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Design safety factor - ASME -1 vs. 2 4
- Thread starter amiragha
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First, in the ASME Booler and Pressure Vessel Codes, the Section numbers are provided in Roman numerals, with the Divisin numbers provided in Arabic numerals. So, your question should be about Section VIII, Divisions 1 and 2.
Second, there is no single Design Margin for each Code. (There is no such thing as a safety factor). Each Division has different rules about how the allowable stresses are calculated. Division 1 uses the lower of 1/1.5 times the yield stress at temperature or 1/3.5 times the UTS, provided that you are not in the creep regime, in which case creep-based criteria apply. Division 2 uses the lower of 1/1.5 times the yield stress at temperature or 1/2.4 times the UTS, again provided that you are not in the creep regime, in which case creep-based criteria would apply.
However, an allowable stress only tells part of the story. One can only have a Design Margin against a particular failure mode. Part 5 of Division 2 discusses some of the failure modes. The above-noted margins used to calculate the allowable stresses are applicable to the Plastic Collapse failure mode. However, for localized plastic collapse, such as those that would generate local primary membrane stress, the design margin may only be 1.0. Likewise, there are various design margins for local failure/triaxiality, buckling failure, ratcheting and fatigue.
So, does that answer your question?
Second, there is no single Design Margin for each Code. (There is no such thing as a safety factor). Each Division has different rules about how the allowable stresses are calculated. Division 1 uses the lower of 1/1.5 times the yield stress at temperature or 1/3.5 times the UTS, provided that you are not in the creep regime, in which case creep-based criteria apply. Division 2 uses the lower of 1/1.5 times the yield stress at temperature or 1/2.4 times the UTS, again provided that you are not in the creep regime, in which case creep-based criteria would apply.
However, an allowable stress only tells part of the story. One can only have a Design Margin against a particular failure mode. Part 5 of Division 2 discusses some of the failure modes. The above-noted margins used to calculate the allowable stresses are applicable to the Plastic Collapse failure mode. However, for localized plastic collapse, such as those that would generate local primary membrane stress, the design margin may only be 1.0. Likewise, there are various design margins for local failure/triaxiality, buckling failure, ratcheting and fatigue.
So, does that answer your question?
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ASME Section VIII Div 2 was first published to provide alternative rules for construction of pressure vessels with additional and more stringent design, construction, inspection, operation and other requirements.
Section VIII div. 1 uses Maximum normal stress theory while Section VIII Div. 2 was based on Maximum shear stress theory.
In 2007 however ASME Div. 2 was changed to use the maximum distortion energy theory (von Mises criteria).
Div 1 is uses a design by rule criteria and Div 2 utilises more detailed (specific) design for certain conditions and is design by analysis with more detailed calculations. One must have an understanding of the subtle differences between the two namely in testing criteria, the experimental analysis behind the formulae and more importantly any specified testing requirements e.g NDE.
Because of the higher detail in Div 2 and more rigorous calculations/testing requirements it generally gives a better estimate of the 'real' stress's and allows for leaner designs with thinner walls.
some useful links summarising the differences and histories of the codes:
There are also many other forum threads on this.
Section VIII div. 1 uses Maximum normal stress theory while Section VIII Div. 2 was based on Maximum shear stress theory.
In 2007 however ASME Div. 2 was changed to use the maximum distortion energy theory (von Mises criteria).
Div 1 is uses a design by rule criteria and Div 2 utilises more detailed (specific) design for certain conditions and is design by analysis with more detailed calculations. One must have an understanding of the subtle differences between the two namely in testing criteria, the experimental analysis behind the formulae and more importantly any specified testing requirements e.g NDE.
Because of the higher detail in Div 2 and more rigorous calculations/testing requirements it generally gives a better estimate of the 'real' stress's and allows for leaner designs with thinner walls.
some useful links summarising the differences and histories of the codes:
There are also many other forum threads on this.
nick262b - some of the things that you said are not quite correct.
Unfortunately, your second link is so horribly out-dated (its from Oct 2000 and refers to a prior Edition of the Division 2 Code) that most of it is no longer valid. The first link is also out of date and is in the process of being revised.
In my opinion, the reason for the lower margin against ultimate is justified by the use of the Users Design Specification (UDS), which requires the user to both communicate the design conditions and information regarding the operation of the vessel, which also requires the user to steward the operation of the vessel to the design specification. There is no mandatory complement in Division 1 (Appendix KK is non-mandatory) and therefore a significant portion of the additional margin against ultimate becomes an ignorance factor against how the vessel will be operated.
While both of these statements are true for the Design By Analysis portions of the old and new Div. 2, neither are correct for the Design By Rules. Part 4, specifically 4.3 uses the maximum normal stress theory - identical to Div. 1. However, for combined loadings, 4.3.10, that uses von Mises (in the 2011 Addenda - however earlier Editions post-2007 used maximum shear stress theory).
Unfortunately, your second link is so horribly out-dated (its from Oct 2000 and refers to a prior Edition of the Division 2 Code) that most of it is no longer valid. The first link is also out of date and is in the process of being revised.
While this used to be true, with the addition of non-unity weld joint efficiency factors into the new Div. 2 it is becoming less true.
In my opinion, the reason for the lower margin against ultimate is justified by the use of the Users Design Specification (UDS), which requires the user to both communicate the design conditions and information regarding the operation of the vessel, which also requires the user to steward the operation of the vessel to the design specification. There is no mandatory complement in Division 1 (Appendix KK is non-mandatory) and therefore a significant portion of the additional margin against ultimate becomes an ignorance factor against how the vessel will be operated.
Could the difference between the material properties on the material certificate and and the Properties specified in Section II be considered a Safety Margin?
Many of the properties specified in Section II cover all thicknesses of a material. However typically thinner plate has higher yeild stress than thicker plate.
So if the yeild stress in Section II is the determining factor (rather than Ultimate Stress which tends to stay constant for all thicknesses) for calculating the allowable design stress, than thinner plate will tend to have some safety margin.
Many of the properties specified in Section II cover all thicknesses of a material. However typically thinner plate has higher yeild stress than thicker plate.
So if the yeild stress in Section II is the determining factor (rather than Ultimate Stress which tends to stay constant for all thicknesses) for calculating the allowable design stress, than thinner plate will tend to have some safety margin.
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