News & Views, Volume 53 | Serviceability Assessment of an L-Grade Stainless Steel Pipe Fitting

By: Terry Totemeier

A client recently ordered a Type 316 stainless steel pipe coupling fitting for use in a high-pressure, high-temperature steam line operating at 1005°F.  The fitting that was received was so-called dual grade Type 316/316L stainless steel.  Given the limitations on using “L” grades of stainless steel at high temperatures, the client requested that SI perform a serviceability assessment for the fitting to determine if it could be safely used until the next scheduled outage when a replacement non-L grade fitting would be available.

The fitting ordered was a ½” nominal diameter (NPS ½), 6000# (Class 6000) full coupling socket-welding fitting in accordance with the ASME B16.11 specification, material ASME SA-182 forging, Type 316 stainless steel (designated as F316 in SA-182).  The fitting supplied was dual grade F316/316L material with a carbon content of 0.023% per the material test certificate.  The designation of this material as “dual grade” means that it meets the requirements of both F316 and F316L material grades.  This is possible because the chemical composition requirements of these two grades overlap, with the primary difference between them being carbon content.  For F316 the carbon content is specified to be 0.08% maximum (no minimum), while for F316L the carbon content is specified to be 0.030% maximum.  Therefore, material with carbon content less than 0.030% will meet the requirements for both grades.  It is worth noting that the carbon content of “H” grade of 316 stainless steel (F316H per SA-182) is specified to be 0.04-0.10%.  The H grade is intended for use at high temperatures.

The received fitting was installed in a main steam valve pressure equalizing line with a steam temperature/pressure of 2750 psia/1015°F at design conditions and 2520 psia/1005°F at operating conditions.  The fitting was welded to Grade P11 pipe on one side and Grade P22 pipe on the other side.  The applicable code was stated to be ASME BPVC Section I.

With a reported carbon content of less than 0.04%, the fitting is technically not permitted for use in ASME Section I construction above a temperature of 1000°F.  Per the ASME Boiler and Pressure Vessel Code (BPVC) Section II, Part D, Table 1A, the allowable stresses for SA-182, F316 material are valid at or above 1000°F only when the carbon content is greater than 0.04% (Note G12).  Per the same table, SA-182, F316L material is only permitted for use in Section I construction up to 850°F.  The reason for this temperature limitation is that the long-term creep-rupture strength of Type 316 stainless steel with lower carbon content is reduced compared to material with higher carbon content because fewer carbides form during service to strengthen the grain boundaries.  There are no other adverse impacts of the lower carbon content, e.g., on fatigue strength or oxidation resistance.

The short-term serviceability of the fitting with low carbon content was assessed by comparing bounding pressure stresses in the fitting with the reported creep-rupture strength for Type 316L material.  Per the ASME B16.11 specification, Class 6000 socket-welding fittings are compatible with NPS Schedule 160 pipe, meaning that pressure stresses in the fitting will be less than those in Sch 160 pipe with minimum wall thickness according to ASME B36.10 (pipe dimension specification), in other words, the fitting will be at least as strong as the pipe.  

The dimensions of NPS ½, Schedule 160 pipe per the ASME B36.10 pipe specification are 0.84” outer diameter (OD), 0.165” minimum wall thickness (MWT).  For an operating steam pressure of 2,520 psi, the reference hoop stress per the equation in ASME BPVC Section I, Appendix A-317 is 5.05 ksi.  Per the general design guidance in ASME B16.11 (Section 2.1.1) the pressure stresses in the fitting must be less than this.  

Figure 1. Schematic diagram for a socket-welding coupling fitting. Per ASME B16.11, an NPS ½, Class 6000 fitting has relevant dimensions B = 0.875” maximum, C = 0.204” minimum, and D = 0.434” minimum.

Since the fitting in question is cylindrical, comparative hoop stresses can also be calculated from dimensions given in ASME B16.11, although these may not be exact due to the varied wall thickness in the fitting.  According to Table I-1 of ASME B16.11, the central body of the fitting is 1.283” OD and 0.395” MWT (Figure 1).  The reference hoop stress calculated using the A-317 equation at 2,520 psi stream pressure and these dimensions is 2.63 ksi, considerably less than 5.05 ksi.  In the female socket ends of the fitting, the OD is also 1.283”, but the minimum wall thickness is 0.204”, leading to a calculated reference hoop pressure stress of 6.58 ksi.  Note that the actual stresses in the socket ends will be much less than this because the pipe will be inserted and welded into the socket, taking up the pressure loading, but the calculated stress can be taken as a bounding value.

Creep-rupture strengths for Type 316L stainless steel have been reported in ASTM Data Series DS 5S2 publication, “An Evaluation of the Yield, Tensile, Creep, and Rupture Strengths of Wrought 304, 316, 321, and 347 Stainless Steels at Elevated Temperatures” (ASTM, 1969).  According to Table 7 in this report, the average 10,000 hour creep-rupture strengths for Type 316L at 1000°F and 1050°F are 34.5 and 25 ksi, respectively.  Minimum creep-rupture strengths are typically taken as 80% of the average strength, so the inferred minimum strengths at 1000°F and 1050°F are 27.6 and 20 ksi, respectively.  

The reported 10,000 hour creep-rupture strengths in the temperature range of interest are more than twice the calculated bounding pressure stresses in the fitting, so it was judged that there is very little risk of failure of the fitting by creep-rupture in the next 10,000 hours of service.

This result is unsurprising since the 1005°F is barely into the creep range for Type 316 regardless of carbon content.  The carbon content effects become more pronounced at higher temperatures (approximately 1100°F and above).

Based on the above assessment, it was SI’s opinion that the Type 316L fitting with carbon content less than 0.03% was suitable for a limited period of service (less than 10,000 hours) until it can be replaced.  Given that the fitting is reportedly welded to low-alloy steel pipe on either side, SI also recommended that a Grade 22 (2.25Cr-1Mo) low-alloy steel fitting be considered as a replacement, which would eliminate dissimilar metal welds (DMWs) between the fitting and pipes.  DMWs are prone to premature failure due to thermal fatigue, weld fusion line cracking, and decarburization of the ferritic material. This voluntary recommendation made by SI, was not part of the original scope of work, but may have been just as critical a finding as it shed light upon a failure risk previously unknown by the client. 

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News & Views, Volume 52 | Online Monitoring of HRSG with SIIQ™

Figure 1. Typical components that are monitored with the pertinent damage mechanisms in mind.


By:  Kane Riggenbach and Ben Ruchte

SI has successfully implemented a real-time, online, damage monitoring system for the Heat Recovery Steam Generators (HRSGs) at a combined cycle plant with a 3×1 configuration (3 HRSGs providing steam to a single steam turbine).  The system is configured to quantify and monitor the life limiting effects of creep and fatigue at select locations on each of the HRSGs (e.g. attemperators, headers, and drums – see Figure 1).  The brand name for this system is SIIQ™, which exists as a monitoring solution for high energy piping (HEP) systems and/or HRSG pressure-part components.  SIIQ™ utilizes off-the-shelf sensors (e.g. surface-mounted thermocouples) and existing instrumentation (e.g. thermowells, pressure taps, flow transmitters, etc.) via secure access to the data historian.  The incorporation of this data into SI’s damage accumulation algorithms generates results that are then displayed within the online monitoring module of SI’s PlantTrack™ data management system (example of the dashboard display shown in Figure 2).  

Figure 2. Example dashboard of the health status and ‘action’ date for a variety of components.

This article will be part of a series discussing items such as the background for monitoring, implementation/monitoring location selection, and future results for the 3×1 combined cycle plant.  

  • Article 1 (current):  Introduction to SIIQ™ with common locations for monitoring within HRSGs (and sections of HEP systems)
  • Article 2: Process of SIIQ™ implementation for the 3×1 facility with a discussion of the technical foundation for damage tracking
  • Article 3: Presentation of results from at least 6+ months, or another appropriate timeframe, of online monitoring data

The owner of the plant implemented the system with the desire of optimizing operations and maintenance expenses by reducing inspections or at least focusing inspections on the highest risk locations.  The system has been in place for a few months now and is continuously updating risk ranking of the equipment and ‘action’ intervals.  The ‘action’ recommended may be operational review, further analysis, or inspections.  This information is now being used to determine the optimum scope of work for the next maintenance outage based on the damage accumulated.  Like many combined cycle plants, attemperators are typically a problem area.  Through monitoring, however, it can be determined when temperature differential events occur and to what magnitude.  Armed with this information aides in root cause investigation but also, if no damage is recorded, may extend the inspection interval.

Many HRSG systems are susceptible to damage due to high temperatures and pressures as well as fluctuations and imbalances.  Attemperators have been a leading cause of damage accumulation (fatigue) through improper design/operation of the spray water stations (Figure 5).  In addition, periods of steady operation can result in accumulation of creep damage in header components (Figure 6) and unit cycling increases fatigue and creep-fatigue damage in stub/ terminal tubes and header ligaments (Figure 7).  Monitoring the damage allows equipment owners to be proactive in mitigating or avoiding further damage.

Traditionally, periodic nondestructive examinations (NDE) would be used to determine the extent of damage, but in HRSGs this can be challenging due to access restraints and, in the case of the creep strength enhanced ferritic (CSEF) materials such as Grade 91, damage detection sensitivity is somewhat limited until near end of life.  Continuous online monitoring and calculations of damage based on unit-specific finite element (FE) models (sometimes referred to as a ‘digital twin’) with live data addresses this issue.

Figure 4. Examples of damage observed by SI on attemperators.

Reliable life consumption estimates are made by applying SI’s algorithms for real-time creep and fatigue damage tracking, which use operating data, available information on material conditions, and actual component geometry.

Figure 5. Examples of creep damage observed by SI on header link pipe connections (olets).

SIIQ tracks trends in damage accumulation to intelligently guide life management decisions, such as the need for targeted inspections, or more detailed “off-line” analysis of anomalous conditions. This marks a quantum leap forward from decision making based on a schedule rather than on actual asset condition. 

Figure 6. Examples of creep/fatigue damage observed by SI at tube-to-header connections.

Figure 7. Examples of online monitoring alerts generated from SIIQ

SIIQ can be configured to provide email alerts (Figure 7) when certain absolute damage levels are reached, or when a certain damage accumulation over a defined time frame is exceeded. In this way, the system can run hands-off in the background, and notify maintenance personnel when action might be required.

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Structural Integrity Associates | Wireless Sensor Node Featured Image

High Energy Piping Monitoring

High Energy Piping Monitoring

SI moves beyond the pilot application of a High Energy Piping monitoring program designed to reduce operational risk and optimize maintenance activities.

Structural Integrity Associates | Wireless Sensor Node 6.17ESI has successfully implemented the initial application of an integrated monitoring solution that provides insight into damage evolution and operational risk using real-time data and automated engineering intelligence. This solution will assist in the optimization of maintenance activities and downtime, helping utilities get the most out of their O&M budgets.  “This is a decisive step toward a more modern asset management approach that will lower O&M cost for our clients,” said Steve Gressler, Vice President, SI Energy Services Group, a division of Structural Integrity Associates, Inc. (SI) focused on power plant asset integrity.

Informed by decades of material performance knowledge, the SI team has refined a proprietary risk-ranking method to optimize sensor placement and deliver a high-value monitoring platform supported by the PlantTrack™ asset data management platform.  The integration of monitoring information into the platform further enhances equipment asset integrity data to simplify stakeholder decision making.   The SI solution incorporates various sensors working on a distributed wireless network to feed real-time data to SI’s state-of-the-art algorithms and is also capable of integrating with existing plant data historians to pull in other valuable operational data. The outcome is a cost-effective damage monitoring approach to focus resources and the timing of comprehensive field inspections.

“The architecture enables asset managers to obtain real-time feedback, alerts, and trends that clearly link actual operating conditions to the lifecycle of critical components.,” said Jason Van Velsor, Director of Integrated Monitoring Technology at SI.

“We have supported clients with asset integrity insights for decades and now offer enhanced monitoring technology that will help automate risk management for high energy piping and help obtain the most value out of field inspection and other maintenance activities during outages.”

Unique Features of the SI Solution include:

  • Design and application of a monitoring program that focuses on safety and reliability and is consistent with guidance contained in the ASME B31.1 regulatory code.
  • Expert assessment (or Gap Analysis) to optimize monitoring including health checkups to validate optimum monitoring for plant operation.
  • Decades of material analysis insights as algorithms to expertly inform decision making.
  • Customized automated alerts to notify operators of abnormal or undesirable operating conditions affecting the life of high-energy components.

Contact Steve or Jason to learn more (

News and Views Volume 49, Attemperator Monitoring with Wireless Sensors 02

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News and Views Volume 49, Attemperator Monitoring with Wireless Sensors

News & Views, Volume 49 | Materials Lab Featured Damage Mechanism - Soot Blower Erosion

News & Views, Volume 49 | Materials Lab Featured Damage Mechanism: Soot Blower Erosion

News & Views, Volume 49 | Materials Lab Featured Damage Mechanism - Soot Blower ErosionBy:  Wendy Weiss

Soot blower erosion (SBE) is caused by mechanical removal of tube material due to the impingement on the tube wall of particles entrained in the “wet” blower steam. As the erosion becomes more severe, the tube wall thickness is reduced and eventually internal pressure causes the tube rupture.


SBE is due to the loss of tube material caused by the impingement of ash particles entrained in the blowing steam on the tube OD surface.  In addition to the direct loss of material by the mechanical erosion, SBE also removes the protective fireside oxide. (Where the erosion only affects the protective oxide layer on the fireside surface, the damage is more properly characterized as erosion-corrosion.) Due to the parabolic nature of the oxidation process, the fireside oxidation rate of the freshly exposed metal is increased. The rate of damage caused by the steam is related to the velocity and physical properties of the ash, the velocity of the particles and the approach or impact angle. While the damage sustained by the tube is a function of its resistance to erosion, its composition, and its operating temperature, the properties of the impinging particles are more influential in determining the rate of wall loss.


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.


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.


News & View, Volume 48 | Bypass Line Spray Issues

News & Views, Volume 48 | Bypass Line Spray Issues

News & View, Volume 48 | Bypass Line Spray IssuesBy:  Ben Ruchte and Kane Riggenbach

To provide operating flexibility, combined cycle plants are typically equipped with bypass systems (high pressure routing steam to cold reheat and hot reheat routing steam to the condenser).  These bypass systems include conditioning valves designed to reduce steam pressures followed by outlet desuperheaters which inject water to reduce steam temperatures.

This service environment exposes the downstream piping to a high frequency of temperature transients making these areas one of the most prominent ‘industry issues’.


News & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process

News & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process

By:  Ben Ruchte, Steve Gressler, and Clark McDonaldNews & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process

Properly inspecting plant piping and components for service damage is an integral part of proper asset management.  High energy systems constructed in accordance with ASME codes require appropriate inspections that are based on established industry practices, such as implementation of complimentary and non-destructive examination (NDE) methods that are best suited for detecting the types of damage expected within the system.  In any instance where NDE is used to target service damage, it is desirable to perform high quality inspections while at the same time optimizing inspection efficiency in light of the need to return the unit to service.  This concept is universally applicable to high energy piping, tubing, headers, valves, turbines, and various other power and industrial systems and components.


News & View, Volume 45 | Life Management for High Energy Piping (HEP)

News & Views, Volume 45 | Life Management for High Energy Piping (HEP)

By:  Matt Freeman

News & View, Volume 45 | Life Management for High Energy Piping (HEP)High Energy Piping systems, including main steam and hot reheat piping, are typically very reliable and can often operate trouble-free for decades.  However, due to the combination of pressure and temperature at which such systems operate, a failure can have catastrophic consequences from a safety perspective and in terms of equipment loss.  Because of this and the requirements of the ASME B31.1 Power Piping code, HEP programs – or as defined by Code, Covered Piping Systems (CPS) – are established to ensure that the integrity of the system is maintained throughout their lifecycle.  This article discusses the steps required to implement an HEP / CPS life management program.

A Life Management Program is not synonymous with an inspection program.  Inspections are an important part of an overall program but should be complimentary to the use of analytical tools, real-time monitoring, and laboratory examinations


News & View, Volume 45 | The Importance of HRSG HP Evaporator Tube Internal Deposit Evaluation

News & Views, Volume 45 | The Importance of HRSG HP Evaporator Tube Internal Deposit Evaluation

By:  Barry Dooley

News & View, Volume 45 | The Importance of HRSG HP Evaporator Tube Internal Deposit EvaluationEvaluation of High Pressure (HP) Evaporator Tube Deposits is important for several reasons:

  • Determining if flow-accelerated corrosion (FAC) might be occurring in the lower pressure circuits.
  • Regular evaluations can provide information on the internal deposit deposition rate, which is information necessary to help prevent under-deposit corrosion damage mechanisms.
  • Provides information necessary to develop an optimized cycle chemistry for HRSGs.
  • Can help determine if the HRSG needs to be chemically cleaned.

The leading heat recovery steam generator (HRSG) tube failure mechanisms are FAC, thermal and corrosion fatigue, and under-deposit corrosion (UDC) and pitting. The corrosion products released by the FAC mechanism are transported from the affected area (typically the feedwater or lower pressure systems) and can eventually reach the HP evaporator tubing, so understanding the deposition in the HP evaporator is an important step in determining if FAC might be occurring. Deposition on the inside of HP evaporator tubing is also a precursor to any of the under-deposit corrosion HRSG tube failure mechanisms.