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CHAPTER

9

9.1 INTRODUCTION

This chapter provides a summary of some of the more signifi-

cant requirements of the ASME Code Section III, SubsectionNE, and a commentary on such requirements. The comments on

and interpretation of the rules are strictly the opinions of the

author and are not to be considered as official ASME Code

Committee interpretations. These opinions are based on several

years of experience in design, analysis, and construction of

containment vessels and participation in various ASME Code

Committees.

The Code requirements cited may be simplified and abbreviated

in this write-up and may not reflect all details of the rules. The

applicable edition of the Code must be consulted for actual appli-

cations. Some comparisons with the rules of Section VIII are

included for information. The analysis procedures are not dealt

with in any great detail, since they are similar to those of

Subsection NB and Section VIII, Division 2 prior to the 2007

Edition. More emphasis has been placed on the unique features ofSubsection NE.

A number of Code Cases and references that pertain to the

rules of this Subsection have been cited. Again, only the points

significant to the item of discussion have been included here. For a

complete understanding, the entire reference should be consulted.

This chapter is based on the 2007 Edition of the Code.

9.2 SCOPE OF SUBSECTION NE

Subsection NE, organized into articles similar to those of other

subsections of Section III, establishes rules for material, design,

fabrication, examination, inspection, testing, overpressure protec-

tion, and documentation of metal containment vessels . Metal

Containments are those vessels with the metal (usually steel)

shell resisting the entire pressure without any help from backingmaterials such as concrete. The metal liners of concrete vessels,

which perform a leak tightness function, are not covered by this

subsection (they are covered by Section III, Division 2). The rules

are for new construction only and do not address service deterio-

ration. It should be pointed out that the rules of Subsection NE are

supplemented by the rules of Subsection NCA, and that the manu-

facturer of a Class MC vessel must be aware of Subsection NCA

requirements as well as the Section III Appendices, Section II,

Section V, and Section IX.

The scope of Subsection NE is specified in paragraph

NE-1100.

This paragraph requires that a vessel classified as Class MC

must be constructed in accordance with the rules of Subsection

NE, with one exception. The exception is provided in NCA-2134,

Optional Use of Code Classes. NCA-2134(c) allows the con-

struction and stamping of a containment vessel, classified by the

Design Specifications as a Class MC vessel, in accordance to the

rules of Subsection NB, provided that the rules of NE-7000 are

applied in lieu of the rules of NB-7000 for protection against

overpressure. This is based on the fact that although the rules of

Subsection NB for Class 1 vessels are considered to be at least as

safe as the rules of Subsection NE, the overpressure protection

requirements of different classes of vessels need to be different

because of the varied functions that they serve.

Class MC may be applied only to containment vessels and their

appurtenances. This class of construction is not applicable to pip-

ing, pumps, and valves that may be a part of the containment sys-

tem. Such piping, pumps, and valves must be classified by theDesign Specifications as Class 1 or Class 2 and constructed to the

rules of Subsections NB or NC, respectively. Figure NE-1120-1

provides some guidelines for classification of typical containment

penetrations.

Like all other Sections of the ASME Code, Subsection NE rec-

ognizes that all different details of construction cannot be cov-

ered. For those cases for which no rules are provided, the

Certificate Holder is responsible to provide details of construction

that are consistent with those provided by the rules of this

Subsection. This is similar to the provisions of the commonly

used paragraph U-2(g) of Section VIII, Division 1. However,

Subsection NE requires that such details be approved by the

owner or his designee and be accepted by the Inspector.

9.3 BOUNDARIES OF JURISDICTIONOF SUBSECTION NE

The boundaries of jurisdiction of Subsection NE are defined in

paragraph NE-1130. The total containment system comprises the

following:

(1) the containment vessel;

(2) penetrations and appurtenances attached to the vessel; and

(3) piping, pumps, and valves

SUBSECTION NE CLASS

MC COMPONENTS

Kamran Mokhtarian and Roger F. Reddy

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2 Chapter 9

The Design Specifications must define the boundaries of the con-

tainment vessel and the classification of each of the items in the

preceding list. Figure NE-1132-1 (reproduced here as Fig. 9.1)

provides some guidelines for specifying jurisdictional boundaries.

The jurisdiction of Subsection NE shall, as a minimum, extend

to the following points:

(1) for welded connections, the first circumferential joint

excluding the connecting weld;

(2) for bolted connections, the face of first flange and excluding

the bolting; and

(3) for screwed connections, the first threaded joint.

The limits in the preceding list are minimums and could be

extended to points further away from the vessel shell by the

Design Specifications.The Code-specified boundaries between the containment vessel

and the attachments depend on the types of attachments.

FIG.9.1 SOME TYPICAL JURISDICTIONAL BOUNDARIES FOR WELDED CONNECTIONS, CLASS MC CONTAINMENT VESSELS


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COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 3

For this purpose, the following four types of attachments are

defined:

(1) pressure-retaining attachments, such as stiffeners and open-

ing reinforcement;

(2) non-pressure retaining attachments, such as thermal sleeves,

supports, and brackets;(3) structural attachments, which perform either a pressure-

retaining function or a support function.

(4) nonstructural attachments, which perform neither pressure-

retaining nor support functions.

Figure NE-1132.2-1, NE-1132.2-2, and NE-1132.2-3 (repro-

duced here as Figs. 9.2, 9.3, and 9.4) are provided to aid in

defining the boundary and construction requirements. Some of

the more significant requirements provided by these figures areas follows.

FIG. 9.2 ATTACHMENTS IN THE CONTAINMENT VESSEL SUPPORT LOAD PATH THAT DO NOT PERFORM A

PRESSURE-RETAINING FUNCTION


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4 Chapter 9

(1) Attachments cast or forged with the vessel or weld buildupare a part of the vessel.

(2) Attachments having a pressure-retaining function are a part

of the vessel.

(3) Within 2tfrom the pressure-retaining portion of the vessel,

the first connecting weld of a non-pressure-retaining struc-

tural attachment to the vessel is a part of the vessel. Beyond

2t, the first weld is a part of the attachment.

(4) The first connecting weld of a non-structural attachment to

the vessel is part of the attachment.

(5) Except in items (3) and (4) above, for a nonpressure-retainingattachment, the vessel boundary is at the surface of the

vessel.

(6) Mechanical fasteners connecting nonpressure-retaining

attachments are part of the attachment.

Again, the vessel boundary may be specified by the Design

Specifications to extend further than the minimums defined in the

preceding list.

FIG. 9.3 ATTACHMENTS THAT DO NOT PERFORM A PRESSURE-RETAINING FUNCTION AND ARE NOT IN THE CONTAIN-

MENT VESSEL SUPPORT LOAD PATH (NONSTRUCTURAL ATTACHMENTS)


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9.4 GENERAL MATERIALREQUIREMENTS

The material requirements for Class MC vessels are specified

in article NE-2000. Additional requirements regarding materials

and materials suppliers are included in Subsection NCA. Metallic

materials used for Class MC construction must be to a SA, SB, or

SFA specification (these specifications are included in Section IIof the ASME Code). Rules for manufacturing, identifying, and

certifying these materials are provided in Subsection NCA.

Tubular products and fittings, which are welded with filler metal,

must be stamped with the nuclear part (NPT) symbol, but name-

plates are not required. Metallic materials produced under an

ASTM designation may be accepted, provided that the material

has been manufactured to an ASTM specification date identified as

acceptable in the applicable Edition of Section II. Likewise, weld-

ing material produced to an AWS designation may be accepted,

provided that the AWS specification is indicated to be identical

with the corresponding ASME specification. If a material does

not have an ASME specification, it may not be used for Class MC

construction unless permitted by a Section III Code Case. The

materials that are allowed for class MC construction are listed in

Table NE-2121(a)-1. This Table is continuously updated to add

new materials or delete materials no longer permitted.

Paragraph NCA-3800 of Subsection NCA contains extensiverequirements regarding certification and marking of materials and

certification of Material Organizations. The term Material

Organization was introduced in the 1994 Addenda to refer to orga-

nizations such as Material Manufacturers and Material Suppliers

that must be accredited by obtaining a Quality System Certificate.

(For definitions of terms, see NCA- 9000.) These quality system

requirements are significantly more extensive than those for other

Sections of the ASME Code and are one of the major contributors

to the high cost of nuclear components. The Material Organizations

FIG. 9.4 ATTACHMENTS THAT PERFORM A PRESSURE-RETAINING FUNCTION


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may be accredited by ASME or by the Certificate Holder. The

Material Organization is responsible for establishing, documenting,

implementing, and maintaining a Quality System Program in

accordance with NCA-3850. Material traceability requirements

are specified in NCA-3856; Material Certification requirements

are described in NCA-3860. The Material Organization must pro-

vide a Certified Material Test Report (CMTR) or a Certificate of

Compliance (COC) at the time of material shipment. A COC may

be provided in lieu of a CMTR only for material in. and less

nominal pipe size and for bolting 1 in. and less. Any material

allowed to be supplied with a COC and all materials defined as

small products are exempt from the quality system requirements

of NCA-3800. Small products are defined as

(1) pipe, tubes, fittings, and flanges 2 in. nominal pipe size and

less;

(2) bolting materials 1 in. nominal diameter and less; and

(3) bars with a nominal cross sectional area of 1 in.2 and less.

The material certification requirements of this Subsection are

generally more restrictive than those of all other Sections of the

ASME Code.

A rather limited number of materials are permitted for use inSubsection NE. These materials are listed in Tables NE-2121(a)-1

and NE-2121(a)-2. Although these tables include a number of

high-alloy materials, containment vessels are usually made of

carbon or low-alloy steels. In the past the most commonly used

materials were SA-516, Grade 70 plate; SA-105 forging; SA-106

piping; and SA-193 bolting. There is usually no need for high-

alloy and nonferritic materials or for high-strength materials in

the construction of containment vessels. Normalized SA-516

material, has the desirable combination of strength and toughness.

Because most containment vessels are inside buildings the tough-

ness requirements can easily be met. Unless the material is impact

tested, pressure-retaining material of ferritic steel with thickness

greater than in. must be normalized or quenched and tempered,

fully killed, and melted to a fine grain melting practice.

Any examination, test, or treatment required by both the MaterialSpecifications and Subsection NE need to be performed only once.

Any of these operations may be performed by the Certificate Holder,

but if any operations beyond those of the Material Specifications

are to be performed by the Material Organization, they must be

specified by the Certificate Holder in his/her purchase order.

Requirements pertaining to material test coupons for ferritic steels

are given in NE-2200. When such steel is subjected to heat treatment

during fabrication or installation of a containment vessel, the material

used for the tensile and impact test specimens shall be heat treated in

the same manner as the vessel. (The only exception to this rule is for

test specimens of P-No. 1 materials with a nominal thickness of 2 in.

or less.) The Certificate Holder must provide the Material

Organization with the temperature and heating/cooling rates to be

used. In the case of postweld heat treatment, the total time at temper-

ature for the test material must not be less than 80% of the total time

at temperature during actual post-weld heat treatment (PWHT) of the

material. Any PWHT time that is anticipated to be applied to

the material after the vessel has been stamped must be specified by

the owner in the Design Specification. The Certificate Holder, in turn,

is required to specify this additional time to the Material

Organization. When ferritic material is subjected to quenching from

the austenitizing temperature, the test coupons representing those

materials shall be cooled at a rate similar to but no faster than the

main body of the material, with some minor exceptions. Using ther-

mal barriers or insulation to achieve this may be necessary.

14

3

4

All welding materials used in the construction of Class MC

containment vessels must conform to the requirements of the

welding material specification. In addition, welding materials

must meet all the requirements of Subsection NE that are related

to these materials. One such requirement is the identification of

welding materials. Such materials must be controlled during fab-

rication so that they are identifiable as acceptable until the mate-

rial is actually consumed in the process. For the testing of welding

material, the Certificate Holder must provide the following infor-

mation:

(1) the welding process;

(2) the SFA specification and classification;

(3) the minimum tensile strength in the as-welded and/or heat-

treated condition;

(4) the drop weight testing requirements;

(5) the Charpy V-notch testing requirements;

(6) the preheat and interpass temperatures for welding of test

coupons;

(7) the postweld heat treatment requirements;

(8) the elements for which chemical analysis is required; and

(9) the minimum delta ferrite.

Detailed requirements are specified for testing of weld material

that go far beyond those required by other Sections of the ASME

Code NE-2420 and NE-2430 must be thoroughly studied to

assure that all requirements are complied with.

Rules for examination and repair of pressure retaining material

are provided in paragraph NE-2530. Examinations required by the

Material Specifications must be performed at the time of material

manufacture. Surface defects must be removed by grinding or

machining. After defect elimination, the depression must be

blended uniformly into the surrounding surfaces to minimize

stress concentration. If the elimination of the defect reduces the

thickness below the minimum required thickness, the material

must be repaired by welding. Such material repair may be per-

formed by the Material Organization provided the depth of the

repair cavity does not exceed one-third of the nominal thicknessand that the prior approval of the Certificate Holder has been

obtained. The welding procedure and welders must be qualified in

accordance with the rules of NE-4000 and Section IX. Each repair

weld must be examined by using the magnetic-particle or liquid-

penetrant method. In addition, when the depth of the repair cavity

exceeds certain limits, the repair weld must be radiographically

examined. Those repairs that are required to be radiographically

examined must be described in a Certified Material Test Report.

Normally, manufacturers perform all weld repairs themselves and

do not allow such repairs to be made by the Material Organization.

9.5 CERTIFIED MATERIAL TESTREPORTS

Appendix P provides recommendations for the contents ofCertified Material Test Reports (CMTR). Although this is a Non-

Mandatory Appendix, complying with it is generally expected.

The materials requirements may change with each edition or

addenda of the Code, and the purchaser must identify the applica-

ble document. For metallic materials, the following information is

recommended to be shown in the CMTRs:

Name of certifying organization.

Number and expiration date of the organizations Certificate

of Authorization.


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Purchasers order or contract number.

Description of material including specification number,

grade, class, and type.

Description of material identification marking.

Actual results of chemical analyses, tests, and examinations

required.

Reports of weld repairs performed.

Charpy V-notch and drop weight test results.

Nondestructive examinations performed and accepted.

Under certain circumstances, additional information, such as

heat treatment data, hydrostatic test data, ferrite number, and

grain size, also may have to be included in the CMTR. It is not

required that all the Material Specifications and Code require-

ments be performed by the same organization. When treatments

and tests are performed by different organizations, the Certificate

Holder must ensure that each organization providing services be

identified on the CMTR along with the activities for which that

organization is responsible. Alternatively, CMTRs may be provided

by the other organizations for the services they perform; these

should be referenced on and attached to the CMTR furnished by

the material supplier. All CMTRs must include a dated statementaffirming that the contents of the report are correct and accurate;

however, a CMTR is not required to be signed or notarized, which

makes the electronic submittal of such reports possible.

9.6 MATERIAL TOUGHNESSREQUIREMENTS

All pressure-retaining materials must be impact tested unless

exempted by the provisions of this subsection. The following are

some of the provisions of this subsection. The following are some

of the exemption provided:

(1) Materials with nominal thickness of in. or less.

(2) Bolting with a nominal size of 1 in. or less.

(3) Bars with a nominal cross-sectional area of 1 in.2 or less.

58

(4) Pipe, tube, and fittings with a nominal pipe size 6 in.diame-

ter or less.

(5) Austenitic stainless steels.

(6) Nonferrous materials.

(7) Materials for which TNDT is lower than the Lowest Service

Metal Temperature (LSMT) at least by an amount estab-

lished by Appendix R of the Appendices to Section III.

(8) Materials with LSMT exceeding 150F.

The LSMT, which is required to be specified by the Design

Specifications, is defined as the lowest temperature that the metal

may experience in service while a plant is in operation. This

exemption does not apply to the weld metal or the welding proce-

dure qualification. The TNDT is the temperature at or above the nil-

ductility transition temperature and is 10F below the temperature

at which at least two specimens show no-break performance. The

values of TNDT for some commonly used materials are listed in

Table NE-2311(a)-1. The values listed in this table were obtained

from data on heavy section steels and are conservative for the

range of thicknesses commonly used for containment vessels.

Appendix R provides rules for determining the permissible

lowest service metal temperatures for materials, relative to the nil-ductility transition temperatures. Figure R-1200-1 (reproduced

here as Fig. 9.5) provides the values for factor A as a function of

material thickness. The permissible LSMT is obtained by adding

this factor to the nil-ductility transition temperature for the mater-

ial. For the material to be acceptable, the permissible LSMT must

not be higher than the specified lowest service metal temperature.

Non-Mandatory Appendix G presents a procedure for obtaining

the allowable loadings for ferritic pressure-retaining materials to

protect against nonductile failure. This procedure is based on

principles of linear elastic fracture mechanics. To apply this pro-

cedure, a maximum postulated flaw has to be assumed at each

location. The postulated flaw must be consistent with the NDE

method and acceptance criteria used at that location. The mode I

stress intensity factors for various loadings are then calculated at

each of the points under consideration. Such factors are then

FIG. 9.5 DETERMINATION OF PERMISSIBLE LSMT (LOWEST SERVICE METAL TEMPERATURE)


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added, and the sum is compared with the critical (reference) stress

intensity value, which is a function of the material and tempera-

ture at the point. Different procedures are recommended for dif-

ferent components and operating conditions. It should be noted

that the term stress intensity factor, as used in fracture mechanics,

has a meaning totally different from the term stress intensity used

for stress evaluations.

Figure G-2210-1 (reproduced here as Fig. 9.6) provides a con-

servative relationship between the critical stress intensity factor

and the difference between the temperature and the nil-ductility

temperature of the material. This curve is based on the lower

bound of test results for a number of commonly used ferritic

steels. As expected, the values of this factor increase with increas-

ing temperature, and the rate of increase becomes very significant

for temperatures that exceed the nil-ductility temperature by more

than 100F or 120F. It is generally assumed that at these tempera-

tures, nonductile failure is not of concern. For a temperature equal

to the NDT temperature, the given value of the stress intensity

factor is 40 ksi; this value drops gradually with colder tempera-

tures, approaching an asymptotic value of about 25. The values

obtained from this curve may be used for ferritic steels with spec-

ified minimum yield strength at room temperature not exceeding50 ksi. For other materials, or whenever the designer chooses, the

actual values for a certain material may be obtained and used

from the test.

The postulated defect used to derive the recommended proce-

dure of Appendix G is a sharp surface defect normal to the direc-

tion of maximum stress. Note that only tensile stresses are of any

significance in brittle fracture, and compressive stresses need not

be considered. The assumed flaw, for thicknesses in the range of

thicknesses between 4 in. and 12 in., has a depth of one-fourth the

section thickness and a length of 1.5 times the section thickness.

For greater thicknesses, the postulated defect for the 12 in. thick

section is used. For sections less than 4 in. thick, the 1 in. deep

defect is conservatively postulated. If the designer chooses, smaller

defects may be used on an individual basis if such smaller defect

can be ensured.

If impact testing is required, paragraphs NE-2320 and NE-2330

provide test procedures and acceptance criteria. Two options are

provided for impact testing of pressure boundary materials. The

first option is Charpy V-notch testing at or below the LSMT.

The second option is drop weight testing to show that the LSMT is

satisfied in accordance with the rules of Appendix R. Materials

may be used at temperatures colder than those established by

either test method, provided that their use is justified by an

Appendix G analysis or equivalent. This provision is similar to

Section VIII provisions that allow the operation of a vessel at tem-

peratures colder than the Minimum Design etal Temperature

(MDMT) provided that the stresses are low enough to meet the

specified rules. If the Design Specifications require a vessel hydro-

static or pneumatic test temperature that is lower than the LSMT,

the impact testing for pressure-retaining materials must be per-

formed at or below the lowest specified vessel test temperature.

The user also has the option of specifying a Lowest OverpressureTest Metal Temperature, which is defined as the lowest tempera-

ture the metal may experience during the overpressure test. In this

case, in addition to the tests required for the operating conditions,

Charpy V-notch testing must be performed at a temperature not

more than 30F below the Lowest Overpressure Test Metal

Temperature. The specific test methods and acceptance standards

for these tests are different from those for tests based on LSMT

(See NE-2333.)

The test temperature and the results of all impact testing must

be reported in the CMTR. For Charpy V-notch testing, the results

FIG. 9.6


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must include both the lateral expansion and absorbed energy. The

impact test specimens must be removed from a depth within the

material that is at least as far from the material surface as that

specified for tensile test specimens in the material specification.

The orientation of specimens for drop weight tests may be in any

direction. For Charpy V-notch specimens the orientation of speci-

mens should be in accordance with SA-370, except for quenched

and tempered materials. Specific requirements for quenched and

tempered materials regarding the orientation of the specimens are

provided in paragraph NE-2220.

It should be noted that the specific test methods and acceptance

standards specified for tests based on the LSMT and those speci-

fied for tests based on Lowest Overpressure Test Metal

Temperature are different. For the test case, only Charpy V-notch

testing is required, and the required average energy value for any

thickness is either 15 ft. lb. or 20 ft. lb., depending on the speci-

fied minimum tensile strength. For the service conditions, the

requirements are more refined and more restrictive. The impact

test requirements of Article NE-2000 must be used in conjunction

with the test requirements of Article NE-4000. The requirements

for impact testing of welds and heat-affected zones are explained

in the fabrication rules. It has long been recognized that thewelding process causes a reduction in the toughness of the heat-

affected zone. Vessel manufacturers used to have their own rules

for ensuring that the HAZ meets the material toughness require-

ments for qualifying the weld procedure. The 2002 Addenda to

NE-4335 provides rules for compensating for the toughness

decrease during welding. Three different methods are offered,

which may be used individually or in combination.

9.7 GENERAL DESIGN REQUIREMENTS

Design and analysis requirements are contained in Article

NE-3000. The loadings that must be taken into account for design

are essentially the same as those listed in Section III for Classes 1,

2 and 3 pressure vessels with the addition of reactions to steam

and water jet impingement. Jet impingement is, of course, one of

the major loadings to which a containment vessel will be subjected

if an accidental pipe break occurs.

The allowable stress value used for design-by-formula analysis

and the basic allowable stress intensity value used for design-

by-stress analysis (with the exception of that used for limiting

secondary stress intensity) are each 1.1 times the allowables for

Section VIII, Division 1 vessels given in Tables 1A and 1B of

Section II, Part D. For the limit on secondary stress intensity, the

basic allowable is the same as that for Section III, Class 1 compo-

nents given in Tables 2A and 2B of Section II, Part D. Up to the

1998 edition (which includes the 1998 Addenda), the stress allow-

ables for temperatures below the creep range, listed in Tables 1A

and 1B, essentially have been based on the smaller of two-thirds

of the specified minimum yield strength and one-quarter of the

specified minimum ultimate strength. (Other factors are involved;for details, see Appendix 1 of Section II, Part D.) With the 1999

addenda, the design margin on tensile strength was reduced from

4.0 to 3.5, resulting in higher allowable stresses, for most materi-

als. The allowable stress values of Tables 1A and 1B reflect the

reduction in design margin on ultimate strength. The ASME Code

Committee has approved the use of these higher allowables for

nuclear containment vessels built to Subsection NE, keeping the

factor of 1.1. The justification for the reduction in design factor

on ultimate strength is included in WRC Bulletin 435 [1].

Basically, the justification is based on the improvements made in

the Code since 1967. These included improved steel making

processes and improvements in nondestructive examination capa-

bilities. The reduction in design factor was made in Sections I, III

(for Classes 2, 3, and MC), and VIII, Division 1.

The reason for the 1.1 multiplier on allowable stress, which

allows stresses 10% higher than those for Section VIII, Division 1

vessels is based on the fact that, unlike most pressure vessels,

containment vessels do not have pressure relief devices. The

110% is equivalent to the pressure accumulation permitted prior

to the full discharge of the pressure relief devices.

If clad material is used for construction, certain design and

analysis requirements must be met. No structural strength may

be attributed to the cladding material for satisfying primary

stress limits. For certain design calculations, credit may be taken

for clad thickness of integrally clad plates. If such credit is taken

for clad thickness, the Maximum Service Metal Temperature

(MSMT) used must be the lower of the values allowed for base

plate material and the cladding material. If credit is not taken for

the clad thickness, the MSMT of the vessel is that allowed for

the base plate material. For components subject to internal pres-

sure, the inside diameter of clad material may be used in thedesign equations, regardless of whether clad material is included

in design thickness or not. When checking for bearing stresses,

the presence of cladding on bearing surfaces must always be

considered.

9.8 QUALIFICATIONS OFPROFESSIONAL ENGINEERS

The rules of Section III Code, including Subsection NE, call

for certification of certain documents by registered professional

engineers. Such certifications are required for the Design

Specifications, the Design Report, and the Overpressure Protection

Report. These certifications must be performed by engineers who

meet certain specified requirements, which are specified in the

Appendix XXIII. The qualifications of the professional engineers

in meeting the requirements of this Appendix must be evaluated

and verified by the owner or the N Certificate Holder, as applica-

ble, responsible for the activity being certified. A record of the

qualifications of professional engineers must be maintained. The

first requirement for certifying engineers is registration in at least

one state of the United States or one province of Canada. In addi-

tion, four years of varied applicable experience, of which at least

two years are spent in the specialty field, is required. The engi-

neers must keep current their knowledge of the Codes and continue

professional development in their specialty fields. Detailed require-

ments are specified for certifiers of various documents. Engineers

must assure themselves that they are qualified to perform the spe-

cific certification activities and must prepare a statement attesting to

that fact. Appendix XXIII also presents the items to be specifically

reviewed in each document for a valid certification. Forms andexamples for these certification activities are also provided.

These detailed certification and personnel qualification require-

ments are among the factors that contribute to the quality of a

Section III vessel as compared with a Section VIII vessel. Section

VIII, Division 1, has no certification requirements other than that

for the data reports. Section VIII, Division 2, requires the certifi-

cation of the Design Specifications and the Design Report by a

professional engineer. However, no detailed requirements for the

experience and documentation of the qualifications are provided.


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There is some belief that additional certification and qualification

requirements are needed for Section VIII and, in particular, for

Division 1.

The purpose of requiring a Registered Professional Engineer

was to assure that only qualified engineers would be responsible

for design of equipment that requires stress analysis. The ASME

Code had permitted anyone to design equipment when the design

consisted of merely substituting numbers in equations to deter-

mine the required thickness. However, the Committee felt that a

detailed stress analysis required the engineering judgment of a

Registered Professional Engineer (RPE), because they were con-

vinced that the Rules of Ethics would prevent the engineer from

certifying the design if he didnt understand the provisions of the

Code. There have been times when engineers have certified

designs of pressure retaining equipment without a full under-

standing of Code requirements, but these engineers are subject to

heavy punishments such as jail and large fines. The purpose of

requiring an RPE is to protect the public.

9.9 OWNERS DESIGN SPECIFICATIONS

Non-Mandatory Appendix B provides detailed guidelines for

the preparation and contents of the Design Specifications. This

Appendix refers to paragraph NCA-3252 for the minimum contents

of the certified Design Specifications. The Design Specifications

must contain sufficient detail to provide a complete basis for

design and construction. As a minimum it must include functions

and boundaries of items, all design and overpressure protection

requirements, the environmental conditions, the Code classifica-

tion of the items, the material requirements, the operating require-

ments, and the effective Code edition and addenda. It is the

responsibility of the owner to provide the Design Specifications.

The Design Specifications must be certified as correct and com-

plete and also certified to be in compliance with the requirements

of the Code by one or more Registered Professional Engineers

(RPEs) competent in the applicable field of design. The RPEs

must also be qualified in accordance with the requirements of

Appendix XXIII (formerly ANSI/ASME N626.3). This Appendix

has detailed requirements for qualifications and duties of special-

ized professional engineers. Such engineers must have a mini-

mum of four years of varied application experience, at least two

of which must have been in the specialty field for which they per-

form certification activity. In addition, they must keep current

their knowledge of Code requirements and continue professional

development in their specialty fields through personal study and

experience. Such qualifications must be reviewed by their

employers at least once every three years to assure that the qualifi-

cations have been maintained. A continuing record of such activi-

ties must be included in the qualification records of the individual.

The filing requirements for the Design Specifications are also

included in Subsection NCA. A copy of the Design Specifications

must be made available to the Inspector at the manufacturing sitebefore the fabrication begins. A copy must also be filed at the

location of installation and be made available to the enforcement

authorities having jurisdiction over the plant installation.

Appendix B lists the loads to be addressed in the Design

Specifications. This list is similar to that of paragraph UG-22 in

Section VIII, Division 1, which lists the loads to be addressed by

the designer. But specific details are provided here for the deter-

mination of each load and how loads are assigned to various ser-

vice limits. Guidelines for load combinations are provided, and it

is noted that the Code stress allowables are not intended to pro-

vide limits on deformations.

The materials requirements that should be addressed in the

Design Specifications are also listed. Also to be included are

any unusual fabrication requirements, whether a hydrotest or

pneumatic test is to be performed, any restrictions on testing

requirements, and leak-tightness requirements. In addition to the

requirements for vessels, Appendix B includes specific require-

ments for pumps, valves, and piping. Article B-11000 lists refer-

ences, which contain regulatory requirements. This list is very

helpful to the engineer preparing the Design Specifications. The

regulatory requirements need to be included either by description

or by reference.

9.10 CERTIFIED DESIGN REPORT

Appendix C provides non-mandatory guidelines for the prepa-

ration of the design report by the Certificate Holder. In general,

the Design Report should be based upon analysis to demonstrate

the adequacy of the structural design to meet all the requirements

of the certified Design Specifications and the Code. At a mini-mum, the report should include the results, conclusions, and

other considerations showing that all requirements related to

structural design are met. Guidelines are provided for the prepa-

ration of the Design Report, with the intent being to cause uni-

formity of such reports to the extent possible. The design report

must be reviewed by the owner or his designee to determine that

all the design and service loadings as stated in the Design

Specifications have been evaluated and that the acceptance crite-

ria of the Code have been met. The responsibility for the method

of analysis and the accuracy of the Design Report remains with

the Certificate Holder.

Suggestions for distribution and retention of the report are also

provided. The owner is responsible for designating the records to

be maintained. The owner is also responsible for continued main-

tenance of the records required by the Code. The owner must

advise the enforcement authority in writing regarding the location

of the records, including the Design Report.

9.11 DESIGN BY ANALYSIS

The rules for design by analysis are specified in Paragraph

NE-3200. These rules are very similar to those of Subsection NB

and Section VIII, Division 2 (prior to the 2007 Edition). The theory

of failure used is the maximum shear stress theory. At a given

point, the maximum shear stress is equal to one-half the differ-

ence between the algebraically largest and the algebraically small-

est of the three principal stresses. The term stress intensity is

defined as twice the maximum shear stress, having a value equal

to the difference between the algebraically largest and smallest

principal stresses at a given point.Terms related to stress analysis are defined in NE-3213.

Calculated stresses need to be categorized as primary, secondary,

or peak, with each category having different allowables, depend-

ing on the mode of failure that they may cause. Primary stresses

are those developed by the imposed loadings and are necessary to

satisfy the laws of equilibrium between external and internal

forces. Such stresses, which are not self-limiting, may cause gross

distortion or collapse. Primary stresses are further divided into

general membrane, local membrane, and bending.


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A general membrane stress extends over large enough portion

of the vessel shell so that an effective redistribution of load is not

possible when the material yields. An example of such stress cate-

gory is the membrane stress due to pressure. These stresses are

limited to the basic stress allowable, which provides a minimum

margin of 1.5 on the minimum specified yield strength. Since the

actual yield strength of most materials may be substantially higher

than the specified minimum value, the true safety margins are

generally greater than those apparent from the basis for allowable

stresses. Another factor contributing to the conservatism of this

stress category is the strain-hardening properties exhibited by

most materials.

A local primary stress is that produced by pressure or mechani-

cal loading and is associated with a discontinuity effect. Such

stresses are sufficiently localized to have the ability to redistribute

to the adjoining areas that have lower stresses. The primary con-

cern with this stress category is excessive local distortion. The

allowable stress for this category of stress is 1.5 times the basic

stress allowable. This value may exceed the yield strength for cer-

tain materials, but since it is localized and able to redistribute, a

collapse failure is not of concern. Criteria are provided for deter-

mining when a stress distribution may be considered local. The dis-tance over which the membrane stress intensity exceeds 1.1 times

the basic allowable shall not extend more than 1.0 , in the

meridional direction. Another limitation is that any two locally

stressed regions may not be closer to each other than 2.5 , to

avoid overlap between the stress fields. The Code does not specify

a limit on the extent of the region with stresses between 1.0 and 1.1

times the basic stress allowable. This may appear as a loophole,

which will allow exceeding the stress allowable by up to 10% for

unlimited distances. But if applied properly to the locally stressed

discontinuities, the shell attenuation will cause a reduction of such

local stresses to basic allowables within a reasonable distance.

A primary bending stress is the through thickness bending at a

point caused by external loadings. A prime example of such stress

is that due to pressure on a flat plate. The limit on the surface stress

for such stress (including the primary membrane stress) is 1.5

times the basic allowable. The factor of 1.5 in this case is the shape

factor to cause through thickness yielding. It should be pointed out

that the Code rules are for rectangular cross sections of normally

encountered pressure vessel shells. If a cross section other than a

rectangular one is encountered, this allowable should be adjusted

by replacing the 1.5 factor with the appropriate shape factor.

A secondary stress is that developed by the constraint of adja-

cent material or by self-constraint of the structure. The basic char-

acteristic of a secondary stress is that it is self-limiting. When the

material yields or distorts locally, the conditions that cause the

stress are satisfied, and one application of the load cannot cause

failure. The primary concern with secondary stresses is the possi-

bility of excessive deformation and fatigue failure. The limit on

the range of primary plus secondary stress intensity at any point is

3.0 times the basic stress allowable. This value is limited to two

times the yield strength of the material, at temperature. The sig-nificance of this limit is that if the range of primary plus sec-

ondary stress intensity does not exceed twice yield, incremental

distortion cannot take place when the stresses are cycled, and also

that the stress intensity range will shake down to elastic action.

This will validate the Code specified fatigue analysis, which is

based on equivalent elastic stresses. For further explanation, see

the ASME published document on the criteria of Section III [2].

A peak stress is that increment of stress that is additive to the

primary plus secondary stresses, caused by local discontinuities

2RT

2RT

and local thermal gradients, including the effects of stress concen-

trations. Such stress does not cause significant distortion; the pri-

mary concern with it is the formation of fatigue crack or that it

will cause brittle fracture. Examples of peak stresses are thermal

stresses due to differential expansion of cladding material, stresses

at local structural discontinuities, and surface stresses produced

by thermal shock. There is no limit specified on peak stresses;

they are included in the fatigue analysis and the number of cycles

is limited by the specified procedure.

The aforementioned stress allowables are for Service Level A

and have been simplified in this chapter. For details of deriving

the stress intensities of various categories and detailed stress lim-

its for various Service Levels, see NE-3200.

The Code does not provide mandatory rules for the methods of

calculating stresses. However, Non-Mandatory Appendix A pro-

vides formulas for calculating stress in some commonly encoun-

tered shells. Appendix A was commonly used for stress analysis

of pressure vessels before use of computers became common-

place. Appendix A is still useful to help engineers understand and

verify the results of computer solutions. Appendix A is consistent

with the design theory of Section III and Section VIII, Division 2

(prior to the 2007 Edition), but finite element computer programsare not.

In spite of the fact that finite element analysis (FEA) is inap-

propriate and not consistent with the design theories of Section

III, some engineers, who do not understand the Code, still use

FEA for stress analysis of Section III pressure vessels. Some of

the problems associated with FEA are that if elements other than

shell elements are used, nonlinear through-thickness stresses may

result. At transition elements between shells, the radial direction

is not defined and categorizing stresses is difficult. At local struc-

tural discontinuities, some but not all of the peak stresses will be

included in the results. How much of peak stresses have been

included depends on the fineness of the model mesh, and to

separate out the secondary and peak stresses is very difficult. The

PVRC has been working on a project called 3-D Stress

Categorization to give recommendations on how to categorize

stresses from a three-dimensional finite element analysis. The

results of this project recently have been published as WRC

Bulletin 429. In addition to this Bulletin, there are a number of

published papers that address the problem of stress classification.

Table NE-3217-1 was put into the Code to assist the designer in

categorizing the calculated stresses. However, this table does not

address the questions that are raised when finite element models

are used. Some of the recommendations of the WRC Bulletin 429

are very briefly summarized in the following list:

(1) General primary membrane stresses should be calculated by

equilibrium equations and not by FEM.

(2) Primary and secondary bending stresses should be lin-

earized for comparing with the allowables.

(3) Linearized distributions should develop the same net forces

and moments.(4) Primary and secondary stress evaluations should be per-

formed in basic structural elements only.

(5) For fatigue analysis in transition regions, the primary plus

secondary stress intensity range should be evaluated in the

nearest structural element.

(6) For primary membrane stress evaluation, the six stress com-

ponents should be averaged across thickness. The average

principal stress should be calculated from the average stress

components.


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12 Chapter 9

The stress intensity limits for the design condition and specified

service loadings are summarized in Table NE-3221-1. For the

design condition, the limits for primary stresses are the same as

those for Service Level A, and secondary and peak stresses need

not be addressed. The allowables for Service Level B are the same

as those for Service Level A. For Service Levels C and D, an

evaluation of secondary and peak stresses is not required. The pri-

mary stresses for Service Level C are the same as Level A if the

structure is not continuous and integral, in which case the allow-

ables are at least 20% higher. Service Level D limits get the same

20% increase for structures that are not integral and continuous.

Also, Service Level D limits, for the same types of structures,

depend on whether an elastic or inelastic analysis is performed.

For an inelastic analysis, the limit is 85% of the allowable calcu-

lated in accordance with Appendix F; for an elastic analysis, the

allowables are 1.5 times those allowed for inelastic analysis.

9.12 APPENDIX F

If Service Level D Limits are specified by the Design

Specifications, the guidelines of Appendix F are to be used. Eventhough this is a Non-Mandatory Appendix, it provides the only

reasonable approach for the evaluation of Service Level D. The

guidelines are intended to ensure that violation of the pressure

retaining function will not occur, but are not intended to ensure

operability of the components during or after the specified event.

The rules of this Appendix, intended for severe hypothesized

accidents, are not applicable to the portion of a component or sup-

port for which a failure has been hypothesized.

For Service Level D Limits, only those on primary stresses are

prescribed. Peak stresses and secondary stresses (the latter being

self-limiting) are of no consequence for this noncyclic event. The

potential for buckling under compressive loads and for brittle

fracture should be considered. Appendix F provides for either

elastic analysis or plastic analysis, and also provides limits for

each. For both methods, Appendix F calls for consideration of

geometric non-linearities, if appropriate. However, no limits on

strains or deformations are provided. Such limits may need to be

provided in the Design Specifications, depending on the function

of the component and the deformation limit that can adversely

affect the operability of the component.

If an elastic analysis is used, the general primary membrane

stress intensity is to be limited to the lesser of 2.4 times the basic

allowable or 0.7 times the minimum specified ultimate strength.

This assures a certain margin against rupture. The local primary

membrane stress intensity and the primary membrane plus primary

bending stress intensity are to be limited to 150% of the limit for

general primary membrane stress intensity. This limit is justified

by the fact that these local stresses will be redistributed to the

adjacent area before they cause rupture. The average primary

shear stress across a section loaded in pure limited to 0.42 times

the shear is not to exceed 0.42 times the specified minimum ulti-mate strength. In all the preceding limits, some additional conser-

vatism is produced by the fact that the actual material properties

are usually greater than the specified minimum values. For com-

pressive stresses, the recommended allowable stresses are equal to

150% of those obtained from the rules of NE-3133, which means

that the safety margins are reduced by a factor of 1.5. If the criti-

cal buckling value is determined by analysis or test, the recom-

mended design margin to be used is 1.5, as compared to 3.0 for

other service limits.

If a plastic analysis is performed, the general primary mem-

brane stress intensity is to be limited to 0.7 times the specified

minimum ultimate strength for most materials. The maximum pri-

mary stress at any location is limited to 0.9 times the ultimate and

the average primary shear is ultimate. If a collapse analysis is

used, the specified load should not exceed 90% of the limit analy-

sis collapse load or 100% of plastic collapse load or test collapse

load.

9.13 FATIGUE ANALYSIS

Containment vessels normally are designed to provide protec-

tion against release of contaminated materials in case of a severe

accident. As a result, they are not subject to significant cyclic

pressure loadings. However, environmental, cyclic piping, and

thermal loads may be specified and, consequently, may need to be

designed. The rules for fatigue analysis are very similar to those

of Subsection NB and Section VIII, Division 2. Subsection NE

includes fatigue exemption rules, similar to one of the methods

provided in Section VIII, Division 2. If a vessel does not meet all

the exemption conditions, then a detailed fatigue analysis must beperformed. The S-N curves, providing allowable number of cycles

for calculated stress intensity amplitude associated with each type

of cycle, are identical to those of other ASME Codes. These

curves were developed in the early 1960s from data obtained from

strain-controlled testing of smooth, polished small specimens in

air. To account for the size effects, environmental effects, surface

roughness, and many other differences between the test environ-

ment and the actual components, design factors of 2 and 20 were

applied to the mean test result of the stress intensity amplitude

and the number of cycles, respectively. The lower of the two

resulting values was plotted as a design curve. These design fac-

tors of 2 and 20 were also intended to account for data scatter and

to provide safety margins. The design curves conservatively

include the effects of mean stresses, so that mean (noncyclic)

stresses do not need to be included in the calculation of the range

of peak stress intensity. These curves have not been updated with

recent test results and advances in analytical methods. It has been

questioned whether the factors of 2 and 20 are adequate to

account for service conditions (especially in corrosive environ-

ments) and to provide adequate safety margins. For several years,

the PVRC has had a Task Group working on proposals for updat-

ing the Code fatigue rules. A significant part of the work assigned

to the Task Group has been to evaluate the effects of various

environments on the fatigue life of components. The report of the

Task Group was recently submitted to the ASME Code

Committee for consideration and possible updating of the Code

fatigue rules.

The Code rules employ the Miners linear damage law for com-

bining the effects of various types of cycles on the overall fatigue

life. This is a simplified and conservative method that does not

account for the sequence of cycles. In reality, the sequence oflarge amplitude versus small amplitude cycles will affect the

resulting fatigue life. All cycles due to Service Levels A and B

Limits, and due to Service Level C Limits where the structure is

not integral and continuous, need to be accounted for. However,

other Service Level C Limits, Service Level D Limits, and the

overpressure test cycles prescribed by the Code do not need to be

included in the fatigue analysis. A service cycle is defined as the

initiation and establishment of new conditions followed by a

return to the conditions that prevailed at the beginning of the


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cycle. A stress cycle is defined as a condition in which the alter-

nating stress difference goes from an initial value through an alge-

braic maximum value and an algebraic minimum value and then

returns to the initial value. A single operational cycle may result

in one or more stress cycles. Dynamic effects must also be

accounted for in the fatigue analysis.

To arrive at the maximum range of peak stresses at a point, the

stress concentration effects of local discontinuities must be con-

sidered. To use the theoretical stress concentration factors at

these points would be too conservative. The use of fatigue

strength reduction factor is recommended. This factor is a stress

intensification factor that accounts for the effect of a local struc-

tural discontinuity on the fatigue strength. The fatigue strength

reduction factor does not have to exceed a value of 5.0, whereas

a theoretical stress concentration value is unlimited. The Code

recommends that a value of 4.0 be applied to the backside of fil-

let welds. This subsection also has some stress indices suggested

for application to pressure stresses at openings, provided that the

opening meets all the requirements of paragraph NE-3338. For

openings that do not meet these requirements and for other local

structural discontinuities, the fatigue strength reduction factors

must be established by analytical and/or experimental means.The use of theoretical stress concentration factor is convenient

and commonly employed.

Another factor that may be applicable to the range of peak

stress intensity is the Ke value obtained from the simplified elastic-

plastic analysis of paragraph NE-3228.3. If the range of primary

plus secondary stress intensity exceeds the 3Sm allowable, the

Code allows the through-thickness thermal-bending stresses,

which have some characteristics of peak stresses, to be deleted

from the range. However, a penalty factor, Ke, is applied to the

range of peak stresses. This factor is a function of the mate-rial

and the calculated range of primary plus secondary stress inten-

sity, and can have a value of up to 3.33. The stress intensity

amplitudes plotted on S-N curves are calculated by multiplying

the test strain values by the material modulus of elasticity, the

value of which is shown on each curve. If the value of the modu-

lus of elasticity used in the stress analysis (to arrive at the range

of elstically calculated stresses) is different from that used for

plotting the curve, a correction factor must be applied. This factor

is the ratio of the modulus of elasticity given on the design fatigue

curve to the value of the modulus of elasticity used in the analy-

sis. Since an operating cycle could cover a range of temperatures

and the modulus of elasticity is a temperature-dependent value,

the question often arises regarding what value of temperature to

use to establish this ratio. It is commonly believed that using an

average temperature over the range is adequate, and this is com-

monly what is done.

9.14 BUCKLING

Rules for buckling analysis are provided in NE-3133. Theserules cover shells and formed heads under external pressure and

cylindrical shells under uniform axial compression, and include

criteria for sizing of stiffeners. These rules are the same as those

in Section VIII and in other subsections of Section III, and have

been in the Codes for several decades. They were derived by

applying knockdown factors, obtained from tests, to the classical

buckling solutions. These factors are consistent with the out-of-

round-ness and deviation-from-true form tolerances that are

allowed. The design margins are not uniform for all loadings and

all geometries, and they range from approximately 2.0 to 3.0. If

the design-by-formula option of this Code is adopted, the use of

these rules is mandatory.

If the design-by-analysis option of NE-3200 is used, the analy-

sis for compressive stresses must be in accordance with the provi-

sions of NE-3222. This paragraph allows the use of buckling rules

of paragraph NE-3133. It also allows the use of calculating the

critical buckling stress by any one of three methods and the appli-

cation of a design factor of three. One method of calculating the

critical stress is by rigorous analysis, which includes the effects of

geometric imperfections and large deformations, but this is a diffi-

cult and expensive option; the expected imperfections are difficult

to predict and model. The second method of calculating the criti-

cal values is by applying a knockdown factor to the buckling

stresses obtained from a classical linear analysis. The knockdown

factors are to account for the difference in buckling stress of a

perfect shell and a fabricated shell constructed to certain imper-

fection tolerances. These factors are obtained from tests of mod-

els fabricated to tolerances allowed by this Code. This is the most

commonly used method of establishing critical stress values and

is the method by which buckling rules of most Codes have been

derived. The third option is the test of models under the sameconditions as those expected for the actual vessel. Again, this can

be an expensive option and is open to interpretation regarding the

size and details of the model.

With the exception of some of the containment vessels (which

are designed for dynamic loadings) for which a rigorous analysis

has been performed, most containment vessels are designed either

by the rules of NE-3133 or by the alternative rules of Code Case

N-284. It has been recognized for some time that the rules of

NE-3133 are not current and neither cover all geometries and

loadings normally encountered on containment vessels nor pro-

vide uniform and reasonable safety margins for all geometries and

loadings. In the mid-1970s, the ASME formed a group to develop

rules for design of containment vessels under compressive stresses.

Most containment vessel designs are controlled by buckling either

caused by environmental loads such as earthquakes or caused by

specified operating dynamic pressures that generate vacuum con-

ditions. Most such loadings are nonaxisymmetric and nonuniform

in the axial direction, which results in nonuniform compressive

stresses that cannot be easily analyzed by the rules of NE-3133.

This recognition led to the need for a more complete and rigorous

set of rules. The ASME Task Force worked on such alternative

rules for several years, the result of which was Code Case N-284.

The rules of this Code Case were based mostly on the research

performed and papers published by Clarence Miller. The original

Code Case contained many typographical errors. These errors

were corrected, the Code Case was improved, and Revision 1 of

the Case was published.

Code Case N-284 covers many geometries that the rules of the

Code do not address, including ring- and/or stringer-stiffened

cylinders, one-way or two-way stiffened spherical shells and

formed heads, and torroidal/ellipsoidal shells. Many loadingsother than those addressed by the Code are also included, some of

which are overturning moment across the entire vessel cross sec-

tion, in-plane shear, unequal biaxial compressive stresses in

spherical shells and formed heads, and thermal stresses.

Interaction relationships are provided for evaluation of any com-

binations of loads for which allowables are included. The design

margin recommended in the Code Case for both the design condi-

tions and the Service Level A and B Limits is a uniform value of

2.0. For Service Level C Limits, this value may be reduced to


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14 Chapter 9

1.67; for Service Level D Limits, a value of 1.34 is recommended.

These design margins are lower than those of the Code rules and

reflect the more rigorous approach of the Code Case. Some addi-

tional safety margin is introduced into the rules by the fact that

the knockdown factors, which are used to develop the critical

buckling values, are based on the lower bound of the test data.

The use of these design margins (or factors of safety), as well as

the use of the Code Case as an alternative to the Code rules, must

be approved by the owner and the jurisdictional authority. The use

of the aforementioned design margins is controversial and has not

been universally accepted. However, the methodology of the Code

Case has been extensively used not only on containment vessels,

but also on many thin-shell structures such as storage tanks. The

use of the Code Case, especially if the proposed design margins

are used, will result in more economical designs either from less

shell thickness or from a smaller number of stiffeners. The size of

stiffeners will also be significantly smaller than that required by

the Code rules. The Code always uses an n of 2 to determine the

size of rings, whereas the Code Case recognizes the fact that rings

are tied to the shell and, as a result, are forced to buckle in a

greater number of nodes.

The Code Case has some general guidelines for the treatmentof the effect of openings on shell buckling. Based on experience

and test results, it is concluded that reinforced openings will not

deteriorate the buckling capacity of the shell as long as the open-

ing diameter does not exceed 10% of the shell diameter. The

effect of larger diameter openings must be considered in the

design report. The rules of the Code Case are limited to shells

with a radius-to-thickness ratio not greater than 1000 and a shell

thickness not less than in. For thinner shells, it is felt that geo-

metric nonlinear effects become significant and cannot be simply

accounted for in the design rules.

Some guidelines on performing stress analysis and bifurcation

analysis are also provided in the Code Case. One difficulty associ-

ated with performing theoretical buckling analysis is the fact that

the knockdown factors, reflecting the difference between perfect

shells and fabricated shells, need to be accounted for. Such fac-

tors, however, are different for different loadings. For a vessel

subject to combination of loadings, it would be difficult to deter-

mine the appropriate knockdown factor to apply to the theoretical

results for arriving at critical values. One suggestion to bypass

this problem is to apply the appropriate knockdown factor to each

loading component and then use such amplified loads for per-

forming the analysis. The resulting Eigenvalue is the design mar-

gin (safety margin) for the applied loading combination.

Some guidelines are also provided for the treatment of local

discontinuity membrane stresses. For compressive membrane

stresses that are not uniform along the axis of the shell, a conserv-

ative approach would be to assume that the maximum value is

applicable over the entire panel. This in certain cases can result in

overly conservative designs or in fact may be impractical to com-

ply with. An example of such a case is the circumferential mem-

brane stress generated at the fixed end of a cylindrical shell due totemperature increase. In fact, many cylindrical containment ves-

sels have a concrete flat bottom and are subject to a design tem-

perature condition. The fixity of the base in the radial direction

results in very large compressive circumferential membrane

stresses at the base. However, these stresses attenuate as the dis-

tance from the base is increased. The Code Case suggests that an

average value of membrane stress within a distance of from

the point of fixity be used for comparison with the hoop compres-

sion allowables. The reasoning is that a shell does not buckle at

1RT

14

one point, but rather that it will buckle over a distance of at least

one attenuation length. Even this proposed approach is very con-

servative, as the same restraint at the base that causes the high

stresses will stiffen the shell and prevent the formation of buckles

within a reasonable multiple of the attenuation distance. It would

not be difficult to perform a bifurcation analysis of such details,

which model the actual geometry and the calculated distribution

of stresses. Applicable knockdown factors will need to be applied

to the results to determine the critical value for a vessel fabricated

to the tolerances of the Code. The knockdown factors of the Code

Case are based on assuming that Code tolerances are met. If the

actual vessel imperfections are significantly less than those

allowed, credit may be taken for this fact and the higher values of

knockdown factor may be used. A flow chart at the end of the

Code Case N-284 directs the user through various steps needed to

establish the allowables and to demonstrate that interaction rela-

tionships for various loading combinations are met. Establishing

allowables for each individual load is rather straightforward

except for the case of shells stiffened in two directions. For such

shells, an energy approach is used, which may require a number

of iterations to solve. Because hand calculations would be very

tedious, a computer program for such shells is recommended. Theuse of interaction equations would also be very time consuming to

perform by hand and is best done by a computer program. To cre-

ate an economical design for a vessel that is designed for buckling

may involve a number of trials. There are trade-offs between shell

thickness and number of stiffeners, between the number of

stiffen-ers and the size of stiffeners, and between one-way and

two-way stiffening. There are no rules of thumb for such determi-

nations and factors as the unit cost of material versus fabrication

cost, possible need for post-weld heat treatment, shipping clear-

ances, and many others must be considered.

Since the introduction of the Code Case, a great deal of addi-

tional work has been performed by the PVRC and the ASME

Code Committee to develop more current and justifiable rules for

design of shell structures subject to buckling. The results of the

PVRC work are included in WRC Bulletin 406 [3]. The work of

this Bulletin and the recommendations therein have been used to

develop a set of Alternative Rules for the buckling design of

Section VIII vessels, Divisions 1 and 2. These alternative rules

were published in Code Case 2286. The alternative rules of the

Code Cases mostly provide higher stress allowables than those of

the Code, indicating that the Code rules are generally too conser-

vative. The alternative rules were put in the Code Case to provide

a trial period before they were incorporated into Section VIII.

These new rules are anticipated to be eventually adopted by

Section III as well. The buckling stress allowables of the alterna-

tive rules are based on theoretical buckling equations, which have

been reduced by knockdown factors, and on plasticity reduction

factors that are determined from tests (similar to the approach of

Code Case N-284). The allowable stresses are then calculated by

applying a design factor to the buckling stress. The design factors

provided by the Code Case range from 2.0, for stresses up to theproportional limit of the material, to 1.67 at yield stress. In addi-

tion to what is covered in the present Code, equations are provid-

ed for column buckling, which may be needed for tall, slender

vessels, for bending over the full cross section of cylindrical and

conical shells, for ring-stiffened cylinders and cones under axial

compression, for spherical shells and formed heads under nonuni-

form stress, and for combined loadings. (For more details on this

Code Case, see the chapter on Section VIII, Division 2.) Code

Case 2286, being based on theoretical buckling values being


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reduced by knockdown factors based on test results, meets the

provisions of NE-3222 regarding the procedure. However, the

design margins of this Code Case do not meet NE-3222 and must

be adjusted. Case N-284-2 was published in 2007. This revision

corrects numerous errors, mostly in the design formulas, that were

published in N-284-1. Case 2286-1, for Section VIII, Division 1

and 2 applications, was published in 2001. Case N-759 was

copied from Case 2286-1, for use in nuclear applications, and was

published in 2007.

9.15 REINFORCEMENT OFCONE-TO-CYLINDER JUNCTION

Rules for reinforcement of cone-to-cylinder junctions are pro-

vided in Appendix XXII-1000. These rules are very similar to

those of Section VIII, Division 2, and they are applicable to

reducer sections where all elements have a common axis and the

half-apex angle does not exceed 60 deg. Criteria are provided for

determining the need for reinforcement at either end of the transi-

tion cone. A formula is given for calculating the required area of

reinforcement. Any excess material in the cone or cylinder, withincertain distance from the junction, may be counted as reinforce-

ment. Any additional area of reinforcement, which may be

required, must be located within a specified distance from the

junction. If the junction is to be considered as a line of restraint

for vessel external pressure design, a moment of inertia criteria

for the junction is to be met as well. These criteria are based on

ring buckling for a ring composed of the reinforcing element plus

the participating portions of the cone and cylinder. These are very

conservative criteria because they assume an isolated ring that

buckles into two nodes. The actual ring, being tied into the

adjoining shells, is forced into a higher number of nodes, requir-

ing more of an in-plane load to cause it to buckle. The criteria for

the required area at the junction are old and undocumented. More

justifiable rules for cone-to-cylinder reinforcement are being

developed by the PVRC.

For half-apex angles exceeding 60 deg. or whenever the designer

chooses, the design may be based on special analysis as an alter-

native to the rules discussed in the preceding paragraph. Such

special analysis can be easily performed by a shells-of-rev-olution

or finite element program. The calculated stresses at the junction

must meet all the stress allowables of Subsection NE. In such an

analysis, the effects of buckling must also be considered, in which

case a design margin of 3.3 on theoretical buckling pressure is

appropriate.

9.16 PLASTIC ANALYSIS

Some of the basic stress allowables may be relaxed if a plastic

analysis is performed. The rules for plastic analysis are specified

in NE-3228. The limits on local membrane stress intensity, primaryplus secondary stress intensity, thermal stress ratchet, and pro-

gressive distortion of nonintegral connections need not be satis-

fied if a plastic analysis is performed. The results of this analysis

are considered acceptable if shakedown occurs and if the defor-

mations do not exceed specified limits. The limits on local mem-

brane stress intensity and primary plus secondary stress intensity

may also be waived if it can be shown by limit analysis or by tests

that the specified loadings do not exceed two-thirds of the lower-

bound collapse load.

Another exemption to the basic stress allowables is the

Simplified ElasticPlastic Analysis procedure of NE-3228.3. This

provision allows exceeding the specified limit on the range of pri-

mary plus secondary stress intensity under certain conditions. The

limit must be satisfied with the exclusion of thermal bending.

However, the fatigue analysis is penalized by applying a factor to

the calculated peak stress intensity amplitude. The value of the

factor depends on the material as well as on the value of the

calculated range of primary plus secondary stress intensity. The jus-

tification for this provision stems from the fact that thermal bending

stresses have some attributes of peak stress. This exemption usually

does not have to be resorted to for the design of containment ves-

sels. However, for reactor vessels, because of more rapid tempera-

ture fluctuations and thicker sections, the thermal-bending stresses

are significant and the use of this provision is common.

9.17 DESIGN BY FORMULA

Rules for design by formula are provided in NE-3300. These

rules (which are similar to the design rules of Section VIII) are

applicable to the design loadings and Service Levels A and BLoadings if such loadings do not include substantial mechanical

or thermal loads. Substantial loads are defined as those that result

in stresses exceeding 10% of the primary stresses induced by the

design pressure. If the mechanical and thermal loads are substan-

tial, and also for Service Levels C and D, the rules for design by

analysis must be used. The allowable stresses to be used in these

formulas are 1.1 times the allowables for Section VIII, Division 1.

This will result in vessels with about 10% less wall thickness than

Section VIII, Division 1 vessels designed for the same conditions.

The design formulas for formed heads are the same as those for

Section VIII, Division 1. It is generally recognized that these rules

were developed on the basis of experience, rather than research.

For several years, work has been ongoing within both the PVRC

and the ASME Code Committee to update these rules. Two ellip-

soidal head models were tested for the PVRC by Praxair, and

many analytical studies were performed by Professor Kalnins.

Some of this work was published in WRC Bulletin 414 [4]. Based

on this work, Code Cases 2260 and 2261 were developed for

Divisions 1 and 2 of Section VIII. These Cases were issued for a

trial period before they were incorporated into the ASME Codes.

These alternative rules result in generally less thickness require-

ments, but for a few materials, they require a greater thickness

than that of the existing rules.

9.18 OPENINGS

For an opening in a vessel or vessel part not in cyclic service,

the area replacement rules of NE-3330 are allowed in lieu of an

analysis. The same applies to openings in vessels or parts, which

are subject to cyclic service but can be rendered exempt fromdetailed fatigue analysis by the rules of this subsection. The area

replacement rules are similar to those of Section VIII, Division 2

prior to the 2007 Edition, with some of the features of Division 1

also included. These rules assure that the collapse pressure of the

opening area is at least the same as that for the unpenetrated

portion of the shell. However, no conclusions may be drawn regard-

ing the fatigue resistance of the intersection and nozzle attach-

ment details. The fatigue exemption rules are based on assuming

the highest fatigue strength reduction factor consistent with the


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16 Chapter 9

details allowed. Meeting these conservatively derived rules ensures

that the opening area is safe from a fatigue standpoint.

For a vessel or vessel part in cyclic service that does not meet

the fatigue exemption rules, the limits on primary and secondary

stresses need to be met and a fatigue analysis must be performed

in the vicinity of the openings. The primary stress limits may be

considered satisfied if the area replacement rules are met. The

limits on primary plus secondary stresses may also be considered

satisfied if, in the vicinity of the opening, the stress intensity

resulting from external nozzle loads and thermal effects is shown

to be less than 1.5 times the basic stress allowable. This rule is

based on the assumption that the pressure-induced stresses in this

vicinity are limited to 1.5 times the basic stress allowable by

meeting the area replacement rules. This would ensure that the

sum of stresses from pressure, external loadings, and thermal

effects will meet the limit of 3.0 times the basic stress allowable.

If a fatigue analysis is required in the vicinity of the opening, the

peak stresses may be determined by one of three acceptable meth-

ods. The first method is the use of analytical techniques, such as

finite element programs. If this method is used, all loadings need

to be included and the mesh needs to be fine enough to produce

the maximum peak stresses; generating an adequate mesh may bedifficult, particularly in welded areas. The second method allowed

is experimental stress analysis, which is based on the use of data

from experiments. If this option is chosen, the requirements of

Appendix II must be followed and met. The third method is the

stress index method, which is based on the use of available ana-

lytical results and test data in the creation of indices to be applied

to the nominal shell stresses. The term stress index is defined as

the ratio of the stress component under consideration to the com-

puted stress component in the unpenetrated shell. Stress indices

are provided for pressure loading, for a limited range of geometric

parameters, in this Code. For geometries not covered in Subsection

NE and for loadings other than pressure, such indices may be inde-

pendently determined and used. However, the Code does not pro-

vide guidelines on the extent of analytical and/or test results that

are required to establish indices for a range of geometries.

9.19 BOLTED FLANGE CONNECTIONS

Rules for bolted flange connections are provided in Appendix

XI. These rules are applicable to Class 2 and Class 3 components

as well as to Class MC vessels. The design rules only provide for

hydrostatic end loads and gasket-seating loads, and are very simi-

lar to those of Section VIII. A discussion of design considerations

for bolted flange connections is included in Appendix XII. This

Mandatory Appendix is very similar to a Non-Mandatory

Appendix in Section VIII, which provides suggestions for consid-

eration. Only circular flanges designated Class RF (raised face)

are covered by the rules of Appendix XI. Rules for Class FF (flat

face) flanges are provided in Non-Mandatory Appendix L. The

design rules in Appendix XI are used primarily for internal pres-sure, but some rules are also provided for external pressure.

Designing a flange involves the selection of the gasket material,

type, and dimensions; flange facing; bolting; hub proportions;

flange width; and flange thickness. It is recommended that bolted

flange connections conforming to the standards listed in

Subsection NE be used rather than customized designed flanges.

Such standard flanges are usually more economical not only

because of the lack of involvement in the design effort, but mainly

because standard flanges are smaller cross-sectionally than

designed flanges. It is commonly recognized that most standard

flanges do

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ASME_Ch9_p001-024