API530 x ASMEB31.3

Dear Srs.,

Which allowable must be used in a heater tube (coil) stress analysis?

Considering the material A106 Gr.B and a temperature equal to 950ºF, we have the following allowables:

API 530: 89MPa (figure F.2 - curve 5)
ASME B31.3: 31MPa

Why is there a large difference between the codes?

Thank you.

rliberato,

The API 530 standard is heater tube design, and the 89 MPa refers to the rupture stress, which is only one step of the heater design.

The ASME B31.3 Process piping code Table A-2 has the 31 MPa to include in calculations for bending and for fatigue life basic 7,000 cycles to accomodate thermal stresses from expansion, which is only part of the piping design.

These are different types of stresses and failure modes - primary vs secondary - as well as including different design factors for allowable vs yield vs tensile.

See ASME B31.3 Table A-2 Note (57) about caution for carbon steel used above 800 deg F. , which is reason most piping designers would not have carbon steel for use at 950 deg F.