Query on API 579-1 - Part 3 Brittle Fracture - Level 2 assessment

[Carlos121]

Hi, I have a question on API 579-1-Part 3 and hope someone can answer this to me:

On clause 3.4.3.1 Step 3 (Determining Stress Ratio, Rts , there are 3 equations (Thickness basis, Stress Basis and Pressure-Temperature Rating Basis). When I calculate each of those three, I will get three different values i.e. Rts(thickness basis) = 0.53 , Rts(Stress basis) = 0.887, Rts(Pressure-Temperature Rating Basis basis) = 0.707 .
Which of those values I need to select for Step 4 determining TR assuming Rts threshold is 0.3 ?

 

[BJI]

If you have a component covered by a pressure-temperature rating, like a flange, then you would use this P-T equation. Otherwise you could use either the thickness or the stress equation, whichever is convenient. If you think about it they should give you the same result, maybe check your calculation. You also need to include general primary membrane stresses caused by thermal loads, such as restrained thermal expansion causing net section bending stress on nozzle necks. So depending on what you are checking it may be easier to use one equation or the other.

 

[BJI]

At a flange to pipe weld you should choose whatever is limiting.