Annex A API 650 Thickness formula

[inammanj123]

Hello everyone,

I have a question regarding the value of 145 used in thickness formula in Annex A of API 650.
In the thickness formula of annex-A of API 650, a constant value, of 145 mPa, is used instead of allowable stress of a particular material. For material, like A-36, which has allowable stress more than 145 mPa, this approach is understandable, but what about material like 283 Gr C, because allowable stress of 283 gr C is 137 mPa, WHICH IS LESS THAN 145.[highlight #FCE94F]My question is should I use 137 instead of 145 in the Annex-A thickness formula or not?[/highlight]

Regards,
Inam

 

[JStephen]

I believe the same allowable stress is to be used with all materials.

 

[IFRs]

JStephen is correct

A.1.3 This Annex is applicable to any of the Section 4 materials, although the single allowable stress does not provide any advantage to higher strength steels.

However, that does not address the OP's question - why is the Annex A allowable stress greater than the tabulated product allowable stress for A283 material? Is it just that Annex A is so conservative that the 137 vs 145 is essentially rounding error?

 

[JStephen]

I assume the actual stress level would be lower, since you'd still have an 85% maximum joint efficiency.
Otherwise, just a quirk in the code generation process.

 

[inammanj123]

But what if Joint efficiency used is 1. In that case required thickness will come out to less by using 145 instead of 137 mpa

 

[JStephen]

But Annex A uses a joint efficiency of either 0.70 or 0.85, not 1.

 

[inammanj123]

OH Yes my mistake.