Int. J. Pres. Ves. & Piping ,19 (1992) 231-265
:~
" : i . L" 7 ,
Summary of Design of Nuclear Vessels and Piping to
ASME III (NB, NC, ND) and Vessels to BS 5500
L. P. Harrop
Nuclear Installations Inspectorate, Health and Safety Executive, St Peter's House,
Balliol Road, Bootie, Merseyside L20 3LZ, UK
(Received 24 April 1991; accepted 6 June 1991)
ABSTRACT
It is recognised that the general safety significance of pressurised
components for nuclear power systems demands design rules beyond
those for non-nuclear components. A S M E III provides the most widely
used design rules for nuclear components, and provides rules in three
Classes to allow the rules to be matched to the safety significance of
different nuclear components. A S M E Ili Class 1 is provided for
components of the highest nuclear safety significance, Class 3 for those
of lowest nuclear safety significance. There is then a hierarchy of design
code requirements for pressurised components, starting with nonnuclear codes as the minimum and progressing through the A S M E III
nuclear Classes 3, 2, 1. In establishing and assessing the safety
}ustifications of nuclear plants it is important to have an appreciation of
the gradation of requirements in the A S M E III design rules and how
these go beyond non-nuclear component design rules. There are two
broad aspects to the structural integrity of pressurised components,
namely the achievement of integrity and the demonstration of integrity.
The technical requirements of design codes are associated with
achieving integrity while the documentary aspects are usually associated with demonstrating integrity. In practice documents also have
a part in achieving integrity in the communication of information
between different organisations and personnel involved in the design
process.
It is not possible to assign simple numerical measures to the relative
integrity afforded by non-nuclear codes and the three Classes of A S M E
IlL Instead it is necessary to compare the different requirements of the
rules for the various technical and documentary aspects. This paper
231
© 1991 Crown copyright.
232
L. P. Harrop
summarises the most important technical and documentary aspects of
the three Classes of the A S M E III Code for vessels and the non-nuclear
code BS 5500. A similar summary is also provided for the three Classes
of A S M E III rules for piping. The intention is that the paper provides a
basis for appreciating the relative integrity afforded by these various
rules.
1 INTRODUCTION
For new plants the American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code (BVPC) Section III is used throughout the world either directly or as a basis for national standards. The
principal non-nuclear vessel design code in the U K is British Standard
BS 5500. The ASME Code discussed here is the 1989 edition. 1The British
Standard dealt with here is the 1988 edition, z ASME III provides rules for
vessels, piping, valves and pumps under each of the three Classes 1,2 and 3.
This integrated approach is one of the distinct features of ASME III.
The vessel and piping design codes intend to provide designs to
perform some duty without failure of the pressure boundary. There are
various well known modes of failure of steel vessels and piping. Table 1
lists seven modes of failure and notes to what extent design codes deal
with these modes of failure.
There are two broad aspects to the structural integrity of pressurised
components, the achievement of integrity and the demonstration of
integrity. The technical requirements of the design codes are associated
with achieving integrity and the documentation aspects with demonstrating integrity. In fact there is some overlap, for instance the
interaction of the ASME Design Specification, the design process and
ASME Design Report is part of the achievement of integrity (e.g. by
ensuring that all required loading conditions are catered for in the
design). The summary of the technical requirements of the design codes
is provided by a set of tables and notes in Appendices 1-3. The
following sections of the paper deal mainly with documentary aspects of
the design codes.
ASME III mainly covers rules for materials selection, design,
fabrication and testing. This paper does not deal with:
---quality assurance (QA) requirements,
--welding procedures and their qualification, or
--in-service inspection.
Design of nuclear vessels and piping
233
TABLE 1
Important Modes of Failure for Steel Vessels and Piping and How Design Codes Deal
With These Modes of Failure
Mode of failure
How dealt with by design codes
1
Bursting due to plastic rupture of shell
wall or ductile tearing at a
discontinuity
High strain fatigue
3
Brittle fracture
Very well--historically the first failure
mode to be considered by design
codes
Well for high strain, low cycle ratcheting, quite well for low strain, high
cycle fatigue if using S-N design
curve approach
To some extent, generally by requiring
materials to pass notched specimen
impact energy tests, often only for
low temperature operation
Hardly at all. General corrosion allowance does not address these modes
Not at all
For elevated temperature operation design codes provide rules for time
dependent material behaviour
Not at all
Well for external pressure, not so well
for unusual situations, e.g. buckling
under internal pressure of knuckle
transition region of head to cylinder
junction
Corrosion fatigue or stress corrosion
Erosion-corrosion
Creep rupture
7
8
Ductile fracture
Buckling
These are all specialised areas and could each be the subject of separate
summaries. Q A requirements are c o v e r e d in A N S I / A S M E N Q A - 1 . 3
Weld procedures for A S M E III are largely d e t e r m i n e d by A S M E IX, 4
as for A S M E non-nuclear vessels and piping. In-service inspection
requirements for nuclear c o m p o n e n t s are c o v e r e d in A S M E X I 2
For the design of piping, the type and location of piping supports are
important. The design of such supports for nuclear piping is dealt with
in A S M E III Subsection NF. 1 This p a p e r does not cover the design of
piping supports. H o w e v e r , a few c o m m e n t s are provided in the section
which deals with piping.
Both the A S M E C o d e s and British Standards are the result of the
continuing efforts of groups o f individuals with a commercial, regulatory or academic interest in the safety of pressurised equipment. A t any
particular time these codes and standards represent a consensus of the
various committees involved using current engineering science. A S M E
Codes and British Standards do not have statutory authority b y
234
L.P. Harrop
themselves; they are largely the result of the voluntary effort of many
individuals, often supported by their employers. However in the USA,
the Code of Federal Register directly references the ASME III Code as
a requirement for nuclear power plants.
For ASME, the dates of the meetings of the various Boards,
Committees, Subcommittees, Subgroups, Working Groups and Technical Groups are published in Mechanical Engineering (a monthly journal
published by ASME, New York). The meetings are open to the
concerned public as well as to interested members of the engineering
community.
The author has based this paper on his experience in using the design
codes referred to. He is not a member of any of the rule-making
committees of ASME or BSI. A summary such as this inevitably must
omit detail which is necessary for a complete design. For such detail the
rules themselves must be consulted. It is important to note that for
nuclear plant systems, Owners and Vendors sometimes impose extra
requirements beyond those of ASME III. To understand and to
implement the ASME III Code it is necessary to take account of the
Code Interpretations and the Code Cases. For BS 5500 it is similarly
necessary to be aware of its Enquiry Cases.
2 ASME III A N D BS 5500 VESSEL R U L E S
The rules for ASME III Class 1, 2 and 3 vessels are summarised in
Appendix 1. The rules in BS 5500 are summarised in Appendix 3. This
section of the paper is divided into five subsections, i.e.:
2.1
2.2
2.3
2.4
2.5
General observations
ASME III Classes, BS 5500 Construction Categories
Selection of ASME III Class, USNRC Quality Groups
Loads to be considered in design
ASME III Design Specifications and Design Report
BS 5500 information to be documented
and
2.1 General observations
The ASME Code provides rules for new construction; it does not
account for in-service degradation other than a general corrosion
allowance. BS 5500 also provides rules for new construction but it
provides a brief commentary on various forms of corrosion. The Owner
must deal with in-service degradation. Several non-nuclear industries
Design of nuclear vessels and piping
235
have developed collaborative organisations to deal with in-service
degradation, e.g. the oil refining and gas industries and the paper and
pulp industries. In the US, the nuclear industry deals with in-service
ageing through a combination of Owner Groups and the Electric Power
Research Institute (EPRI). The ASME III Code covers components
which operate at temperatures where time dependent material behaviour (i.e. creep) does no occur. For nuclear components operating
at temperatures where time dependent material behaviour occurs, the
rules of ASME Code Case N-476 can be used. BS 5500 does incorporate
rules for design when time dependent material behaviour occurs.
It may be noted, when examining the ASME III inspection requirements at the fabrication stage, that there is not much emphasis on
ultrasonic volumetric inspection. Due to the 'finger printing' requirement of ASME XI, vessels which require in-service ultrasonic inspection will also require a pre-service ultrasonic inspection. This in its turn
produces a commercial incentive for conducting ultrasonic inspections
during fabrication that are similar to those in-service. This is so because
it is easier to rectify faults at the fabrication stage rather than after the
vessel has been completed and perhaps delivered to site.
2.2 ASME III Classes, BS 5500 Construction Categories
ASME III provides rules for three classes of nuclear vessel, i.e. 1, 2 and
3 in Subsections NB, NC and ND, respectively. BS 5500 provides rules
for three Construction Categories, again 1, 2 and 3. There are
significant differences between the ASME III Classes and the BS 5500
Construction Categories.
In the case of A S M E III, a vessel of the same:
--size,
--wall thickness,
-----design pressure, and
--material of construction
could be built to any of the three Classes. In contrast, the BS 5500 rules
limit the wall thickness and range of materials for Construction
Categories 2 and 3 compared with Construction Category 1. The
selection of a Construction Category for BS 5500 is to a large extent
determined by the wall thicknesses and material of construction
required.
In the case of a vessel built to BS 5500, different parts of a single
pressure envelope can be built to different Construction Categories.
ASME III permits a multi-chamber vessel to have individual chambers
236
L. P. Harrop
built to different Classes. There is no explicit permission in A S M E III
for a single chamber to be built to different Classes.
Neither ASME III nor BS 5500 makes any claim for a specifie level of
reliability for components built to the code; and in particular neither
code makes any statement about the absolute or relative difference in
reliability between ASME III Classes or BS 5500 Construction Categories. However for ASME III the assumption in developing the Code
Subsections is that the safety requirements and the amount of analysis
required for Classes 2 and 3 are generally not so stringent nor as
extensive as for Class 1 components.~°
2.3 Selection of ASME III Class, USNRC Quality Groups
The ASME III rules do not prescribe how to select a Class for a
particular vessel. Instead the Owner is left to decide based on system
safety analyses and regulatory guidance. The Code of Federal
Regulations, 7 USNRC Regulatory Guide 1.268 and Section 3.2.2 of the
USNRC Standard Review Plan 9 provide specifications and guidance for
selection of ASME Classes for specific components. All this regulatory
guidance is specific to PWR and BWR systems. So, for instance there is
no guidance for nuclear chemical plant components.
A brief review is given below of the USNRC specifications. USNRC
defines four Quality Groups: A, B, C and D. For the reactor coolant
pressure boundary, Quality Group A applies and ASME III Class 1 is
required. Quality Group B applies to components which are either
(B1) or (B2), below:
(B1)
part of the reactor coolant pressure boundary but excluded
from Quality Group A requirements by 10 CFR 50.55a (c) 2
on the basis that:
(i)
(ii)
in the event of a postulated failure of the component
during normal reactor operation, the reactor can be
shut down and cooled down in an orderly manner,
assuming make-up is provided by the reactor coolant
make-up system; or
the component is or can be isolated from the reactor
coolant system by two valves in series. Each open
valve must be capable of automatic actuation and,
assuming the other valve is open, its closure time must
be such that, in the event of postulated failure of the
component during normal reactor operation, each
valve remains operable and the reactor can be shut
Design of nuclear vessels and piping
237
down and cooled down in an orderly manner, assuming make-up is provided by the reactor coolant makeup system only.
(B2)
not part of the reactor coolant pressure boundary, but part of
(i)
(ii)
(iii)
systems important to safety that are designed for
emergency core cooling, post accident containment
heat removal or post-accident fission product
re moval;
systems important to safety that are designed for
reactor shutdown or residual heat removal;
the steam and feedwater system of PWRs extending
from and including the secondary side of steam
generators up to and including the outermost containment isolation valve and connected piping up to
and including the first valve that is either normally
closed or capable of automatic closure during all
modes of normal rector operation.
There are some other detailed definitions of quality Group B items. For
Quality Group B items, ASME III Class 2 rules apply.
USNRC Quality Group C applies to components which are not part
of the reactor coolant pressure boundary or included in quality Group
B but part of (C1)-(C3), below.
(C1)
cooling and auxiliary feedwater systems or portions of systems
important to safety that are designed for:
(i)
emergency core cooling,
post-accident containment heat removal,
(iii) post-accident containment atmosphere clean-up, and
(iv) residual heat removal from the reactor and from the
spent fuel storage pool.
(ii)
(C2)
(C3)
Portions of these systems that are required for their safety
functions and that do not operate under normal conditions and
cannot be tested adequately should be classified as quality
Group B.
cooling water and seal water systems that are designed for
functioning of components and systems important to safety
such as reactor coolant pumps, diesels.
systems other than radioactive waste management systems that
contain or may contain radioactive material and whose postulated failure would result in conservatively calculated off-site
doses that exceed 0-5 rem to the whole body or its equivalent
to any part of the body.
238
L. P. Harrop
There are some other detailed definitions of quality Group C items. For
quality Group C items, A S M E III Class 3 rules apply.
U S N R C quality Group D applies to components which are not
included in Groups A, B or C but are part of systems that contain or
may contain radioactive material. For quality Group D items U S N R C
specifies non-nuclear codes namely A S M E VIII Division 1 for pressure
vessels and ANSI B31.1.0 for piping.
2.4 Loads to be considered in design
A S M E III provides for the Service Loads acting on vessels to be
divided among four Service Limits. The Service Limits are A, B, C and
D, and cover
Level
Level
Level
Level
A:
B:
C:
D:
normal operation (Normal)
operational fluctuations (Upset)
unusual occurrences (Emergency)
very rare events (Faults)
The Owner decides on the Service Limit to which a particular load
combination is assigned. The allowable stress conditions are associated
with Level A, B, C or D. Service Limits are progressively less
restrictive. However, the integrity of the pressure boundary is to be
maintained in all cases.
Level B Service Limits are set so that when such loads occur the
component suffers no damage requiring repair. The duration of Level B
loads must be included in the Design Specification (See Section 2.5
below for discussion of the Design Specification).
Level C Service Limits allow large deformations in areas of structural
discontinuity which may necessitate removal of the c o m p o n e n t from
service for inspection or repair. The aggregate of all Level C loads
should not cause more than 25 cycles with a stress amplitude above the
allowed amplitude for 1 0 6 cycles. The expectation would be that a
component would be subjected to rather less than 25 occurrences of
Level C loads during its life.
Level D Service Limits allow gross general deformation with some
loss of dimensional stability and damage requiring repair. Loads
assigned Level D Service Limits are very rare events which are unlikely
to occur even once in the design life of the component.
From the set of Level A loads, the A S M E Design Pressure and
Design Temperature are determined. The A S M E Design Pressure is set
to be not less than the maximum pressure which exists under the most
severe loadings for which Level A Service Limits are applicable. The
Design of nuclear vessels and piping
239
ASME Design Temperature is set at not less than the expected mean
metal temperature through the thickness of the part for which Level A
Service Limits are specified. By these definitions, the ASME Design
Pressure and Design Temperature need not come from the same load
combination and are not necessarily coincident in time. The ASME
Design Pressure and Design Temperature provides the parameters for
the Design Loadings for which there are Design Limits. The tests to
which the vessel is subjected must also be considered and the Test
Loadings have corresponding Test Limits. Thus an ASME III vessel is
designed taking the following loads into account:
Design Loads--Design Limits apply;
Service Loads---Service Limits A, B, C or D apply;
Test Loads--Test Limits apply.
The chronological order for determining the ASME III Design, Service
and Test Conditions is:
(i)
(ii)
specify Service Loads;
divide Service Loads into four sets, the sets being for Level A,
B, C and D Service Limits, respectively;
(iii) from the set of Level A loads, determine the Design Pressure
and Design Temperature;
(iv) establish the Test Loads.
ASME III does not set a minimum pressure below which the code does
not apply. ASME III does not define a minimum Design Temperature
but the rules for material toughness refer to the Lowest Service
Temperature (LST).
BS 5500 requires the determination of a design pressure and maximum and minimum design temperatures. There is a minimum stress
below which the rules of BS 5500 do not apply (less than 10% of the
permitted design stress). BS 5500 requires the design pressure to be
determined by:
(i)
(ii)
the pressure at the onset of operation of pressure relieving
devices;
the maximum pressure which can be attained when not limited
by a relieving device.
The BS 5500 maximum design temperature shall not be less than the
actual metal temperature expected in service. If different metal
temperatures can be predicted for different parts of the vessel it is
permissible to base the design temperature for any point in the vessel
on the predicted metal temperature. The minimum design temperature
240
L. P. Harrop
is used in the determination of the ability of the materials of
construction to resist brittle fracture. The minimum design temperature
shall be the lowest metal temperature expected in service.
BS5500 requires that, if during normal operation, a vessel is
subjected to more than one loading/temperature combination, the
thickness of the vessel shall be determined from the combination which
results in greatest thickness.
2.5 ASME !II Design Specification and Design Report and BS 5500
information to be documented
A notable feature of the ASME III Code is the requirement for
documentation called the Design Specification and the Design Report.
These are required for all ASME III items whether Class 1, 2 or 3. The
Design Specification must provide sufficient detail to provide a complete basis for construction and includes:
(i)
(ii)
the functions and boundaries of the items;
the design requirements, i.e. Design, Service and Test Load
conditions and corresponding Design, Service and Test Limits.
Also including over-pressure protection;
(iii) the environmental conditions, including radiation;
(iv) the Code Class;
(v) material requirements, including impact test requirements;
(vi) the effective Code edition, Addenda and Code Cases.
The purpose of the Design Report is stated in Appendix C of ASME
11113 as being to provide a demonstration of the structural integrity of
the item. The Design Report should be structured and written to
provide sufficient detail to permit independent reviews of the design.
Clearly the Design Report depends in part on the Design Specification.
The Design Report and the drawings used for design and construction
are the primary Design Documents.
Another notable feature of ASME III is the certification requirements of the Design Specification and Design Report, and the Owner's
Review of the Design Report. The Design Specification is always
required to be certified. The Design Report must be certified for:
(i)
(ii)
Class 1 components (and component supports); and
Class 2 vessels built to NC 3200 or Class 2 or 3 components
designed to Service Loadings greater than Design Loadings (i.e.
Level B, C and D loadings which exceed the maximum pressure
and temperature of the Level A loadings).
Design of nuclear vessels and piping
241
Certification of the Design Specification and Design Report must be by
suitably qualified professional engineers. The Design Report must be
certified by someone other than the person who certifies the Design
Specification. However there is no requirement for the certifier of the
Design Specification to be in an organisation different from that of the
certifier of the Design Report.
The drawings used for design and construction shall be in agreement
with the Design Report before it is certified and shall be identified and
described in the Design Report. This means that any document
modified for construction compared with the document used for the
design analysis shall be reconciled with the Design Report.
The Owner's Review of the Design Report is intended to determine
that all the Design and Service Loadings as stated in the Design
Specification have been evaluated and that the acceptance criteria
associated with the specified Design and Service Conditions have been
considered. The responsibility for the method of analysis and the
accuracy of the Design Report remains with the Designer.
By contrast with the above for ASME III, BS5500 principally
requires:
(1)
(2)
the purchaser to supply the normal working conditions of the
vessel together with details of any transient cyclic and/or
adverse conditions in which the vessel is required to operate;
the manufacturer to supply the purchaser with a fully dimensioned drawing of the vessel both before commencement of
manufacture and following construction for the vessel as built.
Both drawings must include:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
a statement that the vessel is constructed to BS 5500;
specifications to which materials comply;
welding procedures used;
drawings and dimensions of weld preparations;
heat treatment procedures;
non-destructive testing requirements;
design pressure(s) and temperature(s) and major structural loadings;
(viii) test pressure(s);
(ix) amount and location of corrosion allowance.
The manufacturer also supplies the purchaser with a Certificate which
asserts that the vessel complies with the requirements of BS 5500.
The requirements of BS5500 for documentation are typical of
non-nuclear codes but the documentation is not intended to provide the
242
L. P. Harrop
demonstration of structural integrity or for independent review as
intended by the ASME III Design Specification and Design Report.
3 ASME III PIPING RULES
Appendix 2 summarises the ASME III rules for nuclear piping. As for
ASME III vessels, the rules for piping include:
--Classes 1, 2 and 3;
--Design Loads, Service Loads (A, B, C and D) and Test Loads with
corresponding Design, Service and Test Limits;
--Requirements for Design Specification, Design Report and certification under the same rules as for vessels and the Owner's
Review of the Design Report.
The design requirements for Class 2 and 3 piping under Design Loading
and Level A, B Limit Service Loadings are similar to non-nuclear
piping codes, e.g. BS 806 or A N S I / A S M E B 31.1.11'12
For the design of piping, the type and location of supports are
particularly important. The design of supports for ASME III nuclear
piping is dealt with in ASME III Subsection NF. 1 Subsection NF
includes suggestions for the maximum spacing of vertical supports;
these are the same as the suggestions in A N S I / A S M E B31.1 for
non-nuclear piping. Lateral supports for piping must be positioned on
the basis of the lateral loadings considered in the design and the
resulting stresses in the piping. Lateral design loads are quite likely to
be dominated by seismic forces and loss of coolant accident forces.
Provision of piping supports (dealt with in ASME III Subsection NF)
to cope with seismic loads has been a costly area for nuclear power
plants over the past 20 years. Either expensive analysis or a multitude
of supports (or a combination of the two) have been necessary.
4 CONCLUSION
This paper summarises the ASME III Codes for Classes 1, 2 and 3
nuclear pressure vessels and piping and the BS5500 standard for
non-nuclear, unfired pressure vessels. It is intended that these summaries provide a means of appreciating the relative integrity afforded
by these various rules.
Appendices to the paper provide summaries of the main technical
aspects of the Codes. In the case of ASME III the requirements for
Design of nuclear vessels and piping
243
quality assurance, weld procedures and in-service inspection are
covered by separate Sections of the Code; these aspects are not dealt
with here.
Apart from the technical detail of the Codes, important general
points are as listed in the following paragraphs.
Both the ASME Codes and British Standards are the result of a
continuing, mainly voluntary efforts from interested professionals. The
ASME Codes and British Standards do not have statutory authority by
themselves. ASME Committee meetings are open to the concerned
public and interested members of the engineering community.
Both ASME III and BS 5500 provide rules for new construction.
In-service ageing is dealt with by the Owner frequently through
industry wide co-operative organisations.
A distinct feature of ASME III is that it provides rules for vessels,
piping, pumps and valves for each of the three Classes. In addition
ASME III imposes attention to detail in considering the loadings on
components (divided into Design, Service and Test Conditions) and the
subsequent design for these loads. ASME III does this through
requiring the Design Specification and Design Report documents, and
the certification and review of these documents. One important purpose
of these documents is to demonstrate the adequacy of the design and to
facilitate independent review.
Neither ASME III nor BS 5500 makes any quantitative claim for the
relative (or absolute) reliability of components made to Classes 1, 2 or
3. ASME III does not specify how to select a Code Class for a
particular component.
5 FUTURE DEVELOPMENTS
The summary prosented here is based on the design codes in their
current form. The codes have changed in the past and will change in the
future. The ASME III Code is relatively new compared with nonnuclear vessel and piping codes such as ASME VIII. BS 5500 is also
relatively new, having replaced some earlier, separate British
Standards.
The ASME III Code developed rapidly, mainly in the period
1965-1975 essentially for the design of BWR and PWR nuclear
systems. Recently the ASME III Code has been relatively stable, with
changes being limited to detailed amendments. In future other nuclear
power systems may appear. These could include so-called inherently
safe designs or advanced versions of current systems. Most of these
244
L. P. Harrop
design developments reduce the role of active systems and engineered
safety features. In turn this reduces the overall reliance on the
reliability of components in active, engineered safety systems. One
feature common to all nuclear power systems is their dependence on
high reliability of passive components, especially those for containment,
i.e. pressure vessels and piping. The structural integrity of pressure
vessels and piping and the design codes for these components will
therefore remain important.
ACKNOWLEDGEMENTS
The author is grateful to Mr E. A. Ryder, Chief Inspector, HM
Nuclear Installations Inspectorate, for permission to publish this paper
and for the assistance of Dr Hemsworth and Dr Goodison of Nil.
The views expressed in this paper are those of the author and do not
necessarily represent the views of Nil.
REFERENCES
1. ASME Boiler and Pressure Vessel Code. Section III. Rules for Construction of Nuclear Power Plant Components. Division 1: Subsection NCA
General Requirements for Division 1 and Division 2; Subsection NB Class
1 Components; Subsection NC Class 2 Components; Subsection ND Class
3 Components; Subsection NF Component Supports. American Society of
Mechanical Engineers, New York, 1989 (with Addenda to 31 Dec. 1989).
2. British Standard Specification for Unfired Fusion Welded Pressure Vessels. British Standards Institution BS 5500, London, 1988 (to Issue 2 Aug.
1988).
3. ANSI/ASME NQA-1, Quality Assurance Program Requirements for
Nuclear Facilities, American Society of Mechanical Engineers, New York,
1986.
4. ASME Boiler and Pressure Vessel Code. Section IX. Welding and Brazing
Qualifications. American Society of Mechanical Engineers, New York,
1989.
5. ASME Boiler and Pressure Vessel Code. Section XI. Rules for In-service
Inspection of Nuclear Power Plant Components. American Society of
Mechanical Engineers, New York, 1989.
6. ASME Code Cases. Nuclear Components. N-47 Class 1 Components in
Elevated Temperature Service. Section III Division 1. American Society
of Mechanical Engineers, New York, 1986.
7. The Code of Federal Regulations. Energy. 10 CFR 50.55a Codes and
Standards. Office of the Federal Register, National Archives and Records
Service, General Services Administration, Washington DC, USA.
Design of nuclear vessels and piping
245
8. USNRC Regulatory Guide 1.26. Quality Group Classifications and Standards for Water-, Steam-, and Radioactive-Waste-Containing Components
of Nuclear Power Plants. United States Nuclear Regulatory Commission
(USNRC) Rev. 3, Washington DC, USA, Feb. 1976.
9. Standard Review Plan for the Review of Safety Analysis Reports for
Nuclear Power Plants. LWR edn. Section 3.2.2 System Quality Group
Classification. NUREG-0800 USNRC, July 1981.
10. Yukawa S., Doty W. D. & Landerman E. I., Basis and Development of
Toughness Requirements for Class 2, Class 3, Containment and Component Support Materials in Section III of the ASME Code. J. Pres. Ves.
Tech., 112 (1990), 193-8.
11. British Standard Specification for Design and Construction of Ferrous
Piping Installations for and in Connection with Land Boilers. British
Standard Institution, BS 806, London, 1990.
12. ANSI/ASME B31.1. Power Piping. American Society of Mechanical
Engineers, New York, 1989.
13. ASME Boiler and Pressure Vessel Code. Section III Appendix C.
Certificate Holder's Design Report. American Society of Mechanical
Engineers, New York, 1989.
Vessels to NC3200
as Class 1 (NB).
Vessels to NC3300
As Class 1 (NB).
Rules for radiography
before or after PWHT
as for Class 1 (NB).
Maximum
thickness (mm )
2 (NC)
Permitted
material
In general radiographic
All in Tables I-1.0 None. Thickness
examination of welds
of A S M E II.
limits in specifishall be performed afcations of
ter PWHT. For carbon
A S M E III may
and low allow steels, if
be ignored if
radiographed before
other
PWHT they must be
requirements of
ultrasonically inspected
the specification
after PWHT. In geneare met.
rai magnetic particle or
dye penetrant examination shall be performed after PWHT.
Full radiography and dye
penetrant/magnetic
particle examination
for all butt joints.
Magnetic particle/dye
penetrant examination
of partial penetration
welds, progressively as
weld is made.
NDT requirement
Notes
For vessels to NC3200 as
Class 1 (NB) with additional reduction fac-
Hydrostatic test pressure for vessels to
NC3200 as Class
Design Stress Intensity
Hydrostatic test presSm is the lower of 2Sy/3
sure 1.25 × Design
or Su/3 at temperature,
Pressure.
or specified minimum
values at room
temperature.
Design Stress
1
1, 2, 3 ( N B , N C , N D )
1 (NB)
ASME Class
ASME
APPENDIX
III VESSELS
CLASSES
-~
.~
.~
tO
4~
3 (ND)
all in Tables I 7-0 of ASME II.
tors for some joints,
e.g. 0-5 for fillet welds.
For vessels to NC3300
Allowable Stress Value
S is the lower of 2Sy/3
or S,,/4 at temperature,
or specified minimum
values at room temperature. Fillet weld
joint efficiency 0-55 in
tension, 0.49 in shear.
1 (NB), for vessels to
NC3300 1-5 x Design
Pressure. See Note 6
for NC3200/NC3300.
Full radiography of main All in Tables
Allowable Stress Value S Hydrostatic test presNone. Thickness
seam welds is required Tables 1-7.0 of
limits in specifiis the lower of 2Sy/3 or
sure 1.5 x Design
when thickness exceeds
ASME II plus
cations of
S,,/4 at temperature, or
Pressure.
certain limits and/or
all in Tables I ASME II may
specified minimum
joint efficiency factors
8.0 of ASME II.
values at room
be ignored if
are based on full radiother
temperature.
ography. For carbon
requirements of Joint Efficiency Factors
and stainless steels rathe specification
are given for different
diography is required
are met.
Types of welds within
for joints over 31 mm
each weld Category.
thick.
Joint Efficiency Factor
If full radiography is not
depends upon extent of
required due to thickradiography of the
ness or location then
joint. For vessels despot radiography or no
signed for external
radiography is used
pressure only the Joint
depending on Joint
Efficiency Factor for all
Efficiency and weld
welds is 1.
Category.
Also fillet welds joint
Radiography is required
for all butt joints in
vessels designed to
NC3200 and to all butt
joints over 4-7 mm
thick in vessels designed to NC3300.
Magnetic particle/dye
penetrant examination
of nozzle partial penetration welds on
accessible surfaces.
t,~
ASME Class
Full radiography is required for butt welds in
nozzles attached to
vessel sections or heads
which are required to
be fully radiographed.
Radiography is not required for non-buttwelded joints.
No radiography is required when a vessel or
part is designed for external pressure only.
For ferrous materials
there are no requirements for magnetic
particle/dye penetrant
examination of welded
joints.
N D T requirement
Permitted
material
1--contd.
Maximum
thickness (ram)
APPENDIX
Design Stress
Notes
p.
bo
Design of nuclear vessels and piping
249
1. The code gives rules for new construction and includes consideration of mechanical and thermal stresses due to cyclic operation,
but only Classes 1 and 2 provide rules for fatigue assessment. The
Code does not cover deterioration which may occur in service as a
result of radiation effects, corrosion, erosion or instability of
material.
2. The Code recognises the different levels of importance associated
with the function of each item. Code Classes allow the choice of
rules to provide assurance of structural integrity and quality
commensurate with the relative importance assigned to individual
components.
3. The Code does not provide guidance on selection of Code Classes
for a given component. The Owner is responsible for this and
must apply system safety criteria to decide. In a component with
multiple compartments (e.g. heat exchangers) each compartment
may be assigned different Code Classes provided that interaction
between compartments is taken into account. No guidance is given
in the Code on selection of plant operating conditions of significance in selection of Design, Service or Test Loadings and the
corresponding acceptable Limits. The Code refers to guidance in
systems safety criteria documents and requirements of
regulatory/enforcement authorities.
4. A Design Specification and a Design Report are required for Class
1, 2 and 3 components. The Design Report must be certified by a
Registered Professional Engineer for Class 1 (NB) components,
Class 2 (NC) vessels to NC3200 and Class 2 and 3 components
with Service Loadings greater than Design Loadings.
5. ASME III sets no minimum size or pressure for applicability of
the Code.
6. Design:
Class 1 (NB). Design by Analysis is generally used with some
Design by Rule. In the case of conflict between Design by
Analysis and Design by Rule then Design by Rule takes
precedence.
Class 2 (NC). Alternative Subarticles allow Design by Analysis
(NC3200) on same basis as Class 1 (NB) or purely Design by Rule
(NC3300).
Class 3 (ND). Design by Rule only.
7. Fatigue:
Class 1 (NB). Fatigue needs to be considered based on a set of
given rules (experience not allowed). If fatigue analysis is required
it is based on S-N curves and peak stresses from elastic analysis.
250
L. P. Harrop
Class 2 (NC). For vessels to NC3200 (Design by Analysis) no
fatigue analysis is needed if there is comparable experience or the
conditions of one of two sets of rules are met. If fatigue analysis is
required it is similar to that for Class 1 (NB). For vessels designed
to NC3300 (Design by Rule) there are no rules for establishing if
fatigue analysis is required nor for conducting a fatigue analysis.
Class 3 (ND). There are no rules for establishing if fatigue
analysis is required nor for conducting a fatigue analysis.
.
Service limits based on stress are for Design and Levels A, B, C,
D loadings, the latter four being for progressively less frequent
events.
Class 1 (NB)
Maximum shear stress theory used.
Design conditions are Pm < Sm, PL < 1.5Sin, PL + Pb < 1.5Sin,
Level A conditions PL + P~ + Q < 3Sin, PL + Pb + Q + F < Sa.
Level B conditions as Level A except that if pressure exceeds
the design pressure then allowable stress intensities are 110% of
Design Condition limits. Fatigue is as for Level A.
Level C conditions, primary stress limits of the Design Conditions shall be satisfied using an Sm value equal to the greater of
1.2 × the tabulated Sm or 1 × the tabulated yield strength at
temperature. In addition for ferritic material, the Pm elastic
analysis limits shall be the greater of l'lSm or 0.9Sy. External
pressure up to 1.2 × Design Limits. No requirement for secondary
or peak stresses, fatigue or thermal stress ratchet.
Level D conditions are contained in non-mandatory Appendix
F. Only primary stress limits are controlled, i.e.
~lesser of 2-4Sm, 0.7Su for austenitic stainless steel
Pm< t0"7Su for ferritic steel
flesser for 3-6Sm, 1-05Su for austenitic stainless steel
PL, (PL -t- Pb) < ~ 1"05S~ for ferritic steel
Buckling limit 1.5 x Design Condition or 2-5 x Design Condition
if ovality is 1% or less. Or 2/3 of buckling load determined by
comprehensive analysis or tests.
Class 2 (NC) to NC3200
Maximum shear stress theory used.
Primary stress intensity limits are Pm< kSm
PL < l'5kSm
(Pro or PL) + Pb < 1-5kSm
Design of nuclear vessels and piping
251
where k is given in the table below
Service Limit
k
Design
Level A
Level B
Level C
Level D °
1
1
1-1
1.2
2.0
If complete analysis is performed, stress limits of Appendix F apply.
a
Secondary stresses are dealt with by providing rules for geometry
of design details. Where details are not covered a detailed stress
analysis shall be made. Secondary stresses need be evaluated only
for Level A and B limits.
Class 2 (NC) to NC3300 and Class 3 (ND)
Maximum normal stress limits.
Primary normal stress limits are:
Service Limit
Design and Level A
Stress Limit
am < S
(am or eL) + a. < 1-5S
Level B
Level C
Level D
.
am < 1.1S
(Ore or OL) + trb < 1"65S
trm < 1"5S
(am or aL) + Ob < 1"8S
Om< 2.0S
(am or OL) + Ob < 2"4S
Impact Testing (Toughness):
Class 1 (NB). Impact tests are required unless (i) nominal thickness less than 16 mm (ii) material is austenitic stainless steel. Test
requirements involve drop weight tests to determine TNDT then
Charpy tests to determine RTNDT (which is equal to or greater
than TND'r). At R TND'r+ 33°C Charpy energy absorbed must be at
least 68 J and lateral expansion must be at least 0.9 mm. Protection against non-ductile fracture provided by evaluation using
methods of Appendix G or similar. For a minimum toughness of
about 130 MPaVrm, Appendix G requires a temperature of operation at least 78 °C above RTrqDT, for a minimum toughness of
L.P. Harrop
252
about 1 9 0 M P A V ~ Appendix G requires a temperature of
operation at least 94 °C above R TNDT.
Class 2 (NC). Impact tests required unless similar conditions to
Class 1 apply, with the extra exception of materials for components where Lowest Service Temperature (LST)t exceeds
65 °C. For thickness <63 mm toughness requirement is for Charpy
tests at or below the LST or drop weight tests to determine TNDT
and minimum margin between LST and TNDT (varies from 16 to
44 °C margin between 16 and 254 mm thick, Appendix R). For
thickness >63 mm must have L S T - TNDT margin for base metal
and weld procedure qualification weld metal tests and either
Charpy tests at or below LST or L S T - TNDT margin for base
metal and H A Z of weld procedure qualification tests. For
thickness over 63 mm Charpy energy varies from 54 to 68J
depending on minimum yield strength, plus lateral expansion
requirement of about 1 mm. For thickness less than 63 mm can use
energy or lateral expansion.
Class 3 (ND). Impact tests required unless similar conditions to
Class 2 apply, but LST exception is above 38 °C. For thickness
over 63 mm Charpy energy varies from 34 to 54 J depending on
minimum yield strength, alternatively can use lateral expansion,
minimum is about 0.6 mm.
10. Post-weld Heat Treatment:
Class 1 (NB). PWHT is mandatory for ferrous materials with
specific exceptions. Austenitic stainless steels do not require
PWHT. For carbon and carbon manganese steels joint thickness
must be less than 37 mm for no PWHT except longitudinal welds
have to be less than 15 mm thick for no PWHT.
Class 2 (NC). PWHT mandatory for ferrous materials with specific exceptions. Rules similar to Class 1 (NB) except no additional
limit for longitudinal welds.
Class 3 (ND). PWHT rules same as for Class 2 (NC).
Notation for Appendix 1
F
Peak stress increment added to stress intensity by a
concentration or some thermal stresses which cause fatigue
but not distortion
t For Class 2 and 3, LST defined as minimum temperature of contained fluid or
calculated minimummetal temperature whenever pressure above 20% of preoperation
hydrostatic test pressure.
Design of nuclear vessels and piping
PL
Pm
a
RTNDT
S
Sm
&
Sy
TDT
trb
aL
O"m
253
Primary bending stress intensity due to any combination of
pressure and mechanical loadings. Does not include discontinuity or concentration effects
As Pm but local, includes effect of discontinuities but not
concentrations
Average general primary membrane stress intensity across
thickness of section due to any combination of pressure and
mechanical loadings. Does not include discontinuity or concentration effects (stress intensity is 2x maximum shear stress)
Secondary stress intensity, self-equilibriating stress for continuity at structural discontinuities can be caused by pressure,
mechanical loadings or differential thermal expansion
Reference nil ductility temperature
Maximum allowable stress value (Class 2 using NC3300, Class
3)
Design stress intensity limit (Class 1, Class 2 using NC3200)
Ultimate tensile strength at temperature or specified minimum
value at room temperature
Yield strength at temperature or specified minimum value at
room temperature
Nil ductility temperature
Bending normal stress produced by pressure and other mechanical loadings. Excludes effects of discontinuities and
concentrations
As am but local. Includes effects of discontinuities but not
concentrations
General membrane normal stress produced by pressure a n d
other mechanical loadings. Excludes effects of discontinuities
and concentrations
2 (NC)
1 (NB)
ASME
Class
Permitted
material
Timing of examination as for
Class 1 (NB).
Butt-welded joints shall be
radiographed. Fillet and
partial penetration welds
shall be examined by liquid
penetrant or magnetic par-
All in Tables I-7-0 of
ASME II.
Timing of examination as for All in Tables I-1.0 of
Class 1 (NB) vessels.
ASME II.
Full radiography and dye
penetrant/magnetic particle
examination for all butt
joints, including nozzles,
branches and piping
connections,
Magnetic particle/dye penetrant examination of partial
penetration welds,
Full penetration corner
welded branch and pipe
connections over 4 in nominal size examined by radiography and either liquid
penetrant or magnetic partide. Under 4 in nominal
size examine by liquid penetrant or magnetic particle
only.
ND T requirement
As Class 1 (NB).
None. Thickness
limits in specifications of ASME II
may be ignored if
other requirements
of the specification
are met.
Maximum
thickness (mm )
Allowable Stress Value S is
the lower of 2Sy/3 or Su/4
at temperature, or specified
minimum value at room
temperature.
Design equations for axial
primary stress as for Class
Design Stress Intensity Sm is
the lower of 2Sy/3 or S,/3
at temperature, or specified
minimum value at room
temperature.
Design equations for axial
stress given for primary
stresses, primary plus secondary stress range and include stress intensification
factors (stress indices).
Also equation for peak stress intensity range which
includes local stress indices,
in this case half the range is
limited to Sa.
If design does not meet the
simplified equations then
the more general rules of
Design by Analysis NB3200
may be used.
Design Stress
APPENDIX 2
ASME III PIPING CLASSES 1, 2, 3 (NB, NC, ND)
As for Class 1 (NB) piping
system.
Hydrostatic test pressure
1-25 x lowest Design
Pressure of any component in the system.
Notes
3 (ND)
Timing of examination as for All in Tables I-7-0 of
Class 1 (NB).
ASME II plus all in
Examination only required
Tables I-8-0 of
for piping over 2 in nominal
ASME II.
size. Longitudinal welded
joints in piping are examined in accordance with
the material specification of
the tubular product. All
other welds are examined
by magnetic particle, liquid
penetrant or radiography
(Owner's choice of which
one).
ticle. Welded branch connections over 4 in nominal
size examined by radiography, for 4 in and under
liquid penetrant or magnetic particle.
None. Thickness
limits in specifications of ASME II
may be ignored if
other requirements
of the specification
are met.
Allowable Stress Value S is
As for Class 1 (NB) piping
the lower of 2Sy/3 or Su]4
system.
at temperature, or specified
minimum value at room
temperature.
Joint Efficiency Factors are
given for different types of
longitudinal welds for pressure design equation. Design equations for axial
stresses as for Class 2 (NC).
1 (NB). Also design equation limiting thermal expansion stresses for Level
A and B Service Limit
ioadings.
t,o
t.tt
o~
t~
t~
256
1.
2.
3.
4.
5.
6.
L. P. Harrop
The Code gives rules for new construction and includes consideration of mechanical and thermal stresses due to cyclic operation.
Class 1 rules require a detailed fatigue analysis. Class 2 and 3 rules
only consider start-up/shut-down cycling. The Code does not
cover deterioration which may occur in service as a result of
radiation effects, corrosion, erosion or instability of material.
The Code recognises the different levels of importance associated
with the function of each item. Code Classes allow the choice of
rules to provide assurance of structural integrity and quality
commensurate with the relative importance assigned to individual
components.
The Code does not provide guidance on selection of Code Classes
for a given component. The Owner is responsible for this and
must apply system safety criteria to decide. In a component with
multiple compartments (e.g. heat exchangers) each compartment
may be assigned different Code Classes provided that interaction
between compartments is taken into account. No guidance is given
in the Code on selection of plant operating conditions of significance in selection of Design, Service or Test Loadings and the
corresponding acceptable Limits. The Code refers to guidance in
systems safety criteria documents and requirements of
regulatory/enforcement authorities.
A Design Specification and a Design Report are required for Class
1, 2 and 3 components. The Design Report must be certified by a
Registered Professional Engineer for Class 1 (NB) components,
and also Class 2 and 3 components with Service Loadings greater
than Design Loadings.
ASME III sets no minimum size on pressure for applicability of
the Code.
When piping systems operating at different pressures are connected by a valve or valves the valve(s) shall be designed for the
higher pressure system requirements of pressure and temperature.
The lower pressure system shall be designed in accordance with
points (1), (2) or (3), below:
(1)
(2)
(3)
The requirements of the higher pressure system shall be
met.
Pressure relief devices or safety valves shall be included to
protect the lower pressure system.
(a) Redundant check or remote actuated valves shall be
used in series at the interconnection or a check in series
with a remote actuated valve.
Design of nuclear vessels and piping
257
(b) When mechanical or electrical controls are provided,
redundant and diverse controls shall be installed which will
prevent opening when the pressure in the high pressure
system exceeds the Design Pressure of the low pressure
system.
(c) Means shall be provided such that operability of all
components, controls and interlocks can be verified by
test.
(d) Means shall be provided to ensure that the leakage
rate of the interconnecting valves does not exceed the
relieving capacity of the relief devices in the low pressure
system.
(e) Adequate consideration be given to the control of
fluid pressure caused by heating of the fluid trapped
between two valves.
7.
When pressure reducing valves are used and one or more pressure
relief devices or safety valves are provided, bypass valves may be
provided around the pressure reducing valves. Capacity of relief
system for low pressure system depends on whether there are
interlocks between bypass valves and pressure reduction valves.
Design and Fatigue Assessment Requirements: Design of piping
based on limits to hoop stress produced by pressure and limits on
axial stresses produced by pressure, deadweight, thermal expansion, etc. Design for axial stresses requires a piping system stress
analysis. The Code provides flexibility factors for piping components for use in elastic system stress analyses.
Class 1 (NB). Equation given for design for hoop stress due to
pressure. Also equations given for axial stresses of piping. If these
are not met the designer has the option to use the Design by
Analysis rules of NB3200 instead. Piping of 25 mm nominal size or
less which is classified Class 1 may be designed in accordance with
the design requirements of Class 2 (NC). Also Class 1 piping may
be analysed in accordance with the Class 2 analysis of piping using
Class 2 allowable stresses provided the specified service loads for
which Level A and B Limits apply satisfy certain conditions.
These conditions are intended to limit damage due to cyclic
loading. Fatigue of piping analysed by limiting peak axial stress
intensity range using equation for peak stress intensity which is a
simplification of a full NB3200 analysis. Pairs of load sets which
follow each other in time are considered and damage summed
using Miner's Rule.
258
8.
L.P. Harrop
Class 2 (NC). Equation given for design for hoop stress due to
pressure. Sets of equations given for consideration of axial stress
in piping. These require less detailed analysis than for Class 1
piping. Only cyclic stress (fatigue) consideration is start-up and
shut-down.
Class 3 (ND). As for Class 2 (NC) piping except pressure design
equation includes weld joint efficiency factors.
Service limits based on stress are for Design and Levels A, B, C
and D loadings, the latter four being for progressively less
frequent events.
Class 1 (NB)
Design condition.
Pressure Design P = 2Smt/(Do - 0-8t)
Axial primary stress intensity BIPDo/2t + B2DoMi/21 < 1"5Sm
Level A condition.
Axial primary plus secondary stress intensity range
C1PoDo/2t + C2DoMi/2I + C3Eab IteaTa- tebTbl < 3Sm
(alternative elastic-plastic discontinuity analysis also provided)
Axial peak stress intensity range--for each pair of load sets Sp is
given by
K~C~PoDo/2t + K2C2DoMJ2I + K3Eoc IATd/(2(1 - v))
+ K3C3Eab IocaTa- ~ T . I + Ece IAT~I/(1- v)
The alternating stress intensity Sal t is Sp/2; Salt is entered on the
S-N Design Curve and this provides the permitted number of
cycles for the given load pair. Cumulative damage for several load
pairs is assessed using Miner's Rule.
Level B condition.
Permissible pressure not more than 1.1 x Design Pressure. Axial
primary stress limit as for Design but limit is lesser of l'8Sm, l'5Sy,
otherwise Level A limits apply
Level C condition.
Permissible pressure not more than 1.5 x Design Pressure. Axial
primary stress as for Design but limit is lesser of 2"25Sm, l'8Sy.
Level D condition.
Rules of Appendix F may be used. Or permissible pressure not
more than 2 x Design Pressure, axial primary stress as for Design
but limit is lesser of 3Sin, 2Sy.
Design of nuclear vessels and piping
259
Class 2 (NC)
Design condition.
Pressure Design P = 2St/(Do - 2yt) y = 0.4 or (Do - t)/(2Do - t) if
Do/t < 4. Axial primary stress intensity B1PDo/2t + B2MA/Z <
1.5S.
Level A and Level B condition.
Occasional Level B limit loadings. Permissible pressure not more
than 1 . 1 x D e s i g n Pressure, axial stress limit BiPr,~Do/2t+
B2(MA + MB)/Z < lesser of l'8Sh, l'5Sy. Thermal expansion for
Level A and B Service Limit Loadings satisfy either
iMc/Z < SA or
PDo/4t + 0"75iMA/Z + iMc/Z < (Sh + SA)
Level C condition.
Permissible pressure not more than 1.5 x Design Pressure. Axial
stress: Occasional Level B limit equation applies except limit is
lesser of 2"25Sh, l'8Sy and MB need not include anchor displacements due to earthquake or other secondary effects.
Level D condition.
Permissible pressure not more than 2 x Design. Axial stress as for
Level C except limit is lesser of 3Sh, 2Sy.
Class 3 (ND)
Design condition.
Pressure Design P = 2SEt/(Do- 2yt), where E is the weld joint
efficiency factor. All other equations for Design and Level A, B,
C and D Service Limit loadings are as for Class 2 (NC).
9. Impact Testing (Toughness).
Class 1 (NB). Impact tests are required unless (i) nominal thickness less than 16 mm; (ii) material is austenitic stainless steel; (iii)
pipe of any thickness of nominal size 152 mm or less. For nominal
wall thickness less than 63.5 mm testing requires 3Cv specimens at
a temperature lower than or equal to the Lowest Service
Temperature (LST). All three specimens shall meet lateral
expansion requirements (e.g. 38-63 mm thickness minimum lateral expansion 1.02 mm). Testing applied to base material and weld
metal H A Z , base material and weld metal from weld procedure
qualification tests. Over 63.5 mm thick material shall meet requirements of Class 1 (NB) vessels. For over 63-5 mm thickness
the LST shall not be less than RTNDx+56°C unless a lower
temperature is justified using methods similar to Appendix G.
Class 2 (NC). Impact tests required unless similar conditions to
260
L. P. Harrop
Class 1 apply, plus extra exception of material for components
where LST exceeds 65 °C. Toughness requirements as for Class 2
vessels.
Class 3(ND). Impact tests required unless similar conditions to
Class 2 apply, but LST exception is above 38°C. Toughness
requirements as for Class 3 (ND) vessels.
For Class 1, 2 and 3 LST defined as minimum temperature of
contained fluid or calculated minimum metal temperature (for
Class 1 during normal operation) whenever pressure above 20%
of preoperation hydrostatic test pressure.
10. Post-weld Heat Treatment.
Class 1 (NB). PWHT is mandatory for ferrous materials with
specific exceptions. Austenitic stainless steels do not require
PWHT. For carbon and carbon manganese steels joint thickness
must be less than 37 mm for no PWHT except longitudinal welds
have to be less than 15 mm thick for no PWHT.
Class 2 (NC). P W H T mandatory for ferrous materials with specific exceptions. Rules similar to Class 1 (NB) except no additional
limit for longitudinal welds.
Class 3 (ND). PWHT rules same as for Class 2 (NC).
Notation
Primary stress indices
C1, C2, C3 Secondary stress indices
Pipe outside diameter
Do
Average Young's modulus across a discontinuity
Eab
Stress intensification factor (Class 2 and 3) for moments in
i
thermal expansion equation
Pipe section moment of inertia
I
K1, Kz, K3 Local stress indices
B1, B2
Design of nuclear vessels and piping
MA
P
P0
RTNDT
S
SA
261
Resultant moment due to sustained loads (Class 2 and 3)
Resultant moment due to Occasional loads
Range of resultant moments due to thermal expansion
Resultant range of moment when system goes from one
service load set to another (Class 1)
Design internal pressure
Range of service pressure
Reference nil ductility temperature
Maximum allowable stress value (Class 2, Class 3)
=f(l'25Sc+O'25Sh) (Class 2 and 3); f is stress range
reduction factor for cyclic conditions and depends on the
number of full temperature (hot to cold) cycles for total
service. 7000 or less cycles f = 1; 100 000 or more cycles
f=0.5
&
Sm
&
t
ra(r )
Tr~DT
Z
IAT,I
IAT I
V
Maximum allowable stress level in cold condition (Class 2
and 3)
Maximum allowable stress level in hot condition (Class 2
and 3)
Design stress intensity limit (Class 1)
Ultimate tensile strength at temperature or specified
minimum value at room temperature
Yield strength at temperature or specified minimum value
at room temperature
Pipe wall thickness
Range of average temperature on side a(b) of gross
structural discontinuity
Nil ductility temperature
Pipe section modulus
Coefficient of thermal expansion
Absolute value of range of the temperature difference
between the temperature on the outside and inside surfaces assuming moment generating equivalent linear temperature distribution.
Absolute value of range for that portion of the nonlinear
thermal gradient through the wall not included in IA T~[
Poisson's ratio
Construc lion
category
Permitted
material
All in Section 2 of
After all PWHT (unless
BS 5500.
within Category 2 material
and thickness limits):
Main seam welds 100% ultrasonic or radiographic plus
magnetic particle or dye
penetrant examination by
agreement.
Unless otherwise agreed,
nozzle and branch welds
shall be ultrasonically or
radiographically inspected
above given thickness
(40 mm for common carbon
steels and austenitic steels,
10-15 mm for low alloy
steels).
Nozzle, branch and attachment welds, 100% magnetic particle or dye penetrant examination.
N D T requirement
Design Stress
None except where
C, C Mn and low allow steels
NDT method limits.
lower of RetT)/1.5 or
Rm/2.35
Austenitic lower of
Re~T)/1-35, Rm/2.5
Maximum
thickness (ram)
APPENDIX 3
BS 5500 VESSEL C O N S T R U C T I O N C A T E G O R I E S 1, 2, 3
f~
t--cJ
Hydrostatic test pressure
1.25 x Design Pressure
with correction for
difference in test and design temperatures.
Ptest = 1"25 X
Notes
t~
tO
Visual only, done during initial assembly and preparation of second side.
(Magnetic particle or dye penetrant may be used as
aids.)
Main seam welds, at least
10% ultrasonic or radiographic coverage as early in
fabrication as practical.
Magnetic particle or dye
penetrant inspection of full
length of all welds attaching
nozzles, branches to shell
and end plates and at least
10% of length of attachment welds.
Main seam welds made at
site, 100% radiographic
and/or ultrasonic coverage.
C and C Mn steels
with R m ~<432 MPa
Austenitic
M1
M2
Austenitic
16
25
30
40
40
Away from seam welds, in
regions requiring local stress analysis. Category 1/2
limits apply otherwise:
Rm/5
lower of 120 MPa or
120(450/(400 + t) MPa (t is
temperature in °C)
lower of RctT)/1-35, Rm/2"5
lower of RcCT)/1-5, R J 2 . 3 5
Maximum temperature
300 °C. If specified minimum yield strength (1%)
is <230 MPa. Design strength × 0-8. Hydrostatic
test pressure greater of
Category 1 value and
1.5 x Design Pressure.
Hydrostatic test pressure as
for Category 1.
1".2
Ca.
264
1.
2.
3.
4.
5.
6.
7.
L. P. Harrop
In general BS 5500 is based on design by rule (Section 3). However
for loadings and components not covered by Section 3, Appendix A
provides criteria for stresses to demonstrate design acceptability by
stress analysis. The latter includes design by elastic analysis similar
to ASME III Division 1 Subsection NB (i.e. Class 1).
A detailed fatigue analysis need not be made when the design is
based on previous and satisfactory experience of strictly comparable service, or when all of conditions in Appendix C are
satisfied. Otherwise Appendix C gives rules for analysis for cyclic
operation. Analysis is based on S - N curve which accounts for
reduced endurance of ground-flush butt welds compared to base
metal.
Appendix D deals with requirements for ferritic steels in Categories
M0-M4 inclusive for vessels required to operate below 0 °C. M0 to
M4 covers the carbon and carbon-manganese steels plus some low
alloy steels. The Design Reference Temperature plus the material
thickness set the Material Impact Test Temperature. There are
separate graphs for non-post-weld heat treated parts and for
post-weld heat treated parts. The Design Reference Temperature is
the minimum Design Temperature adjusted to take account of
stress level and Construction Category. For Category adjustment
the minimum Design Temperature is reduced by 0, 10, 20 °C for
Categories 1, 2, 3, respectively, to obtain the Design Reference
Temperature. For a Category 1 vessel, P W H T with membrane
stress over 2/3 the stress limit and wall thickness 60 mm or more,
the Impact Test Temperature equals the minimum Design Temperature. The Charpy impact energy at the specified Impact Test
Temperature is 27J (Rm < 450 MPa) or 40J (Rm -> 450 MPa).
Post-weld heat treatment is required for example for M0 and M1
steels with thickness over 35 mm and for M4 low alloy steel with
thickness over 15 mm.
Purchaser and Manufacturer shall give joint consideration to the
likely effect of corrosion.
BS 5500 Construction Categories are intended to apply to components of a vessel and not necessarily to complete vessels which
may therefore comprise components in two or more such
Categories.
BS 5500 does not apply to vessels whose internal pressure (apart
from hydrostatic head) does not exceed 0-014MPa above atmospheric or 0.06MPa below atmospheric or vessels in which the
stresses calculated are less than 10% of the permitted design
stresses.
Design of nuclear vessels and piping
265
Notation
C
fo
M0,
M2,
M5,
M7,
M1
M4 1
M6,
M9 J
geL
Re(T)
Rm
Rpo.2
t
Corrosion allowance
Nominal design strength of material at temperature of
pressure test
Nominal time independent design strength at design temperature or the highest temperature at which time independent design strengths are given if this is lower than the
design temperature
Carbon and carbon manganese steels
Low alloy steel plates
Minimum value of lower yield point stress
Minimum value of ReL or Rpo.2 (Rpl.O for austenitic steels)
for the grade of material at temperature T
Minimum tensile strength specified for the grade of
material at room temperature
Minimum value of proof stress at 0.2% permanent strain
Nominal thickness of the section under consideration