Allowable Stress in Sour Service Piping
Allowable Stress in Sour Service Piping
(OP)
Hi,
In sour service piping some steel grades need to be heat treated after welding and any cold work (such as bending). During stress analysis of some sour service piping we have reached a relatively high stresses in the loop bends. Although the code stress remain within the allowable range but there are some local yields at the corner of bends. This local yields are permitted by B31.3. But the local yield will be kind of cold work (that should be heat treated) that may cause corrosion and cracking. Do we have to reduce the local stresses to a level below yield stress?
Thanks,
In sour service piping some steel grades need to be heat treated after welding and any cold work (such as bending). During stress analysis of some sour service piping we have reached a relatively high stresses in the loop bends. Although the code stress remain within the allowable range but there are some local yields at the corner of bends. This local yields are permitted by B31.3. But the local yield will be kind of cold work (that should be heat treated) that may cause corrosion and cracking. Do we have to reduce the local stresses to a level below yield stress?
Thanks,





RE: Allowable Stress in Sour Service Piping
Steve Jones
Materials & Corrosion Engineer
http://www.oilandgaspeople.com/cv/11499664
http://www.linkedin.com/pub/8/83b/b04
RE: Allowable Stress in Sour Service Piping
1) If the code stresses are below the allowable, you should be nowhere even remotly close to yielding even in the highly stressed bends in an expansion loop.
2) Yielding is only permissable while forming, nowhere that I have read in the code is there an allowance to yield under operation (If i have missed this allowance please quote chapter and verse).
Just my two cents worth
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RE: Allowable Stress in Sour Service Piping
The allowable displacement stress range in B31.3 (Eqn 1a) is greater than the normal code allowable stress value for use in evaluating pressure or longitudinal stresses.
Eqn 1a is "Sa = f (1.25 Sc + 0.25 Sh)" and if we envision a case where Sc = Sh (say at near ambient temperatures) and where f = 1.0 (less than 7000 cycles) then Sa = 1.5 Sc which means that Sa might equal the SMYS. Eqn 1b is even less conservative in that it permits some of the "unused" longitudinal stress allowable to be added to the permitted Sa value - consequently, Sa can exceed yield.
Displacement stresses are self limiting and so the code permits these to be much higher than the allowable stress permitted for primary stresses. In fact, it is not unheard of for the allowable displacement stress range to be in excess of the yield strength of the material at temperature. Code does expect and allow that you may have local yielding at locations such as expansion loops.
This topic is discussed quite well in Chapter 3 of the book "Practical Guide to ASME B31.3" by Woods & Baguley.