why yield stress is used as allowable in B31.3 chapter IX? 1

[kunal01]

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

 

[EnergyMix]

Because they're two different codes looking at two different things.

 

[kunal01]

hi,

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

 

[BigInch]

Any more than that will cut into the safety factor needed to ensure that the material will sustain the required design pressure, given that the material has been manufactured and fabricated within all permitted tolerance.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[kunal01]

hi BigInch,

could you explain me further? I am not getting the message.

kunal 01

 

[BigInch]

Not safe. A certain amount of safety factor (33% fat) is needed to cover the maximum impact of possible defects in fabrication and workmanship. The material standards are developed with the intent to discover defects within the allowable tolerance of those defects and fabrication tolerances. For example, pipe might be allowed to be produced within standard with a wall thickness of 12.5% less than the actual specified value. In that case a tension load on the pipe calculated using the actual area of steel supplied, mighht be 2% higher than what the engineer was thinking when he designed it. If there is 33% extra from allowable stress to yield point, he would still have a 31% margin of safety. If there were some other similar deviation from actual specified value, say one pipe in the lot was supplied with 99% of SMYS, another 1% would be deducted from the safety margin, which would push that to only a 30% safety factor remaining. A small corrosion pit might reduce it further. etc. However a margin of 33% should be enough to cover as many of the possible undetected deficiencies as necessary to still maintain a safe design.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[auzie5]

I dealt with this issue when addressing B31.3 Chapter IX WT calc. (similar methodology to CSA Z662 Annex I) vs CSA Z662 Clause 14.

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.

 

[kunal01]

Thanks BigInch & Auzie5.

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