CONTENTS
Report No. 1: Evaluation of Design Margins
for ASME Code Section VIII, Division 1
Executive Summary ...................................................... 1
A. Introduction ........................................................... 1
B. Service Experience .................................................... 2
C. Failure Modes Considered ............................................. 4
1. Ductile Rupture ...................................................... 4
2. Brittle Fracture ...................................................... 4
3. Fatigue ............................................................... 5
4. Incremental Collapse ................................................. 5
5. Elastic Instability .................................................... 5
6. Plastic Instability ...............•.................................... 5
7. Creep Rupture ........................................•.............. 6
8. Creep Buckling ....................................................... 6
D. Improvements to the Code Since 1940's
1. Materials ............................................................. 6
2. Design ................................................................ 9
3. Fabrication ......................................................... 10
4. Welding ............................................................. 11
5.PWHT ............................................................... 11
6.NDE ................................................................. 11
7. Brittle Fracture .............•....................................... 12
E. Review of PVRC Burst Tests .......................................... 13
F. Review of Significant Code Paragraphs ............................... 14
G. Conclusions........................................................... 17
H. Suggestions for Revisions to Code Requirements ..................... 17
I. Recommendations ..................................................... 18
Bibliography . . . . . . . . . . . . . . . . . . . . ...................................... 18
ISBN No. 1-58145-442-2
Library of Congress
Catalog Card Number: 99-64559
Copyright© 1999 by
Welding Research Council, Inc.
All Rights Reserved
Printed in U.S.A.
ii
Report No. 2: Evaluation of Design Margins for ASME Code Section VIII,
Divisions 1 and 2-Phase 2 Studies
A. Introduction .......................................................... 20
B. Executive Summary .................................................. 20
C. Design Margins for Section VIII, Division 1 (Phase 1 Follow-up
Studies) ............................................................. 25
1. Design Criteria of Section VIII, Division 1 ........................... 25
2. Effect of Design Margins on Fatigue Strength ....................... 26
3. Review of Code Case 2168 and Nozzle Reinfo~ement
Requirements ....................................................... 27
4. The Effect of Lower Design Margin on Brittle Fracture of Part UHT
Materials ............................................................ 28
D. Study of Vessel Test Results ........................................... 29
1. Purpose ............................................................. 29
2. CBI Reduced Girth Tests ............................................ 29
3. CBI Ellipsoidal Head Tests .......................................... 30
4. Containment Vessel Model Tests ..................................... 30
5. Praxair Torispherical Head Test..................................... 30
6. MPC Service Exposed Test Vessels With Local Thin Areas ........... 30
7. Conclusions ......................................................... 31
iii
E. Evaluation of Design Margins for ASME Section VIII, Division 2 ...... 31
1. Review of the Failure Modes Considered in Section VIII, Division 2 .. 31
a. Ductile Rupture .............. ··· .. ································ 31
b. Brittle Fracture .................... · . · .. · · · · · · · · · · · · · · · · · · · · · · · · · · 32
c. Fatigue ..................... · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 32
d. Incremental Collapse ..................... · · .. · · · · · · · · · · · · · · · · · · · · 33
e. Elastic Instability .................. · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 33
f. Plastic Instability..................... · ... ························· 33
g. Excessive Deformation and Leakage .. : ........ : .. · · · · · · · · · · · · · · · · 33
2. Assessment of Significant Factors on Design Margms ............... 33
a. Design Details ........................ · .. · · · · · · · · · · · · · · · · · · · · · · · · · · 33
b. Ductile Rupture ................................... · .. · · · · · · · · · · · · · 33
c. Brittle Fracture ................................. · · · · · · · · · · · · · · · · · · 35
d. Yield Strength/UTS Ratios and Ductility ...................... · ... 40
e. Fabrication/NDE .............................. · · · · · · · · · · · · · · · · · · · · 44
3. Review of Vessel Failures ................................. ··.········ 45
4. Summary and Conclusions .............................. · · · · · · · · · · · · 45
F. Comparison ofASME Section VIII, Divisions 1and2 With Other
Codes ....................................... ························· 46
1. Design Margins ................................. · · · · · · · · · · · · · · · · · · · · 46
a. General ............................. · · .. · · · · · · · · · · · · · · · · · · · · · · · · · · 46
b. ASME, Section VIII, Division 1 .................................... 48
c.ASME, Section VIII, Division 2 ......................... · · ·. · · · · · · · · 48
d. BS 5549 ................................... · · · · · · · · · · · · · · · · · · · · · · · · · 49
e. AD-Merkblatt................................... · · · · · · · · · · · · · · · · · · · 49
f. Stoomwezen ................................ · .. · · · · · · · · · · · · · · · · · · · · 49
g. CEN .......................................... ·· ········ · · · · ······ · 50
2. Design Rules ............................... · · · . · · · · · · · · · · · · · · · · · · · · · 50
a. Design for Internal Pressure ...................... · ... · · · · · · · · · · · · 50
b. Design of Dished Heads ........................... · . · · · · · · · · · · · · · · 51
c. Design of Flat Covers ........................... ··················· 51
d. Design of Flanges ......................... · · .. · · · · · · · · · · · · · · · · · · · · 51
e. Design of Openings ............................... · · . · · · · · · · · · · · · · · 51
f. Fatigue Analysis ............................ · .. · · · · · · · · · · · · · · · · · · · · 52
g. Design for External Pressure ...................................... 52
3. Design Details ..................................... · · · · · · · · · · · · · · · · · · 52
4. Special Service Considerations ...................................... 54
5. Materials Requirements ...................... · . · · · · · · · · · · · · · · · · · · · · · 55
6. Toughness Requirements ......................... · · · · · · · · · · · · · · · · · · · 57
7. Fabrication Requirements ......................... ·. · · · · · · · · · · · · · · · 61
a. Welded Joints .................................... · .... · · · · · · · · · · · · 61
b. Impact Testing of Welded Joints ................................... 62
c. Inspection of Welded Joints ............................. · .. • · · ·. · · 64
d Tolerances ..................... · ... · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 68
e:PWHT ............................................................. 69
f. Third Party Inspection ............................. · · · · · · · · · · · · · · · 71
8. Overpressure Testing Requirements ...........•.................... 72
9. Quality Control System .......•.............. · ... · · · · · · · · · · · · · · · · · · · 74
10. Summary and Conclusions .............................. ·· .. ······· 74
G. Recommendations for ASME Section VIII, Division 1 and Division 2
Revisions ........................ • · .. · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 79
H. Recommendations for Additional Studies ................ · · · · · · · · · · · · · 83
Bibliography ........................ · · .. · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 84
i"(T
Report No. 1: Evaluation Design Margins
Code Section VIII, Division 1
ASME
E. Upitis and K. Mokhtarian
Executive Summary
Improvements to ASME Section VIII, Division 1 Code
rules over the last 40-50 years, successful experience with
vessels designed to the Code rules, improved materials
and fabrication practices, more sophisticated design rules
and toughness requirements have prompted a further
review of the Section VIII, Division 1 requirements to see if
there is a justification for a reduction in the present Code
design margins from a factor of 4 to 3.5 on the ultimate
tensile strength at temperatures below the creep range.
This report discusses the successful experience with
vessels designed to Section VIII, Division 1 rules, the
failure modes considered in the Code rules and margins on
design, improvements in the Code rules since the last
reduction of the design margin, the Code paragraphs
affected by changes in the design margins, areas which
need a further review, and recommended additional work
as a follow-up to this study to further justify the lower
design margin.
The Code rules apply to new construction only and have
no control over the operation of vessels once they hit\re
been placed in service. Failures have occurred in service
from various causes and during overload testing, mainly in
materials with poor notch toughness or because of poor
details or fabrication practices. Failures during overload
testing have now been largely eliminated because of the
improved Code toughness rules and the recommendation
that test water temperature be at least 30°F above the
minimum design metal temperature.
The main issues affecting the safety of new vessels are
ductile rupture and brittle fracture. The work by B. F.
Langer and burst tests at University of Kansas in the
1970's on test vessels with different strain hardening
exponents have demonstrated that ductile rupture is
highly unlikely with the reduced design margin of 3.5 on
ultimate tensile strength.
One of the most significant changes to the Code rules in
Section VIII, Division 1 was the new toughness rules
introduced in 1987. These are based on fracture mechanics
considerations and past successful practices with Code
vessels. Materials and welded joints are now subject to
impact testing, unless exempted by the impact test exemption curves in Fig. UCS-66 and other Code rules. Although
it is not possible to establish a single margin of safety
against brittle fracture, it is unlikely that the safety of
Section VIII, Division 1 vessels would be jeopardized by
lowering the design margin from 4 to 3.5 on tensile
strength, because of several factors:
1. The new Division 1 toughness rules.
2. Enhanced notch toughness of vessels resulting from
improved steel-making practices and vessel manufacturing practices.
3. The successful experience with Division 2 vessels,
designed to higher stresses, but subject to similar toughness requirements.
4. Improved design rules.
A large part of the report has been devoted to the
numerous improvements in the Code rules since the 1940's
when the design margin was reduced from 5 to 4. Significant improvements have been made to materials, the Code
design rules and design details, fabrication requirements,
welding, postweld heat treat requirements, NDE requirements, and the Code toughness rules. New rules have been
introduced where none existed in the early editions of the
Code. These improvements have led to general improvement to the safety and reliability of Section VIII, Division
1 vessels, but no efforts have been made so far to try to
justify a further reduction to the Code design margins,
taking credit for the revisions and improvements to the
Code rules.
This report includes recommendations for more in-depth
investigation in certain areas to provide further justification for reduction of the present design margins.
Section H gives suggestions to the Code Committee for
revisions to certain Code paragraphs to insure that the
Code rules are consistent with the reduced design margin
of 3.5 and tQiiimplement the necessary changes to the Code
rules.
This report concludes that a reduction in the present
design margin from 4 to 3.5 is justified, based on the
improvements in the Code rules and the excellent past
experience of vessels built to the Code rules.
A. Introduction
The ASME Boiler and Pressure Vessel Code was
established to provide a uniform set of rules which
would result in improved safety of pressure vessels.
The first edition was a set of rules published in 1915
Evaluation of Design Margins Section VIII
1
for power boilers which significantly reduced the
amount of boiler accidents. 4 The firstASME Code for
Pressure vessels was issued as "Rules for the Construction of Unfired Pressure Vessels," Section VIII,
1925 Edition. It had a design margin of 5 on the
specified minimum tensile strength. Sections I, III,
IV, and VIII now provide rules for construction of
power boilers, nuclear power plant components, heating boilers, and unfired pressure vessels. These rules
apply to new construction only.
The present ASME Code, Section VIII, Division 1
(herein referred to as "the Code") design margins
have existed since 1942/1943 when the design margin on the ultimate tensile strength was reduced
from 5 to 4 during the World War II, when there was
a need to conserve critical materials due to national
emergency. Soon after that the hydrostatic test pressure was reduced from 2 to 1.5 times the maximum
allowable working pressure.
The successful service experience, improved materials, more sophisticated design rules, and improved
vessel fabrication practices have now prompted a
further review of the ASME Code, Section VIII,
Division 1 requirements to see if there is a justification for further reduction in the Code design margins.
The purpose of this report is to study the justification for reduction of the design margin on the
ultimate tensile strength from the present value of 4
to a proposed value of 3.5. (A change in other design
factors is not contemplated at this time). This report
will also point out any areas of concern if the design
margin on the ultimate tensile strength is reduced.
Service experience, test results, and revisions to the
Code will be considered in providing recommendations for Code revisions and for follow-up studies.
This study is not intended to apply to the parts of
Section VIII, Division 1 which cover vessels designed
to design margins other than 4 on the specified
minimum tensile strength, which excludes vessels
fabricated to the following rules:
• PartUCI
• PartUCD
B. Service Experience with Section VIII,
Division 1 Vessels
The ASME Code, Section VIII, Division 1 includes
design rules, fabrication and NDE requirements and
specific requirements for pressure relief devices to
insure adequate margins of safety. The ASME Code
rules apply to new construction only and have no
control over the operation of vessel. Although the
Code does not address post-construction issues, it
does include certain comments and suggestions to
avoid service induced problems (Section II, Appendix
6; Section VIII, Appendices NF and HA). These
2
suggestions and recommendations have been added
in the later editions of the Code.
Although literature contains various articles· on
failures of pressure vessels, no comprehensive summary exists on service experience of ASME Code,
Section VIII, Division 1 pressure vessels. Also, in
some cases information is not available on failures
due to ongoing litigation. No attempt has been made
here to review all pressure vessel failures. The
Bibliography at the end of this report, however,
includes several references on failures of pressure
vessels.
Pressure vessel failures generally fall into the
following categories:
1. Failures due to inadequate design or inadequate details (fatigue, etc.)
2. Failures due to poor notch toughness, fabrication defects, or welded repairs.
3. Process or operation related failures (overheating, explosions, etc.)
4. Failures caused by service related degradation
or cracking (corrosion, stress corrosion cracking, etc.).
5. Failures from a combination of the causes listed
above.
The majority of failures seem to be related to poor
notch toughness, service degradation, or operation
related problems. The incidence of catastrophic failures due to design faults or fabrication faults is low.
Very few failures can be attributed. to inadequate
design rules in ASME Code.
Table 1 lists several failures, available from the
published literature, most of which have occurred
during hydrostatic testing. Most failures listed in
Table 1 have occurred in as-rolled, coarse grain
materials with poor notch toughness. Recent revisions to the Code rules recommend hydrostatic testing and require pneumatic testing at temperatures
at least 30F above the minimum design metal temperature (UG-99(h) and UG-lOO(c)). This requires a
minimum metal temperature at least 30F above the
appropriate impact test exemption curve in Fig.
UCS-66. Most of the materials listed in Table 1
would be assigned to Curve A. The present Code
rules limit the use of most Curve A materials (see
UCS-6(b)) and would require the use of better materials or heating the test water to insure an adequate
margin of safety during the overload testing.
Reference 24 describes a brittle fracture of a 14 ft.
diameter, 1.6" thick vessel during stand-by operation
below design pressure. This vessel was built of
French A-42C-35 material, which is equivalent to
SA-201B. This vessel failed after welded repair
without PWHT, when the operating pressure was at
65% of design pressure, at about 41°F. Failure
initiated at the tip of small surface cracks, from
welding residual stresses and localized embrittlement by dynamic strain aging.
The failures which have occurred during hydro-
WRC Bulletin 435
1954
New
1955
20yrs
1956
75mm(3")
A302Gr. B
Process vessel
32mm(l1/4'')
Equiv. to A 212 Gr. B
New
13.6 m (44'--8") dia. x
23.1 m (75'-11") high
reactor. (non-PWHT)
32mm(11/4'')
A285Gr. C
1956
New
2.13 m (7 ft) dia. CO
Converter. (PWHT)
43 mm (1.7")
A204Gr. B
1967
New
Cylindrical drum (PWHT)
75mm(3")
A212 Gr. B
1970
15 yrs
3.657 m (12') l.D. catalytical
reformer reactor
51 mm(2")
A204Gr. B
1970
New
49.2 m (15') dia. X 42.3 mm
(1.625") thk (PWHT) &
8'-10Y2" dia. (non-PWHT)
28 mm(l.l")
(as-rolled)
A515-1967 Gr. 70
NL
Abt. 25 yrs
Coke drum
NL
New
6.1 m (20') dia. x 24.3 m
(80') long. (not PWHT)
lW'
A515-1969 Gr. 70
1972
New
1.524 m (5') 1.D. (PWHT)
178mm (7")
A515 Gr. 70
1978
llyrs
305 cm (12') l.D. (PWHT)
7 cm (2.75")
A212 Gr. B
1979
23 yrs
Heat exchanger
NL
Al82 Gr. F12
NL
New
Cylindrical vessel
NL
A515 Gr. 70
NL
NL
NL
NL
NL
A204Gr.A
A387 Gr. 11Cl.2
1980
17 yrs
2.286 m (7.5') I.D.
desulfurizer reactor
85.5 mm (3.25") +
3.5mmclad
A204Gr.A
1871
20yrs
4.3 m (14.1') l.D. spherical
catalitic reformer
41 mm (1.614")
AFNORA!tJ-205
(similar to A201 Gr. B)
1984
16yrs
2.6m (8'-6") I.D. CsfC4
amine absorber vessel
25.4mm(l")
SA-516, Gr. 70
Failed at 14°C (57°F) and 16.5
MPa (2390 psig) during
hydrostatic test. 30
Failed during hydrostatic test at
16-21°C (61-70°F).so
Failed during hydrostatic test at
1.5 times design pressure and
8 to 11°C (46 to 52°F). Failure
initiated in the knuckle of the
bottom head. 30
Failed during hydrostatic test at
1.5 times design pressure and
7°C (45°F).30
Failed during hydrostatic test at
87% of design pressure and
15°C (60°F).30
Failed during gas test at 90% of
design pressure and 11-21°C
(52-70°F). 30
Failure occurred during hydrostatic testing at 127% of
design pressure and 12°C
(54°F). Failure occurred from
a preexisting defect at a fillet
weld attaching manway reinforcing plate to shell. 22
Failure occurred during hydrostatic testing at 315 psig (abt
94% of max test pressure) and
4 °C (40°F) after welded
repairs and PWHT. 29
Failure occurred during hydrostatic test at near max. test
pressure of260 psig. Test
water temp. not known.
Failure initiated at toe of fillet
weld between manhole reinforcing plate and vessel shell.
Failure occurred during hydrostatic test at 131 % of design
pressure at 18°C (64°F).30
Failure occurred during hydrostatic testing, after repairs, at
880 psig internal pressure
and 25°C (77°F). (Design pressure was 650 psig). Failure
initiated from a partial penetration weld between two
halves of steam dome reinforcing pad. 21
Vessel failed during hydrostatic
test at 154% of design pressure. 30
Vessel failed during hydrostatic
test at 13.8 Mpa (2000 psig)
and l6°C (61°F).23
Vessel failed during hydrostatic
test at 10°C (50°F).30
Vessel failed during inert gas
test at 85% design pressure
and ambient temperature.so
Vessel failed during standby service at about 65% of design
pressure at 5°C (41°F).24
Vessel failed during operation
(at about 200 psi and 100°F)
due hydrogen induced cracks
inhardHAZ
=not listed.
Evaluation of Design Margins Section VIII
3
static testing are by no means limited to coarse grain
steels. Several liquid spheres failed in a brittle
manner during hydrostatic testing in Europe during
1960-1970's. 25 At least four of these are known to be
of fine grain, fully killed, normalized steels. The
failures of these spheres are attributed to poor
details and improper fabrication practices for the
materials used. These spheres were designed to
stresses permitted in European codes, but no detailed reports are available to the authors on these
failures.
C. Failure Modes Considered
in the Code Rules
In 1987-1989 the ASME Subgroup on Design
Analysis (SGDA) performed a study to determine the
failure modes considered in the sections of the Code
and the intended safety margins for a given failure
mode in each section of the Code. Eight failure modes
were identified and discussed in the February 1989
report "Summary of the Design Analysis Factors
Inherent in the Established Failure Modes of the
ASME boiler and Pressure Vessel code," prepared by
Subgroup Design Analysis. The following is a brief
description of each failure mode considered in Section VIII, Division 1. A description of design margins
explicitly introduced into the Code design rules is
included in Appendix 1, Section II, Part D.
1. Ductile Rupture
The following general design margins have been
established to provide an adequate margin of safety
against ductile rupture below the creep range:
• SulS;:::: 4
• SylS ;:::: 1.11 (for some non-ferrous alloys and
austenitic materials)
• Sy!S;:::: 1.5 (for all other materials)
S = Allowable design stress
Sy = Specified minimum yield strength
Su = Specified minimum tensile strength
The following references in the Code give the
allowable stress values for Section VIII, Division 1
and the basis for the allowable stress values.
• Section VIII, Division 1, Appendix P, Basis for
Establishing Allowable Stress Values.
• Section II, Part D: Tables lA and lB.
• Section II, Part D, Appendix 1, Nonmandatory
Basis for Establishing Stress Values in Tables
lAand lB.
The maximum allowable stress is the lowest value
obtained from the section VIII, Division 1 criteria at
the specified design temperature.
For structural materials, when used for pressure
retention applications, and for bars and shapes used
as shell stiffeners or stay bolts, the basis for allowable stresses is subject to a quality factor of 0.92 (see
Paragraph 1-lOO(c) of Appendix 1 of Section II, Part
D). This quality factor is based on judgment and
4
experience for present design margins. With a lowering of the design margin, it is recommended that the
Code Committee review this factor.
The basis for establishing allowables for Parts
ULT, UCI, and UCD of the Code are spelled out in
Appendix P. The design margins for ULT-23 materials are generally the same as for wrought or cast
materials. It is assumed that any reduction in design
margins agreed upon by the Code Committee will be
applicable to these materials. However, the design
margins for UCI-23 and UCD-23 materials are different from that of wrought or cast materials, and it is
assumed that these factors will remain unchanged.
The allowables for welded pipe or tube are generally 85% of the value obtained by using the basis for
wrought or cast materials (see Table 1-100 of Appendix 1 of Section II, Part D). It is assumed that this
factor will be retained and will be applicable to any
revised design margins.
It is assumed that the design margin for bolting
materials will also be revised to remain consistent
with other materials. The present Code uses generally the same basis for establishing bolting allowables, i.e. 114 of ultimate or % of yield, except for
materials whose strength has been enhanced by heat
treatment or by strain hardening (see 2-120 of
Appendix 2 of Section II, Part D).
Section E of this report comments on the burst
tests performed on SA240 Type 304, SA516 Gr. 70
and SA517 Gr. F steel test vessels at University of
Kansas. 12 Based on these tests, it is unlikely that the
true margin of safety against ductile rupture would
be affected by lowering the design margin from 4 to
3.5 on ultimate tensile strength.
2. Brittle Fracture
This failure mode is addressed in the Code by rules
for design, the notch toughness requirements in
UG-84, UCS-66, UCS-67 and UCS-68, fabrication
requirements, and hydrostatic/pneumatic testing requirements. The following are examples of Code
rules which guard against brittle fracture:
UG-16 through UG-35 (design), UG-84 (Charpy impact tests), UG-85 (Heat treatment), UG-95 thru
UG-97 (examinations and inspections during fabrication), UG-99 thru UG-100 (Hydrostatic and pneumatic tests), UG-125 thru UG-136 (Pressure relief
devices).
Part UW (Requirements for pressure vessels fabricated by welding).
UCS-56 (Postweld heat treatment), UCS-65 thru
UCS-68 (Low temperature operation), UCS-75 thru
UCS-85 (Fabrication).
UHA-32 (Postweld heat treatment), UHA-50 thru
UHA-52 (Inspection and tests), Appendix HA (Suggestions on selection and heat treatment of high
alloy steels).
UHT-5 & UHT-6 (Materials), UHT-17 (Weldedjoints),
UHT-18 (Nozzles), UHT-56 (Postweld heat treat-
WRC Bulletin 435
ment), UHT-57 (Examination), UHT-75 thru
UHT-86 (Fabrication).
Appendices 4, 6, 7, 8, 10, 12 (requirements for
non-destructive examination and quality control).
Appendix R (Preheating).
The Section VIII, Division 1 toughness rules are
based on fracture mechanics considerations and
extensive good experience with ASME Code vessels.
It is not possible, however, to establish a single
margin of safety against brittle fracture in the Code
rules because brittle fracture depends on three factors, the applied stress, the flaw dimensions, and the
available toughness of the material. In fracture
mechanics calculations, the value of the stress intensity, Kr, is based on the applied stress and dimensions of the flaw. The calculated Kr must be less than
the critical fracture toughness parameter Kic (or Krd,
depending on the loading rate) to avoid brittle fracture. Also, consideration must be given to both
primary and secondary stresses to evaluate maximum stress intensity, Kr, in the structure.
Work by Dr. Sumio Yukawa 16 attempts to evaluate
the Code margins for resistance to brittle fracture in
carbon and low alloy steels based on:
1. Critical vs. permissible flaw length for a given
stress, and
2. Material fracture toughness vs. applied stress
intensity factor, Kr, for a given flaw size.
The report by Dr. Yukawa gives the following results:
• Calculated critical flaw length vs. maximum
Code permissible flaw size of%" for RT examination, for various flaw depth/thickness (alt) ratios
and applied stress/yield strength ratios (SR).
The size margin for SR = 1.0 and alt = 0.5 is
about 2 Yz in 1-3 in. thick material with 38 ksi
minimum yield strength (and higher for lower
alt ratios and lower stress ratios).
i
• Available fracture toughness (based on mihimum required toughness in the Code) vs. the
applied stress intensity, K, for a flaw with a
length of%" (maximum permitted in UW-51 for
RT examination). This evaluation indicates a
minimum margin of about 1.5 for 1 and 3"
thicknesses (and higher values for lower stress
ratios) for a material with a minimum yield
strength of38 ksi and stress ratio (SR) of 1.0.
• This study also concludes that the relief from
notch toughness testing as permitted in Figure
UCS-66.1 does not compromise these margins.
It should be noted that any indication characterized
as a crack or zone of incomplete fusion or penetration
in welded joints is unacceptable by the Code rules
CUW-51(b)(l) and UW-52(c)(l)).
A further review of the Code margins for resistance to brittle fracture is being planned for the
Phase 2 study. This will include the effect of higher
design stresses (i.e. lower margins against ductile
rupture) on margins against brittle fracture for Part
UCS and Part UHT materials.
It is, however, the opinion of the authors that the
safety of Section VIII, Division 1 vessels would not be
jeopardized by lowering the design margin from 4 to
3.5, because of the following considerations:
1. The Section VIII, Division 1 toughness rules
(introduced in 1987).
2. Enhanced notch toughness resulting from improved steel-making practices (see Sect. D.l).
3. Improved welding practices.
4. The successful experience with Section VIII,
Division 2 vessels designed to higher stresses
but similar toughness requirementp as the present Division 1 vessels.
3. Fatigue
Section VIII, Division 1 presently has no rules for
fatigue analysis. The cyclic use for Division 1 vessels
is assumed to be low. Paragraph UG-22 requires
consideration of cyclic loads, but the Code does not
indicate what is a significant number of cycles for
which fatigue analysis should be performed, therefore, the designer is referred to U-2(g). The exemption from fatigue rules of Section VIII, Div. 2 provide
a quantitative estimate of the number of cycles of
pressure and temperature loading that a vessel can
safely tolerate, when designed to a certain stress
limit. In Phase 2, calculations will be included to
quantify the effect oflower design margins.
4. Incremental Collapse
Incremental collapse is generally caused by ratchetting which is inelastic strain accumulation due to
cyclic loading. The Code does not provide explicit
rules for this mode of failure. Design and fabrication
rules of the Code are intended to minimize the
causes of ratchetting. If a vessel is in a cyclic service
and a fatigue analysis is performed, the required
limit on the range of primary plus secondary stress
intensity (required to be limited to two times yield
strength by Section VIII, Div. 2 and Section III) will
guard against ratchetting and incremental collapse,
therefore, the safety of Section VIII, Division 1
vessels would not be adversely affected by lowering
the design margin to 3.5.
5. Elastic Instability
The C?~ rules on elastic instability are based on
geometric parameters and are independent of the
design stress limits. Any change in the design margin on ultimate strength will not affect the buckling
rules. (A PVRC study has proposed alternative rules
to the present Code rules on buckling. The report has
been published as a WRC Bulletin 406). For Code
basis in developing external pressure curves see
Appendix 3 of Section II, Part D.
6. Plastic Instability
As the R/T ratio of shells get smaller, the failure
mode changes from elastic instability to plastic
Evaluation of Design Margins Section VIII
5
instability. The present Code rules for design of
formed heads are generally based on plastic collapse.
The plastic collapse is a strain induced failure, even
though the Code rules are based on stress allowables. A reduction in design margins on ultimate
strength will reduce the margins for these components. But these rules are generally regarded as very
conservative and, therefore, it is unlikely that decreasing the design margin from 4 to 3.5 on UTS
would affect the safety of the vessel. The Code
external pressure curves, at the upper end, are
limited to % of yield strength and these curves will
not be affected by a change in design margins. The
calculated allowable compressive stresses are not
allowed to exceed the basic Code stress allowables.
7. Creep Rupture
The Code allowable stresses at temperatures in
the creep range provide a reasonable margin of
safety against creep rupture failure. The allowable
stresses at temperatures in the creep range are
based on the lower of the following:
• 100% of average stress to produce a creep rate of
0.01 %/1000 hrs.
• 67% of the average stress to cause rupture at
the end of 100,000 hrs.
• 80% of the minimum stress to cause rupture at
the end of 100,000 hrs.
These limits will not be affected by any reduction in
design margin on ultimate tensile strength, however, the transition from time independent properties to time dependent properties will occur at a
lower temperature.
8. Creep Buckling
The temperature limits on the Code buckling
curves are intended to be limited to the temperature
at which creep becomes a consideration for the
material. As such, the present Code does not intend
to address creep buckling. However, some of the
temperatures on Code external pressure curves do
extend into the creep range and the Code Committee
is looking into this (PVRC has been working on
recommendations for creep buckling rules to be
recommended to the Code Committee).
Non-Mandatory Appendices. As the needs developed
over the years, many new materials and Parts were
added to Section VIII. The Requirements for Pressure Vessels Constructed of Carbon and Low Alloy
Steels are now included in Part UCS.
1. Materials
Table 2 lists the carbon and low alloy steel plate
specifications which were included in the 1950 edition of Section VIII, Table UG-23. Table 3 lists the
carbon steel and low alloy steel plate specifications
included in Table lAofthe 1992 edition of Section II,
Part D, for comparison. It is obvious that there have
significant changes to the type of materials permitted for construction of Section VIII vessels. The
following addresses some of the changes to the
materials since the 1950 edition of Section VIII:
SA-30. SA-30 was as-rolled low strength carbon steel
with specified carbon and manganese contents,
but had no requirements for deoxidation. This
specification has been withdrawn.
SA-129. This was a low strength carbon steel. This
specification has been withdrawn.
SA-201. SA-201, Grades A and B were low strength,
fully killed carbon steels with 0.80% maximum
manganese content. SA-201 had no grain size
requirements, therefore both grades could be produced to fine grain or coarse grain practice. Plates
under 2" thick were typically produced as-rolled.
Plates over 2" thick were required to be normalized. This was a commonly used steel for pressure
vessel construction. This specification has now
been voided. SA-201B has been incorporated in
SA-515, as grade 60.
SA-212. This steel was available in grades A (65 ksi
min. tensile strength) and Grade B (70 ksi min.
tensile strength). These were fully killed steels
with a maximum manganese content of 0.90%.
Grade A had a maximum carbon content of 0.31 %
in thicknesses up to and including l" and 0.33% for
Table 2-Carbon and Low Alloy Steel Plate
Specifications Included in th e1950 Edition
of ASME
Section VIII
6
Min. Tensile
SA-283
SA-36
SA-285
SA-299
SA-455
SA-5I5
SA-5I6
SA-662
SA-537
SA-737
SA-738
SA-84I (CC2130-I)
SA-6I2
SA-225
SA-204
SA-302
SA-30
SA-I29
SA-20I
SA-2I2
SA-285
SA-299
SA-30I
SAI-302
A-283
SA-202
SA-203
SA-204
SA-225
WRC Bulletin 435
Flg,AFbx,B
Fbx
A,B
A,B
A,B,C
A,B
A,B,C,D
A,B
A,B,C
A,B,C
A,B
Carbon
55, 42, 48, resp.
Carbon
Carbon
C-Si
Carbon
C-Mn-Si
I\i,iCr-YzMo
Mn-YzMo
Carbon
44
55, 60, resp.
65, 70, resp.
45, 50, 55, resp.
75
70
75, 80, resp.
45, 50, 55, 60,
resp.
75, 85, resp.
65, 70, 75 resp.
65, 70, 75, resp.
70, 75, resp.
A,B,C,D
A
B,C
Carbon
C-Mn-Si
Carbon
C-Mn-Si
C-Mn
C-Si
C-Si
C-Mn-Si
C-Mn-Si
C-Mn-Si
C-Mn-Si
C-Mn-Si
C-Mn-Si
C-Mn-Si-Cb
C-Mn-Si-V
C-Mn-Si
C-Mn-Si
c
C-Mn-Si
Mn-YzNi-V
A,B,C
60, 65
70
55, 60, 65
70
A,B
c
I
2, 3
B
c
C-YzMo
C-YzMo
A
B,C
A
B
c
SA-533
SA-387
SA-202
SA-542 (CCI961-I)
SA-542CCCI960-4)
SA-542(CC2098-2)
SA-832
SA-517
SA-203
SA-543
SA-353
SA-553
SA-645
D. Improvements to the Code Since 1940's
There have been numerous changes and improvements in the Code rules since the present design
margins on ductile rupture were established in
1942/3. Certain current Code requirements are compared to those in the 1950 edition of Section VIII in
this report for comparison. The 1950 edition of
Section VIII consisted of about 280 pages (5114 X 7%")
and included Parts UG (General Requirements for
Vessels of Carbon and Low Alloy Steels), UW (Requirements for Welded Vessels of Carbon and Low
Alloy Steel), UR (Requirements for Riveted Vessels
of Carbon and Low Alloy Steel) and Mandatory and
Table 3-Carbon and Low Alloy Steel Plate Specifications Included in 1992 Edition of ASME Code Section II,
Part D and Section VIII Code Cases
D
A, Cl.I & Cl.2
A,Cl.3
B, Cl.I
B,Cl.2
B,Cl.3
C, Cl.I & Cl.2
C,Cl.3
2, Cl.I
2,Cl.2
I2, Cl.I & Cl.2
11, Cl.I & Cl.2
22, Cl.I & Cl.2
21, Cl.I & Cl.2
5, Cl.I &Cl.2
9I
A,B
C,Cl.4
B, Cl.4
D,Cl.4
A
B
E
J
p
A,B
D,E,F
C, Cl.I, 2 & 3
B, Cl.I, 2 & 3
I, II
Mn-YzMo
Mn-YzMo
Mn-Y2Mo-%Ni
Mn-%Mo-%Ni
Mn-Y2Mo
Mn-YzMo
Mn-YzMo-YzNi
Mn-YzMo-YzNi
Mn-YzMo-YzNi
Mn-YzMo-%Ni
Mn-YzMo-%Ni
YzCr-YzMo
YzCr-YzMo
ICr-YzMo
I;i.iCr-YzMo
2;/.iCr-IMo
3Cr-1Mo
5Cr-YzMo
9Cr-1Mo-V
YzCr-IY-iMn-Si
3Cr-1Mo-;i.iV-Ti-B
2;1.iCr-lMo
2;1.iCr-IMo-;i.iV
3Cr-IMo-\4V-Ti-B
YzCr-;/.i~o-Si
YzCr-YsMo-Si
Io/.Cr-YzMo-Ti
C-YzMo
l;i.iNi-lCr-YzMo
2YzNi
3YzNi
2%Ni-l VzCr-YzMo
3Cr-lo/4Cr-%Mo
9Ni
9Ni,8Ni
5Ni-;i.iMo
I Gr. I
I Gr. I
I Gr. I
I Gr. 2
I Gr. 2
I Gr. I
I Gr. 2
I Gr. I
1 Gr.2
1 Gr. I
lGr. 2
1Gr.2
1Gr.3
I Gr. 2
I Gr. 3
I Gr. 2
I Gr. 2
I Gr. 2
IOC
lOAGr. I
3Gr. I
3Gr.2
3Gr. 2
3Gr. 3
3Gr. 3
3 Gr. 3
3Gr. 3
llAGr. 3
3 Gr. 3
3 Gr. 3
llAGr. 4
3 Gr. 3
11Gr.4
3Gr. I
3 Gr. 2
4Gr. I
4Gr. l
5AGr. I
5AGr. l
5B Gr. I
5B Gr. 2
4Gr.1
5C Gr. I
5C Gr. I
llB Gr. 1
llB Gr. 4
llB Gr. 2
llB Gr. 6
11B Gr. 8
9AGr. l
9B Gr. I
llAGr. 5
llAGr. 5
llAGr. I
llAGr. I
llAGr. 2
45, 50, 55, 60, resp.
58
45, 50, 55, resp.
75
70
60, 65, resp.
70
55, 60, 65, resp.
70
58, 65, resp.
70
70(65)*
80(75,70)*
70
80
70
85, 80(75,70)*
70
83(8I)*
105
65
70, 75, resp.
75
80
80
80
80, 90, resp.
100
80
90
100
80, 90, resp.
100
55
70
55, 65, resp.
60, 75, resp.
60, 75, resp.
60, 70, resp.
60, 75, resp.
85
75, 85, resp.
85
85
85
85
115
115
115
115
115
65, 70, resp.
65, 70, 80, resp.
105, 115, 90, resp.
105, 115, 90, resp.
100
IOO
95
*Varies with thickness
%Cr-I\4Mn-Si
2%Ni
C-YzMo
Mn-YzNi-V
plates over l" thick. These grades were normally
produced to coarse grain practice. Plates up to and
including 2" thick were typically produced asrolled; thicker plates were required to be normalized. These were commonly used grades for pressure vessel construction. This specification has
now been voided and the steels included in SA-515
as grades 65 and 70.
SA-283. These are low strength structural steels
without any specific deoxidation requirements.
Grades A (45 ksi min. UTS), B (50 ksi min. UTS), C
(55 ksi min. UTS) and D (60 ksi min. UTS) are still
permitted for pressure vessel construction, but
their use is limited to%" maximum thickness for
pressure parts and subject to the toughness requirements in UCS-66.
Evaluation of Design Margins Section VIII
7
SA-285. Grades A (45 ksi minimum UTS), B (50 ksi
min. UTS) and C (55 ksi min. UTS) are low
strength carbon steels with 0.90 max. manganese
content and no requirements for deoxidation. These
grades are still permitted for pressure vessel construction, subject to the notch toughness requirements in UCS-66.
SA-299. This is a fully killed C-Mn steel with 75 ksi
minimum UTS and can be produced to fine grain
or coarse grain practice. Plates 2" thick and thinner can be produced as-rolled. Plates over 2" thick
must be normalized. This steel is still used for
Code pressure vessels, but it's use now is restricted
to Curve A steels in Fig. UCS-66 for notch toughness consideration (unless it is produced to fine
grain practice, in which case it is considered Curve
B steel).
SA-300. This specification included toughness requirements for normalized steel plates for pressure vessels. The impact tests were to meet 15
ft-lbs minimum average, using a Charpy type,
keyhole (or milled "U") notch specimen. This specification has now been voided, as the impact test
requirements are now included in the individual
materials specification and in the ASME code. All
impact tests now must be performed using V-notch
type Charpy specimen conforming to Fig. 11 in
ASTM A370 (which is a more severe test than a
keyhole type specimen).
SA-202. Gr. A (75 ksi min. UTS) and Gr. B (85 ksi
min. UTS) are Yz Cr-1114 Mn-Si fully killed low alloy
steels which can be supplied in as-rolled condition
or normalized. These steels are still permitted for
Code vessels, but their use is restricted Curve A or
Curve B steels (as applicable) for notch toughness
considerations.
SA-203. Grades A and B are fully killed, normalized,
2%% Ni steels with good notch toughness and are
still included in the present edition of Section II,
Part D for Code vessels. Grade Chas been eliminated, but several new 3%% Ni grades have been
added.
SA-204. These are fully killed C-Yz Mo steels and are
available in Grades A, B and C, depending on
strength. These steels can be produced as coarse
grain or fine grain steels and do not need to be heat
treated in thiclmesses up to and including 11/z".
These steels are still permitted for Code vessels,
but have poor notch toughness in the as-rolled
condition, and their use is now restricted to Curve
A or Curve B (as applicable) for notch toughness
considerations.
SA-301. These 1114 Cr-Yz Mo and 1 Cr-Yz Mo low alloy
steels have now been replaced with SA-387 Grades
11and12.
SA-302. Grades A and B are C-Yz Mo steels, similar to
SA-204, but have higher manganese contents and
higher strength. SA-302 Grades A and B are now
subject to the same notch toughness considerations in UCS-66 as the SA-204 steels.
8
Many more steels have now been approved for
Section VIII Division 1 vessels than those listed in
the 1950 edition oftheASME Code. Almost all of the
newer steels are fully killed, and many are made to
fine grain practice and heat treated. This results in
better notch toughness and reduces the risk of brittle
fracture and improves safety. An exception is SA-515
which is produced to coarse grain practice. The
SA-515 grades are permitted for use at design metal
temperatures up to and including 1000°F and have
the same allowable stresses as the comparable SA516 grades (made to fine grain practice). The SA-515
grades, however, have poorer notch toughness because of the coarse grain practice. Grades 65 and 70
are assigned to impact test exemption Curve A and
grade 60 to Curve Bin Figure UCS-66.
All SA-516 grades are fully killed, made to fine
grain practice and are normalized when the thickness exceeds 11/z". As-rolled grades 65 and 70 are
assigned to impact test exemption Curve B and
as-rolled grades 55 and 60 to Curve C. When normalized, all SA-516 grades are assigned to Curve D in
Fig. UCS-66 and can be used at lower design metal
temperatures without impact testing. Another example is SA-537, Classes 1, 2 and 3. These are fine
grain, fully killed steels which are heat treated in all
thicknesses, and therefore suitable for lower metal
design temperatures.
Part UHT includes requirements for pressure
vessels constructed of ferritic steels with tensile
properties enhanced by heat treatment. Most of the
steels included in Part UHT were developed since
the 1950 edition of the Code. Some of the steels in
Part UHT are unique in that they were subjected to
large scale low temperature vessel tests to demonstrate their suitability for Code construction of pressure vessels. Four test vessels of SA-517 Gr. F (USS
Tl steel) were burst tested at about -45°F, and
additional four vessels were pressurized and subjected to falling weight drop-impact tests at temperatures from -33°F to -41°F. Similar tests were
performed on SA-553/SA-353 (9% Ni) steel test vessels at - 320°F to justify the use of these steels at
cryogenic temperatures without post-weld heat treatment.
There have been a number of improvements in
steel making practices since the 1950's. For example,
most steels are now produced by the continuous
casting process. This requires the use of killed steels
with aluminum or aluminum plus silicon additions
to achieve full deoxidation of the steel and insure
good castability. Continuous casting also helps to
achieve more uniform properties in the plates. Improvements have also been made to slag processing,
refractories, secondary refining, temperature control, rolling mills, heat treatment, quality control,
etc., which has resulted in cleaner steels with lower
carbon, sulfur and phosphorus contents and better
quality materials. For instance, in recent years
ASTM and ASME specifications have reduced the
WRC Bulletin 435
maximum phosphorus and sulfur contents by 0.01 %
in several pressure vessel steels. It is well recognized
that reducing carbon and sulfur contents improves
notch toughness (See Figures 1-3).All these improvements have a beneficial effect on notch toughness,
however, none of these improvements have been
reflected so far in the Code design margins.
200
125
150
.0
IOOf
>-"
75 IE
>-"
(,!) 100
a:
w
w
50 ~
z
w 50
2. Design
The design rules of the Code have been extensively
expanded and improved since the 1940's. The additions and revisions to these rules are too numerous
to list here. The following is a brief description of
some of the more significant improvements to the
rules which are judged to add to the safety of vessels
designed and constructed to these rules.
a. Design of cylinders for internal pressure:
A criteria for longitudinal stress has been added
[UG-27(c)(2)] and formulas for thick shells have been
provided [Appendix 1].
b. Design ofspherical shells for internal pressure: The value of "0.2P" has been introduced for
calculating design thickness [UG-27(d)]. For thicker
shells, this will result in a measurable increase in
design thickness. Formulas for thick shells have
been provided [Appendix l].
c. Joint efficiencies: The 1949 Edition of the
Code allowed joint efficiencies of 90% and 80% for
longitudinal joints with full RT or no RT. However,
the use of these values were contingent upon many
other welding and testing requirements. Table 4 lists
the maximum joint efficiencies in the 1950 ASME
Code, Section VIII and in the 1992 edition of Section
VIII, Division 1. The present rules are so extensively
different that it is difficult to make a direct comparison. However, it is obvious that today's rules are
much more detailed and sophisticated.
d. Design of formed heads: The formula for
design ofhemi-spherical heads was almost the same
(it did not include the 0.2P value). The only desiifn
formula for "dished heads" was that for spherically
dished covers of present Code. 1- 6 Extensive formulas
and rules for design of other types of formed heads
25
0
0
.018
.024
SULFUR,%
Fig. 2-Effect of sulfur content on the shelf energy of straightaway
rolled carbon steel plate33
have been added. Limitations on geometric parameters have been added.
e. Design of flat heads: The basic design formula was the same as that of present Code, but did
not have the joint efficiency included. Also the values
of "C" in present Code cover more cases and are
generally more conservative.
f. Design of bolted flanged connections: The
rules were basically the same as those in the present
Code, except a great deal of details have been added
over the years.
g. Design ofopenings: The rules allowed considerably larger openings than those of today's Code
[UG-36(c) 3 ] to be exempt from reinforcement requirements. There were no spacing limitations for multiple unreinforced openings (For calculation of ligament efficiencies, reference was made to Section I).
For those openings not exempt, the reinforcement
requirements were similar to today's rules. Full
pressure area had to be replaced, but the limits of
reinforcement were somewhat different. A great deal
of detail and refinements have been added over the
60
200
- - - - - - - - 0.11%Carbon
...,
>-'
(!)
a:
w
z
w
1--
150
0.20% Carbon
100
0.31% Carbon
(..)
<l:
a.
~
20
50
-100
TEST TEMPERATURE, °C.
Fig. 1-Effect of carbon content on transition curves of ferriteprearlite steels32
-50
0
+50
TEMPERATURE, "F
+100
Fig. 3-Effect of sulfur content 0n transverse impact properties
(Bethlehem Steel research data on HSLA steels)31
Evaluation of Design Margins Section VIII
9
Table 4-Maximum Allowable Joint Efficiencies for Butt Welded Joints
1. From 1950 ASME Code, Section VIII Table UW-12:
Spot RT
Limitations
Type of Joint
Thermally
PWHT'd
None
Single welded butt joint, with backing
strip.
Longitudinal joints not over lYi'' thick.
(No thickness limitation on circumferential joints).
Circumferential joints only, not over%"
(0.80)
2. From 1992ASME Code, Section VIII, Division 1, Table UW-12:
x
Max. Joint
Efficiency
0.80
x
Double welded butt joint.
without
Full RT
x
x
x
x
0.85
0.90
0.95
0.70
Joint Description
Limitations
Joint
Category
Full RT
Spot RT
None
Butt joints as attained by doublewelding or by other means which
will obtain the same quality of
deposited weld metal on inside and
outside weld surfaces to agree with
the requirements ofUW-35. Welds
using metal backing strips which
remain in place are excluded.
Single-welded butt joint with backing
strip other than those included
above.
None
A,B,C,D
1.0
0.85
0.70
Single-welded butt joint without use of
backing strip
(a) None except as in (b) below.
(b) Circumferential butt joints with
one plate offset; see UW-13(b)(4) and
Fig. UW-13.1 and sketch (k).
Circumferential butt joints only, not
over%" thick and not over 24" outside dia.
years, which will result in a better and more conservative design.
h. New rules: In addition to improvements to
the old rules, a great many new design rules have
been added to improve the safety and quality of
vessels. A lot of the recent design rules, although put
into design by formula approach for easy application,
are based on modern analytical methods. If a certain
geometry or loading is not covered by Code rules,
paragraph U-2(g) may be used to analyze. For this
purpose many advanced computational techniques
are available, which were not available when the
present design factors were incorporated. Some of
the design rules added to the Code since the 1940's
are cone-cylinder junction design rules, ligament
design rules, rules for multiple openings, requirements for minimum nozzle neck thickness, design
rules for braced and stayed surfaces, and rules for
large openings in cylindrical shells. A number of
appendices have been added to provide design rules
for specific components such as jacketed vessels,
vessels of non-circular cross section, integral fiat
heads with a large opening, dimpled or embossed
assemblies, integrally forged vessels, clamp connections, and expansion joints. These specific and detailed rules can only contribute to quality and uniformity of vessels built to the Code.
The result of these improvements is that better
and more refined design rules have been established,
which has improved the safety of Division 1 vessels.
3. Fabrication
There have been several improvements in the
fabrication requirements of Section VIII pressure
10
A,B,C,D
0.90
0.80
0.65
A,B,C
0.90
0.80
0.65
A,B,C
NA
NA
0.60
vessels since the 1940's. These involve mainly the
requirements for welding, inspection, postweld heat
treatment and quality control. The following changes
since the 1950 edition oftheASME Code are believed
to have improved the quality of Section VIII pressure
vessels and margins of safety:
Forming: No specific limitations were included
in the early editions of Section VIII (e.g. 1950).
UCS-79(d) now limits the maximum forming strains
to 5% without heat treatment if the vessel will
contain lethal substances or the material is impact
tested. UHT-79(a) 1 limits the maximum forming
strains to 5% for high strength materials without
heat treatment.
Joint Efficiencies for Welded Joints: The joint
efficiencies in Table UW-12 have been further refined. The maximum joint efficiencies are now based
on the type of joint and the degree of radiographic
examination (and not on PWHT). Table 4 provides a
brief overview of the maximum joint efficiencies in
Tables UW-12 of the 1950 Code, Section VIII and the
1992 edition of Section VIII, Division 1 for double
butt welded joints and single butt welded joints with
backing strips.
For Type 1 joints with 100% RT the maximum
joint efficiency has been increased to 1.0 (presumably because of improved NDE techniques). For all
types ofjoints without any RT examination, the joint
efficiencies in the 1950 Edition have been decreased
to lower values.
The present UW-35 includes more detailed requirements for the finished longitudinal and circumferential joints than the earlier editions of the Code.
WRC Bulletin 435
UW-35(a) now specifically states that "Butt welded
joints shall have complete penetration and fusion,"
which insures better quality welds and, therefore,
reliability of vessels.
Attachment Welds and Welded Connections: UW13 has been expanded significantly from the early
editions of the Code. New and more detailed requirements have been added in several areas, e.g.:
• Limitations and specific details for head-tocylindrical shell transitions, as shown in Fig.
UW-13.1 sketches (1), (m), (n), and (o).
• Specific requirements for welded unstayed fiat
heads to shells in UW-13 (e) and (f), and as
shown in Fig. UW-13.2 and Fig. UW-13.3.
The requirements for attachment welds at openings
have been significantly expanded since the early
editions of the Code. Specific requirements are now
included in UW-15(c) for groove welds and fillet
welds. Entire UW-16 and Fig. UW-16.1 have been
added since the 1950 edition of Section VIII and
provide very specific requirements for attachment
welds at openings. (Only some of the acceptable
types of welded nozzles in Fig. UW-16.1 were included in Fig. UW-15 of the 1950 edition of the Code).
A number of fabrication tolerances have been
added over the years which improve the quality of
the vessels. Up to 1949 edition of the Code, the
out-of-roundness was limited to a value similar to
present Fig. UG-80.1. In the 1950 Code, the out-ofroundness tolerances and external pressure tolerance were correctly differentiated and the limit of 1%
on out-of-roundness was introduced. It was also in
this Edition that tolerances for formed heads were
introduced.
4. Welding
Significant advances have been made over the last
40-50 years in improving the welding processes and
procedures. All materials are now assigned Pnumbers in Section IX of the ASME Code. The
welding procedure qualifications requirements in
Section IX and several other requirements in the
other book sections for fabrication are related to
P-numbers. Recommended good practice for preheating various steels is now included in Appendix R of
Section VIII, Division 1.
Few welding electrode specifications existed at the
time of the early editions of the Code. New electrodes
and welding processes have been introduced since
that time and are now covered by AWS specifications.
Standardized specifications now reduce the factors
of uncertainty and improve reliability of vessels.
One of the more significant improvements has
been the use of low hydrogen electrodes which
started in 1950's. This reduced the risk of cracking
and improved notch toughness of welds. Another
recent improvement is moisture resistant electrodes.
Much more knowledge exists today about welding,
weldability and preheat requirements for various
steels than at the time of the early editions of the
Code.
5. Postweld Heat Treatment (PWHT)
The early editions of the Code (e.g. 1950 edition of
Section VIII) included all PWHT requirements in
UW-40. The minimum PWHT temperature for all
materials was llOOF (with longer times at lower
temperatures). The minimum holding time was 1
hr/in., regardless of thickness. Later editions of
Section VIII have expanded the PWHT requirements. UW-40 (Procedures for Postweld Heat Treatment) includes much of the same requirements as
the early editions of the Code, but provides a better
definition of the nominal thicknesses of welded joints
and the depth of repairs. The minimum PWHT
temperatures, holding times, and exemptions for
various classes of materials are included in UCS-56,
UHT-56, UHA-32, and UNF-56, depending on the
P-number of that material. This is an improvement
over the early editions of the Code as the present
requirements consider the chemical composition of
the various materials. The current Code rules in
UCS-56(e) and (f) also include specific requirements
for welded repairs after PWHT.
Other improvements are:
• Heat treatment verification test requirements
in UCS-85 and UHT-81 on test specimens which
are subjected to the same heat treatments as
the vessel material during the fabrication (except where exempted by the Code rules). (This
was not required in the early editions of the
Code).
• Ongoing activities in the Code committees to
develop better rules for local PWHT and allowable temperature gradients.
• Ongoing activities in the Code committees to
reexamine minimum PWHT temperatures and
holding times for certain types of materials.
The present requirements insure that no harmful
distortions or excessive stresses occur in the heat
treated vessel and that the required properties are
met in the vessel materials after all heat treatment
needed for fabrication.
6. NDE
Several important changes to the Code have increased the NDE requirements since the early editions ofth~ode:
UW-50 now requires the examination of the full
length of all welds around openings and all attachment welds having a throat thickness greater than
14" for all vessels to be pneumatically tested.
UW-51(a) and UW-51(b) require radiographic procedures to be in accordance with Article 2 of Section
V of the Code. The summer 1973 Adenda incorporated several important changes to the RT requirements:
• Fine grain film must be used.
• No :fluorescent screens are allowed.
Evaluation of Design Margins Section VIII
11
• The density limits are more restrictive.
• The penetrameters changed in thickness and in
hole size.
• A detailed written procedure must be qualified
in accordance with SNT-TC-lA.
The Winter 1975 Adenda included some changes
in Article 2 of Section V, which made the radiographic requirements somewhat less restrictive than
the summer 1973 Adenda.
UW-51(a)2 requires personnel performing and
evaluating radiographic examinations to be qualified and certified in accordance with a written practice, in accordance with SNT-TC-lA.
UW-51(c)2 adds a requirement that provisions for
training, experience, qualification and certification
of personnel shall be described in the Manufacturer's
Quality Control System U-2(h)).
UW-51(b) 1 and UW-52(c) 1 specify that welds in
which indications are characterized as cracks or
zones of incomplete fusion or penetration shall be
unacceptable (regardless of length).
The more recent editions of Section VIII, Division
1 now include mandatory appendices which include
specific requirements for certification ofNDE personnel, evaluation of indications and acceptance standards. These are:
• Appendix 6, Methods for Magnetic Particle Examination.
• Appendix 8, Methods for Liquid Penetrant Examination.
• Appendix 12, Ultrasonic Examination of Welds.
• Appendix 4, Rounded Indication Charts Acceptance. Standard for Radiographically Determined Rounded Indications in Welds.
All these changes contribute to improved quality of
welds and improved safety of vessels.
7. Brittle Fracture
Notch toughness requirements were included in
the Code some time in 1940's. The 1950 Code included the following requirements for carbon and
low alloy steels:
• No impact testing required on any material for
use at temperatures of-20°F and above.
• No impact testing required when the operating
cycle is such that the pressure at -20°F is less
than one fifth of the maximum allowable working pressure at 100°F.
• No impact testing is required on rivets, nuts, or
material less than 0.098 in. thickness.
• When impact testing is required, the impact
values shall meet 15 ft-lbs, minimum average
on Charpy type, keyhole or U-notch specimens.
Keyhole and U-notch impact test specimens
have a larger notch root radius than the V-notch
test specimens, therefore, these are less severe
impact tests than the Charpy V-notch test in the
current edition of the Code.
12
One of the most significant revisions to Section VIII,
Division 1 were in 1987, when the Code published
the new toughness rules for carbon and low alloy
steels. These rules are based on linear elastic fracture mechanics and the extensive good experience
with the existing pressure vessels. The main features in the new toughness rules in Part UCS are:
• Fig. UCS-66 impact test exemption curves. These
curves take into account the material specification (deoxidation, chemical composition, heat
treatment, etc.), the material thickness and the
minimum design metal temperature. The less
notch tough materials are included in Curves A
and B and the more notch tough materials in
curves C and D. Unless otherwise exempted by
the Code rules, impact testing is required for a
combination of minimum design metal temperature and thickness which is below the curve
assigned to the material in question (except
where specifically exempted by the Code rules).
If the minimum design metal temperature and
thickness combination is above the applicable
curve, impact testing is not required, except as
required by UCS-67(a) 2 for weld metal. Impact
testing is required for all welded thicknesses
over 4" and all unwelded thicknesses over 611 •
• Impact testing requirements in UG-84. Fig.
UG-84.1 requires increased Charpy V-notch energy values based on thickness and yield
strength. These Charpy V-notch values are based
on fracture mechanics consideration and experience.
• Impact testing is required on all carbon and low
alloy materials having a specified minimum
yield strength greater than 65 ksi, unless specifically exempted in Fig. UCS-66 (see UCS-66(f)).
Impact testing is now required in base metal and
welded joints for any thiclmess/design metal temperature below the applicable impact test exemption
curve, except for certain exemptions, and adjustments to the permissible design metal temperature
(based on successful past experience and fracture
mechanics considerations), as listed below:
• UG-20(f) exempts Pl Group 1 and 2 materials
from impact testing at temperatures down to
-20°F (and lower when due to seasonal atmospheric temperatures) providing they do not .
exceed %" thickness for Curve A materials and
l" thickness for Curve B, C and D materials and
provided the vessel is hydrostatically tested per
UG-99.
• UCS-66(b) and Fig. UCS-66.1 permit a further
basis for reduction of the minimum design metal
temperature for stationary vessels (for design
metal temperatures -55°F and warmer) based
on ratio of the design stress in tension to the
allowable tensile stress. Furthermore, impact
WRC Bulletin 435
•
•
•
•
testing is not required when the m1mmum
design metal temperature is colder than -55°F
but no colder than -155°F and the coincident
stress ratio as defined in Fig. UCS-66.1 is less
than 0.4.
Table UG-84.4 permits an increase in the impact test temperature due to slow loading on
vessels as compared to the dynamic loading on
the test specimen during the Charpy test. The
permissible temperature differentials in Table
UG-84.4 are conservative.
UCS-68(c) permits a 30°F reduction in impact
test exemption temperatures to the minimum
permissible temperature from Figure UCS-66
for ASME P-No. 1 materials if postweld heat
treating is performed when not otherwise required by the Code rules.
No impact testing is required for ASME/ANSI
Bl6.5 or ASME B16.47 ferritic steel flanges
used at design metal temperatures no colder
than -20°F.
No impact testing is required for UCS materials
less than 0.10" thick or for SA-194 and SA-540
nuts at design metal temperatures of -55°F
and above.
It is also important to note that the Code (UG-20(a))
now requires the determination of the minimum
design metal temperature to be based on the lowest
temperature to be expected in service including
operational upsets, autorefrigeration, atmospheric
temperature (except for lower seasonal atmospheric
temperatures), and any other sources of cooling.
Additional requirements have been included in
UG-99 and UG-100 to preclude brittle fracture during overload testing. UG-99(h) recommends that the
minimum metal temperature be at least 30°F above
the minimum design metal temperature during the
hydrostatic test. UG-lOO(c) requires the minimum
metal temperature during the pneumatic test to be
at least 30°F above the minimum design melal
temperature.
The 1987 revisions to the Code toughness rules
have significantly reduced the risk of brittle fracture
and improved the safety of Code pressure vessels.
E. Review of PVRC Burst Test Results
Ductile rupture or the bursting mode of failure is
the most fundamental of the various failure modes
considered in the Code design rules. The design
margins of 4 on tensile strength and 1.5 on yield
strength (at temperatures below the creep range)
have been established to guard against ductile rupture in the temperature region in which ductile
rupture, rather than one of the other failure modes,
is the governing criterion.
There was considerable interest in 1960's and
early 1970's to re-evaluate the Code limits on tensile
stresses to see if a new criteria is needed to provide a
margin of safety against bursting which is consistent
over the full range of the available materials and
takes into consideration strain concentrations which
occur at discontinuities, such as nozzles. Because of
this the PVRC Subcommittee on Effective Utilization of Yield Strength sponsored several projects to
provide a better understanding of the behavior of
pressure vessels in the bursting mode of failure and
to evaluate the accuracy of the modified Svensson
equation on smooth cylinders with moderate strain
concentrations. The project at University of Kansas
included burst tests on test vessels of three types of
materials with different strain hardening components. The materials were SA-240 Type 304 stainless
steel, SA-516 Grade 70 carbon steel and SA-517
Grade F quenched and tempered high strength low
alloy steel. These steels have strain hardening exponents of 0.585, 0.189, and 0.085, respectively. Burst
tests were performed on test vessels with nozzles
and with sharp notches.
The earlier work by B. F. Langer11 and the burst
tests at University of Kansas 12 led to the following
conclusions:
1. Design of Section VIII vessels is essentially
based on the bursting mode of failure instead of
yielding. (The yield strength controls the design only if the yield strength/tensile strength
ratio is less than 0.5 for Division 2 vessels, and
0.375 for Division 1 vessels).
2. The bursting pressure of cylindrical and spherical shells can be predicted with reasonable
accuracy if consideration is given to the strain
hardening properties of the material.
3. The modified Svensson equation gives a reasonably accurate prediction of burst pressures in
thin wall pressure vessels despite the presence
of moderate strain concentrations, such as
nozzles and end closures. Moderate strain concentrations do not significantly reduce burst
pressure.
4. For vessels with sharp notches, the test results
showed that a direct correlation between reduction in wall thiclmess (due to a sharp notch)
and reduction in burst pressure exists at least
until the notch depth of 25% of the wall thickness for the steels tested. Additional tests on a
higher strength steel HY-140 indicated that the
linear reduction theory for that steel is applicable}eyond a notch depth of 40% of the vessel
wall thickness.
5. For some vessels, particularly those from steel
with a high value of strain hardening coefficient (n), and which have been designed under
ASME rules, the true margin of safety on
bursting mode of failure can be as low as 2.4.
Decreasing the design margin from 4 to 3.5 on
the ultimate tensile strength would not have a
significant effect on the true margin of safety.
Some European pressure vessel codes use lower
margins on the tensile strength than ASME. The
Evaluation of Design Margins Section VIII
13
British BS 5500 uses a factor of 1.5 on yield strength
and 2.35 on tensile strength for carbon and low alloy
steel vessels. The German AD Merkblatt code uses
1.5 on yield strength at temperatures below the
creep range. A more detailed comparison with international codes will be performed in Phase 2.
3. Paragraph UG-23(d)
F. Review of Significant Code Paragraphs
4. Paragraph UG-24
The following is a brief review of those Code
paragraphs which use Code stress allowables or are
affected by a change in stress allowables. The purpose of this write-up is to outline the logic used to
assure that a safety concern does not arise as a result
of increasing the stress allowables. Those areas
which may be of any concern will be listed in part (H)
of this report.
1. Paragraph UG-20(f)
This paragraph exempts Pl Group 1 and Group 2
materials from impact testing down to and including
-20°F (and lower when due to seasonal atmospheric
temperatures) providing they do not exceed W' for
Curve A materials and l" for Curve B, C and D
materials and providing the vessel is hydrostatically
tested per UG-99. Decreasing the design margin
could invalidate the experience base used for justifying this exemption.
This paragraph requires that general primary
membrane shall not exceed 1.2 times the maximum
allowable stress for load combinations including
wind or earthquake. This will allow such stresses to
go as high as 1.2/3.5 of the specified ultimate strength.
This is not of any concern.
This paragraph specifies casting quality factors to
be applied to the allowable stress values for some
cast materials. These factors are believed to be based
on judgment and experience. If the stress allowables
are increased these factors may have to be revised.
However, it should be pointed out that casting
quality has improved significantly over the years.
5. Paragraph UG-27(c)(l)
The formula in this paragraph is valid only when P
does not exceed 0.385 SE. With a higher value of "S,"
the coefficient "0.385" may have to be revised. Code
Committee to look into. The same comment applies
to paragraphs UG-27(c)(2) and UG-27(d).
6. Paragraph UG-28(c)(2)
The formula in step 3 for determining allowable
external pressure of thick cylindrical shells uses a
value of "2S." The formula should be reviewed by the
Code Committee to assure that a revision is not
required.
2. Paragraph UG-23(c)
7. Paragraph UG-32(e)
This paragraph states that the maximum primary
membrane stress plus primary bending stress shall
not exceed 1112 times the allowable stress value.
Since the limit of % on specified minimum yield
strength will be maintained, this paragraph will still
assure that primary surface stresses do not exceed
yield. These stresses will be allowed to go up to
1.5/3.5 of the specified ultimate strength, but this
should not be of any concern.
The only question regarding this paragraph is the
fact that the limit is put on "stresses" and not on
"stress intensities." Also the fact that a definition of
"primary" stress is not provided in Division 1 is of
some concern. But these are concerns about the
existing paragraph regardless of stress allowables.
The other statement that is of significance in this
paragraph is the following: "It is recognized that
high localized discontinuity stresses may exist in
vessels designed and fabricated in accordance with
these rules. Insofar as practical design rules for
details have been written to limit such stresses to a
safe level consistent with experience."
This statement reinforces the fact that Code details have been developed assuming the present
stress allowables. In this report (and in more detail
in phase 2) the authors will review the Code details
to identify any areas of concern. If the Code stress
allowables are increased, the Code Committees will
also need to review these details to assure that the
above statement is still valid.
This paragraph requires that torispherical heads
made of materials having a specified minimum tensile strength exceeding 80,000 psi be designed using
a value of S equal to 20,000 psi (reduced at high
temperatures). This requirement has been introduced to guard against knuckle buckling, which the
Code does not explicitly address. When using higher
stress allowables, buckling could start becoming of
concern at a lower value of D/t. Therefore, this
requirement may have to be revised by the Code
Committee (The Code Committee has an item to look
into this requirement of the present Code).
14
8. Paragraph UG-33(f)(l)(b)
The formula in step 3 uses a value of "28." This
formula should be reviewed by the Code Committee
to assure that a revision is not required.
9. Paragraph UG-32(f)
The value of 0.665 in this paragraph is a function
of the allowable stress and may have to be revised for
a higher allowable stress.
higher proportion of ultimate strength for these
surface stresses should not be a problem. Paragraph
UG-34(c) provides rules for the minimum thickness
of fiat unstayed heads. As stated by note 21 to this
paragraph, these rules provide safe construction as
far as stress is concerned, and this is basically due to
the fact that surface stresses are not allowed to
exceed the minimum specified yield strength of the
material. However, the statement that "Greater thicknesses may be necessary if deflection would cause
leakage" will still be true and will have to be
considered by the designer.
11. Paragraph UG-36
Note 22, related to section on "OPENINGS AND
REINFORCEMENTS," states that "the rules governing openings as given in this Division are based on
the stress intensification created by the existence of
a hole in an otherwise symmetrical section. They are
based on experience with vessels designed with
safety factors of 4 and 5 applied to the ultimate
strength of the shell material." Any reduction in the
present design factor on ultimate strength will make
this statement invalid. However, it is the opinion of
the authors that the rules in Division 1 on design of
openings for internal and external pressure are
adequately safe with a design margin of 3.5 on
ultimate tensile strength.
The remaining part of note 22, on external loadings, remain true regardless of design factors, since
Division 1 does not provide rules for design due to
these loads.
12. Paragraph UG-36(c)(3)
This paragraph provides exemption from reinforcement requirements of paragraph UG-37. The rules of
this paragraph on the size of exempted openings and
spacing requirements of multiple openings are arbitrary. The rules for exemption in Section VIII, Division 2 and in Section III are based on analytical
justifications and are different. A comparison with
other codes and a justification for present Division 1
rules will be provided in the phase 2 study.
15. Paragraph UG-81
The tolerance requirements of this paragraph for
formed heads are also arbitrary and conservative.
There is no safety concern with reducing the design
margins.
16. Paragraph UG-99
This paragraph requires hydrotesting to a minimum value calculated by UG-99(b) or (c). This value
has been chosen to produce the maximum cold
prestressing and blunting of notches, without producing excessive deformation. However, the Code does
not specify an upper limit for hydrostatic test pressure; and paragraph UG-99(d) provides an option to
the Inspector to reject the vessel if it is subjected to
visible permanent distortion. The vessel deformations are a function of yield strength and not the
ultimate strength. Reducing the design factor on
ultimate strength will allow stresses which are a
higher fraction of yield strength for those materials
whose allowable stresses are controlled by ultimate
strength. However, the present limits on yield
strength still have to be met and this will probably
guard against hydrotest resulting in unacceptable
deformations. Therefore, it is believed that the factor
of 1.5 on MAWP for hydrotest can remain unchanged. It is recommended that the Code add limits
for membrane stresses during hydrotest. For carbon
and low alloy steels, a limit of 90% of the yield
strength at test temperature, on general primary
membrane stresses has been successfully used and is
recommended. This will be consistent with the Division 2 limit on general membrane stresses. However,
since bending stresses are not normally calculated
for a Div. 1 vessel, no limit on surface stress may be
imposed. Somewhat higher allowable test stresses
should be acceptable for austenitic stainless steels
and nonferrous materials, due to their lower yield
strength-to-ultimate strength ratio, better ductility,
and their strain hardening characteristics (provided
permanent distortion is not a problem).
17. Paragraph UG-101
This paragraph provides for the use of standard
flanges and pipe fittings without performing design
calculations. Experience has shown that, in general,
these standard flanges and fittings will not meet the
Code design requirements using the present allowable stresses. Allowing higher stress allowables will
probably result in Code designs being closer to
standard designs which have been shown to be safe
by service experience.
This paragraph allows establishing the MAWP of
the vessel or vessel parts using a test procedure. Of
the various test procedures, the procedure of UGlOl(m) on burst test needs a review. The present
rules basically call for a design factor of 5 on burst
pressure. If design rules are based on a reduced
factor of 3.~n ultimate strength, the factor of 5 on
burst test may be excessive. It is recommended that
the Code Committee review this factor to evaluate
the merits ofreducing the value of 5.
14. Paragraph UG-80
18. Paragraph UF-27(b)
The out-of-roundness tolerances permitted by this
paragraph are arbitrary and generally recognized as
conservative. Even though increased internal pressure allowable will result in higher bending stress
due to out-of-roundness, these stresses are selflimiting and there is no safety concern.
The rules of this paragraph allow a reduction in
design pressure if out-of-roundness exceeds 1%, but
not exceeding 3%. No reduction in design pressure is
required if calculated bending stresses are less than
114 of design stress value, and the reduction in
pressure beyond this value is proportional to the
13. Paragraph UG-44
10. Paragraph UG-34
This paragraph on design of unstayed flat heads
and covers provides values for factor "C." These
values of"C" include a 0.667 factor which effectively
increases the allowable stress to l.5S. This will
continue to limit the surface stress to specified
minimum yield strength. The fact that a lower
design margin on ultimate strength will allow a
WRC Bulletin 435
Evaluation of Design Margins Section VIII
15
ratio of bending stress to the design stress value. If
higher design stress values are allowed, proportionally higher bending stresses will be allowed. However, the bending stresses due to internal pressure
are self-limiting and will not affect the safety of the
vessel.
19. Paragraph UCS-6
No specific changes are needed since the materials
are subject to the toughness requirements ofUCS-66
(see comments on UCS-66(b) 3 ).
20. Paragraph UCS-66
These rules for assuring adequate toughness are
based on certain assumptions regarding the tensile
stresses and flaw sizes present at the minimum
design metal temperature of the material. These
toughness rules have been discussed elsewhere in
this report. A more detailed review of these requirements, in light of the proposed reduction on design
margin, will be performed in Phase 2. However, it is
not expected that any significant revisions to this
paragraph will be needed. The Code Committee has
adopted these rules, with the same exemption curves,
for Div. 2 which has even a lower design margin than
that proposed for Div. 1. (Div. 2 has more restrictive
NDE requirements). A comparison of design, fabrication, NDE, and material requirements relating to
toughness with other codes will be performed in
Phase2.
21. Paragraph UCS-66(b)(3)
This paragraph allows exemption from impact
testing, for certain cases, if the ratio of design stress
to allowable stress is less than or equal to 0.40. It is
generally believed that brittle fracture is of no
concern if the tensile membrane stresses do not
exceed 10% of the ultimate tensile strength. The
factor of 0.40 was based on the assumption that the
allowable stresses did not exceed 0.25 of tensile
strength (0.40 x 0.25 = 0.10). With a reduction in
design margin on ultimate strength, this factor
should be revised.
22. Paragraph UCS-68(a)
See above comment on Paragraph UCS-66(b) 3 •
23. Paragraphs 1-2 and 1-3 of Appendix 1
These paragraphs provide design formulas for
thick shells. These formulas are applicable when
design pressure exceeds a fraction of allowable design stress. With a higher allowable stress the value
of these fractions may have to be revised. It is
recommended that the Code Committee review.
26. Paragraph 1-8(e)
See above comment regarding Paragraph 1-5(g).
27. Appendix 2
It is generally recognized that the rules of this
Appendix are conservative. Standard flanges usually
do not meet the stress allowables of this Appendix
and yet they have performed safely. Therefore a
reduction in design margin will not be of any. concern. (The Code Committee is developing totally
different set of design rules to replace Appendix 2
rules for flange design.)
28. Appendix 27
Paragraph 27-2 allows out-of-roundness values
exceeding those of the Code, for glass lined vessels.
reduction in design pressure, similar to that o
Paragraph UF-27(b) is required. See above commen
on UF-27(b). The rule of Paragraph 27-5 allowin
SA-285 Gr.C material to be assigned to Curve
should also be reviewed. It is suggested that i
addition to the 0.18% maximum carbon restriction:,
the SA-285 Gr.C should also be fully killed to justif
its use on Curve B.
29. Code Case 2176
This Code Case allows exemption from impac
testing in certain cases, provided the ratio of desi
stress to allowable stress is less than 0.40. See abov
comment on Paragraph UCS-66(b). 3
30. Code Case 2168
24. Paragraph 104
Note 1 of this paragraph should be reviewed by the
Code Committee. (There is an item for this review on
SGD/VIII agenda).
25. Paragraph 1-5
This paragraph provides rules for the design of
conical reducer sections and conical heads. These
16
rules are generally conservative and reducing the
design factor on ultimate strength to 3.5 will not
result in any concern, except possibly for the provisions of subparagraph 1-5(g). This subparagraph, for
half apex angles greater than 30°, call for special
analysis. The limit on local hoop membrane stress is
1Yz times that of general primary membrane stress
which is consistent with Div. 2 analysis. However,
the allowable for surface stress at discontinuity is 4
times the basic allowable, whereas for Div. 2 analysis, this factor is 3. This was justifiable in light of
present design factors. If the design factor on ultimate tensile strength is reduced to 3.5, it is recommended that 4SE in 1-5(g) be reduced to SSE. This
will be consistent with other Code sections. (There is
also some concern that some of the bending stresses
at cone-cylinder junctions are primary.) Code Case
2150, which provides analytical results for such
cone-cylinder junctions, calls for a limit of 3S, implicitly accounting for the joint efficiency. This Code
Case should be reviewed to assure consistency with
the Code allowables.
This Code Case provides an alternative method t
area replacement rules, based on recommendation
of WRC Bulletin 335. These rules need to be re
viewed to assure that an increase in stress allowabl
will not invalidate the assumptions and test result
used in WRC Bulletin 335 to come up with th
recommendations therein. It is recommended tha
WRC Bulletin 435
this be investigated in detail as a part of Phase 2
study.
31. Material Code Cases
There are numerous Code Cases allowing certain
materials which are not in Section II, Part D. These
Code Cases provide stress allowable values generally based on present Code criteria. If the basis for
Code stress allowables is revised, all such Code
Cases will have to be reviewed to assure consistency.
G. Conclusions
There have been extensive rev1s1ons to Section
VIII Code rules since the present design margins
were established in 1940's. This has led to general
improvement in the safety and reliability of Section
VIII, Division 1 pressure vessels. No efforts, however, have been made so far to try to justify a further
reduction in the design margins taking credit for the
revisions and improvements in the Code rules.
It is the conclusion of the authors of this report
that a reduction in the present design margins from
4 to about 3.5 at temperatures below the creep range
would be justified based on the improvements in the
Code rules and excellent past experience with vessels built to the Code rules.
A number of recommendations are made for additional work to further assure that a reduction in
design margin will not result in any inconsistent
rules.
Section H lists certain areas suggested for Code
Committee review. These will be supplemented and
finalized in Phase 2.
H. Suggestions for Revisions to Code
Requirements
The following is a list of the Code requirements
which may have been based on the assumption that
stresses are limited to those allowed by the preseD;t
Code. If the stress allowables are increased, thes'e
requirements may have to be revised by the appropriate Code Committee to assure that no revisions to
the Code rules are needed. The authors have the
following suggestions.
1. Paragraph UG-20(f)
T~is paragraph allows exemption from impact
testmg of materials which would have been required
~o be impact tested by UCS-66. This exemption was
mtroduced based on extensive experience with the
use of these materials and thicknesses at temperatures down to - 20°F. Increasing the allowable stress
could invalidate that experience base. This paragraph should be reviewed by the Code Committee
and revised as needed.
It is the opinion of the authors that this exemption
ma?' be continued with the design margin of 3.5 on
ultimate tensile strength. This is based on the fact
that similar good experience exists for relatively thin
wall Ct< 1.0 in.) ASME B 31.1 piping components
where the design margin is 3.0 on ultimate tensile
strength. This piping code, as well as foreign code
experience should also be considered in the Phase 2
effort, but a more detailed evaluation is needed.
2. Paragraph UG-24
The casting quality factors in this paragraph
should be reviewed by the Code Committee, but the
authors believe that there are adequate margins in
these factors to justify the proposed reduction in the
design margin.
3. Paragraph UG-36
Note 22, which indicates rules for design of openings are based on experience with vessels designed
with safety factors of 4 or 5 applied to the ultimate
tensile strength, is no longer needed and should be
deleted.
4. Paragraph UG-53(i)
This paragraph calls for multiplying "the appropriate stress value in tension" by the factor 1.18 for
certain ligament efficiencies. This factor is to be
applied to "the stress allowable in tension," and the
wording should be corrected. The reason for the
introduction of this factor and its particular value is
not known to the authors.
5. Paragraph UG-99
The Code has no limit on stresses due to hydrotest.
The criteria of "visible permanent distortion" of
Paragraph UG-99(d) may not be sufficient with the
lower design margin. The authors recommend limiting the general primary membrane stress to 90% of
the ultimate tensile strength for all ferrous materials (except austenitic stainless steels).
6. Paragraph UG-99(h)
This paragraph recommends that metal temperature during hydrotest be maintained at least 30°F
above the MDMT. With a lower design ma.rgin, it
becomes more essential to require a minimum metal
temperature during hydrotest. The authors recommend a mandatory requirement of at least l0°F
above the MDMT for thicknesses up to and including
1 inch and 30°F above MDMT for thicknesses above
1 inch.
7. Paragraph UW-12
Table UW-12 assigns joint efficiencies of 0.50 and
0.55 to singil'e and double full fillet lap joints. The
design of such joints is normally controlled by fatigue
failure criteria. Even though the rules of Div. 1
require consideration of cyclic loads (UG-22), a vast
majority of Div. 1 vessels are considered not to be in
cyclic service and no fatigue analysis is performed
for such vessels.
If lower design factors are allowed, it is recommended that the Code Committee review the adequacy of joint efficiency values for fillet lap joints. It
may be desirable to slightly reduce these factors so
that the past service experience is not violated.
Evaluation of Design Margins Section VIII
17
8. General
With higher stress allowables, the Code Committee will need to re-evaluate the transition from short
time to long time properties (the temperatures at
which the design in creep range starts governing).
I. Recommendations
The following -is a list of recommendations for
follow-up to this study, as further justification for
lower design factor on ultimate tensile strength in
Section VIII, Division 1.
1. Phase 2 Study
It is recommended that this study be followed by a
more in-depth study. Part of the Phase 2 study could
cover the following areas
a. A more detailed investigation of the basis for
pertinent rules in the present Code, and what
effect a reduction in design margin may have on
vessel safety.
b. A comparison of certain parts of the Code with
Section VIII, Div. 2, the BS5500, AD Merkblatt
and possibly other international codes. Based
on these comparisons, additional recommendations may be provided on design rules, fabrication requirements, NDE requirements, and materials requirements to justify lower design
margins.
c. Amore in~depth investigation of fatigue strength
of vessels fabricated to the Code rules. The
fatigue exemption rules of Section VIII, Div. 2
will be used as a guideline to estimate the
number of pressure and temperature cycles
that could safely be accommodated, without
doing a fatigue analysis, for proposed design
factors.
d. A more in-depth study of the effects oflowering
design margins on margins to brittle fracture.
e. A study of additional vessel test results and,
based on results of these tests, drawing conclusions on actual margins to failure of fabricated
vessels. Some of the test results which will be
reviewed include the two ellipsoidal heads tested
for PVRC by CBI in mid 1980's, the two heads
tested for PVRC by Praxair in 1994, several
reduced girth vessel models tested by CBI,
several fabricated models of nuclear containment vessels tested for the NRC, and the tests
of vessels for the MPC Program on Fitness-ForService by Stress Engineering.
f. A review of Code Case 2168 and WRC Bulletin
335 to assure safety of the alternative rules for
reinforcement.
2. Code Committees
As a part of any reduction in Code design factors, it
is recommended that the following areas be reviewed
by the appropriate Code Committees:
a. The quality factor of 0.92 for structural materials. (See 1-lOO(c) of Appendix 1 of Section II,
18
Part D). With lower design margins, the Code
Committee may want to discourage the use of
these materials, by lowering the quality factor.
b. Paragraph UG-20(£). This exemption is based
on experience with present Code stress allowables. (See H.1 for authors' comments).
c. Note 22 under Paragraph UG-36. This note has
to be revised to reflect experience with safety
factors other than 4 or 5. (See H.3 for authors'
comments).
d. The factor of 1.18 in Paragraph UG-53(i). Does
this factor need to be revised if design margins
are lowered?
e. A limit on stresses due to hydrotest added to
Paragraph UG-99? (See H.5 for authors' comments).
f. Recommended hydrotest temperature in Paragraph UG-99(h). Should this temperature be made
mandatory? (See H.6 for authors' comments).
g. Joint efficiencies for fillet lap joints of Paragraph UW-12. Should these values be reduced?
(See H.7 for authors' comments).
h. Fabrication tolerances, weld detail requirements, repair requirements, and NDE requirements. The Code Committee should make a
general review of these areas to assure that
experience and judgment do not dictate any
revisions iflower design margins are adopted.
i. Review casting quality factors of Paragraph
UG-24. With lowering design margins would
these factors have to be revised? (See H.2 for
authors' comments).
j. Factors on "SE" in Paragraph UG-27.
k. The limit of 20,000 psi in Paragraph UG-32(e).
1. The factor 0.665 in Paragraph UG-32(£).
m. The safety factor of 5 in Paragraph UG-101.
n. Factors in Paragraphs 1-2 and 1-3.
o. Note 1 to Paragraph 1-4.
p. The limit 4SE in Paragraph 1-5.
q. Code Case 2150 to assure that allowable stresses
are consistent with the text of the Code.
r. The factor of0.40 in UCS-66(b), 3 UCS-68(a) and
Code Case 2176. This factor may have to be
adjusted to allow exemption at 10% of ultimate
tensile strength.
s. Appendix 27 and in particular Paragraphs 27-2
and 27-5.
t. All material Code Cases which include material
allow ables.
u. Formula in step 3 ofUG-28(c). 2
v. Formula in step 3 ofUG-33(f)(l)(b).
w. The proposed decrease in the design margin
will not affect the allowable stress values in the
creep range. The Code Committee should, how-_
ever, make a review of the transition from time
independent to time dependent properties.
4. History of ASME Boiler Code, by Arthur M. Greene, Jr., published in
1952-1953 issues of Mechanical Engineering, ASME G00028.
5. ASME Boiler Construction Code Section VIII, Rules for Construction
of Unfired Pressure Vessels, 1950 Ed.
6. Design Criteria of Boilers and Pressure Vessels, papers presented at
the First International Conference on Pressure Vessel Technology, Sept.
99-0ct. 2, 1969, The Netherlands.
~ 7. PVP-Vol. 97-1984, International Design Criteria of Boilers and Pressure Vessels, papers presented at the Fifth International Conference on
Pressure Vessel Technology, Sept. 9-14, 1984, San Francisco, CA.
8 PVP-Vol. 152-1988, Design Criteria of Boilers and Pressure Vessels,
papers presented at International Conference on Pressure Vessel Technol-
0~: WRC Bulletin 101, November 1964, PVRC Interpretive Report of
Pressure Vessel Research, Section 2-Materials Considerations, by J. H.
Gross.
10. WRC Bulletin 158, January 1971, PVRC Interpretive Report of
Pressure Vessel Research, Section 3-Fabrication and Environmental
Considerations, by A. P. Bunk.
11. Design Stress Basis for Pressure Vessels, by B. F. Langer, Experimental Mechanics, January, 1971.
12. Paper No. 74-Mat-1, ASME Journal of Engineering Materials and
Technology, Effect of Strain-Hardening Exponent and Strain Concentrations on the Bursting Behavior of Pressure Vessels, by C. P. Royer and S. T.
Rolfe.
43~:ds,uD~~f~~~~!~f~'f~r i3"';i1~;s 7:~j'Pi'.~fs"~r~~~s;~~·in1ih~ J~~~.. b~
M.D. Bernstein.
14. Summary of Design Analysis Factors Inherent in the Established
Failure Modes of the ASME Boiler and Pressure Vessel Code, ASME
Subgroup Design Analysis (SCD) report, February, 1989.
15. Paper No. 88-PVP-8, New Toughness Rules in Section VIII, Division
1 oftheASME Boiler and Pressure Vessel Code, by A. Selz, paper presented
at the Pressure Vessels and Piping Conference, June 19-23, 1988, Pittsburgh, PA.
16. Brittle Fracture Margins in ASME Code, Section VIII, Division 1, by
S. Yukawa, Jan. 7, 1994 (Prepared for MPC Program on Fitness-for
Service).
17. Structural Analysis and Design of Process Equipment, Second Ed.,
M. H. Jawad and J. R. Farr, 1989, John Wiley & Sons, Inc.
18. The International Journal of Pressure Vessels and Piping, Vol. 1, No.
2, 1973, Failures of Boilers and Pressure Vessels: Their Causes and
Prevention, by J. F. Lancaster.
19. Journal of Pressure Vessel Technology, Vol. 110, August 1988, pp
226-233, Statistics of Pressure Vessel and Piping Failures, by S. H. Bush.
20. BWRA Bulletin (Special Report), Vol. 7, No. 6, June 1966, Brittle
Fracture of a Thick Walled Pressure Vessel.
21. Journal of Engineering Materials and Technology, Vol. 102, October
1980, pp 384-387, Brittle Fracture of a Heat Exchanger Shell: A Case
History, by R. F. Wagner and D. R. Mcintyre.
22. Welding and Metal Fabrication, January 1973, pp 4-12, Pressure
Vessel Failure During Hydrotest, by B. Banks.
23. Materials Pe1formance, Vol. 20 (No. 9), 1981, pp 2.
Failures due to poor notch toughness, fabrication defects, welded or
welded repairs. 21-26, The Application of Fracture Mechanics to Refinery
Failure Analysis, byW. R. Warke.
24. Brittle Fracture of a Pressure Vessel-Study Results and Recommendations, by R. D. Merrick and A. R. Ciuffreda, presented at the API 48th
Midyear Refining Meeting, May 10, 1983, Los Angeles, CA.
25. Failures During Hydrotest, September 20, 1983, CBI in-house con-espondence.
26. NBSIR 86-3049, Examination of a Pressure Vessel That Ruptured at
the Chicago Refinery of the Union Oil Company on July 23, 1984, by H. I.
McHenry, T. R. Shives, D. T. Read, J. D. McColskey, C. H. Brady and P. T.
Purtscher, National Bureau of Standards, March, 1986.
27. Pressure Vessel and Piping Design-Collected Papers, 1927-1959,
ASME Publication, 1960.
28. Pressure Vessel and Piping Technology-A Decade of Progress, 1982.
29. Coke Drum Failure During Hydrostatic Testing, presented at API
Operating Practices Committee, May 14, 1973, Philadelphia, PA.
30. G. G. Karcher,ASME TG-Toughness, Subcommittee VIII, February
13, 1986 (Committee Correspondence).
31. Sulfur-Some Effects on Steel Processing and Steel Properties, by G.
J. Roe, P. R. Slimmon and G. F. Melloy, Proceedings, 55th National Open
Hearth and Basic Oxygen Steel Conference, April 10-12, 1972, Chicago, IL.
32. High-Strength, Low-Alloy Steel-A Decade of Progress, By F. B.
Pickering, Micro Alloying 75.
33. Inclusions and Mechanical Properties, by W. C. Leslie, ISS Transactions, Vol. 2, 1983-1.
34. Experimental Effort on Bursting of Constrained Discs as Related to
the Effective Utilization ofYield Strength, by W. E. Cooper, E. H. Kottcamp,
and G.A. Spiering,ASME 71-PVP-49.
35. Properties and Characteristics of a Quenched and Tempered Steel for
Pressure Vessels, by W. D'Orville Doty, Welding Journal Research Supplement, September 1955, pp. 1-17.
36. Design of Welded Pressure Vessels Using Quenched and Tempered
Steel, by Leonard P. Zick, Welding Journal Research Supplement, September 1955, pp. 18-24.
37. Suitability of Quenched and Tempered Steels for Pressure Vessel
Construction, by Leon C. Bibber, Welding Journal Research Supplement,
September 1955, pp. 25-40.
38. WRC Bulletin 407, December, 1995.
39. NBSAR 86-3049, Examination of a Pressure Vessel that Ruptured at
the Chicago Refinery of the Union Oil Company on July 23, 1984, by H.I.
McHenry, T.R. Shieves, D.T. Read, J.D. McColskey, C.H. Brady, and P.T.
Purtscher.
Bibliography
1. ASME Code, Section VIII, Division 1, 1992 Ed.
2. ASME Code, Section II, Part D, 1992 Ed.
3. ASME Code Cases: Boilers and Pressure Vessels, 1992 Ed_
WRC Bulletin 435
Evaluation of Design Margins Section VIII
19
Report No . 2: Evaluation of Design Margins for ASME
Code Section VIII, Divisions 1 and 2-Phase 2 Studies
E. Upitis and K. Mokhtarian
A. Executive Summary
The Phase 1 Report on Evaluation of Design
Margins for ASME Code Section VIII, Division 1,
May 1996, provided justification for decreasing the
design margin on ultimate tensile strength from 4 to
3.5. n That report endorsed the reduction in the
design margin based on the improvements in the
Code rules over the last 40-50 years and recommended further review of certain other factors to
assure consistency of the existing rules with the
reduction in the design margin from 4 to 3.5 for
Division 1.
The Phase 2 Report includes the Phase 1 follow-up
studies, evaluation of the design margins in ASME
Section VIII, Division 2, and a comparison of the
Section VIII, Division 1 and Division 2 Code requirements with those in several European pressure
vessel codes and standards which use lower design
margins than ASME Section VIII. These include the
British Standard 5500, 5 the German Pressure Vessel
Code (AD-Merkblatt Code), 6 the Netherlands Rules
for Pressure Vessels (Stoomwezen Code)? and the
CEN/TC54 Draft Standard for Unfired Pressure
Vessels. 8
The design rules in ASME Section VIII, Division 1
do not explicitly address the fatigue mode of failure.
Many pressure vessels, however, are exposed to
some pressure :fluctuations. The Draft CEN Pressure
Vessel Standard requires all pressure vessels to
resist 500 full pressure cycles. Section C.2 of this
report attempts to evaluate the effect of reduced
design margins on fatigue strength of Division 1
vessels. The use of SA-516 Gr. 70 material, Division
1 allowable stresses and a stress concentration factor of 4 for reinforcing pad fillet welds, results in
approximately 600 allowable pressure cycles using
the Division 2 fatigue design curves. This reduces to
about 500 for design margin of 3.5 and about 190 for
20
design margin of 3. The use of a design factor less
than 3.5 necessitates the use of improved details or a
restriction on the allowable fatigue cycles.
The basic philosophy behind the area replacement
rules in Division 1 is to assure that the burst
pressure of the vessel is at least equal to a vessel
without any openings. The recent Code Case 2168
allows less than full area replacement, but is limited
to materials with allowable stresses in tension not
exceeding 17 .5 ksi and has a provision that cyclic
loading is not a controlling design requirement. A
PVRC project will include a detailed evaluation of
Code Case 2168 and will review the differences
between ASME Section VIII and the European pressure vessel codes.
The report by Dr. S. Yukawa on Brittle Fracture
Margins in the ASME Code Section VIII, Division 1
evaluated margins on brittle fracture for 38 and 50
ksi yield strength steels based on the ratio of the
available dynamic fracture toughness Km to the
applied K1, using the Division 1 allowable design
stresses and the maximum allowable flaw sizes in
Division 1 for radiographic examination. 27 Section
C.4 of this report provides additional information on
brittle fracture margins for the high strength, Q & T.
SA-517 steels in Part UHT of Division 1 (which are
subject to lateral expansion requirements in the
Charpy test), using a similar approach to that by Dr.
Yukawa, with the additional consideration of residual stresses in welded joints. A total stress (design
membrane stress plus welding residual stress) equal
to the yield strength of the weld was used for
as-welded joints. A residual stress of30% of the yield
strength of the weld was added to the design stresses
for the postweld heat treated (PWHT) joints. With a
design factor of 4 on tensile strength (Suf4), th
calculated margins on brittle fracture were about 1.
for the %" material and 1.2 for the 1W' material i
WRC Bulletin 435
the as-welded condition. In the PWHT condition,
with a residual stress equal to 30% of the yield
strength of the weld, the calculated margins on
brittle fracture were 2.1 for the 1114" weld and 2.0 for
the 2Y:i" weld. Reducing the design factor to 3.5
reduced the margin on brittle fracture to 1.9 for both
the 1114" weld and the 2Y2" weld in the PWHT
condition. The calculated margins on brittle fracture
were considerably higher without considering residual stresses in welded joints.
Section D of this report summarizes burst test
results on several test vessels and heads. The test
vessels described in Section D were all designed to
Section VIII, Division 1 allowable stresses. Some
were scale models and others were full size service
exposed cylindrical vessels with local thin areas
(LTA's). The cylindrical vessels failed at 3.0 to 4.5
times the allowable design stresses, with average
being about 3.7. The safety margins for the formed
heads were higher than those for the cylindrical
vessels.
The failure modes considered in ASME Section
VIII, Division 2 are ductile rupture, brittle fracture,
fatigue, incremental collapse, elastic instability and
plastic instability. 25 Section E of this report includes
an assessment of higher design stresses on ductile
rupture and brittle fracture. It also includes. comments on the significance of yield-to-tensile strength
ratios (for high strength quenched and tempered
pressure vessel steels) and the Code requirements
for fabrication and NDE.
Ductile rupture is the most fundamental mode of
failure considered in the Code rules. PVRC sponsored a project at University of Kansas in 1970's to
develop a better understanding of the bursting mode
of failure of pressure vessels. 19•20 The project included burst tests on test vessels fabricated from
three materials with different strain hardening exponents (Type 304 stainless, SA-516 Gr. 70 and SA-517
Gr. F). One set of test vessels was tested wilth
moderate strain concentrations (nozzles, fiat heads)
and another set with sharp notches at various depths
in the axial direction of the vessels. These tests
confirmed that the modified Svensson equation
(which includes the effect of strain hardening of the
material) can be used to predict the burst pressure in
vessels with moderate strain concentrations and
that there is a direct correlation between reduction
in the wall thickness and reduction in burst pressure
for notch depth up to at least 25% of the wall
thickness.
Using the modified Svensson formula and the
specified minimum tensile properties, the theoretical margins of safety for the University of Kansas
test vessels (without sharp notches) designed to
Section VIII, Division 2 rules, ranged from about 2.8
for the 304 stainless steel vessel (with high strain
hardening exponent) to about 3.2 for the high
strength, quenched and tempered steel SA-517 (with
low strain hardening exponent). The actual margins
were higher because the actual tensile properties
exceeded the specified minimum properties. Increasing the design stress from Su/3 to Su/2.4, reduced the
margin from about 3.2 to 2.6 for the SA-517 steel and
from about 3.0 to 2.75 for the SA-516 Gr. 70 steel.
Therefore, it does not appear that a reduction in the
present ASME Section VIII, Division 2 design margin against the bursting mode of failure to a lower
design margin (similar to those used in some European pressure vessel codes) would compromise the
safety of pressure vessels, providing the other considerations for increased design stresses discussed in
this report are met.
Fracture mechanics calculations were performed
to evaluate the effect of higher design stresses (Su/
3.5 vs. Su/4 and the lesser of Su/2.4 and % and % Sy
vs. Su/3) on critical flaw sizes and to determine the
toughness needed to insure the same critical flaw
sizes with the higher design stresses as with the
present Code allowable design stresses (see Section
E.2). The required fracture toughness (Krc) was
evaluated for base metal and for welded joints. The
required fracture toughness for the increased design
stresses in base metal (with primary membrane
stresses only) was evaluated with two methods, the
BS PD 6493:1991 Level 2 failure assessment diagram (FAD) procedure and the ASME Section XI,
Article A-3000 method.
Increase in design stress from Sul 4 to Sul 3.5: The
increase in the required toughness was 16-23% with
the BS PD 6493 Level 2 FAD procedure and 14.515.5% with the ASME Section XI procedure for the
materials and thicknesses considered in this evaluation. It is recognized, however, that there is a significant amount of successful experience with ASME
Section VIII, Division 2 vessels designed to higher
stresses (Su/3) and essentially the same toughness
requirements as Division 1 vessels. Therefore, no
increase in the Code toughness requirements is
proposed in this report for increasing in design stress
from Su/4 to Su/3.5. However, a further review of the
toughness requirements for some of the materials in
Table UHT-23 is suggested, regardless of the design
stress (e.g. SA-517).
Increase in design stress from Sul 3 to the lesser of
Suf 2.4 and % Sy: This resulted in the following
increases in the required toughness:
SA-516 G~O: 11-13.5% with the PD 6493 Level 2
FAD procedure (9-10% with theASME Section XI
method).
SA-537 and SA-517: 28-35% with the PD 6493 Level
2 FAD procedure (26.5-27.5% with the ASME
Section XI method).
As-welded joints in this case were assumed the have
a residual stress equal to the yield strength of the
weld in the direction of the weld (assumed to be the
same as the yield strength of the base metal for these
calculations). Stress relieved welds were assumed to
have a total stress equal to the design stress plus
Evaluation of Design Margins, Section VIII
21
30% of the yield strength of the weld. The increase in
toughness needed for stress relieved welded joints
was somewhat smaller than for base metal, since the
percent increase in the total stress was lower. The
initial fl.aw sizes in as-welded and in stress relieved
welded joints, however, are also smaller.
The calculations in Section E.2 resulted in small
critical fl.aw sizes for the high strength Q & T SA-517
steels, particularly in the as-welded condition with
high residual stresses. This indicates that a further
review of the Section VIII toughness and/or NDE
requirements may be desirable for some of the high
strength steels_ in Table UHT-23 of Division 1 and
Table AQT-1 of Division 2, regardless of the allowable design stresses.
A significant part of this report consists of review
of the British Standard 5500, 5 German (AD-Merkblatt) Pressure Vessel Code, 6 Netherlands Rules for
Pressure Vessels (Stoomwezen Code) 7 and the CEN/
TC54 Draft Standard for Unfired Pressure Vessels8
and comparison of the pertinent requirements in
these codes and standards with those in ASME
Section VIII, Divisions 1 and 2. A summary of each of
these codes and standards is included in Appendices
1through4 of the PVRC report on design margins. 40
(Appendices 1-4 are not included in this WRC
Bulletin). Section F of this report includes a comparison with ASME Section VIII, Division 1 and 2
requirements.
All these European pressure vessel codes and
standards have more liberal design margins than
ASME Section VIII. They use a factor of 1.5 on the
specified minimum yield strength at design temperature and the following factors on the specified minimum tensile strength at room temperature:
• 2.35 in BS 5500 for Construction Categories 1
and2,
• 2.27 in Stoomwezen Code (for materials with
elongation> 10%),
• 2.4 in Draft CEN/TC54 Standard
The AD-Merkblatt Code does not give a factor on
ultimate tensile strength for materials below the
creep range, except for materials without a guaranteed yield strength or elastic limit (such as gray cast
iron and copper and its alloys).
The basic design formulas for shells are essentially the same in the different codes and standards.
However, the formulas for design of components and
for areas of discontinuity vary a great deal. In
general, the ASME Section VIII Codes (especially
Division 1) are more conservative than those of the
European Codes studied.
In most European codes the rules for design of
cone-cylinder junctions are based on limit analysis
or other analytical methods. The rules for nozzle
reinforcement vary a great deal also. The European
codes and standards use more analytical methods,
such as limit analysis.
The fatigue curves in Section VIII, Division 2 are
22
based on smooth specimen tests, and the fatigue
analysis rules need updating. The European rules
are based on fatigue tests on actual weldments and
are comprehensive.
The Draft CEN Standard and the Stoomwezen
Code classify vessels in risk or hazard categories,
depending on the toxicity of the fluids in the vessel,
design pressure and volume. Vessels in the high risk
categories require 100% NDE of all welds.
Most of the European pressure vessel codes and
standards include special requirements for materials. Some examples are:
• In Holland, only materials specifications reviewed and approved by the Competent Body
can be used for pressure vessels. In Germany,
the suitability of filler metals and consumables
for certain groups of pressure vessels must be
established by an expert's report.
• CEN requires European technical approval of
material manufacturers and a review and approval of the material manufacturer's quality
system by the Authorized Inspection Agency.
• Generally, more testing is required by the European codes. The AD-Merkblatt Code requires
testing both ends of plates if they exceed a
certain length. The AD-Merkblatt and Stoomwezen Codes require testing of plates after
forming.
• The Stoomwezen Code requires all non-alloyed
and low alloy steel materials to be supplied in
the normalized, normalized and tempered, or in
the quenched and tempered condition.
• The AD-Merkblatt and Stoomwezen Codes require all materials to be impact tested
• Several European pressure vessel codes require
the type and extent of repair welding to be
indicated in the test certificate.
• CEN requires review and approval of the material manufacturer's quality system by the Authorized Inspection Agency.
The toughness requirements in the European pres-.
sure vessel codes and standards differ from those in
ASME Section VIII. The toughness requirements in
BS 5500 are based on Charpy V-notch correlations
with wide plate test results. These codes provide
separate impact test exemption curves for as-welded
and for PWHT construction. The impact test tempera-.
ture depends on the calculated reference or assessment temperature, thickness and whether the welded
assembly is stress relieved. Formulas for the refer~
ence or assessment temperature include adjustments for calculated membrane stress, construction
categories (in case of BS 5500) and PWHT of vessel
assemblies. Substantial benefits are given to PWHT.
BS 5500 includes toughness requirements only for
operating temperatures below 0°C. The required
minimum average impact values are 27 J (20 ft-lbs)
for steels with specified minimum UTS < 450 N/mm2
and 40 J for steels with specified minimum UTS?
WRC Bulletin 435
450 N/mm 2 • BS 5500 does not require impact testing
of heat affected zones (HAZ) with multi-pass welding
with welding heat inputs between 1 kJ/mm (25
kJ/in.) and 5 kJ/mm (127 kJ/in.).
Stoomwezen requires two types of impact testing:
e Quality test at 20°C for all non-alloyed and low
alloy steels.
• Extra testing at the impact test temperature as
determined from figures which relate assessment temperatures, thicknesses and impact test
temperatures.
The Draft CEN Standard lists three alternative
methods for impact test requirements:
Method 1 is applicable to steels with specified minimum UTS ::::; 460 N/mm 2 and maximum thickness
t::::; 30 mm (l.18") for as-welded construction and
t ::::; 60 mm (2.36") for PWHT construction. The
impact test temperature is based on the minimum
design metal temperature and an adjustment based
on actual tensile stresses (% of maximum allowable).
Method 2 is applicable to all thicknesses and requires base metal, weld metal and HAZ to meet
the required impact test values at the impact test
temperature obtained from the applicable figures/
exemption curves, based on the reference temperature and thickness. Separate figures/exemption
curves are provided for as-welded assemblies and
for PWHT assemblies. As-welded assemblies generally must be impact tested at lower temperatures than PWHT'd assemblies.
Method 3 is a fracture mechanics analysis and may
be used for materials not included in this CEN
standard and cases not covered by Methods 1 and 2.
Some of the European codes and standards have
more severe toughness requirements than ASME
Section VIII Codes, particularly for thicker plates in
the as-welded condition. ASME Section VIII Cod~,
however, have special toughness requirements for
high strength steels (minimum lateral expansion
and drop weight tests for some of the high strength
steels in Part UHT of Division 1 and Article F-6 of
Division 2). BS 5500 and the Stoomwezen code
generally do not require impact testing of heat
affected zones of procedure qualification and production test plates, whereas this is generally required
for vessels built to ASME Section VIII (if the base
metal must be impact tested).
The NDE requirements in the European codes
depend on material, thickness, type of service, design stress and other considerations. For Construction Category 1 vessels, BS 5500 requires 100% RT
or UT of all shell butt welds, regardless of thiclmess
100% RT or UT of nozzle attachment welds when th~
thinnest part welded exceeds the specified thickness
limit for a particular materials group, and 100% MT
or PT for all attachment welds. A lesser amount of
NDE is required for Construction Category 2 vessels,
and only visual examination is required for Construction Category 3 vessels. The NDE requirements in
BS 5500 appear to be comparable to those in Section
VIII, Division 2.
The type and extent of the NDE in the ADMerkblatt Code depends on the type of material
(material group), thickness and design stress. Generally 100% RT or UT is required for all longitudinal
shell butt welds and weld intersections and 100% or
a lesser amount (e.g. 25% or 10%) for circumferential
butt welds. More examination is required for higher
strength materials and for alloyed steels (e.g. 100%
RT or UT of all shell butt welds and weld intersections). The amount of RT or UT examination, however, can be reduced if the results of previous NDE
on similar vessels reveal no serious deficiencies.
(This is not an option for ASME Section VIII vessels).
AD-Merkblatt requires MT or PT examination of all
nozzle and other attachment welds.
The extent of NDE requirements in the Stoomwezen Code depends on the hazard category, material group and thickness of the pressure part. Generally, more examination is required for higher hazard
categories and thicker material. Full (100%) examination is required for all Hazard Category 3 vessels
and for vessels of Category C and Category CP
materials, which require special care in processing
or are crack sensitive materials.
The NDE requirements in the Draft CEN Standard depend on the Risk Category, Testing Category
of welded joints, group of steel, type of joint and
maximum thickness. Longitudinal and circumferential butt welds require 100% RT or UT for some
Testing Categories and a lesser amount for others.
Butt welds which are RT or UT examined also
require a certain amount of MT or PT examination. A
unique feature in the CEN standard is that in many
cases the required amount of RT (or UT) and MT (or
PT) can be reduced based on satisfactory prior
experience (similar to AD-Merkblatt).
In general, ASME Section VIII, Division 2 appears
to have the most comprehensive and well defined
NDE requirements. The NDE requirements in the
European pressure vessel codes and standards generally are more extensive than those in ASME Section
VIII, Division 1.
The out-of-roundness tolerances for shells in the
European pressure vessel codes and standards appear to be '!timilar to those for ASME Section VIII
vessels. However, some of the European codes and
standards have additional tolerance requirements
for peaking at welded seams, local fl.at areas, vessel
straightness and workmanship requirements which
are not included in Section VIII, Divisions 1 and 2.
The European pressure vessel codes and standards generally have lower thiclmess limits for
non-PWHT construction (for C and C-Mn steels)
than ASME Section VIII. Also, the PWHT temperatures and hold times generally are lower (except for
BS 5500, which has similar requirements to those in
Evaluation of Design Margins, Section VIII
23
ASME Section VIII). BS 5500 and the Draft CEN
Standard require PWHT of C and C-Mn steels over
35 mm (l.38"), AD-Merkblatt Code over 30 mm
(l.18") and Stoomwezen Code over 32 mm (l.26").
Most of these codes and standards also permit
increasing the maximum thickness limit without
PWHT with additional requirements (additional
notch toughness· or fracture mechanics analysis,
additional NDE, etc.).
The European pressure vessel codes and standards (particularlytheAD-Merkblatt and the Stoomwezen Codes and the Draft CEN Standard) generally require more third party involvement than
ASME Section VIII, Divisions 1 and 2. Depending on
the particular Code or Standard, this may include
verification of design reports and materials, approval of welders, welding operators and welding
procedure qualifications, witnessing or supervision
of NDE, review of NDE reports and heat treat
records, conducting or witnessing of pressure tests,
certification of vessels, review and certification of
quality management systems.
The Draft CEN Standard requires the Notified
Body (Inspection Authority) to verify that all parts of
the vessel have been designed in accordance with the
CEN Pressure Vessel Standard and that the vessel
has been constructed and tested in accordance with
this standard. The extent and level of the participation by the Notified Body depends on the vessel risk
category, testing category and the module of conformity assessment. There is a reduced involvement by
the Notified Body for a conformity assessment module involving a certified quality system.
Section G of this report includes recommendations
to the ASME Code Committees for revisions to
Section VIII, Division 1 to justify a further reduction
in the design margin below 3.5 on the ultimate
tensile strength. The authors feel that this margin
can be reduced down to approximately 3.0, depending on the evaluation and improvements adopted by
the appropriate ASME Code Committees. No reduction has been proposed in this report for the design
margin based on yield strength nor for the present
design margins based on creep and rupture failures.
Section G also includes recommendations for revisions to Section VIII, Division 2 to justify reduction
of its design margin below 3 on the ultimate tensile
strength. The authors feel that a design margin in
the range of 2.3 to 2.4 may be justifiable for the new
design margins providing the recommendations in
this section of the report are adopted by the Code
Committee. The actual design margins for both
Division 1 and Division 2, however, are to be selected
by the appropriate Code Committees based on their
judgment and collective experience.
The conclusion of this report is that the design
margin on ultimate strength can be smaller than the
current values for both Division 1 and Division 2 of
Section VIII of the ASME Code. However, several
areas in these codes need to be reviewed and im24
proved when the design margins are reduced below
3.5 on the specified minimum UTS for Division 1 and
below 3 for Division 2 (see Section G). To provide
further information for the Code Committees to
make such decisions, additional studies by PVRC or
other organizations are recommended in Section H
of this report.
Based on the Phase 2 study (including a comparison with the European pressure vessel codes and
standards), specific recommendations are provided
for adopting lower design margins for both Division 1
and Division 2. Recommendations are also provided
for additional studies to further improve the Section
VIII Codes and justify the proposed reduction in
design margins.
B. Introduction
The present ASME Code, Section VIII, Division 11
design margins have existed since 1942/1943 when
the design factors on the ultimate tensile strength
were reduced from 5 to 4. The successful experience
with Section VIII, Division 1, improved materials,
more sophisticated design rules and improved vessel
fabrication practices prompted a review of the rules
in this Code. The Phase 1 report of this project
concluded that the present design margin in Section
VIII, Division 1 can be reduced to 3.5 without
affecting the safety of vessels. 11 The Phase 1 report
also recommended a review of certain paragraphs in
the Code to assure consistency with the proposed
design margin of 3.5. (At the time of this writing, the
reduction of the design factor to 3.5 and the proposed
reviews are underway).
The main purpose of the Phase 2 report is to
review the basis of the present design margins and to
provide justification for future reduction of the design margins in ASME Section VIII, Division 2. This
report also includes discussion of several additional
issues related to the design margins for Division 1
vessels and recommendations for a further reduction
of the design margins in Division 1.
Section VIII, Division 2 (Alternative Rules) was
issued in the 1960's and permits lower design margins than Division 1 (a factor of 3 on tensile strength
and 1.5 on yield strength) and includes more comprehensive requirements than Division 1. 2 This report
reviews the failure modes considered in Division 2
and discusses the significant factors which affect the
design margins. This includes ductile rupture, brittle
fracture, yield-to-tensile strength ratios, fabrication
and NDE requirements.
Several European pressure vessel codes use lower
design margins than ASME Section VIII. This includes British Standard 5500, 5 the German AD-.
Merkblatt Code, 6 the Dutch Stoomwezen Code7 and,
the CEN Draft Standard for Unfired Pressure Ves-.
sels.s These codes and standards (except CEN) have
been in existence for some time and generally have a
good performance record. This report reviews the
pertinent requirements in the European pressure
vessel codes and standards and compares them to
the requirements in the ASME Boiler & Pressure.
Vessel Code, including the design margins, design
rules, design details, materials requirements, toughc
ness requirements, fabrication requirements, overload testing, and quality control requirements and
other requirements important for the safety of the
vessels.
WRC Bulletin 435
C. Design Margins for Section VIII, Division 1
(Phase 1 Follow-up)
1. Design Criteria of Section VIII, Division 1
The rules of ASME Section VIII-1, using design by
formula approach, are based partly on analytical
derivations, partly on test results, and to a great
extent on experience. The rules have evolved over
many decades and improved along the way with
many new requirements. One of the most significant
improvements has been the introduction of impact
testing requirements in the 1987 Addenda. One of
the weak points of this code is the fact that there still
are no fatigue analysis or fatigue exemption rules
introduced. The design rules and details have been
developed to limit the magnitude oflocal discontinuity stresses and implicitly protect against fatigue
failure for a nominal number of cycles. But that
number has never been quantified.
The primary mode of failure considered is ductile
rupture. Other modes of failure have been addressed
(see Phase 1 report) but not on a consistent and
totally analytical basis. The approach to local
stresses, which may result in ratcheting and fatigue,
is summarized in paragraph UG-23 (c) which in part
states that "It is recognized that high localized
discontinuity stresses may exist in vessels designed
and fabricated in accordance to these rules. Insofar
as practical, design rules for details have been
written to limit such stresses to a safe level consistent ;;ith experience." Thi~ paragraph al~o requir<:f.S
that ... loads shall not mduce a combmed maXimum in primary membrane stress plus primary
bending stress across the thickness which exceed 1Yz
times the maximum allowable stress value in tension." However, there is no detail specified for the
application of this rule. Some of the rules (such as
design of flat cover) incorporate this rule, but usually
the local stresses and bending stresses are not
addressed. Some of the more recent rules consider
local stresses and assure shakedown.
Many of this code's design rules are in the process
of being revised or alternatives are being provided
for the older rules. Some others are in need of
updating.
The flange design rules are based on the old Taylor
Forge approach, do not consider intersection of bolts
and flanges, do not address leakage, and use out-ofdate gasket factors. A new set of rules is being
developed which will have a more analytical ap~roach, will have leak tightness factor and will
mcorporate recent gasket test data. Any reduction in
design margins will have to be reflected in these
proposed rules.
The rules for design of formed heads also are based
on some old test results, supplemented by experience
base. These rules provide a non-uniform margin of
safety over the range of parameters and are conservative in may cases. A more analytically based approach is being considered by the ASME Code Committee. This new approach is based on a non-linear
elastic plastic analysis that accounts for stiffening
effects of internal pressure on the shape of the
knuckle. This proposed method assures shake clown
to elastic action and implicitly allows for approximately 500 cycles of pressure application. The proposed rules at this time assume stress limits of the
present code.
The rules for design of cone cylinder junctions
have evolved over the years and many recent improvements have been made to them to assure that
collapse and ratcheting are precluded. A Code Case,
based on shell analysis, has been published to provide design factors for a certain range of parameters.4 The proposed buckling rules, based on the
WRC Bulletin 406, include rules for design of cone
cylinder junctions under external pressure. 16 These
rules, to preclude buckling and plastic collapse of the
junction, are based on analytical results, modified by
knockdown factors based on test results. As a result
of this evolutionary process, the rules are scattered
in different areas, and do not provide a consistent
and analytically defensible set of rules. These rules
could be improved by a totally new look in the light of
recent analytical capabilities and available test data.
The Code rules for external pressure and buckling
design also do not cover all geometries and loadings
of interest. The basis of external pressure design is
summarized in ASME Section II, Part D, Appendix 3.
These rules are based on theory of stability, modified
by test results and experience. The "knockdown
factors" (buckling strength reduction factors) applicable to theoretical solutions, to account for fabrication imperfections, have a very significant effect on
resulting rules. Since the rules were put into the
Code several decades ago, a great deal of test data
has become available. The present rules, mostly
conservative, have a factor of safety which varies
significantly for various geometries and loadings. A
proposed Code Case is being considered by the
ASME Cocft!'Committee which will provide an alternative to basic Code rules and is based on recent
data. 16 It is the intent that after a successful trial
period these alternative rules replace the present
rules. (Code Case 2286, approved July 17, 1998).
In general, while Division 1 can remain a "design
by formula" code, the design rules can be improved
by taking advantage of available analytical methods.
Design rules can still be in the form of simple
formulas or graphs, but would have an analytical
underpinning. The analytical results, where needed,
may be adjusted by factors based on test data and
Evaluation of Design Margins, Section VIII
25
experience. This is the approach that has been taken
by most European codes and standards. With more
confidence in the accuracy and uniformity of the
rules, the design margins may safely be reduced.
2. Effect of Design Margins on Fatigue Strength
Section VIII, Division 1 requires that cyclic loadings be considered; but does not provide any rules for
analysis. There are no guidelines on what constitutes cyclic service. Every pressure vessel is subject
to some cyclic loading, but usually a fatigue analysis
is performed only when the number of cycles of
pressure or temperature is more than a nominal
number. There is no uniformity in the Code rules
when fatigue analysis is required, and this is one of
the weaknesses of Section VIII-1. When a fatigue
analysis is required, normally the rules of Section
VIII-2 are used.
To determine if a vessel may be exempt from
detailed fatigue analysis, sometimes the fatigue
exemption rules of paragraph AD-160 of VIII-2 are
employed. Par. AD-160.1 provides for exemptions
based on operating experience. This is probably the
more common basis used for not requiring fatigue
analysis of Division 1 vessels. However this is not a
very justifiable approach, since the weld details,
local discontinuities, material properties, operating
conditions and many other factors may be different
from one vesselto another.
The method of AD-160.2 is a simplified and conservative method of assuring that a vessel designed to
the rules ofVIII-2 can withstand a certain number of
cycles. The rules of this paragraph are applicable to
integral parts of Division 2 vessels. This means that
this method is not applicable to such details as pad
reinforced nozzles and should not be used directly for
a Division 1 vessel. These exemption rules assume a
stress concentration factor of 2.0 and allowable
stresses of Division 2. Many details in Division 1
vessels produce stress concentration (and fatigue
strength reduction) factors exceeding 2.0. Paragraph
AD-160.2 provides two different conditions for exemption from fatigue. Condition A is satisfied if the total
number of cycles of various cyclic loads do not exceed
1000. These cycles consist of full-range pressure
cycles, operating pressure cycles in which range of
pressure variation exceeds 20% of the design pressure (it is assumed that stresses due to cycles with a
lower range fall below the material's endurance
limit), and effective thermal cycles. The equivalent
number of effective thermal cycles is calculated by a
factor assigned to various values of temperature
differentials. For temperature differentials between
adjacent points (for definition see AD-160.2) up to
50°F, the factor is O and for very large temperature
differentials (in excess of 450°F) the factor is 20. The
number of temperature cycles for components involving welds between materials having different coefficients of thermal expansion is also to be included to
satisfy condition A. Condition B is a more accurate
26
method than condition A. This condition is satisfied
if a number of requirements are met. The design
number of full range pressure cycles may not exceed
the allowable number of cycles which correspond to
the primary plus secondary stress intensity range
allowable (3 Sm) multiplied by an implicit stress
concentration factor of 2.0. Another requirement is
that the range of pressure cycles during normal
operation does not exceed certain value. This assures
that the maximum peak stress due to these operating :fluctuations does not exceed the stress range
allowed for 106 cycles. This was based on the assumption that the stress amplitude at 106 cycles constitutes the endurance limit. (It is now known that this
is not true and that the allowable stress amplitude
declines with increasing number of cycles far beyond
106 in certain environments). Another requirement
is a limit on temperature difference between any two
adjacent points. There is also a limit on the range of
temperature difference between any two adjacent
points, depending on the specified number of significant temperature difference :fluctuations. Another
requirement is a limit on the range of mechanical
loads. All these requirements are again based on
assuming a surface stress of 3Sm multiplied by a
stress concentration factor of 2.
The rules of AD-160.3 allow exemptions from
fatigue analysis for non-integral parts (such as reinforcement pads). These rules require a more detailed
calculation of stresses. This paragraph limits the
total number of cycles of various operating conditions to 400. This is due to the fact that a higher
stress concentration factor is assumed. The cycles
with a range exceeding 15% of the full design pres~
sure range are assumed to contribute to fatigue
damage.
The design rules of VIII-1 do not explicitly 01'
implicitly consider the fatigue mode of failure. Some
of the details allowed by this code, such as nozzle
attachment details, head or flange attachment de
tails, or fillet welds for other attachments, produce
stress concentration factors up to 4.0. (Although the
maximum value of fatigue strength reduction factor
is 5.0 for a crack-like defect, VIII-2 suggests a value
4.0 for the notch due to a fillet weld.) For a vessel
fabricated from commonly used SA-516 Gr.70 mate
rial, allowable stress is Yi of ultimate strength o
17 .5 ksi. Assuming that at discontinuities the maxi
mum surface stress is 3S and the SCF is 4.0, wil
result in a maximum stress range of 210 ksi. This i
equivalent to a stress amplitude of 105 ksi, for whic
the number ofallowable cycles (from figure 5-110.1
VIII-2) is approximately 600. For the proposed d
sign factor on ultimate strength of 3.5, the resultin
stress amplitude would be 120 ksi and the allowabl
number of cycles would be approximately 500. For a
assumed design factor of 3, the allowable number
cycles would be approximately 190. For lower desi
factors, the allowable based on yield strength wi
start controlling and will result in allowable numbe
WRC Bulletin 435
of cycles of about 170. This demonstrates that by
increasing the allowable stresses, the number of
design cycles decline significantly. If a factor on
ultimate strength of less than 3.5 is used, the
allowable details will have to be more restrictive or
definite limits need to be put on the number of cycles
exempt from detailed fatigue analysis. If the details
are such that the resulting maximum stress concentration factor is 3.0, the fatigue strength will be the
same as that of present rules if the design factor on
ultimate is reduced from 4 to 3. This of course is a
very simplistic analysis and does not address various
types of cycles. It provides an insight into the
magnitude of cycles that Division 1 vessels would be
safe to be subjected to.
The fatigue rules and S-N curves of Division 2 are
about 30 years old. Agreat deal of additional technology and data are available today. If the design
margins are reduced, it is recommended that a
review ofVIII-2 fatigue analysis rules be made and,
where necessary, these rules be updated. The fatigue
exemption rules will also have to be revised to reflect
higher allowables, resulting in a lower number of
cycles at which exemption is allowed.
With any increase in stress allowables for Division
1 vessels, some guidelines will need to be provided
for exemption from fatigue. Some suggestions on
operational cycle accounting and in-service inspection may also have to be added, until post construction codes are available to address these issues.
3. Review of Code Case 2168 and Nozzle
Reinforcement Requirements
All codes and standards reviewed contain rules for
reinforcement of openings larger than a certain size.
ASME codes almost always require full replacement
of the area needed to resist pressure, within certain
limits. Most European codes base their rules on more
analytical methods such as limit analysis. For the
comparisons of design rules in this report, the basic
ASME method of full area replacement was used?
However, recently a Code Case has been introduced,
which allows less than full area replacement. This
Code Case 2168 is based on studies of and recommendations from WRC Bulletin 335. 15 The Code Case is
limited to carbon and low alloy steels with allowable
stresses in tension not exceeding 17 .5 ksi. There are
also a number of limitations on geometric parameters, consistent with the available test data used for
the preparation of the WRC Bulletin.
The basic philosophy behind nozzle reinforcement
rules is to assure that the burst pressure of the
vessel with the opening is at least equal to that of a
vessel without any opening. Test results summarized in the WRC Bulletin 335 indicated that full
area replacement was not needed to achieve this
goal. Based on test results, rules were developed so
that with reinforcement considerably less than full
area replacement the burst pressure of the cylindrical vessels could be restored. The analysis of test
results was based on the assumption that the vessel
is subjected to design pressures allowed by present
VIII-1 criteria. For a relatively small increase in
allowable stresses, the burst pressure of the opening
will probably still maintain a safe ratio to the burst
pressure of the penetrated shell. However, since the
stress distribution around the reinforced opening is
not uniform, the ratio of burst pressures for penetrated and unpenetrated shell does not remain
constant. If design margins are reduced beyond that
recommended in phase 1 of this study (margin on
tensile strength of 3.5), the provisions of this Code
Case will have to be re-examined.
One of the conditions of the Code Case is that
"cyclic loading is not a controlling design requirement." This was in recognition of the fact that there
is no attempt to maintain the same fatigue life at a
reinforced opening as that of the unpenetrated shell.
In fact, due to high local stresses and stress raisers, a
reinforced opening almost always is inferior to the
unpenetrated shell from a fatigue standpoint. Allowing higher design pressures will accentuate this
problem. Even though VIII-1 does not explicitly
address the fatigue problem, the threshold of the
number of cycles for vessels for which no fatigue
analysis is performed will be lowered. Any decrease
in design margins will have to take this into account.
E. C. Rodabaugh is in the process of preparing a
report for the PVRC which will address the difference betweenASME codes and European codes. This
study will include a detailed evaluation of Code Case
2168. The comparisons in this report are in the form
of all reinforcement being placed in the nozzle neck.
Comparisons of this report which include VIII-1
rules [including 1-7(a) requirements], VIII-2 (including special requirements of AD-560), Code Case
2168, and BS5500, indicate a very wide range of
results for various geometric parameters. It indicates that reinforcement requirements of Code Case
2168 are considerably less than those of VIII-1 and
VIII-2, but closer to those of BS5500. These results
reiterate the fact the VIII-1 and VIII-2 reinforcement rules are more conservative than those of
BS5500, but Code Case rules are generally less
conservative than BS5500.
Various codes put different limits on the size of an
opening above which reinforcement is required. The
rules ofVIII-1 have a nozzle diameter of 2%" to 3%",
based on shell thickness, but regardless of vessel
diameter. Tl:ft!'rules ofVIII-2, put this limit at a ratio
of dl{DT exceeding 0.14. The limit on this ratio in
BS5500 is 0.10. Code Case 2168 does not have a limit
on the size of openings exempt from reinforcement,
but since this is an alternative to basic VIII-1
reinforcement rules at least the same limits are
applicable. The rules of the Code Case may result in
no reinforcement requirement for penetrations larger
than those exempt by the rules ofUG-36, depending
on vessel geometry. In terms of a multiple of dl{DT,
E. C. Rodabaugh's forthcoming report indicates that
nozzles with ratio up to about 0.3 would be exempt.
Evaluation of Design Margins, Section VIII
27
There is no question that an unreinforced opening,
no matter how small, will result in stress concentration. Even though the effect of such small unreinforced openings may be insignificant on burst pressure, they definitely degrade fatigue capability of a
vessel. With higher allowable stresses in vessel
shells, the effect of unreinforced openings on vessel
fatigue life will have to be addressed.
4. The Effe.ct of Lower Design Margins on Brittle
Fracture in Part UHT Materials
The Report by Dr. S. Yukawa on Brittle Fracture
Margins in ASME Code Section VIII, Division 127
evaluated the margin for 38 ksi and 50 ksi yield
strength materials based on the ratio of the available
fracture toughness Krd to the applied Kr, where Krd
was calculated from the Barsom-Rolfe Km-CVN correlation for dynamic fracture toughness, Krd =
~5E(CVN). The applied Kr was evaluated using the
Section VIII, Division I allowable stresses and the
maximum allowable flaw sizes in Section VIII, Division 1 for radiographic examination. Dr. Yukawa's
evaluation of the Section VIII, Division 1 Code
margins for resistance to brittle fracture in carbon
and low alloy steels was based on:
The margin on brittle fracture for Division 1, Part
UHT high strength, quenched and tempered steel SA
517 Gr. F was evaluated in this report by determining the ratio of the available fracture toughness to
the applied K for the Code permissible flaw sizes for
radiographic examination (Section VIII, Division 1,
UW-51).
High strength steels inASME Section VIII (Su 2:: 95
ksi) must meet at least 15 mils lateral expansion.
Recent revisions to both Division 1 and Division 2
increase the MLE requirements for nominal plate
thickness exceeding 11;4". 10 The following Charpy
energy values were obtained from data provided by
Dr. W. D. Doty34 for SA-517 Gr. F steel with typical
yield strength of about 110-120 ksi and from Fig.
A2-2 and A2-3 in WRC Bulletin 175 14 (Table 1):
Mils Lateral
Expansion, MLE
Estimated Charpy
Energy (CVN), ft-lbs
15
20
25
35
23
30
40
63
1. Critical vs. permissible flaw size for a given
stress, and
2. Material fracture toughness vs. applied stress
intensity factor, Kr, for a given flaw size.
The report by Dr. Yukawa gives the following results
for Section VIII, Division 1 vessels:
• Calculated critical flaw length vs. maximum
Code permissible flaw length of %" for RT
examination, for various flaw depth/thickness
(alt) ratios and applied stress/yield strength
ratios (SR). The size margin for SR
1.0 and
alt = 0.5 is about 2% in 1-3 in. thick material
with 38 ksi minimum yield strength (and higher
for lower alt ratios and lower stress ratios).
• Available fracture toughness (based on minimum required toughness in the Code) vs. the
applied stress intensity factor K for a flaw with
a length of%" (maximum permitted in UW-51
for RT examination). This evaluation indicated
a minimum margin of about 1.5 for 1" and 3"
thicknesses for a material with a minimum
yield strength of 38 ksi and a stress ratio CSR) of
1.0 (and higher values for lower stress ratios).
• The report also states that the relief from notch
toughness testing provided in Division 1 (UCS66.1) by the reduced stress criterion does not
compromise these margins.
The maximum allowable defects for radiographic
examination are given in UW-51 of ASME Section
VIII, Division 1 and in Article I-5 of Division 2. Any
elongated inclusion, such as slag, is limited to:
• 114" fort up to%" (where t =thickness of the weld)
28
Table 2
• Ya t for t from %" to 2114"
• %" for t over 2114"
Static plane strain fracture toughness Kic and the
yield strength of weld Sy, (assumed to be equal to th
specified minimum yield strength of the base metal
were used for these fracture mechanics calculations
The Krc values were obtained from the lower boun
Krc - CVN correlation, Kic = 9.35(CVN) 0 ·6 s. 35 ,3 6 Th
results obtained with ASME Section XI, Articl
A-3000 fracture mechanics procedures for %", 1114
and 2:Y2" thick SA 517 Gr. F are given in Table 2
These results are based on Section VIII, Division
and 2 allowable defect lengths for radiographic exami
nation and a surface flaw across the welded join
with a depth to length ratio (all) of% (which resulte
in the highest value of Kr).
Based on the above calculations, the margin o
brittle fracture is about 1.5 for the %" thick materi
and about 1.2 for the 1114" material in the as-welde
condition (using the UW-51 maximum permissibl
flaw sizes for radiographic examination and assum
ing S = Sy for the weld, acting uniformly across th
thiclmess). Although there is a significant amount o
successful experience with the present Code tough
ness requirements, a further review of the Cod
toughness requirements for these steels is sug
gested, particularly for non-stress relieved welde
joints over a certain thickness (e.g.%"), regardless o
the design stress.
In the stress relieved condition with the desi
stress S = Su/4 and residual stress equal to 30% o
the yield strength of the weld, the margins on brittl
WRC Bulletin 435
S
S
S
S
S
S
2W'
22.5
(35 ft-lbs)
87.8
0.75"
S
S
S
S
=Sy= 100*
Su/4 28.8
Su/3.5 = 32.9
= Sy
100 ksi*
= Su/4 + 0.3Sy
= 58.8** (PWHT)
= Su/3.5 + 0.3Sy
= 62.9** (PWHT)
Su/4 28.8 ksi
= Su/3.5 = 32.9
= Su/4 + 0.3Sy = 58.8**
Su/3.5 + 0.3Sy
= 62.9**
=
44.2
15.6
17.9
57.1
32.4
1.5
4.3
3.8
1.2
2.1
34.7
1.9
20.9
24
43.3
46.5
4.2
3.7
2.0
1.9
Notes: *This represents a non-stress relieved butt weld with the design membrane stress plus welding residual stress equal to the yield
strength of the weld (Sy).
**This represents a stress relieved (PWHT) butt weld with the assumed remaining residual stress after PWHT equal to 30% of the room
temperature yield strength of the weld in the direction of the weld (26, 36). (ASME Code requires PWHT ofSA-517 welds at 1000°F-1100°F
for thickness t > 0.58'', or fort> 1Y4" when preheated 200°F and post-heated 400°F over 0.58").
Sy = specified minimum yield strength.
Su = specs minimum fenride strength.
fracture are about 2.1 for the 1114" thick welded joints
and 2.0 for the 2%" welded joints.
Decreasing the design margin from 4 to 3.5 (by
14.3%) slightly decreases the margin on brittle fracture. In the stress relieved, the margins on brittle
fracture are about 1.9 for the 1114" and the 2%" thick
welds welds.
Gross structural discontinuities and stress concentrations at the edge of the weld, however, can
significantly increase local secondary and peak
stresses in the direction transverse to the weld and
thereby increase the risk of brittle fracture of flaws
which may exist in the weld or heat affected zones in
a plane parallel to the weld. Precautions, therefore,
need to be taken in high strength steels to avoid
structural discontinuities (such as misalignment
and peaking) and stress concentrations at 101al
structural discontinuities (such as notches, sharp
corners at edges of the weld, etc.). More sensitive
NDE techniques, such as wet magnetic particle
surface examination of welds and/or UT, provide
additional means of detecting defects in high stress
areas.
The risk of brittle fracture often is the highest
during overload testing of the vessel, because the
vessel is being subjected to high stresses for the first
time. It is, therefore, important to assure adequate
margin against brittle fracture for both the overload
test and for the design/operating conditions of the
vessel. Section VIII, Division 1 presently recommends that the minimum metal temperature during
the hydrostatic test be at least 30°F above the
minimum metal design temperature. It is a Code
requirement for the pneumatic test. Consideration
should be given to making the 30°F differential
mandatory also for the hydrostatic test when a lower
design margin is adopted.
D. Study of Vessel Test Results
1. Purpose
This section of the report presents a summary of
available pressure vessel test data. Based on the test
results, some conclusions are made regarding the
safety of fabricated vessels designed to various rules.
2. CBI Tests of Pressure Vessels with Reduced
Thickness Girth Seams
In 1987, CBI performed destructive testing of 10
pressure vessel models to justify the use of reduced
thickness girths, without reducing the allowable
pressure. This concept takes advantage of the fact
that, in cylindrical shells, the axial stress is one half
of the circumferential stress. The results of this
project were used to introduce a Code Case in VIII-2
to allow reduction in girth weld thickness, without a
reduction in allowable pressure. This Code Case has
now been incorporated in VIII-2, as paragraph AD200(b). The test models included a reference shell
without any reduction in thickness and other models
with varying degrees of reduction in thiclmess and
various beveling to the reduced thickness. The material for the cylinders was 20 inch diameter schedule
80 seamless pipe (SA-333, Gr. 6). The calculated
allowable design pressure, based on VIII-2 rules,
was 1320""si. The actual burst pressures ranged
from 4276 to 4459 psi. The vessels, having been egg
shaped at failure, split along a longitudinal seam.
This of course proved that the reduction in a girth
thickness has no appreciable effect on failure pressure of a cylindrical shell. But these tests also
indicated that margins to failure for vessels fabricated to VIII-2 rules is in the order of 3.24 to 3.38 for
a ductile failure of cylindrical shells of carbon steel
vessels with a diameter/thickness (d/t) ratio of about
17.4. If VIII-1 allowables were used these margins
would have been 4.32 to 4.50. This of course is for a
Evaluation of Design Margins, Section VIII
29
clean shell, without any penetrations or attachments, constructed of a notch tough material (the
measured expansion in diameters ranged from 25 .1 %
to 34.5%). The details of these tests are included in a
CBI report. This brief explanation points to the
safety of cylindrical vessels fabricated to ASME Code
rules.
3. Ellipsoidal Head Model Tests
In 1984, CBI performed pressure tests to failure of
two models of nuclear containment vessel heads. 21
The two torispherical heads, fabricated from SA-516
Gr. 70 material, had cylindrical diameters of 192 in.,
knuckle radii of 32.64 in., spherical dome radii of
172.8 in., and nominal thicknesses of 3/iG and Y-4 in.
For the o/16 in. thick head, initial buckling was noted
at 58 psi. Further pressurization rounded out the
head until rupture occurred in the knuckle at 229
psi. For the Y-4 in. thick model, initial buckling was
noted at 106 psi and rupture occurred at the top of
the head at 332 psi. The allowable design pressures
calculated by the rules of Subsection NE of Section
III ASME Code were 33.2 psi and 45.5 for models 1
and 2. Using VIII-1 rules, the allowable design
pressures would have been 36.5 and 50.1 for the two
models. This would result in calculated margins to
failure of 6.27 and 6.62. These results indicate the
large margins available on rupture of torispherical
heads. For these thin heads, of course, the failure
mode is considered to be knuckle buckling, with
resulting margins which are much lower. It should
also be pointed out that these observed margins to
rupture may not hold over the range of geometric
parameters. These tests give an indication of safety
of thin torispherical heads. 21
allowables. This indicates a considerably higher
margin for this larger and thicker vessel.
5. Praxair Torispherical Head Test
In 1994, Praxair performed pressure testing of two
torispherical heads for the PVRC. The purpose of
these tests were to evaluate the safety margins in
the ASME Code rules for design of formed heads and
to provide a basis for developing new and updated
rules. Both heads were attached to cylinders with 60
in. diameters and were made of SA-516 Gr. 70
material. The crown radius-to-diameter ratios were
1.0 and the knuckle ratios were 6%. The nominal
head thicknesses were 0.25 in. and 0.32 in. The
allowable design pressure, using the rules of VIII-1
were 85 psi and 110 psi. For Model 1, the pressure
was increased to 700 psi at which time a penetration
on the attached cylinder failed. Model 2 failed at
1080 psi by ductile fracture along a longitudinal
seam of the attached cylinder. These failure pressures indicate minimum margins for the heads of 8.2
and 9.8, respectively. This confirms the widely held
belief that the presentASME Code rules for design of
formed heads is overly conservative, at least over a
certain range of parameters.
6. MPC Service Exposed Test Vessels With Local
Thin Areas
The objective of this test program was to evaluate
during a hydrostatic test the performance of two
service exposed, retired pressure vessels with deliberately prepared local thin areas (LTA's) of varying
geometry and locations. The vessels were originally
designed to ASME, Section VIII, Division 1. 3l The
vessel dimensions, material properties, wall thickness and design conditions are listed below (Table 3):
4. Containment Vessel Model Tests
A series of tests were performed in the mid-80's by
Sandia National Labs, to estimate the margin to
failure of nuclear containment vessels. Four models
had a diameter of 43 in. and thickness of .045", with
varying details; some including major penetrations:
These models were fabricated to the rules of Section
III, Subsection NE, and had calculated allowable
design pressures of 40 psi. The failure (rupture)
pressures were 140 psi, 110 psi, 120 psi, and 120 psi.
The calculated margins to failure, using Subsection
NE rules were 3.5, 2.75, 3.0, and 3.0 respectively. If
the design rules of VIII-1 had been employed the
margins to failure would have been 3.85, 3.02, 3.3
and 3.3. This indicated a lower margin than expected. The fact that these models were fabricated of
thin gage material and included major discontinuities probably contributed to these lower than expected margins.
This test program also included a larger model
with diameter of 168 in. The calculated design
pressure for this vessel, using Subsection NE rules,
was also 40 psi and the burst pressure was 195 psi.
This margin of 4.88 would have been 5.37 for VIII-1
30
Table3
Inside diameter
Length
Wall thickness
Design pressure
Test pressure
Design temperature
Material
Shell nozzles/Manways
vessel 1
Vessel2
48"
144"
7/16"
125 psi
297 psi
600°F
SA-285, Gr. C
4" & 18" diameter
with reinforcing
78"
199"
l" shell, 1.25" heads
300 psi
523 psi
650°F
SA-515, Gr. 65
4" & 18" diameter
with reinforcing
Vessel 1. The vessel had significant internal corro-,
sion, which was removed by sandblasting. The aver~
age wall thickness of the vessel wall ranged froni
0.30 to 0.41 inches, and a nominal wall thickness
was assumed for this vessel for the purpose of this
project. There was also a significant amount o
blistering, as a consequence of wet H 2S service
About 30 local thin areas with tapered edges wer
ground into the vessel shell and heads. The size an
depth of the individual LTA's were selected such tha
the remaining strength factor (RSF) associated wit
WRC Bulletin 435
each LTA would be about 0.8. The intent was to
produce several pads with a net thickness of about
o.1" at the bottom of the LTA. The LTA's were
positioned at various locations on the vessel, including edges of the nozzle reinforcing pads, but away
from areas which were heavily corroded or blistered.
At test pressure of 515 psi (average hoop stress of
about 40,000 psi in 0.31" thick shell) a leak occurred
in one LTA, under a strain gage, which had a strain
gage reading that reached 5.9%. Several other LTA's
had strains ranging from 2.1 % to 5.9%. The vessel
wall did reach nominal yield stress. No deformation
was reported at nozzles or manholes. The test temperature was 55°F.
Vessel 2. The as received vessel was in a better
condition than Vessel 1. There was little or no
corrosion and blistering. To generate about the same
remaining strength (RSF) as for Vessel 1 at the
LTA's, the wall thickness at some LTA's was reduced
to about 0.25". The locations of the LTA's were
similar to Vessel 1, except that some additional LTA's
were located at nozzle-to-shell welds and along a
longitudinal weld seam. Also, some long, narrow
grooves were ground into the shell in the axial direction.
After 1050 psi test pressure some of the strain
gages in the LTA's began to follow a previously
established strain hardening curve. Nearly all strain
gages in the hoop direction had significant plastic deformation. The test was terminated at 1315 psi (about
51,300 psi nominal hoop stress in the 1" thick vessel
shell) due to excessive leakage at a nozzle gasket.
The nominal strain gage reading at a test pressure
of 1300 psi was 0.69%, indicating general yielding in
the vessel wall. The maximum local strain reading at
an LTA was 4.69%. No deformation was reported at
nozzles or manholes. The test temperature was 84 °F
for Vessel 2.
Vessel 1 had a margin of 2.91 on rupture and
Vessel 2 test was terminated at a stress equal to 3.16
times the design stress, based on the nominal hobp
stress in the vessel shell at the end of the test. This
does not account for the RSF of about 0.8 in the LTA's.
No information was provided on the actual tensile
properties of the materials in these test vessels.
E. Evaluation of Design Margins for ASME
Section VIII, Division 2
1. Review of Failure Modes Considered in
Section VIII, Division 2
In 1987-1989 the ASME Subgroup on Design
Analysis (SGDA) performed a study to determine the
failure modes considered in the various sections of
the ASME Code and the intended safety margins for
the given failure mode in each Code section. 25 Eight
failure modes were identified and discussed in the
February, 1989 report "Summary of the Design
Analysis Factors Inherent in the Established Failure Modes of the ASME boiler and Pressure Vessel
code," prepared by SGDA. The following is a brief
summary on each failure mode. A description of
design margins explicitly introduced into the Code
design rules is included in ASME Section II, Part D,
Appendix2.
a. Ductile Rupture
ASME Section VIII, Division 2 uses the Tresca, or
maximum shear stress criteria to establish the allowable stress intensities. The shear stress is equal to
one half of the difference between any two principal
stresses at any given point in a structure. Yielding
will occur when the difference between any two
principal stresses, defined as a stress intensity, S,
equals the yield strength. The stress intensity in
Section VIII, Division 2 is the maximum allowable
value of the stress difference. Appendix 2 in Section
II, Part D gives the following criteria for establishing
design stress intensity values:
Table AD-150. l gives the following k factors for
establishing the maximum stress intensity limits, k
Sm, for various load combinations (Table 4):
Table4
Design (Design pressure, dead load of the vessel,
vessel contents, external attachment loads)
Design + wind
Design + earthquake
Design + wave action
Hydrostatic test
Pneumatic test
1.0
1.2
1.2
1.2
1.25
1.15
7. Conclusions
The available test results indicate that for cylindrical vessels, the safety margins against rupture are
approximately the same as design factors on ultimate tensile strength. The test vessels designed to
Section VIII, Division 1 failed at design margins
ranging from about 3.0 to 4.5, with an average of
about 3.7 on the tensile strength of the material. For
formed heads, the safety margins to rupture are
c?nsiderably higher. This of course is a generalization. The data are very limited and does not cover the
entire range of parameters. The true safety margins
are also a function of fabrication details, quality of
welds and many other factors, which cannot be
covered by a finite number of test models.
The basic stress intensity limits are listed in Table 5.
Tables
General primary membrane
<Pml
Local primary membrane (Pd
Primary local
membrane + primary
bending (PL + Pb)
+ secondary
k Sm :s k (lower of 2/3 Sy or ST/3)
1.5 k Sm
1.5 k Sm
3 Sm
+
ST = specified minimum tensile strength at room temperature.
Sy specified minimum yield strength at room temperature.
Evaluation of Design Margins, Section VIII
31
Table 6 lists the ASME Section II, Part D, Appendix 2 criteria for establishing design stress intensity
values for ferrous and non-ferrous materials (other
than bolting) at temperatures below creep range.
The intent of these design margins is to provide
adequate margins of safety at temperatures below
the creep range against the following:
• Ductile rupture
• Local failure
• Uncontrolled bending deformation
A nominal design margin of 3 is provided against
ductile rupture. The design margin can be as low as
2.5 under some operational loading conditions and
2.4 under the hydrostatic test condition.
The local design margin on ultimate tensile
strength can be as low as 1.6 for the hydrostatic test
condition. (True margins of safety generally are greater,
since local stresses in the vessel can redistribute.)
b. Brittle Fracture
The toughness requirements for ferrous materials
is covered in AM-210 of Section VIII, Division 2.
Toughness requirements for quenched and tempered
steels are included in AM-310. Recently approved
changes to the Division 2 toughness rules have made
them essentially the same as Division 1 toughness
requirements. In both Divisions the materials are
grouped into four groups based on the notch toughness characteristics of the materials. Fig. AM-218.1
shows impact test exemption curves A through D for
materials in the four groups of carbon and low alloy
steels. Curve A includes the least notch tough steels,
including structural shapes and bars. Curve D includes heat treated fine grain steels with improved
toughness. Impact testing is required at minimum
design metal temperature/thickness combinations below the exemption curves and for thicknesses above 4
inches. The required minimum average impact energy
values are given in Fig. AM-211 of the revised toughness
rules and depend on thickness and the specified minimum yield strength of the material. The orientation
of the impact test specimen is not specified.
Fig. AM-218.2 of the revised toughness rules provides for a reduction of the impact test temperature
based on the ratio of the required thickness to the
nominal thickness of the component.
All Table ACS-1 carbon and low alloy steels with
specified minimum ultimate tensile strength >95
ksi and all Table AQT-1 high strength quenched and
tempered steels shall meet 15 mils minimum lateral
expansion for thicknesses up to 1114" and 25 mils for
thicknesses above 3" (Fig. AM-211.2).
Other Section VIII, Division 2 requirements relevant to avoidance of brittle fracture are:
• The Code requires impact testing of welds and
RAZ when base metals are required to be
impact tested (Article T-2).
• Essentially all welded joints required to be
non-destructively examined (TableAF-241.1).
he Section VIII, Divisions 1 and 2 toughness rules
are based on fracture mechanics considerations and
extensive good experience with ASME Code vessels.
It is not possible, however, to establish a single
margin of safety against brittle fracture in the Code
rules because brittle fracture depends on three factors, the applied stress, the flaw dimensions, and the
available toughness of the material.
c. Fatigue
AD-160 in Section VIII, Division 2 gives rules for
determining exemption from fatigue analysis. The
rules for exemption from fatigue analysis are based
on experience with low cycle operation. The procedures for design for cyclic loading are included in
Appendix 5 of the Code. The fatigue curves in
Appendix 5 are limited to a temperature of 700°F for
ferritic and 800°F for austenitic steels.
The fatigue design curves in Article 5-1 were derived
from fatigue tests on smooth specimens tested at room
temperature. Peak stresses are calculated by assuming
or evaluating stress concentration factors at discontinuities and comparing these stresses to the smooth speci-
men design S-N curve. TheASME fatigue design curves
include a factor of 2 on stress in the high cycle regime
and 20 on cycles in the low cycle regime (approximately
<10000 cycles). The factors account for scatter of the
test data, size effects, surface finish, and industrial
atmosphere.
d. Incremental Collapse
The Code imposes the following limitations on the
Primary + Secondary Stress Intensity to provide a
margin against progressive deformation during cyclic loading:
The 3 Sm limit is equal to or less than 2 Sy. The 2 S
limit results in elastic cycling after initial yieldin~
(shakedown).
e. Elastic Instability
The basis for buckling rules is outlined in Appendix 3 of Section II, Part D. The true safety factor built
into these rules varies depending on loading and the
geometry being considered. The value of applied
safety factor ranges from 2.0 to 4.0. This nonuniform safety factor is one of the shortcomings of
these rules. In addition to safety factors, knockdown
factors from test results have been applied to the
theoretical buckling stresses. These knockdown factors, being based on the lower bound of test data
provide additional safety margins. The maximu~
buckling stresses are limited to the allowable tensile
stresses. In addition, for cylindrical shells, these
stresses are limited to Ys of minimum yield strength.
f. Plastic Instability
The Code imposes the following limitations on
stress intensities to avoid plastic instability:
1. General primary membrane stress Pm :5 %
(Appendix 4)
2. Local primary membrane stress, PL :::; Sy
3. Local primary membrane plus primary bending
stress, PL + Pb :5 Sy.
Sy = specified minimum yield strength at tempera-
ture.
Welded pipe or tube, ferrous or non-ferrous
The purpose of these design margins is to provide
a margin against instability where tensile straining
by a primary load can produce large deformations.
1.1
Wrought or cast, ferrous or non-ferrous
3ST~
0.85
(l.lx0.85)
----ST~
3
0.85
LlSy
0.85
LlSyRy
g. Excessive Deformation and Leakage
The Code does not have explicit rules for determination of deflections and rotations. For design of
0.92
(l.lx0.92)
0.92
Ferrous materials, structural quality, pressure
flanges, Division 2 requires that the stress allow----ST~
LlSy
- -ST
retention
3
3
a~les of Division 1 be used, in an attempt to miniFerrous materials, structural quality, non-pres'l:i ST
1.1
% Sy
% SyRy or 0.9
m1ze the probability ofleakage. Division 1 does have
sure retention
3 ST~
SyRy
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - " , I l l ~on-mandatory rules in Appendix 5 for further limitmg the rotation of closure flanges, but Division 2
~=ratio of trend curve value of tensile strength to room temperature tensile strength.
does not have such rules.
Ry= ratio of trend curve value of yield strength to room temperature yield strength.
32
WRC Bulletin 435
or
(0.9 x 0.85) SyRy
0.92
LlSyRy
2. Assessment of Significant Factors on
Design Margins
a. Design Details
Design details have a significant effect on the true
margin of safety for pressure vessels. However, it is
difficult to quantify the effects of various details on
d~ff~rent fa~lure modes. For plastic collapse, an analysis is reqmred. For fatigue, a stress concentration
(fatigue stress reduction factor) may be calculated or
assumed. For brittle fracture, the effect of local
stresses and the notch effects of various details need
to be taken into account. Details that are allowed are
normally based on judgment and experience. For
some details, test results and/or analysis results are
available. All these resources need to be taken
advantage of in establishing acceptable details.
b. Ductile Rupture
!he limits on local membrane stress intensity and
membrane plus primary bending stress
mtens1ty (1.5 Sm) in ASME Section VIII, Division 2
have been set at a level which conservatively assures
the prevention collapse as determined by the principles of limit analysis (Paragraph 4-136(b)). Since
the primary stress intensity limits are based on
factors of yield strength and the ultimate tensile
strength of the material, it is generally understood
that this provides assurance against excessive distortion or bursting mode of failure.
B. F. Langer 17 lists the following modes of failures
related to bursting at temperatures below the creep
range:
~r1ma~y
2. Bursting due to general yielding and rupture of
the vessel shell
3. Ductile tearing at discontinuity such as a nozzle,
or head-to-shell intersection
4. Brittle fracture at a defect
Langer 12 recommended the use of the modified Svensson equation for calculating the maximum burst
pressure based on plastic instability since that correlated well with experiments. The modified Svensson
equations are:
PB = O'u Fey! ln W, for cylindrical shells
PB = O'u Fsph ln W, for spherical shells
O'u ultimate tensile strength of the material
Fey! [0.250/(Eu + 0.227)] (elEu)eu, correction factor for a cylinder and represents the effect
o:rt'train hardening on burst pressure,
Fsph = [0.250/(Eu + 0.227)] (l.155e1Eu)eu, correction
factor for a sphere
W = ratio of the vessel outside diameter (d 0 ) to
the inside diameter (dJ
For relatively thin-walled vessels, the In W
can be approximated by ln W = 2t/(di + t)
E = true strain
e = logarithmic base
Eu = ln(l + E~), true strain at maximum load in
the tensile test (true maximum elongation)
and is used in the modified Svensson equa-
Evaluation of Design Margins, Section VIII
33
0"
0
E'u
tion as an approximation of the strain hardening exponent, n, for materials whose true
stress-strain curve is represented by O" = 0"0 En
= strength coefficient (true stress at E = 1.0)
= engineering strain at maximum load (uniform elongation)
The relationship between Fey! and Fsph and Eu are
presented graphically in Fig. 1. Fey! and Fsph are
numerically greater for lower values of Eu· The burst
pressure is proportional to F cyl and F srh and, therefore, higher for vessels made from high strength
steels because of increased tensile strength and the
lower strain hardening exponent.
2.0
1.9
~
'
1.8
1.7
1.6
' ","
~Foph
'
1.5
1.4
" ""-
cc
~ 1.3
u
<
LL.
LL.
"'
"~
~
1.2
I.I
1.0
'
.......
'-....
"'"
0.9
0.8
........ ~,Fcyl
........... ............_
0.7
.............
!'-.......
0.6
0.5
0
.2
.3
.4
.5
.6
.7
.8
.............
.9
t.0
E:u
Fig. 1-Values of cylinder and sphere factors for modified Svensson
equation
Langer used the modified Svensson equations to
predict the actual margins against bursting mode of
failure in spheres and cylinders of four different
materials and d/t
10, designed to Section VIII,
Division 2 allowable stresses. 17 The margins against
bursting mode of failure (SF), calculated by Langer,
are listed in Table 7.
The results in this table indicate that the predicted
margins on bursting pressure (S.F.) are somewhat higher
for cylinders than for spheres with the same d/t.
PVRC Subcommittee on Effective Utilization of
Yield Strength sponsored several projects in 1960's
34
SA-240, 830400
SA-516, Gr. 55
SA-533, Gr. B
SA-517, Gr. F
75
55
80
115
30
30
50
100
20
18.3
26.7
38.3
0.55
0.31
0.17
0.11
2.73
2.49
2.69
2.79
2.91
2.75
3.03
3.17
and early 1970's to provide a better understanding of
the behavior of pressure vessels in the bursting mode
of failure and to evaluate the accuracy of the modified Svensson equation. 19·20 The project at University
of Kansas included burst tests on test vessels of
three types of materials with different strain hardening exponents. The materials were SA-240 Type 304
stainless steel, SA-516 Grade 70 carbon steel and
SA-517 Grade F quenched and tempered steel. These
steels have strain hardening exponents of 0.585,
0.189, and 0.085, respectively. The burst tests included two types of strain concentrations:
• Vessels with "moderate" strain concentrations
common to pressure vessels, such as nozzles,
welds, and end closures.
• Severe strain concentrations, such as sharp
notches in the axial direction of the test vessels:
The notches were machined to depths about 15;
25, and 35% of the vessel wall thickness and had
a root radius at or below 0.001" (0.025 mm).
The PVRC sponsored work led to the following
conclusions:
1. The yield strength controls the design of Divi~
sion 2 vessels only if the yield strength/tensile
strength ratio is less than 0.5. Design ofSectio
VIII vessels is essentially based on the burstin
mode of failure, instead of yielding, in th
temperature range in which bursting, rathe
than the other failure modes (fatigue, brittl
fracture, etc.) is the controlling criterion.
2. The modified Svensson formulas can be ex
pected in most cases to predict the bursti
pressure of a vessel to within about ± 10<7£
providing the material is sufficiently ductile a
the time of failure to fail in a bursting mode o
failure and not by brittle fracture.
3. The modified Svensson equation gives reason
ably accurate prediction of burst pressures i
thin wall pressure vessels with moderate strai
concentrations. (Moderate strain concentra.
tions did not significantly reduce the calculate
burst pressure in the University of Kansas tes
vessels with nozzles, welds and end closures).
4. For vessels with sharp notches, the test resul
showed that a direct correlation between redu
tion in wall thickness (due to a sharp not
and reduction in burst pressure exists at leas
until the notch depth exceeds 25% of the wa
thickness for the steels tested.
WRC Bulletin 435
The actual burst pressures for the University of Kansas
test vessels with full wall thiclmess (without sharp
notches) and the theoretical margins against the bursting mode of failure for these vessels using the modified
Svensson formula for cylinder are listed in Table 8.
The above predicted bursting pressures are based
on the specified minimum tensile strength values,
whereas the actual bursting pressures are based on
the actual measured tensile values, therefore, the
actual factors of safety are somewhat higher than
predicted.
~aln~ns and Updike discuss the safety margins of
cylmdrical. and spherical shells against bursting
mode of failure without and with edge restraints.28
Ba~ed ?n their anal~sis of the plastic instability of
cylmdncal shells with end closures they conclude
that restrained cylindrical and spherical shells burst
at higher pressures than unrestrained shells.29 Fig.
2 shows the margins against bursting mode of failure
vs. par8:meter UR (L = length of cylindrical shell,
R.= radm~) for a cylindrical shell that is designed
with a pnmary membrane stress Sm = Su/3. The
asymptote for a large UR approaches a safety factor
s.s;1 - , - - , - - , - - . - - , . - - - - . - - - , - - - - -
'
:
5
I.JR
Fig. 2-Safety factors for restrained cylindrical shells
of 3.2 on bursting pressure for the SA-516. Gr. 70
material.
Some European pressure vessel codes use lower
margins on the tensile strength than ASME. The
British BS 5500 uses a factor of 1.5 on yield strength
and 2.35 on tensile strength for carbon and low alloy
steel vessels. The CEN draft Standard for Unfired
Pressure Vessels proposes to use the lower of the
following design stresses for other than austenitic
steel for pressure parts other than bolts:
• The minimum specified yield strength or 0.2%
proof strength (Sy) divided by a safety factor of
1.5.
• T~~ minimum specified tensile strength (Su)
dlVlded by a safety factor of 2.4. (The July 1995
draft of the CEN/TC draft Standard for Unfired
Pressure Vessels included an alternative design
basis with a design stress based on the lower of
Su/1.875 and% Sy. However, this design basis is
not included in the May 1998 draft).
Tl~e predicted margins against the bursting mode of
failure for the cylindrical vessels tested at University of Kansas were re-calculated using the modified
Svensson equation and allowable design stresses
based on the lower of Su/2.4 or % Sy and are listed in
Table 9.
Based on the same allowable stresses (the lower of
Su/2.4 or % Sy) spherical vessels with the same
inside diameters and thicknesses as the University
of Kansas cylindrical test vessels have somewhat
lower margins than cylinders on the predicted burst
pressure. The predicted margins are given in
Table 10.
~though this results in lower predicted margins
agamst the burst pressure than vessels designed to
ASME Section VIII, Division 2, there have been no
indication o&bursting mode of failures, therefore, it can
be concluded that the European codes have shown
adequate safety in practice against this failure mode.
Evaluation of Design Margins, Section VIII
35
lent Kie values for this study are listed in the
following table (Table 11):
Table 11
ASMECode
c. Brittle Fracture
In fracture mechanics calculations, the value of
the stress intensity factor, Kr, is based on the applied
stress and dimensions of the flaw. The calculated Kr
must be less than the critical fracture toughness
parameter Krc (or Krd, depending on the loading rate)
to avoid brittle fracture. Also, consideration must be
given to both primary and secondary stresses to
evaluate maximum stress intensity, Kr, in the structure.
This part of the report attempts to evaluate the
following for vessels of several ASME Section II
materials (SA-516 Gr. 70, SA537 Cl. 1 and Cl. 2, and
SA517 Gr. F and Gr. E):
1. The change (reduction) in critical sizes (depths)
of surface flaws with lower design margins
(higher design stresses) but with the same
toughness requirements as presently in ASME
Section VIII, Division 1 and Division 2.
2. The toughness needed to retain the same margins on brittle fracture (the same critical flaw
size) for vessels designed to higher stresses as
for vessels designed to present Code allowable
design stresses. The following cases were investigated:
• Increasing the design stress from Su/4 to
Su/3.5
• Increasing the design stress from Su/3 to the
lower of Su/2.4 or% Sy
SA-537,Cl. l
SA-537,Cl.2
SA-517, Gr. E &F
SA-517, Gr. E
51.5
51.5
57.8
57.8
51.5
59.8
66.5
66.5
61.7
69.2
79.7
79.7
67.4
78.0
95.5
95.5
Ba~e Metal. The following cases list the design
margms selected for this study:
Case 1: S = Su/4
Case 2: S = Su/3.5
Case 3: S = Su/3
Table 12 lists the materials and stress values (ksi)
for this study.
~he critical depths of surface flaws with an aspect
rat10 of all= 0.15 (l/a = 6%) were obtained for Cases
1-~ with the PD 6493:1991, Level 2 FAD procedure.
Tlus aspect ratio was selected from the computer printouts for convenience, mainly because it is close to the
aspect ratio for the flaw dimensions specified in ASME
Code Section III, Part NB(~ t x 1.5 t smface flaw). The
calculated flaw depths are listed in Table 13.
Table 14 indicates when leak-before-break (as
defined by through thickness flaw with depth a = t
and length 1 = 2t) was achieved for Cases 1-5 with
th~ ASME Section VIII toughness requirements.
!ills shows that leak-before break is possible practically
111 ~l the cases examined in the above table with the
Sect10n VIII, .Division 1 design stresses (Su/4) and in
most cases with a design stress of Su/3.5. None of the
materials investigated (except possibly in lesser thicknesses) have leak-before-break capability when the maximum primary plus secondary stresses are equal to Sy.
Tables 15 and 16 show the toughness needed for
Table 12
Material
SA-516, Gr. 70
SA-537, Cl. l
SA-537, CL 2
55 MILS LATERAL EXP.
15 ft-lbs
15 ft-lbs
18ft-lbs
18 ft-lbs
15 ft-lbs
19 ft-lbs
22.5 ft-lbs
22.5 ft-lbs
20 ft-lbs
24 ft-lbs
30ft-lbs
30ft-lbs
15 MLE (=23 ft-lbs)
19.3 MLE (=29 ft-lbs)
Thickness
Case 1
Case2
Case3
Case4
S = Su/4
S = Su/3.5
S = Su/3
S:::; Su/2.4'1'
Case5
S =Sy
All
ts21;;2''
21;;2''< ts 4"
ts
45
75
40
§
~
35
>-
Table 13
es
Sy = specified minimum yield strength
Su = specified minimum tensile strength
~
:x:
~
The assumed increase in design stresses represent
the following decrease in the design margins(%):
S = Su/4 to S = Su/3.5: 14.3%.
S = Su/3 to lesser of S s Su/2.4 and% Sy: 8.6% for
SA-516, Gr. 70 and 25% for the other steels
included in these calculations.
The fracture mechanics evaluation was done with
the Fracture Graphic® Version 1.1 computer program which incorporates the British PD 6493:1991
failure assessment diagram (FAD) procedures, Levels 2 and 3. 26 Most of the calculations in this section
are based on the Level 2 FAD procedure and the
ASME Section VIII Division 1 and Division 2 notch
toughness requirements.
36
The ASME Section VIII required Charpy energy
values were obtained from Fig. UG-84.1 in Division 1
and the recently revised toughness rules in Division
2, Fig. AM-211. The Code toughness requirements
for high strength steels, such as SA-517 Gr. F and Gr.
E (Su ::::: 95 ksi), must meet at least 15 mils lateral
expansion, or higher, depending on thickness. The
minimum mils lateral expansion (MLE) values were
obtained from the new Fig. AM-211.2 in Division 2.10
Since there are no known Kic - MLE correlations,
CVN values for the SA-517 Gr. F and Gr. E steels
were estimated from Fig. 3 (Fig. A2-1 in WRC
Bulletin 175 14) and the MLE - CVN correlations
34
provided by Dr. W. D. Doty for SA-517 Gr. F. Kic
values for the fracture mechanics calculation were
obtained from the lower bound Kic - CVN correlation Kic = 9.35(CVN) 0·63 , to provide a high degree o
conservatism. 35 •36
The materials, the thicknesses, the ASME Section
VIII notch toughness requirements, and the equiva-
3"
4"
1%"
2"
3"
4"
lY:i'
2"
3"
4"
lW'
2"
3"
4"
Case 4: S = lesser of Su/2.4 and % Sy
5: S = Sy, assuming that in some cases the
prnnary plus secondary stresses could be as high
as the specified yield strength of the material at
local discontinuities.
Cas~
50
0
25
"F
>
~<
:x:
<..,)
25
FOR PLATES. FORGINGS, WELD
METAL AND WELD-HEAT-AFFECTED
ZONES
SA-537, Cl. l
SA-537, CL 2
50
100
150
YIELD STRENGTH IKSll
1%"
2"
3"
4"
1%"
2"
3"
4"
SA-517, Gr. F
lY.,"
SA-517, Gr. E
2"
3"
1.018"
1.212"
1.680''.
1.940"
1.028"
1.451"
2.126"
2.496"
1.129"
1.472"
2.209"
2.601"
0.810"
1.188"
1.946"
0.883"
1.039"
1.429"
1.629"
0.899"
1.265"
1.883"
2.149"
0.857"
1.284"
1.928"
2.246"
0.70"
1.019"
1.467"
0.734"
0.849"
0.759"
1.064"
1.550"
1.775"
0.834"
1.083"
1.624"
1.866"
0.582"
0.840
1.397"
0.656"
0.752"
1.017"
1.122"
0.571"
0.795"
1.152"
1.283"
0.629"
0.812"
1.218"
1.364"
0.427"
0.605"
1.025"
0.287
0.312"
0.408"
0.427"
0.187"
0.256"
0.389"
0.406"
0.180"
0.229"
0.369"
0.382"
0.066"
0.090"
0.169"
Fig. 3-Relationship at 25, 35, 40, 45 and 55 mils lateral expansi
between energy and yield strength. (Fig. A2-1, WRC Bulletin 175).
WRC Bulletin 435
37
Table 14
SA-516, Gr. 70
Yes
Yes
Yes
1=6.50"
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
1%"
2"
3"
4"
SA-537, Cl.1
3"
4"
SA-537, Cl. 2
SA-517, Gr. E & F
SA-517, Gr. E
3"
4"
lW'
2"
3"
4"
1 2.74"*
1 = 2.74"*
1 = 3.45"*
No
Yes
1 = 3.91"*
1 = 5.58"*
1 = 5.58"*
Yes
Yes
Yes
1 = 6.06"*
1 = 1.86"*
1 2.49"*
1 = 4.48"*
1 4.47"*
Yes
Yes
Yes
1 = 7.75"*
1=
No
No
No
1=
1=
1
No
1
1
1=
No
No
No
No
No
2.27"*
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
1.78"*
3.40"*
3.41"*
1.96"*
2.47"*
3.72"*
Note: *This table also lists the calculated critical length 1of the through thickness flaw when t < 1 < 2t.
Table 15
SA-516, Gr. 70
SA-537, Cl.1
SA-537, Cl. 2
SA-517, Gr. E & F
SA-517, Gr. E
lY2"
2"
3"
4"
1%"
2"
3"
4"
lYz"
2"
3"
4"
1\1,i"
2"
3"
4"
15 ft-lbs
15 ft-lbs
18 ft-lbs
18 ft-lbs
15 ft-lbs
19 ft-lbs
22.5 ft-lbs
22.5 ft-lbs
20 ft-lbs
24ft-lbs
30 ft-lbs
30 ft-lbs
15 MLE (=23 ft-lbs)
19.3MLE
19.4 (=29 ft-lbs)
25 MLE (=40 ft-lbs)
25 MLE ( =40 ft-lbs)
63.5 (59.5)
61.7 (59.5)
68.6 (66.7)
67.9 (66.7)
61.4 (59.3)
70.9 (68.8)
78.8 (76.5)
77.8 (76.5)
73.4 (71.0)
82.0 (79.6)
94.6 (91.7)
93.3 (91.6)
78.5 (77.4)
90.5 (89.5)
23.3 (15.5)
19.8 (15.5)
18.7 (15.4)
17.5 (15.4)
19.2 (15.1)
18.6 (15.1)
18.5 (15.0)
17.0 (15.0)
19.0 (15.1)
18.5 (15.0)
18.7 (15.1)
17.1 (14.9)
16.5 (14.5)
16.0 (14.7)
21 (19)
20 (19)
23.5 (22.5)
23.5 (22.5)
20 (19)
25 (24)
29.5 (28)
29 (28)
26.5 (25)
31.5 (30)
39.5 (37.5)
38.5 (37.5)
29.5 (28.5)
37 (36)
111.3 (109.0)
110.7 (109.6)
16.5°(14.8)
15.9 (14.8)
51 (50)
50.5 (50)
Table 16
Material
SA-516, Gr. 70
SA-537, Cl. 1
SA-537, Cl. 2
Thickness,
inches
Sect. VIII, Div. 2
CVN Requirement
Required
Kzc,ksi,jin.
%Increase
inKzc
Equiv. Charpy
Energy, ft-lbs
lYz''
2"
3"
4"
1%"
2"
3"
4"
lYz"
2"
3"
4"
15 ft-lbs
15 ft-lbs
18 ft-lbs
18 ft-lbs
15 ft-lbs
19 ft-lbs
22.5 ft-lbs
22.5 ft-lbs
20 ft-lbs
24 ft-lbs
30 ft-lbs
30 ft-lbs
15MLE
19.3 MLE
25MLE
25MLE
58.4 (56.5)
57.7 (56.4)
64.5 (63.3)
64.2 (63.2)
69.4 (65.6)
80.l (76.0)
89.2 (84.7)
87.6 (84.6)
82.8 (78.4)
92.5 (88.0)
106.6 (101.4)
104.8 (101.2)
87.7 (85.2)
101 (98.5)
124.4 (120.8)
122.4 (120.6)
13.4 (9.7)
12.0 (9.5)
11.6 (9.5)
11.1 (9.3)
34.8 (27.4)
33.9 (27.1)
34.l (27.4)
31.7 (27.2)
34.2 (27.1)
33.7 (27.2)
33.8 (27.2)
31.5 (27.0)
30.1 (26.4)
29.5 (26.3)
30.3 (26.5)
28.2 (26.3)
18.5 (17.5)
18 (17.5)
21.5 (21)
21.5 (21)
24 (22)
30 (28)
36 (33)
35 (33)
32 (29)
38 (35)
47.6 (44)
46.5 (44)
35 (33.5)
44(42)
61 (58)
59.5 (58)
SA-517, Gr. F
SA-517, Gr. E & F
3"
4"
second set of fracture toughness values are listed in
Case 6: S = Su/4 + 0.3 Sy residual stress
parenthesis and were obtained using the ASME Section
Case 7: S = Su/3.5 + 0.3 Sy residual stress
XI, Article A-3000 method for K 1 determination.
Case 8: S Su/3 + 0.3 Sy residual stress
Increase in design stress ·from Suf4 to Suf3.5
Case 9: S Lesser of Su/2.4 + 0.3 Sy and% Sy+ 0.3 Sy
(Table 15): Although this table shows an increase
Case 10: S Su/3 + Sy (for SA-516 and SA-537)
the required K1c with the increase in the design
Case 11:
stresses from Su/4 to Su/3.5, there is extensive
S = Su/3 + 50 ksi residual stress (for SA-516 Gr. 70)
experience with Section VIII, Division 2 vessels
S = Su/3 + 71 ksi residual stress (for SA-517)
(S = Su/3) with essentially the same toughness reSy = specified minimum yield strength (assumed to
quirements as Division 1.
be the same in base metal and in weld metal for
Increase in design stress from Suf3 to lesser of
these calculations).
Su/2.4 and% Sy (Table 16): There is a substantial
Cases 6-9 represent stress relieved welds, and Cases
increase in the required Krc for SA-537 and SA-517
10-11 represent as-welded joints. The critical fl.aw
when the design stress is increased from Su/3 to
sizes in the SA-516 Gr. 70 weld were evaluated with
Su/2.4. There is a lesser increase in the required Krc
a residual welding stress equal to 38 ksi and 50 ksi,
for SA-516 Gr. 70 due to the lower yield strength
for comparison. The Su and Sy values for the SA-516,
(S =%Sy governs).
SA-537 and SA-517 steels are listed in Table 17.
The Level 2 FAD procedure requires somewhat higher
fracture toughness than Article A-3000 because of the
stress ratio (Sr) in the Level 2 failure assessment
diagTam.
Welds. In the as-welded condition the residual
SA-516, Gr 70 All
stresses in some welds can be as high as the yield
SA-537, Cl. 1
ts 2W'
strength of the material. In PWHT structures the
2Yz" < t :5 4"
SA-537,
Cl.
2
t
s
residual stresses generally will not be reduced to
zero. For this evaluation, the residual stresses in the
as-welded joints are assumed to be as high as the
yield strength of the weld for the SA-516 and SA-537
steels and 71 ksi (50 kg/mm2 ) for SA-517.39 In the
Critical depths of surface flaws were evaluated
PWHT condition, the residual stresses in the direcwith the Fracture Graphic® software using the
tion of the weld are assumed to 30% of the yield
Level 2 FAD procedure. The residual stress is asstrength of the weld for all steels. 26,36 The following
sumed to act uniformly through the thickness. The
calculations attempt to evaluate the critical fl.aw
calculated critical flaw sizes (depth a) are given in
sizes and the toughness needed for stress relieved
Table 18 for aspect ratio all 0.15 (l/a 6%).
welds to retain the same margin on brittle fracture
Tables 19 and 20 give the required fracture tough(the same critical fl.aw size) for vessels designed to
ness K 1c values (based on Level 2 FAD approach) for
higher stresses as for vessels designed to the present
stress relieved welds to retain the same critical
Code allowable stresses.
depth of surface flaws (with aspect ratio of all 0.15)
Critical fl.aw sizes transverse to the direction oft}lfe
with increased design stresses as with the present
weld are evaluated for the following cases:
Code design stresses and toughness requirements.
SA-537, Cl.1
SA-537, Cl. 2
surface flaws with an aspect ratio of all = 0.15 if the
design stresses are increased from Su/4 to Su/3.5 and
from Su/3 to the lesser of Su/2.4 and % Sy, to assure
the same critical fl.aw sizes as with the present
Section VIII Division 1 and Division 2 design stresses
38
and toughness requirements. Two sets of values ar
given for the required Kic and the Ch~rpy energy.
The fast set of values designate the required fract
toughness Kic obtained with the Fracture Graphi
computer program and Level 2 FAD procedure. Th
WRC Bulletin 435
SA-517, Gr. F
SA-517, Gr. E
3"
4"
3"
4"
lYz"
2"
3"
4"
0.392
0.422"
0.549"
0.572"
0.304"
0.416"
0.607"
0.635"
0.327"
0.416"
0.633"
0.663"
0.185"
0.251"
Evaluation of Design Margins, Section VIII
0.183
0.185
0.233
0.234
0.116
0.157
0.202
0.202
0.120
0.151
0.211
0.211
0.095
0.128
39
From Su/4 + 0.3 Sy to Su/3.5 + 0.3 Sy (Table 19):
Table 19
SA-516, Gr. 70
lY2
SA-537, Cl.1
2
3
4
1%
2
3
4
SA-537, Cl. 2
SA-517, Gr. F
SA-517, Gr. E
3
4
1%
2
3
4
57.2
57.0
63.9
63.8
56
65.1
72.5
72.4
67
75
86.6
86.6
72.6
84
102.9
102.9
11.1
10.7
10.6
10.4
8.7
8.9
9.0
8.9
8.6
8.4
8.7
8.7
7.7
7.7
7.7
7.7
From Su/3 + 0.3 Sy to lesser of Su/2.4 + 0.3 Sy
and% Sy + 0.3 Sy (Table 20):
Table20
SA-516, Gr. 70
l\;2
SA-537, Cl.1
2
3
4
1%
2
3
4
SA-537, Cl. 2
lY2
SA-517, Gr. F
2
3
4
1%
2
3
4
SA-517, Gr. E
55.7
55.5
62.2
62.1
61.3
71.1
79.4
79.1
73
82
94.6
94.3
78.5
90.8
111.5
111.3
8.2
7.8
7.6
7.4
19
18.9
19.4
18.9
18.3
18.5
18.7
18.3
16.5
16.4
16.8
16.5
These calculations give an indication of critical
fl.aw sizes with various stress levels and the percentage increase in the toughness requirements with
increased design stresses to maintain the present
margins against brittle fracture. There is a somewhat lesser percent increase in the toughness requirements for welded joints than for the base metal to
provide the same margin against brittle fracture.
This is because the percent increase in the total
stress is less in welded joints. However, the critical
fl.aw sizes in welded joints are smaller than those in
base metal (when subject to design membrane
stresses only) due to the additional welding residual
stresses in welded joints.
A decrease in the design margin requires a careful
evaluation of notch toughness and NDE requirements to insure that there is adequate margin
40
against brittle fracture. Additional toughness and/or
NDE requirements need to be imposed when the
critical fl.aw sizes are smaller than the Code acceptance criteria (such as the maximum allowable defect sizes for RT examination in UW-51 of Section
VIII, Division 1 and in Article I-5 of Division 2, the
UT acceptance criteria in Code Case 2235, etc.) or
where the combination of the applied stresses and
the available toughness result in very small critical
fl.aw sizes.
A significant amount of satisfactory experience
exists with vessels designed to Section VIII, Division
1 and Division 2 requirements having essentially the
same toughness requirements, therefore, an increase in the toughness requirements due to increase in the design stress from Su/4 to Su/3.5 does
not appear to be warranted. Additional consideration, however, should be given to the high strength
quenched and tempered steels in Division 1, Part
UHT and Division 2, TableAQT-1, regardless of the
design stresses. These steels have high yield strength/
tensile strength ratios and high residual welding
stresses in the as-welded and in the stress relieved
condition. Both Division 1 and Division 2 already
require essentially 100% volumetric and 100% SUI\
face examination of welds in the high strength
quenched and tempered steels. Although there is
good past performance record with the high strength
steel pressure vessels designed to Division 1 and
Division 2 requirements, additional consideration
should be given to thicker plates (over about %" in
as-welded condition), regardless of the design stress
and for all thicknesses when the design stresses ar
increased over Su/3.
d. Yieldfl'ensile Strength Ratios
Steels with different yield-to-tensile strength ra
tios have different values of the slope of the stress~
strain curve in the inelastic range, local elongation.
(elongation in the necking region) and uniform elon?
gation (total elongation less local elongation). lri
tension, both the local elongation and uniform elon"
gation are important. Yield/tensile strength ratios.
typically increase with increasing strength of steels
High yield/tensile strength ratios limit the amoun
of strain that can be safely tolerated at discontinui
ties in pressure vessels of high strength steels (Par
UHT in Section VIII, Division 1 and Article F -6 ·
Division 2). A significant experience already exist
with pressure vessels constructed of high strengt
steels (e.g. SA-517, SA-543). ASME Section VIII
Divisions 1 and 2 already incorporate the necessar
requirements for design, fabrication and NDE t
insure a safe design.
The following discussion is based mainly on th
report by R. L. Brockenbrough & Associates. 30 Thi.
report reviews the studies in Japan and in USA o
the effect of yield/tensile strength ratios on stru
tural behavior of high performance steels for bridg
construction. The following is a brief summary o..
WRC Bulletin 435
some of the effects of yield/tensile strength ratios
discussed in this report, which are significant to
pressure vessels.
Tension and Bending. Fig. 4 (Fig. 1.1 in Ref. 30)
shows a reduced section of a tensile specimen (to
simulate a bolted splice) and a typical stress-strain
curve, where cry = yield strength, cru
tensile
strength, Ey = yield strain, Eu = strain at tensile
strength, and Est = strain at initial strain hardening.
The elongation of the member at ultimate load
depends on the yield/tensile strength ratio. If the
member is subjected to increasing tensile loads and
the ratio is 1.0, the length of the member that yields
approaches zero and the total elongation is limited.
At smaller yield/tensile strength ratios a longer zone
is able to reach yield strength while the minimum
section is reaching the tensile strength, and thus the
total elongation before rupture increases. Ref. 30
gives the following expression, developed by Kato,
for maximum elongation omax in a tensile test specimen with length 2L:
20
15
e
.§
(.,:)
(.,:)
....
G
CJ
>
0
c
IO
0
~~
g
w
G
GG
= 200mm
I
er-a;.
·-~\
8\
\,
•SS41
xSM50
oSM50Y
OSM58
_J
t= 10 mm
r~ ~: 1
0
0.2
0.4
0.6
0
0.8
l.O
Fig ..5-Effe~t of yie~d/tensile strength ratio on elongation capacity of
tension specimen with a reduced section
Eu Est fX A
Ey .LL A
+---Jn
dx +-dx
0
1
0
Yo
}
YxAx
Ao= area of the cross-section at mid-length, Ax= the
cross-section area at point X which defines the limit
of yielding (Axcry = Aocru or Axcry = Ao/Y, where
Y = yield/tensile strength ratio).
Figure 5 (Fig. 1.2 in Ref. 30) compares the above
equation for a plate with a reduced section with test
data for a 10 mm plate from four steels with yield/
tensile strength ratios from about 0.65 to 0.85. This
figure shows the trend of decreasing elongation with
increasing yield/tensile strength ratios, particularly
at yield/tensile strength ratios over about 0.85.
According to Kulak, 30 in a reduced section,
x
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_ _ _ _:_
(1
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Fig: ~:-Tension specimen with reduced section, and stress-strain
def1nit1on
behavior of a member depends on An/Ag and cry/cru. In
some cases it may be desirable to have gross section
yielding before fracture in the net section, to permit
distortion before failure. This requirement would be
satisfied theoretically with the following relationship: An/Ag::::: cry/cru. (If the Ancru < Agcry, the member
deformation is limited).
An = net cross-section area
Ag = gross cross-section area of plate or tensile test
specimen.
cry= yield strength of the material.
cru =tensile strength of the material.
The above relationship, however, is not always practical since the actual properties, instead of the
specified minimum values need to be considered.
Also, the tensile stress through the net section at
fracture is greater than the tensile strength of the
material. Consequently, special details should be
employed to prevent net section fracture before gross
section yieik!ling in materials with high yield/tensile
strength ratios.
Fig. 6 (Fig. 1.13 in Ref. 30) shows the relation
between yield/tensile strength ratio and uniform
elongation. Total elongation in a tensile specimen
consists of two parts, local elongation in the regions
that necks and fractures, and uniform elongation in
the remainder of the specimen. Uniform elongation
is related to the yield/tensile strength ratio, tending
to decrease with increasing yield/tensile strength
ratios. According to Otani, 30 the most effective way to
increase deformation capacity is to:
Evaluation of Design Margins, Section VIII
41
30
~,.,=Q.6(1-YRI
0
25
~
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20
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co
15
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0
0
f
c:
0
ooo
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00
co
0
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oo
0
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70
60
80
90
100
Yield ratio YR (O!o)
Fig. 6-Relation between yield/tensile strength ratio and uniform
elongation
• Decrease the yield/tensile strength ratio for
steels that have over about 10% uniform elongation,
• Increase the uniform elongation in other cases.
Winter 30
Dhalla and
showed that the most important
factor is the local ductility and that an elongation of
20% in a Vz" gage length (including the necking
region) is sufficient to insure ductile behavior. Other
researchers have shown that the elongation capacity
increases with decreasing yield strength/tensile
strength ratio.
The yield/tensile strength ratio also has a significant effect on the rotational capacity of structural
members with a moment gradient. The rotation
capacity decreases rapidly with increasing yield/
tensile strength ratio, and decreases gradually with
increasing yield point. If the yield strength is considerably less than tensile strength, the plastic region
in a beam can extend over some length of the beam
as bending moment at a critical section increases
above a plastic moment by strain hardening. The
spread of the plastic region contributes greatly to
rotational capacity. If, however, the yield strength/
tensile strength ratio is 1.0, there can be no extension of the plastic region because the tension flange
can rapidly reach its ultimate strain and rupture as
the plastic moment is reached. Thus the inelastic
rotational capacity of a structural member with a
moment gradient approaches zero as the yield/
tensile strength ratio approaches 1.0.
Pressure Vessels. The PVRC sponsored research
at University ofKansas included burst testing oftest
vessels made of three types of materials with different strain hardening components. 19 •20 The materials
were SA-240 Type 304 stainless steel, SA-516 Grade
70 carbon steel and SA-517 Grade F quenched and
tempered steel. These steels have strain hardening
exponents of 0.585, 0.189, and 0.085, respectively.
Burst tests were performed on test vessels with
nozzles and fl.at heads and on test vessels with sharp
42
notches. These studies included the effects of stain
hardening and the yield strength/tensile strength
ratio on the bursting strength of the test vessels.
These tests confirmed the validity of the modified
Svensson eguation (PB= au Fey! ln W) for predicting
burst pressure based on plastic instability and
showed that, ignoring the effects of strain concentrations, pressure vessels of higher strength steels tend
to burst at a higher percentage of their tensile
strength than do lower strength steels.
One set of tests at University of Kansas included
tests of vessels with severe strain concentrations.
The effect of a sharp undetected fl.aw was simulated
with a notch 15" long in the axial direction of the
vessel with a notch root radius of0.001" (0.025, mm).
One set of vessels of each type of steel was tested
with a notch depth of 0.18" (approximately 35% of
wall thickness). Three additional A517 vessels were
tested with a notch depth of 0.075", 0.125" and 0.18"
(15%, 25% and 35% of the wall thickness). The report
by Royer and Rolfe 20 also included the results of
several sets of burst tests by others on test vessels of
HY-140 steel with notches up to 40% of the wall
thiclmess.
The results of all the vessel tests with notches
reviewed by Rolfe and Royer are included Fig. 7,
which plots the burst pressure ratio PAIPB vs. notch
depth in percent of wall thickness. (PA = actual
burst pressure, PB = theoretical burst pressure based
on plastic instability). The results of these tests led
the investigators to believe that the percent reduction in burst pressure from the Svensson prediction
is directly proportional to notch depth as percent of
wall thiclmess.
For the A 517 steel the reduction in burst pressure
exceeded the reduction in wall thiclmess for the 35%
notch depth. At some point beyond a notch depth of
25%, the burst failure could no longer be predicted
accurately from the Svenssen equation by assuming
reduction in burst pressure as a percentage of the
wall thickness. For notch depth greater than about
25% of wall thiclmess the reduction in burst pressure
in high strength steels (with high yield/tensile
strength ratios) appears to be dependent upon ductil-
ity, toughness, and possibly the strength-to-toughness ratio of the steel.
Langer 17 •30 derived expressions for strain concentration factors (K.) in a tapered, axially loaded bar
and at the end of a cantilevered beam in terms of the
strain hardening exponent, n. The strain concentration was defined as the actual peak strain divided by
peak strain calculated for completely elastic behavior, based on the assumption that the maximum
deflections are the same. Fig. 8 shows that the strain
1-t
~!---·-_-_-
y
(b) Cantilever beam.
(a) Tapered bar.
4o.--~~~~~~~~~~~~~~~~--.
30
c 304SS
• A516
A517 - lst Heat
• A517 - 2nd Heat
20
111
o HY-lMXTl
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8
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al
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0 I 0 2 0 3 0 4 0 5 0.6
0.7 0 8 0.9
1.0
n
80
60
NOTCH DEPTH. '1o of WALL THICKNESS
20 25
Fig. ?-Effect of notch depth on burst pressure
WRC Bulletin 435
100
(c) Strain concentration factors.
Fig. 8-Relation of strain hardening exponent n to strain concentration factors K,
Evaluation of Design Margins, Section VIII
43
concentration increases as the strain hardening exponent n decreases in axially loaded bar with tapered
notch and in cantilever beam under bending stresses.
Material with low strain-hardening exponent, such
as high strength steel with high yield/tensile strength
ratio, will tend to have a higher strain concentration
in the presence of a notch. In steels with higher
strain hardening exponents, the material surrounding the notch tends to control the strain in the plastic
zone.
Fatigue. The report by R. L. Brockenbrough30
includes a summary on large scale welded HSLA-80
steel fabricated sections for ship construction. Details included longitudinal fillet welds, transverse
groove welds, bulkhead attachment details, etc. The
lower confidence limits of the S-N curves were not
significantly affected by mean stress, and were not
significantly different than those for similar weld
details in structural steels. It was, therefore, concluded that fatigue strength of welded details in air
can be considered to be independent of the strength
and type of steel.
e. Fabrication and NDE
Although difficult to quantify the Code requirements for fabrication and NDE with respect to
margin on safety, they are essential to providing the
necessary design margin in the structure. Design
details, fabrication procedures and NDE become
even more important with lower design margins.
Parts UCS and UHT in Section VIII, Division 1 and
Part AF in Section VIII, Division 2 include specific
requirements to minimize the risk of degradation of
materials properties during fabrication, limitations
on fabrication defects, etc., some of which are discussed below.
Forming. Excessive forming strains (without subsequent heat treatment) can degrade strength and/or
toughness of steels. Section VIII, Division 1 includes
the following limitations on the extreme fiber elongation during cold forming without subsequent PWHT:
a) UCS-79 limits the maximum elongation to 5%
when:
the vessel contains lethal substances
the material requires impact testing
thickness before forming exceeds %"
the reduction of thickness by cold forming is
more than 10%
5) the temperature of the material during
forming is in the range of250°F to 900°F.
1)
2)
3)
4)
The extreme fiber elongation can be as great as 40%
for P-No. 1Group1 and 2 materials if none of the 5
conditions above exist.
b) UHT-79 (high strength quenched and tempered
steels) limits the maximum elongation to 5% in
all cases.
44
Division 2 has the following requirements:
a) AF-111. All materials for shells sections and
heads shall be fored to the required shape by
any process that will not unduly impair the
mechanical properties of the material. However, no testing is required after forming.
b) AF-605 limits the maximum forming strain to
5% max. for the quenched and tempered steels
in TableAF-630.1.
Some non-USA pressure vessel codes have addi.:
tional requirements. BS 5500 requires stress reliev-.
ing of all cold formed heads unless the manufacturer
demonstrates that the cold formed properties are
adequate and the material properties are not significantly altered. (This is because substantial forming
strains may exist in certain dished and flanged
heads which cannot be calculated by the formulas
given for bending strains). For cold formed cylindrical parts, BS 5500 restricts the minimum inside
radius to 10 t for C and C-Mn steels and to 18 t for
other ferritic steels.
Tolerances. The out-of-roundness tolerances are
similar in ASME Section VIII and in some of the
European pressure vessel codes. Specific require~
ments for maximum deviation from a circular form
are also provided in both Section VIII, Division 1 an
Division 2.
Section VIII, Division 1, UG-79(b) requires th~
adjoining edges oflongitudinaljoints to be shaped to
proper curvature to avoid objectionable flat spo
along the completed joints. Both Division 1 an
Division 2 include specific limitations on the maxi2
mum offset in welded joints. Neither Division 1 nor
Division 2 include specific requirements for maxi~
mum peaking at welded joints, whereas detaileg
requirements are included in the draft of the CE~
pressure vessel standard. Local deviations (e.g. a
longitudinal joints in cylinders, cones, dished head
spherical shells) can increase local stresses whic
may need further evaluation under cyclic loadin
conditions, particularly when considerations is give
to increased allowable design stresses.
NDE. ASME Section VIII, Division 1 permit
either 100% RT, spot RT or no RT examined vessel
The joint efficiencies are assigned accordingly, i.
100% for full RT, 85% for spot RT of double bu
welded joints, etc., depending on the type of welde
joint (Table UW-12). No specific examination requir
ments are given in Section VIII, Division 1 for MT o
PT examination of nozzle welds for C, C-Mn, or lo
alloy steel materials in Part UCS.
Higher design stresses necessitate additional r
quirements for NDE of welded joints. This is r
fleeted in ASME Section VIII, Division 2 requir
ments, where essentially all welded joints must b
non-destructively examined. Division 2 requir
100% RT of all butt welds and 100% surface examin
tion of all nozzle attachment and other attachme
welds. Additional MT or PT examination requir
WRC Bulletin 435
1nents are specified in Division 2 for radially disposed cut edges in vessel walls for nozzles to be
attached by fillet welds, partial penetration welds,
temporary attachment weld locations, as well as PT
examination of austenitic chromium nickel steel
welds exceeding%" thickness.
Fracture mechanics calculations show that critical
fl.aw sizes decrease as the design stresses, yield
strength and thickness increase. The calculations in
Section E.2.c of this report resulted in relatively
small critical flaw sizes for the high strength
quenched and tempered steel SA-517 in the aswelded condition. Stress relieving of welds in the
high strength Q & T steels is done at somewhat
lower temperatures than other carbon and low alloy
steels, and significant residual stresses may remain
in welded joints of these steels after PWHT. (See
Table UHT-56 in Section VIII, Division 1 and Table
AF-630.1 in Division 2 for PWHT requirements.) The
Section VIII toughness requirements and/or NDE
requirements, therefore, need to be re-examined for
these steels when consideration is being given to
reducing the present design margins. (Several European pressure vessel codes require additional surface examination of welded joints, including butt
welds, with MT or PT methods for the higher strength
and more crack sensitive materials and for higher
risk category vessels.)
While ASME Section VIII, Division 2 requires
essentially 100% examination of nozzle attachment
welds, Division 1 vessels presently have no specific
requirements for nondestructive examination of
nozzle welds. NDE requirements for nozzle welds
need to be included also in Division 1 when consideration is given to higher allowable design stresses.
Welded Joints. Various types ofweldedjoints are
permitted in the Code for Division 1 vessels. Only
Type 1 and Type 2 (for Category B joints only) are
permitted for butt joints in Division 2 vessels, and
only Type 1 butt joints are permitted for the high
strength materials in TableAQT-1. Type I butt joints
are double welded full penetration shell joints, and
Type 2 are single welded butt joints with backing
strips which remain in place.
British Standard 5500 also permits single-welded
(with or without backing strips) or double-welded
butt joints for main seams in vessels; however,
single-welded butt welded joints are limited to 20
mm thickness, or less, depending on joint detail.
Also, permanent backing strips must not be used for
Construction Category 1 and 2. 5 This should be an
additional consideration for Division 2 vessels subject to higher design stresses.
PWHT. ASME Section VIII, Divisions 1 and 2
require PWHT of welded joints for thicknesses exceeding Viz" in carbon and C-Mn steel vessels provided welded joints over 1 Yi" are preheated to 200°F.
European pressure vessel codes generally require
PWHT at lesser thicknesses (30 mm in the ADMerkblatt Code and 35 mm in BS 5500 and in the
Draft CEN Standard), but most European pressure
vessel codes also have provisions for increasing the
thickness limits for PWHT based on additional toughness, fracture mechanics analysis and NDE. (Although the benefits of PWHT are well known, the
authors do not think it necessary to have more
restrictive PWHT requirements in Section VIII as a
justification for higher allowable design stresses).
3. Review of Vessel Failures
A general discussion on pressure vessel failures
was included in the Phase 1 Report of this project. An
attempt was made by the authors to gather further
information on pressure vessel failures related to
specific design codes (ASME Section VIII, Divisions
1 and 2, BS 5500 and AD-Merkblatt). Although
published literature contains numerous articles on
pressure vessel and piping failures, 22 no comprehensive summary exists on failures grouped with respect to different codes or standards, nor vessels
designed to different allowable design stresses.
The information available to the authors indicate
that pressure vessel failures generally fall into the
following categories:
1. Failures from inadequate design or poor design
details for the specified design conditions (e.g.
fatigue).
2. Failures due to poor notch toughness of materials, fabrication defects, welded repairs, etc.).
3. Failures caused by service related degradation
of materials and cracking (corrosion, stress
corrosion cracking, etc.).
4. Process or operation related failures of pressure vessels (overheating, explosions, etc.)
5. Failures from a combination of the causes listed
above.
The majority of failures can be attributed to poor
design details or improper fabrication practices for
the materials used, poor notch toughness (particularly during overload testing), service related degradation, and operation related problems. The incidence of catastrophic failures due to design faults is
low. No failures are known to the authors which can
be attributed directly to the design rules in the
recent editions of the codes and standards reviewed
as part of this report.
4. Summary and Conclusions
The authors have reviewed in Section E of this
report the various failure modes considered inASME
Section VIII, Division 2 and certain factors which
influence the design margins. Factors, such as design details, yield-to-tensile strength ratios, ductility, fabrication and non-destructive examination are
difficult to quantify with respect to margins of safety,
and the authors have provided a general discussion
on the significance of these factors.
In absence of other failure modes (such as fatigue
Evaluation of Design Margins, Section VIII
45
and brittle fracture), the design of Section VIII,
Division 2 vessels is generally governed by the
bursting mode of failure when the yield-to-tensile
strength ratio is more than 0.5. The vessel tests at
University of Kansas on test vessels of materials
with three different strain hardening exponents
showed that stainless steel vessels (which have a
high strain hardening exponent) designed to Division 2 stresses can have a true margin of safety less
than 3.0 against the bursting mode of failure. The A
517 vessels with moderate strain concentrations had a
true margin of safety over 3.0 against this failure mode.
Fracture mechanics calculations were performed
on several materials using the BS PD 6493:1991
Level 2 FAD procedure to evaluate critical flaw sizes
based on the present Code design stresses and
toughness requirements and for higher design
stresses (S = Su/3.5, and the lesser of % Sy and
Su/2.4) with the present Code toughness requirements. Two sets of calculations were performed to
evaluate the toughness needed to insure the same
margin of safety on brittle fracture with the increased design stresses. One set was based on the
present Code allowable stresses and with increased
allowable stresses in the base metal. Another set was
performed on welded joints assuming that the remaining residual stresses after PWHT are about
30% of yield strength of the weld. The first set of
calculations resulted in 16-23.3% increase in the
required fracture toughness (Krc) for increasing the
design stress from Su/4 to Su/3.5 for all the steels and
thicknesses considered in this evaluation. The increase in Krc was 26.3-27.4% for SA-537 and SA-517
for increasing the design stress from Su/3 to the
lesser of Su/2.4 and% Sy and 9.3-9.7% for SA 516,
Gr. 70 (because of the lower yield strength of this
material). The second set of calculations resulted in
a somewhat lesser increase in Krc for the welded
joints since the relative increase in the total stresses
(primary plus secondary stresses) was lower.
No specific increase in the required fracture toughness was recommended by the authors for increasing
the design stresses from Su/4 to Su/3.5 due to the
extensive successful experience with ASME Section
VIII, Division 2 vessels which have essentially the
same toughness requirements as Division 1 vessels.
However, a further review of the Code requirements
is suggested for the high strength Q & T steels in
Table UHT-23 of Division 1 and Table AQT-1 of
Division 2. Most of these steels have a high yield
strength-to-tensile strength ratios, less capacity for
strain concentrations at notches and discontinuities
than other materials and can have high residual
stresses in welded joints. Therefore, these materials
need additional considerations for notch toughness
and/or NDE, regardless of design stresses.
Other factors discussed in Section E which can
have an effect on the margin of safety are cold
forming strains, fabrication tolerances, non-destructive examination, welded joint details and PWHT.
46
All these factors need to be considered when increasing the allowable design stresses, and additional
Code requirements need to be imposed (with the
exception of PWHT) to insure that the present safety
margins are not jeopardized.
F. Comparison of ASME Section VIII,
Divisions 1 and 2 with Other Codes and
Standards
This part of the Phase 2 study on margins of safety
in ASME Code, Section VIII includes a review
several international pressure vessel codes and standards, which use lower design margins than ASME
Code, Section VIII, Divisions 1 and 2, and comparison of some of the essential requirements in these
international codes and standards with those in
ASME Section VIII, Divisions 1 and 2. The true
margin of safety depends on all of the essential
requirements in a particular code or standard: the
basic design rules, design details, materials and
toughness requirements, ductility, fabrication requirements, NDE, overload testing, overpressure
protection, quality control, and a number of other
factors. It is not possible to include all considerations
affecting the safety margins in this study, therefore
only those factors deemed more significant to the
safety of pressure vessels are addressed. This study
addresses the following codes and standards:
1.
2.
3.
4.
ASME Section VIII, Division 1
ASME Section VIII, Division 2
British Standard 5500
The German Pressure Vessel Code (AD Merkblatt Code)
5. The Netherlands Rules for Pressure
(The Stoomwezen Code)
6. CEN Draft Standard for Unfired Pressure
sels.
This part of the Phase 2 study compares those
requirements in these codes and standards which
have a significant effect on margins of safety of
welded pressure vessels. The following Code requirements are reviewed and compared in this report:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Design margins
Design rules
Design details
Service considerations
Materials requirements
Toughness requirements
Fabrication requirements
Overpressure testing
Quality control requirements
A more detailed summary of each of the non-USA
codes and standards listed above is included in
Appendices 1 through 4 of the Final Report, PVRC
Project No. 97-2.40
The codes and standards used in this report were
the latest available to the authors at the time of the
report and may not all be the latest editions issued.
WRC Bulletin 435
1. Design Margins
a. General
The design margins, although not a sole measure
of the safety of a pressure vessel, are the most
significant factor in evaluating comparative safety.
Design margins, reflecting an "ignorance factor"
have historically been reduced as the understanding
of behavior, experience base, and methods of design
and analysis have improved. The design margins on
yield strength have universally remained at 1.5
(except for austenitic stainless steels and other
strain hardening materials, lower factors have been
used). The definition of yield strength varies somewhat from one code to another, but they are not
significantly different.
The margin on ultimate tensile strength, however,
has changed over the years and varies a great deal
from one code to another. This margin for the ASME
Code, a pioneer code for pressure vessel construction, was 5 for many years. In 1940's the confidence
with these rules and the urgent needs of the period
were the justification for a reduction of this factor to
4. With the advent of design by analysis, most codes
in the 1960's required more analysis and quality
control but reduced this design margin to 3. One
such code was Division 2 of ASME Section VIII.
Another ASME Code using a design margin of 3 on
ultimate tensile strength is the B31.3 piping code.
B31.3 has about 50 years of satisfactory experience
with the use of that code.
British Standard BS 5500 allows a design margin
of 2.35 on the specified minimum ultimate tensile
strength for Construction Category 1 and 2 vessels. 5
This factor, compared to that of ASME codes is even
more liberal than it appears. While the design
margin on yield strength is applied to the value at
design temperature, the factor of 2.35 is applied to
ultimate strength at room temperature, regardless
of design temperature. (This factor for austenitjc
stainless steels is 2.5.). For Category 3 vessels {if
carbon steel this factor is 5. However, this factor
reflects the joint efficiency for these vessels which get
no volumetric examination of the seams. This fact
will make this factor more liberal than Section VIII,
Division 1 (VIII-1), for joints with no radiographic
examination (RT). For Construction Category 3 austenitic steel vessels, there is a specified value to be
used, regardless of the type of austenitic steel. One
factor which slightly contributes to design margins
of BS 5500 is the fact that material thickness
undertolerance is to be accounted for, whereas such
undertolerances are disregarded in the ASME Codes
and assumed to be covered by design margins.
Another major difference is the fact that BS-5500
provides a great deal of flexibility in rules, even as
compared with Section VIII, Division 2 (VIII-2).
Overall, the design margins on ultimate tensile
strength in BS-5500 are considerably more liberal
than those ofVIII-1 and VIII-2.
In the German AD-Merkblatt (ADM) pressure
vessel code the design margin of 1.5 on yield strength
is generally applicable to rolled and forged steels. 6
Factors on ultimate tensile strength are specified
only for cast steels. Another factor contributing to
these rules being liberal is the fact that the stresses
due to dead loads, supports, wind and snow do not
have to be accounted for, as long as these stresses do
not add up to more than 5% of the material stress
allowable. Overall, the design margins of this code
are considerably more liberal than those of VIII-1
and VIII-2, for those materials with stress allowables based on ultimate tensile strength in the
ASMECodes.
The Dutch Stoomwezen pressure vessel code accounts for the ductility of materials in establishing
design margins. 7 For materials with greater than
10% elongation, the design factor on ultimate tensile
strength is 2.27, and with avoiding details that
result in high peak stresses, this factor may be
reduced to 2.0. These factors are applied to ultimate
tensile strength at room temperature, regardless of
design temperature. However, for materials with
elongations of 10% or less, a factor of 4.0 applied to
ultimate tensile strength at design temperature is
specified. This code also requires that material undertolerances be accounted for. For ferritic steels with
high yield strength or yield to tensile ratios greater
than 0.67, certain requirements on details and fatigue exemption calculations are applicable. A feature of this code is the fact that allowable stresses
are modified for certain components. A direct comparison with ASME Codes is not practicable. Overall the
design margins on ultimate tensile strength of this
code are considerably more liberal than those of
VIII-1 and VIII-2, for ductile materials. For a few
materials with low ductility, these rules may not be
more liberal.
In the draft of proposed CEN unfired pressure
vessel standard, the basic ultimate tensile strength
design factor for ferritic steels is 2.4. 8 However, this
factor may be reduced to 1.875 for the Alternative
Design Basis, which requires certain controls on
details, examination, and quality control. (Alternative Design Basis was included in the July 1995 draft
of the CEN/TC 54 Standard for Unfired Pressure
Vessels but not in the May 1998 draft). Austenitic
steels with elongation less than 30% are treated the
same as ferritic materials. For austenitic steels with
30% or
elongation, there is no limit based on
ultimate tensile strength. It should be pointed out
that the rules of this standard are applicable only to
those materials with a minimum of 14% elongation
and a minimum impact value of 20 ft-lb at 20°C
(68°F). In this standard, the material specification
undertolerances are to be accounted for. A unique
feature of this standard is the fact that stress
allowable tables are not provided. Another unique
feature is that for low probability "exceptional design conditions" lower design margins may be used
by agreement. In general, this proposed standard
Evaluation of Design Margins, Section VIII
47
contains the most recent requirements in Europe but
also the most liberal design margins.
In the creep range, the allowable stresses in the
various documents are based on creep and stress
rupture.
Section VIII, Division 2 does not allow design of
vessels temperatures in the creep range. For Section
VIII-1, the stress. allowable stresses in the creep
range are based on the lowest of the following:
• 100% of the average stress to produce a creep
rate of0.01% per 1000 hours.
• 67% of the average stress to cause rupture at
the end of 100,000 hours.
• 80% of the minimum stress to cause rupture at
the end of 100,000 hours.
For welded pipe or tube, the allowable stress is 85%
of that calculated on the above basis (structural quality
ferrous materials are not allowed in the creep range).
In the BS-5500, for temperatures in the creep
range, the allowable stress is based on the mean
value of the stress required to produce rupture in a
specified time divided by a factor of 1.3. This is about
77% of the mean value compared with 67% of
average value for VIII-1. No limit on creep is specified. This is generally more liberal than rules of
VIII-1, ifthe design life is 100,000 hours. For certain
materials, BS-5500 allows design life ofup to 250,000
hours with the stress basis being the same. For an
VIII-1 vessel that may operate up to 250,000 hours
(VIII-1 does not limit design life), the rules of VIII-1
would be very non-conservative relative to BS-5500.
If the rules ofVIII-1 are made more liberal, a limit on
life of vessels operating in the creep range should be
considered.
In the ADM Code, the stress allowable in the creep
range is lowest of the following:
• The average stress to cause rupture in 100,000
hours
• The minimum yield strength
• 1% strain based on proof test
The rupture criteria is more liberal than that of
VIII-1. The other two factors are unique to this code
and are not expected to control the allowable very
often. A direct comparison with criteria of VIII-1 is
difficult.
In the Stoomwezen code, the allowables stresses in
the creep range are based on the lower of average
stress which causes 1% creep in 100,000 hours or
stress which produces rupture in 100,000 hours.
This appears the same as the creep criteria ofVIII-1,
but is in reality more conservative. The rate of creep
increases with time so that .01%in1000 hours results in
more than 1% in 100,000 hours. The stress rupture
criteria of Stoomwezen is, however, more liberal.
In the draft CEN pressure vessel standard no
basis for stress allowables in the creep range has yet ·
been specified.
48
The following is a brief summary of the bases for
design margins at temperatures below the creep
range, as listed in each of the codes and standards
addressed in this report:
The allowable stresses for bolting materials are
the lower of Yi Su or% Sy, except for quenched and
tempered bolts (Section II, Part D, 2-120 ofAppendix 2).
c. ASME, Section VIII, Division 2
b. ASME, Section VIII, Division 1
The basis for establishing allowable design stress
values are summarized in ASME Section II, Part D,
Appendix 1. At temperatures below the creep range,
the maximum allowable stress value S is the lowest
of the following:
•
•
•
•
Su/4 at room temperature
Su/4 at temperature
2Sy/3 at room temperature
2Sy/3 at temperature
Sy = specified minimum yield strength
Su = specified minimum tensile strength
For austenitic steels and for certain non-ferrous
alloys, two sets of allowable stress values are provided. The alternative allowable stress values exceed the 2Sy/3, but do not exceed the 90% of Sy at
temperature, and are not recommended for use
where higher deformations may be objectionable.
In the application of these criteria, the yield
strength at temperature is considered to be the
specified minimum yield strength at room temperature multiplied by the ratio of the average temperature dependent trend curve value of yield strength to
the room temperature yield strength. The tensile
strength at temperature is considered to be 1.1 times
the specified minimum tensile strength at room
temperature multiplied by the ratio of the average
temperature dependent trend curve value of tensile
strength to the room temperature tensile strength.
It should be noted that elevated temperature
tensile testing of each heat of material used in
Section VIII construction is not required. (Such
testing is required by most European Codes.) It
should also be noted that the trend curves are
derived from ratios of elevated temperature tensile
test data to room temperature tensile test data for
each heat of material (based on average properties).
The curves are then the statistical best fits of these
ratio curves and anchored to the minimum specified
properties to establish the tensile and yield strength
values to which the allowable stress factors are
applied. The elevated temperature tensile and yield
strength values thus established are not the minimums, therefore, there is about 50% probability that
the actual elevated temperature tensile and yield
strength values might be lower than the values used
to establish the allowable stresses. This fact takes
away some of the apparent conservatism indicated
by the factors used.
The allowable stresses for structural materials
used for pressure parts, bars and shapes used for
shell stiffeners, or stay bolts are multiplied by a
quality factor of 0.92.
WRC Bulletin 435
Section VIII, Division 2 uses the Tresca, or maximum shear stress criteria to establish the allowable
stress intensities. The stress intensity is the maximum allowable value of the stress difference. The
following are the basic stress intensity, Sm limits in
Division 2:
Combination of Stresses
General primary membrane
Local primary membrane
(PL)
Primary membrane +
bending (PL + Pb)
Primary + secondary
(PL+ Pb+ Q)
Stress intensity,
Sm limits
k Sm :s k(2/3 Sy,
or Su/3)
1.5 kSm
1.5 kSm
(a) Material with specified elevated temperature
strength values:
(1) Up to and including 50°C (122°F):
fE = Rjl.5 or Rm/2.35, whichever is lower
Rm
specified minimum tensile strength
at room temperature
Re specified minimum yield strength at
room temperature
(2) 150°C (302°F) and above:
fE RecT/1.5 or Rm/2.35, whichever is lower
Re(Tl = 0.2% yield strength (Rpo. 2 ) for carbon, C-Mn and low alloy steels (l.0% yield
strength for austenitic stainless steels)
specified for the material at temperature T
(tested in accordance with BS 3688).
(3) Between 50°C and 150°C:
fE is based on interpolation between the
values at 50°C and 150°C.
(b) Material without specific elevated temperature strength values:
3Sm
The following is a summary of design margins for the
various load combinations (See Section II, Part D,
Appendix 2 for detailed explanation):
• Su/Pm :5 3/k
• Su/PL :s 2/k
• Su/(PL + Pb) :s 2/k
(1) Up to and including 50°C:
fE Rjl.5 or Rm/2.35, whichever is lower
(2) 150°C and above:
fE = Rjl.6 or Rm/2.35, whichever is lower
(3) Between 50°C and 150°C:
fE is based on interpolation between the
values at the two temperatures.
Austenitic Stainless Steels:
Su specified minimum tensile strength
Sy = specified minimum yield strength
Pm = General primary membrane stress
PL = Local primary membrane stress
Pb = Primary bending stress
k = stress intensity factor for various loading conditions
The following table lists the stress intensity k factors
from Section VIII, Division 2, Table AD-150.1 flr
various load combinations:
Load Combination
Design pressure + dead load (DL)
Design pressure + DL + wind load
Design pressure + DL + earthquake load
Design pressure + DL + wave action
k
Factor
1.0
1.2
1.2
1.2
The use of the trend curves is the same as for Section
VIII, Division 1.
d. BS 5500.
The design strength values for various materials
are established in accordance with Appendix Kin BS
5500.
Basis for time independent design strength, fE
(BS 5500, Appendix K):
Carbon, Carbon-Manganese and Low Alloy Steels:
(a) Material with specified elevated temperature
strength values:
(1) Up to and including 50°C:
fE = Rjl.5 or Rj2.5, whichever is lower
(2) 150°C and above:
RecT/1.35 or Rm/2.5, whichever is lower.
(3) Between 50°C and 150°C:
Interpolate between the values at the two
temperatures.
Aluminum and Aluminum Alloys (annealed materials for welded construction):
(a) Time-independent Design Strength: fE = Rpo. 2/
1.5
Design Stresses for the Various Construction
Categories in BS 5500:
Categoriet1Pl and 2:
The nominal design strength values in Table 2.3
(subject to the additional requirements in BS 5500).
Category3:
(a) Carbon and carbon-manganese steels
fE :5 Rm/5
(b) Austenitic steel
fE :s 120 N/mm 2 or 120 (450/(400 + t)), whichever is smaller.
t = design temperature, °C (300°C max. for
Construction Category 3 vessels).
Evaluation of Design Margins, Section VIII
49
For materials with specified yield stress (1 % proof
stress) :5 230 N/mm 2 , the allowable design stress so
calculated shall be multiplied by 0.8.
e. AD Merkblatt
The allowable design stress is given by K/S, where
the K value is the specified strength value at the
design temperature (generally the specified minimum yield point) and S is the safety factor. (See
AD-Merkblatt B 0 and B 1). The safety factors Sare
listed in Table 21.
f. Stoomwezen
At temperatures below creep range, the nominal
design stress shall not exceed the smaller of the
following values:
Materials with elongation As > 10%:
The design stress f1 shall not exceed the lower of
the following values:
Materials with elongation As < 10%:
• f1 :s 0.25 Rm
• fi :5 0.25 RmCvnJ
RmCvm) tensile strength at temperature Vm
g. CEN
The CEN draft Unfired Pressure Vessels Standard
lists the following bases for allowable design stresses:
Design Stresses for Other Than Austenitic Steel
for Pressure Parts Other Than Bolts:
• The yield strength or 0.2% proof strength at the
design temperature, divided by a safety factor
1.5.
• The specified minimum tensile strength at 20°C
(68°F) divided by a safety factor of2.4.
Alternative Design Stresses:
The Alternative Design Basis was included in the
July 1995 draft of CEN/TC54 Standard for Unfired
Pressure Vessels, but not in the May 1998 draft of
this standard. The allowable stress values were not
to exceed the smaller of the following:
• The yield strength or 0.2% proof strength at the
design temperature, divided by a safety factor of
1.5.
• The minimum tensile strength at 20°C divided
by a safety factor 1.875.
50
• 1% proof strength at the design tern perature
divided by safety factor SF 1.5.
(2) With minimum rupture elongation not less
than 35%:
The nominal design stress shall not exceed the
greater of the following two conditions:
• The 1% proof strength at the design temperature divided by SF = 1.5.
• The smaller of the following:
a. The minimum tensile strength at the
design temperature divided by SF 3.0.
b. The 1% proof strength at the
temperature divided by SF = 1.2.
Testing Condition:
• fi:::;:; % ReCvm) or
• f1 :::;:; 0.44 Rm
ReCvm) =yield stress at temperature Vm
Rm = tensile strength at 20°C (68°F)
Rolled and forged steel and Aluminum and its alloys
Cast steel
Cast iron with spheroidal graphite
Gray cast iron, Copper and its alloys
Design Stresses for Austenitic Steels:
(1) With minimum rupture elongation not less
than30%:
(1) Steels other than austenitic and austenitic
steels with rupture elongation not less than
30%:
1.0% proof strength at test temperature divided by safety factor 1.05.
(2) Austenitic steels with minimum rupture elongation not less than 35%: The nominal test
stress shall not exceed the following:
• The 1.0% proof stress at test temperature
divided by safety factor 1.05.
• The minimum tensile strength at test temperature divided by safety factor 2.
Design Stresses for Steel Castings:
The nominal design stress shall not exceed the
smaller of the following:
• The yield strength or 0.2% proof strength
design temperature divided by safety factor 1.9.
• The minimum tensile strength at 20°C divided
by safety factor 3.0.
2. Design Rules
a. Design for Internal Pressure
The basic formulas for design of cylindrical, spherical and conical shells are almost the same in all:
various codes and standards. The allowable stresses
and joint efficiencies, however, result in widely different thickness requirements. The rules of Section
VIII-1 result in much thicker shells than do the rules
of other documents considered here. The rules of
1.5
2.0
2.4-5.0, depending the type of cast iron
See Table 3, AD-Merkblatt B 0
WRC Bulletin 435
1.1
1.5
1.2-2.5, depending the type of cast iron
See Table 3, AD-Merkblatt B 0
Section VIII-2 also result in thicker shells than all
the European documents studied.
The European documents mostly require that
materiel specifications undertolerance be accounted
for in the calculated thickness. They also have more
explicit requirements regarding adequate allowance
for forming and for corrosion (by adding forming and
corrosion allowances to the calculated thickness).
The rules for design of cone-cylinder junctions
vary considerably. The European documents provide
rules based mostly on analysis and consider loads
other than pressure. Their rules are probably more
accurate but their design margins are less.
The boundary between thick and thin shells is also
defined differently in various documents. European
codes offer more guidance in calculation and evaluation of loads other than pressure (such as wind and
seismic). The increase in allowable stresses for these
environmental loads is similar in various documents. Another difference between European Codes
and VIII-1 is the fact that VIII-1 does not have stress
limits for test condition. Section VIII-2 and the
European codes and standards referred to in this
report do have such limits.
b. Design of Dished Heads
The rules for design of hemispherical heads are
generally the same as those for spherical shells, in
each document. The methods for design of ellipsoidal
and torispherical heads, however, vary significantly.
The German Code allows only 2 types of such
heads. The basis for the design rules are not clear,
but safety factors are applied to the stresses in the
dome. Openings are not allowed in the knuckle area,
without a significant penalty factor. Straight skirts
of at least 3.5 times the shell thickness are required.
The proposed CEN rules are mostly based on
recommendations ofWRC Bulletin 364 (It should be
pointed out that the authors of that Bulletin hare
proposed more recent methods which are considerably more liberal. These proposed rules are being
considered for adoption by the ASME Code Committee). The existing ASME rules as well as those of
BS5500 are mostly based on the method proposed by
Shields and Drucker (VIII-1 rules have their roots in
some very old experimental data). A number of
factors have been applied in various sets of rules,
which makes a direct comparison impracticable.
However, it is safe to say that VIII-1 and VIII-2 rules
in this area are very conservative, except for thinner
heads. Buckling of the knuckle is not addressed in
the ASME Codes, and as a result an upper limit on
D/T of 500 is specified. Some of the European codes
do address buckling and allow thinner heads.
The rules for spherically dished covers are somewhat different in various codes. The formulas for the
spherical dome is mostly the same. The formulas for
flange design are different. With the higher design
margins, the rules ofVIII-1 are mostly more conservative than other documents.
c. Design of Flat Covers
The basic formula for design of fiat covers is the
same in all codes. The allowable stresses and "C"
factors representing boundary conditions are different. Unlike the ASME codes, most European codes
do not apply a joint efficiency to the allowable stress.
Since the stresses are primarily bending, applying a
joint efficiency intended for joints subjected to membrane tension is too conservative. The diameter used
for calculating bending stresses is different in some
codes from the shell diameter used by ASME codes.
Even though some of the "C" factors in the European
codes are larger than the corresponding values in
VIII-1 and VIII-2, the higher design margins of the
ASME codes will mostly result in greater thicknesses, specially for higher strength materials or
those with high yield to ultimate ratio. For materials
with yield strength controlling the stress allowable,
ASME rules, specially VIII-2, could result in lower
thicknesses than some European codes.
d. Design of Flanges
Design rules for bolted flange connections, in
BS-5500 and ASME Codes, are based on "Taylor
Forge Method." The same gasket factors are specified in these documents. ADM rules are also based on
this method, but the gasket factors, details of application, and weld detail requirements are different.
This German Code puts a limit on allowable flange
rotations. The method of Stoomwezen has a more
analytical basis, and is quite different from other
codes. This code also has a limit on flange rotation. In
addition to the basic method, this code allows a
number of other design and analysis alternatives,
including the rules of VIII-1. The proposed rules in
the CEN standard are also based on Taylor Forge
method, with a number of differences in application.
This standard provides for a more analytical method
in Annex B. Due to many differences, it would be
difficult to compare all these rules, but in general all
the various sets of rules result in very comparable
flange designs (it should be pointed out that the
ASME Code is considering a totally new method of
flange design).
All of the documents studied, allow the use of
standard flanges and, generally, such standard
flanges are lighter than those which would result
from the use ,?f design rules.
e. Design ;} Openings
The German ADM code includes two methods for
reinforcement of openings. One method, based on
pressure area, is similar to the method ofVIII-1; but
there are a number of differences in details of
application. The second method based on a "weakening" factor is a unique method. The definition of an
isolated opening is different from that ofVIII-1. The
rules for multiple openings, based on pressure area,
are not as detailed but are similar to those ofVIII-1.
The rules in BS-5500 are completely different from
area replacement rules ofVIII-1. However, the use of
Evaluation of Design Margins, Section VIII
51
a simplified area replacement method i~ allowed as
an alternative. The basic rules are derived from a
shakedown analysis and consider some minor piping
loads.
The rules of Stoomwezen are totally different from
those of VIII-1. Definition of a single opening is also
different. Rules for multiple openings are quite detailed and different. The applicability of these rul~s
is limited to dJD ratio up to 0.7. It can only be said
that this method is more analytical and more detailed than that ofVIII-1.
The rules of CEN draft are still different from
other codes and are based on a mix of limit analysis
and pressure area approaches. Use of .reinfor?in.g
pads is limited. The rules include a maximum h~mt
on the ratio of nozzle neck thickness to shell thickness which has no parallel in any other code.
N~ne of the codes studied have requirements for
limiting bending stresses at the junction of large
openings in cylindrical shells'. except _YIII-1. The
rules ofVIII-1 have for a long time reqmred that for
large openings % of stiffening area be p:a~e~ close to
the junction. A second requirement for hm1tmg bending stresses, based on McBride and Jacobs pape:·,
was added (Appendix I-7(b)), but it appears t~at this
requirement may result in overly conservativ~ design for certain cases and may be deleted or mod~fied.
The CEN rules allow unreinforced small opemngs,
which are more liberal than the rules ofVIII-1.
f. Fatigue Analysis
The fatigue analysis of BS-5500 is very similar to
that ofVIII-2. However, the SN curves are different.
The fatigue exemption rules are also similar to those
ofVIII-2. Only 2 SN curves are provided for a range
of materials, resulting in conservatism for some
materials. A major difference with SN curves of
VIII-1 is that these curves have been generated from
testing of specimens with ground flush joint.s. The
design margins of 2 and 20 in VIII-2 are revised to
2.2 and 15. Considering all facts, the SN curves of
BS-5500 are more conservative than those ofVIII-2.
This British Standard also offers an alternative
method based on data for welded connections (In.
quiry Case 5500179).
The method of CEN standard is completely different from VIII-2. The fatigue curves in this standard
are based on testing of specimen with various weld
details. In general, it is felt that these rules are more
conservative than VIII-1 rules.
Stoomwezen offers three different methods of exemption from fatigue. Other than. exemption .by
satisfactory past experience, the fatigue exempt10n
rules are very different from those of VIII-2. If the
exemption rules are not met, a detailed fatigue
analysis needs to be performed.
.
.
The fatigue design rules ofADM are qmte different
from those of VIII-2. A simplified method of exemption from fatigue is specified. Two design curves are
52
provided, one for design with low str~ss c~ncentra­
tion factor details and one for details with large
stress concentration factors. This obviously is an
oversimplification and results in undue conservatism for most cases. A comparison is difficult, but
over a certain range the rules ofVIII-2 appear to be
more conservative. A unique feature of ADM fatigue
rules is that a method is offered to include the effect
of creep.
g. Design for External Pressure
The rules of German ADM are different from those
of VIII-1. Although, the, theoretical buckling pressure would be the same, the factors applied to the
theoretical pressure are different. The Ger::nan code
introduces a number of factors that are not m VIII-1.
The rules for cone-cylinder junctions are also
different. The method and the factors are unique to
this code. Overall, the rules of ADM are generally
more liberal.
The Stoomwezen formula for allowable external
pressure of cylinders is different from that ofVIII-1.
A number of factors are involved that have no
parallels in VIII-1. The allowable out-~f-roundness
tolerances are also different. The reqmrements for
stiffening ring moment of inertia are considerably
less than those ofVIII-1. The formulas for spherical
shells are also different. There are too many differences for a simple comparison.
The rules of draft CEN standard are also different
and mostly more liberal than those of VIII-1. The
safety factor consists of two parts for load and ~or
resistance. This results in more accurate evaluat10n
and more liberal results. The concept of heavy
stiffeners has been introduced in this code (a set of
proposed alternative rules for VIII-1 also includes
this concept). These rules are more up-to-date and
mostly more liberal than those of VIII-1.
The rules ofBS-5500 are similar to those ofVIII-1,
but in the inelastic range they are different. The
basic rules in BS-5500 are based on an out-o~­
roundness tolerance of 0.5% (this value for VIII-l 1s
1%). The resulting allowable pressures using BS5500 rules are considerably higher than those of
VIII-1. The size of ring stiffeners are considerably
less than those of VIII-1. The rules for spherical
shells are less conservative than those ofVIII-1.
3. Design Details
a. Section VIII, Division 1.
The design details allowed by VIII-1 are mostly
based on experience and judgment. Historically, it
has been assumed that Division 1 vessels are not
subject to cyclic service, unless cyclic conditions are
specified and considered as required by UG-~2. !n
allowing design details, the effects on cychc l.1fe
generally have not been taken into account; but ~1th
the design margins of the present Code and typical
WRC Bulletin 435
service considerations for these vessels, there have
not been any significant fatigue cracking problems.
Paragraph UG-30 provides rules for attachment of
stiffening rings. Such attachments are allowed by
either brazing or welding. Other codes known to the
authors do not allow attachment by brazing and
allowing this method should be reviewed by the Code
Committee. The attachment is also allowed by intermittent welding, which produces stress raisers at
the start and stop points. Allowing this practice
should be reviewed for vessels designed to higher
allowable design stresses than presently permitted
in Section VIII of ASME Code.
Paragraph UG-34 provides acceptable details for
unstayed fl.at heads and covers. These details are
similar to those allowed by the European Codes.
They include attachment by fillet welds, which are
not allowed by Division 2 and some other codes.
Paragraph UW-3 defines the welded joint categories. Table UW-12 contains the permissible types of
welded joints. Types 1, 2, and 3 are various types of
butt joints, which are also allowed by other codes.
Type 4 is a double full fillet lap joint which is not
allowed for the pressure boundary in some of the
other codes. Type 5 is single full fillet lap joints with
plug weld, which is pretty much unique to VIII-1.
Type 6 is single full fillet lap joints, which are not
allowed by other codes. The joint efficiencies reflect
the inferiority of such details, but it is recommended
that types 5 and 6 be reviewed for acceptability with
lower design margins.
For tapered transitions, UW-9 requires a minimum taper slope of 3:1. This is consistent with most
other codes.
Figure UW-13.1 provides acceptable details for
attachment of heads to shells. The use of one sided
fillet welds is, in general, disallowed. However, some
single fillet lap welds are allowed provided adequate
verlap is provided. Single fillet lap welds with pldg
welds are also allowed. But weld with one plate edge
offset is also allowed. These details are not provided
by most other codes, but they are not prohibited
either.
Figure UW-13.2 provides acceptable details for
attachment of pressure parts to fl.at plates to form
corner joints. These details require adequate weld to
develop the strength of the plate and prohibit one
sided fillet welds.
Figure UW-16.1 provides a number of acceptable
tYPes of welded nozzles. Inside and outside pad
plates are allowed. Fillet welds and partial penetration welds are allowed as well as full penetration
welds. For other than full penetration attachment
welds, weld strength calculations need to be made.
Some of the details may result in high stress concentration factors and should be reviewed with any
significant reduction in design margins.
Figure UW-20 provides some acceptable types of
tube-to tube sheet welds. These details are similar to
those of the other Codes studied.
h. Section VIII, Division 2
This division offers some acceptable design details,
but certain other details may also be used, as long as
they are justified by analysis or test.
Paragraph AD-200 specifically allows a detail that
is unique to this Code. The detail allows reduced
thickness at girth seams of a cylindrical shell, under
certain conditions. It has been shown by tests and
analysis that the introduction of such reduced thickness girths will not appreciably affect the collapse
pressure of a uniform thickness vessel.
Paragraph AD-333 requires that stiffening ring
attachment welds be continuous and does not allow
intermittent welds which Division 1 allows.
Paragraph AD-400 defines welded joint categories
which are consistent with those of Division 1. However, the only tYPeS of joints allowed for main seams
of the pressure boundary (categories A & B) are butt
joints (seeAD-410). Joints for attachment of flanges
and fl.at heads (category C) are required to be butt
joints or full penetration corner joints, with some
exceptions (see AD-413).
For vessels constructed of materials with properties enhanced by heat treatment (AQT materials),
additional restrictions apply. All welds, in general,
are required to be butt welds (type 1). When the plate
thickness is greater than 2", nozzle attachment
welds are allowed to be full penetration corner welds
(seeAD-415).
or vessels in lethal service, only butt joints are
allowed, except for nozzle attachment welds which
may also be full penetration corner welds (see AD416).
Acceptable nozzle attachment details are shown in
FiguresAD-610.l,AD-612.1, andAD-613.1.All of the
details call for butt welds or full penetration welds.
The partial penetration attachment details of Division 1 are not allowed. However, AD-201 does allow
some partial penetration nozzle attachment details
(see Figure AD-621.1) for fittings which are not
subject to external loadings and are not subject to
local thermal stresses. It is also noted that Division 2
details all specify fairly generous corner radii.
The transitions between sections of different thickness are r~uired to have a minimum 3:1 slope,
unless an analysis is performed. Additional requirements are specified for nozzle neck to piping transitions (see AD-420).
Details for attachment of fl.at heads are shown in
figures AD-701.1, AD-701.2, and AD-701.3. Welded
joints are limited to full penetration butt joints and
full penetration corner joints. Partial penetration
joints or fillet welded joints are not allowed.
Welds attaching stiffeners or other attachments
are required to be continuous (see FigureAD-912.1).
Evaluation of Design Margins, Section VIII
53
Some typical attachment details are shown in Figures AD-912.1.
c. Stoomwezen
Some design details are specified in Sheet W0201 V,
but other details are not prohibited. The acceptability of other design details are assessed from case to
case by the competent body.
Sheet W0301 provides acceptable weld details.
Again, other weld details may be used on a case by
case basis. Main pressure boundary welds may be
either butt joint, or full penetration. Backing strips
are not required to be removed. Joggled joints are
also allowed under certain conditions. For nozzles
and other attachments, fillet welds are allowed in
addition to full penetration welds. For fl.at cover
attachments, partial penetration welds are allowed
in addition to full penetration. Additional requirements are applicable to vessels in hazardous service.
Appendices 1 and 2 of Sheet W0301 contain detailed
rules on weld details, which are adopted from the
"International Institute of Welding" documents.
There are more details provided than any other code
in this study. A lot of nozzle attachment details are
similar to those of VIII-1. Part 3 of Sheet W0301
contains details for studded connections and couplings. Most of details allowed by VIII-1 are included
here as well as a number of other details not offered
by other Codes. Part 4 includes details for flanges.
Again, these are similar to those ofVIII-1, with more
details provided here. Part 5 includes details for
jacketed vessels. A number of details are provided
which have no parallels in VIII-1. Part 6 includes
tube-to tube sheet attachment details. Detailed requirements are spelled out. Part 7 includes several
tube sheet to shell connection details. This code
offers, by far, the greatest number of design details
amongst the codes studied. To further help the
designer choose the appropriate detail, Appendix 2
provides a "suitability grading" table. This table
grades various details as "high," "intermediate," or
"low" suitability for various service conditions, loadings, and method of examination. A great deal of
information is provided in these tables making this a
unique feature of this code.
d. British Standard 5500
This standard offers a number of typical details.
Other details may be used, as long as they meet basic
design or analysis requirements.
Appendix D offers a number of suggestions for
details on vessels in low temperature service (below
32°F), to reduce the probability of brittle fracture.
Weld neck flanges and fl.at ends are to be attached
with full penetration butt welds.
Appendix 4 includes recommendations for welded
connections to pressure parts. Weld prep details and
joint details are included which are not in ASME
54
Codes. Joggle joints are permitted with a number of
qualifications. Rules for weld proximity are included
which have no parallel in the ASME Codes. Several
nozzle attachment details are included, mostly requiring full penetration welds. Some of the pad plate
details ofVIII-1 are not included. In general a great
deal more detail is specified here than there is in
ASME Codes. Details of weld preps are included. For
partial penetration and fillet weld attachments, very
detailed requirements are specified. Details for attachment of flanges are similar to those of VIII-1,
with more detailed requirements. Jacket attachment details are also included in this appendix.
Details for attachment of fl.at ends are similar to
those ofVIII-1, with a few additional details here.
e. AD-Merkblatt
This Code does not provide a lot of details. The
rules are flexible enough to ascertain acceptability of
various details by analysis or test. However, some
recommended details are provided.
AD-Merkblatt B2 offers a number of details for
cone cylinder transitions. These details are similar
to those allowed by theASME Codes.
AD-Merkblatt B5 contains recommended details
for attachment of stayed and unstayed fl.at ends.
These details are again similar to those of Section
VIII (except the specified values for C factors are
somewhat different).
AD-Merkblatt B8 includes a few details for attachment of flanges. These details do not violate any
types in VIII-1 but are not as extensive.
AD-Merkblatt B9, on openings, shows a few very
basic types of reinforcing, including pad plates. The
rules do not prohibit the use of other details.
stances to meet the following requirements:
• All welded joints shall be full penetration welds.
• All Category A, B and C joints shall be fully
radiogra phed.
• All carbon and low alloy steel vessels shall be
stress relieved (PWHT).
• Vessels shall not be made of cast iron.
Section VIII, Division 1 and Section II, Part D also
includes information related to certain types of
service induced degradation (Section VIII, Div. 1,
Appendix HA; Section II, Part D, Appendix 6).
ASME Section VIII, Division 2 vessels intended
for lethal service must meet the following additional
requirements:
5. Materials Requirements
• All carbon and low alloy steel materials and
welded joints must be impact tested.
• All carbon and low alloy steel vessels must be
PWHT.
• Minimum permissible service temperature must
be 20°F higher than the impact test temperature for each additional inch of thickness above
l" (but not more than 60°F above the impact test
temperature).
The German Pressure Vessel Code (AD-Merkblatt)
has no special requirements for aggressive or hazardous service, but the Order on Pressure Vessels, Gas
Pressure Vessels and Filling Plants41 includes requirements for operation of pressure vessels and
requires periodic testing by Authorized Inspector of
pressure vessels belonging to Testing Group I (when
used for combustible, corrosive or toxic gasses vapors or liquids), II, III, IV, VI and VII vessels.' The
f. CEN Draft Pressure Vessel Standard
This draft standard has very few detail sketches. A··• frequency Testing Group II, III and VI vessels is
few of flange details and nozzle details are included. determined by the operator on the basis of experience wit~ the product and the method of operatio11r
in the sections on design rules. These details are
The testmg frequency for Testing Group IV and vt.r
similar to those of VIII-1. It is not known to the
vessels must be within the time periods specified in
authors if design details will be added to this docuthe Pressure Vessel Order.41
ment later.
. Periodic examination/overload testing of in-service pressure vessels is not addressed in Section VIII
4. Special Service Considerations
of ASME Code.
ASME Section VIII and The German Federal
The Stoomwezen code classifies systems in three
Order on Pressure Vessels include additional require~ hazard categories on the basis of the nature and the
ments based on certain types of aggressive or hazard~ quantity of substances they contain. A fl.ow chart in
ous service. 41 The Stoomwezen Code classifies ves- Stoomwezen document G-0701 provides a procedure
sels into four hazard categories based on the nature for determining the appropriate hazard category. All
and quantity of substances they contain and speci- the basic Stoomwezen rules apply to hazard category
fies additional requirements for vessels in more 1 (which is thus the "Normal Case"). Hazard cathazardous service. No requirements are included in egory 2 and 3 vessels contain more hazardous subBS 5500: 1991 for special service conditions.
stances. Additional requirements apply to hazard
The following is a brief summary of the require~ categories 2 and 3.
ments in each code related to special service consid-.
The CEN/TC 54 draft Unfired Pressure Vessel
erations:
Standard groups pressure vessels into four Risk
ASME Section VIII, Division 1 requires all unfired Categories, depending on the fluids contained (danpressure vessels which are to contain lethal sub gerous or non-dangerous), pressure and volume of
WRC Bulletin 435
the vessel (the quantity of pressure times volume),
~nd type of pressure vessel. There is increasing
mvolvemen~ of the Notified Body (Authorized Inspector) from Risk Category I (least risk) to Risk Category IV (most risk).
AD-Merkblatt Code limits the maximum HAZ
hardness in welded joints to 350 I-IV 10. The St00111 _
wezen Code limits the maximum difference between
the HAZ hardness and the base metal hardness (and
the weld metal and base metal hardness) to 100 HV
or 150 I-Iv, depending on the base metal. (It is not
clear whether this is to reduce the risk of service
induced cracking or cracking during welding.)
All of the codes and standards permit only those
materials which have been reviewed and approved
for use in pressure vessels by the appropriate code
bodies or experts. The European pressure vessel
codes and standards generally include national and
international steel specifications. Also ASME B&PV
Code has now established a Special Working Group
on Non-ASTM Specifications for evaluation and acceptance of international steels. The AD Merkblatt
and the Stoomwezen codes accept more structural
grade plate specifications than ASME. Both AD
Merkblatt and Stoomwezen codes in certain cases
require testing of plates after forming. AD Merkblatt
also requires identification of all weld repairs in the
test certificate (similar to ASME, Division 2, but not
required by SA 20 for Division 1 vessels).
T~e ASME B&PV Code permits ASME SA specificat10ns for pressure parts (which generally are
identi~al or similar to the ASTM specifications),
materials approved by ASME Code Cases and those
international materials which have bee~ approved
by the appropriate ASME Code committees.
The approved ASME SA specifications for ferrous
materials are listed in ASME, Section II, Part A and
for non-ferrous materials in Part B. Welding materials are included in Part C of Section II. The allowable
design stresses and stress intensities for materials
accepted by ASME Code for Divisions 1 and 2 are
listed in Section II, Part D.
Materials for pressure parts must conform to the
specification,,, permitted by Division 1 or Division 2
as appropriate. Materials for non-pressure parts of
Division 1 vessels need not conform to a material
specif)_ed in Division 1, but must be of weldable
quality.
Materials for Division 2 vessels must meet the
additional testing requirements in AM-201, test
specimen heat treating requirements in AM-202
and the ultrasonic examination requirements i~
AM-203. The materials manufacturer must certify
that all requirements of the materials specification
Evaluation of Design Margins, Section VIII
55
and all special requirements of Division 2, Part AM
to be fulfilled by the materials manufacturer have
been complied with.
Structural steel plates for pressure parts are limited to SA-36 and SA-283, Grades A, B, C and D.
Except for flanges, bolted covers, and stiffening
rings, the maximum thickness of structural grade
plates shall not exceed%" (15.9 mm) for Division 1
vessels. SA-36 and SA-283 plates are not permitted
for vessels in lethal service.
Certification shall be in accordance with the requirements in the appropriate SA specification.
Repair welding of materials by the manufacturer
must be in accordance with the appropriate SA
specifications. For Division 1 vessels, repair of defective materials by fabricator require acceptance by
the Inspector of the repair method and the extent of
repairs (UG-78). For Division 2 vessels, welded
repairs of materials by the manufacturer or the
vessel fabricator must meet the requirements of
AF-104 or AF-750, as appropriate.
BS 5500. All materials for pressure parts in BS
5500 vessels must comply with:
• the British Standards listed in the Design
Strength Table 2.3 of BS 5500, or
• the appropriate requirements in BS 5500 and
are covered by a material specification which is
at least as comprehensive as the BS materials
listed in the design strength tables.
Several requirements (notch toughness, NDE,
PWHT) in BS 5500 are related to specific groups of
materials. 5
AD-Merkblatt. The materials requirements in
the AD-Merkblatt code are are included in the
W-series oftheAD-Merkblatt code.
Most materials listed in the AD-Merkblatt code
are to DIN standards, including the structural steels
to DIN 17100. AD Merkblatt HP 5/241 lists the
material groups in AD Merkblatt Code and the type
of materials in each material group.
AD-Merkblatt WO specifies additional requirements for materials and for filler metals. 41 For
example, the following additional requirements are
given for all product forms for Testing Group III, IV,
VI and VII vessels:
• The materials manufacturer shall employ equipment ensuring proper manufacture and testing
of the products.
• The materials manufacturer shall employ competent personnel, as well as an inspection authority for non-destructive tests, if such tests
have been stipulated in the material specification.
• The manufacturer shall guarantee by means of
quality control and suitable records, the proper
manufacture and processing of the products as
56
•
•
•
•
•
•
•
•
well as meeting all requirements of the materials specification.
The suitability of the materials shall be established by an expert on the basis of the materials
specification. If the suitability of the material
cannot be established on the basis of the material specification, the expert shall impose additional safety conditions and appropriate tests
for the product.
(This is an example of where an Expert plays an
important role in the AD-Merkblatt Code. For
ASME code vessels generally only the ASME SA
material specifications need to be complied with.
Additional requirements are listed in Article
M-2 for Section VIII Div. 2 vessels).
Where the suitability of the material is intended
for wider application, this can be done in the
form of a VdTUV-Werkstoflblatt (VdTUV materials specification).
The manufacturer of the materials shall prove
to the expert that the requirements in 3.1.2 of
AD-Merkblatt WO (i.e. the first three items in
this list) have been satisfied.
The works inspector may issue Test Certificate
Bin accordance with DIN 50049.
If the manufacturer uses, for a given product
form, a material not fully refined by him, he
must have in his possession certificates supplied by the manufacturer of the initial material.
If DIN 50049 certificates A or C are required,
tests must be carried out by an expert. In ·such
cases the chemical composition of the heat, the
steel making method and the method employed
in the manufacture of the product must be
identified. (Type A certificates are issued by a
Government inspector, type B certificates are
attested by a works' inspector independent from
the production, and type C certificates require
inspection by a third party inspector.)
If the material specification does not cover re~
pair welding, it may only be done in agreement
with the customer, and in presence of an expert
(if an expert is required to test the product).
The nature and the extent of repair welding, the
type of tests to be performed on the repair welds
and the test results must be indicated in a;
certificate.
,
(Although detailed requirements for repair weld";
ing are included inASME SA-20 for steel plates,
there are no requirements in SA-20 for reporting the nature and extent ofrepair welds nor the
test results on repair welds).
AD Merkblatt requires certification of all materials;
(similar to ASME Section VIII).
··
AD-Merkblatt HP 8/1 requires one set of mechanical tests (tensile and V-notch impact tests) from eacl1
formed part having a length or diameter >4 m (13.12,
WRC Bulletin 435
ft) :56 m (19.69 ft) and two sets of mechanical tests
(from opposite ends) of a formed part having a length
or diameter larger than 6 m (19.69 ft), with provisions for reduced amount of testing based on satisfactory prior test results.
ASME Section VIII requires stress relieving of
formed parts when the maximum forming strains
exceed 5% (UCS-79), but there are no requirements
for mechanical testing of formed parts.
Stoomwezen. Stoomwezen requirements for materials are given in the M-series Stoomwezen specifications for the various types of materials and product forms. In general, only those material
specifications are accepted which have been accepted
in the country of issue for manufacture of pressure
vessels. Approved materials are listed in the Appendices to the M-series Stoomwezen specifications.
Materials are classified into various categories (A,
B, B-1, C and CP) based on chemical composition and
other attributes for the purpose of establishing specific requirements in Stoomwezen specifications, i.e.
testing materials in their final (fabricated) condition
(T 0121 and T 0221), NDE of welds (T 0110),
destructive testing of welds (T 0120) and PWHT CW
0701).
All carbon, C-Mn and low alloy steel plates and
forgings must be supplied in the normalized, normalized and tempered or in the quenched and tempered
condition.
Stoomwezen Code (T 0121 and T 0221) requires
mechanical testing (tensile and/or impact tests) in
the following cases of materials after fabrication
(forming):
• Impact testing of those Category A materials
which require extra impact testing, after hot
forming or heat treatment (for properties),
• Tensile and impact testing of Category B and
B-1 materials after hot forming or heat tre7tment,
• Testing for hot yield strength (when required by
T0121).
CEN. Materials for pressure parts shall be in
accordance with the technical requirements of European Materials Standards (EN) and European Technical Approvals. Where appropriate European Standards are not available, this European Standard also
permits other National or International Standards,
subject to written agreement between the interested
parties and approval by Notified Body (Authorized
Inspector).
A material may only be used for pressure parts
within the temperature range for which the properties required are defined in the technical delivery
specifications.
Materials used for non-pressure parts welded to
the vessel must be compatible with the material to
which they attach and shall be supplied to a material
specification covering at least the chemical composition and the tensile properties.
6. Toughness Requirements
ASME Section VIII, Divisions 1 and 2. The
toughness considerations in ASME Section VIII differ from those in the other international codes
reviewed in this report. ASME Section VIII, Divisions 1 and 2 use an approach which is based on
fracture mechanics considerations and practical considerations (based on satisfactory past experience) to
preventing brittle fracture and groups materials
based on their notch toughness characteristics in
impact test exemption curves.
ASME Section VIII, Division 1, Fig. UCS-66 includes impact test exemption curves (A, B, C and D)
for determining the permissible minimum design
metal temperature (MDMT) for carbon and low alloy
steels based on thickness and types of materials.
Curve A typically includes as-rolled, semi-killed or
killed steels with relatively poor notch toughness.
Curve D includes normalized and quenched and
tempered fine grain steels with good notch toughness.
Section VIII, Division 1 permits a 30°F decrease in
the minimum metal design temperature (MDMT) if
PWHT is performed on the vessel in accordance with
the Code rules but not required by the Code.
Fig. UCS-66.1 provides means for reducing the
permissible design metal temperature (MDMT) based
on a ratio of the required thiclmess to the actual
thiclmess, or a ratio of the actual tensile stress in the
component to the allowable tensile stress.All materials with specified minimum yield strength > 65 ksi
must be impact tested. No impact testing is required
on materials at metal design temperatures down to
-155°F when the coincident ratio defined in Fig.
UCS 66.1 is less than 0.4. (Also BS 5500, ADMerkblatt and the Draft CEN Standard have provisions for lower MDMT with reduced stresses.)
The required Charpy V-notch energy values depend on thickness and the specified minimum yield
strength (Fig. UG-84). The minimum average Charpy
value is 15 ft-lbs for steels with yield strength Sy ::::;
38 ksi fort ::::; 2%" and 20 ft-lbs for steels with Sy ::::; 50
ksi fort::::; 1%". ASME Section VIII, Division 1, Part
UCS does ffbt specify the direction of the impact test
specimens in the base metal for the carbon and low
alloy steels.
Welded joints (WM and HAZ) must be impact
tested whenever base metal requires impact testing.
Recent ASME Code committee action has made
Section VIII, Division 2 toughness requirements
essentially the same as for Division 1 vessels. 10 Fig.
AM-218.1 includes impact test exemption curves
which are the same as in Fig. UCS-66 of Division 1,
except that the exemption curves A, B, C and D
Evaluation of Design Margins, Section VIII
57
terminate at 4 inches. The required Charpy V-notch
energy values in Fig. AM-211 are the same as in Fig.
UG-84.1 in Division 1. A reduction is permitted in
the minimum impact test temperature based on the
ration of the required governing thickness to the
nominal thickness of the component for ratios down
to 0.3 (Fig. AM-218.2). No impact testing is required
when the design stress intensity is less than 6000 psi
at temperatures above -55°F.
Table ACS-1 materials in Division 2 with minimum specified UTS > 95 ksi and the high strength
Table AQT-1 materials must meet 0.015" (15 mils)
minimum lateral expansion for thicknesses up to
1 Vt" and 25 mils above 3" in the transverse direction
for both Section VIII, Division 1 and Division 2
vessels (Fig. AM-211.2). Also certain stainless steels
in both Divisions must meet the 15 mils minimum
lateral expansion requirement.
Certain high strength steels for Division 1 vessels
(SA-508, SA-517, SA-543 and SA-592 for MDMT
below -20°F and SA-645 for MDMT below -275°F)
also must be drop weight tested. In Division 2 all
high strength quenched and tempered steels must be
drop weight tested when used at design temperatures below -20°F (except for SA-522 for all temperatures, SA-353 and SA-553 for temperatures
above -325°F, and SA-645 for temperatures above
-280°F).
The toughness considerations in BS 5500, Stoomwezen and the Draft CEN standard are based on
Charpy V-notch correlations with wide plate test
results. These codes use graphs which plot reference
or assessment temperatures vs. impact test temperatures for various thicknesses of materials for nonPWHT and for PWHT construction. Formulas are
provided to calculate the reference or assessn1ent
temperatures, which include adjustments for stress,
construction categories (in case of BS 5500), PWHT
of subassemblies, etc. Substantial benefits are given
to stress relieved assemblies.
BS 5500. Appendix D specifies toughness requirements for ferritic steels for vessels required to operate below 0°C (32°F). Figures D.3(1) and D.3(2) show
graphs of impact test temperatures vs. design reference temperatures for different thicknesses for the
as-welded and for the PWHT conditions. The conditions below the curves in these figures require
impact testing. The application of Appendix D is
limited to ferritic steels in the categories MO to M4
(carbon, C-Mn, and certain low alloy steels) in BS
5500, Table 2.3, except that rimming steels cannot be
used below 0°C. Fig. D.3(2) which is intended for
PWHT assemblies, permits impact testing at higher
temperatures than Fig. D.3(1) (non-PWHT assemblies).
The design reference temperature eR is the temperature to be used in Figures D.3(1) and D.3(2) for
58
Table23
determining the suitability of materials for the intended service.
eR ::s eD + es + 8c
eH
8D is the minimum design temperature (as defined in
BS 5500)
8s is an adjustment depending on the calculated
membrane stress.
8c is an adjustment depending on the construction
category.
8tt is an adjustment where all plates incorporating
subassemblies are PWHT.
Table 22 lists the impact test requirements for
plates, forgings, castings and tubes (from BS 5500,
Table D.4.1(1)).
Table22
Specified Minimum
Tensile Strength
Required Min. Aue. Impact
Values
at the Material Test Temperature
<450 N/mm 2 (65250 psi)
2:450 N/mm 2
27 J (20 ft-lbs)
40 J (30 ft-lbs)
AD Merkblatt. The AD Merkblatt Code requires
impact testing of all carbon and low alloy steels.
Impact test requirements for design temperatures
down to - l0°C ( + 14°F) are generally in accordance
with the applicable material specification (DIN,
VdTUV materials data sheets, etc.). Impact testing
generally is performed at a temperature given in the
material specification which is closest to but below
the lowest specified design temperature (20°C, or
lower).
AD-Merkblatt W 10 includes toughness requirements for low temperature service (below - l0°C, or
14°F) for pressure vessel Testing Groups III, IV, VI
and VII. (No special requirements are included for
vessel Testing Groups I, II and V). Table 1 in AD
Merkblatt W 10 gives the lowest permissible operating temperature and maximum thickness (or diameter) for different grades of steel for Stress Categories I, II and III (100%, 75%, and 25% of maximum
design stress).
Table 23 is included as an example of the requirements in AD-Merkblatt W 10, Table 1, and lists the
lowest permissible operating temperatures for Group
1 and Group 2 steels for Stress Categories I, II and
III.
The reduced toughness of the materials at the
lower permissible operating temperature for stress
categories II and III is intended to provide resistance
to brittle fracture equal to that for Category I.
Table 1 in AD-Merkblatt W 10 also includes ISO-V
impact test specimen requirements, test temperature and the certification requirements. All materials in Table 1 require impact testing at the ti:>Ynn<>r~i­
tures specified in this table, or lower.
WRC Bulletin 435
2
Steels in AD-Merkbl. Wl, Suitable grades ofrolled or
W4, W5, WS, W12, Wl3,
cast steel (as specified
except killed & semiin theAD-Merkblatter
killed steel below -10°C.
listed in the second
column)
Fine grain steels to DIN
Basic & elev. temp. grades
17102 & VdTUV
WStE 255 through
WStE 500
Werkstoftblaetter 351 and Low temp. series TStE 255
358
through
TStE 420
TStE 460
TStE 500
High impact low temp.
steels
EStE 255 through EStE
315
EStE 355 through EStE
420
EStE 460 through EStE
500
-10
-60
-20
-70 -100°c
70mm
-20°c
-60 -110 -140°C
-50 -100 -130°c
-40 -90 -120°c
60mm
20mm
20mm
-40°C
-40
-40
-70 -120 -150°C
60mm
-60°C
-60 -110
-140
60mm
-60°C
-60 -110
-140
20mm
-60°C
Stoomwezen. The toughness rules in the Stoomwezen Code, document M 0110 apply to all parts of
pressure vessels with the exception of:
• Structural parts not directly welded to pressure
section
• Structural parts welded to the pressure section
in which the primary stress is ::s50 N/mm 2 (7250
psi).
The Stoomwezen Code requires two types of impact
testing:
• Quality testing, and
• Extra testing
Quality Testing. Appendix 2 to M 0110 includes
a diagrams and procedures how to determine wMn
the quality testing is or is not required. This testing
is depends mainly on the material type and thickness. The test temperature for quality test is 20°C
(68°F).
Extra Testing. Diagrams 3a, 3b and 3c in Appendix 3 to M 0110, give procedures how to determine
when extra testing is required for ferritic steels and
at what test temperatures (e.g. at ve, vti or at another
temperature). This testing depends on the type of
material and operating temperature of the vessel.
Figures 1 through 4 in M 0110, Appendix 3 show
the relationship between the assessment temperature ve, impact test temperature Vt and the thickness
de and are used to determine the impact test temperature Vt for extra testing of steels with Ni < 1.5%:
• Fig. 1. As-welded condition, Rm ::s 450 N/mm 2
• Fig. 2. As-welded condition, Rm > 450 N/mm 2
• Fig. 3. Stress relieved weldments, Rm ::s 450
N/mm 2
-85°C Acc. to theAD-Merkbl.
Acc. to theAD-Merkbl.
listed in second column
listed in second column
• Fig. 4. Stress relieved weldments, Rm > 450
N/mm 2
The purpose of the extra testing is to prevent brittle
fracture during the use of the vessel and during
pressure testing. The risk, however, is considered
acceptable when:
• the material temperature is higher than the
temperature for crack initiation at 0.5% plastic
strain, as determined by Well's wide plate test.
• the primary membrane stress is lower than the
so-called Robertson plateau stress (approximately 50 N/mm2).
The assessment temperature (the temperature for
assessment of notch toughness) ve, is given by the
following formula:
Vm:min + Av1 + Av2
Vm:min = lowest design metal temperature
Av1 =temperature adjustment based on stress (pressure reduction), in accordance with Stoomwezen,
M 0110, Appendix 1.
Av2 = 15°C when all the following under the following conditions are met:
Ve=
• The p~ssure part is made entirely ofC or C-Mn
steel
• The pressure part is spherical, or cylindrical
with hemispherical heads
• Shell butt welds are not stress relieved
• Nozzles, nozzle reinforcements and supports
welded to the pressure part have been stress
relieved in accordance with W 0701
• The vessel has been subjected to a full hydrostatic test in accordance with T 0240
Av 2 = 0 in all other cases
Evaluation of Design Margins, Section VIII
59
Impact testing may be required in the initial (asdelivered) condition and in final (after fabrication of
the vessel) condition of the material. Impact testing
of material in the initial (as-delivered) condition is
required in the following cases:
• Whenever the material specification requires
impact testing (including test temperature).
• Whenever a Quality test is required by the
diagrams in Appendix 2 to M 0110.
• Whenever Extra testing is required by the diagrams in Appendix 3 to M 0110.
Impact tests on base metal may be transverse or
longitudinal (unless specified by purchaser). No separate quality testing is required if extra impact tests
are performed in the transverse direction. (See M
0110, Table 1).
Tables 24a and 24b list the minimum average
Charpy V-notch energy requirements for test for the
quality test and for extra testing.
M 0110 of the Stoomwezen Code also provides
alternate procedures for material selection which
parallel the assessment rules. Figure 3 (as-welded
condition) or Figure 4 (PWHT condition) show the
relationship between the assessment temperature Ve
and thickness de for various groups of steel listed in
M 0110 (B, C, D, E, F and G, depending on type of
material and the impact test temperature in the
material specification).
The materials selected with the alternate rules
from these figures will usually satisfy the assessment rules in M 0110 (including the Appendices);
however, the use of these Figures is not permitted in
some cases (Hazard Category 3 vessels, etc.).
CEN. Appendix D in the CEN draft Unfired Pressure Vessel Standard outlines three alternative methods for establishing impact test requirements and
avoidance of brittle fracture:
Method 1: A code practice developed from operating
experiences and applicable to all metallic materials.
Method 2: A code of practice developed from fracture
mechanics principles and operating experiences
but applicable only to C, C-Mn and low alloy
ferritic steels with specified minimum yield
strength, Re :::; 460 N/mm2 (66.7 ksi). It can be
applied to a wider range of thicknesses than
Method 1.
Method 3: Application of fracture mechanics analysis. This method is applicable to cases not covered
by Methods 1 and 2 and shall only be used in
agreement with the purchaser, manufacturer and
the Notified Body (Authorized Inspector).
It is only necessary to satisfy the requirements of
any one method.
Methodl:
Design reference temperature TR
Temperature adjustments, ts are given in Table 25.
Non-welded,
PWHT(l)
As-welded and
Percentage of Maximum
Allowable Design Stress
>75% :$ 100% 575% 550%
0°C
0°C
+l0°C +25°C
0°c
Ferritic steels, 1.5-5% Ni steels
9% Ni steels
Welds for austenitic stainless steels
Ferritic-austenitic stainless steels
Test
Ferritic steel with Ni 9%
Austenitic Cr-Ni cast steel
Ve<
Ve 2':
Ferritic cast steel
Austenitic Cr-Ni steel
Austenitic weld metal
v
< -105°C
Ve 2':
Other cases
-105°C
-105°C
-105°C
31
27
43
27
43
27
39
27
Notes: 1. The values in parenthesis apply to seamless pipe.
2. Charpy V-notch test specimens shall be used; however, other
test specimen are also acceptable for Quality testing at 20°C, if
permitted by specifications.
60
Membrane
Stress
:s;50N/mm2
+50°C
0°C
The required Charpy V-notch energy values
Method 1 for base metal, weld metal and HAZ at
impact test temperature tKV are determined from
Table 26 (Table D.l, Annex D in the CEN draft
Standard) using the reference thickness eB and
design reference temperature tR of the component.
Method 3 must be used when the wall thickness
exceeds 30 mm (l.18") in the as-welded condition
and 60 mm (2.36") in the PWHT condition.
Method2:
Parent materials, welds and HAZ shall meet the
impact energy Kv in Table 27 (Table D.4 in
WRC Bulletin 435
For as-welded: eB :s; 30 mm Rp 5310 N/mm2
For PWHT: eB 5 60 mm
310 N/mm2 < Rp
TM
530mm
55mm
None
Table27
Specified Minimum Required Impact
Energy, J
Yield Strength of
tKvfor
Base Material,
(10 X lOmm Non-welded
tg:v for
NI mm2
test pieces)
& PWHT As-welded
5310
>310, 5360
>360
27 J
40J
27 J
40J
27 J
Fig. D.l: PWHT condition; materials with Rp:::; 310
N/mm 2 (27 J energy requirement) and Rp > 310
N/mm 2 ( 40 J energy requirement). (This figure is
the same as Fig. D.3(2) in BS 5500).
Fig. D.2: As-welded condition; materials with Rp :::;
310 N/mm2 (27 J energy requirement) and Rp >
310 N/mm 2 (40 J).
Fig. D.3: PWHT condition; materials with Rp > 310
N/mm 2 (27 J energy requirement).
l
Fig. D.4: As-welded condition; materials with Rp >
310 N/mm2 (27 J energy).
Rp = 0.2% proof strength at the design temperature.
5
460 N/mm2
Limited to Tm 2': -30°C for unwelded
2" - 50°C
Method 2 has no thickness limitations but has more
severe toughness requirements in the non-PWHT
condition than in the PWHT condition.
Method3:
A fracture mechanics analysis may be used to
determine suitability of a particular vessel for the
intended service, when agreed between the purchaser, manufacturer and the notified body, in the
following cases:
1. Materials not currently covered in this CEN
standard.
2. When the requirements for low temperature
application cannot be adhered to.
3. Where defects outside the NDT requirements
are detected.
4. Where it is proposed to use thicker material
than permitted by the low temperature requirements without PWHT.
D.2
D.2
D.4.a
D2
D.4.b
The impact test temperature tKV is determined
from Figures D.1, D.2, D.3 and D.4 in the draft CEN
Standard (as appropriate) using for design reference
temperatures tR and reference thickness tR.
Table 28 includes a comparison ofimpact test requirements for 1" (25 mm) and 1.5" (38 mm) thick as-rolled
and 1.5" and 2" thick normalized SA- 516 Gr. 70
plates to be used at the maximum allowable stress at
minimum metal design temperature (MDMT) of
-4°F (-20°C), based on the toughness rules given in
the codes and standards in this summary (Table 28).
This comparison indicates more severe toughness
requirements by BS 5500, Stoomwezen and the draft
CEN standard than ASME, Section VIII, Division 1
for thick and for non-PWHT weldments. These codes
and standards, however, do not differentiate in their
toughness requirements between as-rolled and heat
Table28
Material
Charpy
V-Notch
TR
TR
-196°C
-196°C
CEN Standard) for design reference temperatures tR
and reference thiclmess eB at the impact test temperature tKV.
Note (1): Also applicable for equipment where all nozzles
permanent welded on attachments are first welded to the
nents and these subassemblies are PWHT'd before being
sembled into the equipment by butt-welding, but the main
are not subsequently PWHT'd.
24b
Extra Testing:
27 J
27 J
40J
40J
40J
None
TM+ Ts
TM = lowest temperature defined for each of the
conditions in para. D.3.1.
Ts = temperature adjustment, depending on the
pressure induced principal membrane stress, as
shown in Table D.2 of the CEN standard.
Condition
Table26
Thickness
PWHT
As-rolled SA 516 l" (25.4 mm)
Gr. 70
1.5" (38.1 mm)
No
1.5" (38.1 mm)
Yes
Norm. SA516 Gr. 1.5" (38.1 mm)
70
Norm. SA516 Gr. 1.5" (38.1 mm)
70
Norm. SA 516 Gr. 2"(50.8mm)
70
Yes
No
No
Yes
ASMEVIII
Diu.1
15 ft-lbs & -4°F
(-20°C)
15 ft-lbs & -4°F
(-20°C)
15 ft-lbs & -4°F
(-20°C)
Testing not
required
Testing not
required
Testing not
required
BS5500
Stoomwezen
CEN
AD-Merkblatt
30 ft-lbs & -40°F 20 ft-'itl's & -22°F 20 ft-lbs & 5°F
20 ft-lbs & -4°F
(-40°C)
(-30°C)
(-15°C)
(-20°C)
Requires PWHT** Requires PWHT* Requires PWHT** Requires PWHT*
20 ft-lbs & 50°F 20 ft-lbs & 39°F
20 ft-lbs & -4°F
30 ft-lbs & 57°F
(10°C)
(-20°C)
(14°C)
C4°C)
Requires PWHT** Requires PWHT* Requires PWHT** Requires PWHT*
30 ft-lbs & 57°F
(14°C)
30 ft-lbs & 14°F
(-10°C)
20 ft-lbs & 50°F
ClO°C)
20 ft-lbs & 32°F
(0°C)
20 ft-lbs & 39°F
C4°C)
20 ft-lbs & l4°F
(-10°C)
20 ft-lbs & -4 °F
(-20°C)
20 ft-lbs & -4°F
(-20°C)
*PWHT may be eliminated for this thickness if the material and impact testing meet the requirements in the applicable codes (see
Appendix 3 for Stoomwezen W 0701, and Appendix 2 for AD Merkblatt 7/2 PWHT requirements/exemptions).
**PWHT may eliminated if justified by fracture mechanics analysis.
Evaluation of Design Margins, Section VIII
61
treated materials and appear to give no benefit to the
improved toughness achieved by normalizing or
quenching and tempering, as indicated by the lower
impact test exemption curves in Fig. UCS-66 of
ASME, Section VIII, Division 1. The Stoomwezen
Code, however, requires the carbon, C-Mn and low
alloy steels to be supplied in the normalized, normalized and tempered or in the quenched and tempered
condition.
7. Fabrication Requirements
a. Welded Joints
Section VIII, Division 1, Fig. UW-3 defines joint
categories A, B, C and D based on location in the
vessel:
Category A. Longitudinal welds in main shell, communicating chambers, transitions and in nozzle
necks, any welded joint within a sphere, or in a
formed or a flat head, circumferential welded
joints connecting hemispherical heads to main
shells, nozzles or to communicating chambers.
Category B. Circumferential welded joints within
the main shell, communicating chambers, in
nozzles, or transitions in diameter, circumferential welded joints connecting formed heads (other
than hemispherical) to main shells, to transitions
in diameter to nozzles or to communicating chambers.
Category C. Welded joints connecting flanges tube
sheets or flat heads to main shell, to formed heads,
to transitions in diameter, to nozzles or to communicating chambers.
Category D. Welded joints connecting communicating chambers or nozzles to main shells, to spheres,
to transitions in diameter, to heads or to flat sided
vessels.
The types of welded joints permitted in Section VIII,
Division 1 are:
Type 1. Butt joints as attained by double-welding or
by other means which will obtain the same quality
of deposited weld metal on the inside and outside
weld surfaces to agree with the requirements of
UW-35. Welds using metal backing strips which
remain in place are excluded.
Type 2. Single-welded butt joint with backing strip
other than those included under Type 1.
Type 3. Single-welded butt joint without use of
backing strip.
Type 4. Double full fillet lap joint.
Type 5. Single full fillet lap joints with plug welds
conforming to UW-17.
Type 6. Single full fillet lap joints without plug
welds.
Table UW-12 in Section VIII, Division 1 lists the
types of butt welded joints permitted for each joint
category, limitations for each type of joint, applicable
joint categories for each type of joint, and joint
efficiencies based on the amount of radiographic
62
examination. Type 1 butt joints are assigned the
joint efficiency of 1.0 based on 100% radiographic
examination (RT), 0.85 based on spot RT and 0.70
based on no radiographic examination.
The same categories of welded joints are used in
ASME, Section VIII, Division 2, however, only Type 1
and Type 2 joints permitted for Division 2 vessels:
Category A: Type 1 butt welded joints.
Category B: Type 1 or Type 2 butt joints. Only Type 1
butt joints are permitted for the high strength
quenched & tempered steels in Table AQT-1.
Category C: Type 1 butt welded joints and full
penetration corner joints, except that Type 2 joints
are permitted in AD-416 for lethal service, and
fillet welded joints are permitted in AD-711.1 for
special applications. Type 1 joints are required for
TableAQT-1 materials.
Category D: Type 1 butt joints, full penetration
corner welds, and full penetration corner welds at
the nozzle neck and/or fillet welds. For Table
AQT-1 materials, Type 1 joints are required for
thickness :s2", but special welded joint details in
Figs. AD-610.1 or AD-613.1 may be may also be
used.
The joint efficiency for Division 2 vessels is 1, since
all butt welds must be 100% RT examined.
BS 5500. BS 5500 categorizes welded joints into
Types A and B based on location in the vessel:
Type A. Main seam welded joints within main shells,
transitions in diameter, communicating chambers,
jackets and nozzles. Main seam welded joints
within a flat or formed head or within a sphere.
Type B. Welded joints connecting flanges, tube sheets
or flat ends to main shells, to nozzles and to
communicating chambers. Welded joints connecting nozzles or communicating chambers to main
shells. Fillet welds or full penetration welds attaching reinforcing plates to shell and to end plates.
Double welded and single welded butt joints (with
and without backing strips) may be used for main
seams in vessels. Permanent backing strips
be used for Construction Category 1 and for Construction Category 2 (except where specifically permL1tt;ea
for Construction Category 2). Single
joints for main vessel welds in Construction Category 1and2 vessels are limited to 20 mm (0.79") or
less, depending on the joint detail. This is
restrictive than Section VIII, Division 1 which
mitsASME Type 2joints (with backing
for
joint categories without a limitation on th:tckness,
but assigns a lower joint efficiency than for
joint. It is also somewhat more restrictive
Section VIII, Division 2 which permits Type 2 joints
(with backing strips) for Category B welds (except for
the TableAQT-1 high strength materials).
Generally, full penetration
joint details may be used for
depending on material, inspection requirements
WRC Bulletin 435
intended service. This is less conservative than
ASME Section VIII, Division 2 which requires full
penetration corner welds at nozzle necks.
Cut edges of ferritic alloy steels cut by a thermal
process shall be shall be ground or machined for a
distance of 1.5 mm unless approved otherwise by the
Inspecting Authority.
AD-Merkblatt. Requirements for welds for pressure vessels built to AD-Merkblatt Code are covered
by DIN standard 8562 "Welding in the construction
of vessels and containers; metal containers: welding
principles."
Stoomwezen. Longitudinal and circumferential
welds in cylinders, cones, spheres and heads, and
welds in flat heads shall be full penetration butt
welds. Backing strip (if any) need not be removed
from a circumferential weld, and joggled type circumferential joint may be used between two cylinders or
between a cylinder and a non-conical head, providing
all of the following conditions are met:
•
•
•
•
•
The inside of the weld is inaccessible,
No crevice corrosion is expected,
Static loading,
Vessel is classified as Hazards Category 1,
Material is not category C or CP.
One sided butt welds with backing strips left in place
are not permitted for ASME Section VIII, Division 2
vessels constructed of Q & Thigh strength materials.
Joggled type joints in ASME Section VIII, Division 1,
Fig. UW-13.l(k) are limited to%" maximum thickness
for circumferential welds vessels and are not permitted
for PartAQT materials in Division 2 vessels.
Recommended weld details for nozzles, studded
connections and couplings are given in W 0301,
Appendix 1, Parts 1, 2, and 3 are essentially the
same as those in BS 5500 and similar to those in
ASME Section VIII, Division 1, however, a larger
variety of weld details are given in the Stoomwezen
Code and in BS 5500.
i
W 0301 Appendix 1 permits partial penetration
and full penetration nozzle attachment welds. This
is less restrictive than ASME Section VIII, Division 2,
which requires full penetration nozzle attachment welds
which must be examined by UT or RT for openings >6"
dia. in shells >2112'' thick (AD-600 andAF-240).
Stoomwezen requires full penetration welds and
complete non-destructive examination of welds for
Hazard Category 3 vessels and for all vessels made
of Category C materials (which require special care
in processing) and for CP materials (which are
sensitive to cracking during their use).
b. Impact Testing of Welded Joints
ASME Section VIII, Division 1. Except as exempted in UG-20(f), the welding procedure shall
include impact tests of weld metal and the HAZ
when:
• either base metal is required to impact tested,
• the minimum design metal temperature
(MDMT) is colder than -55°F and base metals
are exempt from impact testing,
• welds in high strength steels in Part UHT which
are not subject to quenching and tempering or
normalizing and tempering (except high nickel
alloy filler metals in UHT-82(c)).
In addition, Division 1 requires impact testing the
weld metal when:
• joining impact tested materials in various product forms from Table UG-84.3 or Fig. UCS-66
Curve C or D and the MDMT is colder than
-20°F but not colder than -55°F unless welding consumables have been impact tested at
-55°F or colder by the applicable SFA specification,
• welds in UCS materials are made without filler
metal and the thickness exceeds 112'' for all
temperatures or when the thickness exceeds o/16"
and the MDMT is colder than 50°F (UCS-67(b)),
• impact tests are required for the deposited weld
metal but certain high alloy steel base metals is
exempted from impact tests (UHA-51).
Also, the HAZ produced with or without filler metal
shall be impact tested whenever the welds have any
individual pass exceeding 112'' and the MDMT is
colder than 70°F.
Impact tests from production test plates are required for all joints for which impact tests are
required for the welding procedure. Tests shall be
made of the weld metal and HAZ to the extent
required for the procedure test (except that no
production impact tests are required for certain Part
UHT materials welded with high nickel filler metals
per UHT-82(c)).
The impact tests from procedure qualification
plates and production test plates shall meet the
same requirements as base metal.
ASME Section VIII, Division 2 requires welding procedure impact testing of welds and heat
affected zones (HAZ) in the following cases:
• When impact tests are required on the base
material to be welded,
• When the base material is selected from Groups
IV and Vin Fig. AM-218.1, however, no impact
tests are required on the HAZ when the base
exempted from impact testing (AT-202).
In addition, impact tests are required on all high
alloy weld metal included in A nos. 6, 7, 8 and 9 of
QW-422, Section IX.
When the base material is required to be impact
tested, Division 2 requires impact testing of welds
and HAZ on production test plates for each qualified
welding procedure used on each vessel.
BS 5500 requires impact testing weld metal of
welding procedure test plates when the operating
temperature is below 0°C (32°F), but does not require impact testing ofHAZ with multi-pass welding
Evaluation of Design Margins, Section VIII
63
with welding heat inputs between 1 kJ/mm (25
kJ/in.) and 5 kJ/mm (127 kJ/in.).
The required minimum average impact test values
are listed in Table 29 (from BS 5500, Table D.4.1(1)).
Table29
Specified Minimum
Tensile Strength
<450 N/mm 2 (65250 psi)
~450N/mm 2
Required Minimum Average
Impact Values at the
Material Test Temperature
27 J (20 ft-lbs)
40 J (30 ft-lbs)
The required impact test values in BS 5500 are
somewhat higher than in ASME Section VIII, except
for the higher strength materials/thick welds in Fig.
UG-84.1.
Production test plates are required when the
minimum design temperature 0n is within 20°C of
the design reference temperature eR and for all 9%
Ni steel vessels. Impact testing of production test
plates is not required in case of welds in materials
less than lOmm (0.394") thick.
AD Merkblatt Code requires impact testing of
all carbon and low alloy steel welding procedure
qualification and production test plates. Impact testing is required from each procedure qualification test
plate in ferritic materials as follows (from ADMerkblatt 2/1, Table 2):
For service temperatures -10°C (14°F) and above:
The impact tests shall be at the same temperature as
the base metal and shall meet the following minimum average ISO V-notch values:
• Ferritic weld metals: the same impact test requirements as base metal.
• Ferritic-austenitic, austenitic and nickel weld
metals: 2:40 J (30 ft-lbs).
• HAZ: 227 J (20 ft-lbs).
For service temperatures below -10°C:
The impact tests shall be at the lowest service
temperature and shall meet the following minimum
average ISO V-notch values:
• Ferritic weld metals: 2:27 J (20 ft-lbs).
• Ferritic-austenitic, austenitic and nickel based
alloy filler metals: 2:32 J (24 ft-lbs).
• The HAZ: 16 J (12 ft-lbs).
The type and amount of production tests for each
vessel depends on the material group, number of the
cylindrical parts and the design stress level (100% or
85% of maximum allowable). The impact test requirements are (Table 1, AD-Merkblatt 5/2):
Service tempeatures -10°C (14°F) and above:
• Weld metal: The same as for base metal in
transverse direction. (240 J ISO V-notch for
ferritic-austenitic, austenitic and Ni based alloy
filler metals).
64
• HAZ: 27 J ISO V-notch minimum average, at
the lowest service temperature.
Service temperatures below - l0°C:
• Weld metal: :527 J minimum average ISO
V-notch, at the lowest service temperature. (2:32
J for ferritic-austenitic austenitic and Ni based
alloy filler metals).
• HAZ: :516 J, ISO V-notch, at the lowest service
temperature.
Stoomwezen. The Stoomwezen code requires impact testing of welding procedure qualification test
plates only for service temperatures below 0°C (14°F).
Impact testing of production test plates is required
only when extra impact testing is required. Impact
tests shall be in base metal and weld metal in both
the welding procedure test plates and production
test plates and shall meet the requirements of M
0110. There is no requirement in the Stoomwezen
code to test the HAZ.
If extra impact testing is required, one production
test plate must be tested for circumferential butt
welds in cylinders and cones for each pressure part
welded under the same welding procedure qualification and meet the impact test requirements of M
0110. Impact tests are required from base metal and
weld metal.
CEN. The CEN draft standard requires impact
testing of the welding procedure qualification test
plates and production test plates for thicknesses
exceeding 12 mm (0.47''). There is no requirement for
impact testing HAZ of steels in Groups 1.1 and 1.2
(low strength C and C-Mn steels).
Welding procedure qualification testing is in accordance with EN 288-3 and EN 288-4. Impact testing of
production test plates depends of material, thickness
and design reference temperatme. Table 30 lists the
impact test requirements for production test plates
(from Table 12 in Part 4 of the CEN draft standard).
Table30
Steel Group
St 1.1, St 1.2
St 2.1, St 2.2, St 3.1, St
St 4.1, St 4.2, St
St 5.1, St 6, St 7.1,
St7.3
Thickness,
mm
Impact Test
Requirements
e > 12
e > 12
Weld metal impacts
3 Weld metal impact tests
3 HAZ impact tests
The weld metal shall meet the specified transverse
impact test values for the base metal. Steels St 9.1,
St 9.2 and St 9.3 shall meet 40 J minimum.
The HAZ shall normally be the same as for base
metal at ambient temperature, or the specified test
temperature for base metal, whichever is lower.
c. Inspection of Welded Joints
ASME Section VIII, Division 1. The extent of
NDE required in ASME Section VIII, Division 1 is
WRC Bulletin 435
related to the type of service, type of material, type of
weld and thickness of welded joints. Full (100%)
Radiographic Inspection is required in the following
cases (from UW-11):
after the hydrostatic test (or after fabrication, if
not accessible during the hydrotest)
• All Fig. UHT-18.2 type nozzle attachment welds
having inside diameter of 2" or less
• All butt welds in vessel shell and heads which
contain lethal substances
• All butt welds in the vessels in which the least
nominal thickness exceeds 1Yz" or the lesser
thicknesses given in Tables UCS-57, UNF-57,
UHA-22, UCL-35, or UCL-36 for the materials
listed, or otherwise required by ULW-51, ULW52(d), ULW-54, or ULT-57
• All Type No. 1 butt welds in high strength heat
treated steels in Part UHT
• All nozzle attachment welds in high strength
heat treated steels in Part UHT
• All butt welds in shell and heads in unfired steam
boilers with design pressures exceeding 50 psi
• All butt welds in nozzles, communicating chambers, etc. attached to vessel sections or heads
that are required to be fully radiographed
• All Category A and D butt welds in vessel
sections and heads where the design of the joint
is based on the joint efficiency permitted in
Table UW-12 for full radiographic examination
• All butt welds joined by electrogas welding with
any single pass greater than 11/z'' and all butt
welds joined by electroslag welding
• All welded repairs deeper than %" in materials
that are required to be radiographed
No specific requirements are included in Section
VIII, Division 1 for MT or PT of nozzle welds in Part
UCS for C, C-Mn and low alloy steels.
ASME Section VIII, Division 2. The following
NDE is required:
Full (100%) Radiographic Examination is required
for:
Spot Radiography is required for all Type No. 1and2
butt welds of Table UW-12 which are not required to
be fully radiographed (except for Category B and C
butt welds in nozzles and communicating chambers
which do not exceed NPS 10 or 1 Ya").
Ultrasonic Examination (UT) may be substituted
for radiography for the final closure seam of pressurr
vessel if the construction of the vessel does nd't
permit interpretable radiographs. UT is required for
welds made by inertia or continuous drive friction
welding processes, when radiography of the welded
joints is required by the Code.
The ASME Code does not permit the general use of
UT in place of RT as in most European pressure
vessel codes. (A recent ASME Code Case 2235,
however, permits the use UT in place of RT for
welded joints exceeding 4" in thickness).
Magnetic Particle or Liquid Penetrant Testing is
required in the following cases:
• When a pressure part is to be welded to a flat
plate thicker than Yz" to form a comer joint the weld
joint preparation shall be examined before and
after welding (the remaining exposed surface)
• All surfaces of weld metal buildup (UW-42)
• All welds, including welds for attaching nonpressure parts to the high strength heat treated
steels in Part UHT shall be MT or PT examined
• All Type No. 1 butt welds, whether longitudinal
or circumferential
• All Type No. 2 butt welds
• All Category C joints in shell thicker than 2Yz"
and Category D joints greater than 611 in diameter in shell thicker than 21/z'' made by inertia or
by continuous drive friction welding processes
Ultrasonic Examination shall performed for:
• Category C joints when the shell thickness is
greater than 2Yz"
• Category D joints when the opening is greater
than 6" in diameter in shell greater than 21/z" in
thickness
• All welds made by the electroslag and electron
beam welding processes (after any required
heat treatments)
• Corner joint constructions as shown in Fig.
AD-701.3, sketches (b) and (c)
Magnetic Particle or Liquid Penetrant shall be performed in the following cases:
• The radially disposed surface of the opening cut
in the vessel wall as shown in Fig. AD-610-1
sketches (a) and (b)
• Both surfaces of welds in all full penetration
corner joints
• All partial penetration welds
• All radially disposed surfaces cut in the vessel
walls
• All welds attaching non-pressure parts to pressure parts (except where reduced examination
is permitted in note 1 to TableAF-241.1)
Liquid Pen~rant Examination shall be performed
on all austenitic chromium-nickel alloy steel welds,
both butt and fillet, in vessel whose thickness exceeds%".
Table AF-241.1 in ASME Section VIII, Division 2,
includes a detailed summary of permitted types of
welds and examination requirements for Section
VIII, Division 2 vessels.
BS 5500. Table 31 (Table 3.4 in BS 5500) lists
three construction categories, the use of which is to
be agreed between the purchaser and the manufacturer.
Evaluation of Design Margins, Section VIII
65
with n11111mum yield point ;::: 370 N/mm 2 < 430
N/mm 2 ) designed up to 100% of allowable stress:
Construction
Category
NDT
100%
Spot
Permitted BS 5500 Band Material
None, except where limited by NDT
40
30
40
40
16
All
MO&Ml
M2
Austenitic SS
Visual only
The NDT of welded joints for final acceptance
purposes depends on the Construction Category of
the components and type of welded joint:
Construction Category 1: The final NDE shall
be performed after completion of any PWHT and
shall include the following:
1. 100% RT or UT of Type A welds.
2. 100% RT or UT of Type B welds when the
thinnest part welded exceeds the thickness
limits in Table 32.
3. MT or PT the full length of Type Band all other
attachment welds.
Austenitic, MO & Ml
M2
M3
M5-Ml0, inclusive
40 (1.57")
30 (1.18")
20 (0.79")
10 (0.39")
Temperature
Limit
Lower Temp.
Limit
Per BS 5500
Per BS 5500
AppendixD
AppendixD
Per BS 5500
Per BS 5500
300°0
None
None
AppendixD
None
als group, wall thickness and the permissible design
stress. RT or UT is permitted for shell butt welds.
Based on satisfactory prior record, the amount of RT
or UT can be reduced substantially for certain lower
strength materials (which is not permitted for ASME
Code vessels). A certain percentage of shell butt
welds (10 or 25%) must also be subjected to MT or PT
examination. Essentially all nozzle and fillet welds
must be MT or PT examined.
For some groups of steel the extent of the NDE can
be reduced based on satisfactory prior experience.
The following examples illustrate the NDE requirements in AD-Merkblatt HP 5/3 for several groups of
steel listed in this document:
Group 1 materials (C-Mn steels with minimum
yield point 370 < N/mm 2 (53.65 ksi)):
As-welded and heat treated (stress relieved) vessels designed to 100% of allowable tensile stress:
• RT or UT 100% of longitudinal welds and weld
intersections
• RT or UT 25% of circumferential welds
• MT 10% of butt welds with t > 50 mm (1.97")
*See BS 5500 for listing of the various groups of steel.
Construction Category 2. Construction Category 2 welded joints shall be subjected to partial
NDE, except for main weld seams welded at site,
which shall be 100% RT and/or UT.
1. RT and/or UT:
a) of butt welds in shells, formed heads, nozzle
necks, flat ends, communicating chambers
and jackets
b) Intersections oflongitudinal and circumferential butt welds
2. MT or PT all nozzle attachment welds and all
reinforcing plate attachment welds.
Construction Category 3: Only visual examination required. Both the initial assembly of components and the preparation of second side of welded
joints shall be inspected and approved by the IA
before welding second side.
AD-Merkblatt. The type and extent of NDE required by AD-Merkblatt is listed in Tables 1, 2 and 3
of AD-Merkblatt HP 5/3 and depends on the materi-
66
Maximum Thiclmess (mm)
If the results of previous NDT have revealed no
serious deficiencies, UT or RT of welds in Group 1
materials can be reduced to the following:
• As-welded vessels with t:::; 30 mm (1.18"): 10%
oflongitudinal seams and 2%
As-welded and heat treated (PWHT) vessels
1 materials designed to 85% of allowable stress:
• t :::; 15 mm (0.59"): 2% of longitudinal seams,
circumferential seams, and weld intersections
• t > 15 :::; 30 mm (l.18"): 10% of longitudinal
seams, 2% of circumferential seams, and
of weld intersections
Based on satisfactory prior experience, this testing
can be reduced as indicated in AD-Merkblatt HP 5/3
Table 1.
,
As-welded (t:::; 30 mm) and heat treated.
(PWHT) Group 2 steels (fine grain structural steels'y:
WRC Bulletin 435
• RT or UT 100% of longitudinal seams and of
weld intersection
• RT or UT 25% of circumferential seams
• MT butt welds 10% fort> 30 :::; 70 mm and 25%
fort> 70 mm (2. 76")
If the results of previous NDE have revealed no
serious deficiencies, the RT or UT can be reduced for
non-heat treated (non-PWHT) vessels with t :::; 15
mm and heat treated (PWHT) vessels with t :::; 30
mm to 10% oflongitudinal and circumferential seams.
As-welded (t :::; 30 mm) or heat treated (PWHT)
Group 3 steels (fine grain structural steels with
minimum yield point ;::::430 N/mm2 ) designed up to
100% of allowable stress:
• RT or UT 100% oflongitudinal seams, circumferential seams, and all weld intersections
• MT butt welds 10% fort:::; 20 mm (0.79") and
25% fort > 20 mm
There are no provisions for reducing the amount of
RT or UT for Group 3 steels based satisfactory prior
experience.
Nozzle and fillet welds for all material groups
require surface MT or PT examination. Additionally,
UT or RT shall be performed on nozzles having
I.D.;::::: 120 mm when the thiclmess of the connecting
cross-sectional area exceeds 15 mm.
Cracks and lack of side wall fusion are not permissible. Incomplete root fusion is not permissible for
single side welded joints. Linear indications which
are attributed to the separation of the materials are
not permissible.
The extent of NDE in the AD-Merkblatt code is
somewhat more than required for ASME Section
VIII, Division 1 (which has no requirement for MT or
PT of nozzle welds in vessels constructed to Pijlrt
1
UCS requirements) and somewhat less than for
Division 2 (which requires essentially 100% examination ofall welded joints).
Stoomwezen. The materials are classified in material categories A, B, C and CP for the purpose of
non-destructive examination of welds. The majority
of unalloyed and low alloy ferritic steels and austenitic stainless steels belong to Category A. The extent
of non-destructive examination depends on the hazard category, material category and thickness of the
pressure part. Generally, more examination is required for those vessels in higher hazard categories
and for thicker material. Welds in Category C materials (which require more care in processing) and
Category CP materials (which are sensitive to cracking during their use) and all Hazard Category 3
vessels require 100% NDE, including temporary
attachment weld locations.
Butt welds in cylinders, cones, spheres, flat
walls, heads and headers: Table 33 lists the
extent of the examination required for material
groupsAandB.
The extent of limited examination must meet the
following, except for tubes with outside diameter
De< 325 mm (See Stoomwezen code7 for tubes with
O.D. De< 325 mm):
• 10% of weld length and all weld intersections
• All area where an opening intersects a weld, or
where the center-to-center distance between
weld and opening is less than 1.5 times the
diameter of that opening
• One examination of each type of weld (beside
the intersections listed above)
• All welds in the knuckle and straight flange for
dished heads
The extent of extensive examination shall be the
same as that for full examination (per Sheet T 0110)
when:
• The strength reduction factor per D 0201, D
0203 or D 0205 is higher than 0.35
• The vessel part is used in creep range
• Fatigue analysis is required
• Additional notch testing is required in accordance with M 0110
In all other cases, the extensive examination shall at
least meet the following:
• 25% of weld length
• 100% of weld intersections
T 0110 does not define the method of examination. It
is interpreted by the authors as radiographic testing
(RT) in T 0110.
Table33
Wall Thickness:
dd < dmin
dmin $ dd $ 16 mm
16 mm < da ,,; 40 mm
Full
Extensive
>
Full
Full
Full
Full
da = wall thickness shown on construction drawings.
dmin = minimum required wall thickness.
Evaluation of Design Margins, Section VIII
67
Butt welds in dished and knuckled heads shall
have the following NDE:
1. Wall thickness da ::; 16 mm: limited examination (as defined above)
2. Wall thickness da > 16 mm: complete (100%)
examination.
Materials in Category C or CP and Vessels in
Hazard Category 3: The following welds require
100% non-destructive examination:
• All welds in or upon pressure vessels,
• All temporary field weld locations after these
welds have been ground off,
• Certain welds in tubes and attachment welds on
tubes of Category B materials. (See Stoomwezen Code, T 0110.)
Ultrasonic examination in ferritic steels: UT
examination generally is performed only for thickness exceeding 8 mm (0.315"). RT examination must
supplemented by UT (to the same extent as RT) in
the following cases:
• When 60 mm (2.36") < dr :S 90 mm (3.54") and at
the same time da :S 60 mm
da = actual wall thickness
d = material thickness between radiation source
r and recording material
• Category C and CP materials, when 30 mm
(1.18") < dr :S 90 mm and da :S 60 mm
• If dr > 90 mm or da > 60 mm
UT must be done in place or RT in following cases:
• If dr > 90 mm or dd > 60 mm
• Nozzle welds for Category C or CP materials
when the diameter of the cutout in the vessel
wall > 100 mm (3.94") and the nozzle wall
thickness >20 mm (0.79")
• Welds for which RT is not practical
The acceptance criteria for UT examination is the
same as for RT. The UT examination procedure shall
meet the requirements of T 0202.
Surface examination must be performed in the
following cases:
• All welds in or on pressure part of Category C
and CP materials
• All welds on the pressure part of Hazard Category 3 system
• Welds for which the required RT/UT is not
practical, in which case also the root pass must
be examined in double butt welded joints before
welding the other side.
• All areas where material has been thermally
removed by gouging or flame cutting (attachments, tapered edges, etc.)
• Where this is otherwise specified in Stoomwezen Code, Sheet T 0110
CEN. s The NDE of welded joints depends on the
68
Risk Category (four categories), Testing Category or
sub-category of welded joints (total seven categories
and subcategories), group of steel, maximum thickness, and type of weld joint. It is intended that any of
the testing categories will provide adequate integrity
for normal purposes.
The CEN draft Standard lists the following testing
categories: 1, 2, 3 and 4. Testing categories 1, 2 and 3
are further subdivided sub-categories la, lb, 2a, 2b,
3a and 3b as indicated in Table 2 of the CEN
standard.
Testing category 4 is intended to be used only as
single category for the entire vessel and shall not be
combined with any other category.
All butt welds in the vessel shell must be full
penetration welds. RT or UT are acceptable methods
for volumetric examination of the butt welds in the
vessel shell.
Table 34 includes a partial listing of the NDE
requirements for butt welds (from Table 4 of the
CEN draft Standard).
All temporary attachment weld areas must be
100% MT or PT examined. All welded repairs must
be 100% RT or UT examined and 100% MT or PT
examined.
In some respects the NDE requirements in the
CEN draft standard are similar to those in ADMerkblatt code. All welds which require 100% RT or
UT also require a certain amount of surface examination by MT or PT (e.g. 10%). Based on satisfactory
prior experience, the amount of RT or UT for butt
welds can be reduced from 100% to 10% and MT or
PT for nozzle welds from 100% to 10% or 25% for
certain materials.
d. Tolerances
ASME Code, Section VIII, Division 1 includes
requirements for out-of-roundness tolerances for both
internal and external pressure (UG-80). For internal
pressure the differences between the maximum and
minimum inside diameters are limited to 1% of the
nominal diameter at the cross section under consideration. The UG-80 and Fig. UG-80.l include procedures for determining maximum plus-or-minus deviation e from the true circular form for external
pressure.
Most international codes have the same or similar
out-of-roundness requirements. Some of the European codes have additional tolerance requirements.
BS 5500 requires checking of the profile by the
means of a template for vessels:
• which have been subjected to fatigue analysis
• vessels constructed of steel with specified minimum yield strength >400 N/mm2 (58 Ksi)
Irregularity of the profile shall be checked using a
template with an enclosed arc of 20° and the deviations o, referred to the mid-thickness line, shall
limited to:
o::; 0.05e + 0.002D (max. 25 mm), where
WRC Bulletin 435
Table 34
Base Materials
Longitudinal
butt welds
Circumf. butt
welds in
shell and
nozzles
with
di> 150
mm or
e> 16mm
Circumf. butt
welds in
nozzles
with
di :s; 150
mm&
e:s; 16mm
Butt welds in
spheres &
heads&
hemi
heads to
shell
St ltoSt 10
RT or UT
100%
MT or PT
10%
RT or UT
MT or PT
100%
10%
RT or UT
MT or PT
0
100%
RT or UT
MT or PT
100%
10%
St 1.1, 1.2,
8.1
100%
St8.2, 9.1,
9.2, 9.3, 10
100-10%
St 1.1, 1.2,
8.1
100-10%
St8.2, 9.1,
9.2, 10
25%
St 1.1, 1.2,
8.1
10%
10%for
e> 30mm
25%
10%for
e>30mm
10%
10%for
e>30mm
25%-10%*
10%for
e>30mm
10%
10%for
e>30mm
10%
10%for
e>30mm
10%
100%
10%for
e> 30mm
100-10%*
10%
0
100-10%*
10%
100%-10%*
10%*
100-10%*
10%for
e> 30mm
10%
10%
St 1.1, 8.1
0
0
0
0
10%
0
10%
0
0
25%
10%
10%
10%for
e>30mm
0
*The second number indicates the extent of examination permitted after successful experience (as defined in CENtrC 54 draft standard
for Unfired Pressure Vessels, Part 5).
ois the maximum local irregularity;
e is the plate thickness, mm;
Dis the outside diameter of the shell, mm.
BS 5500 also limits the straightness of the vessel
to 0.3% of either the total cylinder length or of any
individual 5 m (16.4 ft) length of the vessel.
AD-Merkblatt specifies limits for out-of-roundness and local under thickness deviations (ADMerkblatt HP 1) but does not include limitations for
peaking or local flat areas. The past experience by
the authors is that the AI (TUV) will require propf
(by calculations or strain gage measurements) during overload testing that all stresses will remain in
the elastic range after the first overload test.
The Stoomwezen Code (T 0230) gives procedures for measuring out-of-roundness of cylindrical
shells and for determining local deviations (for vessels subjected to external pressure), but does not
specify maximum permissible deviations.
The CEN draft standard includes tolerance limits for middle line alignment, surface alignment,
out-of-roundness for vessels subject to internal pressure, peaking at welded joints, local area underthickness, and dished heads.
The maximum offset limits for middle line alignment and surface alignment depend on type of shell
seam (longitudinal or circumferential), location (cylinder, cone, dished head, sphere) and thickness.
The straightness of the vessel shall not exceed 0.5%
max. of total cylindrical length (vs. 0.3% in BS 5500).
Peaking shall be checked by a 20° gage length and
shall not exceed 2% of the length of the gage. This
can be increased by 25% ifthe length of the peaking
does not exceed :Y,. of the length of the shell between
two circumferential joints, with a maximum of 1 m.
Limitations are also specified for peaking associated with flat areas adjacent to the welded joints.
e. Postweld Heat Treatment
ASME Section VIII, Division 1 requires PWHT
of vessels in the following cases:
• All carbon and low alloy steel vessels in lethal
service.
• Cold formed carbon and low alloy steel plates
when the extreme fiber elongation is more than
5% and any one of the following conditions exist:
-the vessel will contain lethal substances
-the material requires impact testing
-the thickness of the part before cold forming
exceeds%"
-the material during forming is in the range
250°i,and 900°F
• When the nominal thickness (including corrosion allowance) exceeds the the thicknesses
given in Tables UCS-56, UHT-56, UHA-32 and
UNF-56. PWHT shall be at the minimum temperatures/temperature ranges given in these
tables for the materials listed
• When the MDMT is colder than -55°F, and for
vessels installed at a stationary location, the
coincident ratio defined in Fig. UCS-66.l is 0.4
or greater
• When welding ferritic materials greater than Ya"
with the electron beam process
Evaluation of Design Margins, Section VIII
69
• When welding P-No. 3, P-No. 4. P-No. 5 and
P-No. 10 materials of any thickness using the
inertia or continuous drive friction welding process
Electroslag welds in ferritic materials over l Yz" thick
and eletrogas welds in ferritic materials with any
single pass greater than 1Yz" shall be given austenitizing heat treatment.
ASME, Section VIII, Division 2 requires PWHT
all ferrous materials in welded pressure vessels or
parts when:
• the nominal thiclmess, including corrosion allowance, of any welded joint exceeds the limits in
TablesAF-402.1andAF631.1
• the vessel is designed for lethal service
• when welding ferritic materials greater than Ys"
thick with electron beam welding
• when welding P-No. 3, P-no. 4, P-No. 5A, 5B and
5C, P-no. 6, P-No. 7 (except Type 405 and Type
410S) and P-No. 10 materials using the inertia
and continuous drive friction welding process.
BS 5500. The PWHT requirements in BS 5500
are similar to those in ASME Section VIII. BS 5500,
however, permits as-welded vessels of C and C-Mn
steels up to 35 mm (1.38"). This can be increased up
to 40 mm (1.57") provided the materials are impact
tested to meet 27 J (20 ft-lbs) at -20°C (-4°F).
PWHT of welded assemblies is required in the
following cases:
• Ferritic steel vessels designed to operate above
0°C (32°F), when the thickness of welded joints
exceed the limitations in BS 5500, Table 4.4.3.1.
When agreed to by the purchaser, manufacturer
and the Inspecting Authority, the thickness
limits in Table 4.4.3.1 may be exceeded, providing a fracture analysis is performed in accordance with Appendix U and PD 6493
• Ferritic steel vessels designed to operate below
0°C (32°F) when PWHT is necessary in accordance with Appendix D
• Vessels intended to operate in service which
could cause stress corrosion cracking
• Where specified by the purchaser
AD Merkblatt. PWHT depends on the materials
group and thickness. Normally, C and C-Mn steel
welded assemblies with thickness exceeding 30 mm
(1.18") must be PWHT. AD-Merkblatt HP 7/2 permits a maximum thickness of 50 mm (1.97") without
PWHT for material groups 1 and 5.1 providing the
following additional requirements are met:
• Impact tested fine grain structural steels to DIN
17102, 17178, 17179, etc., to meet 31 J, transverse, at 0°C (32°F).
• Vessels of simple geometric shape (sphere, cylinder)
• 100%NDE
• Test stress during overload test 2::0.85 Remin
70
(specified minimum yield strength) at room
temperature
• Special brittle fracture investigation is performed
• Components with nozzles and welded on parts
are stress relieved
Stoomwezen Code, W 0701 requires PWHT of
welded joints in carbon and low alloy steels when the
thiclmess exceeds 32 mm (1.26"). The PWHT temperatures and times generally are in accordance with published material specifications. If no information is given
on PWHT, the following requirements shall be met:
• Minimum PWHT temperature for unalloyed
and low alloy steels: 600°C (1112°F)
• Permissible temperature variation: +/- 20°c
(±36°F)
• Minimum hold time: 120 de, sec., with a minimum of 1800 sec. (0.5 hr)
• Maximum hold time: 7200 sec. (2 hrs)
The following examples list PWHT requirements for
Material Groups 1, 2, 3 and 5.1 (from AD-Merkblatt
are from HP 7/2). PWHTcan be omitted when all the
following conditions have been met:
Materials Group 1 (C and C-Mn steels with specified minimum yield point <370 N/mm2):
s < 30 mm (1.18"): No additional requirements.
(s =thickness at welded joint)
s > 30 :;::; 50 mm (2"): Fine grain structural steels to
DIN 17102, 17178 and 17179, and other steel
grades which meet the same minimum impact test
requirements (as listed in DIN 17102, etc.).
s > 38 :;::; 50 mm (2"): All steels with specified impact
test requirement 2::31 J (23 ft-lbs), transverse, at
0°C (32°F). Simple geometrical shape (sphere,
cylinder); 100% NDE; max. overload test stress
2:: 0.85 UTS; brittle fracture assessment; nozzle
assemblies shall be PWHT.
Materials Group 2 (fine grain structural steels with
specified minimum yield point 2::370 <430 N/mm2):
s :;::; 30 mm: Fine grain structural steels to DIN
17102, 17178 and 17179 and other steel grades
which meet the same impact test requirements.
Materials Group 3 (fine grain structural steels with
minimum yield point 2:: 430 N/mm2 ):
s :;::; 30 mm: All grades in this category.
Materials Group 5.1 (fine grain structural steels to
DIN 17102, 17178 and 17179 with specified minimum yield point 2::370 N/mm 2):
s :;::; 30 mm: All grades.
s > 30 :;::; 38 mm: Fine grain steels to DIN 17102,
17178 and 17179 and all grades which meet the
same min. impact test requirements.
s 2:: 38 :;::; 50 mm: All grades with min. impact
requirement 2:: 31 J, transverse, at 0°C. Simple
geometrical shape (sphere, cylinder); 100% NDE;
overload test stress 2:: 0.85 UTS; brittle fracture
assessment; nozzle assemblies PWHT.
Group 5.4 materials (3.5, 5 and 9% Ni steels) need
not be PWHT with thiclmess s :;::; 50 mm, providing
they are welded with austenitic or Ni based alloy
filler metals. (The maximum thickness without
PWHT for these steels is 30 mm if welded with same
type filler metals as the base metal).
The PWHT temperatures and hold times in ADMerkblatt Code generally are in accordance with the
material specifications. The PWHT temperatures for
C and C-Mn steels generally are lower than those in
ASMECode.
WRC Bulletin 435
de is the effective thickness of weld, mm (as defined
in W 0701 for evaluating PWHT requirements).
PWHT is required when the wall thickness de > 32
mm (1.26"). However, PWHT may be required also
for lesser thicknesses, e.g.:
• Steels with carbon equivalent Ceq > 0.45% or
carbon content C > 0.25%
• Fine grain steel with C > 0.23%
• Fine grain steels with Ceq > 0.55% or specified
minimum yield strength Re > 450 N/mm2 and
hardness difference between HAZ and base
metal or HAZ and weld metal ~H > 150 HV
Welded joints with de < 40 mm in fine grain steels
need not be PWHT when all the following requirements are met:
•
•
•
•
c:;::; 023%
Ceq:;::; 0.55%
Re:;::; 370 N/mm2
Base metal meets Charpy V-notch impact test
energy 31 J at 0°C or 27 J at -50°C and weld
metal 2:: 31 J at 0°C and the vessel is subjected
to hydrostatic test.
The CEN drafi Standard, Part 4 (Table 18)
requires PWHTofwelds in low strength C and C-Mii
steels (St 1.1 and St 1.2), normalized fine grain C-Mn
steels (St 2.1), C-Mo steels 16Mo3 (St 1.2), MnNi and
Ni bearing low alloy steels (St 9.1 and St 9.2, except
St 9.3, 9% Ni steel X8Ni9), when the thickness
exceeds 35 mm (1.38").
The PWHT temperature for thicknesses over 35
mm is 550-600°C (1022-ll12°F) for St 1.1, St 1.2
and St 2.1, 550-620°C (1022-1148°F) for St 1.2, and
530-580°C (986-1076°F) for St 9.1 and St 9.2, with
the hold times listed in Table 35.
::;;35
>35 :s; 90
en, nominal thickness of weld, is defined in the CEN draft
standard, Part 4.
The 35 mm maximum thickness without PWHT
may be exceeded when justified by fracture mechanics analysis.
The PWHT temperatures and hold times in the
Draft CEN Standard are lower than those required
by ASME Code.
If repair welding is performed after PWHT or after
the hydrostatic test, these operations shall be repeated.
f. Third Party Inspection
The Authorized Inspector (AI) has the responsibility for monitoring the ASME Section Vlll Division 1 vessel Manufacturer's Quality Cont;ol System as well as for the duties in UG-90(c). The AI for
ASME Section VIII, Division 2 vessels has similar
responsibilities. These are outlined in AG-303 and
AI-102. Some of the Al's duties include verification
that:
• the manufacturer has a valid Certificate of
Authorization and is working to a Quality Control System
• the applicable Design Report, User's Design
Specification, drawings and other related documents are available
• the materials comply with the requirements of
the Code
• all welders, welding operators and welding procedures have been qualified
• all heat treatments, including PWHT, have been
performed
• material imperfections have been properly repaired and examined
• the various inspections and tests have been
performed and that the results are acceptable
• the required marking is provided and that the
name plate is properly attached
• the manufacturer has maintained proper records
The inspector has to perform internal and external
inspections and witness the pressure tests and sign
the Certificate of Inspection on the Manufacturer's
Data Report, but he does not need to verify the
design nor perform or supervise NDE or other types
of tests (as stipulated in some European pressure
vessel codes and standards).
BS 5500. The Inspecting Authority (IA) is responsible for ve:Nit:ying that:
• all parts have been designed in accordance with
requirements of BS 5500 an as specified by the
purchaser
• that the vessel has been constructed and tested
in accordance with BS 5500 and the purchaser's
options (as listed in Table 1.5 of BS 5500)
The specific duties for IA during manufacturing and
testing are given in Table 5.1 of BS 5500 and are
summarized below:
• Correlation of material certificates with materi-
Evaluation of Design Margins, Section VIII
71
•
•
•
•
•
•
•
•
•
•
als and check for conformity with material
specifications
Identification of material and witnessing of
transfer of identification marks at manufacturer's facility
Examination of cut edges and heat affected
zones (Category 3 components).
Approval of welding procedures
Approval of welders and operators
Examination of seams for welding, including
dimensional check, weld preparations, tack
welds, etc. (Category 3 components)
Inspection of second side of weld preparation
after preparation of first side and root cleaning
(Category 3 components)
Examination ofNDE reports and check compliance with the agreed procedure and acceptability of any weld defects.
Examine heat treatment records and check compliance with the agreed procedure
Witness pressure test and, where necessary,
record the amount of permanent set
Examine completed vessel before shipping.
Check marking.
The German Pressure Vessel Code 6, 41 specifies
that certain Testing Group vessels may not be put in
service until the Authorized Inspector has subjected
the vessel to an initial test (inspection) and an
acceptance test (inspection) and has certified that he
has found it in a serviceable condition. These vessels
belong to Testing Groups III, IV, VI and VII. The
initial test consists of a preliminary test, a structural
test and a pressure test. The acceptance test consists
of a service test, an equipment test and an installation test. (See Appendix 2.)
The requirements for involvement of Inspection
Authority (or Authorized Inspector appear throughout the AD-Merkblatt Code. Some of the requirements are:
• NDE of materials, if such tests are stipulated in
the material specification (AD-Merk.blatt WO)
• Verification of materials, testing, marking and
certification of materials for vessels belonging to
Testing Groups III, IV, VI and VII (W-series
AD-Merkblaetter)
• Transfer of marking of materials subject to
construction inspection by the AI
• Review and approval for all modifications and
repair work on finished components and vessels
after NDE and after the construction inspection
or partial construction inspection (AD-Merkblatt HP O)
• Verification of welding procedure qualification
tests for vessels subject to construction inspection (AD-Merk.blatt HP 2/1)
• Witnessing tests on production test plates for
vessels subject to construction inspection (ADMerkblatt HP 5/2).
• Witnessing and supervision of NDE of vessels
72
subject to construction inspection (AD-Merkblatt HP 5/3)
• Review of heat treat procedures for parts and
vessels subject to construction inspection (ADMerkblatt HP 7/1, HP 7/2)
• Testing by Authorized Inspectors; Initial Test
(TRB 511), Pressure Test (TRB512), Acceptance
Test (TRB 513), Repeat Tests (TRB 514)
Sto01nwezen. The requirements for involvement
by the Notified Body (Inspection Authority) appears
throughout the Stoomwezen Code. The following
lists some of the involvement by Notified Body
during manufacturing:
• Approval of material specifications and certification of materials (M 0201throughM0803)
• Supervision during fabrication, examination of
Hazard Category 2 and 3 vessels (T 0101)
• NDE of welded joints (T 0110, T0112).
• Destructive testing of welds CT 0120)
• Mechanical testing of materials CT 0121)
• Welding procedure qualification (T0210 & T
0211)
• Welders' qualification CT 0215)
• Testing of materials in final condition CT 0221)
• Out-of-roundness measuremens CT 0230)
• Pressure test CT 0240)
• Certification of vessels CT 0270)
Drafi CEN Standard. The Notified Body (Al) is
responsible for verifying (where applicable):
l. that all parts of the vessel have been designed
in accordance with the requirements of the
Draft CEN standard to the designated design
criteria.
2. that the vessel has been constructed and tested
in accordance with this standard.
The extent and level of participation by the Notified
Body depends on:
• Risk categories (I, II, III and IV)
• The testing category (1, 2a, 2b, 3a, 3b, and 4)
• The module of conformity assessment (A through
Hl)
The vessel manufacturer has an option of selecting
between a module of conformity assessment involving a certified quality system and one which does
not; however, it shall be agreed with the purchaser
and communicated to the Notified Body. In the case
of conformity assessment module involving a certified quality system, there is a reduced involvement
by the Notified Body.
Table 36 (Table F.2-1 in Section 5 of the CEN draft
Standard) lists the applicable conformity assessment modules for the various hazards categories
without and with a certified quality system (QS).
The various conformity assesment modules are
described in the European Pressure Equipment
rective (PED). 42 The Notified Body is responsible for
WRC Bulletin 435
I
II
III
IV
A
Al
Bl+F
A
Al
B +Cl
DlorEl
Bl+ D,orH
DlorEl
B + E
QS = quality system.
the following activities when the conformity assessment module Hl has been selected:
• Assessing the vessel manufacturer's quality system
• Evaluating any changes to the approved system
• Performance of surveillance and audits
Table H-1 in Section 5 of the CEN draft Standard
provides a detailed summary of the activities and
participation by manufacturer and Notified Body for
individual pressure vessels with respect to conformity assessment modules A, Al, Dl, El, Bl+ F, H,
Bl + D, G and Hl. Table H-2 lists the involvement
by manufacturer and Notified Body for serially produced pressure vessels with respect to the various
conformity assessment modules. The Notified Body
is responsible for inspection operations related to:
• Design, purchase and subcontracting specifications
• Material inspection
• Welding and forming
• Production tests
• PWHT
• NDE
• Pressure test
• Safety devices
• Conformity assessment
least 30°F above the minimum MDMT to minimize
the risk of brittle fracture.
The hydrostatic test pressure for ASME Section
VIII, Division 2 vessels is at least 1.25 times the
maximum allowable working pressure in every part
of the vessel. The pneumatic test pressure shall be at
least 1.15 times the maximum allowable working
pressure (Articles T-3 and T-4).
The primary general membrane stress intensity
Pm shall not exceed the 90% of the specified minimum yield strength Sy at the test temperature for
the hydrostatic test and 0.8Sy for the pneumatic and
hydro-pneumatic test. The test stresses shall also
satisfy the requirements:
Pm+ Pb 5 1.35 Sy for Pm 5 0.67Sy and
Pm+ Pb:$ 2.15Sy - 1.2 Pm for 0.67Sy <Pm:$ 0.9 Sy
Pb = Primary bending stress intensity
Both Division 1 and Division 2 include the following
considerations for the metal temperature during
overload testing to minimize the risk of brittle
fracture:
• The Code recommends that the minimum metal
temperature during the hydrostatic test be maintained at least 30°F above the minimum metal
design temperature (MDMT), but need not exceed 120°F.
• The Code requires that the metal temperature
during the pneumatic test be maintained at
least 30°F above the minimum MDMT), but
need not exceed 120°F.
BS 5500. The test pressure for hydrostatic, pneumatic and combined hydrostatic/pneumatic tests
shall not be less than the following:
Category 1 and 2 construction:
Pt = 1.25 p (fjft) (t/(t
c)]
Category 3 construction:
8. Overpressure Testing Requirements
The hydrostatic test pressure for ASME Section
VIII, Division 1 vessels is at least 1.5 times the
maximum allowable working pressure in every part
of the vessel multiplied by the lowest ratio of the
allowable design stress at the test temperature to
the allowable design stress at the design temperature. (Recent changes to the Division 1 requirements
have reduced the hydrostatic test factor from 1.5 to
1.3). The Code does not specify the maximum upper
limit; however, vessels with visible permanent distortion may be rejected by the Authorized Inspector
(Al).
The pneumatic test pressure shall be at least 1.25
times the maximum allowable working pressure
multiplied by the lowest ratio of the allowable design
stress at the test temperature to the allowable
design stress at the design temperature. The metal
temperature during the pneumatic test shall be at
Pt = 1.25 P (fafft) (t/(t - c)], or Pt = 1.5 p, whichever is
higher
p = design pressure
fa = nominal design strength (stress) at test temperature
ft = nominal design strength (stress) at design
temperature
t = nominl!! thickness of the section under consideration
c = corrosion allowance
The general membrane stress in any part of the
vessel during test shall not exceed 90% of the
minimum specified yield strength of the material.
AD Merkblatt. The maximum overload test pressure at the highest point in the vessel shall be:
Hydrostatic test: 1.3 X p for the following materials: steel and malleable non-ferrous metallic materials, cast steel, cast iron.
Evaluation of Design Margins, Section VIII
73
Two alternates are given for pneumatic test:
c = corrosion allowance
1. The same test pressure as for hydrostatic test
(and the applicable materials) when the pressure vessel is located in a place where nobody is
in the immediate danger while the vessel is
subjected to the test pressure (i.e. explosion
proof test chamber, water basin with the appropriate precautions to prevent flying objects).
2. Pp= l.lp.
The temperature of the shall meet all the following
requirements:
Stoomwezen. The required hydrostatic test
pressure Pt is given in the table below (Table 37):
The maximum allowable hydrostatic test pressure
Pt is:
• Pt = 13.5 Pdi for dished parts with pressure on
the convex side
the lowest "adjusted design pressure" at
P dl
test temperature for each pressure part, calculated without deducting the corrosion allowance
• Pt = 14 Pdi for all other cases
The minimum test temperature must be at least 5°C
(9°F) above the freezing point, but no lower than
l0°C (50°F).
Pneumatic test (including pneumatic/hydrostatic test). The required and allowable test pressures for are:
• The required test pressure Pt = 10 Pd
• The allowable test pressure Pt = 10 Pd1
Pdi = the lowest "adjusted design pressure" at
20°C (68°F) for each pressure part, calculated
without deducting the corrosion allowance.
Toughness evaluation of the vessel shall be made in
accordance with Stoomwezen M 0110 if the test
temperature reduces to below 15°C (59°F) during the
pneumatic test.
CEN. For Testing Category 1, 2 and 3 vessels, the
hydrostatic test pressure Pt shall be the higher of:
Pt = 1.25 Pd (falft), or
Pt= 1.43 Pd
Pd = the design pressure of the vessel part
fa = nominal design stress at test temperature
ft = nominal design stress at design temperature
For Testing Category 4 vessels the test pressure
shall be the higher of the following:
Pt = 1. 75 Pd (fafft)(e/e-c) for St 1.1 steels
Pt= 1.6 Pd (fafft) for St 8.1 steels
e = nominal thiclmess of the section under consideration
1. It shall be above the freezing point
2. It shall be 10 K (18°F) below the atmospheric
boiling point
3. It shall be of sufficient temperature to avoid
brittle fracture
The pneumatic test pressure shall be:
Pt = 1.1 Pd (fa/ft)
Special consideration must be given to vessels subjected to pneumatic test, e.g.:
• MT or PT of all welds which have not been
subjected to 100 RT or equivalent inspection as
required for the main seams.
• Test temperature at least 25°C above the impact
test temperature required by this CEN standard for vessels which have not been previously
subjected to a hydrostatic test at a pressure
exceeding the pneumatic test pressure.
• Absolute necessity of avoiding brittle fracture.
• Remote monitoring during the test.
of the quality system during the initial approval
phase of the facility and review of the quality system
during the production phase.
The CEN draft Standard lists various modules for
conformity assessment. The vessel manufacturer
has the option of selecting an appropriate module of
conformity assessment; however, it shall be agreed
with the purchaser and communicated to the with
the Notified Body. In the case of conformity assessment module involving a certified quality system,
there is a reduced involvement by the Notified Body.
The vessel manufacturer must have a quality
management system for vessels fabricated in accordance with the conformity assessment modules listed
in Table 38.
Table38
Module of
Conformity Assessment
Quality Management
System
E
D
H
ISO 9003 (EN 29003)
ISO 9002 (EN 29002)
ISO 9001(EN29001)
10. Summary and Conclusions
9. Quality Control System
ASME Section VIII, Divisions 1 and 2. Both,
Division 1 and Division 2 require that the vessel
manufacturer or assembler have and maintain a
Quality Control System which establishes that all
Code requirements, including material, design, fabrication, examination (by the manufacturer or assembler) and inspection (by the Inspector) will be met.
The requirements for Quality Control System for
manufacturers of Division 1 vessels are included in
Section VIII, Division 1, Appendix 10. The requirements for Quality Control System for manufacturers
of Division 2 vessels are included in Section VIII,
Division 2, Appendix 18.
No specific requirements are included in BS 5500
for quality control.
Although quality control is mentioned in the ADMerkblatt Code, there are no specific requirements
for a quality control system in the Code.
CEN. The materials manufacturer shall maintain a quality system in accordance with EN ISO
9002 and obtain certification for this system from a
Notified Body (a third party authorized to perform
the conformity assessment tasks).
Materials manufacturers are subject to inspection
This part of the Phase 2 included a review of four
European pressure vessel codes and standards and a
comparison withASME Section VIII, Divisions 1 and
2. The following are the observations and conclusions from this study:
Design Margins
All the European codes and standards included in
this study have lower design margins at temperatures below the creep range than in both Section VIII
Division 1 and Division 2. For un-alloyed and low
alloy steels, all these codes and standards use a
factor of 1.5 on the specified minimum yield strengt)l
at design temperature and different factors on tB:e
specified minimum tensile strength. The factors on
tensile strength are:
• 4 at design temperature for ASME, Section VIII,
Division 1
• 3 at design temperature for ASME, Section VIII,
Division 2
• 2.35 at room temperature for BS 5500 for Construction Categories 1 and 2 (5 for Construction
Category C and C-Mn vessels)
• 2.27 at room temperature for Stoomwezen (with
elongationA5 > 10%, otherwise the factor is 4)
• 2.4 at room temperature for the Draft CEN
Standard (1.875 for the Alternative Design Basis, which was included in the July 1995 CEN/TC
54 Draft Standard for Unfired Pressure Vessels
but not in the May 1998 Draft.)
The AD-Merkblatt Code specifies factors on ultimate
tensile strength only for gray cast iron and for copper
and its alloys.
Pd
14
WRC Bulletin 435
The allowable design stresses for stainless steels
and non-ferrous alloys are the same as for C, C-Mn
and low alloy steels, or somewhat different, but in
any case, more liberal than those for both ASME
Section VIII, Divisions 1and2.
No actual comparisons were made on allowable
design stresses in the creep range; however, the
allowable stress basis for each code and standard is
included in the report.
All the European codes and standards addressed
in this report (except BS 5500) require adding the
material under-thiclmess tolerance to the calculated
shell thickness.
Design Rules
The basic formulas for the design of shells are
almost the same in various codes and standards.
These formulas, being based on simple shell theory,
cannot be modified or improved much. However, the
design rules for special components and for areas of
discontinuity vary a great deal from one code to
another.
In most European Codes the rules for design of
cone-cylinder junctions are based on limit analysis
or other analytical methods. Such rules in the ASME
Codes are scattered in various parts and could be
improved by the use of available analytical methods.
However, the existing rules are conservative and
have resulted in no known problems.
The existing rules in the ASME Codes for the
design of dished heads need to be improved. Alternative rules being considered by the ASME Code
Committee will significantly improve these rules.
Although the present rules have a rather arbitrary
basis, resulting in a varying safety factor over the
range of geometric parameters, they are quite conservative. Over the most of the range of geometries, the
calculated thiclmess of ASME Code (Div. 1 and Div.
2) heads are greater than those of European Codes
investigated in this report. Failure by buckling should
be included in the ASME rules and the range of
geometries extended to thinner shells.
The formulas for design of fl.at covers are similar in
all codes. However, the value of "c" factors vary
significantly. Analytical tools are available to arrive
at more accurate values of the "c" factors. Various
code organizations should cooperate to specify consistent values. Flat covers by ASME rules, would be
generally tmcker than those designed by European
Codes, mostly due to lower allowable stresses.
The rules for design of bolted flanges in theASME
Codes are conservative. A revised set of rules is being
developed which has not been studied for this report.
It is difficult to assert that the ASME rules are
consistently conservative relative to European Codes
and Standards.
Opening reinforcement rules vary a great deal
also. The ASME pressure area method has been
extensively used with excellent results. However,
these rules may be overly conservative. The Euro-
Evaluation of Design Margins, Section VIII
75
pean Codes and Standards have recently used more
analytical approaches, such as limit analysis. A
review of ASME Code approach to penetration design is desirable.
There are no rules for fatigue exemption or fatigue
analysis in ASME Section VIII, Division 1. It would
be desirable to provide some guidelines. The fatigue
curves in Section VIII, Division 2 are based on
smooth specimen tests, and the fatigue analysis
rules require evaluation of stress concentration factors at discontinuities. The Section VIII, Division 2
rules could possibly be unconservative in some cases.
A thorough review and update of these rules is
needed. The European rules are based on fatigue
tests on weldments and generally are more up-todate and comprehensive.
The rules for external pressure in the ASME Codes
are conservative. A set of alternative rules, based on
latest test data, are included in Code Case 2286. In
general, the present ASME buckling rules are more
conservative than those of the European Codes.
Overall, the comparisons of this report indicate
that design rules of the ASME Code Section VIII
(specially Division 1) are considerably more conservative than those of the European Codes studied.
However, some ASME design rules are out-of-date,
do not have adequate analytical basis, and provide a
non-uniform margin of safety from one component to
another over the range of geometries. The ideal
situation, of course, would be for all vessels to have
uniform safety margins for all components. Although
this will not be totally achievable, a more uniform
safety margin should be strived for.
Special Service Considerations
Both ASME Section VIII, Division 1 and 2 require
vessels in lethal service to be fully radiographed
(Category A, Band C joints for Section 1 vessels, and
all butt joints in Division 2 vessels, as specified in
Division 2). AD-Merkblatt Code does not specifically
address lethal or hazardous service, but requires
periodic testing (NDE and pressure testing) of Testing Group I vessels (when used for combustible,
corrosive or toxic gases, vapors or liquids) and all
Testing Group II, III, IV, VI and VII vessels. 41
Stoomwezen Code classifies all vessels in four hazards categories, with the most severe (Category 3)
requiring 100% NDE. The Draft CEN Code also
classifies its vessels in four risk categories, with the
highest risk category vessels requiring additional
quality control and the most involvement by the
Notified Body (Inspection Agency).
Material Requirements
All codes and standards only permit materials
which have been approved by the appropriate code
bodies or experts. The AD Merkblatt and Stoowezen
Codes accept more and higher strength structural
grade plates than ASME, but both these codes have
some additional requirements beyond those in Sec76
tion VIII, Divisions 1 and 2. Some of the requirements are:
AD-Merkblatt.
• Materials used for pressure vessels belonging to
Testing Groups III, IV, VI and VII are subject to
additional special requirements (as specified in
AD-Merkblatt W))
• The suitability of filler metals and consumables
for pressure vessels belonging to Testing Groups
III, IV, VI and VII must be established by an
expert's report
• Certification is required of all materials (similar
to Section VIII, Division 2)
• Use of Inspection Authority for non-destructive
tests (if such tests are required in the material
specification)
• The type and extent of repair welding and the
type of tests performed on repair welds must be
indicated in the test certificate
• Generally, all materials are subject to notch
toughness requirements at the lowest specified
design temperature, however, materials used at
service temperatures above -10°C (14 °F) generally are impact tested at room temperature
• Additional testing of plates after forming. One
set of mechanical tests (tensiles and impact
tests) from one end of each formed part with
length or diameter greater than 4 m (13.12 ft)
and both ends from of a formed part having a
length or diameter greater than 6 m (19.69 ft)
Stoomwezen:
• Only materials specifications reviewed and approved by the Competent Body can be used for
pressure vessels. Materials must meet any additional requirements established by the Competent Body
• All non-alloyed and low alloy steel materials to
be supplied in the normalized, normalized and
tempered, or in the quenched and tempered
condition
• Ferritic materials are subjected to impact testing at 20°C for quality requirements
• Stoomwezen Categories B and B-1 materials
must be impact tested after hot forming and all
heat treatments (including PWHT)
CEN:
• European technical approval of material manufacturers
• Review and approval of the material manufacturer's quality system by the Notified Body
(Authorized Inspection Agency)
ASME Section VIII covers the requirements pertaining to vessels constructed of specific materials groups
in separate parts of the Code (e.g. Parts UCS, UDA,
UHT, etc. Division 1 and Part AQT, etc. in Division
2), whereas the European codes and standards assign materials into various materials groups and
WRC Bulletin 435
cover the requirements pertaining to specific materials groups throughout the codes and standards.
Toughness Requirements
The toughness requirements in the European pressure vessels codes and standards differ from those in
ASME Section VIII. The toughness requirements in
BS 5500, Stoomwezen and the CEN draft pressure
vessel standard generally are based on Charpy Vnotch correlations with wide plate test results. These
codes use graphs of vessel reference or assessment
temperatures vs. impact test temperatures for different thiclmess materials for non-PWHT and for PWHT
construction. The impact test temperature depends
on the calculated reference or assessment temperature, thic~ness and whether the welded assembly is
stress reheved. Formulas are provided to calculate
the reference or assessment temperatures which
include adjustments based on calculated membrane
stress, construction categories (in case of BS 5500),
PWHT of vessel assemblies, etc. Substantial benefits
are given to PWHT.
B~ .5500 includes toughness requirements only for
ferntic steels required to operate below 0°C (32°F).
?'he required minimum average impact values at the
impact test temperature are 27 J (20 ft-lbs) for steels
with specified minimum UTS < 450 N/mm2 (65.25
ksi) and 40 J (30 ft-lbs) for steels with specified
minimum UTS :::: 450 N/mm2.
BS 5500 requires impact testing weld metal of
welding procedure test plates when the operating
temperature is below 0°C (32°F), but does not req1:1ire impa.ct testing ofHAZ with multi-pass welding
with weldmg heat inputs between 1 kJ/mm (25
kJ/in.) ~nd 5 kJ/mm (127 kJ/in.). Impact testing of
product10n test plates is not required for welds in
material less than 10 mm (0.394") thick.
Stoomwezen requires two types of impact testing:
• Quality test at 20°C (68°F) for all non-alloytd
and low alloy steels. (The required impact test
values are 31 J (23 ft-lbs) for carbon, C-Mn, and
fine grain C-Mn steels with minimum specified
UTS < 450 N/mm 2 and 27 J for UTS :::: 450
N/mm 2)
• Extra testing at the impact test temperature
determined from figures which relate assessment temperatures, thicknesses and impact test
temperatures. (The required impact test values
are 27 J)
!mpact ~esting of procedure qualification test plates
is required for service temperatures below 0°c.
Production test plates need to be impact tested only
when extra testing is required. Impact tests are
required for base metal and weld metal, but not
HAZ. (This is less restrictive than ASME Section
VIII, which requires impact testing of weld metal
and HAZ in carbon and low alloy steels whenever
base metal needs to be impact tested).
The Draft CEN. Standard
lists three alternat·ive
.
meth od s to estabhsh impact test requirements·
Method 1,. developed from past operating e~peri­
e~c~s, and is applicable to steels with specified
mmimum UTS :s 460 N/mm 2 (66.7 ksi) and maximum ~hickness :s 30 mm (l.18") for as-welded constru~t10n and 60 mm (2.36") for PWHT construction.
The impact test temperature is based on the minimurr: MDMT and an adjustment based on actual
tensile stresses (% of maximum allowable). Th
re~':ired impact test values are 27 J (20 ft-lbs~
mmimum average.
Method 2. The base metal, weld metal and HAZ
~hall meet the required impact test values at the
impact test temperature obtained from Fig's D 1
D.2, D.3 or D.4, based on the reference temper.at~r~
and thickness. Fig. D.1 and Fig. D.3 are for PWHT
assemblies and Fig. D.2 and Fig. D.4 are for aswelded assemblies. The required impact test values
are 27 J minimum average for materials with specified minimum UTS:::: 310 N/mm2 and 40 J (30 ft-lbs)
for materials with specified minimum UTS > 310
N/mm 2 (45 ksi).
Method 3 is a fracture mechanics analysis and
may be used for materials not included in this CEN
standard and cases not covered by Methods 1 and 2.
Welding procedure qualification and production
test plates must impact tested in thicknesses exceeding 12 mm (0.47''). Impact tests are required in base
metal, weld metal and HAZ, except that no impact
tests are required in the HAZ of low strength C and
C-Mn steels (Steels in Groups 1.1and1.2).
TheAD-Merkblatt Code requires impact testing of
all carbon and low alloy steel base metals and
welding procedure qualification and production test
plates. For service temperatures - l0°C (l4°F) and
above, the impact test temperatures and impact
values ~or b~se metal ~nd weld metal are generally
as specified m the apphcable material specifications.
The HAZ shall meet 27 J (20 ft-lbs) ISO V-notch
minimum average.
For service temperatures below -10°C (14 °F), the
base metal shall meet the impact test values in the
applicable material specification (generally not less
than 27 J (20 ft-lbs) minimum average) at the lowest
service temperature. The weld metal shall meet 27 J
minimum average and the HAZ 16 J (12 ft-lbs) at the
lowest service temperature.
In gener:f; most of the European codes and standards rev~ewed appear to have more severe toughness requir~ments thanASME Section VIII, particularly the thicker plates in as-welded condition. (See
the comparison of impact test requirements for l"
1.5" and 2" ~hick plates in Section F.6 of this report)'.
AS~E Sect10n VIII, however, have special toughness
requirements (mils lateral expansion and also drop
weight tests for some steels). In some cases the BS
550? and Stoomwezen codes do not require impact
t~stmg of heat affected zones of procedure qualification and production test plates, whereas this is
Evaluation of Design Margins, Section VIII
77
generally required for vessels built to ASME Section
VIII, Division 1 and 2, when the base metal must be
impact tested.
Welded Joints
Whereas various types of joints are permitted for
ASME Section VIII Division 1 vessels, generally only
Type 1 and Type 2 butt joints (for Category B joints
only) can be used for Division 2 vessels. Type 1 butt
joints are required for the high strength materials in
Table AQT-1 in Division 2. Section VIII, Division 2
has very detailed requirements for the types of
welded joints permitted, particularly for high
strength steels.
BS 5500 permits single-welded (with or without
backing strips) or double-welded butt joints for main
seams in vessels; however, single-welded butt joints
are limited to 20 mm thickness, or less, depending on
joint detail, for Construction Category 1 vessels. Also
permanent backing strips must not be used for
Construction Category 1 and 2 vessels. BS 5500
permits full penetration and partial penetration
nozzle attachment welds and includes detailed information on nozzle attachment details and weld preps.
Most nozzle attachment details are full penetration
welds. Rules are also included for weld proximity.
Appendix D includes suggestions for details to be
used on vessels in low temperature service (below
0°C) to reduce the risk of brittle fracture.
Stoomwezen permits one sided butt welds with
backing strips left in place for girth welds for all
materials. Also joggle joints are permitted under
certain conditions. Full penetration and partial penetration welds are permitted for nozzle attachment
welds, but Stoomwezen requires full penetration
welds for all Hazards Category 3 welds and certain
materials which require more care in processing and
are more prone to cracking during their use (Stoomwezen Category C and CP materials).
The Stoomwezen Code offers the greatest amount
of design details amongst the pressure vessel codes
studied in this report. Many of the weld details are
adopted from recommendations by the International
Institute of Welding. The Stoomwezen Code also
includes a "suitability grading" table to help the
designer select the appropriate design details for the
intended service, loading conditions and the method
of examination.
Although most of the European pressure vessel
codes and standards have provisions for use of
various joint efficiencies in their designs (depending
on the type of joints, welding processes and NDE),
they are not as detailed as in ASME Section VIII,
Division 1. Division 2 only uses joint efficiency of 1
since it requires a 100% NDE of welded joints.
Inspection of Welded Joints
ASME Section VIII, Division 1 permits full, spot
and no radiographic examination for butt welded
joints. Division 1 has no specific requirements for
NDE of nozzle attachment welds for Part UCS
78
materials. Division 2 requires 100% RT of all Type 1
and 2 butt welds, UT of certain types of Category C
and Category D joints, and 100% MT or PT of all
nozzle and other attachment welds.
The NDE requirements in the European codes
depend on material, thickness, type of service, design stress and other considerations. For Construction Category 1 vessels, BS 5500 requires 100% RT
or UT all shell butt welds, regardless of thickness,
and 100% RT or UT of nozzle attachment welds
when the thinnest part welded exceeds the specified
thickness limit for a particular materials group.
100% MT or PT is required for all attachment welds.
A lesser amount of NDE is permitted for Construction Category 2 vessels; however only certain materials are permitted for Construction Category 2 vessels and are limited to 40 mm thickness (30 mm for
C-Mo steels). Only visual examination is required for
Construction Category 3 vessels, but these are further restricted to the types of materials and thickness.
The type and extent of the NDE in the ADMerkblatt Code depends on the type of material
(material group), thickness and design stress. Generally 100% of RT or UT is required for all longitudinal
shell butt welds and weld intersections and 100% or
a lesser amount (e.g. 25% or 10%) for circumferential
butt welds. More examination is required for higher
strength materials and for alloyed steels (e.g. 100%
RT or UT of all shell butt welds and weld intersections). The amount of RT or UT examination, however, can be reduced if the results of previous NDE
(on similar vessels) reveals no serious deficiencies.
(This is not an option for ASME Section VIII vessels).
AD-Merkblatt requires MT or PT examination of
all nozzle and other attachment welds. Nozzle welds
also require RT or UT examination for nozzles
exceeding certain size and connecting thickness.
The extent of NDE requirements in the Stoomwezen Code depends on the hazard category, material group and thickness of the pressure part. Generally, more examination is required for higher hazard
categories, more crack sensitive, and thicker material. Full (100%) examination is required for vessels
of Category C or CP materials and for all Hazard
Category 3 vessels. Hazard Category 2 vessels require either full or "extensive" examination depending on the material group and thickness. Hazard
Category 1 require limited examination for Group A
materials and either limited or extensive examination for Group B materials, depending on thickness.
UT must be used in addition to RT for thicknesses
exceeding 60 mm (30 mm for Category C and CP
materials). UT must be used in place of RT for
thicknesses exceeding 60 mm and for nozzle welds
for all Category C and CP materials when the shell
cutouts exceed 100 mm and nozzle wall thickness
exceeds 20 mm.
Surface examination (MT or PT) must be per-
WRC Bulletin 435
formed on all material Category C and CP welds and
on all welds of Hazard Category 3 vessels.
The NDE requirements in the CEN draft Standard depend on the Risk Category, Testing Category
of welded joints, group of steel, type of joint and
maximum thickness. It has probably the most complicated set of requirements of all the European codes
and standards discussed in this report. Longitudinal
butt welds require 100% RT or UT for some Testing
Categories and a lesser amount for others. The same
applies to circumferential butt welds in vessel shells
and to full penetration nozzle attachment welds with
nozzle I.D. > 150 mm and thickness e > 16 mm. Butt
welds which are RT or UT examined also require a
certain amount of MT or PT examination.
The amount or MT or PT of nozzle attachment
welds again depends on the Testing Category, material group, the type of attachment weld and nozzle
size/thickness. Some Testing Categories require 100%
MT or PT, some less.
A unique feature in the CEN standard is that in
many cases the required amount of RT (or UT) and
MT (or PT) can be reduced based on satisfactory
prior experience (similar to AD-Merkblatt). This is
not permitted in the ASME pressure vessel codes.
The ASME Section VII, Division 2 appears to have
the strictest and most well defined NDE requirements. The NDE requirements in BS 5500 appear to
be comparable to those in Section VIII, Division 2.
Also the AD-Merkblatt and Stoomwezen appear to
have similar NDE requirements to Section VIII,
Division 2; however the AD-Merkblatt Code permits
a lesser amount of NDE in certain cases based on
satisfactory prior experience, and the NDE requirements for nozzle welds in the Stoomwezen Code are
not as well defined. It is difficult to compare the CEN
draft Standard requirements to those for Section
VIII vessels because of the many Testing Categories
in the CEN Standard. The NDE requirements in t}ae
European pressure vessel codes and standards generally are more extensive than those in ASME Section
VIII, Division 1.
Tolerances
The out-of-roundness tolerance requirements in
the European pressure vessel codes and standards
appear to be similar to those for ASME Section VIII
vessels; however, some of the European codes and
standards have certain local tolerance requirements
which are not included in Section VIII, Divisions 1
and 2. Some of these are:
BS 5500 has limitations on local deviations (20°
arc) for vessels subject to fatigue analysis and vessels constructed of steels with specified minimum
UTS > 400 N/mm 2 . The Draft CEN Standard limits
peaking at welded joint to 2% of a 20° are gage
length. This standard also has detailed maximum
offset limits for middle line alignments and surface
alignment.
Postweld Heat Treatment (PWHT)
The European pressure vessel codes and standards generally have lower thiclmess limits for
non-PWHT construction (at least for C and C-Mn
steels) than ASME Section VIII. Also, the PWHT
temperatures and hold times are generally lower
(except for BS 5500, which has similar requirements
to those in ASME Section VIII). Table 39 includes a
summary of the thickness limitations for C and
C-Mn steels.
Third Party Inspection
Generally more third party involvement is required by the European pressure vessel codes and
standards, particularly the AD-Merkblatt and the
Stoomwezen Codes and the CEN draft Standard
thanASME Section VIII, Divisions 1and2.
'
BS 5500. The Inspection Authority is commissioned by the Purchaser and is responsible for verifying the design and that the vessel has been constructed and tested in accordance with BS 5500. Its
duties involve verification of materials, approval of
welders and welding operators and welding procedure qualifications, examination of welds and NDE
reports, heat treat records, and witnessing of pressure tests. (This is similar to the requirements in
Section VIII, except verification of the design).
The Authorized Inspector for AD-Merkblatt Code
vessels has additional duties and responsibilities,
such as NDE of materials (where needed), witnessing and supervision of NDE, conducting construction
test (examination) and pressure test, and certification of vessels. The duties of the Notified Body
(Inspection Authority) for vessels built to the Stoomwezen Code are similar to those for the ADMerkblatt Code.
The CEN draft Standard requires the Notified
Body (Inspection Authority) to verify that all parts of
the vessel have been designed in accordance with the
Table39
Code or Standard
Maximum Thickness Without PWHT
ASME VIII, Div. 1 1.5", providing welded joints over 1.25"
thick are preheated 200°F
ASME VIII, Div. 2 Same as for Division 1
35 mm (1.38"); 40 mm (1.57") provided the
BS 5500
materials are impact tested to meet 27 J
(20 ft-lbs) at -20°c (-4°F).
30 mm (1.18"); 38 mm with fine grain,
AD-Merkbl~
impact tested material with minimum
yield strength :s370 N/mm2 ; 50 mm provided material is fine grain steel with
yield strength :s370 N/mm2, impact
tested to meet 31 J at 0°C, welded
assemblies with nozzles and attachments stress relieved, and brittle fracture assessment is performed.
32 mm (1.26"); 40 mm (1.57") provided
Stoomwezen
C :s 0.23%, CE :s 0.55, yield strength
:s370 N/mm2 , base metal and WM meet
31 J at 0°C, and the vessel is subjected to
hydrostatic test.
35 mm (1.38") >35
CEN draft Std
mechanics
Evaluation of Design Margins, Section VIII
79
CEN Pressure Vessel Standard and that the ves~el
has been constructed and tested in accordance. ~1th
this standard. The extent and level of the partici~a­
tion by the Notified Body depends on the vessel nsk
category, testing category and the m?dule of conformity assessment. There is a reduced mvolvement by
the Notified Body for a conformity assessment module involving a certified quality system.
G. Recommendations for ASME Section VIII
Division 1 and Division 2 Revisions
a. Section VIII, Division 1
.
Phase 1 of the PVRC report on design margms
recommended that the design margin on ultimate
tensile strength be reduced from 4.0 to 3.5, mostly on
the basis of the improvements made to that document over the last three or four decades. The change
is in the process of being implemented by the ASME
Code Committee. (Code Case 2278, approved May
20, 1998; Code Case 2290, approved June .17, 199.8).
Based on this Phase 2 study and comparisons with
a number of European Codes, it is felt that f~1:t~er
reduction in design margin for Section VIII, Div1s1on
1 is justifiable if a number of improvements are
made to this Code. The value of design margin on
ultimate tensile strength may be reduced ?elow
down to approximately 3.0 for various service conditions, depending on how many of th~ .proposed
improvements are adopted. The final revisions to be
made and the value of the margin to use is, of c?urse,
left to the discretion oftheASME Code Com1'.1ittee~.
It is suggested that the following areas be mvestigated and, when deemed necessary, the Code .rules
modified by the appropriateASME Code Committees
for adopting a lower design margin:
3.?
1. Simplified fatigue analysis rules or fatigue
exemption rules should be added. One of the
weaknesses of the present rules is the fa~t
that no guidelines are provided on what con~ti­
tutes cyclic service. Simple fatigue exemption
rules, similar to those of ParagraphAD-160 of
Division 2 would be helpful. If a vessel can not
be exempted from fatigue analysis, then the
analysis rules of Division 2 may be referenced.
A number of cycles for exemption will have ~o
be assumed (in the range of 500 to 2000 is
recommended) and the design details and
design rules made consistent for adequacy
with that many cycles.
2. Design details should be reviewed and revised as needed. A number of details allowed
by the existing rules, with a reduction in
design margins, would provide a very low
number of allowable cycles. Depending on the
final design margin adopted and the nun_iber
of cycles used for exemption. from ~ati~e
analysis (item 1 above), the design det~Ils ':ill
have to be modified. Some of the details with
partial penetration and fillet attachment welds
80
will have to be disallowed. In particular, the
details for nozzle attachments will have to be
made more stringent.
3. The rules for design of formed heads should be
updated. The existing rules are out of date and
do not have a rational basis. These rules do
not provide uniform safety margins over the
range of parameters. Analytical methods are
available to prepare a more rational set of
rules. Test results and past experience can be
used to justify analytical results. A proposed
Code Case is being considered by the ASME
Code Committee which will provide alternative design rules for formed heads). (Code
Cases 2260 and 2261, approved May 20, 1998).
4. The rules for design of cone-cylinder junctions
should be improved. The existing rules have
evolved over a long time, without the basis for
various rules having been documented. With
today's analytical means, a more rational a:id
consistent set of rules can be prepared. With
reduced design margins, the satisfactory past
experience will not be adequate justification
for keeping the existing rules.
5. Design rules for reverse knuckles should be
provided. Existing rules do not provide such
rules and rules consistent with the reduced
design margins are needed to assure that
unsafe designs are not used.
6. Design rules for bolted flanges should be improved. The existing design rules an~ gasket
factors have been in existence a long time and
need to be updated. (At the time of this
writing, the ASME Code Committee is ~ork­
ing on a revised set of rules for flange design).
7. Material toughness requirements should be
reviewed and revised as needed. The existing
toughness rules are based on fracture mechanics calculations, modified by successful past
experience. For fracture mechanics c~lcula­
tions the stress in the region surroundmg the
flaw is generally assumed as yield stress, to
account for residual stresses in the as-welded
condition. Therefore, the reduction in design
margin may not provide significantly different
results for the as-welded condition. However,
a number of provisions are based on past
experience with the existing design ma~gins.
Such past experience will not be applicable
with a significant reduction in design margins. Examples of such provisions are Par.
UG-20 (which exempts certain carbon steels
up to l" thickness from impact testing for
minimum design metal temperatures as cold
as -20°F) and paragraph UCS-66(c), (which
allows exemption from impact testing for ferric steel standard flanges for temperatures as
cold as -20°F). Other considerations are:
a. The credit allowed by Figure UCS-66.1 for
WRC Bulletin 435
vessels stressed to a value less tha~ the
allowable stress will also have to be revised.
b. The provisions of paragraph UCS-68(c),
which allows a 30°F credit for PWHT (if not
required by the Code) also needs refinement. Most European codes allow a credit
for PWHT in their impact test exemption
curves. It is recommended that also Section
VIII, Division 1 includes provisions in the
Code rules for impact testing to give benefit
for any PWHT'd vessel.
c. A general review of the toughness requirements in Division 1 for higher allowable
stresses to insure the same margin of safety
on brittle fracture for steels subject to Code
toughness requirements, including the high
strength Q & T steels in Part UHT. (See
Sections C.4 and E.2.c of this report).
8. The required hydrostatic and pneumatic test
pressures should be reviewed. With increased
allowable design stresses, the stresses at the
hydrostatic test pressure (1.5 times design
pressure) may be excessive. The 1.5 factor is
larger than most other codes and will probably
have to be reduced.
9. Allowable stresses for over-pressure testing
should be added. One shortcoming of the
present code is the fact that no limits are
specified for test stresses.
10. The joint efficiencies of paragraph UW-12
should be reviewed and, as needed, revised.
With increased stress allowable design
stresses, some of the joint types, specially fillet
lap joints, may have to be disallowed or further limited. The types of joints allowed will
influence the fatigue exemption rules.
11. The non-destructive examination requirements need to be reviewed and updated for
higher allowable design stresses. For ex/.
ample, Division 1, Part UCS presently has no
specific requirements for NDE of nozzle attachment welds. With higher allowable design
stresses, requirements need to be included in
Division 1 for non-destructive examination of
nozzle welds.
12. Further requirements on the qualifications of
designers may have to be imposed. There is
now a requirement in the Code "Foreword"
that the designer be familiar with the computer programs used for design. This requirement may have to be expanded in general or
for certain types of design. Considerations
may include certification by a Professional
Engineer, a listing of items to be included in
the design specifications, and documentation
of design calculations.
13. The role of the "third party" in reviewing
design calculations may need to be expanded.
Most European codes place the design respon-
14.
15.
16.
17.
18.
19.
sibility with a party other than the vessel
manufacturer. Under the existing ASME Code
rules, the Authorized Inspector is only responsible to assure that design calculations exist.
This responsibility may need to be expanded
to some degree. This of course will have an
effect on the qualification requirements of the
inspectors.
The requirements for environmental loads
should be reviewed and inclusion of provisions
for low probability accident conditions should
be considered. The stresses allowed for conditions such as wind and earthquake may have
to be revised, in light of reduced design margins. A number of European codes include
certain provisions for accidental conditions.
(In the draft CEN pressure vessel standard,
these are called "exceptional design conditions.") For such conditions, lower design margins are justifiable. For such conditions, progressive plastic deformation, fatigue, and creep
do not have to be considered in setting the
allowable. Safe shutdown would normally be
the criteria.
The requirements for award ofASME's Certificates of Authorization may be reviewed and, if
needed, expanded. Under the present arrangements, there are no specific requirements for
design qualifications. With more sophisticated
design rules, it will become more important
for the Certificate holder to possess a certain
degree of design capability.
The rules for external pressure design should
be updated. The present rules are old and do
not cover all geometries and loadings of interest. A Code Case containing alternative buckling rules is being considered by the ASME
Code Committee. (Code Case 2286, approved
July 17, 1998).
Design rules for penetrations, and particularly those for large penetrations should be
reviewed. Code Case 2168 which provides an
alternative to full area replacement rules and
is based on existing allowable stresses will
also have to be reviewed and revised. (At the
time of this writing, PVRC is sponsoring a
study by E.C. Rodabaugh to come up with
design rules for large openings). Advantage
shoullfl'be taken of the great deal of analytical
work performed on shell intersections by PVRC
and others.
The desirability of allowing a lower design
margin for materials with certain degree of
ductility may be investigated. Most European
codes differentiate between materials of low
and high ductility. The design margin on
ultimate tensile strength for ductile materials
may be relatively low and still provide adequate safety margin.
The desirability of basing the design margin
Evaluation of Design Margins, Section VIII
81
on the consequences of failure may be investigated. Some of European codes differentiate
among vessels in various types of service and
classify vessels in different risk or hazard
categories, depending on the toxicity of the
fluids, pressure and volume of the vessel. The
Stoomwezen Code and the proposed CEN rules
provide such cla,ssifications.
20. Knuckle buckling for thin heads (D/t > 500)
should be addressed. The present rules and
the proposed Code Case for design of formed
heads do not address buckling. PVRC is performing a project to come up with proposed
rules. With higher allowable values of internal
pressure, the knuckle buckling becomes more
of a controlling factor.
21. External pressure curves should be extended
into the creep range. The temperature limitations on these curves is a shortcoming of
present code. PVRC has performed a great
deal of work in this area which can be taken
advantage of.
22. Although Section VIII, Division 1 includes
requirements for out-of-roundness (UG-80) and
edge alignment (UW-33), additional workmanship requirements may have to be added. With
higher allowable design stresses considerations should be given to the following:
a. Peaking at weld seams, local surface irregularities (flat spots) and verticality. (Examples of such tolerances are given in 4.2.3
of BS 5500 and in CEN draft Pressure
Vessel Standard).
b. Local imperfections at welds, such as weld
undercut, excess gap at fillet welds, root
concavity, etc. (examples of such limits are
given in Table 5. 7(3) of the British Standard
5500).
23. Additional rules and cautions for vessels operating in the high temperature range may have
to be added. Certain details and material
combinations may not be advisable for high
temperature operations. TheASME Code Committee and PVRC are working on such rules
for high temperature design. With increased
allowables, the effects of non-linearities will
be more pronounced.
24. No consideration has been given in this report
for increasing the allowable stresses in the
creep range. The Code Committees, however,
may want to make a review of the transition
from time independent to time dependent
properties.
25. Section VIII, Division 1 presently permits
structural quality plates (SA-36 and SA-283)
for pressure parts (except for lethal service) in
thicknesses up through %11 with SA-6 delivery
requirements and frequency of testing. With
higher design stresses, these structural grade
82
26.
27.
28.
29.
30.
31.
plates may have to be eliminated for use on
pressure parts.
SA-20 requires purchaser approval for repair
welding of plates by the manufacturer. Some
of the European pressure vessel codes require
that all welded repairs on material by the
manufacturer shall be indicated on test reports. With higher design stresses, this may
also need to be included in the Division 1
requirements.
The forming requirements in UCS-79 should
be revised to include stress relieving of cold
spun heads (unless the manufacturer can
demonstrate that the material properties are
not significantly altered).
Higher design stresses are likely to increase
distortion of bolted flanges. In addition to the
stress criteria, there should be also a criteria
for deformation and rotation of bolted flanges
to prevent leakage. Presently there are some
non-mandatory requirements in Appendix 5.
More detailed mandatory requirements should
be considered.
Quality control requirements may have to be
expanded. Appendix 10 of this Code includes a
number ofrequirements that are to be incorporated into the Manufacturer's QC plan. Additional requirements are normally included in
most manufacturers' QC plans which are not
spelled out in this appendix. With lower design margins, an updating of this appendix
will be advisable.
The effect of lower design margins on allowables for shear stress and bearing stress should
be investigated.
Consideration should be given to providing
values of yield strength and ultimate tensile
strength for all Code allowed materials in
Section II, Part D. Each Code section could
then specify design factors to be applied to
these values at design temperatures below the
creep range. This will eliminate the need for
generating and maintaining voluminous allowable stress tables.
b. Section VIII, Division 2
Based on comparisons with the other codes and
standards in this report and satisfactory past experience with vessels fabricated to these codes, it is
concluded that the design margin on ultimate
strength may be reduced from the present value of
3.0. The European codes have successfully used
values as low as 2.35 on tensile strength and % on
yield strength. A number of improvements in the
Section VIII, Division 2 rules and additional requirements related to a lower design margin are recommended below. Depending on the improvements to
be incorporated in the Code, it is felt that a design
margin in the range of 2.3 to 2.4 on ultimate tensile
strength may be justifiable. No reduction has been
WRC Bulletin 435
proposed for the present design margin on yield
strength. For materials with low ductility, a penalty
factor may be warranted. The following is a list of
some of the areas suggested for review by the ASME
Code Committee, if a reduction in design margin is
adopted:
1. Fatigue analysis rules should be improved.
The fatigue rules of this code, which were
developed in the 1960's, need to be updated. A
great deal of data and design information has
been generated since these rules were adopted.
With higher stress allowables, it will be essential that a reliable fatigue analysis is performed for vessels in cyclic service.
2. Flange design rules should be improved. Comments made for flange design rules in Division
1 are applicable to this code as well.
3. All the European pressure vessel codes and
standards reviewed as part of this study add
the under-thickness tolerance for plates to the
required thickness. With higher design
stresses, consideration should be given in this
practice for Division 2 also.
4. Design rules for formed heads should be improved. Although Division 2 rules are an improvement over those of Division 1, they still
do not reflect today's understanding of behavior of such heads.
5. Design rules for cone-cylinder junctions should
be improved. Analytical tools are available to
develop more reliable rules than are presently
in this Code.
6. External pressure design rules should be improved. These rules are presently the same as
those of Division 1 and the comments made for
Division 1 apply here also.
7. Notch toughness requirements should be reviewed and updated. Toughness rules of Division 2 are consistent with those of Division J·
With increased allowables, these rules will
need to be updated. Consideration should be
given to:
a. Notch toughness requirements for increased
allowable stresses to provide the same (or
greater) margin of safety on brittle fracture
as presently in Division 2. (See Section
E.2.c). Consideration should also be given
to residual stresses when evaluating the
toughness requirements, particularly for
the high strength quenched and tempered
(Q&T) steels in TableAQT-1.
b. The quenched and tempered steels in Table
AQT-1 have a high yield strength-to-tensile
strength ratios and, therefore, have less
tolerance for flaws and discontinuities. Additional NDE should be considered for
welded joints when these steels used at
lower design margins than are presently in
Division 2.
c. Section VIII, Division 2 presently gives no
credit for PWHT in toughness requirements. Most European pressure vessel codes
have less severe impact test requirements
for stress relieved than for non-stress relieved weldments. Consideration should be
given to including the credit for PWHT also
in the Division 2 toughness rules.
8. The role of the "third party" in reviewing
design calculations may need to be expanded.
Most European codes place the design responsibility with a party other than the vessel
manufacturer. Under the existing ASME Code
rules, the Authorized Inspector is only responsible to assure that design calculations exist.
This responsibility may need to be expanded.
9. Design rules for penetrations, and particularly large penetrations, should be reviewed.
More up-to-date methods are available that
could be taken advantage of.
10. Knuckle buckling for thin heads should be
addressed.
11. The analysis methods of Appendix 4 should be
updated. The rules were developed in the
1960's and were intended for analysis by the
shells-of-revolution method. With today's widespread use of finite element analysis, the rules
of this appendix are difficult to apply. The
analysis rules should provide for finite element and other recent methods of analysis
and specify acceptance criteria general enough
to be applicable to various methods.
12. Section VIII, Division 2 includes requirements
for shells (AF-130), heads (AF-135) and edge
alignment (AF-140), however, additional tolerance and workmanship requirements may
have to be considered. With higher allowable
design stresses considerations should be given
to the following (and possibly others):
a. Peaking at weld seams, local surface irregularities (fiat spots) and verticality. (Examples of such tolerances are given in 4.2.3
of BS 5500 and in CEN draft Pressure
Vessel Standard).
b. Local imperfections at welds, such as weld
undercut, excess gap at fillet welds, root
co1'.cavity, etc. (examples of such limits are
given in Table 5.7 (3) of the British Standard 5500).
13. Section VIII, Division 2 permits structural
quality plates (SA-36 and SA-283 Gr. B & D)
for pressure parts with SA-6 delivery requirements and frequency of testing. With higher
design stresses, these structural grade plates
may have to be eliminated for use on pressure
parts.
14. SA-20 requires purchaser approval for repair
welding of plates by the manufacturer. Some
Evaluation of Design Margins, Section VIII
83
of the European pressure vessel codes require
that all welded repairs on material by the
manufacturer shall be indicated on test reports. With higher design stresses, this should
also be considered for inclusion in the Division
2 requirements.
15. AF-111 in Division 2 permits any process for
forming as long it does not unduly impair the
mechanical properties of the material. AF-605
in Division 2 permits a maximum fiber elongation of 5% for the high strength materials in
Table AQT-1 without stress relieving. Additional consideration should be given to loss of
toughness due to cold forming for:
a. Cold spun heads. (Cold spun heads should
be stress relieved unless the manufacturer
demonstrates that the material properties
meet the specified requirements).
b. Microalloyed steels. (See WRC Bulletin 322).
16. Higher design stresses are likely to increase
distortion of bolted flanges. In addition to the
stress criteria, there should be also a criteria
for deformation and rotation of bolted flanges
to prevent leakage.
17. The desirability of basing the design margin
on the consequences of failure should be considered. Some European codes differentiate
among vessels in various types of service and
classify vessels in different risk or hazard
categories, depending on the toxicity of the
fluids, pressure and volume of the vessel. (See
Appendix 3 on Stoomwezen Code and Appendix 4 on CEN draft Pressure Vessel Standard).
18. The addition of ultimate load design concept
as a design tool should also be considered. In
that case the acceptance criteria would need
to be provided in Appendix 4 for that method.
19. As a long term project, an effort should be
made to make the safety margins consistent
for various components and for the different
failure modes. With the existing rules, the
true safety margins vary considerably from
component to component and for different
failure modes. It is recognized that this would
be a very ambitious task and may not be
totally feasible.
H. Recommendations for Additional Studies
The conclusion of this report is that the design
margin on ultimate strength should be reduced for
both Divisions 1 and 2 of Section VIII Code. However, a number of areas in these codes need to be
reviewed and improved. The final values of the
design factors and the extent of improvements made
to these codes will of course depend on the judgment
and collective experience of many Code Committees.
To help the Code Committees make such decisions,
additional studies by PVRC or other organizations
84
may be helpful. The following is a list of some of the
areas that are recommended for additional studies:
1. Development of a more detailed list of recom-
mendations for revisions to Division 1 (for a
further reduction in the design margin below
3.5) and for Division 2. This phase 2 study has
been fairly broad and the recommendations
therein are mainly based on comparisons with
other codes. As a result, the recommendations
for revisions to the ASME Codes are not very
specific. A follow up study, could assume definitive values of design margins and based on
these values provide recommendations for specific revisions.
2. Notch toughness requirements should be reviewed and updated. With increased allowable
stresses, the notch toughness requirements
need to be increased to insure at least the
same margin of safety on brittle fracture as
presently in Division 1 and in Division 2.
Consideration should be given to impact testing requirements both in the as-welded condition and in the PWHT condition. Particular
attention should be given to the high strength
Q & T steels in Table AQT-1, which have a
high yield strength-to-tensile strength ratios
and, therefore, have less tolerance for flaws
and discontinuities.
3. A study of joint efficiency factors and NDE
requirements for Division 1 to assure consistency of such rules. The joint efficiency used in
design and the impact test requirements depend on the extent and effectiveness of NDE
methods and acceptance criteria. With many
revisions to these areas over the years, there
may not be adequate consistency.
4. Development of design rules for openings,
including large penetrations. A great deal of
data and analytical results are available to
develop more up-to-date and consistent set of
rules for both Divisions.
5. Development of analysis rules and acceptance
criteria. The rules of Appendix 4 of Division 2
should be updated. More up-to-date rules are
needed to allow the more advanced analysis
methods. Some European codes have adopted
such rules.
6. Development of up-to-date fatigue rules. Technology exists for preparing rationale and up-todate fatigue design rules.
7. Review of criteria for high temperature design
margins. This phase 2 report includes a comparison of rules for high temperature design
in various codes. However, no recommendations are made for revisions. A further study
could concentrate on this area and provide
recommendations for updating the rules and
design margins.
8. A detailed review of fabrication requirements
WRC Bulletin 435
which are consistent with higher design
stresses for both Division 1 and Division 2.
9. Review of Sections I, III and IV and B31
piping codes. To assure consistency amongst
the various ASME documents, a review of
various pressure vessel, boiler and piping
codes under the jurisdiction of ASME may be
advisable. This would be a long term project
which may result in recommended revisions to
these documents.
10. A qualitative cost-benefit evaluation of some
of the more significant changes required to
justify lower design margins.
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'
·
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'
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28. SME PVP, Vol. 277, 1994, p. 79-88, Safety Margins of Pressure
Vess.els When Designed by Primary Stress Limits, A. Kalnins and D. P.
Updike.
29. {'!?ME PVP, Vol. ?77, 19.94, p. 89-94, Burst by Tensile Plastic
Insta.bihty of Vessels With Tonspherical Heads, D. P. Updike and A.
Kalmns.
3~. ONR-AISI Agreeme~t No. N00014-94-2-0-002, Effect ofYield-Tensile
~at10 on Structural Behavior-High Performance Steels for Bridge Construction, R. L. Brockenbrough & Associates, Inc. (Expanded Draft Final
Report), June 6, 1995.
31. ASME PVP, Vol. 315, 1995, p. 253-284, Hydro-test of 'I\vo Retired
Pressure Vessels With Local Thin Areas, L. M. Connelly.
32. The Draft GEN Standard for Unfired Pressure Vessels, Seminar
Papers, 25th September, 1996, IMechE, London, England.
33. D~velopment of a New Pressure Vessel System in the European Union,
F. Osweiller, Sept. 20, 1996. (ASME B & PV Code: Main Committee
Meeting).
34. W. D. Doty, Committee correspondence to G. Karcher September 21
1997.
,
,
35. WRC Bullet~n 265! February 1981, Interpretive Report on Small
Scale Test Correlat10ns with Kro Data, R. Roberts and C. Newton.
36. API 579 Draft, Issue 6, February 15, 1997.
37. ASME Code, Section IX, 1995 Ed.
38. ASME Code Case 2235.
39. A Study of StreifS Relief Heat 1}-eatment for 89 kg I mm2 Class High
!?trength Steels, M. fiiromatsu, Y. Kasamatsu and K Horikawa, Proceedings of an Internat10nal Conference on Welding of HSLA (Microalloyed)
Structural Steels? 9-12 November, 1976, Rome, Italy, p. 503-529.
40. P~C Project.J'!o: 97-2, Evaluation of Design Margins for ASME
Code, Section VII~, D!Vls10ns 1 and 2 Phase 2 Studies, including Appendices
1-4, Final Report, November 1997.
41. Order on Pressure Vessels, Gas Pressure Vessels and Filling Plants
(Germany), November 1992.
41. European Pressure Equipment Directive 97/23/EC, 29 May 1997.
Evaluation of Design Margins, Section VIII
85