Design by Analysis in the Modernized Boiler Code

Proceedings of the ASME 2018 Symposium on Elevated Temperature Application of Materials for Fossil, Nuclear, and Petrochemical Industries ETAM2018 April 3-5, 2018, Seattle, WA, USA

ETAM2018-6749 Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021

DESIGN BY ANALYSIS IN THE MODERNIZED BOILER CODE

David I Anderson David J Dewees Doosan Babcock Limited Becht Engineering Crawley, West Sussex, United Kingdom Medina OH, USA

ABSTRACT boilers do and will operate at higher temperatures and pressures In general Section I of the ASME Boiler Code was originally under cyclic loading, requiring a more detailed assessment and developed for industrial boilers through to sub-critical boilers examination to ensure safe and reliable operation. Essentially, at operating at relatively low temperatures and pressures under present, ASME Section I is supplemented by additional steady state conditions. Current and future boilers do and will requirements based on manufacturers’ own experience and operate at higher temperatures and pressures under cyclic expertise to ensure safety and reliability. loading requiring a more detailed assessment and examination to ensure safe and reliable operation. Other Codes and Standards, such as the European Standard EN12952, being a more recent Standard, include some of the Design by Analysis (DBA) methods will be fundamental to the rules and guidance to meet these requirements, as illustrated by assessment process for key boiler components. It is intended Figure 1. ASME Section VIII Division 2 is also being revised to that the Code will incorporate several DBA methods, ranging in add Code rules to allow Design by Analysis in the time complexity, to allow the user some flexibility to select the dependent regime combined with fatigue. To ensure ASME method appropriate to the design conditions. Section I remains as the pre-eminent Code of choice for pressure equipment some form of modernization is required. The methods currently being considered include an elastic approach based on Section VIII Division 2, a simplified inelastic approach, an inelastic approach based on the Omega method from API 579, the Section VIII Division 2 Code Case 2843 based on the Section III Part NH rules utilizing the strain deformation method and a new Section III Code Case based on the EN 13445 approach.

This paper will look at the key aspects of the methods and highlight the limitations of each. Figure 1 – Illustration of the differences between Codes INTRODUCTION THE MODERNIZATION PROCESS In general Section I of the ASME Boiler Code [1] was ASME charged the main Section I (SC-I) Committee with the originally developed for industrial boilers through to sub- task of investigating the needs and preparing a roadmap for the critical boilers operating at relatively low temperatures and future development of the Code. It was this Committee that pressures under steady state conditions. It also only really formed the special Task Group (TG) for Modernization from addresses pressure containment and not other loadings such as expert members of the SC-I Committees. external loads, thermal loads and fatigue. Current and future

Each SC-I Sub-Group (SG) was required to review and same quality specifications and at similar costs. This is not only compare the text of both Section I and Section VIII Division 2 applicable to current plants (both HRSG and USC plants), but is to identify an initial view on what text was relevant for the also key to progression of the development of HSC plants for modernized Code and where any technical gaps lay and whether the future. the gaps could be covered by reference to other Codes or Standards. Figure 2 illustrates an example of the initial gap analysis focused on the fabrication rules. Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021

Figure 2 – Example Gap Analysis

Figure 4 – Data Sources for Design by Analysis Early on in the process two key technical gaps were identified that required external input to fill. These were (a) guidelines on Various Design by Analysis methodologies are being reviewed how to address the effects of high temperature erosion, for inclusion in the modernized Code. corrosion and oxidation [4] and (b) rules for incorporation of design by analysis methods into boiler design [5]. Other gaps, Whilst Section I is written for new boiler construction the such as NDE acceptance requirements were also identified but introduction of Design by Analysis will open up the Code to it was thought that the SG could provide the necessary input to other applications for fitness for service and life assessments. fill them. Figure 3 shows the two ASME published reports. By its very nature it will introduce the concept of “design life”;

something that is currently not defined by the Code.

Additionally, by adopting the industry best practices for areas

such as advanced NDE and life monitoring systems it will also

aid both the owner/operator and the OEM to ensure the best

availability and life is obtained for plants based on the more

onerous operating conditions that current and future plants will

be subjected to.

Currently these advanced techniques are used either to

supplement the existing Code rules or for fitness for service Figure 3 – ASME Published Reports assessments.

The modernized Code will include essential technical additions DESIGN BY ANALYSIS such as more prescriptive heat treatment and NDE as well as enabling the effects of creep-fatigue to be addressed with the Design by Analysis (DBA) methods will be fundamental to the introduction of Design by Analysis enabling state of the art assessment process for key boiler components. It is intended boiler components to be designed and operated. that the Code will incorporate several DBA methods, ranging in

complexity, to allow the user some flexibility to select the The principle is that there will be a harmonized approach that method appropriate to the design conditions. enables Design by Rule and Design by Analysis to be integrated such that only those components of the boiler that would benefit The methods currently being considered include an elastic from the more rigorous Design by Analysis methods need be approach based on Section VIII Division 2, an inelastic subjected to these methods. Figure 4 illustrates some of the approach based on the Omega method from API 579, the sources being used. These proposals are based on best practice Section VIII Division 2 Code Case 2843 based on the Section and when included will require all manufacturers to work to the

III Part NH rules utilizing the strain deformation method and a Creep-Fatigue Interaction – To use this Part of the Code a new Section III Code Case based on the EN 13445 approach. fatigue screening process must be undertaken to demonstrate no creep-fatigue interaction. Method 1: Elastic Approach (based on Section VIII Division 2, New (DRAFT) Part 5.6) Two fatigue screening criteria which must be met (1) the number of full-range pressure cycles must not exceed 250 and

Part 5.6 is organised with each sub-paragraph addressing one of (2) the total number of cycles (including full-range and Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021 the potential failure modes that are addressed in the rest of part significant pressure cycles and significant temperature cycles) 5: rupture, buckling, creep/fatigue interaction, and ratcheting. must not exceed 500. If both these criteria are met then a detailed creep-fatigue analysis is not required. The procedure evaluates protection against stress rupture using elastic stress analysis. It also includes a fatigue screening Creep Ratcheting – the summation of local primary membrane, method. Figure 5 illustrates the traditional stress categories and primary bending and secondary stress range must be kept within associated stress limits for time independent conditions. The the sum of the cold yield strength and hot allowable stress. new draft Part 5.6 invokes more restrictive primary stress limits and requires that secondary stresses due to primary loading (e.g. pressure-induced discontinuity stresses) be treated as primary. This is based on the well-established differences in relaxation behavior between time dependent creep and time independent plastic action.The full Stress Rupture is addressed in a 7 step process, as given in Table 1.

STEP 1 Define the loads and load combinations, evaluating those associated with “load- controlled” loads (e.g. pressure or weight) and “strain-controlled” loads (e.g. thermal gradients or imposed displacements). Tables 5.1 and 5.3 give guidance. STEP 2 At the location of interest calculate the stress tensor (6 components of stress) and assign to either (1) General primary membrane, (2) Local primary membrane, (3) Primary bending, or (4) Secondary as defined by Figure 5.1 (Noting that Service Stress is currently not considered). STEP 3 Sum the stress tensors for each stress category Figure 5 – Traditional Stress Category Assessment Limits STEP 4 Determine the principal stress for each stress tensor and compute the equivalent Method 2 Inelastic Approach (based on the Omega method stress from API 579) STEP 5 Apply appropriate weld strength reduction factor This method is based on the Omega method of API 579 Part 10, STEP 6 Determine the time dependent allowable currently applied to fitness for service evaluations but equally stress applicable to new construction design, see Figure 6. While STEP 7 Evaluate protection against plastic Part 10 is a complete fitness-for-service procedure and draws on collapse (time independent regime) or existing Code methods for failure modes such as plastic stress rupture (time dependent regime) collapse, the unique portion exploited for time-dependent design is the creep/damage material model (the “MPC Omega Table 1 – Stress Rupture 7 Step Assessment Process Model”). This model will be included as a Code Case that allows detailed inelastic analysis to support both stress rupture Creep Buckling – is considered for external pressure, generally and local damage estimates. Note that creep-fatigue is utilising the isochronous stress-strain curve approach. addressed in this method by (un-coupled) detailed creep and plasticity inelastic analysis; the output damage fractions are used with the same interaction diagrams of CC2843. Figure 7 illustrates the simplified concept of evaluating both creep and

fatigue life usage. The “knuckle” point is set at 0.15 for all The Code Case also addresses creep-fatigue criterion which carbon and low alloy steels (note some other Codes only restrict further brings in the lifetime specification for components. sum to be less than unity). Figure 8 illustrates the application within a FE model and Additionally, the model will allow generation of isochronous typical areas of consideration for assessment. stress-strain curves.

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Material data for the MPC Omega model will be provided in the Code Case as illustrated in Figure 6.

Figure 8 – Illustrates of the Application within a FE model As with the Section VIII Division 2 Part 5, this Code Case uses the different loads and load combinations, which are assessed for adequacy against different stress limits. This is illustrated in Figure 9.

Figure 6 – API 579 / ASME FFS-1 Material Data Illustration

Figure 9 – Flow Diagram for Load-Controlled Stress Limits

The concept of design life is also introduced into this methodology with design curves being specified in ASME Section IID and life fraction rules being specified within the Code Case.

Figure 7 Illustration of Evaluation of Ceep and Fatigue Life Figure 10 illustrates these curves for creep evaluation. Usage

Method 3 – Elastic Approach Utilizing Section VIII Division 2 Code Case 2843 (based on the Section III Part NH rules utilizing the strain deformation method)

This recently published Code Case includes for time dependent cases. It uses load controlled limits and strain controlled limits.

Load controlled limits are applied to ensure stress levels are maintained below Code allowable values, extended to specific design lives. Figure 10 Illustrations of Design Curves for Creep Life Evaluation Strain controlled limits are used to ensure protection against racheting.

Creep-Fatigue is evaluated using a modified interaction diagram from Section III Part NH and is illustrated in Figure 11, noting Design Checks for Time Dependent Conditions cover: the variability in the location of the “knuckle” of the interaction  Creep Rupture diagram with other methods.  Excessive Creep strain  Creep-Fatigue Interaction

The model basis for each assessment is as outlined below. Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021

 Gross Plastic Deformation check – linear-Elastic ideal plastic law using Tresca’s yield condition (maximum shear stress).

 Progressive Plastic Deformation (Ratcheting), Creep Rupture and Excessive Creep Strain checks a linear-elastic ideal plastic law is used with von Mises’ yield condition

Figure 11 Creep-Fatigue Interaction Diagram (maximum distortion energy).

Currently there is a comparison of CC 2843 with Section I  Instability check – either a linear-elastic or linear-elastic Design being undertaken. Section I makes use of wall thickness ideal-plastic law. or pressure capacity equations for component sizing, supplemented by rules for compensation of openings.  Fatigue check - a linear-elastic law

CC 2843 uses a combination of hand calculations for basic Note von Mises yield condition may be used for the Gross stresses (termed General Primary Stresses) and finite element Plastic Deformation check if the strength parameter is modified analysis (FEA) with linearized through-thickness stress results by √3 / 2. at key locations (Local Primary as well as Secondary and Peak stresses and limits) which take the place of compensation rules. Figures 12, 13 and 14 illustrate, pictorially, the modeling It also requires a definition of a specific design life, in addition process output. to consideration of both Design and Operating cases.

Method 4 – Simplified Inelastic Approach Utilizing a Draft Section III Code Case (based on EN 13445)

This final case is still being investigated for Section I use and was recommended by the authors of the ASME funded research project for Section I Modernization – STP-PT-070 “Design Guidelines for the Effects of Creep, Fatigue and Creep-Fatigue Interaction”.

It is the method mandated by EN12952 for design by analysis as defined in EN 13445 Annex B. It assumes that the material is sufficiently ductile / creep ductile and it characterizes design (ultimate) loads and also service load conditions. Figure 12 – Tresca Elastic Stress

This method addresses both time independent and time dependent conditions as required by both the design and operating parameters of the component.

Design Checks for Time Independent Conditions cover:  Gross Plastic Deformation  Progressive Plastic Deformation (Ratchetting)  Instability  Fatigue  Static Equilibrium

MATERIALS DATA

One of the key gaps identified as part of the modernisation process was the requirement to identify all the material data

required for the application of the more advanced DBA Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021 methods. A research project was therefore put in place to document the required data and identify any gaps in available data. A unified material property compilation and development project will ensure baseline consistency between all parts of the Code, while still allowing industry-specific design methods. It should be noted that this project was not to undertake any testing work but only to document existing data. Figure 13 – Von Mises Stress Range

The project is to compile existing material property data up to the current Code material use limit for:

1. Creep rupture, average and minimum. 2. Creep ductility. 3. Creep strain vs. time curves. 4. Elevated temperature yield, tensile strength and physical properties. 5. Elevated temperature continuous cycling fatigue curves. 6. Elevated temperature hold time fatigue curves.

Figure 14 – Plastic Strain The above properties are listed in order of priority, and are needed for the following materials (also listed in order of Examples to Illustrate the Different Methods priority). It should be noted that the materials selected were not just required for HSC type boiler applications but also those of A number of practical worked examples have been completed, interest to users of ASME Section III and ASME Section VIII: based on existing boiler components, all with known operating conditions and in some instances failure analysis. This has 1. Grade 91 enabled the different analysis methods to be bench marked to 2. Inconel ® 740H ® both ensure that the methods reflect the real life component 3. Type 304H history and also to identify the benefits of each, as illustrated in 4. Type 347H Table 2. 5. Grade 22 6. Grade 92 7. Grade 22V 8. Grade 9 (9Cr) 1. As available and permitted by funding, additional materials have also been identified for property compilation.

All creep data will be presented in parametric (equation) form as a function of temperature and stress. Creep ductility data is meant to allow quantification of damage tolerance, which is rapidly becoming a key aspect of effective elevated temperature Table 2 – Illustration of variability in pressure capacity design. Recognized high temperature parametric representations normalised to ASME Section I such as Larson-Miller will be utilized, and all underlying material characterization data will be reported in addition to This work is on-going as part of the validation process and will details of all data analysis. Materials will also be addressed by form part of the background to the modernized ASME Code product form (if applicable) and include data on typical welded rules once completed. joints and processes.

Temperature, stress and time limitations will be specified in all The defined conditions (including transients) are intended to be cases; data spanning a range of stress levels is desired, representative of a typical ultra-supercritical (USC/HSC) power supporting loading typical of short term local stress relaxation plants. to long term gross rupture. The data provided must be The analysis methods to be applied are those specified in API consistent with ASME allowable stresses to facilitate baseline 579-1/ASME FFS-1 Part 10 (including material models and compatibility with traditional Design-by-Formula. However, the data), EDF Recommended Procedure R5 V4/5 (including R66 Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021 parametric form of the basic data itself will support material models and date) and Electric Power Research Institute development of new design methods. BLESS Code (including embedded material models and data).

The configuration being considered is a girth weld in typical boiler components (superheater and reheater tubes and headers), with both circumferential and longitudinal flaws NDE ACCEPTANCE STANDARDS located at the inside surface, outside surface and mid-wall. Flaw Geometries considered are infinite length/full circumferential As introduced earlier, the move to using ultrasonic test methods 6:1 (2c vs. a) semi-elliptical. in lieu of radiography requires special consideration in developing rational flaw acceptance criteria for equipment The materials being assessed are typical grades found in current operating in the creep regime. Therefore another ASME ST power plants (Grade 22, Grade 91, Type 304H and Grade 23). LLC research project has been developed to provide the necessary extension to the current Section I Code Case 2235 for This creates a matrix of 4 components x 4 materials = 16 using ultrasonic test methods in lieu of radiography, and directly component models for each of the 3 analysis methods. It is supports Section I modernization. expected that different contractors will be needed for each of the 3 analysis methods. For each component models, there are Flaw Growth 16 x 2 flaw orientations x 3 flaw locations x 2 flaw sizes = 192 flaw analysis cases (per analysis method). The output from the The proposed research project: Creep-Fatigue Flaw Growth analysis of each of the flaw case is to be the largest permitted Analysis to Support Elevated Temperature Flaw Size starting flaw, and the results of the analysis must be documented Acceptance Criteria has been agreed but requires funding and in a formal technical report. Acceptance criteria should be probably will not start until 2018. consistent with the given analysis method. If no acceptance criteria are given, then failure shall be defined as either a flaw The scope of this project is to analyze a matrix of typical growing to 75% through-wall at its deepest point or gross elevated temperature components using recognized creep- rupture due to loss of section. fatigue flaw growth analysis methods and data. The key deliverable will be the largest initial flaw size for each CURRENT STATUS case that satisfies the specified transient operating conditions (temperature, pressure, time, cycles). Considerable progress has been made to date, with a number of This information will then be used in developing rational flaw key additions to the 2017 Edition of the BPV Code. A new Part acceptance criteria for equipment operating in the creep regime. PA, along with a new NMA for guidelines on corrosion, erosion and oxidation has been published in Section I. Additionally a Specific details of the requested analysis are as follows: new Code Case for Section VIII Division 2 has been published introducing the concept of design life. Specified Inputs: The remaining work is being finalized to cover Design by  Operating Duration: 200,000 hours (22.8 years) Analysis methods which are being evaluated using boiler  Operating Conditions: component worked examples. The necessary data for the o Cold Starts (> 48 hours shutdown) = 100 material models is being compiled along with creep-fatigue flaw o Warm Start (8 to 48 hours shutdown) = 1,000 growth analysis, associated NDE requirements and allowable o Hot Start (<8 hours shutdown) = 6,000 stress limits and margins.  Stresses o Pressure-induced o Welding residual equal to 35% of average 0.2% yield strength o Thermal

ACKNOWLEDGMENTS ASME BPV I TG on Modernization ASME BPV III WG on Elevated Temperature Construction

George Komora, Robert Diekemper and Luther Krupp - Downloaded from http://asmedigitalcollection.asme.org/PT/proceedings-pdf/ETAM2018/40764/V001T04A006/2381228/v001t04a006-etam2018-6749.pdf by guest on 27 September 2021 Nooter/Eriksen Mike Cooch - Babcock & Wilcox

REFERENCES [1] ASME, BPV - I (2015). [2] ASME, BPV Section VIII Division 2 (2015) [3] CEN, EN 13445, (2011) [4] Livingston, W.R., Davis, C., Fry, T., Wright, I., STP- PT-066 (2014) [5] Perrin, I., Parker, J., Shingledecker, J., Peters, D., Cofie, N., STP-PT-070 (2014) [6] Cameron, S.W., PVP Conference, Modernization Key Note Paper (2014)