asme companion guide.pdf

8/16/2019 ASME COMPANION GUIDE.pdf

1/18

CHAPTER

SUBSECl\ION

NB NC ND-3600

PIPING

8 1

BACKGROUND

Previous discussion in Chapter 5 indicates that three classes

of components are provided in the requirements of Section III,

Division 1 [1]. Each class can be considered a quality level, with

Class 1 being the highest and Class 3 the lowest. These levels of

quality exist because of the various requirements for each class

in Section III that relate to the following: materials, fabrication,

installation, examination, and design. Design is listed last because

there is sufficient evidence to indicate that the other considerations

listed are

o

more importance than (or at least equal to) the design

requirements. The following discussion will address service limits

onald F

Landers

Categories I), (2) and (3) provide protection against cata

strophic failure. Categories (4) and (5) provide protection against

fatigue failure [14]. Definitions

o

these terms are provided in NB-

3213

o

Section III; however, the main characteristics

o

these

stresses are discussed

in

the following paragraphs.

A primary stress is a stress developed

by

the imposed load

ing that

is

necessary to satisfy the laws

o

equilibrium between

forces and moments external and internal to the component. A

primary stress is not self-limiting. Once a primary stress exceeds

the yield strength of the material through the entire thickness

o

a

component, the prevention

o

failure

is

entirely dependent on the


2/18

266 • Chapter 8

A peak stress is the maximum stress that exists in the location

o

a component being evaluated. A peak stress causes no signif

icant distortion and is a concern only with respect to continued

cycling resulting in fatigue failure. With the exception of some

thermal stresses, peak stresses are normally a result of a stress

concentration factor.

Thermal stresses are classified as secondary and peak stresses

and are never considered primary stresses since they are self

limiting. Thermal stresses that produce distortion are placed in

the secondary category and those that result from almost complete

suppression o differential expansion and cause no significant dis

tortion are classified

as

peak stresses. The peak classification o

some thermal stresses is worthy

o

discussion since one will find

that the Class 1 rules o NB-3600 [13] treat these stresses differ

ently than the design-by-analysis rules of NB-3200 [12].

To

bet

ter understand this phenomenon, a little history is required. When

Section III was first issued in 1965 the stress from a radial ther

mal gradient through the component wall was classified

as

a peak

stress. This is explained in the Criteria Document

[2]

o Section

III

a discussion

o

which follows.

A special exception to the thermal rules is the case of the stress

due to a radial gradient in a cylindrical shell. It is specifically

stated that this stress may be considered a local thermal stress (i.e.,

peak stress). In reality, the linear portion of this gradient can cause

deformation, but it was the opinion o the Special Committee that

this exception could be safely made.

Therefore, the design-by-analysis rules, when first published,

considered the radial gradient

as

a peak stress. It must be noted

that the Section III vesssel rules had a special stress limit require

ment

to

prevent thermal stress ratchet o a shell subjected to ther

mal stress cycling in the presence

o

a static mechanical load.

These rules provide a limit on the linear and parabolic varia

tion

o

temperature through the wall. During the development

of ANSI B31.7, Nuclear Power Piping [3], their Design Task

Group decided

to

place the linear portion of the radial gradient

into the secondary stress category rather than the peak stress cat

egory. It was recognized that this approach did not agree with

the existing Section III requirements for vessels, which placed the

stress resulting from this loading into the peak stress category.

However, inclusion o the radial gradient stress in the secondary

category in B3 l.7 eliminated the ne ed to be overly concerned

with thermal stress ratchet. When the rules for Class l piping in

B31.7 were adopted into Section III in 1971, the vessel commit

tee agreed, although grudgingly, to include the radial gradient in

the secondary stress category. Inclusion o the radial gradient in

the secondary category resulted in a number o cases where the

limit on primary plus secondary stress intensity was exceeded,

which resulted in the need to develop a simplified approach to

elastic-plastic analysis. Again,

B3

l.7 had developed a technique

for addressing this concern because

o

the need resulting from the

decision to include the radial gradient in the secondary stress cate

gory [5]. When Section III adopted the piping rules o

B3

l.7, they

adopted a different approach to simplified elastic-plastic analysis

[15] . The Section III approach is,

o

course, applicable to all com

ponents and the B31.7 technique was abandoned. The effect on

industry of including the radial gradient

as

a secondary stress in

piping will be discussed when the details o the applicable equa

tions in NB-3600 are addressed.

A more current area in the piping rules in Section III, Division

1, that has resulted in significant controversy concerns seismic

requirements. Again, this will be addressed when the details of

the applicable equations are discussed.

8.2 NUCLEAR CLASS 1 NB 3600

With the acceptance o ANSI B31.7 and the adoption o piping

design rules in Section III, the piping design industry was sub

jected to dramatic changes that resulted in a significant increase in

manpower, a proliferation

o

computer analysis, and the loss o the

piping designer. Despite the fact that the rules have been in exis

tence since the publication of B31.7 in 1969, there still remains

some confusion regarding the proper application of these rules. It

is hoped that this discussion will help

to

alleviate that confusion.

Based on a review o NB-3600 and NB-3200, the two sections

appear to be completely different. In NB-3200, little guidance is

provided in the areas o load or stress calculation, but considerable

discussion and guidance is given concerning stress characteristics,

categories, allowables, and criteria. In NB-3600 there is basically

no discussion o these issues, but equations are provided to calcu

late the required stresses to be compared with allowables.

As

the

requirements o NB-3600 are discussed, the important areas will

be compared to those o NB-3200. We will see that for primary

stress protection, NB-3200 and NB-3600 are quite different, but

in the secondary and peak stress areas, including fatigue, they are

essentially the same.

In developing rules for piping design in ANSI B31.7, the com

mittee took advantage o the existence o industrywide standards

for pipe and pipe fittings [4], [6], [21] and [22]. These stan

dards provide a reasonable control on geometry and therefore it

is

assumed that a specific fitting will respond to an applied load in a

known manner. For example, a 4 in. schedule 40 elbow purchased

to

the same standard will deform in the same manner when sub

jected to an in-plane bending moment, whether it is in a nuclear

or a fossil plant. This led to the use o stress indices (B, C, and K

factors) that allow the engineer to calculate the stress in a straight

piece of pipe o the same diameter and wall thickness

as

the fitting

and multiply that value times an index to calculate the stress in the

fitting for the intended protection (primary, secondary, or peak).

The use o standard fittings that provide control on geometry and

allow the use

o

stress intensification factors

(i

values) was first

introduced in B3 l to calculate thermal expansion stresses. Signif

icant discussion of stress indices and stress intensification factors

is provided in Chapter 38.

8.2.1 Satisfaction o Primary

Stresses

for Design

Conditions

The NB-3600 approach to the satisfaction

o

primary stresses

relies on both design-by-rule and design-by-analysis methods. The

first task is to determine wall thickness. The equations in NB-

3600 for required wall thickness are

no

different than they were

in the past in the

ANSI B3 Codes [8] and [21]. One issue that

is proven to be critical in the industry is the failure to consider

service environment and flow conditions when determining wall

thickness. The required information for making this determination

must be in the Design Specification and should include proper

material selection to preclude attack or at least sufficient warning

to the user. The piping analyst cannot be expected to make these

decisions; his/her responsibility is

to

satisfy the Code by using the

Design Specification requirements. An inadequate Design Speci

fication used by the analyst to satisfy Code requirements does not

assure pressure boundary integrity.

a) Primary Membrane NB-3640)

The equations provided in

NB-3641.1 for deter mining the minimum wall thickness of piping

are

as

follows:

COMPANION GUIDE

T

THE ASME BOILER PRESSURE VESSEL CO

PD

0

tm

=

+A

2 Sm

+Py

(8.1)

Pd+ 2A Sm +Py

tm =

2 Sm

+P y

- P)

(8.2)

Definitions

o

the equation terms are found in NB-3641.1. Note

that the term A is partially defined as an additional thickness to

provide for material removed in threading, corrosion, or erosion

allowance.

It is

this A factor that requires sufficient input and guid

ance from others for the analyst to provide an appropriate value to

increase the wall thickness to preclude failure. With the exception

o rolled and welded pipe, once tm is determined the next higher

schedule size

is

usually used.

Determining minimum thickness satisfies a portion

o

the pri

mary stress requirements - that is, primary membrane stress.

For standard fittings purchased and used in accordance with the

requirements of NB-3691.1, no minimum thickness analysis is

required. The basis for this is that fittings that satisfy the ANSI

standards [22], [6], [23] are considered acceptable because their

pressure-temperature ratings are based on burst tests, thereby

assuring the fitting will withstand the design pressure. The engi

neer must assure that short radius elbows manufactured in accor

dance with ANSI B16.28 [4] have a minimum thickness in the

crotch region 20% greater than required

by

the pipe schedule. This

is required to maintain the pressure stresses in the crotch region

at an acceptable level. The pressure stress in a toroidal shape,

such

as

an elbow, varies linearly across the toroid, being mini

mum at the extrados and maximum at the intrados. This variation

and maximum value is a function o the toroidal radius - that

is, a short radius elbow has higher stresses due

to

the same pres

sure in the intrados than does a long radius elbow

o

the same

diameter and wall thickness. There is some question whether this

is required since the maximum stress is so localized; however,

because the committee has made this a requirement, it must be sat

isfied. For pipe bends, the wall thickness after bending must sat

isfy the minimum thickness requirements. Table NB-3642.1 (b -1

provides guidance on the wall thickness to be used prior to bend

ing. Note that bending pipe results in a thinning o the wall on

the extrados.

It

is important not to use excessive wall thickness

since crimping o the metal can occur in the intrados, resulting in

the potential for stress concentrations

to

exist.

For intersections, a set o different rules applies. Tees manu

factured in accordance with an ANSI or MSS standard listed in

Table NB-3132-1 [13] are acceptable for satisfying primary mem

brane stress requirements. Branch connections not manufactured

to

a standard listed in Table NB-3132-1, and those that are fabri

cated, must satisfy the reinforcement requirements

o

NB-3643.3.

These reinforcement rules require that the area o metal removed

for the branch connection must be available in a limited distribu

tion area around the opening - that is, the portion o pipe mate

rial that has been removed that carried membrane stress must be

replaced in close proximity to the area removed. These rules for

intersections and other standard fittings have been in place for

many years and are based on the ANSI B31.1 approach

to

pro

tection against burst-type failure.

The fact that minimum thickness and the use o standard

(pressure-temperature-rated) fittings both satisfy the primary

membrane requirements is a departure from the design-by-anal

ysis requirements. This should make the user aware that piping

Codes have been based historically on pressure protection and the

use o pressure-temperature consideration has alway

important and successful.

In

design by analysis, the a

intensity in the component wall must be calculated

sure and other design mechanical loads, and this st

is

compared

to

the allowable stress value,

Sm.

For N

average stress in the pipe wall due to design mechan

not considered a primary membrane stress. The str

from these external loads is considered in the next

mary stress protection. The following paragraphs add

olution o primary stresses for Design Conditions o

stress requirements for Levels

A,

B, C, and D operatin

are discussed later.

b) Primary Membrane plus Bending NB-3652) T

bit misleading since the approach in piping is again q

than that for design by analysis. In piping, equation (9

to satisfy what is defined

in

NB-3200

as

primary me

primary bending stress intensity. This equation is r

load-based equation since it provides the maximum

stress, rather than a maximum shear stress, in a piping

For example, in a thin shell cylinder, the stresses du

are shown in the equations that follow.

For axial stress:

For hoop stress:

For radial stress:

PDo

Ix =

4t

PDo

YH

=

t

Y =

- P

The maximum shear stress intensity is aH - aR or

In equation (9), the pressure term is

PD

0

i

2t

On the surface this appears

to

represent the hoop

cylinder. However, the

B1

factor is important; for a s

o

pipe (a cylinder), B1 = 0.5. Therefore, the press

equation (9) for a straight piece of pipe represents the

due

to

pressure, PD

0

/4t.

Equation (9)

is

provided

in

NB-3652

as

follows:

where

B1,

B = primary stress indices for the specific p

investigation (NB-3680)

P

= design pressure, psi

D

0

=

outside diameter o pipe,

in.

(NB-3683)

t = nominal wall thickness o product,

in.

(N

I =

moment

o

inertia, in.

4

(NB-3683)

M;

=

resultant moment due

to

a combinatio

mechanical loads, in. lb. All design mech


3/18

268 • Chapter 8

and combinations thereof shall be provided in the

Design Specification. In the combination of loads, all

directional moment components in the same direction

shall be combined before determining the resultant

moment - that is, resultant moments from different

load sets shall not be used in calculating the moment

M;

).

f he method of analysis for earthquake or other

dynamic loads is such that only magnitudes without

relative algebraic signs are obtained, the most conser

vative combination shall be assumed.

Sm = allowable design stress intensity value, psi.

The pressure term was discussed previously. The moment term,

without the stress index B2, is merely the axial bending stress in a

straight cylinder due to an external moment. Therefore, equation

(9) provides the axial stress in a piping component. The limit on

this stress is based on limit load design theory and assumes that

the material is elastic-perfectly plastic with no strain hardening.

Using this assumption, the stress limit was developed by provid

ing a margin on the actual limit load stress curve for combined

tension and bending on a rectangular section. This is shown in

Fig. 8.1. Quite often, the point is made that 1.5 is the shape fac

tor for a particular section and should not be used for hollow

circular sections. This point is usually made in reference to the

allowable stress of l

5Sm

for primary bending for Class 1 compo

nents.

t

is important to recognize that the 1.5 factor used with

Sm

is not an attempt to provide a shape factor ; rather, it is a factor

that, when taken with the allowable stress Sm), provides a mar

gin on the theoretical limit load for any elastic-perfectly plastic

material. For ferritic materials,

Sm

can never be higher than

~ S y ;

therefore, l.5Sm results

in

an allowable stress for primary mem

brane plus primary bending that is never higher than Sy on the

outside wall

of

the pipe. Figure

8.1

indicates that the theoretical

limit stress varies from 1.5Sy with no membrane stress present

to

l.OSy

with only membrane stress present. It should be noted

that the theoretical limit stress peaks at approximately 1.65 with

a combination

of

bending and membrane stress when the mem

brane stress

is

approximately ~ S y . The theoretical limit of 1.65

should not be used for design purposes since it would negate the

margins of safety.

In reality, Fig.

8.1

does not represent the margin between Code

allowable and theoretical limit load for a straight cylinder that has

a shape factor of about 1.33 instead of 1.5. However, it should be

recognized that for elbows, the addition of membrane stresses due

to pressure increases the capacity of the elbow to carry a bending

moment. This is recognized in the winter 1981 addenda to NB-

3600 of Section III that provided an equation to determine B2,

which considered the effect of pressure on collapse of an elbow.

This effect varies from zero to a value equal to that for a straight

pipe. Normally, in most piping systems, the elbows are consid

ered the weak link with respect to collapse resulting from external

mechanical loads.

(c)

Moment Calculation

The fact that the stress calculated in

equation (9) is a longitudinal stress and the allowable is based on limit

load restrictions assuming elastic-perfectly plastic material with no

strain hardening is one part of he picture; the other part s calculation

ofloads. The value

ofPis

given as the design pressure and is provided

in the Design Specification. The moment M; must be calculated

and considers those design mechanical loading conditions provided

in the Design Specification.

t

must be noted that NCA-2142.1 [1]

is quite specific in indicating that Design Loadings are those that

exist under the most severe Level A loading conditions. Based on

BENDING

STRESS

s,

165

1.5

1.0

0.5

;:

ACCEPTABLE

STRESS

0.5

MEMBR NE

STRESS

s,

I

I

I

I

I

FIG 8 1 LIMIT LOAD CURVE

1.0

this, design pressure, design temperature, and design mechanical

loads may not be the maximum values that the piping system is

subjected to since higher conditions may exist for othe r Service Level

loadings. This is unique to Section III design, and exists because

a number of postulated events must be considered to provide the

necessary protection associated with nuclear power generation. That

is, conditions other than those associated with the operation of the

plant must be considered. Some of hese conditions are earthquakes

of

such magnitude that they have a low probability of occurrence, as

well as large pipe rupture and small pipe breaks.

Load calculation, which is of major importance, is the item that

is not addressed in NB-3600 just as it is not addressed in NB-

3200. One can find great detail and limitations on the calcula

tion

of

stress indices [18] and [19], including those to be used

for detailed analysis (NB-3684), but little if any guidance on load

calculation. Of course, the determination of the moment values to

be used in the individual equations

of

NB-3650 is critical to the

value of calculated stress, and some in the industry believe that it

is more important than the stress indices.

For example, conservative assumptions made with respect to

thermal expansion or anchor point loads may not be conserva

tive when considering dynamic loading. For thermal expansion,

the assumption that anchors, equipment nozzles, and supports are

rigid is a conservative one. In general, this is not the case for seis

mic loading where the most standard approach is to have a design

where the fundamental frequency

of

the piping system is above

the peak of the spectra. Flexible anchors, equipment nozzles, and

supports will result in a lower frequency than calculated with

those items rigid, which could result in the actual design frequency

being in the peak of the broadened spectra. In f;;ict, the accurate

calculation of piping moment loading is one of the most diffi-

COMPANION GUIDE TO THE ASME BOILER PRESSURE VESSEL C

cult analytical problems that exist in meeting Code compliance.

There are many variables that have an effect on the actual moment

value, some of which were mentioned previously. In many cases

where the results of standard analytical techniques for determining

thermal expansion loads were compared to experimental results,

there have been large variations. In some cases, the analytical val

ues were greater than the experimental; in others, they were less.

This is rather ironic when compared to the details provided in

the Code with respect to calculating stress indices. Piping design

history, including that

in

the nuclear power field, indicates that

failures as a result of thermal expansion are highly extraordinary.

The majority

of

failures in piping in nuclear plants have resulted

from erosion, erosion-corrosion, or vibration. Note that the first

two causes can be minimized by material selection, additional wall

thickness beyond Code requirements, chemistry control, or other

measures that essentially are outside the scope of the piping ana

lyst. Although NB-3613.1, Corrosion or Erosion, states that the

wall thickness of the piping shall be increased over that required

by other design requirement and shall be consistent with the spec

ified design life of the piping, the analyst/designer normally is not

qualified to determine this. The Design Specification must provide

the required information or require the use

of

a material that resists

these phenomena. The third cause, vibration, usually results from

poor design of such items as drains, strainers, and other small

branch connections that were not supported properly. NB-3622.3

requires that piping shall be arranged and supported so that vibra

tion will be minimized. It further requires that the designer shall be

responsible, by design and by observation under start-up or initial

service conditions, for ensuring that vibration is within acceptable

levels. Experience indicates that this does not occur in most cases.

With respect to the moment term in equation (9) for design con

ditions, the designer is dealing only with weight loading; the solu

tion is not a concern, nor is it affected considerably by the stiff

ness assumptions discussed earlier. In reality, the designer will

have located supports prior to satisfying equation (9) by µsing a

standard hanger spacing table, most of which are based on simple

supported beams using an allowable stress in the order

of

5000 psi

or less. In locating supports, the designer should attempt to place

them where vertical displacement is minimal so that their effect

on thermal expansion will be slight. Doing so at this point in the

design stage is a judgment call that relies on the experience of the

designer. The thermal expansion analysis will verify that experi

ence. A more detailed discussion of this issue and a procedure for

design process is provided in Section 8.4.

In the definition

of M;

there is discussion

of

earthquake loading

and how the moment values generated as a result of that analysis

must be handled. As discussed previously, for Design Conditions

the designer should not find a requirement to analyze for a seismic

event in the Design Specification. The discussion under the defini

tion of M;

is

there to deal with protection associated with dynamic

events that are classified

as

Level B, C, or D in the Design Spec

ification.

In

summary, primary stress protection for Design Conditions in

NB-3600 is as follows:

(1) calculation of minimum wall thickness;

(2) use of standard fittings;

(3) area replacement rules for intersections;

(4) 20% increase in short radius wall thickness;

(5) assurance that the extrados of bends satisfies minimum wall

thickness; and

(6) satisfaction of equation (9).

8.2.2

Fatigue, Elastic Action Requireme

NB-3653 and NB-3654)

The NB-3650 requirements for fatigue protectio

design-by-analysis requirements of NB-3200 [9]. T

ference is that NB-3650 defines how the stresses w

lated.

t

is critical for the user to recognize that th

of fatigue requirements involves all operating cond

Design Specification that are classified as Levels A

written in NB-3650, all of the requirements, includi

for fatigue are provided under Consideration

of

Lev

Limits (NB-3653). t is not until the Code user read

ation of Level B Service Limits (NB-3654) that he

these conditions must be included to satisfy all

of

the

ofNB-3653.1 through NB-3653.7. Perhaps this notifi

be better placed in NB-3653. In addition, some Ser

loadings must be analyzed to assure primary stress

The fatigue rules

of

both NB-3200 and NB-36

determination of whether the piping

is

cycling ela

some elastic-plastic cycling is occurring. This is

in NB-3653.1 using equation (10) by considering e

ing condition listed in the Design Specification as

Level B condition. This equation is as follows:

P

0

D

0

D

0

Sn = C1

[

C2

2J M;

+

C3EablaaTa

- CXbT

where

C1, C2,

C3 = secondary stress indices for the spe

nent under investigation (NB-3680).

Do,

t,

I, Sm =

as defined for equation (9)

Po =

range

of

pressure, psi

M;

=

resultant range of moment that occu

system goes from one service load s

in. lb. Service loads and combinat

shall be provided in the Design Speci

combination includes earthquake effe

be either (1) the resultant range of m

the combination of all loads conside

of the range of the earthquake, or (

tant range of moment due to the full

earthquake alone, whichever is greate

Ta Tb) =

range of average temperature on side

structural discontinuity or material d

O

F.

t

is important to understand what the Code me

tic cycling. The allowable stress value of

3Sm,

assu

rial where

Sm

=

~ S y

can be defined

as 2Sy.

This

since a

2Sy

limit will ensure shakedown to elastic a

few cycles. Figure 8.2 shows this graphically for elas

plastic material. A calculated elastic stress of

2Sy,

po

in a strain of

2ey.

Once the stress reaches the yield p

ther increase in stress will occur, but strain increase

This is shown along the line OAB'. This value of stra

the strain associated with the fictitious elastic stress o

unloading occurs, the fictitious stress is assumed to re

from

R

However, the existing strain of

2ey

must b

dated. This happens by self-springing of the materi

into a compressive stress. During the subsequent cycl


4/18

270 • Chapter 8

(a) ELASTIC SHAKEDOWN

I

I

S

- - - - - -

;

1

1

C

·Sy

I I I

I I I

(/)

rJ

(/)

UJ

a

1-

/)

(b)

PLASTIC SHAKEDOWN

8 - - - - - - - -

)

I

I

I

I

I

, 1

I

I I

FIG. 8.2 SHAKEDOWN FIGURE

ponent will cycle elastically between C and B . This phenomenon

is called shakedown to elastic action.

It

is

important

to

recognize that in most cases, the number

of stress cycles, exclusive of vibration and poor fluid mixing,

applied to a piping system in a nuclear power plant is only a few

thousand. This

is

quite different from rotating machinery or air

craft where the expected number

of

cycles is in the millions and

can be assumed to be infinite. Design for so many cycles (high

cycle fatigue) usually requires limiting the stress range that can be

applied

to

the component

to

the endurance limit of the material.

Based on that, there is little or no plastic action associated with

high cycle fatigue. For most nuclear power applications the low

number of imposed cycles results in designs that can allow strains

in

excess of the yield strain. Note that strain, rather than stress, is

the important value when cycling above the yield strength where

fatigue damage is a function of plastic strain. For convenience,

the Section III fatigue curves have been developed using fictitious

stress values (actual strain x the elastic modulus) so that they can

be

directly comparable to the stress calculated on the assumption

of elastic behavior. Differences between NB-3200 and NB-3600

are discussed in the following paragraph.

There are a number of issues to understand when solving equa

tion (10). The first is that every load dealt with represents a range

of loads - this is the difference at this point between NB-3200

and NB-3600. Under NB-3200, the designer is required to cal

culate ranges of stress (using maximum shear stress theory) for

each possible set

of

operating conditions and compare the results

to 3Sm. In NB-3600, the designer is required to calculate ranges of

loads for each possible set of operating conditions and

to

use the

loads

as

input

to

equation (10) to determine compliance, which

is

not

as

readily accomplished

as

it may seem.

In

addition to calcu

lating ranges of loads, the designer must be sure that the number

of cycles for each condition considered is accounted

for.

The set

of conditions used to calculate the ranges of load do not have to

occur in sequence. For example, the most severe range of Ta

-

Tb

can occur between a set of operating conditions, one of which

occurs in the first year

of

operation and the other in the last.

This process of load range determination requires a significant

amount of time with involvement of a number of disciplines. The

variations of pressure and thermal expansion moments versus time

are fairly simple since each Level A and B Service Condition in

the Design Specification provides the linear change in pressure

and temperature versus time. Based on that, the pressure is a given

value and the thermal expansion moment

is

merely the calculated

value at some temperature chosen by the analyst times the ratio

of the temperature of the Service Condition under consideration

divided by the calculated value temperature. The determination

of Ta - Tb

values

is

much more involved. In the first place, the

definition of Ta Tb) in NB-3653.l is

as

follows:

Ta Tb)

= range of average temperature on side

a b)

of gross

structural discontinuity or material discontinuity,

0

F

For generally cylindrical shapes, the v e r g ~ f T

(NB-3653.2) shall be over a distance of vi

dat a

for

Ta

and over a distance

of V bib

for

Tb.

ta tb) = v e ~ wall thickness through the length ./d ;;t;,

(vidbtb),

in.

A trial-and-error solution for ta and tb

may be necessary.

The first task

is to

understand what a gross structural disconti

nuity is. NB-3213.2 defines this

as

follows:

Gross structural discontinuity is a geometric or material

discontinuity which affects the stress or strain distribution

through the entire wall thickness of the pressure retaining

member. Gross discontinuity type stresses are those portions

of the actual stress distributions that produce net bending

and membrane force resultants when integrated through the

wall thickness. Example of gross structural discontinuities

are head-to-shell and flange-to-shell junctions, nozzles (NB-

3331), and junctions between shells of different diameters or

thicknesses.

In a piping system,

we

therefore can expect

to

be concerned

about Ta Tb) values at components such

as

branch connections,

reducers, flanges , changes in wall thickness, and changes in mate

rial. To understand what is required by equation (10) with respect

to the

Ta - Tb

value, let us consider a very simple situation, such

as

a change of material

in

a straight piece of pipe. Different mate

rials will result in stresses

as

the fluid temperature changes since

their thermal diffusivity and conductivity are different, resulting

in variations in average temperature. Also, the coefficients of ther

mal expansion and Young's Modulus are different , which would

result

in

stresses even if both materials were at the same temper

ature. To determine the values of Ta Tb), it is necessary to know

the

flow

rate related to the condition under consideration since

this will determine the film coefficient and therefore the transfer

of

heat between the fluid and the pipe. Two conditions having the

same change in fluid temperature over the same time, but with

different

fl.ow

rates, would result

in

different values of Ta Tb)

COMPANION GUIDE TO THE ASME BOILER PRESSURE VESSELCO

and therefore different stresses. The analytical techniques used to

determine Ta Tb) could have a significant effect. For example, if

axial heat transfer between the two materials

is

ignored the results

will be conservative. The issues that can affect this one load set

for equation (10) are significant: fl.ow rate, time, modeling, ther

mal diffusivity, and conductivity. Once the load set is known, dif

ferences

in

coefficient of thermal expansion and Young 's Modu

lus will affect the stresses. These types

of

consideration in piping

analysis are far different from what the piping designer faced prior

to

the issuance of B3 l.7 .

For those nuclear units that have piping systems designed to

Section III, Class 1, or to ANSI B31.7, some of the issues dis

cussed above are important. The role of the piping analyst during

plant design is

to

satisfy the Code, which in the case of fatigue

evaluation means having a cumulative usage factor

of

le

ss

than

1.0.

In

many cases, conservative assumptions may be made, which

is acceptable if the Code requirements are met. Some of these are

directly related to the

Ta Tb)

term,

as

well

as

other thermal load

terms covered in equation

(ll)

.

Film coefficients may be taken

as

infinite, axial heat transfer may be ignored, and the conserva

tive grouping of conditions with different changes in temperature,

fl.ow

rate, and time may occur. This is perfectly acceptable and

demonstrates, in a conservative ~ a n n e r that the Code has been

satisfied. However,

an

operating unit concerned about extending

life or dealing with piping systems subjected to conditions not

considered in the original design may want to review the design

analysis to eliminate conservatisms.

Figure 8.3 demonstrates the difference in approach between

NB-3200 and NB-3600

in

satisfying the elastic cycling require

ments of Class 1 Figure 8.3(a) demonstrates the stress range that

would be obtained using the design-by-analysis rules of NB-3200,

which require the calculation of the range of maximum shear

stress versus time. Figure 8.3(b) demonstrates the ranges of loads

that must be determined prior to solving equation (10). Figure

8.3(c) demonstrates the type of differences that can occut when

comparing the NB-3200 and NB-3600 results. Note that the results

of

equation (10) will usually result

in

a larger range

of

stress than

design by analysis, which is why Fig. 8.3(c) indicates that result.

It

is

important

to

recognize that the example used deals with only

two conditions. Even for three conditions , this

is

relatively simple

+

STRESS

RANGE 0

LEVEL A

CONDITION 4

- that is, combine 1 and 2, l and 3, and 2 and 3. For

conditions, however, the task is much more difficult

8.2.3

Fatigue Elastic-Plastic Requiremen

NB-3653.6)

NB-3653.1 states that equation (10) shall be sat

pairs of load sets, that is, for the ranges of loads re

the consideration of each pair of Level A and Lev

ing conditions. Satisfaction of this requirement mea

piping component will cycle elastically for all spe

tions. However,

NB

-3653.6, Simplified Elastic- Plas

nuity Analysis, provides an alternate analysis that

for those load set pairs that do not satisfy equation (10

nate analysis limits two stresses, thermal expansion

plus secondary membrane plus primary membrane,

imposes a factor Ke) on the alternating stress that

elastically calculated value to account for plastic cycl

Ke

value and alternating stress will be discussed w

(ll)

is addressed. The first requirement under NB-3

assure that the thermal expansion stresses satisfy

3S

(12) provides this assurance; it

is as

follows:

where

M = same

as in

equation (10), except that here it

moments due to thermal expansion and th

movements.

This requirement does not exist for other comp

design by analysis. It exists for piping to assure tha

expansion requirements in place since

B3

l.1 develop

met.

An

explanation of the comparison between this

B31. l is provided.

The second requirement under NB-3653 .6(b) is

the primary plus secondary membrane plus primary b

intensity is equal

to

or less than

3S,,,.

Thermal bend

LEVEL B

CONDITION 9

1

NB-3200

STRESS

RANGE

FIG. 8.3 a) NB-3200 STRESS RANGE


5/18

272 • Chapter 8

LOAD

RANGE

+

LEVEL

A

LEVEL B

CONDITION 4

CONDITION 9

FIG. 8.3(b) NB-3600 LOAD RANGE

+

NB-3200 NB-3600

/ . /

EQ. (10)

RANGE

0

\

\ \

/

\

J

/

- - . . - - - - - - - ~

_ . . _ · - - - - 1 ~ - - - - - - -

LEVEL A

LEVEL B

CONDITION4

CONDITION9

FIG. 8.3(c) NB-3200 VERSUS NB-3600

ma expansion stresses are excluded from this requirement. This

requirement is the same as that in NB-3228.5(a) , except that NB-

3653.6(b) provides equation (13) as follows:

(8.10)

With the exception of the

C3

value, this equation looks identi

cal to equation (10); however, the

M;

value includes only those

moments required to satisfy design conditions as defined in NB-

3652 since

we

are evaluating membrane stress only. The C3 val

ues are essentially one-half the

C3

values for most components

except girth butt welds. The reason for these lower values

is

that

the bending component

of

the Ta -

Tb

stresses are not included in

this calculation. Since there are few discontinuity bending stresses

at a girth butt weld except for any weld reinforcement effects, the

C3

and C3 values are relatively close (0.60 and 0.50 respectively)

for that component. Satisfying equation (13) limits the membrane

stresses, averaged through the wall,

to

3Sm, which satisfies shake

down criteria. There has generally been continuous discussion

concerning the Ta - Tb term - that is, whether

it

should be

included since displacement of the component would relieve the

membrane stresses that exist due to the Ta

- Tb

loading. However,

since the criteria is in place to assure that progressive distortion

with each cycle will not occur, it

is

imperative that any membrane

stresses resulting from a thermal discontinuity be considered. The

displacement that would relieve the thermal membrane stress is

the progressive distortion that is being protected against.

Prior to solving equation (13) of NB-3653 .

6,

the thermal stress

ratchet requirement

of

NB-3653.7 must be satisfied. This require

ment limits the allowable range of linear radial gradient as a func

tion of the ratio of the hoop stress due to pressure over the yield

strength of the material.

8.2.4 Fatigue Evaluation (NB-3653.2)

Having satisfied either equation (10) or the simplified elastic

plastic requirements of NB-3653 .6(a) and (b), the .fatigue evalu

ation can now be completed. The fatigue approach

in

NB-3600

requires the determination of the alternating stress intensity for

each pair of Level A and Level B operating conditions . For those

COMPANION GUIDE TO THE ASME BOILER

&

PRESSURE VESSEL

pairs of conditions that satisfied equation (10), the alternating

stress intensity

Sa)

can be used directly. For those pairs that did

not satisfy equation (10), but did satisfy NB-3653.6(a) and (b), and

NB-3653.6, the alternating stress intensity value must be modified

to account for the fact that plastic cycling is occurring .

The first step in the fatigue evaluation is to determine the value

of the peak stress intensity Sp). This

is

provided by equation (11)

as follows:

where

IAT1

I

K3C3EablO aTa

-

O bTbl

1

+ E°IAT2

l - v

(8

.11)

K

1,

K

2 ,

K

3 = local stress indices for the specific component

under investigation (NB-3680)

Ea

=

modulus of elasticity

E)

times the mean coef

ficient of thermal expansion (a) both at room

temperature, psij°F

IAT I

= absolute value

of

the range for that portion

of

the nonlinear thermal gradient through the wall

thickness not included in AT

1

, °F

IAT I

= absolute value of the range of the temperature

difference between the temperature

of

the out

side surface and the temperature of the inside

surface of the piping product assuming moment

generating equivalent linear temperature distri

bution,

0

F.

All other terms are as defined for equation (10).

Terms 1, 2, and 4 in equation (11) are the same

as

in equa

tion (10) with the inclusion of a K value. The K value is defined

as

a local stress index in NB-3653.2; it can be considered a stress

concentration or a fatigue strength reduction factor. No distortions

are associated with the local stress indices, which result in very

localized stresses that are a concern

in

fatigue crack initiation and

propagation. An example would be the very local stresses associ

ated with a sharp geometric corner.

Terms 3 and 5 in equation (11) are new. These terms determine

the stress in a component resulting from a radial thermal gradient

- that is, a gradient through the wall thickness. NB-3653.2(b)

provides detailed quantitative definitions of the values of jAT1 I

and of IAT2I. NB-3653.2(a) defines IAT1 as the absolute value

of the range of temperature difference between the temperature

of the outside surface

T

0

)

and the temperature

of

the inside sur

face T;) of the piping product, assuming a moment generating

equivalent linear temperature distribution ,

0

F.

jAT

2j

is defined

as

the absolute value of the range for that portion of the nonlinear

thermal gradient through the wall thickness not included in

I

AT

1 ,

0

F. Based on these definitions, for any radial gradient through the

piping product wall thickness, there is

an

equivalent linear radial

gradient that will result in the same moment as the actual gradient.

Using the linear radial gradient,

AT

1

is

the difference between the

temperature on the outside wall

T

0

)

and on the inside wall

T;).

AT2

is the difference between the actual temperature on the inside

wall and the value obtained to determine

AT

1

using an equiva

lent moment generating linear gradient. In a practical sense these

two terms are generated when a thermal transie

fluid in a piping component. The magnitudes a

the change in temperature, the time over which

occurs , and the value of the film coefficient, w

the amount of heat transferred from the fluid to

For a given load condition, the higher its tempera

film

coefficient and the shorter the time over wh

ture change occurs, the larger the values of AT

Recognize that these values change with time. Fo

perfect insulated exterior surface on a pipe, these

at zero, increase

to

a maximum value over time,

to zero.

It

should be noted that the maximum va

AT2

do not normally occur at the same time

in

a

Figure 8.4 is provided

to

show the effect of a

values of AT 1 and AT2, assuming a perfectly in

surface and a high film coefficient. Figure 8.4(

there

is no

gradient through the wall thickness

tiation of the transient. Figure 8.4(b) indicates

transient only a small portion of the wall thickne

and there

is

no through-the-wall gradient. Based

zero

in

Fig. 8.4(b) and AT2 is the difference bet

inside surface temperature and the isothermal tem

rest of the wall thickness. Figure 8.4(c) indicates

transient the entire wall thickness is responding to

change. The actual radial gradient shown results i

is equal to the linear gradient shown. Using the

AT1

is

the difference between the outside and in

values obtained from the linear gradient and

AT

1

A

PRIOR TO

TRANSIENT

J

I

B

VERY EARLY

~ A T ~ ? J

\

T

1

c

LATER

D

AFTER TRANSIENT

FIG. 8.4

GRAPHIC DESCRIPTION OF a

T


6/18

27 4 • Chapter 8

ence between the actual inside wall temperature and that obtained

from the linear gradient. Figure 8.4(d) represents the fact that, over

time, the entire wall thickness has become isothermal and that no

through-the-wall gradient exists.

With respect

to

calculating Sp it

is

necessary

to

determine

the ranges of load sets for each combination of conditions. The

approach

is

the same as was required for determining

Sn

using

equation (10).

Having determined the values of Sp for each load set, the value

of the alternating stress intensity, Sa, can now be determined. For

those pairs of load sets that satisfy the

3Sm

limit of equation (10),

the value

of Sa is

equal

to

one-half the value

of Sp

calculated

by

using equation (11). For those pairs of load sets that do not satisfy

the 3Sm limit of equation (10), a penalty factor must be applied

to

the alternating stress intensity. This

is

required since the fatigue

rules in Section III were written assuming elastic cycling. Since

these load sets do not satisfy the elastic cycling requirements, the

Sa

value must be calculated using equation (14) applying a

Ke

factor greater than 1.0.

As discussed in Section 8.1, B31.7 included the radial gradi

ent in the secondary stress category; this was adopted

by

Section

III

when nuclear piping was included. This resulted in a number

of

load sets exceeding

3Sm.

The greatest impact

of

this was the

fact that the Nuclear Regulatory Commission (NRC) established

requirements for postulating pipe break locations based on the

value

of

S

11

(equation 10) as well as the calculated fatigue life. The

NRC approach resulted in the need to postulate a significant num

ber of break locations because the inclusion of /j.T1 in calculating

Sn

increased this stress beyond the NRC limit. Postulated break

locations required that the broken pipe be kept from whipping

due to the large forces generated by the sudden release of inter

nal pressure. The pipe whip restraints required

to

do this were,

in many cases, massive structural members that could interfere

with displacement

of

the piping during normal plant operation and

interfere with normal plant maintenance and inspections required

by Section XI. A resolution to this concern was to remove /j.T1

from equation (10), which was a return

to

the original Section III

criteria document. This change resulted in a significant reduction

in postulated break locations. The NRC followed with the leak

before-break concept for primary coolant systems, which elimi

nated the requirement for most

of

the massive pipe whip restraint

structures. The leak-before-break concept is,

of course, the basis

for the Section III rules. That is, for the pressure-retaining mate

rial allowed

by

Class 1 and for the design approach, flaws of sizes

significant enough

to

be detected will be detected with sufficient

time margin prior to catastrophic severances.

Also in Section 8.1, there

is

a discussion of the approach devel

oped by B3 l.7 for applying a factor

to

the alternating stress when

the

3S

limit for primary plus secondary stress was not satis

fied. The B3 l.7 approach applied an A factor greater than 1.0 to

the calculated alternating stress. A good description of the need

and technical basis for this technique

is

provided in reference [5].

When Section III adopted piping rules

in

1971, the approach

to

fatigue correction for plastic cycling was modified. The A factor

was replaced with a

Ke

factor, which is discussed in reference

[16]. The concept

is no

different than that presented in reference

[5]; however, the values

of Ke

are different than the

B3 l.7 A

val

ues. It is interesting to note that the maximum A values in B3 l.7

are lower than the maximum

Ke

values in Section III. In addi

tion, for some materials the

A

value

in

B31.7 had an initial value

at

3S

greater than 1.0. When the French first published their

Nuclear Pressure Vessel Code, the correction factor they applied

to

the alternating stress for plastic cycling was closer

to

the

B3

l.7

A

values than the Section III

Ke

values [28], [29]. At that time,

the Design Subgroup of Section III reviewed this difference and

the consensus was

to

stay with the

Ke

correction factor, which

is

conservative. The

Ke

factor is dependent on the material strain

hardening exponent

n)

and a material factor (m) to determine the

rate of increase in

Ke.

(See Figs. 8.5 and 8.6.)

Having determined the Sa values for each set

of

loading con

ditions, the cumulative usage factor must be determined. This

requires the use of the Code fatigue curves, and NB-3600 uses

the same curves as NB-3200. The basis for these curves

is

dis

cussed in Chapters 4 and 6 .

The approach used is to evaluate the various stress cycles to

which a piping component

is

subjected and determine its accept

ability. The cumulative effects of the various stress cycles are

evaluated using a linear damage relationship. This assumes that if

a certain number of cycles N 1 would produce failure at a given

stress level, then a fewer number of cycles

(n1)

at the same stress

level would use a fraction

of

the life

n1/N1 .

The

Sa

value for a

given set of conditions is entered into the fatigue curve and an

allowable number of cycles N

1

is determined for that Sa value.

K

c

K,

A

DEFLECTION

K

ELASTIC

STRESS

CONCENTRATION FACTOR

K MAXIMIM STRAIN CONCENTRATION FACTOR, APPROXIMATELY EQUAL TO lin

STRAIN HARDENING EXPONENT OF THE MATERIAL

FIG 8 5 BASIS FOR

e

VALUE

REGION

A

K

I /

l/n

s. 13

s.

IS

EASILY OBTAINED

FROM

A STATIC TENSILE TEST BY MEASURll'lG THE UNIFORM

ELONGATION

AT MAXIMUM

LOAD

m

CAN ONLY

BE

DETERMINED BY PERFORMING FATIGUE

TESTS

AND IS USED

TO

PRODUCE

THE APPROPRIATE

SLOPE

IN

REGION

A

FOR

THE MATERIAL

OF

CONCERN.

FIG 8 6

e

VALUES


This number is divided into the number of imposed cycles for the

set of conditions under consideration,

an

approach that continues

for each set of loading conditions. The cumulative usage factor

is

the sum

of

the individual usage factors determined for each set

of

conditions; it cannot exceed 1.0.

This approach for determining fatigue acceptability is based on

Miner's Hypothesis. The basis

is

that fatigue damage is accu

mulated independent

of

time sequence or the order in which

the events occur. There has been some suggestion that Miner's

approach may not be fully appropriate since it

is

most accurate

when the larger stress ranges are equally distributed with small

stress ranges throughout the specified life. Since start-up and shut

down results in the full range of loading for a significant number

of

cycles and

is

equally dispersed over the life a

of

a nuclear plant,

the Code approach using Miner's Hypothesis

is an

appropriate

design tool.

8.2.5 Satisfaction o Primary Stresses for Levels B

C and D Service Limits

Paragraph 8.2.1 discusses the design philosophy and back

ground related

to

primary stress calculation for design conditions.

Paragraph 8.2.l(c) points out that the moment term, M;, used

in equation (9)

is

based on Level A loading conditions. How

ever, there are other Service Levels for which protection against

catastrophic failure must be addressed. Section III does not define

which plant operating conditions or postulated events shall be

placed into which Service Level, this being the responsibility of

the owner. Once established, the Code allowable stresses associ

ated with each Service Level shall be satisfied.

a) Level B Service Limits NB-3654)

Level B events are those

that are not associated with normal power operation but are

expected

to

occur during the lifetime

of

the plant. They are nor

mally associated with conditions such as operator error and plant

trip. Since Level B events are expected

to

occur with sufficient

frequency, they must be included in any required fatigue·evalua

tion. In addition, protection against catastrophic failure must also

be

provided. The first step is

to

demonstrate that the maximum

pressure does not exceed the following:

(

S

t )

Pa = l l Do

-

yt

(8.12)

The preceding equation results in a 10 increase of the allow

able working pressure calculated

by

using equation (3) of NB-

3641.1.

The second step is

to

determine whether or not equation (9)

must be satisfied. This is required if any nonreversing moment

loadings other than those used

in

satisfying equation (9) are spec

ified for Level B events. A detailed description

of

reversing and

nonreversing dynamic loads is provided in NB-3622 and Fig. NB-

3622-1. The basis for treating these loads differently is that revers

ing dynamic loads, which satisfy NB-3622.2, do not result

in

catastrophic failure and are objectionable

in

a fatigue sense only.

Therefore, they must only be included in the fatigue evaluation

required by NB-3653. On the other hand, nonreversing loads can

result in catastrophic failure of the piping system, and appropri

ate protection must be provided . This protection for nonreversing

loads is accomplished by determining primary stresses using equa

tion (9) with

an

allowable stress of l .8Sm, but not greater than

l.5Sy. The allowable of l.8S

is

an increase of 20 over that

allowed for Design Conditions and is based on the

occurrence of Level B events versus Design Condi

J) Seismic Loading

Although the following

is

discussion

of

Level B Service Limits, it is really

Levels C and D Service Limits. It

is

placed he

is the first time that reversing and nonreversing d

have been addressed. The issue

of

reversing dynam

are objectionable in a fatigue sense rather than col

from a significant testing program funded by the E

Research Institute (EPRI) and the U.S. Nuclear Reg

mission (NRC). A Technical Core Group was establ

uate the results of the test data and

to

make recomm

Code changes related

to

these type loads

to

Section

dynamic loads are those dynamic loads which cycle

value. The positive and negative values of the rever

not

be

(and usually are not) consistent, but they alw

through the mean value. Figure

NB

-3622- l b) show

graphically. A number

of

piping components were

was subjected to a number of earthquake-type loads

failure of the component was a fatigue or a fatigue-r

Based on these results, the Technical Core Group

the rules that currently exist in Section III for Level

reversing dynamic loads. Nonreversing dynamic loa

be treated as they always have been - that is, prot

catastrophic failure must be provided by satisfying

with varying allowable stresses related to probabi

rence of the event.

The initial concern with respect

to

the EPRI and

program was the Level D earthquake referred to as t

down Earthquake (SSE). Evaluation by the Technica

and Section

III

demonstrated that the failure mode

jected

to

building filtered loads such as the SSE w

fatigue ratchet. Rather than write a set of new ru

fatigue-ratchet, Section III decided

to

control the pri

a level that would preclude fatigue failure. Fatigue

low number

of

high stress cycles associated with th

occur when stresses are well into the nonlinear reg

rent Section III rules for Level D events limit the s

enough level (not much above the linear range) that f

failure

is

controlled and loads and displacements ca

tically are reasonably well predicted. Discussion of

in general can be found

in

Chapter 34. It should be

date, the NRC has not adopted these seismic rules. W

ing

in

Section III to study what changes, if any, a

satisfy NRC objections.

During the attempt

to

resolve differences with the

III

has had a number

of

discussions related

to

stress

area that is becoming more involved is the B ind

used to satisfy bending stress protection in equatio

nally, the

B

index for piping components was gene

0.75 times the

C2

index. Over time, the

B

values ha

been reduced as a result of further study. As discuss

above, when using

B

indices the basis for protection

The biggest example of this is for the elbow that sta

a

B

equal to 0.75C2 but never

Jess

than 1.0.

Of

cou

B

2

index for elbows ranges between 0.0 and 0.5 b

effect of internal pressure increasing the resistance

is

anticipated that the seismic rules for piping wil

from that shown

in

8.2.5(b) and (c). The

B

valu

ing dynamic loads will be based on the dynamic tes

performed

by

EPRI and NRC rather than the static

which they are currently based. This could result


7/18

276 • Chapter 8

in most of the B2 values by a factor

of

approximately 1.5 . Based

on

this approach the allowable stress for reversing dynamic loads

would return to 2.25Sm(l .8Sy) for Level C and 3.0Sm 2.0Sy) for

Level D. Therefore, the seismic stress allowables for Levels C

and D would be the same for reversing and nonreversing dynamic

loading; however, the B2 indices would be different.

(b)

Level

C Service Limits NB-3655) NCA-2142.4(b)(3) states

that Level C Service Limits permit large deformations in areas of

structural discontinuities that may necessitate the removal of the

component from service for repair or damage to the component.

Based on this, the owner shall review the selection of this Ser

vice Limit for compatibility with established system safety crite

ria. This is a warning

to

the owner that conditions placed in Level

C Service Limits could result in the inability of a component to

perform its intended function when subjected to these loads and

therefore could impact any safety function of that component.

The rules for Level C Service Limits were developed based

on the final determination of the limits prescribed for Level D to

maintain stresses to a level low enough

to

preclude fatigue-ratchet

when reversing dynamic loads not in combination with nonrevers

ing dynamic loads are evaluated. When evaluating nonreversing

dynamic loads, the Level C Service Limits are unchanged. The

permissible pressure shall not exceed the following:

Pa =

1.5

(

2Smt )

D

0

- 2yt

(8.13)

where

Pa

= the calculated maximum allowable internal pressure for

a straight pipe that shall at least equal the Design Pres

sure, psi. It may be used for piping products with pressure

ratings equal to that of straight pipe (see ANSI B 16.9).

For standard flanged joints, the rated pressure shall be

used instead of Pa For other piping products where the

pressure rating may be less than that of the pipe (e.g.,

flanged joints designed to Appendix XI and reinforced

branch connections (NB-3643), where part of the required

reinforcement is

in

the run pipe), the Design Pressure of

those products shall be used instead of Pa. Pa may be

rounded to the next higher unit of

10

.

t

=

the specified or actual wall thickness minus,

as

appro

priate, material removed in threading, corrosion or ero

sion allowance, material manufacturing tolerances, bend

ing allowance (NB-3642.1), or material to be removed by

counterboring, in.

All other definitions can be found in NB-3641.1.

Primary stress limits using equation (9) must be satisfied using

an allowable stress of 2.25Sm but not greater than l.8Sy . Satisfy

ing these two limits will preclude catastrophic failure

of

the piping

when it is subjected to nonreversing dynamic loads specified as

Level C Service Loadings.

For Level C Service Loadings that include reversing dynamic

loads that are not required to be combined with nonreversing

dynamic loads, the following shall be satisfied. The piping must

be fabricated from material designated P-No. 1 through P-No.

9 in Table 2A, Section II, Part D [30], and must be limited

to

D t

50. The limitation on material is based on two issues . The

first is that the experimental work enveloped the type of material

allowed above and the second is that this material is considered

very ductile and therefore not subject to brittle-type failure. For

any other materials, or

D t

ratios> 50, the rules for nonreversing

dynamic loading must be used .

For weight loading Mw):

(8.14)

where

Mw = resultant moment from weight effects (NB-3613)

The pressure occurring coincident with the reversing load shall

not exceed the design pressure .

For weight and inertial loading due to reversing dynamic loads

ME) in combination with the Level C coincident pressure, the

following shall be satisfied:

(8.15)

where

ME

=

the amplitude of the resultant moment due to the

inertial loading from the earthquake, other reversing

type dynamic events, and weight. Earthquake and other

reversing dynamic loads shall be computed from a lin

ear elastic response spectrum analysis as defined in

Appendix N-1226, except the spectrum peak broaden

ing value

f:: Jg

in N-1226.3 sh all not be less than 15%.

The ground motion design input for generating the floor

response spectrum to be used in the linear elastic analy

sis shall meet the requirements of Appendix N-12ll a)

and N-12ll b). Moments and forces may be computed

using a methodology other than that prescribed above

if the alternate methodology

is

demonstrated to produce

results that envelope the prescribed methodology results.

In the combination of oads, all directional moment com

ponents in the same direction shall be combined before

determining the resultant moment. f he method of anal

ysis is such that only magnitude without algebraic signs

is

obtained, the most conservative combination shall be

assumed.

Pc

=

the pressure coincident with the reversing dynamic load

The range of the resultant moment

MAM)

and the amplitude

of the longitudinal force FAM) resulting from the anchor motions

due to earthquake and other reversing dynamic loading shall not

exceed the following:

8 .16)

(8.17)

where

AM =

cross-sectional area

of

metal m the piping component

wall

The basis for the last two equations involving MAM and FM

is discussed in the following . The limit on MAM

is

a true fatigue

COMPANION GUIDE TO THE ASME BOILER PRESSURE VESSEL

limit since anchor motions are treated as secondary stresses in

Section III. The stress limits are set at a level that would con

tribute essentially zero

to

the system usage factor, considering that

Appendix N states that 10 significant cycles can be expected dur

ing an earthquake. Certainly more than 10 cycles can occur, but

their range of stress would not be significant. The allowable stress

for Level D Service Limits, on which the Level C Service Limits

are based, is set at a level that would contribute less than 0.1 to

an allowable usage factor of 1.0, assuming 20 significant cycles

occur.

The limit of FM/AM prevents failure due to axial loading.

Although this is not expected since the moment resulting from

axial loads usually controls, the extra protection was considered

reasonable.

(c)

Level

D Service Limits NB-3656) As noted in 8.2.4(b)

above, NCA-2142.4(b)(4) states that the owner must review the

selection of this Service Limit for compatibility with established

system safety criteria since the primary stress limits permit gross

general deformation with some consequent loss of dimensional

stability and damage requiring repair, which may require removal

of the component from service. Conditions placed in Level D usu

ally have a very low probability ,of occurrence since the repair

effort associated with using Level D primary stress limits could

be overwhelming.

Level D Service Limits is the area

to

which the EPRI and NRC

testing, and the evaluation thereof, was directed. In developing

and accepting final rules, Section III first dealt with the proposed

approach for Level D and then modified the allowables for Level

C.

In design of a nuclear power plant the designer must deal

with two types of dynamic loadings: nonreversing and reversing.

Whenever nonreversing dynamic loads occur, even in combina

tion with reversing dynamic loads, they must be treated

to

pro

vide protection against catastrophic failure due to one application

of load. Primary stress limits using equation (9) must be satisfied

using an allowable stress of 3Sm but not greater than 2Sy. Revers

ing loads, acting without nonreversing loads, are considered

to

be

a concern for fatigue and fatigue ratchet only. More detail on this

has been provided in 8.2.5(a).

For reversing dynamic loads under Service Limit D that are not

required

to

be combined with nonreversing loads, the following

must be satisifed. The piping must be fabricated from material

designated P-No. 1 through P-No. 9 in Table 2A, Section II, Part

D [30] and limited

to

D t 50. The limitation on material is based

on two issues: The first is that the experimental work enveloped

the aforementioned type of material allowed and the second is that

this material is considered very ductile and therefore not subject

to brittle-type failure. For other materials, or D t ratios > 50, the

rules for nonreversing dynamic loads must be used.

The sustained stress due to dead weight loading Mw) shall not

exceed

(8.18)

The pressure occurring coincident with the reversing loading

cannot exceed the Design Pressure.

The stress due to weight and inertial loading due to reversing

dynamic loads ME) in combination with the coincident pressure

shall not exceed

The range of the resultant moment due to anchor m

and the amplitude of the longitudinal force

FAM)

sh

C2MAMD0

21

6Sm

FM

AM l.OSm

Definitions for all equations are provided in 8 .2.5

The stress limits are set at a level that would c

little to the cumulative usage factor and in no case m

The FM/AM limit

is

discussed in 8.2.5(b).

As an alternative to the preceding rules for both

and reversing dynamic loads, the rules contained in

may be used in evaluating these service loadings i

of all other design and service loadings.

8.2.6

Test Loadings (NB-3657)

Paragraph NB-3657 indicates that test loadings sh

ated in accordance with NB-3226 . The requirements

provide protection against catastrophic failure from te

limiting the primary membrane, primary membrane

stress intensities, and controlling any external press

sion of these limits can be found

in

Chapter 3.3. T

is often missed

is

that the first

10

hydrostatic or pne

need only be evaluated

to

the above primary stress

beyond those

10

must be included in the fatigue eval

component. Both ASME Section XI and the O&M C

testing associated with inspection, repairs, replacem

operability of pumps and valves. These tests should

to

determine whether they need to be included in t

fatigue evaluation of piping components.

8.2. 7 Other Issues

of

Importance

a) Flanged Joints NB-3658) The rules for flang

essentially the same for all classes of piping. Class 1

that flanged joints that do not satisfy ANSI B 16 .5 s

lyzed in accordance with NB-3200. In addition, flange

do satisfy ANSI Bl6 .5 but use a bolt material having

at

lOOF

that is less than 20 ksi must also be analyz

dance with NB-3200. The rules for flanged joints in NB

changed little over the years

[7]

and therefore do not

cussion. One issue is that the new Levels C and D s

for piping may result in flanges being the limiting com

to the current Code maximum allowed moment for fl

b) Expansion

and

Flexibility NB-3672) Detailed d

this issue is provided in 8.3 for Class 2 and 3 piping

it is worth noting here that the calculation of therma

moments requires the consideration of the full range

ature that can be expected from service or shutdown

For example, if it is assumed that 70°F is the zero stre

tion and that the maximum temperature a system is s

during service or shutdown conditions is 300 °F and th

is 45

°F, then the range

of

temperature

to

be conside

-

25

°F to +230°F.


8/18

278 • Chapter 8

(c)

Stress Indices and Flexibility Factors NB-3680)

A detailed

discussion

of

these indices and factors related to moment load

ing is provided in Chapter 38. The following discussion addresses

those indices that may appear unique to the user.

The primary stress index, Bt

,

is 0 .5 for all components except

reducers and elbows. This is because the approach in piping is

to control the primary stress to a level below the limit moment

value. This approch requires that axial stresses be calculated and

compared

to an

allowable value less than the limit stress value.

Since the pressure stress in equation (9)

is

written as the hoop

stress , it is necessary to multiply that value by 0.5 to obtain the

axial stress.

For elbows, it is important to note that pressure in

an

elbow

increases its ability to withstand a bending moment without fail

ing. This results from the fact that an elbow ovalizes when sub

jected to a bending moment. The greater the ovalization, the

higher the stress. Internal pressure does not allow the elbow to

ovalize as much as an elbow with zero internal pressure, which

thereby reduces the stresses. This is recognized by allowing the

Bi factor

to

range from 0 .0 for small bend radii to 0.5 for large

bend radii . The larger the bend radii, the smaller the ovalization.

The

C3

indices are used to determine the stress resulting from dif

ferences in temperature

of

the metal

of

adjoining members Ta

-

Tb). For example, a cylinder built into a wall

of

the same material

with no consideration

of

local flexibility (the most severe case pos

sible) has a stress that can be determined using shell theory

of

S

=

1.83Ea Ta - Tb)

(8.22)

The value 1.83 can be considered the

C3

stress index. Local

flexibility would,

of

course , reduce the value

of

1.83. It is inter

esting to note that maximum values

of

C3 of 2.0 are provided in

Table

NB-368l a)-l

for socket welds and transitions . Based on

shell theory, these values are conservative.

8.3 NUCLEAR CLASS 2 AND 3

NC/ND 3600

This section

is

written

to

cover both Class 2 (NC-3600) [13) and

Class 3 (ND-3600) [13) piping design rules. The differences in

these design rules are at best slight. The major areas

of

differences

in these two Classes occur in the material and examination rules,

with Class 2 being more stringent than Clas s 3. In many cases in

the area

of

materials and fabrication , Class 2 rules are the same

as

those

of

Class

1.

Where the rules for Class 2 and 3 design are

different, it will be noted clearly in the following discussion.

In 8.1 it was noted that the Code rules assure protection against

violation

of

the pressure boundary as long as all

of

the loads and

conditions to which the component is being subjected are appro

priately defined

in

the Design Specification.

The approach to protection against catastrophic failure

of

the

pressure boundary for Class 2 and 3 piping is quite similar

to

NB-

3600, Class

1

rules. The approach to fatigue protection - that is,

the initiation and propagation of a crack or the propagation

of

an

existing flaw through the wall - is quite different.

When ASME B3

l.7

was first published the rules for design

of

Class 2 and 3 piping referenced B3 l . with a few additional require

ments . When piping rules were first incorporated into Section

III

n

1971

, the same rules for Class 2 and 3 that were in B3 l.7 existed . It

was not until the rules had to be expanded to cover other than nor-

ma operating conditions that significant changes between NC / ND-

3600 and B31.l began to occur. Note that prior to the use

of

Ser

vice Limits (A, B, C, and D), Section III used Operating Conditions

(Normal, Upset, Emergency, and Faulted) .

8.3.1 Protection Against Catastrophic Failure

The NC/ ND-3600 approach to satisfaction

of

protection against

catastrophic failure relies on both design by rule and design by

analysis. The first task is to determine wall thickness and, based

on that thickness, select a schedule size for the piping. This step is

critical in current operating nuclear plants since there can be sig

nificant erosion and erosion-corrosion attack in Class 2 and 3 pip

ing. However, the analyst who

is

satisfying Code design require

ments cannot be the individual making the determination

of

how

to protect against such attack . The thickness equation has an

A

factor that

is

defined in NC-3641.l as follows:

An additional thickness to provide for material removed

in threading, corrosion or erosion allowance, and material

required for structural strength

of

the pipe during erection,

as appropriate.

The portion

of

he definition related to erosion-corrosion requires

a knowledge

of

fluid and material-fluid interaction beyond that

of

most piping designers

or

analysts . Input is required from metallur

gists and fluids experts for the proper protection to be provided. The

information required to make the proper decision should be con

tained in the Design Specification. This is required, although NC-

3613.1, which addresses corrosion or erosion allowance, provides

no more guidance than the A factor definition.

The equations provided in NC/ ND-3641.1 for determining the

minimal wall thickness

of

piping are

PDo

fm

=

2(S

+Py)

+A

(8.23)

tm =

Pd+

2SA + 2yPA

(8.24)

2(S +Py

-

P

These equations are the same as those

in

NB-3641. l for Class I

piping, with one exception: NC/ ND uses S rather than Sm for the

allowable stress . These values are different since the design factor,

based on the ultimate strength of the material, is 3.5 for Class 2

and 3 and 3.0 for Class

1

(Note that this change in design factor

from 4.0 to 3.5 for Class 2 and 3 occurred in 1999.)

Once the minimum thickness is determined, an appropriate pipe

schedule

is

selected. In addition, standard piping products having

specific pressure-temperature ratings that satisfy the system pres

sure and temperature conditions are selected. For these products

listed in Table NC/ ND-3132-I , no minimum thickness calcula

tion

is

required since their acceptability is based on burst testing

to develop the pressure-temperature ratings specified. Class 2 and

3 design rules do not require that short radius elbows manufac

tured in accordance with ANSI Bl6 .28 have an increased thick

ness in the crotch region as do the Class l rules. As pointed out

in 8.2. l for Clas s l piping, there is some question whether this

requirement is necessary since the pressure stress increase on the

intrados

of

the elbow is so localized. In Class 1 it is conservatively

required. For pipe bends, the wall thicknesses after bending must

satisfy the minimum thickness requirements

of

the straight pipe.

Table NC-3642. (

c)-l

is provided as guidance on the w all thick-


ness to be used prior to bending. Note that bending pipe results

in thinning

of

the wall on the extrados but that excess thickness

prior to bending can result in crimping and therefore stress con

centrations in the extrados.

Branch connections manufactured to one of the standards listed

in Table NC/ ND-3132-1 and used within the speci.fied

p ~ e ~ s u r e -

temperature rating are acceptable without s ~ t 1 s f y 1 ~ g mm1_mum

thickness requirements other than that the nommal t h . c ~ n e s s .

is

not

less than the nominal thickness required

of

the adJmmng pipe. A

2 in. or less pipe size coupling or half-coupling is acceptable

as

Jong as the wall thickness meets extra heavy 01: 3,000 lb. nommal

rating. Extruded outlets that are 2 in . in pipe size

or

less, or one

quarter

of

the run pipe diameter, w ~ i c h e v e r is ~ e s s are acceptable

as Jong as the abutting end wall thickness

s a t 1 s f i e ~

the thickness

requirements

of

the branch pipe.

Bra_nch

con.nections not satis

fying the requirements above , includmg f ~ n c t e d branch con

nections, must satisfy the reinforcement reqmrements

of

NC/ ND-

3643. .

These reinforcements rules are essentially the same as those m

Class I and require that the area

of

metal removed for the opening

must be available in a limited distribution area around the open

ing. However, a major difference between Class 2 and 3 .class

l is that reinforcing pads can be used in Class

asme companion guide.pdf

Documents