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Page 1




Power Boilers, Section I of the ASME Boiler and Pressure
Vessel Code, provides rules for the construction of power boilers,
but since it is neither a textbook nor a design handbook, its rules
are accompanied by very little explanation. The objective of this
chapter is to provide an overview of Section I rules, their intent,
and how they are applied and enforced.

This chapter is an abbreviated version of the book Power
Boilers, A Guide to Section I of the ASME Boiler and Pressure
Vessel Code [1]. That comprehensive guide was used as the text-
book for a two-day ASME Professional Development Department
course on Section I, developed and taught by Martin D. Bernstein
and Lloyd Yoder. Accordingly, only some of the more important
aspects of Section I construction are covered here; these are:

• History and Philosophy of Section I
• How the ASME Code Works (the System of ASME Code

• Organization of Section I
• Scope of Section I
• Distinction between Boiler Proper Piping and Boiler External

• How and Where Section I Is Enforced
• Fundamentals of Section I Construction:

• Permitted Materials
• Design
• Fabrication
• Welding and Postweld Heat Treatment
• Nondestructive Examination
• Hydrostatic Testing
• Third-Party Inspection
• Certification by Stamping & Data Reports

The design and construction of power boilers involves the use of
other sections of the ASME Code besides Section I, and the use of
those other book sections is mentioned in this chapter when appro-
priate. Section II, Materials, provides detailed specifications for
materials and welding consumables, as well as tabulations of
design stresses and material properties, such as yield strength and
tensile strength as a function of temperature. Section V,
Nondestructive Examination, contains a series of standards that
provide the methodology for conducting the various nondestructive
examinations used in Section I construction. Section IX, Welding

and Brazing Qualifications, provides the information necessary to
qualify the weld procedures and the welders required for Section I
construction. In a rather unusual arrangement, the construction
rules for boiler piping are found partly in Section I and partly in the
B31.1 Power Piping Code. This has led to considerable misunder-
standing and confusion, as explained in section 1.5 of this chapter,
Distinction between Boiler Proper Piping and Boiler External
Piping. For a fuller description of those other Code sections, refer
to the specific chapters in this volume that cover them.

Those unfamiliar with the ASME Code may be confused at
first by a number of terms in it. Examples include third-party
inspection, Authorized Inspector, Authorized Inspection Agency,
jurisdiction, Maximum Allowable Working Pressure (MAWP),
boiler proper, boiler proper piping, boiler external piping, inter-
pretation, Code Case, accreditation, Manufacturers’ Data Report,
and Certificate of Authorization (to use a Code symbol stamp).
These terms are explained in the text wherever appropriate.

Although the ASME Boiler and Pressure Vessel Code changes
very slowly, it does change continuously. The rate of change in
recent years seems to have increased, perhaps due to technologi-
cal innovation and international competition. Thus, although this
chapter provides a substantial body of information and explana-
tion of the rules as they now exist, it can never provide the last
word. Nevertheless, it should provide the user with a very useful
introduction and guide to Section I and its application.


It is helpful to begin the study of Section I of the ASME Boiler
& Pressure Vessel Code with some discussion of its character and
philosophy. According to the dictionary, the term code has several
meanings: a system of principles or rules; a body of laws arranged
systematically for easy reference; a systematic statement of a
body of law, especially one given statutory force. Section I is pri-
marily a system of rules. When the ASME decided in 1911 that
the country needed a boiler code, it assigned a committee and
gave it a mandate to formulate standard rules for the construction
of steam boilers and other pressure vessels. The first edition of
what is now known as Section I was finally approved by the
ASME in 1915, and incorporated what was considered at the time
to be the best practice in boiler construction. However, the guid-
ing principle, then as now, was that these are safety rules.

Lloyd W. Yoder and John R. MacKay1

1Late Martin D. Bernstein was the originator of his Chapter for the 1st edition.

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2 • Chapter 1

Part of the foreword of Section I explains the guiding principles
and philosophy of Section I, and also of the Boiler and Pressure
Vessel Committee (the Committee), which continues to adminis-
ter the Code. Here are some excerpts from that foreword:

The American Society of Mechanical Engineers set up a com-
mittee in 1911 for the purpose of formulating standard rules
for the construction of steam boilers and other pressure ves-
sels. This committee is now called the Boiler and Pressure
Vessel Committee.

The Committee’s function is to establish rules of safety, relat-
ing only to pressure integrity, governing the construction2 of
boilers, pressure vessels, transport tanks and nuclear compo-
nents, and inservice inspection for pressure integrity of
nuclear components and transport tanks, and to interpret
these rules when questions arise regarding their intent. This
code does not address other safety issues relating to the con-
struction of boilers, pressure vessels, transport tanks and
nuclear components, and the inservice inspection of nuclear
components and transport tanks. The use, of the Code should
refer to other pertinent codes, standards, laws, regulations, or
other relevant documents. With few exceptions, the rules do
not, of practical necessity, reflect the likelihood and conse-
quences of deterioration in service related to specific service
fluids or external operating environments. Recognizing this,
the Committee has approved a wide variety of construction
rules in this Section to allow the user of his designee to select
those which will provide a pressure vessel having a margin
for deterioration in service so as to give a reasonably long,
safe period of usefulness. Accordingly, it is not intended that
this Section be used as a design handbook; rather, engineer-
ing judgment must be employed in the selection of those sets
of Code rules suitable to any specific service or need.

This Code contains mandatory requirements, specific prohi-
bitions, and non-mandatory guidance for construction activ-
ities. The Code does not address all aspects of these activities
and those aspects which are not addressed should not be con-
sidered prohibited. The Code is not a handbook and cannot
replace education, experience, and the use of engineering
judgment. The phrase engineering judgment refers to techni-
cal judgments made by knowledgeable designers experienced
in the application of the Code. Engineering judgments must
be consistent with Code philosophy and such judgments must
never be used to overrule mandatory requirements of specific
prohibitions of the Code.

The Boiler and Pressure Vessel Committee deals with the care
and inspection of boilers and pressure vessels in service only
to the extent of providing suggested rules of good practice as
an aid to owners and their inspectors.

The rules established by the Committee are not to be inter-
preted as approving, recommending, or endorsing any
proprietary or specific design or as limiting in any way the
manufacturer’s freedom to choose any method of design or
any form of construction that conforms to the Code rules.

Certain points in these paragraphs should be stressed. Section
I covers the design, fabrication, and inspection of boilers during
construction, that is, it covers new construction only. Other rules
cover repair and alteration of boilers and pressure vessels in ser-
vice, for example, the National Board Inspection Code [2] (see
also section 1.6 in this chapter) and the API Pressure Vessel
Inspection Code, API 510 [3]. Although there is general agree-
ment that Section I should apply to new replacement parts, and
such parts are usually specified that way, until the appearance of
the 1996 Addenda (see Effective Dates of the Code and Code
Revisions in section 1.6.5), Section I had no clear provisions
dealing with replacement parts other than how they should be
documented. Those addenda included changes to PG-106.8 and
PG-112.2.4 that require the manufacturers of replacement parts
to state on the data report form (the documentation that accom-
panies the part, see section 1.7.8, Certification by Stamping and
Data Reports) whether or not the manufacturer is assuming
design responsibility for those replacement parts. Also men-
tioned in the Foreword is the objective of the rules: reasonably
certain protection of life and property, but with a margin for dete-
rioration in service to provide a reasonably long, safe period of
usefulness. This is an acknowledgment of the fact that no equip-
ment lasts forever, and that boilers do have a finite life.

The Foreword (of all book sections) now includes cautions that
the designer using computers is responsible for assuring that any
programs used are appropriate and are used correctly. It is also
now mentioned that material specifications of recognized national
or international organizations other than ASTM and AWS may be
acceptable in ASME construction.

The Section I rules have worked well over many years. They
were based on the best design practice available when they were
written, and have evolved further on the same basis. Rules have
been changed to recognize advances in design and materials, as
well as evidence of satisfactory experience. The needs of the
users, manufacturers, and inspectors are considered, but safety is
always the first concern. Today, the committee that governs, inter-
prets, and revises Section I is called the Subcommittee on Power
Boilers, also known as Subcommittee I, or SC I. (It is also some-
times referred to as the Section I Committee, but that is an unoffi-
cial designation.)

Another basis for the success of Section I is the Committee’s
insistence that the rules are to be understood as being general and
are not to be interpreted as approving, recommending, or endors-
ing any proprietary or specific design, or interpreted as limiting a
manufacturer’s freedom to choose any design or construction that
conforms to the Code rules. The Committee deems the manufac-
turer to be ultimately responsible for the design of its boiler and
leaves certain aspects not covered by Section I to the manufactur-
er. Traditionally, the manufacturer has recognized this and borne
the responsibility for such things as functional performance of the
boiler, thermal expansion and support of the boiler and its associ-
ated piping, and the effects of thermal stress, wind loading, and
seismic loading on the boiler. Further evidence of the flexibility
and reasonableness of Section I— and another key to its success
as a living document— is found in the second paragraph of the

The Code does not contain rules to cover all details of design
and construction. Where complete details are not given, it is

2Construction, as used in this Foreword, is an all-inclusive term comprising materials, design, fabrication, examination, inspection, testing, certifica-
tion, and pressure relief.

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20 • Chapter 1

parts may cause impaired service performance when the compo-
nent operates in the creep range (above 540°C). Heat treatment
after cold forming at temperature given in the material specifica-
tion will restore the intended properties of the material and will
minimize the threat of premature failure due to recrystallization
during the time of operation.”

For this reason Section I introduced PG-19 COLD FORMING
OF AUSTENITIC MATERIALS with the 1999 Addendum. For
cold bending operations Section I permit’s exemption from the
post cold-forming heat treatment requirements when the forming
strains are less than the proscribed maximum strain limits in
Table PG-19. However, PG-19.2 requires heat treatment in accor-
dance with Table PG-19 for flares, swages and upsets regardless
of the amount of strain.

PG-19 includes the formulas for calculating the forming strain in
cylinders, spherical or dished heads, pipes and tubes, Table PG-19
lists 22 austenitic materials permitted by Section I which may be
subject to post cold-forming heat treatment.

1.7.4 Welding and Postweld Heat Treatment
An important factor in the evolution of today’s efficient high-

pressure boilers was the development of welding as a replacement
for the riveted construction used in the 19th and first half of the
20th centuries. The ASME has recognized the importance of
welding by instituting Section IX (Welding and Brazing
Qualifications, first published as a separate Code section in 1941)
and including special welding rules in each of the book sections
covering boilers or pressure vessels. The reader is also directed to
the separate chapter on Section IX in this volume for further
information on welding. Section I rules for welding are found in
Part PW (Requirements for Boilers Fabricated by Welding),
which refers to Section IX for qualification of weld procedures
and welder performance and, in addition, provides rules specifi-
cally applicable to boilers and their components, including boiler
proper piping. The rules for the welding of boiler external piping
(as opposed to boiler proper piping) are not found in Section I;
they are found in B31.1, Power Piping. Power Piping also invokes
Section IX for the qualification of weld procedures and welder
performance, but is somewhat more liberal with respect to the
transfer of procedures and welders from one organization to

Among the many aspects of welding covered by Part PW are
the following: responsibilities of the manufacturer or other orga-
nization doing the welding; the materials that may be welded; the
design of welded joints; radiographic and ultrasonic examination
of welds and when such examination is required; the welding of
nozzles; attachment welds; welding processes permitted; qualifi-
cation of welding; preparation, alignment, and assembly of parts
to be joined; the use of backing strips; advice on preheating;
requirements for postweld heat treatment; repair of weld defects;
exemptions from radiography; the design of circumferential
joints; the design of lug attachments to tubes; duties of the
Authorized Inspector related to welding; acceptance standards for
the radiography and ultrasonic examination required by Section I;
preparation of test plates for tension and bend tests; and welding
of attachments after the hydrostatic test. Some of these topics are
discussed here.

An important aspect of welding, compared to other means of
construction, is the absolute need for careful control of the weld-
ing process to achieve sound welds. The ASME Code attempts to
achieve this control by allowing welds to be made only by quali-
fied welders using qualified procedures. Section IX provides the

rules by which the welders and the weld procedures may be quali-
fied. However, there are many other aspects of welded construc-
tion besides qualification, and rules covering these other aspects
are found in the other ASME book sections covering welded con-
struction (Sections I, III, IV, and VIII).

The ultimate goal of welding procedure and performance quali-
fication is to achieve a weldment with properties that are at least
the equivalent of the base material being joined, as demonstrated
by certain tests. Qualification of the procedure establishes that the
weldment will have the necessary strength and ductility when it is
welded by an experienced welder following that procedure.
Qualification of the welder establishes that he or she can deposit
sound weld metal in the positions and joints to be used in produc-
tion welding.

An assumption implicit in the welding rules of Section I is that
a weld is the equivalent of the base material it joins; accordingly,
a properly made weld does not weaken the vessel. Consequently,
there is no bar to superimposing attachments on welds or having
nozzles or other openings placed where they intersect welds (this
is stated explicitly in PW-14). There is also no rule that would
prevent a manufacturer from making an opening for a nozzle in a
vessel, then deciding the nozzle isn’t needed, and replacing the
material removed for the opening with a properly designed and
welded patch. Qualification of Welding Procedures It is a general rule
under any Section of the ASME Boiler and Pressure Vessel Code
and the ASME Code for Pressure Piping, B31, that all welding must
be done by qualified welders using qualified welding procedures.
With some rare exceptions, every manufacturer (or other organiza-
tion doing the welding, such as an assembler or parts manufacturer)
is responsible for the welding it does and is also responsible for con-
ducting the tests required by Section IX to qualify the welding pro-
cedures used and the performance of the welders who apply those
procedures. The manufacturer does this by preparing certain docu-
mentation, known as a Welding Procedure Specification (WPS)
and a Procedure Qualification Record (PQR) that documents and
supports the WPS. The WPS comprises a set of instructions, pri-
marily for the welder (but also for the Authorized Inspector), regard-
ing how to make a production weld to Code requirements. These
instructions are written in terms of certain welding parameters that
are called essential, nonessential, and, when required, supplemental
essential variables (see below).

The Procedure Qualification Record (PQR) is a formal record
of the welding data (the actual value of the variables recorded
during the welding of the test coupons) for each welding process
used and the results of mechanical test specimens cut from the
test coupons. Nonessential variables used during the welding of
the test coupon may be recorded at the manufacturer’s option. The
tests (usually tensile and bend tests) are used to demonstrate that a
weldment made using the WPS has the required strength and duc-
tility for its intended application. Various other tests are some-
times used instead of the bend and tensile tests. All of these tests
are more fully explained in QW-140 and QW-202 of Section IX.
The variables used in production welding may vary somewhat
from those used during the welding of the test coupons. Note that
the purpose of the Procedure Qualification test is just to establish
the properties of the weldment and not the skill of the welder,
although it is presupposed that the welder performing the proce-
dure qualification test is a skilled worker.

With the 2000 addenda, all book sections of the ASME Code
began to permit the use of a number of ANSI/AWS Standard

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Welding Procedure Specifications (SWPS) as an alternative to
requiring the manufacturer or contractor to qualify its own proce-
dures. Requirements were added to ensure that each manufacturer
would have some experience with standard welding procedures
before using them. The standard procedures accepted (33 in the
group that appeared in the 2007 Edition; more may follow) are
listed in Section IX, Appendix E. Paragraph PW-1.2 and
Appendix paragraph A-302.7 of Section I advise that the use of
Standard Welding Procedure Specifications is acceptable, provid-
ed the welding meets the additional requirements of Section I.
Section IX simultaneously added a new Article V, Standard
Welding Procedure Specifications, which provides details and
restrictions on the use of standard welding procedures. The Code
Committee wanted to make sure that any organization using an
SWPS would first establish its competence with respect to key
aspects of the welding. To that end, an employee of the manufac-
turer or contractor must sign and date the SWPS, as evidence that
the organization is acknowledging responsibility for its use. With
some exceptions, the organization must then demonstrate its abili-
ty to control welding using an SWPS by welding and testing one
groove weld coupon, and must record detailed information about
the welding variables used (see below). For further details on the
use of SWPS, see the chapter in this volume on Section IX. Qualification of Welder Performance To complete the
qualification process for welded construction, performance tests
must be conducted for each welder to establish the welder’s abili-
ty to deposit sound weld metal. In general, this welder perfor-
mance qualification is accomplished by mechanical bend tests of
performance test coupons, radiography of a test coupon, or radi-
ography of the welder’s initial production weld. When welders are
qualified, identification marks are assigned to them so that all
welded joints can be identified by the identity of the person who
made them.

For various reasons, performance qualification tests do not
qualify welders to weld for other manufacturers or contractors,
except in the case of similar welding work on piping using the
same procedure (see PW-28.5). Among the reasons for this prohi-
bition is a desire to ensure that each manufacturer will take full
responsibility for welding done by his or her organization. The
Committee apparently believed that requiring performance quali-
fication for each new employer would achieve better results than a
system that permits one manufacturer to rely on performance test-
ing supposedly carried out by another organization. Some manu-
facturers share a related view; if they are to be held responsible
for their welding, they want to conduct their own welder perfor-
mance qualifications rather than relying on others. Welding Variables In the development of the WPS, it is
necessary to establish which variables of the welding process are
so-called essential variables in the production of qualifying welds
and those that are not (nonessential variables). An essential vari-
able is one that, if changed, would affect the properties of the
weldment and thus require requalification of the procedure, that is,
additional testing and issuance of a new PQR to support the
changed WPS. A nonessential variable for a weld procedure is one
that can be changed without requiring requalification, although it
would require a change in the WPS. Essential variables for weld
procedures include material thickness, P-Number (which refers to
material category; see below), filler metal alloy, the use or omis-
sion of backing, and the use or omission of preheat or postweld
heat treatment. Examples of nonessential variables for procedure

qualification are groove design, root spacing, method of back
gouging or cleaning, change in electrode size, and the addition of
other welding positions to any already qualified.

Variables may be categorized differently, depending on whether
they are applied to weld procedure or welder performance qualifi-
cation. For example, the addition of other welding positions to
any already qualified is an essential one for performance, since
welding in a new position, such as vertical instead of horizontal,
could certainly affect the ability of the welder to deposit a sound
weld. However, it is a nonessential variable for procedure qualifi-
cation because, in the words of QW-100.1, “Welding procedure
qualification establishes the properties of the weldment, not the
skill of the welder or welding operator.” Such a welder would pre-
sumably deposit sound metal in any position, and the properties
of the resulting weldment would be unaffected. Section IX has a
great number of tables summarizing procedure and performance
variables for all common welding processes. Material Categories (P-Numbers) One important
essential variable is the type of base material being welded. There
are several hundred different materials permitted for Code
Construction by the various book sections. If every change in base
material meant that weld procedure specifications and welder per-
formance had to be requalified, the qualification of procedures and
welders for all these materials would be an enormous task. To
reduce this task to manageable proportions, the ASME has adopt-
ed a classification system in which material specifications are
grouped into categories, based on similar characteristics such as
composition, weldability, and mechanical properties. Each catego-
ry is defined by a P-Number. Within the P-Number category are
subcategories called Group Numbers for ferrous base metals that
have specified impact requirements. Although Section I does not
use Group Numbers in relation to impact testing, it does some-
times establish different postweld heat treatment requirements for
the different Group Numbers within a given P-Number category. A
numerical listing of P-Numbers and Group Numbers can be found
for all Section II materials, by specification number, in Table
QW/QB-422 of Section IX.

A welding procedure that has been qualified for a particular
base material may be used for any other base material with the
same P-Number. (On the other hand, if a base material has not
been assigned a P-Number, it requires an individual procedure
qualification.) Similarly, a welder who has been qualified for a
base metal of a particular P-Number may be qualified for quite a
few other P-Number base materials (see QW-423).

QW-423.2 permits assigning a P- or S- Number to materials con-
forming to national or International Standards or Specifications,
providing the material meets the mechanical and chemical require-
ments of the assigned material.

Welding filler metals (electrode, bare wire, cored wire, etc.) are
categorized by F-Number in Table QW-432 of Section IX. Part C
of Section II lists American Welding Society filler metal specifi-
cations that have been adopted by the ASME and designated as
SFA specifications.

Although a specific definition of each P-Number in terms of
chemical composition is not given in the Code, the P-Number cat-
egories for commonly used Section I materials are as follows:

• P-No. 1 covers plain carbon steels, C–Mn–Si, C–Mn, and C–Si

• P-No. 3 covers low-alloy steels obtained by additions of upto
of Mn, Ni, Mo, Cr, or combinations of these elements.54%

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the inside radius, and instead, the following formula shall be


Joints in full-hemispherical heads including the joint to the
shell shall be governed by and meet all the requirements for lon-
gitudinal joints in cylindrical shells, except that in a buttwelded
joint attaching a head to a shell the middle lines of the plate thick-
nesses need not be in alignment.

If a flanged-in manhole which meets the Code requirements is

placed in a full-hemispherical head, the thickness of the head

Y =
2(S + P)

2S - P

t = L(Y1>3 - 1)

shall be the same as for a head dished to a segment of a sphere
(see PG-29.1 and PG-29.5), with a dish radius equal to eight-
tenths the diameter of the shell and with the added thickness for
the manhole as specified in PG-29.3.

The corner radius of an unstayed dished head measured on the

concave side of the head shall be not less than three times the
thickness of the material in the head; but in no case less than 6%
of the diameter of the shell. In no case shall the thinning-down
due to the process of forming, of the knuckle portion of any
dished head consisting of a segment of a sphere encircled by a
part of a torus constituting the knuckle portion (torispherical),
exceed 10% of the thickness required by the formula in PG-29.1.
Other types of heads shall have a thickness after forming of not
less than that required by the applicable formula.

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