm for many years, remains the industry’s most prevalent design. It is characterized by individual nozzles without sidewalls, typically made of stainless steel, that are inserted into profiled holes in thin stainless-steel spacer bands that form the inner and outer flow paths for the steam.

The spacer band and nozzle subassembly are structurally welded to the inner and outer rings, normally in four places (inner and outer ring, inlet and exit sides) to form the diaphragm. The weld process typically used is MIG or submerged arc; stick and electron beam welding are less common.

Integral sidewall construction (GE singlet, Alstom platform, etc) generally is found in impulse turbines installed since the late 1990s (Fig 4). It consists of individual nozzles with integral sidewalls, typically of stainless steel. The nozzles are structurally welded to the rings, normally in four places (inner and outer ring, inlet and exit sides). Weld process used usually is MIG or submerged arc; stuck and electron beam welding are less common.

Fillet fabrication construction (Fig 5) is very common on the last few stages of LP steam turbines. It consists of individual nozzles, usually of stainless steel, welded directly to the inner and outer rings using full-perimeter fillet welds. A TIG weld, using an Inconel or stainless filler wire, is typical. The nozzles can be solid or hollow and may incorporate moisture-removal features.

Mechanical assemblies are less common but are being offered by some OEMs because they are said to eliminate distortion from welding. In theory, they allow you to replace individual components in the diaphragm. Fig 6 shows two patented concepts.

Failure modes

Steam-path distortion and/or blade/nozzle mechanical damage (including foreign-object damage, solid-particle erosion, cracking, and moisture erosion) are the most common failure modes for diaphragms.

Nozzle mechanical damage and blade flow-path distortion can contribute to three significant failure modes for the steam turbine. They are:

• • Distortion at the nozzle opening adversely affects turbine thermodynamic performance and efficiency because it suboptimizes flow from the nozzle to the downstream rotating bucket.
  • For long blades, changes in nozzle exit openings are conducive to variations in the impulse on the downstream bucket and can lead to bucket resonance/vibration and premature failure of the rotating blades.
  • Nozzle damage can lead to stress risers and mechanical failure of the nozzles themselves, which, in turn, can cause severe or catastrophic downstream damage to rotating buckets.

Structural weld failure. When the structural welds that tie the nozzles to the inner and outer rings fail, the diaphragm is likely to collapse and move into the downstream rotating buckets. This can cause significant damage to steam-path internals, up to and including catastrophic failure of the turbine. Failures often can be attributed to inadequate weld tie-in, improper weld application, and undercut erosion (Fig 7).

Nozzle failures. Although not a common cause of failure, a temperature difference between the diaphragm’s upper- and lower-half components can create forces strong enough to crack and liberate nozzle partitions. Condensate introduction typically is the cause of a thermal mismatch (Fig 8).

Dishing (creep). Diaphragms designed and manufactured with less-than-desirable materials or inadequate structural-weld depths, can, over time, and at elevated temperatures, distort (creep) in the direction of stress. Eventually, this causes the diaphragm to rub against the downstream rotating bucket stage, often resulting in significant damage to steam-path internals, and possibly catastrophic turbine failure.

Seal degradation. Diaphragms typically incorporate seals to prevent performance-robbing steam leakage around bucket tips and along the rotor shaft. When seals are damaged—especially seal tips—their effectiveness at keeping steam in the as-designed pathway is greatly diminished and efficiency suffers. Worse still, if the seals rub against mating components, they can cause surface cracking on the bucket covers and/or rotor shaft. Such cracks can propagate and lead to bucket or shaft failures.

Most diaphragm designs incorporate replaceable seal strips, which should be inspected, straightened and sharpened, and/or replaced at major-inspection intervals. Proper planning is required to ensure the correct spares are on hand to support component replacement.

Repair challenges, risks

Steam-path sections of all diaphragm types. The goals for repairing nozzles and sidewalls of a diaphragm are the following:

• • Remove stress risers in the steam path that could lead to mechanical failure of the diaphragm.
  • Restore the flow path (nozzle opening) to reduce opening-to-opening variations that can cause undue stresses on downstream buckets and premature failure.
  • Ensure diaphragm steam flow into downstream buckets is optimized for thermodynamic performance.

The repair challenges are to understand and apply the correct repair techniques and to preserve flow-path integrity with respect to the three goals above. When the repair involves welding, selection of weld process, weld materials, and proper application of post-weld heat treatment (PWHT) is critical to a successful and lasting repair.

Structural repairs to welded diaphragms. Repairs involving the structural weld require a finite determination of the construction type and the axial depth of the OEM’s structural weld, as well as a basic understanding of the stress fields in the diaphragm.

Additionally, for structural repairs to rings and/or sidewalls, welding must account for the very thin nature of the space band (or hollow ring) and its tendency to distort. Keeping the sidewall in the correct radial location is critical to unit performance; minimizing distortion of the steam path is critical to unit performance and the diaphragm’s fit into the turbine.

Weld repairs also require knowledge of the material types for each component. Material verification testing is recommended before any welding is performed. PWHT must be evaluated based on material types and weld repairs.

Repair of mechanically assembled diaphragms. For blade and steam-path repairs, the challenges noted above apply. For structural repairs to any diaphragm components (rings, etc), knowledge of the component material is necessary. Material verification testing is recommended before any welding is performed. PWHT must be evaluated based on material types and weld repairs. One more concern: Make sure the proposed repair does not negatively impact the mechanical joining feature at the ring-to-nozzle interface.

Best practice: Review proposed repairs on a case-by-case basis with a full understanding of the mechanical stress transfer mechanism of the diaphragm assembly.

Seal repair/replacement. While some refurbishment of seals is possible, replacement typically is the best solution. Depending on the type of seals, challenges can range from procurement cycles to installation techniques; knowledge of the seal design is critical.

The first thing to note about the GE presentations is that you’re probably going to want to listen to the recordings, if you’re approved to access them at the OEM’s MyDashboard website.

A blizzard of information swept through the virtual room, along with a virtual army of presenters and technical support folks at the ready. The overarching message was that if you’re seeking to adapt your plant to changing market or operating circumstances, GE can help.

To set the stage, Tom Freeman, chief customer consultant, and Blair Van Dyne, South Region sales director, reviewed basic market forces—mainly the continued surge of renewable energy into the market and decline of coal. Van Dyne said, “US wind developers will add 23 GW of new capacity this year. The previous record was 13.2 GW in 2012. Solar PV accounted for 3.5% of US total energy generation in July. About 31 GW of US coal has retired since 2018. Coal generation dropped 30% in the first half of 2020.”

John Sholes, value solutions leader, discussed myriad ways to “make your plant better,” breaking down the options into short-, mid-, and long-term frames of concern.

A common short-term issue is getting sufficient turndown without exceeding attemperator limits. Options include software changes to lower exhaust temperatures, upgrading the HRSG attemperator to increase spray-water flow, and adding a ring-style second-stage attemperator, which could require up to a one-week outage. GE now offers a standalone “smart attemperator” box that biases the attemperator to current operating data by “buffering” the current instrumentation.

Sholes then discussed a variety of upgrades to “nudge ahead,” including a dry low NOx (DLN) 2.6+ combustor upgrade, advanced gas path (AGP) technology, and extended-turndown valves. Such “technology infusions” on the gas-turbine side can boost capacity by up to 20% but will require balance-of-plant (BOP) upgrades, possibly even a transmission-line upgrade and generator nameplate capacity uprate.

The mods can be phased in over time for plants seeking significant performance-shift goals. Generators have lots of design margin, Sholes added, although cooling capacity is critical.

Responses to audience questions addressed the following:

• • Changes required to condensate recirculation systems.
  • Wet compression on hot days.
  • Retrofitting safety valves versus replacing them.
  • When to involve the HRSG OEM.
  • Excitation-system and voltage-control limitations.
  • Steam-bypass considerations in event of a steam-turbine trip.

John Korsedal, product manager, GE Digital Worker Solutions, talked on flexible mobile and remote operations to achieve mission goals. Digital solutions are available to assist leaner, less-experienced workforces with expertise accessed remotely, inside or external to the owner/operator organization.

Korsedal dwelled on GE Remote Operations, a “packaged software and appliance solution” with much of its recent functionality derived from Covid-19 responses. Goals of the package are to enhance worker location flexibility, promote reliable remote operations, extend monitoring and contingency operations, and achieve multi-plant control flexibility. The package includes a “purpose-built” mobile HMI with view-only access as the default mode and a “pop-out for control management.”

Communications capabilities available with GE’s Remote Operations includes voice, video, text, content-sharing, hands-free broadcast, and group messaging through WIFI or cellular access.

In addition to Remote Operations, GE’s Asset Performance Management Portfolio will shortly introduce a next-generation Rounds product which will be called Rounds Pro. This will be available for large and small BYOD (bring your own device) screens with a new, intuitive, interface to complement work processes.

The presentation includes remote operations use cases.

To close the CCUG portion, Freeman returned to the virtual stage for “Adapting Plant Maintenance with Operational Changes in an Aging Portfolio.” Broad topics included:

• • Conducting a pressure-wave (HRSG cleaning technique) value study by monitoring differential pressure across the HRSG. Freeman noted that typically 0.25 MW (0.35 MW on 7FA) is gained for every 1.0 in. H2O recovered in HRSG differential pressure. An easy check in your historian.
  • Generator reliability and uprates. Freeman cautioned that you don’t want to be forced into a rewind at the wrong time as this requires a lengthy outage. Generally, rotors need a rewind after 15 to 20 years, stators between 25 and 30 years.
  • Impacts on the steam turbine/generator from greater cycling, such as the following: inlet-valve throttling and solid-particle erosion; low-cycle fatigue on HP/IP shells, last-stage blades (LSB), and valve casings; and moisture-induced erosion of LSBs and the turbine casing.
  • Subsystems, including valves, fire protection/hazardous gas, lube oil, and inlet/exhaust plenums.

The annual conference of the Combined Cycle Users Group (CCUG) was conducted online for the first time in 2020. The presentations summarized below took place during Week 3 of the conference and are available to owner/operators on the Power Users website. Access Week 1 (User presentations) and Week 2 (GE Day) recaps here. More to follow…

Subjects covered during the CCUG’s Week Three session ran the gamut from what you can bring into (or catch at) the plant to what your facility discharges out the plant—and much in-between. ALL presentations—both user and vendor—are available on the Power Users website for on-demand viewing.

Pandemic viruses. Probably nothing was top of mind like Covid-19 and so the day began with the presentation, “Covid Best Practices.” First slides reviewed the Covid personal practices we’ve been seeing and hearing about for eight months in the news.

Then the presenter drilled down to in-plant practices, specifically changes to outage execution. Some of the basic steps include the following:

• • Daily site employee and worker temperature monitoring for fever.
  • Segregating day and night shift staffs and decreasing shifts by one hour to avoid overlap.
  • Phone- or digital-based shift turnover.
  • Increased social distancing during the shift by holding morning toolbox and shift turnover meetings outside (weather permitting) or in rented trailer, separating crews into teams with different break schedules, and adding a separate trailer for work crews.
  • Increased personal hygiene and addition of wash stations around the site.
  • Wipe down of tools at the end of each shift for the next crew.
  • Additional personnel protection equipment (PPE) in areas where 6-ft distance could not be maintained (confined spaces, for example).
  • All vehicles limited to one worker per trip.
  • Frequent cleaning of all high-traffic surface areas like refrigerators, microwaves, coffee pots, door handles, etc.

The presenter underscored the need to be aware of heat-related stress from wearing masks for long periods in hot environments (such as above 90F), and a need for a solution to crowding at emergency muster points.

The Q&A session got interesting. Illustrating a non-obvious tradeoff of one safety issue for another, one plant rep noted that they had to back off on safety audits and suspend fire drills to minimize person-to-person contact. One user expressed frustration that they couldn’t get the right tech-support folks into plants because of local, state, and national restrictions. In an extreme case, this caused scheduled hot-gas-path (HGP) maintenance to be deferred.

Another facility modified smoking areas and port-a-potty units to keep groups isolated. At least two plants added portable heated-water hand-washing facilities, one said to include a tankless water heater, to encourage longer hand-washing.

Unfortunately, no one had any good way to track workers offsite, behavior that could nullify whatever good practices were occurring onsite. Craft-labor supervisors, the presenter said, were responsible for ensuring that crew members only traveled from hotel to site. Lunches were served onsite to avoid unnecessary offsite travel.

Safety. The next presentation, titled “You Have to Be Present to Win,” addressed safety issues, with Covid-19 being the most recent challenge added to the safety basket. It’s worth getting the slides for the photos of a GT major outage during a pandemic. Some of the specific steps taken at this plant:

• • Substituted a safety-orientation video and a downloadable Excel spreadsheet for the in-person site orientation session.
  • Replaced break trailers with tents, provided by a local contractor, equipped with lights, heat, tables, chairs, floor, etc.
  • Added two remote hot-water hand-washing stations.
  • Held daily contractor meetings in an open, ventilated shop area.

Speaker opened with the personal experience of an injury during a family outing to illustrate “what we do at home affects our work,” and then recounted the experience of a serious accident at the plant as a reminder that “we work in a dangerous environment.”

That set the stage for a discussion of six enemies of safety: complacency, stepping through the motions without thinking about what you’re doing; poor housekeeping, taking the time to clean up and avoid shortcuts; fatigue/lack of focus, especially during long outages and when workers are offsite; deadlines, distinguishing between real and implied; lack of training, “feeling” what’s happening with the equipment in addition to “knowing”; and trusting without verifying, such as taking the time to know what is going on in a LOTO area.

Reporting “near-misses” is key, the speaker stressed, and with follow-up training on the precursors to them.

Two slides list a baker’s dozen of “items to consider.” While most are the usual reminders when plant safety is addressed, a few of the most salient are these:

• • Require workers to state what they are doing instead of just doing it when signing off on work permits.
  • Consider taking the most conservative option when making an on-the-spot decision about safety.
  • Encourage an open mind when personnel suggest solutions and “hear” employees’ safety concerns rather than just listening to them.

One listener encouraged attendees to adopt OSHA’s Voluntary Protection Program (VPP) process to strengthen their safety programs. VPP plants invite OSHA representatives into the plant to guide them in how to do things more safely. Presentations at previous CCUG conferences have addressed the OSHA VPP process. Access them on the Power Users website.

Another attendee conceded that training new employees during a pandemic presents opportunities for improvement. One concrete idea is to upload a virtual orientation to YouTube with a QR code for workers who didn’t see a video before they arrive at the site.

Inspections. Remotely inspecting high-risk areas is another facet of safety. The next speaker presented experience with remote camera inspections of LM6000 and 7EA peaking-unit compartments. This is a specific solution based on a GoPro camera and a digital monitoring device.

Craft labor in this utility’s peaking-turbines department sought a system that would be safe, avoid unnecessary tagouts for things like oil leaks, and not violate the gas-turbine OEM’s requirements—such as the prohibition of entering a turbine package during operation. The solution selected features off-the-shelf components costing at most $1500, rather than expensive stationary cameras. The camera sits in a mag base with remote mounts, while the monitor stands outside the GT housing.

This was also a case where putting the minds of your younger workers to bear on the problem pays dividends, the speaker noted.

The apparatus has already proved its value in detecting water leakage in a GT package following a shutdown and confirming its source (NOx injection water line), detecting smoke emitted from the turbine compartment and confirming its source (vent fan), as well as for conducting condition assessments of inlet-house fogging nozzles and evaporative cooler media, and for monitoring an oil-consumption sight gage.

Generally, any piece of equipment inside a housing can be monitored and recorded externally during operation for an extended period.

Questions included whether the components are explosion-rated and “intrinsically safe” and what the high-temperature limit is (answer, 400F for direct contact, but does not actually contact hot surfaces). One commenter noted that such cameras have also been used in place of borescopes for “troubleshooting insight.”

Market competition. As if it wasn’t yet clear, the presentation titled “Renewables Are Coming” made unassailable the coming competition to gas-fired plants from solar and wind. And if you don’t like that, you can no longer blame it solely on government mandates.

Eight states now require 100% renewable energy by 2045 and five others have 100% renewable “goals.” Large high-profile corporations like Facebook, Google, Microsoft, and other digital-economy leaders, the speaker noted, plan to either build or buy 72 GW of renewable energy by 2030 for their electricity-hungry server farms and other needs.

That’s the demand side. On the supply side, the presenter noted that solar photovoltaic (PV) systems have dropped in cost from $3.50/watt to 50 cents over the last 12 years, and their active-power control capabilities have greatly improved. Grid-scale battery systems, which assist in load management, also have dropped in price by 70% between 2015 and today.

“Lots of states already show solar and wind to be the least-cost capacity options,” observed the presenter, “and only a handful of states show gas-fired generation as the least cost option in 2030.”

There are unintended consequences, however. For example, smoke from the California wildfires this past year decreased solar generation from existing facilities by around 30%. Guess which plants would be making up that loss on a moment’s notice? Yup, gas plants.

Another consequence of the strange year called 2020: Utility system loads shifted dramatically, because of COVID, from commercial facilities to residential units. Zooming takes electrons.

Most of the bulleted items on four slides about how to adapt GT units to this coming onslaught are probably more than obvious to most users and have all been topics of one or more presentations during prior CCUG conferences, including the future potential for hydrogen produced by renewable sources as a GT fuel.

HRSG drum wall cracks. Anand Gopa Kumar, analysis manager, HRST Inc, coached the audience through a relatively new onsite crack-size assessment technique, conducted along with ALS Industrial Services, that has now been demonstrated “on a few HRSGs.” The technique, which follows API 579 and ASME-FFS-1 standards, combines transient thermal simulation (based on finite-element analysis) crack growth under drum operating conditions with standard NDE crack inspection methods—including magnetic-particle and ultrasonic testing.

The deliverable, if you will, is a failure assessment diagram (FAD) of the areas under investigation which reveals critical crack size (Fig 1) as a basis for decisions about remaining life, additional run time, etc. In other words, measure the crack dimensions (length and depth) and project their growth (assuming other variables are fixed) over the next operating cycle.

“All high-pressure drums should be periodically inspected but the thicker HP drums are most at risk,” Kumar said, with the area of greatest risk being the surfaces exposed to the 0-400-psig pressure range where the fastest temperature rises are experienced. “Thick cold drums plus fast pressure ramp equals stresses at the large nozzles,” Kumar noted. The shell-to-head area is also susceptible to cracking.

A typical F-class HRSG HP drum needs around eight different FADs, one for each of the major weld locations. The technique is best performed before an outage, so that relatively quick decisions can be made on repair during the outage versus continued monitoring.

The technique is applied to ID wall cracks, since removing insulation from the OD side usually is impractical or not possible. However, Kumar said, some cracks at OD weld areas can be detected from the inside. The analyst also has to consider adjacent cracks and the potential for crack interaction. “Sometimes cracks close together should be considered a single larger crack,” he said.

Many of Kumar’s slides were devoted to pre-outage, start of outage, and in-testing work.

Pre-outage work includes organizing the information—such as design drawings, operating profile data, historical repair procedures, photos, and any other previous inspection results or condition reports. Drum weld areas need to be properly exposed, cleaned, and prepped for NDT, and drum internals removed. Less obvious: Install snug-fitting foam plugs in nozzles to protect them from foreign objects and install lanyards on all tools if open holes exist.

At the start of the outage, inspect surface prep before the NDT crew arrives, label each weld location with paint stick per the drum weld location map (Fig 2), and protect nozzles from falling objects.

During testing, the technician performs mag-particle tests first, then the phased-array ultrasonic tests to accurately document the start of cracks, while being aware of multiple crack interactions. Length and depth of cracks must both be determined to decide whether more run time without repair is prudent. Decisions whether to leave as is or grind out shallow cracks must be made as well. Minimum wall thicknesses should also be calculated ahead of the outage, using ASME methods.

In response to questions, Kumar stated that fatigue-life calculations are not part of this exercise—these components typically do not operate in the creep temperature range—and it is uncommon to see cracks slow down or stop rather than continue to grow. Performing this technique before a unit enters cycling service can be especially valuable.

Catalyst and turndown. Moving through the combined-cycle system to the NOx and CO emissions catalysts, Andy Toback, Environex, asked in his presentation title, “Is Your SCR/CO System Ready for Turndown?” If your SCR was designed for baseload operation, the answer is probably not.

Chemical constituents change at temperatures typical of low-load operation. NO2 from the gas turbine gets elevated and CO from the GT exhaust can “grow exponentially at low loads,” because the operating-temperature range has shifted. Toback then turned to two case studies to illustrate his points.

The plant in the first case was experiencing ammonia flows higher than design, sometimes twice as much, even at low loads, and low NH3 vaporizer temperatures at high NH3 flows. The NO2 fraction of NOx was measured consistently higher than 50%, and as high as 70%, during startup. Normally, it should be around 20% NO2/NOx.

Nevertheless, both CO and NOx catalysts were performing well. However, the CO catalyst was also oxidizing NO to NO2, so the SCR catalyst had to work harder neutralizing the elevated NO2 levels, acting as if it was near the end of life, asking (through the control logic) for additional NH3 spray.

“The catalyst was behaving perfectly for the baseload conditions it was designed for,” Toback reported, “but to operate at lower loads and meet permit limits, it would require 20% additional volume and 0.6 in. H20 additional pressure drop.”

Toback called the second case study a “turndown field exercise.” The test crew measured steady state catalyst operating temperatures and CO and NOx concentrations in the GT exhaust down to 8% load. The goal was to determine what turndown levels the plant could run at with available catalyst configurations. It turned out that 35% was the load limit with the present catalyst. Both turbine-exit NOx and CO levels (Fig 3) rose precipitously below this point.

“This plant could get to a 28% load limit if they replaced the present CO catalyst with a dual-purpose formulation,” Toback concluded. While this approach could prove worthwhile for some plants facing extended operation at extreme turndown levels, this plant opted to stick with what it had. In addition to the larger catalyst volume, there is a pressure-drop penalty.

One questioner asked what the catalyst concerns would be running the plant at higher-than-design loads. Answer was that the anticipated life of the catalyst would have to be modified based on the operating data post-uprate.

Another asked if catalyst degradation is gradual or “falls off a cliff.” Answer was that NH3 consumption tends to increase exponentially and catalyst deactivates quickly near end of life. Short answer, probably. Catalyst needs to be tested periodically, and “married up to operating data,” to keep from approaching the cliff, especially after the OEM’s warranty period, to establish a baseline. “Early catalysts were over-designed,” Toback noted, “while later CC/GT facilities have catalyst supplied more competitively on volume.

In the Environex virtual breakout room, discussion continued on topics such as these:

• • How often to clean NH3 heaters. One plant cleans with acid every three years, while another plots wattage to vaporizer exit temperature to predict when the next cleaning should be.
  • Options for running at lower capacity factors when your NH3 flow is capped. Check for plugged nozzles in the ammonia spray array, and try to tune the unit by measuring NOx and ammonia slip at each point in a traverse (assuming you can reach the sampling ports or add a sampling port grid), and selectively increase the ammonia flow in trouble spots.
  • Plants having issues with operation below 40F and above 85F can consider seasonal tuning