Tag Archive for: fossil power

News & Views, Volume 49 | Rapid Assessment of Boiler Tubes Using Guided Wave Testing

News & Views, Volume 49 | Rapid Assessment of Boiler Tubes Using Guided Wave Testing

News & Views, Volume 49 | Rapid Assessment of Boiler Tubes Using Guided Wave TestingBy:  Jason Ven Velsor, Roger Royer, and Ben Ruchte

Tubing in conventional boilers and heat-recovery steam generators (HRSGs) can be subject to various damage mechanisms.  Under-deposit corrosion (UDC) mechanisms have wreaked havoc on conventional units for the past 40-50 years and have similarly worked their way into the more prevalent combined cycle facilities that employ HRSGs.  Water chemistry, various operational transients, extended outage periods, etc. all play a detrimental role with regards to damage development (UDC, flow-accelerated corrosion, pitting, etc.).

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News & View, Volume 49 | Piping Fabricated Branch Connections

News & Views, Volume 49 | Piping Fabricated Branch Connections

By:  Ben Ruchte

Fabricated branch connections represent a common industry issue in combined cycle plants. Many are vulnerable to early damage development and have experienced failures.  Despite these challenges, a well-engineered approach exists to ensure that the baseline condition is fully documented and a life management plan is put in place to help reduce the overall risk to personnel and to help improve plant reliability.

Fabricated branch connections between large bore pipes (including headers and manifolds) are often fabricated with a reinforced branch commonly in the form of a “catalogue” (standard size) fitting, such as an ‘o-let’. These are more prevalent in today’s combined cycle environment as compared to conventional units that used forged blocks or nozzles rather than welded-on, integrally reinforced pipe fittings. The fittings are typically thicker than the pipes in which they are installed to provide compensating reinforcement for the piping run penetration. Full reinforcement is often not achieved as the current Code requirements place all of the reinforcement on the branch side of the weld joint.  As a result,  higher sustained stresses are generated and, particularly in the case of creep strength enhanced ferritic (CSEF) steels, early formation creep cracking in the weld heat-affected zone (HAZ) can occur (known as Type IV damage – see Figure 1). The well documented challenges of incorrect heat treatment of the o-let weld can also add to the likelihood of damage in CSEF components.  Damage is therefore most likely to occur in fabricated branches that operate with temperatures in the creep range.

Figure 1. Examples of cavities located within the fine-grained HAZ (a few of the cavities are highlighted in red).

Damage is primarily in the form of creep cracking at the toe of the weld on the main run side of the connection (flank position), as shown in Figure 2. The susceptibility to damage early in life (in some cases, before 50,000 hours of service) has been widely reported. As early as 2008, a warning was issued by an architect engineering (AE) company to advise on the known problems. Despite that warning, use of these fittings with their associated inadequacies remains prevalent.  

Figure 2. Example of cracking along the flank positions of o-let connections.

Figure 3. Example of a cross-section through a weld-o-let showing the small size of the weld compared to the thickest part of the nozzle fitting.

Several key factors contribute to early damage development for these components:

Temperature – Most combined cycle plants operate near the 1,040-1,050°F range, which increases the susceptibility to creep damage in Grade 91 HAZs.  Some combined cycle plants operate at much lower temperatures (1,005-1,030°F), which can result in a marked increase in the cross-weld strength.

Geometry – Experience has shown that the size of the branch relative to the main run of piping can have a pronounced effect on the damage vulnerability.  The larger the opening the more reinforcement that is needed at the weld joint.  Current code  requirements place all of the reinforcement on the branch side of the fitting.  The amount of required integral reinforcement is defined only by consideration of the crotch location, not the flank location.  This is a known limitation of the code which in many cases leaves the flank location with insufficient strength.  Figure 3 shows an example of this with a cross-section through a weld-o-let where the small size of the weld compared to the thickest part of the nozzle fitting is evident. SI has performed detailed calculations of these types of cases and found that local stresses at the weld exceeded the allowable stress, even without consideration of weld strength reduction factors (WSRFs).  The use of Grade 91 has highlighted this code deficiency both because of the weakness of the fine-grained HAZ in Grade 91 and because of its greater stress sensitivity (higher stress exponent) compared to common low-alloy steels.

Figure 4. Example of ‘set-through’ left and ‘set-on’ right fabricated connection configurations that shows the orientation of the HAZ (red dashed lines) compared to the hoop stress.

It is also important to mention the various styles of welded configurations (Figure 4):

  • ‘Set-on’ represents a more standard o-let connection where the HAZ of the saddle weld follows the OD of the main run pipe and is oriented parallel to the internal hoop stress from pressure.
  • ‘Set-through’ is less common and has mostly been associated with HRSG-supplied piping.  In this configuration, the HAZ of the saddle weld traverses through the thickness of the main run pipe and is oriented mostly normal to the internal hoop stress from pressure.  
  • λ This can result in much more rapid damage propagation.

Figure 5. Example of common o-let locations within high energy piping (HEP) systems.

Chemistry – As defined by EPRI, select impurity or tramp elements in high enough concentrations can reduce the damage tolerance of Grade 91 material resulting in greater cavitation susceptibility.  

Added System Loads – Damage can become non-uniform and develop more rapidly across the flank positions when malfunctioning supports are in the vicinity of these connections (e.g. bending). 

Despite the numerous issues, there are several simple approaches to screen these connections:

  1. Determine the piping systems that operate within the creep regime (typically high pressure/main steam, hot reheat, gas turbine transition cooling, etc.).
  2. Review detailed isometrics on both the architect engineering (AE), HRSG-supplied, and turbine-supplied piping looking for specific junctions (see Figure 5).
    • Bypass take-offs
    • HRSG-to-HRSG connection points
    • Drains
    • Turbine lead splits
    • Link piping from HRSG-exit-to-collection manifolds
  3. ‘Golden ratio’ of branch OD/main run OD >0.5, where damage susceptibility increases as the ratio approaches 1 – SI has experience with damage development at ratios ≥0.5.
  4. Verify materials of construction.  The problem is intensified by the creep-weak nature of the Type IV location (fine-grained HAZ) in Grade 91 steel; however, low-alloy steels such as Grade 11 and Grade 22 are not immune.

Figure 6. Example of a replication location at a flank position for a weld-o-let. A close-up of the replication site shows a macro-crack (red arrows) located within the Type IV zone (bound by the yellow lines).

If fabricated connections are identified, a baseline condition assessment through nondestructive examinations should be performed via several techniques:

  1. Positive material identification (PMI) via X-ray fluorescence spectrometry (XRF) to assess general material compositions.
  2. Ultrasonic wall thickness testing (UTT) to check thicknesses of the o-let, branch pipe, and main run pipe.
  3. Wet fluorescent magnetic particle testing (WFMT) for identification of surface-connected defects.
  4. Hardness testing of the surrounding area to detect possible anomalies from heat treating.  
  5. Metallurgical replication can be used to determine if creep cavities are present and should be performed at the main run pipe side toe at the flank locations on both sides of the connection (Figure 6).
  6. Metal shavings can be collected from the main run of piping for a more detailed chemical analysis to determine if impurities or tramp elements are present at levels that could reduce the overall damage tolerance.
  7. Laser surface profilometry (LSP) is a technique that can be used to capture a detailed 3D model of a component for an accurate geometry for computational modeling.  While this technique does not provide any quantitative data itself, it is very useful in analytical techniques to determine potential geometric constraints that could result in additional sustained stresses on the component, which could significantly increase damage accumulation.  

Figure 7. Example LSP rendering that can be used for finite element analyses.

Several steps can be considered to mitigate damage in these types of joints:

  1. Weld build-up at the saddle, and in some cases the crown, can be applied to improve the strength of the connection.  Finite element analysis, completed via the 3D model captured from the LSP scan (Figure 7), can be used to estimate the amount of weld build-up required to appropriately decrease stresses; however, the amount of weld buildup necessary is very often impractically large. 
  2. Replacing (or specifying) fabricated joints with forged fittings (Figure 8), which eliminates welding at the branch connection and provides a more balanced reinforcement, is the best method of dealing with these components.
  3. Pipe support modifications to reduce bending and other system loads.
  4. Re-normalizing and tempering the component after fabrication can minimize the detrimental effects of the HAZ and reduce the likelihood of Type IV cracking. 

Figure 8. Example of a fully contoured, uniform forging that can eliminate these problematic saddle weld joints.

Summary

In summary, HEP systems should be globally reviewed to determine if these fabricated connections exist and to what level that they may pose a problem for safety and reliability of the plant.  Once identified, a baseline condition assessment should be performed, and a life management plan should be implemented.  Detailed engineering analyses that use models with the appropriate Grade 91 creep damage mechanics can be used to determine whether these components need true mitigation (repair/replacement) or if appropriate re-inspection intervals are a sufficient mitigation step.  Consideration should also be given to assessing continuous operating data (temperatures and pressures) to help understand life consumption with actual operation.  

Footnotes

(1)   ASME B31.1 (requirements for integrally-reinforced branch fittings defined in Paragraph 127.4.8 and the associated Figure 127.4.8(E).  Some requirements for the pressure design of such fittings are also provided in Paragraph 104.3.1 of ASME B31.1.

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News & Views, Volume 49 | Attemperator Monitoring with Wireless Sensors - Risk and Cost Reduction in Real Time

News & Views, Volume 49 | Attemperator Monitoring with Wireless Sensors: Risk and Cost Reduction in Real Time

News & Views, Volume 49 | Attemperator Monitoring with Wireless Sensors - Risk and Cost Reduction in Real TimeBy: Jason Van Velsor, Matt Freeman and Ben Ruchte

Installed sensors and continuous online monitoring are revolutionizing how power plants manage assets and risk by facilitating the transformation to condition-based maintenance routines. With access to near real-time data, condition assessments, and operating trends, operators have the opportunity to safely and intelligently reduce operations and maintenance costs and outage durations, maximize component lifecycles and uptime, and improve overall operating efficiency.

But not all data is created equal and determining what to monitor, where to monitor, selecting appropriate sensors, and determining data frequency are all critical decisions that impact data value. Furthermore, sensor procurement, installation services, data historian/storage, and data analysis are often provided by separate entities, which can lead to implementation challenges and disruptions to efficient data flow.

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Structural Integrity Associates Announces the Appointment of Steve Gressler as Vice President, Fossil

Structural Integrity Associates Announces the Appointment of Steve Gressler as Vice President, Fossil

Structural Integrity Associates | Steve Gressler, Vice President, FossilStructural Integrity Associates, Inc. (SI), a leader in engineering and technology in the power and oil & gas industries, today announced the appointment of Steve Gressler as Vice President of its Fossil business. Gressler will report to Mark Marano, SI President and Chief Executive Officer.

Mr. Gressler is a metallurgical engineer with over 30 years of experience in metallurgy, failure analysis, and the damage assessment of fossil power generation equipment and distributed heating and cooling systems with an emphasis on piping, boilers, headers, and balance of plant equipment. He is a principal contributor to engineering life cycle programs and procedures, risk assessments, analysis tools, and technologies to assess service degradation in low alloy and creep-strength enhanced ferritic steels.  His current work focuses on the development, integration, and deployment of nondestructive testing and damage tracking technology in addition to engineering consulting and materials assessments.

Since joining Structural Integrity Associates in 1997, Steve has been a principal contributor to the methodologies applied for life assessment of high temperature and critical piping components. This includes characterization of creep and fatigue damage in longitudinal seam welds, girth welds, and header ligaments.  It also extends to the engineering of data management systems used for aiding decision-making processes.

Marano noted, “Steve brings metallurgical leadership and technology development experience to the SI leadership team and is an integral part of our new customer focused strategy. Steve’s knowledge of the fossil business, tenure with SI, and relationships with our clients will accelerate our journey to create value for power and oil industry customers as a highly valued engineering and technology partner.”

Gressler received a Bachelor of Science in Metallurgical Engineering from the University of Pittsburgh and is a Registered Professional Engineer. Steve is also a prior recipient of the SI Riccardella Innovation Award for Prioritization Method for Fossil Plant Piping.

News & View, Volume 47 | Materials Lab Featured Damage Mechanism- SH:RH Fireside Corrosion in Conventional Coal Fired Boilers

News & Views, Volume 47 | Materials Lab Featured Damage Mechanism: SH/RH Fireside Corrosion in Conventional Coal Fired Boilers

By:  Wendy Weiss

Superheater/reheater fireside corrosion is also known as coal ash corrosion in coal fired units.

News & View, Volume 47 | Materials Lab Featured Damage Mechanism- SH:RH Fireside Corrosion in Conventional Coal Fired Boilers

MECHANISM
Coal ash corrosion generally occurs as the result of the formation of low melting point, liquid phase, alkali-iron trisulfates. During coal combustion, minerals in the coal are exposed to high temperatures, causing release of volatile alkali compounds and sulfur oxides. Coal-ash corrosion occurs when flyash deposits on metal surfaces in the temperature range of 1025 to 1200oF. With time, the volatile alkali compounds and sulfur compounds condense on the flyash and react with it to form complex alkali sulfates such as K3Fe(SO4)3 and Na3Fe(SO4)3 at the metal/deposit interface, which are low melting point compounds. The molten slag fluxes the protective iron oxide covering the tube, exposing the metal beneath to accelerated oxidation.

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News & View, Volume 46 | Metallurgical Lab Featured Damage Mechanism- Waterwall Fireside Corrosion (WFSC) in Conventional Boilers

News & Views, Volume 46 | Metallurgical Lab Featured Damage Mechanism: Waterwall Fireside Corrosion (WFSC) in Conventional Boilers

By: Wendy Weiss

News & View, Volume 46 | Metallurgical Lab Featured Damage Mechanism- Waterwall Fireside Corrosion (WFSC) in Conventional BoilersIndustry experience shows that waterwall tubing in conventional boilers can be susceptible to fireside corrosion, depending on fuel type, firing practice, etc. In boilers where fireside corrosion has been identified as a maintenance issue, wastage rates of 5 to 25 mils/year are not uncommon. Since the mid 1990s, the installation of low NOx burner systems designed to lower NOx emissions has significantly increased the wastage rates in some boilers. Operators of subcritical boilers have reported wastage rates as high as 30 mils/year, while those operating supercritical boilers have reported rates exceeding 100 mils/year in the worst cases. These higher damage rates have resulted in an increase in tube failures, and operators have struggled to accurately define the extent of the damage and install the appropriate mitigating technologies.

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News & View, Volume 46 | Plant Integrity Assistant – There’s an App for That! PlantTrack App Now Available

News & Views, Volume 46 | Plant Integrity Assistant – There’s an App for That!

By:  Matt Freeman

PlantTrack App now available

News & View, Volume 46 | Plant Integrity Assistant – There’s an App for That! PlantTrack App Now AvailablePlant engineers must often answer ‘what damage is this piece of equipment susceptible to, and what can we do about it?’ or ‘how much longer can this component continue operating’?   A free app is now available in both the Google Play store and the Apple App store to help answer these and other equipment integrity questions (see figure 1).

The PlantTrack App is a plant integrity assistant that provides life calculators, damage mechanism guides, technical articles, lab sample submittal form, and other functions relevant for plant engineers.  While it is geared towards fossil (both coal and gas) power plants, it has applicability for equipment at other plants as well.  For subscribers to the PlantTrack web application, the mobile app also provides a dashboard view of inspection history, action items, damage tracking results, etc.  Users of the PlantTrack software will recognize some of the features from the ‘PlantTrack Tools’ that have previously been available through a web browser.

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