Design by Analysis in the Modernized Boiler...

DESIGN BY ANALYSIS IN THE MODERNIZED BOILER CODE

David I Anderson Doosan Babcock Limited

Crawley, West Sussex, United Kingdom

David J Dewees Becht Engineering Medina OH, USA

ABSTRACT In general Section I of the ASME Boiler Code was originally

developed for industrial boilers through to sub-critical boilers

operating at relatively low temperatures and pressures under

steady state conditions. Current and future boilers do and will

operate at higher temperatures and pressures under cyclic

loading requiring a more detailed assessment and examination

to ensure safe and reliable operation.

Design by Analysis (DBA) methods will be fundamental to the

assessment process for key boiler components. It is intended

that the Code will incorporate several DBA methods, ranging in

complexity, to allow the user some flexibility to select the

method appropriate to the design conditions.

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.

INTRODUCTION In general Section I of the ASME Boiler Code [1] was

originally developed for industrial boilers through to sub-

critical boilers operating at relatively low temperatures and

pressures under steady state conditions. It also only really

addresses pressure containment and not other loadings such as

external loads, thermal loads and fatigue. Current and future

boilers do and will operate at higher temperatures and pressures

under cyclic loading, requiring a more detailed assessment and

examination to ensure safe and reliable operation. Essentially, at

present, ASME Section I is supplemented by additional

requirements based on manufacturers’ own experience and

expertise to ensure safety and reliability.

Other Codes and Standards, such as the European Standard

EN12952, being a more recent Standard, include some of the

rules and guidance to meet these requirements, as illustrated by

Figure 1. ASME Section VIII Division 2 is also being revised to

add Code rules to allow Design by Analysis in the time

dependent regime combined with fatigue. To ensure ASME

Section I remains as the pre-eminent Code of choice for

pressure equipment some form of modernization is required.

Figure 1 – Illustration of the differences between Codes

THE MODERNIZATION PROCESS ASME charged the main Section I (SC-I) Committee with the

task of investigating the needs and preparing a roadmap for the

future development of the Code. It was this Committee that

formed the special Task Group (TG) for Modernization from

expert members of the SC-I Committees.

1 Copyright © 2018 ASME

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

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Each SC-I Sub-Group (SG) was required to review and

compare the text of both Section I and Section VIII Division 2

to identify an initial view on what text was relevant for the

modernized Code and where any technical gaps lay and whether

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.

Figure 2 – Example Gap Analysis

Early on in the process two key technical gaps were identified

that required external input to fill. These were (a) guidelines on

how to address the effects of high temperature erosion,

corrosion and oxidation [4] and (b) rules for incorporation of

design by analysis methods into boiler design [5]. Other gaps,

such as NDE acceptance requirements were also identified but

it was thought that the SG could provide the necessary input to

fill them. Figure 3 shows the two ASME published reports.

Figure 3 – ASME Published Reports

The modernized Code will include essential technical additions

such as more prescriptive heat treatment and NDE as well as

enabling the effects of creep-fatigue to be addressed with the

introduction of Design by Analysis enabling state of the art

boiler components to be designed and operated.

The principle is that there will be a harmonized approach that

enables Design by Rule and Design by Analysis to be integrated

such that only those components of the boiler that would benefit

from the more rigorous Design by Analysis methods need be

subjected to these methods. Figure 4 illustrates some of the

sources being used. These proposals are based on best practice

and when included will require all manufacturers to work to the

same quality specifications and at similar costs. This is not only

applicable to current plants (both HRSG and USC plants), but is

also key to progression of the development of HSC plants for

the future.

Figure 4 – Data Sources for Design by Analysis

Various Design by Analysis methodologies are being reviewed

for inclusion in the modernized Code.

Whilst Section I is written for new boiler construction the

introduction of Design by Analysis will open up the Code to

other applications for fitness for service and life assessments.

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

assessments.

DESIGN BY ANALYSIS

Design by Analysis (DBA) methods will be fundamental to the

assessment process for key boiler components. It is intended

that the Code will incorporate several DBA methods, ranging in

complexity, to allow the user some flexibility to select the

method appropriate to the design conditions.

The methods currently being considered include an elastic

approach based on Section VIII Division 2, 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.

Method 1: Elastic Approach (based on Section VIII Division 2,

New (DRAFT) Part 5.6)

Part 5.6 is organised with each sub-paragraph addressing one of

the potential failure modes that are addressed in the rest of part

5: rupture, buckling, creep/fatigue interaction, and ratcheting.

The procedure evaluates protection against stress rupture using

elastic stress analysis. It also includes a fatigue screening

method. Figure 5 illustrates the traditional stress categories and

associated stress limits for time independent conditions. The

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

STEP 4 Determine the principal stress for each

stress tensor and compute the equivalent

stress

STEP 5 Apply appropriate weld strength reduction

factor

STEP 6 Determine the time dependent allowable

stress

STEP 7 Evaluate protection against plastic

collapse (time independent regime) or

stress rupture (time dependent regime)

Table 1 – Stress Rupture 7 Step Assessment Process

Creep Buckling – is considered for external pressure, generally

utilising the isochronous stress-strain curve approach.

Creep-Fatigue Interaction – To use this Part of the Code a

fatigue screening process must be undertaken to demonstrate no

creep-fatigue interaction.

Two fatigue screening criteria which must be met (1) the

number of full-range pressure cycles must not exceed 250 and

(2) the total number of cycles (including full-range and

significant pressure cycles and significant temperature cycles)

must not exceed 500. If both these criteria are met then a

detailed creep-fatigue analysis is not required.

Creep Ratcheting – the summation of local primary membrane,

primary bending and secondary stress range must be kept within

the sum of the cold yield strength and hot allowable stress.

Figure 5 – Traditional Stress Category Assessment Limits

Method 2 Inelastic Approach (based on the Omega method

from API 579)

This method is based on the Omega method of API 579 Part 10,

currently applied to fitness for service evaluations but equally

applicable to new construction design, see Figure 6. While

Part 10 is a complete fitness-for-service procedure and draws on

existing Code methods for failure modes such as plastic

collapse, the unique portion exploited for time-dependent

design is the creep/damage material model (the “MPC Omega

Model”). This model will be included as a Code Case that

allows detailed inelastic analysis to support both stress rupture

and local damage estimates. Note that creep-fatigue is

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

carbon and low alloy steels (note some other Codes only restrict

sum to be less than unity).

Additionally, the model will allow generation of isochronous

stress-strain curves.

Material data for the MPC Omega model will be provided in

the Code Case as illustrated in Figure 6.

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

Figure 7 Illustration of Evaluation of Ceep and Fatigue Life

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.

Strain controlled limits are used to ensure protection against

racheting.

The Code Case also addresses creep-fatigue criterion which

further brings in the lifetime specification for components.

Figure 8 illustrates the application within a FE model and

typical areas of consideration for assessment.

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 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 10 illustrates these curves for creep evaluation.

Figure 10 Illustrations of Design Curves for Creep Life

Evaluation



Creep-Fatigue is evaluated using a modified interaction diagram

from Section III Part NH and is illustrated in Figure 11, noting

the variability in the location of the “knuckle” of the interaction

diagram with other methods.

Figure 11 Creep-Fatigue Interaction Diagram

Currently there is a comparison of CC 2843 with Section I

Design being undertaken. Section I makes use of wall thickness

or pressure capacity equations for component sizing,

supplemented by rules for compensation of openings.

CC 2843 uses a combination of hand calculations for basic

stresses (termed General Primary Stresses) and finite element

analysis (FEA) with linearized through-thickness stress results

at key locations (Local Primary as well as Secondary and Peak

stresses and limits) which take the place of compensation rules.

It also requires a definition of a specific design life, in addition

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.

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

Design Checks for Time Dependent Conditions cover:

Creep Rupture

Excessive Creep strain

Creep-Fatigue Interaction

The model basis for each assessment is as outlined below.

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

(maximum distortion energy).

Instability check – either a linear-elastic or linear-elastic

ideal-plastic law.

Fatigue check - a linear-elastic law

Note von Mises yield condition may be used for the Gross

Plastic Deformation check if the strength parameter is modified

by √3 / 2.

Figures 12, 13 and 14 illustrate, pictorially, the modeling

process output.

Figure 12 – Tresca Elastic Stress



Figure 13 – Von Mises Stress Range

Figure 14 – Plastic Strain

Examples to Illustrate the Different Methods

A number of practical worked examples have been completed,

based on existing boiler components, all with known operating

conditions and in some instances failure analysis. This has

enabled the different analysis methods to be bench marked to

both ensure that the methods reflect the real life component

history and also to identify the benefits of each, as illustrated in

Table 2.

Table 2 – Illustration of variability in pressure capacity

normalised to ASME Section I

This work is on-going as part of the validation process and will

form part of the background to the modernized ASME Code

rules once completed.

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

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.

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.

The above properties are listed in order of priority, and are

needed for the following materials (also listed in order of

priority). It should be noted that the materials selected were not

just required for HSC type boiler applications but also those of

interest to users of ASME Section III and ASME Section VIII:

1. Grade 91

2. Inconel ® 740H

®

3. Type 304H

4. Type 347H

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

design. Recognized high temperature parametric representations

such as Larson-Miller will be utilized, and all underlying

material characterization data will be reported in addition to

details of all data analysis. Materials will also be addressed by

product form (if applicable) and include data on typical welded

joints and processes.



Temperature, stress and time limitations will be specified in all

cases; data spanning a range of stress levels is desired,

supporting loading typical of short term local stress relaxation

to long term gross rupture. The data provided must be

consistent with ASME allowable stresses to facilitate baseline

compatibility with traditional Design-by-Formula. However, the

parametric form of the basic data itself will support

development of new design methods.

NDE ACCEPTANCE STANDARDS

As introduced earlier, the move to using ultrasonic test methods

in lieu of radiography requires special consideration in

developing rational flaw acceptance criteria for equipment

operating in the creep regime. Therefore another ASME ST

LLC research project has been developed to provide the

necessary extension to the current Section I Code Case 2235 for

using ultrasonic test methods in lieu of radiography, and directly

supports Section I modernization.

Flaw Growth

The proposed research project: Creep-Fatigue Flaw Growth

Analysis to Support Elevated Temperature Flaw Size

Acceptance Criteria has been agreed but requires funding and

probably will not start until 2018.

The scope of this project is to analyze a matrix of typical

elevated temperature components using recognized creep-

fatigue flaw growth analysis methods and data.

The key deliverable will be the largest initial flaw size for each

case that satisfies the specified transient operating conditions

(temperature, pressure, time, cycles).

This information will then be used in developing rational flaw

acceptance criteria for equipment operating in the creep regime.

Specific details of the requested analysis are as follows:

Specified Inputs:

Operating Duration: 200,000 hours (22.8 years)

Operating Conditions:

o Cold Starts (> 48 hours shutdown) = 100

o Warm Start (8 to 48 hours shutdown) = 1,000

o Hot Start (<8 hours shutdown) = 6,000

Stresses

o Pressure-induced

o Welding residual equal to 35% of average

0.2% yield strength

o Thermal

The defined conditions (including transients) are intended to be

representative of a typical ultra-supercritical (USC/HSC) power

plants.

The analysis methods to be applied are those specified in API

579-1/ASME FFS-1 Part 10 (including material models and

data), EDF Recommended Procedure R5 V4/5 (including R66

material models and date) and Electric Power Research Institute

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

located at the inside surface, outside surface and mid-wall. Flaw

Geometries considered are infinite length/full circumferential

6:1 (2c vs. a) semi-elliptical.

The materials being assessed are typical grades found in current

power plants (Grade 22, Grade 91, Type 304H and Grade 23).

This creates a matrix of 4 components x 4 materials = 16

component models for each of the 3 analysis methods. It is

expected that different contractors will be needed for each of

the 3 analysis methods. For each component models, there are

16 x 2 flaw orientations x 3 flaw locations x 2 flaw sizes = 192

flaw analysis cases (per analysis method). The output from the

analysis of each of the flaw case is to be the largest permitted

starting flaw, and the results of the analysis must be documented

in a formal technical report. Acceptance criteria should be

consistent with the given analysis method. If no acceptance

criteria are given, then failure shall be defined as either a flaw

growing to 75% through-wall at its deepest point or gross

rupture due to loss of section.

CURRENT STATUS

Considerable progress has been made to date, with a number of

key additions to the 2017 Edition of the BPV Code. A new Part

PA, along with a new NMA for guidelines on corrosion, erosion

and oxidation has been published in Section I. Additionally a

new Code Case for Section VIII Division 2 has been published

introducing the concept of design life.

The remaining work is being finalized to cover Design by

Analysis methods which are being evaluated using boiler

component worked examples. The necessary data for the

material models is being compiled along with creep-fatigue flaw

growth analysis, associated NDE requirements and allowable

stress limits and margins.



ACKNOWLEDGMENTS ASME BPV I TG on Modernization

ASME BPV III WG on Elevated Temperature

Construction

George Komora, Robert Diekemper and Luther Krupp -

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)



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