SIF
SIF
(OP)
Hello !
I would like to know some comments about SIF applications,
In Asme B31.1 to sustained stresses it is indicated to apply the factor 0.75*i on bending moments but in B31.3 I did not find the similar requirement so I thing that is not indicated this and I could develop the analysis with the basic values to bending moments, is this ?
In Asme, the SIF are factors indicateds from fadigue tests but I already did some researchs and the procedures for theses tests (in the past) are a quite uncompletes, for exemple, are not indicates the bending moments applieds or the number of cycles, etc., besides this in a elbow for example, we have others components/stresses besides bending moments. In other words is frequent to get situations where the SIFs are higher compared with a finit element analyses ( except materials faults from manufacturer conditions ).
Thanks
I would like to know some comments about SIF applications,
In Asme B31.1 to sustained stresses it is indicated to apply the factor 0.75*i on bending moments but in B31.3 I did not find the similar requirement so I thing that is not indicated this and I could develop the analysis with the basic values to bending moments, is this ?
In Asme, the SIF are factors indicateds from fadigue tests but I already did some researchs and the procedures for theses tests (in the past) are a quite uncompletes, for exemple, are not indicates the bending moments applieds or the number of cycles, etc., besides this in a elbow for example, we have others components/stresses besides bending moments. In other words is frequent to get situations where the SIFs are higher compared with a finit element analyses ( except materials faults from manufacturer conditions ).
Thanks





RE: SIF
The ASME B31 Code for Pressure Piping, B31.3, Process Piping requires the use of Stress Intensification Factors for the calculation of longitudinal stresses for sustained loadings in the B31 Code Case 178 which was approved on May 6, 2005. Code Case 178 defines SIF's that are to be used in the absence of more applicable data when calculating longitudinal stresses for sustained loadings. If you do not have a copy of Code Case 178 you can go here:
http://www.becht.com/Content.aspx?cid=21&lvl=2
.......and read Dr. Charles Becht's description of Technical Changes in ASME B31.3 for 2004. In this discussion Dr. Becht has included the equation for the calculation of longitudinal stresses for sustained loadings as it is provided in Code Case 178.
It should be noted that SIF's are used in B31.3 ONLY for application to bending stresses and the equations are those for beam bending.
It should be understood that stresses calculated using ASME B31 equations for bending stresses (which mandate the use of B31 SIF's) SHOULD ONLY BE COMPARED TO THE MAXIMUM ALLOWABLE STRESSES (and stress ranges) provided by the applicable B31 section Code. It must be further understood that when ASME B31 Stress Intensification Factors are used with B31 equations the resulting calculated stresses ARE NOT TRUE ELASTIC STRESSES (note that the stress intensifiers provided by ASME B&PV Code, Section III piping stress equations are nearly TWICE the magnitudes of B31 SIF's). Therefore, these B31 calculated stresses cannot be compared to stresses calculated by using other methodologies.
The B31 Codes for Pressure Piping allow more applicable SIF data to be used when it is available. I would however recommend that any such "more applicable" SIF data be documented in the design brief. In May 30, 2008, ASME B31 issued ASME B31J, Standard Test Method for Determining Stress Intensification Factors (i-Factors) for Metallic Piping Components. Reference is also made to several papers by the staff of Paulin Research Group (PRG) regarding the use of FE/Pipe for determining B31 type SIF's.
The current ASME B31 SIF data is based upon testing done at Tube Turns Company in the late 1940's and early 1950's (with a little data incorporated later). If you want to read and understand the way the testing was done and how the fatigue data was incorporated into the ASME B31 Codes you will want to read the classic papers by A.R.C. Markl, "Fatigue Tests of Piping Components" and "Piping-Flexibility Analysis". These papers (and other equally interesting papers) were published in the Tube Turns Company book, "Piping Engineering".
http://rapidshare.com/files/39115048/pipeng.rar
The ASME B31.3 Code for Process Piping provides in Appendix "D" Stress Intensification Factor data for several types of metallic piping components. The Committee is aware of the limitations of this data and is moving ahead to provide more and better data in future issues of the Code.
Regards, John
RE: SIF
S longitudinal = ((Sa + Sb)^2 + (2*St)^2)^0.5
Where:
Sa is the ABSOLUTE VALUE of stresses due to axial loads
Sb is the resultant INTENSIFIED bending stress dues to pressure, weight and other sustained loadings
Sb = ((Ii*Mi)^2 + (Io*Mo)^2)^0.5
Ii is the in-plane SIF and Mi is the in-plane moment
Io is the out-of-plane SIF and Mo is the out-of-plane moment
Z is the section modulus
St is torsional stress due to pressure, weight and other sustained loadings
St is defined as Mt / 2Z, where Mt is the torsional moment and Z is the section modulus
John
RE: SIF
By the way, please understand that for B31.1 or B31.3 in calculating the longitudinal stresses for sustained loadings
you use THE GREATER OF 0.75 times the SIF OR 1.0 times the SIF in calculating the INTENSIFIED bending stress component.
John
RE: SIF
The correct equation for the INTENSIFIED bending stress component of the Code Case 178 equation for the calculation of longitudinal stresses for sustained loadings is:
Sb = ((Ii*Mi)^2 + (Io*Mo)^2)^0.5 / Z
again, where Z is he section modulus
Gotta get a cuppa in me before I post in the future <grin>
John
RE: SIF
To try close the basic idea for sustained stress, B31.1: I should to apply 0.75*i (>=1) on bending moments and B31.3 I should apply 1.0*i on bending moments (like thermal dysplacements stresses !)Is this ?
Making use of this oportunity, Is correct to compare the stresses (sustaneids and by thermal dysplacements) in separeted: Ssustaineds => Sh Sth.dyspl => Sa
but I noticed in some softwares that is frequent to make "a combined analyse" ( Ssust+Sth =>Sa ). This would not be only to nuclear cases ?
Regards
RE: SIF
I am sorry in my posting above I misstated the calculation of the SIF to be applied (I should have been ore careful). For BOTH B31.1 AND B31.3 the bending moments are to be intensified using 0.75*i but never less than 1.0. In other words, the product of 0.75*i shall never be less than 1.0. The bending moment will never be less than 1.0 times the bending moment.
The difference between B31.1 and B31.3 is that B31.3 uses the in-plane SIF to intensify the in-plane moment and the out-of-plane SIF to intensify the out-of-plane moment. B31.1 uses ONLY the out-of-plane SIF to intensify BOTH moments.
Stresses due to sustained loadings are primary stresses and they are compared to the allowable stress at temperature from Appendix A (Sh). Calculated stresses due to sustained loadings must be less than the Code allowable stress at temperature from Appendix A (Sh). This analysis is required by the B31 Codes.
For secondary stresses due to thermal expansion/contraction (displacement) the TOTAL RANGE of stresses must be compared to the Code allowable stress RANGE, SA. The absolute value of the calculated stress at the coldest temperature the pipe will experience must be added to the absolute value of the calculated stress at the hottest temperature the pipe will experience to obtain the total stress range. This total range of stress must be compared to the Code allowable stress range (SA). Note this is a range of stresses and not a single stress state. This analysis is required by the B31 Codes.
Pipe stress analysis software will let you calculate stresses for any combined loading that you want to look at, however there may not be a Code maximum allowable stress to which the calculated stresses can be compared. You will notice that software will calculate the W + P + T combination (operating) but there is NO B31 MAXIMUM ALLOWABLE Stress for this loading case. The software will also calculate the W + P loading case and for this there is a B31 Code maximum allowable stress and it is Sh. Calculated W + P loading case stresses must be less than Sh. The software will subtract the moments from the W + P case from the moments from the W + P + T case and that will leave the moments from which to calculate thermal stress that can be compared to the Code maximum allowable stress range (SA).
Note that in the calculation of stresses (and stress ranges) the B31 Codes currently require the use of the modulus of elasticity at 70 degrees F (21 degrees C). If you wanted to evaluate the LOADINGS on the equipment to which the piping is attached you might want to run a case where the modulus of elasticity AT THE OPERATING TEMPERATURE is used and where the calculated thermal (displacement) will reflect only the range from the installed temperature to the operating temperature. This will give you more accurate loads (forces and moments) on the equipment at temperature, however the stresses calculated during this run cannot be validly compared to any allowable stress limit because there is no such allowable stress.
The analyst should ignore B31.3 Appendix P until all the concepts above are completely understood.
Regards, John
RE: SIF
It has been very useful your help !
I would like to make one more question: To some cases, I could not use the specifics SIF predicted in Asme code and to present a FEA analyse eventually associated with fadigue check ( at least a compare with SxN curve )showing the local stress level below the allowable stress. This would have acceptation or cover by codes and piping technical rules ?
Thanks
RE: SIF
I agree with John Breen.
RE: SIF
Technical Reference: PVP2008-Hinnant_and_Paulin.pdf Page 38 see attach.
Regards
Leonard Stephen Thill