20th International Conference on Structural Mechanics in Reactor Technology (SMiRT 20)
Espoo, Finland, August 9-14, 2009
SMiRT 20-Division 3, Paper 2575
DETAILED FEA OF LOCALLY THINNED PIPE BENDS
Usama Abdelsalam & Dk Vijay
Operations Engineering, AMEC Power &Process Americas, Toronto, Canada
e-mail: [email protected]
Keywords: FEA, Pipe Bends, ASME SEC III, LTA, Class 1 Piping.
1
ABSTRACT
Carbon Steel pipe bends experience wall thinning due to flow accelerated corrosion (FAC) resulting in
locally thinned areas (LTA) superimposed on general thinning. The continued wall thinning deteriorates the
structural integrity of the piping component and fitness for continued service assessment, repair, or
replacement is required. This paper presents the structural integrity assessment of tight radius pipe bends
under internal pressure loading using the finite element method. Both general and local wall thinning is
included in the three dimensional models through detailed axial and circumferential thickness profiles. The
developed thickness profiles conservatively maintain a lower bound on the measured thicknesses. A location
dependent thinning rate function is developed and used to provide a basis for a prediction of the wall
thickness distribution at the end of an arbitrary evaluation period. The finite element analysis is conducted
using ANSYS v10.0 and the results are post-processed to show compliance with the ASME Code SEC III
NB-3221 criteria. Results are also presented for the elastic-plastic limit load analysis following NB-3228.1.
This paper presents a new approach of modelling wall thinning and demonstrates the effectiveness of using
detailed FEA to extend the useful life of aging nuclear components. It is also demonstrated that the linear
elastic criteria of the ASME SEC III NB-3221 could be overly conservative compared to the limit load
analysis.
2
INTRODUCTION
The pressure requirement in the ASME Code SEC III protects against the catastrophic collapse of the
designed components due to a single application of the primary load. The basic criterion for internal pressure
loading under the ASME Code SEC III NB-3600 “Piping Design” rules is given in NB-3640 “Pressure
Design“. This criterion is applicable for straight pipe segments as described in NB-3641.1 and the wall
thickness calculated from this equation is often called the pressure-based thickness, tmin. For curved segments
of pipe, NB-3642.1 “Pipe Bends” adopts the same equation for determining the wall thickness for the
straight segments of pipe with three limitations. Of special relevance to this paper, the second limitation,
expressed in the form of Table NB-3642.1(b)-1 “Bend Radius Versus Thickness”, guides the designer when
ordering a pipe to use a higher than tmin thickness for the pipe prior to bending. For instance, for a pipe bend
with a bend radius equals 3 pipe diameters, the recommended minimum thickness prior to bending is
1.25tmin. When a straight pipe is bent, the extrados thins out and the intrados thickens. As such, the thickness
on the intrados will be even higher than 1.25tmin after the bending operation. In other words, the code
acknowledges that a pipe bend with uniform thickness having the pressure based thickness of NB-3640.1
equation is not acceptable.
Under NB-3630 “Piping Design And Analysis Criteria” it is stated that “(c) When a design does not
satisfy the requirements of NB-3640 “Pressure Design” and NB-3650 “Analysis of Piping Products”, the
more detailed alternative analysis given in NB-3200 or the experimental stress analysis of Appendix II may
be used to obtain stress values for comparison with the criteria of NB-3200 “Design By Analysis”.
Considering the design pressure loading, the design by analysis rules of NB-3221 requires that general
primary membrane stress intensity, Pm, meet the Sm limit (NB-3221.1), the local membrane stress intensity,
PL, meet the 1.5Sm limit (NB-3221.2) and the primary membrane (Pm or PL) plus primary bending stress
intensity, Pb meet the 1.5Sm limit (NB-3621.3).
During operation, the wall thinning may be general, local or both depending on the piping geometry and
the fluid flow characteristics. For fitness for service assessments, the wall loss needs to be assessed. The
1
ASME Code SEC III, being a construction code, does not provide explicit guidance as to how to deal with
locally thinned areas (LTA). EPRI [5] provides guidance and acceptance criteria for the evaluation of Carbon
Steel piping erosion/corrosion wall thinning. A three-step evaluation process is proposed based on the ASME
code design requirements and defines the degree of wall thinning (depth and extent) which can be safely left
in service. This guidance is consistent with the ASME Code SEC XI Code Case N-597-2 that provides a
criterion to assess the LTA for Class 2 and Class 3 pipes. Osage et al. [6] provided an overview of the latest
technology at the time for the assessment of non-crack like flaws including erosion/corrosion, pitting,
blisters, shell out-of-roundness, weld misalignment, bulges and dents. With regard to LTAs, they provided a
review and evaluation of available methodologies for LTA assessment including effective area methods
(ASME B31G), extensions to the effective area method, and thickness averaging approaches. Zhang et al. [7]
calculated the plastic collapse load of a single elbow using a finite element analysis model. Only pressure
loading is considered and as such, both the geometry and loading are symmetric with respect to the in-plane
plane of the elbow. To preserve the model symmetry, only symmetric thinning patches could be modelled.
The thinned regions were modelled by removing elements resulting in sharp transitions from the LTA and
the surrounding material. The effects of the LTA location, bend radius, and LTA size are investigated. S.
Iyer and R. Kumar [8] presented an assessment of LTA in class 1 piping components using the finite element
method and following the ASME Code SEC III NB-3221 criteria. Two tight radius bends connected by a
straight piece of pipe were considered. S. Iyer [9] re-iterated the methodology and procedure in his previous
paper with more focus on the calculation of stress indices considering the local nature of the thinned areas to
be used in the piping analysis. On the Canadian regulatory side, J. Jin [12] summarized the regulatory view
point regarding the fitness for service assessments employing the ASME Code elastic and plastic
methodologies emphasizing the opinion of maintaining the same margin of safety. Abdelsalam and Vijay
[13] presented the results of an earlier assessment of local and general wall thinning of pipe bends with a
focus on the detailed modelling of the thickness profiles. Double tight radius bends were considered under
pressure loading. The idealized profiles were combined with a location dependent thinning rate function to
develop the predicted thickness profiles corresponding to a target operation period. These idealized profiles
were implemented in detailed finite element models to perform the pressure assessment of typical CANDU
feeder pipe bends according to the ASME Code SEC III NB-3221 criteria.
In a typical CANDU reactor, 380 or 480 Fuel Channels (FC) are arranged horizontally in a lattice inside
the Calandria Vessel. The nuclear fuel bundles are placed inside the Fuel Channels. The heavy water flowing
inside the Fuel Channels transports the heat energy generated from the nuclear reaction to the steam
generators. The flow of the heavy water coolant through the Fuel Channels is provided by Primary Heat
Transport (PHT) pumps and carried through pipes running from the inlet headers and removed through pipes
connecting to the outlet header. Each Fuel Channel is connected to two pipes called inlet and outlet feeders.
Feeders are made of Low Carbon Steel SA-106 Gr. B pipes with tight radius bends/elbows welded to the
Grayloc hub that is assembled to the end-fittings at the ends of the Fuel Channels with a bolted connection.
The pipe sizes used for feeders in a typical CANDU reactor are in the 2-3.5” outer diameter range with
thickness in the 0.218-0.3” range. Feeders as part of the Primary Heat Transport system (PHT) are classified
as Class 1 piping components. Therefore, feeders were designed according to ASME B&PV Code Section III
Division 1 Subsection NB-3600 (Piping Design). It is observed that the outlet feeders encounter considerable
wall thinning due to the flow accelerated corrosion (FAC) phenomenon. The wall thinning is more
pronounced at the tight radius elbow/bend regions close to the Grayloc hub. The reduced wall thickness leads
to higher stresses that need to be assessed to demonstrate feeders’ fitness for continued service under the
specified loads.
This paper builds on the analysis and results presented in [13] where more detailed thickness profiles
are used to extend the safe operation period of the feeder pipes. Also, the concept of a location dependent
thinning rate function is introduced. In addition, results from elastic plastic limit load analyses are presented
to establish the base line for the allowable pressure defined by the ASME Code. As highlighted by Rao [14],
the intent as stated in the criteria document is to ensure that the allowable pressure is equal to or less than the
pressure obtained by the limit load analysis. The relative margin on the calculated allowable pressure using
the NB-3221 linear elastic criteria is explored.
3
FINITE ELEMENT MODEL
In the finite element modelling presented in this paper, the full feeder length is considered. Solid brick
elements are used at the tight radius bends region to provide flexibility in introducing wall thinning profiles
2
on the inner surface only. The rest of the piping away from the area of interest is modelled using pipe
elements.
3.1
Feeder Pipe Geometry
Figure 0 illustrates a typical CANDU feeder model with a close up view showing the lower portion with the
tight radius bends and the Grayloc hub. The focus of this paper is the tight radius bends where the most
significant wall thinning is observed. As such, the feeder extension portion and the attached straight segment
of pipe are modelled using ANSYS SOLID95 elements. Three through-thickness layers of ANSYS
SOLID95 brick elements were used along with seventy two elements along the circumferential direction.
The rest of the feeder up to the header nozzle weld is modelled using pipe elements. The general purpose
finite element program, ANSYS is used to setup the numerical models, perform the stress analyses, and postprocess the results. The nominal piping cross-section dimensions used to build the geometric models are:
Outer Diameter, Do = 2.361 in (60 mm)
Nominal Thickness, tnom= 0.218 in (5.54 mm)
Bend Angle, = 50o
Bend Radius, R = 3 in (76.2 mm)
3.2
Material Model
A linear-elastic isotropic material model, using
the properties of ASME material SA-106 Grade
B [1], is used for the analysis (at 605 oF).
Elastic Modulus, E = 26.7x106 psi
Poisson’s Ratio, ν = 0.3
Allowable stress Intensity, Sm = 17.26 Ksi
3.3
Pressure-based Thickness
taloc/tmin
The pressure based allowable thickness for a
straight pipe, tmin, is calculated in accordance
Figure 0: Typical CANDU Feeder Model
with the ASME Section III, NB-3641.1(1) as
1.5
follows:
2" Extrados:
PDo
1.4
/t
= 0.863
t
t min =
+A
2" Intrados:
2( S m + Py )
2" Feeders
1.3
/t
= 1.303
t
P is the internal Design Pressure (1455psig), Do
2.5" Extrados:
1.2
/t
= 0.878
t
is the outside diameter of the pipe, A is the
2.5" Intrados:
1.1
corrosion allowance, y is equal to 0.4, and Sm is
/t
= 1.238
t
2.5" Feeders
the max allowable stress intensity for the
1.0
material at the design temperature. Therefore,
0.9
the pressure-based thickness for a straight pipe
0.8
segment calculated using the above formula is,
tmin = 0.096 in  2.45 mm
0.7
t
0.5
In the central area of a pipe bend, Osage et al
" 0.5 +
0.6
R
t
[6] presented a criterion for the allowable
1+
Cos !
Intrados
Extrados
Cheek
R
0.5
thickness that is developed using the required
0
15
30
45
60
75
90
105
120
135
150
165
thickness of a toroidal vessel compared to a
_L
straight pipe. This criterion is illustrated
Figure 0: Allowable Thickness in Pipe Bends
graphically in Figure 0 for 2” and 2.5” pipe
bends where,
taloc
is the allowable local wall thickness
Rmin
is the mean radius of the piping item based on tmin.
Rb
is the bend radius of curvature
is the circumferential angle.
L
aloc
min
aloc
min
aloc
min
aloc
min
aloc
min
min
L
b
This criterion is adopted by SEC XI Code Case N-597-2 [3 & 4].
3
180
3.4
Axial & Circumferential Thickness Profiles
Thickness measurements of CANDU feeder bends are performed using a bracelet tool that has 14 probes
equally spaced along the circumference covering a 140o angle. For the intrados scan, the tool is centred with
the intrados of the first bend and moved axially to cover both bends. As such, the second half of the intrados
scan represents the extrados scan for the second bend. Similarly, the second half of the extrados scan
represents the intrados scan over the second bend. Error! Reference source not found. shows the
developed minimum axial thickness profile of the second tight radius bend (solid line is the new refined
profile while the dashed line represent the previous attempt [13]). Similarly, smooth functions are used to
bound the measured data in the circumferential direction. The developed idealized functions are used to
construct a finite element model where the thickness anywhere in the bends is equal to or less than the
measured thickness at that location.
6
5.5
5
14-probe Extrados
14-probe Intrados
14-probe Left Cheek
14-probe Right Cheek
6-Probe Extrados
6-Probe Intrados
Wall Thickness (mm)
4.5
4
3.5
Idealized
Profile
3
tmin
2.5
2
67
77
87
97
107
Ax ia l Dista nce Along Fe e de r Ce nte rline (m m )
117
127
Figure 3: Measured Thickness Data & Idealized Profile
3.5
Location Dependent Thinning Rate
The idealized axial and circumferential profiles and
the corresponding FEA models are used as a basis to
develop
the
projected
idealized
profiles
corresponding to a target evaluation period.
Applying a uniform thinning rate leads to a simple
shift of the thickness distribution to lower values.
This uniform treatment could be either conservative
or non-conservative considering that not all locations
started with the same thickness (extrados started
thinner than the intrados as a result of the bending
operation during manufacturing of the bend). For
instance, if the thinnest spot is at the intrados,
assuming a uniform thinning rate produces a too
conservative estimate of the projected thickness on
the extrados. To reduce the conservatism, a location
dependent thinning rate function is developed.
The location dependent thinning rate function is
Figure 4: Non-Uniform Original Thickness Distribution
developed assuming that the percentage increase in
the intrados wall thickness is equal to the percentage
decrease in the extrados wall thickness preserving the material volume. Figure 4 shows a pipe bend cross
section depicting the non uniform wall thickness distribution where the intrados is 50% thicker and the
extrados is 50% thinner (for illustration purposes only). This non-uniform thickness distribution is assumed
to represent the initial pipe bend condition after manufacturing. During subsequent operation, wall thinning
4
occurs and the thinning rate is calculated based on the non-uniform thickness distribution representing the
original state and the idealized thickness profiles representing the current state. In the axial direction along
the pipe bend, it is assumed that the middle of the bend has the maximum non-uniformity and the ends have
a uniform thickness distribution in the hoop direction.
Thinning Rate (mm/EFPY)
In this investigation, a 5% thicker than nominal intrados and 5% thinner than nominal extrados are used to
approximate the original thickness. As such the thinning rates at the bend middle section extrados and
intrados are as follows:
Extrados Thinning Rate, tr,Ext = (0.95tnom-tmeas,Ext)/EFPYc
Intrados Thinning Rate, tr, Int = (1.05tnom-tmeas,Int)/EFPYc
Where, tnom is the nominal thickness and EFPYc is the time measure corresponding to tmeasured. In general, at
any location, i, the thinning rate is given by:
tr,i = (to,i-tmeas,i)/EFPYc where to,i is the
0.2
0.18
original thickness at the location i.
0.16
The predicted thinckness, tp,i, at a specific location is
0.14
calculated as follows:
0.12
tp,i = tmeas,i – (EFPYp – EFPYc)* tr,i
where
0.1
EFPYp is the time measure for the end of the
0.08
evaluation period, tmeas,i is the measured thickness at
0.06
location i, and tr,i is the thinning rate at location i.
0.04
0.02
Close to the cheeks, both the intrados and extrados
0
have original thickness equal to the nominal thickness.
0
10
20
30
40
50
60
Axial Distance (Deg)
This increase and decrease in the initial wall thickness
is consistent with data obtained from thickness
Figure 5: FEA Model Thinning Rate Distribution
measurements of inlet feeders (where in-significant
thinning occurs) and spare bends. Figure shows the axial distribution of the thinning rate along the second
tight radius bend. The maximum thinning rate in the
model is 0.17 mm/EFPY at the intrados of the second
bend. EFPY stands for Effective Full Power Year of
continuous operation. The maximum thinning rate at
the extrados is 0.11mm/EFPY. Figure 6 shows the
distributions of the wall thickness over the entire
t
second tight radius bend region at each throughthickness path from one node on the inner surface to a
corresponding node on the outer surface. The top graph
shows the axial distribution illustrating the local thin
spot downstream from the beginning of the bend in the
axial direction while the second graph shows the
circumferential thickness distribution. As can be
observed from the circumferential distribution of the
thickness, the thin spot is centred at the intrados and
extends over a considerable portion of the
circumference.
t
7
6
Wall Thickness (mm)
5
4
3
min
2
1
0
0
5
10
15
20
25
30
35
40
45
50
Ax ia l Dista nce Along Fe e de r Ce nte rline (de g.)
7
6
Wall Thickness (mm)
5
4
3
min
2
3.6
Out of Roundness
1
LC
Intrados
RC
During the manufacturing process out of roundness of
the pipe bend cross section is introduced. This out-ofroundness affects the local stress distributions. In this
Figure 6: Idealized Thickness Profiles
paper, the out-of-roundness is represented by an
elliptical cross section with the major axis connecting
the left and right cheeks of the bend. 8% out of roundness is used and the maximum out of roundness is
conservatively placed at the middle of each bend. The beginning and end of each bend is modelled as perfect
circles.
0
0
30
60
90
120
150
180
210
Circumferential Angle (deg.)
5
240
270
300
330
360
3.7
Boundary Conditions & Loading
The finite element model is fixed in all six degrees of freedom at the two terminal ends; at the Grayloc hub
flange and at the header nozzle weld. The location of the rigid hanger support is constrained in the vertical
direction using a linear spring with a very large spring constant. A linear spring is attached at the spring
hanger location with the appropriate spring constant as per the design specifications.
4
ASME SEC III NB-3221 - DESING LOADINGS
The traditional way to obtain the solution for the primary stress intensity is organized as follows:
• Apply the internal pressure as a static load and perform a linear elastic finite element static analysis.
• Linearize the resulting stress solution across the wall thickness to obtain the corresponding membrane,
and membrane plus bending stress intensities.
• Examine the membrane and membrane-plus-bending stress intensities against the ASME SEC III NB3221 criteria. The maximum general membrane stress intensity, Pm, is checked against Sm and the
maximum local membrane plus bending, PL+Pb, stress intensity is checked against 1.5Sm.
• In the locally thinned areas, the local membrane stress intensity, PL, is checked against 1.5Sm.
The question often arise at what point should the linearization be performed since the maximum output from
the linearization operation is not necessarily at the point having the maximum stress. In this paper, the
linearization operation is performed over the whole tight radius bend region and all the points are checked
for compliance with the NB-3221 criteria.
30
1.5 S
Primary Membrane Stress Intensity (Ksi)
The above procedure is followed and the results for
the second bend (limiting) are presented in Figure
7 where the horizontal axes represent the axial
direction starting from the Grayloc hub weld (top
graph) or the circumferential direction starting
from the centre of the extrados (bottom graph).
The left vertical axis represents the membrane or
the membrane-plus-bending stress intensities. Each
point on these graphs represents the result of stress
linearization along a path going from one node on
the inner surface to a corresponding node on the
outer surface of the second tight radius bend.
m
25
1.1 S m
20
Sm
15
10
5
0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
Axial Distance Along Feeder Centerline (deg.)
30
The compliance with the ASME Code SEC III NB1.5 S
3221 elastic criteria for pressure loading is
illustrated graphically in Figure 7. The primary
1.1 S
membrane stress intensity in the general thinned
S
areas (Pm) is compared to Sm represented by a
horizontal line. Stress intensity points below the Sm
line meet the general membrane stress intensity
criterion of ASME SEC III NB-3221.1. The local
LC
Intrados
RC
primary membrane stress intensity (PL) in the
locally thinned areas is compared to 1.5Sm
represented by a second horizontal line. Stress
intensity points below the 1.5Sm line meet the local
Figure 7: NB-3221.1 (Pm) & NB-3221.2 (PL)
primary membrane stress criterion of ASME SEC
III NB-3221.2 as long as the extent of the region with stress intensity higher than 1.1Sm is limited by
√(Rmintmin) in both the axial and circumferential directions (Rmin is the mean radius and tmin is the pressure
based wall thickness). The primary membrane (Pm or PL) plus primary bending (Pb) stress intensity
everywhere is compared to 1.5Sm. Stress intensity points below the 1.5Sm line meet the ASME SEC III NB3221.3.
Primary Membrane Stress Intensity (Ksi)
m
25
20
m
m
15
10
5
0
0
30
60
90
120
150
180
210
240
270
300
330
360
Circumferential Angle (deg.)
5
ASME SEC III NB-3228 - APPLICATION OF PLASTIC ANALYSIS
The ASME SEC III NB-3228.1 Limit Analysis states that “The limits on General Membrane Stress Intensity
(NB-3221.1), Local Membrane Stress Intensity (NB-3221.2), and Primary Membrane Plus Primary Bending
6
Stress intensity (NB-3221.3) need not be satisfied at a specific location if it can be shown by limit analysis
the specified loadings do not exceed two-thirds of the lower bound collapse load“. This statement is
converted to the following mathematical form:
PD ≤ (2/3) PC
OR
(2/3) PC /PD ≥ 1
Where,
PD
Design Pressure
PC
Collapse Pressure
The same finite element analysis models used for the linear elastic analyses are used for the limit load
analysis. The finite element analysis results are post-processed to produce the load-displacement curves for
all nodal points in the tight radius bend region. For each load-displacement curve, a tangent line (elastic line)
is developed starting from the origin (0, 0). The angle that this line is making with the vertical axis is . A
second line (double slope line) is developed with an angle where tan() = 2 tan(). The intersection point
between the double slope line (making an angle
with the vertical axis) and the load-displacement curve
defines the limit load. Figure 8 shows the load displacement curves and the limit loads for the nodal point
with the maximum von-Mises stress. The left graph corresponds to a minimum local thickness of 2.22 mm
with a calculated limit load of 2.03 PD (1.35*3/2 PD). The right graph is for a minimum local wall thickness
of 1.71 mm (70% tmin) with a calculated limit load of 1.76 PD (1.17*3/2 PD). In both cases, there still margin
between the calculated limit load and the allowable (3/2 PD).
1.5
1.5
1.25
1.25
1.35
1.17
1
Pressure, (3/2) PD
Pressure, (3/2) PD
1
0.75
_
tp,m in=2.22 mm
0.5
0.75
_
tp,m in=1.71 mm
0.5
_
_
Load Curve
DBL Slope Line
Criterion
Limit Load
Elastic Slope
0.25
Load Curve
DBL Slope Line
Criterion
Limit Load
Elastic Slope
0.25
0
0
0
0.001
0.002
0.003
0
0.004
0.002
0.004
0.006
0.008
0.01
Displacem ent, in
Displacem ent, in
Figure 8: Double Angle Limit Load Criterion
Figure 9 shows the development of the plastic zone as the pressure increases for the second case with tp,min =
1.71 mm. At the design pressure, the plastic zone is limited to a small local area centered at the thin spot. As
the pressure increases, two plastic zones at the intrados of the first and second bends grow independently. As
the pressure increases further, the two plastic zones join together leading to unstable solution with very large
deformation indicating structural collapse. It should be noted that the stress contours in Figure 9 are plotted
on the deformed model with a displacement magnification of 5.
As expected, the limit load analysis provides relieve from the more restrictive conditions of the linear elastic
rules of NB-3221. To put both approaches in perspective, the maximum allowable pressure obtained from
each approach is compared at different minimum local wall thickness values. Figure 10 shows two lines
representing the allowable pressure at different minimum local wall thicknesses for the two approaches. As
observed in this figure, the linear elastic approach of NB-3221 with through thickness linearization is very
conservative compared to the plastic limit load analysis.
7
Figure 9: Equivalent Stress at Different Pressure Loading Levels (tp,min=1.71 mm)
3.0
6
CONCLUSIONS
LE_Through Thickness
The ASME Code SEC III NB-3000 is used to
assess the structural integrity of thinned tight
radius pipe bends under internal pressure loading.
Three dimensional finite element models are
constructed to simulate general and local inner
wall thinning. The thinning profiles are smoothly
varied in both the axial and circumferential
directions and are constructed as a lower bound to
the measured wall thickness distribution over the
entire tight radius bends region. A location
dependent thinning rate function is developed to
provide an estimate for the wall thickness at the
end of an arbitrary evaluation period. The
allowable pressure is calculated based on the linear
elastic criteria of NB-3221 and the plastic analysis
limit load criteria of NB-3228.1. It is demonstrated
that the use of the linear elastic rules of the ASME
Code SEC III NB-3221 are overly conservative
compared to the limit load analysis.
Limit Load
2.5
Pall/PD
2.0
1.5
1.0
0.5
0.0
0.6
0.8
1.0
1.2
1.4
t/tmin
Figure 10: Maximum Allowable Pressure
REFERENCES
8
1.6
1. ASME Boiler and Pressure Vessel Code, Section II, Part D, Material Properties, 2004.
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2004.
3. ASME Boiler and Pressure Vessel Code, “Case N-597-2: Requirements for Analytical Evaluation of
Pipe Wall Thinning, Section XI, Division 1”, Approval Date: November 18, 2003, ASME Boiler and
Pressure Vessel Code.
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