STRESS ITENSIFICATION FACTORS STRESS ITENSIFICATION FACTORS

S I FS I F• SIF: SIF is define as the ratio of the maximum stress intensity to the normal stress,

calculated by the ordinary formula of mechanics. It is used as a safety factor to account for the effect of localized stress on piping under a repetitive loading. In piping design this factor is applied to weld fittings branch connection, and other piping components where stress concentrations and possible fatigue failure might occur. Usually experimental methods are used to determine these factors.

In some cases different editions of the same code provide different SIF for a given component.

The way that the SIF are applied to moment loading is also different for different codes.

The B31.1 and ASME section iii codes require that the same SIF be applied to all the three directional moments while the B31.3,B31.4,B31.5and B31.8 require that different SIF be applied to the in plane and out plane moments with no SIF required for torsion.

SIF: A stress intensification factor is a multiplier on nominal stress for typically bend

and intersection components so that the effect of geometry and welding can be

considered in a beam analysis. Stress Intensification Factors (SIFs) form the basis of

most stress analysis of piping systems. The stress intensification factor is used in a

pipe stress analysis as shown in the equation below:

( Beam Stress) (SIF) < (Allowable Stress)

Expanding the terms gives the following equation using the SIF:

(M/Z)(SIF) < 6N-0.2(1.25(Sc + Sh) – Sl)

Why we use S I F.Why we use S I F.

• Classical pipe stress analysis programs use piping beam elements that are

constrained to the limitations inherent with beam elements. For example, beam

elements do not capture the effect of ovalization where the cross section of the pipe

deforms away from the unloaded round geometry. To compensate for the limitations,

correction factors are applied to the beam models to more closely match the flexibility

and stresses of the pipe to reality. The correction factors, flexibilities and stress

intensification factors (SIFs), are based on analytical and empirical relations

correlated to piping component geometries. The accuracy of the correlations dependson the geometries of the piping components.

The geometrical parameter h characterizes the ability to ovalize of an elbow.

But end effect may inhibit ovalization of the cross –section. Moreover pressure ,

by inducing a stabilizing term in the rigidity matrix decreases both flexibility and

maximal stress of the bend. These two effects will be the principal matter of study.

In short we can say that for elbow adjacent straight parts and internal pressure inhibitovalization of the cross section .

Table D3001 Flexibility Factor, Table D3001 Flexibility Factor, kk, and Stress Intensification Factor, , and Stress Intensification Factor, i i (Cont(Cont’’dd

MOMENT IN BENDSMOMENT IN BENDS

MOMENTS IN BRANCH CONNECTIONMOMENTS IN BRANCH CONNECTION

B31.3 SUSTAINED SIF MULTIPLIERB31.3 SUSTAINED SIF MULTIPLIER

• B31.3 CODE Interpretation 1-34 dated February 23,1981 file;1470-1 states that

for sustained and occasional loads an SIF of .75i, but not less than 1.0 may

be used. This setup directive allow the user to enter their co-efficient the

default is 1.0. to comply with this interpretation the user would interpretation

the user would enter 0.75. b31.3 code interpretation 6-03 dated December 14,

1987 permitted users to ignore the stress intensification for sustained and

occasional loads. To comply with this interpretation, the user would enter 0.0.

STRESS INTENSIFICATION FACTORS DETAILSSTRESS INTENSIFICATION FACTORS DETAILS

Stress intensification factors are calculated automatically for bends and defined intersection as specified by the applicable piping code.

The user may enter specific stress intensification factor for any point in the piping system by activating the SIF and tees check box on the piping spread sheet. The node number where the stress is to be intensified is entered in the first available node field, and the in plane and out plane stress intensification factors are entered in the SIF(i) and the SIF(O) fields, respectively. The only exception is that users cannot specify SIFs for bend elements (unless the allow users bend SIF directive is activated in the configuration file) code defined SIFs always apply.

CAESAR II will not allow user- defined stress intensification factors to be less than 1.0.

The node to be intensified must be the TO or the FROM node on the current element.

CAESAR II will not calculate stress intensification factors for intersection having more than three piping framing into it.

USER BEND SIFUSER BEND SIF

• The user is not permitted to override code –calculated stress intensification factors for bend element (unless the user bend SIF directive is activated in the configuration file).

This characteristic allows the user to apply code- calculated intersection stress intensification factors to dummy legs without disturbing the normal bend stress intensification factors. The node on dummy leg that is also on the bend curvature, is defined as an intersection on the intersection SIF scratchpad. The intersection stress intensification factors will be calculated and can be applied to the dummy leg end that connects to the bend. Bend stress intensification factors are unchanged.

Stress intensification factors can be calculated for intersection having one, two, or three pipes framing into it. Where two pipes from a partial intersection ,CAESAR II assume that the larger pipe is the header and smaller the branch.

SIF SCRATCHPADSIF SCRATCHPAD

The stress intensification factors calculated by CAESARII can be viewed interactively

from the pipe spreadsheet by selecting either the KAUX- Review SIFs at intersection

node menu item or the KAUX- REVIEW SIFs at bend node menu item. One of the

following SIF SCRATCHPADS will appear after typing in the node number to review

when prompted. Note that the node must be a valid bend node when reviewing SIF at

bends. At this point the user may interactively change any of the spreadsheet data and

recalculate the SIF. This feature allows the user to see the effect that changing

geometrics and properties have on code stress intensification factors.

CLASS 1 BRANCH FLEXIBILITYCLASS 1 BRANCH FLEXIBILITY

Activate the class 1 flexibility calculation. The appearance of this parameter in the setup file will completely change the modeling of intersection in the analysis. For intersection not satisfying the reduced branch rules that d/D<.5 AND THAT D/T<100, the branch will start at the surface of header pipe. A perfectly rigid junction between the centerline of the header and surface will be formed automatically by CAESARII USING ELEMENT OFFSET CALCULATIONS.SIF acts at the surface point for the branch. When the reduced branch rule are satisfied, the local flexibility of the header is also inserted at this surface point. Intersection not satisfying the reduced intersection rule will stiffer and carrymore load, while intersection satisfying the reduced intersection rule will be more flexible

and will carry less load.

The offset necessary to form the class 1 intersection are automatically generated by Caesar ii. There is no need to extra input required by the user to have Caesar II. Build these intersection.

Note- when a bend curved element is part of an intersection model, the offset and flexibility calculation will not be performed.

SIF FOR REDUCERSSIF FOR REDUCERS

Even though the reducer is a standard fitting and the formula for SIF is available in the codes ,the cone angle is not readily available. the following SIF formula from the code

can be used for large –bore concentric and eccentric reducers, using data ,the following expression can be used for the cone angle @.

i = 0.5 + 0.01@(d2/t2)^2 for 1.0<i<2.0

@1=(80.4*D/d)-62.9 for 1<D/d<1.3

@1=(28.3*D/d) +2.9 for D/d>1.3

@ = @1/2 for concentric reducers.

@= @1 for eccentric reducers.

where @=cone angle in degree

D, d = mean diameter of large and small ends of reducer respectively.

d2,t2= outside diameter and thickness of small end of reducer, respectively.

SIF APPROHSIF APPROH

In this method ,the local stresses are not evaluated directly but are indirectly accounted for by applying a SIF to the general piping stresses.

The SIF approach has the following limitation. applicable to some specific configuration Only not applicable to irregular shape.

The SIF approach is applicable in the following situations.

360 FULL WRAPPER PLATE– This configuration is no longer a load stress problem.

A SIF of 2.1 or 1.3 can be applied depending on the applicable code.

180 WRAPPER PLATES - THE FOLLOWING SIF ARE RECOMMENDED BY RODABAUGH.

I = 4.2 for the run pipe torisional moment component

I = 2.1 for the run pipe out of plane bending moment component

I = 1.3 for the run pipe in plane bending moment component

USE SCHNEIDERUSE SCHNEIDER

This directive activates the Schneider reduced intersection assumption. It was because of

observation by Schneider that much of the work on WRC 329 WAS STARTED.

Schneider pointed out that the code SIFs could be in error when the d/D ratio at the

intersection was less than 1.0 and greater than .5 .In this d/D range of the SIFs could be

in error by a factor as high as 2.0. Using the SCHNEIDER option in CAESAR II results in

a multiplication of the out of plane branch stress intersection by a number between .5 and

1 for B31.3 and other codes that do not differentiate between IN and OUT of plane SIFs.

The multiplication will be used for the single stress intensification given.

ADD TORSION IN STRESSADD TORSION IN STRESS

Some piping code include torsion in the sustained and occasional stress by explicitly

including it in the stress equation (i.e b31.1),and some don’t include torsion in the

sustained and occasional stresses by implicitly calling for “longitudinal stresses” only

(i.e b 31.3). Setting the add torsion in SL stress directive CAESAR II to include the torsion

term in those codes that don’t include it already by default.

In sustained analysis of a very hot piping system subject to creep, it is recommended that

the user include torsion in the sustained stress calculation via this parameter in the setup

file.