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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
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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
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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
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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
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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.
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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.
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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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