ASME B31_1 Power Piping

CHAPTER

16

16.1 BACKGROUND AND GENERALINFORMATION

This chapter is based on the 2007 edition of ASME B31.1,Power Piping Code. As changes, some very significant, are madeto the Code every year, the reader should refer to the Code forany specific requirements. This chapter should be considered toprovide background information and not specific, current Coderules.

References herein to Sections I, II, III, V, VIII, and IX are ref-erences to Sections of the ASME Boiler and Pressure VesselCode. References to a para. are generally to a paragraph in ASMEB31.1 or to a paragraph in this book.

The equations that are numbered in this chapter use the samenumbers as are used in ASME B31.1. Equations that are not num-bered are either not in ASME B31.1 or are not numbered therein.

Published references are listed at the end of each major sectionof this chapter. Reference documents other than codes and stan-dards are numbered. Codes and standards, such as those providedby the ASME, API, AWWA, and ASTM, are simply listed at theend of each reference section.

16.1.1 History of B31.1 In 1926, the American Standards Institute initiated Project B31

to develop a piping Code. The ASME was the sole administrativesponsor. The first publication of this document, AmericanTentative Standard Code for Pressure Piping, occurred in 1935.From 1942 through 1955, the Code was published as theAmerican Standard Code for Pressure Piping, ASA B31.1. It con-sisted of separate sections for different industries.

These sections were split off, starting in 1955, with the GasTransmission and Distribution Piping Systems, ASA B31.8. ASAB31.3, Petroleum Refinery Piping Code, was first published in1959. A number of separate documents have been prepared, mostof which have been published. The various designations follow:

(1) B31.1, Power Piping (2) B31.2, Fuel Gas Piping (withdrawn in 1988) (3) B31.3, Process Piping (4) B31.4, Pipeline Transportation Systems for Liquid

Hydrocarbons and Other Liquids (5) B31.5, Refrigeration Piping (6) B31.6, Chemical Plant Piping (never published; merged into

B31.3) (7) B31.7, Nuclear Piping (moved to B&PV Code Section III) (8) B31.8, Gas Transmission and Distribution Piping Systems (9) B31.9, Building Services Piping

(10) B31.10, Cryogenic Piping (never published; merged intoB31.3)

(11) B31.11, Slurry Piping With respect to the initials that appear in front of B31.1, these

have been ASA, ANSI, and ASME. It is currently correct to referto the Code as ASME B31.1. The initial designation ASA referredto the American Standards Association. This became the UnitedStates of America Standards Institute and then the AmericanNational Standards Institute (ANSI) between 1967 and 1969; thus,ASA was changed to ANSI. In 1978, the Standards Committeewas reorganized as a committee operating under ASME proce-dures with ANSI accreditation. Therefore, the initials ASME nowappear in front of B31.1. These changes in acronyms have notchanged the committee structure or the Code itself.

16.1.2 Scope of B31.1 The ASME B31.1 Code was written with power piping in

mind. It was intended to cover the fuel gas and oil systems in theplant (downstream of the meters), central and district heating sys-tems, in addition to the water and steam systems in power plants.The 1998 edition specifically listed systems that are included andthose that are excluded. However, the ASME B31 StandardsCommittee has directed that the B31 Codes be revised to permitthe Owner to select the piping code most appropriate to their pip-ing installation; this change is incorporated in the 1999 addenda.The Introduction to ASME B31.1 (as well as the Introductions tothe other B31 Codes) states the following:

It is the Owners responsibility to select the Code Sectionwhich most nearly applies to a proposed piping installation.Factors to be considered by the Owner include: limitations ofthe Code Section; jurisdictional requirements; and the applic-ability of other Codes and Standards. All applicable require-ments of the selected Code Section shall be met.

The applications considered in the preparation of ASME B31.1include piping typically found in electric-generating stations,industrial and institutional plants, geothermal heating systems,and central and district heating and cooling systems. It alsoincludes the following:

(1) central and district heating systems for the distribution ofsteam and hot water away from the plant; and

(2) fuel gas or fuel oil piping from where it is brought into theplant site from a distribution system, downstream from theoutlet of the plant meter set assembly, unless the meter setassembly is located outside of the plant property.

B31.1, POWER PIPINGCharles Becht IV

ASME_Ch16_p001-052.qxd 11/11/09 12:19 PM Page 1

2 Chapter 16

The following items are excluded from coverage:

(1) pressure equipment covered by the ASME Boiler andPressure Vessel Code;

(2) building heating and distribution steam piping designed for15 psig [100 kPa (gage)] or less, or hot-water heatingsystems designed for 30 psig [200 kPa (gage)] or less;

(3) piping for hydraulic or pneumatic tools and their compo-nents downstream of the first block or stop valve off thesystem distribution header; and

(4) piping for marine or other installations under federalcontrol.

Note that piping for nuclear power installations is covered bythe ASME Boiler and Pressure Vessel Code Section III. ASMEB31.1 is also not intended to be applied to the following items,which were listed as exclusions in the 1998 edition:

(1) roof and floor drains, plumbing, sewers, and sprinkler andother fire protection systems;

(2) building services piping within the property limits or build-ings of industrial and institutional facilities, which is withinthe scope of ASME B31.9 (piping outside of the scope ofB31.9, such as due to pressure and/or temperature limita-tions, falls within ASME B31.1.);

(3) fuel gas piping inside industrial and institutional buildings,which is within the scope of ANSI Z223.1, National FuelGas Code; and

(4) pulverized fuel piping, which is within the scope of NFPA8503.

These exclusions were removed in the 1999 addenda andreplaced by the general statement that it is the Owners responsi-bility to select the most applicable Code Section. While ASMEB31 now permits the Owner to select the Code Section that he orshe thinks is most appropriate to the piping installation, theASME B31.1 Section Committee has generally considered indus-trial and institutional piping, other than process piping, to bewithin the scope of ASME B31.1. In process facilities, most allpiping, including utilities, generally is constructed in accordancewith ASME B31.3. In other industrial and institutional facilities,ASME B31.9 should generally be the Code of choice unless thesystem is not within the coverage limitations of ASME B31.9.Some of these limits are given below.

(1) Maximum size and thickness limitations, depending onmaterial: (a) Carbon steel: NPS 30 (DN 750) and 0.50 in. (12.5 mm) (b) Stainless steel: NPS 12 (DN 300) and 0.50 in. (12.5 mm) (c) Aluminum: NPS 12 (DN 300) (d) Brass and copper: NPS 12 (DN 300) [12.125 in. OD

(308 mm) for copper tubing] (e) Thermoplastics: NPS 14 (DN 350) (f) Ductile iron: NPS 18 (DN 450) (g) Reinforced thermosetting resin: 14 in. (DN 350)

(2) Maximum pressure limits: (a) Boiler external piping for steam boilers: 15 psig

(105 kPa) (b) Boiler external piping for water heating units: 160 psig

(1,100 kPa) (c) Steam and condensate: 150 psig (1,035 kPa) (d) Liquids: 350 psig (2,415 kPa) (e) Vacuum: 1 atm external pressure (f) Compressed air and gas: 150 psig (1,035 kPa)

(3) Maximum temperature limits: (a) Boiler external piping for water heating units: 250F

(120C) (b) Steam and condensate: 366F (185C) (c) Other gases and vapors: 200F (95C) (d) Other nonflammable liquids: 250F (120C)

The minimum temperature for ASME B31.9 piping is 0F(18C). Toxic and flammable gases and toxic liquids are alsoexcluded from the scope of ASME B31.9.

High pressure and/or temperature steam and water piping with-in industrial and institutional buildings should generally be con-structed to ASME B31.1. One of the reasons that B31.1 is per-haps a better choice for these facilities than B31.3 is that B31.3places significant responsibility on the Owner. For users of B31.3,the Owner should have a depth of knowledge that may wellexceed what the Owners of many industrial and institutional facil-ities have. B31.1, on the other hand, is more prescriptive and doesnot place the same responsibility for decisions on the Owner.

A boiler has three types of piping: boiler proper piping, boilerexternal piping, and nonboiler external piping. A discussion ofboiler piping classification and the history behind it is providedby Bernstein (1998) [1]. Boiler proper piping is entirely coveredby Section I of the Boiler and Pressure Vessel Code. Boiler properpiping is actually part of the boiler (e.g., downcomers, risers,transfer piping, and piping between the drum and an attachedsuperheater). It is entirely within the scope of Section I and is notcovered at all by ASME B31.1.

Boiler external piping includes piping that is considered to bepart of the boiler, but is external to the boiler. It covers pipingfrom the boiler to the valve or valves that are required by Section I.Example systems include feedwater, main steam, vent, drain,blowoff, and chemical feed piping. It includes the connection tothe boiler proper piping and the valves, beyond which is the non-boiler external piping. The technical requirements for this pipingwere transferred from Section I to ASME B31.1 in 1972.However, the administrative requirements remain with Section I,as this piping is considered to be part of the boiler. Because thetechnical requirements differ between Section I and ASMEB31.1, this sometimes results in confusion and error. Reference[1] provides a detailed comparison of key differences.

Nonboiler external piping is the piping beyond the boiler thatis, the balance of plant piping beyond the block valve(s) thatdefine the boundary of the boiler. For this piping, the rules fallentirely within ASME B31.1.

Figures 16.1.1 and 16.1.2 illustrate the jurisdictional limits ofboiler proper, boiler external, and nonboiler external piping.

Because the Code is written for a very specific application power plant pipingvery detailed piping systemspecific rulesare provided. This differs, for example, from ASME B31.3, whererules are written with respect to service conditions (e.g., pressure,temperature, flammable, and toxic) rather than specific systems(e.g., main steam, hot reheat, blowoff, and blowdown).

16.1.3 Intent The ASME B31.1 Code provides minimum requirements for

safety. It is not a design handbook; furthermore, it is for design ofnew piping. However, it is used for guidance in the repair, replace-ment, or modification of existing piping. See NonmandatoryAppendix V, Recommended Practice for Operation, Maintenance,and Modification of Power Piping Systems, para. V-8.1, whichstates the following:


COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 3

Piping and piping components which are replaced, modified, oradded to existing piping systems are to conform to the editionand addenda of the Code used for design and construction ofthe original systems, or to later Code editions or addenda asdetermined by the Operating Company. Any additional pipingsystems installed in existing plants shall be considered as newpiping and shall conform to the latest issue of the Code. Further clarification on the issue of using a more recent edition

of the Code for replacement, modification, or addition is providedin Interpretation 26-1, Question (2).

Question (2): If a Code edition or addenda later than theoriginal construction edition (and applicable addenda) isused, is a reconciliation of the differences required?

Reply (2): No. However, the Committee recommends that theimpact of the applicable provisions of the later edition oraddenda be reconciled with the original Code edition andapplicable addenda.

Some of the philosophy of the Code is discussed in theForeword. ASME B31.1 is intended to parallel the Boiler andPressure Vessel Code Section I, Power Boilers, to the extent thatit is applicable to power piping.

The Foreword states that the Code is more conservative thansome other piping Codes; however, conservatism consists ofmany aspects, including allowable stress, fabrication, examina-tion, and testing. When comparing ASME B31.1 with ASMEB31.3, covered in Chapter 17 herein, one will find that ASME

FIG. 16.1.1 CODE JURISDICTION LIMITS FOR PIPINGFORCED-FLOW STEAM GENERATOR WITHOUT FIXED STEAM ANDWATER LINE [Source: ASME B31.1, Fig. 100.1.2(A)]


4 Chapter 16

B31.1 is more proscriptive and, depending on the circum-stances, more or less conservative. For example, wall-thicknessof ASME B31.1 will generally be the same or greater. Degree ofexamination will be more or less, depending on the service.Hydrotest pressure will be lower, but pneumatic test pressurewill be higher.

The Foreword also contains the following additional paragraph:

The Code never intentionally puts a ceiling limit on conser-vatism. A designer is free to specify more rigid requirementsas he feels they may be justified. Conversely, a designer whois capable of a more rigorous analysis than is specified in theCode may justify a less conservative design, and still satisfythe basic intent of the Code.

In the Introduction, the following paragraph is provided:

The specific design requirements of the Code usually revolvearound a simplified engineering approach to a subject. It isintended that a designer capable of applying more completeand rigorous analysis to special or unusual problems shallhave latitude in the development of such designs and the eval-uation of complex or combined stresses. In such cases, thedesigner is responsible for demonstrating the validity of hisapproach.

Thus, while ASME B31.1 is largely very proscriptive, it pro-vides the latitude for good engineering practice when appropriateto the situation. Note that designers are essentially required to

FIG. 16.1.2 CODE JURISDICTIONAL LIMITS FOR PIPINGDRUM-TYPE BOILERS [Source: ASME B31.1, Fig. 100.1.2(B)]



demonstrate the validity of their approach to the Owners and, forboiler external piping, the Authorized Inspectors satisfaction.This is addressed in Interpretation 1113, Question (1).

Question (1): To whom should a designer justify a less con-servative design by more rigorous analysis to satisfy the basicintent of the Code as allowed in the Foreword andIntroduction?

Reply (1): The Owner of a piping installation has overallresponsibility for compliance with the B31.1 Code, and forestablishing the requirements for design, construction, exam-ination, inspection, and testing. For boiler external piping,the requirements of para. 136.3 shall also be satisfied. Adesigner capable of more rigorous design analysis than isspecified in the B31.1 Code may justify less conservativedesigns to the Owner or his agent and still satisfy the intentof the Code. The designer is cautioned that applicable juris-dictional requirements at the point of installation may have tobe satisfied.

Chapter VII, Providing Operation and Maintenance require-ments, was added in the 2007 edition. See 16.16.

16.1.4 Responsibilities (a) Owner The Owners first responsibility is to determine

which Code Section should be used. The Owner is also responsiblefor imposing requirements supplementary to those of the selectedCode Section, if necessary, to ensure safe piping for the proposedinstallation. These responsibilities are contained in the Introduction.

The Owner is responsible for inspection of nonboiler externalpiping to ensure compliance with the engineering design and withthe material, fabrication, assembly, examination, and test require-ments of ASME B31.1.

(b) Designer While not specifically stated in ASME B31.1, thedesigner is responsible to the Owner for assurance that the engineer-ing design of piping complies with the requirements of the Codeand with any additional requirements established by the Owner.

(c) Manufacturer, Fabricator, and Erector While not specificallystated in ASME B31.1, the manufacturer, fabricator, and erectorof piping are responsible for providing materials, components,and workmanship in compliance with the requirements of theCode and of the engineering design.

(d) Inspector The inspector is responsible to the Owner, fornonboiler external piping, to ensure compliance with the engi-neering design and with the material, fabrication, assembly,examination, and test requirements of the Code.

An Authorized Inspector, which is a third party, is required forboiler external piping. The manufacturer or assembler is requiredto arrange for the services of the Authorized Inspector. TheAuthorized Inspectors duties are described in para. 16.13.1 here-in. The qualifications of the Authorized Inspector are specified inSection I, PG-91, as follows:

An Inspector employed by an ASME accredited AuthorizedInspection Agency, that is, the inspection organization of astate or municipality, of the United States, a Canadianprovince, or of an insurance company authorized to writeboiler and pressure vessel insurance. They are required tohave been qualified by written examination under the rules ofany state of the United States or province of Canada whichhas adopted the Code (Section I).

16.1.5 How Is B31.1 Developed and Maintained? ASME B31.1 is a consensus document. It is written by a com-

mittee that is intended to contain balanced representation from avariety of interests. Membership includes the following:

(1) Manufacturers (2) Owners/Operators (3) Designers/Constructors (4) Regulatory Agents (5) Insurers/Inspectors (6) General Interest Parties The members of the committee are not intended to be represen-

tatives of specific organizations; their membership is consideredbased on qualifications of the individual and desire for balancedrepresentation of various interest groups.

B31.1 is written as a consensus Code and is intended to reflectindustry practice. This differs from a regulatory approach inwhich rules may be written by a government body.

Changes to the Code are prepared by the B31.1 SectionCommittee. Within the Section Committee, responsibility forspecific portions of the Code are split among Task Groups. Theseare the following:

(1) Task Group on General Requirements (TG/GR) (2) Task Group on Materials (TG/M) (3) Task Group on Design (TG/D) (4) Task Group on Fabrication, Examination, and Erection

(TG/FEE) (5) Task Group on Intercode Liaison (TG/IL) (6) Task Group on Special Assignments (TG/SA) (7) Task Group on Piping System Performance (TG/SA) To make a change to the Code, the responsible Task Group pre-

pares documentation of the change, which is then sent out as a bal-lot to the entire Section Committee to vote on. Anyone who votesagainst the change (votes negatively) must state their reason fordoing so, which is shared with the entire Section Committee. Theresponsible Task Group usually makes an effort to resolve anynegatives. A two-thirds majority is required to approve an item.

Any changes to the Code are forwarded to the B31 StandardsCommittee along with the written reasons for any negative votes.In this fashion, the Standards Committee is given the opportunityto see any opposing viewpoints. If anyone on the B31 StandardsCommittee votes negatively on the change, on first consideration,the item is returned to the Section Committee with written rea-sons for the negative. The Section Committee must consider andrespond to any negatives, either by withdrawing or modifying theproposed change or by providing explanations that respond to thenegative. If the item is returned to the Standards Committee forsecond consideration, it requires a two-thirds approval to pass.

Once an item is passed by the Standards Committee, it is for-warded to the Board on Pressure Technology Codes andStandards, which is the final level at which the item is voted onwithin ASME. Again, any negative vote at this level returns theitem to the Section Committee, and a second considerationrequires two-thirds approval to pass.

While the Board on Pressure Technology Codes and Standardsreports to the Council on Codes and Standards, the Council doesnot vote on changes to the Code.

The final step is a public review process. Availability ofdocument drafts is announced in two publications: ANSIsStandards Action and ASMEs Mechanical Engineering. Copies


6 Chapter 16

of the proposed changes are also forwarded to the B31Conference Group and B31 National Interest Review Group forreview. Any comments from the public or the Groups are consid-ered by the Section Committee.

While there are a lot of steps in the process, an item can bepublished as a change to the Code within one year of approval bythe Section Committee, assuming it is passed on first considera-tion by the higher committees. The procedures provide for carefulconsideration and public review of any change to the Code.

16.1.6 Code Editions and Addenda A new edition of the B31.1 Code is issued every three years.

Addenda are issued every year except the year in which a newedition is published.

Addenda are designated a and b. Addenda and the new edi-tion include the following:

(1) technical changes that have been approved by letter ballot; (2) editorial changes, which clarify the Code but do not change

technical requirements; and (3) errata items. The issuance of only two addenda was a change instituted as of

the 1998 edition. Prior policy was to issue three addenda, withone addenda being issued in the same year as that in which thenew edition was published. All technical changes were made inaddenda, and only editorial changes and errata were included inany new edition.

This chapter is prepared based on the 2007 edition. Significant changes can occur each addenda and, naturally,

between editions. An engineer whose practice includes powerpiping should keep current Codes. ASME sells new editions ofthe B31.1 Code, which include delivery of the associated addenda,errata, and interpretations.

16.1.7 How Do I Get Answers to Questions About the Code?

The B31.1 Section Committee responds to all questions aboutthe Code via the inquiry process. Instructions for writing a requestfor an interpretation are provided in Appendix H. The Committeewill provide a strict interpretation of the existing rules.

However, as a matter of policy, the Committee will not approve,certify, rate, or endorse any proprietary device, nor will it act as aconsultant on specific engineering problems or the general under-standing or application of Code rules. Furthermore, it will not pro-vide explanations for the background or reasons for Code rules. Ifyou need any of the above, you should engage in research or edu-cation, read this chapter, and/or hire a consultant, as appropriate.

The Section Committee will answer any request for interpreta-tion with a literal interpretation of the Code. It will not createrules that do not exist in the Code, and will state that the Codedoes not address an item if it is not specifically covered by ruleswritten into the Code.

Inquiries are assigned to a committee member who develops aproposed question and reply between meetings. Although the pro-cedures permit these to be considered between meetings, the prac-tice is for the Section Committee as a whole to consider andapprove interpretations at the Section Committee meetings. Theapproved question and reply is then forwarded to the inquirer bythe ASME staff. Note that the inquiry may not be considered atthe next meeting after it is received (the person responsible forhandling the inquiry may not have prepared a response yet).

Interpretations are published with addenda for the benefit of allCode users.

16.1.8 How Can I Change the Code? The simplest means for trying to change the Code is to write a

letter suggesting a change. Any requests for revision to the Codeare considered by the Code Committee.

To be even more effective, the individual should come to themeeting at which the item will be discussed. ASME B31.1Section Committee meetings are open to the public, and participa-tion of interested parties is generally welcomed. Having a personexplain the change and the need for it can be more effective than aletter alone. If you become an active participant and have appro-priate professional and technical qualifications, you could beinvited to become a member.

Your request for a Code change may be passed to one of twotechnical committees under ASME B31. These are the Fabricationand Examination Technical Committee and the Mechanical DesignTechnical Committee, which are technical committees intended toprovide technical advice to and consistency among the variousCode Sections.

16.1.9 References 1. Bernstein, M.D., and Yoder, L. W., Power Boilers: A Guide to Section I

of the ASME Boiler and Pressure Vessel Code; The American Societyof Mechanical Engineers, 1998.

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

ASME B31.3, Process Piping; The American Society ofMechanical Engineers.

ASME B31.4, Pipeline Transportation Systems for LiquidHydrocarbons and Other Liquids; The American Society ofMechanical Engineers.

ASME B31.5, Refrigeration Piping; The American Society ofMechanical Engineers.

ASME B31.8, Gas Transmission and Distribution PipingSystems; The American Society of Mechanical Engineers.

ASME B31.9, Building Services Piping; The American Society ofMechanical Engineers.

ASME B31.11, Slurry Piping; The American Society ofMechanical Engineers.

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

ASME Boiler and Pressure Vessel Code Section III, Rules forConstruction of Nuclear Power Plant Components; The AmericanSociety of Mechanical Engineers.

16.2 ORGANIZATION OF B31.1 16.2.1 Boiler External and Nonboiler External Piping

The Code has separate requirements for boiler external andnon-boiler external piping. Boiler external piping is actually with-in the scope of Section I of the Boiler and Pressure Vessel Code.Section I refers to ASME B31.1 for technical requirements.Nonboiler external piping falls entirely within the scope of ASMEB31.1. Thus, boiler external piping is treated as part of the boiler



and subject to the Boiler and Pressure Vessel Code, whereas non-boiler external piping is not.

Boiler external piping is considered to start at the first weld forwelded pipe, flange-face for flanged piping, or threaded joint forthreaded piping outside of the boiler. It extends to the valve orvalves required by Section I (and B31.1 para. 122). Both the jointwith the boiler proper piping and the valve(s) at the end of thepiping fall within the scope of boiler external piping.

16.2.2 Code Organization Since the systems in a power plant are well defined, require-

ments are given for specific piping systems. This differs fromB31.3, which describes requirements in terms of more generalfluid services. Specific requirements for a piping system, includ-ing the basis for determining the design pressure and temperaturefor specific systems, can be found in Chapter II, Part 6 (para.122). The following systems are covered:

(1) boiler external piping including steam, feedwater, blowoff,and drain piping;

(2) instrument, control, and sampling piping; (3) spray-type desuperheater piping for use on steam generators

and reheat piping; (4) piping downstream of pressure-reducing valves; (5) pressure-relief piping; (6) piping for flammable and combustible liquids; (7) piping for flammable gases, toxic gases or liquids, or

nonflammable nontoxic gases; (8) piping for corrosive liquids and gases; (9) temporary piping systems;

(10) steam-trap piping; (11) pump-discharge piping; and (12) district heating and steam distribution systems. The Code consists of six chapters and 13 appendices.

Appendices with a letter designation are mandatory; those with aRoman numeral designation are nonmandatory.

The paragraphs in the Code follow a specific numbering scheme.All paragraphs in the Code are in the 100 range. The 100-seriesparagraphs are the ASME B31.1 Code Section of the ASME B31Code for Pressure Piping.

16.2.3 Nonmandatory Appendices ASME B31.1 contains several nonmandatory appendices.

These are described below, but are not covered in detail, except asotherwise noted.

Appendix II: Nonmandatory Rules for the Design of SafetyValve Installations provides very useful guidance for the design ofsafety-relief-valve installations. In addition to general guidanceon layout, it provides specific procedures for calculating thedynamic loads that occur when these devices operate.

Appendix III: Nonmandatory Rules for Nonmetallic Pipingprovides rules for some of the services in which nonmetallic pip-ing is permitted by ASME B31.1. It does not cover all potentialnon-metallic piping system applications within the scope ofASME B31.1. Appendix III is discussed in greater detail inSection 16.15.

Appendix IV: Nonmandatory Corrosion Control for ASMEB31.1 Power Piping Systems contains guidelines for corrosioncontrol both in the operation of existing piping systems and thedesign of new piping systems. Though nonmandatory, Appendix IVis considered to contain minimum requirements. It includes

discussions of external corrosion of buried pipe, internal corro-sion, external corrosion of piping exposed to the atmosphere, anderosioncorrosion.

Appendix V: Recommended Practice for Operation, Maintenance,and Modification of Power Piping Systems provides minimumrecommended practices for maintenance and operation of powerpiping. It includes recommendations for procedures; documenta-tion; records; personnel; maintenance; failure investigation andrestoration; piping position history and hanger/support inspection;corrosion and/or erosion; piping addition and replacement; safety,safety-relief, and relief valves; considerations for dynamic loadand high-temperature creep; and rerating.

Appendix VI: Approval of New Materials offers guidanceregarding information generally required to be submitted to theASME B31.1 Section Committee for the approval of new materials.

Appendix VII: Nonmandatory Procedures for the Design ofRestrained Underground Piping provides methods to evaluate thestresses in hot underground piping where the thermal expansionof the piping is restrained by the soil. It includes not only theaxial compression of fully restrained piping, but also the calcula-tion of bending stresses that occur at changes of direction, wherethe piping is only partially restrained by the soil.

16.2.4 References ASME B31.1, Power Piping; The American Society ofMechanical Engineers.

ASME B31.3, Process Piping; The American Society ofMechanical Engineers.


ASME Boiler and Pressure Vessel Code Section VIII, Division 1,Pressure Vessels; The American Society of Mechanical Engineers.

16.3 DESIGN CONDITIONS AND CRITERIA 16.3.1 Design Conditions

Design conditions in ASME B31.1 are specifically intended forpressure design. The design pressure and temperature are themost severe coincident conditions that result in the greatest pipewall-thickness or highest required pressure class or other compo-nent rating. Design conditions are not intended to be a combina-tion of the highest potential pressure and the highest potentialtemperature unless such conditions occur at the same time.

While it is possible for one operating condition to govern thedesign of one component in a piping system (and be the designcondition for that component) and another to govern the design ofanother component, this is a relatively rare event. If this case wereencountered, the two different components in a piping systemwould have different design conditions.

16.3.1.1 Design Pressure In determining the design pressure,all conditions of internal pressure must be considered. Theseinclude thermal expansion of trapped fluids, surge, and failure ofcontrol devices. The determination of design pressure can besignificantly affected by the means used to protect the pipe fromoverpressure. An example is the piping downstream of a pressure-reducing valve. Per para. 122.5, this piping must either be provid-ed with a pressure-relief device or the piping must be designed forthe same pressure as the upstream piping.


8 Chapter 16

In general, piping systems are permitted to be used withoutprotection of safety-relief valves. However, in the event that noneare provided on the pipe (or attached equipment that would alsoprotect the pipe), the piping system must be designed to safelycontain the maximum pressure that can occur in the piping sys-tem, including consideration of failure of any and all controldevices.

ASME B31.1 dictates how the design pressure is determinedin para. 122 for specific systems. For example, for boiler exter-nal feedwater piping, the design pressure is required to exceedthe boiler design pressure by 25% or 225 psi (1,550 kPa),whichever is less. These requirements are based on system-specific experience. For example, the aforementioned 25% high-er pressure is required because this piping is considered to be inshock service and subject to surge pressure from pump tran-sients.

While short-term conditions such as surge must be considered,they do not necessarily become the design pressure. The Codepermits short-term pressure and temperature variations per para.102.2.4. If the event being considered complies with the Coderequirements of para. 102.2.4, the allowable stress and/or compo-nent pressure rating may be exceeded for a short time, as dis-cussed in 16.3.3. While this is often considered to be an allowablevariation above the design condition, the variation limitations arerelated to the maximum allowable working pressure of the piping,not the design conditions, which could be lower than the maxi-mum allowable pressure at temperature.

16.3.1.2 Design Temperature It is the metal temperature that isof interest in establishing the design temperature. The design tem-perature is assumed to be the same as the fluid temperature, unlesscalculations or tests support use of other temperatures. If a lowertemperature is determined by such means, the design metal tem-perature is not permitted to be less than the average of the fluidtemperature and the outside surface temperature.

Boilers are fired equipment and therefore subject to possibleovertemperature conditions. Paragraph 101.3.2(C) requires thatsteam, feedwater, and hot-water piping leading from fired equip-ment have the design temperature based on the expected continu-ous operating condition plus the equipment manufacturers guar-anteed maximum temperature tolerance. Short-term operation attemperatures in excess of that condition fall within the scope ofpara. 102.2.4 covering permitted variations.

ASME B31.1 does not have a design minimum temperature forpiping, as it does not contain impact test requirements. This isperhaps because power piping generally does not run cold.Certainly, operation of water systems below freezing is not a real-istic condition to consider.

16.3.2 Allowable Stress The Code provides allowable stresses for metallic piping in

Appendix A. These are, as of addend a to the 2004 edition, thelowest of the following with certain exceptions:

(1) 1/3.5 times the specified minimum tensile strength (which isat room temperature);

(2) 1/3.5 times the tensile strength at temperature (times 1.1); (3) two-thirds specified minimum yield strength (which is at

room temperature); (4) two-thirds minimum yield strength at temperature; (5) average stress for a minimum creep rate of 0.01%/1,000 hr.;

(6) two-thirds average stress for creep rupture in 100,000 hr.;and

(7) 80% minimum stress for a creep rupture in 100,000 hr. Specified values are the minimum required in the Material

Specifications. The minimum at temperature is determined bymultiplying the specified (room temperature) values by the ratioof the average strength at temperature to that at room temperature.The allowable stresses listed in the Code are determined by theASME Boiler and Pressure Vessel Code Subcommittee II, and arebased on trend curves that show the effect of strength on yield andtensile strengths (the trend curve provides the aforementionedratio). An additional factor of 1.1 is used with the tensile strengthat temperature.

An exception to the above criteria is made for austenitic stain-less steel and nickel alloys with similar stressstrain behavior,which can be as high as 90% of the yield strength at temperature.This is not due to a desire to be less conservative, but is a recogni-tion of the differences between the behaviors of these alloys. Thequoted yield strength is determined by drawing a line parallel tothe elastic loading curve, but with a 0.2% offset in strain. Theyield strength is the intercept of this line with the stressstraincurve. Such an evaluation provides a good yield strength value ofcarbon steel and alloys with similar behavior, but it does not rep-resent the strength of austenitic stainless steel, which has consid-erable hardening and additional strength beyond this value.However, the additional strength is achieved with the penalty ofadditional deformation. Thus, the higher allowable stresses rela-tive to yield are only applicable to components that are not defor-mation sensitive. Thus, while one might use the higher allowablestress for pipe, it should not be used for flange design.

The allowable stress for Section I of the ASME Boiler andPressure Vessel Code was revised to change the factor on tensilestrength from to in 1999. Code Case 173 was issued in 2001to permit use of the higher allowable stresses, while new allow-able stress tables were under preparation for B31.1 The newallowable stress tables were issued with addenda 2005a (issued in2006) to the 2004 edition.

The increase in allowable stress for Section I was not applied tobolting. Bolting remains at one-fourth tensile strength.

For cast and ductile iron materials, the behavior is brittle andthe allowable stress differs accordingly. For cast iron, the basicallowable stress is the lower of one-tenth of the specified mini-mum tensile strength (at room temperature) and one-tenth of theminimum strength at temperature, also based on the trend ofaverage material strength with temperature. For ductile iron, afactor of one-fifth is used rather than a factor of one-tenth, and thestress is also limited to two-thirds times the yield strength. Theseare in accordance with Section VIII, Division 1, Appendix P, andTables UCI-23 and UCD-23.

16.3.3 Allowances for Temperature and PressureVariations

While the Code does not use the term maximum allowableworking pressure, the concept is useful in discussion of theallowances for variations. Pressure design of piping systems isbased on the design conditions. However, since piping systemsare an assembly of standardized parts, there is quite often signifi-cant pressure capacity in the piping beyond the design conditionsof the system. The allowances for variations are relative to themaximum permissible pressure for the system. The allowances

13.5

14



for variations are not used in sustained (longitudinal), occasional(wind, earthquake), nor displacement (thermal expansion) stressevaluations. They are only used in pressure design.

Increases in pressure and temperature above the design condi-tions are permitted for short-term events as long as several condi-tions are satisfied, one of which is that this maximum allowableworking pressure is not exceeded by more than some percentage.Thus, the variation can be much higher than the design condi-tions, yet remain permissible.

ASME B31.1 does not allow use of the variations provision ofthe Code to override limitations of component standards or thosegiven by manufacturers of components.

The circumferential pressure stress may exceed the allowablestress provided by ASME B31.1, Appendix A, by the following:

(1) 15% if the event duration occurs for no more than 8 hr atany one time and no more than 800 hr/year; or

(2) 20% if the event duration occurs for no more than 1 hr atany one time and no more than 80 hr/yr.

There is no provision requiring Owners approval, nor anyrequiring the designer to determine that the system is safe withthe variations.

Use of the variations for piping containing toxic fluid is prohib-ited [see para. 122.8.2(F)].

16.3.4 Overpressure Protection As discussed in the prior section on design pressure, the piping

system must either be designed to safely contain the maximumpossible pressure, considering such factors as failure of controldevices and dynamic events such as surge, or be provided withoverpressure protection such as a safety-relief valve. Specificexamples are provided in the Systems (Part 6) part of Chapter IIfor pressure-reducing valves (para. 122.5) and pump dischargepiping (para. 122.13), as well as elsewhere in specific system dis-cussions.

For example, if a 600 psi system goes through a pressure let-down valve (irrespective of fail-closed features or other safe-guards) to a 300 psi system, if no safety-relief devices are provid-ed, the 300 psi system would have to be designed to safelycontain 600 psi.

If a pressure-relieving device is used, ASME B31.1 refers toSection I for boiler external piping and nonboiler external pipingreheat systems, and to Section VIII, Division 1, for nonboilerexternal piping. See para. 16.5.2 herein.

Block valves are prohibited from the inlet lines to pressure-relieving safety devices, and diverter or changeover valves forredundant protective devices are permitted under certain condi-tions (para. 122.6.1). Block valves are also prohibited from use inpressure-relieving device discharge piping (para. 122.6.2).

16.3.5 References ASME B31.1, Power Piping; The American Society ofMechanical Engineers.


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

ASME Boiler and Pressure Vessel Code Section VIII, Division 1,Pressure Vessels; The American Society of Mechanical Engineers.

16.4 PRESSURE DESIGN 16.4.1 Methods for Internal Pressure Design

The ASME B31.1 Code provides four basic methods for designof components for internal pressure, as described in para. 102.2.

(1) Components in accordance with standards listed in Table 126.1for which pressure ratings are provided in the standard, suchas ASME B16.5 for flanges, are considered suitable byASME B31.1 for the pressure rating specified in the standard.Note that the other methods of pressure design provided inASME B31.1 can be used to determine pressure ratings abovethe maximum temperature provided in the standard if thestandard does not specifically prohibit that.

(2) Some listed standards, such as ASME B16.9 for pipe fittings,state that the fitting has the same pressure rating as matchingseamless pipe. If these standards are listed in Table 126.1,the components are considered to have the same allowablepressure as seamless pipe of the same nominal thickness.Note that design calculations are not usually performed forthese components; design calculations are performed for thestraight pipe, and matching fittings are simply selected.

(3) Design equations for some components such as straight pipeand branch connections are provided in para. 104 of ASMEB31.1. These can be used to determine the required wall-thickness with respect to internal pressure of components.Also, some specific branch connection designs are assumedto be acceptable.

(4) Specially designed components that are not covered by thestandards listed in Table 126.1 and for which design formu-las and procedures are not given in ASME B31.1 may bedesigned for pressure in accordance with para.104.7.2. Thisparagraph provides accepted methods, such as burst testingand finite element analysis, to determine the pressure capac-ity of these components.

The equations in the Code provide the minimum thicknessrequired to limit the membrane and, in some cases, bendingstresses in the piping component to the appropriate allowablestress. To this thickness must be added mechanical and corro-sion/erosion allowances. Finally, the nominal thickness selectedmust be such that the minimum thickness that may be provided,per specifications and considering mill tolerance, is at least equalto the required minimum thickness.

Mechanical allowances include physical reductions in wall-thickness such as from threading and grooving the pipe.Corrosion and erosion allowances are based on the anticipatedcorrosion and/or erosion over the lifespan of the pipe. Suchallowances are derived from estimates, experience, or referencessuch as NACE publications. These allowances are added to thepressure design thickness to determine the minimum requiredthickness of the pipe or component when it is new.

For threaded components, the nominal thread depth (dimension hof ASME B1.20.1, or equivalent) is used for the mechanicalallowance. For machined surfaces or grooves, where the toleranceis not specified, the tolerance is required to be assumed as in.(0.40 mm) in addition to the depth of the cut.

Mill tolerances are provided in specifications. The most commontolerance on wall-thickness of straight pipe is 12.5%. This means thatthe wall-thickness at any given location around the circumference ofthe pipe must not be less than 87.5% of the nominal wall-thickness.Note that the tolerance on pipe weight is typically tighter, so that

164


10 Chapter 16

volume of metal and its weight may be there but a thin region wouldcontrol design for hoop stress from internal pressure.

Note that the appropriate specification for the pipe must beconsulted to determine the specified mill tolerance. For example,plate typically has an undertolerance of 0.01 in. (0.25 mm).However, pipe formed from plate does not have this undertoler-ance; it can be much greater. The pipe specification, which canpermit a greater undertolerance, governs for the pipe. The manu-facturer of pipe can order plate that is thinner than the nominalwall-thickness for manufacturing the pipe, as long as the pipespecification mill tolerances are satisfied.

16.4.2 Pressure Design of Straight Pipe for Internal Pressure

Equations for pressure design of straight pipe are provided inpara. 104.1. The minimum thickness of the pipe selected, consid-ering manufacturers minus tolerance, must be at least equal to tm,as calculated using equation (3) or (3A).

(3)

where

additional thicknesspipe outside diameter (not nominal diameter)internal design gage pressuremaximum allowable stress in material from internalpressure and joint efficiency (or casting quality factor)at design temperature from Appendix A minimum required thickness including additional thick-ness, Acoefficient provided in Table 104.1.2(A) of the Code andTable 16.4.1 herein

The additional thickness, A, is to compensate for materialremoved in threading and grooving; to allow for corrosion and/or

y =

tm =

SE = P =

Do = A =

tm =PDo

2(SE + Py) + A

erosion; to account for cast iron pipe, [0.14 in. (3.56 mm) for cen-trifugally cast and 0.18 in. (4.57 mm) for statically cast]; and toaccommodate other variations, as described in para. 102.4.4, suchas local stresses from pipe support attachments.

When equation (3) or (3A) is used for a casting, SF (basicmaterial allowable stress, S, multiplied by casting quality factor, F),is used rather than SE.

Note that the equation is based on the outside, rather than theinside diameter, which is used in pressure vessel Codes. This isfor a very good reason: the fact that the outside diameter of pipeis independent of wall-thickness that is, an NPS 6 pipe will havean outside diameter of 6.625 in. regardless of the wall-thickness.Therefore, the wall-thickness can be directly calculated when theoutside diameter is used in the equation.

The foregoing equation is an empirical approximation of themore accurate and complex Lam equation. The hoop or circum-ferential stress is higher toward the inside of the pipe than towardthe outside. This stress distribution is illustrated in Fig. 16.4.1.The Lam equation can be used to calculate the stress as a func-tion of location through the wall-thickness. Equation (3) is theBoardman equation [1]. While it has no theoretical basis, it pro-vides a good match to the more accurate and complex Lam equa-tion for a wide range of diameter-to-thickness ratios. It becomesincreasingly conservative for lower D/t ratios (thicker pipe).

The Lam equation for hoop stress on the inside surface of pipeis given in the following equation. Note that for internal pressure,the stress is higher on the inside than the outside. This is becausethe strain in the longitudinal direction of the pipe must be con-stant through the thickness, so that any longitudinal strain causedby the compressive radial stress (from Poissons effects and con-sidering that the radial stress on the inside surface is equal to thesurface traction of internal pressure) must be offset by a corre-sponding increase in hoop tensile stress to cause an offsettingPoissons effect on longitudinal strain.

sh = P c0.5(Do>t)2 - (Do>t) + 1

(Do>t) - 1 d

TABLE 16.4.1 VALUES OF COEFFICIENT y [Source: ASME B31.1, Table 104.1.2(A)]



where

sh = hoop stress

The Boardman empirical representation of this simply basesthe calculation of pressure stress on some intermediate diame-ter between the inside and outside diameters of the pipe, asfollows:

where

y = 0.4

Simple rearrangement of the above equation, and substitutingSE for sh, leads to the Code equation (3). Furthermore, insidediameterbased formulas add 0.6 times the thickness to the insideradius of the pipe rather than subtract 0.4 times the thickness fromthe outside radius. Thus, the inside diameterbased formula in thepressure vessel Codes and equations (3) and (3A) of the pipingCode are consistent.

A comparison of hoop stress calculated using the Lam equa-tion versus the Boardman equation (3) is provided in Fig. 16.4.2.Remarkably, the deviation of the Boardman equation from theLam equation is less than 1% for D/t ratios greater than 5.1.Thus, the Boardman equation can be directly substituted for themore complex Lam equation.

For thicker wall pipe, ASME B31.1 provides the followingequation for the calculation of the y factor in the definition of y inNote (b) of Table 104.1.2(A). Use of this equation to calculate yresults in equation (3) matching the Lam equation for heavy wallpipe as well.

The factor y depends on temperature. At elevated tempera-tures, when creep effects become significant, creep leads to a

y =d

Do + d

sh = P cDo - 2yt2t dmore even distribution of stress across the pipe wall-thickness.Thus, the factor y increases, leading to a decrease in the calculat-ed required wall-thickness (for a constant allowable stress).

The following additional equation is in ASME B31.1.

(3A)

where

d = inside diameter

Equation (3A) is the same as (3) but with (d + 2t) substitutedfor D and the equation rearranged to keep thickness on the leftside. This equation can provide a different thickness than equation(3) because equation (3A) implicitly assumes that the additionalthickness, A, is on the inside, whereas equation (3A) implicitlyassumes it is on the outside. If it were assumed to be on theinside, there would be an additional P2A added to the numeratorof equation (3A). Alternatively, d could be taken as the insidediameter in the corroded condition.

The thickness of gray and ductile iron pipe in other than steamservice may, as an alternate to equation (3), be determined fromrelevant standards. See para. 104.1.2(B). The thickness in steamservice must be determined using equation (3).

The following additional minimum thickness requirements arespecified to provide added mechanical strength, beyond what isrequired to satisfy burst requirements, in para. 104.1.2(C):

tm =Pd + 2SEA + 2yPA

2[SE + Py - P]

FIG. 16.4.1 STRESS DISTRIBUTION THROUGH PIPEWALL-THICKNESS FROM INTERNAL PRESSURE

FIG.16.4.2 COMPARISON OF LAME AND BOARDMANEQUATIONS


12 Chapter 16

16.4.3 Pressure Design for Straight Pipe UnderExternal Pressure

For straight pipe under external pressure, there is a membranestress check in accordance with equation (3) or (3A) of ASMEB31.1 (the equation for internal pressure) as well as a bucklingcheck in accordance with the external pressure design rules of theB&PV Code Section VIII, Division 1 (paras. UG-28, UG-29, andUG-30).

Flanges, heads, and stiffeners that comply with Section VIII,Division 1, para. UG-29 are considered stiffeners. The lengthbetween stiffeners is the length between such components. Thebuckling pressure is a function of geometry parameters and mate-rial properties.

Buckling pressure calculations in Section VIII, Division 1require first calculation of a parameter A, which is a function ofgeometry, and then a parameter B, which depends on parameterA and a material property curve. The charts that provide the

parameter B account for plasticity that occurs between theproportional limit of the stressstrain curve and the 0.2% offset yieldstress. The chart for determination of parameter A is provided inFig. 16.4.3. A typical chart for B is provided in Fig. 16.4.4.

Two equations are provided for calculating the maximum permis-sible external pressure. The first uses the parameter B, as follows:

where:

parameter from material curves in Section II, Part D,Subpart 3 inside diameter (note that the B&PV Code takes dimen-sions as in the corroded condition) allowable external pressure pressure design thickness

The second equation is for elastic buckling and is necessary to usewhen the value of parameter A falls to the left of the material prop-erty curves that provide parameter B. This equation is as follows:

p =4AE

3

t = p =

D =

B =

p =4B

3D>t

where:

parameter from geometry curves in Section II, Part D,Subpart 3, Fig. G (included herein as Fig. 16.4.3) elastic modulus from material curves in Section II, PartD, Subpart 3.

The second equation is based on elastic buckling, so the elasticmodulus is used. Note that a chart of parameter B could be used,with the linear elastic portion of the curve extended to lowervalues of B, but this would unnecessarily enlarge the charts. Thecharts provided in ASME B31.5 have this form, with the elasticlines extended.

The Section VIII procedures include consideration of the allow-able out-of-roundness in pressure vessels, and use the design mar-gin of 3. While pipe is not generally required to comply with thesame out-of-roundness tolerance as is required for pressure vessels,this has historically been ignored, and has not led to any apparentproblems.

The basis for the Section VIII approach is provided in refs.[2][6].

A new buckling evaluation procedure, provided in Code Case2286, is more relevant to piping as it permits consideration of com-bined loads, including external pressure, axial load, and gross bend-ing moment. It is not presently explicitly recognized in ASMEB31.1, but could be considered as permitted by the Introduction.

16.4.4 Pressure Design of Welded BranchConnections

The pressure design of branch connections is based on a rathersimple approach, although the resulting design calculations arethe most complex of the design-by-formula approaches providedin the Code. A branch connection cuts a hole in the run pipe. Themetal removed is no longer available to carry the forces due tointernal pressure. An area replacement concept is used for thosebranch connections that do not either comply with listed standardsor with certain designs (see para. 16.4.7 herein). The area of metalremoved by cutting the hole, to the extent that it was required forinternal pressure, must be replaced by extra metal in a regionaround the branch connection. This region is within the limits ofreinforcement, defined later.

The simplified design approach is limited to branches where theangle (angle between branch and run pipe axes) is at least 45 deg.

Where the above limitations are not satisfied, the designer isdirected to para. 104.7 (see para. 16.4.15 herein). Alternatives inthat paragraph include proof testing and finite element analysis.

The area A7 is the area of metal removed and is defined as follows:

A7 = (tmh - A)d1(2 - sina)where:

inside centerline longitudinal dimension of the finishedbranch opening in the run of the pipe required minimum thickness of run pipe as determinedfrom equation 3angle between branch and run pipe axes

In this equation, d1 is effectively the largest possible insidediameter of the branch pipe. It is appropriate to use the insidediameter of the pipe in the fully corroded condition.

The angle is used in the evaluation because a lateral connec-tion, a branch connection with an a other than 90 deg., creates alarger hole in the run pipe. This larger hole must be considered in

a =

tmh =

d1 =

E =

A =



FIG. 16.4.3 TYPICAL CHART TO DETERMINE A (Source: Fig. G, Section II, Part D, Subpart 3 of the ASME B&PV Code)


14 Chapter 16

d1. For a lateral, d1 is the branch pipe inside diameter, consideringcorrosionerosion allowance, divided by sin a. The (2 - sina)term in the equation for A7 is used to provide additional reinforce-ment that is considered to be appropriate because of the geometryof the branch connection.

The required minimum thickness, tmh, is the pressure designthickness of the run pipe per equation (3), with one exception. Ifthe run pipe is welded and the branch does not intersect the weld,the weld quality factor E should not be used in calculating thewall-thickness. The weld quality factor only reduces the allowablestress at the location of the weld.

Only the pressure design thickness is used in calculating therequired area since only the pressure design thickness wasrequired to resist internal pressure. Corrosion allowance and milltolerance at the hole are obviously of no consequence.

The area removed, A7, must be replaced by available area aroundthe opening. This area is available from excess wall-thickness thatmay be available in the branch and run pipes as well as added rein-forcement, and the fillet welds that attach the added reinforcement.This metal must be relatively close to the opening of the run pipe toreinforce it. Thus, there are limits, within which any metal areamust be to be considered to reinforce the opening. The areas andnomenclatures are illustrated in Fig. 16.4.5.

The limit of reinforcement along the run pipe, taken as adimension from the centerline of the branch pipe where it inter-sects the run pipe wall is d2, defined as follows:

(However, d2 is not permitted to exceed Dh.) where

allowance (mechanical, corrosion, erosion) outside diameter of header pipe measured or minimum thickness of branch permissibleunder purchase specification

Tb = Dn = A =

d2 = greater of [d1, (Tb - A) + (Th - A) + d1>2]

measured or minimum thickness of header permissibleunder purchase specification half-width or reinforcing zone

The limit of reinforcement along the branch pipe measuredfrom the outside surface of the run pipe is L4. L4 is the lesser of2.5(Th - A) and 2.5(Tb - A) + tr ,

where:

thickness of attached reinforcing pad (when the rein-forcement is not of uniform thickness, it is the height ofthe largest 60 deg. right triangle supported by the run andbranch outside diameter projected surfaces and lyingcompletely within the area of integral reinforcement; seeFig. 16.4.5, Example C)

The reinforcement within this zone is required to exceed A7.This reinforcement consists of excess thickness available in therun pipe (A1); excess thickness available in the branch pipe (A2);additional area in the fillet weld metal, (A3); metal area in ring,pad, or integral reinforcement (A4); and metal in a reinforcingsaddle along the branch (A5). (See Fig. 16.4.5, Example A.) Thesecan be calculated as follows:

A3 is the area provided by deposited weld metal beyond the out-side diameter of the run and branch and for fillet weld attachmentsof rings, pads, and saddles within the limits of reinforcement.

A4 is the area provided by a reinforcing ring, pad, or integralreinforcement.

A5 is the area provided by a saddle on 90 deg. branch connec-tions. See Fig. 16.4.5, Example A.

A2 = 2L4(Tb - tmh)>sin a A1 = (2d2 - d1)(Th - tmh)

tr =

d2 =

Th =

FIG. 16.4.4 TYPICAL CHART TO DETERMINE B (Source: Fig. CS-2, Section II, Part D, Subpart 3 of the ASME B&PV Code)



FIG

.16.

4.5

BR

AN

CH C

ONN

ECTI

ON

NOM

ENCL

ATUR

E [S

ourc

e:AS

ME

B31.

1, F

ig.1

04.3

.1 (D

)]


16 Chapter 16

FIG

.16.

4.5

(CON

TINUE

D)



The area A4 is the area of properly attached reinforcement andthe welds that are within the limits of reinforcement. For the to beconsidered effective, it must be welded to the branch and runpipes. Minimum acceptable weld details are provided in Fig. 127.4.8(D). The ASME B31.1 Code does not require thedesigner to specify branch connection weld size because general-ly acceptable minimum sizes are specified by the Code.Furthermore, the ASME B31.1 Code differs from the B&PVCode in that strength calculations for load paths through the weldjoints are not required.

If metal with a lower allowable stress than the run pipe is usedfor reinforcement, the contributing area of this reinforcementmust be reduced proportionately. No additional area credit is pro-vided for reinforcement materials with a higher allowable stress.

Note that branch connections of small bore pipe by creating asocket or threaded opening in the run pipe wall are permitted withcertain limitations, as stated in 104.3.1(B.3) and (B.4).

16.4.5 Pressure Design of Extruded Outlet Header An extruded outlet header is a branch connection formed by

extrusion, using a die or dies to control the radii of the extrusion.Paragraph 104.3.1(G) provides area-replacement rules for suchconnections; they are applicable for 90 deg. branch connectionswhere the branch pipe centerline intercepts the run pipe center-line,and where there is no additional reinforcement. Figure 16.4.6[ASME B31.1, Fig. 104.3.1(G)] shows the geometry of an extrudedoutlet header.

Extruded outlet headers are subject to minimum and maximumexternal contour radius requirements, depending on the diameterof the branch connection.

A similar area-replacement calculation as described in para.16.4.4 for fabricated branch connections is provided, exceptthat the required replacement area is reduced for smallerbranch-to-run diameter ratios. The replacement area is from addi-tional metal in the branch pipe, additional metal in the run pipe,and additional metal in the extruded outlet lip.

16.4.6 Additional Considerations for BranchConnections Under External Pressure

Branch connections under external pressure are covered in para.104.3.1. The same rules described in paras. 16.4.4 and 16.4.5 aboveare used. However, only one-half of the area described in para.16.4.4, covering welded branch connections, requires replacement.In other words, only one-half of the area A7 requires replacement.Also, the thicknesses used in the calculation are the requiredthicknesses for the external pressure condition.

16.4.7 Branch Connections That Are Presumed to Be Acceptable

Some specific types of branch connections are presumed to beacceptable. This includes fittings listed in Table 126.1 (e.g.,ASME B16.9 tees, MSS SP-97 branch outlet fittings) and the fol-lowing [para. 104.3.1(C)]:

(1) For branch connections NPS 2 or less that do not exceedone-fourth of the nominal diameter of the run pipe, thread-ed or socket welding couplings or half couplings (Class3000 or greater) are presumed to provide sufficient rein-forcement as long as the minimum thickness of the couplingwithin the reinforcement zone is at least as thick as theunthreaded branch pipe.

(2) Small branch connections, NPS 2 or smaller as shown inASME B31.1 Fig. 127.4.8(F) (these are partial penetra-tion weld branch connections for NPS 2 and smallerbranch fittings), provided the thickness of the weld joint(not including the cover fillet) is at least equal to thethickness of schedule 160 pipe of the branch size, areacceptable.

Integrally reinforced fittings and integrally reinforced extrudedoutlets that satisfy the area replacement requirements or are qualifiedby burst or proof tests or calculations substantiated by successfulservice of similar design [para. 104.3.1(D.2.7)] are also acceptable.

16.4.8 Pressure Design of Bends and Elbows Bends are required to have, after bending, a wall-thickness at

least equal to either the required wall-thickness for straight pipein para. 104.1.2(A) (para. 104.2 refers to para. 102.4.5 whichrefers to para. 104.1.2(A), or to satisfy equations 3B and 3C.These equations are based on the Lorenz equation. Paragraph17.4.8, herein, discusses the Lorenz equation, which provides theactual pressure stresses in a pipe bend or elbow.

Because of the bending process, the thickness tends to increasein the intradors, or inside curve of the elbow, and decrease on theextrados, or outside curve of the elbow. ASME B31.1 providesminimum recommended thickness of the pipe, prior to bending, inTable 102.4.5, which, based on experience, results in a pipe thick-ness after bending that is at least equal to the required wall thick-ness of straight pipe.

Elbows in accordance with standards listed in Table 126.1 (e.g.,B16.9 elbows) are acceptable for their rated pressure temperature.

16.4.9 Pressure Design of Miters Miter joints and miter bends are covered by para. 104.3.3.

Miters in a miter bend are either widely spaced or closely spaced.The criteria for closely spaced versus widely spaced are containedin Table D-1. If the following equation is satisfied, the miter isclosely spaced; otherwise it is widely spaced.

where mean radius of pipe chord length between miter joints, taken along pipe centerline one-half angle between adjacent miter axes (see Fig. 16.4.7;the axes are the extension of the line of miter cuts towhere they intercept)

If the miters are widely spaced and the half-angle satisfies thefollowing equation, no further consideration is required. Themiter cut is simply considered to be equivalent to a girth butt-welded joint.

where

nominal wall-thickness of the pipe

The required wall-thickness of other miters depends onwhether they are closely spaced or widely spaced. For closely

tn =

u 6 9Atnr

u = s = r =

s 6 r (1 + tan u)


18 Chapter 16

spaced miter bends, the required pressure design wall-thickness isper the following equation:

where

bend radius of miter bend minimum required thickness for straight pipe tm =

R =

ts = tm 2 - r>R

2(1 - r>R)

For widely spaced miters, the following equation provides therequired pressure design wall-thickness:

This equation must be solved iteratively since the requiredthickness is on both sides of the equation.

There are additional pressure limitations for miters. These are10 psi (70 kPa) and less, above 10 psi (70 kPa) but not exceeding100 psi (700 kPa), and above 100 psi (700 kPa). The above

ts = tm(1 + 0.641r>ts tan u)

FIG. 16.4.6 EXTRUDED OUTLET HEADER NOMENCLATURE [Source: ASME B31.1, Fig. 104.3.1 (G)]



equations can be used for design of the miter bends up to 100 psiunder the following conditions: the thickness is not less thanrequired for straight pipe; the contained fluid is nonflammable,nontoxic, and incompressible, except for gaseous vents to atmos-phere; the number of full pressure cycles is less than 7,000 duringthe expected lifetime of the piping system; and full penetrationwelds are used in joining miter segments.

For above 100 psi, or when the above conditions are notsatisfied, the design is required to be qualified per para. 104.7,with additional qualifications to the para. 104.7 requirements statedin para. 104.3.3(C).

For use up to 100 psi, the following requirements must besatisfied:

(1) Angle must not exceed 22.5 deg. (e.g., 2 cut miter for 90deg bend, minimum).

(2) The minimum length of the miter segment at the crotch (theshortest length in a miter segment), B, must be at least 6tnwhere tn is the pipe nominal wall thickness.

The above two conditions need not be satisfied if the pressureis limited to 10 psi (70 kPa).

16.4.10 Pressure Design of Closures Closures are covered in para. 104.4.1. Components in accor-

dance with standards listed in Table 126.1, such as ASME B16.9pipe caps, can be used for closures within their specified pressure-temperature ratings. The other options provided in ASME B31.1are to either design the closure in accordance with either Section I,PG-31, or to Section VIII, Division 1, UG-34 and UW-13, or toqualify it as an unlisted component in accordance with para.104.7 (see para. 16.4.15 herein).

Openings in closures are covered in para. 104.4.2. Theserequirements are summarized as follows:

(1) If the opening is greater than one-half of the inside diame-ter of the closure, it is required to be designed as a reducerper para. 104.6. While not an ASME B31.1 requirement, theASME B31.3 requirement that if the opening is in a flat clo-sure, it be designed as a flange, is appropriate and should beconsidered.

(2) Small openings and connections using branch connectionfittings that comply with para. 104.3.1(C) (by the referenceto para. 104.3.1) are considered to be inherently adequatelyreinforced.

(3) The required area of reinforcement is the inside diameter ofthe finished opening times the required thickness of the

u

closure. The Section VIII, Division 1 rules that only requireone-half of that area for flat heads are not applicable.

(4) The available area of reinforcement should be calculated perthe rules in ASME B31.1 contained in para. 104.3.1.

(5) Rules for multiple openings follow para. 104.3.1(D.2.5) rulesfor multiple openings (by the reference to para. 104.3.1).

16.4.11 Pressure Design of Flanges (para. 104.5.1) Most flanges are in accordance with standards listed in

Table 126.1, such as ASME B16.5 and, for larger flanges, ASMEB16.47. When a custom flange is required, design by analysis ispermitted by para. 104.5.1. ASME B31.1 refers to the rules forflange design contained in Section VIII, Division 1, Appendix 2,but uses the allowable stresses and temperature limits of ASMEB31.1. In addition, the fabrication, assembly, inspection, and test-ing requirements of ASME B31.1 are governing.

16.4.12 Pressure Design of Blind Flanges (para. 104.5.2)

Most blind flanges are in accordance with standards listed inTable 126.1, such as ASME B16.5. When designing a blindflange, the rules of Section I for bolted flat cover plates are applic-able (these are contained in PG-31). Additionally, the ASMEB31.1 design pressure and allowable stresses are to be used.

16.4.13 Pressure Design of Blanks Blanks are flat plates that get sandwiched between flanges to

block flow. A design equation for permanent blanks is provided inpara. 104.5.3, as follows:

(7)

where

inside diameter of gasket for raised or flat-face flanges, orthe gasket pitch diameter for retained, gasketed flanges

Other terms are as defined in para. 16.4.2 herein. Mechanical and corrosionerosion allowances must be added

to the pressure design thickness calculated from equation (7). Blanks used for test purposes are required to be designed per

the foregoing equation, except that the test pressure is used andSE may be taken, if the test fluid is incompressible (e.g., not apneumatic test), at 95% of the specified minimum yield strengthof the blank material.

16.4.14 Pressure Design of Reducers Most reducers in piping systems are in accordance with the

standards listed in Table 126.1. This is the only provision forreducers in para. 104.6 (which is not helpful when one is referredfrom para. 104.4.2 to this paragraph for large-diameter openingsin closures). However, pressure design per 104.7 is also an option.

16.4.15 Specially Designed Components If a component is not in accordance with a standard listed in

Table 126.1, and the design rules provided elsewhere in para. 104are not applicable, para. 104.7.2 is applicable. This paragraphrequires that some calculations be done in accordance with thedesign criteria provided by the Code and be substantiated by one

d6 =

t = d6A3p

16 SE

FIG. 16.4.7 ILLUSTRATION OF MITER BEND SHOWINGNOMENCLATURE (Source: ASME B31.1, Table D-1)


20 Chapter 16

of several methods. The most important element of this paragraph isconsidered to be the substantiation; the aforementioned calculationsare not generally given much consideration. The methods to verifythe pressure design include the following: (Note that this paragraphwas substantially changed in the 1999 addenda, including the addi-tion of detailed stress analysis as an option for substantiation.

(1) Extensive, successful service experience under comparableconditions with similarly proportioned components of thesame or like material.

(2) Experimental stress analysis, such as described in theB&PV Code Section VIII, Division 2, Appendix 6.

(3) A proof test conducted in accordance with ASME B16.9,MSS SP-97, or Section I, A-22. The option for witnessingby the Authorized Inspector was removed in the 1999addenda. This prior provision was not necessarily practicaland could create difficulty for the manufacturer, since prooftests may be conducted to qualify a line of components wellbefore being sold for any specific piping system.

(4) Detailed stress analysis (e.g., finite element method) withresults evaluated in accordance with Section VIII, Division 2,Appendix 4 except the basic allowable stress fromAppendix A is required to be used in place of Sm. These arethe design-by-analysis rules in the B&PV Code.

Of the above, the methods normally used to qualify new unlist-ed components are proof testing and detailed stress analysis.

It should be noted that the Code permits interpolation betweensizes, wall-thicknesses, and pressure classes, and also permitsanalogies among related materials. Extrapolation is not permitted.

The issue of how to determine that the above has been done ina satisfactory manner is addressed in the 1999 addenda. Earliereditions of the Code only provided for witnessing of the proof testfor boiler external piping. However, this is not practical when themanufacturer performs proof tests to qualify a line of piping com-ponents. Obviously, all the potential future Authorized Inspectorscould not be gathered for this event. Furthermore, the other meth-ods are of at least equal concern, and their review may be moreappropriately done by an engineer rather than an Inspector. As aresult of these concerns, the requirement was added that docu-mentation showing compliance with the above means of pressuredesign verification must be available for the Owners approvaland, for boiler external piping, available for the AuthorizedInspectors review. The Owners review could be done by anInspector or some other qualified individual.

While MSS SP-97 and ASME B16.9 provide a clear approachfor determining that the rating of a component is equivalent orbetter to matching straight pipe, they do not provide defined pro-cedures for determining a rating for a component that may have aunique rating, which may differ from matching straight pipe. Theprocedure generally used here is to establish a pressure-temperaturerating by multiplying the proof pressure by the ratio of the allow-able stress for the test specimen to the actual tensile strength ofthe test specimen. In the proposed ASME B31H Standard, thiswould be reduced by a testing factor depending on the number oftests. An example of this approach is provided in ref. [7].

The proposed standard ASME B31H, Standard Method toEstablish Maximum Allowable Design Pressure for PipingComponents, is under development by the ASME and will even-tually add to or replace the existing proof test alternatives in para.104.7.2. This standard provides procedures to either determine ifa component has a pressure capacity at least as great as a matching

straight pipe, or to determine a pressure-temperature rating for acomponent.

16.4.16 References 1. Boardman, H.C., Formulas for the Design of Cylindrical and

Spherical Shells to Withstand Uniform Internal Pressure, The WaterTower, Vol. 30, 1943.

2. Bergman, E. O., The New-Type Code Chart for the Design of VesselsUnder External Pressure, Pressure Vessel and Piping Design,Collected Papers 19271959, The American Society of MechanicalEngineers, 1960, pp. 647654.

3. Holt, M., A Procedure for Determining the Allowable Out-of-Roundness for Vessels Under External Pressure, Pressure Vessel andPiping Design, Collected Papers 19271959, The American Societyof Mechanical Engineers, 1960, pp. 655660.

4. Saunders, H. E., and Windenburg, D., Strength of Thin CylindricalShells Under External Pressure, Pressure Vessel and Piping Design,Collected Papers 19271959, The American Society of MechanicalEngineers, 1960, pp. 600611.

5. Windenburg, D., and Trilling, C., Collapse by Instability of ThinCylindrical Shells Under External Pressure, Pressure Vessel andPiping Design, Collected Papers 19271959, The American Societyof Mechanical Engineers, 1960, pp. 612624.

6. Windenburg, D., Vessels Under External Pressure: Theoretical andEmpirical Equations Represented in Rules for the Construction ofUnfired Pressure Vessels Subjected to External Pressure, PressureVessel and Piping Design, Collected Papers 19271959, TheAmerican Society of Mechanical Engineers, 1960, pp. 625632.

7. Biersteker, M., Dietemann, C., Sareshwala, S., and Haupt, R. W.,Qualification of Nonstandard Piping Product Form for ASME Codefor Pressure Piping, B31 Applications, Codes and Standards andApplications for Design and Analysis of Pressure Vessels and PipingComponents, PVP-Vol. 210-1, The American Society of MechanicalEngineers, 1991.

ASME B1.20.1, Pipe Threads, General Purpose (Inch); The AmericanSociety of Mechanical Engineers.

ASME B16.5, Pipe Flanges and Flanged Fittings; The American Societyof Mechanical Engineers.

ASME B16.9, Factory-Made Wrought Steel Butt-Welding Fittings; TheAmerican Society of Mechanical Engineers.

ASME B16.47, Large-Diameter Steel Flanges: NPS 26 through NPS 60;The American Society of Mechanical Engineers.

ASME B31.1, Power Piping; The American Society of MechanicalEngineers.

ASME B31.3, Process Piping; The American Society of MechanicalEngineers.

ASME B31.5, Refrigeration Piping; The American Society of MechanicalEngineers.

ASME B31H, Standard Method to Establish Maximum Allowable DesignPressures for Piping Components; The American Society of MechanicalEngineers (to be published). ASME Boiler and Pressure Vessel Code Section I, Power Boilers; TheAmerican Society of Mechanical Engineers.

ASME Boiler and Pressure Vessel Code Section II, Part D, Materials,Properties; The American Society of Mechanical Engineers.

ASME Boiler and Pressure Vessel Code Section VIII, Division 1, PressureVessels; The American Society of Mechanical Engineers.

ASME Boiler and Pressure Vessel Code Section VIII, Divisions 1 and 2,Code Case 2286, Alternative Rules for Determining Allowable



Compressive Stresses for Cylinders, Cones, Spheres, and Formed Heads;The American Society of Mechanical Engineers.

MSS SP-97, Integrally Reinforced Forged Branch Outlet FittingsSocketWelding, Threaded, and Butt-Welding Ends; The ManufacturersStandardization Society of the Valve and Fittings Industry, Inc.

16.5 LIMITATIONS ON COMPONENTSAND JOINTS

16.5.1 Overview ASME B31.1 includes limitations on components and joints in

the design chapter, Chapter II. These are contained in Part 3,Selection and Limitations of Piping Components; and in Part 4,Selection and Limitations of Piping Joints. This section (16.5)combines the limitations with pressure design and other consider-ations, on a component-by-component basis.

16.5.2 Valves Most valves in ASME B31.1 piping systems are in accordance

with standards listed in Table 126.1. These standards include thefollowing:

(1) ASME B16.10, Face-to-Face and End-to-End Dimensionsof Valves

(2) ASME B16.34, ValvesFlanged, Threaded, and WeldingEnd

(3) AWWA C500, Metal-Seated Gate Valves for Water SupplyService (with limitation regarding stem retention)

(4) AWWA C504, Rubber-Seated Butterfly Valves (5) MSS SP-42, Class 150 Corrosion-Resistant Gate, Globe,

Angle, and Check Valves With Flanged and Butt-WeldedEnds (with limitation regarding stem retention)

(6) MSS SP-67, Butterfly Valves (with limitation regardingstem retention)

(7) MSS SP-80, Bronze Gate, Globe, Angle and Check Valves Listed valves are accepted for their specified pressure ratings.

Valves that are not in accordance with one of the listed standardscan be accepted as unlisted components in accordance with para.102.2.2. The pressure-temperature rating for such valves shouldbe established in accordance with para. 104.7.2. The manufacturersrecommended rating is not permitted to be exceeded.

Additional requirements are provided in para. 107. Theseinclude the following:

(1) requirements for marking (para. 107.2); (2) requirement for use of outside screw threads for valves NPS

3 (DN 75) and larger for pressure above 600 psi (4,150 kPa)(para. 107.3);

(3) prohibition of threaded bonnet joints where the seal dependson the thread tightness for steam service at pressure above250 psi (1,750 kPa) (para. 107.5); and

(4) requirements for bypasses (para. 107.6). Additional requirements for valves in boiler external piping

(steam-stop valves, feedwater valves, blowoff valves, and safetyvalves) are provided in para. 122.1.7.

Requirements for safety-relief valves for ASME B31.1 pipingare also covered in para. 107.8. Safety-relief valves on boilerexternal piping are required to be in a accordance with Section I(by reference to para. 122.1.7(D.1). Safety-relief valves for non-boiler external piping are required to be in accordance with

Section VIII, Division 1, paras. UG-126 through UG-133. Anexception for valves wit set pressures 15 psig (100 kPa (gage))and lower is that ASME Code Stamp and capacity certificationare not required. Safety-relief valves for nonboiler external reheatpiping are required to be in accordance with Section I, PG-67through PG-73.

Appendix II provides nonmandatory rules for the design ofsafety-valve installations.

For piping containing toxic fluids [para. 122.8.2(D)], steelvalves are required, and bonnet joints with tapered threads areprohibited. Also, special consideration should be given to valvedesign to prevent stem leakage. Permitted bonnet joints includeunion, flanged with at least four bolts; proprietary, attached bybolts, lugs, or other substantial means, and having a design thatincreases gasket compression as fluid pressure increases; orthreaded with straight threads of sufficient strength, with metal-to-metal seats and a seal weld.

16.5.3 Flanges Most flanges in ASME B31.1 piping systems are in accordance

with listed standards. These listed standards include the following:

(1) ANSI B16.1, Cast Iron Pipe Flanges and Flanged Fittings (2) ASME B16.5, Pipe Flanges and Flanged Fittings (3) ASME B16.24, Cast Copper Alloy Pipe Flanges and Flanged

Fittings Class 150, 300, 400, 600, 900, 1500, and 2500 (4) ASME B16.42, Ductile Iron Pipe Flanges and Flanged

Fittings, Classes 150 and 300 (5) ASME B16.47, Large Diameter Steel Flanges, NPS 26

through NPS 60 (6) AWWA C115, Flanged Ductile-Iron Pipe with Threaded

Flanges (7) AWWA C207, Steel Pipe Flanges for Water Works Service,

Sizes 4 Inch through 144 Inch (100 mm through 3,600 mm) (8) MSS SP-51, Class 150LW Corrosion-Resistant Cast

Flanges and Flanged Fittings (9) MSS SP-106, Cast Copper Alloy Flanges and Flanged

Fittings, Class 125, 150, and 300

Flanges that are listed in Table 126.1 are accepted for theirspecified pressure ratings. Flanges that are not in accordance withone of the listed standards can be designed using the rules ofSection VIII, Division 1, Appendix 2, with appropriate allowablestress and design pressure (see para. 104.5.1), or qualified usingpara. 104.7.2. ASME B31.1 states that the Section VIII rules arenot applicable when the gasket extends beyond the bolt circle,which is also a limitation stated in Section VIII.

Paragraphs 104.5, 108, 112, and 122.1.1 provide additionalrequirements for flanges, including the following:

(1) ASME B16.5 slip-on flanges are not permitted for higherthan Class 300 flanges [para. 104.5.1(A)].

(2) When bolting Class 150 steel flanges to matching cast ironflanges, the steel flange is required to be flat face to preventoverloading the cast iron flange (para. 108.3). Use of full-face gaskets with flat-face flanges helps the flange resistrotation from the bolt load.

(3) Class 250 cast iron are permitted to be used with raised-faceClass 300 steel flanges (para. 108.3).

(4) Table 112 provides detailed requirements for flange bolt-ing, facing, and gaskets. These depend on f

Date post:	24-Nov-2015
Category:	Documents
Upload:	luis-santana
View:	351 times
Download:	19 times

ASME B31_1 Power Piping

Documents