CHAPTER

1

1.1 INTRODUCTION

Power Boilers, Section I of the ASME Boiler and PressureVessel Code, provides rules for the construction of power boilers,but since it is neither a textbook nor a design handbook, its rulesare accompanied by very little explanation. The objective of thischapter 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 PowerBoilers, A Guide to Section I of the ASME Boiler and PressureVessel Code [1]. That comprehensive guide was used as the text-book for a two-day ASME Professional Development Departmentcourse on Section I, developed and taught by Martin D. Bernsteinand Lloyd Yoder. The Second Edition [2] of the book, PowerBoilers, A Guide to Section I of the ASME Boiler and PressureVessel Code was published in May 2011. After a 10 year hiatusthe ASME Professional Development course on Section I hasbeen reinstated and scheduled for early in 2012. The courseinstructor, Robert G. McLaughlin, Vice-Chairman of BPVI, willbe using the second edition of The Power Boiler Guide Book asthe text book for this ASME Professional Development course.

Some of the more important aspects of ASME Section I con-struction are covered here, these are:

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

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

Piping• 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 ofother 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 formaterials and welding consumables, as well as tabulations ofdesign stresses and material properties, such as yield strength andtensile strength as a function of temperature. Section V, Non-destructive Examination, contains a series of standards that providethe methodology for conducting the various nondestructive exami-nations used in Section I construction. Section IX, Welding andBrazing Qualifications, provides the information necessary to qualifythe weld procedures and the welders required for Section I con-struction. In a rather unusual arrangement, the construction rulesfor boiler piping are found partly in Section I and partly in theB31.1 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 ExternalPiping. For a fuller description of those other Code sections, referto the specific chapters in this volume that cover them.

Those unfamiliar with the ASME Code may be confused at firstby a number of terms in it. Examples include Manufacturer third-party inspection, Authorized Inspector, Authorized InspectionAgency, jurisdiction, Maximum Allowable Working Pressure(MAWP), boiler proper, boiler proper piping, boiler external piping,interpretation, 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 changesvery slowly, it does change continuously. The rate of change inrecent years seems to have increased, perhaps due to technologi-cal innovation and international competition. Thus, although thischapter provides a substantial body of information and explana-tion of the rules as they now exist, it can never provide the lastword. Nevertheless, it should provide the user with a very usefulintroduction and guide to Section I and its application.

1.2 HISTORY AND PHILOSOPHY OF SECTION I

It is helpful to begin the study of Section I of the ASMEBoiler & Pressure Vessel Code with some discussion of its char-acter and philosophy. According to the dictionary, the term codehas several meanings: a system of principles or rules; a body oflaws arranged systematically for easy reference; a systematicstatement of a body of law, especially one given statutory force.Section I is primarily a system of rules. When the ASME decidedin 1911 that the country needed a boiler code, it assigned a com-mittee and gave it a mandate to formulate standard rules for the

INTRODUCTION TO POWER BOILERS

edition and Lloyd W. Yoder updated this chapter for the second edition thatwas revised by John R. MacKay for the third edition.

John R. MacKay1

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

1-2 • Chapter 1

construction of steam boilers and other pressure vessels. Thefirst edition of what is now known as Section I was finallyapproved by the ASME in 1915, and incorporated what was con-sidered at the time to be the best practice in boiler construction.However, the guiding principle, then as now, was that these aresafety rules.

Part of the foreword of Section I explains the guiding principlesand philosophy of Section I, and also of the Boiler and PressureVessel 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 rulesfor the construction of steam boilers and other pressure ves-sels. This committee is now called the Boiler and PressureVessel Committee.

The Committee’s function is to establish rules of safety, relat-ing only to pressure integrity, governing the construction2 ofboilers, pressure vessels, transport tanks and nuclear compo-nents, and inservice inspection for pressure integrity ofnuclear components and transport tanks, and to interpretthese rules when questions arise regarding their intent. Thiscode does not address other safety issues relating to the con-struction of boilers, pressure vessels, transport tanks andnuclear components, and the inservice inspection of nuclearcomponents and transport tanks. The user of the Code shouldrefer to other pertinent codes, standards, laws, regulations, orother relevant documents. With few exceptions, the rules donot, of practical necessity, reflect the likelihood and conse-quences of deterioration in service related to specific servicefluids or external operating environments. Recognizing this,the Committee has approved a wide variety of constructionrules in this Section to allow the user or his designee to selectthose which will provide a pressure vessel having a marginfor deterioration in service so as to give a reasonably long,safe period of usefulness. Accordingly, it is not intended thatthis Section be used as a design handbook; rather, engineer-ing judgment must be employed in the selection of those setsof 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 activitiesand those aspects which are not addressed should not be con-sidered prohibited. The Code is not a handbook and cannotreplace education, experience, and the use of engineeringjudgment. The phrase engineering judgment refers to techni-cal judgments made by knowledgeable designers experiencedin the application of the Code. Engineering judgments mustbe consistent with Code philosophy and such judgments mustnever be used to overrule mandatory requirements of specificprohibitions of the Code.

The Boiler and Pressure Vessel Committee deals with the careand inspection of boilers and pressure vessels in service only

to the extent of providing suggested rules of good practice asan aid to owners and their inspectors.

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

Certain points in these paragraphs should be stressed. SectionI covers the design, fabrication, and inspection of boilers duringconstruction, that is, it covers new construction only. Other rulescover repair and alteration of boilers and pressure vessels in ser-vice, for example, the National Board Inspection Code [3] (seealso section 1.6 in this chapter) and the API Pressure VesselInspection Code, API 510 [4]. Although there is general agree-ment that Section I should apply to new replacement parts, andsuch parts are usually specified that way, until the appearance ofthe 1996 Addenda (see Effective Dates of the Code and CodeRevisions in section 1.6.5), Section I had no clear provisionsdealing with replacement parts other than how they should bedocumented. Those addenda included changes to PG-106.8 and PG-112.2.4 that require the manufacturers of replacement partsto state on the data report form (the documentation that accom-panies the part, see section 1.7.8, Certification by Stamping andData Reports) whether or not the manufacturer is assumingdesign responsibility for those replacement parts. Also men-tioned in the Foreword is the objective of the rules: reasonablycertain protection of life and property, but with a margin for dete-rioration in service to provide a reasonably long, safe period ofusefulness. 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 thatthe designer using computers is responsible for assuring that anyprograms used are appropriate and are used correctly. It is alsonow mentioned that material specifications of recognized nationalor international organizations other than ASTM and AWS may beacceptable in ASME construction.

The Section I rules have worked well over many years. Theywere based on the best design practice available when they werewritten, and have evolved further on the same basis. Rules havebeen changed to recognize advances in design and materials, aswell as evidence of satisfactory experience. The needs of the users,manufacturers, and inspectors are considered, but safety is alwaysthe first concern. Today, the committee that governs, interprets,and revises Section I is called the Standards Committee on PowerBoilers, also known as BPVI, (It is also sometimes referred to asthe Section I Committee, but that is an unofficial designation.)

Another basis for the success of Section I is the Committee’sinsistence that the rules are to be understood as being general andare not to be interpreted as approving, recommending, or endors-ing any proprietary or specific design, or interpreted as limiting amanufacturer’s freedom to choose any design or construction thatconforms to the Code rules. The Committee deems the manufac-turer to be ultimately responsible for the design of its boiler andleaves certain aspects not covered by Section I to the manufacturer.Traditionally, the manufacturer has recognized this and borne theresponsibility for such things as functional performance of theboiler, thermal expansion and support of the boiler and its associ-ated piping, and the effects of thermal stress, wind loading, andseismic loading on the boiler. Further evidence of the flexibilityand reasonableness of Section I— and another key to its success

2Construction, as used in this Foreword, is an all-inclusive term com-prising materials, design, fabrication, examination, inspection, testing, cer-tification, and pressure relief.

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-3

as a living document— is found in the second paragraph of thePreamble:

The Code does not contain rules to cover all details of designand construction. Where complete details are not given, it isintended that the manufacturer, subject to the acceptance ofthe Authorized Inspector, shall provide details of design andconstruction which will be as safe as otherwise provided bythe rules in the Code.

This important paragraph has provided a way to accept new orspecial designs for which no rules are provided by allowing thedesigner to prove to the satisfaction of an Authorized Inspector(AI); (see definition in section 1.7.7, Third-Party Inspection) thatthe safety of the new design is equivalent to that of traditionaldesigns. If necessary, the AI can seek the assistance of the organiza-tion by which he or she is employed, a so-called AuthorizedInspection Agency, in determining the acceptability of new designs.

According to one of the definitions cited above, a code is abody of laws arranged systematically for easy reference.Although this may be true of Section I, it is not at first easy to useor understand, as it is a collection of rules that have been revisedand expanded over the years with very little accompanying expla-nation. These rules mandate the fundamental construction fea-tures considered necessary for a safe boiler (one that is a safepressure container), but typically do not provide any advice onhow to design a boiler from the standpoint of what size orarrangement of components should be used. There are no provi-sions, for example, dealing with the thermal performance and effi-ciency of the boiler or how much steam it will produce (other thanfor some approximate guidelines for judging the adequacy of thesafety-valve discharge capacity). It is assumed that the boilermanufacturer or designer already has this knowledge, presumablyfrom experience or available technical literature. Many rulesseem, and indeed are, arbitrary; but as explained before, they wereoriginally written to incorporate what was considered good prac-tice in the industry.

Code construction under the rules of Section I takes place asfollows: The ASME accredits a manufacturer (i.e., after appropri-ate review and acceptance of the manufacturer’s quality controlsystem, the ASME authorizes the manufacturer to engage in Codeconstruction). The manufacturer then constructs, documents, cer-tifies, and stamps the boiler in compliance with the rules ofSection I. The manufacturer’s activities are monitored andinspected by a third party (the Authorized Inspector). The boileris then acceptable to jurisdictions with laws stipulating ASMEconstruction of boilers. Section VIII construction (of pressurevessels) and Section IV construction (of heating boilers) is carriedout in similar fashion.

1.3 THE ORGANIZATION OF SECTION I

The original edition of Section I was not well organized. Overthe years the contents changed as portions were moved into othersections of the Code and more and more provisions were added.In 1965, following a precedent set by Subcommittee VIII,Subcommittee I completely reorganized Section I into its present,improved arrangement of parts, in which each part covers a majortopic or particular type of boiler or other Section I device. Thatreorganization was merely a reshuffling of the existing require-ments, with no technical changes from the previous version. Hereis that arrangement:

• Foreword• Statements of Policy• Personnel• Preamble• Part PG, General Requirements for All Methods of Construction• Part PW, Requirements for Boilers Fabricated by Welding• Part PR, Requirements for Boilers Fabricated by Riveting• Part PB, Requirements for Boilers Fabricated by Brazing• Part PWT, Requirements for Watertube Boilers• Part PFT, Requirements for Firetube Boilers• Part PFH, Optional Requirements for Feedwater Heater

(When Located within Scope of Section I)• Part PMB, Requirements for Miniature Boilers• Part PEB, Requirements for Electric Boilers• Part PVG, Requirements for Organic Fluid Vaporizers• Part PHRSG Requirements for Heat Recovery Steam

Generators• Appendix I, (Mandatory) Preparation of Technical Inquiries

to the Boiler and Pressure Vessel Committee• Appendix II, (Mandatory) Standard Units for use in Equations• Appendix III, (Mandatory) Criteria for Reapplication of a

Certification Mark• Appendix IV, (Mandatory) Local Thin Areas in Cylindrical

Shells and in Spherical Segments of Heads• Appendix A, (Nonmandatory) Explanation of Code Contain-

ing Matter not Mandatory Unless Specifically Referred to inthe Rules of the Code

• Appendix B, (Nonmandatory) Positive Material IdentificationPractice

• Endnotes• Index

The front of the volume contains the Foreword and Preamble,which provide a fundamental description of Section I and itsapplication. First-time users of Section I often skip the Forewordand the Preamble and proceed directly to the body of the book.This is a mistake because this introductory material contains agood deal of useful information about Section I, its scope, howthe Code works, and definitions of various types of power boilers.Also in the front of the book is a current listing of the personnelof the Boiler and Pressure Vessel Committee, which administersall sections of the Code.

1.3.1 The Parts of Section I

1.3.1.1 Part PG Part PG is the first major section of Section Iand provides general requirements for all methods of construction.It covers such important topics as scope and service limitations;permitted materials; design; cold forming of austenitic materials;requirements for piping, valves, fittings, feedwater supply, and safetyvalves; permitted fabrication methods; inspection; hydrostatic test-ing; and certification by stamping and data reports. The generalrequirements of Part PG must be used in conjunction with specificrequirements given in the remainder of the book for the particulartype of construction or type of boiler used. A number of Part PGtopics are covered in later sections of this chapter.

1.3.1.2 Part PW The second major section is called Part PW,Requirements for Boilers Fabricated by Welding. Since almost allboilers are now welded, this is a broadly applicable part containingmuch important information. It covers such topics as responsibil-ity for welding; qualification of welding procedures and welders;

1-4 • Chapter 1

acceptable weld joint designs; types of welding permitted; post-weld heat treatment of welds; radiography, inspection, and repairof welds; and testing of welded test plates. The rules of Part PWare described and explained further in section 1.7.4, Welding andPostweld Heat Treatment.

1.3.1.3 Part PR With the gradual replacement of riveting bywelding, Subcommittee I decided that no purpose was served bycontinuing to maintain and reprint in each edition the special rulesapplicable to riveted construction, Part PR. Accordingly, the 1974and all subsequent editions of Section I mandate the use of Part PRrules as last published in the 1971 edition.

1.3.1.4 Part PB The next section, Part PB, Requirements forBoilers Fabricated by Brazing, first appeared in the 1996 Addendato the 1995 edition of Section I. Although brazing had long beenused in the construction of certain low-pressure boilers, no brazingrules had ever been provided in Section I. Part PB brazing rulesresemble Part PW welding rules, but are much less extensive. Onenotable difference is that the maximum design temperaturedepends on the brazing filler metal being used and the base metalsbeing joined. Maximum design temperatures for the various braz-ing filler metals are given in Table PB-1. Brazing procedures andthe performance of brazers must be qualified in accordance withSection IX by methods similar to those described in section 1.7.4for qualifying weld procedures and welders. The design approachused for determining the strength of brazed joints is given in PB-9:the manufacturer must determine from suitable tests or from expe-rience that the specific brazing filler metal selected can provide ajoint of adequate strength at design temperature. The strength ofthe brazed joint may not be less than that of the base metals beingjoined. This strength is normally established by the qualification ofthe brazing procedure. If the manufacturer desires to extend thedesign temperature range normally permitted by Table PB-1 forthe brazing filler metal selected, the manufacturer must conducttwo tension tests of production joints: one at design temperature,T, and one at 1.05T. The joints must not fail in the braze metal.Some acceptable types of brazed joints are illustrated in Fig. PB-15.Nondestructive examination of brazed construction relies prima-rily on visual examination, supplemented by dye-penetrant inspec-tion if necessary. PB-49 provides guidance on inspection and anynecessary repairs.

The remainder of Section I is composed of parts providing spe-cial rules applicable to particular types of boilers or other SectionI components, such as feedwater heaters.

1.3.1.5 Part PWT Part PWT, Requirements for WatertubeBoilers, is a very brief collection of rules for this type of boiler,with some of the rules pertaining to construction details rarelyused today. All large high-pressure boilers are watertube boilers.Most construction rules for these boilers are found in Parts PG andPW, and Part PWT is merely a brief supplement. It is, however, theonly place where rules for the attachment of tubes to shells andheaders of watertube boilers can be found. Part PWT is an exam-ple of the retention by Section I of certain old rules and construc-tion details because some manufacturers might still use them.

1.3.1.6 Part PFT Part PFT, Requirements for Firetube Boilers,is a much more extensive set of rules, still very much in use for thispopular type of boiler, which is quite economical for generatinglow-pressure steam. Except in special cases, the large-diametershell places a practical limit on design pressure at about 400 psi,

although most firetube boilers have lower pressures. Part PFTcovers many variations within the type in considerable detail.Requirements cover material, design, combustion chambers andfurnaces, stayed surfaces, doors and openings, domes, setting,piping, and fittings. Part PFT is the only part of Section I wheredesign for external pressure (on the tubes and on the furnace) isconsidered.

A good deal of Part PFT dates back to the original 1915 editionof Section I. However, extensive revisions were made in the 1988Addenda. A much larger selection of materials was permitted, andallowable stresses for the first time became a function of tempera-ture, as is the case elsewhere in the Code. In addition, the designrules for tubes and circular furnaces under external pressure weremade consistent with the latest such rules in Section VIII, fromwhich they were taken. In the mid-1990s, Part PFT was furtherrevised and updated.

1.3.1.7 Part PFH Part PFH, Optional Requirements for Feed-water Heater, applies to feedwater heaters that fall within thescope of Section I by virtue of their location in the feedwater pip-ing between the Code-required stop valve and the boiler. Underthese circumstances, the heater may be constructed in compliancewith the rules in Section VIII, Pressure Vessels, Division 1, forunfired steam boilers, which are more strict in a number ofrespects than those applicable to ordinary Section VIII vessels, asexplained in section 1.4.4, Use of Section VIII Vessels in a Section IBoiler.

The primary side of the heater must be designed for a higherpressure than the Maximum Allowable Working Pressure(MAWP) of the boiler, per 122.1.3 of Power Piping, ASMEB31.1. (Remember: This heater is in the feedwater piping, andrules for the design of feedwater piping are within the scope ofB31.1.) Part PFH also stipulates how the heater is to be stampedand documented.

If a feedwater heater within the scope of Section I is equippedwith isolation and bypass valves, there is a possibility that itcould be exposed to the full shut-off head of the boiler feedpump. PG-58.3.3 cautions about this and notes that control andinterlock systems are permitted in order to prevent excessivepressure. (It is impractical to provide sufficient safety-valvecapacity in these circumstances.)

1.3.1.8 Part PMB Part PMB, Requirements for Miniature Boilers,contains special rules for the construction of small boilers that do notexceed certain limits (16 in. inside diameter of shell, 20 sq ft of heat-ing surface, 5 cu ft gross volume, 100 psi MAWP). Because of thisrelatively small size and low pressure, many requirements normallyapplicable to power boilers are waived. These have to do withmaterials, material marking, minimum plate thickness, postweldheat treatment and radiography of welds, and feedwater supply. Tocompensate somewhat for this relaxation of the normal rules, andto provide an extra margin of safety, PMB-21 stipulates that minia-ture boilers are to be given a hydrostatic test at a pressure equal tothree times the MAWP.

1.3.1.9 Part PEB This part, Requirements for Electric Boilers,was added to Section I in 1976. Until that time, manufacturers hadbeen building these boilers as Section VIII devices. However,Section VIII had no specific rules for electric boilers and the vari-ous openings, valves, and fittings normally mandated by Section I(blowoff, drain, water gage, pressure gage, check valve, etc.).Also, the manufacturers were not formally assuming design

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-5

responsibility for the boiler. The rules of Part PEB remedied theseshortcomings.

Part PEB covers electric boilers of the electrode and immersion-resistance type only and doesn’t include boilers in which the heatis applied externally by electrical means. The boiler pressure vesselmay be a Section I vessel or may have been constructed as anunfired steam boiler under special rules of Section VIII, Division1. Note that those rules require radiography and post-weld heattreatment, as explained later, in section 1.4.4, under Use ofSection VIII Vessels in a Section I Boiler.

Under the provisions of Part PEB, a manufacturer can obtainfrom the ASME a Certificate of Authorization to use the Certifi-cation Mark and the “E” designator (see Section 1.7.9 Certificationby Stamping and Data Reports), which authorizes the manufacturerto assemble electric boilers (by installing electrodes and trim on apressure vessel). The manufacturer must obtain the pressure ves-sel from an appropriate symbol stamp holder. The E-designatorholder is limited to assembly methods that do not require weldingor brazing. Electric boilers may, of course, also be constructed byholders of an S or M designators. The E-designator holder becomesthe manufacturer of record, who is responsible for the design ofthe electric boiler. This is stipulated in PEB-8.2.

The Data Report Form P-2A (see section 1.7.9, Certification byStamping and Data Reports) for electric boilers is in two parts:one for the vessel manufacturer and one for the manufacturerresponsible for the completed boiler, who may be a holder of anE, S, or M designator. If the vessel is constructed to the Section VIII,Division 1, rules for unfired steam boilers, it must be stampedwith the Code symbol U and documented with a U-1 or U-lAData Report (these are Section VIII data report forms). In such acase, the Section I master Data Report P-2A must indicate thisfact, and the U-lA form must be attached to it.

Electric boilers of the resistance-element type must beequipped with an automatic low-water cutoff, which cuts thepower before the surface of the water falls below the visible partof the gage glass. Such a cutoff is not required for electrode-typeboilers (which use the water as a conductor), since these in effectcut themselves off when the water level falls too low.

1.3.1.10 Part PVG This part provides rules for organic fluidvaporizers, which are boilerlike devices that use an organic fluid(such as DowthermTM) instead of steam as the working fluid. Theprincipal advantage of using these organic fluids is that they havemuch lower vapor pressures than water at a given temperature. Thusthey are particularly suitable for heating in industrial processesrequiring high temperatures at low pressure. On the other hand,these liquids are both flammable and toxic, and Part PVG containsa number of special provisions because of these drawbacks.

To prevent the uncontrolled discharge of organic fluid or vaporto the atmosphere, the use of gage cocks is prohibited. Safetyvalves must be of a totally enclosed type that will discharge into apipe designed to carry all vapors to a safe point of discharge. Thesafety-valve lifting lever normally required on Section I safetyvalves is prohibited, and valve body drains are not mandatory.Because the polymerization of organic fluids can cause clogging orotherwise adversely affect the operation of safety valves, and alsobecause the valves have no lifting levers by which they can be testedperiodically, PEB 12.2 requires the removal and inspection ofthese valves at least yearly. This is a rare instance of Section Ireaching beyond its normal new-construction-only coverage.

As a further means of reducing unintentional discharge oforganic fluid to the environment by leakage through safety valves,

and to prevent the gumming up of these valves, Section I permitsinstallation of rupture disks under the valves. This is the only useof such disks permitted by Section I. Special rules are also pro-vided for calculating safety-valve capacity. The required capacityof these valves is based on the heat of combustion of the fuel andthe latent heat of vaporization of the organic fluid. This capacityis determined by the manufacturer.

1.3.1.11 Part PHRSG This part provides rules for a heat recov-ery steam generator, HRSG, that has as it’s principal source ofthermal energy a hot gas stream having high ramp rates and tem-peratures such as the exhaust of a gas turbine. Such an HRSG mayutilize supplemental firing and may have one or more super-heaters, reheaters, evaporators, economizers, and/or feedwaterheaters, which are housed in a common gas path enclosure. Thesections cannot be individually isolated from the gas stream.

1.3.1.12 Mandatory Appendices Following is a list of contentsin the 2010 Code Edition, 2011 Addenda.

• Mandatory Appendix I – Submittal of Technical inquires tothe Boiler and Pressure Vessel Committee (Note: This mate-rial has been moved to the Front matter)

• Mandatory Appendix II – Standard Units for use in Equations• Mandatory Appendix III – Criteria for Reapplication of a

Certification mark• Mandatory Appendix IV – Local Thin Areas in Cylindrical

Shells and in Spherical Segements of Heads

1.3.1.13 Nonmandatory Appendix A Contains a great deal ofmiscellaneous information, some of it dating from the first edition.Much of this is nonmandatory explanatory material, unless it isspecifically referred to in the main body of Section I. In recentyears, the Boiler and Pressure Vessel Standards Committee I(BPVI) has been doing some long overdue housekeeping and hasremoved material in the Appendix considered obsolete or redun-dant. The diversity of the remaining subjects can be seen from thislist of contents:

• Braced and Stayed Surfaces• Method of Checking Pressure Relief Valve Capacity by

Measuring Maximum Amount of Fuel that can be Burned• Automatic Water Gages• Proof Tests to Establish Maximum Allowable Working

Pressure• Suggested Rules Covering Existing Installations • Pressure Relief Valves for Power Boilers• Repairs to Existing Boilers• Examples of Methods of Computation of Openings in Vessel

Shells• Examples of Computation of Allowable Loading on

Structural Attachment to Tubes• Preheating• Heating and Cooling Rates for Postweld Heat Treatment• Maximum Allowable Working Pressure – Thick Shells• Rounded Indication Charts• Methods for Magnetic Particle Examination (MT)• Methods for Liquid Penetrant Examination (PT)• Quality Control System• Acceptance of Testing Laboratories and Authorized

Observers for Capacity Certification of Pressure Relief Valves• Cylindrical Components Under Internal Pressure

1-6 • Chapter 1

• Data Report Forms and Guides• Codes, Standards and Specifications Referenced in the Text• Guide to Information Appearing on Certificate of Authorization• Sample Calculations for External Pressure Design• Guidance for the Use of U.S. Customary and S.I. Units in the

ASME Boiler and Pressure Vessel Code• Guidelines Used to develop S.I. Equivalents• Soft Conversion factors

Nonmandatory Appendix B Positive Material IdentificationPractice

This appendix covers the case where a Manufacturer maydetermine that a situation warrants positive material identificationfor a specific material or item. The rules contained in thisAppendix permits the Manufacturer to use the specific material oritem in the construction of his ASME Code certified component.

• Endnotes

All footnotes have been compiled and relocated to a singlelocation at the end of the Codebook. This complementation isnow entitled “Endnotes.”

• Index

1.3.2 InterpretationsSince 1977, the ASME has published all the replies from the

ASME staff on behalf of the ASME Boiler and Pressure VesselCommittee to inquiries on the interpretation of Section I (and alsothe other book sections). These interpretations, issued twice ayear, have a cumulative index by subject and paragraph number.A purchaser of a new edition of any Code section receives theInterpretations for that section until a new edition of the Code ispublished, which is every two years (see Effective Dates of theCode and Code Revisions in section 1.6.5). Many Section I usersfind it convenient to bind the interpretations in the back of thebook for easy reference. Despite the fact that interpretations aresaid not to be a part of the Code, they can be very useful inexplaining its application and in resolving disputes regardingwhat the Code intends, for Code users and Authorized Inspectorsgenerally accept interpretations as the equivalent of Code rules.

An example of the designation of a Section I Interpretation isI-98-27. The “I” signifies that the interpretation pertains to therules of Section I; the “98,” that it is an interpretation of the 1998Edition of Section I; and the “27,” that it is the 27th publishedinterpretation of that edition. Before 1983, the year of the interpre-tation denoted the year it was issued rather than the edition ofSection I to which it pertained. For example, Interpretation I-81-26was the 26th interpretation issued in 1981.

1.3.3 Code CasesOne of the many definitions of the word case is “a special situa-

tion,” which is a good description of the circumstances covered bywhat the Committee calls a Code Case. In the early application ofthe first edition of the Code, users sought Committee guidance forcircumstances not specifically covered by the Code or when theintent of the Code rules was not clear. The Committee considereda number of these special situations and issued formal guidance inthe form of what it called a Case. This is the origin of the practiceof issuing numbered Code Cases. In those days, no distinction wasmade between a Case and what we now call an Interpretation.

Today, Code Cases are used for several purposes:

• To provide for early implementation of new or revised Coderules, since a Code Case can be approved and published morequickly than the text of the Code can be revised. A textchange can take well over a year after approval by the ASMEBoiler and Pressure Vessel Standards Committee before it ispublished, while a Code Case can be approved in a singleCodeweek, after which the only further approval needed isfrom the Board of Pressure Technology Codes and Standards.(Codeweek is the name given to the week of Code Committeemeetings held four times a year.) Thus a Code Case may beusable two or three months after approval by the ASMEBoiler and Pressure Vessel Standards Committee.

• To permit the use of new materials or new forms of construc-tion not covered or not otherwise permitted by existing Coderules, when the need is urgent.

• To gain experience over a period of time with new materials,new forms of construction, or new design rules before chang-ing the Code to include them. This is particularly useful forrapidly evolving technology.

Initially Code Cases had a limited life, usually three years.They automatically expire at that point unless the Committeereaffirms them for another three years.

In March 2005, the Standards Committee took action to elimi-nate Code Case Expiration dates. This means that all Code Caseslisted in Supplement 3 of the 2004 Code Edition and beyond willremain available for use until annulled by the ASME Boiler andPressure Vessel Standards Committee.

Code Cases may be used beginning with the date of approvalshown on the Case. Annulled Code Cases will remain in theNumeric Index until the next Edition, at which time they will bedeleted.

The digit following a Case Number is used to indicate the num-ber of times a Code Case has been revised. Code Cases arearranged in numerical order, and each page of a Case is identifiedat the top with the appropriate Case Number.

Code Cases may also be annulled at any time by the Committee.Usually a Code Case is annulled six months after its contents havebeen incorporated and published in the Code. Note that Code Casesare nonmandatory; they are permissive only. In effect, they are anextension of the Code. Most Code Cases require the Code Casenumber to be listed on the Manufacturers’ Data Report (see sec-tion 1.7.9 for a description of data reports). Although there used tobe pressure to incorporate all Code Cases into the Code as soon asadequate experience had been gained with their use, it is now rec-ognized that some Cases do not lend themselves to incorporationand should be left as Cases indefinitely. One problem with CodeCases that has recently emerged is that some jurisdictions will notallow their use. Manufacturers are thus cautioned to check accept-ability of Code Cases with the jurisdiction where the boiler will beinstalled.

With each new Code edition, the ASME publishes all currentCode Cases in two Code Case books: Boilers and Pressure Vesselsand Nuclear Components. Supplements with the latest Cases andannulments are now sent quarterly to purchasers of the Code Casebooks until the publication of the next edition of the Code, when anew Code Case book must be purchased. Theoretically, issuing thenew Cases quarterly enables the publication of new Cases as soonafter each ASME Boiler and Pressure Vessel Standards Committeemeeting as possible.

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-7

1.4 SCOPE OF SECTION I: PRESSURELIMITS AND EXCLUSIONS

1.4.1 ScopeSection I applies to several types of boilers and components of

boilers, such as economizers, superheaters, reheaters, and in somecircumstances feedwater heaters. Although its title is PowerBoilers, the scope of Section I is somewhat broader. The Preambleto Section I explains that it covers power boilers, electric boilers,miniature boilers, high-temperature water boilers, and organicfluid vaporizers. Since the precise definitions of these varioustypes of boilers are not generally known, the following definitions,found in footnotes to the Preamble, are helpful:

(1) Power boiler— a boiler in which steam or other vapor isgenerated at a pressure of more than 15 psi (100 kPa) foruse external to itself.

(2) Electric boiler— a power boiler or a high-temperaturewater boiler in which the source of heat is electricity.

(3) Miniature boiler— a power boiler or high-temperaturewater boiler in which the limits in PMB-2 are not exceeded.

(4) High-temperature water boiler— a water boiler intendedfor operation at pressures in excess of 160 psi (1.1 MPa)and/or temperatures in excess of 250°F (120°C).

(5) Heat recovery steam generator (HRSG) – a boiler that hasas its principal source of thermal energy a hot gas streamhaving high ramp rates and temperatures such as theexhaust of a gas turbine.

(6) Fired pressure vessel – reheaters, isolable superheaters,and nonintegral separately fired superheaters.

Section I doesn’t provide an explicit definition of an organicfluid vaporizer, which is a boilerlike device that uses an organicfluid instead of steam as the working fluid. However, the last para-graph of the Preamble states that a pressure vessel in which anorganic fluid is vaporized by the application of heat resulting fromthe combustion of fuel shall be constructed under the provisionsof Section I. (Those provisions are found in Part PVG.) Thus, sofar as Section I is concerned, an organic fluid vaporizer is a boiler-like device in which an organic fluid is vaporized as justdescribed. (Note that a key factor is the vaporization of the organicfluid. If the organic fluid is merely heated without vaporizing, thedevice does not fall within the scope of Section I; it might fallinstead under the scope of Section VIII as a pressure vessel.) ThePreamble then provides a notable exception to the Section I defin-ition of an organic fluid vaporizer: “Vessels in which vapor is generated incidental to the operation of a processing system, con-taining a number of pressure vessels such as are used in chemicaland petroleum manufacture, are not covered by the rules ofSection I.” Again, if Section I rules do not cover these vessels,what rules do? The answer is the rules of Section VIII, PressureVessels. Those rules cover all kinds of pressure vessels, includingin some cases fired pressure vessels.

Although the origin of the above exception to Section I domin-ion is uncertain, a possible explanation can be surmised. Note thatthe vessel in question would probably be used in a chemical plantor petroleum refinery. Such plants are normally owned and oper-ated by large companies with a capable engineering staff andwell-trained operators. Those companies can usually demonstratea good record of maintenance and safety. Furthermore, vaporizingorganic liquids inside pressure vessels is a routine matter forthem. They might also argue that it really does not make much

difference whether a properly designed vessel is built to the rulesof Section I or Section VIII, nor does it matter whether the sourceof heat is from direct firing, from hot gases that may have givenup some of their heat by having passed over a heat-transfer sur-face upstream, or from a hot liquid that is being processed. Thereare also economic reasons why an Owner might prefer a Section VIIIvessel over a Section I vessel, as explained later in the discussionof fired versus unfired boilers.

The Preamble does not explain the precise meaning of “vaporgeneration incidental to the operation of a processing system.” Itapparently means the generation of vapor in a vessel or heatexchanger that is part of a processing system in a chemical plantor petroleum refinery where this vapor generation is only a minoror secondary aspect of the principal business of the plant, such asrefining oil. Thus, certain equipment normally constructed toSection I rules could, under this exception, be constructed insteadto the rules of Section VIII, provided the appropriate authoritiesin the jurisdiction where the equipment is to be installed have noobjection. These so-called jurisdictional authorities have the lastword in deciding which Code section applies (see How andWhere Section I Is Enforced in section 1.6).

There is also some imprecision in the use and meaning of theterm power boiler. This term is sometimes understood to mean aboiler with steam that is used for the generation of power, asopposed, for example, to a boiler with steam that is used forchemical processing or high-pressure steam heating. However,according to the definition in the Preamble, a boiler that generatessteam or other vapor at a pressure greater than 15 psi for externaluse is considered by Section I to be a power boiler, irrespective ofhow the steam might be used. Although exceptions exist, mostjurisdictional authorities follow the ASME Code in defining andcategorizing boilers and pressure vessels.

Note from the definition of a power boiler that the steam orother vapor generated is for use external to the boiler. This issupposed to distinguish a power boiler from certain other pres-sure vessels, such as autoclaves, that may similarly generatesteam or vapor at a pressure greater than 15 psi but not generallyfor external use. These pressure vessels, often used as processequipment in the chemical and petroleum industries, and forcooking or sterilization in other industries, are designed to meetthe rules of Section VIII.

From the Preamble definitions, it is apparent that a high-temperature water boiler, which generally produces pressurized hotwater for heating or process use, is not considered a power boiler.However, as a practical matter, the particular characterization of adevice by Section I as a power boiler or something else is lessimportant than the fact that it is indeed covered by Section I rules.

The Preamble explains that the scope of Section I covers thecomplete boiler unit, which is defined as comprising the boilerproper and the boiler external piping. This very important dis-tinction needs further explanation. The term boiler proper is anunusual one, chosen to distinguish the boiler itself from its exter-nal piping. The boiler proper consists of all the pressure partscomprising the boiler, such as the drum, the economizer, thesuperheater, the reheater, waterwalls, steam-generating tubesknown as the boiler bank, various headers, downcomers, risers,and transfer piping connecting these components. Any such pip-ing connecting parts of the boiler proper is called boiler properpiping. The boiler external piping is defined by its extent: it is thepiping that begins at the first joint where the boiler proper termi-nates and extends to and includes the valve or valves required bySection I.

1-8 • Chapter 1

The importance of all these definitions and distinctions is thatthe construction rules that apply to the boiler proper and boilerproper piping are somewhat different from those that apply to theboiler external piping. This is explained in section 1.5, DistinctionBetween Boiler Proper Piping and Boiler External Piping.

The Preamble also explains that the piping beyond the valvesrequired by Section I is not within the scope of Section I. Thus

these valves define the boundary of the boiler, and Section I juris-diction stops there. Note that the upstream boundary of the scopeof Section I varies slightly, depending on feedwater valve arrange-ments. Different valve arrangements are required for a single boilerfed from a single source, as opposed to two or more boilers fedfrom a common source, with and without bypass valves aroundthe required regulating valve (see the solid and dotted lines shown

Integral

Single installation

Multiple installationCommon header

Drain

Drain

DrainDrain Common

header

Drain

Boiler no. 2

Boiler no. 2

Boiler no. 1

Boiler no. 1

Part PFH

Vent

PG-58.3.6

PG-58.3.1

PG-58.3.7

PG-68.1

PG-68.2

PG-58.3.2

PG-58.3.7

PG-71

PG-58.3.2

Water drum

economizer

Level indicators PG-60

superheater

Inlet header (if used)

Steam drum

Vent

Vent

Vents and instrumentation

Blow-off single and multiple installations

Two or more boilers fed from a common source

Two or more boilers fed from a common source

Regulating valves

Single boilerSingle boiler

Feed

wat

er s

yste

ms

P

G-5

8.3.

3

Control device PG-60

Multiple installation

Main steamPG-58.3.1 Soot blowers PG-68.5

Soot blowers PG-68.5

Surface blowContinuous blowChemical feedDrum sample

Single installation

Administrative Jurisdiction & Technical ResponsibilityBoiler Proper — The ASME Boiler and Pressure Vessel Code (ASME BPVC) has totaladministrative jurisdiction and technical responsibility (refer to Section I Preamble)Boiler External Piping and Joint — The ASME BPVC has total administrative jurisdiction(mandatory certification by stamping the Certification Mark with appropriate Designator, ASME Data Forms, and Authorized Inspection) of Boiler External Piping and Joint. The ASME Section Committee B31.1 has been assigned technical responsibilty.Non-Boiler External Piping and Joint — Not Section I jusidiction (see applicable ASMEB31 Code).

(if used)

Integral

(if used)

PG-58.3.7

PG-58.3.7

PG-58.3.7

GENERAL NOTE: This figure provides references to other paragraphs of the Code for information only.

FIG. PG-58.3.1(a) CODE JURISDICTIONAL LIMITS FOR PIPING - DRUM-TYPE BOILERS (Source: ASME SECTION I)

in Fig. PG-58.3.1(a), reproduced in this chapter). The requiredboiler feed check valve is typically placed upstream of the feedstop valve, except that on a single boiler-turbine unit installation,the stop valve may be located upstream of the check valve.Changes in the Section I boundary can have some significance inthe choice of design pressure for the feed water piping and valves,which is governed by the rules of B31.1, Power Piping (see para-graph 122.1.3).

An exception to this coverage of boiler piping is found in thetreatment of the hot and cold reheat piping between the boiler anda turbine, (see Fig. PG-58.3.1(c), reproduced in this chapter)which is excluded from the scope of Section I. Occasionally,someone asks how and why reheat piping was left outside thescope of Section I. The explanation offered some years ago by asenior member of the Committee is as follows: Althoughreheaters date back to the earliest days of Section I, the risingsteam pressures employed in large utility boilers in the 1940s led

to their increased use. The reheat piping became larger, heavier,and more complex. In those days, the General Electric Companyand Westinghouse made virtually all of the turbines used in theUnited States, and it was customary for those turbine manufactur-ers to take responsibility for the design of the reheat piping.Whenever some Committee members suggested that it might betime to consider bringing reheat piping into the scope of Section I,those two companies objected. On the basis of their long, success-ful experience, they convinced the Committee that such a changewas unnecessary and that they were perfectly capable of design-ing that piping. Thus the reheat piping remained outside the scopeof Section I.

In the late 1980s, failures of hot reheat piping occurred at twomajor utilities, with injuries and loss of life. This reopened thequestion of whether Section I should cover reheat piping andwhether the failed piping, designed in the 1960s, might nothave failed had it been within the scope of Section I. (Current

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-9

FIG. PG-58.3.1(c) CODE JURISDICTIONAL LIMITS FOR PIPING - REHEATERS AND NONINTEGRAL SEPARATELY FIRED SUPERHEATERS (Source: ASME SECTION I)

1-10 • Chapter 1

Section I rules call for all boiler components to be manufactured,inspected, certified, and stamped under a quality control system,a requirement imposed in 1973. Moreover, any piping within thescope of Section I is generally inspected yearly, with the rest ofthe boiler, although finding potential leaks or failures under theinsulation is not so readily accomplished.) However, investiga-tors were not able to agree on the causes of the failures, and theidea of bringing the reheat piping within Section I jurisdictiondied for lack of support.

Code coverage of this reheat piping and also of piping beyondthe boiler external piping varies. Normally in power-plant design,the owner or the architect-engineer will select the B31.1, PowerPiping Code, for the reheat piping, and either that Code or theB31.3, Process Piping Code (formerly called the Chemical Plantand Petroleum Refinery Piping Code), for the piping beyond theboiler external piping. Or the jurisdiction (state or province) mayhave laws mandating use of one or the other of these two Codes tocover piping that isn’t within the scope of Section I.

Some unfired steam boilers are constructed to the requirementsof Section VIII, as permitted by the Preamble. Section VIII rulesdeal only with the vessels themselves; they do not deal with anypiping attached to the vessels. In such cases, the choice of anappropriate design Code for the piping may be left to the plantdesigners; otherwise, the jurisdictional authorities may mandate aparticular piping Code.

1.4.2 Pressure Range of Section I BoilersAs explained in the Preamble, Section I covers boilers in which

steam or other vapor is generated at a pressure of more than 15 psi(100 kPa) for use external to the boiler. (All pressures used inSection I are gage pressure.) For high-temperature water boilers,Section I applies if either the pressure exceeds 160 psi (1.1 MPa)or the temperature exceeds 250°F (120°C). The vapor pressure ofwater at 250°F (120°C) is approximately 15 psi (100 kPa). Thus,if the water temperature is less than 250°F (120°C), saturationpressure is less than 15 psi (100 kPa). Boilers designed for pres-sures below 15 psi (100 kPa) are usually constructed to the rulesof Section IV, Heating Boilers. However, such units could bebuilt, certified, and stamped as Section I boilers if all the require-ments of Section I are met. Section I has no upper limit on boilerdesign pressure. The design pressure (also called by Section I theMaximum Allowable Working Pressure, or MAWP) of large, natural-circulation boilers used by electric utilities can be as high as 2975psi (20 MPa). At higher pressures, approaching the critical pointof water and steam 3206 psia (21.4 MPa) and 705.4°F (372.5°C),the difference in density between steam and water becomes sosmall that natural circulation in the boiler would be inadequate.The design pressure of what is known as a forced-flow steam gen-erator with no fixed steam and water line can be substantiallyhigher, approaching 4000 psi (26.7 MPa).

1.4.3 Fired versus Unfired BoilersSection I rules are intended primarily for fired steam or high-

temperature water boilers and organic fluid vaporizers. There are,however, boilers called unfired steam boilers that do not derivetheir heat from direct firing. These unfired steam boilers may beconstructed under the provisions of either Section I or Section VIII,Pressure Vessels. The definition of an unfired steam boiler is notas clear as it might be, which has led to some confusion and occa-sional disagreement between Subcommittees I and VIII. ThePreamble to Section I states that:

A pressure vessel in which steam is generated by the applica-tion of heat resulting from the combustion of fuel (solid, liquid,or gaseous) shall be classified as a fired steam boiler.Unfired pressure vessels in which steam is generated shall beclassed as unfired steam boilers with the following exceptions:

(a) vessels known as evaporators or heat exchangers;(b) vessels in which steam is generated by the use of heat

resulting from operation of a processing system containinga number of pressure vessels such as used in the manufac-ture of chemical and petroleum products.

It is these exceptions that cause the confusion, for several rea-sons. Sometimes it is not apparent whether a boiler using wasteheat is fired or unfired. Furthermore, it may be difficult to distin-guish between an unfired steam boiler using waste heat and cer-tain heat exchangers also using waste heat. Another difficultyarises from the source of the waste heat. If it stems from the com-bustion of fuel, the device is generally considered fired. However,some jurisdictions have permitted boilers to be considered unfiredif they use waste heat from a combustion turbine. The presence ofauxiliary burners in a boiler using waste heat from another sourcewould cause that boiler to be considered fired. This was affirmedin Interpretation I-92-20. All ASME Code sections routinely cau-tion users that laws or regulations at the point of installation maydictate which Code section applies to a particular device and thatthose regulations may be different from or more restrictive thanCode rules. The jurisdictions may also have rules that definewhether a device is considered a boiler or a heat exchanger andwhether it is considered fired or unfired.

Electric boilers can be considered fired or unfired, dependingon the circumstances. An inquiry on this subject was answered byInterpretation I-81-01, as follows:

Question: The Preamble of Section I defines an electric boileras “a power boiler or a high-temperature water boiler inwhich the source of heat is electricity.” Would you definewhether or not an electric boiler is considered to be a firedor an unfired steam boiler?

Reply: PEB-2 provides criteria for determining if an electricboiler is considered to be a fired or an unfired steam boiler.An electric boiler where heat is applied to the boiler pres-sure vessel externally by electric heating elements, inductioncoils, or other electrical means is considered to be a firedsteam boiler.

An electric boiler where the medium (water) is directly heatedby the energy source (electrode type or immersion elementtype) is considered to be an unfired steam boiler.

An unfired steam boiler constructed to Section VIII must meetrules more stringent than run-of-the-mill Section VIII vessels. Forexample, these rules require additional radiographic examination,postweld heat treatment, and possibly impact testing of materialsand welds, as more fully described in the next section, The Use ofSection VIII Vessels in a Section I Boiler.

Some owners of steam-generating vessels would prefer toavoid, if possible, the classification of boiler or fired boilerbecause most states and provinces require an annual shutdown forinspection of boilers, although some states permit longer periodsof operation. The interval between inspections mandated forSection VIII vessels is typically much longer, permitting longeruninterrupted use of the equipment.

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-11

One of the criteria some Committee members use to distin-guish between fired and unfired boilers is whether a flame from apoorly adjusted burner can impinge directly on pressure partsand over-heat them. This is clearly a possibility in a fired boiler,and it is one reason for some of the conservativeness of Section Irules, and for the typical practice of requiring annual outages forinspection. By contrast, hot gases from some upstream source areless likely to cause overheating of those same pressure partsbecause they are less likely to be misdirected and, presumably,no actual flames can reach the downstream steam-generatingpressure parts.

1.4.4 The Use of Section VIII Vessels in a Section I Boiler

Usually all parts of a Section I boiler are constructed to SectionI rules, but there are a few exceptions worth noting. Pressurevessels for electric boilers (see PEB-3), drums or other parts ofunfired steam boilers (see Code Case 1855, below), and feedwaterheaters that fall under Section I jurisdiction by virtue of theirlocation in the feedwater piping (see PFH-1.1) are among thoseboiler components that may be constructed under the special rulesfor unfired steam boilers in UW-2(c) of Section VIII. One othertype of Section VIII vessel that can be used in a Section I boiler ismentioned in the next-to-last paragraph of the Preamble. Thatvessel is an expansion tank used in connection with what isdefined in footnote 4 to the Preamble as a high-temperature waterboiler. This is the only mention of such tanks in Section I, andnothing is said about their construction having to meet the specialSection VIII requirements for unfired steam boilers. Thus it seemsthat an ordinary Section VIII vessel would serve the purpose.

Although it might seem that components built to Section VIIIare just as safe as those built to Section I and therefore should bevirtually interchangeable, this is unfortunately not the case. WhenSection I does permit the use of Section VIII vessels, it insists oninvoking certain special Section VIII rules for unfired steam boil-ers. These rules are a little more stringent than those for run-of-the-mill Section VIII vessels. The rules are found in U-1(g)(1),UG-16(b)(3), UG-125(b) and UW-2(c) of Section VIII, Division 1.The most important requirements called out in these several para-graphs are the following:

• Safety valves are to be furnished in accordance with therequirements of Section I insofar as they are applicable.

• For design pressures exceeding 50 psi (350 kPa), radiographyof all butt-welded joints is required, except for circumferen-tial welds that meet the size and thickness exemptions of UW-11(a)(4). (Those exemptions cover welds in nozzles that areneither greater than NPS 10 (DN 250) nor thicker than in.(29 mm)).

• Postweld heat treatment is required for vessels constructed ofcarbon and low-alloy steel.

• A minimum thickness of in. (6 mm) is required for shellsand heads, exclusive of any corrosion allowance.

Another extra requirement imposed on an unfired steam boilerconstructed to Section VIII rules applies to all Section VIII ves-sels made of carbon and low-alloy steels. The designer mustestablish a Minimum Design Metal Temperature (MDMT), thelowest temperature expected in service at which a specifieddesign pressure may be applied. Unless the combination of mater-ial and thickness used is exempt by the curves of UCS-66 at the

14

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MDMT, impact testing of the material is required, and the WeldProcedure Qualification would also have to include impact testingof welds and heat-affected zones. This is not explicitly mentionedin the paragraphs dealing with unfired steam boilers because it isa requirement that was added to Section VIII in the mid-1980s,long after those other paragraphs were written and probablybecause Subcommittee VIII members assumed that everyoneknew that impact testing was routinely required unless the vesselmaterials met certain exemptions provided in UCS-66.

What is also notable about these rules for unfired steam boilersis the fact that the ordinary Section VIII exemptions for postweldheat treatment (PWHT) are not available, even for welds in rela-tively thin P-No. 1 materials. Moreover, the Section I PW-41.1exemption from radiography of circumferential butt welds insteam-containing parts up to NPS 16 (DN 400) or in. (41 mm)wall thickness is also not available.

An example of the consequences of overlooking these some-what obscure rules occurred recently when a boiler manufacturerplaced an order for a sweetwater condenser with a manufacturerof Section VIII pressure vessels without specifying that the con-denser, as a type of feedwater heater, had to meet the UW-2(c)rules for unfired steam boilers. (A sweetwater condenser is ashell-and-tube heat exchanger connected directly to the boilerdrum that receives saturated steam from the drum on the shellside and has feedwater going to the drum inside the tubes. Thesteam condenses on the tubes, giving up its heat to the feedwaterand providing a supply of water pure enough to use as spray waterin the superheater. These condensers are used where the regularfeedwater supply is not of sufficient quality to use as spray water.)

After the condenser was installed and the boiler was ready forthe hydrostatic test, the Authorized Inspector (AI) inquired aboutthe presence of a Section VIII vessel in a Section I boiler. He wastold that it was a feedwater heater furnished to the rules of PartPFH of Section I. The AI reviewed those rules and asked aboutthe required radiography and postweld heat treatment, which itturned out had not been done because the boiler manufacturer hadordered the condenser as an ordinary Section VIII vessel. Thecondenser had to be cut out of the installation, shipped back to themanufacturer, radiographed, and given a postweld heat treatment.This PWHT had to be conducted very slowly and carefully, usingthermocouples during heating and cooling to avoid too great atemperature difference between shell and tubes, which couldcause excessive loading on the tubes, tubewelds, or tubesheet.Fortunately, the radiography and PWHT were quickly and suc-cessfully accomplished and the condenser was reinstalled in theboiler with minimal delay.

Three circumstances have been mentioned in which Section VIIIvessels can be used as part of a Section I boiler: feedwaterheaters, under Part PFH; pressure vessels for electric boilers,covered under Part PEB; and a “Section VIII Unfired SteamBoiler in a Section I System,” covered under the provisions ofCode Case 1855. The title of Case 1855 is a little confusing, asis the case itself. Its ostensible purpose is to permit in a Section Iboiler the inclusion of parts built to Section VIII rules and con-strued to be components of an unfired steam boiler. Case 1855was originally developed for application to waste-heat boilers inpetroleum refineries, which use one or more drums and one ormore arrays of heat-exchange surface to generate steam. Case1855 permits what might be called a hybrid Section I boiler,with some portions built to Section VIII rules. The separate por-tions must be stamped and documented as called for by therespective sections. In addition, a P-5 Summary Data Report

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1-12 • Chapter 1

(see section 1.7.9, Certification by Stamping and Data Reports)for Process Steam Generators must be completed in accordancewith PG-112.2.6 of Section I.

1.5 DISTINCTION BETWEEN BOILERPROPER PIPING AND BOILEREXTERNAL PIPING

For many years, Section I made no distinction regarding the pip-ing of a boiler; it was all considered just boiler piping. In the late1960s, some members of the Committee decided that Section Irules were no longer adequate to cover the design of modern high-pressure, high-temperature steam piping because the Section I rulesfor the design of boiler proper piping are actually quite limited,covering the design of pipe for internal or external pressure only.Far more extensive rules are provided in the B31.1 Power PipingCode. In addition to pressure, those rules cover many other loadsthat piping might be subject to, such as mechanical loads that maydevelop from thermal expansion and contraction of the piping,impact loading, gravity loads, and seismic loads. Although suchcomprehensive rules are not provided by Section I for the designof the boiler proper piping, boiler manufacturers over the yearshave managed to design this piping— often by using the designmethods of B 31.1— so that it serves its purpose in a satisfactorymanner.

After due consideration, the Committee decided to divide allpiping within the scope of Section I into two categories. The pip-ing that was actually part of the boiler (such as downcomers, ris-ers, and transfer piping) was designated as boiler proper piping.Construction rules for this piping were retained in Section I. Thepiping that led to or from the boiler (such as feedwater, steam,vent, drain, blowoff, and chemical feed piping) was designatedinto a new category, boiler external piping, an appropriate namesince it is indeed external to the boiler. This piping was defined byits extent: from the boiler to the valve or valves required bySection I (the boundary of the complete boiler unit). These valveswere also defined as part of the boiler external piping. In 1972,responsibility for most aspects of the construction of this pipingwas transferred from Section I to the Power Piping Code, ASMEB31.1. That is, rules for material, design, fabrication, installation,and testing of boiler external piping were now contained in B31.1,but Section I retained its usual certification requirements (withAuthorized Inspection and stamping). Designers of boiler piping,whether boiler proper piping or boiler external piping, are thusfaced with a mixed bag of interrelated rules and should be awareof a number of special provisions and potential pitfalls.

To clarify the distinction between the two categories of piping,Subcommittee I expanded Fig. PG-58.3.1(a) to show each categoryof piping. A comparable figure appears in the front of B31.1.Complicating matters further, a new category of piping— callednonboiler external piping— had to be established within B31.1.Nonboiler external piping is the piping beyond the scope ofSection I that is connected to the boiler external piping. Fig-ure PG-58.3.1(a) is reproduced in this chapter for ready reference.

Transferring coverage of the boiler external piping to B31.1made available, at a stroke, detailed and comprehensive rules notprovided by Section I for such important aspects of piping designas flexibility analysis and hanger design. By retaining completecoverage of boiler proper piping, Section I continued its policy ofproviding minimal guidance in the design of this piping, whichhas traditionally been left to the boiler manufacturers.

The transfer of the design of boiler external piping to B31.1 hasresulted in some negative consequences. For one thing, an entirelydifferent committee governs the Power Piping Code. Inquiries aboutboiler external piping typically are answered by that committee, andoccasional differences develop between it and Subcommittee I. Inthe early 1990s, this problem was addressed by an action of theBoard of Pressure Technology Codes and Standards, which reaf-firmed that the B31.1 section committee had primary responsibilityfor the rules governing the design of boiler external piping. Also,despite the best efforts of both committees, minor differences thatexist between the respective rules result in different treatment of pip-ing on the same boiler, depending on whether it is boiler externalpiping or boiler proper piping (e.g., requirements for postweld heattreatment and radiography differ for the two categories of piping).While both sets of rules are more than adequate, unwary designerswho do not pay sufficient attention to what type of piping they aredesigning can run into expensive delays should an AuthorizedInspector notice at the last minute that the piping does not meet theappropriate rules.

1.6 HOW AND WHERE SECTION I IS ENFORCED AND EFFECTIVEDATES

1.6.1 United States and CanadaSection I is a set of rules for the construction of boilers. These

rules are not mandatory unless they are adopted into the laws of agovernment. The applicable territorial and political units within agovernment are usually referred to as jurisdictions, a term thatderives from the range of their governmental authority. A jurisdic-tion can be a government subdivision, that is, state, province,county, or city. If there happen to be no jurisdictional require-ments regarding Code coverage, the purchaser of a boiler mayspecify to the manufacturer that construction to Section I rules isa requirement.

Currently (2011), laws requiring Section I construction of boil-ers have been adopted by all but one state (Wyoming being theexception) and by all of the provinces of Canada. Because ofWyoming’s relatively sparse population and limited industrialfacilities, that state’s legislature apparently does not consider theexpense of implementing a boiler law to be justified because, as apractical matter, all boilers in the state probably meet the ASMECode. In addition to the states and provinces, about 17 cities andcounties also have laws requiring Section I construction.

Except as noted above, states and provinces with boiler lawstypically enforce the Code by means of an appointed board orcommission that writes and revises boiler rules and an appointedchief inspector with the necessary enforcement staff. The jurisdic-tional authorities also depend on the efforts of AuthorizedInspectors working for Authorized Inspection Agencies, whosefunction is to ensure that the boiler manufacturer has compliedwith Section I (see section 1.7.8, Third-Party Inspection.) Thesesame authorities also have the responsibility for overseeing thecare and operation of boilers once they are completed.

In 1997, because of international trade agreements that requireremoval of nontechnical barriers to trade, the situation justdescribed, in which most states and provinces require boilers andpressure vessels installed within their jurisdictions to meet theASME Code, began to change. At that time, the National Boardof Boiler and Pressure Vessel Inspectors (a regulatory agencycomprising the chief inspectors of those states and provinces that

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-13

adopt and enforce at least one section of the ASME Code)changed its policy regarding equipment that could be registeredwith the National Board.

The National Board has established criteria for the acceptanceof Codes of construction, quality systems, and third-party inspec-tion in a document entitled “Criteria for Registration of Boilers,Pressure Vessels and Other Pressure-Retaining Items,” NB264,Rev.6 October 2007. The introduction to this document explainsthat the purpose of registration with the National Board is toprovide owners, users, and jurisdictional authorities charged withpublic safety with certification by the manufacturers of boilers,pressure vessels, and other pressure-retaining items that those reg-istered items have been manufactured in accordance with anationally or internationally recognized code of constructionaccepted by the National Board.

Before a manufacturing organization may register its equip-ment with the National Board, it must obtain a Certificate ofAuthorization to Register from the National Board. Such a certifi-cate is issued only after the National Board determines thatthe NB264 requirements for construction, quality system, andthird-party inspection have been met. (ASME construction codesmeet those requirements.) By mid-2000, the National Board hadaccepted certain construction codes (other than ASME) fromCanada, Great Britain, France, and the European Community. TheNational Board website, www.nationalboard.org, lists all thecodes so far approved for registration and where those codes canbe obtained. The telephone number of the National Board head-quarters in Columbus, Ohio, is (614) 888-8320. Codes from othercountries are being considered, and it is likely that some of thesewill be accepted by the National Board. Since the National Boardis governed by influential state officials, a number of jurisdictionshave indicated that they would accept items manufactured tointernational standards and registered with the National Board.Those jurisdictions (as of mid-2000) are Alaska, Iowa, Kansas,Massachusetts, Michigan, Minnesota, New Jersey, North Dakota,Oregon, Tennessee, Vermont, Virginia, Washington, Wisconsin,and the city of Milwaukee. These jurisdictions may not accept allboilers or vessels accepted for registration by the National Board,and it is advisable to verify acceptability by contacting jurisdic-tional officials whose names and contact information may beobtained from the National Board Website (www.nationalboard.org). It remains to be seen how soon the European Communityand other foreign jurisdictions will reciprocate and accept equip-ment built to the ASME Code.

1.6.2 International AcceptanceSection I is recognized internationally as an acceptable design

Code, along with other national Codes, such as the German CodeTRD (Technische Regeln fur Dampfkessel, which may be trans-lated as Technical Regulations for Steam Boilers) and variousJapanese Codes that fall under the jurisdiction of the Japaneseministry MITI (Ministry of International Trade and Industry). Atone time the Mexican mechanical engineering society AMIMEtranslated Section I into Spanish for use in Mexico, but it wasapparently little used, in part due to the difficulty of keeping upwith the ongoing changes in Section I. Before the 1970s, theASME allowed only U.S. and Canadian companies to engage inASME Code Construction. However, as a result of legal action atthat time, the ASME has been under specific instructions from aconsent decree requiring cooperation with non-U.S. manufactur-ers to allow their accreditation as ASME Code symbol stampholders, provided those manufacturers’ quality control systems

pass the ASME review process. There are now many foreignmanufacturers who are ASME Code symbol stamp holders andcan engage in Code Construction of boilers and pressure vessels.More recently, the provisions of NAFTA (North American FreeTrade Agreement) and GATT (General Agreement on Tariffs andTrade) require a level playing field for domestic and foreign man-ufacturers, and the ASME has been active in seeking acceptanceof its Code by the European Community.

For field-assembled boilers (those too large to be completed inthe shop), full compliance with the ASME Code requires thatfield assembly be accomplished by appropriate Code symbolstamp holders and also that an Authorized Inspection Agency pro-vide the necessary field inspection. Foreign jurisdictions otherthan Canada do not generally insist on full compliance with theASME Code. Since foreign purchasers want to reduce costs, theyusually accept so-called ASME boilers that have not been fieldassembled by appropriate ASME Code symbol stamp holdersunder the surveillance of an Authorized Inspector and thus cannotbe furnished with full ASME Code stamping and certification.Consequently, although the components of such boilers may havebeen designed and manufactured in accordance with Section I, thecompleted boilers do not meet all the requirements that wouldpermit full Code stamping and certification.

1.6.3 Adoption of Uniform Requirements by Jurisdictional Authorities

To provide the advantages of uniformity of construction andinspection, the Uniform Boiler and Pressure Vessel Laws Society,Inc., was founded in 1915, just three months following the officialadoption of the first ASME Boiler Code. This society providedmodel legislation to jurisdictions and supported nationally acceptedcodes, such as Section I and Section VIII, as standards for theconstruction of boilers and pressure vessels. In general, the soci-ety encouraged adoption of uniform requirements by the jurisdic-tional authorities. Manufacturers traditionally strongly supportedthis effort because it promoted the manufacture of economicalstandard products and obviated custom-designed features for dif-ferent jurisdictions. Besides the model legislational promotionactivity of the society, it provided a publication entitled SYNOP-SIS of Boiler and Pressure Vessel Laws, Rules and Regulations[4], which was arranged by states, cities, counties, provinces andother international jurisdictional entities. Originally this coveredjust the United States and Canada but was expanded to includedinformation about the laws in a number of other countries.

Now that most states have boiler laws and many have pressurevessel laws, the original mandate of the UBPVLS society can beconsidered essentially fulfilled. As a consequence of the low vol-ume sale of boilers, resulting from a recent recession, funding forthe society’s efforts diminished substantially and it was foundnecessary to dissolve this historical significant organization.Many of the functions it provided have been picked up by theNational Board (see 1.6.4 below). A National Board Synopsis ofBoiler and Pressure Vessel Laws, Rules and Regulations is avail-able by accessing the National Board Web site, calling 614-888-2463 or via e-mail www.orders@nationalboard.org.

1.6.4 National Board Inspection CodeSection I covers only new construction, and once all of its

requirements for a new boiler have been met and the necessarydata reports have been signed, Section I no longer applies. Injurisdictions that accept the National Board Inspection Code(NBIC), that code is applicable for installation of new boilers

1-14 • Chapter 1

and inservice inspection, repair, or alteration. The stated purposeof the NBIC is to “maintain the integrity of pressure-retainingitems after they have been placed into service by providing rulesand guidelines for inspection, repair, and alteration, therebyensuring that these objects may continue to be safely used.” Aguiding principle of the NBIC is to continue to follow the rulesof the original code of construction insofar as practicable. Whenthere is no such code, or when it is unknown, the ASME Code isused as a default. Any welding used in repairs or alterationsmust be done in accordance with the code used for the originalconstruction. For example, for ASME Section I construction, theNational Board requires that any welding used in repairs oralterations must be done to the same strict standards as requiredby Section I, using welders and weld procedures qualified inaccordance with Section IX; the exception to this is that theNBIC has permitted the use of some ANSI/AWS StandardWelding Procedure Specifications since 1995 but the ASME didnot until mid-2000. (In a fairly significant change, the 2000Addenda to Section I and Section IX, and the other book sec-tions of the Code, provided for the use of a number ofANSI/AWS Standard Welding Procedure Specifications withsome additional restrictions, as explained in section 1.7.4,Welding and Postweld Heat Treatment.) The National BoardInspection Code also requires that all the safety appurtenancesof a Section I boiler (e.g., safety valves and gages) be main-tained in good order. Thus it could be said that the NationalBoard Inspection Code takes up where Section I leaves off. Asof 2008, 45 states of the United States and 12 provinces and ter-ritories of Canada have made the NBIC mandatory, and mostother jurisdictions will accept it.

1.6.5 Effective Dates of the Code and Code References

Since the Code is in a constant state of evolution, to apply itproperly, the user needs to understand the effective dates of theCode and Addenda. In 2010 the ASME decided to change themethod and frequency of publishing the Code. Up until that time,the Code was issued every three years (2004, 2007 etc.) with anaddendum issued annually. The date of publication was July 1st ofeach calendar year. The change proposed in 2010 was to issue theCode on a two-year cycle, without an interim addenda. To imple-ment this change, the 2010 edition, which contains everything inthe 2007 edition (including the 2008 and 2009 addenda), plusadditional changes that were approved subsequent to the 2009addenda but in time for the 2010 edition publication. Thesechanges were noted in the front matter as “Summary of Changes”and noted in the Code book with (10). In 2011 the Code book wasre-published in its entirety with new Code changes noted with (a). This edition of the Code was entitled was entitled “2010 ASMEBoiler & Pressure Vessel Code, 2011a Addenda. There will be noaddenda issued in 2012, however, to implement some changesthat have been approved by the Standards Committee and areawaiting publication, the following paragraphs ware added to the“Summary of Changes” in the 2011 Addenda to the 2010 Edition.

A Special Notice may be posted on the ASME Web site inadvance of the next edition of the Boiler and Pressure VesselCode to provide approved revisions to Code requirements.Such revisions may be used on the date posted and willbecome mandatory 6 months after the date of issuance in thenext edition. A special notice may also include a revision to a

Code Case. The superseded version of the Code Case shallnot be used.

Errata to the BPV Code may be posted on the ASME Web siteto provide corrections to incorrectly published items, or tocorrect typographical or grammatical errors in the BPVCodes. Such errors shall be used on the date published.

Information regarding Special Notices and Errata is pub-lished on the ASME Web site under the Boiler and PressureVessel Code Resources Page at http//www.asme.org/standards/publications/bpvc-resources.

The next edition of the Boiler and Pressure Code will be pub-lished in 2013. This Code edition will remain in effect until thepublication of the 2015 Code edition.

The new Code editions will continue to be issued in July on atwo-year cycle. There will be no addenda issued in the interimperiod between Code editions. All changes to the new Code edi-tion will appear in the “Summary of Changes”, as noted above,and as indicated in the margins of the Code book by (year) or (a), which ever ia appropriate. Revisions in a new edition areidentified and a description of each change is provided. Becauserevisions do not become mandatory immediately upon issuance,this publication schedule has led to some confusion and uncer-tainty as to which rules are mandatory in a new edition of the Code.

Revisions become mandatory six months after the date of issue,except for boilers contracted for before the end of the six-monthperiod. In effect, this gives a manufacturer a six-month grace pe-riod during which any Code changes affecting new contracts areoptional. Note that the key date is the contract date; this can be aformal contract, a letter of intent, or other legally accepted con-tractual date. Since boiler external piping may be contracted forlong after the boiler, the parties involved may use either theSection I edition as of the boiler contract date or any mutuallyacceptable subsequent addenda for this piping. This wasaddressed in Interpretation I-78-13, Applicable Code Edition forBoiler External Piping:

Question: When boiler external piping is contracted for laterthan the boiler is, to what rules is the boiler external pipingto be built?

Reply: Boiler external piping is a part of the boiler and, assuch, the applicable Code Editions are those which corre-spond to the contract date for the boiler. However, whencontracts for boiler external piping are placed later than theboiler contract date, the parties have the option of usingSection I as of the boiler contract date or any subsequentAddenda of Section I.

The edition and addenda of B31.1 to which the boiler externalpiping is ordinarily constructed (unless a later version is selected)is found in the Appendix of Section I in Table A-360: Codes,Standards, and Specifications Referenced in Text.

Note that once the edition of Section I to be used for the con-struction of the boiler is established, the manufacturer is notobliged to implement any changes made in subsequent addenda oreditions. This is a practical approach that recognizes that anychanges to Section I are now incremental and are very unlikely tohave any influence on safety. One exception to this general princi-ple is if the allowable stresses for a material were significantlyreduced. In such cases, it might be prudent for the manufacturer

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-15

of as yet uncompleted components to use the new, lower allow-able stresses.

1.7 FUNDAMENTALS OF SECTION ICONSTRUCTION

The technique used by Section I to achieve safe boiler design isa relatively simple one. It requires all those features considerednecessary for safety (e.g., water glass, safety valve, pressure gage,check valve, and drain) and then provides detailed rules governingthe construction of the various components comprising the boiler.This approach is analogous to the old saying that a chain is nostronger than its weakest link. For a boiler, the links of the figura-tive chain are the materials, design (formulas, loads, allowablestress, and construction details), fabrication techniques, welding,inspection, testing, and certification by stamping and data reports.If each of these elements meets the appropriate Section I rules, asafe boiler results. The boiler can then be described as a Section Iboiler, meaning one constructed to meet all the requirements ofSection I of the ASME Boiler & Pressure Vessel Code. An impor-tant element of this construction process is a quality control pro-gram intended to ensure that the Code has been followed. Eachaspect of the process will now be discussed.

1.7.1 Materials

1.7.1.1 How Materials Are Ordered Materials are a fundamen-tal link in the chain of Code construction, and great care is taken toensure their quality. The Code does this by adopting material spec-ifications that have first been developed by the American Societyfor Testing Materials, the ASTM. The ASME material specifica-tions are thus usually identical to those of the ASTM. The ASTMissues specifications designated by letter and number; for example,A-106, Seamless Carbon Steel Pipe for High-Temperature Service.When the ASME adopts ASTM specifications, it adds the letter S.The equivalent ASME specification is thus SA-106.

The ASME, or SA, specifications are used as purchase specifi-cations. Each specification contains a variety of informationappropriate to that product and deals with how the material isordered, the manufacturing process, heat treatment, surface condi-tion, chemical composition requirements, tensile and yield require-ments, hardness requirements, various test requirements, and howthe material is to be marked. The purchaser may also invoke sup-plementary, nonmandatory requirements dealing with such thingsas stress relief, nondestructive examination, limits on chemistry(Within the range permitted by the specification) and additionaltesting. All of these requirements have evolved over the years inresponse to the needs of the users.

The purchaser orders by specification number, and the suppliercertifies that the material complies with that specification. Inmost cases, a Certified Material Test Report, giving the results ofvarious tests required by the specification, is furnished with thematerial.

1.7.1.2 Using Section II Section II of the Code, Materials, is acompilation of all the material specifications adopted by theASME for use by the various book sections of the Code. However,as a general rule, a material cannot be used for pressure partsunless it is also adopted and listed within the section of the Codecovering the construction. Section I lists permitted materials inPG-5 through PG-14. PW-5 adds further requirements for materi-

als used in welded construction. In addition, Section I sets temper-ature limits for the use of the various materials; they must be usedwithin the temperature range for which allowable stress values(often called design stresses) are provided.

With the publication of the 1992 edition of the Code on July 1,1992, the allowable stress tables that formerly appeared inSections I, III, and VIII were consolidated and reformatted into asingle volume, Section II, Part D. This consolidation was sup-posed to facilitate uniformity among those Code sections that usethe same criteria for establishing allowable stress. (Note that somesections, such as Section VIII, Division 2, use different criteria,which generally result in higher allowable stresses than those ofSection I and Section VIII, Division 1). The new arrangement lostthe convenience of having the allowable stresses included inSection I. The new arrangement was also quite difficult to usebecause of multiple listings of the same materials, spread over fourpages. Fortunately the ASME has reduced this difficulty by improv-ing the format of the tables and by issuing a CD-ROM that containsthe tables of Section II, Part D. Unlike the actual paper copy ofSection II, Part D, the CD-ROM has a search feature, called a filter,that can find a given specification and grade quickly.

1.7.1.3 Use of Non-ASME Materials and Material Not FullyIdentified Both Section I and Section VIII, Division 1, have longhad provisions (PG-10 and UG-10) dealing with the acceptance ofmaterial that is not fully identified. These provisions involvedemonstrating that the material in question complies with chemi-cal requirements and physical properties within the permittedrange of an acceptable ASME material specification. In 1987,these provisions were expanded to provide more guidance on whatmust be done and who may do it when a material is requalified asthe equivalent of an acceptable ASME material. These changesalso aid in the recertification of material manufactured to foreign(i.e., international) specifications, such as DIN or JIS, as the equiv-alent of ASME specifications suitable for Section I or Section VIIIconstruction.

1.7.1.4 Use of International Material Specifications With theincreasing emphasis on global competition, the ASME Boiler andPressure Vessel Committee has sought ways in which to gainwider international acceptance of ASME Code construction. Thisis a two-way street, and the ASME recognizes the need to removeunnecessary barriers to the use of ASME construction in the hopethat other countries will reciprocate. One of these barriers was theASME’s previous insistence that only ASME or ASTM materialsbe used in ASME construction. Recognizing that many areas of theworld have economic constraints or local rules that may necessi-tate the use of non-ASME materials, the ASME relaxed its policy,and in 1997 approved the first two foreign material specificationsfor inclusion in the 1998 addenda to Section II. Those two specifi-cations cover a Canadian structural steel, CSA-G40.21 (similar toSA-36), and a European steel plate, EN 10028-2 (similar to SA-516 Grade 65). At the time of this writing (2007), several othercarbon steel specifications have been approved, and their allowablestress values have been added to Section II, Part D. The ASMEestablishes the allowable stresses for these foreign materials on thesame basis used for domestic materials. For further discussion ofthe acceptance and use of international material specifications, seethis volume’s chapter on Section II.

1.7.1.5 Special Concerns There are a number of Section I mate-rial applications that deserve special mention. The use of austenitic

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stainless steels is one of these and is explained in footnote 1 to PG-5.5(note that all footnotes are located near the back of the book in asection entitled “ENDNOTES”). Austenitic steels are particularlysusceptible to chloride stress corrosion. Consequently, paragraphPG-5.5 of Section I limits the use of these steels to what it callssteam-touched service and prohibits their use for normally water-wetted service where chlorides might be present, except for gageglass bodies (PG-12) and miniature electric boilers operated ondeionized water (PEB-5.3). Those stainless steels listed in PG-9.1for boiler parts (normally water-wetted) are ferritic stainless steels,which are not susceptible to chloride corrosion. Another concern isthe problem of graphitization that occurs in some carbon steels afterlong service at temperatures above 800°F (425°C). Graphitizationcan severely affect the strength and ductility of these steels, particu-larly the heat-affected zone of welds. Notes in the allowable stresstables caution about this problem.

The use of cast iron and nonferrous metals is limited by SectionI to those applications mentioned in PG-8.

Material requirements for boiler external piping are contained inB31.1, Power Piping, 123.2.2. That paragraph permits a manufac-turer to use ASTM Specifications equivalent to those of the ASMEby demonstrating that the requirements of the latter have been met.

1.7.1.6 Miscellaneous Pressure Parts Miscellaneous pressureparts (see PG-11), such as valves, fittings, and welding caps, are often furnished as standard parts. When those parts meet therequirements of the ASME product standards adopted by referencein PG-42, Section I accepts the materials listed in those standardsunless they are specifically prohibited or are beyond the use limitsof Section I. However, standard parts made to a parts manufactur-er’s standard (defined in footnote 4 to PG-11) must be made ofmaterials permitted by Section I. This had never presented a prob-lem until 1992, when Section II, Part D, introduced the compositestress tables for Sections I, III, and VIII. In those tables, the lettersNP (not permitted) that appear in the columns on applicability ofmaterial for a given book section were applied by default if thatmaterial had not been listed for use in that book section before theadvent of Part D. Therein lies a problem, because there are manymaterials listed in the ASME product standards accepted bySection I by reference in PG-42 that had never been listed in theSection I stress tables. No Authorized Inspector had previouslychallenged the use of those materials in these product standardsbecause their use was (and still is) explicitly sanctioned by PG-11.However, a problem arose once the designation NP appeared inPart D. One company found itself facing the following situation: Ithad designed and subcontracted to another stamp holder the fabri-cation of an austenitic stainless steel superheater with return bendsthat were purchased as standard elbows made in accordance withASME B16.9, Factory-Made Wrought Steel Buttwelding Fittings.The manufacture of B16.9 elbows starts with pipe or tube madefrom a material that is designated only by class and grade (e.g.,WP 304) and that meets the chemical and tensile requirements ofSA-403, Specification for Wrought Austenitic Stainless Steel PipeFittings. When the elbow is completed in accordance with the pro-visions of SA-403 and ASME B16.9, it is given a new materialdesignation, A-403 or SA-403, as a fitting. Unfortunately, Section Ihad never listed that material, but it had been listed in Section VIII.Accordingly, the Section II, Part D column for Section I applica-bility listed SA-403 as NP (not permitted); apparently, it was per-mitted only for Section VIII construction.

The AI at the fabricator saw that listing and proclaimed that theelbows could not be used in Section I construction, a decision that

stopped fabrication of the superheater whose contract specifiedheavy daily penalties for late completion. Fortunately, theAuthorized Inspection Agency providing the AI at the job site hadknowledgeable Code committee members who quickly agreedthat the designation NP should not apply in these circumstancesor, rather, should not mean NP if PG-11 provided otherwise.Section I subsequently issued Interpretation I-92-97, whichsolved the problem for these particular fittings but did nothingabout the generic problem of the potentially misleading NP desig-nation in Section II, Part D. An attempt to have Subcommittee IImodify the NP designation with a note referring to PG-11 provedunavailing, so the unwary designer faces a potential trap in situa-tions similar to the one just described.

Interpretation I-92-97 is as follows:

Question: May SA-403 austenitic fittings made to ASME/ANSI standards accepted by reference in PG-42 be used forSection I steam service?

Reply: Yes.

1.7.1.7 Electric Resistance Welded (ERW) Materials Whenthe Code first accepted tubes and pipes formed of electric resis-tance welded materials, it imposed a 15% penalty on allowablestress, except for tubes within the boiler setting whose design tem-perature was 850°F (455°C) or lower. There was apparently somedoubt about the welding, and normal allowable stress was permit-ted only for tubes inside the protective barrier of the setting.Because of the good service record of these tubes in recent years,some thought is being given to removing the penalty on ERWtubes. As a first step in this direction, in 1988, several Code Caseswere passed permitting the use of ERW tubes at any temperaturewithout the 15% reduction in allowable stress, provided the tubesare given extra nondestructive examination (angle beam ultrasonicinspection and an electric test in accordance with ASME specifi-cation SA-450), are no larger than in. outside diameter, and areenclosed within the setting. Subsequently the Code Cases havebeen incorporated and annulled. See notes G4 and W13 for theparticular materials in Section II, Part D, Table A1.

Code users often wonder just what the term “within the setting”means, since this term is not defined in Section I. However, vari-ous books on boiler construction typically define the setting as theconstruction surrounding the boiler and or the tubes: refractory,insulation, casing, lagging, or some combination of these. In for-mer years, heavy casing and refractory were used so that a casecould be made that a failed tube would not represent much of adanger to someone standing nearby. With the evolution of simplerand lighter construction, there now may be only insulation andlagging outside the tubes or enclosing a header. Although suchconstruction may not provide so strong a barrier as was the caseformerly, in the author’s opinion— which is shared by otherCommittee members— the tubes are still “enclosed within thesetting,” and if they meet the other provisions of the Code Casescited, they would be entitled to the full allowable stress withoutthe 15% penalty of former years. That penalty was, in effect, avote of no confidence in the electric resistance weld and/or theinspection of the weld. However, it was always an arbitrary pen-alty. If the weld were truly bad, it would leak. Reducing the stressby 15% is not likely to help. There has now been quite a lot ofgood experience with ERW tubes, and the additional NDE calledfor in Note W13 of Section II, Part D, Table A1 should henceforthprovide some further assurance that any poor welds will be

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detected. After further satisfactory use, the provisions of the CodeCases may be incorporated into Section I, although some thinkthat the cost to a utility of even a single tube failure in service,requiring shutdown for repair, far outweighs any initial costadvantage of ERW tubing.

1.7.1.8 Nonpressure Part Materials Nonpressure parts, suchas lugs, hangers, brackets, and fins, are not limited to being madeof materials listed in the stress tables, but must be of weldablequality. “Weldable quality” is a rather vague term, and the choiceof nonpressure-part material is thus left to the designer. The factthat a material is of weldable quality is established when the pro-cedure used to weld it is qualified. For carbon and low-alloy steels,PW-5.2 also stipulates another requirement for weldability: thecarbon content may not exceed 0.35%.

1.7.2 Design

1.7.2.1 Design Methods For the most part, Section I uses anexperience-based design method known as design-by-rule.(Certain other sections of the Code, namely Section III, SubsectionNB, and Section VIII, Division 2, use a newer method, known asdesign-by-analysis.) Design-by-rule is typically a process requiringthe determination of loads, the choice of a design formula, and theselection of an appropriate design stress for the material to be used.Rules for this kind of design are found throughout Section I, withmost being in Part PG (the general rules). Other design rules arefound in those parts of Section I dealing with specific types of boil-ers or particular types of construction.

The principal design rules are found in Part PG, paragraphsPG-16 through PG-55. There are formulas for the design of cylin-drical components under internal pressure (tube, pipe, headers,and drums), heads (dished, flat, stayed, and unstayed), stayed sur-faces, and ligaments between holes. Rules are also provided foropenings or penetrations in any of these components, based on asystem of compensation in which the material removed for theopening is replaced as reinforcing in the region immediatelyaround the opening, called the limits of compensation (see PG-36). All of these formulas involve internal pressure except for therules for support and attachment lugs of PG-55, for which thedesigner chooses the design loads on the basis of the anticipatedweight or other loads to be carried.

Another method of design permitted by Section I is a hydrosta-tic deformation or proof test (PG-18, Appendix A-22). This isanother experience-based method used to establish a safe designpressure for components for which no rules are given or whenstrength cannot be calculated with a satisfactory assurance ofaccuracy. In this type of proof test, a full-size prototype of thepressure part is carefully subjected to a slowly increasing hydro-static pressure until yielding or bursting occurs (depending on thetest). The maximum allowable working pressure is then estab-lished by an appropriate formula that includes the strength of thematerial and a suitable safety factor. In the 1999 addenda, Section Ijoined Section VIII in allowing a burst test to be stopped beforeactual bursting occurs, when the test pressure justifies the desireddesign pressure. The particular component so tested may never beused for Code Construction (because it might have been on theverge of failure). The design factor, or so-called safety factor,used in the burst test formula for ductile materials has been 5since the 1930s, when one of the factors used to establish allow-able design stress was one-fifth of the Ultimate Tensile Strength(UTS). That factor on UTS has been reduced over the years from

5 to 4 (circa 1950) and from 4 to 3.5 in the 1999 addenda. Thusthe design factor used in the burst test was seen to be out of dateand due for a reduction. In 2000, as an interim measure, bothSection I and Section VIII approved a reduction in this factor to4.0 for ductile materials only. The Subcommittee on Design wascontinuing to study whether any further reduction was warranted,and also whether a similar change might be appropriate for non-ductile cast materials. Proof testing may not be used if Section Ihas design rules for the component, and in actual practice, suchtesting is seldom employed. However, it can be a simple andeffective way of establishing an acceptable design pressure forunusual designs, odd shapes, or special features that would be dif-ficult and costly to analyze, even with the latest computer-basedmethods. A common application of proof testing before theadvent of sophisticated analytical methods was in the design ofmarine boiler headers of D-shaped or square cross section.

Tests that are used to establish the maximum allowable work-ing pressure of pressure parts must be witnessed and approved bythe Authorized Inspector, as required by Appendix A-22.10. Thetest report becomes a permanent reference to justify the design ofsuch parts should the manufacturer want to use that design againfor other boilers.

1.7.2.2 Design Loads When Section I was written, its authorsunderstood so well that the internal working pressure of the boilerwas the design loading of overriding importance that virtually noother loading was mentioned. To this day there is very little dis-cussion of any other kind of loading, except for paragraph PG-22,which cautions that this Section does not fully address additionalloadings other than those from working pressure and static head,and provides no guidance regarding how to take such loads intoaccount, thus leaving it to the designer. Actually, internal pressureis the principal loading of concern, and it appears in all Section Idesign formulas. The pressure inside the boiler is called the work-ing pressure. For design purposes, Section I uses the termMaximum Allowable Working Pressure, or MAWP, to define whatengineers usually understand to mean design pressure.

There are many other loads on a boiler in addition to pressure.Among these are the weight of the boiler and its contents, seismicloads, wind loads, and thermal loads. Section I has no provisionsspecifically dealing with the last three, but it is the responsibilityof the designer to consider them. As boiler designs evolved overthe years, the manufacturers have recognized and made provisionsfor these loads.

In recent years, as nuclear plants have assumed more of the util-ities’ base load, fossil-fired boilers are increasingly being used incyclic service, with frequent start-ups and shutdowns. Cyclic ser-vice also occurs with the increasing use of heat-recovery steamgenerators (HRSGS) fired by combustion turbines. This type ofoperation has the potential for causing repeated, relatively hightransient thermal stress and may require careful analysis forfatigue and creep damage. Although Section I has successfullyconsidered creep and creep rupture in setting allowable stresses, ithas no specific rules for creep-fatigue interaction or even for sim-ple fatigue. Nevertheless, designing against fatigue, creep, andtheir interaction remains the responsibility of the manufacturer.Elevated-temperature design for fatigue and creep is a complexsubject for which no complete rules have yet been developed. SuchCode rules that are available appear in Section III, Subsection NH,Class 1 Components in Elevated Temperature Service. SubsectionNH is based on Code Case N-47, which dealt with nuclear powerplant components in elevated-temperature service and whose earliest

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version dates back to the late 1960s. Elevated-temperature servicemeans service above that temperature when creep effects becomesignificant. This threshold is usually taken as 700°F (370°C) formany ferritic materials (carbon and low-alloy materials) and 800°F(425°C) or higher for austenitic materials (the high-alloy materialssuch as stainless steel). With the exception of parts of some super-heaters, most boilers are made of ferritic materials.

1.7.2.3 Choice of Design Pressure and Temperature Section Idoes not tell the designer what the design pressure (MAWP) anddesign temperature of a boiler should be. These design conditionsmust be chosen by the manufacturer or specified by an architect-engineer, user, or others. Section I requires only the MAWP to bestamped on the boiler, not the design temperature. (When dis-cussing design temperature, it is necessary to distinguish betweenmetal temperatures and steam temperatures, either of which mayvary within the boiler.) As a practical matter, the designer mustknow and choose an appropriate design (metal) temperature foreach element of the boiler, because the allowable stress values arebased on these temperatures. In many cases, the design tempera-ture of an individual element such as a superheater tube may bemuch higher than the design steam temperature of the boiler. Thisstems from the fact that the metal temperature of a heated tube isalways higher than that of the steam within and from the need toprovide a design margin for potential upsets in gas temperature orsteam flow.

In a natural-circulation boiler, most of the pressure parts aredesigned for the MAWP. However, slightly different design pressuresare used for some components, since paragraph PG-22 stipulatesthat hydrostatic head shall be taken into account in determiningthe minimum thickness required “unless noted otherwise.” Theformulas for determining the thickness of cylindrical componentsunder internal pressure found in paragraph PG-27 are of twotypes: one described as applicable to “tubing— up to and includ-ing 5 in. outside diameter,” and the other applicable to “piping,drums, and headers.” Having only these two categories of designformula sometimes puzzles inexperienced designers who are notsure which formula to use for tubing larger than 5 in. outsidediameter. They soon learn the accepted practice for such tubing,which (by the process of elimination) is to use the formula for“piping, drums, and headers.” Advice to this effect is stated in thefirst paragraph of PG-27.1 as follows: “. . . the formulas under thisparagraph shall be used to determine the minimum required thick-ness or the maximum allowable working pressure of piping,tubes, drums and headers in accordance with the appropriatedimensional categories as given in PG-27.2.1, Pg-27.2.2 and PG-27.2.3 . . . ”. The two types of wall thickness formulas haveother differences worth noting, as they are a source of confusionto designers and have led to quite a number of inquiries toSubcommittee I. See 1.7.2.6, Other Factors Applicable to Tubeand Pipe Design.

With the 2006 Addendum, Appendix A-317 CYLINDRICALCOMPONENTS UNDER INTERNAL PRESSURE was added toSection I. The designer may use the requirements of this appendixto determine the minimum required thickness or the maximumallowable working pressure of piping, tubes, drums and headersin place of the requirements of PG-27.

1.7.2.4 Design Stresses: Past, Present, and Future TheSection I basis for determination of allowable stress is explained inSection II, Part D, Appendix 1, where the various safety factors, ordesign factors, and quality factors applied to the various tensile,

yield, and creep strengths are presented in Table 1-100. See thechapter on Section II in this volume for further details.

The so-called safety factors, or design margins, now used inestablishing allowable stresses with respect to the various failuremodes, such as yielding or creep rupture, have evolved over the life of the Code. Before World War II, the factor used on ten-sile strength was 5. It was temporarily changed to 4 in order tosave steel during the war, and that change was made permanentaround 1950. Starting in the late 1970s, the factor on yieldstrength was changed from to , a change that was carried outover quite a long period. The factor on the 100,000 hr creep rup-ture strength was formerly 0.6, but around 1970, this was changedto the current factor of 0.67. These reductions in design margins,or safety factors, were adopted over time as improvements intechnology permitted. These improvements included the develop-ment of newer and more reliable methods of analysis, design, andnondestructive examination. The imposition of quality controlsystems in 1973 and a record of long, satisfactory experience alsohelped justify reducing some of the design conservatism.

One of the design factors not used by most other countries thatthe ASME used until 1999 in setting allowable stress was a factorof approximately 4 on ultimate tensile strength. It happens thatthis design factor is a significant one because it controlled theallowable stress for many ferritic (carbon and low-alloy) steelsbelow the creep range. However, it put users of the ASME Codeat a disadvantage in world markets, where competing designs areable to utilize higher allowable stresses based just on yieldstrength. The inequity of this situation caused the CodeCommittees to reconsider the usefulness and necessity of using afactor of 4 on tensile strength as one of the criteria for settingallowable stress. In 1996, the Pressure Vessel Research Council(PVRC), a research group closely associated with the CodeCommittees, was asked to determine whether the design factor ontensile strength could safely be reduced. The PVRC prepared areport reviewing all the technological improvements in boiler andpressure vessel construction that have occurred since the early1940s, which was when the design factor on tensile strength wasfirst reduced from 5 to 4. On the basis of that report’s favorablerecommendation, Subcommittee VIII decided, as an initial step,to change the factor on tensile strength from about 4 to about 3.5 forpressure vessels constructed under the provisions of Section VIII,Division 1.

Subcommittee I decided to make the same change for Section Iand, in 1997, established a task group to investigate the potentialeffects of such a change and how best to implement it. That taskgroup concluded that Section I could safely join Section VIII inincreasing its allowable stresses. The actual determination andpublishing of new allowable stresses took Subcommittee II sometime, because it was quite a task. To expedite the process whileSubcommittee II completed its work, new stresses for a limitedgroup of materials were introduced by means of Code Cases, onefor Section I and two for Section VIII, since Code Cases canbe issued far more quickly than Code addenda. The three Caseswere approved for use in mid-1998. One problem with the newCases was that some jurisdictions were reluctant to accept them,or had no ready mechanism that would permit their prompt adop-tion. However, by 1999, Subcommittee II had completed itsreview and revision of stresses, permitting incorporation of thenew stress values into Section II, Part D, in the 1999 addenda.

In the future, it is possible that the design factor on tensilestrength may be reduced further or eliminated altogether, depend-ing on the results of these first steps and further studies. Note that

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COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-19

the change in design factor applied to tensile strength from to is a 14% change. However, few allowable stresses changed thatmuch, because the higher allowable stress may be determined andcontrolled by the factor applied to yield strength, rather thantensile strength. Also note that the higher stresses are applicablebelow the creep range only.

The above-described methods of setting maximum allowable(design) stress values have been used by the Code Committeessince the mid-1950s. During that time, new data have beenobtained and analyzed to revise design stresses as appropriate,based both on new laboratory tests and reported experience fromequipment in service. There have been times when the analysis ofnew data has resulted in a significant lowering of the allowablestresses at elevated temperature. In all but a few instances, however,the fine safety record of equipment built to the ASME Code hasdemonstrated the validity of the material data evaluation, designcriteria, and design methods used.

1.7.2.5 Hydrostatic Head The tubing design formula refers toNote 11, which states that the hydrostatic head need not be includedwhen this formula is used. Thus, the design pressure, P, is taken as just the MAWP for all tubes having a 5 in. and under outsidediameter. The additive term 0.005D in the tube design formula waschosen by Subcommittee I when the current version of the tubedesign formula was adopted in the 1960s. The new formula ini-tially proposed at that time gave somewhat thinner walls than theprevious formula, leading to a concern on the part of someCommittee members that tubes for low-pressure boilers wouldlack adequate strength to sustain any occasional mechanical load-ing to which they might be subjected. The 0.005D term was acompromise value selected by the Committee in response to thisconcern, an attempt to provide a little extra strength in the tubes.No similar note excluding consideration of hydrostatic headapplies to the design formulas for piping, drums, and headers.Accordingly, the design pressure, P, used in those formulas mustinclude the hydrostatic head, which on a large utility boiler couldadd as much as 50 psi to the MAWP for a lower waterwall headeror a downcomer, since the height of such units may equal that of a15-story building. Of course, the additional hydrostatic headwould be much less for a header near the top of the boiler, such asa roof-outlet header. Incidentally, traditionally the hydrostatic headis measured from the elevation of the drum centerline, since that isthe normal waterline.

The exclusion of hydrostatic head in the tube-design formulabut not in the pipe-design formula came about from the adoptionof slightly different formulas, which in one case (tubing) hadevolved for equipment of modest height, where hydrostatic headwas considered negligible, and in the other case (piping), where itwas thought necessary to include hydrostatic head. This arbitrarilydifferent treatment of hydrostatic head has little significance inmost practical instances. A much larger potential difference inwall thickness for pipe versus tube results from a number of fac-tors prescribed by PG-27 to account for the effects of expandingtubes, threading pipe, or the need for structural stability, asdescribed next.

1.7.2.6 Other Factors Applicable to Tube and Pipe DesignThe first of these several factors is e, defined as a thickness factorfor expanded tube ends, that is applied to tubes below certain min-imum sizes. Note 4 of PG-27.4 lists e values required for varioussizes of tubes. The thickness factor is an extra 0.04 in. (1.0 mm)thickness required over “the length of the seat plus 1 in. (25 mm)

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14

for tubes expanded into tube seats.” This provides a slightly heavierwall for that portion of the tube that is rolled, or expanded, into thetube hole. In the rolling process, the tube undergoes considerablecold-working and some thinning, and the extra 40 mils (1.0 mm)wall thickness was intended to ensure a sound joint no weaker thanthe unexpanded portion of the tube. This extra 40 mils (1.0 mm)applies only to very thin tubes, as stipulated in Note 4. Such tubesare usually used only for very-low-pressure boilers.

Another of these factors is C, described as a “minimumallowance for threading and structural stability.” It is intended forapplication to pipe, but it can sometimes also apply to tubes. Whenused as a threading allowance, C provides extra wall thickness tomake up for material lost in threading. For threaded pipe, Note 3of paragraph PG-27.4 stipulates a C value of 0.065 in. (1.65 mm)for pipe NPS (DN 20) and smaller, and a C value equal to thedepth of the thread for pipe NPS 1 (DN 25) and larger. Forunthreaded pipe, an extra wall thickness of 0.065 in. (1.65 mm),called an allowance for structural stability, must be provided forpipe smaller than NPS 4 (DN 100). This allowance is traditionallyexplained to new members of Subcommittee I as having been cho-sen to provide sufficient bending resistance in the smaller sizepipes to withstand loads imposed by a 200 lb (90 kg) worker on aladder leaning against the middle of a 10 ft (3 m) span of pipe todo some maintenance on the boiler. The larger size pipes presum-ably are strong enough to take care of themselves under such occa-sional mechanical loads. While the story of the worker on the laddermay not be exactly true, the effect of these several factors is tostrengthen the tubes of very-low-pressure boilers, whose wallsmight otherwise be so thin that the boiler would be too flimsy toresist any occasional mechanical loads. However, Note 3 of PG-27.4 makes clear that the allowances for threading and structuralstability are not intended to provide for conditions of misappliedexternal loads or for mechanical abuse.

1.7.2.7 Some Design Differences: Section I versus B31.1 Asexplained in section 1.5 on the distinction between boiler properpiping and boiler external piping, design rules for the latter arefound in the ASME B31.1 Power Piping Code. The formula for thedesign of straight pipe under internal pressure in paragraph 104.1of B31.1 looks the same as the design formula for piping, drums,and headers in paragraph PG-27.2.2 of Section I, except that theadditive term C of Section I is called A by B31.1. In either case (Aor C ), the intention is to account for material removed in thread-ing, to provide some minimum level of mechanical strength and, ifneeded, to provide an allowance for corrosion or erosion. Thereare, however, some slight differences between Section I and B31.1that should be mentioned, since occasionally designers forget thatcertain piping, such as piping from the drum to water level indica-tors, or vent and drain piping, is boiler external piping for whichB31.1 rules apply. Unlike Section I, B31.1 does not specificallymandate a 0.065 in. (1.65 mm) extra thickness for mechanicalstrength or structural stability of plain-end pipes smaller than NPS 4(DN 100). Rather, paragraph 102.4.4 of B31.1 recommends anincrease in wall thickness where necessary for mechanical strengthor, alternatively, requires the use of design methods to reduce oreliminate superimposed mechanical loading. Thus, B31.1 gives thedesigner a choice: provide some (unspecified) increase in wallthickness or take measures to limit mechanical loads. In eithercase, the details are left to the judgment of the designer. Also,according to 104.1.2(C) of B31.1, pipe used for service above cer-tain pressures or temperatures must be at least a schedule 80 pipe.This paragraph is similar to Note 5 of Section I, PG-27.4.

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1.7.2.8 Bend Design Another difference in the design of pipingaccording to Section I versus B31.1 is in their respective treatmentof bends. The B31.1 rule, found in paragraph 102.4.5, is that theminimum wall thickness at any point in a bend must not be lessthan that required for the equivalent straight pipe. Ordinary bend-ing methods usually result in some thinning of the wall on the out-side of bends and some thickening on the inside. Consequently,under the B31.1 rule, a bend thinning allowance must be added tothe wall of a straight pipe so that the finished bend will meet therequired minimum thickness. Table 102.4.5 of B31.1 providesrecommendations for minimum thickness prior to bending as afunction of the radius of the pipe bend. If good shop practices arefollowed, these allowances should result in bends of the requiredminimum thickness. Section I has no similar rule, and over theyears, quite a few inquiries have been received regarding guidanceon this subject. Before 1977, Subcommittee I responded to ques-tions by a letter known as an informal reply, but since January1977, Committee responses have been published as Interpretations(see section 1.3.2). Section I’s design approach to bends, for bothpipe and tube, is explained in Interpretation I-83-23, a compositeof earlier, informal replies. Because the acceptance of pipe or tubebends by the Authorized Inspector as meeting the rules of eitherSection I or B31.1 is crucial to the timely completion of the boiler,the designer should always be aware of which rules apply. Here isInterpretation I-83-23:

Question: What rules does Section I impose with respecttowall thinning and flattening of bends in boiler tubing?

Reply: Section I does not provide specific rules directlyapplicable to wall thinning and flattening of bends in boilertubing. However, the Subcommittee on Power Boilersintends that the manufacturer shall select a combination ofthickness and configuration such that the strength of thebend under internal pressure will at least equal the mini-mum strength required for the straight tube connected tothe bend.

The Subcommittee on Power Boilers’ position, based onmany years of service experience, has been that thinning andflattening of tube bends are permissible within limits estab-lished by the manufacturer, based upon proof testing.

To understand the reason for the difference in design approachused by B31.1 and Section I for bends, remember thatSubcommittee I has always recognized satisfactory experience asan acceptable basis for design. When the issue of the wall thick-ness of tube and pipe bends was raised in the late 1960s, theCommittee had available a long record of satisfactory experienceto justify the design approach traditionally used for bends by themajor boiler manufacturers. There are, however, further argu-ments to support such an approach. First, bends are componentswith two directions of curvature, having inherently greaterstrength than straight pipe, which has only one direction of curva-ture. Second, a pipe bend resembles a portion of a torus.Membrane stresses in a torus happen to be lower on the outsidediameter than on the inside, so the change in wall thickness frombending a straight pipe tends to put the material just where it isneeded. Third, the bending process usually increases the yieldstrength of the metal by work hardening. Unless the bend is sub-sequently annealed, it retains this extra strength in service. Whenbends are proof tested to failure, they typically fail in the straightportion, not in the bend, showing that the bend is stronger. This is

the basis on which Section I accepts them. It is also true thatbends in Section I tubes and pipe are more likely to be within thesetting of the boiler, where there is less chance that a bend failurewould be dangerous to people.

1.7.2.9 Corrosion/Erosion Allowance Section I notes in para-graph PG-27.4 that the design formulas do not include anyallowance for corrosion and/or erosion and that additional thick-ness should be provided where they are expected. Thus, thedesigner is given the responsibility for deciding whether suchallowances are needed and, if so, their amount and location.

Boiler manufacturers in the United States do not normally adda corrosion or erosion allowance when calculating the designthickness of boiler pressure parts. Experience has shown suchallowances to be unnecessary under normal service conditions.Ordinarily, the boiler water treatment controls oxygen levels, dis-solved solids, and pH so that corrosion on the waterside isinsignificant. Similarly, fireside corrosion has not been a problemexcept under unusual circumstances attributable to certain tem-perature ranges and some particularly corrosive constituents in thefuel. Even in these cases, it is not usually practical to solve theproblem by a corrosion allowance. In the unusual circumstance ofsevere local corrosion or erosion, any extra wall thicknessallowance might at best serve to prolong component life for onlya short time. Furthermore, extra thickness added to the wall of aheated tube raises the temperature of the outside surface, therebytending to increase the rate of corrosion. Some other solution tothe problem is usually necessary, such as elimination of the corro-sive condition, a change in material, or the provision of shieldingagainst fly-ash erosion.

It is interesting to compare similar provisions for corrosion orerosion found in Pressure Vessels, Section VIII of the ASMECode, and in the Power Piping Code, B31.1. Section VIII pro-vides similar but somewhat more specific guidance on corrosionand erosion than Section I in paragraphs U-2(a)(1) and UG-25.Allowances for corrosion or erosion are left to the designer’sjudgment based on information provided by the user of the vesselor the user’s agent. Section VIII also states that extra materialprovided for this purpose need not be of the same thickness for allparts of the vessel if different rates of attack are expected for thevarious parts. Finally, Section VIII provides that no additionalthickness needs to be provided when previous experience in likeservice shows that corrosion does not occur or is only of a superfi-cial nature.

The Power Piping Code, B31.1, 102.4, requires a corrosion orerosion allowance only when corrosion or erosion is expected andin accordance with the designer’s judgment. Thus, the same gen-eral approach to corrosion and erosion is seen in Section I,Section VIII, and B31.1.

1.7.3 FabricationFabrication is not specifically defined in Section I, but it is gen-

erally understood to mean all those activities by which the manu-facturer converts material (plate, tube, pipe, etc.) into completedboiler components. Such activities include welding, bending,forming, rolling, cutting, machining, punching, drilling, broach-ing, reaming, and others. Section I permits the manufacturer verybroad latitude in fabrication because of the wide variations inmanufacturing practice, machinery, and trade methods.

Section I provides only a limited number of general rules forfabrication in PG-75 through PG-82. (Welding is covered exten-sively in Part PW, and a brief discussion of those rules is given

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-21

next, in section 1.7.4, Welding and Postweld Heat Treatment.) Thegeneral rules in Part PG specify tolerances for drums and ellip-soidal heads, and limit certain cutting, drilling, and punchingprocesses so that subsequent machining will remove all metalwith mechanical and metallurgical properties that have beenaffected by the earlier process.

The manufacturer is permitted to repair defects in material ifacceptance of the Authorized Inspector is first obtained for boththe method and the extent of repairs.

1.7.3.1 Cold Forming of Austenitic Materials Appendix A370of Section II, Part D states in part: “Cold forming operations per-formed during manufacture of austenitic stainless steel pressureparts may cause impaired service performance when the compo-nent operates in the creep range [above 1000°F (540°C)]. Heattreatment after cold forming at temperature given in the materialspecification will restore the intended properties of the materialand will minimize the threat of premature failure due to recrystal-lization during the time of operation.”

For this reason Section I introduced PG-19 COLD FORMINGOF AUSTENITIC MATERIALS with the 1999 Addendum. Forcold bending operations Section I permit’s exemption from thepost cold-forming heat treatment requirements when the formingstrains 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 regardlessof the amount of strain.

PG-19 includes the formulas for calculating the forming strain incylinders, spherical or dished heads, pipes and tubes, Table PG-19lists 26 austenitic materials permitted by Section I which may besubject to post cold-forming heat treatment.

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

pressure boilers was the development of welding as a replacementfor the riveted construction used in the 19th and first half of the20th centuries. The ASME has recognized the importance of weld-ing by instituting Section IX (Welding and Brazing Qualifications,first published as a separate Code section in 1941) and includingspecial welding rules in each of the book sections covering boilersor pressure vessels. The reader is also directed to the separatechapter on Section IX in this volume for further information onwelding. Section I rules for welding are found in Part PW(Requirements for Boilers Fabricated by Welding), which refers toSection IX for qualification of weld procedures and welder perfor-mance and, in addition, provides rules specifically applicable toboilers and their components, including boiler proper piping. Therules for the welding of boiler external piping (as opposed to boilerproper piping) are not found in Section I; they are found in B31.1,Power Piping. Power Piping also invokes Section IX for the quali-fication of weld procedures and welder performance, but is some-what more liberal with respect to the transfer of procedures andwelders from one organization to another.

Among the many aspects of welding covered by Part PW arethe following: responsibilities of the manufacturer or other orga-nization doing the welding; the materials that may be welded; thedesign of welded joints; radiographic and ultrasonic examinationof welds and when such examination is required; the welding ofnozzles; attachment welds; welding processes permitted; qualifi-cation of welding; preparation, alignment, and assembly of partsto 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 circumferentialjoints; the design of lug attachments to tubes; duties of theAuthorized Inspector related to welding; acceptance standards forthe radiography and ultrasonic examination required by Section I;preparation of test plates for tension and bend tests; and weldingof attachments after the hydrostatic test. Some of these topics arediscussed here.

An important aspect of welding, compared to other means ofconstruction, is the absolute need for careful control of the weld-ing process to achieve sound welds. The ASME Code attempts toachieve this control by allowing welds to be made only by quali-fied welders using qualified procedures. Section IX provides therules 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 aspectsare 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 leastthe equivalent of the base material being joined, as demonstratedby certain tests. Qualification of the procedure establishes that theweldment will have the necessary strength and ductility when it iswelded by an experienced welder following that procedure.Qualification of the welder establishes that he or she can depositsound 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 thata 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 havingnozzles or other openings placed where they intersect welds (thisis stated explicitly in PW-14). There is also no rule that wouldprevent a manufacturer from making an opening for a nozzle in avessel, then deciding the nozzle isn’t needed, and replacing thematerial removed for the opening with a properly designed andwelded patch.

1.7.4.1 Qualification of Welding Procedures It is a general ruleunder any Section of the ASME Boiler and Pressure Vessel Codeand the ASME Code for Pressure Piping, B31, that all welding mustbe 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 thoseprocedures. The manufacturer does this by preparing certain docu-mentation, known as a Welding Procedure Specification (WPS)and a Procedure Qualification Record (PQR) that documents andsupports 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. Theseinstructions are written in terms of certain welding parameters thatare called essential, nonessential, and, when required, supplementalessential variables (see below).

The Procedure Qualification Record (PQR) is a formal recordof the welding data (the actual value of the variables recordedduring the welding of the test coupons) for each welding processused and the results of mechanical test specimens cut from thetest coupons. Nonessential variables used during the welding ofthe test coupon may be recorded at the manufacturer’s option. Thetests (usually tensile and bend tests) are used to demonstrate that a

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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 testsare more fully explained in QW-140 and QW-202 of Section IX.The variables used in production welding may vary somewhatfrom those used during the welding of the test coupons. Note thatthe purpose of the Procedure Qualification test is just to establishthe 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 Codebegan to permit the use of a number of ANSI/AWS StandardWelding Procedure Specifications (SWPS) as an alternative torequiring the manufacturer or contractor to qualify its own proce-dures. Requirements were added to ensure that each manufacturerwould have some experience with standard welding proceduresbefore using them. The standard procedures accepted (33 in thegroup that appeared in the 2007 Edition; more may follow) arelisted in Section IX, Appendix E. Paragraph PW-1.2 andAppendix paragraph A-302.7 of Section I advise that the use ofStandard Welding Procedure Specifications is acceptable, providedthe welding meets the additional requirements of Section I.Section IX simultaneously added a new Article V, StandardWelding Procedure Specifications, which provides details andrestrictions on the use of standard welding procedures. The CodeCommittee wanted to make sure that any organization using anSWPS would first establish its competence with respect to keyaspects of the welding. To that end, an employee of the manufac-turer or contractor must sign and date the SWPS, as evidence thatthe organization is acknowledging responsibility for its use. Withsome exceptions, the organization must then demonstrate its abili-ty to control welding using an SWPS by welding and testing onegroove weld coupon, and must record detailed information aboutthe welding variables used (see below). For further details on theuse of SWPS, see the chapter in this volume on Section IX.

1.7.4.2 Qualification of Welder Performance To complete thequalification process for welded construction, performance testsmust be conducted for each welder to establish the welder’s abilityto deposit sound weld metal. In general, this welder performancequalification is accomplished by mechanical bend tests of perfor-mance test coupons, radiography of a test coupon, or radiographyof the welder’s initial production weld. When welders are qualified,identification marks are assigned to them so that all welded jointscan be identified by the identity of the person who made them.

For various reasons, performance qualification tests do notqualify welders to weld for other manufacturers or contractors,except in the case of similar welding work on piping using thesame procedure. Among the reasons for this prohibition is adesire to ensure that each manufacturer will take full responsibil-ity for welding done by his or her organization. The Committeeapparently believed that requiring performance qualification foreach new employer would achieve better results than a systemthat permits one manufacturer to rely on performance testing sup-posedly carried out by another organization. Some manufacturersshare a related view; if they are to be held responsible for theirwelding, they want to conduct their own welder performancequalifications rather than relying on others.

1.7.4.3 Welding Variables In the development of the WPS, it isnecessary to establish which variables of the welding process areso-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 theweldment and thus require requalification of the procedure, that is,additional testing and issuance of a new PQR to support thechanged WPS. A nonessential variable for a weld procedure is onethat can be changed without requiring requalification, although itwould require a change in the WPS. Essential variables for weldprocedures include material thickness, P-Number (which refers tomaterial category; see below), filler metal alloy, the use or omis-sion of backing, and the use or omission of preheat or postweldheat treatment. Examples of nonessential variables for procedurequalification are groove design, root spacing, method of backgouging or cleaning, change in electrode size, and the addition ofother welding positions to any already qualified.

Variables may be categorized differently, depending on whetherthey are applied to weld procedure or welder performance qualifi-cation. For example, the addition of other welding positions toany already qualified is an essential one for performance, sincewelding in a new position, such as vertical instead of horizontal,could certainly affect the ability of the welder to deposit a soundweld. However, it is a nonessential variable for procedure qualifi-cation because, in the words of QW-100.1, “Welding procedurequalification establishes the properties of the weldment, not theskill of the welder or welding operator.” Such a welder would pre-sumably deposit sound metal in any position, and the propertiesof the resulting weldment would be unaffected. Section IX has agreat number of tables summarizing procedure and performancevariables for all common welding processes.

1.7.4.4 Material Categories (P-Numbers) One importantessential variable is the type of base material being welded. Thereare several hundred different materials permitted for CodeConstruction by the various book sections. If every change in basematerial meant that weld procedure specifications and welder per-formance had to be requalified, the qualification of procedures andwelders for all these materials would be an enormous task. Toreduce this task to manageable proportions, the ASME has adopteda classification system in which material specifications aregrouped into categories, based on similar characteristics such ascomposition, weldability, and mechanical properties. Each categoryis defined by a P-Number. Within the P-Number category are sub-categories called Group Numbers for ferrous base metals that havespecified impact requirements. Although Section I does not useGroup Numbers in relation to impact testing, it does sometimesestablish different postweld heat treatment requirements for thedifferent Group Numbers within a given P-Number category. Anumerical listing of P-Numbers and Group Numbers can be foundfor all Section II materials, by specification number, in TableQW/QB-422 of Section IX.

A welding procedure that has been qualified for a particularbase material may be used for any other base material with thesame P-Number. (On the other hand, if a base material has notbeen assigned a P-Number, it requires an individual procedurequalification.) Similarly, a welder who has been qualified for abase metal of a particular P-Number may be qualified for quite afew 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.) arecategorized by F-Number in Table QW-432 of Section IX. Part C

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-23

of Section II lists American Welding Society filler metal specifi-cations that have been adopted by the ASME and designated asSFA specifications.

Although a specific definition of each P-Number in terms ofchemical 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–Sisteels.

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

• P-No. 4 alloys have higher additions of Cr or Mo or some Nito a maximum of 2% Cr.

• P-No. 5 covers a broad range of alloys varying from 2.25 Cr-1Mo to 9 Cr-1 Mo. This category is divided into three group-ings: 5A, 5B, and 5C. The 5A grouping covers alloys up to3% Cr; 5B covers alloys with from 3–9% Cr; and 5C coversalloys with strength that has been improved by heat treatment.

• P-No.15E (former P-No.5B. Group 2) covers alloy having9Cr, 1Mo and V.

• P-No. 6 covers 12-15% Cr. Ferratic Stainless Steels includingTypes 403, 410 and 429.

• P-No. 7 covers 11-18% Cr. Ferratic Stainless Steels includingTypes 405, 409, 410s and 430.

• P-No. 8 contains the austenitic stainless steels.

1.7.4.5 Weldability Materials are said to have good weldabilityif they can be successfully joined, if cracking resulting from thewelding process can be avoided, and if welds with approximatelythe same mechanical properties as the base material can beachieved. PW-5.2 prohibits welding or thermal cutting of carbon oralloy steel with a carbon content of over 0.35%. Higher carbon cancause zones of high hardness that impart adverse properties to theweld and heat-affected zone (HAZ) and may increase susceptibil-ity to cracking, for regions of high hardness often lack ductility. Infact, carbon strongly affects weldability because of its influence onhardenability during heat treatment. Heat treatment includes notonly deliberate heat treatment such as solution annealing, but alsothe thermal cycle that the weld and HAZ undergo in the weldingprocess.

Various expressions, known as carbon equivalency (CE) formu-las, have been developed to predict a steel’s hardenability by heattreatment. Thus they have also proven to be rough indicators ofthe relative weldability of steels, their susceptibility to crackingduring welding, and the need for preheat to prevent cracking dur-ing or after welding. This type of cracking is variously describedas cold cracking, hydrogen cracking, and delayed cracking(because it may occur some hours after welding is completed).ASME Boiler and Pressure Vessel Code – Section I has adoptedthe following version of Carbon Equivalent (CE) in Ta-ble PW-39.1:

CE � C � (Mn � Si)/6 � (Cr � Mo � V)/5 � (Ni � Cu)/15

Note: If the chemistry values of the last two terms are not avail-able, 0.15% shall be substituted as follows:

CE � C � (Mn � Si)/6 � 0.15

Thus carbon equivalency (CE) is the sum of weighted alloy con-tents, where the concentration of alloying elements is given inweight percent. From this equation, it is seen that carbon is indeed

54%

the most potent element affecting weldability. The formula alsoshows the relative influence of other elements that might be pre-sent in pressure vessel steels, compared to that of carbon.

Associated with the various carbon equivalency formulas areweldability guidelines. A certain value of carbon equivalency,perhaps 0.35% or less, might indicate good weldability; a highervalue might indicate that preheat and/or postweld heat treatmentare required to avoid excessive hardness and potential cracking inthe welds; and a still higher value might mean that the material isvery difficult to weld and both high preheat and postweld heattreatment will be needed to obtain satisfactory welds. The maxi-mum permitted CE for the application in Section I is 0.45%.However to use this exemption in Section I, a minimum preheatof 250°F (120°C) is required. The same carbon equivalency for-mula as shown above is given in the supplementary (nonmandatory)requirements of ASME Specifications SA-105 and SA-106. ForSA-105 forgings, the carbon equivalent is limited to 0.47 or 0.48,depending on thickness; for SA-106 pipe, the limit is 0.50. Therewould be little likelihood of making successful welds in thesematerials with CE values higher than those limits, even with highpreheat.

Although various carbon equivalency formulas have beendeveloped over the years for specific alloys, no universal formulahas been devised that works well for all the low-alloy steels. Infact the more complex the alloy becomes, the more difficult it isto devise an equivalency formula. More recent formulas for low-alloy steels involve more complex interactive terms rather thanthe simple additive terms of the formula cited above.

1.7.4.6 Bending Stress on Welds Most of the design formulasin both Section I and Section VIII are based on the use of mem-brane stress as the basis for design. In a boiler, the tubes, the pip-ing, the headers, and the shell of the drum are all designed for themembrane stress in their cylindrical walls. Bending stress and theeffects of stress concentration are not calculated, although they areknown to be present to some extent. This is explained not inSection I, but in UG-23(c) of Section VIII. In fact, there are bend-ing stresses in such pressure vessel components as flat heads,stayed heads, dished heads, hemispherical heads, and cylinder-to-cylinder junctions.

In recognition of the existence of these largely unavoidablebending stresses, and perhaps still reflecting some lack of confi-dence in the soundness of welded construction, Section I containsa number of design rules intended to minimize bending stresseson welded joints. For instance, PW-9.3 requires a gradual taper,no greater than 1 : 3, between plates of unequal thickness. A 1 : 3slope, or taper, for plate materials represents an angle of a littleover 18 degrees. PW-9.3 advises that its rules for transition slopesdo not apply to joint designs specifically covered elsewhere inSection I, and less restrictive rules are provided for the weld endtransition where components such as valves and pipes or fittingsare welded together (see Fig. PG-42.1). For such applications, amaximum slope of 30 deg. is permitted.

Long after these rules were formulated, finite element analyseshave demonstrated their validity. That is, the discontinuity stre-sses developed at these gradual changes in thickness have gener-ally been found to be unremarkable and well within the limits thatwould apply if the design-by-analysis methods of Section VIII,Division 2, were to be applied.

For decades, paragraph PW-9.4 had prohibited construction inwhich bending stresses were “brought directly on a welded joint.”This paragraph had also specifically prohibited a single-welded

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butt joint if the joint would be subject to bending stress at the rootof the weld, as illustrated in Fig. PW-9.2. In 1996, SubcommitteeI recognized that bending stresses routinely occur on weldedjoints in all boilers and pressure vessels with no untoward resultsand that this broad prohibition against bending stresses wasunwarranted. Accordingly, the general prohibition against subject-ing weld joints to bending stress was deleted, and PW-9.4 wasrevised so that it now prohibits only the single-welded cornerjoints illustrated in Fig. PW-9.2.

1.7.4.7 Welded Nozzles For welded connections such as noz-zles, Section I requires sufficient weld on either side of the con-nection to develop the strength of any nonintegral reinforcement(compensation) through shear or tension in the weld. The allow-able stress values for weld metal are a function of the type of weldand loading, varying from 49% to 74% of the allowable stress forthe vessel material. Fig. PW-15 EXAMPLES OF WELDSTRENGTH CALCULATIONS illustrates how the required weld strength (see PG-37.2) may be calculated. A number ofacceptable types of connections to boiler vessels are illustrated inFig. PW-16.1.

Weld strength calculations for nozzle attachments are notrequired (PW-15.1.6) for certain configurations shown in Fig. PW-16.1.

1.7.4.8 Some Acceptance Criteria for Welds Part PWincludes fabrication rules covering welding processes, base-metalpreparation, assembly, alignment tolerances, and the amount ofexcess weld that may be left on the weld joint (so-called rein-forcement ). In general, butt welds in pressure-containing partsmust have complete penetration, and the weld groove must becompletely filled. To ensure that it is filled and that the surface ofthe weld metal is not below the surface of the adjoining basemetal, extra weld metal may be added as reinforcement on eachface of the weld. A table in PW-35 gives the maximum amount ofreinforcement permitted, as a function of nominal thickness andwhether the weld is circumferential or longitudinal. Requirementsfor butt welds are found in PW-9, PW-35, and PW-41.2.2. PW-9and PW-35 stipulate that butt welds must have full penetration.PW-41.2.2 adds that complete penetration at the root is requiredfor single-welded butt joints and that this is to be demonstrated bythe qualification of the weld procedure. Lack of penetration is areason for rejection of welds requiring radiography. PW-35.1 lim-its undercuts to the lesser of in. (0.8 mm) or 10% of the wallthickness, and they may not encroach on the required wall thick-ness. Some concavity in the weld metal is permitted at the root ofa single-welded butt joint if it does not exceed the lesser of in.(2.5 mm) or 20% of the thinner of the two sections being joined.When concavity is permitted, its contour must be smooth, and theresulting thickness of the weld, including any reinforcement onthe outside, must not be less than the required thickness of thethinner section being joined.

1.7.4.9 Preheating Deposited weld metal is cooled very quickly by its surroundings. Once it solidifies, it forms variousmicrostructural phases— solid solutions of iron and carbon (andother alloying elements). These phases include ferrite, pearlite,bainite, martensite, and austenite (in the higher alloy materials).The formation of any particular phase depends on the cooling rateand alloying elements such as chrome or nickel that may be pre-sent. Certain phases, such as untempered martensite or bainite, canimpart unfavorable properties (e.g., high hardness, lack of ductility,

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and lack of fracture toughness) to the resulting weldment. One wayto address this problem is through the use of preheating, which, byraising the temperature of the surrounding base metal, slows thecooling of the weld and may prevent the formation of undesirablephases.

Preheat serves another useful function: It ensures that the weldjoint is free of any moisture, which can otherwise serve as asource of hydrogen contamination in the weld. Hydrogen can dis-solve in liquid weld metal and by various mechanisms can causecracking in the weld (known variously as cold cracking, hydrogen-assisted cracking, and delayed cracking). Maintaining the weld-ment at preheat temperature during and after welding promotesthe evolution of dissolved hydrogen.

Although preheating a weld joint can be beneficial, Section Idoes not mandate the use of preheat, except as a condition for theomission of postweld heat treatment (as provided in Table PW39).However, it is often specified for new construction welding andfor repair welding, especially when the walls are relatively thick.A brief guide to preheating practices is included in Non-mandatoryAppendix A-100.

1.7.4.10 Postweld Heat Treatment As explained under pre-heating, rapidly cooling weld metal and the adjacent heat-affectedzone are subject to the formation of adverse phases that can resultin zones of high hardness, lack of ductility, and poor fracturetoughness in the weldment. In addition, the differential coolingof the weld metal compared to the surrounding base metal can leadto the formation of very high residual stresses. Again, the extent towhich any of these effects take place is a function of the coolingrate, alloy composition, relative thickness of the parts being joined,and whether preheat is applied. Experience has shown that pre-heating and postweld heat treatment (PWHT) are often unneces-sary for relatively thin weldments, especially those of plain carbonsteel. However, the general rule imparted in paragraph PW-39 isthat all welded pressure parts of power boilers must be given apostweld heat treatment unless otherwise specifically exempted—for example, by PFT-29 for staybolts or PMB-9 for miniature boil-ers, or by the notes in Table PW-39. The materials in Table PW-39are listed in accordance with the P-Number grouping of TableQW/QB-422 of Section IX. See section 1.7.4.4, MaterialCategories (P-Numbers), above for details.

While preheating is aimed at preventing or limiting the forma-tion of adverse phases, postweld heat treatment is used to amelio-rate their effects. Postweld heat treatment is capable of temperingor softening hard phases, and it can restore a large measure ofductility and fracture toughness in the weld and heat-affectedzone. At the same time, it relieves the residual stress caused bythe cooling of the weld metal. The goal of PWHT, therefore, is torestore the properties of the weldment as nearly as possible tothose of the original base metal.

In PWHT the components to be treated are placed in a heat-treatment furnace and slowly heated to the temperature speci-fied in Table PW-39. They are held at that temperature for thetime specified, usually 1 hr/in. (1 hr/25 mm) of thickness plusan additional 15 min/in. (15 min/25 mm) over a certain thick-ness. This rule is intended to ensure that the centermost portionsof thick sections have sufficient time to reach the minimumholding temperature. The nominal thickness in Table PW-39used to determine PWHT requirements is the thickness of theweld, the pressure-retaining material, or the thinner of the sec-tions being joined, whichever is the least. The time-at-tempera-ture requirement can be satisfied by an accumulation of post-weld

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-25

heat treatment cycles. When it is impractical to heat-treat at thespecified temperature, it is permissible to heat-treat at lowertemperatures for longer periods of time, but only for P-No. 1,P-No. 3, P-No. 9A, and P-No. 9B materials. Table PW-39.1lists these alternative longer times and shows that for a 50°F(28°C) decrease in the minimum specified holding temperature,the holding time doubles. This shows how strongly temperature-dependent the underlying creep relaxation process is.

Unlike some other book sections, Section I usually does notspecify any particular heating or cooling rate to be used in PWHT.PW-39.3 stipulates only that the weldment shall be heated slowlyto the required holding temperature. It goes on to say that afterbeing held at that temperature for the specified time, the weld-ment shall be allowed to cool slowly in a still atmosphere to atemperature of 800°F (425°C) or less. This is to avoid high ther-mal stresses and potential distortion caused by large differentialtemperatures throughout the component.

The subject of welding and postweld heat treatment is a com-plex one. For further guidance, see the chapter on Section IX inthis volume.

1.7.5 Brazing RulesAlthough brazing had long been used in construction of certain

low pressure boilers, until recently, no brazing rules had ever beenprovided in Section I. Thus Part PB brazing rules resemble thePart PW welding rules, but are much less extensive. One notabledifference is that the maximum design temperature is dependenton the brazing filler metal used and base metals being joined.Brazing procedures and the performance of brazers must be quali-fied in accordance with Section IX by methods similar to thoseused for qualifying weld procedures and welders. The designapproach used for determining the strength of brazed joints isgiven: the manufacturer must determine from suitable tests or fromexperience that the specific brazing filler metal selected can pro-vide a joint which will have adequate strength at design tempera-ture. The strength of the brazed joint may not be less than that ofthe base metals being joined. This strength is normally establishedby the qualification of the brazing procedure. Some acceptabletypes of brazed joints are illustrated and art PB provides guidanceon inspection and any necessary repairs. Nondestructive examina-tion of brazed construction relies primarily on visual examinationsupplemented by dyepenetrant inspection if necessary.

1.7.6 Nondestructive ExaminationThere are several related terms applied to Code Construction

that are somewhat imprecisely used. These are examination,inspection, and testing. Within the context of Section I, examina-tion usually describes activities of the manufacturer; inspectionrefers to what the Authorized Inspector does; and testing refers toa variety of activities, usually performed by the manufacturer.Some confusion arises from the fact that nondestructive examina-tion (NDE) includes activities defined as examinations but whichare often called tests. NDE is an indispensable means of ensuringsound construction because, when properly used, these examina-tions are capable of discovering hidden flaws in material or welds.

The examinations referenced by Section I in the NDE categoryare the following: radiographic examination, ultrasonic examina-tion (which is mandated as an alternative or supplement toradiographic examination), magnetic-particle examination, andliquid-penetrant examination (also called dyepenetrant examina-tion). Although not explicitly mentioned in Section I, the use ofvisual inspection is certainly implied and is called out in B31.1

for application to the boiler external piping. These examinationsare often referred to in the industry by the shorthand terms RT(radiographic test), UT (ultrasonic test), MT (magnetic-particletest), PT (liquid-penetrant test), and VT (visual examination).

Section I generally follows the Code practice of referring toSection V, Nondestructive Examination, for the rules on how toconduct the various examinations and also provides its ownacceptance standards. For example, Table PW-11 calls for certainwelds to be examined in accordance with Article 2, Section V, tothe acceptance standards of PW-51. Similarly, PG-25, QualityFactors for Steel Castings, calls for magnetic-particle or dyepene-trant examination of all surfaces of castings in accordance withSection V, but provides acceptance criteria within its own domain(i.e., in paragraph PG-25).

Personnel performing and evaluating radiographic, ultrasonic,and other nondestructive examinations are required to be qualifiedand certified as examiners in those disciplines, in accordance witha written practice of their employer (see PW-50). This writtenpractice must be based on a document called RecommendedPractice for Nondestructive Testing Personnel Qualification andCertification, SNT-TC-1A, published by the American Society forNondestructive Testing. Note the use of the word testing, ratherthan examination in this title, showing again that the industrytends to use these terms interchangeably. SNT-TC-1A establishesthree categories of examiners, depending on experience and train-ing, designating them as Level I, II, or III, with Level III being thehighest qualification. The lowest ranking examiner, Level I, isqualified to perform NDE following written procedures developedby Level II or Level III personnel. A Level I individual can alsointerpret and accept the results of nondestructive examinations inaccordance with written criteria (except for RT and UT examina-tions). A Level II individual has sufficient additional training andexperience so that under the direction of a Level III person, he orshe can teach others how to conduct, interpret, and accept theresults of nondestructive examinations. A Level III examiner isthe most qualified and can develop and write procedures as wellas establish a written practice for his or her employer. A Level IIIexaminer can also administer examinations to qualify Level I, II,and III examiners.

The American Society for Nondestructive Testing has anotherdocument, entitled Standard for Qualification and Certification ofNondestructive Testing Personnel, CP-189, which is an alterna-tive to SNT-TC-1a (remember, the latter is only a recommendedpractice). CP-189 requires Level III individuals to pass an exami-nation given by the ASNT and calls for the employer to give afurther examination, since the first examination may not assessthe full range of capability needed. In 1997 Subcommittee I votedto allow CP-189 to be used as an alternative to SNT-TC-1A, withthe choice left to the certificate holder. This alternative standardappeared in the 1997 addenda to Section I.

PW-51 requires a complete set of radiographs for each job to beretained and kept on file by the manufacturer for at least fiveyears. This requirement is based on the reasonable idea that theradiographs might be of assistance in determining responsibility(or lack of responsibility) for any defects, alleged or actual, subse-quently discovered, or other problems that might occur in service.Similarly, PW-52 requires a manufacturer who uses ultrasonicexamination to retain a report of that examination for a minimumof five years. Again, these records might prove useful shouldproblems occur in service.

In 1990 Subcommittee I added an explanation at the beginningof paragraph PW-11 to guide users in resolving disagreements

P

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that sometimes arise under the following circumstances. Section Inormally exempts certain welds from radiography, such as cir-cumferential welds in tubes complying with limits specified inPW-41. Occasionally, a user will have these welds radiographed,revealing imperfections that would have been cause for rejectionif the welds were ones for which Section I required radiography.The explanation added to PW-11 is intended to help resolve thedilemma posed by such a situation, and illustrates Section I’s phi-losophy of accepting long satisfactory experience as a basis forCode rules. Here is that explanation: “This Section exemptsselected welds from radiographic and ultrasonic examination.Experience has demonstrated that such welds have given safe andreliable service even if they contain imperfections which may bedisclosed upon further examination. Any examination and accep-tance standards beyond the requirements of this Section arebeyond the scope of the Code and shall be a matter of agreementbetween the Manufacturer and the user.” This guidance, whilehelpful, has unfortunately not sufficed to prevent disputes arisingfrom the provisions of PW-11.

1.7.7 Hydrostatic TestingThe hydrostatic test is one of the last steps in the construction

of the boiler. Hydrostatic test requirements are given in PG-99and PW-54. These tests may be made either in the manufacturer’sshop or in the field using water. Unlike Section VIII, Section Idoes not permit the use of other fluids or pneumatic testing.

The hydrostatic test serves a number of purposes. Many mem-bers of the Code Committees believe that its major purpose is toestablish that the boiler (or pressure vessel) has been properlyconstructed and that it has a significant design margin, or safetymargin, above and beyond its nominal maximum allowable work-ing pressure. (The hydrostatic test pressure is normally 1.5 timesMAWP.) In this sense, the hydrostatic test is seen to demonstratethe validity of the design as a pressure container. Another impor-tant aspect of the hydrostatic test is that it serves as a leak test.Any leaks revealed by the test must be repaired, and the boilermust be retested (see PW-54).

The first edition of Section I in 1915 called for a hydrostatictest at 1.5 times the Maximum Allowable Working Pressure, abasic rule essentially unchanged to this day. We may surmise thatthe choice of this pressure followed the practice of the time andwas seen to provide evidence that the boiler had been constructedwith a significant design margin. If a higher test pressure hadbeen chosen, the stress in the components might have begun toapproach their yield strength, a situation to be avoided.

Because of the increase in allowable stresses (see 1.7.2.4) therewas some concern that the hydrostatic test at 1.5 times the designpressure could bring stresses during the test close to the yieldstrength of the material. Consequently the committee changedPG-99 to limit the primary membrane stress during a hydrostatictest of any kind of boiler to 90% of the yield strength of the pres-sure parts.

1.7.7.1 Prevention of Brittle Fracture PG-99 requires thehydrostatic test to be conducted using water “at no less than ambienttemperature, but in no case less than 70°F (20°C).” This stipulationabout the water temperature is intended to minimize the possibilityof catastrophic brittle fracture of heavy-walled pressure parts dur-ing the test. Brittle fracture is a type of behavior that can occurwhen metal is under tensile stress when its temperature is at orbelow its so-called nil-ductility transition temperature (NDTT).Above this temperature, the metal behaves in a ductile manner;

below this temperature, its behavior is brittle. Contrasting examplesof these two types of behavior are the bending of a wire hangercompared with the bending of a glass rod. The wire, which is duc-tile, bends easily when its yield strength is exceeded and can berestored to its original shape. The glass rod can carry a certainamount of bending load, but then suddenly fractures in a brittlemanner when its yield strength is reached.

Any flaw, notch, or other discontinuity can raise stress at somelocal area to the yield point. If the material is ductile, it can yieldlocally with little harm done. If the material is below its NDTT, itbehaves in a brittle manner; when stress reaches the yield point,the material may tear or form a crack, which can then grow sud-denly through the thickness, causing a catastrophic failure. Theability of a material to resist tearing or cracking is a measure of itsfracture toughness.

Brittle fracture is generally not a concern for relatively thinmaterials. The manufacturers of steels used in boilers havedeveloped melting practices that usually result in nil-ductilitytransition temperatures well below 70°F (20°C), ensuring ade-quate fracture toughness during hydrostatic tests. The compo-nent of greatest concern in a large utility boiler (so far as brittlefracture is concerned) is the drum, since it has very thick walls.(It happens that thick vessels and headers are more susceptibleto brittle fracture than thinner components.) After a number ofbrittle failure accidents in the 1970s, the manufacturers of largeboilers took steps to lower the NDTT of heavy-walled parts suchas drums (by slightly modifying specifications for materialordered to achieve a finer grained material with greater fracturetoughness). Also, in some instances the manufacturers of largeboilers recommended to their customers that any future hydro-static tests of existing boilers be conducted using warm or evenhot water, to ensure ductile behavior at the time of the test. Thisbecame potentially hazardous for the Authorized Inspector, andSubcommittee I placed a 120°F (50°C) limit on metal temperatureduring the hydrostatic test at the request of the National Board.Also around this time, most boiler manufacturers switched fromthe use of SA-515 to SA-516 plate for heavy-walled boiler drums,because the latter has greater fracture toughness. Even so, heavySA-516 plate is often examined metallurgically by boiler manu-facturers to determine its NDTT, and if that temperature is notwell below 70°F (20°C), a hydrostatic test using water warmerthan 70°F (20°C) is recommended.

The concerns described above can be illustrated by consideringthe design of a thick-walled drum for a large high-pressure boiler.Such a drum would typically be made of SA-516 grade 70 plate.The allowable stress used for designing the drum is about 18 ksi(after an increase of 9% in 1999; see discussion in DesignStresses in section 1.7.2.4, above, and Test Pressure for Drum-Type Boilers, below). Accordingly, during the hydrostatic test, theactual average membrane stress in the drum wall is approximately1.5 times 18 ksi (124 MPa), or 27 ksi (186 MPa). At first glance,this seems to be a reasonable margin below the minimum coldyield strength of this material, which is about 38 ksi (262 MPa).Unfortunately, this is not the case, because in a real vessel thereare always irregularities of various kinds present that act as stressraisers. Examples include welds with undercuts, grooves, orridges. Ligaments between openings can also act as stress raisers,as can changes in vessel geometry at transitions between materi-als of different thickness and at nozzle-to-shell junctions. Thesestress-raising irregularities, sometimes loosely called notches,may cause a stress concentration of two or more. Thus in a drumundergoing hydrostatic testing, it is very likely that local surface

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-27

stress at some of these notches may approach or exceed the yieldstress. At such a time, it is important that the boiler plate materialbe warm enough for its behavior to be ductile so that local yield-ing can occur without significant danger of initiating a brittle failure.When the boiler is in normal service, it is, of course, at a tempera-ture that ensures ductile behavior. It is also at a pressure muchlower than that during the hydrostatic test, so that the stress evenat notches is likely to be well below the yield stress.

1.7.7.2 Test Pressure for Drum-Type Boilers For most boilers,the hydrostatic test is conducted by slowly raising the pressure to1.5 times the MAWP. Close visual inspection is not required dur-ing this stage, in the interest of safety of the Inspector. The pres-sure is then reduced to the MAWP, and the boiler is carefullyexamined for leaks or other signs of distress.

In 1999, after careful consideration, certain allowable designstresses were increased below the creep range by changing in bothSections VIII and I the design factor on tensile strength fromabout 4 to about 3.5. There had been some concern that to avoidyielding in any of the pressure parts, this might require a reduc-tion in the hydrostatic test pressure. Subcommittee I investigatedthe ramifications of what was really only a modest increase insome design stresses and concluded that a reduction in hydrostatictest pressure was unnecessary. However, the rules for hydrostatictests in PG-99 were modified in the 1999 addenda to stipulate that“At no time during the hydrostatic test shall any part of the boilerbe subjected to a general primary membrane stress greater than90% of its yield strength (0.2% offset) at test temperature.” Thenew words characterizing the limit as applicable to general primarymembrane stress were the first use of such terminology bySection I. They were chosen to make clear that the goal is to limitaverage membrane stress through the wall of the components, andnot the bending stress or peak stress at the surface. A subsequentreview of the materials used in Section I construction showed thatthere is still a significant margin between actual stresses during ahydrostatic test and yield strength, as just noted in the discussionunder Prevention of Brittle Fracture.

Hydrostatic tests for Section VIII vessels can be conducted at amuch higher pressure than those for Section I boilers, because thefactor used to determine test pressure includes the ratio of theallowable stress at room temperature to the allowable stress atdesign temperature. Because of this higher pressure, SubcommitteeVIII was concerned about potential yielding during a hydrostatictest and reduced the longstanding hydrostatic test pressure factorfrom 1.5 to 1.3 at about the same time as the new, somewhat higherstresses were adopted in the 1999 addenda.

In addition to the 90% of yield strength limit for membranestress during a hydrostatic test introduced in the 1999 addenda,there is another, much older limit found in PG-99.1. That para-graph requires the test pressure to “be under proper control at alltimes so that the required test pressure is never exceeded by morethan 6%.”

The rule that the test pressure must not be exceeded by morethan 6%, that is, not more than (1.06)(1.50)(MAWP) � (1.59)(MAWP), while arbitrary, is not unreasonable. As explained underthe topic Design Stresses in section 1.7.2.4, one of the criteria forsetting allowable stress is two-thirds of the yield strength of thematerial. Therefore, a hydrostatic test at 1.5 times MAWP couldtheoretically utilize 100% of the yield strength of those compo-nents with allowable stress that is based on the yield strength atroom temperature. There would then be little margin for over-shooting the prescribed test pressure. However, this is not the case

for several reasons. First of all, for most Section I materials, theallowable stress at room temperature is actually based on of theultimate tensile strength, which is quite a bit lower than two-thirds of the yield strength at that temperature. Moreover, theyield strength (and ultimate tensile strength) of most materials asreceived from the mill is somewhat stronger, often by 10% ormore, than the minimum called for by the material specification,although no credit may be taken for this extra strength. Also, anycomponents intended for service at elevated temperature aredesigned using allowable stresses lower than those at room tem-perature (allowable stress for many Section I materials is relativelyconstant from room temperature to about 500°F (260°C) and thenbegins to fall off as the temperature increases). Thus at test temper-ature, these components have extra strength available to provide amargin against yielding.

During a typical hydrostatic test, a pump capable of providing alarge volume of water at relatively low pressure is used to fill theboiler. Various vents are left open to permit all the air to escape,and when the boiler is full of water, all valves are closed and a dif-ferent kind of pump is used, one that can achieve the high pressurerequired. Often the pump used for the hydrostatic test is a piston-type pump that slowly builds pressure as any remaining air pocketsare filled with water. When the water-holding volume of the boilerreaches the stage where it is essentially solid water, each pistonstroke raises the pressure substantially due to the essentiallyincompressible nature of water. It thus may be difficult to achievethe desired test pressure without overshooting the mark. Test per-sonnel must be attentive at this time to avoid exceeding this testpressure tolerance. (established by the designer).

As might be expected, the maximum permitted hydrostatic testpressure has occasionally been exceeded, and in a few cases, theAuthorized Inspector had refused to accept the boiler because PG-99.1 states that the desired test pressure is never to be exceededby more than 6%. What to do? Inquiries came to Subcommittee I,where the sentiment was on the side of common sense, namely,that if the manufacturer could demonstrate to the AI, by somemeans, perhaps through calculations, that stresses during the testhad not exceeded the yield strength of the parts and that no dam-age had been done to the boiler, the AI could accept the boiler. In1981 Interpretation I-81-27, Maximum Hydrostatic Test Pressure,was issued, as follows:

Question: Under what circumstances may the hydrostatictest pressure stated in PG-99 be exceeded?

Reply: The test pressure may be exceeded when the manu-facturer demonstrates to the satisfaction of the AuthorizedInspector that no component has been overstressed.

This Interpretation is also consistent with Section VIII’s designphilosophy, as expressed in UG-99, the counterpart to PG-99 inSection I. Section VIII does not establish an upper limit forhydrostatic test pressure. Section VIII, UG-99(d) says that if thetest pressure exceeds the prescribed value, either intentionally oraccidentally, to the degree that the vessel is subjected to visiblepermanent distortion, the Inspector shall reserve the right to rejectthe vessel.

Subcommittee I considered such an approach to be reasonableand, at one point in the 1980s, actually voted to add a similar pro-vision to Section I in order to settle future inquiries on this topic.However, the Main Committee would not approve the change.Among the objections offered was the assertion that it was too

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difficult for an Inspector to see the slight deformation that wouldindicate yielding had taken place. The fact that Section VIII hadfor years granted the Inspector the discretion to make such a judg-ment was insufficient to convince the negative voters, andSubcommittee I abandoned the effort. Interpretation I-81-27 citedabove should serve to resolve most problems about overshootingthe prescribed hydrostatic test pressure. With the 2004 Edition,the 6% overpressure limit was removed from the Code. Section Ihas only the requirement that, at no time during the hydrostatictest, the primary membrane stress may not exceed 90% of theyield strength at test temperature. This would permit a reasonabledegree of the test pressure exceeding the 1.5 times the MaximumAllowable Working Pressure (MAWP) without damage to thecomponent. This is true with most materials utilized in Section Iconstruction. The designer should be aware that when the ratio oftensile strength to yield strength of a material exceeds 2.0, heshould evaluate the component for the possibility of exceeding thedesignated hydrostatic test pressure.

1.7.7.3 Forced-Flow Steam Generators with No Fixed Steamand Waterline The usual hydrostatic testing procedure is modifiedfor forced-flow steam generators with no fixed steam and water-line. These boilers are designed for different pressure levels alongthe path of water-steam flow, with a significant difference betweeneconomizer inlet and superheater outlet. In one such boiler, forexample, those design pressures were 4350 psi (29 MPa) and 3740 psi (25 MPa), respectively. The design pressure or MAWP atthe superheater outlet is the design pressure stamped on this typeof boiler and is called the master stamping pressure. In the firststage of the hydrostatic test, a pressure equal to 1.5 times the mas-ter stamping pressure (but no less than 1.25 times the MAWP ofany other part of the boiler proper) must be applied. In the exam-ple given, the 1.5 factor controls, at 5610 psi (37 MPa). For thesecond or close-examination stage, the pressure may be reduced tothe MAWP at the superheater outlet.

1.7.7.4 Welding After Hydrostatic Test Formerly, the hydro-static test completed the construction of the boiler, and no furtherwork (i.e. welding) was permitted as part of the ASME new-construction process. However, around 1980, the Committeeadded provisions to PW-54 permitting nonpressure parts to bewelded to the pressure parts after the hydrostatic test if certain con-ditions are met. (Welding is limited to P-No. 1 materials; attachmentis done by stud welds or small fillet welds; 200°F (95°C) preheat isapplied when the thickness of the pressure part exceeds in. (19 mm);and the completed weld is inspected by the Authorized Inspectorbefore he or she signs the Manufacturers’ Data Report Form forthe completed boiler.) This change granted relief from delays thatarose when miscellaneous structural steel parts to be welded to thepressure parts were not available, but the pressure parts were readyfor the hydrostatic test.

In 1999 Code Case permission was granted for the welding ofcarbon steel attachments not classified as P-No. 1 material, buthaving a maximum carbon content of 0.2%, to P-No. 1 pressureparts after the hydrostatic test. Incorporation of the Code Caseinto Section I was recently approved and it was published in the2004 Addenda.

1.7.8 Third-Party InspectionThird-party inspection refers to the system evolved by the Code

Committee to ensure that manufacturers or other Code symbolstamp holders will actually follow the Code rules. In the simplest

34

situation, a boiler manufacturer and the boiler purchaser are thefirst two parties, and an Authorized Inspector (AI) is the indepen-dent third party. It is the function of the AI to ensure and verifythat the manufacturer complies with the Code.

An Authorized Inspector is defined by Section I in PG-91 asan inspector employed by a state or municipality of the UnitedStates, a Canadian province, or an insurance company authorizedto write boiler and pressure vessel insurance. The employer of anAuthorized Inspector is called an Authorized Inspection Agency(AIA). Thus an AIA can be either the inspection agency of aninsurance company or of a jurisdiction that has adopted andenforced at least one section of the Code. In the United States, theAuthorized Inspection Agencies providing authorized inspectionfor ASME Code symbol stamp holders traditionally have beenprivate insurance companies such as the Hartford Steam BoilerInspection & Insurance Company, Factory Mutual, or One CISamong others. In Canada, until 1996, the provincial governmentshad provided authorized inspection services through offices suchas the department of labor. However, in an effort to cut the cost ofgovernment, two provinces (Alberta and Ontario) have recentlyprivatized their authorized-inspection activities by spinning themoff into self-sustaining private companies. The new Alberta orga-nization is called the Alberta Boilers Safety Association (ABSA),and the new Ontario organization is called the TechnicalStandards and Safety Authority (TSSA). The Canadian govern-ment passed a bill allowing the delegation of authority over thispublic safety program, formerly vested in the provincial govern-ments, to not-for-profit nongovernment organizations. The neworganizations are Crown Corporations, the administrators ofwhich are dual employees of the jurisdiction and the newly priva-tized corporations. The National Board of Boiler and PressureVessel Inspectors has accepted the new corporations as represent-ing the jurisdiction. The chief inspectors of the new corporationsare National Board members, as they were before the change.Other provinces may follow the example set by Alberta andOntario. As Code Construction becomes increasingly an interna-tional activity, the National Board may recognize and accept otherforeign jurisdictions and their inspection agencies, provided theymeet certain criteria.

The Authorized Inspector must be qualified by written exami-nation under the rules of the National Board NB-263. When anInspector is so qualified, he or she may obtain a commission, orcertificate of competency, from the jurisdiction where they areworking. The National Board of Boiler and Pressure VesselInspectors (the National Board) also grants commissions to thosewho meet certain qualifications and pass a National Board exami-nation. The qualifications are these: The applicant must beemployed by an Authorized Inspection Agency and must have ahigh school diploma plus satisfy a 5 point education and experi-ence as the applicant must also pass the National Board examina-tion. Finally, the AIA that employs the Inspector must apply tothe National Board on the Inspector’s behalf for issuance of aNational Board commission.

As a condition of obtaining from ASME a Certificate ofAuthorization to use an ASME Code symbol stamp, each Section Imanufacturer or assembler must have in force a contract with anAuthorized Inspection Agency spelling out the mutual responsi-bilities of the manufacturer or assembler and the AuthorizedInspector. The manufacturer or assembler is required to arrangefor the Authorized Inspector to perform the inspections called forby Section I. Paraphrasing the words of A-300 (the QualityControl System): the manufacturer shall provide the Authorized

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-29

Inspector access to all drawings, calculations, specifications,process sheets, repair procedures, records, test results, and anyother documents necessary for the Inspector to perform his or herduties in accordance with Section I. Section I lists many duties ofthe Authorized Inspector, all of which are intended to ensureCode compliance. In 1997 Subcommittee I approved the expan-sion of PG-90 on Inspection and Tests to include a comprehensivelist of the AI’s duties and references to the paragraphs wherethose duties are further described. The revised PG-90 appeared inthe 1998 addenda. However, even when Section I does not specifi-cally describe the duties of an AI with respect to some provisionof the Code, it is understood that the AI has very broad latitude incarrying out the mandate to verify compliance by the Code sym-bol stamp holder with all applicable Code rules.

During Committee deliberations, a number of members whohappen to be Authorized Inspectors have explained that oncethey have established that a manufacturer is following itsapproved quality control system, they merely spot-check themanufacturer’s activities. As a practical matter, an AI cannot be expected to check every detail of a manufacturer’s CodeConstruction operation.

It may be recalled from the discussion of the Preamble in sec-tion 1.2 of this chapter that “the Code does not contain rules tocover all details of design and construction” and that when com-plete details are not given, the manufacturer must provide detailsof design and construction as safe as those the Code does providein its rules, subject to the acceptance of the Authorized Inspector.An AI may have sufficient experience to make a decision in sucha situation, in which case approval may be a simple matter. Atother times, because of new or unusual construction, the AI mayseek guidance from a higher authority within his or her organiza-tion (many Authorized Inspection Agencies maintain an engineer-ing staff) or via an inquiry to the Code Committee, to determinewhether the proposed construction is acceptable.

If a symbol stamp holder fails to comply with the Code, the AIhas several powerful remedies. The Authorized Inspector canrequire rework, additional NDE, or simply refuse to accept theboiler (or other component). Without the AI’s signature on theData Report Form, the boiler is not complete and cannot be soldor used where the Code is enforced. For repeated violations, theAI might also recommend that the contract for AuthorizedInspection not be renewed. Finally, if the violations were flagrant,the matter could be brought to the attention of the Subcommitteeon Boiler and Pressure Vessel Accreditation. The Subcommitteewould then conduct a hearing that could lead to the revocation ofthe offender’s Certificate of Authorization to use a Code symbolstamp. Since the manufacturer needs its stamp to stay in business,third-party inspection provides a very effective means of assuringCode compliance.

1.7.9 Certification by Stamping and Data Reports

1.7.9.1 Certification and Its Significance For the Code to beeffective, there must be some means of ensuring that it has beenfollowed. Each phase of the work of designing, manufacturing,and assembling the boiler must be done in accordance with theCode. Assurance that a boiler is constructed to the Code is provid-ed in part by a process called Code certification, described inPG-104. In the simplest case of a complete boiler unit made by asingle manufacturer, the manufacturer must certify on a formcalled a Manufacturers’ Data Report Form (see below) that allwork done by the manufacturer or others responsible to the manu-

facturer complies with all requirements of the Code. When someportion is performed by others not responsible to the manufacturer,the manufacturer must obtain the other organization’s Code certi-fication that its work complied with the Code. In addition, themanufacturer must stamp the boiler with a Code symbol, whichsignifies that it has been constructed in accordance with the Code.Certification thus is an integral part of a quality assurance pro-gram, which establishes that

(1) the organization that did the work held an appropriateASME Certificate of Authorization to use a Code symbol;

(2) the organization has certified compliance with the Coderules by signing and furnishing the appropriateManufacturers’ Data Report Form;

(3) the organization has applied the Code symbol stamp toidentify the work covered by its Data Report Form; and

(4) a qualified Inspector has confirmed by his or her signatureon the Data Report Form that the work complied with theapplicable Code rules.

In the case of a complete boiler unit that is not manufacturedand assembled by a single manufacturer, the same principles arefollowed. There is always one manufacturer who must take theoverall responsibility for ensuring through proper Code certifica-tion that all the work complies with the requirements of the Code.If, for example, the manufacturer of a boiler buys the drum (orany other part) of the boiler from another manufacturer, that othermanufacturer must follow similar certification and stamping pro-cedures. The part manufacturer must have a Certificate ofAuthorization to use the appropriate Code symbol, certify compli-ance with all the Code rules on a Manufacturers’ Data ReportForm, and stamp or otherwise identify the part; in addition, anAuthorized Inspector must sign the form as confirmation that thework complied with the applicable Code rules. That formbecomes part of the complete documentation assembled by themanufacturer of record. In general, the manufacturer of record hasthe duty of obtaining from all organizations which have done anyCode work on the boiler their proper Code certification for thatwork.

The same approach is used to certify that portion of the workcalled field assembly for those boilers that are too large to becompletely assembled in the shop. The Manufacturers’ DataReport Form typically has a certification box for use by theassembler of a boiler.

1.7.9.2 Manufacturers’ Data Report Forms and TheirDistribution Section I has 12 different Manufacturers’ DataReport Forms (MDRFs) that have evolved over the years to covervarious types of boilers and related components. These formsserve several purposes, one of which is to provide a documentedsummary of certain important information about the boiler: itsmanufacturer, purchaser, location, and identification numbers.Most forms also provide a concise summary of the constructiondetails used: a list of the various components (drum, heads, head-ers, tubes, nozzles, and openings), their material, size, thickness,type, etc., and other information such as the design and hydrostatictest pressures and maximum designed steaming capacity.

The forms and their use are described in PG-112, in PG-113,and in Appendix A-350, which contains examples of all the formsand a guide for completing each one. Also in the Appendix (pageA-357) is a so-called Guide to Data Report Forms Distribution.This guide explains which forms should be used in ten differentcircumstances involving several types of boilers that may be

1-30 • Chapter 1

designed, manufactured, and assembled by different stamp hold-ers. There are so many forms and combinations of forms that canbe used that the subject can often be quite confusing. What isimportant to remember is the purpose of the forms: to provide asummary of essential information about the parts composing theboiler and to provide for certification, as appropriate, by all thoseinvolved (the various manufacturers and inspectors) that all com-ponents of the boiler were constructed to the rules of Section I. Atthe same time, it is also important to understand that the MDRFswere never intended to be all-inclusive catalogs or parts lists forthe boiler or substitutes for the manufacturers’ drawings. If moreinformation is needed than is found on the forms, it can be foundon those drawings.

Certain of the forms are sometimes called Master Data ReportForms. A Master Data Report Form, as the name implies, is thelead document when several different forms are used in combi-nation to document the boiler. The P-2, P-2A, P-2B, P-3, P-3A,and P-5 forms can be used as Master Data Report Forms. Theother forms (the P-4, P-4A, P-4B, and P-6) usually supplementthe information on the Master Data Report Forms and areattached to them. Manufacturers Data Report Forms P-7 and P-8cover the manufacturer and assembly of Safety and PressureRelief Valves.

PG-112.3 requires copies of the Manufacturers’ Data ReportForms to be furnished to the purchaser, the inspection agency,and the municipal, state, or provincial authority at the place ofinstallation. In addition, many jurisdictions require registration ofthe boiler with the National Board, which would then be sentcopies of the Manufacturers’ Data Report Forms. Many boilermanufacturers routinely register all their boilers with theNational Board. Thus the concerned organizations have availablea valuable summary of the design data and the technical detailsof the boiler. This can be very helpful many years later for inves-tigations, repairs, or alterations when original plans may havedisappeared or are not readily available. Surprisingly, this isoften the case. Having such information as the applicable Codeedition and the details of original components can facilitate anysubsequent work on the boiler. The designer then knows thedesign pressure, the size, thickness, material, allowable stress,and design formulas originally used and can make appropriatechoices for repair, replacement, or alteration. Copies of DataReport Forms on file with the National Board can be readilyobtained from that organization, and this is a major benefit ofNational Board registration (a service available for both boilersand pressure vessels).

1.7.9.3 Certification Marks and How They Are ObtainedThe ASME committee that formulated the first edition of theCode in 1915 recognized a need to identify in a unique way anyboiler constructed to meet the Code. They decided to do this byhaving the manufacturer stamp the boiler with a Code symbol.Paragraph 332 of that original edition stipulates, in part, “Eachboiler shall conform in every detail to these Rules, and shall bedistinctly stamped with the symbol shown in Fig. 19, denoting thatthe boiler was constructed in accordance therewith. Each boilershall also be stamped by the builder with a serial number and withthe builder’s name either in full or abbreviated. . . .” The officialsymbol for such a boiler was then, an S enclosed in a cloverleaf.That symbol is evidence that the boiler complies with the Codeand represents an assurance of safe design and construction. Agreat many provisions of Section I are directed to making surethat this will be so.

Over the years, Section I has added other symbol stamps, andthere are now a total of six covering different aspects of boilerconstruction. Until the late 1970s, there was also an L stamp, usedfor locomotive boilers, but by that time there were only two man-ufacturers who were still authorized to use the L stamp. TheCommittee decided to abolish what had become an obsoletestamp, and those last two manufacturers were given S stampsinstead.

In 2011 ASME changed the Code Symbol Stamp to a Certifi-cation Mark (see Figure PG-105.1 – included here for reference) tobe used with a Designator. The Designator will be located immedi-ately below the Certification Mark in the 6 o’clock position. TheDesignators applicable to Section I construction are listed below:

(a) S – power boiler symbol stamp(b) M – miniature boiler symbol stamp(c) E – electric boiler symbol stamp(d) A – boiler assembly symbol stamp(e) PP – pressure piping symbol stamp(f) V – boiler pressure relief valve symbol stamp

Except under certain carefully controlled circumstances, noorganization may do Code work (e.g., fabricate or assemble anySection I boiler components) without having first received fromthe ASME a Certificate of Authorization to use the Certifica-tion Mark with one of the Designators. Before an organizationcan obtain such a certificate, it must meet certain qualifications,among which are the following.

(1) The organization (manufacturer, assembler, or engineer-ing-contractor) must have in force at all times an agree-ment with an inspection agency (the Authorized InspectionAgency, or AIA), usually an insurance company autho-rized to write boiler and pressure vessel insurance, or witha government agency or government authorized agency(see Section 1.7.8, Third Party Inspection) that administersa boiler law in that jurisdiction. This agreement covers theterms and conditions of the authorized Code inspection tobe provided and stipulates the mutual responsibilities ofthe manufacturer or assembler and an inspector providedby the inspection agency. These inspectors, calledAuthorized Inspectors, must be qualified by written exami-nation under the rules of any state of the United States orprovince of Canada whose boiler laws have adoptedSection I. Authorized Inspectors provide the inspectionrequired by Section I during construction or assembly and,after completion, of any Section I component.

(2) A further requirement for obtaining a Certificate ofAuthorization to use one of the ASME Code symbol stamps

FIG. PG-105.1 OFFICIAL CERTIFICATION MARK TODENOTE THE AMERICAN SOCIETY OF MECHANICAL

ENGINEERS’ STANDARD FOR BOILERS

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-31

is that the manufacturer or assembler must have, anddemonstrate, a quality control system to establish that allCode requirements will be met. These pertain to material,design, fabrication, examination by the manufacturer orassembler, and inspection by the Authorized Inspector. Anoutline of the features required in the quality control systemis provided in paragraph A-300 of the Appendix and is dis-cussed in section 1.7.10 below.

(3) Finally, before the ASME issues (or renews) a Certificateof Authorization, the manufacturer’s or assembler’s facili-ties and organization are reviewed by a team consisting ofa representative of the inspection agency (AIA) and anindividual certified as an ASME Designee, who is selectedby the legal jurisdiction involved. The manufacturer orassembler must make available to the review team a writ-ten description of the quality control system that explainswhat documents and procedures will be used to produce orassemble a Code item. On recommendation of the reviewteam, the ASME issues (or renews) a Certificate ofAuthorization to use a particular Code symbol stamp, nor-mally for a period of three years.

1.7.10 Quality Control SystemIn 1973, the ASME Boiler and Pressure Vessel Committee

adopted comprehensive and detailed rules for what is called aquality control (QC) system in Sections I, IV, and VIII, the boilerand pressure vessel sections of the Code. In Section I, these rulesare found in PG-105.4 and Appendix A-300, both entitled QualityControl System. Each manufacturer (or other certificate holder,such as an assembler) is required to have a documented qualitycontrol system that is fully implemented into its manufacturingoperations. See Appendix J of B31.1 Power Piping Code QualityControl Requirements for Boiler External Piping (BEP).

The QC system is intended to control the entire manufacturingprocess from design to final testing and certification. The scope ofsuch systems may vary significantly among manufacturers, sincethe complexity of the work determines the program required;however, the essential features of a QC system are the same foreveryone. Each manufacturer is free to determine the length andcomplexity of its Quality Control Manual, making it as general ordetailed as desired. It is not advisable to include detailed practicesin the manual that are not actually followed in the shop. Also,when shop practices are revised, the manual must be updated toreflect the revisions. Otherwise the Authorized Inspector is likelyto find nonconformities, which will cause considerable extra workbefore full compliance with the Code can be demonstrated.Implicit in the Code QC requirements is recognition that eachmanufacturer has its own established practices and that these areacceptable if a documented system can be developed showingcontrol of the operation in conformance with Code requirements.It is also recognized that some of the information included in awritten description of a quality control system is proprietary, andno distribution is required other than to the Authorized Inspector.

An outline of features to be incorporated into a Section I qualitycontrol system is found in the A-302 paragraphs. These para-graphs are a list of requirements that have proven to be relativelyeasy to understand and have worked well. To begin, the authorityand responsibility of those performing quality control functionsmust be established. That is, the manufacturer must assign anindividual with the authority and organizational freedom to identifyquality problems and to recommend and implement corrective

actions should they be needed. In addition, the written systemmust include the following:

• The manufacturer’s organization chart, showing the relation-ship between management, engineering, and all other groupsinvolved in the production of Code components and what theprimary responsibility of each group is. The Code does notspecify how to set up the organization, nor does it prevent themanufacturer from making changes, so long as the resultantorganization is appropriate for doing Code work.

• Procedures to control drawings, calculations, and specifica-tions. That is, the manufacturer must have a system to ensurethat current drawings, design calculations, specifications, andinstructions are used for the manufacture of Code components.

• A system for controlling material used in Code fabrication toensure that only properly identified and documented materialis used.

• An examination and inspection program that provides refer-ences to Nondestructive Examination Procedures and to per-sonnel qualifications and records needed to comply with theCode. Also required is a system agreed upon with theAuthorized Inspector for correcting nonconformities. A non-conformity is any condition that does not comply with therules of the Code, whether in material, manufacture, or theprovision of all required appurtenances and arrangements.Nonconformities must be corrected before the component canbe considered to comply with the Code.

• A program to ensure that only weld procedures, welding oper-ators, and welders that meet Code requirements are used toproduce Code components. This is normally one of the mostdetailed and important sections of the QC Manual, and itdescribes how qualified weld procedures are maintained andused and who within the organization is responsible for them.It also includes details on how welder qualifications are estab-lished and maintained.

• A system to control postweld heat treatment of welded partsand any other heat treatment, such as might be required fol-lowing tube bending or swaging. This system must also pro-vide means by which the Authorized Inspector can verify thatthe heat treatment was applied.

• A system for calibration of examination, measurement, andtest equipment used in Code Construction to ensure its accuracy.The Code does not require such calibrations to be traceable toa national standard such as those maintained at the NationalInstitute of Science and Technology (NIST). It merely statesthat the equipment must be calibrated.

• A system of record retention for such items as radiographsand Manufacturers’ Data Reports.

• Procedures covering certain other activities of the manufac-turer, such as hydrostatic testing.

• Procedures and controls for any intended subcontracting ofany aspect of Code Construction, such as design, radiography,or heat treatment.

Appendix A-300 concludes with the admonition that the qualitycontrol system shall provide for the Authorized Inspector to haveaccess to all drawings, calculations, records, procedures, test results,and any other documents necessary for him or her to perform theinspections mandated by Section I. The objective is to ensure thequality of the construction and compliance with the Code. Notealso that the quality control system may not be changed withoutobtaining the concurrence of the Authorized Inspector.

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As explained in section 1.7.9, under Certification Marks, one of the conditions for the issuance or renewal of a Certificate ofAuthorization to use one of the ASME Certification Mark with theapprobate Designator is that the manufacturer or assembler musthave a quality control system that is intended to ensure all Coderequirements will be met. Furthermore, the QC system must bedemonstrated to a review team.

The issuance by the ASME of a new Certificate of Authorizationor the required triennial renewal of an existing Certificate ofAuthorization is based on a favorable recommendation after a jointreview of the written quality control system by a representative ofthe manufacturer’s Authorized Inspection Agency and a representa-tive of the legal jurisdiction involved. In those areas where there areno jurisdictional authorities or when the jurisdiction declines to par-ticipate, the second member of the review team is a qualifiedASME review team leader.

1.8 REFERENCES1. Bernstein, M. D., and Yoder, L. W., Power Boilers, A Guide to Section I

of the ASME Boiler and Pressure Vessel Code, New York, ASMEPress, 1998.

2. MacKay J. R and Pillow J. T., Power Boilers, A Guide to Section I ofthe ASME Boiler and Pressure Vessel Code. Second Edition, NewYork, ASME Press 2011

3. ANSI/NB-23, 1998 ed., 1999 addenda, National Board InspectionCode; The American National Standards Institute/The National Boardof Boiler and Pressure Vessel Inspectors.

4. API 510, 1997 ed., 2000 addenda, API Pressure Vessel InspectionCode; The American Petroleum Institute.

5. SYNOPSIS of Boiler and Pressure Vessel Laws, Rules andRegulations, Uniform Boiler and Pressure Vessel Laws Society,Louisville, KY, 2000.

6. ASME Boiler and Pressure Vessel Code Section I, Power Boilers; TheAmerican Society of Mechanical Engineers.

7. ASME Boiler and Pressure Vessel Code Section II, Materials, PartD— Properties; The American Society of Mechanical Engineers.

8. ASME Boiler and Pressure Vessel Code Section V, NondestructiveExamination; The American Society of Mechanical Engineers.

9. ASME Boiler and Pressure Vessel Code Section VIII, Rules for theConstruction of Pressure Vessels; The American Society ofMechanical Engineers.

10. ASME Boiler and Pressure Vessel Code Section IX, Welding andBrazing Qualifications; The American Society of MechanicalEngineers.

11. ASME Boiler and Pressure Vessel Code, Code Cases: Boilers andPressure Vessels; The American Society of Mechanical Engineers.

12. ASME B31.1, Power Piping Code; The American Society ofMechanical Engineers.

13. ASME B36.10M, Welded and Seamless Wrought Steel Pipe; TheAmerican Society of Mechanical Engineers.

14. ASME B16.34, Valves—Flanged, Threaded, and Welding End; TheAmerican Society of Mechanical Engineers.

1.9 DESIGN EXERCISES

Design Exercise No. 1, Design of a HeaderThis first problem illustrates the use of Section I’s method of

design-by-rule, using the rules of paragraph PG-27 to design a

header. The reader is advised to review PG-27 before attemptingto solve the problem or looking at the solution. When reviewingPG-27, note that slightly different formulas are used for designingpipe and tubes.

It has been decided to make a certain cylindrical superheaterheader from pipe, using 10 in. nominal pipe size (NPS) of appropri-ate wall thickness for a maximum allowable working pressure of1020 psi. Applying Section I formula PG-27.2.2 with a 10.75 in.outside diameter and an allowable stress of 10,800 psi (at an 800°Fdesign temperature) for SA-106 Grade B pipe material, determinethe minimum wall thickness required. Then select the lightest pipeadequate for this design pressure, from a table showing standardpipe sizes and wall thicknesses varying according to schedule num-ber. Assume the constant C (for threading and structural stability)in formula PG 27.2.2 is zero, and determine the temperature coeffi-cient y from the table in Note 6 of PG-27.4. Assume also that nocorrosion allowance is needed, that the hydrostatic head, whichnormally must be included in the design pressure when designingpipe, is negligible in this case, and that there are no openings orconnections that would require compensation or ligament efficiencycalculations. For ready reference, paragraph PG-27 and a table ofpipe sizes have been included at the end of this chapter.

Solution to Design Exercise No. 1: After using Note 1 of PG-27.4 to determine the efficiency E for seamless pipe to be 1.00,Note 3 of PG-27.4 to verify that the coefficient C is zero, and thetable in Note 6 of PG-27.4 to determine the temperature coeffi-cient y to be 0.4, we have all factors needed to solve formula PG-27.2.2. We can then find from that formula that the minimumthickness required is 0.49 in., as follows:

We now go to a standard table of pipe sizes and thickness thatis derived from ASME Standard B36.10, Welded and SeamlessWrought Steel Pipe. Note that for any diameter, the wall thicknessvaries in accordance with the so-called schedule number of thepipe. The table shows that schedule 60 (formerly called extra-strong) NPS 10 appears to have just about the wall thickness weneed: 0.50 in. However, this is not the case because the thicknesslisted in the table is nominal and is subject to an undertolerance,as explained in Note 7 of PG-27.4. This 12.5% undertolerance onthickness is given in ASMEASTM Specification SA-530 coveringgeneral requirements for carbon and alloy steel pipe. It is alsofound in the individual pipe specifications listed in PG-9, forexample, in paragraph 19.3 of the SA-106 pipe specification.Accordingly, we must go to the next commonly available wallthickness among the specifications approved by the ASME:schedule 80, with a nominal wall thickness of 0.594 in. Even withan undertolerance of 12.5%, the minimum thickness at any pointin this pipe would be which satis-fies our required minimum of 0.49 in. Our final choice wouldtherefore be an NPS 10 schedule 80 SA-106 Grade B pipe ofwhatever length is required.

Design Exercise No. 2, Design of a HeadDesign Exercise No. 1 consisted of the design of a cylindrical

header, which turned out to be an NPS 10 schedule 80 SA-106Grade B pipe (with a 10.75 in. OD and a nominal wall thicknessof 0.594 in.). In this exercise, various heads for that header will bedesigned starting with a so-called dished head and using the rules

0.875 * 0.594 in. = 0.52 in .,

t =

PD

2SE + 2yP+ C =

(1002)(10.75)

2(10,800) + 2(0.4)(1020)= 0.49 in .

.

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-33

of paragraph PG-29.1. (Dished heads are usually standard catalogitems furnished as standard pressure parts under the rules of para-graph PG-11). Again the reader is advised to review PG-29 beforeproceeding. For ready reference, PG-29 is included at the end ofthis chapter.

(A) For the same maximum allowable working pressure of1020 psi and design temperature of 800°F, determine therequired thickness of a blank, unstayed dished head with a10 in. radius L on the concave side of the head. Assumethe head is made of SA-516 Grade 70 material. The maxi-mum allowable working stress, S, from Section II, Part D,is determined to be 12,000 psi. Verify this value if youhave available a copy of Section II, Part D, or its CD-ROMequivalent. Notice that the allowable stress for this materialis higher than that of the pipe material.

(B) PG-29.7 provides a rule for the design of a semiellipsoidal-shaped head. What minimum thickness would be needed ifthis type of head were used?

(C) PG-29.11 provides the rules for designing a full hemispherical-shaped head. What minimum thickness would be requiredif this type of head were used?

Solution to Design Exercise No. 2(A):

Solution to Design Exercise No. 2(B):

The explanation is that PG-29.7 says that a semiellipsoidalhead shall be at least as thick as the required thickness of a seam-less shell of the same diameter. Thus, the answer derived inDesign Exercise No. 1 applies.

Solution to Design Exercise No. 2(C):

Note that L is the inside radius of the head. In this case, it waschosen to match the nominal inside radius of the piece of pipeused for the header in Design Exercise No. 1.

From the results of these three head-thickness calculations, it isclear that the hemispherical head is the most efficient shape,requiring a thickness only a fraction of that required for a dishedor an ellipsoidal head, and only about half that of the header towhich it is attached. This is because a hemispherical head carriespressure loads predominantly by developing membrane stresses,while the ellipsoidal and especially the dished heads develop sig-nificant bending stresses in addition to the membrane stresses.

Note also that paragraph PG-16.3 establishes the minimumthickness of boiler plate under pressure as in. in most cases.

Design Exercise No. 3, Choice of Feedwater Stop Valve

From the provisions of PG-42, it is seen that most boiler valvesare furnished in compliance with the ASME product standard forvalves, ASME B16.34, Valves— Flanged, Threaded, and WeldingEnd. This standard provides a series of tables for various materialgroups, giving pressure-temperature ratings according to what are

14

t =

PL

1.6S=

1020[10.75 - (2 * 0.594)]

2(1.6)(12,000)= 0.25 in.

t = 0.49 in.

t =

5PL

4.8S=

5(1020)(10)

4.8S(12,000)= 0.89 in.

called pressure classes. The classes typically vary from the 150class (the weakest) to the 4500 class. For each class, the allowablepressure is tabulated as a function of temperature. The allowablepressure at any temperature is the pressure rating of the valve atthat temperature. Temperatures range from the lowest zone, –20°Fto 100°F, to 850°F and higher, depending on the material group.The allowable pressure for the valve falls with increasing temper-ature, since it is based on the allowable stress for the material.

The designer of a valve for Section I service must select a valvewhose pressure rating at design temperature is adequate for theMaximum Allowable Working Pressure (MAWP) of the boiler.However, feedwater and blowoff piping and valves are designedfor a pressure higher than the MAWP of the boiler, because theyare in a more severe type of service called shock service. Designrules for feedwater piping are found in paragraph 122 of B31.1,Power Piping. The shock service somewhat complicates thechoice of the valve; to clarify the choice, Subcommittee I issuedInterpretation I-83-91. The following example illustrates thechoice of a feedwater stop valve following the procedure outlinedin the fourth reply of that interpretation.

The problem here is to select a feedwater stop valve for a boilerwith a Maximum Allowable Working Pressure (MAWP) of 1500 psi.Note that the design rules for a valve that is part of the boiler exter-nal piping are found in paragraph 122 of B31.1, Power Piping.

Solution to Design Exercise No. 3: The first step in this prob-lem is to determine the design pressure and temperature requiredfor this valve. According to paragraph 122.1.3(A.1) of B31.1, thedesign pressure of a feedwater valve must exceed the MAWP ofthe boiler by 25% or 225 psi, whichever is the lesser.

The design pressure PF is taken as the lesser value, 1725 psi. Thedesign temperature is found from paragraph 122.1.3(B) of B31.1 tobe the saturation temperature of the boiler. That temperature for aMAWP of 1500 psi is found to be 596°F from any steam table.Thus we must now find a valve designed for 1725 psi at 596°F.

We next turn to the tables of valve materials and ratings fromthe governing valve standard, ASME B 16.34, Valves— Flanged,Threaded, and Welding End (for ready reference, these pages areincluded at the end of this chapter). Assume the valve material isa forging, SA-105, which is a carbon steel suitable for the rela-tively low temperature of feedwater service. Notice that the stan-dard uses the ASTM designation A-105, rather than the ASMEdesignation SA-105 (it makes no difference in this case, becausePG-11 permits use of the materials in the ASME product stan-dards accepted by reference in PG-42). It is seen from the Table 1list of material specifications that A-105 falls in what is known asmaterial group 1.1.

The task now is to find the lowest valve class adequate for thedesign conditions we have established. Turning to Table 2-1.1,the ratings for group 1.1 materials, we enter with the design tem-perature, rounded up to 600°F, and look for a valve class ade-quate for 1725 psi. The first such class is a 1500 (standard class)valve. However, an alternative choice is available from the rat-ings table for special class valves. From that table, it is seen thata 900 special class valve would also be adequate. (Note that thedifference between the ratings of these two classes is based onthe amount of NDE used in their manufacture. The additionalNDE required for special class valves provides ensurance that

1500 psi * 1.25 = 1875 psi

1500 psi + 225 psi = 1725 psi

critical locations are free of significant defects and justifies theirhigher pressure ratings.) The final choice of valve then becomesone of cost and delivery time.

Design Exercise No. 4, Design of a Tube for Lug Loading

PW-43 provides a method for determining the allowable loadon a tube lug. When PW-43 was revised in 1992, the sample prob-lems in the Section I Appendix paragraphs A-71 to A-74, whichillustrate the determination of allowable loading on tube lugs,were revised to show the new method. Somehow, the explanationof the examples as printed was not as clear as the Subcommitteehad intended, and the examples are a little hard to follow.Accordingly, the first of the sample problems, that shown in A-71,is explained here to serve as a guide to the method. The problemis described as follows.

A tube is suspended by a welded attachment with a 1500 lbdesign load and the dimensions shown in Fig. A-71. This is a con-dition of direct radial loading on the tube. The allowable lug load-ing is calculated for the following conditions:

• Tube material is SA-213 T-22• MAWP � 2258 psi

• Tube design temperature T � 800°F• Tube diameter D � 4.0 in.• Tube wall thickness t � 0.30 in.• Lug thickness � in.• Lug attachment angle (the angle subtended by the lug width;

see Table PW-43.1) � 7 deg.

Design stress for tube at 800°F, psi (increasedfrom its former value of 15,000 psi by the new stress criteriaadopted in the 1999 addenda).

Sa = 16,600

14

1-34 • Chapter 1

TABLE 1 MATERIAL SPECIFICATION LIST (Reproduced From ASME B16.34)APPLICABLE ASTM SPECIFICATION

FIG. A-71 LUG LOAD ON TUBE

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-35

TABLE 2 DIMENSIONS AND WEIGHTS OF WELDED AND SEAMLESS WROUGHT STEEL PIPE (Excerpt—Reproduced From ASME B36.10m)

1-36 • Chapter 1

TABLE 2-1.1 RATINGS FOR GROUP 1.1 MATERIALS (Reproduced From ASME B16.34)

TABLE 2-1.1A STANDARD CLASS (Reproduced From ASME B16.34)

TABLE 2-1.1B SPECIAL CLASS (Reproduced From ASME B16.34)

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-37

Solution to Design Exercise No. 4: The first step in the solu-tion to this problem is to determine K, the tube-attachment-angledesign factor, by interpolation in Table PW-43.1. K is found to be1.07. Also needed is the value of the variable X, which is definedin PW-43.2. In this example,

The load factor, Lf, is now determined either by reading it fromthe plot in Fig. PW-43.1 or by using the appropriate load-factorequation in PW-43.2.1 or PW-43.2.2, depending on whether thelug is applying tension or compression loading to the tube. Theload-factor equations follow.

For compression loading,

For tension loading,

In either case, the load factor is a function of X, which hasalready been determined to be 44.4. It is hard to read the plot withan accuracy greater than two significant figures (which ought tobe good enough for the design of a tube lug). However, if theequations are used, greater accuracy can be obtained. Note thatthe log terms in the equations are logarithms to the base 10.Substituting in those equations gives a tension load fac-tor and a compression load factor . Theproblem as stated concerns a tension load on the lug, but the val-ues of both load factors are derived in order to illustrate that therecan be a significant difference in allowable load, depending on thedirection of the load.

The next step is to determine St, the amount of the allowablestress that is available for the lug loading. St is found from theequation in PW-43.2.3:

Thus, St is seen to be twice the allowable tube stress less themembrane pressure stress, S, at MAWP determined from equationPG-27.2.1. (Notice that if there were no stress due to pressure,equation PW-43.2.3 would give a value of St equal to twice theallowable membrane pressure stress. This is not unreasonablebecause the bending stress in the wall of the tube caused by lugloading is considered to be a secondary bending stress.) In thepresent example, S is found to be 15,000 psi. Thus

La, the maximum allowable unit load on the attachment, cannow be determined from equation PW-43.2.4:

The comparable unit load in compression is

Tension La = K(Lf)St = (1.07)(0.0405)(18,200) = 789 Ib/in.

St = 2(16,600) - 15,000 = 18,200 psi

St = 2.0Sa - S

Lf = 0.0326Lf = 0.0405X = 44.4

Lf = 49.937X[-2.978-0.898(log X)-0.139(log X)2]

Lf = 1.618X[-1.020-0.014(log X)+0.005(log X)2]

X = D/t2 = (4.0/0.32) = 44.4

PG-27 CYLINDRICAL COMPONENTSUNDER INTERNAL PRESSURE(Reproduced from ASME 2011a, Section I)

PG-27.1 GeneralUnless the requirements of A-317 of Appendix A are selected,

the formulas under this paragraph shall be used to determine theminimum required thickness or the maximum allowable workingpressure of piping, tubes, drums, Shells, and headers in accor-dance with the appropriate dimensional categories as given in PG-27.2.1, PG-27.2.2, and PG-27.2.3 for temperatures not exceed-ing those given for the various materials listed in Tables 1A and1B of Section II, Part D.

The calculated and ordered thickness of material must includethe requirements of PG-16.2, PG-16.3, and PG-16.4. Stress calcu-lations must include the loadings as defined in PG-22 unless theformula is noted otherwise.

When required by the provisions of this Code, allowance mustbe provided in material thickness for threading and minimumstructural stability (see PWT-9.2 and PG-27.4, Notes 3 and 5).

If local thin areas are present in cylindrical shells, the requiredthickness may be less than the thickness determined in PG-27provided the requirements of mandatory Appendix IV are met.

PG-27.2 Formulas for Calculation

PG-27.2.1 Tubing—Up to and Including 5 in. (125 mm)Outside Diameter For bare tubes or bimetallic tubes when thestrength of the clad is not included,11 use the following equations:

See PG-27.4.2, PG-27.4.4, PG-27.4.8, and PG-27.4.9.For bimetallic tubes when the strength of the clad is included,11

use the following equations:

See PG-27.4.4, PG-27.4.8, PG-27.4.9, and PG-27.4.10.

PG-27.2.1.2 The wall thickness of the ends of tubes strength-welded to headers or drums need not be made greater than the runof the tube as determined by these formulas.

PG-27.2.1.3 The wall thickness of the ends of tubes permittedto be attached by threading under the limitations of PWT-9.2 shallbe not less than t as determined by this formula, plus 0.8/n (20/n),where n equals the number of threads per inch (per mm).

PG-27.2.1.4 A tube in which a fusible plug is to be installedshall be not less than 0.22 in. (5.6 mm) in thickness at the plug inorder to secure four full threads for the plug (see also A-20).

P = Sb c 2(tb + tc¿) - 0.01D - 2e

D - [(tb + tc¿) - 0.005D - e]d

t = tb + tc

tc¿ = tc c Sc

Sbd

tb + tc¿ =

PD

2Sb + P+ 0.005D + e

P = Sw c 2t - 0.01D - 2e

D - (t - 0.005D - e)d

t =

PD

2Sw + P+ 0.005D + e

Compression La = K(Lf)St = (1.07)(0.0326)(18,200) = 635 Ib/in.

Since the design load is 1500 lb in tension, applied at the centerof a 3 in. long lug, the actual unit load is only 500 lbin., which isless than the maximum permitted. The design is thus seen to besatisfactory.

1-38 • Chapter 1

PG-27.2.1.5 Bimetallic tubes for which the strength of the cladis not included and meeting the requirements of PG-9.4 shall usean outside diameter, D, in the appropriate equation in PG-27.2.1 noless than the calculated outside diameter of the core material. Theoutside diameter of the core material shall be determined by sub-tracting twice the minimum thickness of the cladding from the out-side diameter of the bimetallic tube, including the maximum plustolerance of the core tube. The minimum required thickness, t,shall apply only to the core material.

Tubes for which the strength of the clad is included and meet-ing the requirements of PG-9.4 shall use an outside diameter, D,in the appropriate equation in PG-27.2.1 equal to the outsidediameter of the bimetallic tube, including the maximum plus tol-erance for both the core tube diameter and clad thickness.

PG-27.2.2 Piping, Drums, Shells, and Headers. (based onstrength of weakest course)

see PG-27.4.1, PG-27.4.3, and PG-27.4.5 through PG-27.4.9.

PG-27.2.3 Thickness Greater Than One-Half the InsideRadius of the Component The maximum allowable working pres-sure for parts of boilers of cylindrical cross section, designed for tem-peratures up to that of saturated steam at critical pressure [705.4oF(374.1oC)], shall be determined by the formulas in A-125.

PG-27.3 SymbolsSymbols used in the preceding formulas are defined as follows:

C � minimum allowance for threading and structural stability(see PG-27.4.3)

D � outside diameter of cylinderE � efficiency (see PG-27.4.1)e � thickness factor for expanded tube ends (see PG-27.4.4)P � maximum allowable working pressure (see PG-21)R � inside radius of cylinderS � maximum allowable stress value at the design tempera-

ture of the metal, as listed in the tables specified in PG-23(see PG-27.4.2)

Sb � maximum allowable stress value at the design tempera-ture of the base metal, as listed in the tables specified inPG-23, for a bimetallic tube in which the clad strength isto be included (see PG-27.4.10)

Sc � maximum allowable stress value at the design tempera-ture of the clad metal, as listed in Section II, Part D,Tables 1A or 1B, for a bimetallic tube in which the cladstrength is to be included (see PG-27.4.10)

t � minimum required thickness (see PG-27.4.7)tb � minimum required thickness of the base metal for a

bimetallic tube in which the clad strength is to be included(see PG-27.4.10)

tc � minimum required thickness of the clad for a bimetallictube in which the clad strength is to be included (see PG-27.4.10)

tc’� minimum effective clad thickness for strength purposesfor a bimetallic tube in which the clad strength is to beincluded (see PG-27.4.10)

P =

2SE(t - C)

D - 2y(t - C) or SE(t - C)

R + (1 - y)(t - C)

t =

PD

2SE + 2yP+ C or PR

SE - (1 - y)P+ C

w � weld joint strength reduction factor per PG-26y � temperature coefficient (see PG-27.4.6)

PG-27.4 NotesNotes referenced in the preceding formulas are as follows:

PG-27.4.1 Note 1E � 1.0 for seamless cylinders without openings spaced to

form ligaments� the ligament efficiency per PG-52 or PG-53 for seamless

cylinders with ligaments� w, the weld joint strength reduction factor per PG-26, for

longitudinally welded cylinders without ligaments

For longitudinally welded cylinders with ligaments locatedsuch that no part of the longitudinal weld seam is penetrated bythe openings forming the ligament, E shall be taken as the lesserof w or the ligament efficiency from PG-52 or PG-53. If any partof the longitudinal seam weld is penetrated by the openings thatform the ligaments, E shall be taken as the product of w times theligament efficiency.

PG-27.4.2 Note 2The temperature of the metal to be used in selecting the S value

for tubes shall not be less than the maximum expected mean walltemperature, i.e., the sum of the outside and inside tube surfacetemperatures divided by 2. For tubes that do not absorb heat, themetal temperature may be taken as the temperature of the fluidwithin the tube but not less than the saturation temperature.

PG-27.4.3 Note 3Any additive thickness represented by the general term C may

be considered to be applied on the outside, the inside, or both. Itis the responsibility of the designer using these formulas to makethe appropriate selection of diameter or radius to correspond tothe intended location and magnitude of this added thickness. Thepressure- or stress-related terms in the formula should be evaluatedusing the diameter (or radius) and the remaining thickness whichwould exist if the “additive” thickness had not been applied or isimagined to have been entirely removed.

The values of C below do not include any allowance for corro-sion and/or erosion, and additional thickness should be providedwhere they are expected. Likewise, this allowance for threadingand minimum structural stability is not intended to provide forconditions of misapplied external loads or for mechanical abuse.

Threaded Pipea Value of Cb in. (mm)

D � 3/4 in. (19 mm) nominal 0.065 (1.65)D � 3/4 in. (19 mm) nominal Depth of thread hc

(a) Steel or nonferrous pipe lighter than Schedule 40 of ASMEB36.10M, Welded and Seamless Wrought Steel Pipe, shallnot be threaded.

(b) The values of C stipulated above are such that the actualstress due to internal pressure in the wall of the pipe is nogreater than the values of S given in Table 1A of SectionII, Part D, as applicable in the formulas.

(c) The depth of thread h in in. (mm) may be determined fromthe formula h� 0.8/n (h � 20/n), where n is the number ofthreads per inch (25 mm) or from the following:

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-39

n h

8 0.100 (2.5)0.0696 (1.77)

(a) PG-27.4.4 Note 4e � 0.04 (1.0) over a length at least equal to the length of the

seat plus 1 in. (25 mm) for tubes expanded into tubeseats, except

� 0 for tubes expanded into tube seats provided the thick-ness of the tube ends over a length of the seat plus 1 in.(25 mm) is not less than the following:

� 0 for tubes strength-welded to tubesheets, headers, anddrums. Strength-welded tubes shall comply with the min-imum weld sizes of PW-16.

PG-27.4.5 Note 5While the thickness given by the formula is theoretically ample

to take care of both bursting pressure and material removed inthreading, when steel pipe is threaded and used for steam pres-sures of 250 psi (1.7 MPa) and over, it shall be seamless and of aweight at least equal to Schedule 80 in order to furnish addedmechanical strength.

PG-27.4.6 Note 6y � a coefficient having values as follows:

1112

PG-27.4.7 Note 7If pipe is ordered by its nominal wall thickness, as is customary

in trade practice, the manufacturing tolerance on wall thicknessmust be taken into account. After the minimum pipe wall thick-ness t is determined by the formula, this minimum thickness shallbe increased by an amount sufficient to provide the manufacturingtolerance allowed in the applicable pipe specification. The nextheavier commercial wall thickness may then be selected fromStandard thickness schedules as contained in ASME B36.10M.The manufacturing tolerances are given in the several pipe speci-fications listed in PG-9.

PG-27.4.8 Note 8When computing the allowable pressure for a pipe of a definite

minimum wall thickness, the value obtained by the formulas maybe rounded up to the next higher unit of 10 psi (0.1 MPa).

PG-27.4.9 Note 9The maximum allowable working pressure P need not include

the hydrostatic head loading, PG-22, when used in this equation.

PG-27.4.10 Note 10This note has additional requirements for bimetallic tubes for

which the strength of the clad is included. For additional fabrica-tion requirements, see PW-44. For such bimetallic tubes, the ther-mal conductivity of the base metal shall be equal to or greaterthan the thermal conductivity of the clad material. The claddingprocess shall achieve a metallurgical bond between the clad andthe base metal (core tube).

The temperature of the metal to be used in selecting the Sbvalue for core tubes shall not be less than the maximum expectedmean wall temperature calculated using the base metal thermalproperties for a tube with the same outside diameter and total wallthickness as the clad tube, i.e., the sum of the outside and insidetube surface temperature of an equivalent core tube, divided by 2.

The temperature of the metal to be used in selecting the Scvalue for the clad shall not be less than the maximum expectedmean wall temperature of the clad, i.e., the sum of the outside sur-face temperature and the base metal-clad interface temperature,divided by 2.

The value of Sc shall be taken as that for an annealed wroughtmaterial with nominally equivalent strength and composition asthe clad. Values applicable to either Section I or Section VIII,Division 1 may be used. If two stress values are listed for a mater-ial, the higher value may be used.

The sizing equation is subject to the following constraints:

tb � tc (excludes clads thicker than core tube)

t � (excludes thick-walled tubes)

If � 1, the ratio is set to 1 in the calculation

If � 1, the actual ratio is used in the calculation

PG-29 DISHED HEADS (Reproduced from ASME2011a, Section I)

PG-29.1The thickness of a blank unstayed dished head with the pres-

sure on the concave side, when it is a segment of a sphere, shallbe calculated by the following equation:

a Sc

Sbb

a Sc

Sbb

D

4

Temperature, °F (°C)

900 (480) and

below950

(510)1,000 (540)

1,050 (565)

1,100 (595)

1,150 (620)

1,200 (650)

1,250 (675) and

above

Ferritic 0.4 0.5 0.7 0.7 0.7 0.7 0.7 0.7Austenitic 0.4 0.4 0.4 0.4 0.5 0.7 0.7 0.7Alloy 800,

8010.4 0.4 0.4 0.4 0.4 0.4 0.5 0.7

800H, 800HT

0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.7

825 0.4 0.4 0.4 … … … … …230 Alloy 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.7N06022 0.4 0.4 0.4 0.4 0.5 0.7 0.7 0.7N06045 0.4 0.4 0.4 0.4 0.5 0.7 0.7 0.7N06600 0.4 0.4 0.4 0.4 0.5 0.7 0.7 …N06601 0.4 0.4 0.4 0.4 0.5 0.7 0.7 …N06625 0.4 0.4 0.4 0.4 0.4 … … …N06690 0.4 0.4 0.4 0.4 0.5 0.7 0.7 …Alloy 617 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.7S31803 0.4 … … … … … … …

Values of y between temperatures listed may be determined byinterpolation. For nonferrous materials not listed, y � 0.4.

(a) 0.095 in. (2.41 mm) for tubes 11/4 in. (32 mm) O.D.and smaller

(b) 0.105 in. (2.67 mm) for tubes above 11/4 in. (32 mm)O.D. and up to 2 in. (50 mm) O.D., incl.

(c) 0.120 in. (3.05 mm) for tubes above 2 in. (50 mm)O.D. and up to 3 in. (75 mm) O.D., incl.

(d) 0.135 in. (3.43 mm) for tubes above 3 in. (76 mm)O.D. and up to 4 in. (100 mm) O.D., incl.

(e) 0.150 in. (3.81 mm) for tubes above 4 in. (100 mm)O.D. and up to 5 in. (125 mm) O.D., incl.

1-40 • Chapter 1

whereL � radius to which the head is dished, measured on the con-

cave side of the head P � maximum allowable working pressure (hydrostatic head

loading need not be included) S � maximum allowable working stress, using values given in

Table 1A of Section II, Part D t � minimum thickness of head

w � weld joint strength reduction factor per PG-26

PG-29.1.1 If local thin areas are present in the spherical portionof the dished head, the required thickness may be less than thethickness determined in PG-29.1 provided the requirements ofMandatory Appendix IV are met.

PG-29.2The radius to which a head is dished shall be not greater than

the outside diameter of flanged portion of the head. Where tworadii are used the longer shall be taken as the value of L in theequation.

PG-29.3When a head dished to a segment of a sphere has a flanged-in

manhole or access opening that exceeds 6 in. (150 mm) in anydimension, the thickness shall be increased by not less than 15%of the required thickness for a blank head computed by the aboveformula, but in no case less than 1/8 in. (3 mm) additional thick-ness over a blank head. Where such a dished head has a flangedopening supported by an attached flue, an increase in thicknessover that for a blank head is not required. If more than one man-hole is inserted in a head, the thickness of which is calculated bythis rule, the minimum distance between the openings shall be notless than one-fourth of the outside diameter of the head.

PG-29.4Except as otherwise provided for in PG-29.3, PG-29.7, and PG-

29.12, all openings which require reinforcement, placed in a headdished to a segment of a sphere, or in an ellipsoidal head, or in afull-hemispherical head, including all types of manholes exceptthose of the integral flanged-in type, shall be reinforced in accor-dance with the rules in PG-33.

When so reinforced, the thickness of such a head maybe thesame as for a blank unstayed head.

PG-29.5Where the radius L to which the head is dished is less than 80%

of the outside diameter of the head, the thickness of a head with aflanged-in manhole opening shall be at least that found by makingL equal to 80% of the outside diameter of the head and with theadded thickness for the manhole. This thickness shall be the mini-mum thickness of a head with a flanged-in manhole opening forany form of head and the maximum allowable working stress shallnot exceed the values given in Table 1A of Section II, Part D.

PG-29.6No head, except a full-hemispherical head, shall be of a lesser

thickness than that required for a seamless shell of the same diameter.

t = 5PL>4.8Sw PG-29.7A blank head of a semiellipsoidal form in which half the minor

axis or the depth of the head is at least equal to one-quarter of theinside diameter of the head shall be made at least as thick as therequired thickness of a seamless shell of the same diameter asprovided in PG-27.2.2. If a flanged-in manhole that meets theCode requirements is placed in an ellipsoidal head, the thicknessof the head shall be the same as for a head dished to a segment ofa sphere (see PG-29.1 and PG-29.5) with a dish radius equal toeight-tenths the outside diameter of the head and with the addedthickness for the manhole as specified in PG-29.3.

PG-29.8When heads are made to an approximate ellipsoidal shape, the

inner surface of such heads must lie outside and not inside of atrue ellipse drawn with the major axis equal to the inside diameterof the head and one-half the minor axis equal to the depth of thehead. The maximum variation from this true ellipse shall notexceed 0.0125 times the inside diameter of the head.

PG-29.9Unstayed dished heads with the pressure on the convex side

shall have a maximum allowable working pressure equal to 60%of that for heads of the same dimensions with the pressure on theconcave side.

Head thicknesses obtained by using the formulas in PG-29.11for hemispherical heads and PG-29.7 for blank semiellipsoidalheads do not apply to heads with pressure on the convex side.

PG-29.11The thickness of a blank unstayed full-hemispherical head with

the pressure on the concave side shall be calculated by the follow-ing equation:

whereL � radius to which the head was formed, measured on the

concave side of the head P � maximum allowable working pressure S � maximum allowable working stress, using values given in

Table 1A of Section II, Part Dt � minimum thickness of head

w � weld joint strength reduction factor per PG-26

The above equation shall not be used when the required thick-ness of the head given by this formula exceeds 35.6% of theinside radius, and instead, the following equation shall be used:

where

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

Y =

2(Sw + P)

2Sw - P

t = L(Y1>3- 1)

t =

PL

2Sw - 0.2P

COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE • 1-41

If local thin areas are present in the full-hemispherical head, therequired thickness may be less than the thickness determined aboveprovided the requirements of Mandatory Appendix IV are met.

PG-29.12If a flanged-in manhole that meets the Code requirements is

placed in a full-hemispherical head, the thickness of the headshall 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 outside diameter of the head and with the added thick-ness for the manhole as specified in PG-29.3.

PG-29.13The corner radius of an unstayed dished head measured on the

concave side of the head shall be not less than three times thethickness of the material in the head; but in no case less than 6%of the outside diameter of the head. In no case shall the thinning-down due to the process of forming, of the knuckle portion of anydished head consisting of a segment of a sphere encircled by apart 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 notless than that required by the applicable equation.