PWHT effect on vessel stability 2

[ssylvest]

We are trying to confirm the stability of a vessel made with SA-516-70N which will have girth welds PWHT to 1200 deg F. ASME Section II, Appendix 3 give some insight on calculating allowable stresses at various temperatures. At higher temperatures, it seems to be governed by the Creep/Rupture properties. But since the PWHT is very short term (about 2 hours holding, or total of only about 6 hour above 800 deg F), would it be safe to ignore creep/rupture issues? The maximum actual axial compressive stress is only about 600 psi. But the available data for yield, E, etc., do not go out to 1200 degrees. So it is not clear how to definitively verify whether it will be okay.

 

[metengr]

Thatr is correct, you really don't use allowable stresses for this type of analysis. What you need for buckling analysis is elevated temperature tensile strength data versus creep/stress rupture data.

I would suggest you review Section II, Part D, Table Y-1 and U for this material. The tables only go to 1000 deg F, but you can extrapolate the 1200 deg F values and use these values for your buckling analysis under PWHT, unless you find published elevated temperature tensile data.

 

[TGS4]

As metengr says, don't use the allowable stresses. However, creep buckling is a serious concern and should be addressed by someone experienced in this type of work.

I recommend reviewing an old paper by Griffin (1974) and ASME Section III, Subsection NH for creep buckling. I use isochronous stress-strain curves that use MPC Project Omega data, per ASME FFS-1/API 579 when I perform this type of evaluation.