ASME VIII-2 2004 & 2007 safety factor 3 & 2.4

[yudhichen]

Dear ASME experts,

I need some enlightment regarding safety factor (SF) of ASME viii-2 which change from '3' to '2.4'.

Initially, i saw ASME viii-2 2004 edition that refers to ASME ii-D mandatory appendix 2,
which i thought coefficient '3' was mentioned on tensile strength limit St/'3' at
Table 2.100(a) under "design stress intensity value for wrought, cast, ferrous & non-ferrous".

i heard the SF value was change since 2007 edition to '2.4' instead of '3'.
apparently, when i look into ASME viii-2 2007 edition, ASME change the structure how it was written
and it seems they change the term 'design intensity' as mentioned in 2004.
as i look ASME ii-D 2007 edition in the same way as i found Table 2.100(a), i still found the same table in 2007.

Does anyone can enlight me how and at what passage i could found this coefficient at both edition (2004 & 2007) apple-to-apple? I'm not familiar with ASME code but i'm curious with this change SF value since this particular ASME code was cited on API 6A for wellhead structure design.

Thank you

 

[TGS4]

First off, let's get the semantics out of the way. There is no such thing as a safety factor. What you are referring to is the Design Margin against ultimate. In determining the allowable stress, there is also a design margin of 1.5 against yield.

Furthermore, the 2004 Edition used the Tresca stress criteria (hence stress intensity) whereas the 2007 Edition uses maximum principal stress for much of the Design By Rules and von Mises (equivalent stress) for some of the Design By Rules and all of the Design By Analysis.

The allowable stress tables in Section II, Part D are also used by other sections in the Boiler and Pressure Vessel Code. So, Table 2 was used for VIII-2 and III, and is used only for III. Now, Table 5 is used for VIII-2.

The current design margins for VIII-2 are found in Appendix 10 of II-D.

What calculation or analysis techniques are you using to evaluate your API 6A wellhead?

 

[yudhichen]

Dear TGS4,

Thank you for the correction and some enlightment.
at this moment, i dont have any specific calculation that i want to finish till i come across with ASME.

In one of API 6A clause, it was mention that ASME method could be used as a method to qualify the design.
One time, i was told that in this ASME method for API 6A, ASME calculate the SF (in which you corrected as design margin) as 2.4.

I just got a copy of ASME 2007 edition from my colleague and I'm curious to know more about the difference how it will benefit me in case i need to use ASME method, while API 6A cited specifically for 2004 ed with 2005-2006 addenda (in this case, supposed to be the design margin still 3).

Thanks

 

[TGS4]

If you are going to be doing design by analysis, I would recommend using the latest edition. The older editions had issues that were fixed in the newer editions.

The reason that I use the term design margin against a particular failure mode is that, especially in Design By Analysis there are many different failure modes with different design margin for each failure mode.