Allowable Stress in Sour Service Piping 1

[chakaad]

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,

 

[SJones]

Under ISO 15156, the level of outer fibre deformation that triggers the need for thermal stress relief is 5%. However, the wording doesn't deal with welds where localised concentrations can occur. Work at TWI in the UK has shown that the ISO 15156 hardness limits may need to be tightened for carbon steels that are subjected to even very small amounts of plastic strain. Therefore, in terms of a very qualitative sulphide stress cracking risk assessment, the answer to your question would have to be: yes, especially if a weld zone is involved.

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

http://www.linkedin.com/pub/8/83b/b04

 

[ColonelSanders83]

Something is amiss here.

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

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[rneill]

ColonelSanders83,

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.