Guidance from consultants, suppliers benefits HRSG owner/operators

Most of the users groups serving generating plants powered by gas turbines are managed by steering committees comprised of like-minded volunteers, and the sessions at these meetings often are open only to owner/operators. By contrast, conferences conducted by the Australasian HRSG Users Group (AHUG) are open to all attendees, the steering committee believing consultants and equipment/services providers can contribute significantly to resolving plant-level challenges.

The AHUG annual meeting, held November 15-17 in Sydney, Australia, featured participation of non-users on the topics below, likely of interest to most owner/operators:

      • Dry-ice cleaning of HRSG heat-transfer surfaces. (go)

      • Maximize the life of penetration and casing seals. (go)

      • The importance of accurate steam and water sampling. (go)

      • HRSG designs and trends. (go)

      • Damage tolerance of materials and codes. (go)

      • Cycle chemistry update. (go)

      • HRSG thermal performance assessments. (go)

      • Online analysis of “ultra-trace” iron and copper. (go)

      • Using ultrasonic technology to control superheater and reheater drains. (go)

      • IAPWS guidelines for cycle chemistry. (go)

      • IAPWS updates on Technical Guidance Documents. (go)

      • Pitfalls to avoid in steam/water analysis. (go)

Dry-ice cleaning of HRSG heat-transfer surfaces

Joel Williams, Precision Iceblast Corp, provided a specific strategy for increasing combined-cycle efficiency and output, and decreasing costs: Periodic dry-ice blasting of HRSG tubes. He said CO2 is effectively removes the four primary types of deposits in HRSGs: ammonia salts, sulfate deposits, iron oxide deposits (corrosion), and insulation materials.  

If such fouling is not removed, consequences often include increased gas-turbine backpressure, reduced thermal efficiency, and particulate emissions during startup.  

Examples were reviewed, including common methods of determining when to clean:

      • Visual inspection.

      • Backpressure readings.

      • Stack temperatures.

Maximize the life of penetration and casing seals

There are multiple expansion-joint locations in combined-cycle plants with common problems, increasingly aggravated by two-shift and cycling operation. HRSG penetration- and casing-seal degradation was addressed by Dekomte’s Jon Tarrant.  

Topics included locations, applications, and OEM designs. Then came the identification of typical failures, followed by repairs, replacements, and retrofits. Example: For steam-line side-wall metal-bellows penetrations, Tarrant suggested, “Fabric retrofits offer advantages of lower cost, quick installation, and no requirements for NDT or heat treatment.”  

For OEM axial metallic bellows, which allow only minimal lateral movement, distortions and corrosion are common. Replacement costs are also high, so fabric retrofits can become a viable option. Retrofit details (bolsters, fabric, clamp bands) were presented.  

Fabric retrofits are also suitable for packed-gland labyrinth seals, Tarrant added, an upgrade to counter high maintenance, gas leakage, and typical water ingress. Design specifics were reviewed.  

HRSG insulation topics included typical formats and maintenance procedures, and an update on pumpable insulation. Typically, work on liner plates (and insulation) requires unit shutdown. One option is pumpable insulation, injected from outside of HRSG (after ensuring liner plates are properly in place). Such material is a mix of short fibers dispersed in high-temperature binders which, upon drying, produces a strong insulation structure with low thermal conductivity (with good adhesion, high melting point, and low shrinkage).  

The unit remains online while the product is pumped into place. Pumpable mastic is available in both regular and bio-soluble forms.  

An interesting caution followed during discussions, supporting close inspection and possible component replacement. If bellows have failed near HRSG drains, the area could be an unsafe work environment with hot gas leakage. Hot gases can also damage electrical equipment.

The importance of accurate steam and water sampling

John Powalisz, Sentry Equipment Corp, covered current steam and water sampling issues related to cycling operation, reduced plant staffing, and corrosion products (metals) transport leading to erosion and corrosion.  

Sampling issues specific to combined-cycle plants, he explained, are:

      • Flow is too high during startups.

          • High temperature impact on equipment.

          • Inconsistent data (pH, for example).

          • Sample flow interruption by thermal valve trips.

      • Flow is too low during low load, startups, or with plugging.

          • Erroneous data, no flow to analyzers.

          • Air ingress causing false numbers (carbon content).

The goal is to control the important parameters of velocity, pressure, and temperature for both online instrumentation and grab samples. Automated sampling racks, for example, should offer temperature control monitoring, pressure-reducing-valve (PRV) control, and the ability to set flow rates. “People are scarce; pressure and load changes are frequent,” said Powalisz. Limited staff is available to operate the sample panel during transients. Therefore, it is good to automate (and/or outsource) as many functions as possible.  

Iron-transport sampling continues as a significant issue. During comments and discussions, it was noted that iron transport indicates suboptimal cycle chemistry control and should be controlled at the source in the first instance through the correct application of cycle-chemistry guidelines. The technical guidance documents (TGDs) developed by the International Association for the Properties of Water and Steam (IAPWS) were suggested.    

Trying to address iron-transport issues at the sampling rack does not address their root cause, it was stated, and the risk of major component failures caused by FAC will still remain. An optimized cycle-chemistry program for the feedwater and evaporators of a combined-cycle plant will result in very low total-iron transport rates and very low total-iron levels in the sample lines.

HRSG designs and trends

John Roberts of Jacobs, a global engineering consulting firm, provided a comprehensive overview of HRSG design improvements and trends, touching on the major global OEM suppliers. Many trends are driven by the global emphasis on green options, creating variabilities in load that must be absorbed quickly by traditional power. Fluctuating gas prices add to the uncertainty.  

Fast starts and flexibility carry the weight; combined cycles are becoming “mid-merit” vehicles to meet demand. HRSG designers must therefore address:

      • Faster ramp rates to match green-power unavailability.

      • Ongoing demand for higher efficiency and lower emissions.

      • Lower capital and lifetime maintenance costs.

      • High reliability under all operating regimes.

“One of the earliest problems faced by HRSG designers,” stated Roberts, “was large volumes of condensate.” This requires good drains and correct drain operation to prevent thermal transients.

He continued, “The steam drum is seen by many as the limiting factor to a fast start,” because of problems caused by the differential temperatures (steam and cold feed). For this, OEM designers are offering two solutions: a patented DrumPlus™ design by NEM with secondary separators outside the drum, and vertical steam separators by Babcock & Wilcox. Single-row harp designs also were discussed.  

Materials also remain a common issue. Criteria for progress include the following:

      • Materials with greater allowable stress at high temperature and pressure.

      • Properties of thermal conductivity and thermal expansion.

      • Grain structure and heat treatment.

      • Weldability and transitions.

      • These items were reviewed and discussed by attendees.

Almost unanimously, OEMs are recommending and offering online real-time monitoring programs for critical components (drums, high-temperature headers, etc). Specific commercially available examples were given.  

For superior efficiency, there is a possible move toward supercritical once-through (Benson-type) units, but design benefits and limitations must be carefully explored. Also in development mode is reduced backend temperature for more heat/energy extraction (stack temperature of perhaps 160F). Discharge corrosion issues are also under investigation.

Damage tolerance of materials and codes

Traditionally, the ASME Boiler and Pressure Vessel Code uses mechanical strength as its primary basis for construction rules. Other material considerations stem from recognized material specifications (ASTM, for example). Yet with more moves toward critical service (higher temperatures and pressures, and cycling), the current Code approach may be limited. Perhaps damage tolerance should be part of it.

Mike Drew, Australian Nuclear Science and Technology Organisation (Ansto), began his presentation by looking back to a 1983 main-steam piping failure at Philadelphia Electric Co’s 325-MW Eddystone Unit 1, the world’s first truly ultra-supercritical coal-fired utility boiler. Original steam outlet conditions were 1210F and 5000 psig; temperature later was reduced and held at 1135F until unit retirement in 2014.

A steam-pipe failure occurred after 130,000 hours of operation; and there were no indications that the leak might develop. Failure analysis showed intergranular cracking; extensive creep-induced cavitation was observed at grain boundaries in damage areas, with sigma damage (a loss of fracture toughness caused by exposure to high temperature) but no evidence of swelling. Piping material met specification requirements (both mechanical properties and chemistry).

Cracking was directly related to residual stress induced by thermal shock events (condensate flow) during shutdowns. The material’s ability to tolerate abnormal operating stresses was compromised.

For the replacement piping, primary emphasis was on chemical composition control to improve the material’s damage tolerance—specifically:

      • P91. EPRI recommends more restrictive compositional controls for P91 material. The example given was a P91 header failure in the UK at 60,000 hours, and high variability in ductility within the header. The EPRI specifications are now much tighter than the original for SA-335 P91. Examples were given showing differences in both strength and rupture ductility at different temperatures.

      • TP304H and TP347H. The National Institute for Materials Science (NIMS) has evaluated creep-life variability factors for its Creep Data Sheet Project. Studies were conducted up to 200,000 hours. Drew presented data on the long-term effects of nitrogen and boron on material properties at high temperatures.

The bottom line: Damage tolerance should be incorporated into design. Damage tolerant means that a material can uniformly incorporate damage and will normally show signs of distress (during inspections) prior to failure. Damage intolerant means that there is localized accumulation of damage or strain, often at grain boundaries.  

Drew offered the following recommendations:

      1. 1.Critical service should be defined for each construction code, with failure of a pressure part defined as one that would threaten personnel safety and/or result in extended downtime.

      2. 2. For critical service, a damage-tolerant material should be required, or if a damage-intolerant material is used, a penalty factor on allowable stress should be imposed.

      3. 3. For grades of materials in the power industry, codes should be updated to define a damage-tolerant class for a given alloy, perhaps labeled the DT Class. This can be added to ASTM requirements as supplementary.

Cycle chemistry update

AHUG Chairman Barry Dooley, recognized internationally for his expertise in water chemistry and metallurgy, updated the group on cycle chemistry and FAC. “The base issues are global,” he said, “and many plants are experiencing repeat failures.”

Repeat failure situations include:

      • Corrosion product transport.

      • HP evaporator deposits.

      • Instrumentation (insufficient).

      • Drum carryover.

      • Shutdown protection (inadequate).

      • Contaminant ingress.

      • Failure to challenge the plant’s chemistry status quo.

Dooley based his comments on data he gathered during 70 HRSG and 114 fossil plant assessments worldwide.

Next, the chairman presented the case study of a failure in the phase transition zone (PTZ) of a steam turbine serving an F-class 2 × 1 combined cycle. It occurred at 95,000 hours. Cycle chemistry was an amine blend with reducing agent, and phosphate to all drums. There were multiple leaks in the titanium condenser tubes. The LP turbine experienced an L-0 blade failure and pitting was extensive (transition from pit to micro-crack to failure).

Seven root-cause chemistry situations were identified (five directly related to PTZ damage):

      1. 1. Total-iron corrosion products not measured.

      2. 2. No HP evaporator tubes removed to assess deposits.

      3. 3. Instrumentation at low level (53%) compared to recommendations in IAPWS technical guidance documents.

      4. 4. Drum carryover not measured.

      5. 5. Shutdown protection (dehumidification) not applied.

      6. 6. Repetitive contaminant ingress.

      7. 7. Status quo on plant guidance and action levels.

Avoiding repeat situations is critical, explained Dooley: “Cycle-chemistry-influenced failure and damage always can be related back to multiples of repeat cycle-chemistry situations in fossil and combined-cycle plants.”

HRSG thermal performance assessments

Bob Anderson, principal, Competitive Power Resources, a Florida-based consultancy, and a member of the AHUG steering committee, updated the group on the scores of HRSG thermal performance assessments he has conducted globally. Anderson is well known in the US for his work on EPRI projects related to HRSGs and the HRSG Forum with Bob Anderson, a new group formed to help users.

First step in a thermal assessment, Anderson said, is to review HRSG design and component configurations. Primary dangers, well understood, are:

      • Drain pipes too small.

      • Blowdown tank above the HRSG’s headers.

      • Attemperator leakage or overspray.

      • Drain pipes not having continuous downward flow.

      • Drain operation not based on reliable condensate detection.

Plant DCS data can help identify condensate migration, RH and superheater (SH) overspray conditions, and HP pressure ramps set too high during startup. The data, however, must be reviewed carefully.

Operating choices are also important:

      • Are drains opened during purges?

      • Is there manual manipulation of outlet steam-temperature set points?

      • Is a routine attemperator inspection program in place?

      • Is a tube-failure root-cause program in use?

Participants then discussed drum thickness and ramp rates.

Online analysis of “ultra-trace” iron and copper

Tomi Maatta of ANZ-based MEP Instruments, addressed online analyzers for what he called “ultra-trace” iron and copper. The purpose: To understand the correlations between operating conditions, FAC, and metals transport.

This presentation of a promising new technology discussed separation of dissolved and particulate metals, voltammetrics, and repeatability of results.

Also discussed in detail were sampling points, analyzer layout, and detailed online test results. Corrosion examples also were shown.

Using ultrasonic technology to control superheater and reheater drains

Anderson returned to the podium and presented on ultrasonic control of superheater and reheater drains. Large amounts of condensate form in SH and RH tubes during pressurized startup, he said, and drain flow varies greatly with pressure. Small transients are noticeable in bulk steam, but at the tube level these transients (stresses) are very large. The solution is optimum drain operation.

One requirement is to detect water to minimize steam release during purge. Thermocouples cannot do this; high-flow drain-pot arrangements can, but they are expensive and will not fit (physically) under many existing HRSGs.

Ultrasonic detection can determine water in drain pipes, using the transit-time technique, but must be adaptable to high temperatures. Such a design, the WaveInjector®, was reviewed in detail.

It is installed on four HRSGs for testing—various drain-pipe sizes, different materials, and various pipe configurations and control-valve types. Anderson said that “once installed, the WaveInjector is very reliable,” but performance can be impacted by many variables. Testing continues.

IAPWS guidelines for cycle chemistry

David Addison, principal, Thermal Chemistry, and a member of the AHUG steering committee, followed with proper application of IAPWS guidelines for effective cycle chemistry. “Effective cycle chemistry minimizes deposition and corrosion within the whole water/steam circuit of a CCGT plant,” Addison stated. “But it must be a continuous process.”

With that introduction, he reviewed the common results of poor chemistry, including the following:

      • Evaporator iron oxide deposits.

      • Under-deposit corrosion and tube failures.

      • Mechanical and vaporous carryover.

      • Deposition in superheaters and reheaters.

For the steam turbine, this also means:

      • Deposits.

      • Pitting and cracking.

“However,” Addison warned, “cycle chemistry often is overlooked, carried out incorrectly, or not carried out at all.” Such issues often lead to both equipment failure and plant personnel safety concerns. They can present high-risk situations.

Addison also pointed to a problem not often considered: “A unit is designed in country A, manufactured in country B, built in country C, and uses specifications and guidelines from country D.” Therefore, “the world now needs truly international guidance on cycle chemistry that can be the foundation for guidelines in each country, and for manufacturers and other organizations worldwide.”

IAPWS is uniquely placed to offer such guidance, with active participation by knowledgeable representatives from nearly two-dozen nations and drawing on the resources of several international standards-setting organizations—EPRI, VGB, ASME, and others. The IAPWS TGD process is comprehensive, specific, and robust, with an extensive technical review component.

Addison then reviewed several examples, ending with two that were new for 2016:

      • HRSG HP-evaporator sampling for internal-deposit identification and determining the need to chemically clean, and

      • Application of film-forming amines in fossil, combined-cycle, and biomass powerplants.

He wrapped up the presentation with an optimization path for cycle chemistry.

IAPWS updates on Technical Guidance Documents

Dooley followed Addison to explain both the TGD process and the various IAPWS technical groups—including the Power Cycle Chemistry Group—and offered an overview of current activities of the Australian National Association.

He also listed TGDs in development and under consideration:

      • Ensuring the integrity and reliability of demineralized makeup water supply to the unit cycle (2017).

      • Neutralizing amines (2017).

      • Film-forming products for nuclear plants (2017).

      • Air in-leakage (2017).

      • Steam chemistry for geothermal plants (white paper 2017).

      • Corrosion products in flexible (cycling) plants (white paper 2017).

Pitfalls to avoid in steam/water analysis

Chris Wellard, Swan Australia, ended the day with a review of outdated versus modern sampling and analysis designs. The fundamental instrumentation differences:

      • Component-oriented design (dry rack/wet rack), or

      • Modular process-oriented design—online instruments featuring integrated sample preparation (degassed cation conductivity, cation conductivity).

With changes to plant operations (flexibility), new components, and changes to original water chemistry details and methods, traditional systems are no longer adapted to actual monitoring requirements. Even in some new plants, outdated dry/wet-type rack/panel arrangements can prevail, in part to reduce capital cost. But there are consequences:

      • High O&M costs.

      • Upgrade or modification difficulty.

      • Low reliability.

      • Collateral damage to plant components.

The speaker closed with a quick commercial: Swan’s new modular Steam/Water Analysis System is driven by both process and function; modules can be grouped by sample line for:

      • Improved O&M (less cost).

      • Upgrades/modifications.

      • Instrumentation giving perspectives on the process.

Discussions then centered on conductivity measurement and degassing devices. “Swan’s position,” stated Wellard, “is that the fully controlled thermal degasses system, with automatic boiling control, is the optimal method.” Examples of upgrades and replacement systems were discussed.

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