why yield stress is used as allowable in B31.3 chapter IX?
why yield stress is used as allowable in B31.3 chapter IX?
(OP)
hi,
In B31.3, Appendix-A is providing allowable stress based on 1/3 of tensile stress or 2/3 of yield stress, whichever is lower.
why Appendix-K for chapter IX piping is considering 2/3 yield stress is allowable stress? Any specific reason?
Kunal01
In B31.3, Appendix-A is providing allowable stress based on 1/3 of tensile stress or 2/3 of yield stress, whichever is lower.
why Appendix-K for chapter IX piping is considering 2/3 yield stress is allowable stress? Any specific reason?
Kunal01





RE: why yield stress is used as allowable in B31.3 chapter IX?
RE: why yield stress is used as allowable in B31.3 chapter IX?
you are correct. both are different codes.
some specific reason will be there for considering only 2/3 of Yield strength. i am looking for that reason.
kunal 01
RE: why yield stress is used as allowable in B31.3 chapter IX?
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: why yield stress is used as allowable in B31.3 chapter IX?
could you explain me further? I am not getting the message.
kunal 01
RE: why yield stress is used as allowable in B31.3 chapter IX?
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: why yield stress is used as allowable in B31.3 chapter IX?
Here's a link to the thread: http://www.eng-tips.com/viewthread.cfm?qid=320575
Here's a (not so brief) excerpt:
The benefits of using Annex I are encountered when we begin to use higher strength materials (i.e. CSA Gr. 550 Q&T material; ~API 5L X80).
From mechanical test it has been shown that at 344 deg. C YS of Gr. 550 Q&T pipe = 520 MPa. Minimum TS at both room temp. and 344 deg. C are the same, TS = 645 MPa. (This is the key point!!)
As you can see, this thermo-mechanically control processed (TMCP) pipe maintains a relatively high YS at elevated temperatures which is why Annex I (i.e. design using 2/3 YS) is used in these instances to significantly reduce material costs (savings in 1000's of tons on a pipeline).
Example:
CSA Gr. 550 Q&T: WT Calculations using CSA Z662 Clause 14 vs. Annex I
• Ratio of Elevated Temp. YS : Room. Temp. YS = 520 MPa/550 MPa = 95%
• 2/3*Elevated YS = (2/3)*520 MPa= 346.7MPa (used to calc. WT using Annex I) – slightly more conservative WT formula but much higher allowable stress. Therefore smaller minimum acceptable WT calculated.
• 1/3*Elevated TS = (1/3)*645 MPa= 215 MPa (used to calc. WT using Clause 14) – slightly less conservative WT formula but much lower allowable stress. Therefore larger minimum acceptable WT calculated.
Whereas, a low yield strength ASTM material (i.e. A106-B) does not maintain as good a YS at elevated temperatures.
ASTM A106-B: WT Calculations using CSA Z662 Clause 14 vs. Annex I
• Ratio of Elevated Temp. YS : Room Temp. YS = 179MPa/241 MPa = 74%
• 2/3*Elevated YS = (2/3)*179 MPa= 119.3 MPa (used to calc. WT using Annex I) – slightly more conservative WT formula with approximately the same allowable stress. Therefore slightly larger minimum acceptable WT calculated. Also note that for low yield strength ASTM materials like A106-B, 2/3 elevated temp. YS is approximately equal to 1/3 elevated temp. tensile (119.3 MPa vs. 117 MPs).
• 1/3*Elevated TS = 117 MPa (B31.3, Table A-1) (used to calc. WT using Clause 14) – slightly less conservative WT formula with approximately the same allowable stress. Therefore slightly smaller minimum acceptable WT calculated.
Good luck!
K.D.W.
RE: why yield stress is used as allowable in B31.3 chapter IX?
Auzie5,
you mean that Chapter IX method of calculation will be highly advantageous for the material having high yield strength. Am I correct?
regards,
kunal01